Nutrient-Gene Interaction: Folic Acid and Prenatal Neural
Author: Christe Marbbn
Published: April 2, 2009
Folic Acid; Neural Tube Defects; Homocysteine; Methionine;
J. Ugrad. Biol. S.
Nutrients play a major role in influencing gene
expression and protein interaction. The effect of dietary
changes on phenotypes differs significantly between
individuals. Some individuals appear to
be relatively insensitive to dietary
interventions, while others are highly sensitive (Roche,
2006). This phenomenon has been extensively researched in
relation to the regular dietary consumption of folic acid
and the prevention of neural tube defects during
embryogenesis in humans. Folic acid or folate is a
water-soluble vitamin of the B complex which plays a
critical role in cell division and the formation of new
cells - two main processes that are vital during prenatal
development and the maturation of undifferentiated cells.
Folate is important to numerous bodily functions, ranging
from nucleotide synthesis to the reduction of elevated
plasma homocysteine levels. It naturally occurs in leafy
vegetables such as spinach and lettuces, but as more
research is conducted regarding the nutrient’s benefits on
health and wellness, its discovery has influenced food
scientists and processing experts to add folate to a variety
nutraceutical supplements and fortified foods. Furthermore,
numerous reports have suggested that nutritional deficiency
in general, and folate deficiency in particular, can cause
adverse birth outcomes such as neural tube defects (Bendich
& Butterworth, 1996).
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
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
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
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
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.,
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
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).
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
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
incorporation of uracil into DNA, with the subsequent
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.
It is quite evident that folate is an important factor in the biochemical
processes that lead to neural tube formation; however, if
supplementation is not taken at the right time, pregnant
women cannot reap its full benefits. The neural tube closure
generally occurs on days 22-28 after ovulation (Economides &
Kadir, 2002). Since more than 40% of pregnancies are
unplanned most of these women are unaware of being pregnant
up until the first or second month in their pregnancy
(Economides & Kadir, 2002). In a study performed by Schorah
et al. (1976),
vitamin levels in blood of first trimester women who had a
baby with neural tube defects were measured. They discovered
that red blood cell folate were significantly lower compared to mothers
without neural tube defected infants, even though their
serum folate was normal. Recall that red blood cells have a life
span of 120 days; this suggests that the red blood cells of
these women originated well before conception, whereas serum
folate reflects recent dietary intake. Therefore, even if
adequate amounts of folate are ingested during the first few
weeks into pregnancy, what matters more is if adequate
amounts of the vitamin are ingested prior to pregnancy than
during pregnancy (Schorah
et al., 1976).
This implies that periconception ingestion of folate can
reduce the number of pregnancies affected by neural tube
defects (Berry et al.,
1988). Currently, 400 µg of folic acid is recommended for
women in the general population who are trying to conceive
(Economides & Kadir, 2002).
Studying folate metabolism helps scientists and medical physicians
understand why neural tube defects arise in certain
pregnancies. These studies help women who are trying to
conceive understand the risks of exercising certain diets
before and during their pregnancy. Given the available
evidence, women who take these precautions can feel relaxed
that they have done all they can to ensure that their
developing child has a lower chance of being inflicted with
a folate-deficient cause of a neural tube defect. In fact,
knowing the culprit behind these defects potentially avoids
or minimizes the number of pregnancy terminations in the
second trimester because of these anomalies that prevent the
child from developing normally. On the basis of these
findings, the government can also take the immediate
precautions of determining the proper recommended amounts
for people of the general public and persuade food makers to
fortify foods with folic acid as a justifiable means of
preventing neural tube defects from occuring in women with
unplanned pregnancies who, at the same time, are uneducated
about this phenomenon.
Future studies should focus on identifying polymorphisms
or mutations in genes involved in the synthesis of thymidylate, purines,
regulatory proteins, or substrates involved in folate and homocysteine
metabolism, such as serine hydroxymethyltransferase and
rather than simply focusing on 5,10-methylenetetrahydrofolate reductase as the
main cause. These studies should investigate mutations within the coding regions
of the genes, quantification of mRNA levels, examination of untranslated regions
of the gene, and test promoter functions
et al., 2001).
Moreover, most of the current studies only verify the mechanism of an impaired
homocysteine metabolism in relation to a defective neural tube closure, but
still give no insight
in the mechanisms that are affected
during neurulation. Henceforth, in order to be able to
stipulate a vivid link between a disruption in homocysteine metabolism and
neural tube defects, studies should focus on the involvement of homocysteine in
microfilament synthesis or processes leading to DNA methylation. Lastly,
research should also focus on non-folate alternatives in preventing neural tube
defects, since only 30% of neural tube defected pregnancies are due to dietary
folate deficiencies (Blom
et al., 2001). If
these factors are not taken into full consideration and detailed accurately,
major pieces of the puzzle still await discovery and explanation.
Overall, dietary folate should be
accepted as an important means of helping to keep a developing infant and
his/her mother healthy.
Alberto J.M., Daval J.L., Forges T., Guéant J.L., Guéant-Rodriguez R.M.,
Monnier-Barbarino P. (2007). Impact of folate and
homocysteine metabolism on human reproductive health.
Update, 13(3): 225-238.
Ames B.N., Blount B.C., Everson R.B., Hiatt R.A., Mack M.W., MacGregor
J.T., & Wickramasinghe S.N. (1997). Folate deficiency causes
uracil misincorporation into human DNA and chromosome
breakage: implications for cancer and neuronal damage.
The Proceedings of the
National Academy of Sciences USA, 94: 3290-3295.
Bendich A., & Butterworth C.E. (1996). Folic Acid and the Prevention of
Birth Defects. Annual
Reviews of Nutrition, 16: 73-79.
Berry R.J., Cordero J.F., Erickson J.D., & Mulinare J. (1988).
Periconceptional use of multivitamins and the
occurrence of neural tube defects. The Journal of the
American Medical Association, 260: 3141-3145.
Blom H.J., Boers G.J.H., Den Heijer M., Frosst P., Goyette
P., Matthews R.G., Milos R., Sheppard C.A., & Van Den Heuve
A candidate genetic risk factor
disease: a common
mutation in methylenetetrahydrofolate reductase.
Blom H.J., Esker T.K., Graaf-Hess A., Mariman E.C.M.,
Smeitinik J.A.M., Steegers-Theunissen R.P.M., Thomas C.M.G.,
& Can Der Put N.M.J. (1997).
Altered folate and
vitamin B12 metabolism in
families with spina
bifida offspring. QJM:
Journal of Medicine,
Blom H.J., Trijbels F.J.M., Van Der Put N.M.J., & Van
Straaten H.W.M. (2001).
Folate, Homocysteine and Neural Tube Defects:
and Medicine, 226(4): 243-270.
Boers G.H.J., Eskes T.K., Nelen W.L., Steegers
E.A., & Thomas C.M. (1998).
Methylenetetrahydrofolate reductase polymorphism
affects the change in homocysteine and folate
from low dose folic acid supplementation in women
Jounral of Nutrition,
Boubelik M., Lehnert S., & Monk M. (1987). Temporal and regional changes
methylation in the embryonic, extraembryonic and germ cell lineages during mouse embryo development.
Choi S.W., Mason J.B., Selhub J., & Stern L.L. (2000). Genomic DNA
hypomethylation, a characteristic of most cancers, is
present in peripheral leukocytes of individuals who are
homozygous for the C677T polymorphism in the
methylenetetrahydrofolate reductase gene.
Cancer epidemiology, biomarkers & prevention, 9: 849-853.
Dehner L.P., & Stocker T.J. (2001). Pediatric
Pathology (1st ed.).
Lippincott Williams &
Economides D.L., & Kadir R.A. (2002). Neural
Tube Defects & Periconceptional Folic Acid.
Canadian Medical Association Journal, 167(3): 255-256.
Fowler B. (2001). The folate cycle and disease in humans.
Herrmann W. (2001). The importance of hyperhomocysteinemia as a risk
factor for diseases: an overview.
Clinical Chemistry and
Laboratory Medicine, 39: 666-74.
Roche H.M. (2006). Nutrigenomics - new
approaches for human nutrition research.
Journal of the Science
of Food and Agriculture, 86(8): 1156-1163.
Schorah C.J., Sheppard S., & Smithells R.W. (1976). Vitamin deficiencies
tube defects. Archives
of Disease in Childhood,