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 environmentaltoxin which has been found to be non-genotoxic in short termin vitro tests but strongly carcinogenic in two stage modelsof hepatocellular carcinogenesis in female rats. Many recentstudies have shown that after treatment of mice with variousgenotoxic or non-genotoxic compounds, the H-ras oncogene mutationalpatterns exhibited by hepatocellular tumors appear to vary specificallywith the chemical. To gain insight into the mechanism of TCDD-associatedcarcinogenesis, susceptible B6C3F1 mice and resistant C57BL/6mice were treated with a single dose of vinyl carbamate (VC)or vehicle, and TCDD was administered once every 2 weeks for1 year to half of the animals in each group. Liver tumor prevalencewas assessed and found to be highest in the VC+TCDD treatmentgroups, reaching nearly 100% at 600 days in both sexes and bothstrains of mice. DNA was isolated from 20 or more frozen livertumors (if available) from each exposure group and analyzedfor H-ras mutations in codon 61 by sequencing after PCR amplificationof exon 2. Fifty-one percent of tumors analyzed from B6C3F1mice treated with TCDD alone had H-ras codon 61 mutations witha pattern similar to that detected in spontaneous tumors. Seventy-eightpercent of tumors from B6C3F1 mice treated with both VC andTCDD had codon 61 mutations, and most mutations were A