Increased Resection at DSBs in G2-Phase Is a Unique Phenotype Associated with DNA-PKcs Defects That Is Not Shared by Other Factors of c-NHEJ.

The load of DNA double-strand breaks (DSBs) induced in the genome of higher eukaryotes by different doses of ionizing radiation (IR) is a key determinant of DSB repair pathway choice, with homologous recombination (HR) and ATR substantially gaining ground at doses below 0.5 Gy. Increased resection and HR engagement with decreasing DSB-load generate a conundrum in a classical non-homologous end-joining (c-NHEJ)-dominated cell and suggest a mechanism adaptively facilitating resection. We report that ablation of DNA-PKcs causes hyper-resection, implicating DNA-PK in the underpinning mechanism. However, hyper-resection in DNA-PKcs-deficient cells can also be an indirect consequence of their c-NHEJ defect. Here, we report that all tested DNA-PKcs mutants show hyper-resection, while mutants with defects in all other factors of c-NHEJ fail to do so. This result rules out the model of c-NHEJ versus HR competition and the passive shift from c-NHEJ to HR as the causes of the increased resection and suggests the integration of DNA-PKcs into resection regulation. We develop a model, compatible with the results of others, which integrates DNA-PKcs into resection regulation and HR for a subset of DSBs. For these DSBs, we propose that the kinase remains at the break site, rather than the commonly assumed autophosphorylation-mediated removal from DNA ends.