Neural tube defects and folate: case far from closed

Abstract

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 approximately 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.

Figure 1. Neural tube closure in the developing mouse

A | Four phases of primary neurulation in neural tube closure. Panels a–c are dorsal views, whereas panel d is a lateral view. a–c | Bidirectional fusion beginning from the original initiation site in the mid-cervical region (a), followed by continued bidirectional fusion (b and c) rostrally towards the anterior neuropore (blue shading) and caudally towards the posterior neuropore (green shading), which progresses as a result of convergent extension and apical constriction. Panel d shows a closed neural tube, with complete fusion over the rhomobencephalon, which has yet to be covered externally. Failure of the posterior neural tube to close can result in spina bifida. Defects in the closure of the anterior neural tube could result in anencephaly. B | Stages of neural tube closure in a transverse section. The neural plate elevates to form neural folds, which progressively appose each other, ultimately fusing to create a closed neural tube. This occurs just before the end of the first month of pregnancy. Panel B modified, with permission, from REF. © (2001) Society for Experimental Biology and Medicine.

Figure 2. Structures of folates

The bioactive form of folate is 5,6,7,8-tetrahydrofolate. Although folic acid is not present in nature, it is more stable than other forms of folate and so is the form typically used in tablets and fortified food. Folic acid is reduced to 5,6,7,8-tetrahydrofolate by dihydrofolate reductase via 7,8-dihydrofolate. The function of folate is to provide one-carbon moieties for biosynthetic purposes. The most reduced form of folate is 5-methyltetrahydrofolate (5-MeTHF) and the most oxidized forms are 5-and 10-formyltetrahydrofolate. Polyglutamation prevents transport of folate out of the cell. Modified, with permission, from REF. © (2001) Society for Experimental Biology and Medicine.

Figure 3. Simplified folate metabolism

In the circulation, folate is mainly present as 5-methyltetrahydrofolate (5-MeTHF), which is taken up by cells via specific carriers or receptors. In the cell, folate donates one-carbon units for use in methylation reactions and in purine and thymidine synthesis. Alternatively, folate can also accept one-carbon units from several amino acids, including serine. Methylenetetrahydrofolate reductase (MTHFR) competes with thymidylate synthase (TS) and methylenetetrahydrofolate dehydrogenase (MTHFD) for the one-carbon units of 5-MeTHF. MTHFR activity is regulated by S-adenosylmethionine (AdoMet), which inhibits enzyme activity. Vitamin B12 is a cofactor of methionine synthase (MTR) and is involved in the transfer of the methyl group from 5-MeTHF to homocysteine. AdoHcy, S-adenosylhomocysteine; AIRCARFT, aminoimidazolecarboxamide ribonucleotide transformylase; DHF, dihydrofolate; DHFR, DHF reductase; dTMP, deoxythymidine monophosphate; dUMP, deoxyuridine monophosphate; FR, folate receptor; GARFT, glycinamide ribonucleotide transformylase; MTRR, MTR reductase; RFC, reduced folate carrier; SHMT, serine hydroxymethyltransferase; THF, tetrahydrofolate. Modified, with permission, from REF. © (2003) Oxford University Press.

Figure 4. The three MTHFR genotypes affect methylation and DNA synthesis to different extents

Variations in methylenetetrahydrofolate reductase (MTHFR) activity influence the flux through the different folate pathways. The MTHFR 677 CC genotype produces the variant with the highest enzymatic activity. This results in lower concentrations of 5,10-methylenetetrahydrofolate (5,10-methylene THF) and 10-formyl THF available for DNA synthesis and higher concentrations of 5-methyl THF available for adequate levels of methylation. The CT genotype produces an intermediate effect. Finally, the TT genotype produces the variant with the lowest MTHFR activity and leads to higher concentrations of 5,10-methylene THF and 10-formyl THF for DNA synthesis, and lower concentrations of 5-methyl THF. Consequently, there is a reduction in the extent of methylation, which could result in an increased risk of neural tube defects.

Figure 5. Folate metabolism is influenced by environmental and genetic factors

Intake of B vitamins other than folate, such as vitamin B12, B6 and B2, also affects folate metabolism. This process can also be disrupted by genetic variants, with an elegant example being the MTHFR 677C>T polymorphism. Disruption of the methylation of lipids, DNA and proteins during early embryogenesis could lead to neural tube defects (NTDs). It is proposed that folate prevents NTDs by increasing methylation of various molecules that are essential to cellular processes.