Mutations induced by aminofluorene-DNA adducts during replication in human cells

To gain insight into the mechanisms by which carcinogens induce mutations in human cells, we treated a shuttle vector, pZ189, carrying the supF gene as the target for mutations with N-acetoxy-N-trifluoroacetyl-2-aminofluorene (N-AcO-TFA-AF). The plasmids were allowed to replicate in human cell line 293, and the progeny plasmids were examined for the frequency and kinds of mutations induced in supF, as well as their specific location in the sequence of the supF gene. The plasmids were reacted with N-AcO-TFA-AF so as to obtain the deacetylated adduct N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF), the principal adduct formed in DNA when mammalian cells are exposed to reactive derivatives of 2-acetylaminofluorene (AAF), including N-acetoxy-2-acetylaminofluorene. The results showed there was a linear relationship between the number of dG-C8-AF adducts per plasmid and the frequency of supF mutants induced. DNA sequencing of 47 independent mutants obtained from doses of N-AcO-TFA-AF that increased the frequency of mutants 9-15 times the background frequency and three independent mutants from lower doses showed that 92% contained point mutations, i.e. changes affecting one, or two, or three nearby bases, and that all of these point mutations involved G.C base pairs. Ninety eight percent of the point mutations were base substitutions, predominantly G.C----T.A transversions. 46% of these mutations occurred at four out of the 85 bp in the target gene (hot spots). The most prominent mutation hot spot was also the most prominent hot spot for adduct formation as judged by the frequency of termination of in vitro polymerization by the Klenow fragment on N-AcO-TFA-AF-treated plasmids.