CHO mutant UV61 removes (6–4) photoproducts but not cyclobutane dimers

Abstract

The CHO mutant UV61 was previously assigned to complementation group 6 of UV-sensitive rodent cell mutants. UV61 is less sensitive to killing by UV radiation than mutants such as UV5, which is highly defective in the incision process that acts on UV-induced lesions. The D₃₇ for cell survival is ∼4 J/m² for UV61, compared with 10 J/m² for the parental AA8 line and ∼2 J/m² for UV5. Similarly, mutation induction at the hprt and aprt loci shows an intermediate response to UV61. In a post-replication recovery assay, the kinetics of maturation of pulse-labelled nascent DNA were normal after UV irradiation in UV61. Data from alkaline elution and alkaline unwinding assays showed that the rates of break accumulation and reseating, measured 0–120 min after irradiation, were also normal in the mutant. This repair incision correlated with the rapid, normal removal of pyrimidine(6–4)pyrimidone photoproducts in UV61 measured using a radioimmunoassay that is specific for this class of damage. In contrast, after exposure to 10 or 15 J/m², no detectable removal of cyclobutane dimers from DNA was found in UV61 while AA8 cells removed 32% by 24 h. We suggest that the mutation in UV61 specifically lowers the affinity of a repair protein for cyclobutane dimers, which are also inefficiently removed from the bulk DNA of normal CHO cells. The resistance of UV61 to killing by the direct acting chemical 7-bromomethylbenz[a]anthracene was only slightly greater than that of UV5, indicating defective repair of bulky chemical adducts in addition to cyclobutane dimers. We conclude that (6–4) photoproducts are both cytotoxic and mutagenic in CHO cells and that the incision events observed early after UV-irradiation result from the repair of (6–4) photoproducts rather than cyclobutane dimers.