Impact of the MTHFR C677T polymorphism on risk of neural tube defects: case-control studyBMJ 2004; 328 doi: https://doi.org/10.1136/bmj.38036.646030.EE (Published 24 June 2004) Cite this as: BMJ 2004;328:1535
- Peadar N Kirke, specialist in public health medicine ()1,
- James L Mills, chief, pediatric epidemiology section2,
- Anne M Molloy, senior experimental officer3,
- Lawrence C Brody, senior investigator4,
- Valerie B O'Leary, postdoctoral geneticist5,
- Leslie Daly, associate professor6,
- Sharon Murray, research coordinator1,
- Mary Conley, information technology specialist2,
- Philip D Mayne, consultant chemical pathologist7,
- Owen Smith, consultant haematologist8,
- John M Scott, professor of experimental nutrition5
- 1Child Health Epidemiology Division, Health Research Board, Dublin 2, Ireland
- 2 Division of Epidemiology, Statistics and Prevention Research, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MA 20892, USA
- 3Department of Clinical Medicine, Trinity College Dublin, St James's Hospital, Dublin 8
- 4Molecular Pathogenesis Section, Genome Technology Branch, National Human Genome Research Institute, National Institutes of Health,
- 5Department of Biochemistry, Trinity College Dublin, Dublin 2
- 6Department of Public Health Medicine and Epidemiology, University College Dublin, National University of Ireland, Dublin 2
- 7The Children's University Hospital, Dublin 1
- 8National Centre for Hereditary Coagulation Disorders, St James's Hospital, Dublin 8
- Correspondence to: P N Kirke
- Accepted 3 February 2004
Homozygosity for the T allele of the C677T polymorphism of the gene encoding the folate dependent enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR) is a risk factor for neural tube defects.1 Both the homozygous (TT) and heterozygous (CT) genotypes are associated with lower tissue concentrations of folate, higher homocysteine concentrations, and lower enzyme activity than the wild type (CC) genotype; these effects are more marked in homozygotes. Low folate and raised homocysteine levels in early pregnancy are risk factors for neural tube defects.2 We investigated the possibility that the CT genotype would also increase the risk of these malformations.
Participants, methods, and results
We recruited 397 individuals with spina bifida aperta (380) or encephalocele (17) throughout Ireland. Participants were aged between 5 months and 52 years (mean 16.8 years). We drew blood for analysis of DNA. We derived the controls from a random sample of 1000 newborn screening cards collected on all Irish births. Of these 1000, DNA was successfully extracted from 855 cards. We successfully genotyped 395 (99.5%) cases and 848 (99.2%) controls.
We calculated population attributable fractions, broadly interpreted as the percentage of the disease in a population that is “caused by” a risk factor, for heterozygotes and homozygotes separately comparing each to the wild type.
The heterozygous genotype is associated with an increased risk of neural tube defects (odds ratio 1.52; P = 0.0015; table). Risk is also raised for the homozygous TT genotype (2.56; P < 0.0001), confirming our earlier finding.3
Population attributable fraction calculations reveal that the CT genotype is responsible for at least as many neural tube defects in the population as the TT genotype (14.9% ν 11.3%; table). This arises because a much greater proportion of the population are heterozygous for this allele (about 38% of the general population are CT compared with 10% who are TT; table).
Heterozygosity for the MTHFR polymorphism, which is present in 38% of the population, increases the risk of neural tube defects. Most studies of MTHFR C677T and neural tube defects and other conditions have focused on the risk associated with T allele homozygosity. The possibility that heterozygosity might also increase neural tube defect risk has gone unrecognised except for a small study in which an association between CT and these malformations was thought to be due to the higher than expected proportion of CC control subjects.4
The combined CT and TT genotypes account for about 26% of neural tube defects in Ireland. Folate or folic acid is estimated to be involved in about 50% to 70% of these defects. Thus up to a half of the folate related neural tube defects may be explained by this single genetic variant.
These findings have two important implications. Firstly, MTHFR C677T heterozygosity needs to be considered as a risk factor for other conditions where homozygosity has been shown to be associated with increased risk, for example, ischaemic heart disease.5 Secondly, the population at risk, and the population that will benefit from food fortification, is much larger than previously believed. Based on pooled data from published studies, about 59% of the European population and 53% of the North American population have either CT or TT genotypes.1
Both the lower folate and increased homocysteine concentrations associated with CT and TT genotypes can be corrected by folic acid, even in relatively small doses. Therefore, our study provides new data underscoring the importance of public health intervention programmes of folic acid supplementation and food fortification targeted at all women of childbearing age to prevent neural tube defects. Such intervention may also turn out to have other public health benefits—for example, in the prevention of cardiovascular disease.
We thank Deborah Watson, Marie Sutton, Maeve Royston, Helen Burke, and Mary-Patricia McKeever for subject recruitment and data collection and the Irish Association for Spina Bifida and Hydrocephalus for their help with subject recruitment.
Contributors PNK, JLM, AMM, LCB, LD, MC, and JMS designed the study, analysed and interpreted the data, and drafted the report. VBO'L, AMM, LCB, and OS did the genotyping. PNK and SM recruited some of the cases and PDM provided the Guthrie cards for the controls. LD did the statistical analysis. All authors contributed to writing revisions of the report and approved the final manuscript. PNK is guarantor.
Funding National Institute of Child Health and Human Development, National Institutes of Health (Contract No. NOIHD 23163) and the Health Research Board of Ireland.
Competing interests None declared
Ethical approval Research Ethics Committee of the Health Research Board. We got written informed consent from all participants