Level of DNA double-strand break rejoining in Chinese hamster xrs-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. 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 37oC 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. (author)