Titia de Lange

TRF2 Protects Human Telomeres from End-to-End Fusions

The mechanism by which telomeres prevent end-to-end fusion has remained elusive. Here, we show that the human telomeric protein TRF2 plays a key role in the protective activity of telomeres. A dominant negative allele of TRF2 induced end-to-end chromosome fusions detectable in metaphase and anaphase cells. Telomeric DNA persisted at the fusions, demonstrating that TTAGGG repeats per se are not sufficient for telomere integrity. Molecular analysis suggested that the fusions represented ligation of telomeres that have lost their single-stranded G-tails. Therefore, TRF2 may protect chromosome ends by maintaining the correct structure at telomere termini. In addition, expression of mutant forms of TRF2 induced a growth arrest with characteristics of senescence. The results raise the possibility that chromosome end fusions and senescence in primary human cells may be caused by loss by TRF2 from shortened telomeres.

How Shelterin Protects Mammalian Telomeres

The genomes of prokaryotes and eukaryotic organelles are usually circular as are most plasmids and viral genomes. In contrast, the nuclear genomes of eukaryotes are organized on linear chromosomes, which require mechanisms to protect and replicate DNA ends. Eukaryotes navigate these problems with the advent of telomeres, protective nucleoprotein complexes at the ends of linear chromosomes, and telomerase, the enzyme that maintains the DNA in these structures. Mammalian telomeres contain a specific protein complex, shelterin, that functions to protect chromosome ends from all aspects of the DNA damage response and regulates telomere maintenance by telomerase. Recent experiments, discussed here, have revealed how shelterin represses the ATM and ATR kinase signaling pathways and hides chromosome ends from nonhomologous end joining and homology-directed repair.

Control of telomere length by the human telomeric protein TRF1.

Abstract Human telomeres, the nucleoprotein complexes at chromosome ends, consist of tandem arrays of TTAGGG repeats bound to specific proteins. In normal human cells, telomeres shorten with successive cell divisions1,2, probably due to the terminal sequence loss that accompanies DNA replication. In tumours and immortalized cells, this decline is halted through the activation of telomerase3–5, a reverse transcriptase that extends the telomeric TTAGGG-repeat arrays6–7. Telomere length is stable in several immortal human-cell lines3, suggesting that a regulatory mechanism exists for limiting telomere elongation by telomerase. Here we show that the human telomeric-repeat binding factor TRF1 (ref. 8) is involved in this regulation. Long-term overexpression of TRF1 in the telomerase-positive tumour-cell line HT1080 resulted in a gradual and progressive telomere shortening. Conversely, telomere elongation was induced by expression of a dominant-negative TRF1 mutant that inhibited binding of endogenous TRF1 to telomeres. Our results identify TRF1 as a suppressor of telomere elongation and indicate that TRF1 is involved in the negative feedback mechanism that stabilizes telomere length. As TRF1 does not detectably affect the expression of telomerase, we propose that the binding of TRF1 controls telomere length in cis by inhibiting the action of telomerase at the ends of individual telomeres.

DNA Damage Foci at Dysfunctional Telomeres

We report cytologic and genetic data indicating that telomere dysfunction induces a DNA damage response in mammalian cells. Dysfunctional, uncapped telomeres, created through inhibition of TRF2, became associated with DNA damage response factors, such as 53BP1, gamma-H2AX, Rad17, ATM, and Mre11. We refer to the domain of telomere-associated DNA damage factors as a Telomere Dysfunction-Induced Focus (TIF). The accumulation of 53BP1 on uncapped telomeres was reduced in the presence of the PI3 kinase inhibitors caffeine and wortmannin, which affect ATM, ATR, and DNA-PK. By contrast, Mre11 TIFs were resistant to caffeine, consistent with previous findings on the Mre11 response to ionizing radiation. A-T cells had a diminished 53BP1 TIF response, indicating that the ATM kinase is a major transducer of this pathway. However, in the absence of ATM, TRF2 inhibition still induced TIFs and senescence, pointing to a second ATM-independent pathway. We conclude that the cellular response to telomere dysfunction is governed by proteins that also control the DNA damage response. TIFs represent a new tool for evaluating telomere status in normal and malignant cells suspected of harboring dysfunctional telomeres. Furthermore, induction of TIFs through TRF2 inhibition provides an opportunity to study the DNA damage response within the context of well-defined, physically marked lesions.

Protection of mammalian telomeres.

Telomeres allow cells to distinguish natural chromosome ends from damaged DNA. When telomere function is disrupted, a potentially lethal DNA damage response can ensue, DNA repair activities threaten the integrity of chromosome ends, and extensive genome instability can arise. It is not clear exactly how the structure of telomere ends differs from sites of DNA damage and how telomeres protect chromosome ends from DNA repair activities. What are the defining structural features of telomeres and through which mechanisms do they ensure chromosome end protection? What is the molecular basis of the telomeric cap and how does it act to sequester the chromosome end? Here I discuss data gathered in the last few years, suggesting that the protection of human chromosome ends primarily depends on the telomeric protein TRF2 and that telomere capping involves the formation of a higher order structure, the telomeric loop or t-loop.Telomeres allow cells to distinguish natural chromosome ends from damaged DNA. When telomere function is disrupted, a potentially lethal DNA damage response can ensue, DNA repair activities threaten the integrity of chromosome ends, and extensive genome instability can arise. It is not clear exactly how the structure of telomere ends differs from sites of DNA damage and how telomeres protect chromosome ends from DNA repair activities. What are the defining structural features of telomeres and through which mechanisms do they ensure chromosome end protection? What is the molecular basis of the telomeric cap and how does it act to sequester the chromosome end? Here I discuss data gathered in the last few years, suggesting that the protection of human chromosome ends primarily depends on the telomeric protein TRF2 and that telomere capping involves the formation of a higher order structure, the telomeric loop or t-loop.

A Human Telomeric Protein

Telomeres are multifunctional elements that 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. A major protein component of human telomeres has been identified and cloned. This factor, TRF, contains one Myb-type DNA-binding repeat and an amino-terminal acidic domain. Immunofluorescent labeling shows that TRF specifically colocalizes with telomeric DNA in human interphase cells and is located at chromosome ends during metaphase. The presence of TRF along the telomeric TTAGGG repeat array demonstrates that human telomeres form a specialized nucleoprotein complex.

Control of Human Telomere Length by TRF1 and TRF2

ABSTRACT 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.

Cell-cycle-regulated association of RAD50/MRE11/NBS1 with TRF2 and human telomeres

Telomeres allow cells to distinguish natural chromosome ends from damaged DNA and protect the ends from degradation and fusion. In human cells, telomere protection depends on the TTAGGG repeat binding factor, TRF2 (refs 1–4), which has been proposed to remodel telomeres into large duplex loops (t-loops). Here we show by nanoelectrospray tandem mass spectrometry that RAD50 protein is present in TRF2 immunocomplexes. Protein blotting showed that a small fraction of RAD50, MRE11 and the third component of the MRE11 double-strand break (DSB) repair complex, the Nijmegen breakage syndrome protein (NBS1), is associated with TRF2. Indirect immunofluorescence demonstrated the presence of RAD50 and MRE11 at interphase telomeres. NBS1 was associated with TRF2 and telomeres in S phase, but not in G1 or G2. Although the MRE11 complex accumulated in irradiation-induced foci (IRIFs) in response to γ-irradiation, TRF2 did not relocate to IRIFs and irradiation did not affect the association of TRF2 with the MRE11 complex, arguing against a role for TRF2 in DSB repair. Instead, we propose that the MRE11 complex functions at telomeres, possibly by modulating t-loop formation.

Mammalian Telomeres Resemble Fragile Sites and Require TRF1 for Efficient Replication

Telomeres protect chromosome ends through the interaction of telomeric repeats with shelterin, a protein complex that represses DNA damage signaling and DNA repair reactions. The telomeric repeats are maintained by telomerase, which solves the end replication problem. We report that the TTAGGG repeat arrays of mammalian telomeres pose a challenge to the DNA replication machinery, giving rise to replication-dependent defects that resemble those of aphidicolin-induced common fragile sites. Gene deletion experiments showed that efficient duplication of telomeres requires the shelterin component TRF1. Without TRF1, telomeres activate the ATR kinase in S phase and show a fragile-site phenotype in metaphase. Single-molecule analysis of replicating telomeres showed that TRF1 promotes efficient replication of TTAGGG repeats and prevents fork stalling. Two helicases implicated in the removal of G4 DNA structures, BLM and RTEL1, were required to repress the fragile-telomere phenotype. These results identify a second telomere replication problem that is solved by the shelterin component TRF1.

Protection of telomeres through independent control of ATM and ATR by TRF2 and POT1

When telomeres are rendered dysfunctional through replicative attrition of the telomeric DNA or by inhibition of shelterin, cells show the hallmarks of ataxia telangiectasia mutated (ATM) kinase signalling. In addition, dysfunctional telomeres might induce an ATM-independent pathway, such as ataxia telangiectasia and Rad3-related (ATR) kinase signalling, as indicated by the phosphorylation of the ATR target CHK1 in senescent cells and the response of ATM-deficient cells to telomere dysfunction. However, because telomere attrition is accompanied by secondary DNA damage, it has remained unclear whether there is an ATM-independent pathway for the detection of damaged telomeres. Here we show that damaged mammalian telomeres can activate both ATM and ATR and address the mechanism by which the shelterin complex represses these two important DNA damage signalling pathways. We analysed the telomere damage response on depletion of either or both of the shelterin proteins telomeric repeat binding factor 2 (TRF2) and protection of telomeres 1 (POT1) from cells lacking ATM and/or ATR kinase signalling. The data indicate that TRF2 and POT1 act independently to repress these two DNA damage response pathways. TRF2 represses ATM, whereas POT1 prevents activation of ATR. Unexpectedly, we found that either ATM or ATR signalling is required for efficient non-homologous end-joining of dysfunctional telomeres. The results reveal how mammalian telomeres use multiple mechanisms to avoid DNA damage surveillance and provide an explanation for the induction of replicative senescence and genome instability by shortened telomeres.