Proto-oncogene activation in liver tumors of hepatocarcinogenesis-resistant strains of mice

Abstract

Activation of the ras family of oncogenes occurs frequently in liver tumors of the B6C3F1 mouse, a strain which is highly sensitive to hepatocarcinogenesis. Many other mouse strains are much more resistant to hepatocarcinogenesis; the aim of this study was to determine the frequency and pattern of oncogene activation in spontaneous and chemically induced liver tumors of three such strains, the C57BL/6J, the C57BL/6 × DBA/2 F1 hybrid (B6D2F1) and the C57BL/6 × Balb/c F1 hybrid (B6BCF1). The C57BL/6, DBA/2 and Balb/c strains are all relatively resistant to spontaneous hepatocarcinogenesis (1.5–3.6% of animals develop liver tumors in 2 years); with regard to chemically induced hepatocarcinogenesis the Balb/c is highly resistant, the C57BL/6 has low susceptibility and the DBA/2 has low to moderate susceptibility. The nude mouse tumorigenidty assay was used to search for activated oncogenes in 15 C57BL/6J liver tumors induced by a single neonatal dose of vinyl carbamate (VC, 0.15 μmol/g body weight). Three tumors contained H-ras genes activated by point mutations at codon 61 and one contained a non-ras oncogene. The polymerase chain reaction and allele-spedfic oligonucleotide hybridization were used to study H-ras mutations in spontaneous and VC-induced tumors from all three strains of mice. The frequency of H-ras codon 61 mutations in tumors induced by 0.15 μmol/g body weight VC in the C57BL/6J mouse (5/37) was similar to that in spontaneous tumors (2/9); surprisingly, tumors induced by a lower dose of VC (0.03 μmol/g body weight) had a higher frequency of H-ras mutations (12/28). The frequencies of H-ras activation detected in VC (0.03 μmol/g body weight)-induced tumors from the two F1 hybrids studied differed markedly. Only one VC-induced B6BCF1 tumor contained a mutated H-ras gene (1/10), whereas the majority of B6D2F1 tumors contained such mutations (23/33). Several spontaneous B6D2F1 liver tumors contained H-ras codon 61 mutations (6/15). Thus, H-ras activation frequency does not determine susceptibility to hepatocarcinogenesis in inbred mice and then- F1 hybrids, since a relatively high frequency of H-ras mutations was observed in two resistant strains and a low frequency was found in the other strain.