Cell killing effects of low-dose-rate γ-rays and repair of γ-ray-induced DNA damage

This study was performed to elucidate the relationship between repair of DNA damage and cellular survival after irradiation of gamma-rays at different dose rates to cells of the saturated confluence. Cells were the primarily cultured human normal diploid HE-4 cells, MRC-5/hTERT cells which had been immortalized by transduction of human teromerase transcriptase (hTERT) gene in human normal diploid MRC-5 cells, and mouse C3H 10T1/2 cells. They were cultured till the confluence to stop their division (G1/G0 stage) and irradiated by gamma rays at the high dose rate of 1.0 Gy/min and low rate of 0.347 mGy/min (0.5 Gy/day) from a Cs-137 source to observe their colony formation for survival and potentially lethal damage (PLD). DNA double strand break was detected by H2AX foci with fluorescent immuno-staining. All cell lines irradiated at low dose rate were found to show the marked resistance to radiation, indicating the rate dependency of lethality, which was confirmed by PLD restoration in cells irradiated with total 2 or 5 Gy at the high rate. H2AX foci were found to increase linearly with the dose range 0.17-2.0 Gy either at the high or low rate although the number of foci was much larger at the high rate. Cell survival vs number of H2AX foci was rather parallel in all cell lines irradiated at the high rate while more foci relative to survival were observed in those at the low rate. Thus this study revealed the dose rate dependency of lethality and of DNA double strand break, and at the low rate irradiation, repair of damaged DNA was not parallel to the survival, suggesting the possible existence of another system of repair or damage fixation after the low rate irradiation. (T.T.)