Methyl deficiency, DNA methylation, and cancer: Studies on the reversibility of the effects of a lipotrope-deficient diet

Abstract

Methylation of C residues in CpG sites in the regulatory regions of a wide variety of genes has been linked to silencing of their expression. During normal mammalian development, loss of methylation at specific sites accompanies tissue-specific activation of genes. Overall decreases in the level of DNA methylation and alterations in the pattern of methylation of specific genes are also closely linked to tumor development in humans and other mammals. Dietary methyl deficiency sufficient to cause hepatocarcinogenesis in male rats induces profound and rapid changes in the morphology and metabolic activity of liver cells. As we have previously reported, these changes include a decrease in the overall level of DNA methylation and alternations in the patterns of methylation and levels of transcripts of specific growth-related genes. These alterations persist as long as the rats are maintained on a methyl-deficient diet. The starting hypothesis for the studies summarized here is that methyl deficiency induced changes in liver cells that persist, even when dietary sources of methyl groups are restored, are more likely to be critical for establishment of neoplasia than those that are reversible. We find that loss of methylation at specific sites in liver DNA persists for at least 9 weeks after restoration of methionine, choline, folate, and vitamin B 12 to the diet of rats previously deprived of these nutrients for 4 weeks. Other molecular changes are reversed in less than 3 weeks. This suggests that exposure of rats to alternating periods of dietary methyl deficiency and sufficiency may provide an experimental model for determining whether persistent alterations in methylation of growth regulatory genes allow affected hepatocytes to escape constraints on cell division because they respond to growth stimuli differently than cells in which the genes are normally methylated.