Sequence specificity of mutations induced by benzo[a]pyrene-7,8-diol-9,10-epoxide at endogenous aprt gene in CHO cells.

We have determined the spectrum of mutations induced by +/--trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene (BPDE) at the endogenous aprt locus in an hemizigous Chinese hamster ovary cell line exposed to 0.7 microM BPDE. Southern analysis of 59 independent mutants revealed no major genomic alterations, indicating that gene inactivation was the result of a point mutation. This conclusion was confirmed by the cloning and sequencing of 21 of these mutants. The predominant mutation, the G:CT----T:A transversion, comprised 62% of the spectrum, but other base pair substitutions and frameshifts were recovered. An examination of the target sequences for BPDE mutation revealed that mutations were localized within runs of G:C base pairs. However, approximately half of these G:C runs involved a particular sequence--a run of guanines flanked by adenine residues. Of seven such sites within the coding sequence of aprt, mutations were clustered within five of them. This class of sequence occurs at codon 61 of the human C-Ha-ras 1 protooncogene and may account for the selective activation of this codon by BPDE.