HGNC Approved Gene Symbol: POT1
Cytogenetic location: 7q31.33 Genomic coordinates (GRCh38) : 7:124,822,386-124,929,825 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 7q31.33 | ?Cerebroretinal microangiopathy with calcifications and cysts 3 | 620368 | Autosomal recessive | 3 |
| ?Pulmonary fibrosis and/or bone marrow failure syndrome, telomere-related, 8 | 620367 | Autosomal dominant | 3 | |
| Tumor predisposition syndrome 3 | 615848 | Autosomal dominant | 3 |
The POT1 gene encodes a protein conserved across widely diverged eukaryotes that binds the G-rich strand of its own telomeric repeat sequence, consistent with a direct role in protecting chromosome ends (Baumann and Cech, 2001).
By searching databases for homologs of S. pombe Pot1, followed by PCR of ovary cDNA, Baumann and Cech (2001) cloned human POT1. Human POT1 encodes a 71-kD protein whose N-terminal region shares 26% identity with the N terminus of S. pombe Pot1. PCR analysis showed ubiquitous expression of human POT1, consistent with the idea that POT1 is a housekeeping gene required to ensure the integrity of chromosome ends in all cells.
By PCR of human ovary and peripheral blood leukocyte cDNA, Baumann et al. (2002) cloned POT1 splice variants 1 through 5, which encode proteins of 71, 38, 58, 4, and 52 kD, respectively. Isoform 1 contains 634 amino acids. All POT1 isoforms except isoform 4 have a putative N-terminal DNA-binding domain. PCR analysis showed that POT1 variants 1 through 4 were expressed in all tissues examined, whereas variant 5 was expressed only in peripheral blood leukocytes. Highest expression of POT1 was in testis, and lowest expression was in skeletal muscle and colon. Immunohistochemical analysis showed that epitope-tagged POT1 colocalized with telomeric proteins at distinct nuclear foci in interphase nuclei of human embryonic kidney cells and a HeLa cell subclone with exceptionally long telomeres.
Independently, Wu et al. (2006) and Hockemeyer et al. (2006) cloned 2 mouse orthologs of POT1, which they called Pot1a and Pot1b. Hockemeyer et al. (2006) reported that the Pot1a and Pot1b proteins both contain 640 amino acids and share 71 to 75% amino acid identity with the 634-amino acid human POT1 protein and with one another. Two N-terminal OB folds and a C-terminal telomere/TPP1 (ACD; 609377)-binding domain are conserved in the human and mouse POT1 proteins. RT-PCR by Wu et al. (2006) and Hockemeyer et al. (2006) showed ubiquitous expression of mouse Pot1a and Pot1b during embryonic development and in adult tissues. Indirect immunofluorescence and chromatin immunoprecipitation analyses by Hockemeyer et al. (2006) revealed that Pot1a and Pot1b were specifically associated with telomeres.
Baumann and Cech (2001) found that human POT1 bound to the G-rich strand of human telomeric DNA. In contrast, binding was not observed with the complementary C-rich strand or with double-stranded telomeric DNA. Deletion of the fission yeast Pot1 gene had an immediate effect on chromosome stability, causing rapid loss of telomeric DNA and chromosome circularization.
Baumann et al. (2002) found that POT1 isoforms 1, 2, 3, and 5 bound telomeric DNA in vitro. Isoform 2 produced 6-fold more protein-DNA complexes than isoform 1, and isoforms 3 and 5 showed much weaker DNA binding.
Colgin et al. (2003) found that POT1 isoforms 1, 2, and 3 lengthened telomeres following overexpression in a telomerase (see 187270)-positive human cell line. Isoform 1 was most effective at lengthening telomeres, and POT1 was unable to lengthen telomeres in telomerase-negative cells unless telomerase activity was induced.
Human telomere length is regulated by the TTAGGG-repeat-binding protein TRF1 (600951) and its interacting partners tankyrase-1 (603303), TIN2 (604319), and PINX1 (606505). Loayza and de Lange (2003) demonstrated that the TRF1 complex interacts with a single-stranded telomeric DNA-binding protein, POT1, and that human POT1 controls telomerase-mediated telomere elongation. The presence of POT1 on telomeres was diminished when the amount of single-stranded DNA was reduced. Furthermore, POT1 binding was regulated by the TRF1 complex in response to telomere length. A mutant form of POT1 lacking the DNA-binding domain abrogated TRF1-mediated control of telomere length, and induced rapid and extensive telomere elongation. Loayza and de Lange (2003) proposed that the interaction between the TRF1 complex and POT1 affects the loading of POT1 on the single-stranded telomeric DNA, thus transmitting information about telomere length to the telomere terminus, where telomerase is regulated.
By SDS-PAGE of fractionated HeLa cell nuclear extracts and immunoprecipitation analysis, Liu et al. (2004) determined that the endogenous TIN2 complex contains TIN2, POT1, TRF2 (TERF2; 602027), and PTOP (ACD; 609377). Deletion analysis showed that the central domain of PTOP interacted with the C-terminal half of POT1. PTOP was essential for targeting POT1 to telomeres, and expression of the POT1 recruitment domain of PTOP functioned in a dominant-negative manner, blocking the interaction of POT1 with telomeres and permitting telomere extension.
Ye et al. (2004) found that PTOP, which they called PIP1, bound both POT1 and TIN2 and could tether POT1 to the TRF1 complex. Reduction of PTOP or POT1 levels with short hairpin RNAs led to telomere elongation, indicating that PTOP contributes to telomere length control through recruitment of POT1.
Lei et al. (2005) found that the crystal structure of the human POT1-TTAGGGTTAG complex suggested that the complex would not be extended by telomerase, and they confirmed this hypothesis by in vitro assays with recombinant telomerase. On the other hand, when POT1 was bound at a position 1 telomeric repeat before the 3-prime end, leaving an 8-nucleotide 3-prime tail, the complex was extended with improved activity and processivity. Lei et al. (2005) concluded that POT1 can either inhibit telomerase action or form a preferred substrate for telomerase depending on its location relative to the DNA 3-prime end.
Hockemeyer et al. (2005) found that a 10-fold reduction of POT1 protein in tumor cells by RNA interference (RNAi) induced neither telomere fusions nor cell cycle arrest. However, the 3-prime overhang DNA was reduced, and all telomeres elicited a transient DNA damage response in G1, indicating that extensive telomere damage can occur without cell cycle arrest or telomere fusions. RNAi to POT1 also changed the recessed 5-prime end of the telomere, which normally ends on the sequence ATC-5-prime, to a random position within the AATCCC repeat. Hockemeyer et al. (2005) concluded that POT1 determines the structure of the 3-prime and 5-prime ends of human chromosomes.
Opresko et al. (2005) found that POT1 isoforms 1 and 2 stimulated the WRN (RECQL2; 604611) and BLM (RECQL3; 604610) helicases to unwind telomeric forked duplex and D-loop structures that were poor substrates for these helicases alone. Optimal stimulation was dependent on the presence of telomeric sequence in the duplex regions of the substrates. Opresko et al. (2005) also showed that WRN and BLM physically interacted with POT1.
Xin et al. (2007) demonstrated that TPP1-POT1 association enhanced POT1 affinity for telomeric single-stranded DNA. In addition, the TPP1 oligonucleotide/oligosaccharide-binding (OB) fold, as well as POT1-TPP1 binding, seemed critical for POT1-mediated telomere length control and telomere end protection in human cells. Disruption of POT1-TPP1 interaction by dominant-negative TPP1 expression or RNA interference resulted in telomere length alteration and DNA damage responses. Furthermore, Xin et al. (2007) offered evidence that TPP1 associates with the telomerase in a TPP1-OB-fold-dependent manner, providing a physical link between telomerase and the telosome/shelterin complex (a 6-protein complex thought to protect the telomeres of human chromosomes). Xin et al. (2007) concluded that their findings highlighted the critical role of TPP1 in telomere maintenance, and supported a yin-yang model in which TPP1 and POT1 function as a unit to protect human telomeres, by both positively and negatively regulating telomerase access to telomere DNA.
Miyoshi et al. (2008) reported that Tpz1, the TPP1 (609377) homolog in fission yeast, forms a complex with Pot1. Tpz1 binds to Ccq1 and Poz1 (Pot1-associated in Schizosaccharomyces pombe), which protect telomeres redundantly and regulate telomerase in positive and negative manners, respectively. Thus, Miyoshi et al. (2008) concluded that the Pot1-Tpz1 complex accomplishes its functions by recruiting effector molecules Ccq1 and Poz1. Moreover, Poz1 bridges Pot1-Tpz1 and Taz1-Rap1, thereby connecting the single-stranded and double-stranded telomeric DNA regions. Miyoshi et al. (2008) suggested that such molecular architectures are similar to those of mammalian shelterin, indicating that overall DNA-protein architecture is conserved across evolution.
Maintenance of telomeres requires both DNA replication and telomere capping by shelterin. These 2 processes use 2 single-stranded DNA (ssDNA)-binding proteins, replication protein A (RPA; see 179835) and POT1. POT1 ablation leads to activation of the ataxia-telangiectasia and Rad3-related checkpoint kinase (ATR; 601215) at telomeres, suggesting that POT1 antagonizes RPA binding to telomeric ssDNA. Unexpectedly, Flynn et al. (2011) found that purified POT1 and its functional partner TPP1 are unable to prevent RPA binding to telomeric ssDNA efficiently. In cell extracts, they identified a novel activity that specifically displaces RPA, but not POT1, from telomeric ssDNA. Using purified protein, Flynn et al. (2011) showed that the heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1; 164017) recapitulates the RPA displacing activity. The RPA displacing activity is inhibited by the telomeric repeat-containing RNA (TERRA) in early S phase, but is then unleashed in late S phase when TERRA levels decline at telomeres. Interestingly, TERRA also promotes POT1 binding to telomeric ssDNA by removing hnRNPA1, suggesting that the reaccumulation of TERRA after S phase helps to complete the RPA-to-POT1 switch on telomeric ssDNA. Flynn et al. (2011) concluded that hnRNPA1, TERRA, and POT1 act in concert to displace RPA from telomeric ssDNA after DNA replication, and promote telomere capping to preserve genomic integrity.
Crystal Structure
Lei et al. (2003) described the 1.9-angstrom resolution crystal structure of the N-terminal DNA-binding domain of S. pombe Pot1 protein complexed with single-stranded DNA. The protein adopts an oligonucleotide/oligosaccharide-binding (OB) fold with 2 loops that protrude to form a clamp for single-stranded DNA binding. The structure explains the sequence specificity of binding: in the context of the Pot1 protein, DNA self-recognition involving base-stacking and unusual G-T basepairs compacts the DNA. Any sequence change disrupts the ability of the DNA to form this structure, preventing it from contacting the array of protein hydrogen-bonding groups. Lei et al. (2003) concluded that the structure also explains how Pot1 protein avoids binding the vast excess of RNA in the nucleus.
Wang et al. (2007) demonstrated that the crystal structure of a domain of human TPP1 (609377) reveals an OB fold that is structurally similar to the beta-subunit of the telomere end-binding protein of a ciliated protozoan, suggesting that TPP1 is the missing beta-subunit of human POT1 protein. Telomeric DNA end-binding proteins have generally been found to inhibit rather than stimulate the action of the chromosome end-replicating enzyme, telomerase. In contrast, Wang et al. (2007) found that TPP1 and POT1 form a complex with telomeric DNA that increases the activity and processivity of the human telomerase core enzyme. Wang et al. (2007) proposed that POT1-TPP1 switches from inhibiting telomerase access to the telomere, as a component of shelterin, to serving as a processivity factor for telomerase during telomere extension.
Baumann et al. (2002) determined that the POT1 gene contains 22 exons and spans 120 kb. Translation starts in exon 6.
By genomic sequence analysis, Baumann et al. (2002) mapped the POT1 gene to chromosome 7. They mapped the mouse Pot1 gene to chromosome 6.
Hockemeyer et al. (2006) mapped the mouse Pot1a gene to chromosome 6 in region that shows homology of synteny to human chromosome 7. They mapped the mouse Pot1b gene to chromosome 17.
Stumpf (2023) mapped the POT1 gene to chromosome 7q31.33 based on an alignment of the POT1 sequence (GenBank BC002923) with the genomic sequence (GRCh38).
Tumor Predisposition Syndrome 3
In 9 affected members of 4 unrelated families with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Robles-Espinoza et al. (2014) identified 4 different heterozygous mutations in the POT1 gene (606478.0001-606478.0004). The mutations were found by whole-exome sequencing. Two mutation carriers developed nonmelanoma cancers, and several untested family members had a history of nonmelanoma cancers, suggesting a possible role for germline POT1 variants in susceptibility to a range of cancers in addition to melanoma. In vitro functional expression studies showed that the mutations disrupted POT1 telomere binding, resulting in significantly longer telomere length in mutation carriers compared to melanoma patients without POT1 mutations. Robles-Espinoza et al. (2014) suggested that the mutations may promote uncapping of telomeres, telomere length extension, and chromosomal aberrations, thereby promoting tumorigenesis. The families were among 105 families with melanoma studied, thus accounting for 4% of the cohort.
In affected members of 7 Italian families with TPDS3 manifest as cutaneous malignant melanoma, Shi et al. (2014) identified heterozygous mutations in the POT1 gene (see, e.g., 606478.0005-606478.0007). The mutations were found by whole-exome sequencing. One mutation (S270N; 606478.0005) showed a founder effect in 5 Italian families. Cells from mutation carriers showed increased telomere lengths and numbers of fragile telomeres compared controls, suggesting that perturbation of telomere maintenance is involved in tumorigenesis. Sequencing the POT1 gene in 768 Italian melanoma cases and 768 controls showed a significant increase in burden for rare exonic variants among cases compared to controls (16 carriers among cases and 3 carriers among controls; odds ratio of 5.4, p = 0.0021). Subsequent sequencing of the POT1 gene in 3 other populations identified germline missense variants (see, e.g., 606478.0008) in families of American and French origin. Functional studies were not performed for any variant identified by Shi et al. (2014).
In affected members of 3 unrelated families with TPDS3 manifest as glioma, Bainbridge et al. (2015) identified 3 different heterozygous mutations in the POT1 gene (606478.0009-606478.0011). Two families had members with histories of other cancers, although none had melanoma, and several unaffected family members carried the mutation, consistent with incomplete penetrance. Functional studies of the variants were not performed, but there was evidence that mutation carriers had a higher telomere content compared to those without the mutation. The families were ascertained from a larger cohort of 55 glioma families who underwent whole-exome sequencing.
In 4 affected individuals from 3 unrelated families of Jewish descent with TPDS3, Wong et al. (2019) identified a heterozygous I78T mutation in the POT1 gene (606478.0006). The mutation segregated with the disorder in the families, although there was evidence of incomplete penetrance. In vitro functional studies showed that the variant impaired POT1 binding to telomere-like probes. Expression of the mutation into cells in vitro resulted in elongated telomeres. The probands in all families had melanoma, and some affected family members also developed chronic lymphocytic leukemia, thyroid cancer, and cutaneous T-cell lymphoma. All tumors were adult-onset. Examination of melanoma and nevi tissue from 2 of the patients showed accumulation of somatic events in several driver genes, many of which were involved in the MAPK pathway. Haplotype analysis suggested a founder effect.
In affected members of 8 unrelated families with TPDS3 with various types of hematopoietic and nonhematopoietic benign and malignant neoplasms, DeBoy et al. (2023) identified heterozygous germline mutations in the POT1 gene (see, e.g., 606478.0014-606478.0016). The mutations, which were confirmed by Sanger sequencing, segregated with the disorder in the families, with some evidence of incomplete penetrance. The mutations were either absent from or found at a low frequency in the gnomAD database. Patient cells showed decreased POT1 expression and defective binding to telomere DNA, consistent with a loss of function and haploinsufficiency. Mutation carriers, even those without tumors, had long telomeres compared to nonmutation carriers. DeBoy et al. (2023) concluded that loss of the tumor-suppressor mechanism of telomere shortening resulting from POT1 haploinsufficiency may support the expansion of clonal populations, leading to an elevated risk of cancer.
Telomere-Related Pulmonary Fibrosis and/or Bone Marrow Failure Syndrome 8
In 4 affected members of a family with telomere-related pulmonary fibrosis and/or bone marrow failure syndrome-8 (PFBMFT8; 620367), Kelich et al. (2022) identified a heterozygous missense mutation in the POT1 gene (L259S; 606478.0012). The mutation, which was found by next-generation panel sequencing, segregated with the disorder in the family and was not present in the gnomAD database. The patients had lung and liver manifestations associated with shortened telomeres. Detailed studies of patient cells and of cells transfected with the mutation in vitro showed that it caused detrimental effects, including decreased DNA binding affinity, reduced nuclear localization, shortened lagging strands during synthesis, an increase in the telomeric overhang signal, and sister telomere loss during metaphase. There was evidence of a defect in telomere capping that could lead to shortened telomeres, and patient cells showed premature senescence.
Cerebroretinal Microangiopathy with Calcifications and Cysts 3
In 2 sisters, born of consanguineous parents, with cerebroretinal microangiopathy with calcifications and cysts-3 (CRMCC3; 620368), Takai et al. (2016) identified a homozygous missense mutation in the POT1 gene (S322L; 606478.0013). The unaffected parents were heterozygous for the mutation. Patient fibroblasts showed premature senescence and contained telomere dysfunction-induced foci, indicating a DNA damage signal at the telomeres. The growth arrest could be mitigated by activation of telomerase. The mutant POT1 protein was able to interact with TPP1, localize to telomeres, and repress ATR signaling in POT1-null HeLa cells that contained telomerase. However, the mutant protein was unable to limit telomerase-mediated telomere elongation in vitro, indicating dysfunction with a separation-of-function effect. Cells carrying the mutation were defective in regulating the telomeric 5-prime C strand, causing extended 3-prime G overhangs. This resulted in stochastic telomere truncations that could be healed with expression of telomerase. Consistent with shortening of the telomeric C strand, metaphase chromosomes showed loss of telomeres synthesized by the leading DNA strand. The truncation of the telomeres and sister telomere loss during metaphase caused excessively short telomeres that likely lost the ability to protect chromosome ends. The findings were consistent with a defect in telomere end fill-in that generates truncated telomeres, which halt proliferation in cells lacking telomerase.
Wu et al. (2006) conditionally deleted the Pot1a gene in mouse embryonic fibroblasts and found that Pot1a was necessary for telomere protection. Lack of Pot1a elicited a DNA damage response that initiated replicative senescence in a p53 (191170)-dependent manner. Pot1a deletion resulted in aberrant cytogenetic products and aberrant telomeric homologous recombination, which manifested as increased sister chromatid exchanges, telomere circle formation, and chromosomal instability. Telomeric homologous recombination following Pot1a loss required Nbs1 (602667). Wu et al. (2006) concluded that POT1 is crucial for maintenance of telomere integrity and genomic stability.
Hockemeyer et al. (2006) generated single- and double-knockout cells lacking Pot1a and/or Pot1b and demonstrated the importance of these proteins in preventing DNA damage at chromosome ends. However, Pot1a and Pot1b were dispensable for repression of telomere fusions. Pot1b -/- mice were viable and fertile, whereas Pot1a -/- mice died in utero. Pot1a, but not Pot1b, was required to repress a DNA damage signal at telomeres. Conversely, Pot1b, but not Pot1a, had the ability to regulate the amount of single-stranded DNA at the telomere terminus. Hockemeyer et al. (2006) concluded that mouse telomeres require 2 distinct POT1 proteins, whereas humans need only 1, a finding that may have implications for modeling human telomere biology in mice.
Autosomal dominant forms of dyskeratosis congenita (DC; 127550), a bone marrow failure syndrome, can be caused by mutations in the genes encoding the RNA (TERC; 602322) or reverse transcriptase (TERT; 187270) components of telomerase. However, mice lacking components of telomerase have failed to show phenotypes typical of DC. Hockemeyer et al. (2008) developed a mouse model in which key characteristics of DC were induced by enhanced telomere degradation. Mice lacking Pot1b (Pot1b -/-) and also deficient in Terc (Terc +/-) developed progressive bone marrow failure, hyperpigmentation, and nail abnormalities. Bone marrow failure was fatal between 4 and 5 months of age in Pot1b -/- Terc +/- mice.
In 4 affected members of a 4-generation family with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Robles-Espinoza et al. (2014) identified a heterozygous c.266A-G transition in exon 8 of the POT1 gene, resulting in a tyr89-to-cys (Y89C) substitution at a highly conserved residue in the OB1 domain. The mutation, which was found by whole-exome sequencing, was not found in the Exome Sequencing Project database or in 2,922 controls. In vitro studies showed that the mutation resulted in a complete abolition of POT1-DNA complex formation, thus disrupting the interaction with telomeres. Additional nonmelanoma cancers occurred in several untested family members.
In 3 affected members of a 3-generation family with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Robles-Espinoza et al. (2014) identified a heterozygous G-to-A transition in intron 17 of the POT1 gene (c.1687-1G-A), resulting in a splice site mutation and premature termination. The mutation, which was found by whole-exome sequencing, was not found in the Exome Sequencing Project database or in 2,922 controls. Additional nonmelanoma cancers occurred in several untested family members.
In a patient with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Robles-Espinoza et al. (2014) identified a heterozygous c.280C-G transversion in exon 8 of the POT1 gene, resulting in a gln94-to-glu (Q94E) substitution at a highly conserved residue in the OB1 domain. The mutation, which was found by whole-exome sequencing, was not found in the Exome Sequencing Project database or in 2,922 controls. In vitro studies showed that the mutation resulted in a complete abolition of POT1-DNA complex formation, thus disrupting the interaction with telomeres. The patient had 4 melanomas and died at age 45. An affected sib also had melanoma, but DNA analysis was not performed on the sib.
In a patient with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Robles-Espinoza et al. (2014) identified a heterozygous c.818G-T transversion in exon 10 of the POT1 gene, resulting in an arg273-to-leu (R273L) substitution at a highly conserved residue near the end of the OB2 domain. The mutation, which was found by whole-exome sequencing, was not found in the Exome Sequencing Project database or in 2,922 controls. A cousin of the patient had late-onset melanoma, but did not carry the mutation. In addition, several untested family members had nonmelanoma cancers, including brain cancer, stomach cancer, and pancreatic cancer. The R273L mutation was also found by direct sequencing in 1 of 1,739 patients with sporadic melanoma. In vitro studies showed that the mutation resulted in a complete abolition of POT1-DNA complex formation, thus disrupting the interaction with telomeres.
In affected members of 5 unrelated families from Romagna, Italy, with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Shi et al. (2014) identified a heterozygous G-to-A transition in the POT1 gene, resulting in a ser270-to-asn (S270N) substitution at a highly conserved residue in the OB2 domain. The mutation was found by whole-exome sequencing and confirmed by Sanger sequencing, and was not present in several large control exome databases, including dbSNP, 1000 Genomes Project, Exome Sequencing Project, an in-house database, or in 2,038 Italian controls. Genotyping for this variant among 1,824 Italian melanoma cases identified 1 additional carrier, who was also from Romagna. Haplotype analysis indicated a founder effect for this mutation. Cells from mutation carriers showed significantly increased telomere length compared to controls, suggesting that the S270N variant may perturb telomere length maintenance. Mutation carriers also had increased numbers of fragile telomeres, but did not have chromosome breaks or loss of telomere signals. The mutation did not affect telomerase (see 187270) activity. Functional studies of the variant were not performed.
In an Italian mother and son with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Shi et al. (2014) identified a heterozygous G-to-A transition in the POT1 gene, resulting in an arg137-to-his (R137H) substitution in the alpha-helix in the OB1 domain. The mutation, which was found by exome sequencing, was not present in public databases or in 3,489 controls. Cells from mutation carriers showed increased telomere intensity signals and telomere fragility compared to controls. Functional studies of the variant were not performed.
In an Italian mother and daughter with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Shi et al. (2014) identified a heterozygous G-to-C transversion in the POT1 gene, resulting in a glu623-to-his (Q623H) substitution in the C terminus of the protein, near the binding region for ACD (609377), which forms a heterodimer with POT1. The mutation, which was found by exome sequencing, was not present in public databases or in 3,489 controls. Cells from mutation carriers showed increased telomere intensity signals and telomere fragility compared to controls. Functional studies of the variant were not performed.
In 4 of 5 members of an American family with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as cutaneous malignant melanoma, Shi et al. (2014) identified a heterozygous G-to-A transition in the POT1 gene, resulting in an asp224-to-asn (D224N) substitution at a highly conserved residue in the OB2 domain near DNA-binding sites. The mutation was also found in an Italian patient with sporadic melanoma and in 1 of 6,500 controls in the Exome Sequencing Project. Functional studies of the variant were not performed.
In 2 affected members of a family (family A) with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as glioma, Bainbridge et al. (2015) identified a heterozygous mutation (chr7.124,503,667C-A, GRCh37) in the POT1 gene, resulting in a gly95-to-cys (G95C) substitution at a highly conserved residue in the OB1 DNA-binding domain. The mutation was found by whole-exome sequencing. Another family member with late-onset lung cancer was found to carry the mutation, and there was an obligate carrier who died of astrocytoma at age 38 years. Three family members without cancer also carried the mutation, indicating incomplete penetrance. There was a strong family history of other cancers, including lung cancer and kidney cancer. Functional studies of the variant were not performed, but there was evidence that mutation carriers had a higher telomere content compared to those without the mutation.
In 2 affected members of a family (family B) with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as glioma, Bainbridge et al. (2015) identified a heterozygous mutation (chr7.124,481,048C-A, GRCh37) in the POT1 gene, resulting in a glu450-to-ter (E450X) substitution. The mutation was found by whole-exome sequencing. One family member with leukemia also carried the mutation, and there were 2 unaffected carriers, indicating incomplete penetrance. The mutation was predicted to delete highly conserved residues in the C terminus, which would affect binding to TPP1 (ACD; 609377) and impair association with the shelterin complex and telomeres. The mutation may also be subject to nonsense-mediated mRNA decay, resulting in haploinsufficiency. Functional studies of the variant were not performed, but there was evidence that mutation carriers had a higher telomere content compared to those without the mutation.
In a man with tumor predisposition syndrome-3 (TPDS3; 615848) manifest as glioma, Bainbridge et al. (2015) identified a heterozygous mutation (chr7.124,464,068A-T, GRCh37) in the POT1 gene, resulting in premature termination (Asp617GlufsTer8). The mutation was found by whole-exome sequencing. DNA from additional family members was not available, but the patient's father died of an oligodendroglioma at age 57 years. The mutation was predicted to delete highly conserved residues in the C terminus, which would affect binding to TPP1 (ACD; 609377) and impair association with the shelterin complex and telomeres. Functional studies of the variant were not performed, but there was evidence that mutation carriers had a higher telomere content compared to those without the mutation.
In 4 affected members of a family with telomere-related pulmonary fibrosis and/or bone marrow failure syndrome-8 (PFBMFT8; 620367), Kelich et al. (2022) identified a heterozygous c.776T-C transition (c.776T-C, NM_015450.2) in the POT1 gene, resulting in a leu259-to-ser (L259S) substitution at a conserved residue. The mutation, which was found by next-generation panel sequencing, segregated with the disorder in the family and was not present in the gnomAD database. In vitro functional expression studies showed that the mutant protein had decreased DNA binding affinity compared to wildtype, although it was able to interact with TPP1 (ACD; 609377) and localize to telomeres. Nuclear localization was reduced, suggesting that less POT1 is bound to DNA. Although the patients were noted to have short telomeres, the mutant protein did not negatively affect telomerase extension activity in vitro. Patient fibroblasts harboring the mutation had shortened lagging strands during synthesis, an increase in the telomeric overhang signal, and sister telomere loss during metaphase. The L259S mutation led to an increase in telomere dysfunction-induced foci in patient cells, suggesting a defect in telomere capping that could lead to shortened telomeres. Finally, the L259S mutation induced premature cellular senescence in patient cells, likely a consequence of telomere-induced DNA damage resulting from the mutation. Of note, HEK293 cells expressing the mutation in vitro developed longer telomeres compared to wildtype, suggesting that mutant POT1 is defective in limiting telomere elongation in cells.
In 2 sisters, born of consanguineous parents, with cerebroretinal microangiopathy with calcifications and cysts-3 (CRMCC3; 620368), Takai et al. (2016) identified a homozygous c.965C-T transition (chr7:124,487,037G-A) in the POT1 gene, resulting in a ser322-to-leu (S322L) substitution at a highly conserved residue in a region of unknown function. The unaffected parents were heterozygous for the mutation, which was not present in the dbSNP or ExAC databases. Patient fibroblasts cells showed premature senescence and contained telomere dysfunction-induced foci, indicating a DNA damage signal at the telomeres. The growth arrest could be mitigated by activation of telomerase. The mutant POT1 protein was able to interact with TPP1, localize to telomeres, and repress ATR signaling in POT1-null HeLa cells that contained telomerase. However, the mutant protein was unable to limit telomerase-mediated telomere elongation in vitro, indicating dysfunction with a separation-of-function effect. Cells carrying the mutation were defective in regulating the telomeric 5-prime C strand, causing extended 3-prime G overhangs. This resulted in stochastic telomere truncations that could be healed with expression of telomerase. Consistent with shortening of the telomeric C strand, metaphase chromosomes showed loss of telomeres synthesized by the leading DNA strand. The truncation of the telomeres and sister telomere loss during metaphase caused excessively short telomeres that likely lost the ability to protect chromosome ends. The findings were consistent with a defect in telomere end fill-in that generates truncated telomeres, which halt proliferation in cells lacking telomerase.
In 8 members of a 3-generation family (family 5) and in 5 patients from 2 additional unrelated families from a second cohort (pedigrees 1 and 3) with tumor predisposition syndrome-3 (TPDS3; 615848), DeBoy et al. (2023) identified a germline heterozygous c.818G-A transition (chr7.124,493,077C-T, GRCh37) in the POT1 gene, resulting in an arg273-to-gln (R273Q) substitution at a conserved domain that interacts with telomere DNA. The mutation, which was confirmed by Sanger sequencing, segregated with the disorder in the families, with evidence of incomplete penetrance. The mutation was not present in the gnomAD database. Patient cells showed decreased POT1 expression and defective binding to telomere DNA, consistent with a loss of function and haploinsufficiency. Mutation carriers, even those without tumors, had long telomeres compared to nonmutation carriers.
In 2 affected members of a family (family 2) and in 2 affected members of an unrelated family from a second cohort (pedigree 2) with tumor predisposition syndrome-3 (TPDS3; 615848), DeBoy et al. (2023) identified a germline heterozygous c.233T-C transition (chr7.124,510,987A-G, GRCh37) in the POT1 gene, resulting in an ile78-to-thr (I78T) substitution at a conserved domain in the OB1 domain that interacts with telomere DNA. The mutation, which was confirmed by Sanger sequencing, segregated with the disorder in the families. The mutation was present at a low frequency in the gnomAD database (4 of 246,852 alleles, frequency of 1.6 x 10(-5)). Patient cells showed decreased POT1 expression and defective binding to telomere DNA, consistent with a loss of function and haploinsufficiency. Mutation carriers, even those without tumors, had long telomeres compared to nonmutation carriers.
Wong et al. (2019) identified a heterozygous I78T mutation (chr7.124,870,933A-G, GRCh38) in the POT1 gene in 4 affected individuals from 3 unrelated families with TPDS3. The mutation segregated with the disorder in the families, although there was evidence of incomplete penetrance. In vitro functional studies showed that the variant impaired POT1 binding to telomere-like probes. Expression of the mutation into cells in vitro resulted in elongated telomeres. The probands in all families had melanoma, and some affected family members had chronic lymphocytic leukemia, thyroid cancer, and cutaneous T-cell lymphoma. All tumors were adult-onset. Examination of melanoma and nevi tissue from 2 of the patients showed accumulation of somatic events in several driver genes, many of which were involved in the MAPK pathway. All families were of Jewish descent, and haplotype analysis suggested a founder effect.
In 2 members of a family (family 4) with tumor predisposition syndrome-3 (TPDS3; 615848), DeBoy et al. (2023) identified a germline heterozygous intronic G-to-A transition, c.1164-1G-A (chr7.124,481,233C-T, GRCh37) in the POT1 gene, predicted to result in a splicing defect. The mutation, which was confirmed by Sanger sequencing, was present at a low frequency in the gnomAD database (1 of 31,346 alleles, frequency of 3.2 x 10(-5)). Patient cells showed decreased POT1 expression compared to controls, suggesting haploinsufficiency. One patient studied had longer telomeres than controls.
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