CELLULAR AND CHROMOSOMAL HYPERSENSITIVITY TO DNA CROSS-LINKING AGENTSAND TOPOISOMERASE INHIBITORS IN THE RADIOSENSITIVE CHINESE-HAMSTER IRS MUTANTS - PHENOTYPIC SIMILARITIES TO ATAXIA-TELANGIECTASIA AND FANCONIS ANEMIA CELLS

The mutants irs1, irs2 and irs3 were previously isolated from the Chin ese hamster line V79-4 on the basis of their hypersensitivity (2-3-fol d) to cell inactivation by X-rays. Here the cross-sensitivities of the irs mutants to em array of chemical mutagens and topoisomerase inhibi tors was determined in a differential cytotoxicity assay. Irs2 showed moderate hypersensitivity (2-3-fold) to simple alkylating agents and o xidative mutagens but was most sensitive (8-fold) to the topoisomerase I inhibitor camptothecin. In contrast irs2 showed little or no increa sed sensitivity to four topoisomerase II inhibitors. Irs3 proved to be particularly hypersensitive to DNA crosslinking agents (5-15-fold) su ch as 1,3-butadiene diepoxide and mitomycin C. Irs1 was hypersensitive (3-fold or greater) to simple alkylating agents, oxidative mutagens a nd topoisomerase I and II inhibitors and exhibited extreme sensitivity (20-100-fold) to DNA crosslinking agents. The cellular hypersensitivi ties of irs2 and irs3 were reflected at the level of the chromosome. C amptothecin induced chromosomal aberrations in irs2 consisted almost e xclusively of chromatid deletions and exchanges, whilst in irs3 1,3 bu tadiene diepoxide induced a 50-fold increase in chromatid exchanges co mpared with V79-4. The nature of irs2's camptothecin hypersensitivity was investigated. Analysis of the protein associated DNA single strand breaks produced by camptothecin indicated that there was no differenc e between V79-4 and irs2 in either the number of breaks induced or in the rate of their reversal following drug removal. In addition, levels of topoisomerase I activity in V79-4 and irs2 were indistinguishable. The data presented suggest that irs3 is likely to be defective in som e aspect of DNA cross-link removal and irs2, whilst showing no gross d efect in DNA strand break repair may fail to correctly respond to or r epair certain types of strand breaks, possibly these associated with r eplicating DNA. Tile phenotypes of irs2 and irs3 respectively show sim ilarities to those of cultured cells from the syndromes ataxia telangi ectasia and Fanconi's anaemia.