The terminus of most eukaryotic chromosomes forms a specialized structure called the telomere, which resolves two challenges faced by linear chromosomes: the end replication problem and the necessity to protect chromosome ends from erroneous recognition and repair as DNA double-strand breaks (DSBs). The mammalian telomere is hypothesized to be organized into a unique lariat-like structure (t-loop) (Griffith et al., 1999
), formed by the invasion of the 3′ single-stranded G-rich overhang (G-tail) into the double-stranded (ds) region of the telomere, which prevents exposure of free DNA ends. Formation and maintenance of this t-loop structure is promoted by the shelterin complex that is itself telomere-associated (Figure A, Palm and de Lange, 2008
and references therein). Shelterin is a multi-protein complex that is composed of six core components: TRF1, TRF2, RAP1, POT1, TPP1 and TIN2. (The mouse contains two POT1 isoforms: POT1a and POT1b, whereas humans have just one.) In addition to maintaining the t-loop structure, shelterin also controls telomerase-mediated telomere elongation, aids semi-conservative replication of telomeres and suppresses the DNA damage response (DDR). The shelterin component TRF2 binds to the double-stranded portion of the telomere and promotes formation of t-loops in vitro
(Griffith et al., 1999
). Another shelterin component, the POT1a–TPP1 complex, is bound to the single-stranded portion of the telomere and helps maintain t-loop structure. Loss of TRF2 activates ataxia telangiectasia-mutated (ATM)-dependent DDR and leads to massive chromosome end-to-end fusions by the non-homologous end joining (NHEJ) pathway, whereas loss of POT1a or TPP1 results in the activation of Ataxia telangiectasia- and Rad3-related (ATR)-dependent DDR and aberrant HR events at telomeres (Denchi and de Lange, 2007
; Guo et al., 2007
Figure 1 (A) Schematic representation of mammalian telomeric complex. The shelterin components are shown in bold text. (B) An updated model for the suppression of DNA damage response at the telomere end based on the data in Deng et al. (2009).
The Mammalian Mre11–Rad50–Nbs1 (MRN) complex, and its homolog in the budding yeast Saccharomyces cerevisiae
, Mre11–Rad50–Xrs2 (MRX) complex, plays a critical role in the repair of and checkpoint signaling triggered by DSBs (Lisby and Rothstein, 2005
and references therein). It senses DSBs and is the first cellular component to arrive at DSB sites, tethers dsDNA ends, recruits and activates ATM at the DSB end and coordinates DSB end resection (Mimitou and Symington, 2009
and references therein), a step that precedes and is necessary for HR and ATR activation. Defects in the MRN/MRX complex result in reduced efficiency of both HR (gene conversion and strand annealing) and NHEJ. However, whether the MRN complex has a direct role in NHEJ in higher eukaryotes remains unclear. Moreover, only recently was ATM-dependent DDR implicated in the NHEJ repair (Denchi and de Lange, 2007
Previously, it was shown that in the conditionally deleted TRF2 cells, NHEJ-dependent telomere fusions were greatly reduced by inactivation of ATM (Denchi and de Lange, 2007
), suggesting a role for ATM-dependent DDR in promoting NHEJ. Because the MRN complex localizes to dysfunctional telomeres and recruits ATM, in a recent Nature
paper, Chang and colleagues sought to determine the role of MRN in NHEJ at uncapped telomeres (Deng et al., 2009
). They observed that both ATM activation and chromosome fusions were compromised after TRF2 loss in the absence of Mre11. Importantly, in nuclease-defective Mre11H129N/Δ
cells depleted for TRF2, chromosome fusions were similarly reduced despite normal ATM activation, indicating a separate function for Mre11 in NHEJ independent of ATM activation that relies on Mre11 nuclease activity. The Chang group also observed that the G-tails of uncapped telomeres remained intact in Mre11 null or nuclease-deficient cells, and the majority of fusions observed in these cells are those from the leading strand telomeres of sister chromatids, which are blunt-ended immediately after DNA replication. These data suggest a role for the MRN complex in resection of the uncapped telomere 3′-overhang to create a suitable substrate for NHEJ to act on, a step that is downstream of ATM activation. Furthermore, the MRN complex may also play a role in processing newly synthesized leading strand telomeres to generate G-tails, which are essential for functional telomeres.
POT1a–TPP1 levels at the telomere are greatly reduced upon TRF2 depletion. However, the Chang group observed the persistent presence of POT1a–TPP1 complex at telomeres in TRF2-depleted, Mre11-defective cells, presumably due to the lack of G-tail processing after telomere uncapping. This result led to the hypothesis that the unprocessed G-tail is protected by the POT1a–TPP1 complex. Indeed, depletion of both TRF2 and POT1a–TPP1 complex in Mre11-defective cells triggered robust NHEJ and ATR activation. Nevertheless, when ATR activation was also inhibited, telomere fusions were reduced, suggesting a role for ATR in NHEJ, presumably in the processing of G-tails. Interestingly, Chang and colleagues also observed that removal of POT1a–TPP1 alone in Mre11-defective cells did not lead to increased homologous recombination (HR) as it is seen in MRN wild-type cells, indicating a role for MRN in stimulating HR.
Taken together, this study elegantly reveals the involvement of the MRN complex in multiple pathways in response to telomere deprotection and advances the knowledge of how the shelterin complex represses the DDRs at telomeres (Figure B). These roles are highly reminiscent of the roles of MRN in DSB repair, supporting the notion that dysfunctional telomeres are recognized as DSBs by the cell. Together with two recent papers studying the repair of endonuclease-generated DSBs (Rass et al., 2009
; Xie et al., 2009
), the Chang group firmly establishes a direct contribution of the MRN complex to NHEJ by both activating ATM and processing the 3′-G-tail. Although Mre11 itself exhibits single-stranded DNA endonuclease and 3′ → 5′ exonuclease activities, in DSB repair, the MRN/MRX complex primarily functions in degrading the 5′-terminated strand. This degradation is achieved by interacting and coordinating with other cellular factors, such as CtIP (Sae2 in S. cerevisiae
) and Exo I (Mimitou and Symington, 2009
and references therein). It is possible that by regulating the interaction partners of MRN, the 3′ or 5′ strand is differentially degraded, and the decision of which repair pathway to use is made. It is also noteworthy that the nuclease activity of MRN is important in promoting HR at POT1a–TPP1-depleted telomeres. Because the POT1a–TPP1 complex binds to the single-stranded portion of the telomere and the t-loop resembles a recombination intermediate, removal of POT1a–TPP1 might facilitate access of RPA and recombination factors to the t-loop, so that strand processing is not needed. The requirement of the MRN nuclease activity for HR suggests that MRN may participate in t-loop processing, such as disassembly of t-loops to allow invasion of sister chromatids or for t-circle generation.
A convergence of discoveries in the fields of DNA damage repair, checkpoint signaling and telomere biology has advanced our understanding of how the cell responds to an uncapped telomere, either due to natural telomere attrition or loss of shelterin components induced experimentally. Contrary to previous notions, association with the shelterin complex does not render the telomere invisible to the cellular surveillance mechanism. Rather, it creates a unique identity for the telomere, so that the telomere is still recognized in the cell by the DDR machinery without triggering adverse consequences. ATM, HR and NHEJ proteins have all been reported to localize at telomeres, and their presence is required for functional telomeres (Verdun and Karlseder, 2007
; Rog and Cooper, 2008
and references therein). For example, in S. cerevisiae
, activation of the ATM homolog Tel1 is required for telomerase access to telomere ends and for the preferential elongation of short telomeres, and deletion of both TEL1
(the S. cerevisiae
ATR homolog) leads to the inability to maintain telomeres by telomerase and increased chromosome fusions. HR proteins are required for the proper replication of chromosome ends and the generation of t-loops. Only when its identity is lost is a dysfunctional telomere recognized as a DSB for repair. How the shelterin complex establishes this telomere identity remains an interesting topic for further investigation. Given the involvement of the MRN complex in multiple downstream pathways, it is likely to be a major target of shelterin regulation.