DNA base changes in benzo[a]pyrene diol epoxide-induced dihydrofolate reductase mutants of Chinese hamster ovary cells.

We previously treated Chinese hamster ovary (CHO) cells with benzo[a]pyrene diol epoxide (BPDE) and mutants at the dihydrofolate reductase (dhfr) locus were isolated. On the basis of Southern blotting and RNA heteroduplex mapping experiments, 14 of the 15 mutants were presumed to carry point mutations. Two restriction fragment length polymorphism mutants were cloned and sequenced; one carried a point mutation, the other a -1 frameshift mutation. Using polymerase chain reaction techniques and direct sequencing of amplified mutant DNA, we have now determined the induced changes in the remaining 12 cell lines. All changes occurred at guanine bases; all target guanines except one were on the non-transcribed coding strand. Most mutants (79%) contained base substitutions; the rest (3/14) carried frameshift mutations. Of the point mutations, all but one (91%) were GC----TA transversions either in the dhfr coding sequence or at splice sites. The single exception was a GC----AT transition. Of the frameshift mutations, two were deletions of a GC pair and the other was an insertion of an AT pair. Four different mutations (29%) were clustered in a 3 bp region in exon 4. Tandem guanine bases adjacent to adenine were favored sites for mutation occurring in 9/14 cases (64%). These results are consistent with data previously obtained by others in the supF shuttle vector system and the CHO aprt gene.