Most human somatic cells can undergo only a limited number of population doublings in vitro1. This exhaustion of proliferative potential, called senescence, can be triggered when telomeres—the ends of linear chromosomes—cannot fulfil their normal protective functions. Here we show that senescent human fibroblasts display molecular markers characteristic of cells bearing DNA double-strand breaks. These markers include nuclear foci of phosphorylated histone H2AX and their co-localization with DNA repair and DNA damage checkpoint factors such as 53BP1, MDC1 and NBS1. We also show that senescent cells contain activated forms of the DNA damage checkpoint kinases CHK1 and CHK2. Furthermore, by chromatin immunoprecipitation and whole-genome scanning approaches, we show that the chromosome ends of senescent cells directly contribute to the DNA damage response, and that uncapped telomeres directly associate with many, but not all, DNA damage response proteins. Finally, we show that inactivation of DNA damage checkpoint kinases in senescent cells can restore cell-cycle progression into S phase. Thus, we propose that telomere-initiated senescence reflects a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres.
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We thank R. Abraham, C. Aquaviva, J. Bartek, R. Laskey, N. La Thangue, T. de Lange, S. Elledge, T. Halazonetis, M. Kastan and T. Kouzarides for sharing reagents; K. Woodfine for constructing the chromosome 22 array; M. Goldberg for help with immunofluorescence; B. Williams for generating pCHK2-KD; J. Bradbury and K. Dry for editorial help; and D. Baird, J. Bartek, M. Foiani, J. Pines and all the S.P.J. laboratory for suggestions. F.d'A.d.F. was supported by a Cancer Research UK grant, and P.M.R. is supported by a studentship from Cancer Research UK. T.v.Z. acknowledges support from Research into Ageing UK.
The authors declare that they have no competing financial interests.
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Fagagna, F., Reaper, P., Clay-Farrace, L. et al. A DNA damage checkpoint response in telomere-initiated senescence. Nature 426, 194–198 (2003). https://doi.org/10.1038/nature02118
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