A DNA damage checkpoint response in telomere-initiated senescence

Abstract

Most human somatic cells can undergo only a limited number of population doublings in vitro. 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.

Mitochondria are required for pro‐ageing features of the senescent phenotype

“…Activation of a DDR is a prominent initiator of senescence during stress‐induced (Passos et al , 2010), replicative (d'Adda di Fagagna et al , 2003) and oncogene‐induced senescence (OIS) (Suram et al , 2012). In order to understand mitochondria regulation during senescence, we tested the impact of DDR activators on mitochondrial content in a variety of human and mice cell types.…”

Telomere dysfunction promotes transdifferentiation of human fibroblasts into myofibroblasts

“…Since telomere dysfunction activates a p53 dependent DNA damage checkpoint (d’Adda di Fagagna et al, 2003; Herbig et al, 2004; Takai et al, 2003), it is possible that TGFβ1‐induced telomere dysfunction contributes to α‐SMA upregulation and transdifferentiation of myofibroblasts by promoting p53‐dependent α‐SMA expression. Consistent with this interpretation, we demonstrate, using ChIP, that p53 binds to the α‐SMA promoter in BJ fibroblasts following TGF‐β1‐treatment (Figure 6a).…”

“…Progressive telomere erosion eventually generates one or more telomeres that are so short that the protective structure can no longer be formed. This causes telomeres to become dysfunctional which activates a persistent DDR and promotes cells to enter replicative senescence (d’Adda di Fagagna et al, 2003; Herbig, Jobling, Chen, Chen, & Sedivy, 2004). Significantly, telomere dysfunction‐induced senescence (TDIS) can also be triggered in the absence of continuous cell proliferation and in the absence of critical telomere shortening.…”

Targeting Tankyrase 1 as a therapeutic strategy for BRCA-associated cancer

“…Therefore, we analysed whether the synthetic lethality between Tankyrase 1 and BRCA was due to a cumulative negative impact on telomere maintenance. Loss of functional telomeres results in the appearance of DNA damage foci (known as TIFs or telomeredysfunction-induced foci) and eventually end-to-end fusions (d'Adda di Fagagna et al, 2003). We monitored the presence of TIFs by immunofluorescence and the frequency of chromosome fusions by fluorescent in situ hybridization in ARCV and MOCK cells transfected with either BRCA-silencing plasmids or the control plasmid.…”