Vitamin B12 and folate are essential co-enzymes in the remethylation of homocysteine to methionine, thereby ensuring the supply of S-adenosylmethionine (SAM), the major methyl group donor for biological methylation, including DNA and histones. Diets deficient in methyl group donors (vitamin B12, folate and methionine) are known to increase the risks of various cancers in both humans and rodents, and have been consistently observed to induce DNA hypomethylation. This hypomethylation is thought to be responsible for the induction of some cancers. Few studies have been conducted on the effect of methyl group deficiency on histone methylation, which, like DNA methylation, plays an important role in the regulation of gene expression. In our studies, we focused on the effect of methyl group deficiency on histone H3-lysine 9 (K9) methylation in several mouse models, which were defective in either folate metabolism or histone H3-K9 methylation.

We investigated the effects of vitamin B12 deficiency on mice heterozygous for deletion of methionine synthase (MS) or methylene-tetrahydrofolate reductase (MTHFR). We found that vitamin B12 depletion significantly increased plasma homocysteine levels in both mouse models, but it did not reduce histone H3-K9 methylation in major mouse tissues, including liver, brain and kidney. In the MTHFR heterozygous mouse, histone H3-K9 was not hypomethylated, although DNA hypomethylation has been observed. Microarray analysis showed that B 12-deficiency caused changes in the expression of many genes in MS and MTHFR heterozygous mouse brain, but not in wild type mice. Among the subset of induced and repressed genes were a number encoding lipoproteins, proteins involved in iron metabolism and stress related proteins.

We also characterized a new mouse model in which the euchromatic histone methyltransferase 1 (GLP) was knocked down, and studied the influence of methyl group deficiency on histone H3-K9 methylation in this model. GLP knockdown animals died soon after birth. Methionine and folate deficiency did not affect histone H3-K9 methylation in primary mouse embryonic fibroblasts (PMEF) derived from the animals, although it has been shown previously that folate depletion alone was able to trigger DNA hypomethylation under similar conditions. Previous studies found that GLP was the major histone H3-K9 mono- and di- methyltransferase in ES cells. Our data indicated that GLP deficiency did not influence global histone H3-K9 methylation in fetal liver in later development stages, and that its functional importance may be tissue-specific in the mouse.