Five UV-sensitive mutant strains of CHO cells representing different genetic complementation groups were analyzed for their ability to perform the incision step of nucleotide excision repair after UV exposure. The assay utilized inhibitors of DNA synthesis to accumulate the short-lived strand breaks resulting from repair incisions. After 6 J/m2, each of the mutants showed <10% of the incision rate of the parental AA8 cells. After 50 J/m2, the rate in AA8 was similar to that at 6 J/m2, but the rates in the mutants were significantly higher (∼20% of the rate of AA8). Thus by this incision assay the mutants were phenotypically indistinguishable. Each of the mutants were hypersensitive to mutation induction at both the hprt and aprt loci by a factor of 10, and in the one strain tested ouabain resistance was induced sevenfold more efficiently than in AA8 cells. Sister chromatid exchange was also induced with sevenfold increased efficiency in the two mutant strains examined. Thus, these CHO mutants resemble xeroderma pigmentosum cells in terms of their incision defects and their hypersensitivity to DNA damage by UV.
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