DNA-BREAK REPAIR, RADIORESISTANCE OF DNA-SYNTHESIS, AND CAMPTOTHECIN SENSITIVITY IN THE RADIATION-SENSITIVE IRS MUTANTS - COMPARISONS TO ATAXIA-TELANGIECTASIA CELLS

Induction and rejoining of DNA single-strand breaks (ssb) and double-strand breaks (dsb) after γ-irradiation were measured, respectively, by alkaline and neutral sucrose gradien sedimentation methods. The radiosensitive mutants irs1, irs2, and irs3 showed no significant difference from wild-type V79 hamster cells in ability to rejoin either ssb or dsb, while the previously-described xrs-1 mutant showed the expected defect in rejoining dsb. The resistance of DNA synthesis to γ-irradiation was measured in the 3 irs mutants and, for comparative purposes, in transformed human cell lines from normal and ataxia- telangiectasia (A-T) individuals. The irs2 mutant was found to be very similar in response to the A-T lines, showing a marked decrease in inihition of DNA synthesis, compared to V79 cells, in both time-course and dose-response experiments. However, irs1 also had some decrease in inhibition at the higher doses used, while irs3 was similar to the wild-type V79 cells. Both irs1 and irs2 were found to be considerably more sensitive to the DNA topoisomerase I-inhibitor camptothecin, while irs3 was only slightly more sensitive than the parent V79 line. These data place the irs1 mutants in a similar category of radiosensitive phenotype to A-T cells, but we view this as only the beginning of a useful classification of this type of mutant. The irs2 mutant has the strongest links to A-T cells, through its sensitivity profile to DNA-damaging agents and radioresistant DNA synthesis, but irs1 in particular has other similarities to A-T. © 1990.