Abstract
DNA double strand break (DSB) is thought to be the most critical damage induced by ionizing radiation (IR), and non-homologous end joining (NHEJ) pathway is regarded as the most prevalent mechanism to repair DSBs in mammalian cells. Although several key proteins such as DNA-PK and ligase IV/XRCC4 have been identified for NHEJ, it is not clear if another critical protein, ATM is directly involved in this pathway. Some researchers suggested the involvement of ATM in repair of complex (or dirty) type DSBs. We observed kinetics of gammaH2AX foci in normal human cells irradiated with X-rays, carbon (290 MeV/n, 70 keV/um) and iron ions (500MeV/n, 200 keV/um). These heavy ions are a good source to induce complex DSBs. The appearance of gammaH2AX foci is similar for all three radiation sources, but the disappearance is significantly delayed in carbon and iron irradiation when compared that with X-rays. In contrast, when AT heterozygote cells were used, significant delays were observed in gammaH2AX foci appearance in carbon and iron irradiated cells. Moreover, we were not able to observe the phosphorylation of DNA-PKcs in AT cells irradiated with heavy ions, while X-irradiation induced DNA-PKcs phosphorylation in AT cells. In addition, we observed a significant delay in ATM phosphorylation in normal cells irradiated with 200keV/um iron ions. These studies indicate that 1) high LET irradiation induces complex type DSBs which significantly affects NHEJ repair proteins, 2) ATM seems to play an active role in NHEJ process, and may be critical in repairing complex type DSBs.
