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Alternative titles; symbolsINVERTED CCAAT BOX-BINDING PROTEIN, 90-KD; ICBP90NUCLEAR PHOSPHOPROTEIN, 95-KD; NP95HGNC Approved Gene Symbol: UHRF1Cytogenetic locati...

Alternative titles; symbols


HGNC Approved Gene Symbol: UHRF1

Cytogenetic location: 19p13.3 Genomic coordinates (GRCh38): 19:4,903,079-4,962,153 (from NCBI)

▼ Description
UHRF1 binds to an inverted CCAAT box in the promoter of topoisomerase II-alpha (TOP2A; 126430). TOP2A introduces transient double-stranded breaks in DNA, which are required during the cell cycle for DNA replication, chromosome condensation, and segregation. UHRF1 is likely involved in TOPO2A expression (Hopfner et al., 2000).

▼ Cloning and Expression
By yeast 1-hybrid analysis of a Jurkat leukemia cell line cDNA library using the second inverted CCAAT box (ICB2) of the TOP2A promoter as bait, followed by screening an adult thymus cDNA library, Hopfner et al. (2000) cloned UHRF1, which they called ICBP90. The deduced 793-amino acid protein has a calculated molecular mass of about 90 kD. ICBP90 contains an N-terminal ubiquitin-like domain partially overlapped by a leucine zipper domain, followed by a central zinc finger of the PHD finger type and a C-terminal zinc finger of the ring finger type. It also has 2 nuclear localization signals, phosphorylation sites for several kinases, and a consensus Rb1 (614041)-binding sequence. RNA dot blot analysis detected expression of ICBP90 at highest levels in adult and fetal thymus, followed by fetal liver, adult bone marrow, testis, lung, heart, and fetal kidney. Highly differentiated tissues, such as central nervous system and peripheral blood leukocytes, showed no detectable expression. Immunohistochemical analysis revealed nuclear expression of both endogenous HeLa cell ICBP90 and ICBP90 ectopically expressed in COS-1 cells. Western blot analysis detected ICBP90 at an apparent molecular mass of 97 kD in confluent HeLa cells; a minor 50-kD band was also detected. Other bands were observed in proliferating HeLa cells. Histochemical analysis of appendix showed expression restricted to areas of cell proliferation (germinal centers and glandular crypts), where ICBP90 colocalized with TOP2A. Expression of both proteins was elevated in high-grade breast carcinomas and slightly elevated in low-grade breast carcinomas.

By Northern blot analysis, Hopfner et al. (2001) detected transcripts of 5.1 and 4.3 kb in several cancer cell lines. The relative amount of each transcript depended upon the tissue of origin.

▼ Gene Function
Hopfner et al. (2000) determined that ICBP90 bound the ICB2 of TOP2A in vivo and in vitro, and that expression of ICBP90 was concomitant with expression of TOP2A in human lung fibroblasts and several cell lines. Transfection of ICBP90 into COS-1 cells resulted in enhanced expression of both ICBP90 and TOP2A.

Bonapace et al. (2002) determined that mouse Np95 is an early target of the oncogenic DNA virus, adenovirus E1A, during induced proliferation. E1A induced Np95 expression in terminally differentiated mouse myotubes. The concomitant expression of Np95 and of cyclin E (123837)-Cdk2 (116953) was sufficient to induce S phase in these cells. In mouse fibroblasts, expression of Np95 was tightly regulated during the cell cycle, and its functional ablation abrogated DNA synthesis. Bonapace et al. (2002) concluded that Np95 is essential for S phase entry.

Unoki et al. (2004) demonstrated that ICBP90 bound to methylated CpG and to HDAC1 (601241) through its SET and RING finger-associated (SRA) domain. ICBP90 expression was increased by binding of E2F1 (189971) to its intron 1. Unoki et al. (2004) proposed that ICBP90 is involved in cell proliferation through methylation-mediated gene regulation.

Bostick et al. (2007) showed that the protein UHRF1, also known as NP95 in mouse and ICBP90 in human, is required for maintaining DNA methylation. UHRF1 colocalizes with the maintenance DNA methyltransferase protein DNMT1 (126375) throughout S phase. UHRF1 appears to tether DNMT1 to chromatin through its direct interaction with DNMT1. Furthermore, UHRF1 contains a methyl DNA binding domain, the SRA domain, that shows strong preferential binding to hemimethylated CG sites, the physiologic substrate for DNMT1. Bostick et al. (2007) concluded that UHRF1 may help recruit DNMT1 to hemimethylated DNA to facilitate faithful maintenance of DNA methylation.

Sharif et al. (2007) demonstrated that localization of mouse Np95 to replicating heterochromatin is dependent on the presence of hemimethylated DNA. Np95 forms complexes with Dnmt1 and mediates the loading of Dnmt1 to replicating heterochromatic regions. By using Np95-deficient embryonic stem cells and embryos, Sharif et al. (2007) showed that Np95 is essential in vivo to maintain global and local DNA methylation and to repress transcription of retrotransposons and imprinted genes. Sharif et al. (2007) concluded that the link between hemimethylated DNA, Np95, and Dnmt1 thus establishes key steps of the mechanism for epigenetic inheritance of DNA methylation.

Using in vitro pull-down assays in HeLa cell nuclear extracts, Karagianni et al. (2008) showed that ICBP90 bound directly to histone H3 (see 602810) peptides with methylated lys9 (H3K9). In transfected NIH-3T3 cells, human ICBP90 localized to pericentromeric heterochromatin, which is highly enriched in trimethylated H3K9 (H3K9me3) peptides. Endogenous mouse Np95, the ortholog of ICBP90, exhibited similar localization with heterochromatin protein-1 (HP1)-alpha (CBX5; 604478) in NIH-3T3 cells. H3K9me3 was required for localization of ICBP90 and Np95 to heterochromatin. Binding analysis showed that the PHD of ICBP90 was a major determinant for its binding to H3K9me3, with the SRA domain contributing to overall H3 binding affinity. Both the PHD and the SRA domain were required for proper heterochromatic localization of ICBP90. Downregulation of ICBP90 in HeLa cells and Np95 in NIH-3T3 cells disrupted distribution of H3K9me3 and Hp1-alpha, respectively, in interphase nuclei. ICBP90 could function as a ubiquitin ligase both in vitro and in transfected cells and promoted ubiquitination of histone H3. Overexpression of an enzymatically inactive ICBP90 mutant disrupted higher order organization of heterochromatin in interphase NIH-3T3 cells.

Sen et al. (2010) showed that DNMT1 (126375) is essential for epidermal progenitor cell function. DNMT1 protein was found enriched in undifferentiated cells, where it was required to retain proliferative stamina and suppress differentiation. In tissue, DNMT1 depletion led to exit from the progenitor cell compartment, premature differentiation, and eventual tissue loss. Genomewide analysis showed that a significant portion of epidermal differentiation gene promoters were methylated in self-renewing conditions but were subsequently demethylated during differentiation. Furthermore, UHRF1, a component of the DNA methylation machinery that targets DNMT1 to hemimethylated DNA, is also necessary to suppress premature differentiation and sustain proliferation. In contrast, Gadd45A (126335) and B (604948), which promote active DNA demethylation, are required for full epidermal differentiation gene induction. Sen et al. (2010) concluded that proteins involved in the dynamic regulation of DNA methylation patterns are required for progenitor maintenance and self-renewal in mammalian somatic tissue.

Nishiyama et al. (2013) showed that UHRF1-dependent histone H3 ubiquitylation has a prerequisite role in the maintenance of DNA methylation. Using Xenopus egg extracts, Nishiyama et al. (2013) successfully reproduced maintenance DNA methylation in vitro. DNMT1 depletion resulted in a marked accumulation of UHRF1-dependent ubiquitylation of histone H3 at lysine-23. DNMT1 preferentially associates with ubiquitylated H3 in vitro through a region previously identified as a replication foci targeting sequence. The RING finger mutant of UHRF1 failed to recruit DNMT1 to DNA replication sites and to maintain DNA methylation in mammalian cultured cells. Nishiyama et al. (2013) concluded that their findings represented the first evidence of the mechanistic link between DNA methylation and DNA replication through histone H3 ubiquitylation.

Li et al. (2018) demonstrated that the loss of Stella (DPPA3; 608408) in mouse oocytes led to ectopic nuclear accumulation of the DNA methylation regulator UHRF1, which resulted in the mislocalization of maintenance DNA methyltransferase DNMT1 in the nucleus. Genetic analysis confirmed the primary role of UHRF1 and DNMT1 in generating the aberrant DNA methylome in Stella-deficient oocytes. Li et al. (2018) concluded that Stella therefore safeguards the unique oocyte epigenome by preventing aberrant de novo DNA methylation mediated by DNMT1 and UHRF1.

Chen et al. (2018) found that Uhrf1 was specifically upregulated by the Myc (190080)-Ap4 (TFAP4; 600743) axis in mouse germinal center (GC) B cells compared with naive follicular B cells. Mice lacking Uhrf1 specifically in active B cells had a reduced splenic GC response to model antigen and virus infection compared with wildtype. Analysis in Uhrf1-deficient GC B cells showed that Uhrf1 enhanced GC B-cell proliferation by promoting G1-to-S transition but had no role in cell survival. Uhrf1 promoted these effects partially through maintaining DNA methylation of Cdkn1a (116899) and through methylation of the Slfn1/Slfn2 locus (see 614955) to inhibit Slfn1 and Slfn2 expression. Uhrf1 expression in GC B cells was essential for somatic hypermutation (SHM) and antibody affinity maturation, and Uhrf1 expression was required for chronic virus clearance.

Using quantitative real-time PCR, Elia et al. (2018) identified Uhrf1 as the most upregulated gene during differentiation of vascular smooth muscle cells (VSMCs) in mice. Analysis of knockout mice and VSMCs in vitro showed that Mir145 (611795) contributed to Uhrf1 expression. Uhrf1 expression was upregulated in diseased vascular tissues in mice. Localized reduction of Uhrf1 improved stenosis development in mice. Uhrf1 silencing and overexpression studies revealed that Uhrf1 mediated VSMC proliferation and migration and thereby regulated VSMC plasticity in vitro. Knockdown of Uhrf1 resulted in an increased contractile phenotype and demonstrated that Uhrf1 modulated expression of critical VSMC differentiation genes by interacting with their promoters. Uhrf1 regulated the VSMC phenotype by mediating dedifferentiation triggered by PDGF-BB (see 190040), as well as by antagonizing differentiation triggered by Tgf-beta (TGFB1; 190180). UHRF1 was upregulated in human and mouse aneurysms, but Uhrf1-knockout mice lacked major structural defects of aorta and instead showed marked reduction of aneurysm formation.

▼ Biochemical Features
Avvakumov et al. (2008) reported the 1.7-angstrom crystal structure of the SRA domain of human UHRF1 and the 2.2-angstrom structure of the SRA domain in complex with a 12-bp double-stranded DNA with a central hemimethylated CpG. In the SRA-DNA complex, the 5-methylcytosine was flipped out of the duplex DNA and interacted with a binding pocket of the SRA domain. Two loops in the SRA domain reached through the resulting gap in the DNA from both the major and minor grooves to read the other 3 bases of the CpG duplex. The major groove loop appeared to confer both specificity for the CpG dinucleotide and discrimination against methylation of deoxycytidine of the complementary strand.

Arita et al. (2008) and Hashimoto et al. (2008) determined the crystal structure of the SRA domain of mouse Uhrf1 in complex with hemimethylated CpG and presented findings similar to those of Avvakumov et al. (2008).

▼ Gene Structure
Hopfner et al. (2001) determined that the ICBP90 gene contains 8 exons and spans about 35 kD. Exons 1 and 2 are noncoding. The promoter region contains 3 Sp1 (189906)-binding sites.

▼ Mapping
By FISH, Hopfner et al. (2001) mapped the UHRF1 gene to chromosome 19p13.3.

▼ Animal Model
Muto et al. (2002) determined that inactivation of mouse Np95 was lethal to midgestation embryos. Using Np95 null embryonic stem cells, they found that lack of Np95 expression increased cell sensitivity to inhibition of DNA replication and to DNA damaging agents, including x-rays, ultraviolet light, and alkylation.

Tags: 19p13.3