Analysis of chromatid-break-repair detects a homologous recombination to non-homologous end-joining switch with increasing load of DNA double-strand breaks.

We recently reported that when low doses of ionizing radiation induce low numbers of DNA double-strand breaks (DSBs) in G-phase cells, about 50 % of them are repaired by homologous recombination (HR) and the remaining by classical non-homologous end-joining (c-NHEJ). However, with increasing DSB-load, the contribution of HR drops to undetectable (at ∼10 Gy) as c-NHEJ dominates. It remains unknown whether the approximately equal shunting of DSBs between HR and c-NHEJ at low radiation doses and the predominant shunting to c-NHEJ at high doses, applies to every DSB, or whether the individual characteristics of each DSB generate processing preferences. When G-phase cells are irradiated, only about 10 % of the induced DSBs break the chromatids. This breakage allows analysis of the processing of this specific subset of DSBs using cytogenetic methods. Notably, at low radiation doses, these DSBs are almost exclusively processed by HR, suggesting that chromatin characteristics awaiting characterization underpin chromatid breakage and determine the preferential engagement of HR. Strikingly, we also discovered that with increasing radiation dose, a pathway switch to c-NHEJ occurs in the processing of this subset of DSBs. Here, we confirm and substantially extend our initial observations using additional methodologies. Wild-type cells, as well as HR and c-NHEJ mutants, are exposed to a broad spectrum of radiation doses and their response analyzed specifically in G phase. Our results further consolidate the observation that at doses <2 Gy, HR is the main option in the processing of the subset of DSBs generating chromatid breaks and that a pathway switch at doses between 4-6 Gy allows the progressive engagement of c-NHEJ. PARP1 inhibition, irrespective of radiation dose, leaves chromatid break repair unaffected suggesting that the contribution of alternative end-joining is undetectable under these experimental conditions.