Increased frequency of formation of interphase ring-chromosomes in radiosensitive irs-1 cells exposed to X-rays.

Induction and rejoining of interphase chromosome breaks were measured, after exposure to X-rays, in plateau-phase Chinese hamster V79 cells and in a radiosensitive mutant cell line derived from them, iris-1, using premature chromosome condensation (PCC). There was no difference in the induction of interphase chromosome breaks per Gy between the radiosensitive mutant cells and wild-type V79 cells despite the large differences in their radiosensitivity; an induction of 2.85 +/- 0.05 breaks/cell/Gy was measured in both cell lines. Also, rejoining of interphase chromosome breaks proceeded in the two cell lines with similar kinetics (t1/2 = 26.2 min). In contrast, ring chromosome formation was higher in irs-1 cells, as compared to wild-type V79 cells (0.78 versus 0.44 after 6 h of repair). These results confirm previous observations suggesting that a general deficiency in the rejoining of DNA dsb is unlikely to be a direct cause of the increased radiosensitivity of irs-1 cells, and are consistent with the hypothesis that the increased radiosensitivity of these cells derives from an increase in the probability of misrepair.