Analysis of the MTHFR 1298A-->C and 677C-->T polymorphisms as risk factors for neural tube defects

Dynamic inter-domain transformations mediate the allosteric regulation of human 5, 10-methylenetetrahydrofolate reductase

5,10-methylenetetrahydrofolate reductase (MTHFR) commits folate-derived one-carbon units to generate the methyl-donor S-adenosyl-L-methionine (SAM). Eukaryotic MTHFR appends to the well-conserved catalytic domain (CD) a unique regulatory domain (RD) that confers feedback inhibition by SAM. Here we determine the cryo-electron microscopy structures of human MTHFR bound to SAM and its demethylated product S-adenosyl-L-homocysteine (SAH). In the active state, with the RD bound to a single SAH, the CD is flexible and exposes its active site for catalysis. However, in the inhibited state the RD pocket is remodelled, exposing a second SAM-binding site that was previously occluded. Dual-SAM bound MTHFR demonstrates a substantially rearranged inter-domain linker that reorients the CD, inserts a loop into the active site, positions Tyr404 to bind the cofactor FAD, and blocks substrate access. Our data therefore explain the long-distance regulatory mechanism of MTHFR inhibition, underpinned by the transition between dual-SAM and single-SAH binding in response to cellular methylation status.

Metabolic Analysis of Methylenetetrahydrofolate Reductase Single Nucleotide Polymorphisms (MTHFR 677C<T and MTHFR 1298A<C), Serum Folate and Vitamin B12 in Neural Tube Defects

Neural tube defects (NTDs) are among the most prevalent and debilitating birth defects with their causes are still unknown, despite mounting evidence that genetic and/or environmental factors may play a role. We aimed to analyze two single nucleotide polymorphisms of methylenetetrahydrofolate reductase (MTHFR) gene, serum folate and vitamin B12 status among a cohort of Egyptian children with NTDs and their mothers. A case-control study has been conducted on 50 Egyptian children with various types of NTDs and their mothers. They were comparable with 50 unrelated healthy, age and sex matched children and their mothers (50) selected as controls. Pediatric and neurosurgical assessments were performed to the included cases. Serum folate and vitamin B12 were measured using ELISA kits. MTHFR 677C Collapse

Key Words

• Folic acid
• Neural tube defects
• Vitamin B12
• rs1801131
• rs1801133

Collapse

MESH Headings Collapse

Grants Collapse

Affiliation(s)

Mohammed H. Hassan Department of Medical Biochemistry, Faculty of Medicine, South Valley University, Qena, 83523 Egypt
Mohamed A. Raslan Department of Chemistry, Faculty of Science, Aswan University, Aswan, 81528 Egypt
Mena Tharwat Department of Chemistry, Faculty of Science, Aswan University, Aswan, 81528 Egypt
Hala M. Sakhr Department of Pediatrics, Faculty of Medicine, South Valley University, Qena, 83523 Egypt
Eslam El-Sayed El-Khateeb Department of Neurosurgery, Faculty of Medicine, South Valley University, Qena, 83523 Egypt
Shimaa Fathy Sakr Molecular Biology Unit, Medical Technology Center, Medical Research Institute, Alexandria University, Alexandria, Egypt
Hesham H. Ameen Department of Clinical Pathology, Faculty of Medicine, Al-Azhar University (Assiut Branch), Assiut, Egypt
Ali R. Hamdan Department of Neurosurgery, Faculty of Medicine, South Valley University, Qena, 83523 Egypt

Collapse

Polymorphisms in Maternal Selected Folate Metabolism-Related Genes in Neural Tube Defect-Affected Pregnancy

Background

Neural tube defects (NTDs) are abnormalities of the brain and spinal cord, which occur as a result of failure in neural tube closure during embryogenesis. Causes of NTDs are complex and multiple, with hereditary, lifestyle, and environmental factors appearing to play a role. In spite of their impact on public health, the role genetics play on NTDs in Ethiopia is lacking. In this study, the role of polymorphisms in MTHFR 677C > T (rs1801133), MTHFR 1298A > C (rs1801131), MTRR 66A > G (rs1801394), RFC1 80A > G (rs1051266), and TCN2 776C > G (rs1801198) on the risk of having NTD-affected pregnancy was investigated.

Materials and Methods

One hundred women with NTD-affected pregnancy and 100 women with normal pregnancy were included in the study. DNA was extracted from saliva and genotyping for five polymorphisms in four genes was analyzed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). The departure of the genotype's distribution from Hardy-Weinberg equilibrium (HWE) was evaluated using the x2 goodness-of-fit test. Frequencies of genotypes and alleles in case and control mothers were determined and differences between relative frequencies were evaluated by the x2 or the Fisher's exact test.

Results

The statistically significant difference was absent in the genotype and allele frequencies for all the analyzed polymorphisms between cases and controls (P > 0.05).

Conclusion

MTHFR 677C > T, MTHFR 1298A > C, MTRR 66A > G, RFC1 80A > G, and TCN2 776C > G polymorphisms lack association with the risk of having a pregnancy affected by NTD. The role of other genes or environmental factors in NTD etiology needs to be investigated.

Folate Pathway Gene Single Nucleotide Polymorphisms and Neural Tube Defects: A Systematic Review and Meta-Analysis

Neural tube defects (NTDs) are congenital abnormalities in the central nervous system. The exact etiology of NTDs is still not determined, but several genetic and epigenetic factors have been studied. Folate supplementation during gestation is recommended to reduce the risk of NTDs. In this review we examine single nucleotide polymorphisms (SNPs) of the genes in the folate pathway associated with NTD. We reviewed the literature for all papers discussing both NTDs and SNPs in the folate pathway. Data were represented through five different genetic models. Quality assessment was performed using the Newcastle-Ottawa Scale (NOS) and Cohen's Kappa inter-rater coefficient assessed author agreement. Fifty-nine papers were included. SNPs in MTHFR, MTRR, RFC genes were found to be highly associated with NTD risk. NOS showed that high quality papers were selected, and Kappa Q-test was 0.86. Our combined results support the notion that SNPs significantly influence NTDs across the population, particularly in Asian ethnicity. Additional high-quality research from diverse ethnicities is needed and meta-regression analysis based on a range of criteria may provide a more complete understanding of the role of folate metabolism in NTDs.

Association between Fetal MTHFR A1298C (rs1801131) Polymorphism and Neural Tube Defects Risk: A Systematic Review and Meta-Analysis

BACKGROUND

The association of the fetal MTHFR A1298C (rs1801131) polymorphism and neural tube defects (NTDs) susceptibility has been widely demonstrated, but the results remain inconclusive. Thus, we performed a meta-analysis to investigate the association between fetal MTHFR A1298C polymorphism and NTDs risk.

METHODS

An electronic search of PubMed, web of science, SciELO, CNKI database for studies on the fetal MTHFR A1298C polymorphism and NTDs risk was performed up to March 30, 2020.

RESULTS

A total of 22 case-control studies with 3,224 fetuses with NTDs and 3,295 controls were selected. Overall, pooled data showed that the fetal MTHFR A1298C polymorphism was not significantly associated with risk an increased risk of NTDs in the global population. When stratified analysis by ethnicity, country of origin and NTDs type, still no statistically significant association was found.

CONCLUSIONS

Our pooled data emerged no evidence for significant association between fetal MTHFR A1298C polymorphism and NTDs risk.

No evidence for association of MTHFR 677C>T and 1298A>C variants with placental DNA methylation

Background

5,10-Methylenetetrahydrofolate reductase (MTHFR) is a key enzyme in one-carbon metabolism that ensures the availability of methyl groups for methylation reactions. Two single-nucleotide polymorphisms (SNPs) in the MTHFR gene, 677C>T and 1298A>C, result in a thermolabile enzyme with reduced function. These variants, in both the maternal and/or fetal genes, have been associated with pregnancy complications including miscarriage, neural tube defects (NTDs), and preeclampsia (PE), perhaps due to altered capacity for DNA methylation (DNAm). In this study, we assessed the association between MTHFR 677TT and 1298CC genotypes and risk of NTDs, PE, or normotensive intrauterine growth restriction (nIUGR). Additionally, we assessed whether these high-risk genotypes are associated with altered DNAm in the placenta.

Results

In 303 placentas screened for this study, we observed no significant association between the occurrence of NTDs (N = 55), PE (early-onset: N = 28, late-onset: N = 20), or nIUGR (N = 21) and placental (fetal) MTHFR 677TT or 1298CC genotypes compared to healthy pregnancies (N = 179), though a trend of increased 677TT genotype in PE/IUGR together was observed (OR 2.53, p = 0.048). DNAm was profiled in 10 high-risk 677 (677TT + 1298AA), 10 high-risk 1298 (677CC + 1298CC), and 10 reference (677CC + 1298AA) genotype placentas. Linear modeling identified no significantly differentially methylated sites between high-risk 677 or 1298 and reference placentas at a false discovery rate < 0.05 and Δβ ≥ 0.05 using the Illumina Infinium HumanMethylation450 BeadChip. Using a differentially methylated region analysis or separating cytosine-guanine dinucleotides (CpGs) by CpG density to reduce multiple comparisons also did not identify differential methylation. Additionally, there was no consistent evidence for altered methylation of repetitive DNA between high-risk and reference placentas.

Conclusions

We conclude that large-scale, genome-wide disruption in DNAm does not occur in placentas with the high-risk MTHFR 677TT or 1298CC genotypes. Furthermore, there was no evidence for an association of the 1298CC genotype and only a tendency to higher 677TT in pregnancy complications of PE/IUGR. This may be due to small sample sizes or folate repletion in our Canadian population attenuating effects of the high-risk MTHFR variants. However, given our results and the conflicting results in the literature, investigations into alternative mechanisms that may explain the link between MTHFR variants and pregnancy complications, or in populations at risk of folate deficiencies, are warranted.

Electronic supplementary material

The online version of this article (10.1186/s13148-018-0468-1) contains supplementary material, which is available to authorized users.

Variants in maternal COMT and MTHFR genes and risk of neural tube defects in offspring

Methylenetetrahydrofolate reductase (MTHFR) C677T and catechol-O-Methyltransferase (COMT) G158A are associated with a risk of neural tube defects (NTDs) in offspring. This study examined the effect of a MTHFR × COMT interaction on the risk of NTDs in a Chinese population with a high prevalence of NTDs. A total of 576 fetuses or newborns with NTDs and 594 controls were genotyped for MTHFRrs1801133, MTHFRrs1801131, and COMTrs4680 and COMTrs737865. Information on maternal sociodemographic characteristics, reproductive history, and related behavior was collected through face-to-face interviews. Possible interactions between genetic variants of MTHFR and COMT were examined. MTHFR C677T homozygous TT was associated with an elevated risk of total NTDs (odds ratio [OR] = 1.37, 95 % confidence interval [CI] = 0.93-2.03) and of anencephaly (OR = 1.67, 95 % CI = 0.98-2.84) compared with the CC genotype. There was a COMT rs737865 CC × MTHFR rs1801133 TT interaction for total NTDs (OR = 3.02, 95 % CI = 1.00-9.14) and for anencephaly (OR = 3.39, 95 % CI = 0.94-12.18). No interaction was found between COMT rs4680 AA/AG and MTHFR CT/TT genotypes for total NTDs or any subtype of NTD. The interaction of COMT rs737865 and MTHFR C677T was associated with an increased risk of NTDs, especially anencephaly, in a Chinese population with a high prevalence of NTDs.

Association between MTHFR C677T polymorphism and neural tube defect risks: A comprehensive evaluation in three groups of NTD patients, mothers, and fathers

BACKGROUND

The C677T polymorphism in the methylenetetrahydrofolate reductase gene (MTHFR) gene has been reported to play a critical role in the pathogenesis of neural tube defects (NTDs). The association of the C677T polymorphism in the MTHFR gene and NTD susceptibility has been widely demonstrated, but the results are inconclusive. In this study, we performed a meta-analysis in three groups to investigate the association between the MTHFR C677T polymorphism and NTD risk.

METHODS

A computer retrieval of PubMed, Cochrane Library, CBM, and Embase for papers on the MTHFR C677T polymorphism and NTD risk was performed. All data were analyzed with STATA (Version 13.0). Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated to assess the association. A test for heterogeneity, a sensitivity analysis, and an assessment of publication bias were performed in our meta-analysis.

RESULTS

Forty articles were included in this meta-analysis: 13 studies for Group A: 1329 NTD patients versus 2965 healthy controls; 34 studies for Group B: 3018 mothers with NTD progeny versus 8746 healthy controls; three studies for Group C: 157 fathers with NTD progeny versus 705 healthy controls. The analysis results show: allele contrast in NTD patients: OR = 1.445, 95% CI [1.186, 1.760]; allele contrast in mothers: OR = 1.342, 95% CI [1.166, 1.544]; allele contrast in fathers: OR = 1.062, 95% CI [0.821, 1.374].

CONCLUSION

We found no association between any of the fathers' genotypes and NTDs, whereas a significant correlation between MTHFR C677T polymorphism and NTD risk was found in NTD patients and in their mother.

"Polymorphisms in folate metabolism genes as maternal risk factor for neural tube defects: an updated meta-analysis"

Epidemiological studies have evaluated the association between maternal methylenetetrahydrofolate reductase (MTHFR) C677T, A1298C and methionine synthase reductase (MTRR) A66G polymorphisms and risk of neural tube defects (NTDs) in offspring. However, the results from the published studies on the association between these three polymorphisms and NTD risk are conflicting. To derive a clearer picture of association between these three maternal polymorphisms and risk of NTD, we performed meta-analysis. A comprehensive search was conducted to identify all case-control studies of maternal MTHFR and MTRR polymorphisms and NTD risk. We used odds ratios (ORs) with 95% confidence intervals (CIs) to assess the strength of the association. Overall, we found that maternal MTHFR C677T polymorphism (OR(TvsC) =1.20; 95% CI = 1.13-1.28) and MTRR A66G polymorphism (OR(GvsA) = 1.21; 95% CI = 0.98-1.49) were risk factors for producing offspring with NTD but maternal MTHFR A1298C polymorphism (OR(CvsA) = 0.91; 95% CI = 0.78-1.07) was not associated with NTD risk. However, in stratified analysis by geographical regions, we found that the maternal C677T polymorphism was significantly associated with the risk of NTD in Asian (OR(TvsC) = 1.43; 95% CI: 1.05-1.94), European (OR(TvsC) = 1.13; 95% CI: 1.04-1.24) and American (OR(TvsC) = 1.26; 95% CI: 1.13-1.41) populations. In conclusion, present meta-analysis supports that the maternal MTHFR C677T and MTRR A66G are polymorphisms contributory to risk for NTD.

Professor John Scott, folate and neural tube defects

John Scott (1940-2013) was born in Dublin where he was to spend the rest of his career, both as an undergraduate and subsequently Professor of Biochemistry and Nutrition at Trinity College. His research with the talented group of scientists and clinicians that he led has had a substantial impact on our understanding of folate metabolism, mechanisms of its catabolism and deficiency. His research established the leading theory of folate involvement with vitamin B12 in the pathogenesis of vitamin B12 neuropathy. He helped to establish the normal daily intake of folate and the increased requirements needed either in food or as a supplement before and during pregnancy to prevent neural tube defects. He also suggested a dietary supplement of vitamin B12 before and during pregnancy to reduce the risk of neural tube defects. It would be an appropriate epitaph if fortification of food with folic acid became mandatory in the UK and Ireland, as it is in over 70 other countries.

Folate-related gene variants in Irish families affected by neural tube defects

Periconceptional folic acid use can often prevent neural tube defects (NTDs). Variants of genes involved in folate metabolism in mothers and children have been associated with occurrence of NTDs. We identified Irish families with individuals affected by neural tube defects. In these families, we observed that neural tube defects and birth defects overall occurred at a higher rate in the maternal lineage compared with the paternal lineage. The goal of this study was to look for evidence for genetic effects that could explain the discrepancy in the occurrence of these birth defects in the maternal vs. paternal lineage. We genotyped blood samples from 322 individuals from NTD-affected Irish families, identified through their membership in spina bifida associations. We looked for differences in distribution in maternal vs. paternal lineages of five genetic polymorphisms: the DHFR 19 bp deletion, MTHFD1 1958G>A, MTHFR 1298A>C, MTHFR 677C>T, and SLC19A1 80A>G. In addition to looking at genotypes individually, we determined the number of genotypes associated with decreased folate metabolism in each relative (“risk genotypes”) and compared the distribution of these genotypes in maternal vs. paternal relatives. Overall, maternal relatives had a higher number of genotypes associated with lower folate metabolism than paternal relatives (p = 0.017). We expected that relatives would share the same risk genotype as the individuals with NTDs and/or their mothers. However, we observed that maternal relatives had an over-abundance of any risk genotype, rather than one specific genotype. The observed genetic effects suggest an epigenetic mechanism in which decreased folate metabolism results in epigenetic alterations related to the increased rate of NTDs and other birth defects seen in the maternal lineage. Future studies on the etiology of NTDs and other birth defects could benefit from including multigenerational extended families, in order to explore potential epigenetic mechanisms.

Neural tube defects, folic acid and methylation

Neural tube defects (NTDs) are common complex congenital malformations resulting from failure of the neural tube closure during embryogenesis. It is established that folic acid supplementation decreases the prevalence of NTDs, which has led to national public health policies regarding folic acid. To date, animal studies have not provided sufficient information to establish the metabolic and/or genomic mechanism(s) underlying human folic acid responsiveness in NTDs. However, several lines of evidence suggest that not only folates but also choline, B12 and methylation metabolisms are involved in NTDs. Decreased B12 vitamin and increased total choline or homocysteine in maternal blood have been shown to be associated with increased NTDs risk. Several polymorphisms of genes involved in these pathways have also been implicated in risk of development of NTDs. This raises the question whether supplementation with B12 vitamin, betaine or other methylation donors in addition to folic acid periconceptional supplementation will further reduce NTD risk. The objective of this article is to review the role of methylation metabolism in the onset of neural tube defects.

Genetic variants in the folate pathway and the risk of neural tube defects: a meta-analysis of the published literature

BACKGROUND

Neural Tube Defects (NTDs) are among the most prevalent and most severe congenital malformations worldwide. Polymorphisms in key genes involving the folate pathway have been reported to be associated with the risk of NTDs. However, the results from these published studies are conflicting. We surveyed the literature (1996-2011) and performed a comprehensive meta-analysis to provide empirical evidence on the association.

METHODS AND FINDINGS

We investigated the effects of 5 genetic variants from 47 study populations, for a total of 85 case-control comparisons MTHFR C677T (42 studies; 4374 cases, 7232 controls), MTHFR A1298C (22 studies; 2602 cases, 4070 controls), MTR A2756G (9 studies; 843 cases, 1006 controls), MTRR A66G (8 studies; 703 cases, 1572 controls), and RFC-1 A80G (4 studies; 1107 cases, 1585 controls). We found a convincing evidence of dominant effects of MTHFR C677T (OR 1.23; 95%CI 1.07-1.42) and suggestive evidence of RFC-1 A80G (OR 1.55; 95%CI 1.24-1.92). However, we found no significant effects of MTHFR A1298C, MTR A2756G, MTRR A66G in risk of NTDs in dominant, recessive or in allelic models.

CONCLUSIONS

Our meta-analysis strongly suggested a significant association of the variant MTHFR C677T and a suggestive association of RFC-1 A80G with increased risk of NTDs. However, other variants involved in folate pathway do not demonstrate any evidence for a significant marginal association on susceptibility to NTDs.

Association of the maternal MTHFR C677T polymorphism with susceptibility to neural tube defects in offsprings: evidence from 25 case-control studies

Background

Methylenetetrahydrofolate reductase (MTHFR) is a critical enzyme in folate metabolism and is involved in DNA methylation, DNA synthesis, and DNA repair. In addition, it is a possible risk factor in neural tube defects (NTDs). The association of the C677T polymorphism in the MTHFR gene and NTD susceptibility has been widely demonstrated, but the results remain inconclusive. In this study, we performed a meta-analysis with 2429 cases and 3570 controls to investigate the effect of the MTHFR C677T polymorphism on NTDs.

Methods

An electronic search of PubMed and Embase database for papers on the MTHFR C677T polymorphism and NTD risk was performed. All data were analysed with STATA (version 11). Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated to assess the association. Sensitivity analysis, test of heterogeneity, cumulative meta-analysis, and assessment of bias were performed in our meta-analysis.

Results

A significant association between the MTHFR C677T polymorphism and NTD susceptibility was revealed in our meta-analysis ( TT versus CC: OR  = 2.022, 95% CI: 1.508, 2.712; CT+TT versus CC: OR  = 1.303, 95% CI: 1.089, 1.558; TT versus CC+CT: OR  = 1.716, 95% CI: 1.448, 2.033; 2TT+CT versus 2CC+CT: OR  = 1.330, 95% CI: 1.160, 1.525). Moreover, an increased NTD risk was found after stratification of the MTHFR C677T variant data by ethnicity and source of controls.

Conclusion

The results suggested the maternal MTHFR C677T polymorphism is a genetic risk factor for NTDs. Further functional studies to investigate folate-related gene polymorphisms, periconceptional multivitamin supplements, complex interactions, and the development of NTDs are warranted.

Evaluation of common genetic variants in 82 candidate genes as risk factors for neural tube defects

BACKGROUND

Neural tube defects (NTDs) are common birth defects (~1 in 1000 pregnancies in the US and Europe) that have complex origins, including environmental and genetic factors. A low level of maternal folate is one well-established risk factor, with maternal periconceptional folic acid supplementation reducing the occurrence of NTD pregnancies by 50-70%. Gene variants in the folate metabolic pathway (e.g., MTHFR rs1801133 (677 C > T) and MTHFD1 rs2236225 (R653Q)) have been found to increase NTD risk. We hypothesized that variants in additional folate/B12 pathway genes contribute to NTD risk.

METHODS

A tagSNP approach was used to screen common variation in 82 candidate genes selected from the folate/B12 pathway and NTD mouse models. We initially genotyped polymorphisms in 320 Irish triads (NTD cases and their parents), including 301 cases and 341 Irish controls to perform case-control and family based association tests. Significantly associated polymorphisms were genotyped in a secondary set of 250 families that included 229 cases and 658 controls. The combined results for 1441 SNPs were used in a joint analysis to test for case and maternal effects.

RESULTS

Nearly 70 SNPs in 30 genes were found to be associated with NTDs at the p < 0.01 level. The ten strongest association signals (p-value range: 0.0003-0.0023) were found in nine genes (MFTC, CDKN2A, ADA, PEMT, CUBN, GART, DNMT3A, MTHFD1 and T (Brachyury)) and included the known NTD risk factor MTHFD1 R653Q (rs2236225). The single strongest signal was observed in a new candidate, MFTC rs17803441 (OR = 1.61 [1.23-2.08], p = 0.0003 for the minor allele). Though nominally significant, these associations did not remain significant after correction for multiple hypothesis testing.

CONCLUSIONS

To our knowledge, with respect to sample size and scope of evaluation of candidate polymorphisms, this is the largest NTD genetic association study reported to date. The scale of the study and the stringency of correction are likely to have contributed to real associations failing to survive correction. We have produced a ranked list of variants with the strongest association signals. Variants in the highest rank of associations are likely to include true associations and should be high priority candidates for further study of NTD risk.

Association between MTHFR A1298C polymorphism and neural tube defect susceptibility: a metaanalysis

OBJECTIVE

To determine whether 5,10-methylenetetrahydrofolate reductase A1298C polymorphism is associated with neural tube defect susceptibility.

STUDY DESIGN

Computerized literature searches of the PubMed, Embase, and Medline database were conducted to identify all eligible articles. Study subjects were classified as neural tube defect patients, mothers with neural tube defect offspring and fathers with neural tube defect offspring. The pooled odds ratios with 95% confidence interval (CI) were calculated by fixed-effects model or random-effects model.

RESULTS

Twenty-three case-control studies were finally included. The pooled results showed no significant association between 5,10-methylenetetrahydrofolate reductase A1298C polymorphism and neural tube defect susceptibility in any genetic contrast among the 3 groups, except 1 (AC vs AA in neural tube defect patients: odds ratio, 1.16; 95% CI, 1.10-1.32; P = .03), which, however, turned out to be of no statistical significance in the subgroup of the white population (odds ratio, 1.14; 95% CI, 0.98-1.31; P = .08).

CONCLUSION

This metaanalysis suggests that 5,10-methylenetetrahydrofolate reductase A1298C polymorphism is not associated with neural tube defect susceptibility in the white population.

Genetic aspects of folate metabolism

The vitamin folate functions within the cell as a carrier of one-carbon units. The requirement for one-carbon transfers is ubiquitous and all mammalian cells carry out folate dependent reactions. In recent years, low folate status has been linked to risk of numerous adverse health conditions throughout life from birth defects and complications of pregnancy to cardiovascular disease, cancer and cognitive dysfunction in the elderly. In many instances inadequate intake of folate seems to be the primary contributor but there is also evidence that an underlying genetic susceptibility can play a modest role by causing subtle alterations in the availability, metabolism or distribution of intermediates in folate related pathways. Folate linked one-carbon units are essential for DNA synthesis and repair and as a source of methyl groups for biological methylation reactions. The notion of common genetic variants being linked to risk of disease was relatively novel in 1995 when the first functional folate-related polymorphism was discovered. Numerous polymorphisms have now been identified in folate related genes and have been tested for functionality either as a modifier of folate status or as being associated with risk of disease. Moreover, there is increasing research into the importance of folate-derived one-carbon units for DNA and histone methylation reactions, which exert crucial epigenetic control over cellular protein synthesis. It is thus becoming clear that genetic aspects of folate metabolism are wide-ranging and may touch on events as disparate as prenatal imprinting to cancer susceptibility. This chapter will review the current knowledge in this area.

Pharmacogenetics of drug-induced birth defects: what is known so far?

A literature review was performed to collect information on the role of pharmacogenetics in six proposed teratogenic mechanisms associated with drug use during pregnancy: folate antagonism, oxidative stress, angiotensin-converting enzyme inhibition and angiotensin II receptor antagonism, cyclooxygenase-1 and -2 inhibition, 5-hydroxytryptamine-reuptake inhibition and drug transporters in the placenta. Data on the direct relationship between pharmacogenetics and drug-induced birth defects were found for folate metabolism, oxidative stress caused by phenytoin exposure and drug transporters in the placenta. Although no specific data to support pharmacogenetic-related birth defects were found for the NSAIDs, paroxetine and fluoxetine, it might be expected that polymorphisms modify their teratogenic effects. The usually low prevalence of drug-induced malformations impedes the demonstration of the contribution of pharmacogenetics. Large-scale studies, preferably case-control studies, are needed.

Maternal one-carbon metabolism, MTHFR and TCN2 genotypes and neural tube defects in India

BACKGROUND

Neural tube defects (NTDs) are among the most common severe congenital malformations, representing a long-term public health burden in India. A deranged one-carbon metabolism and genes regulating this metabolism have been linked to NTDs. Vitamin B(12) deficiency is reported to be more prevalent than folate deficiency in the Indian population. We investigated the role of maternal nutritional and genetic markers related to one-carbon metabolism in the etiology of NTDs.

METHODS

We conducted a multicenter case-control study to compare plasma folate, vitamin B(12) , homocysteine and holo-transcobalamin levels, and polymorphisms in methylenetetrahydrofolate reductase (MTHFR, 677C>T, 1298A>C, 1781G>A and 236+724A>G) and transcobalamin (TCN2, 776C>G) genes, in 318 women with NTD-affected offspring (cases) and 702 women with apparently healthy offspring (controls). The samples were collected at diagnosis in cases and at delivery in controls.

RESULTS

We observed a significant association of high maternal plasma homocysteine concentrations with NTDs in the offspring (p = 0.026). There was no association of maternal folate or B(12) levels with NTDs (p > 0.05) but low maternal holo-transcobalamin predicted strong risk of NTDs in the offspring (p = 0.003). The commonly associated maternal polymorphism 677C>T in the MTHFR gene did not predict risk of NTDs in the offspring (p > 0.05) and 1298A>C and 1781G>A polymorphisms in MTHFR were protective (p = 0.024 and 0.0004 respectively). Maternal 776C>G polymorphism in TCN2 was strongly predictive of NTD in the offspring (p = 0.006).

CONCLUSION

Our study has demonstrated a possible role for maternal B(12) deficiency in the etiology of NTDs in India over and above the well-established role of folate deficiency.

Evaluation of 64 candidate single nucleotide polymorphisms as risk factors for neural tube defects in a large Irish study population

Individual studies of the genetics of neural tube defects (NTDs) contain results on a small number of genes in each report. To identify genetic risk factors for NTDs, we evaluated potentially functional single nucleotide polymorphisms (SNPs) that are biologically plausible risk factors for NTDs but that have never been investigated for an association with NTDs, examined SNPs that previously showed no association with NTDs in published studies, and tried to confirm previously reported associations in folate-related and non-folate-related genes. We investigated 64 SNPs in 34 genes for association with spina bifida in up to 558 case families (520 cases, 507 mothers, 457 fathers) and 994 controls in Ireland. Case-control and mother-control comparisons of genotype frequencies, tests of transmission disequilibrium, and log-linear regression models were used to calculate effect estimates. Spina bifida was associated with over-transmission of the LEPR (leptin receptor) rs1805134 minor C allele [genotype relative risk (GRR): 1.5; 95% confidence interval (CI): 1.0-2.1; P = 0.0264] and the COMT (catechol-O-methyltransferase) rs737865 major T allele (GRR: 1.4; 95% CI: 1.1-2.0; P = 0.0206). After correcting for multiple comparisons, these individual test P-values exceeded 0.05. Consistent with previous reports, spina bifida was associated with MTHFR 677C>T, T (Brachyury) rs3127334, LEPR K109R, and PDGFRA promoter haplotype combinations. The associations between LEPR SNPs and spina bifida suggest a possible mechanism for the finding that obesity is a NTD risk factor. The association between a variant in COMT and spina bifida implicates methylation and epigenetics in NTDs.

Testing reported associations of genetic risk factors for oral clefts in a large Irish study population

BACKGROUND

Suggestive, but not conclusive, studies implicate many genetic variants in oral cleft etiology. We used a large, ethnically homogenous study population to test whether reported associations between nonsyndromic oral clefts and 12 genes (CLPTM1, CRISPLD2, FGFR2, GABRB3, GLI2, IRF6, PTCH1, RARA, RYK, SATB2, SUMO1, TGFA) could be confirmed.

METHODS

Thirty-one single nucleotide polymorphisms (SNPs) in exons, splice sites, and conserved non-coding regions were studied in 509 patients with cleft lip with or without cleft palate (CLP), 383 with cleft palate only (CP), 838 mothers and 719 fathers of patients with oral clefts, and 902 controls from Ireland. Case-control and family-based statistical tests were performed using isolated oral clefts for the main analyses.

RESULTS

In case-control comparisons, the minor allele of PTCH1 A562A (rs2066836) was associated with reduced odds of CLP (odds ratios [OR], 0.29; 95% confidence interval [CI], 0.13-0.64 for homozygotes), whereas the minor allele of PTCH1 L1315P (rs357564) was associated with increased odds of CLP (OR, 1.36; 95% CI, 1.07-1.74 for heterozygotes; and OR, 1.56; 95% CI, 1.09-2.24 for homozygotes). The minor allele of one SUMO1 SNP, rs3769817 located in intron 2, was associated with increased odds of CP (OR, 1.45; 95% CI, 1.06-1.99 for heterozygotes). Transmission disequilibrium was observed for the minor allele of TGFA V159V (rs2166975) which was over-transmitted to CP cases (p = 0.041).

CONCLUSIONS

For 10 of the 12 genes, this is the largest candidate gene study of nonsyndromic oral clefts to date. The findings provide further evidence that PTCH1, SUMO1, and TGFA contribute to nonsyndromic oral clefts.

The search for genetic polymorphisms in the homocysteine/folate pathway that contribute to the etiology of human neural tube defects

In this paper, we trace the history of current research into the genetic and biochemical mechanisms that underlie folate-preventable neural tube defects (NTDs). The inspired suggestion by Smithells that common vitamins might prevent NTDs ignited a decade of biochemical investigations-first exploring the nutritional and metabolic factors related to NTDs, then onto the hunt for NTD genes. Although NTDs were known to have a strong genetic component, the concept of common genetic variance being linked to disease risk was relatively novel in 1995, when the first folate-related polymorphism associated with NTDs was discovered. The realization that more genes must be involved started a rush to find polymorphic needles in genetic haystacks. Early efforts entailed the intellectually challenging and time-consuming task of identifying and analyzing candidate single nucleotide polymorphisms (SNPs) in folate pathway genes. Luckily, human genome research has developed rapidly, and the search for the genetic factors that contribute to the etiology of human NTDs has evolved to mirror the increased level of knowledge and data available on the human genome. Large-scale candidate gene analysis and genome-wide association studies are now readily available. With the technical hurdles removed, the remaining challenge is to gather a sample large enough to uncover the polymorphisms that contribute to NTD risk. In some respects the real work is beginning. Although moving forward is exciting, it is humbling that the most important result-prevention of NTDs by maternal folic acid supplementation-was achieved years ago, the direct result of Smithells' groundbreaking studies.

Folic acid, methylation and neural tube closure in humans

This review provides a brief description of folate use and folic acid metabolism in relation to neural tube defect (NTD) risk. First, a meta-analysis of reduction in NTD recurrence and occurrence risk with periconceptional folic acid supplementation is presented. Second, an overview of the complex folate metabolism is given. Third, SNPs for genes involved in folate and homocysteine metabolism that have been studied in relation to NTD riskare discussed. Fourth, the questions whether folate receptor autoantibodies or hampered methylation are mechanisms underlying NTDs are briefly discussed.

Testing for genetic association with constrained models using triads

It has been shown that it is preferable to use a robust model that incorporated constraints on the genotype relative risk rather than rely on a model that assumes the disease operates in a recessive or dominant fashion. Previous methods are applicable to case-control studies, but not to family based studies of case children along with their parents (triads). We show here how to implement analogous constraints while analyzing triad data. The likelihood, conditional on the parents genotype, is maximized over the appropriately constrained parameter space. The asymptotic distribution for the maximized likelihood ratio statistic is found and used to estimate the null distribution of the test statistics. The properties of several methods of testing for association are compared by simulation. The constrained method provides higher power across a wide range of genetic models with little cost when compared to methods that restrict to a dominant, recessive, or multiplicative model, or make no modeling restriction. The methods are applied to two SNPs on the methylenetetrahydrofolate reductase (MTHFR) gene with neural tube defect (NTD) triads.

Folate-related gene polymorphisms as risk factors for cleft lip and cleft palate

BACKGROUND

Cleft lip with or without cleft palate (CLP) and cleft palate only (CPO) have an inherited component and, many studies suggest, a relationship with folate. Attempts to find folate-related genes associated with clefts have, however, often been inconclusive. This study examined four SNPs related to folate metabolism (MTHFR 677 C-->T, MTHFR 1298 A-->C, MTHFD1 1958 G-->A, and TC II 776 C-->G) in a large Irish population to clarify their relationship with clefts.

METHODS

Cases and their parents were recruited from major surgical centers performing cleft repairs in Ireland and a support organization. Data on risk factors, medical history, and DNA were collected. Controls were pregnant women from the greater Dublin area (n = 1,599).

RESULTS

CLP cases numbered 536 and CPO cases 426 after exclusions. CPO mothers were significantly more likely than controls to be MTHFR 677 TT, OR 1.50 (95% CI: 1.05-2.16; p = .03). Log-linear analysis showed a borderline association (p = .07). Isolated CPO case mothers were significantly more likely than controls to be homozygous for the MTHFD1 1958 G-->A variant, OR 1.50 (95%CI: 1.08-2.09; p = .02). When multiple cases were added, both CPO cases and case mothers were significantly more likely to be AA (p = .02 and p = .007, respectively). The CLP case-control and mother-control analyses also showed significant effects, ORs 1.38 (95% CI: 1.05-1.82; p = .03) and 1.39 (95% CI: 1.04-1.85; p = .03), respectively.

CONCLUSIONS

Associations were found for both CPO and CLP and MTHFD1 1958 G-->A in cases and case mothers. MTHFR 677 C-->T could be a maternal risk factor for clefts but the association was not strong. Because multiple comparisons were made, these findings require additional investigation. Given the known association between MTHFD1 1958 G-->A and NTDs, these findings should be explored in more detail.

Non-Latin European descent could be a requirement for association of NTDs and MTHFR variant 677C > T: a meta-analysis

There are several studies that have found a positive association between neural tube defects (NTDs) and the common mutation 677C > T of 5,10-methylenetetrahydrofolate reductase (MTHFR), and others that have not found such an association. We updated the meta-analyses of the published data about NTDs and MTHFR 677C > T variant from January 1994 to October 2005 identifying 170 potentially relevant studies. After applying pertinent exclusion criteria, 37 different populations from 32 studies were included in the meta-analysis, with a total of 3,530 cases and 6,296 controls. Further we stratified the data according to geographical region and ethnicity, and produced two separated meta-analyses for non-Latin European and Latin European descent populations. The general (odds ratio 1.41; 95% confidence interval 1.24-1.59), and the non-Latin European meta-analyses (1.62; 1.38-1.90) indicate an association of TT genotype and NTDs; no association was demonstrated for Latin European populations (1.16; 0.95-1.43). The examination of non-Latin European studies revealed that the association of TT genotype with NTD has only been proven for Irish populations, both by case-control studies, and by family-based tests, such as the allele transmission disequilibrium test (TDT).

The MTR A2756G polymorphism is associated with an increase of plasma homocysteine concentration in Brazilian individuals with Down syndrome

Individuals with Down syndrome (DS) present decreased homocysteine (Hcy) concentration, reflecting a functional folate deficiency secondary to overexpression of the cystathionine ss-synthase gene. Since plasma Hcy may be influenced by genetic polymorphisms, we evaluated the influence of C677T and A1298C polymorphisms in the methylenetetrahydrofolate reductase gene (MTHFR), of A2756G polymorphism in the methionine synthase gene (MTR), and of A80G polymorphism in the reduced folate carrier 1 gene on Hcy concentrations in Brazilian DS patients. Fifty-six individuals with free trisomy 21 were included in the study. Plasma Hcy concentrations were measured by liquid chromatography_tandem mass spectrometry with linear regression coefficient r(2) = 0.9996, average recovery between 92.3 to 108.3% and quantification limits of 1.0 micromol/L. Hcy concentrations >15 micromol/L were considered to characterize hyperhomocystinemia. Genotyping for the polymorphisms was carried out by polymerase chain reaction followed by enzyme digestion and allele-specific polymerase chain reaction. The mean Hcy concentration was 5.2 +/- 3.3 micromol/L. There was no correlation between Hcy concentrations and age, gender or MTHFR C677T, A1298C and reduced folate carrier 1 A80G genotype. However, Hcy concentrations were significantly increased in the MTR 2756AG heterozygous genotype compared to the MTR 2756AA wild-type genotype. The present results suggest that the heterozygous genotype MTR 2756AG is associated with the increase in plasma Hcy concentrations in this group of Brazilian patients with DS.

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.

MTHFR 677C-->T and 1298A-->C polymorphisms: evaluation of maternal genotypic risk and association with level of neural tube defect

BACKGROUND

Neural tube defects (NTDs) are common birth defects (1 in 1,000) leading to significant morbidity and mortality. Periconceptional folic acid supplementation helps in prevention of 70% of NTDs. Recently, polymorphisms in genes encoding enzymes of the folate pathway have been implicated in causation of NTDs. Since the closure of neural tube occurs at multiple sites, the etiology of defect at different sites may be different - which explains the failure of folic acid supplementation to prevent all NTDs.

METHODS

Molecular analysis of methylenetetrahydrofolate reductase polymorphisms was carried out using polymerase chain reaction and restriction enzyme digestion. We studied the association of these polymorphisms in mothers with a previous child with NTD and further refined the risk by stratification based on level of defect.

RESULTS

The frequency of 677C-->T homozygotes was higher in mothers with a previous child with NTD than the controls (OR = 1.6 (0.38-6.7), 95% CI, p = 0.72) but the difference was statistically insignificant. There was a significant difference in frequency of T alleles among mothers with a previous child with a 'lower' type of defect compared to controls (OR = 2.15 (1.13-4.1), 95% CI, p = 0.02). We did not find any significant association of 1298A-->C polymorphism with the level of NTDs.

CONCLUSIONS

We conclude that in the North Indian population, the 677C-->T allele of the MTHFR gene may be associated with the occurrence of a lower type of NTD. This points towards the differential role of thermolabile MTHFR at different sites of neural tube closure.

Genetic variation in genes of folate metabolism and neural-tube defect risk

Neural-tube defects (NTD) are common congenital malformations that can lead to severe disability or even death. Periconceptional supplementation with the B-vitamin folic acid has been demonstrated to prevent 50-70% of NTD cases. Since the identification of the first genetic risk factor of NTD, the C677T single-nucleotide polymorphism (SNP) in the methylenetetrahydrofolate reductase (MTHFR) gene, and the observation that elevated plasma homocysteine levels are associated with NTD, research has focused on genetic variation in genes encoding for enzymes of folate metabolism and the closely-related homocysteine metabolism. In the present review relevant SNP in genes that code for enzymes involved in folate transport and uptake, the folate cycles and homocysteine metabolism are summarised and the importance of these SNP discussed in relation to NTD risk.

Folate gene polymorphisms and the risk of Down syndrome pregnancies in young Italian women

Maternal impairments in folate metabolism and elevated homocysteinemia are known risk factors for having a child with Down syndrome (DS) at a young age. The 80G>A polymorphism of the reduced folate carrier gene (RFC-1) has been recently demonstrated to affect plasma folate and homocysteine levels, alone or in combination with the 677C>T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene. We performed the present study on 80 Italian mothers of DS individuals, aged less than 35 at conception, and 111 Italian control mothers, to study the role of the RFC-1 80G>A, MTHFR 677C>T, and MTHFR 1298A>C genotypes to the risk of a DS offspring at a young maternal age. When polymorphisms were considered alone, both allele and genotype frequencies did not significantly differ between DS mothers and control mothers. However, the combined MTHFR677TT/RFC-1 80GG genotype was borderline associated with an increased risk (OR 6 (CI 95%: 1.0-35.9), P = 0.05), and to be MTHF1298AA/RFC-1 80(GA or AA) was inversely associated with the risk (OR 0.36 (CI 95%: 0.14-0.96), P = 0.04). Present results seem to indicate that none of the RFC-1 80G>A, MTHFR 677C>T, and MTHFR 1298A>C polymorphisms is an independent risk factor for a DS offspring at a young maternal age; however, a role for the combined MTHFR/RFC-1 genotypes in the risk of DS pregnancies among young Italian women cannot be excluded.

The MTHFR 1298CC and 677TT genotypes have opposite associations with red cell folate levels

Individuals homozygous for the thermolabile variant (677TT) of methylenetetrahydrofolate reductase exhibit reduced folate status as evidenced by a drop in the biomarker red cell folate (RCF) compared to those who carry at least one 677C allele. We now report that a different polymorphism in the same enzyme, namely 1298A>C, is associated with increased RCF levels. Thus, these two common polymorphisms change a metabolic phenotype in opposite directions suggesting that their cancer protective associations are by different mechanisms.

Acquired and inherited disorders of cobalamin and folate in children

Cobalamin deficiency in the newborn usually results from cobalamin deficiency in the mother. Megaloblastic anaemia, pancytopenia and failure to thrive can be present, accompanied by neurological deficits if the diagnosis is delayed. Most cases of spina bifida and other neural tube defects result from maternal folate and/or cobalamin insufficiency in the periconceptual period. Polymorphisms in a number of genes involved in folate and cobalamin metabolism exacerbate the risk. Inborn errors of cobalamin metabolism affect its absorption, (intrinsic factor deficiency, Imerslund-Gräsbeck syndrome) and transport (transcobalamin deficiency) as well as its intracellular metabolism affecting adenosylcobalamin synthesis (cblA and cblB), methionine synthase function (cblE and cblG) or both (cblC, cblD and cblF). Inborn errors of folate metabolism include congenital folate malabsorption, severe methylenetetrahydrofolate reductase deficiency and formiminotransferase deficiency. The identification of disease-causing mutations in specific genes has improved our ability to diagnose many of these conditions, both before and after birth.

Etiology, pathogenesis and prevention of neural tube defects

Spina bifida, anencephaly, and encephalocele are commonly grouped together and termed neural tube defects (NTD). Failure of closure of the neural tube during development results in anencephaly or spina bifida aperta but encephaloceles are possibly post-closure defects. NTD are associated with a number of other central nervous system (CNS) and non-neural malformations. Racial, geographic and seasonal variations seem to affect their incidence. Etiology of NTD is unknown. Most of the non-syndromic NTD are of multifactorial origin. Recent in vitro and in vivo studies have highlighted the molecular mechanisms of neurulation in vertebrates but the morphologic development of human neural tube is poorly understood. A multisite closure theory, extrapolated directly from mouse experiments highlighted the clinical relevance of closure mechanisms to human NTD. Animal models, such as circle tail, curly tail, loop tail, shrm and numerous knockouts provide some insight into the mechanisms of NTD. Also available in the literature are a plethora of chemically induced preclosure and a few post-closure models of NTD, which highlight the fact that CNS malformations are of hetergeneitic nature. No Mendelian pattern of inheritance has been reported. Association with single gene defects, enhanced recurrence risk among siblings, and a higher frequency in twins than in singletons indicate the presence of a strong genetic contribution to the etiology of NTD. Non-availability of families with a significant number of NTD cases makes research into genetic causation of NTD difficult. Case reports and epidemiologic studies have implicated a number of chemicals, widely differing therapeutic drugs, environmental contaminants, pollutants, infectious agents, and solvents. Maternal hyperthermia, use of valproate by epileptic women during pregnancy, deficiency and excess of certain nutrients and chronic maternal diseases (e.g. diabetes mellitus) are reported to cause a manifold increase in the incidence of NTD. A host of suspected teratogens are also available in the literature. The UK and Hungarian studies showed that periconceptional supplementation of women with folate (FA) reduces significantly both the first occurrence and recurrence of NTD in the offspring. This led to mandatory periconceptional FA supplementation in a number of countries. Encouraged by the results of clinical studies, numerous laboratory investigations focused on the genes involved in the FA, vitamin B12 and homocysteine metabolism during neural tube development. As of today no clinical or experimental study has provided unequivocal evidence for a definitive role for any of these genes in the causation of NTD suggesting that a multitude of genes, growth factors and receptors interact in controlling neural tube development by yet unknown mechanisms. Future studies must address issues of gene-gene, gene-nutrient and gene-environment interactions in the pathogenesis of NTD.

Folate and human reproduction

The influence of folate nutritional status on various pregnancy outcomes has long been recognized. Studies conducted in the 1950s and 1960s led to the recognition of prenatal folic acid supplementation as a means to prevent pregnancy-induced megaloblastic anemia. In the 1990s, the utility of periconceptional folic acid supplementation and folic acid food fortification emerged when they were proven to prevent the occurrence of neural tube defects. These distinctively different uses of folic acid may well be ranked among the most significant public health measures for the prevention of pregnancy-related disorders. Folate is now viewed not only as a nutrient needed to prevent megaloblastic anemia in pregnancy but also as a vitamin essential for reproductive health. This review focuses on the relation between various outcomes of human reproduction (ie, pregnancy, lactation, and male reproduction) and folate nutrition and metabolism, homocysteine metabolism, and polymorphisms of genes that encode folate-related enzymes or proteins, and we identify issues for future research.

Homocysteine metabolism in families from southern Italy with neural tube defects: role of genetic and nutritional determinants

OBJECTIVE

To evaluate the role of different polymorphic gene variants involved in homocysteine metabolism and plasma levels of homocysteine, folate and vitamin B12 in families from southern Italy with neural tube defects (NTDs).

METHODS

Eighteen fathers, 15 NTD children and 60 women who had conceived NTD foetuses were investigated. A group of 100 adults and 43 apparently healthy children was used as control. At the time of blood draw, none were taking vitamin pills or nutritional supplements.

RESULTS

Among controls, 79 (55.2%) were heterozygous for C677T MTHFR variant and 26 (18.2%) were TT homozygous. Among the cases, 35 (61.4%) out of 57 mothers and 7 (38.9%) out of 18 fathers carried the T allele; 12 (21.1%) mothers and 2 (11.1%) fathers had the TT genotype. Four (26.7%) out of 15 probands were TT homozygous and 11 (73.3%) were heterozygous (Fisher exact test p = 0.025). No significant difference between groups was observed for the 1298C MTHFR variant and CBS haplotypes. Median homocysteine in NTD children was significantly higher (10.0 micromol/L) than that of controls (median 4.5 micromol/L, Mann-Whitney p < 0.05). Folate and B12 were not different among groups.

CONCLUSIONS

The T677 MTHFR allele is significantly associated with the occurrence of NTDs; no significant association has been observed with other genetic determinants analysed. Homocysteine levels in children with NTDs are significantly higher than those of the paediatric population from the same geographical area.

Screening for new MTHFR polymorphisms and NTD risk

The enzyme, 5,10-methylenetetrahydrofolate reductase (MTHFR) plays a key role in cellular folate metabolism. The A222V (677C->T) polymorphism is a confirmed neural tube defect (NTD) risk factor within Irish and other populations. To search for other unknown single nucleotide polymorphisms (SNPs) that might play a role in the etiology of NTDs, we examined the entire MTHFR coding region in healthy individuals (n = 100). SNPs were identified using sequencing and database analysis and allele frequencies were determined in our Irish population. We identified P39P (116C->T; T allele frequency 0.13) and previously reported R594Q (1793G->A; Q allele frequency 0.07). We screened a large ethnically homogeneous Irish NTD cohort (n>1,300) for P39P and R594Q. A possible association between NTD cases and P39P (P = 0.034) was found but this was not confirmed by transmission disequilibrium testing. R594Q also showed some evidence of a NTD case association (P = 0.07). Further analysis indicated these observations are due to linkage disequilibrium with A222V (677C->T), and therefore these new SNPs are unlikely to be independent risk factors for NTDs. As rates of NTDs differ between ethnic groups, we examined allele and genotype frequencies of P39P and R594Q within African-American and American-Caucasian populations. This is the first NTD association study of both R594Q and the novel P39P. The association with NTD risk reported for these SNPs is driven by the linkage disequilibrium with the A222V (677C->T) NTD risk factor.

Congenital heart defects and genetic variants in the methylenetetrahydroflate reductase gene

BACKGROUND

Most non-syndromic congenital heart defects (CHD) are caused by a complex interaction between maternal lifestyle factors, environmental exposures, and maternal and fetal genetic variants. Maternal periconceptional intake of folic acid containing vitamin supplements is reported to decrease the risk of CHD. The 677C-->T and 1298A-->C polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene decrease enzyme activity.

OBJECTIVE

To examine the relation between CHD and maternal and fetal MTHFR polymorphisms.

METHODS

375 nuclear families were studied. The transmission/disequilibrium test was used to test for transmission distortion in complete triads. A log-linear approach was used to test for associations between CHD and maternal and offspring polymorphisms, and to estimate independently the contributions of maternal and fetal variants to relative risks. Haplotype frequencies were estimated and a haplotype transmission disequilibrium test carried out.

RESULTS

The 1298C allele was transmitted less often than expected (p = 0.0013). There was no distortion in the transmission of the 677T allele, neither was there evidence of a parent of origin effect in the transmission of either of the single nucleotide polymorphisms. The 677C-1298C haplotype was also transmitted less often than expected (p = 0.0020). The relative risk associated with inheriting one copy of the 1298C allele was 0.64 (95% confidence interval, 0.48 to 0.87) and the that associated with inheriting two copies of the 1298C allele, 0.38 (0.21 to 0.70).

CONCLUSIONS

The apparent protective effect of the MTHFR 1298C allele against CHD could have several explanations and further study is needed.

Thrombophilic Polymorphisms and Intrauterine Growth Restriction

BACKGROUND

The relationship between thrombophilic polymorphisms and intrauterine growth restriction remains unclear. Whereas a subset of these polymorphisms have received some attention, others have not.

METHODS

We conducted a case-control study of 493 cases of intrauterine growth restriction (birthweight less than the 10th percentile for gestational age and sex) and 472 controls (greater than the 10th percentile). We also conducted a family study including approximately 250 case trios (affected newborn, mother, and father) for each studied gene. Plasminogen activator inhibitor-1(PAI-1) 4G/5G and Factor XIII Val134Leu variants were compared between maternal and newborn cases and controls. Relative risks for newborn and maternal PAI-1 and Factor XIII genotypes in case trios were estimated using a log-linear model. Transmission of MTHFR C677T and 1298C haplotypes was analyzed in case trios, and the estimated haplotype distribution compared between cases and controls. Finally, we explored pairwise gene-gene interactions between all measured polymorphisms, including Factor V Leiden G1691A and Prothrombin G20210A.

RESULTS

PAI-1 and Factor XIII polymorphisms were not associated with an increased risk of intrauterine growth restriction in the case-control analysis, and the case-parent analysis showed no newborn or maternal excess genotype relative risk. No excess transmission of the MTHFR haplotypes was observed in case newborns, and the distribution of MTHFR haplotypes was similar between cases and controls. Some results were suggestive of interactions between genes.

CONCLUSIONS

Overall, there seems to be little or no indication that thrombophilic genes, at least individually, have an effect on intrauterine growth restriction.

MTHFD1 R653Q polymorphism is a maternal genetic risk factor for severe abruptio placentae

This study examined the relationship between folate/homocysteine-related genetic polymorphisms: MTHFD1 1958G --> A (R653Q), MTHFR 677C --> T (A222V), MTHFR 1298A --> C (E429A), and risk of severe abruptio placentae. We genotyped 62 women with a pregnancy history complicated by severe abruptio placentae and 184 control pregnancies. Analysis of the MTHFD1 1958G --> A (R653Q) polymorphism showed increased frequency of the 'QQ' homozygote genotype in pregnancies affected by severe abruptio placentae compared to control pregnancies (odds ratio 2.85 (1.47-5.53), P = 0.002). In contrast to previous reports, the MTHFR polymorphisms 677C --> T (A222V) and 1298A --> C (E429A) were not associated with abruptio placentae risk in our cohort, when analyzed either independently or in combination. We conclude that women who are 'QQ' homozygote for the MTHFD1 1258G --> A (R653Q) polymorphism are almost three times more likely to develop severe abruptio placentae during their pregnancy than women who are 'RQ' or 'RR.'

Folate and homocysteine interrelationships including genetics of the relevant enzymes

PURPOSE OF REVIEW

Inadequate folate status has been linked to risk of a wide range of adverse health conditions throughout life, from birth defects and complications of pregnancy to cardiovascular disease, cancer and cognitive dysfunction in the elderly. In many instances these risks are manifested through elevated plasma homocysteine. This review focuses on current research into the contribution of genetic variability to folate status and disease predisposition.

RECENT FINDINGS

Some dozen potentially important polymorphisms in folate-related genes have been examined for disease associations or for their role in determining the level of plasma homocysteine. In most instances, the effects are either modest, not significant, or undetectable. However, the mechanism by which the 677C-->T variant of methylenetetrahydrofolate reductase determines homocysteine status has become clearer with the elucidation of a critical role for riboflavin in modulating the plasma homocysteine of TT homozygotes. Moreover, several new metaanalyses have confirmed an association of this variant with vascular disease, probably through low folate status and elevated plasma homocysteine.

SUMMARY

There are enormous difficulties in attempting to assess the contribution of minor genetic variability to nutrient status, against major background differences due to ethnicity, age, gender, lifestyle, dietary habits and disease status. Nevertheless, this is an important goal in the future management of chronic multifactorial disease. The present research into the genetic components of folate and homocysteine variability is paving the way towards an eventual capacity to ensure optimal folate status in every individual and, consequently, to reduce their risk of developing such diseases.

Polymorphisms within the vitamin B12 dependent methylmalonyl-coA mutase are not risk factors for neural tube defects

Methionine synthase and methylmalonyl-CoA mutase (mutase) are the only two known vitamin B(12) (B(12)) dependent enzymes in humans. A lower level of B(12) has been shown to be an independent maternal risk factor for neural tube defects (NTDs) prompting an investigation of common genetic variants within B(12) dependent enzymes. To investigate the role of methylmalonyl-CoA mutase variants we studied 279 complete NTD triads (NTD affected case and both parents) and 256 controls. Based on case-control and family based (transmission disequilibrium test) analyses we did not find an association between the mutase single nucleotide polymorphisms (SNPs) K212K (636A-->G), H532R (1595A-->G) and V671I (2011G-->A) and NTDs. However, there was a significant difference in the frequencies of these polymorphisms between a group of African Americans and American Caucasians (K212K, P=0.002; H532R, P</=0.001; V671I, P=0.006). In conclusion, common variants in the mutase gene do not appear to be risk factors for NTDs but their allele frequencies are significantly different between ethnic groups.