Alterations in expression and methylation of specific genes in livers of rats fed a cancer promoting methyl-deficient diet

Abstract

We have reported earlier that hypomethylated DNA is rapidly induced in the livers of male Fischer rats fed an extremely methyl-deficient diet (MDD). The early effects of dietary methyl deficiency on the expression of several genes in the livers of such animals have now been investigated. Poly(A)+ RNA was isolated from the livers of rats fed MDD or a similar diet supplemented with adequate supplies of choline, methionine, folic acid and vitamin B12 (CSD) for periods ranging from 1 to 4 weeks. The levels of mRNAs for the c-myc and c-fos protooncogenes in livers of rats given either MDD or the liver carcinogen, 2-acetylaminofluorene (AAF), were compared by Northern blot analysis with those in livers of animals given control diets. Both AAF and MDD induced significant elevations in levels of mRNAs specific for these two genes. After 1 week of MDD intake, large increases in the levels of c-myc and c-fos mRNAs and a smaller increase in the levels of c-Ha-ras mRNAs were observed. In contrast, there were marked decreases in the levels of mRNAs for epidermal growth factor receptor and for epidermal growth factor. These effects on mRNA accumulation persisted and were further enhanced during a 4 week period of MDD feeding. The appearance of hypomethylated DNA in the livers of these MDD-fed rats coincided with the observed changes in levels of mRNA for these genes associated with the regulation of cell growth. Increases in levels of mRNA for c-fos, c-Ha-ras and c-myc were correlated with loss of methylation at specific sites within these genes as early as 1 week after the start of MDD feeding. These combined observations are consistent with the hypothesis that methyl-deficient diets are cancer promoting and/or carcinogenic, at least in part, because they induce hypomethylation of DNA with concomitant alterations in the regulation of gene expression.