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Alternative titles; symbolsP53TRANSFORMATION-RELATED PROTEIN 53; TRP53HGNC Approved Gene Symbol: TP53Cytogenetic location: 17p13.1 Genomic coordinates (GRCh3...

Alternative titles; symbols

  • P53

HGNC Approved Gene Symbol: TP53

Cytogenetic location: 17p13.1 Genomic coordinates (GRCh38): 17:7,668,420-7,687,489 (from NCBI)

▼ Description
The transcription factor p53 responds to diverse cellular stresses to regulate target genes that induce cell cycle arrest, apoptosis, senescence, DNA repair, or changes in metabolism. In addition, p53 appears to induce apoptosis through nontranscriptional cytoplasmic processes. In unstressed cells, p53 is kept inactive essentially through the actions of the ubiquitin ligase MDM2 (164785), which inhibits p53 transcriptional activity and ubiquitinates p53 to promote its degradation. Numerous posttranslational modifications modulate p53 activity, most notably phosphorylation and acetylation. Several less abundant p53 isoforms also modulate p53 activity. Activity of p53 is ubiquitously lost in human cancer either by mutation of the p53 gene itself or by loss of cell signaling upstream or downstream of p53 (Toledo and Wahl, 2006; Bourdon, 2007; Vousden and Lane, 2007).

▼ Cloning and Expression
Vogelstein and Kinzler (1994) stated that the central region (amino acids 100 to 300, approximately) of the 393-amino acid p53 protein contains the DNA-binding domain. This proteolysis-resistant core is flanked by a C-terminal end mediating oligomerization and an N-terminal end containing a strong transcription activation signal.

Yin et al. (2002) found that MDM2 (164785) induced translation of p53 mRNA from 2 alternative initiation sites. Translation from the second site resulted in an N-terminally truncated protein with an apparent molecular mass of 47 kD that the authors designated p53/47. The p53/47 isoform lacks the MDM2-binding site and the most N-terminal transcriptional activation domain of full-length p53.

Bourdon et al. (2005) showed that the p53 gene has a complex transcriptional expression pattern encoding different p53 mRNA variants through the use of alternative splicing and an internal promoter in intron 4. The C terminus can be alternatively spliced to produce 3 isoforms, p53, p53-beta, and p53-gamma, where the last 2 isoforms lack the oligomerization domain. The alternative promoter leads to expression of an N-terminally truncated protein (del133p53) initiated at codon 133. RT-PCR detected the full-length p53 transcript in all normal human tissues examined, and 5 variants were expressed in a tissue-specific manner. All variants tested were translated in transfected cells, and the proteins had apparent molecular masses of 28 to 53 kD. Western blot analysis detected endogenous p53 isoforms of 28 and 45 kD in an osteosarcoma cell line. The 6 isoforms also showed distinct subcellular localizations following transfection.

Using RT-PCR, Nikoshkov and Hurd (2006) identified 8 novel p53 transcripts in human brain regions. Almost all alternative splice events occurred due to atypical splicing of direct repeats at splice sites. The pattern of p53 splicing was specific for brain areas and for individuals. In contrast to brain, human kidney and heart expressed only full-length p53.

Reisman et al. (1996) cloned a cDNA representing an mRNA apparently initiated from a second promoter located in intron 1 of the p53 gene. They designated the gene encoding the transcript HP53INT1. The cDNA was polyadenylated downstream from a consensus poly(A) addition site and was derived entirely from intron 1 of the p53 gene. Reisman et al. (1996) concluded that HP53INT1 may be a pseudogene. Alternatively, they suggested that it may have a function, since transcripts were present in human cells and their levels were induced during terminal differentiation of myeloid leukemia cells.

▼ Gene Structure
Reisman et al. (1988) identified 2 promoters in the p53 gene. The first is located 100 to 250 bp upstream of the noncoding first exon, and the second, a stronger promoter, is located within the first intron.

Bourdon et al. (2005) stated that the TP53 gene contains 11 exons. It has 2 transcriptional start sites in exon 1, and alternative splicing occurs in intron 2 and between exons 9 and 10. The gene also contains an internal promoter and transcription initiation site in intron 4.

▼ Mapping
By analyzing man-rodent hybrid cells, McBride et al. (1985, 1986) mapped the P53 gene to chromosome 17. They regionalized the gene to chromosome 17p13 using hybrids with a chromosome 17 translocation and in situ hybridization. By somatic cell hybrid analysis, Benchimol et al. (1985) also assigned the P53 gene to the short arm of chromosome 17. Isobe et al. (1986) assigned the TP53 gene to chromosome 17p13.

By in situ hybridization with a mouse DNA probe, Le Beau et al. (1985) mapped the human P53 gene to chromosome 17q21-q22. Subsequently, this group concluded that the TP53 gene is on the short arm (Rowley, 1986).

By Southern filter hybridization of DNAs from human-rodent hybrids, Miller et al. (1986) localized the P53 gene to chromosome 17p. They suggested that the use of the mouse gene as the probe in the work of Le Beau et al. (1985) may have been responsible for their inaccurate results, since the murine and human genes are not completely homologous.

Somatic cell hybrid studies by vanTuinen and Ledbetter (1987) narrowed the assignment of the TP53 gene to chromosome 17p13.105-p12.

▼ Gene Function

Levine et al. (1991) reviewed p53 function and how alteration or inactivation of p53 by mutation or by interaction with oncogene products of DNA tumor viruses can lead to cancer.

Vogelstein and Kinzler (1992) reviewed function and dysfunction of the p53 gene and outlined 5 mechanisms for p53 inactivation, including disruption of its negative regulator, MDM2 (164785).

Science magazine designated p53 the 'Molecule of the Year' for 1993. Culotta and Koshland (1993) and Harris (1993) gave an extensive account of its discovery and elucidation of function, as well as the use of p53 in cancer risk assessment.

Harris and Hollstein (1993) reviewed molecular mechanisms of p53 function and highlighted the clinical implications of changes in the p53 gene in the pathogenesis, diagnosis, prognosis, and therapy of human cancer.

Levine (1997) reviewed all aspects of p53, which he referred to as the cellular gatekeeper for growth and division.

Artandi and Attardi (2005) reviewed the role of p53 in enforcing senescence and apoptotic responses to dysfunctional telomeres. They stated that loss of p53 creates a permissive environment in which critically short telomeres are inappropriately joined to generate chromosomal end-to-end fusions. These fused chromosomes result in cycles of chromosome fusion bridge breakage, which can lead to cancers, especially in epithelial tissues, by facilitating changes in gene copy number.

Toledo and Wahl (2006) reviewed in vitro studies, human tumor data, and mouse models to deduce p53 regulatory mechanisms. They concluded that p53 posttranslational modifications have modulatory roles, whereas MDM2 and MDM4 (602704) have more profound roles in p53 regulation.

Bourdon (2007) reviewed p53 isoforms and their roles in p53 regulation and cancer.

Vousden and Lane (2007) reviewed ways in which p53 can contribute to the development, life expectancy, and overall fitness of an organism outside of its role in protecting against cancer development.

Green and Kroemer (2009) reviewed the cytoplasmic functions of p53.

Role of p53 in Transcriptional Regulation

Fields and Jang (1990), Unger et al. (1992), and Chumakov et al. (1993) discussed the DNA-binding properties of wildtype and mutant p53 and their roles in transcriptional transactivation.

By sequencing 18 human genomic clones that bound p53 in vitro, El-Deiry et al. (1992) identified a consensus binding site with striking internal symmetry, consisting of 2 copies of a 10-bp motif separated by 0 to 13 bp. One copy of the motif was insufficient for p53 binding, and subtle alterations of the motif, even when present in multiple copies, resulted in loss of affinity for p53. Mutants of p53 representing each of the 4 'hotspots' that are altered frequently in human cancers failed to bind the consensus dimer.

Vogelstein and Kinzler (1992) proposed a model in which p53 binds as a tetramer to a p53-binding site (PBS) and activates expression of downstream genes that inhibit growth and/or invasion. Pavletich et al. (1993) stated that tetramerization occurs by interactions between the p53 monomers through a C-terminal domain comprising amino acid residues 325 to 356.

Foster et al. (1999) identified multiple classes of small molecules (300 to 500 daltons) that promoted conformational stability of the wildtype p53 DNA-binding domain and of full-length p53. These compounds also allowed mutant p53 to maintain an active conformation. A prototype compound caused accumulation of conformationally active p53 in cells with mutant p53, enabling it to activate transcription and to slow tumor growth in mice.

Yu et al. (2000) showed that loss of the ERCC6 protein (609413) or overexpression of the C-terminal domain of p53 in human cells induced fragility of the RNU1 (180680), RNU2 (180690), and RN5S (180420) genes and the ancient PSU1 locus, which consists entirely of pseudogenes. Moreover, they found that p53 interacted with ERCC6 in vivo and in vitro. The authors proposed that ERCC6 functions as an elongation factor for transcription of structured RNAs, including some mRNAs. Activation of p53 inhibits ERCC6, stalling transcription complexes and locally blocking chromatin condensation.

To determine whether TP53 gene dosage affects transcriptional regulation of target genes, Yoon et al. (2002) performed oligonucleotide array gene expression analysis by using human cells with wildtype p53 or with 1 or both TP53 alleles disrupted by homologous recombination. They identified 35 genes whose expression was significantly correlated with TP53 dosage, including genes involved in signal transduction, cell adhesion, transcription regulation, neurogenesis, and neural crest migration. Motif search analysis revealed that of the genes highly expressed in wildtype and heterozygous p53 cells, several had a putative p53 consensus binding site, suggesting that they may be directly regulated by p53. From these genes, Yoon et al. (2002) chose CSPG2 (118661) for further study, and in vitro and in vivo assays showed that CSPG2 was directly transactivated by p53.

Using systems reconstituted with recombinant chromatin templates and coactivators, An et al. (2004) showed that p300 (EP300; 602700), PRMT1 (602950), and CARM1 (603934) acted both independently and cooperatively in mediating gene activation by p53. Overexpression of p53 or ultraviolet (UV) irradiation-induced DNA damage in human cell lines led to targeted recruitment of these and other coactivators, as well as accumulation of histone acetylation and methylation marks, on the p53 target gene GADD45 (126335).

Bourdon et al. (2005) showed that human p53 and the p53-beta isoform bound differentially to p53-responsive promoters and differentially activated p53-responsive reporter genes. The del133p53 isoform impaired p53-mediated apoptosis. Bourdon et al. (2005) concluded that the functions of p53 are mediated by the interplay between p53 isoforms and full-length p53.

Van Nostrand et al. (2014) found that a knockin mutant mouse strain expressing a stabilized and transcriptionally dead variant of the tumor suppressor protein p53 (p53(25,26,53,54)), along with a wildtype allele of p53, revealed late gestational embryonic lethality associated with a host of phenotypes characteristic of CHARGE syndrome (214800), including coloboma, inner and outer ear malformations, heart outflow tract defects, and craniofacial defects. Van Nostrand et al. (2014) also found that the p53(25,26,53,54) mutant protein stabilized and hyperactivated wildtype p53, which then inappropriately induced its target genes and triggered cell cycle arrest or apoptosis during development. Importantly, these phenotypes were only observed with a wildtype p53 allele, as p53(25,26,53,54)-null embryos were fully viable. Furthermore, Van Nostrand et al. (2014) found that CHD7 (608892) can bind to the p53 promoter, thereby negatively regulating p53 expression, and that CHD7 loss in mouse neural crest cells or in samples from patients with CHARGE syndrome results in p53 activation. Strikingly, Van Nostrand et al. (2014) found that p53 heterozygosity partially rescued the phenotypes in Chd7-null mouse embryos, demonstrating that p53 contributes to the phenotypes that result from CHD7 loss. The authors concluded that inappropriate p53 activation during development can promote CHARGE phenotypes, supporting the idea that p53 has a critical role in developmental syndromes and providing insight into the mechanisms underlying CHARGE syndrome.

Role of p53 in MicroRNA Processing

MicroRNAs (miRNAs) are key posttranscriptional regulators of gene expression that are involved in diverse physiologic and pathologic processes. Suzuki et al. (2009) found that p53 enhanced posttranscriptional maturation of several miRNAs with growth-suppressive functions, including miR16-1 (609704), miR143 (612117), miR145 (611795), and miR206 (611599), in response to DNA damage. In HCT116 human colon cancer cells and human diploid fibroblasts, p53 interacted with the Drosha (RNASEN; 608828) miRNA-processing complex through association with the DEAD-box RNA helicase p68 (DDX5; 180630) and facilitated processing of primary miRNAs (pri-miRNAs) to precursor miRNAs (pre-miRNAs). Suzuki et al. (2009) introduced several tumor-derived, transcriptionally inactive p53 mutants, including arg175 to his (R175H; 191170.0030) and arg273 to his (R273H; 191170.0020), into p53-null HCT116 cells and found that they suppressed the pre-miRNA and mature miRNA levels of miR16-1, miR143, and miR206 compared with the constant level of pri-miRNAs. In contrast, wildtype p53 increased the pre-miRNA and mature miRNA expression levels of these miRNAs. The p53 mutants also decreased production of mature and precursor miR16-1 and miR143 from ectopically expressed pri-miR16-1/pri-miR143, suggesting that the p53 mutants hindered miRNA processing in a transcription-independent manner. Further experiments suggested that the p53 mutants interfered with functional assembly between Drosha complex and p68, leading to attenuation of miRNA-processing activity. Suzuki et al. (2009) concluded that transcription-independent modulation of miRNA biogenesis is intrinsically embedded in a tumor-suppressive program governed by p53.

Role of p53 in Cell Cycle Control

Using transgenic mice, Lee and Bernstein (1993) found that expression of either of 2 mutant p53 alleles significantly increased cellular resistance of a variety of hematopoietic cell lineages to gamma radiation. They speculated that wildtype p53 may serve as a 'guardian of the genome,' preventing proliferation of a cell that has sustained genetic damage. Thus, cells lacking wildtype p53 protein due to a dominant-negative action of mutant p53 might not undergo radiation-induced cell death, thereby increasing radiation resistance.

El-Deiry et al. (1993) found that induction of WAF1 (CDKN1A; 116899) was associated with wildtype but not mutant p53 gene expression in a human brain tumor cell line. WAF1 is also called CIP1 or p21, and Harper et al. (1993) showed that it binds to cyclin complexes and inhibits the function of cyclin-dependent kinases. El-Deiry et al. (1993) suggested that p53 is not required for normal development, but its expression is stimulated in certain cellular environments, such as DNA damage or cellular stress. In turn, p53 binds to WAF1 regulatory elements and transcriptionally activates its expression. WAF1 subsequently binds to and inhibits cyclin-dependent kinase activity, preventing phosphorylation of critical cyclin-dependent kinase substrates and blocking cell cycle progression. In tumor cells with inactive p53, this pathway would thereby be defective, permitting unregulated growth.

After DNA damage, many cells appear to enter a sustained arrest in the G2 phase of the cell cycle. Bunz et al. (1998) demonstrated that this arrest could be sustained only when p53 was present in the cell and capable of transcriptionally activating p21. After disruption of either p53 or p21, gamma-radiated cells progressed into mitosis and exhibited G2 DNA content only due to failure of cytokinesis. Bunz et al. (1998) concluded that p53 and p21 are essential for maintaining the G2 checkpoint in human cells.

The centrosome plays a vital role in mitotic fidelity, ensuring establishment of bipolar spindles and balanced chromosome segregation. Centrosome duplication occurs only once during the cell cycle and is therefore highly regulated. Fukasawa et al. (1996) showed that in mouse embryonic fibroblasts lacking p53, multiple copies of functionally competent centrosomes were generated during a single cell cycle. In contrast, mouse embryonic fibroblasts from normal mice or mice deficient in the retinoblastoma tumor suppressor gene product (RB1; 614041) did not display these abnormalities. The abnormally amplified centrosomes profoundly affected mitotic fidelity, resulting in unequal segregation of chromosomes. These observations implicated p53 in the regulation of centrosome duplication and suggested a possible mechanism by which loss of p53 may cause genetic instability.

Raj et al. (2001) reported that adeno-associated virus (AAV) selectively induced apoptosis in human cells lacking active p53. Cells with intact p53 activity were not killed, but underwent arrest in the G2 phase of the cell cycle. This arrest was characterized by increased p53 activity and p21 levels and by targeted destruction of CDC25C (157680). Neither cell killing nor arrest depended upon AAV-encoded proteins. Rather, AAV DNA, which is single stranded with hairpin structures at both ends, elicited in cells a DNA damage response that, in the absence of p53, led to cell death. AAV also inhibited tumor growth in mice. Raj et al. (2001) concluded that viruses can be used to deliver DNA of unusual structure into cells to trigger a DNA damage response without damaging cellular DNA and to selectively eliminate cells lacking p53 activity.

Aylon et al. (2006) found that LATS2 (604861) had a role in the p53-dependent G1/S arrest following damage to the mitotic spindle and centrosome dysfunction. LATS2 interacted physically with MDM2 (164785) to inhibit p53 ubiquitination and to promote p53 activation.

Xue et al. (2007) used RNA interference to conditionally regulate endogenous p53 expression in a mosaic mouse model of liver carcinoma. Brief reactivation of endogenous p53 in p53-deficient tumors could produce complete tumor regressions. The primary response to p53 was not apoptosis, but instead involved induction of a cellular senescence program associated with differentiation and upregulation of inflammatory cytokines. This program, although producing only cell cycle arrest in vitro, also triggered an innate immune response that targeted tumor cells in vivo, thereby contributing to tumor clearance. Xue et al. (2007) concluded that p53 loss may be required for maintenance of aggressive carcinomas and that the cellular senescence program can act together with the innate immune system to potently limit tumor growth.

Using semiquantitative RT-PCR of wildtype and p53-null mouse embryonic fibroblasts, He et al. (2007) found that expression of miR34a (611172), miR34b (611374), and miR34c (611375) correlated precisely with p53 status. These miR34 genes were direct transcriptional targets of p53 in human and mouse cells, and their induction by DNA damage and oncogenic stress depended on p53 in vitro and in vivo. Ectopic expression of miR34 induced cell cycle arrest in both primary and tumor-derived cell lines, consistent with the ability of miR34 to downregulate a program of genes promoting cell cycle progression.

Qian et al. (2008) identified DEC1 (BHLHB2; 604256) as a p53 family target gene that mediates p53-induced cellular senescence in response to DNA damage.

Lee et al. (2012) found that starved mouse embryonic fibroblasts lacking the essential autophagy gene product Atg7 (608760) failed to undergo cell cycle arrest. Independent of its E1-like enzymatic activity, Atg7 could bind to the tumor suppressor p53 to regulate the transcription of the gene encoding the cell cycle inhibitor p21(CDKN1A) (116899). With prolonged metabolic stress, the absence of Atg7 resulted in augmented DNA damage with increased p53-dependent apoptosis. Inhibition of the DNA damage response by deletion of the protein kinase Chk2 (604373) partially rescued postnatal lethality in Atg7 -/- mice. Thus, Lee et al. (2012) concluded that when nutrients are limited, Atg7 regulates p53-dependent cell cycle and cell death pathways.

Using a computational model, Purvis et al. (2012) identified a sequence of precisely timed drug additions that altered p53 pulses to instead produce a sustained p53 response. This led to the expression of a different set of downstream genes and also altered cell fate: cells that experienced p53 pulses recovered from DNA damage, whereas cells exposed to sustained p53 signaling frequently underwent senescence. Purvis et al. (2012) concluded that protein dynamics can be an important part of a signal, directly influencing cellular fate decisions.

Jiang et al. (2013) showed that p53 represses the expression of the tricarboxylic acid cycle-associated malic enzymes ME1 (154250) and ME2 (154270) in human and mouse cells. Both malic enzymes are important for NADPH production, lipogenesis, and glutamine metabolism, but ME2 has a more profound effect. Through the inhibition of malic enzymes, p53 regulates cell metabolism and proliferation. Downregulation of ME1 and ME2 reciprocally activates p53 through distinct MDM2- (164785) and AMP-activated protein kinase (AMPK; see 602739)-mediated mechanisms in a feed-forward manner, bolstering this pathway and enhancing p53 activation. Downregulation of ME1 and ME2 also modulates the outcome of p53 activation, leading to strong induction of senescence, but not apoptosis, whereas enforced expression of either malic enzyme suppresses senescence. Jiang et al. (2013) concluded that their findings defined physiologic functions of malic enzymes, demonstrated a positive-feedback mechanism that sustains p53 activation, and revealed a connection between metabolism and senescence mediated by p53.

Role of p53 in Apoptosis

Caelles et al. (1994) developed immortalized somatotropic progenitor cells expressing a temperature-sensitive p53 mutant. In these cells, induction of apoptosis by DNA damage depended strictly on p53 function. Temperature-shift experiments showed that the extent of apoptotic DNA cleavage was directly proportional to the period during which p53 was functional. A shift to the permissive temperature triggered apoptosis following UV radiation-induced DNA damage independently of new RNA or protein synthesis. Caelles et al. (1994) suggested that, rather than activating apoptosis-mediator genes, p53 either represses genes necessary for cell survival or is a component of the enzymatic machinery for apoptotic cleavage or repair of DNA.

Polyak et al. (1997) used serial analysis of gene expression (SAGE) to examine transcripts induced by p53 before the onset of apoptosis. Of 7,202 transcripts identified, only 14 (0.19%) were markedly increased in p53-expressing cells compared with controls. Many of these genes were predicted to encode proteins that could generate or respond to oxidative stress. Additional biochemical and pharmacologic experiments suggested that p53 triggers apoptosis through transcriptional induction of redox-related genes, followed by formation of reactive oxygen species and oxidative degradation of mitochondrial components.

Sablina et al. (2005) found that p53 had an antioxidant function associated with highly responsive p53 target genes induced during nonlethal oxidative stress in several human cell lines. Prooxidant effects of p53 in gravely damaged cells were associated with delayed induction of proapoptotic genes. The p53-dependent increase in reactive oxygen species was secondary to induction of apoptosis and originated from mitochondrial leakage.

Conseiller et al. (1998) constructed a 'chimeric tumor suppressor-1' (CTS1) gene from wildtype p53 by removing the domains that mediate p53 inactivation. CTS1 enhanced transcriptional activity, was resistant to inactivation by MDM2 (164785), had the ability to suppress cell growth, and showed faster induction of apoptosis. Conseiller et al. (1998) considered CTS1 to be an alternative for use in gene therapy for wildtype p53-resistant tumors.

Ryan et al. (2000) examined the effect of p53 induction on activation of NF-kappa-B (NFKB; see 164011), a transcription factor that can protect from or contribute to apoptosis. In human cells without NFKB activity, p53-induced apoptosis was abrogated. Ryan et al. (2000) found that p53 activated NFKB through the RAF (164760)/MEK1 (176872)/p90(rsk) (see 601684) pathway rather than the TNFR1 (191190)/TRAF2 (601895)/IKK (e.g., 600664) pathway used by TNFA (191160). Inhibition of MEK1 blocked p53-induced NFKB activation and apoptosis, but not cell cycle arrest.

Ollmann et al. (2000) identified a Drosophila homolog of p53, which they called Dmp53. Like mammalian p53, Dmp53 bound specifically to human p53-binding sites, and overexpression of Dmp53 induced apoptosis. Inhibition of Dmp53 function rendered cells resistant to x-ray-induced apoptosis, suggesting that Dmp53 is required for the apoptotic response to DNA damage. Unlike mammalian p53, Dmp53 appeared unable to induce a G1 cell cycle block when overexpressed, and inhibition of Dmp53 activity did not affect x-ray-induced cell cycle arrest. These data revealed an ancestral proapoptotic function for p53 and identified Drosophila as an ideal model system for elucidating the p53 apoptotic pathway(s) induced by DNA damage.

Brodsky et al. (2000) also identified a Drosophila p53 homolog and demonstrated that it could activate transcription from a promoter containing binding sites for human p53. Dominant-negative forms of Dmp53 inhibited transactivation in cultured cells and radiation-induced apoptosis in developing tissues. The cis-regulatory region of the proapoptotic gene 'reaper' contains a radiation-inducible enhancer that includes a consensus p53-binding site. Dmp53 could activate transcription from this site in yeast, and a multimer of this site was sufficient for radiation induction in vivo. These results indicated that reaper is a direct transcriptional target of Dmp53 following DNA damage.

Robles et al. (2001) identified a classic p53-responsive element upstream of the APAF1 (602233) transcription start site that bound p53 and induced APAF1 gene expression. Apoptosis in a lymphoblastoid cell line, caused by DNA damage due to exposure to ionizing radiation or to doxorubicin, induced APAF1 mRNA and protein expression and was strictly dependent on wildtype p53 function. Robles et al. (2001) concluded that APAF1 is an essential downstream effector of p53-mediated apoptosis.

Fortin et al. (2001) identified 2 p53 consensus binding sites in the mouse Apaf1 promoter and showed that both sites were used by p53. Primary cultures of Apaf1-deficient neurons were significantly protected from p53-induced apoptosis.

Castedo et al. (2001) delineated the apoptotic pathway resulting from human immunodeficiency virus (HIV)-1 envelope glycoprotein (Env)-induced syncytia formation in vitro and in vivo. Immunohistochemical analysis demonstrated the presence of phosphorylated ser15 of p53 as well as the preapoptotic marker tissue transglutaminase (TGM2; 190196) in syncytium in the apical light zone (T-cell area) of lymph nodes, as well as in peripheral blood mononuclear cells, from HIV-1-positive but not HIV-1-negative donors. The presence of these markers correlated with viral load (HIV-1 RNA levels). Quantitative immunoblot analysis showed that phosphorylation of ser15 of p53 in response to HIV-1 Env was mediated by FRAP (601231) and was accompanied by downregulation of protein phosphatase-2A (see 176915). The phosphorylation was significantly inhibited by rapamycin. Immunofluorescence microscopy indicated that FRAP was enriched in syncytial nuclei and that the nuclear accumulation preceded phosphorylation of ser15 of p53. Castedo et al. (2001) concluded that HIV-1 Env-induced syncytium formation leads to apoptosis via a pathway that involves phosphorylation of ser15 of p53 by FRAP, followed by activation of BAX (600040), mitochondrial membrane permeabilization, release of cytochrome C, and caspase activation.

Derry et al. (2001) identified Cep1, a C. elegans homolog of mammalian p53. Cep1 was ubiquitously expressed in embryos, promoted DNA damage-induced apoptosis, and was required for normal meiotic chromosome segregation in the germline. Although somatic apoptosis was unaffected, Cep1 mutants showed hypersensitivity to hypoxia-induced lethality and decreased longevity in response to starvation-induced stress. Overexpression of Cep1 promoted widespread caspase-independent cell death, demonstrating the critical importance of regulating p53 function at appropriate levels.

Sax et al. (2002) presented evidence that BID (601997) belongs to a subset of p53-upregulated targets whose induction and subsequent processing mediates p53-induced apoptosis.

Brantley et al. (2002) examined expression of p53 and Rb (614041) in 12 eyes containing posterior uveal melanomas following plaque radiotherapy. All cases showed tumor cell loss with residual tumor cells. Strong p53 staining was observed in 6 cases (50%) and was significantly associated with recent radiotherapy. Abnormal cytoplasmic Rb staining was observed in 4 cases (33%). Brantley et al. (2002) concluded that plaque radiotherapy damaged DNA, inhibited cell division, and promoted cell death, at least in part, due to induction of p53.

Seoane et al. (2002) identified MYC (190080) as a principal determinant of whether DNA damage-induced activation of p53 results in cell cycle arrest or apoptosis. MYC was directly recruited to the p21 (CDKN1A; 116899) promoter by the DNA-binding protein MIZ1 (604084). This interaction blocked p21 induction by p53 and other activators. As a result, MYC switched the p53-dependent response of colon cancer cells to DNA damage from cytostatic to apoptotic. MYC did not modify the ability of p53 to bind the p21 or PUMA (605854) promoters, but it selectively inhibited bound p53 from activating p21 transcription. By inhibiting p21 expression, MYC favored initiation of apoptosis, thereby influencing the outcome of a p53 response in favor of cell death.

Yin et al. (2003) showed that p53 activated transcription of PAC1 (603068) by binding to a palindromic site in the PAC1 promoter during apoptosis. PAC1 transcription was induced in response to serum deprivation and oxidative stress, which resulted in p53-dependent apoptosis, but not in response to gamma irradiation, which caused cell cycle arrest. Reduction of PAC1 transcription using small interfering RNA inhibited p53-mediated apoptosis, whereas overexpression of PAC1 increased susceptibility to apoptosis and suppressed tumor formation. Moreover, Yin et al. (2003) found that activation of p53 significantly inhibited MAP kinase (see 602425) activity. They concluded that, under specific stress conditions, p53 regulates transcription of PAC1 through a novel p53-binding site, and that PAC1 is necessary and sufficient for p53-mediated apoptosis.

Takaoka et al. (2003) found that IFNA (147660)/IFNB (147640) induced transcription and translation of p53. IFNA/B signaling itself did not activate p53, but it contributed to boosting p53 responses to stress signals. Takaoka et al. (2003) provided examples in which p53 gene induction by IFNA/B contributed to tumor suppression. Furthermore, they showed that p53 was activated in virally infected cells to evoke an apoptotic response and that p53 was critical for antiviral defense of the host. IFNA/B transcriptionally induced p53 through ISGF3 (147574). Whereas IFNA/B induced p53 mRNA and increased its protein level, p53-mediated responses such as cell cycle arrest or apoptosis were not observed in cells treated with IFNA/B alone.

Mihara et al. (2003) provided evidence that p53 translocation to mitochondria occurred in vivo in irradiated thymocytes. They showed that p53 could directly induce permeabilization of the outer mitochondrial membrane by forming complexes with the protective BCLXL (see 600039) and BCL2 (151430) proteins, resulting in cytochrome c release. p53 bound BCLXL via its DNA-binding domain. Tumor-derived transactivation-deficient mutants of p53 concomitantly lost the ability to interact with BCLXL and promote cytochrome c release. Mihara et al. (2003) concluded that p53 mutations might represent 'double hits' by abrogating the transcriptional and mitochondrial apoptotic activities of p53.

Jin et al. (2003) found that CIAP1 (BIRC2; 601712), an inhibitor of apoptosis, was involved in the p53-dependent response to apoptotic stimuli. In both primary mouse thymocytes and HeLa cells, the mitochondrial serine protease HTRA2 (606441) cleaved CIAP1. HTRA2 expression was induced by p53, and cleavage of CIAP1 by HTRA2 was required to relieve caspase inhibition and activate apoptosis.

Chipuk et al. (2004) found that cytosolic localization of endogenous wildtype or transactivation-deficient p53 was necessary and sufficient for apoptosis. p53 directly activated the proapoptotic BCL2 protein BAX in the absence of other proteins to permeabilize mitochondria and engage the apoptotic program. p53 also released both proapoptotic multidomain proteins and BH3-only proteins that were sequestered by BCLXL. Transcription-independent activation of BAX by p53 occurred with similar kinetics and concentrations to those produced by activated BID. Chipuk et al. (2004) proposed that when p53 accumulates in the cytosol, it can function analogously to the BH3-only subset of proapoptotic BCL2 proteins to activate BAX and trigger apoptosis.

Leu et al. (2004) found that after cell stress, p53 interacted with BAK (600516), resulting in oligomerization of BAK and release of cytochrome c from mitochondria. Formation of the p53-BAK complex coincided with loss of interaction between BAK and the antiapoptotic protein MCL1 (159552). Leu et al. (2004) suggested that p53 and MCL1 have opposing effects on mitochondrial apoptosis by modulating BAK activity.

Liver is generally refractory to apoptosis induced by p53. Leu and George (2007) found that p53 activation led to enhanced expression of IGFBP1 (146730) in human hepatoma cells. A portion of intracellular IGFBP1 localized to mitochondria, where it bound the proapoptotic protein BAK. Binding of IGFBP1 to BAK impaired formation of the proapoptotic p53/BAK complex and induction of apoptosis in cultured human and mouse cells and in mouse liver. In contrast, livers of Igfbp1-deficient mice exhibited spontaneous apoptosis accompanied by p53 mitochondrial accumulation and evidence of Bak oligomerization. Leu and George (2007) concluded that IGFBP1 is a negative regulator of the p53/BAK-dependent pathway of apoptosis.

Schultz et al. (2004) showed that TIAF1 (609517) and p53 induced apoptosis in human U937 myocytoma cells in both synergistic and antagonistic manners. At optimal levels, both TIAF1 and p53 mediated apoptosis cooperatively. Both proteins also suppressed adherence-independent growth in a mouse fibroblast cell line. In contrast, initiation of apoptosis by overexpressed TIAF1 was blocked by low doses of p53, and vice versa. Ectopic p53 blocked apoptosis in U937 cells stably expressing TIAF1. TIAF1 and p53 did not appear to physically interact; however, nuclear translocation of phosphorylated p53 was significantly reduced in TIAF1-silenced cells. Schultz et al. (2004) concluded that TIAF1 likely participates in the nuclear translocation of activated p53.

Johnson et al. (2005) generated a Trp53 knockin mouse strain carrying mutations of 2 residues crucial for transactivation: leu25 to gln (L25Q) and trp26 to ser (W26S). The mutant protein was designated p53(QS). These mutations had selective effects on the biologic functions of p53 in mouse embryonic fibroblasts. Although its ability to activate various p53 target genes was largely compromised, the p53(QS) protein retained the ability to transactivate Bax. The ability of the p53(QS) protein to elicit a DNA damage-induced G1 cell cycle arrest response was also partially impaired. The p53(QS) protein had selective defects in its ability to induce apoptosis: it was completely unable to activate apoptosis in response to DNA damage and was partially unable to do so when subjected to serum deprivation, but it retained substantial apoptotic activity upon exposure to hypoxia. These findings suggested that p53 acts through distinct, stimulus-specific pathways to induce apoptosis.

Nuclear p53 regulates proapoptotic genes, whereas cytoplasmic p53 directly activates proapoptotic BCL2 proteins to permeabilize mitochondria and initiate apoptosis. Chipuk et al. (2005) found that a tripartite nexus between BCLXL, cytoplasmic p53, and PUMA coordinated these distinct p53 functions in mouse and human cells. After genotoxic stress, BCLXL sequestered cytoplasmic p53. Nuclear p53 caused expression of PUMA, which then displaced p53 from BCLXL, allowing p53 to induce mitochondrial permeabilization. Mutant BCLXL that bound p53, but not PUMA, rendered cells resistant to p53-induced apoptosis irrespective of PUMA expression. Thus, Chipuk et al. (2005) concluded that PUMA couples the nuclear and cytoplasmic proapoptotic functions of p53.

Esteve et al. (2005) found that DNMT1 (126375) bound p53 and that the 2 proteins colocalized in nuclei of human colon carcinoma cell lines. DNMT1 and p53 cooperated in methylation and repression of endogenous survivin (BIRC5; 603352), an antiapoptotic gene containing p53-binding sites in its promoter region.

Raver-Shapira et al. (2007) showed that p53 overexpression in a human colon cancer cell line led to a 20-fold increase in MIR34A (611172), paralleling induction of p21. Exposure to whole-body irradiation induced both Mir34a and p21 mRNA in wildtype mice, but it only induced p21 mRNA in p53-knockout mice. Inactivation of MIR34A attenuated p53-mediated apoptosis in cells exposed to genotoxic stress, whereas overexpression of MIR34A mildly increased apoptosis. Independently, Chang et al. (2007) also identified MIR34A as a direct target of p53.

Cuadrado et al. (2007) found that ZNHIT1 (618617) was an unstable protein that accumulated in response to DNA damage and that this accumulation induced apoptosis. Phosphorylation of ZNHIT1 by p38 (see 600289) was essential for ZNHIT1 accumulation and induction of apoptosis. Accumulation of ZNHIT1 upregulated the p53-dependent proapoptotic gene NOXA (PMAIP1; 604959) and induced p53-mediated apoptosis. Coimmunoprecipitation analysis revealed that the C-terminal region of p53 interacted with the zinc finger domain of ZNHIT1, a different part of ZNHIT1 than what interacted with p38. ZNHIT1 stimulated p53-induced transactivation by increasing the ability of p53 to bind to proapoptotic target promoters. The C-terminal domain of ZNHIT1 was required for p53 transactivation. ZNHIT1 also interacted and colocalized with cyclin G1 (CCNG1; 601578), and ZNHIT1 levels were strictly regulated by cyclin G1 to avoid triggering improper apoptotic responses under normal growth conditions.

Godar et al. (2008) found that p53 negatively regulated CD44 (107269) expression in normal human mammary epithelial cells by binding to a noncanonical p53-binding sequence in the CD44 promoter. Inhibition of CD44 enabled the cells to respond to stress-induced, p53-dependent cytostatic and apoptotic signals that would have otherwise been blocked by CD44. In the absence of p53, CD44 promoted growth in a highly tumorigenic mammary epithelial cell line.

Sendoel et al. (2010) showed that C. elegans HIF1, homologous to human HIF-alpha, 603348, protects against DNA damage-induced germ cell apoptosis by antagonizing the function of CEP1, the homolog of p53. The antiapoptotic property of HIF1 is mediated by means of transcriptional upregulation of the tyrosinase family member TYR2 in the ASJ sensory neurons. TYR2 is secreted by ASJ sensory neurons to antagonize CEP1-dependent germline apoptosis. Knockdown of the TYR2 homolog TRP2 (also called DCT, 191275) in human melanoma cells similarly increased apoptosis, indicating an evolutionarily conserved function. Sendoel et al. (2010) concluded that their findings identified a novel link between hypoxia and programmed cell death, and provided a paradigm for HIF1 dictating apoptotic cell fate at a distance.

Yoon et al. (2015) demonstrated that p53 controls signaling-mediated phagocytosis of apoptotic cells through its target, Death Domain 1-alpha (DD1-alpha; 615608), which suggests that p53 promotes both the proapoptotic pathway and postapoptotic events. DD1-alpha appears to function as an engulfment ligand or receptor that engages in homophilic intermolecular interaction at intercellular junctions of apoptotic cells and macrophages, unlike typical scavenger receptors that recognize phosphatidylserine on the surface of dead cells. DD1-alpha-deficient mice showed in vivo defects in clearing dying cells, which led to multiple organ damage indicative of immune dysfunction. Yoon et al. (2015) concluded that p53-induced expression of DD1-alpha thus prevents persistence of cell corpses and ensures efficient generation of precise immune responses.

Role of p53 in Angiogenesis

As normal cells progress toward malignancy, they must switch to an angiogenic phenotype to attract the vasculature that they depend on for growth. Dameron et al. (1994) found that the angiogenic switch in cultured fibroblasts from patients with Li-Fraumeni syndrome (151623) coincided with loss of the wildtype p53 allele and resulted from reduced expression of thrombospondin-1 (TSP1, or THBS1; 188060), a potent inhibitor of angiogenesis. Transfection assays revealed the p53 could stimulate the endogenous TSP1 gene and positively regulate TSP1 promoter sequences. Dameron et al. (1994) concluded that wildtype p53 inhibits angiogenesis in fibroblasts through regulation of TSP1 synthesis.

The earliest genetic alteration in human astrocytoma (see 137800) progression is mutation of the p53 gene, and one of the earliest phenotypic changes is stimulation of neovascularization. Van Meir et al. (1994) tested the role of p53 in angiogenesis by introducing an inducible wildtype p53 gene into p53-null human glioblastoma cells. The parental cells exhibited strong angiogenic activity, but upon induction of wildtype p53 expression, the cells secreted a factor that could neutralize the angiogenic factors produced by parental cells, as well as the angiogenic activity of FGF2 (134920).

Teodoro et al. (2006) showed that p53 transcriptionally activated the alpha-2 collagen prolyl-4-hydroxylase (P4HA1; 176710) gene, resulting in extracellular release of antiangiogenic fragments of collagen types IV (see 120130) and XVIII (see 120328). Conditioned media from cells ectopically expressing either p53 or P4HA1 selectively inhibited growth of primary human endothelial cells. When expressed intracellularly or exogenously delivered, P4HA1 significantly inhibited tumor growth in mice. Teodoro et al. (2006) concluded that there is genetic and biochemical linkage between the p53 tumor suppressor pathway and synthesis of antiangiogenic collagen fragments.

Sano et al. (2007) found that cardiac angiogenesis was crucially involved in the adaptive mechanism of cardiac hypertrophy and that p53 accumulation was essential for transition from cardiac hypertrophy to heart failure. Pressure overload in mice initially promoted vascular growth in the heart by hypoxia-inducible factor-1 (HIF1; see 603348)-dependent induction of angiogenic factors, and inhibition of angiogenesis prevented development of cardiac hypertrophy and induced systolic dysfunction. Sustained pressure overload induced an accumulation of p53 that inhibited Hif1 activity and thereby impaired cardiac angiogenesis and systolic function. Conversely, promoting cardiac angiogenesis by introducing angiogenic factors or by inhibiting p53 accumulation developed hypertrophy further and restored cardiac dysfunction under chronic pressure overload. Sano et al. (2007) concluded that the antiangiogenic property of p53 may have a crucial function in the transition from cardiac hypertrophy to heart failure.

Role of p53 in Aging

Matheu et al. (2007) showed that genetically manipulated mice with increased but otherwise normally regulated levels of Arf (600160) and p53 had strong cancer resistance and decreased levels of aging-associated damage. They proposed that the spectra of genes activated by p53 under normal physiologic conditions have a global antioxidant effect, thus decreasing aging-associated oxidative damage.

Role of p53 in Induced Pluripotent Stem Cell Generation

Induced pluripotent stem (iPS) cells, which have the capacity to form complete embryos, can be generated from somatic cells by the introduction of Oct3/4 (164177), Sox2 (184429), Klf4 (602253), and c-Myc (190080) in mouse and in human. This process is extremely insufficient. Pluripotency can be induced without c-Myc but with even lower efficiency. Zhao et al. (2008) demonstrated that an siRNA directed at p53 was able to promote human iPS cell generation. Hong et al. (2009) reported that up to 10% of transduced mouse embryonic fibroblasts lacking p53 become iPS cells, even without the Myc retrovirus. The p53 deletion also promoted the induction of integration-free mouse iPS cells with plasmid transfection. Furthermore, in the p53-null background, iPS cells were generated from terminally differentiated T lymphocytes. The suppression of p53 also increased the efficiency of human iPS cell generation. DNA microarray analyses identified 34 p53-regulated genes that are common in mouse and human fibroblasts. Functional analyses of these genes demonstrated that the p53-p21 (CDKN1A; 116899) pathway serves as a barrier not only in tumorigenicity, but also in iPS cell generation.

Li et al. (2009) showed that the Ink4/Arf locus, comprising Cdkn2a (600160)-Cdnk2b (600431), is completely silenced in iPS cells as well as in embryonic stem cells, acquiring the epigenetic marks of a bivalent chromatin domain, and retaining the ability to be reactivated after differentiation. Cell culture conditions during reprogramming enhance the expression of the Ink4/Arf locus, further highlighting the importance of silencing the locus to allow proliferation and reprogramming. Indeed, Oct4, Klf4, and Sox2 together repress the Ink4/Arf locus soon after their expression and concomitant with the appearance of the first molecular markers of 'stemness.' This downregulation also occurs in cells carrying the oncoprotein simian virus-40 'large-T' antigen, which functionally inactivates the pathways regulated by the Ink4/Arf locus, thus indicating that the silencing of the locus is intrinsic to reprogramming and not the result of a selective process. Genetic inhibition of the Ink4/Arf locus has a profound positive effect on the efficiency of iPS cell generation, increasing both the kinetics of reprogramming and the number of emerging iPS cell colonies. In murine cells, Arf, rather than Ink4a, is the main barrier to reprogramming by activation of p53 and p21, whereas in human fibroblasts, INK4a is more important than ARF. Furthermore, organismal aging upregulates the Ink4/Arf locus, and accordingly, reprogramming is less efficient in cells from old organisms, but this defect can be rescued by inhibiting the locus with a short hairpin RNA. Li et al. (2009) concluded that the silencing of Ink4/Arf locus is rate-limiting for reprogramming, and its transient inhibition may significantly improve the generation of iPS cells.

Kawamura et al. (2009) demonstrated that reprogramming factors can activate the p53 pathway. Reducing signaling to p53 by expressing a mutated version of one of its negative regulators, by deleting or knocking down p53 or its target gene p21, or by antagonizing reprogramming-induced apoptosis in mouse fibroblasts increases reprogramming efficiency. Notably, decreasing p53 protein levels enabled fibroblasts to give rise to iPS cells capable of generating germline-transmitting chimeric mice using only Oct4 and Sox2. Furthermore, silencing of p53 significantly increased the reprogramming efficiency of human somatic cells.

Utikal et al. (2009) noted that the reprogramming potential of primary murine fibroblasts into iPS cells decreases after serial passaging and the concomitant onset of senescence. They demonstrated that cells with low endogenous p19(Arf) protein levels and immortal fibroblasts deficient in components of the Arf-Trp53 pathway yield iPS cell colonies with up to 3-fold faster kinetics and at a significantly higher efficiency than wildtype cells, endowing almost every somatic cell with the potential to form iPS cells. Notably, the acute genetic ablation of p53 in cellular subpopulations that normally fail to reprogram rescues their ability to produce iPS cells. The results of Utikal et al. (2009) showed that the acquisition of immortality is a crucial and rate-limiting step towards the establishment of a pluripotent state in somatic cells and underscored the similarities between induced pluripotency and tumorigenesis.

Marion et al. (2009) showed that p53 is critically involved in preventing the reprogramming of cells carrying various types of DNA damage, including short telomeres, DNA repair deficiencies, or exogenously inflicted DNA damage. Reprogramming in the presence of preexisting, but tolerated, DNA damage is aborted by the activation of a DNA damage response and p53-dependent apoptosis. Abrogation of p53 allows efficient reprogramming in the face of DNA damage and the generation of iPS cells carrying persistent DNA damage and chromosomal aberrations. Marion et al. (2009) concluded that during reprogramming, cells increase their intolerance to different types of DNA damage and that p53 is critical in preventing the generation of human and mouse pluripotent cells from suboptimal parental cells.

Using a genomewide screen in murine induced pluripotent stem cells, Dejosez et al. (2013) identified a network of genes, centered on p53, topoisomerase (126420), and olfactory receptors (see 164342), whose downregulation caused the cells to replace wildtype cells in vitro and in the mouse embryo, but without perturbing normal development. Dejosez et al. (2013) suggested that these genes appear to fulfill an unexpected role in fostering cell cooperation.

Regulation of p53 by MDM2 and Ubiquitination

Fuchs et al. (1998) stated that direct association of p53 with the cellular protein MDM2 (164785) results in ubiquitination and subsequent degradation of p53.

Yin et al. (2002) found that MDM2 induced translation of p53 mRNA from 2 alternative initiation sites, resulting in full-length p53 and an N-terminally truncated protein, p53/47. p53/47 lacks the MDM2-binding site and the most N-terminal transcriptional activation domain of full-length p53. Translation induction required MDM2 to interact directly with the nascent p53 polypeptide and led to a change in the ratio of p53 to p53/47 by inducing translation of both proteins followed by selective degradation of full-length p53.

By mass spectrometry of affinity-purified p53-associated factors, Li et al. (2002) identified the herpesvirus-associated ubiquitin-specific protease (HAUSP; 602519) as a novel p53-interacting protein. HAUSP strongly stabilized p53, even in the presence of excess MDM2, and induced p53-dependent cell growth repression and apoptosis. HAUSP had an intrinsic enzymatic activity that specifically deubiquitinated p53 both in vivo and in vitro. Expression of a catalytically inactive point mutation of HAUSP in cells increased the levels of p53 ubiquitination and also destabilized p53. Li et al. (2002) concluded that p53 can be stabilized by direct deubiquitination and suggested that HAUSP may function as a tumor suppressor in vivo through stabilization of p53.

Both p53 and MDM2 interact with p300 (EP300; 602700)/CREB-binding protein (CBP; 600140) transcriptional coactivators. Grossman et al. (2003) observed that purified p300 exhibited intrinsic ubiquitin ligase activity. In vitro, p300 with MDM2 catalyzed p53 polyubiquitination, whereas MDM2 alone catalyzed p53 monoubiquitination. Grossman et al. (2003) concluded that generation of the polyubiquitinated forms of p53 that are targeted for proteasome degradation requires the intrinsic ubiquitin ligase activities of MDM2 and p300.

Using an in vitro ubiquitination assay with mouse and human UBE4B (613565) and MDM2, Wu et al. (2011) showed that either UBE4B or MDM2 alone led to monoubiquitination of p53, while UBE4B in combination with MDM2 promoted p53 polyubiquitination. Overexpression and knockdown studies in mouse and human cell lines revealed that interaction of UBE4B with MDM2 reduced the half-life of p53 via proteasome-mediated degradation and caused repression of p53-dependent transactivation and apoptosis.

Colaluca et al. (2008) described a previously unknown function for human NUMB (603728) as a regulator of tumor protein p53. NUMB enters in a tricomplex with p53 and the E3 ubiquitin ligase MDM2 thereby preventing ubiquitination and degradation of p53. This results in increased p53 protein levels and activity, and in regulation of p53-dependent phenotypes. In breast cancers there is frequent loss of NUMB expression. Colaluca et al. (2008) showed that, in primary breast tumor cells, this event causes decreased p53 levels and increased chemoresistance. In breast cancers, loss of NUMB expression causes increased activity of the receptor Notch (190198). Thus, in these cancers, a single event--loss of NUMB expression--determines activation of an oncogene (NOTCH1) and attenuation of the p53 tumor suppressor pathway. Biologically, this results in an aggressive tumor phenotype, as witnessed by findings that NUMB-defective breast tumors display poor prognosis.

Le Cam et al. (2006) found that human E4F1 (603022) functioned as as a ubiquitin E3 ligase for p53 both in vitro and in vivo. E4F1-mediated ubiquitylation of p53 occurred at sites distinct from those targeted by MDM2, competed with PCAF (602303)-induced acetylation of p53, and did not target p53 for proteasomal degradation. E4F1-stimulated p53-ubiquitin conjugates were associated with chromatin, and their stimulation coincided with induction of a p53-dependent transcriptional program specifically involved in cell cycle arrest, but not apoptosis. Le Cam et al. (2006) concluded that E4F1 is a key posttranslational regulator of p53 that plays an important role in the cellular life-or-death decision controlled by p53.

Impeding ribosomal biogenesis generates ribosomal stress that activates p53 to stop cell growth. Dai et al. (2006) stated that the ribosomal proteins L5 (RPL5; 603634), L11 (RPL11; 604175), and L23 (RPL23; 603662) interact with MDM2 and inhibit MDM2-mediated p53 ubiquitination and degradation in response to ribosomal stress. They found that L5 and L23 inhibited ubiquitination of both p53 and MDM2 in human cell lines. In contrast, L11 inhibited proteasome-mediated degradation of ubiquitinated MDM2, but not p53, resulting in stabilization of p53.

By immunoblot analysis and immunoprecipitation, Hu et al. (2011) found that the nucleolar protein ZNF668 (617103) interacted with p53 and MDM2 in human osteosarcoma cells. Mutation analysis showed that ZNF668 bound MDM2 and p53 via regions in its N-terminal half, and these regions were also required for nucleolar localization of ZNF668. ZNF668 regulated p53 stability and activity by disrupting MDM2-mediated ubiquitination and degradation of p53. Overexpression of ZNF668 repressed proliferation of a breast cancer cell line and prevented tumor formation in mice in both p53-dependent and -independent manners. Hu et al. (2011) concluded that ZNF668 is a breast tumor suppressor gene that regulates p53 stability.

By image-based screening, followed by synthesis of derivatives of an autophagy inhibitor, Liu et al. (2011) identified a compound, spautin-1, that inhibited autophagy without also inhibiting PDE5 (603310). Spautin-1 selectively promoted degradation of VPS34 (602609) complexes by inhibiting the deubiquitinating enzymes USP10 (609818) and USP13 (603591), resulting in an increase in ubiquitinated BECN1 (604378). Knockdown of BECN1 or VPS34 reduced expression of USP10 and USP13. In addition, knockdown of USP10 or USP13 resulted in reduced expression of the other enzyme, because the enzymes regulate deubiquitination of each other, either directly or indirectly. Spautin-1 treatment also led to a reduction of expression of p53, which is also deubiquitinated by USP10. Liu et al. (2011) found that Becn1 +/- mice had reduced levels of Vps34 complex proteins and p53, likely an important factor in the increased susceptibility of Becn1 +/- mice to tumorigenesis. Liu et al. (2011) concluded that spautin-1 targets the deubiquitination activity of USP10 and USP13, leading to increased ubiquitination of VPS34 complexes and the tumor suppressors, BECN1 and p53.

Using Tctp (TPT1; 600763)-haploinsufficient mice and mouse cells and human cell lines, Amson et al. (2012) found that TCTP had an antiapoptotic function by promoting MDM2-dependent ubiquitination and proteasome-dependent degradation of p53. TCTP also interacted directly with NUMB, and Amson et al. (2012) suggested that TCTP may compete with NUMB for binding to the MDM2-p53 complex. On the other hand, p53 bound to the promoter region of TCTP and repressed TCTP transcription, suggesting a negative-feedback loop between TCTP and p53 for the control of cell and tumor growth.

Using yeast 2-hybrid screens and protein interaction assays, Suh et al. (2013) showed that endogenous human ECD (616464) interacted directly with TXNIP (606599). Overexpression of ECD and TXNIP, either individually or together, inhibited binding of MDM2 to p53, reducing MDM2-dependent p53 ubiquitination and increasing p53 stability and activity. Overexpression of ECD or TXNIP also increased actinomycin D-mediated cell death in human cell lines in a p53-dependent manner. Conversely, knockdown of ECD or TXNIP reduced p53-dependent cell death. Suh et al. (2013) concluded that ECD and TXNIP cooperatively regulate p53 stability and activity.

Regulation of p53 by Neddylation

Abida et al. (2007) found that the F-box protein FBXO11 (607871) coprecipitated with p53 from H1299 human lung carcinoma cells, and that endogenous p53 and FBXO11 coimmunoprecipitated from HCT116 human colorectal carcinoma cells. FBXO11 also coimmunoprecipitated with the SCF (SKP1 (601434), cullin (see 603134), F-box) ubiquitin ligase complex. FBXO11 did not promote p53 ubiquitination and degradation, but it promoted p53 neddylation (see NEDD8, 603171). Conjugation of NEDD8 to p53 was lost with deletion of the F-box domain of FBXO11, or when 8 lysines of p53, including lys320 and lys321 within a nuclear localization signal, were mutated to arginine. Knockdown of FBXO11 in U2OS cells resulted in enhanced levels of p21, a primary p53 transcriptional target. Abida et al. (2007) concluded that full-length FBXO11 functions within the SCF complex in p53 neddylation, inhibiting p53 transcriptional activity.

Regulation of p53 by Phosphorylation

Shieh et al. (1997) showed that, upon DNA damage, p53 was phosphorylated at ser15 and that this event led to reduced interaction of p53 with its negative regulator, MDM2 (164785). Furthermore, phosphorylation of p53 at ser15 and ser37 by purified DNA-dependent protein kinase (see 600899) impaired the ability of MDM2 to inhibit p53-dependent transactivation. Shieh et al. (1997) concluded that these effects were most likely due to a conformational change induced by phosphorylation of p53. Shieh et al. (1997) proposed that under normal unstressed conditions, p53 associates with MDM2, and p53-dependent transactivation is repressed. Upon DNA damage, p53 is phosphorylated at ser15, which induces a conformational change that makes MDM2 unable to bind p53, relieving the inhibitory effect of MDM2 on p53.

Oda et al. (2000) identified an apoptosis-inducing gene, p53AIP1 (605426), whose expression was induced by wildtype p53. Upon severe DNA damage, ser46 on p53 was phosphorylated, leading to induction of apoptosis. Substitution of ser46 inhibited the ability of p53 to induce apoptosis and selectively blocked expression of p53AIP1. Oda et al. (2000) concluded that p53AIP1 mediates p53-dependent apoptosis and that phosphorylation of ser46 on p53 regulates transcriptional activation of p53AIP1.

Okamura et al. (2001) found that overexpression of P53DINP1 (606185) and DNA damage induced by double-strand breaks synergistically enhanced ser46 phosphorylation of p53, induction of p53AIP1, and apoptotic cell death. P53DINP1 interacted with a protein complex that phosphorylated p53 on ser46.

Hirao et al. (2000) found that Chk2 (604373) -/- mouse embryonic cells were defective for p53 stabilization and for induction of p53-dependent transcripts, such as p21, in response to gamma irradiation. Reintroduction of the Chk2 gene restored p53-dependent transcription in response to gamma irradiation. Human CHK2 directly phosphorylated p53 on ser20, a modification known to interfere with MDM2 binding. Hirao et al. (2000) concluded that phosphorylation of p53 by CHK2 increases p53 stability by preventing ubiquitination in response to DNA damage. The results provided a mechanistic link between CHK2 and p53 to explain the phenotypic similarity of Li-Fraumeni syndrome-1 (LFS1; 151623), which is caused by mutations in p53, and Li-Fraumeni syndrome-2 (LFS2; 609265), which is caused by mutations in CHK2.

Phosphorylation of the human p53 protein at ser392 is responsive to UV but not gamma irradiation. Keller et al. (2001) identified and purified a mammalian UV-activated protein kinase complex that phosphorylated ser392 in vitro. This kinase complex contained casein kinase-2 (CK2; see 115441) and the chromatin transcriptional elongation factor FACT, a heterodimer of SPT16 (605012) and SSRP1 (604328). In vitro studies showed that FACT altered the specificity of CK2 in the complex such that it selectively phosphorylated p53 over other substrates, including casein, and phosphorylation by the kinase complex enhanced p53 activity.

Zhang and Xiong (2001) identified a nuclear export signal in the N terminus of p53 containing 2 serines that were phosphorylated after DNA damage. The N-terminal signal was required for p53 nuclear export in collaboration with the C-terminal nuclear export signal. Serine-15-phosphorylated p53 induced by UV irradiation was not exported. Zhang and Xiong (2001) concluded that DNA damage-induced phosphorylation may achieve optimal p53 activation by inhibiting both MDM2 binding to, and nuclear export of, p53.

Expression of oncogenic RAS (HRAS; 190020) mutants, such as HRASV12 (see 190020.0001), in primary human cells activates p53, thereby protecting cells from transformation. Bulavin et al. (2002) showed that p38 MAPK (MAPK14; 600289) phosphorylated p53 at ser33 and ser46 in a human fibroblast cell line expressing oncogenic RAS. The activity of p38 MAPK was regulated by the p53-inducible phosphatase PPM1D (605100), creating a potential feedback loop. Expression of oncogenic Ras suppressed PPM1D mRNA induction, leaving p53 phosphorylated at ser33 and ser46 and in an active state. Overexpression of PPM1D reduced p53 phosphorylation at these sites, which abrogated RAS-induced apoptosis and partially rescued cells from cell-cycle arrest.

Hofmann et al. (2002) and D'Orazi et al. (2002) found that HIPK2 (606868) colocalized and interacted with p53 and CBP (CREBBP; 600140) within promyelocytic leukemia nuclear bodies. Activation of HIPK2 by UV radiation led to phosphorylation of p53 at ser46, facilitating CBP-mediated acetylation of p53 at lys382 and promoting p53-dependent gene expression.

Rinaldo et al. (2007) stated that phosphorylation of p53 on ser46 shifts the affinity of p53 for promoters of genes involved in cell cycle arrest to promoters of genes involved in apoptosis. They observed that lethal DNA damage increased HIPK2 expression, whereas sublethal DNA damage repressed HIPK2 expression. Rinaldo et al. (2007) identified HIPK2 as a target for MDM2-mediated ubiquitin-dependent degradation and found that HIPK2 degradation only occurred in growth-arresting conditions when MDM2 was efficiently induced by p53.

Taira et al. (2007) found that DYRK2 (603496) phosphorylated p53 on ser46 in vitro and in human cells. Upon exposure to genotoxic stress, DYRK2 translocated into the nucleus and phosphorylated p53 on ser46, inducing P53AIP1 expression and apoptosis in a ser46 phosphorylation-dependent manner.

Cordenonsi et al. (2007) found that RTK/Ras/MAPK activity induced p53 N-terminal phosphorylation, enabling interaction of p53 with TGF-beta (190180)-activated SMADs (see 601595). This mechanism confined mesoderm specification in Xenopus embryos and promoted TGF-beta cytostasis in human cells.

Regulation of p53 by Acetylation

Luo et al. (2000) found that deacetylation of p53 was mediated by a histone deacetylase-1 (HDAC1; 601241)-containing complex, and they purified a p53 target protein, MTA1L1 (603947), in the deacetylase complexes. MTA1L1, a component of the nucleosome remodeling and histone deacetylation (NURD) complex, specifically interacted with p53 in vitro and in vivo. Expression of MTA1L1 reduced steady-state levels of acetylated p53, repressed p53-dependent transcriptional activation, and modulated p53-mediated cell growth arrest and apoptosis. Luo et al. (2000) concluded that deacetylation and functional interactions between the MTA1L1-associated NURD complex may represent an important pathway to regulate p53 function.

Pearson et al. (2000) found that the tumor suppressor PML (102578) regulated the p53 response to oncogenic signals. Oncogenic RAS (HRAS; 190020) upregulated PML expression, and overexpression of PML induced senescence in a p53-dependent manner. p53 was acetylated at lys382 upon RAS expression, an event essential for its biologic function. RAS induced relocalization of p53 and the CBP (CREBBP; 600140) acetyltransferase within PML nuclear bodies and induced formation of a trimeric p53-PML-CBP complex. RAS-induced p53 acetylation, p53-CBP complex stabilization, and senescence were lost in PML -/- fibroblasts. Pearson et al. (2000) concluded that their there is a link between PML and p53 and that integrity of PML bodies is required for p53 acetylation and senescence upon oncogene expression.

Vaziri et al. (2001) found that SIRT1 (604479) bound and deacetylated p53 specifically at lys382, modification of which is implicated in activation of p53 as a transcription factor. Expression of wildtype SIRT1 in human cells reduced p53 transcriptional activity. In contrast, expression of a catalytically inactive SIRT1 protein potentiated p53-dependent apoptosis and radiosensitivity.

Luo et al. (2001) found that nicotinamide (vitamin B3) inhibited NAD-dependent p53 deacetylation induced by SIRT1 and also enhanced p53 acetylation levels in vivo. SIRT1 repressed p53-dependent apoptosis in response to DNA damage and oxidative stress, whereas expression of a SIRT1 point mutant increased the sensitivity of cells in the stress response.

Using a yeast p53 dissociator assay with a HeLa cell expression library, Wang et al. (2001) identified ADA3 (TADA3L; 602945), a part of histone acetyltransferase (HAT) complexes, as a cofactor for p53 activity. ADA3 and p53 interacted directly in cotransfected cells. Mutation analysis showed that the N terminus of ADA3 interacted with the N terminus of p53, while the C terminus of ADA3 interacted with ADA2 (TADA2L; 602276) and p300 (EP300; 602700), components of HAT complexes. Following DNA damage, p53 was phosphorylated at its N terminus, and this enhanced the amount of p53 that could be coimmunoprecipitated with ADA3. The N terminus of ADA3 alone could inhibit p53 transcriptional activity and prevent p53-mediated apoptosis. Wang et al. (2001) concluded that ADA3 function is essential for full transcriptional activity of p53 and p53-mediated apoptosis.

Tang et al. (2006) found that lys120 (K120) within the DNA-binding domain of p53 was acetylated in several human cell lines, and that acetylation of K120 was significantly enhanced upon DNA damage. This modification of p53 was catalyzed by TIP60 (601409). A tumor-derived p53 mutant defective for TIP60-mediated acetylation, lys120 to arg (K120R), abrogated p53-dependent activation of apoptosis but had no significant effect on cell growth arrest.

Sykes et al. (2006) showed that the p53 K120R mutation selectively blocked transcription of proapoptotic target genes such as BAX (600040) and PUMA (605854). Depletion of TIP60 or MOF (MYST1; 609912), another enzyme that can acetylate p53 at K120, inhibited the ability of p53 to activate BAX and PUMA transcription. Sykes et al. (2006) showed that the acetyl-K120 form of p53 specifically accumulated at proapoptotic target genes.

Upon DNA damage, p53 is acetylated by CBP at K373 and K382, by PCAF (602303) at K320, and by TIP60 at K120. This acetylation enhances the ability of p53 to bind DNA and recruit transcriptional coactivators to p53-responsive promoters. Li et al. (2007) showed that acetylation of K373 and K382 on p53 led to their direct interaction with the tandem bromodomains of TAF1 (313650). p53 recruited TAF1 to a distal p53-binding site on the p21 (CDKN1A; 116899) promoter prior to the DNA looping that brings TAF1 to the TATA box-containing core promoter.

Tang et al. (2008) identified K164 as an additional site for acetylation of full-length human p53 by CBP/p300. K164 is a conserved residue located in the L2 loop of the DNA-binding core domain of p53. Although acetylation defects at each individual site (K164, K120, and the 6 C-terminal lysines) could be compensated by acetylation at other sites, loss of acetylation at all of these major sites completely abolished the ability of p53 to activate p21 and suppress cell growth. Acetylation blocked the interaction of p53 with its repressors MDM2 and MDMX (MDM4; 602704) on the p21 promoter, and this directly resulted in p53 activation regardless of its phosphorylation status. In addition, inactivation of MDM2 and MDMX restored the transcriptional functions of unacetylated p53.

Tian et al. (2009) found that APAK (ZNF420; 617216) interacted with p53 and KAP1 (TRIM28; 601742) via its zinc fingers and KRAB domain, respectively, in unstressed human cells. KAP1 recruited ATM (607585) and HDAC1 to attenuate acetylation of p53, thereby repressing p53 activity and expression of proapoptotic genes. APAK, KAP1, and ATM did not regulate p53 targets that induce cell cycle arrest. In response to DNA damage, ATM phosphorylated APAK, causing dissociation of APAK and HDAC1 from p53, allowing expression of proapoptotic p53 target genes and apoptosis.

Wang et al. (2016) found that acidic domain-containing proteins, including SET (600960), DAXX (603186), PELP1 (609455), and VPRBP (DCAF1; 617259), bound the deacetylated C-terminal domain of p53 in human cell lines and repressed p53 function. Acetylation of p53 upon DNA damage disrupted the p53-SET interaction and activated p53.

Regulation of p53 by Methylation

Chuikov et al. (2004) reported that SET9 (606594) specifically methylated p53 at lys372 within the C-terminal regulatory region in human cells. Methylated p53 was restricted to the nucleus, and the modification stabilized p53. SET9 regulated expression of p53 target genes in a manner dependent on the p53 methylation site.

Huang et al. (2006) reported that SMYD2 (610663) methylated lys370 in p53. In contrast to methylation of lys372, methylation of lys370 repressed p53-mediated transcriptional regulation by maintaining low concentrations of promoter-associated p53. Reduction of SMYD2 by siRNA enhanced p53-mediated apoptosis. SET9-mediated methylation of lys372 inhibited SMYD2-mediated methylation of lys370, in part, by blocking interaction between p53 and SMYD2. Huang et al. (2006) concluded that, similar to histones, p53 is subject to both activating and repressing lysine methylation.

Huang et al. (2007) demonstrated that in human cells the histone lysine-specific demethylase LSD1 (609132) interacts with p53 to repress p53-mediated transcriptional activation, and to inhibit the role of p53 in promoting apoptosis. They found that in vitro, LSD1 removes both monomethylation (K370me1) and dimethylation (K370me2) at K370, a SMYD2-dependent monomethylation site (Huang et al., 2006). However, in vivo, LSD1 showed a strong preference to reverse K370me2, which is performed by a distinct methyltransferase. Huang et al. (2007) concluded that K370me2 has a different role in regulating p53 from that of K370me1: K370me1 represses p53 function, whereas K370me2 promotes association with the coactivator 53BP1 (605230). The observations of Huang et al. (2007) showed that p53 is dynamically regulated by lysine methylation and demethylation and that the methylation status at a single lysine residue confers distinct regulatory output.

Shi et al. (2007) showed that SET8 (SETD8; 607240) monomethylated p53 in human cell lines. This monomethylation suppressed p53-mediated transcriptional activation of highly responsive target genes, such as p21 (CDKN1A; 116899) and PUMA (BBC3; 605854), but it had little influence on weak p53 targets. Depletion of SET8 augmented the proapoptotic and checkpoint activation functions of p53, and SET8 expression was downregulated upon DNA damage.

Regulation of p53 by MicroRNAs

Le et al. (2009) identified highly conserved miRNA response elements in the 3-prime UTRs of zebrafish and human p53 transcripts and showed that MIR125B (see 610105) bound directly to these elements. MIR125B repressed translation of endogenous p53, reduced expression of p53 target genes, and countered drug-induced apoptosis in human cells. Knockdown of mir125b in zebrafish embryos resulted in severe developmental defects, particularly accumulation of dead cells in the brain, and loss of mir125b increased p53 protein and p53-dependent apoptosis. Treatment of zebrafish embryos with DNA-damaging agents resulted in downregulation of mir125b and a rapid increase in p53 protein. Le et al. (2009) concluded that MIR125B is an important negative regulator of p53 and p53-induced apoptosis during development and during the stress response.

Swarbrick et al. (2010) identified 2 highly conserved putative miR380-5p (MIR380*; 613654)-binding regions in the 3-prime UTR of the p53 transcript. Using knockdown and overexpression studies with mouse and human cells and cell lines, they showed that miR380-5p negatively regulated p53 expression at the translational level and countered the apoptotic function of p53 in neuroblastoma cells.

Using overexpression and inhibition studies in human cancer cell lines, Wang et al. (2017) found that MIR766 (301062) increased p53 protein expression at the posttranscriptional level. MIR766 reduced cell proliferation and colony formation and caused G2/M arrest in cancer cells, consistent with a role in promoting p53 signaling. MIR766 bound the 3-prime UTR of MDM4 (602704), a negative regulator of p53, and reduced MDM4 mRNA and protein expression. Wang et al. (2017) concluded that MIR766 induces p53 accumulation and G2/M arrest by directly targeting MDM4.

Other p53 Regulators

By immunoprecipitation and binding analyses, Lu and Levine (1995) showed that TAF9 (600822) interacted with the N-terminal domain of p53 at sites identical to those bound by MDM2 (164785). Antibodies to TAF9 inhibited p53-activated transcription. Lu and Levine (1995) concluded that p53 activity is regulated by MDM2 and TAF9 competing for the same region of the p53 protein.

Based on evidence for JNK (602896) association with p53, Fuchs et al. (1998) sought to elucidate the role of nonactive JNK2 in regulating p53 stability. The amount of p53-JNK complex was inversely correlated with the p53 level in nonstressed mouse fibroblasts. A peptide corresponding to the JNK-binding site on p53 inhibited JNK binding and efficiently blocked ubiquitination of p53. Similarly, p53 lacking the JNK-binding site exhibited a longer half-life than wildtype p53. Outcompeting JNK association with p53 increased the level of p53, whereas overexpression of a phosphorylation mutant form of JNK inhibited p53 accumulation. JNK-p53 and MDM2-p53 complexes were preferentially found in G0/G1 and S/G2M phases of the cell cycle, respectively. Fuchs et al. (1998) concluded that JNK is an MDM2-independent regulator of p53 stability in nonstressed cells.

Bernal et al. (2002) identified securin (PTTG1; 604147) as a negative regulator of p53. Assays demonstrated that p53 interacted specifically with securin both in vitro and in vivo, and this interaction blocked specific binding of p53 to DNA and inhibited its transcriptional activity. Securin also inhibited the ability of p53 to induce cell death. Transfection of human non-small cell lung carcinoma cells with securin induced an accumulation of cells in G2 that compensated for the loss of G2 cells caused by transfection with p53. Both apoptotic and transactivating functions of p53 were potentiated in securin-deficient human tumor cells cells compared with parental cells.

Zacchi et al. (2002) found that, on DNA damage, p53 interacted with PIN1 (601052), a peptidyl-prolyl isomerase that regulates proteins involved in cell cycle control and apoptosis. The interaction was strictly dependent on DNA damage-induced p53 phosphorylation and required ser33, thr81, and ser315. On binding, PIN1 generated conformational changes in p53 that enhanced its transactivation activity. Stabilization of p53 was impaired in UV-treated Pin1 -/- mouse cells owing to the inability of p53 to efficiently dissociate from MDM2. As a consequence, Pin1 -/- cells exhibited a reduced p53-dependent response to DNA damage that correlated with diminished transcriptional activation of p53-regulated genes. Zheng et al. (2002) presented similar findings and showed that PIN1-mediated p53 activation required the WW domain and isomerase activity of PIN1.

Fernandez-Fernandez et al. (2005) found that S100B (176990) and S100A4 (114210) bound the C-terminal tetramerization domain of p53 when the domain was exposed in lower oligomerization states, disrupting p53 tetramerization. S100B also bound to the negative regulatory and nuclear localization domains of p53, resulting in tight binding. Because trafficking of p53 depends on its oligomerization state, Fernandez-Fernandez et al. (2005) proposed that S100B and S100A4 may regulate subcellular localization of p53 but with different effects on p53 function in cell cycle control due to their differences in binding p53.

Barral et al. (2005) showed that E1BAP5 (HNRNPUL1; 605800), a heterogeneous nuclear ribonucleoprotein family member, interacted directly with p53 and inhibited induction of p53-regulated genes following UV irradiation.

By coimmunoprecipitation and pull-down analyses of transfected HEK293 cells, followed by nanoporous optical interferometry, Sperandio et al. (2009) showed that TOE1 (613931) interacted with the C-terminal tetramerization domain of p53. Reporter analysis revealed that coexpression of both proteins resulted in TOE1-dependent enhancement of p53-induced transactivation of the PTEN (601728) and p21 promoters. Sperandio et al. (2009) proposed that TOE1 is a coregulator of p53.

Zhang et al. (2013) showed that the long intergenic noncoding RNA ROR (LINC-ROR; 615173) suppressed induction of cellular p53 after DNA damage in human cell lines and inhibited p53-mediated cell cycle arrest and apoptosis. ROR had little effect on p53 in the absence of DNA damage. ROR repression of p53 depended upon direct interaction of ROR with heterogeneous nuclear ribonucleoprotein I (hnRNP I, or PTBP1; 600693). ROR predominantly interacted with phosphorylated hnRNP I in the cytoplasm. Knockdown of ROR increased DNA damage-induced p53 expression, whereas knockdown of hnRNP I reduced DNA damage-induced p53 expression. Qualitative RT-PCR revealed that p53 transcriptionally induced expression of ROR, resulting in a negative-feedback loop.

Activation of p53 by Meiotic Recombination

Using genetic reporters as proxies to follow in vivo activation of the p53 network in Drosophila, Lu et al. (2010) discovered that the process of meiotic recombination instigates programmed activation of p53 in the germline. Specifically, double-stranded breaks in DNA generated by the topoisomerase Spo11 (605114) provoked functional p53 activity, which was prolonged in cells defective for meiotic DNA repair. This intrinsic stimulus for the p53 regulatory network is highly conserved, as Spo11-dependent activation of p53 also occurs in mice. Lu et al. (2010) concluded that their findings established a physiologic role for p53 in meiosis and suggested that tumor-suppressive functions may have been co-opted from primordial activities linked to recombination.

Role of p53 in Carcinogenesis

Chen et al. (1990) introduced single copies of exogenous p53 genes containing either point-mutated or wildtype versions of the p53 cDNA sequence into a human osteosarcoma cell line lacking endogenous p53 by infecting the cells with recombinant retroviruses. Expression of wildtype p53 suppressed the neoplastic phenotype. In a 2-allele configuration, wildtype p53 was phenotypically dominant to mutated p53.

Halevy et al. (1990) demonstrated that the ability of a p53 mutant to bind endogenous p53 is not the sole determinant of its oncogenic potential. They concluded that p53 mutants involved in the neoplastic process display various properties, including gain of function.

In tumors showing rapid growth, hexokinase-2 (HK2; 601125) is highly expressed to facilitate high rates of glucose catabolism, which promote rapid tumor proliferation. Mathupala et al. (1997) cloned p53 from the AS-30D rat hepatoma cell line and identified 2 point mutations at the periphery of the p53 core DNA-binding domain. Using coexpression studies, they showed that overexpressed mutant p53 significantly and reproducibly activated the HK2 promoter and increased gene expression. The findings were consistent with reports describing the transactivating effects of p53 on various genes (Unger et al., 1992; Chumakov et al., 1993; Zhang et al., 1993), but they contrasted with reports that mutant p53 functions in tumor cells only to prevent wildtype p53 from transactivating genes involved in suppressing cell proliferation (Fields and Jang, 1990; Farmer et al., 1992).

Raman et al. (2000) found low p53 mRNA levels in a large proportion of breast tumors. They identified consensus HOX-binding sites in the p53 promoter and found that transient transfection of HOXA5 (142952) activated the p53 promoter. Expression of HOXA5 in epithelial cancer cells expressing wildtype p53, but not in isogenic variants lacking p53, led to apoptotic cell death. Moreover, breast cancer cell lines and patient tumors displayed a coordinate loss of p53 and HOXA5 mRNA and protein expression. The HOXA5 promoter region was methylated in 16 of 20 p53-negative breast tumor specimens. Raman et al. (2000) concluded that loss of p53 expression in human breast cancer may be primarily due to lack of HOXA5 expression.

Constitutive activation of JAK2 (147796) is frequently detected in human cancers. Reid et al. (2004) found that reintroduction of p53 in 2 human ovarian cancer cell lines with mutant p53 and high levels of phosphorylated JAK2 upregulated protein tyrosine phosphatase-1B (PTPN1; 176885), reduced JAK2 tyrosine phosphorylation, and induced apoptosis.

Using mouse and human cells, Insinga et al. (2004) showed that the acute promyelocytic leukemia-associated fusion proteins PML/RAR (see PML; 102578) and PLZF/RAR (see ZNF145; 176797) directly inhibited p53, allowing leukemic blasts to evade p53-dependent cancer surveillance pathways. PML/RAR expression led to p53 deacetylation and destabilization, resulting in MDM2 (164785)-mediated p53 degradation. Protection of PML/RAR-expressing cells from the p53-dependent genotoxic stress response depended on the presence of wildtype PML, suggesting that PML/RAR acts as a gain-of-function mutation.

Bartkova et al. (2005) showed that in clinical specimens from different stages of human tumors of urinary bladder, breast, lung, and colon, the early precursor lesions, but not normal tissues, commonly expressed markers of an activated DNA damage response. These included phosphorylated kinases ATM (607585) and CHK2 (604373) and phosphorylated histone H2AX (601772) and p53. Similar checkpoint responses were induced in cultured cells upon expression of different oncogenes that deregulate DNA replication. Together with genetic analyses, including a genomewide assessment of allelic imbalances, Bartkova et al. (2005) concluded that early in tumorigenesis, before genomic instability and malignant conversion, human cells activate an ATR/ATM-regulated DNA damage response network that delays or prevents cancer. Mutations compromising this checkpoint, including defects in the ATM-CHK2-p53 pathway, might allow cell proliferation, survival, increased genomic instability, and tumor progression.

Gorgoulis et al. (2005) analyzed a panel of human lung hyperplasias that retained wildtype p53 genes and had no signs of gross chromosomal instability and found signs of a DNA damage response, including histone H2AX and CHK2 phosphorylation, p53 accumulation, focal staining of p53 binding protein-1 (53BP1; 605230), and apoptosis. Progression to carcinoma was associated with p53 or 53BP1 inactivation and decreased apoptosis. A DNA damage response was also observed in dysplastic nevi and in human skin xenografts, in which hyperplasia was induced by overexpression of growth factors. Both lung and experimentally-induced skin hyperplasias showed allelic imbalance at loci prone to DNA double-strand break formation when DNA replication is compromised (common fragile sites). Gorgoulis et al. (2005) proposed that, from its earliest stages, cancer development is associated with DNA replication stress, which leads to DNA double-strand breaks, genomic instability, and selective pressure for p53 mutations.

Fujiwara et al. (2005) transiently blocked cytokinesis in p53-null mouse mammary epithelial cells, enabling isolation of diploid and tetraploid cultures. Tetraploid cells had an increased frequency of whole-chromosome missegregation and chromosomal rearrangements, and only tetraploid cells were transformed in vitro after exposure to carcinogen. In the absence of carcinogen, only tetraploid cells gave rise to malignant mammary epithelial cancers when transplanted subcutaneously into nude mice. These tumors all contained numerous nonreciprocal translocations and an 8- to 30-fold amplification of a chromosomal region containing a cluster of matrix metalloproteinase (MMP) genes, overexpression of which had been linked to mammary tumors in humans and in animal models (Egeblad and Werb, 2002). Fujiwara et al. (2005) concluded that tetraploidy enhances the frequency of chromosomal alterations and promotes tumor development in p53-null mouse mammary epithelial cells.

Chen et al. (2005) showed that conditional inactivation of Trp53 in mouse prostate failed to produce a tumor phenotype, whereas complete Pten (601728) inactivation in prostate triggered nonlethal invasive prostate cancer after long latency. Strikingly, combined inactivation of Pten and Trp53 elicited invasive prostate cancer as early as 2 weeks after puberty and was invariably lethal by 7 months of age. Acute Pten inactivation induced growth arrest through the p53-dependent cellular senescence pathway both in vitro and in vivo, which could be fully rescued by combined loss of Trp53. In addition, Chen et al. (2005) detected evidence of cellular senescence in specimens from early-stage human prostate cancer. They concluded that cellular senescence plays a role in restricting tumorigenesis in vivo and that p53 is an essential failsafe protein of Pten-deficient tumors.

Laurie et al. (2006) showed that the tumor surveillance pathway mediated by ARF (see 600160), MDM2, MDMX (602704), and p53 was activated after loss of RB1 during retinogenesis in mouse and human. RB1-deficient retinoblasts underwent p53-mediated apoptosis and exited the cell cycle. Subsequently, amplification of the MDMX gene and increased expression of MDMX protein were strongly selected for during tumor progression as a mechanism to suppress the p53 response in RB1-deficient retinal cells. Laurie et al. (2006) concluded that the p53 pathway is inactivated in retinoblastoma and that this cancer does not originate from intrinsically death-resistant cells, as had been thought.

Matoba et al. (2006) found that p53 modulated the balance between the use of respiratory and glycolytic pathways. They identified SCO2 (604272), which is critical for regulating the COX complex, the major site of oxygen use in the eukaryotic cell, as the downstream mediator of this effect in mice and human cancer cell lines. Disruption of the SCO2 gene in human cancer cells with wildtype p53 recapitulated the metabolic switch toward glycolysis exhibited by p53-deficient cells. Matoba et al. (2006) concluded that the coupling of p53 to mitochondrial respiration by SCO2 provides a possible explanation for the Warburg effect, in which cancer cells preferentially use glycolytic pathways for energy generation while downregulating their aerobic respiratory activity.

Ventura et al. (2007) showed that restoring endogenous p53 expression led to regression of autochthonous lymphomas and sarcomas in mice without affecting normal tissues. The main consequence of p53 restoration was apoptosis in lymphomas and suppression of cell growth with features of cellular senescence in sarcomas. Ventura et al. (2007) concluded that sustained p53 inactivation is required for tumor maintenance.

Feng et al. (2007) found evidence that increased tumor incidence with age may be due to reduced p53 function in older populations. They showed that p53 responses to gamma irradiation and other stresses were reduced in aging mice and in cultured splenocytes from older mice, which included decreased p53 transcriptional activity and p53-dependent apoptosis. The function of Atm declined significantly with age, which may be responsible for reduced p53 activity. The time of onset of decreased p53 response correlated with the life span of mice; mice that lived longer delayed their onset of decreased p53 activity.

Foo et al. (2007) noted that only about one-half of cancers have p53 loss-of-function mutations. They demonstrated that the apoptotic function of wildtype p53 was inactivated by binding to ARC (NOL3; 605235) in the nucleus of human cancer cell lines. ARC bound to the p53 tetramerization domain, which inhibited p53 tetramerization and exposed a nuclear export signal in p53, leading to CRM1 (XPO1; 602559)-dependent relocation of p53 to the cytoplasm. Knockdown of endogenous ARC in breast cancer cells resulted in spontaneous tetramerization of endogenous p53, accumulation of p53 in the nucleus, and activation of endogenous p53 target genes. In primary human breast cancers with nuclear ARC, p53 was almost always wildtype. Conversely, nearly all breast cancers with mutant p53 lacked nuclear ARC. Foo et al. (2007) concluded that nuclear ARC is induced in cancer cells and negatively regulates p53.

The Cancer Genome Atlas Research Network (2008) reported the interim integrative analysis of DNA copy number, gene expression, and DNA methylation aberrations in 206 glioblastomas (137800) and nucleotide sequence alterations in 91 of the 206 glioblastomas. The authors found that p53 itself showed mutation or homozygous deletion in 35% of tumors and that there was altered p53 signaling in 87% of tumors, as demonstrated by homozygous deletion or mutations in CDKN2A in 49% of tumors, amplification of MDM2 in 14%, and amplification of MDM4 in 7%.

Zheng et al. (2008) showed that concomitant central nervous system-specific deletion of p53 and Pten in the mouse central nervous system generates a penetrant acute-onset high grade malignant glioma phenotype with notable clinical, pathologic, and molecular resemblance to primary glioblastoma in humans. This genetic observation prompted TP53 and PTEN mutation analysis in human primary glioblastoma, demonstrating unexpectedly frequent inactivating mutations of TP53 as well as the expected PTEN mutations. Integrated transcriptomic profiling, in silico promoter analysis, and functional studies of murine neural stem cells established that dual, but not singular, inactivation of p53 and Pten promotes an undifferentiated state with high renewal potential and drives increased Myc (190080) protein levels and its associated signature. Functional studies validated increased Myc activity as a potent contributor to the impaired differentiation and enhanced renewal of neural stem cells doubly null for p53 and Pten (p53-/-Pten-/-) as well as tumor neurospheres derived from this model. Myc also serves to maintain robust tumorigenic potential of p53-/-Pten-/- tumor neurospheres. These murine modeling studies, together with confirmatory transcriptomic/promoter studies in human primary glioblastoma, validated a pathogenetic role of a common tumor suppressor mutation profile in human primary glioblastoma and established Myc as an important target for cooperative actions of p53 and Pten in the regulation of normal and malignant stem/progenitor cell differentiation, self-renewal, and tumorigenic potential.

Junttila et al. (2010) modeled the probable therapeutic impact of p53 restoration in a spontaneously evolving mouse model of nonsmall cell lung cancer (NSCLC) initiated by sporadic oncogenic activation of the endogenous KRAS (190070) developed by Jackson et al. (2001). Surprisingly, p53 restoration failed to induce significant regression of established tumors, although it did result in a significant decrease in the relative proportion of high-grade tumors. This was due to selective activation of p53 only in the more aggressive tumor cells within each tumor. Such selective activation of p53 correlates with marked upregulation in Ras signal intensity and induction of the oncogenic signaling sensor p19(ARF) (600160). Junttila et al. (2010) concluded that p53-mediated tumor suppression is triggered only when oncogenic Ras signal flux exceeds a critical threshold. Importantly, the failure of low-level oncogenic Kras to engage p53 reveals inherent limits in the capacity of p53 to restrain early tumor evolution and in the efficacy of therapeutic p53 restoration to eradicate cancers.

Feldser et al. (2010) showed that restoration of p53 in established murine lung tumors leads to significant but incomplete tumor cell loss specifically in malignant adenocarcinomas, but not in adenomas. They defined amplification of MAPK signaling as a critical determinant of malignant progression and also a stimulator of Arf tumor suppressor expression. The response to p53 restoration in this context is critically dependent on the expression of Arf. Feldser et al. (2010) proposed that p53 not only limits malignant progression by suppressing the acquisition of alterations that lead to tumor progression, but also, in the context of p53 restoration, responds to increased oncogenic signaling to mediate tumor regression. Their observations underscored that the p53 pathway is not engaged by low levels of oncogene activity that are sufficient for early stages of lung tumor development. Feldser et al. (2010) concluded that restoration of pathways important in tumor progression, as opposed to initiation, may lead to incomplete tumor regression due to the stage-heterogeneity of tumor cell populations.

Maddocks et al. (2013) showed that human cancer cells rapidly use exogenous serine and that serine deprivation triggered activation of the serine synthesis pathway and rapidly suppressed aerobic glycolysis, resulting in an increased flux to the tricarboxylic acid cycle. Transient p53-p21 (CDKN1A; 116899) activation and cell-cycle arrest promoted cell survival by efficiently channeling depleted serine stores to glutathione synthesis, thus preserving cellular antioxidant capacity. Cells lacking p53 failed to complete the response to serine depletion, resulting in oxidative stress, reduced viability, and severely impaired proliferation. The role of p53 in supporting cancer cell proliferation under serine starvation was translated to an in vivo model, indicating that serine depletion has a potential role in the treatment of p53-deficient tumors.

To investigate the relationship between epigenetic changes and mitochondrial DNA (mtDNA) alterations in breast cancer patients lacking a TP53 mutation, Barekati et al. (2010) screened triple-matched samples (cancerous tissues, matched adjacent normal tissues, and serum samples) from breast cancer patients for TP53 mutations, and analyzed the promoter methylation profile of p14(ARF) (CDKN2A; 600160), MDM2 (164785), TP53, and PTEN (601728) genes. They also analyzed mtDNA alterations, including D-loop mutations and mtDNA content. No mutation was found in the TP53 DNA-binding domain. Comparison of p14(ARF) and PTEN methylation patterns showed significant hypermethylation levels in tumor tissues, whereas the TP53 tumor suppressor gene was not hypermethylated. The proportion of PTEN methylation was significantly higher in serum than in the normal tissues and it had a significant correlation to tumor tissues. mtDNA analysis revealed 36% somatic and 91% germline mutations in the D-loop region and also significant mtDNA depletion in tumor tissues. In addition, the mtDNA content in matched serum was significantly lower than in the normal tissues. Barekati et al. (2010) concluded that hypermethylation could break down the p14(ARF)/MDM2/TP53 and PTEN regulatory pathways, resulting in p53 inactivation in breast cancer patients lacking TP53 mutation in the DNA-binding domain.

In a humanized genetically modified mouse model of pancreatic ductal adenocarcinoma (PDAC), Rosenfeldt et al. (2013) showed that autophagy's role in tumor development is intrinsically connected to the status of the tumor suppressor p53. Mice with pancreases containing an activated oncogenic allele of Kras (190070), the most common mutational event in PDAC, developed a small number of precancerous lesions that stochastically developed into PDAC over time. However, mice also lacking the essential autophagy genes Atg5 (604261) or Atg7 (608760) accumulated low-grade, premalignant pancreatic intraepithelial neoplasia lesions, but progression to high-grade pancreatic intraepithelial neoplasias and PDAC was blocked. In marked contrast, in mice containing oncogenic Kras and lacking p53, loss of autophagy no longer blocked tumor progression but actually accelerated tumor onset, with metabolic analysis revealing enhanced glucose uptake and enrichment of anabolic pathways, which can fuel tumor growth. Rosenfeldt et al. (2013) also show that treatment of mice with the autophagy inhibitor hydroxychloroquine significantly accelerates tumor formation in mice containing oncogenic Kras but lacking p53.

Viros et al. (2014) showed that sunscreen (UVA superior, UVB sun protection factor (SPF) 50) delayed the onset of ultraviolet radiation (UVR)-driven melanoma in mice expressing the BRAF V600E (164757.0001) mutation in melanocytes, but provided only partial protection. The UVR-exposed tumors showed increased numbers of single-nucleotide variants, and Viros et al. (2014) observed mutations in Trp53 (TP53) in approximately 40% of cases. TP53 is an accepted UVR target in human nonmelanoma skin cancer but was not thought to play a major role in melanoma. However, Viros et al. (2014) showed that in mice, mutant Trp53 accelerated BRAF(V600E)-driven melanomagenesis, and that in humans TP53 mutations are linked to evidence of UVR-induced DNA damage in melanoma. Thus, the authors provided mechanistic insight into epidemiologic data linking UVR to acquired nevi in humans. Furthermore, they identified TP53/Trp53 as a UVR target gene that cooperates with BRAF(V600E) to induce melanoma, providing molecular insight into how UVR accelerates melanomagenesis. Viros et al. (2014) stated that their study validated public health campaigns that promote sunscreen protection for individuals at risk of melanoma.

Jiang et al. (2015) showed that p53 inhibits cystine uptake and sensitizes cells to ferroptosis, a nonapoptotic form of cell death, by repressing expression of SLC7A11 (607933), a key component of the cystine/glutamate antiporter. Notably, p53(3KR), an acetylation-defective mutant that fails to induce cell-cycle arrest, senescence, and apoptosis, fully retains the ability to regulate SLC7A11 expression and to induce ferroptosis upon reactive oxygen species (ROS)-induced stress. Analysis of mutant mice showed that these noncanonical p53 activities contribute to embryonic development and the lethality associated with loss of Mdm2 (164785). Moreover, SLC7A11 is highly expressed in human tumors, and its overexpression inhibits ROS-induced ferroptosis and abrogates p53(3KR)-mediated tumor growth suppression in xenograft models.

Zhu et al. (2015) demonstrated that p53 gain-of-function mutants bind to and upregulate chromatin regulatory genes, including the methyltransferases MLL1 (KMT2A; 159555), MLL2 (KMT2D; 602113), and acetyltransferase MOZ (KAT6A; 601408), resulting in genomewide increases of histone methylation and acetylation. Analysis of The Cancer Genome Atlas showed specific upregulation of MLL1, MLL2, and MOZ in p53 gain-of-function patient-derived tumors, but not in wildtype p53 or p53-null tumors. Cancer cell proliferation was markedly lowered by genetic knockdown of MLL1 or by pharmacologic inhibition of the MLL1 methyltransferase complex. Zhu et al. (2015) concluded that their study revealed a novel chromatin mechanism underlying the progression of tumors with gain-of-function p53, and suggested possibilities for designing combinatorial chromatin-based therapies for treating individual cancers driven by prevalent gain-of-function p53 mutations.

Li et al. (2019) reported that the tumor suppressor p53 regulates ammonia metabolism by repressing the urea cycle. Through transcriptional downregulation of CPS1 (608307), OTC (300461), and ARG1 (608313), p53 suppresses ureagenesis and elimination of ammonia in vitro and in vivo, leading to the inhibition of tumor growth. Conversely, downregulation of these genes reciprocally activates p53 by MDM2 (164785)-mediated mechanism(s). Furthermore, the accumulation of ammonia causes a significant decline in mRNA translation of the polyamine biosynthetic rate-limiting enzyme ODC (ODC1; 165640), thereby inhibiting the biosynthesis of polyamine and cell proliferation. Li et al. (2019) conclude that together, their findings linked p53 to ureagenesis and ammonia metabolism, and further revealed a role for ammonia in controlling polyamine biosynthesis and cell proliferation.

Wellenstein et al. (2019) used a panel of 16 distinct genetically engineered mouse models for breast cancer and uncovered a role for cancer-cell-intrinsic p53 as a key regulator of prometastatic neutrophils. Mechanistically, loss of p53 in cancer cells induced the secretion of WNT (see 164820) ligands that stimulate tumor-associated macrophages to produce IL1-beta (147720), thus driving systemic inflammation. Pharmacologic and genetic blockade of WNT secretion in p53-null cancer cells reversed macrophage production of IL1-beta and subsequent neutrophilic inflammation, resulting in reduced metastasis formation. Collectively, Wellenstein et al. (2019) demonstrated a mechanistic link between the loss of p53 in cancer cells, secretion of WNT ligands, and systemic neutrophilia that potentiates metastatic progression. Wellenstein et al. (2019) concluded that their insights illustrated the importance of the genetic makeup of breast tumors in dictating prometastatic systemic inflammation, and set the stage for personalized immune intervention strategies for patients with cancer.

Morris et al. (2019) found that p53 remodels cancer cell metabolism to enforce changes in chromatin and gene expression that favor a premalignant cell fate. Restoring p53 function in cancer cells derived from KRAS (190070)-mutant mouse models of pancreatic ductal adenocarcinoma led to the accumulation of alpha-ketoglutarate (alpha-KG), a metabolite that also serves as an obligate substrate for a subset of chromatin-modifying enzymes. p53 induced transcriptional programs that are characteristic of premalignant differentiation, and this effect could be partially recapitulated by the addition of cell-permeable alpha-KG. Increased levels of the alpha-KG-dependent chromatin modification 5-hydroxymethylcytosine (5hmC) accompanied the tumor cell differentiation that was triggered by p53, whereas decreased 5hmC characterized the transition from premalignant to dedifferentiated malignant lesions that is associated with mutations in Trp53. Enforcing the accumulation of alpha-KG in p53-deficient pancreatic ductal adenocarcinoma cells through the inhibition of oxoglutarate dehydrogenase, an enzyme of the tricarboxylic acid cycle, specifically resulted in increased 5hmC, tumor cell differentiation, and decreased tumor cell fitness. Conversely, increasing the intracellular levels of succinate, a competitive inhibitor of alpha-KG-dependent dioxygenases, blunted p53-driven tumor suppression. Morris et al. (2019) concluded that their data suggested that alpha-KG is an effector of p53-mediated tumor suppression, and that the accumulation of alpha-KG in p53-deficient tumors can drive tumor cell differentiation and antagonize malignant progression.

Amit et al. (2020) compared the transcriptomes of cancer-associated trigeminal sensory neurons with those of endogenous neurons in mouse models of oral cancer and identified an adrenergic differentiation signature. They showed that loss of TP53 leads to adrenergic transdifferentiation of tumor-associated sensory nerves through loss of the microRNA miR34a (611172). Tumor growth was inhibited by sensory denervation or pharmacologic blockade of adrenergic receptors, but not by chemical sympathectomy of preexisting adrenergic nerves. A retrospective analysis of samples from oral cancer revealed that p53 status was associated with nerve density, which was in turn associated with poor clinical outcomes. This crosstalk between cancer cells and neurons represents mechanism by which tumor-associated neurons are reprogrammed towards an adrenergic phenotype that can stimulate tumor progression, and is a potential target for anticancer therapy.

Role in Insulin Resistance

Using a mouse model of type 2 diabetes, Minamino et al. (2009) found that p53 has a role in regulating insulin resistance. Excessive caloric intake led to the accumulation of oxidative stress in adipose tissue, development of insulin resistance, increased expression of p53, and increased production of proinflammatory cytokines. Inhibition of p53 markedly ameliorated these changes, and conversely, upregulation of p53 in adipose tissue caused an inflammatory response that led to insulin resistance.

▼ Biochemical Features
Crystal Structure

Cho et al. (1994) co-crystallized the core domain of p53 bound to DNA. They found that the structure of p53 is unique, consisting of a large beta sandwich that acts as a scaffold for 3 loop-based elements. The sandwich is composed of 2 anti-parallel beta sheets containing 4 and 5 beta strands, respectively. The first loop binds to DNA within the major groove, the second loop binds to DNA within the minor groove, and the third loop packs against the second loop to stabilize it. Vogelstein and Kinzler (1994) noted that the p53 residues most frequently mutated in cancers are all at or near the protein-DNA interface, and more than two-thirds of the missense mutations are in 1 of the 3 DNA loops.

Jeffrey et al. (1995) reported the crystal structure of the p53 tetramerization domain at 1.7-angstrom resolution and described the physical properties of the tetrameric interaction.

Chuikov et al. (2004) reported the crystal structure of a ternary complex of SET9 (606594) with a p53 peptide and the cofactor product S-adenosyl-L-homocysteine. The structure provided the molecular basis for recognition of p53 by SET9.

▼ Molecular Genetics

In a review, Frebourg and Friend (1992) presented information on 18 germline mutations of the P53 gene. The mutations were widely distributed over the P53 gene, resulting in changes between amino acid residues 72 and 325.

In a review, Levine et al. (1991) noted that there are at least 3 mutation 'hotspots' affecting residues 175, 248, and 273 of p53. The highest percentage of mutations (13%) had been found at position 273.

Hollstein et al. (1991) reviewed the repertoire of mutations in the evolutionarily conserved codons of P53 in diverse types of human cancer. Transitions predominated in colon, brain, and lymphoid malignancies, whereas G:C to T:A transversions were the most frequent substitutions observed in lung and liver cancers. Mutations at A:T basepairs were more frequent in esophageal carcinomas than in other solid tumors. Most transitions in colorectal carcinomas, brain tumors, leukemias, and lymphomas were at CpG dinucleotide mutation hotspots. G-to-T transversions in lung, breast, and esophageal carcinomas were dispersed among numerous codons. In liver tumors in persons from geographic areas in which both aflatoxin B1 (AFB1) and hepatitis B virus (HBV) are cancer risk factors, most mutations were at 1 nucleotide pair of codon 249.

In a review, Varley (2003) stated that nearly 250 independent germline TP53 mutations had been reported, most of which were associated with Li-Fraumeni syndrome (LFS1; 151623) or Li-Fraumeni-like syndrome (151623). They discussed the spectrum of mutations, methods for mutation detection, tumors associated with germline mutations, and ethical and clinical issues related to patients with germline TP53 mutations. They noted that the most striking association between germline TP53 mutations and cancer occurred in cases of childhood adrenocortical carcinoma (ADCC), which was identified as a component tumor of Li-Fraumeni syndrome from the earliest studies. Varley et al. (1999) had found that more than 80% of a cohort of children with ADCC unselected for family history had a germline TP53 mutation. In addition, all 12 LFS or LFS-like families with a case of ADCC that Varley (2003) studied had a germline TP53 mutation. They estimated that TP53 mutations are present in 88% of ADCC cases.

Li-Fraumeni and Li-Fraumeni-Like Syndromes

Li-Fraumeni syndrome (LFS) is an inherited cancer syndrome characterized by autosomal dominant inheritance and early onset of tumors, multiple tumors within an individual, and multiple affected family members. The most common types of tumors are soft tissue sarcomas and osteosarcomas, breast cancer, brain tumors, leukemia, and adrenocortical carcinoma. Classic Li-Fraumeni syndrome (LFS1; 151623) is defined as a proband with a sarcoma before the age of 45 years with a first-degree relative with any cancer before the age of 45 years and 1 additional first- or second-degree relative in the same lineage with any cancer before the age of 45 years or a sarcoma at any age (Li et al., 1988). Li-Fraumeni-like syndrome (LFL; 151623) is defined as a proband with any childhood cancer, or a sarcoma, brain tumor, or adrenocortical tumor before the age of 45 years, plus a first- or second-degree relative in the same lineage with a typical LFS tumor at any age, and an additional first- or second-degree relative in the same lineage with any cancer before the age of 60 years (Birch et al., 1994). A less restrictive definition of LFL is 2 different LFS-related tumors in first- or second-degree relatives at any age (Eeles, 1995). Approximately 70% of LFS cases and 40% of LFL cases contain germline mutations in the p53 gene (Bachinski et al., 2005).

Malkin et al. (1990) detected germline mutations in the TP53 gene in all 5 families with Li-Fraumeni syndrome analyzed.

Malkin et al. (1992) identified germline mutations in the p53 gene in 3 of 59 children and young adults with a second primary cancer whose family histories were not indicative of Li-Fraumeni syndrome.

Wang et al. (2013) reported on members of families with Li-Fraumeni syndrome who carried germline mutations in the TP53 gene. As compared with family members who are not carriers and with healthy volunteers, family members with these mutations have increased oxidative phosphorylation of skeletal muscle. Basic experimental studies of tissue samples from patients with the Li-Fraumeni syndrome and a mouse model of the syndrome supported this in vivo finding of increased mitochondrial function. Wang et al. (2013) concluded that their results suggested that p53 regulates bioenergetic homeostasis in humans.

Hepatocellular Carcinoma

Hsu et al. (1991) analyzed p53 for mutations in hepatocellular carcinomas from patients in Qidong, China, an area of high incidence in which both hepatitis B virus and aflatoxin B1 (AFB1) are risk factors. Eight of 16 tumors had a point mutation at the third base position of codon 249 (191170.0006). The G-to-T transversion in 7 of the DNA samples and the G-to-C transversion in the eighth were consistent with mutations caused by aflatoxin B1 in mutagenesis experiments. No mutations were found in exons 5, 6, 8, or the remainder of exon 7. These results contrasted with p53 mutations previously reported in carcinomas and sarcomas of lung, colon, esophagus, and breast; these were scattered over 4 of the 5 evolutionarily conserved domains, including codon 249.

Studying hepatocellular carcinoma in sub-Saharan Africa, where hepatitis B virus and aflatoxins are risk factors, Bressac et al. (1991) found allelic deletions from chromosome 17p and mutations of the P53 gene in 50% of tumors. G-to-T substitutions, with clustering at codon 249, were found in 4 of 5 mutations. The G-to-T mutation at codon 249 led to a change from arginine to serine (AGG to AGT). Bressac et al. (1991) also identified a G-to-T substitution in codon 157 resulting in a change from valine to phenylalanine (191170.0007). They noted that Foster et al. (1983) had shown that aflatoxin B1 induces G-to-T substitutions almost exclusively. (Cigarette smoke appears to induce predominantly C-to-A mutations, whereas sunlight produces G-to-A mutations, and replication error results in C-to-T mutations.)

On the basis of their experience, Patel et al. (1992) suggested that the contrast in the frequency rates of the arg249-to-ser mutation between areas of high and low aflatoxin exposure was less striking than inferred by earlier reports. Buetow et al. (1992) arrived at a similar conclusion.

Aguilar et al. (1993) studied mutagenesis of codons 247 to 250 of p53 by rat liver microsome-activated AFB1 in human hepatocellular carcinoma cells of the HepG2 line. AFB1 preferentially induced C-to-T transversions in the third position of codon 249, but it also induced G-to-T and C-to-A transversions into adjacent codons, albeit at lower frequencies. Since the latter mutations are not observed in human hepatocellular carcinoma, it follows that both mutability on the DNA level and altered function of the mutant ser249 p53 protein are responsible for the observed mutation hotspot.

To investigate the role of AFB1 and of the AGG-to-AGT mutation at codon 249 in hepatocarcinogenesis, Aguilar et al. (1994) examined TP53 for mutations in normal liver samples from the United States, Thailand, and Qidong, China, where AFB1 exposures are negligible, low, and high, respectively. The frequency of the arg249-to-ser mutation paralleled the level of AFB1 exposure, supporting the hypothesis that AFB1 has a causative and probably early role in hepatocarcinogenesis.

Osteogenic Sarcoma

Masuda et al. (1987) surveyed 134 human carcinomas, sarcomas, leukemias, and lymphomas obtained at surgery or from peripheral blood and found rearrangements of the P53 gene only in osteogenic sarcomas (259500). The change was identified in 3 of 6 osteogenic sarcomas examined. Normal tissue from 1 of these patients had an unrearranged gene, indicating that the genetic abnormality in the tumor was acquired. Two of the sarcomas with rearranged genes expressed levels of p53 protein that were elevated relative to other tumors. Alterations of the P53 gene were also found in 3 human osteogenic sarcoma cell lines.

Romano et al. (1989) reported a G-to-C mutation in codon 156 of the human P53 gene, resulting in an arg-to-pro substitution, in an osteosarcoma cell line.

In osteosarcomas, Mulligan et al. (1990) detected homozygous deletion and lack of expression of p53 RNA or aberrant expression of p53 protein. Since other, primary mutations had been defined in these tumors, they suggested that the change in p53 played a progressional role in tumorigenesis. In 26 retinoblastoma tumors, no change in the P53 gene was found, despite the frequency with which such alterations were found in the clinically associated tumor, osteosarcoma. Mulligan et al. (1990) concluded that retinoblastoma and osteosarcoma may have common requirements for an initiating mutation, whereas different progressional mutations, isochromosome 6p in retinoblastoma, are involved in progression.

Using SSCP analysis, Iavarone et al. (1992) identified p53 point mutations in tumor DNA from 4 patients with multifocal osteogenic sarcoma without familial histories of increased tumor predisposition. A germline p53 mutation was detected in 1 of the patients, whose tumor tissue showed a further rearrangement of the residual wildtype allele.

Toguchida et al. (1992) identified a germline p53 mutation in 8 of 15 patients selected from a total of 196 patients with sarcoma because they had had multiple primary cancers or had a family history of cancer. Three of the patients had no known family history of cancer, and the other 5 had an unusual personal or family history of cancer. The tumor was osteosarcoma in 7 of the 8 patients and malignant fibrous histiocytoma in the eighth. Four mutations caused amino acid substitutions, and 4 caused stop codons. The authors concluded that the group of patients with cancer and germline p53 mutations appears to be more diverse than suggested by the clinical definition of Li-Fraumeni syndrome.

Smith-Sorensen et al. (1993) noted that osteosarcoma frequently occurs in patients with Li-Fraumeni syndrome and in transgenic mice carrying a mutant p53 allele (Lavigueur et al., 1989). Having previously used constant denaturant gel electrophoresis (CDGE) followed by direct sequencing to identify mutations in conserved domains II to V of TP53, Smith-Sorensen et al. (1993) reported conditions for screening more of the codons in the frequently mutated region of exons 5 to 8 of TP53 and for detecting mutations in sequences encoding functional domains in the N- and C-terminal parts of the p53 protein. Of 28 osteosarcomas examined, 6 had a TP53 mutation, 2 of which had previously been identified in osteosarcomas: ser241-to-phe (191170.0013) and arg282-to-trp (191170.0018).


Mulligan et al. (1990) studied 241 patients with various neoplasms and found changes in p53 in 31 and 29 cases of rhabdomyosarcoma and osteosarcoma, respectively. The p53 alterations in rhabdomyosarcomas included complete deletion of both p53 alleles, complete deletion of 1 allele with or without point mutation of the remaining allele, and absence of detectable RNA.

Colorectal Cancer

Baker et al. (1989) concluded that TP53 mutations may be involved in colorectal cancer (114500), perhaps through inactivation of a tumor suppressor function of the wildtype gene. Monpezat et al. (1988) found loss of alleles on chromosomes 17 and 18 in polyploid colorectal carcinomas.

By immunohistologic staining of primary colorectal carcinomas with antibodies specific for p53, Rodrigues et al. (1990) demonstrated gross overexpression of the protein in 50% of cases. Benign adenomas were all negative for p53 overexpression. By direct sequencing and chemical-mismatch-cleavage analysis of p53 cDNA using PCR in 6 cell lines expressing high levels of p53, they showed that all were synthesizing mRNAs encoding mutant p53 proteins. In 2 of 4 cell lines in which p53 expression was lower, point mutations were still detected. An arg273-to-his mutation was found in 4 of the 7 instances in which specific point mutations were identified.

The histopathologic features of adenocarcinoma of the small intestine and colorectal cancer are similar, and patients with one cancer are considered at risk for developing the other (Neugut and Santos, 1993). Wheeler et al. (2002) examined the possible contribution of TP53 to sporadic small intestinal adenocarcinoma. TP53 protein expression was assessed immunohistochemically in 21 nonfamilial, nonampullary small intestinal adenocarcinomas, and overexpression was demonstrated in 5 (24%). Arai et al. (1997) had previously demonstrated TP53 overexpression in 8 (53.3%) of 15 small intestinal adenocarcinomas and found missense mutations in 4 of these. This and similar studies led Wheeler et al. (2002) to suggest that TP53 mutation plays an important role in pathogenesis of small intestinal adenocarcinoma.

Liu and Bodmer (2006) analyzed the TP53 gene and its protein status in 56 colorectal cancer cell lines and detected 46 mutations in 43 of the cell lines, of which almost half were truncating mutations. The frequency of TP53 mutations (76.8%) in this study was higher than the usually reported average of 50%. Protein product was detectable in 32 (74%) of 43 mutant cell lines. Although only 4 cell lines made no TP53 transcript, no protein was detected in 6 cell lines with truncating mutations and also in 1 cell line with a missense mutation. Liu and Bodmer (2006) suggested that truncating mutations may have dominant-negative effects, even when no truncated protein can be detected by standard methods.

McMurray et al. (2008) showed that a large proportion of genes controlled synergistically by loss-of-function p53 and Ras activation are critical to the malignant state of murine and human colon cells. Notably, 14 of 24 'cooperation response genes' were found to contribute to tumor formation in gene perturbation experiments. In contrast, only 1 of 14 perturbations of the genes responding in a nonsynergistic manner had a similar effect. McMurray et al. (2008) concluded that synergistic control of gene expression by oncogenic mutations thus emerges as an underlying key to malignancy, and provides an attractive rationale for identifying intervention targets in gene networks downstream of oncogenic gain- and loss-of-function mutations.

Vermeulen et al. (2013) quantified the competitive advantage during tumor development of Apc (611731) loss, Kras (190070) activation, and p53 mutations in the mouse intestine. Their findings indicated that the fate conferred by these mutations is not deterministic, and many mutated stem cells are replaced by wildtype stem cells after biased but still stochastic events. Furthermore, Vermeulen et al. (2013) found that p53 mutations display a condition-dependent advantage, and especially in colitis-affected intestines, clones harboring mutations in this gene were favored. Vermeulen et al. (2013) concluded that their work confirmed the notion that the tissue architecture of the intestine suppresses the accumulation of mutated lineages.

Lung Cancer

Since lung cancer frequently shows loss of heterozygosity on 17p, Takahashi et al. (1989) examined the p53 gene and found that it was frequently mutated or inactivated in all types of human lung cancer. The genetic abnormalities included gross changes, such as homozygous deletions and abnormally sized mRNAs, along with a variety of point or small mutations that changed amino acids in a region highly conserved between man and mouse. Low or absent expression of p53 mRNA in lung cancer cell lines compared with normal lung was observed. In all of 10 small cell lung cancer cell lines and 9 non-SCLC cell lines, Takahashi et al. (1989) found coexistence of 3 abnormalities involving chromosome 3p, RB (614041) on chromosome 13, and p53.

Iggo et al. (1990) suggested that P53 is the protooncogene most commonly mutated in lung cancer. They identified several G-to-T transversions that resulted in missense changes in evolutionarily conserved amino acids.

Chiba et al. (1990) identified p53 mutations in 23 (45%) of 51 early stage, primary, resected non-small cell lung cancer specimens, but not in corresponding normal lung. G-to-T transversions were a common result of p53 mutations in lung cancer compared with other cancers, suggesting exposure to different mutagens. In univariate and multivariate analysis, p53 mutations were associated with younger age and squamous histology. However, p53 mutations were not significantly associated with tumor stage, nodal status, or sex and were found in all histologic types of lung cancer.

Takahashi et al. (1990) identified intronic point mutations as a mechanism for inactivation of the tumor suppressor function of P53 in lung cancer. They identified point mutations at the splice acceptor site in the third intron and in the splice donor site in the seventh intron accounting for abnormal mRNA splicing. In 1 patient, the same intronic point mutation was found in the tumor cell line derived from a bone marrow metastasis and in multiple liver metastases, but not in normal DNA.

Roth et al. (1996) investigated the effects of injecting a retroviral vector containing wildtype p53 under control of a beta-actin promoter into non-small cell lung carcinomas. Following injection, tumor regression occurred in 3 patients, and tumor growth stabilized in 3 other patients.

Individuals with 1 malignancy of the aerodigestive tract have a high incidence of second primary aerodigestive tumors. Franklin et al. (1997) studied an individual with widespread dysplastic changes in the respiratory epithelium but no overt carcinoma. The patient was a 66-year-old man with a 50-pack-per-year history of smoking and chronic obstructive pulmonary disease. His sputum cytology had shown moderate atypia. He died unexpectedly after laparotomy for small bowel obstruction. The tracheobronchial tree was obtained at autopsy and embedded in paraffin, and bronchial epithelial cells were isolated by microdissection. A single, identical point mutation in the p53 gene, a transversion of G:C to T:A in codon 245, was identified in bronchial epithelium from 7 of 10 sites in both lungs. Epithelium at sites containing the p53 mutation was morphologically abnormal, exhibiting squamous metaplasia and mild-to-moderate atypia. No invasive tumor was found in the tracheobronchial tree or any other location, and cells from the peripheral blood, kidney, liver, and lymph node exhibited no abnormality in the p53 gene. Franklin et al. (1997) hypothesized that a single progenitor bronchial epithelial clone may expand to populate broad areas of the bronchial mucosa, a novel mechanism for field carcinogenesis in the respiratory epithelium.

Head and Neck Cancer

Hollstein et al. (1990) studied 4 human esophageal carcinoma cell lines and 14 human esophageal squamous cell carcinomas and identified a mutated p53 allele (1 frameshift and 6 missense mutations) in 2 cell lines and in 5 of the tumor specimens. All missense mutations occurred at G:C basepairs in codons at or adjacent to mutations previously reported in other cancers.

Chakrani et al. (1995) studied 41 primary tumors of the undifferentiated nasopharyngeal cancer type from Hong Kong and the Guangxi province of southeastern China. Four point mutations were found clustered in exon 5 from codon 175 to 177.

Brennan et al. (1995) presented results pointing to the molecular target of tobacco and alcohol, which epidemiologic studies show are associated with squamous cell carcinoma of the head and neck. They found P53 mutations in 42% of patients (54 of 129); in 58% of patients who smoked cigarettes and used alcohol (37 of 64); in 33% of patients who smoked but abstained from alcohol (13 of 39); and in 17% of patients who neither smoked nor drank alcohol (4 of 24). All mutations in patients who neither drank nor smoked occurred at sites containing CpG dinucleotides (potentially representing endogenous mutations) within the P53 gene, whereas only 23% of those in cigarette smokers consisted of such changes.

Poeta et al. (2007) found TP53 mutations in tumors from 224 (53.3%) of 420 patients with squamous cell carcinoma of the head and neck. Compared with wildtype TP53, the presence of any TP53 mutation was associated with decreased overall survival, with an even stronger association with disruptive mutations and no significant association with nondisruptive mutations. Poeta et al. (2007) defined disruptive mutations as nonconservative mutations inside the key DNA-binding domain (L2-L3 region), or stop codons in any region, and nondisruptive mutations as conservative or nonconservative mutations outside the L2-L3 region, excluding stop codons.

Brain Tumors

Chung et al. (1990) found that human glioblastomas (see GLM1, 137800) with P53 mutations had an earlier age of onset than did tumors without P53 mutations. The average postoperative survival among patients with P53 mutations was considerably longer than that of the group without such mutations.

In a family in which many members had died of cancers of various types, consistent with Li-Fraumeni syndrome, Metzger et al. (1991) identified a patient with a malignant ependymoma of the posterior fossa who had a cys242-to-tyr mutation (C242Y; 191170.0008) of the P53 gene in both the germline and the tumor.

Schiffer et al. (1995) used SSCP analysis of exons 5 to 8 and direct sequence analysis to search for p53 mutations in 30 cases of childhood astrocytic tumors. Somatic mutations were found in 2 of 8 glioblastomas and in 1 of 9 anaplastic astrocytomas, but in none were found in the more benign pilocytic astrocytomas.

Bladder Cancer

Sidransky et al. (1991) identified alterations of p53 in 11 of 18 invasive bladder cancers (109800). Point mutations resulting in single amino acid substitutions were present in 10, and a 24-bp deletion was found in 1. In all but 1, the mutations were associated with 17p allelic deletions, leaving the cells with only mutant forms of the p53 gene product. Using PCR and oligomer-specific hybridization, p53 mutations were identified in 1 to 7% of cells in urine sediment from each of 3 patients tested.

Skin Cancer

In a series of New England and Swedish patients, Brash et al. (1991) found that 14 (58%) of 24 invasive squamous cell carcinomas of skin contained mutations in the P53 gene, each altering the amino acid sequence. Involvement of UV light in these mutations was indicated by the presence in 3 tumors of a CC-to-TT change, which is only induced by UV. UV was also implicated by a UV-like occurrence of mutations exclusively at dipyrimidine sites, including a high frequency of C-to-T substitutions. In internal malignancies, p53 mutations do not show these UV-specific mutations.

Dumaz et al. (1993) used RT-PCR and SSCP to analyze more than 40 skin tumors, mainly basal and squamous cell carcinomas, from patients with xeroderma pigmentosum (see 278700). They found that 17 of 43 contained at least 1 point mutation in the TP53 gene. All mutations were located at dipyrimidine sites, essentially at CC sequences, which are hotspots for UV-induced DNA lesions. In 14 of 19 mutations at CC sites, a tandem CC-to-TT transition, unique to UV-induced lesions, was found. The mutations were considered to be due to translesion synthesis of unrepaired dipyrimidine lesions left on the untranscribed strand.

Ziegler et al. (1994) pointed out that squamous cell carcinoma of the skin can progress by stages: sun-damaged epidermis, with individual disordered keratinocytes; actinic keratosis, spontaneously regressing keratinized patches having aberrant cell differentiation and proliferation; carcinoma in situ; squamous cell carcinoma of the skin; and metastasis. They showed that p53 mutations induced by UV radiation and found in more than 90% of human squamous cell carcinomas were present in actinic keratoses. Inactivating p53 in mouse skin reduced the appearance of sunburn cells (apoptotic keratinocytes generated by overexposure to UV radiation). Ziegler et al. (1994) concluded that skin possesses a p53-dependent 'guardian-of-the-tissue' response to DNA damage that aborts precancerous cells. If this response is reduced in a single cell by a prior p53 mutation, sunburn can select for clonal expansion of the p53-mutated cell into an actinic keratosis. Thus, sunlight can act twice: as tumor initiator and tumor promoter.

DNA-damaged cells can either repair the DNA or be eliminated through a homeostatic control mechanism mediated by p53 termed 'cellular proofreading.' Elimination of DNA-damaged cells after UV radiation through sunburn cell (or apoptotic keratinocyte) formation is thought to be pivotal for removal of precancerous skin cells. Hill et al. (1999) demonstrated that sunburn cell formation depended upon Fas ligand (FASL; 134638), a proapoptotic protein induced by DNA damage. Chronic exposure to UV radiation caused 14 (70%) of 20 FasL-deficient mice and 1 (5%) of 20 wildtype mice to accumulate p53 mutations in the epidermis. Hill et al. (1999) concluded that FASL-mediated apoptosis is important for skin homeostasis, suggesting that dysregulation of FAS-FASL interactions may be central to development of skin cancer.

Cervical and Anal Cancer

Development of cervical and anal cancers has been linked with infection by human papillomaviruses (HPVs), most commonly HPV16 or HPV18 (167960). The virally encoded oncoproteins E6 and E7 form complexes with cell-encoded p53 and RB, respectively, likely resulting in loss of the negative growth control normally exerted by p53 and RB (614041). E6 can direct rapid proteolytic degradation of p53, and E6 expression following HPV infection is likely to be a loss of functional wildtype p53 protein within the cell. Crook et al. (1992) found that HPV-negative cervical cancers had point mutations in the TP53 gene. They also showed that loss of wildtype p53 function was critical in the pathology of anogenital cancer and that, in the absence of a virally encoded E6 protein mediating p53 degradation, this loss of function occurred via somatic mutation. Prognosis of HPV-negative cervical cancers was found to be worse than that of HPV-positive cancers. Kaelbling et al. (1992) likewise found loss of heterozygosity on chromosome 17p and mutant p53 in HPV-negative cervical carcinomas.

Breast Cancer

Borresen et al. (1991) devised a modification of denaturing gradient gel electrophoresis (DGGE), termed 'constant denaturant gel electrophoresis' (CDGE), and used it to screen the 4 conserved regions of the P53 gene where most mutations had been found. CDGE detected P53 mutations in 11 of 32 breast carcinomas (114480).

Using a monoclonal antibody, Moll et al. (1992) demonstrated that 10 (37%) of 27 breast cancers showed a pattern of staining indicating that the p53 protein was limited to the cytoplasm and absent from the nucleus. In 8 cases (30%), the nuclei of cancer cells showed high levels of P53, and in 9 cases (33%) there was complete lack of staining. Sequencing of P53 cDNAs from the samples with cytoplasmic staining revealed only wildtype p53 alleles in 6 of 7 cases. An eighth case was determined to be wildtype by SSCP. In contrast, the samples containing nuclear p53 contained a variety of missense mutations and a nonsense mutation. Tumors that lacked detectable p53 staining had wildtype nucleotide sequences. A case of normal lactating breast tissue also showed intense cytoplasmic staining with nuclear sparing. Moll et al. (1992) concluded that some breast cancers inactivate the tumor-suppressing activity of p53 by sequestering the protein in the cytoplasm, away from its site of action in the nucleus. Detection of cytoplasmic p53 in normal lactating breast tissue suggested that this mechanism may be employed in specific physiologic situations to permit transient cell proliferation. Moll et al. (1992) referred to the phenomenon as nuclear exclusion.

Borresen et al. (1992) reported a germline arg181-to-his (R181H) mutation in the TP53 gene in a case of familial breast cancer. However, one sister with breast and colon cancer had not inherited the mutation, and another with breast cancer and Hodgkin disease had inherited the mutation, but the mutant allele was lost somatically in the breast tumor, suggesting that the mutation at codon 181 was not responsible for the cancer. On transfection of the mutant TP53 into malignant cells, Frebourg et al. (1992) found that it behaved like wildtype TP53. Frebourg et al. (1992) emphasized the importance of genetic or biologic analysis of germline mutations in tumor suppressor genes as a prerequisite to counseling about cancer risks.

Patocs et al. (2007) hypothesized that mutational inactivation of TP53 and genomic alterations in stromal cells of a tumor's microenvironment contribute to clinical outcome. They performed TP53 mutation analysis and genomewide analysis of loss of heterozygosity and allelic imbalance on DNA from isolated neoplastic epithelial and stromal cells from 43 patients with hereditary breast cancer due to BRCA1 (113705) or BRCA2 (600185) mutations and 175 patients with sporadic breast cancer. TP53 mutations were associated with an increased loss of heterozygosity and allelic imbalance in both hereditary and sporadic breast cancers, but samples from patients with hereditary disease had more frequent mutations than did those from patients with sporadic tumors. Only 1 microsatellite locus in stromal cells from hereditary breast cancers was associated with mutated TP53, whereas there were 66 such loci in cells from sporadic breast cancers. Somatic TP53 mutations in stroma, but not epithelium, of sporadic breast cancers were associated with regional nodal metastases (P of 0.003). A specific set of 5 loci linked to an increased loss of heterozygosity and allelic imbalance in the stroma of sporadic tumors was associated with nodal metastases in the absence of TP53 mutations. In hereditary breast cancer, no associations were found between any of the clinical or pathologic features and somatic TP53 mutations. Several authors disputed the findings of Patocs et al. (2007) based on their own studies (Campbell et al., 2008), comparison with known p53 mutation databases (Zander and Soussi, 2008 and Zalcman et al., 2008), or selection of the patient population (Roukos, 2008).

Leukemia and Lymphoma

Felix et al. (1992) used SSCP analysis to study the P53 gene in 10 families with multiple members affected with leukemia. The diagnoses included acute and chronic leukemias and Hodgkin disease. Felix et al. (1992) concluded that p53 mutations are not a primary event responsible for hereditary susceptibility to familial leukemia.

Felix et al. (1992) examined the p53 gene in primary lymphoblasts of 25 pediatric patients with acute lymphoblastic leukemia (ALL) using the RNase protection assay and SSCP analysis. In 4 of 25, p53 mutations were found. In 1 pedigree consistent with Li-Fraumeni syndrome, a germline G-to-T transversion at codon 272 (valine to leucine) was found. The proband died at age 19 years of ALL.

Although malignant lymphomas had been described as part of the spectrum of neoplasias in Li-Fraumeni syndrome, Potzsch et al. (1999) found no constitutional p53 mutations in 12 lymphoma patients with a family history of lymphoma and/or metachronous lymphoma. The results implied that outside the clinical spectrum of LFS, constitutional p53 mutations are rare in patients with lymphomas.

Wong et al. (2015) sequenced the genomes of 22 patients with therapy-related acute myeloid leukemia (t-AML) and showed that the total number of somatic single-nucleotide variants and the percentage of chemotherapy-related transversions are similar in t-AML and de novo AML, indicating that previous chemotherapy does not induce genomewide DNA damage. Wong et al. (2015) identified 4 cases of t-AML/t-MDS in which the exact TP53 mutation found at diagnosis was also present at low frequencies (0.003-0.7%) in mobilized blood leukocytes or bone marrow 3 to 6 years before the development of t-AML/t-MDS, including 2 cases in which the relevant TP53 mutation was detected before any chemotherapy. Moreover, functional TP53 mutations were identified in small populations of peripheral blood cells of healthy chemotherapy-naive elderly individuals. Finally, in mouse bone marrow chimeras containing both wildtype and Tp53 +/- hematopoietic stem/progenitor cells (HSPCs), the Tp53 +/- HSPCs preferentially expanded after exposure to chemotherapy. Wong et al. (2015) concluded that these data suggested that cytotoxic therapy does not directly induce TP53 mutations. Rather, they supported a model in which rare HSPCs carrying age-related TP53 mutations are resistant to chemotherapy and expand preferentially after treatment. The early acquisition of TP53 mutations in the founding HSPC clone probably contributes to the frequent cytogenetic abnormalities and poor responses to chemotherapy that are typical of patients with t-AML/t-MDS.

Metastatic Cancer

Robinson et al. (2017) performed whole-exome and transcriptome sequencing of 500 adult patients with metastatic solid tumors of diverse lineage and biopsy site. The most prevalent genes somatically altered in metastatic cancer included TP53, CDKN2A (600160), PTEN (601728), PIK3CA (171834), and RB1 (614041). Putative pathogenic germline variants were present in 12.2% of cases, of which 75% were related to defects in DNA repair. RNA sequencing complemented DNA sequencing to identify gene fusions, pathway activation, and immune profiling.

Bone Marrow Failure Syndrome 5

In 2 unrelated patients with bone marrow failure syndrome-5 (BMFS5; 618165), Toki et al. (2018) identified de novo heterozygous mutations in the TP53 gene (191170.0043 and 191170.0044) that resulted in the same truncation of the protein with a loss of 32 residues from the C-terminal end (Ser362AlafsTer8). The mutations were found by exome sequencing and confirmed by Sanger sequencing. In vitro functional expression studies showed that both TP53 mutants had increased transcriptional activity compared to controls. Human induced pluripotent stem cells expressing a CRISPR/Cas9-derived C-terminal truncated TP53 showed significantly elevated expression of downstream TP53 targets, as well as impaired erythroid differentiation. The findings indicated that augmented p53 function, not loss of function, was responsible for the phenotype. Expression of a C-terminal truncated tp53 in zebrafish resulted in developmental defects with severe morphologic abnormalities, reduced erythrocyte production, and increased lethality. Toki et al. (2018) noted that mouse models with animals lacking the C-terminal end of Tp53 show similar abnormalities (Simeonova et al., 2013, Hamard et al., 2013).

Rheumatoid Arthritis

Firestein et al. (1997) proposed that genetic changes caused by local oxidative damage is a mechanism that permanently alters or imprints synovial cells in rheumatoid arthritis (RA; 180300). In contrast, mutations in TP53 had not been observed in osteoarthritis synovial samples. Using microdissected RA synovial tissue sections, Yamanishi et al. (2002) observed abundant TP53 transition mutations characteristic of DNA damage caused by oxidative stress. TP53 mutations, as well as TP53 mRNA expression, were located mainly in the synovial intimal lining rather than the sublining (P less than 0.01). Clusters of TP53 mutant subclones were observed in some microdissected regions, suggesting oligoclonal expansion. Because expression of the IL6 gene (147620) is regulated by wildtype p53, Yamanishi et al. (2002) quantified IL6 mRNA expression in microdissected tissues. The regions with high rates of TP53 mutations contained significantly greater amounts of IL6 mRNA compared with the low mutation samples (P less than 0.02). Yamanishi et al. (2002) concluded that TP53 mutations are induced in RA synovial tissues by inflammatory oxidative stress. This process, as in sun-exposed skin and inflamed colonic epithelium, provides some of the mutant clones with a selective growth advantage. A relatively low percentage of cells containing TP53 mutations can potentially affect neighboring cells and enhance inflammation through elaboration of proinflammatory cytokines.


Ara et al. (1990) reported a pro72-to-arg (P72R; 191170.0005) polymorphism in p53. Dumont et al. (2003) found that R72 had up to 15-fold increased apoptotic ability compared with P72 in both inducible cell lines and cells with endogenous p53 homozygous for each variant. For further information on the P72R polymorphism, see 191170.0005.

Wildtype p53 has a proline at codon 47 (P47) that acts as a recognition site for phosphorylation of ser46 by p38 MAPK (MAPK14; 600289), which enhances induction of apoptosis. However, fewer than 5% of African Americans have a serine at codon 47 (S47) due to a C-to-T SNP. Li et al. (2005) showed that the S47 variant of p53 was a poorer substrate for phosphorylation by p38 MAPK, resulting in up to 5-fold reduced ability to induce apoptosis compared with wildtype p53. This decreased ability to induce apoptosis was accompanied by decreased ability to transactivate p53AIP1 (605246) and PUMA (BBC3; 605854), but not other p53 target genes, in transfected human cells.

Drug and Chemotherapy Resistance

Aas et al. (1996) presented data linking specific mutations in the P53 gene to primary resistance to doxorubicin therapy and early relapse in breast cancer patients. The L2 domain (codons 163 to 195) and the L3 domain (codons 236 of 256) contain zinc finger sequences and are involved in p53 DNA-binding function. Of 63 patients studied, 11 had mutations affecting L2 and/or L3. Four of the 11 patients with mutations affecting L2/L3 became progressively worse during doxorubicin treatment versus 2 of 52 patients without mutations or with mutations not affecting the L2/L3 domain. P53 mutations in patients with progressive disease all affected the L3 domain. Of 8 patients with L3 mutations, 4 expressed primary resistance to anthracycline therapy. Aas et al. (1996) reported that patients with P53 mutations affecting the zinc-binding domain who responded to primary chemotherapy initially in most cases had a relapse of disease within 3 months.

Yu et al. (2002) reported that mice bearing tumors derived from p53 -/- HCT116 human colorectal cancer cells were less responsive to antiangiogenic combination therapy than mice bearing tumors derived from p53 +/+ HCT116 cells. They concluded that genetic alterations that decrease the vascular dependence of tumor cells can influence the response of tumors to antiangiogenic therapy.

p53 and INK4A/ARF (600160) mutations promote tumorigenesis and drug resistance in part by disabling apoptosis. Schmitt et al. (2002) showed that primary murine lymphomas responded to chemotherapy by engaging a senescence program controlled by p53 and p16(Ink4a). Hence, tumors with p53 or Ink4a/Arf mutations, but not those lacking Arf alone, responded poorly to cyclophosphamide therapy in vivo. Moreover, tumors harboring a Bcl2 (151430)-mediated apoptotic block underwent a drug-induced cytostasis involving accumulation of p53, p16(Ink4a), and senescence markers, and they typically acquired p53 or Ink4a mutations upon progression to a terminal stage. Mice bearing tumors capable of drug-induced senescence had a much better prognosis following chemotherapy than those harboring tumors with senescence defects. Schmitt et al. (2002) concluded that cellular senescence contributes to treatment outcome in vivo.

Mechanisms of Somatic Mutations

The role of cytosine methylation in CpG dinucleotides in germline mutations that cause mendelian disorders was pointed out by Cooper and Youssoufian (1988). Rideout et al. (1990) emphasized the contribution of 5-methylcytosine to somatic mutations resulting in human disease, particularly tumorigenesis. They suggested that as much as 43% of P53 somatic mutations (9 of 21 observed mutations in tumors) may be due to the presence of 5-methylcytosine. Rideout et al. (1990) noted that CpGs make up 3.5% of the P53 double-stranded coding sequence and may be contributing to as much as 43% of the point mutations, each of which is a transition from 5-methylcytosine to thymine (or a corresponding transition from guanine to adenine).

Krawczak et al. (1995) compared the spectrum of somatic TP53 single basepair substitutions detected in 955 cancers with that of 2,224 different germline mutations in 279 different human genes (other than TP53), reported as the cause of inherited disease. The comparison demonstrated that, disregarding a relatively small subset (12%) of TP53 mutations that probably resulted from the action of exogenous mutagens, both the relative rates and the nearest-neighbor spectra of single basepair substitutions were similar in the 2 datasets. This resemblance suggested that a substantial proportion of cancer-associated somatic TP53 mutations result from endogenous cellular mechanisms. The likelihood of clinical observation of a particular mutation type differs, however, between tumors and genetic diseases, when the chemical properties of the resulting amino acid substitutions are considered. Together with a 6-fold higher observation likelihood for mutations at evolutionarily conserved residues, this finding argues that selection is a critical factor in determining which TP53 mutations are found to be associated with human cancer.

Since cigarette smoke carcinogens such as benzo(a)pyrene are implicated in development of lung cancer, Denissenko et al. (1996) investigated the relationship between benzo(a)pyrene diol epoxide (BPDE) adduct formation and P53 mutations. (BPDE is the ultimate carcinogenic metabolite of benzo(a)pyrene.) HeLa cells and normal bronchial epithelial cells were treated with BPDE, and DNA was isolated and cleaved at the sites of modified bases with UvrABC nuclease complex from E. coli. They mapped the distribution of BPDE adducts along the P53 gene using a modification of ligation-mediated PCR (LMPCR) with P53 oligonucleotide primers. They also examined isolated genomic DNA. Denissenko et al. (1996) demonstrated strong and selective adduct formation at guanine positions in codons 157, 248, and 273. They noted that these positions are the major mutation hotspots in human lung cancers and stated that the coincidence of mutational hotspots and adduct hotspots suggests that benzo(a)pyrene metabolites or structurally related compounds are involved in transformation of human lung tissue.

To investigate possible mechanisms underlying the selectivity of BPDE binding, Denissenko et al. (1997) mapped the adducts in plasmid DNA containing genomic P53 sequences. They found that when cytosines in CpG sequences were converted to 5-methylcytosines by the CpG-specific methylase, SssI, and the DNA was subsequently treated with BPDE, adduct hotspots were created that were similar to those seen in genomic DNA when all CpGs were methylated. A strong positive effect of 5-methylcytosine on BPDE adduct formation at CpG sites was also documented with sequences of the PGK1 gene (311800) derived from an active or inactive human X chromosome and having differential methylation patterns. These results showed that methylated CpG dinucleotides, in addition to being an endogenous promutagenic factor, may represent a preferential target for exogenous chemical carcinogens.

Hemochromatosis (HFE; 235200) and Wilson disease (WND; 277900), characterized by excess hepatic deposition of iron and copper, respectively, produce oxidative stress and increase the risk of liver cancer. Hussain et al. (2000) studied nontumorous liver tissue from WND and HFE patients for the frequency of p53 mutated alleles. When compared with the liver samples from normal controls, higher frequencies of G:C to T:A transversions at codon 249 (see 191170.0006), and C:G to A:T transversions and C:G to T:A transitions at codon 250 were found in liver tissue from WND cases, and a higher frequency of G:C to T:A transversions at codon 249 was also found in liver tissue from hemochromatosis cases. These results were consistent with the hypothesis that the generation of oxygen/nitrogen species and unsaturated aldehydes from iron and copper overload in hemochromatosis and WND causes mutation in the p53 gene.

Boettcher et al. (2019) used CRISPR-Cas9 to generate isogenic human leukemia cell lines of the most common TP53 missense mutations. Functional, DNA-binding, and transcriptional analyses revealed loss-of-function but no gain-of-function effects. Comprehensive mutational scanning of p53 single-amino acid variants demonstrated that missense variants in the DNA-binding domain exert a dominant-negative effect. In mice, the dominant-negative effect of p53 missense variants conferred a selective advantage to hematopoietic cells on DNA damage. Analysis of clinical outcomes in patients with acute myeloid leukemia showed no evidence of gain of function for TP53 missense mutations. Boettcher et al. (2019) concluded that a dominant-negative effect is the primary unit of selection for TP53 missense mutations in myeloid malignancies.

Mutation Detection

Tonisson et al. (2002) described an arrayed primer extension-based TP53 gene test as an accurate and efficient tool for DNA sequence analysis for both research and clinical applications.

Mutation Databases

Hollstein et al. (1996) gave an update on a listing of point mutations in the p53 gene of human tumors and cell lines compiled from the literature and made available electronically through the file server at the EMBL Data Library. In July 1995, the database contained records on almost 4,500 mutations. In their Figure 1, they mapped the growth of mutations in the database since 1989. The data were made available from the European Bioinformatics Institute (EBI) network server.

Beroud et al. (1996) described software and a database for p53 gene mutations available for both MS-DOS and Macintosh platforms. Their database contained more than 4,200 mutations as of September 1995.

De Vries et al. (1996) described a database of p53 mutations.

Hainaut et al. (1997) reported on the p53 mutation database maintained at the International Agency for Research on Cancer (IARC) in Lyon, France, reported on earlier by Hollstein et al. (1996). The current version contained records on 5,091 published mutations and was expected to surpass the 6,000 mark in the January 1997 release.

Hernandez-Boussard et al. (1999) stated that the IARC database included p53 somatic mutations (more than 10,000 entries), p53 germline mutations (144 entries), and p53 polymorphisms (13 entries), with somatic mutations organized into a relational database. Included in the database were annotations on individual characteristics and oncogenic exposures and a classification of pathologies based on the International Classification of Diseases for Oncology (ICD-O). In addition, several interfaces had been developed to analyze the data in order to produce mutation spectra, codon analyses, or visualization of the mutation with the tertiary structure of the protein.

Olivier et al. (2002) provided an update on the IARC TP53 database. More detailed annotations on patients, including carcinogen exposure, virus infection, and genetic background, had been added.

Beroud and Soussi (2003) described development of the UMD-p53 database maintained in Montpellier, France. Soussi et al. (2005) stated that more than 1,500 different TP53 mutants had been described in this database. The frequency of mutants was highly heterogeneous, with 11 hotspot mutants found more than 100 times, and 306 mutants found only once. Soussi et al. (2005) demonstrated high diversity in terms of loss of transactivation activity by testing TP53 mutants representing all possible amino acid substitutions caused by a point mutation. While the most frequent TP53 mutants sustained a clear loss of transactivation activity, more than 50% of rare TP53 mutants displayed significant activity.

▼ Other Features

Patrier-Sallebert et al. (2015) reported a gestational choriocarcinoma (CC) that developed in a female partner of a male patient with Li-Fraumeni syndrome (LFS1; 151623); the CC carried a germline TP53 mutation initially detected in this LFS patient. The authors then identified 78 fathers who were carriers of a germline TP53 mutation. Among the 213 corresponding pregnancies, Patrier-Sallebert et al. (2015) found 2 other cases of gestational CC in the female partners, and estimated that gestational CC occurs in approximately 1% of the deliveries in female partners of TP53 mutation carriers.

Mutations in Pluripotent Stem Cells

Merkle et al. (2017) sequenced the exomes of 140 independent human embryonic stem cell lines, including 26 lines prepared for potential clinical use, and identified 5 unrelated human embryonic stem cell lines that carried 6 mutations in the TP53 gene. The mutations were dominant-negative and were the mutations most commonly seen in human cancers. Merkle et al. (2017) found that the TP53 mutant allelic fraction increased with passage number under standard culture conditions, suggesting that the mutations confer selective advantage. The authors then mined published RNA sequencing data from 117 human pluripotent stem cell lines, and observed another 9 TP53 mutations, all resulting in coding changes in the DNA-binding domain. In 3 lines, the allelic fraction exceeded 50%, suggesting additional selective advantage resulting from the loss of heterozygosity at the TP53 locus. Merkle et al. (2017) concluded that, as the acquisition and expansion of cancer-associated mutations in pluripotent stem cells may go unnoticed during most applications, careful genetic characterization of human pluripotent stem cells and their differentiated derivatives should be carried out before clinical use.

▼ Animal Model
Donehower et al. (1992) found that development was normal in mice lacking 1 or both p53 alleles, but spontaneous tumors, specifically lymphomas and sarcomas, occurred in high frequency.

Kemp et al. (1994) reported that a single dose of 4 Gy radiation dramatically decreased the latency for tumor development in p53 heterozygous mice. The pattern of genetic alterations at the remaining wildtype allele in these tumors differed substantially from that in spontaneous tumors from similar mice, indicating that p53 itself may have been a target for radiation-induced alterations. Radiation at a lower dose, 1 Gy, of preweanling p53-null mice also significantly decreased tumor latency, suggesting that additional genetic targets are involved in radiation-induced malignancy.

Mutations in the p53 gene are found in only 25 to 33% of hepatocellular carcinoma cases, and the majority of hepatocellular carcinoma cases are associated with chronic hepatitis B virus (HBV) infections. Ueda et al. (1995) developed a transgenic mouse model in which expression of a single HBV gene product, the HBx transcriptional transactivator protein, led to progressive neoplastic changes in liver. Tumor development correlated precisely with p53 binding to HBx in the cytoplasm and complete blockage of p53 entry into the nucleus. Analysis of tumor DNA revealed p53 mutations in only a small proportion of advanced tumors, suggesting that p53 mutations were not the cause of tumors but may have contributed to tumor progression. Transgenic mice lacking the p53 gene were indistinguishable from wildtype litter mates except for early onset of tumor formation.

Sah et al. (1995) found that a variable percentage of p53-deficient animals exhibited midbrain exencephaly, a neural tube defect incompatible with postnatal survival.

DNA damage is thought to initiate the teratogenicity caused by numerous drugs and environmental chemicals, collectively termed xenobiotics. Nicol et al. (1995) hypothesized that p53 deficiencies may enhance susceptibility to chemical teratogenesis. To test this hypothesis, they chose benzo[a]pyrene, a known teratogen, and tested its effects on pregnant heterozygous p53-deficient mice. Such mice exhibited between 2- and 4-fold higher embryo toxicity and teratogenicity than normal p53 controls. Fetal resorptions reflecting in utero death increased 2.6- and 3.6-fold with heterozygous and homozygous p53-deficient embryos, respectively. Nicol et al. (1995) concluded that p53 is an important teratologic suppressor gene that protects the embryo from DNA-damaging chemicals and developmental oxidative stress.

Nakamura et al. (1995) generated transgenic mice expressing wildtype human p53 in lens, a tissue entirely composed of epithelial cells that differentiate into elongated fiber cells. These mice developed microphthalmia due to a defect in fiber formation that occurred shortly after birth. Apoptotic cells were observed that failed to undergo proper differentiation. The normal lens phenotype was restored in double-transgenic mice carrying both wildtype human p53 and a mutant human p53 lacking wildtype function. Nakamura et al. (1995) concluded that normal tissue differentiation requires proper balance of p53 expression.

XRCC4 (194363) is involved in DNA double-strand break repair and in V(D)J recombination. Xrcc4 -/- mice die late in embryonic development with extensive neuronal apoptosis and arrested lymphocyte development. Gao et al. (2000) generated mice lacking both Xrcc4 and p53 and found that p53 deficiency rescued several aspects of Xrcc4 deficiency, including embryonic lethality, neuronal apoptosis, and impaired cellular proliferation. However, p53 deficiency did not rescue impaired V(D)J recombination or lymphocyte development. Mice lacking both Xrcc4 and p53 appeared healthy until postnatal week 6, but then most succumbed to pro-B-cell lymphomas with chromosomal translocations linking amplified Myc oncogene (190080) and IgH locus (see 147100) sequences. Gao et al. (2000) concluded that the increased neuronal apoptosis and cellular proliferation defects of Xrcc4 -/- mice result from a p53-dependent response to unrepaired DNA damage.

Marino et al. (2000) generated a mouse model for medulloblastoma (155255) by conditional inactivation of Rb and p53 in cerebellar external granular layer (EGL) cells. Inactivation of Rb in a p53-null background produced mice that developed highly aggressive embryonal tumors of cerebellum with typical features of medulloblastoma. These tumors were identified as early as 7 weeks of age on the outer surface of the molecular layer, corresponding to the location of EGL cells during development.

Aged humans sustain a high rate of epithelial cancers, whereas mice with common tumor suppressor gene mutations typically develop soft tissue sarcomas and lymphomas. Artandi et al. (2000) found that telomere attrition in aging telomerase (TERC; 602322)-deficient p53 mutant mice promoted development of epithelial cancers by a process of fusion-bridge breakage that led to formation of complex nonreciprocal translocations, a classic cytogenetic feature of human carcinomas.

Jimenez et al. (2000) generated mice with an allele of the Trp53 gene encoding changes at leu25 and trp26, residues essential for transcriptional transactivation and binding with Mdm2 (164785). Mutant Trp53 was abundantly expressed, its level was not affected by DNA damage, and it bound DNA constitutively; however, it showed defects in cell cycle regulation and apoptosis. Both mutant and Trp53-null mouse embryonic fibroblasts were readily transformed by oncogenes, and the corresponding mice were prone to tumors. Jimenez et al. (2000) concluded that Trp53-mediated tumor suppression in mice requires the Trp53 transactivation domain.

Reilly et al. (2000) described a mouse model of astrocytoma involving mutation of Nf1 (613113) and Trp53. Humans with neurofibromatosis-1 due to mutations in the NF1 gene have an increased risk of optic gliomas, astrocytomas, and glioblastomas. TP53 is often mutated in a subset of astrocytomas that develop at a young age and progress slowly to secondary glioblastomas. The mouse model developed by Reilly et al. (2000) showed a range of astrocytoma stages, from low-grade astrocytoma to glioblastoma multiforme, and they suggested that it may accurately model human secondary glioblastoma involving TP53 loss.

Jonkers et al. (2001) developed conditional mutants for Brca2 (600185) and/or p53 inactivated in various epithelial tissues, including mammary gland epithelium. Although no tumors arose in mice carrying conditional Brca2 alleles, mammary and skin tumors developed frequently in females carrying conditional Brca2 and Trp53 alleles. The presence of 1 wildtype Brca2 allele resulted in a markedly delayed tumor formation; loss of the wildtype Brca2 allele occurred in a subset of these tumors. Jonkers et al. (2001) concluded that inactivation of BRCA2 and of p53 combine to mediate mammary tumorigenesis and that disruption of the p53 pathway is pivotal in BRCA2-associated breast cancer.

Tyner et al. (2002) inadvertently generated mice with a deletion of the first 6 exons of p53, resulting in expression of a truncated RNA capable of encoding a C-terminal p53 fragment. They termed the defective p53 allele the 'm' allele. Tyner et al. (2002) could not identify any p53 m protein, but the mutant allele conferred phenotypes consistent with activated rather than inactivated p53. Mutant (p53 +/m) mice exhibited enhanced resistance to spontaneous tumors compared with wildtype littermates, and they displayed early aging phenotypes. A second line of transgenic mice containing a temperature-sensitive mutant allele of p53 also exhibited early aging phenotypes. Tyner et al. (2002) concluded that p53 has a role in regulating organismal aging.

Garcia-Cao et al. (2002) developed mice carrying supernumerary copies of the p53 gene in the form of large genomic transgenes. Mice carrying the p53 transgene alleles in addition to the 2 endogenous alleles exhibited enhanced response to DNA damage and were significantly protected from cancer compared with normal mice.

Zhu et al. (2002) reported that mice deficient for both p53 and nonhomologous end-joining (NHEJ) succumbed to pro-B cell lymphomas that featured complex translocations harboring coamplified Myc and IgH sequences, and they elucidated the molecular mechanism by which these translocations arose.

Deletion of Trp53 greatly accelerates Myc-induced lymphomagenesis, resulting in highly disseminated disease (Schmitt et al., 2002). To determine whether RNA interference (RNAi)-mediated suppression of Trp53 could produce a similar phenotype, Hemann et al. (2003) introduced Trp53 short hairpin RNAs (shRNAs) into hematopoietic stem cells derived from transgenic E-mu-Myc transgenic mice and monitored tumor onset and overall pathology in lethally irradiated recipients. Different Trp53 shRNAs produced distinct phenotypes in vivo, ranging from benign lymphoid hyperplasias to highly disseminated lymphomas, that paralleled Trp53 -/- lymphomagenesis in E-mu-Myc transgenic mouse. In all cases, the severity and type of disease correlated with the extent to which specific shRNAs inhibited p53 activity.

Olive et al. (2004) engineered the structural mutant arg172 to his (R172H) and the contact mutant arg270 to his (R270H) into the endogenous p53 locus in mice; the mutations correspond to those of codons 175 (191170.0030) and 273 (191170.0020) in human. p53 R270H/+ and p53 R172H/+ mice were models of Li-Fraumeni syndrome and developed allele-specific tumor spectra distinct from p53 +/- mice. p53 R270H/- and p53 R172H/- mice developed novel tumors compared with p53 -/- mice, including a variety of carcinomas and more frequent endothelial tumors. Dominant effects that varied by allele and function were observed in primary cells derived from p53 R270H/+ and p53 R172H/+ mice. Olive et al. (2004) concluded that point mutant p53 alleles expressed under physiologic control have enhanced oncogenic potential beyond simple loss of p53 function.

Lang et al. (2004) generated mice harboring the R172H mutation. p53 +/R172H mice displayed a similar tumor spectrum and survival curve as p53 +/- mice, but tumors from p53 +/R172H mice metastasized with high frequency. Embryonic fibroblasts from p53 R172H/R172H mice displayed enhanced cell proliferation, DNA synthesis, and transformation potential. p53 containing R172H bound p63 (TP63; 603273) and p73 (TP73; 601990) in mouse tumor cell lines, and downmodulation of p63 and p73 in p53 -/- cells increased transformation capacity and reinitiated DNA synthesis to levels observed in p53 R172H/R172H cells.

Terzian et al. (2008) showed that mice homozygous for the R172H mutation had unstable mutant p53 in normal cells and stabilized mutant p53 in some, but not all, tumors. Deletion of Mdm2 or p16(Ink4a), a member of the Rb tumor suppressor pathway, stabilized mutant p53 and caused an earlier age of tumor onset, a gain-of-function metastatic phenotype, and defects in the Rb pathway. Additionally, ionizing radiation stabilized both wildtype and mutant p53. Terzian et al. (2008) concluded that stabilization of mutant p53 is a prerequisite for its gain-of-function phenotype.

Sablina et al. (2005) found that daily dietary supplementation of Trp53-null mice with the antioxidant N-acetylcysteine reduced the number of animals that developed lymphomas.

Erker et al. (2005) showed that treatment of p53-deficient mice with the nitroxyl antioxidant tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) resulted in a small but significant (25%) increase in life span by prolonging latency to tumorigenesis, demonstrating that existing oxidative stress and damage may not be necessary for the chemopreventative effects of tempol. The relatively small effect on latency in p53-deficient mice and the finding that tempol-mediated resistance to oxidative insult was p53-dependent suggested a more direct role of p53 in the chemopreventative effects of tempol. Tempol treatment specifically increased serine-18 phosphorylation of p53, but not gamma-H2AX (H2AFX; 601772) and p21 (CDKN1A; 116899) expression in primary thymocytes in vitro in a p53-dependent fashion. Inhibition of PI3K (see 171834) family members suggested that SMG1 (607032) was responsible for the tempol-mediated enhancement of p53 serine-18 phosphorylation. Erker et al. (2005) suggested that the chemopreventative effect of tempol may not be solely due to the reduction of oxidative stress and damage, but may also be related to redox-mediated signaling functions that include p53 pathway activation.

Using the knockin mouse model developed by Christophorou et al. (2005) in which p53 status can be reversibly switched in vivo between functional and inactive states, Christophorou et al. (2006) found that the p53-mediated pathologic response to whole body irradiation, a prototypic genotoxic carcinogen, was irrelevant for suppression of radiation-induced lymphoma. In contrast, delaying restoration of p53 function until the acute radiation response subsided abrogated all radiation-induced pathology yet preserved much of the protection from lymphoma. Such protection was absolutely dependent on p19(Arf), a tumor suppressor induced not by DNA damage, but by oncogenic disruption of the cell cycle.

Efeyan et al. (2006) found that mice lacking Arf responded normally to DNA damage. Mice with an additional transgenic copy of the p53 gene, or p53(super) mice, showed the same enhancement of apoptosis irrespective of whether Arf was present or absent. However, Arf-null cells were unable to respond effectively to oncogenic signaling and underwent neoplastic transformation by oncogenes in vitro, irrespective of the presence or absence of the p53(super) allele. p53(super)/Arf-null mice succumbed to spontaneous tumors at the same rate as wildtype p53/Arf-null mice and produced the same profile of sarcomas, lymphomas, and histiocytic sarcomas. When treated with a DNA-damaging agent, p53(super)/Arf-null mice did not benefit from the extra p53 allele. Efeyan et al. (2006) concluded that oncogenic signaling is the critical event that elicits p53-dependent protection and that DNA damage stimulus is less important.

Inactivation of both p53 and Brca1 (113705) in mouse mammary gland mimics the majority of human BRCA1-associated tumors, which also harbor mutations in both p53 and BRCA1. Poole et al. (2006) found that mammary glands of nulliparous Brca1/p53-deficient mice accumulated lateral branches and underwent extensive alveologenesis, a phenotype that occurs only during pregnancy in wildtype mice. Progesterone receptors, but not estrogen receptors, were overexpressed in mutant mammary epithelial cells because of a defect in their degradation by the proteasome pathway. Treatment of Brca1/p53-deficient mice with a progesterone antagonist prevented mammary tumorigenesis.

Hu et al. (2007) showed that p53 is important in reproduction in a gender-specific manner. Significant decreases in embryonic implantation, pregnancy rate, and litter size were observed in matings with p53 -/- female mice but not with p53 -/- male mice. Hu et al. (2007) identified the gene encoding leukemia inhibitory factor (LIF; 159540), a cytokine critical for implantation, as a p53-regulated gene that functions as the downstream mediator of this effect. p53 can regulate both basal and inducible transcription of LIF. Loss of p53 decreased both the level and function of LIF in uteri. Lower LIF levels were observed in the uteri of p53 -/- mice than in those of p53 +/+ mice, particularly at day 4 of pregnancy, when transiently induced high levels of LIF were crucial for embryonic implantation. Hu et al. (2007) suggested that this observation probably accounts for the impaired implantation of embryos in p53 -/- female mice. Administration of LIF to pregnant p53 -/- mice restored maternal reproduction by improving implantation. These results demonstrated a function for p53 in maternal reproduction through the regulation of LIF.

Bmi1 (164831) is necessary for the maintenance of adult hematopoietic stem cells (HSCs) and neural stem cells. Akala et al. (2008) demonstrated that bone marrow cells from mice with triple deletion of p16(Ink4a), p19(Arf) (alternative reading frames of the Cdkn2a gene, 600160), and Trp53, all genetically downstream of Bmi1, have an approximately 10-fold increase in cells able to reconstitute the blood long term. This increase was associated with the acquisition of long-term reconstitution capacity by cells of the phenotype c-kit+Sca1+Flt3+CD150-CD48-Lin-, which defines multipotent progenitors in wildtype mice. The pattern of triple mutant multipotent progenitor response to growth factors resembled that of wildtype multipotent progenitors but not wildtype HSCs. Akala et al. (2008) concluded that p16(Ink4a)/p19(Arf) and Trp53 have a central role in limiting the expansion potential of multipotent progenitors. The authors commented that these pathways are commonly repressed in cancer, suggesting a mechanism by which early progenitor cells could gain the ability to self-renew and become malignant with further oncogenic mutations.

In a study of mouse pigment mutants Dsk3 and Dsk4, caused by mutation in the ribosomal proteins Rps19 (603474) and Rps20 (603682), respectively, McGowan et al. (2008) identified a common pathophysiologic program in which stabilization of p53 stimulates KIT ligand (184745) expression and, consequently, epidermal melanocytosis via a paracrine mechanism. Accumulation of p53 also caused reduced body size and erythrocyte count. McGowan et al. (2008) concluded that their results provided a mechanistic explanation for the diverse collection of phenotypes that accompany reduced dosage of genes encoding ribosomal proteins, and have implications for understanding normal human variation and human disease.

Begus-Nahrmann et al. (2009) analyzed the functional consequences of conditional deletion of p53 in late-generation telomerase (Terc; 602322) knockout mice. Intestinal deletion of p53 shortened the life span of telomere-dysfunctional mice without inducing tumor formation. In contrast to deletion of p21 (116899), which elongates life span of telomere-dysfunctional mice, the deletion of p53 impaired the depletion of chromosomal-instable intestinal stem cells in aging telomere-dysfunctional mice. These instable stem cells contributed to epithelial regeneration leading to an accumulation of chromosomal instability, increased apoptosis, altered epithelial cell differentiation, and premature intestinal failure. Begus-Nahrmann et al. (2009) concluded that their results provided the first experimental evidence for an organ system in which p53-dependent mechanisms prevent tissue destruction in response to telomere dysfunction by depleting genetically instable stem cells.

Ruzankina et al. (2009) reported that p53 deficiency severely exacerbates tissue degeneration caused by mosaic deletion of the essential genome maintenance regulator Atr (601215). Combined loss of Atr and p53 led to severe defects in hair follicle regeneration, localized inflammation (Mac1+Gr1+ infiltrates), accelerated deterioration of the intestinal epithelium, and synthetic lethality in adult mice. Tissue degeneration in double-null mice was characterized by the accumulation of cells maintaining high levels of DNA damage. Moreover, the elevated frequency of these damaged cells in both progenitor and downstream compartments in double-null mouse skin coincided with delayed compensatory tissue renewal from residual Atr-expressing cells. Ruzankina et al. (2009) concluded that, taken together, their results indicated that the combined loss of Atr and Trp53 in adult mice leads to the accumulation of highly damaged cells, which, consequently, impose a barrier to regeneration from undamaged progenitors.

Acute exposure to ionizing radiation can cause lethal damage to the gastrointestinal (GI) tract, a condition called the GI syndrome. Whether the target cells affected by radiation to cause the GI syndrome are derived from the epithelium or endothelium and whether the target cells die by apoptosis or other mechanisms are controversial issues. Studying mouse models, Kirsch et al. (2010) found that selective deletion of the proapoptotic genes Bak1 (600516) and Bax (600040) from the GI epithelium or from endothelial cells did not protect mice from developing the GI syndrome after subtotal-body gamma irradiation. In contrast, selective deletion of p53 from the GI epithelium, but not from endothelial cells, sensitized irradiated mice to the GI syndrome. Transgenic mice overexpressing p53 in all tissues were protected from the GI syndrome after irradiation. Kirsch et al. (2010) concluded that the GI syndrome is caused by the death of GI epithelial cells and that these epithelial cells die by a mechanism that is regulated by p53 but independent of apoptosis.

Sahin et al. (2011) used transcriptomic network analyses in mice null for either Tert or Terc, which exhibit telomere dysfunction, to identify common mechanisms operative in hematopoietic stem cells, heart, and liver. Their studies revealed profound repression of peroxisome proliferator-activated receptor-gamma (PPARG; 601487), coactivator-1 alpha and beta (PCG1-alpha, 604517 and PGC1-beta, 608886), and the downstream network. Consistent with PGCs as master regulators of mitochondrial physiology and metabolism, telomere dysfunction was associated with impaired mitochondrial biogenesis and function, decreased gluconeogenesis, cardiomyopathy, and increased reactive oxygen species. In the setting of telomere dysfunction, enforced Tert or PGC1-alpha expression or germline deletion of p53 substantially restored PGC network expression, mitochondrial respiration, cardiac function, and gluconeogenesis. Sahin et al. (2011) demonstrated that telomere dysfunction activates p53 which in turn binds and represses PGC1-alpha and PGC1-beta promoters, thereby forging a direct link between telomere and mitochondrial biology. Sahin et al. (2011) proposed that this telomere-p53-PGC axis contributes to organ and metabolic failure and to diminishing organismal fitness in the setting of telomere dysfunction.

Elyada et al. (2011) showed that casein kinase I-alpha (CSNK1A1; 600505), a component of the beta-catenin (116806) destruction complex, is a critical regulator of the Wnt signaling (see 164820) pathway. Inducing the ablation of Csnk1a1 in the gut triggers massive Wnt activation, surprisingly without causing tumorigenesis. CKI-alpha-deficient epithelium shows many of the features of human colorectal tumors in addition to Wnt activation, in particular the induction of the DNA damage response and cellular senescence, both of which are thought to provide a barrier against malignant transformation. The epithelial DNA damage response in mice is accompanied by substantial activation of p53, suggesting that the p53 pathway may counteract the protumorigenic effects of Wnt hyperactivation. Notably, the transition from benign adenomas to invasive colorectal cancer in humans is typically linked to p53 inactivation, underscoring the importance of p53 as a safeguard against malignant progression; however, the mechanism of p53-mediated tumor suppression is unknown. Elyada et al. (2011) showed that the maintenance of intestinal homeostasis in CKI-alpha-deficient gut requires p53-mediated growth control, because the combined ablation of Csnk1a1 and either p53 or its target gene p21 (116899) triggered high-grade dysplasia with extensive proliferation. Unexpectedly, these ablations also induced nonproliferating cells to invade the villous lamina propria rapidly, producing invasive carcinomas throughout the small bowel. Furthermore, in p53-deficient gut, loss of heterozygosity of the gene encoding CKI-alpha caused a highly invasive carcinoma, indicating that CKI-alpha caused a highly invasive carcinoma, indicating that CKI-alpha functions as a tumor suppressor when p53 is inactivated. Elyada et al. (2011) identified a set of genes (the p53-suppressed invasiveness signature, PSIS) that is activated by the loss of both p53 and CKI-alpha and which probably accounts for the brisk induction of invasiveness. PSIS transcription and tumor invasion were suppressed by p21, independently of cell cycle control. Restraining tissue invasion through suppressing PSIS expression is thus a novel tumor suppressor function of wildtype p53. PROX1 (601546), IFITM2 (605578), and IFITM3 (605579) are all PSIS genes.

Spehlmann et al. (2013) found that loss of p53 or of its upstream activating kinase, Atm, protected mice against acute intestinal inflammation through increased survival of epithelial cells and lamina propria macrophages, higher Il6 expression due to enhanced glucose-dependent Nfkb activation, and increased mucosal Stat3 (102582) activation. Blocking Il6 signaling reversed the protective effects of p53 deficiency, whereas Il6 treatment protected against acute colitis in a Stat3-dependent manner. Spehlmann et al. (2013) concluded that p53 promotes inflammation in the intestinal tract by suppressing epithelial protective factors.

▼ ALLELIC VARIANTS ( 44 Selected Examples):

Malkin et al. (1990) demonstrated that alterations of the TP53 gene occur not only as somatic mutations in human cancers, but also as germline mutations in some cancer-prone families. In 2 families with Li-Fraumeni syndrome-1 (151623), they identified a C-to-T mutation at the first nucleotide of codon 248, changing arginine to tryptophan (R248W). Loss of the wildtype allele was found in the tumor in some cases.

By transfection of the R248W mutant into malignant cells, Frebourg et al. (1992) demonstrated loss of tumor suppressor activity.

In a family with the Li-Fraumeni syndrome-1 (151623), Malkin et al. (1990) identified a G-to-A mutation at the first nucleotide of codon 258, resulting in substitution of lysine for glutamic acid (E258K).

In a family with the Li-Fraumeni syndrome-1 (151623), Malkin et al. (1990) identified a G-to-T mutation at the first nucleotide of codon 245, resulting in substitution of cysteine for glycine (G245C).

Frebourg et al. (1992) showed that the germline G245C mutation resulted in loss of tumor suppressor activity in malignant cells.

TP53, 1-BP DEL, T, CODON 184
In a family with the Li-Fraumeni syndrome-1 (151623), Malkin et al. (1990) identified deletion of a thymidine at the third nucleotide of codon 184, resulting in a frameshift and a novel stop at codon 246. This mutation was reported in the erratum for the article in which Malkin et al. (1990) had reported that this family had a germline T-to-C change at the first position of codon 252, resulting in substitution of proline for leucine (LEU252PRO; L252P).

TP53, PRO72ARG (rs1042522)
Ara et al. (1990) reported that the pro72-to-arg (P72R) change in p53 is caused by polymorphism rather than mutation. Olschwang et al. (1991) assessed the frequency of the pro72-to-arg (P72R) polymorphism and, from its frequency in colon cancer patients and control subjects, concluded that there was no strong association with colon cancer. In both the cancer group and the control group, the frequencies of the pro72 and arg72 alleles were about 31 and 69%, respectively.

The E6 oncoprotein derived from tumor-associated human papillomaviruses (HPVs) binds to and induces degradation of p53. Storey et al. (1998) investigated the effect of the P72R polymorphism on susceptibility of p53 to E6-mediated degradation and found that the arg72 form of p53 was significantly more susceptible than the pro72 form. Moreover, allelic analysis of patients with HPV-associated tumors revealed a striking overrepresentation of homozygous arg72 p53 compared with the normal population, indicating that individuals homozygous for arg72 are about 7 times more susceptible to HPV-associated tumorigenesis than heterozygotes.

Using immunoprecipitation followed by SDS-PAGE, Thomas et al. (1999) found that the arg72 and pro72 p53 variants did not differ in their ability to bind DNA in a sequence-specific manner. They concluded that arg72 and pro72 are conformationally indistinguishable and that both can be considered wildtype. However, Thomas et al. (1999) noted that p53(pro) was a stronger inducer of transcription than p53(arg), whereas p53(arg) induced apoptosis faster and was a more potent suppressor of transformation than p53(pro).

Marin et al. (2000) found that some tumor-derived p53 mutants bound and inactivated p73 (601990). The binding of such mutants was influenced by whether TP53 codon 72 encoded arginine or proline. The ability of p53 to bind p73, neutralize p73-induced apoptosis, and transform cells in cooperation with EJ-Ras (see 190020) was enhanced when codon 72 encoded arg. Marin et al. (2000) found that the arg-containing allele was preferentially mutated and retained in squamous cell tumors arising in arg/pro germline heterozygotes. They concluded that inactivation of p53 family members may contribute to the biologic properties of a subset of p53 mutants, and that a polymorphic residue within p53 affects mutant behavior.

Laryngeal papillomatosis is caused by human papillomavirus and is associated with malignant transformation in 3 to 7% of cases. Aaltonen et al. (2001) found no difference in the prevalence of the P72R polymorphism between a group of patients with laryngeal papillomas and a control group.

The pro72-to-arg polymorphism occurs in the proline-rich domain of p53, which is necessary for the protein to fully induce apoptosis. Dumont et al. (2003) found that in cell lines containing inducible versions of alleles encoding the pro72 and arg72 variants, and in cells with endogenous p53, the arg72 variant induced apoptosis markedly better than the pro72 variant. They suggested that at least 1 source of this enhanced apoptotic potential is the greater ability of the arg72 variant to localize to mitochondria; this localization was accompanied by release of cytochrome c into the cytosol.

In 92 Caucasian MLH1 (120436) or MSH2 (609309) mutation carriers, including 47 with colorectal cancer, Jones et al. (2004) analyzed the p53 codon 72 genotype and found that arg/pro heterozygotes were 1.94 times more likely to get colorectal cancer during any age interval and developed it 13 years earlier than arg/arg homozygotes. The number of pro/pro homozygotes was too small to provide meaningful results.

Kruger et al. (2005) studied the p53 genotype of 167 unrelated patients with hereditary nonpolyposis colon cancer (HNPCC; see 120435) with germline mutations in either MSH2 or MLH1 and found that the median age of onset was 41 years for arg/arg, 36 years for arg/pro, and 32 years for pro/pro individuals (p less than 0.0001). There was no difference in age of onset in 126 patients with microsatellite stable colorectal cancers. Kruger et al. (2005) concluded that in a mismatch repair-deficient background, p53 codon 72 genotypes are associated with the age of onset of colorectal carcinoma in a dose-dependent manner.

Bougeard et al. (2006) studied the effect of the MDM2 SNP309 polymorphism (164785.0001) and the arg72-to-pro polymorphism of the p53 gene on cancer risk in 61 French carriers of the p53 germline mutation. The mean age of tumor onset in p53 codon 72 polymorphism arg allele carriers (21.8 years) was different from that of pro/pro patients (34.4 years, p less than 0.05). Bougeard et al. (2006) also observed a cumulative effect of both polymorphisms because the mean ages of tumor onset in carriers of MDM2 G and p53 arg alleles (16.9 years) and those with the MDM2 T/T and p53 pro/pro genotypes (43 years) were clearly different (p less than 0.02). The results confirmed the impact of the MDM2 SNP309 G allele on the age of tumor onset in germline p53 mutation carriers, and suggested that this effect may be amplified by the p53 arg72 allele.

IASPP (607463) is among the most evolutionarily conserved inhibitors of p53, whereas ASPP1 (606455) and ASPP2 (602143) are activators of p53. Bergamaschi et al. (2006) showed that, in addition to the DNA-binding domain, the ASPP family members also bound to the proline-rich region of p53 containing the codon 72 polymorphism. Furthermore, the ASPP family members, particularly IASPP, bound to and regulated the activity of p53 pro72 more efficiently than that of p53 arg72.

Orsted et al. (2007) stated that arg72 increases the ability of p53 to locate to mitochondria and induce cell death, whereas pro72 exhibits lower apoptotic potential but increases cellular arrest in G1 of the cell cycle. In a study of 9,219 Danish individuals, they found that overall 12-year survival was increased in p53 arg/pro heterozygotes by 3% (P of 0.003) and in pro/pro homozygotes by 6% (P of 0.002) compared with arg/arg homozygotes, corresponding to an increase in median survival of 3 years for pro/pro versus arg/arg homozygotes. Pro/pro homozygotes also showed increased survival after development of cancer, or even after development of other life-threatening diseases, compared with arg/arg homozygotes. The arg72-to-pro change was not associated with decreased risk of cancer.

Among 254 patients with glioblastoma multiforme (see 137800), El Hallani et al. (2009) found an association between the pro72 allele and earlier age at onset. The pro/pro genotype was present in 20.6% of patients with onset before age 45 years, compared to in 6.5% of those with onset after age 45 years (p = 0.002) and 5.9% among 238 controls (p = 0.001). The findings were confirmed in an additional cohort of 29 patients. The variant did not have any impact on overall patient survival. Analysis of tumor DNA from 73 cases showed an association between the pro allele and a higher rate of somatic TP53 mutations.

In a study of 863 individuals with European grandparents from an unselected New Zealand birth cohort, Hancox et al. (2009) analyzed lung function (FEV1 and FEV1/FVC) between ages 18 and 32 in relation to cumulative history of cigarette smoking and the rs1042522 SNP, and found that the G allele was associated with smoking-related accelerated rate of decline in lung function (see 608852) (FEV1, p = 0.020; FEV1/FVC, p = 0.037).

Hsu et al. (1991) analyzed for mutations in p53 in hepatocellular carcinomas (see 114550) from patients in Qidong, an area of high incidence in China, in which both hepatitis B virus and aflatoxin B1 are risk factors. Eight of 16 tumors had a G-to-T mutation at the third base position of codon 249, changing arginine to serine (R249S).

The R249S mutation was found by Crook et al. (1992) in cervical cancer (603596). They noted that p53 mutations were found in only cervical cancers that did not show HPV sequences.

In cases of hepatocellular carcinoma (114550) in southern Africa, Bressac et al. (1991) identified a G-to-T substitution in codon 157 of TP53, changing valine to phenylalanine (V157F).

In a patient with Li-Fraumeni syndrome-1 (151623) presenting as a malignant ependymoma of the posterior fossa, Metzger et al. (1991) identified a germline cys242-to-tyr (C242Y) substitution in the TP53 gene. Tumor tissue from the patient carried the same mutation. Family history revealed that many members had died of various cancers, including osteosarcoma and other brain tumors. The mutation was in exon 7 in an area highly conserved across species and a region involved in several other mutations in neoplasms, including in other families with Li-Fraumeni syndrome-1. Ependymoma had not previously been observed as a feature of Li-Fraumeni syndrome. Eeles (1995) noted that this family had tumors characteristic of Li-Fraumeni-like syndrome, but only among relatives with a third-degree relationship.

Srivastava et al. (1990) reported a family with Li-Fraumeni syndrome-1 (151623) in which noncancerous skin fibroblasts from affected individuals showed an unusual radiation-resistant phenotype. They found that these cells from 4 family members, spanning 2 generations, had the same point mutation in codon 245 of the P53 gene. A change from GGC to GAC predicted substitution of aspartic acid for glycine (G245D). The fibroblast cell lines retained the normal P53 allele as well. A different mutation of codon 245 has been observed in a different Li-Fraumeni family (191170.0003).

In 2 of 8 families with Li-Fraumeni syndrome-1 (151623), Santibanez-Koref et al. (1991) identified mutations in the TP53 gene. One was the previously described arg248-to-trp mutation (191170.0001). The second was a novel mutation in the same codon: a CGG-to-CAG change resulting in substitution of glutamine for arginine (R248Q). Each family had 2 individuals affected. In the arg248-to-trp family, one individual had breast cancer at age 33, and the other had rhabdomyosarcoma at age 3 and chondrosarcoma at age 16. In the arg248-to-gln family, one had bilateral breast cancer at age 25 and leiomyosarcoma at age 44, and the other had medulloblastoma at age 3 and osteosarcoma at age 8.

Toguchida et al. (1992) also identified an arg248-to-gln change as a novel germline mutation in a patient with osteosarcoma who had had 2 primary tumors in her lifetime. At 17 years of age she was found to have osteosarcoma of the right femur, and 2 years later had an osteosarcoma of her right forearm. She was disease-free until the age of 28 years, when bilateral breast cancer was diagnosed. Orbital rhabdomyosarcoma developed in her daughter at the age of 5 years. Both the mother and the daughter had the same variant band on SSCP analysis of exon 7. The proband's parents lacked the abnormal band.

In a family with features of Li-Fraumeni syndrome, Tachibana et al. (2000) identified a germline R248Q mutation in the p53 gene. Several family members developed glioblastoma multiforme (see 137800).

In 9 members of an extended family with Li-Fraumeni syndrome-1 (151623), Law et al. (1991) found that a germline mutation at codon 133 (ATG-to-ACG), resulting in substitution of threonine for methionine (M133T), completely cosegregated with the cancer syndrome. An ATG-to-TTG mutation at codon 133, resulting in substitution of leucine for methionine, had been reported previously in a sporadic cancer of the colon (Nigro et al., 1989).

Hung et al. (1999) identified the same M133T mutation in the TP53 gene in 2 large, apparently unrelated African American families, both of which had a high incidence of breast cancer and other tumors characteristic of Li-Fraumeni syndrome. Haplotype analysis revealed that the 2 families shared an identical haplotype. Loss of heterozygosity at the TP53 locus in tumor tissue from each family was observed; in each case, the retained allele carried the common haplotype. The frequency of this haplotype in the general African American population is less than 0.003. This unique haplotype, combined with the rare TP53 mutation, suggested that these African American families share a common ancestry. The second proband of Hung et al. (1999) was from the same family as that in which Law et al. (1991) had originally described the M133T mutation in relation to Li-Fraumeni syndrome.

Felix et al. (1992) examined the p53 gene in primary lymphoblasts of 25 pediatric patients with acute lymphoblastic leukemia by the RNase protection assay and by SSCP analysis. In 4 of 25, p53 mutations were found. In 1 pedigree consistent with Li-Fraumeni syndrome-1 (151623), a germline G-to-T transversion at codon 272, changing valine to leucine (V272L), was found. The proband died at age 19 of ALL. A brother died of osteogenic sarcoma at the age of 17. Their mother died of uterine cancer at age 37. Bone cancer was the cause of death in a maternal uncle at age 33, and uterine cancer in the maternal grandmother at the age of about 40.

Toguchida et al. (1992) identified a ser241-to-phe (S241F) mutation due to a TCC-to-TTC change in exon 7 of the p53 gene in a patient whose hepatoblastoma (see 114550) was diagnosed at the age of 3 months. At the age of 8 years, multiple foci of osteosarcoma (259500) were discovered both within and outside the field of radiation therapy for the hepatoblastoma. SSCP analysis in the family suggested that this was a novel germline mutation. The same mutation was identified in an osteosarcoma by Smith-Sorensen et al. (1993).

TP53, 1-BP INS, 151C
Toguchida et al. (1992) identified a 1-bp insertion involving codons 151 and 152 of exon 6 of the p53 gene, resulting in a stop codon at position 180, in a patient in whom osteosarcoma was diagnosed at the age of 19 years. He had a family history of cancer consistent with Li-Fraumeni syndrome-1 (151623). The insertion was a cytosine in a stretch of 5 cytosines spanning codons 151 through 152. The change was predicted to lead to truncation of 212 amino acids from the p53 protein. His apparently healthy 4-year-old daughter and 12-year-old nephew also carried the mutant allele.

TP53, 2-BP DEL, CODONS 209-210
Toguchida et al. (1992) identified a 2-bp deletion at codons 209 and 210 of p53, resulting in a premature stop at codon 214, in an 8-year-old girl with malignant fibrous histiocytoma. Although the father was well, his brother had died of brain tumor at age 31, his sister of neurofibrosarcoma at age 17, and his father of pancreatic cancer. Eeles (1995) classified this family as having classic Li-Fraumeni syndrome (151623).

TP53, 1-BP INS, CODONS 71-72
Toguchida et al. (1992) identified an insertion of 1 cytosine in a stretch of 6 cytosines spanning codons 71 and 72 of the p53 gene in a girl who died at the age of 15 of osteosarcoma. The mother had died at age 25 years of a brain tumor. Eeles (1995) classified this family as having Li-Fraumeni-like syndrome (see 151623).

Toguchida et al. (1992) identified an AAG-to-TAG change of codon 120 of the p53 gene, resulting in conversion of lysine to a stop codon (K120X). The patient had an osteosarcoma and adenocarcinoma of the lung at age 18 years, and brain tumor at the age of 27 years. The patient's mother had breast cancer at age 25 years. Eeles (1995) classified this family as having Li-Fraumeni-like syndrome (see 151623).

Toguchida et al. (1992) identified a CGG-to-TGG change at codon 282 of the p53 gene, resulting in substitution of tryptophan for arginine (R282W). The proband had osteosarcoma at age 10 years and had an extensive family history of malignant tumors with an unusual prevalence of gastric cancer on the paternal side. The germline mutation in this family was demonstrated not only by the proband, but also by the affected father and by 2 apparently healthy sisters, aged 15 and 9 years at the time of the study. Eeles (1995) classified this family as having Li-Fraumeni-like syndrome (see 151623).

Iavarone et al. (1992) identified the R282W mutation. in a patient with multifocal osteogenic sarcoma. Further rearrangement of the residual wildtype allele was detected in tumor tissue.

The germline R282W mutation was identified by Malkin et al. (1992) in a proband who had liposarcoma diagnosed at the age of 7 years and osteosarcoma at the age of 12 years.

The R282W mutation was identified in an osteosarcoma by Smith-Sorensen et al. (1993).

Toguchida et al. (1992) identified a GGC-to-AGC mutation in the p53 gene, resulting in a gly245-to-ser (G245S) substitution, in a patient whose osteosarcoma was diagnosed at the age of 18 years. The disease pursued a rapid course with multiple foci of osteosarcoma and unsuccessful treatment. The same gly245-to-ser mutation was identified in his father and younger brother. The father, who was in his mid-fifties, was healthy but had numerous pigmented, benign nevi. The brother had a single osteosarcoma at the age of 18 years that was successfully treated; he also had skin lesions like those of his father. Eeles (1995) classified this family as having Li-Fraumeni-like syndrome (see 151623).

Malkin et al. (1992) identified a germline CGT-to-CAT mutation in exon 8 of the p53 gene that converted arg273 to his (R273H). The proband was a male in whom soft-tissue sarcoma was discovered at the age of 22 years and gastric carcinoma at the age of 30 years (see LFS; 151623).

In 5 of 6 anaplastic carcinomas of the thyroid and in an anaplastic carcinoma thyroid cell line ARO, Fagin et al. (1993) identified the R273H mutation. The presence of p53 mutations almost exclusively in poorly differentiated thyroid tumors and thyroid cancer cell lines suggested that inactivation of p53 may confer these neoplasms with aggressive properties and may further loss of differentiated function.

Malkin et al. (1992) identified a germline GGA-to-GTA mutation in exon 9 of the p53 gene, resulting in a change of gly325 to val (G325V). The proband had non-Hodgkin lymphoma (605027) diagnosed at age 17 years and colon carcinoma (114500) at age 26 years. The patient had a number of cafe-au-lait spots suggestive of neurofibromatosis. The mother and 1 sister had the same mutation; neither had had cancer, but both had cystic changes in the breast or ovary, and the sister had evidence of cervical dysplasia.

.0022 MOVED TO 191170.0018


Nasopharyngeal carcinoma (607107) occurs with a particularly high frequency in southern China and Southeast Asia. It has been proposed that initiation of nasopharyngeal carcinoma requires expression of the Epstein-Barr virus, but that induction of preneoplastic events and maintenance of the tumor-cell phenotype require critical cellular genes. Sun et al. (1992) found a heterozygous G-to-C transversion at codon 280 (exon 8), position 2, of the TP53 gene, predicted to change arginine to threonine (R280T), in a nasopharyngeal carcinoma cell line originating from Guangdong, a province in the People's Republic of China that leads the world in NPC incidence. However, the mutation was found in only 1 of 12 NPC samples from Hunan, another province in the PRC with high NPC incidence, and in none of 10 biopsies from Taiwan. Sun et al. (1992) concluded that alterations in the TP53 gene are not common in NPC. Normal expression of p53 mRNA in NPC cells as well as no loss of heterozygosity or gross structural alteration of the TP53 gene was observed in NPC cell lines and biopsies.

In a breast cancer (114480), Carrere et al. (1993) identified a CCC-to-ACC transversion in codon 151 of the p53 gene, resulting in a substitution of proline by threonine (P151T).

In a breast cancer (114480), Chen et al. (1991) identified a CCC-to-TCC transition in codon 151 of the p53 gene, resulting in substitution of proline by serine (P151S). (In the article by Chen et al. (1991), the codon was erroneously cited as 149 (Smith, 1993).) A different mutation in the same codon (191170.0025) has also been identified in a breast cancer.

Casey et al. (1993) found that the TP53 gene was mutated in 8 of 24 pancreatic cancers (260350) examined. One mutation was a G-to-T transversion at codon 35, resulting in a change from TTG (leu) to TTT (phe) (L25F). Casey et al. (1993) found no P53 mutations in 8 cases of chronic pancreatitis.

Mazoyer et al. (1994) found constitutional heterozygosity for a CTG-to-CAG transversion at codon 257 of the TP53 gene, leading to substitution of glutamine for leucine (L257G). The proband developed osteosarcoma at age 11 years, phyllodes tumor at the age of 15 years, and soft-tissue sarcoma at the age of 22 years. No DNA was available from the deceased mother, who had developed breast cancer at the age of 31 years. Both brothers of the proband, who were healthy at the ages of 22 and 19 years, had the same mutation. Mazoyer et al. (1994) also identified a different mutation at the same nucleotide of codon 257 (191170.0029) in an unrelated family with Li-Fraumeni syndrome (151623). Eeles (1995) classified the family with the L257G mutation as having Li-Fraumeni-like syndrome (see 151623).

TP53, 1-BP DEL, CODON 257
Mazoyer et al. (1994) found deletion of a single base (CTG to CG) at codon 257 of the TP53 gene. The deletion predicted a shift in the open reading frame, producing a mutant protein with 87 C-terminal amino acids not present in the wildtype protein. Breast cancer, with diagnosis at age 34 years, was present in the proband. A brother who developed osteosarcoma at age 31 years had the same mutation. A third sib had the mutation, but was healthy at age 41 years, although his son had developed a medulloblastoma at the age of 4 years. The family had features of Li-Fraumeni syndrome (151623).

Varley et al. (1995) studied an extensively affected 4-generation family with Li-Fraumeni syndrome-1 (151623). The structure of the family was sufficient to establish linkage to TP53. Subsequent DNA sequence analysis showed a CGC-to-CAC transition in exon 5 of the TP53 gene, resulting in an arg175-to-his (R175H) substitution that altered a recognition site for the restriction enzyme HhaI. This LFS family was unusual for the presence of 2 gastric carcinomas; endometrial cancers were absent, and malignancies were of early onset and particularly severe. Two persons developed a childhood sarcoma, and brain tumors were present in 4. An increase in the risk of breast cancer in mothers of children with osteosarcoma and chondrosarcoma had been reported and was a phenomenon not demonstrated in this family.

By in vitro studies, Capponcelli et al. (2005) found that R175H fibroblasts showed increased resistance to doxorubicin treatment with decreased nuclear localization of the p53 protein compared with wildtype cells.

Varley et al. (1996) described a family with classic Li-Fraumeni syndrome (151623) in which a leu344-to-pro (L344P) mutation was identified in the TP53 gene. Codon 344 is a key residue within the tetramerization domain, and the mutation had profound implications for tetramerization and potentially for DNA binding. This was the first report of a mutation in this residue in either sporadic tumors or in the germline, and it was the first report of a germline mutation within the tetramerization domain. The family did not appear to be remarkable in the spectrum of tumors, and there was loss of the wildtype allele in leiomyosarcoma in the proband. The proband presented at the age of 44 years with a retroperitoneal leiomyosarcoma. Previously he had had leg amputation for osteosarcoma. One brother had died of pancreatic cancer at the age of 49 years, and a second brother had died of osteosarcoma under the age of 40 years. The father had died at age 27 years of esophageal cancer. Many of the family members lived in India.

In a family with Li-Fraumeni syndrome-1 (151623), Sedlacek et al. (1998) detected a change of codon 138 from GCC (ala) to CCC (pro) (A138P). The family was remarkable for 2 early childhood cases of adrenocortical tumors occurring in sisters at the ages of 1.5 and 0.5 years. The older sister had also developed rhabdomyosarcoma at the age of 2.5 years. The girl's grandfather died of renal cell carcinoma at the age of 45 years and the great-grandparents died of gastric cancer and osteosarcoma at ages 37 and 45 years, respectively.

TP53, 1-BP DEL, CODON 178
In a family with Li-Fraumeni syndrome-1 (151623), Sedlacek et al. (1998) found deletion of C from codon 178 (CAC to AC) of the TP53 gene, resulting in a frameshift and premature chain termination. Three of 6 tumors examined from this family and the family with the ala138-to-pro mutation (191170.0032) showed loss of heterozygosity and contained only the mutant p53 allele. The remaining 3 neoplasms, 2 adrenocortical tumors and a choroid plexus tumor, retained heterozygosity. Immunohistochemistry with anti-p53 antibody confirmed accumulation of p53 protein in tumors with loss of heterozygosity, while the remaining tumors were p53 negative. These results were interpreted as supporting the view that complete loss of activity of the wildtype p53 need not be the initial event in formation of all tumors in LFS. The proband in the 1-bp deletion family had choroid plexus carcinoma at the age of 2 years. Breast cancer, ovarian cancer, osteosarcoma, liposarcoma, leukemia, astrocytoma, meningioma, gastric cancer, uterine cancer, and pharyngeal cancer occurred in other members of the family.

In a Turkish family with the diagnosis of Li-Fraumeni syndrome (151623), Guran et al. (1999) analyzed the mutation pattern of TP53, p57(KIP2) (CDKN1C; 600856), p15(INK4B) (CDKN2B; 600431), and p16(INK4A) (CDKN2A; 600160) in the peripheral blood, and loss of heterozygosity (homo/hemizygous deletion) pattern of TP53 and p15(INK-4B)/p16(INK4A) in 2 tumor tissues. The propositus had a seminoma, his daughter had a medulloblastoma, and one of his healthy cousins (aged 6 years) had a TP53 codon 292 missense point mutation, AAA (lys) to ATA (ile) (K292I), in the peripheral blood cells. Tumor tissue obtained from the propositus with the seminoma revealed loss of heterozygosity in the TP53 gene. In the analyses of tumor tissues from the propositus and his daughter, a CDKN2A codon 94 missense point mutation, GCG (ala) to GAG (glu) (600160.0011), was observed with the hereditary TP53 mutation. This was the first time that a mutation in CDKN2A had been observed in Li-Fraumeni syndrome.

The incidence of pediatric adrenocortical carcinoma (202300) in southern Brazil is 10 to 15 times higher than that of pediatric ADCC worldwide. Because childhood ADCC is associated with Li-Fraumeni syndrome (151623), Ribeiro et al. (2001) examined the cancer history and p53 status of 36 Brazilian patients and their families. Remarkably, 35 of 36 patients had an identical germline point mutation in exon 10 of the p53 gene, a G-to-A transition at nucleotide 1010 encoding an arg337-to-his (R337H) amino acid substitution. Differences within intragenic polymorphic markers demonstrated that at least some mutant alleles arose independently, thus eliminating the possibility of a founder effect. In tumor cells, the wildtype allele was deleted, and mutant p53 protein accumulated within the nuclei. Although these features are consistent with Li-Fraumeni syndrome-associated adrenal tumors, there was no history of increased cancer incidence among family members. Therefore, this inherited R337H p53 mutation represents a low-penetrance p53 allele that contributes in a tissue-specific manner to the development of pediatric ADCC.

DiGiammarino et al. (2002) demonstrated that the mutant tetramerization domain of p53 harboring the R337H mutation adopts a native-like fold but is less stable than the wildtype domain. Furthermore, the stability of the p53 R337H-bearing tetramer is highly sensitive to pH in the physiologic range; this sensitivity correlates with the protonation state of the mutated his337. DiGiammarino et al. (2002) concluded that their results demonstrated a pH-sensitive molecular defect of p53, suggesting that the pH-dependent p53 dysfunction is the molecular basis for these cases of ADCC in Brazilian children.

Latronico et al. (2001) studied this mutation in a larger series of 55 patients (37 adults and 18 children) with benign and malignant sporadic adrenocortical tumors. None of the patients had family cancer histories that conformed to the criteria for Li-Fraumeni syndrome. Among the 19 patients with the R337H mutation, only 1 boy and 3 adults showed fatal evolution or recurrent metastases. This mutation was also identified in heterozygous state in asymptomatic first-degree relatives of the patients, indicating that R337H mutation was inherited in most cases. The authors concluded that the germline R337H mutation of p53 protein is present at a high frequency (approximately 78%) in children with benign or malignant sporadic adrenocortical tumors, but it is not restricted to the pediatric group, since about 14% of adults with adrenocortical tumors also had this mutation. The presence of this mutation was related to unfavorable prognosis in most of the adults but not in the children with adrenocortical tumors.

Longui et al. (2004) investigated the inhibin-alpha (INHA; 147380) gene in 46 Brazilian children with ADCC, 39 of whom were heterozygous carriers of R337H. Six patients were heterozygous for 3 INHA mutations, and Longui et al. (2004) concluded that INHA may be one of the contributing factors needed for adrenocortical tumor formation in pediatric patients with the R337H TP53 mutation.

Figueiredo et al. (2006) identified the R337H mutation in 40 children from southern Brazil with ADCC. The mutation was also identified in 34.5% of relatives tested in parental carrier lines. The penetrance of ADCC among carriers of R337H was estimated at 9.9%.

Pinto et al. (2005) studied deletion mapping of chromosome 17 in 30 adrenocortical tumors from 29 Brazilian patients (15 children and 14 adults). Sixteen patients had the germline R337H mutation. Loss of heterozygosity (LOH) analysis using 6 polymorphic microsatellite markers disclosed loss of the entire chromosome 17 in 18 tumors (10 adenomas and 8 carcinomas) from 17 patients. The R337H mutation was present in 13 of them. The authors demonstrated a high frequency of biallelic inactivation of p53 derived from 2 distinct events occurs, the germline R337H mutation and the acquired loss of the entire chromosome 17. The isolated loss of the entire chromosome 17 did not correlate with aggressive tumor behavior in these patients with adrenocortical tumors.

TP53, 7-BP INS, NT13160
In a 29-year-old woman with a rare choroid plexus papilloma (260500) who had had, at the age of 22 years, an osteosarcoma (259500), Rutherford et al. (2002) detected a germline 7-bp insertion in exon 5 of the p53 gene. The alteration was predicted to produce amino acid substitutions beginning with alanine to glycine at position 161 and a stop codon at position 182 in the mutated protein. Two assays of p53 function gave apparently wildtype results on peripheral blood lymphocytes from this woman. The mutant allele was expressed either at very low levels or not at all in phytohemagglutinin-stimulated lymphocytes. Furthermore, the mutant protein was completely nonfunctional in terms of its ability to transactivate a series of p53-responsive genes, to transrepress a target gene, and to inhibit colony growth in transfected cells. However, data from irradiated peripheral blood lymphocytes and transfected cells suggested that this truncated, mutant protein retained significant ability to induce apoptosis.

TP53, 11-BP DEL/5-BP INS
Most p53 mutations have been found in the DNA-binding domain of the protein, which results in the loss of p53 transcriptional function. Birch et al. (1994), however, reported a family with Li-Fraumeni syndrome-1 (151623) with a p53 mutation in exon 4, outside of the DNA-binding domain. This mutation involved deletion of 11 bp and insertion of 5 bp that corresponded to a change in codons 108-111 from gly-phe-arg-leu to ile-gln, but did not lead to an alteration of the reading frame. The same mutation was detected in the proband and his affected mother, indicating that this mutation indeed accounts for the high incidence of cancer in the family. Gu et al. (2001) investigated how this mutation affected p53 function and led to malignant transformation. The mutation resided in the region of the protein necessary for p53 degradation, which is mediated by MDM2 (164785). Gu et al. (2001) created an equivalent deletion in a p53 expression construct and functionally characterized it. They demonstrated that a mutation in this region is associated not only with resistance of the mutant p53 to MDM2-mediated degradation, but also with an impaired response of mutant protein to DNA damage. In addition, the mutant protein was defective in its transactivation function, which correlated with its inability to suppress cell growth and to induce apoptosis. The molecular basis for the mutant form of p53 responsible for LFS in this family appears to be its predominantly cytoplasmic localization caused by a faulty nuclear import mechanism which, at least in part, results from the mutant's decreased affinity to importin (602738).

During a search for causative genes in patients with concurrent multiple primary colon tumors (see 114500), Miyaki et al. (2003) identified a germline mutation of the p53 gene, from GCC (ala) to GTC (val) at codon 189 (A189V), in a 73-year-old man. Of the 6 primary colon tumors that this patient had, 1 large advanced carcinoma exhibited a somatic mutation in the p53 gene and a somatic mutation in the APC gene (611731) in addition to the germline p53 mutation. Two early carcinomas and 3 adenomas had somatic APC mutations but no somatic p53 mutation or loss of the p53 allele. Mutations of the KRAS2 gene (190070) were detected in an advanced carcinoma and an early carcinoma. The findings were interpreted as indicating that certain types of germline p53 mutations predispose to concurrent multiple colon tumors. The results also suggested that in patients with such mutations, a somatic APC mutation is involved in tumor formation and that an additional somatic p53 mutation contributes to tumor progression.

In a mother and her 3 children with Li-Fraumeni syndrome (151623), Capponcelli et al. (2005) identified a heterozygous germline 659A-C transversion in exon 6 of the TP53 gene, resulting in a tyr220-to-ser (Y220S) substitution. Loss of heterozygosity for wildtype p53 was observed in all available tumor samples. All affected family members had an aggressive clinical phenotype associated with resistance to doxorubicin and early death from cancer. Supernatant from Y220S fibroblasts induced significantly increased neoangiogenesis on gelatin sponge chorioallantoic membranes compared to wildtype. In vitro, Y220S fibroblasts showed increased resistance to doxorubicin with decreased nuclear p53 localization and increased levels of peroxiredoxin II (PRDX2; 600538) and thioredoxin (TXN; 187700), both of which reduce reactive oxygen species. The findings suggested a mechanism for chemoresistance conferred by the Y220S mutation.

In a male infant who developed an adrenocortical carcinoma (202300) and a choroid plexus carcinoma (see 260500) by age 1.5 years, Russell-Swetek et al. (2008) identified a germline heterozygous de novo A-to-T transversion in the TP53 gene, resulting in a glu285-to-val (E285V) substitution in the DNA binding domain. Immunohistochemical analysis showed strong positive staining for p53 in the nuclei of both types of cancer cell, consistent with these tumors expressing the mutant p53 protein. Functional analyses of E285V revealed significant defects in its ability to regulate promoter activity, suppress tumor cell growth, and trigger apoptosis. The mutant protein also functioned efficiently as a dominant-negative regulator that neutralized wildtype p53 activity.

TP53, A-C, 3-PRIME UTR (rs78378222)
In the discovery phase of a genomewide association study of 16 million SNPs identified through whole-genome sequencing of 457 Icelanders, Stacey et al. (2011) identified association of a single-nucleotide polymorphism (SNP) in the TP53 gene, rs78378222C, with susceptibility to basal cell carcinoma (BCC7; 614740). Stacey et al. (2011) then confirmed this association in non-Icelandic samples (OR = 1.75, p = 0.0060; overall OR = 2.16, p = 2.2 x 10(-20)). The SNP rs78378222 is in the 3-prime untranslated region of TP53 and changes the AATAAA polyadenylation signal to AATACA. Studies of RNA from rs78378222A/C heterozygotes and A/A homozygotes suggested that the rs78378222C variant impairs proper termination and polyadenylation of the TP53 transcript.

In a patient with a multifocal anaplastic astrocytoma (GLM1; 137800), Kyritsis et al. (1994) identified a germline G-to-T transversion in the TP53 gene, resulting in an arg181-to-leu (R181L) substitution. The patient had no family history of cancer, except for a maternal aunt with cervical cancer.

TP53, 1-BP DEL, 1083G
In a 20-year-old man with bone marrow failure syndrome-5 (BMFS5; 618165), Toki et al. (2018) identified a de novo heterozygous 1-bp deletion (c.1083delG, NM_001126112.2) in exon 10 of the TP53 gene, predicted to result in a frameshift and premature termination (Ser362AlafsTer8). The mutation was found by whole-exome sequencing and confirmed by Sanger sequencing. Analysis of patient cells indicated that the mutant transcript escaped nonsense-mediated mRNA decay and produced a mutant protein. In vitro functional expression studies showed that the TP53 mutant had increased transcriptional activity compared to controls. Human induced pluripotent stem cells expressing a CRISPR/Cas9-derived C-terminal truncated TP53 showed significantly elevated expression of downstream TP53 targets, as well as impaired erythroid differentiation. Toki et al. (2018) postulated that the deletion may compromise binding of negative transcriptional regulators. The findings indicated that augmented p53 function, not loss of function, was responsible for the phenotype. An unrelated patient with the disorder had a different mutation that resulted in the same truncated protein (see 191170.0044).

TP53, 1-BP DEL, 1077A
In a 5-year-old boy with bone marrow failure syndrome-5 (BMFS5; 618165), Toki et al. (2018) identified a de novo heterozygous 1-bp deletion (c.1077delA, NM_001126112.2) in exon 10 of the TP53 gene, predicted to result in a frameshift and premature termination (Ser362AlafsTer8). The mutation was found by whole-exome sequencing and confirmed by Sanger sequencing. In vitro functional expression studies showed that the TP53 mutant had increased transcriptional activity compared to controls. An unrelated patient with the disorder had a different mutation that resulted in the same truncated protein (see 191170.0043).

Tags: 17p13.1