The methylenetetrahydrofolate reductase C677T mutation induces cell-specific changes in genomic DNA methylation and uracil misincorporation: a possible molecular basis for the site-specific cancer risk modification

Abstract

The C677T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene is associated with a decreased risk of colon cancer although it may increase the risk of breast cancer. This polymorphism is associated with changes in intracellular folate cofactors, which may affect DNA methylation and synthesis via altered one-carbon transfer reactions. We investigated the effect of this mutation on DNA methylation and uracil misincorporation and its interaction with exogenous folate in further modulating these biomarkers of one-carbon transfer reactions in an in vitro model of the MTHFR 677T mutation in HCT116 colon and MDA-MB-435 breast adenocarcinoma cells. In HCT116 cells, the MTHFR 677T mutation was associated with significantly increased genomic DNA methylation when folate supply was adequate or high; however, in the setting of folate insufficiency, this mutation was associated with significantly decreased genomic DNA methylation. In contrast, in MDA-MB-435 cells, the MTHFR 677T mutation was associated with significantly decreased genomic DNA methylation when folate supply was adequate or high and with no effect when folate supply was low. The MTHFR 677T mutation was associated with a nonsignificant trend toward decreased and increased uracil misincorporation in HCT116 and MDA-MB-435 cells, respectively. Our data demonstrate for the first time a functional consequence of changes in intracellular folate cofactors resulting from the MTHFR 677T mutation in cells derived from the target organs of interest, thus providing a plausible cellular mechanism that may partly explain the site-specific modification of colon and breast cancer risks associated with the MTHFR C677T mutation.

Figure 1

Simplified scheme of the role of 5,10-methylenetetrahydrofolate reductase (MTHFR) in folate metabolism and one-carbon transfer reactions involved in DNA synthesis and biological methylation reactions, including that of DNA. MTHFR catalyzes the irreversible conversion of 5,10-methylenetetrahydrofolate (5,10-methyleneTHF) to 5-methyltetrahydrofolate (5-methylTHF), and hence the MTHFR C677T polymorphism, which results in decreased MTHFR activity and increased thermolability of MTHFR, leads to lower levels of 5-methylTHF and an accumulation of 5,10-methyleneTHF. SAM is both an allosteric inhibitor of MTHFR and an activator of cystathionine β-synthase. B12, vitamin B-12; BHMT, betaine:homocysteine methyltransferase; CβS, cystathionine β-synthase; CH₃, methyl group; CpG, cytosine-guanine dinucleotide sequence, DHF, dihydrofolate; DHFR, dihydrofolate reductase; DNMT, DNA methyltransferase; MS, methionine synthase; SAH, S-adenosylhomocysteine; SAHH, S-adenosylhomocysteine hydrolase; SAM, S-adenosylmethionine; SHMT, serine hydroxymethyltransferase; THF, tetrahydrofolate; TS, thymidylate synthase.