5-methyldeoxycytidine and Precancerous-Conditions

The protooncogene c-myc was investigated in N-nitrosomorpholine-induced rat liver nodules to elucidate the role of altered DNA methylation in chemical carcinogenesis. Furthermore, Micrococcus luteus DNA and chicken erythrocyte DNA were modified in vitro by reactive metabolites of N-nitrosomorpholine, generated by P450-dependent monooxygenases. The modified DNAs were less methylated in vitro than control DNAs by DNA-(cytosine-5)-methyltransferase (DNA methylase). The DNA methylase assay and 32P-postlabeling analysis revealed lowered levels of DNA methylation in nodular DNA. In nodular tissue, c-myc messenger RNA levels were found to be increased compared to normal liver. DNA methylation analysis using the restriction endonucleases HpaII/MspI indicated hypomethylation in the first intron of c-myc DNA in liver nodules. The results suggest that genotoxic lesions may cause stably inherited, aberrant DNA methylation patterns which may be responsible for site-specific hypomethylation of the c-myc protooncogene in liver nodules.

Topics: Animals; Deoxycytidine; DNA Modification Methylases; DNA, Bacterial; DNA, Neoplasm; Liver Neoplasms; Male; Methylation; Nitrosamines; Precancerous Conditions; Proto-Oncogenes; Rats; Rats, Inbred Strains; RNA, Neoplasm

DNA from hepatocyte nodules induced in rats with dietary DL-ethionine and from the surrounding non-nodular liver contained less 5-methyldeoxycytidine per deoxycytidine when compared with that from normal adult liver. The degree of apparent hypomethylation, 37% in nodules and 20% in the surrounding liver, decreased somewhat (29% and 16% respectively) at 2 weeks after terminating the exposure to ethionine. Nodules and surrounding liver, like normal liver, responded to partial hepatectomy with a decrease in the 5-methyldeoxycytidine level at 24 hrs and a return to the level at the time of partial hepatectomy by 38 hrs. These findings indicate the need for careful control of cell proliferation in comparing the levels of a post-replicative DNA modification, methylation, in proliferating and non-proliferating cell populations. These findings also suggest that a portion of the hypomethylation in preneoplastic nodules may be due to a bona fide decrease in the level of cytosine methylation in the parental strand of DNA. This hypomethylation could be one basis for the altered gene expression in hepatocyte nodules, possible precursors for liver cancer.

Topics: Animals; Deoxycytidine; DNA; Ethionine; Liver; Liver Neoplasms, Experimental; Male; Methylation; Precancerous Conditions; Rats; Rats, Inbred F344; Reference Values