A neural tube is an early embryonic precursor of the central nervous system, comprising of the brain and spinal cord. After a short period prior to its complete formation, the neural tube is open both cranially and caudally (Dehner & Stocker, 2001). Improper closure during the fourth week of pregnancy can result in neural tube defects such as spina bifida, anencephaly, and encephalocele (Dehner & Stocker, 2001). It is during this crucial point in embryogenesis where folate acts as a substrate for enzymes involved in DNA and RNA biosynthesis – biological processes that are essential to these nascent tissues. Overall, the underlying purpose of this report is to distinguish the relationship between folate consumption and neural tube formation and to discuss many of today’s accepted mechanisms describing the role of folate in the prevention of neural tube defects.

Folates are involved in a large number of biochemical processes, such as amino acid metabolism, purine and pyrimidine synthesis, and methylation of a large number of nucleic acids, deoxyribonucleic acid, proteins, and lipids (Alberto et al., 2007). Folate is particularly important in the homocysteine/methionine cycle. Over the past decade, there has been growing evidence that even a moderately elevated increase in homocysteine can increase the risk of prenatal abnormalities, particularly neural tube defects (Herrmann, 2001). Homocysteine is a homologue of the amino acid cysteine which is used by the body to help manufacture proteins and carry out cellular metabolism. In order to fully comprehend the link between folate and early prenatal development, it is necessary to examine the biochemical roles of folate in homocysteine metabolism. When homocysteine accumulates in cells, it is removed by remethylation into the amino acid methionine or by a process known as trans-sulfuration into cysteine (Alberto et al., 2007). In the latter mechanism, homocysteine is condensed with the amino acid serine in a reaction catalyzed by the enzyme cystathionine-β-synthase to form cystathionine, which in turn, is reduced to cysteine and α-ketobutyrate by the enzyme cystathionine lyase. Both of these enzymes depend on an active form of vitamin B₆ (Alberto et al., 2007).

During the remethylation into methionine, a methyl group is transferred from 5-methyltetrahydrofolate to homocysteine via the enzyme methionine synthase. For this reaction to take place, vitamin B₁₂ must be present since it acts as an intermediate molecule that carries the methyl group. The resulting methionine will then be used either in the formation of peptides during translation or transformed by the enzyme methionine adenosyltransferase into S-adenosylmethionine, which transfers an adenosyl group from ATP to methionine. S-adenosylmethionine plays a significant role in cellular functions because it serves as a universal methyl donor in a large number of methylation reactions. This cycle is highly dependent on the folate cycle; in fact, this cycle could not turn without the folate cycle. The methyl-donating 5-methyltetra-hydrofolate is converted from 5,10-methylenetetrahydrofolate via the enzyme 5,10-methylenetetrahydrofolate reductase. After homocysteine has been remethylated to methionine, tetrahydrofolate lacking a methyl group will be converted back into 5,10-methylenetetrahydrofolate (Alberto et al., 2007). Therefore, methylenetetrahydro-folate reductase has a central function in the folate cycle, especially since it directs dietary folate towards the remethylation of homocysteine for DNA and RNA synthesis (Fowler, 2001). In addition, methylenetetrahydrofolate reductase can also be used in several one-carbon transfer reactions during the synthesis of thymine, as well as during the synthesis of purines (Alberto et al., 2007).

Since all these metabolic processes are interwoven, any imparity would lead to dramatic biochemical variations in terms of homocysteine levels circulating within the growing fetus. In a study conducted by Blom et al. (1995), it was discovered that a single nucleotide polymorphism exists for the enzyme methylenetetrahydrofolate reductase gene, characterized by an alanine-to-valine substitution at position 222, resulting in a 50% reduction in enzyme activity. Consequently, the polymorphism has been associated with elevated levels of homocysteine, particularly in patients with insufficient folate intake. When the frequency of this mutation was further analyzed by Blom et al. (1997), it was observed that this gene was more prevalent in mothers who gave birth to infants with neural tube defects. In fact, it was shown that if the offspring also inherited this gene, that is, both mother and her child are homozygous for the mutation, the risk of spina bifida increased 6 to 7-fold (Blom et al., 1997). However, the effect of this gene can be reversed by additional folic acid intake (Boers et al., 1998).

Recall that if the reductase enzyme is unavailable, tetrahydrofolate can no longer attain another methyl group in order convert homocysteine into methionine. Methionine is an essential amino acid that cannot be synthesized by the body. In fact, nearly all mRNA encode methionine as their initial amino acid to begin translation. Besides protein synthesis, methionine is involved in the formation of S-adenosylmethionine, which is a substrate for many transmethylation reactions, such as DNA methylation (Blom et al., 2001). DNA methylation can change the properties of chromatin, such as its structure and activity, and is associated with a number of key processes including the repression of gene expression - a process that allows cells to become determined and possess specialized roles in varying tissues. It is during the early stages of embryogenesis where DNA methylation is undergoing its most rapid changes (Boubelik et al., 1987).

In an experiment conducted by Choi et al. (2000), it was discovered that in homozygous methylenetetrahydrofolate reductase patients, the resulting deficit in 5-methylenetetrahydrofolate was associated with a lack of DNA methylation in peripheral blood mononuclear cells. Therefore, a shortage in the supply of methyl groups or any small delays as a result of dietary folate deficiencies or folate-associated enzyme deficiencies could result in impaired methylation of DNA, which in turn, could lead to improper gene expression of undifferentiated cells encoding for the receptors associated in cell adhesion during neural tube closure.

During neural tube formation, cells destined to becoming the neural tube undergo a primary and secondary neurulation phase. In primary neurulation, the cells of the neural plate invaginate from a flat surface into a cylindrical neural tube. Secondary neurulation refers to the sequential process where the cells of the neural plate form a massive neural cord that migrates inside the embryo and hollows to form the tube (Blom et al., 2001). In an experiment performed by Ames et al. (1997), uracil levels in DNA in folate-deficient and folate-sufficient groups were accurately determined to study the role of uracil misincorporation in DNA damage induced by folate deficiency. It was discovered that folate deficiency caused massive incorporation of uracil, as opposed to thymine, into human DNA (4 million per cell) and induced chromosome breaks. They proposed that the likely mechanism is due to the deficient methylation of deoxyuridine monophosphate to deoxythymidine monophosphate and subsequent incorporation of uracil into DNA by polymerase. Therefore, if folate is deficient in a person’s diet, namely one who is pregnant, there is likely to be a decrease in the cellular synthesis of 5,10-methylenetetrahydrofolate, as well as reduced methylenetetrahydrofolate reductase activity, which in turn, would lead to an increase in deoxyuridine monophosphate, and thus to an excessive incorporation of uracil into DNA, with the subsequent repair mechanisms increasing the risk of chromosome breakage. Without adequate amounts of thymine, DNA cannot be produced in cells that make up the neural tube, which inevitably would lead to a defective end-product. Therefore, although folate does not have a direct effect on DNA, its presence indirectly allows processes which lead to DNA synthesis to take place, which is why adequate folate is crucial for neural development and function.