Hyperhomocysteinaemia has been identified as a risk factor for cerebrovascular, peripheral vascular, and coronary heart disease. 1-4 Elevated levels of plasma homocysteine can result from genetic or nutrient-related disturbances in the trans-sulphuration or re-methylation pathways for homocysteine metabolism. 1,5-7 5,10-Methylenetetrahydrofolate reductase (MTHFR) catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the predominant circulatory form of folate and carbon donor for the re-methylation of homocysteine to methionine. Reduced MTHFR activity with a thermolabile enzyme has been reported in patients with coronary and peripheral artery diseases. 6 We have identified a common mutation in MTHFR which alters a highly-conserved amino acid; the substitution occurs at a frequency of approximately 38% of unselected chromosomes. The mutation in the heterozygous or homozygous state correlates with reduced enzyme activity and increased thermolability in lymphocyte extracts; in vitro expression of a mutagenized cDNA containing the mutation confirms its effect on thermolability of MTHFR. Finally, individuals homozygous for the mutation have significantly elevated plasma homocysteine levels. This mutation in MTHFR may represent an important genetic risk factor in vascular disease.

Recently, we showed that homozygosity for the common 677(C-->T) mutation in the methylenetetrahydrofolate reductase (MTHFR) gene, causing thermolability of the enzyme, is a risk factor for neural-tube defects (NTDs). We now report on another mutation in the same gene, the 1298(A-->C) mutation, which changes a glutamate into an alanine residue. This mutation destroys an MboII recognition site and has an allele frequency of .33. This 1298(A-->C) mutation results in decreased MTHFR activity (one-way analysis of variance [ANOVA] P < .0001), which is more pronounced in the homozygous than heterozygous state. Neither the homozygous nor the heterozygous state is associated with higher plasma homocysteine (Hcy) or a lower plasma folate concentration-phenomena that are evident with homozygosity for the 677(C-->T) mutation. However, there appears to be an interaction between these two common mutations. When compared with heterozygosity for either the 677(C-->T) or 1298(A-->C) mutations, the combined heterozygosity for the 1298(A-->C) and 677(C-->T) mutations was associated with reduced MTHFR specific activity (ANOVA P < .0001), higher Hcy, and decreased plasma folate levels (ANOVA P <.03). Thus, combined heterozygosity for both MTHFR mutations results in similar features as observed in homozygotes for the 677(C-->T) mutation. This combined heterozygosity was observed in 28% (n =86) of the NTD patients compared with 20% (n =403) among controls, resulting in an odds ratio of 2.04 (95% confidence interval: .9-4.7). These data suggest that the combined heterozygosity for the two MTHFR common mutations accounts for a proportion of folate-related NTDs, which is not explained by homozygosity for the 677(C-->T) mutation, and can be an additional genetic risk factor for NTDs.

Individuals with the MTHFR 677 TT genotype had a significantly higher risk of CHD, particularly in the setting of low folate status. These results support the hypothesis that impaired folate metabolism, resulting in high homocysteine levels, is causally related to increased risk of CHD.

M a r t i n d e n H e u e r , M.D., T e d K o s t e r , M.D., H e n k J. B lo m , Ph.D., G e r a r d M.J. Bos, M.D., E r n e s t B r i ë t , M.D., P i e t e r H. R e its m a , P h .D ., J a n P. V a n d e n b r o ljo k e , M.D., and F rits R. R o sen d a a l, M .D .

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Neural tube closure takes place during early embryogenesis and requires interactions between genetic and environmental factors. Failure of neural tube closure is a common congenital malformation that results in morbidity and mortality. A major clinical achievement has been the use of periconceptional folic acid supplements, which prevents ~50-75% of cases of neural tube defects. However, the mechanism underlying the beneficial effects of folic acid is far from clear. Biochemical, genetic and epidemiological observations have led to the development of the methylation hypothesis, which suggests that folic acid prevents neural tube defects by stimulating cellular methylation reactions. Exploring the methylation hypothesis could direct us towards additional strategies to prevent neural tube defects.Neural tube defects (NTDs) are complex congenital malformations of the CNS. Anencephaly and spina bifida are the most common and severe forms of NTD, with a prevalence of about 1 in 1,000 births, although this varies throughout the world 1 . Infants with anencephaly are stillborn or die shortly after birth, whereas infants with spina bifida can survive but are likely to have severe lifelong disabilities and are at risk of psychosocial maladjustment. The estimated lifetime medical costs are high; for example, in California, USA, the costs for children born with spina bifida exceed US$81 million per year 2 .It has been known for more than 20 years that women can reduce their risk of having an NTDaffected pregnancy by taking folic acid supplements. However, the underlying mechanisms byCorrespondence to H.J.B. H.Blom@cukz.umcn.nl. Competing interests statementThe authors declare no competing financial interests. DATABASES NIH Public Access Author ManuscriptNat Rev Neurosci. Author manuscript; available in PMC 2010 November 2. Neurulation and neural tube defectsNeurulation has long fascinated scientists, as evidenced by a devoted literature that extends back hundreds of years. The process of neurulation occurs during early embryogenesis, starting with the formation of the neural plate from specialized ectodermal cells and culminating in the closure of the neural tube (FIG. 1), the precursor of both the CNS and most of the PNS. Considered most simply, the neural plate develops bilateral neural folds, which elevate and fuse at the midline to create the neural tube. Subsequent development, together with vesiculation within the tube, gives rise to the brain and spinal cord. Further details of this complex process are beyond the scope of this review (for reviews, see REFS 4,5 ).Neurulation seems to be highly complex and tightly regulated. The development and closure of the neural tube is usually completed by 28 days post-conception. This means that by the time a woman finds out that she is pregnant, the neural tube is almost completely closed. For the neural tube to close properly, genes that regulate various morphogenetic activities must function cooperatively. These activities include programmed cell death, neural crest...

Hyperhomocysteinaemia has been regarded as a new modifiable risk factor for atherosclerosis and vascular disease. Homocysteine is a branch-point intermediate of methionine metabolism, which can be further metabolised via two alternative pathways: degraded irreversibly through the transsulphuration pathway or remethylated to methionine by the remethylation pathway. Both pathways are B-vitamin-dependent. Plasma homocysteine concentrations are determined by nongenetic and genetic factors. The metabolism of homocysteine, the role of B vitamins and the contribution of nongenetic and genetic determinants of homocysteine concentrations are reviewed. The mechanisms whereby homocysteine causes endothelial damage and vascular disease are not fully understood. Recently, a link has been postulated between homocysteine, or its intermediates, and an alterated DNA methylation pattern. The involvement of epigenetic mechanisms in the context of homocysteine and atherosclerosis, due to inhibition of transmethylation reactions, is briefly overviewed.

This overview addresses homocysteine and folate metabolism. Its functions and complexity are described, leading to explanations why disturbed homocysteine and folate metabolism is implicated in many different diseases, including congenital birth defects like congenital heart disease, cleft lip and palate, late pregnancy complications, different kinds of neurodegenerative and psychiatric diseases, osteoporosis and cancer. In addition, the inborn errors leading to hyperhomocysteinemia and homocystinuria are described. These extreme human hyperhomocysteinemia models provide knowledge about which part of the homocysteine and folate pathways are linked to which disease. For example, the very high risk for arterial and venous occlusive disease in patients with severe hyperhomocysteinemia irrespective of the location of the defect in remethylation or transsulphuration indicates that homocysteine itself or one of its “direct” derivatives is considered toxic for the cardiovascular system. Finally, common diseases associated with elevated homocysteine are discussed with the focus on cardiovascular disease and neural tube defects.