Age-dependent usage of double-strand-break repair pathways

A DNA double-strand break (DSB) can be repaired by any of several alternative and competing mechanisms. The repaired sequences often differ from the original depending on which mechanism was used so that the cell's "choice" of repair mechanism can have profound genetic consequences. DSBs can accumulate with age [1, 2], and human diseases that mimic some of the effects of aging, such as increased susceptibility to cancer, are associated with certain defects in DSB repair [3, 4]. The premeiotic germ cells of Drosophila provide a useful model for exploration of the connection between aging and DNA repair because these cells are subject to mortality and other age-related changes [5], and their DNA repair process is easily quantified. We used Rr3, a repair reporter system in Drosophila [6], to show that the relative usage of DSB repair mechanisms can change substantially as an organism ages. Homologous repair increased linearly in the male germline from 14% in young individuals to more than 60% in old ones, whereas two other pathways showed a corresponding decrease. Furthermore, the proportion of longer conversion tracts (> 156 bp) also increased nearly 2-fold as the flies aged. These findings are relevant to the more general question of how DNA damage and repair are related to aging.