Folate is a generic term for a naturally occurring family of B-group vitamins composed of an aromatic pteridine ring linked to p
-aminobenzoic acid and a glutamate residue (Shane, 1995
). Following absorption in the small intestine dietary food folates undergo hydrolysis to methyltetrahydrofolate (methylTHF), the predominant form of folate in plasma (Shane, 1995
). Folic acid also undergoes conversion to methylTHF, but this process becomes saturated at doses of ~270μ
g and at higher levels folic acid is transported directly into the plasma (Sweeney et al, 2007
). Hence, daily ingestion of 400μ
g, a dose commonly used in supplements, produces a sustained level of plasma folic acid.
Cellular folates act as donors and acceptors of methyl groups in the biosynthesis of nucleotide precursors used for DNA synthesis, and provision of methyl groups for methylation of DNA, RNA, and proteins (; Shane, 1995
). These important cellular processes lie at opposite ends of folate metabolism linked by methylenetetrahydrofolate reductase (MTHFR) which catalyses the irreversible conversion of 5,10-methyleneTHF to 5-methylTHF. The MTHFR substrate, 5,10-methyleneTHF, is also a substrate for the thymidylate synthase (TS) enzyme in the methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), which is the sole de novo
source of thymidine and the rate limiting step in DNA synthesis in mammalian cells (Choi and Mason, 2002
). 5,10-methyleneTHF is also used in the production of formylTHF, which is in turn used in de novo
purine synthesis. The MTHFR product, 5-methylTHF, is the methyl group donor for the remethylation of homocysteine to methionine catalysed by the enzyme methionine synthase (MTR). Methionine is adenylated to form S
-adenosylmethionine (SAM), which is the methyl group donor in methylation reactions, whereas SAM inhibits the MTHFR enzyme providing a negative feedback control loop.
Figure 1 Schematic representation of folate metabolism illustrating the entry of natural folates and folic acid into the pathway, and flow of methyl groups towards either DNA synthesis or DNA methylation. RFC, reduced folate carrier; hFR, human folate receptor; (more ...)
Variation in the distribution of methyl groups through altered folate metabolism impacts on both DNA methylation and DNA synthesis, two crucial cellular processes in relation to neoplastic transformation (Choi and Mason, 2002
). Methylation of the cytosine residues of cytosine–guanine dinucleotide pairs is an important epigenetic determinant of gene expression and also has a role in maintaining DNA stability. Hypermethylation of gene promoter regions results in loss of tumour suppressor gene function, whereas reduced ‘global' methylation results in chromosomal instability and an increase in mutational events (Eden et al, 2003
). Long-term dietary folate deficiency in humans results in global DNA hypomethylation in lymphocytes, which is reversible on repletion of folate status (Jacob et al, 1998
). Paradoxically studies in rodents have shown promoter-specific hypermethylation increases in response to folate deficiency (Pogribny et al, 1997
). Through a reduction in the availability of methyl groups for methylation of homocysteine to methionine, folate deficiency leads to a rise in homocysteine levels which in turn results in raised levels of S
-adenosylhomocysteine (SAH). S
-adenosylhomocysteine is an inhibitor of methylation reactions, and this is thought to underlie the reduced global DNA methylation observed in individuals with low folate status.
Reduced deoxythymidylate synthesis leading to uracil misincorporation during DNA synthesis provides a second mechanism through which folate deficiency can disrupt DNA integrity and promote carcinogenesis. Removal of a uracil base involves creation of a single-strand DNA break, and where two adjacent uracil bases lie on opposite strands a double-strand DNA break will occur which are difficult to repair and are associated with an increased cancer risk (McKinnon and Caldecott, 2007
). Increased DNA uracil content and chromosomal breaks occur in folate-deficient humans, and both can be reversed by restoration of adequate folate status (Blount et al, 1997
The understanding that folate metabolism can reciprocally influence both DNA synthesis and methylation has made environmental and genetic variants that impact on this pathway attractive candidates as cancer susceptibility factors. These include dietary intakes of folate and folic acid, and functional polymorphisms in the genes coding for folate metabolism enzymes.