Base-excision repair increases DNA double-strand break clustering within heavy-ion tracks and modulates repair at δ-electron-induced breaks

Space radiation poses a threat to human health during space missions. Its biological effect largely depends on heavy ions. These induce highly clustered DNA damage along their tracks, which is difficult to repair. If this damage is not repaired correctly, or not at all, mutations and possibly cancer can occur in the long term. δ-electrons induced by fast heavy ions lead to further DNA damage outside ion tracks, resembling that of sparsely-ionising radiation. Using inhibitors of the crucial base-excision repair factors OGG1 and APE1, we show that the repair of DNA base-lesions within heavy-ion tracks causes DNA double-strand breaks (DSBs), which increases difficult-to-repair in-track DSB clustering. We further found that DSBs induced by δ-electrons of fast heavy ions are more often decorated by the resection factor RPA, which suggests that they are more often repaired in a resection-dependent manner than X-ray-induced DSBs, despite their resemblance. Using γH2AX assays to assess DSB repair kinetics, we found that δ-electron-induced DSBs are repaired faster than those induced by X-rays in G1-phase cells, despite the fact that δ-electron-induced DSBs are frequently resected, which typically entails slower repair processes. These findings on δ-electron-induced DSBs imply that the quantity of clustered DSBs in irradiated cells affects the overall response of cells to DNA-damage. Based on our results on base-excision repair and the processing of δ-electron-induced DSBs, we conclude that the interplay between DNA-damage repair processes is a pivotal factor in the course of DNA repair and, consequently, genomic integrity.