Targeting assay to study the cis functions of human telomeric proteins: evidence for inhibition of telomerase by TRF1 and for activation of telomere degradation by TRF2

We investigated the control of telomere length by the human telomeric proteins TRF1 and TRF2. To this end, we established telomerase-positive cell lines in which the targeting of these telomeric proteins to specific telomeres could be induced. We demonstrate that their targeting leads to telomere shortening. This indicates that these proteins act in cis to repress telomere elongation. Inhibition of telomerase activity by a modified oligonucleotide did not further increase the pace of telomere erosion caused by TRF1 targeting, suggesting that telomerase itself is the target of TRF1 regulation. In contrast, TRF2 targeting and telomerase inhibition have additive effects. The possibility that TRF2 can activate a telomeric degradation pathway was directly tested in human primary cells that do not express telomerase. In these cells, overexpression of full-length TRF2 leads to an increased rate of telomere shortening.     

Telomerase inhibitor PinX1 provides a link between TRF1 and telomerase to prevent telomere elongation

Telomere maintenance is essential for protecting chromosome ends. Aberrations in telomere length have been implicated in cancer and aging. Telomere elongation by human telomerase is inhibited in cis by the telomeric protein TRF1 and its associated proteins. However, the link between TRF1 and inhibition of telomerase elongation of telomeres remains elusive because TRF1 has no direct effect on telomerase activity. We have previously identified one Pin2/TRF1-interacting protein, PinX1, that has the unique property of directly binding and inhibiting telomerase catalytic activity (Zhou, X. Z., and Lu, K. P. (2001) Cell 107, 347-359). However, nothing is known about the role of the PinX1-TRF1 interaction in the regulation of telomere maintenance. By identifying functional domains and key amino acid residues in PinX1 and TRF1 responsible for the PinX1-TRF1 interaction, we show that the TRF homology domain of TRF1 interacts with a minimal 20-amino acid sequence of PinX1 via hydrophilic and hydrophobic interactions. Significantly, either disrupting this interaction by mutating the critical Leu-291 residue in PinX1 or knocking down endogenous TRF1 by RNAi abolishes the ability of PinX1 to localize to telomeres and to inhibit telomere elongation in cells even though neither has any effect on telomerase activity per se. Thus, the telomerase inhibitor PinX1 is recruited to telomeres by TRF1 and provides a critical link between TRF1 and telomerase inhibition to prevent telomere elongation and help maintain telomere homeostasis.     

Telomerase enzymatic component hTERT shortens long telomeres in human cells

Telomere lengths are tightly regulated within a narrow range in normal human cells. Previous studies have extensively focused on how short telomeres are extended and have demonstrated that telomerase plays a central role in elongating short telomeres. However, much about the molecular mechanisms of regulating excessively long telomeres is unknown. In this report, we demonstrated that the telomerase enzymatic component, hTERT, plays a dual role in the regulation of telomere length. It shortens excessively long telomeres and elongates short telomeres simultaneously in one cell, maintaining the optimal telomere length at each chromosomal end for efficient protection. This novel hTERT-mediated telomere-shortening mechanism not only exists in cancer cells, but also in primary human cells. The hTERT-mediated telomere shortening requires hTERT’s enzymatic activity, but the telomerase RNA component, hTR, is not involved in that process. We found that expression of hTERT increases telomeric circular DNA formation, suggesting that telomere homologous recombination is involved in the telomere-shortening process. We further demonstrated that shelterin protein TPP1 interacts with hTERT and recruits hTERT onto the telomeres, suggesting that TPP1 might be involved in regulation of telomere shortening. This study reveals a novel function of hTERT in telomere length regulation and adds a new element to the current molecular model of telomere length maintenance.     

Control of human telomere length by TRF1 and TRF2

Telomere length in human cells is controlled by a homeostasis mechanism that involves telomerase and the negative regulator of telomere length, TRF1 (TTAGGG repeat binding factor 1). Here we report that TRF2, a TRF1-related protein previously implicated in protection of chromosome ends, is a second negative regulator of telomere length. Overexpression of TRF2 results in the progressive shortening of telomere length, similar to the phenotype observed with TRF1. However, while induction of TRF1 could be maintained over more than 300 population doublings and resulted in stable, short telomeres, the expression of exogenous TRF2 was extinguished and the telomeres eventually regained their original length. Consistent with their role in measuring telomere length, indirect immunofluorescence indicated that both TRF1 and TRF2 bind to duplex telomeric DNA in vivo and are more abundant on telomeres with long TTAGGG repeat tracts. Neither TRF1 nor TRF2 affected the expression level of telomerase. Furthermore, the presence of TRF1 or TRF2 on a short linear telomerase substrate did not inhibit the enzymatic activity of telomerase in vitro. These findings are consistent with the recently proposed t loop model of telomere length homeostasis in which telomerase-dependent telomere elongation is blocked by sequestration of the 3' telomere terminus in TRF1- and TRF2-induced telomeric loops.     

Homologous recombination generates T-loop-sized deletions at human telomeres

The t-loop structure of mammalian telomeres is thought to repress nonhomologous end joining (NHEJ) at natural chromosome ends. Telomere NHEJ occurs upon loss of TRF2, a telomeric protein implicated in t-loop formation. Here we describe a mutant allele of TRF2, TRF2DeltaB, that suppressed NHEJ but induced catastrophic deletions of telomeric DNA. The deletion events were stochastic and occurred rapidly, generating dramatically shortened telomeres that were accompanied by a DNA damage response and induction of senescence. TRF2DeltaB-induced deletions depended on XRCC3, a protein implicated in Holliday junction resolution, and created t-loop-sized telomeric circles. These telomeric circles were also detected in unperturbed cells and suggested that t-loop deletion by homologous recombination (HR) might contribute to telomere attrition. Human ALT cells had abundant telomeric circles, pointing to frequent t-loop HR events that could promote rolling circle replication of telomeres in the absence of telomerase. These findings show that t-loop deletion by HR influences the integrity and dynamics of mammalian telomeres.     

Suppression of hPOT1 in diploid human cells results in an hTERT-dependent alteration of telomere length dynamics

POT1 is a 3' telomeric single-stranded overhang binding protein that has been implicated in chromosome end protection, the regulation of telomerase function, and defining the 5' chromosome terminus. In human cancer cells that exhibit constitutive hTERT activity, hPOT1 exerts control over telomere length. Primary human fibroblasts express low levels of catalytically active hTERT in an S-phase-restricted manner that fails to counteract telomere attrition with cell division. Here, we show that diploid human fibroblasts in which hPOT1 expression has been suppressed harbor telomeres that are longer than control cells. This difference in telomere length delays the onset of replicative senescence and is dependent on S-phase-restricted hTERT expression. These findings are consistent with the view that hPOT1 promotes a nonextendable telomere state resistant to extension by S-phase-restricted telomerase. Manipulating this function of hPOT1 may thus hasten the cytotoxic effects of telomerase inhibition.     

MRE11-RAD50-NBS1 and ATM function as co-mediators of TRF1 in telomere length control

Human telomeres are associated with ATM and the protein complex consisting of MRE11, RAD50 and NBS1 (MRN), which are central to maintaining genomic stability. Here we show that when targeted to telomeres, wild-type RAD50 downregulates telomeric association of TRF1, a negative regulator of telomere maintenance. TRF1 binding to telomeres is upregulated in cells deficient in NBS1 or under ATM inhibition. The TRF1 association with telomeres induced by ATM inhibition is abrogated in cells lacking MRE11 or NBS1, suggesting that MRN and ATM function in the same pathway controlling TRF1 binding to telomeres. The ability of TRF1 to interact with telomeric DNA in vitro is impaired by ATM-mediated phosphorylation. We propose that MRN is required for TRF1 phosphorylation by ATM and that such phosphorylation results in the release of TRF1 from telomeres, promoting telomerase access to the ends of telomeres.     

Requirement of DDX39 DEAD box RNA helicase for genome integrity and telomere protection

Human chromosome ends associate with shelterin, a six-protein complex that protects telomeric DNA from being recognized as sites of DNA damage. The shelterin subunit TRF2 has been implicated in the protection of chromosome ends by facilitating their organization into the protective capping structure and by associating with several accessory proteins involved in various DNA transactions. Here we describe the characterization of DDX39 DEAD-box RNA helicase as a novel TRF2-interacting protein. DDX39 directly interacts with the telomeric repeat binding factor homology domain of TRF2 via the FXLXP motif (where X is any amino acid). DDX39 is also found in association with catalytically competent telomerase in cell lysates through an interaction with hTERT but has no effect on telomerase activity. Whereas overexpression of DDX39 in telomerase-positive human cancer cells led to progressive telomere elongation, depletion of endogenous DDX39 by small hairpin RNA (shRNA) resulted in telomere shortening. Furthermore, depletion of DDX39 induced DNA-damage response foci at internal genome as well as telomeres as evidenced by telomere dysfunction-induced foci. Some of the metaphase chromosomes showed no telomeric signal at chromatid ends, suggesting an aberrant telomere structure. Our findings suggest that DDX39, in addition to its role in mRNA splicing and nuclear export, is required for global genome integrity as well as telomere protection and represents a new pathway for telomere maintenance by modulating telomere length homeostasis.     

Cell cycle-dependent recruitment of telomerase RNA and Cajal bodies to human telomeres

Telomerase is a ribonucleoprotein enzyme that counteracts replicative telomere erosion by adding telomeric sequence repeats onto chromosome ends. Despite its well-established role in telomere synthesis, telomerase has not yet been detected at telomeres. The RNA component of human telomerase (hTR) resides in the nucleoplasmic Cajal bodies (CBs) of interphase cancer cells. Here, in situ hybridization demonstrates that in human HeLa and Hep2 S phase cells, besides accumulating in CBs, hTR specifically concentrates at a few telomeres that also accumulate the TRF1 and TRF2 telomere marker proteins. Surprisingly, telomeres accumulating hTR exhibit a great accessibility for in situ oligonucleotide hybridization without chromatin denaturation, suggesting that they represent a structurally distinct, minor subset of HeLa telomeres. Moreover, we demonstrate that more than 25% of telomeres accumulating hTR colocalize with CBs. Time-lapse fluorescence microscopy demonstrates that CBs moving in the nucleoplasm of S phase cells transiently associate for 10-40 min with telomeres. Our data raise the intriguing possibility that CBs may deliver hTR to telomeres and/or may function in other aspects of telomere maintenance.     

Rescue of an hTERT mutant defective in telomere elongation by fusion with hPot1

The protein hPot1 shares homology with telomere-binding proteins in lower eukaryotes and associates with single-stranded telomeric DNA in vitro as well as colocalizing with telomere-binding proteins in vivo. We now show that hPot1 is coimmunoprecipitated with telomeric DNA and that stable expression of this protein in telomerase-positive cells results in telomere elongation, supporting the idea that hPot1 is a bona fide mammalian telomere-binding protein. We previously found that mutations in the N-terminal DAT domain of the hTERT catalytic subunit of telomerase rendered the enzyme catalytically active but unable to elongate telomeres in vivo. This phenotype could be partially rescued by fusion with the double-stranded telomeric protein hTRF2. Given that hPot1 binds to single-stranded DNA in vitro (at the same site that hTERT binds to in vivo), we addressed whether fusion of hPot1 can rescue the DAT mutations more efficiently than that of hTRF2. We now report that a DAT mutant of hTERT is indeed efficiently rescued upon fusion to hPot1. However, this rescue depended on the ability of hPot1 to localize to telomeres rather than binding to DNA per se. These data support a model whereby the DAT domain of hTERT is implicated in telomere-telomerase associations.     

Telomerase maintains telomere structure in normal human cells

In normal human cells, telomeres shorten with successive rounds of cell division, and immortalization correlates with stabilization of telomere length. These observations suggest that human cancer cells achieve immortalization in large part through the illegitimate activation of telomerase expression. Here, we demonstrate that the rate-limiting telomerase catalytic subunit hTERT is expressed in cycling primary presenescent human fibroblasts, previously believed to lack hTERT expression and telomerase activity. Disruption of telomerase activity in normal human cells slows cell proliferation, restricts cell lifespan, and alters the maintenance of the 3' single-stranded telomeric overhang without changing the rate of overall telomere shortening. Together, these observations support the view that telomerase and telomere structure are dynamically regulated in normal human cells and that telomere length alone is unlikely to trigger entry into replicative senescence.     

PTOP interacts with POT1 and regulates its localization to telomeres

Telomere maintenance has been implicated in cancer and ageing, and requires cooperation between a multitude of telomeric factors, including telomerase, TRF1, TRF2, RAP1, TIN2, Tankyrase, PINX1 and POT1 (refs 1-12). POT1 belongs to a family of oligonucleotide-binding (OB)-fold-containing proteins that include Oxytricha nova TEBP, Cdc13, and spPot1, which specifically recognize telomeric single-stranded DNA (ssDNA). In human cells, the loading of POT1 to telomeric ssDNA controls telomerase-mediated telomere elongation. Surprisingly, a human POT1 mutant lacking an OB fold is still recruited to telomeres. However, the exact mechanism by which this recruitment occurs remains unclear. Here we identify a novel telomere protein, PTOP, which interacts with both POT1 and TIN2. PTOP binds to the carboxyl terminus of POT1 and recruits it to telomeres. Inhibition of PTOP by RNA interference (RNAi) or disruption of the PTOP-POT1 interaction hindered the localization of POT1 to telomeres. Furthermore, expression of the respective interaction domains on PTOP and POT1 alone extended telomere length in human cells. Therefore, PTOP heterodimerizes with POT1 and regulates POT1 telomeric recruitment and telomere length.     

Telomeric recombination induced by dysfunctional telomeres

Telomere maintenance is essential for cellular immortality, and most cancer cells maintain their telomeres through the enzyme telomerase. Telomeres and telomerase represent promising anticancer targets. However, 15% of cancer cells maintain their telomeres through alternative recombination-based mechanisms, and previous analyses showed that recombination-based telomere maintenance can be activated after telomerase inhibition. We determined whether telomeric recombination can also be promoted by telomere dysfunction. We report for the first time that telomeric recombination can be induced in human telomerase-positive cancer cells with dysfunctional telomeres.