H-RAS ONCOGENE MUTATION SPECTRA IN B6C3F1 AND C57BL 6 MOUSE-LIVER TUMORS PROVIDE EVIDENCE FOR TCDD PROMOTION OF SPONTANEOUS AND VINYL CARBAMATE-INITIATED LIVER-CELLS/

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent environmental t oxin which has been found to be nongenotoxic in short term in vitro te sts but strongly carcinogenic in two stage models of hepatocellular ca rcinogenesis in female rats. Many recent studies have shown that after treatment of mice with various genotoxic or non-genotoxic compounds, the H-ras oncogene mutational patterns exhibited by hepatocellular tum ors appear to vary specifically with the chemical. To gain insight int o the mechanism of TCDD-associated carcinogenesis, susceptible B6C3F1 mice and resistant C57BL/6 mice were treated with a single dose of vin yl carbamate (VC) or vehicle, and TCDD was administered once every 2 w eeks for 1 year to half of the animals in each group. Liver tumor prev alence was assessed and found to be highest in the VC+TCDD treatment g roups, reaching nearly 100% at 600 days in both sexes and both strains of mice. DNA was isolated from 20 or more frozen liver tumors (if ava ilable) from each exposure group and analyzed for H-ras mutations in c odon 61 by sequencing after PCR amplification of exon 2, Fifty-one per cent of tumors analyzed from B6C3F1 mice treated with TCDD alone had H -ras codon 61 mutations with a pattern similar to that detected in spo ntaneous tumors, Seventy-eight percent of tumors from B6C3F1 mice trea ted with both VC and TCDD had codon 61 mutations, and most mutations w ere A-->T transversions in the second base as observed similarly with VC alone, In the C57BL/6 strain comparable results were found in the r espective exposure groups, These data suggest that TCDD is acting as a promoter of lesions previously initiated either spontaneously or by V C. Moreover, the intrinsic resistance of both male and female C57BL/6 mice to liver tumor formation seemed to disappear after treatment with TCDD.