Abstract
We studied demethylation within the transgene promoter in transgenic mice carrying the N‐ras proto‐onco‐gene driven by the mouse mammary tumor long terminal repeat (MMTV/N‐ras^N^) and the relationship of demethy!ation to transgene overexpression and tumorigenesis. Demethylation at Fspl or Clal sites correlated with age of the animal and transgene expression in nontumorous mammary gland. Demethylation preceded expression in this tissue. In lymphomas and mammary tumors, the promoter Fspl and Clal sites were significantly more demethylated than in nontumorous control tissues. The Aval, Cfol, and Hpall sites were also found to be undermethylated in older animals and showed differences between tumor and control tissues. Two additional sites (Eagl and Narl) remained fully methylated in all tissues. In contrast with normal tissue, demethylation at the Fspl and Clal sites and expression were not correlated in tumor tissue. An increase in expression in normal tissue initially occurred and was correlated with the level of promoter demethylation; this increase was followed by a further increment in transgene expression when tumors developed. Thus, promoter demethylation leading to transgene overexpression was associated with long‐latency tumorigenesis in MMTV/N‐ras^N^ transgenic mice. Demethylation of proto‐oncogene promoters may therefore be a mechanism of carcinogenesis that requires further investigation in human tumors.