title: DNA-break repair, radioresistance of DNA synthesis, and camptothecin sensitivity in the radiation-sensitive irs mutants: comparisons to ataxia-telangiectasia cells.
abstract: Induction and rejoining of DNA single-strand breaks (ssb) and double-strand breaks (dsb) after gamma-irradiation were measured, respectively, by alkaline and neutral sucrose gradient 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 gamma-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 inhibition 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 irs 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.
authors.affiliation: MRC Radiobiology Unit, Chilton, Didcot, Oxon, Great Britain.
authors.forename: J
authors.initials: J
authors.lastname: Thacker
authors.forename: A N
authors.initials: AN
authors.lastname: Ganesh
mesh_terms: D000818:Animals; D001260:Ataxia Telangiectasia / Q000235:genetics*; D002166:Camptothecin / Q000494:pharmacology*; D002460:Cell Line; D002461:Cell Line, Transformed; D006224:Cricetinae; D003412:Cricetulus; D004247:DNA / Q000187:drug effects / Q000528:radiation effects; D004260:DNA Repair / Q000187:drug effects; D004261:DNA Replication / Q000528:radiation effects*; D006801:Humans; D009154:Mutation; D011836:Radiation Tolerance / Q000235:genetics*
chemical_list: D004247:DNA; D002166:Camptothecin