INDUCTION OF DOUBLE-STRAND BREAKS IN CHINESE-HAMSTER OVARY CELLS AT 2DIFFERENT DOSE-RATES OF GAMMA-IRRADIATION

Using pulsed-field gel electrophoresis (PFGE) analysis we investigated the existence of a dose rate effect of gamma-irradiation on the measu red presence of DNA double-strand breaks (DSB) in a repair competent ( K1) and a repair deficient (mutant xrs6) Chinese hamster ovary (CHO) c ell line. The fraction of DNA fragments released from cells embedded i n agarose during PFGE after gamma-irradiation was taken as a measure o f DSB induction. In CHO-K1 cells DSB were present at a significantly h igher rate when gamma-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 degrees C. However, the same amount of DSB was found when irradiation was performed at the two dose rates at 4 degrees 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 gamma-ray induced DSB in repair competent CHO cells due to partial repair of DSB taking place during gamma-ray exposures at MDR but not at HDR. This repair of DSB was inhibited upon irradiation at 4 degrees C and in repair deficient xrs6 cells.