The role of ataxia telangiectasia-mutated and Nijmegen breakage syndrome protein-1 in the accumulation of UVC-induced DNA replication-dependent double strand breaks

Maintaining the integrity of any genome is a critical function that requires proteins involved in DNA replication, damage recognition and repair. Particularly deleterious to chromosomal integrity are agents such as ionizing radiation (IR) that cause DNA double strand breaks (DSBs) directly. It has also been suggested that UV-induced DNA damage contributes to the formation of DSBs in dividing cells. UV light does not break DNA directly; it is possible that replication fork arrest at UV-induced lesions leads to the formation of DSBs. Here we examine whether UVC causes DSBs, the role of DNA replication in the formation of breaks, and signal transduction pathways involved in UV-induced DNA damage and repair responses. Both ataxia telangiectasia-mutated (ATM) and a downstream target of ATM, Nijmegen breakage syndrome protein or nibrin (NBS1) play major roles in sensing and responding to DNA damage caused by DSBs mediated through IR. Here we show that A-T cells (cells lacking ATM) accumulated more UVC-induced DSBs. These data suggest that ATM is required for signaling the repair of DSBs generated by replication of UV-damaged templates. By contrast, NBS cells (cells lacking NBS1) showed fewer DSBs, suggesting that NBS1 may be required to induce the initial DSB. The NBS1 protein was bound more tightly in the nucleus after treatment with UVC. The nuclear retention of NBS1 protein was seen as early as 4 h after UVC. We also found that the nuclear retention of NBS1 was increased after UVC radiation in a time-dependent manner. The increased retention of NBS1 after UVC suggests its possible activation after UVC and that this treatment stimulates the association with the NBS1/MRE11/RAD50 DSB repair complex. Our data from the PFGE analysis and neutral comet assay may suggest the NBS1 protein may participate in the generation of the DSBs produced after replication of UV-damaged templates. Taken together, our results suggest that ATM and NBS1 are involved in response to UVC-induced DSBs.