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

Abstract
The telomeres that cap normal chromosomes share virtually the same structure as damaged DNA sequences within chromosomes. So how do telomeres evade recognition by the DNA-repair machinery? Simple: two telomere binding proteins, TRF2 and POT1, block the activation of two key DNA damage response pathways. With TRF2 and POT1 inactivated, telomeres can be 'repaired' into the chromosomes where they don't belong. At a gross level, telomere ends resemble the ends of a double-strand break caused by DNA damage, yet telomeres do not activate the DNA damage response. An insight is offered as to how this response is suppressed. Two telomere-binding proteins, TRF2 and POT1, inhibit the activities of two upstream 'sensor' damage-responsive kinases, ATM and ATR, respectively. As these kinases initiate the signalling cascades that provoke the repair of damaged DNA, their inhibition by TRF2 and POT1 makes the telomere ends 'invisible' to the DNA repair machinery. When telomeres are rendered dysfunctional through replicative attrition of the telomeric DNA or by inhibition of shelterin1, cells show the hallmarks of ataxia telangiectasia mutated (ATM) kinase signalling2,3,4. 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 cells2,5 and the response of ATM-deficient cells to telomere dysfunction6,7. 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.