Strand-specific mutation spectra in repair-proficient and repair-deficient hamster cells

The mutation spectrum induced by UV light has been determined at the hprt locus for both cultured normal (AA8) and UV-sensitive (UV-5) Chinese hamster ovary cells to investigate the effect of DNA repair on the nature of induced mutations. DNA base-pair changes of 23 hprt mutants of AA8 and of 28 hprt mutants of UV-5 were determined by sequence analysis of in vitro amplified hprt cDNA. Almost all mutants in AA8 carried single-base substitutions, transitions and transversions accounting for 38% and 62% of the base changes, respectively. In contrast, in repair-deficient cells (UV-5) tandem and nontandem double mutations represented a considerable portion of the mutations observed (30%), whereas the vast majority of base-pair substitutions were GC greater than AT transitions (87%). Moreover, 5 splice mutants and 2 frameshift mutations were found in the UV-5 collection. In almost all mutants analyzed base changes were located at dipyrimidine sites where UV photoproducts could have been formed. In AA8 the photolesions causing mutations were predominantly located in the nontranscribed strand whereas a strong bias for mutation induction towards photolesions in the transcribed strand was found in UV-5. We hypothesize that preferential removal of lesions from the transcribed strand of the hprt gene accounts for the observed DNA strand specificity of mutations in repair-proficient cells. Furthermore, differences in the degree of misincorporation opposite a lesion for lagging and leading strand DNA synthesis may dictate the pattern of UV-induced mutations in the absence of DNA repair.