Methylenetetrahydrofolate reductase C677T polymorphism does not alter folic acid deficiency-induced uracil incorporation into primary human lymphocyte DNA in vitro

Abstract

MnTHF is available to provide methyl groups for the conversion of uracil to thymidine. Folic acid deficiency homocysteine concentrations (4,7,8), which is thought to be causes the intracellular accumulation of dUMP and the due to the inefficient recycling of homocysteine to methionine. subsequent incorporation of uracil into DNA. The removal The TT genotype is associated with an elevated risk for of uracil from DNA may result in double-stranded DNA cardiovascular disease (8,9), which may be linked to homocyst- breaks, the accumulation of which is a putative risk factor eine. In addition, TTs are reported to have a 2.8-fold higher for cancer. We tested whether human lymphocytes taken risk for endometrial cancer (10), a 2.7- and 2.8-fold higher from TTs (n 10) were more able to resist uracil incorpora- risk for Crohn's disease and ulcerative colitis, respectively tion into DNA than controls (n 14 CCs and 6 CTs) when (11), and a 2.6-3.2-fold higher risk for Down syndrome in cultured in medium containing 12-120 nM folic acid for 9 offspring (12,13). Various reports also show that homozygotes days. DNA uracil content of these lymphocytes was meas- have a 1.6-, 1.8- or 2.2-fold higher risk for neural tube defects ured by CG-MS. TTs and controls showed a dose-dependent (14-16). increase in DNA uracil content during folic acid deficiency In contrast, various reports document that TTs have a 1.2-, (P < 0.0001, R2 0.23 for TTs and P < 0.0001, R2 0.19 1.7- and 3.0-fold reduced risk for colorectal cancer (carcinoma) for controls). DNA uracil content was not different between (17-19). Despite this evidence relating to carcinomas, the TT the two groups at any of the folic acid concentrations (two- genotype does not seem to afford any protection against the way ANOVA: media (folic acid), P < 0.0001; genotype, P formation of colorectal adenomas (20,21) and may even 0.4). The results show that, in this in vitro system, the increase risk for adenoma if dietary intakes of folic acid, MTHFR C677T polymorphism does not affect the cell's vitamin B12, vitamin B6 and methionine are low (22). Recently, ability to resist uracil incorporation into DNA. Chromo- the TT genotype has also been associated with a 4.3-fold some breakage, as measured by micronuclei, was also reduction in the risk for developing acute lymphocytic leukemia shown to correlate with folic acid concentration in a (23). It is hypothesized that the reduced risk of some cancers preliminary experiment (P < 0.0001). Although the results afforded by the polymorphism is due to a diversion of folic appear not to support the hypothesis that a reduced risk acid to thymidine synthesis. Reduced MTHFR activity is for certain cancers in TTs is due to diversion of folic acid thought to increase the intracellular concentration of 5,10- to thymidine synthesis, differences between the in vivo and MnTHF and, hence, the amount of methyl groups available in vitro situation make this conclusion not definitive. for the conversion of dUMP to dTMP (Figure 1).