Frequent Ha-ras mutations in murine skin and liver tumors induced by 7H-dibenzo[c,g]carbazole.

7H-dibenzo[c,g]carbazole (DBC) is a potent liver and skin carcinogen when topically applied to the back skin of mice. This compound is found in complex mixtures produced during incomplete combustion of fossil fuels as well as in wood and tobacco smoke. The objective of this study was to elucidate the mechanism of action of this compound by assessing the Ha-ras mutational spectra of skin and liver tumors induced in a mouse model system. Low doses (50 nmol) and high doses (100 nmol) of DBC were applied topically to the backs of Hsd:ICR(Br) mice twice weekly. No treatment and solvent application were used as controls. After the mice were killed, the skin and liver tumors were removed, DNA was isolated, and tumor DNA was screened for Ha-ras codon 12, 13, and 61 mutations by using an enriched polymerase chain reaction method. Mutations were confirmed by reverse cyclic dideoxy sequencing. No mutations were found in codons 12 and 13 of DBC-induced tumors, whereas one acetone-control tumor had a codon 13 mutation. Sixty-seven percent of skin tumors and 45% of liver tumors induced by high doses of DBC and 67% of skin tumors induced by low doses of DBC contained codon 61 mutations, whereas liver tumors induced by low doses of DBC did not. The codon 61 mutations were exclusively A:T-->T:A transversions within the second base (CAA-->CTA). These results indicate that DBC is a unique polycyclic aromatic hydrocarbon in that it induces both skin and liver tumors upon topical application and that the mutational spectra are the same in tumors from two target organs, skin and liver, yet different from tumors from a third target organ, lung.