Although administration of a single dose of N-nitroso-N-
methylurea (NMU) to young virgin rats induces a high rate of mammary
carcinomas, precise histogenesis of the
carcinomas has not been well characterized. In this study, we investigated the alterations of H-ras gene in early focal lesions as well as
carcinomas in the mammary glands of F344 rats treated with NMU. At 2 weeks
after treatment, intraductal proliferation (
IDP) was occasionally observed, and mammary
carcinomas emerged at 12 and 36 weeks. The individual lesions of
IDP and
carcinomas were scooped out from the tissue sections under a stereomicroscope, and the DNA-sequence-spanning
codon 12 of H-ras gene was amplified from the tissue sections by polymerase chain reaction (PCR). The analysis of amplified
DNA by
oligonucleotide hybridization revealed that 65% (11/17) of
IDP and 89% (16/18) of
carcinomas had a point mutation (G-to-A transition) at the 2nd position of H-ras
codon 12. However, the
DNA amplified from the areas, which appear histologically normal, never showed such mutation. These results indicate that
IDP is a very early change for NMU-induced mammary
carcinogenesis.