A mammary cancer suppressor gene and its site of action in the rat

Fifty-day-old female rats of the inbred Osborne-Mendel (OM) and Copenhagen (COP) strains were exposed to a single dose of either of 2 highly effective mammary chemical carcinogens, 7,12-dimethylbenz[a] anthracene (DMBA) or 1-methyl-1-nitrosourea (MNU). Female OM rats are highly susceptible to both of these carcinogens developing greater than 5 mammary adenocarcinomas per rat following a single exposure to either chemical. In contrast, female COP rats are completely resistant to both DMBA and MNU mammary cancer induction. Genetic breeding analysis of the F1 and F2 hybrids produced by crossing COP to OM rats demonstrated that the resistance of the female COP rat to DMBA and MNU is due to the presence of a single dominant autosomal allele in the germ line of the COP rat. Transplantation experiments demonstrated that the site of action of this COP gene is within the mammary epithelial cells themselves, not systemically or at the local mammary gland level within nonepithelial mammary cells. The resistance to DMBA-induced mammary carcinogenesis affected by the COP gene does not involve prevention of the initial interaction of DMBA with the mammary epithelial cells, but suppression of the progression of these initiated mammary cells to full cancer. This suppression does not involve paracrine release of diffusible factor(s). This gene does not suppress the development of MNU-induced renal or bladder cancers in the COP female rats. Thus, this newly identified autosomal dominant gene is specifically a mammary cancer suppressor gene. Analysis of the response of female feral rats to DMBA or MNU exposure demonstrates that this mammary cancer suppressor gene is also functional in feral rats. This suggests that this mammary cancer gene is functionally inactivated either by mutation or deletion in the germ line of highly susceptible strains of rats like the OM and inbred Sprague-Dawley rats, but functionally retained in resistant strains like the COP.