Induction of double-strand breaks in Chinese hamster ovary cells at two different dose rates of γ-irradiation

Using pulsed-field gel electrophoresis (PFGE) analysis we investigated the existence of a dose rate effect of γ-irradiation on the measured presence of DNA double-strand breaks (DSB) in a repair competent (K1) and a repair deficient (mutant xrs6) Chinese hamster ovary (CHO) cell line. The fraction of DNA fragments released from cells embedded in agarose during PFGE after γ-irradiation was taken as a measure of DSB induction. In CHO-K1 cells DSB were present at a significantly higher rate when γ-irradiation was delivered at a high dose rate of 22 Gy/min (HDR) than at a medium dose rate of 0.45 Gy/min (MDR) at 37°C. However, the same amount of DSD was found when irradiation was performed at the two dose rates at 4°C. The DSB yield was also identical at both dose rates in the DSB repair deficient mutant xrs6. The results indicate that there is an apparent dose rate effect for γ-ray induced DSB in repair competent CHO cells due to partial repair of DSB taking place during γ-ray exposures at MDR but not at HDR. This repair of DSB was inhibited upon irradiation at 4°C and in repair deficient xrs6 cells.