Inducible response required for repair of low-dose radiation damage in human fibroblasts.

Ionizing radiation (IR) induces a variety of DNA lesions among which DNA double-strand breaks (DSBs) are the biologically most significant. It is currently unclear if DSB repair is equally efficient after low and high doses. Here, we use gamma-H2AX, phospho-ATM (pATM), and 53BP1 foci analysis to monitor DSB repair. We show, consistent with a previous study, that the kinetics of gamma-H2AX and pATM foci loss in confluent primary human fibroblasts are substantially compromised after doses of 10 mGy and lower. Following 2.5 mGy, cells fail to show any foci loss. Strikingly, cells pretreated with 10 microM H(2)O(2) efficiently remove all gamma-H2AX foci induced by 10 mGy. At the concentration used, H(2)O(2) produces single-strand breaks and base damages via the generation of oxygen radicals but no DSBs. Moreover, 10 microM H(2)O(2) up-regulates a set of genes that is also up-regulated after high (200 mGy) but not after low (10 mGy) radiation doses. This suggests that low radical levels induce a response that is required for the repair of radiation-induced DSBs when the radiation damage is too low to cause the induction itself. To address the in vivo significance of this finding, we established gamma-H2AX and 53BP1 foci analysis in various mouse tissues. Although mice irradiated with 100 mGy or 1 Gy show efficient gamma-H2AX and 53BP1 foci removal during 24 h post-IR, barely any foci loss was observed after 10 mGy. Our data suggest that the cellular response to DSBs is substantially different for low vs. high radiation doses