Altered Folate Metabolism and Disposition in Mothers Affected by a Spina Bifida Pregnancy: Influence of 677c → t Methylenetetrahydrofolate Reductase and 2756a → g Methionine Synthase Genotyp
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文摘
Periconceptional folate prevents spina bifida although the mechanisms involved are unclear. We present the genotype frequency for the 677 ct methylenetetrahydrofolate reductase (MTHFR) and 2756ag methionine synthase (MetSyn) polymorphisms. Calculated odds ratios (OR) show that neither the homozygous recessive genotype, carriage of the mutant allele, nor frequency of the mutant allele represent significantly increased risk for neural tube defect (NTD). This is true for both polymorphisms. Simultaneous carriage of t and g alleles is also not a significantly increased risk for NTD. OR and 95 % CI for carriage of (i) t allele, (ii) g allele, and (iii) simultaneous carriage of t and g alleles in NTD are 0.89 (0.28–2.82), 0.97 (0.28–3.30), and 0.61 (0.11–3.52), respectively. OR and 95 % CI for frequency of t and g alleles are 0.94 (0.42–2.13) and 0.88 (0.29–2.67), respectively. Unlike some previous studies, we could not detect a significantly increased risk for NTD conferred by the 677ct MTHFR tt genotype; OR 0.98 (0.19–6.49). Differences were found to exist in the circulating whole blood folate profile: total formyl-H4PteGlu was significantly higher than total 5-methyl-H4PteGlu in control (P = 0.036) but not NTD blood. When broken down into the various 677 ct MTHFR and 2756ag MetSyn genotypes, carriage of the 677ct MTHFR allele appears to affect formyl-H4PteGlu metabolism in non-NTD mothers. In addition, NTD mothers exhibited noticeably lower formyl-H4PteGlu levels compared to controls; these effects, however, were not significant. 2756ag MetSyn is similarly associated with an altered formyl-H4PteGlu disposition. The ag genotype had significantly more formyl-H4PteGlu relative to 5-methyl-H4PteGlu than wildtype 2756ag MetSyn (P = 0.024). This heterozygous increase in the relative formyl-H4PteGlu level holds true for controls only; no such relationship occurred in NTD samples. Folyl hexaglutamates are the active cellular coenzyme forms. We showed that where 5-methyl-H4PteGlu6 predominates, Hcy levels are highest. As the relative abundance of formyl-H4PteGlu6 increased, so Hcy decreased, presumably due to increased Hcy remethylation, a process in which 5-methyl-H4PteGlu6 is demethylated and downstream folates like formyl-H4PteGlu6 are produced. The negative linear association between the hexaglutamate ratio (formyl-H4PteGlu6/5-methyl-H4PteGlu6) and Hcy is significant for control (r = −0.64, P = 0.003) but not NTD samples. This effect, centering on Hcy remethylation, is supported by a statistically elevated formyl-H4PteGlu6 to 5-methyl-H4PteGlu6 level in controls relative to NTDs (P = 0.047). The overall (polymorphism independent) effect of exogenous 5,10-methenyl-H4PteGlu1 substrate on the cellular folate profile was to preferentially increase formyl-H4PteGlu, while exogenous 5-methyl-H4PteGlu1 substrate dramatically increased metabolic production of 5,10-methylene-H4PteGlu. The following differences were observed between NTD and control samples: (i) a reduced expansion of the formyl-H4PteGlu6 pool in NTD with exogenous 5,10-methenyl-H4PteGlu1 (P = 0.0005 for control expansion, NS for NTD increase); (ii) a reduced initial expansion of the 5,10-methylene-H4PteGlu pool in NTD following treatment with exogenous 5-methyl-H4PteGlu1 substrate (difference between subject groups; P = 0.031). In addition, taking polymorphisms into account, lysate from NTD-MTHFR wildtypes utilized less exogenous 5-methyl-H4PteGlu1 substrate than control-MTHFR wildtypes in the short (P = 0.011) and long term (P = 0.036). Commensurate with this latter effect, the initial production of 5,10-methylene-H4PteGlu due to exogenous 5-methyl-H4PteGlu1 substrate was significantly reduced in the NTD–MTHFR wildtype (P = 0.037). These two MTHFR wildtype effects imply that the 677 ct polymorphism is not the only mutation affecting folate metabolism in NTD mothers. The effect of this non-677ct MTHFR mutation could be to reduce 1-C channeling into nucleotides, particularly purines. Our data are not inconsistent with the remethylation cycle nexus (B12/MetSyn/MSR) being involved in NTD, although other sites in 1-C metabolism cannot be ruled out, particularly those responsible for maintaining the formylfolate pool.