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.
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We thank D. White for invaluable help in maintaining our mouse colonies, D. Argibay for assistance with genotyping, E. Brown for providing the ATR conditional knockout mice, H. Takai for technical advice and suggestions, D. Hockemeyer for providing Pot1 double knockout cells and the Tpp1 and Pot1a shRNA constructs, S. Soll for his contributions to the initial stages of this research and members of the de Lange laboratory for critical comments on the manuscript. This work was supported by a grant from the NIH.
This file contains Supplementary Figures S1-S9 with Legends. Figures S1-S5 show γH2AX TIFs (S1), ChIP for shelterin (S2), ATR-independent DNA damage response (S3), ATM-dependent NEHJ (S4-S5) following TRF2 deletion. Figures S6-S7 show activation of ATR but not ATM upon POT1 inhibition. Figures S8-S9 show TIF formation and NHEJ in TRF2-/- ATM-/- cells upon POT1 inhibition.