Interaction of DNA repair proteins in response to DNA double strand break induced by ionizing irradiation

DNA double strand break (DSB) is the most serious damages and causes cell death, somatic mutation, and cellular malignancy. To maintain genomic stability, cells have various repair machineries which are activated by DNA damage responding proteins. DSB is known to be repaired by homologous recombination (HR) or by non-homologous end joining (NHEJ). HR uses homologous DNA sequences as a template for repair synthesis, whereas NHEJ simply rejoins the DSB ends. In this study, to examine the interaction of DNA repair proteins following DNA damage by radiation, we analyzed phenotypes of Ku70 knockout cells, Nbs1 knockout cells, Rad54/Ku70 double knockout cells, and Nbs1/Ku70 double knockout cells which are established by using chicken DT40 cells. Both the Nbs1/Ku70 and Rad54/Ku70 cells were extremely sensitive to radiation, and they displayed slower DSB rejoining compared to wild type cells. However, 50~60 % of the DSBs were rejoined within 30 minutes after irradiation even in those double knockout cell lines. This suggests that there is another DSB repair pathway(s) different from typical HR or NHEJ. The rate of DSB rejoining in Nbs1/Ku70 cell line was slower than that in Rad54/Ku70 cell line, suggesting that Nbs1 might be involved in a part of the another DSB repair pathway. In addition, Nbs1/Ku70 double knockout cells showed higher rate of spontaneously induced apoptosis. The kinetics of apoptosis induction in Nbs1/Ku70 cells by radiation was similar to that in wild type cells, whereas apoptosis induction in Nbs1 knockout cells was significantly suppressed.