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Ras gene mutation-independent tumours in the intestine of the rat by a single dose of N-methyl-N-nitrosourea.

Aiming at a sequential analysis of the role of ras gene point mutations during intestinal carcinogenesis, we established an experimental rat tumour model using N-methyl-N-nitrosourea (MNU) as an initiating agent as this carcinogen has been found to induce rat mammary carcinomas with a high prevalence of ras gene mutations. MNU treatment of a total of 249 rats (25 or 50 mg/kg i.p.) in various combinations with partial hepatectomy, hydroxyurea infusion and/or phenobarbital exposure resulted in a high incidence of intestinal adenomas and carcinomas of different histological types, besides liver, soft tissue and auditory sebaceous gland tumours. With PCR-amplified DNA the prevalence of mutations of codon 12 and 61 of H-, K- and N-ras was determined in dot blots by hybridization with 32P-labelled allele-specific oligonucleotides. Ras gene point mutations were not observed in any of the 41 intestinal rat tumours randomly selected from various experimental groups. Considering the high prevalence of ras mutations in MNU-induced mammary carcinomas of the rat the observed complete lack of ras mutations in intestinal tumours induced in the rat by the same carcinogen suggests that organ-specific intraspecies differences in the mechanism of malignant transformation exist even for a heterolytically decomposing, direct acting carcinogen like MNU.