ISOLATION AND PRELIMINARY CHARACTERIZATION OF AN X-RAY-SENSITIVE MAMMALIAN MUTANT-CELL LINE (WMXRS-1)

Mammalian cell lines that are sensitive to particular genotoxic agents have proved the most effective starting point for the cloning of huma n DNA-repair genes. After ethyl methanesulphonate mutagenesis of the p arent murine fibroblast L-cell line, a new mammalian X-ray-sensitive c ell line (WMXRS-1) was isolated. For selection of the mutant, a novel detection method was used: putative X-ray-sensitive clones were identi fied by their lack of incorporation of the DNA precursor, bromodeoxyur idine, after irradiation. The WMXRS-1 cell line was collaterally sensi tive to ultraviolet radiation and some other agents known to be remove d from DNA by the nucleotide excision repair pathway, but not to bleom ycin or hydrogen peroxide. In relation to the wild-type strain, WMXRS- 1 showed a similar pattern of induction of micronuclei up to an X-ray dose of 4 Gray and a similar DNA double-strand break (dsb) induction p rofile. The overall level of dsb rejoining was the same in the parent and mutant lines. However, WMXRS-1 demonstrated a reduced initial rate of dsb-rejoining, perhaps accounting for its radiosensitivity. WMXRS- 1 also showed a greater G2 cell cycle phase accumulation after treatme nt with mitomycin-C. The cross-sensitivity profile and strand-break re joining deficiency phenotype of WMXRS-1 is unique amongst previously c haracterised mammalian mutant cell lines.