The induction of azaguanine-resistant mutants in cultured Chinese hamster cells by reactive derivatives of carcinogenic hydrocarbons

Abstract

A study was made of the induction by two chemically reactive hydrocarbon derivatives of mutations to 8-azaguanine (8-AzG) resistance in the Chinese hamster cell line V79-379A. The compounds, 7-bromomethylbenz[ a]anthracene (7-BrMeBA) and 7-bromomethyl-12-methylbenz[ a]anthracene (7-BrMe12MeBA), were chosen since they had exhibited a considerable variation in carcinogenicity in animal experiments 10. The use of radioactively-labelled compounds enabled the extents of reaction of the derivatives with cellular macromolecules to be determined. Following treatment with the bromo-derivatives, the induced mutation frequency increased arithmetically with the number of cell divisions which occurred prior to the addition of azaguanine, and reached a maximum after 3–4 divisions. With still longer expression times, the mutant yield declined sharply. The time required to achieve 3–4 cell divisions was dependent on the dose of mutagen used. To allow comparison between mutant yields after different mutagen doses, azaguanine was added to the dishes after the cells had undergone 3 divisions. A plot of induced mutation frequency against extent of DNA reaction for both agents indicated that no significant mutation resulted from that extent of reaction corresponding to the “shoulder” on the survival curves. At somewhat higher levels of reaction, both compounds induced about 5.0·10 −5 mutants per μmole hydrocarbon derivative reacted per mole DNA phosphorus. At still higher reaction levels, an abrupt increase in mutant yield per extent of reaction with DNA occurred, 7-BrMeBA and 7-BrMe12MeBA inducing 1.3·10 −3 and 3.0·10 −4 mutants per μmole per mole DNA phosphorus reaction, respectively. The results are discussed in relation to those of others with X-ray, UV and other chemical agents, and a model is proposed which considers the role of DNA repair processes in mutation induction. The reported difference in the carcinogenicity of the two hydrocarbon derivatives studied was not paralleled by their mutagenicity in the present system.