Strand-specific mutation induction by 1,2-dibromomethane at the hypoxanthine-guanine phosphoribosyltransferase locus of Chinese hamster ovary cells.

The nature of mutations induced by 1,2-dibromoethane (DBE) at the hprt (hypoxanthine-guanine phosphoribosyl-transferase) gene was analysed in Chinese hamster ovary (CHO-9) cells. Molecular characterization of 36 hprt mutants at the cDNA level yielded 19 GC-->AT transitions, two AT-->CG transversions, three frameshift mutations, two identical small deletions and 10 exon deletions. Further analysis of the deletion mutants by amplification of specific exons from genomic DNA showed two more GC-->AT transitions at splice sites and an approximately 70 bp deletion. Assuming that the S-[2-(N7-guanyl)ethyl]glutathione adduct is responsible for the GC-->AT transitions, 90% of the affected guanines were located in the non-transcribed strand of the hprt gene, suggesting a strand bias in repair of this adduct. Nearest neighbour analysis of induced GC-->AT transitions indicates a preference for a 5'-PyPuG DNA sequence, i.e. 15/21 mutated guanines were located in either a TGG or a CAG DNA sequence. These molecular data on DBE-induced mutations showed similar features as data from a study by Graves et al. (Mutagenesis, 11, 229-233, 1996) in which they analyzed 13 hprt mutants induced by DBE in CHO-K1 cells. Six of the seven GC-->AT mutations were on positions mutated more than once among the 36 hprt mutants in the present study. The combined findings suggest that some positions seem to be hot spots for DBE-induced mutations. Concerning the relevance of these in vitro studies for germ cell mutagenesis the conclusion may be that these data lend further support to the view that mutation spectra derived from in vitro systems have little predictive value for the nature of mutations induced in post-meiotic germ cells in vivo, as demonstrated for other alkylating agents in both Drosophila and mice.