* 600951

TELOMERIC REPEAT-BINDING FACTOR 1; TERF1


Alternative titles; symbols

TRF1
TRF
PIN2


HGNC Approved Gene Symbol: TERF1

Cytogenetic location: 8q21.11     Genomic coordinates (GRCh38): 8:73,008,864-73,048,123 (from NCBI)


TEXT

Description

Human telomeres are nucleoprotein complexes at chromosome ends that consist of tandem arrays of TTAGGG repeats bound to specific proteins. Telomeres shield chromosome ends from degradation and end-to-end fusions, prevent activation of DNA damage checkpoints, and modulate the maintenance of telomeric DNA by telomerase (TERT; 187270). TERF1 is a major telomeric protein involved in the regulation of telomere length (Chong et al., 1995; van Steensel and de Lange, 1997).


Cloning and Expression

Chong et al. (1995) noted that protein components of the telomeric complex had been identified in ciliates and yeast, but not in vertebrates. A candidate vertebrate telomeric protein, TRF, had been found to associate with double-stranded TTAGGG repeat arrays in vitro and to display strong specificity for vertebrate telomere DNA. Chong et al. (1995) purified human TRF from HeLa cell nuclear extracts to near homogeneity. Three independent preparations of purified TRF contained a protein in the 60-kD apparent molecular mass range that copurified with TRF activity over a column containing double-stranded TTAGGG repeats. Chong et al. (1995) identified a partial TRF cDNA by database analysis, and using this sequence, they obtained a full-length TRF cDNA from a HeLa cell cDNA library. Northern blot analysis detected 2 mRNAs of approximately 1.8 and 3.0 kb that were expressed in all human tissues examined. The cDNA derived from the larger mRNA revealed an ORF encoding a 439-amino acid protein with a calculated molecular mass of 50.3 kD. In vitro transcription and translation of the cloned cDNA produced a protein of the same size as purified HeLa TRF. TRF contains an N-terminal acidic domain, a nuclear localization signal, and a C-terminal MYB (189990)-like DNA-binding repeat. Immunofluorescence analysis showed that TRF colocalized with telomeric DNA in human metaphase cells and was located at chromosome ends during metaphase.

Broccoli et al. (1997) characterized the genes encoding the human and mouse TRF1 proteins. The mouse Trf1 mRNA was ubiquitously expressed as a single 1.9-kb polyadenylated transcript in mouse somatic tissues. High levels of a 2.1-kb transcript were found in testis. In vitro translation of the mouse cDNA resulted in a 56-kD protein. Expression of epitope-tagged mouse Trf1 showed that it localized to the ends of murine metaphase chromosomes. Conceptual translation indicated that the mouse and human proteins are of similar size, with nearly identical C-terminal Myb-related DNA-binding motifs.

Young et al. (1997) identified 2 alternatively spliced TERF1 transcripts in fetal brain that differed by the presence or absence of a 60-bp exon.


Gene Function

Using mobility shift assays and competition experiments, Chong et al. (1995) showed that in vitro-expressed human TRF and purified HeLa TRF bound telomeric DNA with the same sequence specificity and did not require a DNA end for binding.

Broccoli et al. (1997) showed that in vitro-translated mouse Trf1 bound to TTAGGG repeat arrays.

Telomeres shorten with successive cell divisions in normal human cells, but this decline is halted through activation of telomerase in tumors and immortalized cells. Telomere length is stable in several human immortalized cell lines, suggesting that a regulatory mechanism limits telomere elongation by telomerase. Van Steensel and de Lange (1997) found that overexpression of human TRF1 in the telomerase-positive tumor cell line HT1080 resulted in gradual and progressive telomere shortening. Conversely, expression of a dominant-negative TRF1 mutant induced telomere elongation and inhibited binding of endogenous TRF1 to telomeres. Van Steensel and de Lange (1997) concluded that TRF1 is a suppressor of telomere elongation that is involved in a negative-feedback mechanism that stabilizes telomere length.

In yeast, Marcand et al. (1997) demonstrated a protein-counting mechanism for regulation of the length of telomeres. This mechanism involved the telomere repeat-binding protein Rap1p (see 605061). Because the structural and functional properties of telomeres appear to be highly conserved, Marcand et al. (1997) suggested that their findings may be relevant to telomere length regulation in humans, which has been associated with aging and cancer.

Using electron microscopy, Griffith et al. (1999) demonstrated that TRF2 could remodel linear telomeric DNA into large duplex loops (t loops) in vitro. Electron microscopy analysis of psoralen cross-linked telomeric DNA purified from human and mouse cells revealed abundant large t loops with a size distribution consistent with their telomeric origin. Binding of TRF1 and single-stranded DNA-binding protein (600439) suggested that t loops are formed by invasion of the 3-prime telomeric overhang into the duplex telomeric repeat array. Griffith et al. (1999) concluded that t loops may provide a general mechanism for the protection and replication of telomeres.

By interaction cloning using TRF1 as bait, Kim et al. (1999) isolated a novel human telomere-associated protein, TIN2 (604319). TIN2 interacted with TRF1 in vitro and in cells, and colocalized with TRF1 in nuclei and metaphase chromosomes. A mutant TIN2 that lacked N-terminal sequences effected elongated human telomeres in a telomerase-dependent manner. The findings suggested that TRF1 is insufficient for control of telomere length in human cells, and that TIN2 is an essential mediator of TRF1 function.

Okabe et al. (2000) investigated cellular factors required for telomere formation using the frequency of telomere seeding as an index and identified TRF1 as an essential transacting factor. The exogenous telomere repeat induced telomere formation at a frequency determined by the availability of TRF1, even in telomerase-negative cells. The authors concluded that TRF1 has a novel physiologic significance distinct from its role as a regulator of telomere length in the endogenous chromosome.

Smucker and Turchi (2001) determined that mammalian Trf1, when bound to duplex telomeric DNA, inhibited DNA polymerase-alpha (312040)-catalyzed DNA synthesis. Inhibition was specific for Trf1 but not specific for the polymerase catalyzing synthesis. Trf1 had no effect on nontelomeric, random DNA substrates.

Forwood and Jans (2002) determined that nuclear accumulation of mammalian Trf1 is mediated by importin beta-1 (KPNB1; 602738) and Ran (601179). The showed that Kpnb1 binds directly to Trf1. Importin alpha (see 600685) inhibited Kpnb1-mediated nuclear accumulation, although it did not affect the Kpnb1 and Trf1 interaction.

Human telomere length is regulated by the TTAGGG-repeat-binding protein TRF1 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, protection of telomeres-1 (POT1; 606478), 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.

Kanauchi et al. (2003) examined whether apoptosis-regulating genes, BCLXL (see 600039) and FAS (134637), and the telomere-related gene TERF1 differ in expression between adrenal cortical cancers and benign adrenal tumors. Tissues from 4 adrenal cortical cancers were compared with 7 normal adrenal tissues, 17 cortical adenomas, 4 cortical hyperplasias, and 20 pheochromocytomas for expressions of BCLXL and FAS by RT-PCR, and for expressions of TERF1 by real-time quantitative RT-PCR. All benign adrenal tissues expressed both the antiapoptosis gene BCLXL and the proapoptosis gene FAS, but the adrenal cortical cancers expressed only BCLXL and not FAS. Expression of TERF1 was increased by more than 30-fold in the adrenal cortical cancers compared with benign adrenal tissues, and was inversely correlated with the prognosis of patients with the adrenal cortical cancers. This lack of expression of FAS in adrenal cortical cancer may help to distinguish it from benign adrenal tumors. The authors concluded that level of TERF1 expression may be helpful prognostically for patients with adrenal cortical cancers.

Okabe et al. (2004) investigated the role of TRF1 in the proliferation of human fibroblasts. TRF1 expression was upregulated in a large variety of immortal human cells and supported de novo telomere formation in a dose-dependent manner. However, primary fibroblasts ectopically overexpressing TRF1 were unable to avoid senescence. Exogenously expressed TRF1 in primary fibroblasts neither supported de novo telomere formation nor bound to the nuclear matrix as tightly as observed in immortal cells that show upregulated TRF1 expression. Okabe et al. (2004) suggested that mortal human cells may lack specific ligands that anchor TRF1 to the nuclear matrix and that this may contribute to their limited life span.

In addition to increased DNA strand exchange, another cytogenetic feature of Bloom syndrome (210900) cells lacking BLM (RECQL3; 604610) is the tendency for telomeres to associate. Lillard-Wetherell et al. (2004) reported that BLM colocalized and complexed with TERF2 (602027) in cells that employ recombination-mediated telomere lengthening (ALT, alternative lengthening of telomeres). BLM colocalized with TERF2 in foci actively synthesizing DNA during late S and G2/M; colocalization increased in late S and G2/M when ALT is thought to occur. TERF1 and TERF2 interacted directly with BLM and regulated its unwinding activity in vitro. Whereas TERF2 stimulated BLM unwinding of telomeric and nontelomeric substrates, TERF1 inhibited its unwinding of telomeric substrates only. TERF2 stimulated BLM unwinding with equimolar concentrations of TERF1 but not when TERF1 was added in molar excess. Lillard-Wetherell et al. (2004) proposed a function for BLM in recombination-mediated telomere lengthening and a model for the coordinated regulation of BLM activity at telomeres by TERF1 and TERF2.

Telomere repeats in the fission yeast Schizosaccharomyces pombe are bound by Taz1, a regulator of diverse telomere functions. Miller et al. (2006) showed that Taz1 is crucial for efficient replication fork progression through the telomere. Using 2-dimensional gel electrophoresis, Miller et al. (2006) found that loss of Taz1 leads to stalled replication forks at telomeres and internally placed telomere sequences, regardless of whether the telomeric G-rich strand is replicated by leading- or lagging-strand synthesis. In contrast, the Taz1-interacting protein Rap1 is dispensable for efficient telomeric fork progression. Upon loss of telomerase, Taz1-delta telomeres are lost precipitously, suggesting that maintenance of Taz1-delta telomere repeats cannot be sustained through semiconservative replication. As the human telomere proteins TRF1 and TRF2 are Taz1 orthologs, Miller et al. (2006) predicted that 1 or both of the human TRFs may orchestrate fork passage through human telomeres. Stalled forks at dysfunctional human telomeres are likely to accelerate the genomic instability that drives tumorigenesis.

Mammalian telomeres are protected by a 6-protein complex, shelterin. Shelterin contains 2 closely related proteins, TRF1 and TRF2 (602027), which recruit various proteins to telomeres. Chen et al. (2008) dissected the interactions of TRF1 and TRF2 with their shared binding partner TIN2 (604319) 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).

By coimmunoprecipitation analysis of HeLa cell lysates, Lee et al. (2009) found that the peptidyl-prolyl isomerase PIN1 (601052) interacted with TRF1 during mitosis, but not during interphase. Mutation analysis showed that the WW domain of PIN1 bound the phosphorylated motif thr149-pro150 in TRF1. Inhibitor studies revealed that CDK (see CDK1; 116940) phosphorylated TRF1 on thr149, and this phosphorylation was required for interaction of PIN1 with TRF1. Knockdown or inhibition of PIN1 stabilized TRF1 against degradation, resulting in elevated binding of TRF1 to telomeres and gradual, progressive telomere shortening. Furthermore, Pin1 -/- mice exhibited accelerated aging in association with accelerated telomere loss within a single generation. Lee et al. (2009) concluded that PIN1 functions to protect telomeres by inducing TRF1 instability and degradation.

To define the telomere end-protection problem, Sfeir and de Lange (2012) removed the whole shelterin complex from mouse telomeres through conditional deletion of TRF1 and TFR2 in nonhomologous end-joining (NHEJ)-deficient cells. The data revealed 2 DNA damage response pathways not theretofore observed upon deletion of individual shelterin proteins. The shelterin-free telomeres are processed by microhomology-mediated alternative-NHEJ when Ku70/80 (152690/194364) is absent and are attacked by nucleolytic degradation in the absence of 53BP1 (605230). Sfeir and de Lange (2012) concluded that their data established that the end-protection problem is specified by 6 pathways (ATM and ATR signaling, classical-NHEJ, alt-NHEJ, homologous recombination, and resection), and showed how shelterin acts with general DNA damage response factors to solve this problem.

Reviews

Zakian (1995) reviewed telomere structure, function, and replication.


Biochemical Features

TRF1 and TRF2 are key components of vertebrate telomeres and bind to double-stranded telomeric DNA as homodimers. Dimerization involves the TRF homology (TRFH) domain, which also mediates interactions with other telomeric proteins. Fairall et al. (2001) determined the crystal structures of the dimerization domains from human TRF1 and TRF2 at 2.9- and 2.2-angstrom resolution, respectively. Despite a modest sequence identity, the 2 TRFH domains have the same entirely alpha-helical architecture, resembling a twisted horseshoe. The dimerization interfaces feature unique interactions that prevent heterodimerization. Mutational analysis of TRF1 corroborated the structural data and underscored the importance of the TRFH domain in dimerization, DNA binding, and telomere localization.


Mapping

By fluorescence in situ hybridization (FISH), Broccoli et al. (1997) assigned human TERF1 to chromosome 8q13. The assignment was confirmed by PCR analysis of a panel of human/rodent somatic cell hybrids containing different human chromosomes. FISH was also used to map the Terf1 gene to mouse chromosome 17. No part of mouse chromosome 17 has been reported to be syntenic with human chromosome 8. By somatic cell hybrid analysis, Young et al. (1997) confirmed the mapping of the TERF1 gene to chromosome 8.


Animal Model

Martinez et al. (2009) stated that Terf1 deletion in mice results in early embryonic lethality. They developed mice with targeted deletion of Terf1 in skin after embryonic day 11.5. These mutant mice were born at the expected mendelian ratio, but they died perinatally with severe skin hyperpigmentation and reduced skin development. Defects included complete absence of mature skin hair follicles and sebaceous glands, focal development of preneoplastic lesions with abnormally high and ubiquitous expression of cytokeratin K6 (see KRT6A; 148041), and defective barrier function. Stratified epithelia of the tongue, palate, esophagus, and nongrandular stomach showed severe hyperkeratosis and dysplasia. Deletion of Terf1 in mouse embryonic fibroblasts (MEFs) resulted in massive induction of telomeric DNA damage, activation of Atm (607585) and Atr (601215) and their downstream checkpoint kinases Chek1 (603078) and Chek2 (604373), and cell cycle arrest. Terf1-null MEFs also showed abundant telomere fusions and multitelomeric signals. Martinez et al. (2009) concluded that TERF1 protects telomeres from eliciting a DNA damage response.


REFERENCES

  1. Broccoli, D., Chong, L., Oelmann, S., Fernald, A. A., Marziliano, N., van Steensel, B., Kipling, D., Le Beau, M. M., de Lange, T. Comparison of the human and mouse genes encoding the telomeric protein, TRF1: chromosomal localization, expression and conserved protein domains. Hum. Molec. Genet. 6: 69-76, 1997. [PubMed: 9002672, related citations] [Full Text]

  2. Chen, Y., Yang, Y., van Overbeek, M., Donigian, J. R., Baciu, P., de Lange, T., Lei, M. A shared docking motif in TRF1 and TRF2 used for differential recruitment of telomeric proteins. Science 319: 1092-1096, 2008. [PubMed: 18202258, related citations] [Full Text]

  3. Chong, L., van Steensel, B., Broccoli, D., Erdjument-Bromage, H., Hanish, J., Tempst, P., de Lange, T. A human telomeric protein. Science 270: 1663-1667, 1995. [PubMed: 7502076, related citations] [Full Text]

  4. Fairall, L., Chapman, L., Moss, H., de Lange, T., Rhodes, D. Structure of the TRFH dimerization domain of the human telomeric proteins TRF1 and TRF2. Molec. Cell 8: 351-361, 2001. [PubMed: 11545737, related citations] [Full Text]

  5. Forwood, J. K., Jans, D. A. Nuclear import pathway of the telomere elongation suppressor TRF1: inhibition by importin alpha. Biochemistry 41: 9333-9340, 2002. [PubMed: 12135354, related citations] [Full Text]

  6. Griffith, J. D., Comeau, L., Rosenfield, S., Stansel, R. M., Bianchi, A., Moss, H., de Lange, T. Mammalian telomeres end in a large duplex loop. Cell 97: 503-514, 1999. [PubMed: 10338214, related citations] [Full Text]

  7. Kanauchi, H., Wada, N., Ginzinger, D. G., Yu, M., Wong, M. G., Clark, O. H., Duh, Q.-Y. Diagnostic and prognostic value of Fas and telomeric-repeat binding factor-1 genes in adrenal tumors. J. Clin. Endocr. Metab. 88: 3690-3693, 2003. [PubMed: 12915656, related citations] [Full Text]

  8. Kim, S., Kaminker, P., Campisi, J. TIN2, a new regulator of telomere length in human cells. Nature Genet. 23: 405-412, 1999. [PubMed: 10581025, images, related citations] [Full Text]

  9. Lee, T. H., Tun-Kyi, A., Shi, R., Lim, J., Soohoo, C., Finn, G., Balastik, M., Pastorino, L., Wulf, G., Zhou, X. Z., Lu, K. P. Essential role of Pin1 in the regulation of TRF1 stability and telomere maintenance. Nature Cell Biol. 11: 97-105, 2009. [PubMed: 19060891, images, related citations] [Full Text]

  10. Lillard-Wetherell, K., Machwe, A., Langland, G. T., Combs, K. A., Behbehani, G. K., Schonberg, S. A., German, J., Turchi, J. J., Orren, D. K., Groden, J. Association and regulation of the BLM helicase by the telomere proteins TRF1 and TRF2. Hum. Molec. Genet. 13: 1919-1932, 2004. [PubMed: 15229185, related citations] [Full Text]

  11. Loayza, D., de Lange, T. POT1 as a terminal transducer of TRF1 telomere length control. Nature 424: 1013-1018, 2003. [PubMed: 12768206, related citations] [Full Text]

  12. Marcand, S., Gilson, E., Shore, D. A protein-counting mechanism for telomere length regulation in yeast. Science 275: 986-990, 1997. [PubMed: 9020083, related citations] [Full Text]

  13. Martinez, P., Thanasoula, M., Munoz, P., Liao, C., Tejera, A., McNees, C., Flores, J. M., Fernandez-Capetillo, O., Tarsounas, M., Blasco, M. A. Increased telomere fragility and fusions resulting from TRF1 deficiency lead to degenerative pathologies and increased cancer in mice. Genes Dev. 23: 2060-2075, 2009. [PubMed: 19679647, images, related citations] [Full Text]

  14. Miller, K. M., Rog, O., Cooper, J. P. Semi-conservative DNA replication through telomeres requires Taz1. Nature 440: 824-828, 2006. [PubMed: 16598261, related citations] [Full Text]

  15. Okabe, J., Eguchi, A., Masago, A., Hayakawa, T., Nakanishi, M. TRF1 is a critical trans-acting factor required for de novo telomere formation in human cells. Hum. Molec. Genet. 9: 2639-2650, 2000. [PubMed: 11063723, related citations] [Full Text]

  16. Okabe, J., Eguchi, A., Wadhwa, R., Rakwal, R., Tsukinoki, R., Hayakawa, T., Nakanishi, M. Limited capacity of the nuclear matrix to bind telomere repeat binding factor TRF1 may restrict the proliferation of mortal human fibroblasts. Hum. Molec. Genet. 13: 285-293, 2004. [PubMed: 14681297, related citations] [Full Text]

  17. Sfeir, A., de Lange, T. Removal of shelterin reveals the telomere end-protection problem. Science 336: 593-597, 2012. [PubMed: 22556254, images, related citations] [Full Text]

  18. Smucker, E. J., Turchi, J. J. TRF1 inhibits telomere C-strand DNA synthesis in vitro. Biochemistry 40: 2426-2432, 2001. [PubMed: 11327863, related citations] [Full Text]

  19. van Steensel, B., de Lange, T. Control of telomere length by the human telomeric protein TRF1. Nature 385: 740-743, 1997. [PubMed: 9034193, related citations] [Full Text]

  20. Young, A. C., Chavez, M., Giambernardi, T. A., Mattern, V., McGill, J. R., Harris, J. M., Sarosdy, M. F., Patel, P., Sakaguchi, A. Y. Organization and expression of human telomere repeat binding factor genes. Somat. Cell Molec. Genet. 23: 275-286, 1997. [PubMed: 9542529, related citations] [Full Text]

  21. Zakian, V. A. Telomeres: beginning to understand the end. Science 270: 1601-1607, 1995. [PubMed: 7502069, related citations] [Full Text]


Ada Hamosh - updated : 5/30/2012
Matthew B. Gross - updated : 5/20/2010
Patricia A. Hartz - updated : 5/19/2010
Patricia A. Hartz - updated : 12/11/2009
Ada Hamosh - updated : 4/4/2008
George E. Tiller - updated : 1/16/2007
Ada Hamosh - updated : 6/1/2006
George E. Tiller - updated : 2/17/2006
John A. Phillips, III - updated : 11/4/2004
Ada Hamosh - updated : 5/29/2003
Patricia A. Hartz - updated : 5/5/2003
Stylianos E. Antonarakis - updated : 10/29/2001
George E. Tiller - updated : 1/25/2001
Patti M. Sherman - updated : 5/30/2000
Victor A. McKusick - updated : 11/30/1999
Stylianos E. Antonarakis - updated : 7/2/1999
Victor A. McKusick - updated : 2/25/1997
Victor A. McKusick - updated : 2/12/1997
Creation Date:
Victor A. McKusick : 12/7/1995
alopez : 06/01/2012
alopez : 6/1/2012
terry : 5/30/2012
mgross : 5/20/2010
terry : 5/19/2010
mgross : 1/5/2010
terry : 12/11/2009
alopez : 4/11/2008
terry : 4/4/2008
wwang : 1/25/2007
terry : 1/16/2007
alopez : 6/5/2006
terry : 6/1/2006
alopez : 3/28/2006
wwang : 3/9/2006
terry : 2/17/2006
alopez : 11/4/2004
alopez : 11/4/2004
alopez : 7/28/2003
alopez : 5/29/2003
terry : 5/29/2003
mgross : 5/5/2003
terry : 12/7/2001
mgross : 11/27/2001
mgross : 10/29/2001
mgross : 10/29/2001
mcapotos : 2/1/2001
mcapotos : 1/25/2001
mcapotos : 6/20/2000
psherman : 5/30/2000
alopez : 11/30/1999
terry : 11/30/1999
mgross : 7/9/1999
kayiaros : 7/2/1999
psherman : 9/14/1998
carol : 6/26/1998
jenny : 9/30/1997
mark : 2/25/1997
terry : 2/12/1997
terry : 2/7/1997
joanna : 12/7/1995

* 600951

TELOMERIC REPEAT-BINDING FACTOR 1; TERF1


Alternative titles; symbols

TRF1
TRF
PIN2


HGNC Approved Gene Symbol: TERF1

Cytogenetic location: 8q21.11     Genomic coordinates (GRCh38): 8:73,008,864-73,048,123 (from NCBI)


TEXT

Description

Human telomeres are nucleoprotein complexes at chromosome ends that consist of tandem arrays of TTAGGG repeats bound to specific proteins. Telomeres shield chromosome ends from degradation and end-to-end fusions, prevent activation of DNA damage checkpoints, and modulate the maintenance of telomeric DNA by telomerase (TERT; 187270). TERF1 is a major telomeric protein involved in the regulation of telomere length (Chong et al., 1995; van Steensel and de Lange, 1997).


Cloning and Expression

Chong et al. (1995) noted that protein components of the telomeric complex had been identified in ciliates and yeast, but not in vertebrates. A candidate vertebrate telomeric protein, TRF, had been found to associate with double-stranded TTAGGG repeat arrays in vitro and to display strong specificity for vertebrate telomere DNA. Chong et al. (1995) purified human TRF from HeLa cell nuclear extracts to near homogeneity. Three independent preparations of purified TRF contained a protein in the 60-kD apparent molecular mass range that copurified with TRF activity over a column containing double-stranded TTAGGG repeats. Chong et al. (1995) identified a partial TRF cDNA by database analysis, and using this sequence, they obtained a full-length TRF cDNA from a HeLa cell cDNA library. Northern blot analysis detected 2 mRNAs of approximately 1.8 and 3.0 kb that were expressed in all human tissues examined. The cDNA derived from the larger mRNA revealed an ORF encoding a 439-amino acid protein with a calculated molecular mass of 50.3 kD. In vitro transcription and translation of the cloned cDNA produced a protein of the same size as purified HeLa TRF. TRF contains an N-terminal acidic domain, a nuclear localization signal, and a C-terminal MYB (189990)-like DNA-binding repeat. Immunofluorescence analysis showed that TRF colocalized with telomeric DNA in human metaphase cells and was located at chromosome ends during metaphase.

Broccoli et al. (1997) characterized the genes encoding the human and mouse TRF1 proteins. The mouse Trf1 mRNA was ubiquitously expressed as a single 1.9-kb polyadenylated transcript in mouse somatic tissues. High levels of a 2.1-kb transcript were found in testis. In vitro translation of the mouse cDNA resulted in a 56-kD protein. Expression of epitope-tagged mouse Trf1 showed that it localized to the ends of murine metaphase chromosomes. Conceptual translation indicated that the mouse and human proteins are of similar size, with nearly identical C-terminal Myb-related DNA-binding motifs.

Young et al. (1997) identified 2 alternatively spliced TERF1 transcripts in fetal brain that differed by the presence or absence of a 60-bp exon.


Gene Function

Using mobility shift assays and competition experiments, Chong et al. (1995) showed that in vitro-expressed human TRF and purified HeLa TRF bound telomeric DNA with the same sequence specificity and did not require a DNA end for binding.

Broccoli et al. (1997) showed that in vitro-translated mouse Trf1 bound to TTAGGG repeat arrays.

Telomeres shorten with successive cell divisions in normal human cells, but this decline is halted through activation of telomerase in tumors and immortalized cells. Telomere length is stable in several human immortalized cell lines, suggesting that a regulatory mechanism limits telomere elongation by telomerase. Van Steensel and de Lange (1997) found that overexpression of human TRF1 in the telomerase-positive tumor cell line HT1080 resulted in gradual and progressive telomere shortening. Conversely, expression of a dominant-negative TRF1 mutant induced telomere elongation and inhibited binding of endogenous TRF1 to telomeres. Van Steensel and de Lange (1997) concluded that TRF1 is a suppressor of telomere elongation that is involved in a negative-feedback mechanism that stabilizes telomere length.

In yeast, Marcand et al. (1997) demonstrated a protein-counting mechanism for regulation of the length of telomeres. This mechanism involved the telomere repeat-binding protein Rap1p (see 605061). Because the structural and functional properties of telomeres appear to be highly conserved, Marcand et al. (1997) suggested that their findings may be relevant to telomere length regulation in humans, which has been associated with aging and cancer.

Using electron microscopy, Griffith et al. (1999) demonstrated that TRF2 could remodel linear telomeric DNA into large duplex loops (t loops) in vitro. Electron microscopy analysis of psoralen cross-linked telomeric DNA purified from human and mouse cells revealed abundant large t loops with a size distribution consistent with their telomeric origin. Binding of TRF1 and single-stranded DNA-binding protein (600439) suggested that t loops are formed by invasion of the 3-prime telomeric overhang into the duplex telomeric repeat array. Griffith et al. (1999) concluded that t loops may provide a general mechanism for the protection and replication of telomeres.

By interaction cloning using TRF1 as bait, Kim et al. (1999) isolated a novel human telomere-associated protein, TIN2 (604319). TIN2 interacted with TRF1 in vitro and in cells, and colocalized with TRF1 in nuclei and metaphase chromosomes. A mutant TIN2 that lacked N-terminal sequences effected elongated human telomeres in a telomerase-dependent manner. The findings suggested that TRF1 is insufficient for control of telomere length in human cells, and that TIN2 is an essential mediator of TRF1 function.

Okabe et al. (2000) investigated cellular factors required for telomere formation using the frequency of telomere seeding as an index and identified TRF1 as an essential transacting factor. The exogenous telomere repeat induced telomere formation at a frequency determined by the availability of TRF1, even in telomerase-negative cells. The authors concluded that TRF1 has a novel physiologic significance distinct from its role as a regulator of telomere length in the endogenous chromosome.

Smucker and Turchi (2001) determined that mammalian Trf1, when bound to duplex telomeric DNA, inhibited DNA polymerase-alpha (312040)-catalyzed DNA synthesis. Inhibition was specific for Trf1 but not specific for the polymerase catalyzing synthesis. Trf1 had no effect on nontelomeric, random DNA substrates.

Forwood and Jans (2002) determined that nuclear accumulation of mammalian Trf1 is mediated by importin beta-1 (KPNB1; 602738) and Ran (601179). The showed that Kpnb1 binds directly to Trf1. Importin alpha (see 600685) inhibited Kpnb1-mediated nuclear accumulation, although it did not affect the Kpnb1 and Trf1 interaction.

Human telomere length is regulated by the TTAGGG-repeat-binding protein TRF1 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, protection of telomeres-1 (POT1; 606478), 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.

Kanauchi et al. (2003) examined whether apoptosis-regulating genes, BCLXL (see 600039) and FAS (134637), and the telomere-related gene TERF1 differ in expression between adrenal cortical cancers and benign adrenal tumors. Tissues from 4 adrenal cortical cancers were compared with 7 normal adrenal tissues, 17 cortical adenomas, 4 cortical hyperplasias, and 20 pheochromocytomas for expressions of BCLXL and FAS by RT-PCR, and for expressions of TERF1 by real-time quantitative RT-PCR. All benign adrenal tissues expressed both the antiapoptosis gene BCLXL and the proapoptosis gene FAS, but the adrenal cortical cancers expressed only BCLXL and not FAS. Expression of TERF1 was increased by more than 30-fold in the adrenal cortical cancers compared with benign adrenal tissues, and was inversely correlated with the prognosis of patients with the adrenal cortical cancers. This lack of expression of FAS in adrenal cortical cancer may help to distinguish it from benign adrenal tumors. The authors concluded that level of TERF1 expression may be helpful prognostically for patients with adrenal cortical cancers.

Okabe et al. (2004) investigated the role of TRF1 in the proliferation of human fibroblasts. TRF1 expression was upregulated in a large variety of immortal human cells and supported de novo telomere formation in a dose-dependent manner. However, primary fibroblasts ectopically overexpressing TRF1 were unable to avoid senescence. Exogenously expressed TRF1 in primary fibroblasts neither supported de novo telomere formation nor bound to the nuclear matrix as tightly as observed in immortal cells that show upregulated TRF1 expression. Okabe et al. (2004) suggested that mortal human cells may lack specific ligands that anchor TRF1 to the nuclear matrix and that this may contribute to their limited life span.

In addition to increased DNA strand exchange, another cytogenetic feature of Bloom syndrome (210900) cells lacking BLM (RECQL3; 604610) is the tendency for telomeres to associate. Lillard-Wetherell et al. (2004) reported that BLM colocalized and complexed with TERF2 (602027) in cells that employ recombination-mediated telomere lengthening (ALT, alternative lengthening of telomeres). BLM colocalized with TERF2 in foci actively synthesizing DNA during late S and G2/M; colocalization increased in late S and G2/M when ALT is thought to occur. TERF1 and TERF2 interacted directly with BLM and regulated its unwinding activity in vitro. Whereas TERF2 stimulated BLM unwinding of telomeric and nontelomeric substrates, TERF1 inhibited its unwinding of telomeric substrates only. TERF2 stimulated BLM unwinding with equimolar concentrations of TERF1 but not when TERF1 was added in molar excess. Lillard-Wetherell et al. (2004) proposed a function for BLM in recombination-mediated telomere lengthening and a model for the coordinated regulation of BLM activity at telomeres by TERF1 and TERF2.

Telomere repeats in the fission yeast Schizosaccharomyces pombe are bound by Taz1, a regulator of diverse telomere functions. Miller et al. (2006) showed that Taz1 is crucial for efficient replication fork progression through the telomere. Using 2-dimensional gel electrophoresis, Miller et al. (2006) found that loss of Taz1 leads to stalled replication forks at telomeres and internally placed telomere sequences, regardless of whether the telomeric G-rich strand is replicated by leading- or lagging-strand synthesis. In contrast, the Taz1-interacting protein Rap1 is dispensable for efficient telomeric fork progression. Upon loss of telomerase, Taz1-delta telomeres are lost precipitously, suggesting that maintenance of Taz1-delta telomere repeats cannot be sustained through semiconservative replication. As the human telomere proteins TRF1 and TRF2 are Taz1 orthologs, Miller et al. (2006) predicted that 1 or both of the human TRFs may orchestrate fork passage through human telomeres. Stalled forks at dysfunctional human telomeres are likely to accelerate the genomic instability that drives tumorigenesis.

Mammalian telomeres are protected by a 6-protein complex, shelterin. Shelterin contains 2 closely related proteins, TRF1 and TRF2 (602027), which recruit various proteins to telomeres. Chen et al. (2008) dissected the interactions of TRF1 and TRF2 with their shared binding partner TIN2 (604319) 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).

By coimmunoprecipitation analysis of HeLa cell lysates, Lee et al. (2009) found that the peptidyl-prolyl isomerase PIN1 (601052) interacted with TRF1 during mitosis, but not during interphase. Mutation analysis showed that the WW domain of PIN1 bound the phosphorylated motif thr149-pro150 in TRF1. Inhibitor studies revealed that CDK (see CDK1; 116940) phosphorylated TRF1 on thr149, and this phosphorylation was required for interaction of PIN1 with TRF1. Knockdown or inhibition of PIN1 stabilized TRF1 against degradation, resulting in elevated binding of TRF1 to telomeres and gradual, progressive telomere shortening. Furthermore, Pin1 -/- mice exhibited accelerated aging in association with accelerated telomere loss within a single generation. Lee et al. (2009) concluded that PIN1 functions to protect telomeres by inducing TRF1 instability and degradation.

To define the telomere end-protection problem, Sfeir and de Lange (2012) removed the whole shelterin complex from mouse telomeres through conditional deletion of TRF1 and TFR2 in nonhomologous end-joining (NHEJ)-deficient cells. The data revealed 2 DNA damage response pathways not theretofore observed upon deletion of individual shelterin proteins. The shelterin-free telomeres are processed by microhomology-mediated alternative-NHEJ when Ku70/80 (152690/194364) is absent and are attacked by nucleolytic degradation in the absence of 53BP1 (605230). Sfeir and de Lange (2012) concluded that their data established that the end-protection problem is specified by 6 pathways (ATM and ATR signaling, classical-NHEJ, alt-NHEJ, homologous recombination, and resection), and showed how shelterin acts with general DNA damage response factors to solve this problem.

Reviews

Zakian (1995) reviewed telomere structure, function, and replication.


Biochemical Features

TRF1 and TRF2 are key components of vertebrate telomeres and bind to double-stranded telomeric DNA as homodimers. Dimerization involves the TRF homology (TRFH) domain, which also mediates interactions with other telomeric proteins. Fairall et al. (2001) determined the crystal structures of the dimerization domains from human TRF1 and TRF2 at 2.9- and 2.2-angstrom resolution, respectively. Despite a modest sequence identity, the 2 TRFH domains have the same entirely alpha-helical architecture, resembling a twisted horseshoe. The dimerization interfaces feature unique interactions that prevent heterodimerization. Mutational analysis of TRF1 corroborated the structural data and underscored the importance of the TRFH domain in dimerization, DNA binding, and telomere localization.


Mapping

By fluorescence in situ hybridization (FISH), Broccoli et al. (1997) assigned human TERF1 to chromosome 8q13. The assignment was confirmed by PCR analysis of a panel of human/rodent somatic cell hybrids containing different human chromosomes. FISH was also used to map the Terf1 gene to mouse chromosome 17. No part of mouse chromosome 17 has been reported to be syntenic with human chromosome 8. By somatic cell hybrid analysis, Young et al. (1997) confirmed the mapping of the TERF1 gene to chromosome 8.


Animal Model

Martinez et al. (2009) stated that Terf1 deletion in mice results in early embryonic lethality. They developed mice with targeted deletion of Terf1 in skin after embryonic day 11.5. These mutant mice were born at the expected mendelian ratio, but they died perinatally with severe skin hyperpigmentation and reduced skin development. Defects included complete absence of mature skin hair follicles and sebaceous glands, focal development of preneoplastic lesions with abnormally high and ubiquitous expression of cytokeratin K6 (see KRT6A; 148041), and defective barrier function. Stratified epithelia of the tongue, palate, esophagus, and nongrandular stomach showed severe hyperkeratosis and dysplasia. Deletion of Terf1 in mouse embryonic fibroblasts (MEFs) resulted in massive induction of telomeric DNA damage, activation of Atm (607585) and Atr (601215) and their downstream checkpoint kinases Chek1 (603078) and Chek2 (604373), and cell cycle arrest. Terf1-null MEFs also showed abundant telomere fusions and multitelomeric signals. Martinez et al. (2009) concluded that TERF1 protects telomeres from eliciting a DNA damage response.


REFERENCES

  1. Broccoli, D., Chong, L., Oelmann, S., Fernald, A. A., Marziliano, N., van Steensel, B., Kipling, D., Le Beau, M. M., de Lange, T. Comparison of the human and mouse genes encoding the telomeric protein, TRF1: chromosomal localization, expression and conserved protein domains. Hum. Molec. Genet. 6: 69-76, 1997. [PubMed: 9002672] [Full Text: https://doi.org/10.1093/hmg/6.1.69]

  2. Chen, Y., Yang, Y., van Overbeek, M., Donigian, J. R., Baciu, P., de Lange, T., Lei, M. A shared docking motif in TRF1 and TRF2 used for differential recruitment of telomeric proteins. Science 319: 1092-1096, 2008. [PubMed: 18202258] [Full Text: https://doi.org/10.1126/science.1151804]

  3. Chong, L., van Steensel, B., Broccoli, D., Erdjument-Bromage, H., Hanish, J., Tempst, P., de Lange, T. A human telomeric protein. Science 270: 1663-1667, 1995. [PubMed: 7502076] [Full Text: https://doi.org/10.1126/science.270.5242.1663]

  4. Fairall, L., Chapman, L., Moss, H., de Lange, T., Rhodes, D. Structure of the TRFH dimerization domain of the human telomeric proteins TRF1 and TRF2. Molec. Cell 8: 351-361, 2001. [PubMed: 11545737] [Full Text: https://doi.org/10.1016/s1097-2765(01)00321-5]

  5. Forwood, J. K., Jans, D. A. Nuclear import pathway of the telomere elongation suppressor TRF1: inhibition by importin alpha. Biochemistry 41: 9333-9340, 2002. [PubMed: 12135354] [Full Text: https://doi.org/10.1021/bi025548s]

  6. Griffith, J. D., Comeau, L., Rosenfield, S., Stansel, R. M., Bianchi, A., Moss, H., de Lange, T. Mammalian telomeres end in a large duplex loop. Cell 97: 503-514, 1999. [PubMed: 10338214] [Full Text: https://doi.org/10.1016/s0092-8674(00)80760-6]

  7. Kanauchi, H., Wada, N., Ginzinger, D. G., Yu, M., Wong, M. G., Clark, O. H., Duh, Q.-Y. Diagnostic and prognostic value of Fas and telomeric-repeat binding factor-1 genes in adrenal tumors. J. Clin. Endocr. Metab. 88: 3690-3693, 2003. [PubMed: 12915656] [Full Text: https://doi.org/10.1210/jc.2002-020965]

  8. Kim, S., Kaminker, P., Campisi, J. TIN2, a new regulator of telomere length in human cells. Nature Genet. 23: 405-412, 1999. [PubMed: 10581025] [Full Text: https://doi.org/10.1038/70508]

  9. Lee, T. H., Tun-Kyi, A., Shi, R., Lim, J., Soohoo, C., Finn, G., Balastik, M., Pastorino, L., Wulf, G., Zhou, X. Z., Lu, K. P. Essential role of Pin1 in the regulation of TRF1 stability and telomere maintenance. Nature Cell Biol. 11: 97-105, 2009. [PubMed: 19060891] [Full Text: https://doi.org/10.1038/ncb1818]

  10. Lillard-Wetherell, K., Machwe, A., Langland, G. T., Combs, K. A., Behbehani, G. K., Schonberg, S. A., German, J., Turchi, J. J., Orren, D. K., Groden, J. Association and regulation of the BLM helicase by the telomere proteins TRF1 and TRF2. Hum. Molec. Genet. 13: 1919-1932, 2004. [PubMed: 15229185] [Full Text: https://doi.org/10.1093/hmg/ddh193]

  11. Loayza, D., de Lange, T. POT1 as a terminal transducer of TRF1 telomere length control. Nature 424: 1013-1018, 2003. [PubMed: 12768206] [Full Text: https://doi.org/10.1038/nature01688]

  12. Marcand, S., Gilson, E., Shore, D. A protein-counting mechanism for telomere length regulation in yeast. Science 275: 986-990, 1997. [PubMed: 9020083] [Full Text: https://doi.org/10.1126/science.275.5302.986]

  13. Martinez, P., Thanasoula, M., Munoz, P., Liao, C., Tejera, A., McNees, C., Flores, J. M., Fernandez-Capetillo, O., Tarsounas, M., Blasco, M. A. Increased telomere fragility and fusions resulting from TRF1 deficiency lead to degenerative pathologies and increased cancer in mice. Genes Dev. 23: 2060-2075, 2009. [PubMed: 19679647] [Full Text: https://doi.org/10.1101/gad.543509]

  14. Miller, K. M., Rog, O., Cooper, J. P. Semi-conservative DNA replication through telomeres requires Taz1. Nature 440: 824-828, 2006. [PubMed: 16598261] [Full Text: https://doi.org/10.1038/nature04638]

  15. Okabe, J., Eguchi, A., Masago, A., Hayakawa, T., Nakanishi, M. TRF1 is a critical trans-acting factor required for de novo telomere formation in human cells. Hum. Molec. Genet. 9: 2639-2650, 2000. [PubMed: 11063723] [Full Text: https://doi.org/10.1093/hmg/9.18.2639]

  16. Okabe, J., Eguchi, A., Wadhwa, R., Rakwal, R., Tsukinoki, R., Hayakawa, T., Nakanishi, M. Limited capacity of the nuclear matrix to bind telomere repeat binding factor TRF1 may restrict the proliferation of mortal human fibroblasts. Hum. Molec. Genet. 13: 285-293, 2004. [PubMed: 14681297] [Full Text: https://doi.org/10.1093/hmg/ddh032]

  17. Sfeir, A., de Lange, T. Removal of shelterin reveals the telomere end-protection problem. Science 336: 593-597, 2012. [PubMed: 22556254] [Full Text: https://doi.org/10.1126/science.1218498]

  18. Smucker, E. J., Turchi, J. J. TRF1 inhibits telomere C-strand DNA synthesis in vitro. Biochemistry 40: 2426-2432, 2001. [PubMed: 11327863] [Full Text: https://doi.org/10.1021/bi001871o]

  19. van Steensel, B., de Lange, T. Control of telomere length by the human telomeric protein TRF1. Nature 385: 740-743, 1997. [PubMed: 9034193] [Full Text: https://doi.org/10.1038/385740a0]

  20. Young, A. C., Chavez, M., Giambernardi, T. A., Mattern, V., McGill, J. R., Harris, J. M., Sarosdy, M. F., Patel, P., Sakaguchi, A. Y. Organization and expression of human telomere repeat binding factor genes. Somat. Cell Molec. Genet. 23: 275-286, 1997. [PubMed: 9542529] [Full Text: https://doi.org/10.1007/BF02674418]

  21. Zakian, V. A. Telomeres: beginning to understand the end. Science 270: 1601-1607, 1995. [PubMed: 7502069] [Full Text: https://doi.org/10.1126/science.270.5242.1601]


Contributors:
Ada Hamosh - updated : 5/30/2012
Matthew B. Gross - updated : 5/20/2010
Patricia A. Hartz - updated : 5/19/2010
Patricia A. Hartz - updated : 12/11/2009
Ada Hamosh - updated : 4/4/2008
George E. Tiller - updated : 1/16/2007
Ada Hamosh - updated : 6/1/2006
George E. Tiller - updated : 2/17/2006
John A. Phillips, III - updated : 11/4/2004
Ada Hamosh - updated : 5/29/2003
Patricia A. Hartz - updated : 5/5/2003
Stylianos E. Antonarakis - updated : 10/29/2001
George E. Tiller - updated : 1/25/2001
Patti M. Sherman - updated : 5/30/2000
Victor A. McKusick - updated : 11/30/1999
Stylianos E. Antonarakis - updated : 7/2/1999
Victor A. McKusick - updated : 2/25/1997
Victor A. McKusick - updated : 2/12/1997

Creation Date:
Victor A. McKusick : 12/7/1995

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