Altered folate metabolism and disposition in mothers affected by a spina bifida pregnancy: Influence of 677c -> t methylenetetrahydrofolate reductaseand 2756a -> g methionine synthase genotypes

Periconceptional folate prevents spina bifida although the mechanisms invol ved are unclear. We present the genotype frequency for the 677 ct methylene tetrahydrofolate 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 mu tant 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 carriag e of t and g alleles in NTD are 0.89 (0.28-2.82), 0.97 (0.28-3.30), and 0.6 1 (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 previ ous studies, we could not detect a significantly increased risk for NTD con ferred by the 677ct MTHFR tt genotype; OR 0.98 (0.19-6.49). Differences wer e found to exist in the circulating whole blood folate profile: total formy l-H(4)PteGlu was significantly higher than total 5-methyl-H(4)PteGlu in con trol (P = 0.036) but not NTD blood. When broken down into the various 677 c t MTHFR and 2756ag MetSyn genotypes, carriage of the 677ct MTHFR allele app ears to affect formyl-H(4)PteGlu metabolism in non-NTD mothers. In addition , NTD mothers exhibited noticeably lower formyl-H(4)PteGlu levels compared to controls; these effects, however, were not significant. 2756ag MetSyn is similarly associated with an altered formyl-H(4)PteGlu disposition. The ag genotype had significantly more formyl-H(4)PteGlu relative to 5-methyl-H(4 )PteGlu than wildtype 2756ag MetSyn (P = 0.024). This heterozygous increase in the relative formyl-H(4)PteGlu level holds true for controls only; no s uch relationship occurred in NTD samples. Folyl hexaglutamates are the acti ve cellular coenzyme forms. We showed that where 5-methylH(4)PteGlu(6) pred ominates, Hey levels are highest. As the relative abundance of formyl-H(4)P teGlu(6) increased, so Hey decreased, presumably due to increased Hey remet hylation, a process in which 5-methyl-H(4)PteGlu(6) is demethylated and dow nstream folates like formyl-H(4)PteGlu(6) are produced. The negative linear association between the hexaglutamate ratio (formyl-H(4)PteGlu(6)/5-methyl -H(4)PteGlu(6)) and Hcy is significant for control (r = -0.64, P = 0.003) b ut not NTD samples. This effect, centering on Hey remethylation, is support ed by a statistically elevated formyl-H(4)PteGlu(6) to 5-methylH(4)PteGlu(6 ) level in controls relative to NTDs (P = 0.047). The overall (polymorphism independent) effect of exogenous 5,10-methenyl-H(4)PteGlu(1) substrate on the cellular folate profile was to preferentially increase formyl-H(4)PteGl u, while exogenous 5-methyl-H(4)PteGlu(1) substrate dramatically increased metabolic production of 5,10-methylene-H(4)PteGlu. The following difference s were observed between NTD and control samples: (i) a reduced expansion of the formyl-H(4)PteGlu(6) pool in NTD with exogenous 5,10-methenyl-H(4)PteG lu(1) (P = 0.0005 for control expansion, NS for NTD increase); (ii) a reduc ed initial expansion of the 5,10-methylene-H(4)PteGlu pool in NTD following treatment with exogenous 5-methyl-H(4)PteGlu(1) substrate (difference betw een subject groups; P = 0.031). In addition, taking polymorphisms into account, lysate from NTD-MTHFR wild- types utilized less exogenous 5-methyl-H(1)PteGlu(1) substrate than control -MTHFR wildtypes in the short (P = 0.011) and long term (P = 0.036). Commen surate with this latter effect, the initial production of 5,10-methglene-H( 4)PteGlu due to exogenous 5-methyl-H(4)PteGlu(1) substrate was significantl y reduced in the NTD-MTHFR wildtype (P = 0.037). These two MTHFR wildtype e ffects imply that the 677 ct polymorphism is not the only mutation affectin g folate metabolism in NTD mothers, The effect of this non-677ct MTHFR muta tion could be to reduce 1-C: channeling into nucleotides, particularly puri nes, Our data are not inconsistent with the remethylation cycle news (B-12/ MetSyn/MSR) being involved in NTD, although other sites in 1-C metabolism c annot be ruled out, particularly those responsible for maintaining the form ylfolate pool. (C) 2000 Academic Press.