The effect of dietary folate on genomic and p53-specific DNA methylation in rat colon.

Folate is an important mediator in the transfer of methyl groups for DNA methylation, abnormalities of which are considered to play an important mechanistic role in colorectal carcinogenesis. This study investigated the time-dependent effects of dietary folate on genomic and p53 (in the promoter region and exons 6-7) DNA methylation in rat colon, and how these changes are related to steady-state levels of p53 transcript. Despite a marked reduction in plasma and colonic folate concentrations, a large increase in plasma homocysteine (an accurate inverse indicator of folate status), and a progressive decrease in colonic S-adenosylmethionine (SAM; the primary methyl donor for methylations) to S-adenosylhomocysteine (SAH; a potent inhibitor of methylations) ratio, isolated folate deficiency did not induce significant genomic DNA hypomethylation in the colon. Paradoxically, isolated folate deficiency increased the extent of genomic DNA methylation in the colon at an intermediate time point (P = 0.022). Folate supplementation did not modulate colonic SAM, SAH and SAM to SAH ratios, and genomic DNA methylation at any time point. The extent of p53 methylation in the promoter and exons 6-7 was variable over time at each of the CpG sites examined, and no associations with time or dietary folate were observed at any CpG site except for site 1 in exons 6-7 at week 5. Dietary folate deprivation progressively decreased, whereas supplementation increased, steady-state levels of p53 transcript over 5 weeks (P < 0.05). Steady-state levels of p53 mRNA correlated directly with plasma and colonic folate concentrations (P = 0.41-0.49, P < 0.002) and inversely with plasma homocysteine and colonic SAH levels (r = -0.37-0.49, P < 0.006), but did not significantly correlates with either genomic or p53 methylation within the promoter region and exons 6-7. The data indicate that isolated folate deficiency, which significantly reduces steady-state levels of colonic p53 mRNA, is not associated with a significant degree of genomic or p53 DNA hypomethylation in rat colon. This implies that neither genomic or p53 hypomethylation within exons 6-7 nor aberrant p53 methylation within the promoter region is likely a mechanism by which folate deficiency enhances colorectal carcinogenesis in the rat.