Alternative titles; symbolsTIN2HGNC Approved Gene Symbol: TINF2Cytogenetic location: 14q12 Genomic coordinates (GRCh38): 14:24,239,639-24,242,673 (from NCBI)...
Alternative titles; symbols
HGNC Approved Gene Symbol: TINF2
Cytogenetic location: 14q12 Genomic coordinates (GRCh38): 14:24,239,639-24,242,673 (from NCBI)
TINF2 is a subunit of the 6-protein shelterin/telosome complex. This complex protects telomere ends and cooperates with telomerase (see TERT; 187270) to maintain telomeres. TINF2 plays a central role in the assembly and function of the shelterin/telosome complex by connecting the double-stranded DNA-binding proteins TRF1 (TERF1; 600951) and TRF2 (TERF2; 602027) to the single-stranded DNA-binding unit TPP1 (ACD; 609377)/POT1 (606478) (summary by Yang et al., 2011).
▼ Cloning and Expression
By interaction cloning using the telomeric DNA-binding protein TRF1, Kim et al. (1999) isolated a novel human telomere-associated protein that they named TIN2. The TIN2 protein contains 354 amino acids. A wide variety of human tissues and cell types expressed a 2.4-kb TIN2 transcript on Northern blot analysis, and expression did not vary with growth state, immortalization, or transformation. TIN2 colocalized with TRF1 in nuclei and metaphase chromosomes.
Yang et al. (2011) stated that the 354-amino acid TINF2 protein contains an N-terminal domain of about 200 amino acids that binds the shelterin/telosome subunits TPP1 and TRF2, followed by an approximately 20-amino acid TRF1-binding motif.
▼ Gene Function
Kim et al. (1999) showed that TIN2 interacted with TRF1 in vitro and in cells. Expression of a TIN2 mutant lacking N-terminal sequences resulted in elongated human telomeres in a telomerase-dependent manner. Kim et al. (1999) interpreted the findings as suggesting that TRF1 is insufficient for control of telomere length in human cells and that TIN2 is an essential mediator of TRF1 function. The mutant TIN2 had the properties of a dominant-negative mutant. The findings suggested that wildtype TIN2 is a negative regulator of telomerase length.
Telomere length in humans is partly controlled by a feedback mechanism in which telomere elongation by telomerase is limited by the accumulation of the TRF1 complex at chromosome ends. TRF1 itself can be inhibited by the poly(ADP-ribose) polymerase (PARP) activity of its interacting partner tankyrase-1 (603303), which abolishes its DNA binding activity in vitro and removes the TRF1 complex from telomeres in vivo. Ye and de Lange (2004) reported that the inhibition of TRF1 by tankyrase is in turn controlled by a second TRF1-interacting factor, TIN2. Partial knockout of TIN2 by small hairpin RNA in a telomerase-positive cell line resulted in telomere elongation, which is typical of reduced TRF1 function. Transient inhibition of TIN2 with small interfering RNA led to diminished telomeric TRF1 signals. These and other data identified TIN2 as a PARP modulator in the TRF1 complex and explained how TIN2 contributes to the regulation of telomere length.
Liu et al. (2004) found that TIN2 was part of a high molecular mass protein complex in HeLa cells that mediates telomere end-capping and length control. Other members of this complex are TRF1, TRF2, RAP1 (TERF2IP; 605061), POT1, and TPP1.
Through reconstitution and fractionation experiments, O'Connor et al. (2006) found that TPP1 and TIN2 were essential mediators of telomeric complex formation and that TPP1-TIN2 interaction regulated bridging of TRF1 and TRF2. Overexpression of TPP1 enhanced TIN2-TRF1 association, and conversely, knockdown of TPP1 reduced the ability of endogenous TRF1 to associate with the TRF2 complex. O'Connor et al. (2006) concluded that coordinated interaction among TPP1, TIN2, TRF1, and TRF2 is required for assembly of the telomere complex and ultimately for telomere maintenance.
Mammalian telomeres are protected by a 6-protein complex, shelterin. Shelterin contains 2 closely related proteins, TRF1 and TRF2, which recruit various proteins to telomeres. Chen et al. (2008) dissected the interactions of TRF1 and TRF2 with their shared binding partner TIN2 and other shelterin accessory factors. TRF1 recognizes TIN2 using a conserved molecular surface in its TRF homology domain. However, this same surface does not act as a TIN2-binding site in TRF2, and TIN2 binding to TRF2 is mediated by a region outside the TRF homology domain. Instead, the TRF homology domain docking site of TRF2 binds a shelterin accessory factor, Apollo, also known as SNM1B (609683), which does not interact with the TRF homology domain of TRF1. Conversely, the TRF homology domain of TRF1, but not of TRF2, interacts with another shelterin-associated factor, PINX1 (606505).
▼ Molecular Genetics
Patients with dyskeratosis congenita (DKC), a heterogeneous inherited bone marrow failure syndrome, have abnormalities in telomere biology including very short telomeres. Whereas germline mutations in DKC1 (300126), TERC (602322), and TERT (187270) have been found in DKC patients, approximately 60% of DKC patients lack an identifiable mutation. With the very short telomere phenotype and a highly penetrant, rare disease model, Savage et al. (2008) performed a linkage scan on a family with autosomal dominant dyskeratosis congenita-3 (DKCA3; 613990) in which affected members did not have mutations in DKC1, TERC, or TERT. Evidence favoring linkage was found at 14q11.2, which led to the identification of the TINF2 mutations K280E (604319.0001) in the proband and her 5 affected relatives. Three additional unrelated DKC probands, including 1 with Revesz syndrome (268130), carried TINF2 R282H (604319.0002). A fifth DKC proband had an R282S mutation (604319.0003). All 3 mutations were in extremely close proximity and near the end of the TRF1 (600951)-binding domain of the TIN2 protein. This study provided the first example of a shelterin complex mutation linked to human disease and confirmed the role of very short telomeres as a diagnostic test for dyskeratosis congenita.
Walne et al. (2008) identified heterozygous mutations in the TINF2 gene in 33 (18.9%) of 175 cases of uncharacterized DKC. Of these 33 samples, 21 were found to have a mutation in arg282 in exon 6 (see, e.g., R282H; 604319.0002). The remaining 12 mutations were all in a tight cluster between residues 280 and 298. No additional mutations were found elsewhere in the gene. Most of the mutations were de novo. Clinically, all the DKC patients with a TINF2 mutation had severe disease associated with shorter telomeres compared to patients with DKC1 mutations.
Yang et al. (2011) found that expression of TIN2 with the K280E, R282H, or R282S mutation in human cell lines recapitulated the telomere shortening defect in DKC. These mutations did not affect total telomerase activity, TIN2 localization at telomeres, telomere end protection, or expression or stability of other core telomeric proteins. Despite the clustering of these mutation near the TRF1-binding motif, they did not affect interaction of TIN2 with TRF1. However, all 3 mutations reduced the level of telomerase activity and the amount of TERC protein that immunoprecipitated with TIN2. Yang et al. (2011) concluded that mutation of K280 or R282 in TIN2 results in defective targeting of telomerase to telomere ends.
The 2 truncating mutations identified by Sasa et al. (2012) in 2 unrelated children with severe DKCA3 (Q269X, 604319.0005 and Q271X, 604319.0007, respectively) both occurred in exon 6, but affected the more N-terminal region compared to earlier reported mutations and thus extended the affected segment of the gene to amino acid 269. In vitro functional expression studies in HEK293 cells showed that the Q269X mutant protein was markedly impaired in its ability to interact with TERF1. This was in contrast to R282H (604319.0002), which retained substantial ability to interact with TERF1. These findings indicated that disrupted TERF1 binding is not the main factor driving disease pathogenesis, but may contribute to a more severe phenotype.
Vulliamy et al. (2012) identified 16 new families with mutations in exon 6 of the TINF2 gene ascertained from 224 consecutive patients with different forms of bone marrow failure, including 46 with criteria meeting dyskeratosis congenita, 122 with aplastic anemia, and 57 with some features of DKC. Seven of the 46 patients with DKC carried mutations, 5 of whom had the R252H mutation (604319.0002). Nine of 57 patients with bone marrow failure and some features of DKC were found to carry mutations, including 2 with R282H, and 4 with nonsense or frameshift mutations (see, e.g., 604319.0005 and 604319.0008). Telomere length was only available for 7 of the mutation carriers, 6 of whom had shortened telomeres. The seventh patient had a missense variant that was also found in an asymptomatic individual, and both had normal telomere lengths, suggesting that this variant was not disease causing. Vulliamy et al. (2012) concluded that TINF2 mutations can cause a spectrum of clinical features and that telomere length should be able to distinguish pathogenic mutations from polymorphic variants in the absence of functional data. Most of the mutations appeared to occur de novo.
▼ ALLELIC VARIANTS ( 8 Selected Examples):
.0001 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
In 4 members of a family with autosomal dominant dyskeratosis congenita-3 (DKCA3; 613990), Savage et al. (2008) identified heterozygosity for a A-to-G transition in exon 6 of the TINF2 gene (Ex6+234A-G) that resulted in a lys-to-glu substitution at codon 280 (K280E). The mutation was also found in 2 family members who were classified as silent carriers because, while they showed some clinical features, did not meet the clinical definition of DKC; these 2 individuals as well as the 4 with a DKC diagnosis had telomere lengths below the first percentile for age.
Yang et al. (2011) showed that the K280E mutation caused defective targeting of telomerase to telomere ends.
.0002 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
REVESZ SYNDROME, INCLUDED
In 3 individuals with autosomal dominant dyskeratosis congenita-3 (613990), Savage et al. (2008) identified heterozygosity for a G-to-A transition in exon 6 of the TINF2 gene (Ex6+241G-A) that resulted in an arg282-to-his (R282H) amino acid substitution. Two of these patients had the 'DKC triad' of abnormal nails, reticular pigmentation, and oral leukoplakia, with other features; 1 of these patients had Revesz syndrome (268130). One patient without the DKC triad had dystrophic nails and IgA deficiency. All 3 patients had telomere lengths below the first percentile for age.
Tsangaris et al. (2008) identified a de novo heterozygous R282H mutation in an 18-month-old girl with DKC who presented with pancytopenia and ataxia. Brain imaging showed cerebellar hypoplasia. She later developed a small area of leukoplakia, but had no nail dystrophy or skin hyperpigmentation. Laboratory studies showed shortened telomeres and decreased telomerase activity (92.5% reduction compared to control values). The authors noted that the phenotype was consistent with the ataxia-pancytopenia syndrome (159550), which may be part of the phenotypic spectrum of DKC.
Yang et al. (2011) showed that the R282H mutation caused defective targeting of telomerase to telomere ends.
By in vitro functional expression studies in HEK293 cells, Sasa et al. (2012) demonstrated that the R282H mutant protein retained substantial ability to interact with TERF1 (600951).
Vulliamy et al. (2012) identified a heterozygous R282H mutation in 7 unrelated patients with DKC. Two had a severe form of the disorder, with additional features including growth retardation, retinopathy, ataxia, developmental delay, and cerebellar hypoplasia, consistent with a clinical diagnosis of Hoyeraal-Hreidarsson syndrome. Three patients had classic DKC, with bone marrow failure and skin and nail involvement; 2 had leukoplakia. The last 2 patients had bone marrow failure with variable features overlapping DKC, mainly nail dystrophy; 1 had intracranial calcifications. Telomere length was only available from 4 of these patients, but all showed significantly shortened telomeres. The findings indicated that TINF2 mutations can be associated with a broad spectrum of phenotypes.
.0003 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
In a patient with autosomal dominant dyskeratosis congenita-3 (613990) and very short telomeres, Savage et al. (2008) identified a C-to-A transversion in exon 6 of the TINF2 gene (Ex6+240C-A) that resulted in an arg282-to-ser substitution (R282S).
Yang et al. (2011) showed that the R282S mutation caused defective targeting of telomerase to telomere ends.
.0004 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
In 7 patients with DKCA3 (613990), Walne et al. (2008) identified a heterozygous 844C-T transition in exon 6 of the TINF2 gene, resulting in an arg282-to-cys (R282C) substitution.
.0005 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
In a 4-year-old Hispanic boy with DKCA3 (613990), Sasa et al. (2012) identified a de novo heterozygous 805C-T transition in exon 6 of the TINF2 gene, resulting in a gln269-to-ter (Q269X) substitution. He presented with pancytopenia, and was found to have nail dystrophy, oral leukoplakia, and lacy hyperpigmentation of the neck and genital regions. He also had undescended testis and phimosis. Telomere flow-FISH analysis showed telomere lengths below the first percentile. He underwent hematopoietic stem cell transplantation. The truncated protein was expressed, but at lower levels than wildtype, suggesting decreased stability. In vitro functional expression studies in HEK293 cells showed that the Q269X mutant protein was markedly impaired in its ability to interact with TERF1 (600951). This was in contrast to R282H (604319.0002), which retained substantial ability to interact with TERF1. These findings indicated that disrupted TERF1 binding is not the main factor driving disease pathogenesis, but may contribute to a more severe phenotype.
Vulliamy et al. (2012) reported a 4-year-old boy with bone marrow failure and mucocutaneous features of DKC who had a Q269X mutation in the TINF2 gene. Telomere length was significantly decreased compared to controls.
.0006 REVESZ SYNDROME
TINF2, 1-BP DEL, 839A
In a 21-month-old Hispanic boy with Revesz syndrome (268130), Sasa et al. (2012) identified a heterozygous 1-bp deletion (839delA) in exon 6 of the TINF2 gene, resulting in a frameshift and premature termination. The mutation was not present in the mother, but the reportedly normal father was not available. The child presented with severe aplastic anemia and was noted to have bilateral exudative retinopathy at age 9 months. He also had poor growth, nail dystrophy, oral leukoplakia, and delayed development with wide-based gait, suggesting cerebellar involvement. He underwent cord blood transplantation, but died 92 days later. Telomere lengths were shortened. A truncated protein was expressed, but at lower levels than wildtype, suggesting decreased stability.
.0007 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
In a Caucasian girl with DKCA3 (613990), Sasa et al. (2012) identified a heterozygous 811C-T transition in exon 6 of the TINF2 gene, resulting in a gln271-to-ter (Q271X) substitution. She presented at age 21 months with severe aplastic anemia and underwent hematopoietic stem cell transplantation. Subsequently, she developed skin hyperpigmentation, nail dystrophy, and oral leukoplakia, as well as epiphora, esophageal stricture, and osteopenia-related fractures. At age 10 years, she had progressive interstitial lung disease with fibrosis, gastrointestinal bleeding secondary to enteropathy, and noncirrhotic portal hypertension. She died at age 12 years from multiorgan failure.
.0008 DYSKERATOSIS CONGENITA, AUTOSOMAL DOMINANT 3
TINF2, 1-BP DEL, 826A
In a 3-year-old girl with bone marrow failure, nail dystrophy, lichenoid tongue, dry skin, and intrauterine growth retardation consistent with DKCA3 (613990), Vulliamy et al. (2012) identified a heterozygous 1-bp deletion (826delA) in the TINF2 gene, resulting in a frameshift and premature termination (Arg276GlyfsTer41). Telomere length was decreased compared to controls.