See corresponding article on page 109.
Folate-responsive birth defects: of mice and women1,2
The recognition that periconceptional folate supplementation
in addition to normal dietary folate intake reduced the incidence
of neural tube defects (NTDs), one of the most common birth
defects, has led to mandatory folate fortification in the United
States and ;50 other countries (1, 2). Fortification programs
have reduced the incidence of NTDs by ;20–80%, with the
largest reductions occurring in populations with the highest pre-
fortification rates (3). The benefits of folate fortification in re-
ducing birth defects and in reducing the incidence of folate
deficiency in the general population have been clearly shown
(3, 4). However, concerns have been raised about possible ad-
verse effects of high folate intake on segments of the population
other than women of childbearing age. The level of fortification
in the United States was estimated to provide, on average, 100
lg additional folic acid daily with only a very small proportion
of the population receiving .1 mg to prevent masking of symp-
toms of vitamin B-12 deficiency (anemia), primarily in the el-
derly population. More recently, concerns have been raised that
high folate may promote tumor growth (5). Although poor folate
status is a risk factor for various cancers, some studies have
suggested that high folate intakes have increased the incidence
of some cancers, and it has been speculated that high folate may
increase the growth rate of existing neoplasms.
It would be beneficial if it were possible to identify those
women most at risk of NTDs. However, the reason why folate
influences NTD rates is not known. NTDs are a low-penetrance
condition influenced by environmental factors such as folate in-
take and genetic factors such as polymorphisms in folate genes,
among others. A large number of human gene variants have now
is a common polymorphism in the methylenetetrahydrofolate re-
ductase (MTHFR 677C/T) gene (6). However, the vast major-
ity of fetuses that are homozygous for the 677T allele do not
develop NTDs. Mouse models for NTDs have the potential for
providing more mechanistic information. A large number of
mouse models for birth defects have been described, but few
are folate responsive. Disruption of the mouse Folbp1 gene,
which encodes a folate receptor involved in folate transport,
results in spina bifida and embryonic lethality; both can be
prevented by very high folate, indicating, in this model at least,
that an impairment of folate-dependent one-carbon metabolism
is sufficient to generate the NTD phenotype (7).
In this issue (8) and in a separate recent article (9) of the
Journal, Beaudin et al report on the first folate-responsive mouse
model for NTDs due to a disruption in a gene encoding an
enzyme directly involved in one-carbon metabolism. Mice lack-
ing Shmt1, which encodes cytosolic serine hydroxymethyltrans-
ferase (cSHMT), developed exencephaly, a form of anencephaly,
when fed a diet deficient in folate and choline but did not de-
velop spina bifida (9). Choline deficiency was used because of
epidemiologic evidence suggesting an effect on NTDs (10). Fol-
low-up studies showed that this phenotype was due to reduced
folate and not to reduced choline (8). Folate-dependent one-
carbon metabolism is required for the synthesis of precursors
for DNA synthesis (thymidylate and purines) and for methionine
synthesis (homocysteine remethylation) and consequently for
methylation reactions involved in a host of processes including
epigenetic events (11). Because choline is also involved in
methyl group status and homocysteine remethylation, their data
suggest that the NTD phenotype and folate responsiveness of
this mouse model were due to a defect in DNA replication rather
than a defect in methylation reactions. cSHMT in the cytosol can
provide one carbons for methionine synthesis. However, during
the S phase of the cell cycle, it translocates to the nucleus
together with the 2 other folate-dependent enzymes in the thy-
midylate synthesis cycle, thymidylate synthase and dihydrofo-
late reductase, to support DNA replication (12).
The involvement of defective DNA synthesis in NTD gener-
ation was further reinforced by the authors’ studies in mice with
the Splotch mutation in the Pax3 gene (9). Pax3 encodes a tran-
scription factor and Splotch has high penetrance for NTDs,
mainly spina bifida but also some exencephaly. The Splotch
mutant had previously been shown to be impaired in de novo
thymidylate synthesis and that its NTD phenotype was folate
responsive and could also be rescued with thymidine (13, 14).
In the current studies, by using embryonic fibroblasts from the
Pax3 mutants, the impairment in de novo thymidylate synthesis
was reconfirmed and, interestingly, de novo purine synthesis
was also impaired. Methylation potential, as measured by the
S-adenosylmethionine:S-adenosylhomocysteine ratio, was unim-
paired and actually increased. Deletion of Shmt1 on the Pax3Sp/Sp
background exacerbated the NTD phenotype, and the effect was
enhanced by combined maternal folate and choline deficiency.
Embryonic fibroblasts from Splotch mutants expressed higher
1From the Department of Nutritional Sciences and Toxicology, University
of California, Berkeley, CA.
2Address correspondence to B Shane, Department of Nutritional Sciences
and Toxicology, 329 Morgan Hall, University of California, Berkeley, CA
94720-3104. E-mail: firstname.lastname@example.org.
First published online December 7, 2011; doi: 10.3945/ajcn.111.029595.
Am J Clin Nutr 2012;95:1–2. Printed in USA. ? 2012 American Society for Nutrition
by guest on October 19, 2015
amounts of cSHMT protein and lower amounts of thymidylate
synthase, whereas Pax3 amounts increased in embryos from
Shmt1-mutant mice fed the folate- and choline-deficient diet.
These data indicate that, in these mouse models at least, the
NTD phenotype shares a common etiology of defective DNA
synthesis and the genetic disruptions appear to be cross-regulating
the same subset of enzymes.
How relevant is this to human NTDs? No genetic variants in the
human folate receptor a gene (equivalent to mouse Folbp1), in the
cSHMT gene or in the Pax3 gene have been identified that are risk
factors for human NTDs. Conversely, mice with deletions in the
MTHFR gene do not develop NTDs. Although some may question
the usefulness of mouse models for identifying risk factors for
human NTDs, there has been a paucity of studies investigating
posttranscription risk factors, which may also play a role in NTD
etiology. For example, some studies have found that maternal
circulating antibodies to the folate receptor are a risk factor for
NTDs (15), although other studies have been unable to confirm
this (16). cSHMT is also an example of an enzyme that is regu-
lated posttranscriptionally by a variety of factors (17).
It is reasonable to suppose that the underlying mechanism of
defective DNA replication suggested by the mouse models of
NTDs would be shared by human NTDs. This mechanism is par-
coordinated regulation of cell division of layers of cells growing at
different rates. Whereas a multitude of factors can cause defective
DNA synthesis, the folate responsiveness of most human NTDs
would limit the possibilities to a much smaller subset.
The author had no conflicts of interest to report.
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by guest on October 19, 2015