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A single dose of N-methyl-N-nitrosourea given to sexually immature female Buf/N rats produces a high incidence of mammary adenocarcinomas. A large percentage of these tumors contain the Ha-ras oncogene, activated by a G----A transition at the second nucleotide of codon 12. Copenhagen rats, on the other hand, are completely resistant to mammary tumor induction by a number of carcinogens, including N-methyl-N-nitrosourea. Here we show, using a sensitive method involving PCR, that codon 12 Ha-ras mutations occur in the mammary glands of both Buf/N and Copenhagen rats 30 days after N-methyl-N-nitrosourea treatment. These mutations were evenly distributed among individual mammary glands and were present in purified mammary epithelial cells. In Buf/N rats, the fraction of cells containing a mutated Ha-ras allele increased by a factor of 10-100 between 30 and 60 days, whereas in Copenhagen rats, there was no such increase during this time period. We conclude that the resistance of the Copenhagen rat to mammary carcinogenesis is not due to a defect in initiation but rather appears to be due to the inability of cells containing a mutated ras allele to undergo sustained clonal expansion.

Ha-ras oncogene activation in mammary glands of N-methyl-N-nitrosourea-treated rats genetically resistant to mammary adenocarcinogenesis.