Abstract
The high biological effectiveness of accelerated high LET heavy ions could be caused predominantly by the induction of complex clustered DNA damages (CCDs), leading to non-repairable DNA double-strand breaks (DSBs). NBS1 is a causative gene for Nijmegen breakage syndrome (NBS), and is essential for DSB repair by homologous recombination (HR) and cell cycle checkpoint. We have shown that NBS1 recognizes the induction of CCDs, but cannot restore cell survival and abnormal G2 checkpoint after exposure to high LET heavy ions. NBS1 recruited to the vicinity of DSBs induced along the track of heavy ions and formed indissoluble large foci through the dynamic movement in the cell nucleus even 16 h after exposure. On the other hand, X-ray-induced discrete NBS1 foci were almost dispersed within 8 h after irradiation. Abnormal G2 checkpoint and failed Chk2 phosphorylation were observed in the cells derived from NBS patients after both X-ray and heavy ion irradiation. In NBS cells expressing full length NBS1, these defects could be complemented after X-irradiation but not after exposure to heavy ions. In addition, sensitivity to X-rays of NBS cells was restored in NBS cells expressing NBS1. However, the sensitivity to heavy ions at high LET of NBS cell expressing NBS1 was not significantly different from that of NBS cells. These results suggest that heavy ion-induced CCDs are recognized by NBS1 but are difficult to repair by HR. NBS1 regulates G2 checkpoint through Chk2 phosphorylation to avoid abnormal cell cycle progression in response to the severity of DNA damages.
