Pathogenic characterization of 1-methyl-1-nitrosourea-induced mammary carcinomas in the rat.

The induction of mammary carcinogenesis in the rat by 1-methyl-1-nitrosourea (MNU) is widely used in experimental breast cancer research. In the experiments reported, the Ha-ras codon 12 (ras12) mutation (GGA-->GAA) was used as a molecular marker to address issues of the clonality of carcinomas induced, pathogenetic independence among multiple carcinomas within the same animal and topographic distribution of mutant ras12 carcinomas in different mammary gland chains. In order to determine whether the frequently observed morphologically distinguishable lobules within carcinomas originate from the coalescence of independent lesions or whether cancerous cells within a carcinoma share a common origin, 44 randomly selected MNU-induced mammary carcinomas were genotyped for two to four lobules each for the ras12 mutation. A total of 43 carcinomas out of 44 (97.7%) had concordant ras12 genotypes among the multiple sites within each tumor, which is consistent with the latter possibility. Next, it was observed that as carcinoma multiplicity increased, the discordance rate of ras12 genotypes among multiple carcinomas within the same animal increased in a manner that was in excellent agreement with the expected discordance rate based on an assumption of no pathogenetic association among carcinomas. Furthermore, a significant difference was observed in the occurrence of mutant ras12 carcinomas between the cervical-thoracic and the abdominal-inguinal mammary glands in that three times as many carcinomas were mutant in the former as in the latter glands, whereas the occurrence of wild-type carcinomas was approximately the same in both regions. Taken together, the data are consistent with (i) carcinomas induced by MNU and detected by palpation are monoclonal in origin, (ii) independently-initiated cells emerge as distinct mammary carcinomas in the same animal, and (iii) the anatomical location of the gland may affect the prevalence of mammary carcinomas that harbor a mutant ras12.