Abstract
DNA double-strand break (DSB) formation is essential in several normal cellular processes, but accidental DSB formation may lead to cancer and death. A universal cellular response to a DSB is the phosphorylation of histone protein H2AX to form γ-H2AX in the chromatin flanking the break site. Immunostaining with anti-γ-H2AX reveals each nuclear DSB as a focus of γ-H2AX (γ- focus). Normal human cells exhibit increasing numbers of γ-foci as they senescence---from ~0.4 per early passage cell to >2 per late passage cell---and similar increases in the incidence of γ-foci occur in both the somatic and germline tissues of mice as they age. When nascent DSBs are generated in hu-man cell cultures with ionizing radiation, the numbers of nascent γ-foci are similar in cells at different stages of se-nescence but the rates of dimensional focal growth and focal accumulation of DSB-repair proteins are substantially slower in late passage cells and even slower in cells taken from Werner Syndrome patients, who exhibit premature aging and genome instability. Young fibroblast cultures induced to prematurely senesce by exposure to DNA damaging agents exhibit increases in DSB incidence similar to those found in replicative senescence, indicating that accumulating unrepairable DSB-containing lesions may be a casual factor in aging. The results demonstrate that mammalian cells aging in vitro and in vivo accumulate DSB-containing lesions that may play a causal role in aging.
