Hot spots of frameshift mutations induced by the ultimate carcinogen N- acetoxy-N-2-acetylaminofluorene

An important step in carcinogenesis is thought to be the initial attack of the DNA molecule by a so-called ultimate carcinogen. More than 90% of the carcinogens tested have been found to be mutagens in bacterial systems 1 . The covalent binding of the ultimate carcinogen to the DNA bases or phosphate groups creates a premutational lesion that in vivo is processed by the repair, replication and recombination enzymes, and eventually may be converted into a mutation. Being interested in the way that an initial premutational event is converted into a stable heritable mutation, we have sequenced stable mutations in a gene that has formed covalent adducts in vitro with N- acetoxy- N- 2-acetylaminofluorene ( N- AcO-AAF, a model for the ultimate metabolite of the rat liver carcinogen 2-acetylaminofluorene AAF). We show here that the mutations are mainly frameshifts involving G · C base pairs, and that certain base pairs (hotspots) are affected at relatively high frequences. These results fit a model in which N- AcO-AAF-modified guanine acts as a non-coding base that during replication results in deletion of the modified residue.