Role of ATM and the damage response mediator proteins 53BP1 and MDC1 in the maintenance of G(2)/M checkpoint arrest.

ATM-dependent initiation of the radiation-induced G(2)/M checkpoint arrest is well established. Recent results have shown that the majority of DNA double-strand breaks (DSBs) in G(2) phase are repaired by DNA nonhomologous end joining (NHEJ), while approximately 15% of DSBs are slowly repaired by homologous recombination. Here, we evaluate how the G(2)/M checkpoint is maintained in irradiated G(2) cells, in light of our current understanding of G(2) phase DSB repair. We show that ATM-dependent resection at a subset of DSBs leads to ATR-dependent Chk1 activation. ATR-Seckel syndrome cells, which fail to efficiently activate Chk1, and small interfering RNA (siRNA) Chk1-treated cells show premature mitotic entry. Thus, Chk1 significantly contributes to maintaining checkpoint arrest. Second, sustained ATM signaling to Chk2 contributes, particularly when NHEJ is impaired by XLF deficiency. We also show that cells lacking the mediator proteins 53BP1 and MDC1 initially arrest following radiation doses greater than 3 Gy but are subsequently released prematurely. Thus, 53BP1(-/-) and MDC1(-/-) cells manifest a checkpoint defect at high doses. This failure to maintain arrest is due to diminished Chk1 activation and a decreased ability to sustain ATM-Chk2 signaling. The combined repair and checkpoint defects conferred by 53BP1 and MDC1 deficiency act synergistically to enhance chromosome breakage.