Level of DNA double-strand break rejoining in Chinese hamster xrs-5 cells is dose-dependent: implications for the mechanism of radiosensitivity.

Rejoining of DNA double-strand breaks (dsb) was measured in a dsb repair-deficient mutant of CHO cells, xrs-5, after exposure to various doses of X-rays in the range between 15 and 50 Gy. For the experiments plateau-phase cultures were employed and dsb assayed by a pulsed field gel electrophoresis assay, the asymmetric field inversion gel electrophoresis (AFIGE). The half-times of dsb rejoining were larger in xrs-5 than in parental CHO cells and increased in both cell lines with increasing dose of radiation. The fraction of dsb remaining unrejoined after 240 min incubation at 37 degrees C was also higher in xrs-5 than in CHO cells, but decreased with decreasing dose of radiation. Although a decrease in the fraction of unrepaired dsb with decreasing dose has also been reported for repair-proficient cell lines, the extent of the phenomenon and its dependence on dose are entirely different in xrs-5 cells. We propose that this decrease in the fraction of unrejoined dsb with decreasing dose of radiation derives from the genetic alterations underlying the increased sensitivity to radiation of xrs-5 cells, and should be considered whenever results at the DNA level are correlated to results at the cell level. It is likely that similar responses will also be observed in other radiation-sensitive mutant cell lines deficient in dsb repair. There was no difference in the induction of dsb per Gy and dalton, as measured with AFIGE, between CHO and xrs-5 cells tested either in the exponential or in the plateau phase of growth.