Abstract
DNA-PK is composed of a catalytic subunit (DNA-PKcs) and DNA end-binding Ku70 and Ku86, and is essential to the repair of DNA double-strand breaks (DSBs) via by non-homologous end joining (NHEJ). Accelerated heavy ions with high LET can induce complex clustered DNA damage involving two or more DSBs. The complexity of clustered DNA damage shows a strong LET dependence, which makes it a good candidate prime determinant of the higher RBE of high-LET heavy ions than that of low-LET X-rays. Here we show the cell killing effect of heavy ions and the response of DNA-PK to heavy-ion-induced DSBs. We determined that the surviving fractions using human DNA-PKcs-defective M059J and control M059K cell lines irradiated with X-rays and heavy ions. The peak RBE value of M059J cells was significantly lower than that of M059K cells. Similar results were obtained using HeLa cells treated with an inhibitor of DNA-PK. Next we observed that phosphorylated DNA-PKcs was recruited to the vicinity of DSBs induced along the track of heavy ions. In addition, phosphorylation level of XRCC4 after heavy-ion irradiation was higher than that after X-ray irradiation, according to LETs. The time course of XRCC4 phosphorylation, however, showed no difference between X-rays and heavy ions. These results suggest that DNA-PK can efficiently recognize DSBs induced by high-LET heavy ions and is activated, but NHEJ cannot precisely repair because of the complexity of DNA damage. The relationship of phosphorylation of DNA-PKcs and XRCC4 will be presented.
