Overexpression of human Ku70/Ku80 in rat cells resulting in reduced DSB repair capacity with appropriate increase in cell radiosensitivity but with no effect on cell recovery

The effect of an overexpression of human Ku70/80 was studied using cells of the rat cell lines Rat-1 and R7080, the latter being transfected with the human cDNAs for Ku70 and Ku80, The overexpression was found to result in a 20% reduction of the DNA-PK activity. The kinetics of DSB repair, which was studied after exposure of the cells to 30 Gy of X rays, was biphasic and h ad identical half-times for Rat-1 and R7080 cells (t(fast) = 7 min and t(sh ow) = 135 min). However, there was a significant difference between the cel l lines in the fractions of DSBs repaired with slow and fast kinetics. In R 7080 cells, about twice as many DSBs were repaired with slow kinetics compa red to Rat-1 cells (34% compared to 16%). A similar difference was found in the number of residual DSBs (3.6% compared to 2.0%). R7080 cells also show ed a reduced capacity to repair chromosome damage as detected by the PCC te chnique. Concerning cell killing, R7080 cells were clearly more radiosensit ive than Rat-1 cells (D0.1 = 6.4 compared to 10.5 Gy), and this increase in sensitivity correlated well with the increase in residual DSBs. The two ce ll lines, however, did not vary in cell recovery. For sublethal as well as potentially lethal damage, Rat-1 and R7080 cells showed identical recovery ratios. These data demonstrate that the overexpression of human Ku70/Ku80 l ed to a reduced capacity for DSB repair with an associated increase in cell sensitivity but with no effect on cell recovery. 1999 by Radiation Researc h Society).