Abstract
Folic acid and natural folates are B group vitamins obtained exclusively from the diet. Biochemically, folate derivatives participate in the transfer of single carbon units, acting as methyl group donors for numerous cellular processes, including the addition of methyl moieties to genomic CpG dinucleotides, a process termed DNA methylation. This is an important epigenetic mechanism that contributes to the control of gene expression and maintenance of genomic integrity. Low folate intake can result in hypomethylation of DNA, which can disrupt gene expression and promote genetic instability. Genomic hypomethylation is associated with the progression of numerous diseases, including multiple cancers and atherosclerosis. Conversely, limited evidence suggests excessive folate intake may also enhance development of certain carcinomas.
The enzyme methylenetetrahydrofolate reductase (MTHFR) commits folate-bound one-carbon moieties to the DNA methylation pathway. A common enzyme variant of reduced catalytic function may modify the relationship between dietary folate intake, DNA methylation and disease propensity. However, there is a paucity of studies in humans that wholly link these events. Additionally, New Zealand is considering introducing mandatory folic acid fortification to reduce the incidence of neural tube defects. Accordingly, the current study sought to assess changes in DNA methylation as a result of modulated folate intakes in a normal, healthy New Zealand population, which might infer a propensity towards disease.
Peripheral blood was obtained from 93 participants (equally matched by number, age and sex across all MTHFR genotypes) following a thirteen week folic acid avoidance period and a subsequent eleven week folic acid supplementation period (400 µg folic acid/day). Mean plasma folate fell across all MTHFR genotypes by 7 nmol/L (p < 0.001) and increased by 26 nmol/L (p < 0.001) following folic acid avoidance and supplementation, respectively. Total homocysteine, an inverse and intermediate indicator between folate status and DNA methylation, decreased significantly following supplementation (p < 0.001). This was most pronounced in individuals homozygous for the MTHFR polymorphism with a low folate status (< 12 nmol/L) during the avoidance period.
Despite global hypomethylation being observed under similar conditions by other researchers, no significant change in leukocyte DNA total methylation was found in the current study, either between MTHFR genotypes, or between folate avoidance and supplementation periods. This was measured using high performance liquid chromatography and mass spectrometry methods.
Microarray analysis of leukocyte-derived RNA revealed a small number of genes differentially expressed between genotypes and dietary regimes; however fold change was slight and follow-up analysis indicated none were probable disease candidates. The methylation status of repetitive satellite 2 elements was queried using a methylation-sensitive restriction digest followed by quantitative PCR. Participants homozygous for the MTHFR polymorphism displayed a small but significantly lower level of Sat2 methylation. This difference was not observed following supplementation.
These results suggested a negligible DNA methylation response to modulated folate status and no indication of disease propensity – DNA methylation is particularly immune from medium term perturbations to folate intake in healthy New Zealanders.