Telomere shortening alters the kinetics of the DNA damage response after ionizing radiation in human cells

Studies of telomerase-deficient mice and human cell lines have showed that telomere shortening enhances sensitivity to ionizing radiation (IR). The molecular basis for this observation remains unclear. To better understand the connection between telomere shortening and radiation sensitivity, we evaluated components of the DNA damage response pathway in normal human fibroblasts with short and long telomeres. Late-passage cells with short telomeres showed enhanced sensitivity to IR compared with early-passage cells with longer telomeres. Compared with early-passage cells, late-passage cells had a higher baseline level of phosphorylated H2AX protein (γH2AX) before IR but diminished peak levels of H2AX phosphorylation after treatment with IR. Both the appearance and disappearance of γH2AX foci were delayed in late-passage cells, indicative of delayed DNA repair. In contrast to the situation with H2AX, ATM and p53 phosphorylation kinetics were similar in early- and late-passage cells, but phosphorylation of the chromatin-bound ATM targets SMC1 and NBS1 was delayed in late-passage cells. Because impaired phosphorylation associated with short telomeres was restricted to chromatin-bound ATM targets, chromatin structure was assessed. DNA from cells with short telomeres was more resistant to digestion with micrococcal nuclease, indicative of compacted chromatin. Moreover, cells with short telomeres showed histone acetylation and methylation profiles consistent with heterochromatin. Together our data suggest a model in which short telomeres induce chromatin structure changes that limit access of activated ATM to its downstream targets on the chromatin, thereby providing a potential explanation for the increased radiation sensitivity seen with telomere shortening.