Mechanistic insights into folate supplementation from Crooked tail and other NTD-prone mutant mice

The DNA demethylase TET1 modifies the impact of maternal folic acid status on embryonic brain development

Folic acid (FA) is well known to prevent neural tube defects (NTDs), but we do not know why many human NTD cases still remain refractory to FA supplementation. Here, we investigate how the DNA demethylase TET1 interacts with maternal FA status to regulate mouse embryonic brain development. We determined that cranial NTDs display higher penetrance in non-inbred than in inbred Tet1-/- embryos and are resistant to FA supplementation across strains. Maternal diets that are either too rich or deficient in FA are linked to an increased incidence of cranial deformities in wild type and Tet1+/- offspring and to altered DNA hypermethylation in Tet1-/- embryos, primarily at neurodevelopmental loci. Excess FA in Tet1-/- embryos results in phospholipid metabolite loss and reduced expression of multiple membrane solute carriers, including a FA transporter gene that exhibits increased promoter DNA methylation and thereby mimics FA deficiency. Moreover, FA deficiency reveals that Tet1 haploinsufficiency can contribute to DNA hypermethylation and susceptibility to NTDs. Overall, our study suggests that epigenetic dysregulation may underlie NTD development despite FA supplementation.

Epigenetic regulation by TET1 in gene-environmental interactions influencing susceptibility to congenital malformations

The etiology of neural tube defects (NTDs) involves complex gene-environmental interactions. Folic acid (FA) prevents NTDs, but the mechanisms remain poorly understood and at least 30% of human NTDs resist the beneficial effects of FA supplementation. Here, we identify the DNA demethylase TET1 as a nexus of folate-dependent one-carbon metabolism and genetic risk factors post-neural tube closure. We determine that cranial NTDs in Tet1 -/- embryos occur at two to three times higher penetrance in genetically heterogeneous than in homogeneous genetic backgrounds, suggesting a strong impact of genetic modifiers on phenotypic expression. Quantitative trait locus mapping identified a strong NTD risk locus in the 129S6 strain, which harbors missense and modifier variants at genes implicated in intracellular endocytic trafficking and developmental signaling. NTDs across Tet1 -/- strains are resistant to FA supplementation. However, both excess and depleted maternal FA diets modify the impact of Tet1 loss on offspring DNA methylation primarily at neurodevelopmental loci. FA deficiency reveals susceptibility to NTD and other structural brain defects due to haploinsufficiency of Tet1. In contrast, excess FA in Tet1 -/- embryos drives promoter DNA hypermethylation and reduced expression of multiple membrane solute transporters, including a FA transporter, accompanied by loss of phospholipid metabolites. Overall, our study unravels interactions between modified maternal FA status, Tet1 gene dosage and genetic backgrounds that impact neurotransmitter functions, cellular methylation and individual susceptibilities to congenital malformations, further implicating that epigenetic dysregulation may underlie NTDs resistant to FA supplementation.

Folic Acid Fortification Prevents Morphological and Behavioral Consequences of X-Ray Exposure During Neurulation

Previous studies suggested a causal link between pre-natal exposure to ionizing radiation and birth defects such as microphthalmos and exencephaly. In mice, these defects arise primarily after high-dose X-irradiation during early neurulation. However, the impact of sublethal (low) X-ray doses during this early developmental time window on adult behavior and morphology of central nervous system structures is not known. In addition, the efficacy of folic acid (FA) in preventing radiation-induced birth defects and persistent radiation-induced anomalies has remained unexplored. To assess the efficacy of FA in preventing radiation-induced defects, pregnant C57BL6/J mice were X-irradiated at embryonic day (E)7.5 and were fed FA-fortified food. FA partially prevented radiation-induced (1.0 Gy) anophthalmos, exencephaly and gastroschisis at E18, and reduced the number of pre-natal deaths, fetal weight loss and defects in the cervical vertebrae resulting from irradiation. Furthermore, FA food fortification counteracted radiation-induced impairments in vision and olfaction, which were evidenced after exposure to doses ≥0.1 Gy. These findings coincided with the observation of a reduction in thickness of the retinal ganglion cell and nerve fiber layer, and a decreased axial length of the eye following exposure to 0.5 Gy. Finally, MRI studies revealed a volumetric decrease of the hippocampus, striatum, thalamus, midbrain and pons following 0.5 Gy irradiation, which could be partially ameliorated after FA food fortification. Altogether, our study is the first to offer detailed insights into the long-term consequences of X-ray exposure during neurulation, and supports the use of FA as a radioprotectant and antiteratogen to counter the detrimental effects of X-ray exposure during this crucial period of gestation.

Advances in Nutritional Epigenetics-A Fresh Perspective for an Old Idea. Lessons Learned, Limitations, and Future Directions

Nutritional epigenetics is a rapidly expanding field of research, and the natural modulation of the genome is a non-invasive, sustainable, and personalized alternative to gene-editing for chronic disease management. Genetic differences and epigenetic inflexibility resulting in abnormal gene expression, differential or aberrant methylation patterns account for the vast majority of diseases. The expanding understanding of biological evolution and the environmental influence on epigenetics and natural selection requires relearning of once thought to be well-understood concepts. This research explores the potential for natural modulation by the less understood epigenetic modifications such as ubiquitination, nitrosylation, glycosylation, phosphorylation, and serotonylation concluding that the under-appreciated acetylation and mitochondrial dependant downstream epigenetic post-translational modifications may be the pinnacle of the epigenomic hierarchy, essential for optimal health, including sustainable cellular energy production. With an emphasis on lessons learned, this conceptional exploration provides a fresh perspective on methylation, demonstrating how increases in environmental methane drive an evolutionary down regulation of endogenous methyl groups synthesis and demonstrates how epigenetic mechanisms are cell-specific, making supplementation with methyl cofactors throughout differentiation unpredictable. Interference with the epigenomic hierarchy may result in epigenetic inflexibility, symptom relief and disease concomitantly and may be responsible for the increased incidence of neurological disease such as autism spectrum disorder.

Mouse Models of Neural Tube Defects

During embryonic development, the central nervous system forms as the neural plate and then rolls into a tube in a complex morphogenetic process known as neurulation. Neural tube defects (NTDs) occur when neurulation fails and are among the most common structural birth defects in humans. The frequency of NTDs varies greatly anywhere from 0.5 to 10 in 1000 live births, depending on the genetic background of the population, as well as a variety of environmental factors. The prognosis varies depending on the size and placement of the lesion and ranges from death to severe or moderate disability, and some NTDs are asymptomatic. This chapter reviews how mouse models have contributed to the elucidation of the genetic, molecular, and cellular basis of neural tube closure, as well as to our understanding of the causes and prevention of this devastating birth defect.

High levels of iron supplementation prevents neural tube defects in the Fpn1ffe mouse model

BACKGROUND

Periconception maternal nutrition and folate in particular are important factors influencing the incidence of neural tube defects (NTDs). Many but not all NTDs are prevented by folic acid supplementation and there is a pressing need for additional strategies to prevent these birth defects. Other micronutrients such as iron are potential candidates, yet a clear role for iron deficiency in contributing to NTDs is lacking. Our previous studies with the flatiron (ffe) mouse model of Ferroportin1 (Fpn1) deficiency suggest that iron is required for neural tube closure and forebrain development raising the possibility that iron supplementation could prevent NTDs.

METHODS

We determined the effect of periconception iron and/or folic acid supplementation on the penetrance of NTDs in the Fpn1^ffe mouse model. Concurrently, measurements of folate and iron were made to ensure supplementation had the intended effects.

RESULTS

High levels of iron supplementation significantly reduced the incidence of NTDs in Fpn1^ffe mutants. Fpn1 deficiency resulted in reduced folate levels in both pregnant dams and embryos. Yet folic acid supplementation did not prevent NTDs in the Fpn1^ffe model. Similarly, forebrain truncations were rescued with iron. Surprisingly, the high levels of iron supplementation used in this study caused folate deficiency in wild-type dams and embryos.

CONCLUSION

Our results demonstrate that iron supplementation can prevent NTDs and forebrain truncations in the Fpn1^ffe model. Surprisingly, high levels of iron supplementation and iron overload can cause folate deficiency. If iron is essential for neural tube closure, it is possible that iron deficiency might contribute to NTDs. Birth Defects Research 109:81-91, 2017. © 2016 The Authors Birth Defects Research Published by Wiley Periodicals, Inc.

Prevention of neural tube defects in Lrp2 mutant mouse embryos by folic acid supplementation

BACKGROUND

Neural tube defects (NTDs) are among the most common structural birth defects in humans and are caused by the complex interaction of genetic and environmental factors. Periconceptional supplementation with folic acid can prevent NTDs in both mouse models and human populations. A better understanding of how genes and environmental factors interact is critical toward development of rational strategies to prevent NTDs. Low density lipoprotein-related protein 2 (Lrp2) is involved in endocytosis of the folic acid receptor among numerous other nutrients and ligands.

METHODS

We determined the effect of iron and/or folic acid supplementation on the penetrance of NTDs in the Lrp2^null mouse model. The effects of supplementation on folate and iron status were measured in embryos and dams.

RESULTS

Periconceptional dietary supplementation with folic acid did not prevent NTDs in Lrp2 mutant embryos, whereas high levels of folic acid supplementation by intraperitoneal injection reduced incidence of NTDs. Importantly, Lrp2^null/+ dams had reduced blood folate levels that improved with daily intraperitoneal injections of folate but not dietary supplementation. On the contrary, iron supplementation had no effect on the penetrance of NTDs in Lrp2 mutant embryos and negated the preventative effect of folic acid supplementation in Lrp2^null/null mutants.

CONCLUSION

Lrp2 is required for folate homeostasis in heterozygous dams and high levels of supplementation prevents NTDs. Furthermore, high levels of dietary iron supplementation interfered with folic acid supplementation negating the positive effects of supplementation in this model. Birth Defects Research 109:16-26, 2017. © 2016 The Authors Birth Defects Published by Wiley Periodicals, Inc.

Untargeted metabolite profiling of murine embryos to reveal metabolic perturbations associated with neural tube closure defects

BACKGROUND

Neural tube closure defects (NTDs) are among the most common congenital malformation in human, typically presenting in liveborns as spina bifida. At least 240 gene mutations in mouse are known to increase the risk of NTD. There is a growing appreciation that environmental factors significantly contribute to NTD expression, and that NTDs likely arise from complex gene-environment interactions. Because maternal folic acid supplementation reduces human NTD risk in some populations by 60 to 70%, it is likely that NTD predisposition is often associated with a defect in folate-dependent one-carbon metabolism. A comprehensive, untargeted metabolic survey of NTD-associated changes in embryo metabolism would provide a valuable test of this assumption. We sought to establish a metabolic profiling platform that is capable of broadly assessing metabolic aberrations associated with NTD-promoting gene mutations in early-stage mouse embryos.

METHODS

A liquid chromatography/mass spectrometry-based untargeted metabolite profiling platform was used to broadly identify significant differences in small molecule levels (50-1000 Da) in NTD-affected embryonic day (E) 9.5 mouse embryos (Lrp6(-) (/) (-) ) versus unaffected (Lrp6(+/+) ) control embryos.

RESULTS

Results provide proof-of-principal feasibility for the broad survey of the metabolome of individual E9.5 mouse embryos and identification of metabolic changes associated with NTDs and gene mutations. Levels of 30 different metabolites were altered in association with Lrp6 gene deletion. Some metabolites link to folate-dependent one-carbon transfer reactions, as anticipated, while others await structure elucidation and pathway integration.

CONCLUSION

Whole-embryo metabolomics offers the potential to identify metabolic changes in genetically determined NTD-prone embryos.

Mouse as a model for multifactorial inheritance of neural tube defects

Neural tube defects (NTDs) such as spina bifida and anencephaly are some of the most common structural birth defects found in humans. These defects occur due to failures of neurulation, a process where the flat neural plate rolls into a tube. In spite of their prevalence, the causes of NTDs are poorly understood. The multifactorial threshold model best describes the pattern of inheritance of NTDs where multiple undefined gene variants interact with environmental factors to cause an NTD. To date, mouse models have implicated a multitude of genes as required for neurulation, providing a mechanistic understanding of the cellular and molecular pathways that control neurulation. However, the majority of these mouse models exhibit NTDs with a Mendelian pattern of inheritance. Still, many examples of multifactorial inheritance have been demonstrated in mouse models of NTDs. These include null and hypomorphic alleles of neurulation genes that interact in a complex fashion with other genetic mutations or environmental factors to cause NTDs. These models have implicated several genes and pathways for testing as candidates for the genetic basis of NTDs in humans, resulting in identification of putative pathogenic mutations in some patients. Mouse models also provide an experimental paradigm to gain a mechanistic understanding of the environmental factors that influence NTD occurrence, such as folic acid and maternal diabetes, and have led to the discovery of additional preventative nutritional supplements such as inositol. This review provides examples of how multifactorial inheritance of NTDs can be modeled in the mouse.

The epigenetic effects of a high prenatal folate intake in male mouse fetuses exposed in utero to arsenic

Inorganic arsenic (iAs) is a complete transplacental carcinogen in mice. Previous studies have demonstrated that in utero exposure to iAs promotes cancer in adult mouse offspring, possibly acting through epigenetic mechanisms. Humans and rodents enzymatically convert iAs to its methylated metabolites. This reaction requires S-adenosylmethionine (SAM) as methyl group donor. SAM is also required for DNA methylation. Supplementation with folate, a major dietary source of methyl groups for SAM synthesis, has been shown to modify iAs metabolism and the adverse effects of iAs exposure. However, effects of gestational folate supplementation on iAs metabolism and fetal DNA methylation have never been thoroughly examined. In the present study, pregnant CD1 mice were fed control (i.e. normal folate, or 2.2 mg/kg) or high folate diet (11 mg/kg) from gestational day (GD) 5 to 18 and drank water with 0 or 85 ppm of As (as arsenite) from GD8 to 18. The exposure to iAs significantly decreased body weight of GD18 fetuses and increased both SAM and S-adenosylhomocysteine (SAH) concentrations in fetal livers. High folate intake lowered the burden of total arsenic in maternal livers but did not prevent the effects of iAs exposure on fetal weight or hepatic SAM and SAH concentrations. In fact, combined folate-iAs exposure caused further significant body weight reduction. Notably, iAs exposure alone had little effect on DNA methylation in fetal livers. In contrast, the combined folate-iAs exposure changed the CpG island methylation in 2,931 genes, including genes known to be imprinted. Most of these genes were associated with neurodevelopment, cancer, cell cycle, and signaling networks. The canonical Wnt-signaling pathway, which regulates fetal development, was among the most affected biological pathways. Taken together, our results suggest that a combined in utero exposure to iAs and a high folate intake may adversely influence DNA methylation profiles and weight of fetuses, compromising fetal development and possibly increasing the risk for early-onset of disease in offspring.

Use of laboratory studies for the design, explanation, and validation of human micronutrient intervention studies

Many micronutrient supplementation trials have led to important new findings relevant to public health, but some outcomes have been unclear or concerning. Can and should laboratory studies and animal models be used more extensively to pretest the proposed designs of human studies? This paper illustrates, as examples, the contributions that animal models have made to several major advances in understanding the biology of the micronutrients vitamin A and carotenoids, and it proposes that animal studies can play a more integrated role in public health nutrition by serving as a first line of interrogation for study designs and thereby as a means of refining the designs of human studies so that large, expensive, and logistically difficult human trials will yield the best possible information.

Neural tube closure in mouse whole embryo culture

Genetic mouse models are an important tool in the study of mammalian neural tube closure (Gray & Ross, 2009; Ross, 2010). However, the study of mouse embryos in utero is limited by our inability to directly pharmacologically manipulate the embryos in isolation from the effects of maternal metabolism on the reagent of interest. Whether using a small molecule, recombinant protein, or siRNA, delivery of these substances to the mother, through the diet or by injection will subject these unstable compounds to a variety of bodily defenses that could prevent them from reaching the embryo. Investigations in cultures of whole embryos can be used to separate maternal from intrinsic fetal effects on development. Here, we present a method for culturing mouse embryos using highly enriched media in a roller incubator apparatus that allows for normal neural tube closure after dissection (Crockett, 1990). Once in culture, embryos can be manipulated using conventional in vitro techniques that would not otherwise be possible if the embryos were still in utero. Embryo siblings can be collected at various time points to study different aspects of neurulation, occurring from E7-7.5 (neural plate formation, just prior to the initiation of neurulation) to E9.5-10 (at the conclusion of cranial fold and caudal neuropore closure, Kaufman, 1992). In this protocol, we demonstrate our method for dissecting embryos at timepoints that are optimal for the study of cranial neurulation. Embryos will be dissected at E8.5 (approx. 10-12 somities), after the initiation of neural tube closure but prior to embryo turning and cranial neural fold closure, and maintained in culture till E10 (26-28 somities), when cranial neurulation should be complete.

Maternal intake of folic acid and neural crest stem cells

Maternal folic acid (FA) intake has beneficial effects in preventing neural tube defects and may also play a role in the prevention of adult onset diseases such as Alzheimer's disease, dementia, neuropsychiatric disorders, cardiovascular diseases, and cerebral ischemia. This review will focus on the effects of maternal FA intake on neural crest stem cell proliferation and differentiation. Although FA is generally considered beneficial, it has the potential of promoting cell proliferation at the expense of differentiation. In some situations, this may lead to miscarriage or postnatal developmental abnormalities. Therefore, a blind approach such as "FA for everyone" is not necessarily the best course of action. Ultimately, the best approach for FA supplementation, and potentially other nutritional supplements, will include customized patient genomic profiles for determining dose and duration.

An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure

The number of mouse mutants and strains with neural tube defects (NTDs) now exceeds 240, including 205 representing specific genes, 30 for unidentified genes, and 9 multifactorial strains. These mutants identify genes needed for embryonic neural tube closure. Reports of 50 new NTD mutants since our 2007 review (Harris and Juriloff, 2007) were considered in relation to the previously reviewed mutants to obtain new insights into mechanisms of NTD etiology. In addition to null mutations, some are hypomorphs or conditional mutants. Some mutations do not cause NTDs on their own, but do so in digenic, trigenic, and oligogenic combinations, an etiology that likely parallels the nature of genetic etiology of human NTDs. Mutants that have only exencephaly are fourfold more frequent than those that have spina bifida aperta with or without exencephaly. Many diverse cellular functions and biochemical pathways are involved; the NTD mutants draw new attention to chromatin modification (epigenetics), the protease-activated receptor cascade, and the ciliopathies. Few mutants directly involve folate metabolism. Prevention of NTDs by maternal folate supplementation has been tested in 13 mutants and reduces NTD frequency in six diverse mutants. Inositol reduces spina bifida aperta frequency in the curly tail mutant, and three new mutants involve inositol metabolism. The many NTD mutants are the foundation for a future complete genetic understanding of the processes of neural fold elevation and fusion along mechanistically distinct cranial-caudal segments of the neural tube, and they point to several candidate processes for study in human NTD etiology.

Functional interactions between the LRP6 WNT co-receptor and folate supplementation

Crooked tail (Cd) mice bear a gain-of-function mutation in Lrp6, a co-receptor for canonical WNT signaling, and are a model of neural tube defects (NTDs), preventable with dietary folic acid (FA) supplementation. Whether the FA response reflects a direct influence of FA on LRP6 function was tested with prenatal supplementation in LRP6-deficient embryos. The enriched FA (10 ppm) diet reduced the occurrence of birth defects among all litters compared with the control (2 ppm FA) diet, but did so by increasing early lethality of Lrp6(-/-) embryos while actually increasing NTDs among nulls alive at embryonic days 10-13 (E10-13). Proliferation in cranial neural folds was reduced in homozygous Lrp6(-/-) mutants versus wild-type embryos at E10, and FA supplementation increased proliferation in wild-type but not mutant neuroepithelia. Canonical WNT activity was reduced in LRP6-deficient midbrain-hindbrain at E9.5, demonstrated in vivo by a TCF/LEF-reporter transgene. FA levels in media modulated the canonical WNT response in NIH3T3 cells, suggesting that although FA was required for optimal WNT signaling, even modest FA elevations attenuated LRP5/6-dependent canonical WNT responses. Gene expression analysis in embryos and adults showed striking interactions between targeted Lrp6 deficiency and FA supplementation, especially for mitochondrial function, folate and methionine metabolism, WNT signaling and cytoskeletal regulation that together implicate relevant signaling and metabolic pathways supporting cell proliferation, morphology and differentiation. We propose that FA supplementation rescues Lrp6(Cd/Cd) fetuses by normalizing hyperactive WNT activity, whereas in LRP6-deficient embryos, added FA further attenuates reduced WNT activity, thereby compromising development.

Folic acid remodels chromatin on Hes1 and Neurog2 promoters during caudal neural tube development

The mechanism(s) behind folate rescue of neural tube closure are not well understood. In this study we show that maternal intake of folate prior to conception reverses the proliferation potential of neural crest stem cells in homozygous Splotch embryos (Sp(-/-)) via epigenetic mechanisms. It is also shown that the pattern of differentiation seen in these cells is similar to wild-type (WT). Cells from open caudal neural tubes of Sp(-/-) embryos exhibit increased H3K27 methylation and decreased expression of KDM6B possibly due to up-regulation of KDM6B targeting micro-RNAs such as miR-138, miR-148a, miR-185, and miR-339-5p. In our model, folate reversed these epigenetic marks in folate-rescued Sp(-/-) embryos. Using tissue from caudal neural tubes of murine embryos we also examined H3K27me2 and KDM6B association with Hes1 and Neurog2 promoters at embryonic day E10.5, the proliferative stage, and E12.5, when neural differentiation begins. In Sp(-/-) embryos compared with WT, levels of H3K27me2 associated with the Hes1 promoter were increased at E10.5, and levels associated with the Neurog2 promoter were increased at E12.5. KDM6B association with Hes1 and Neurog2 promoters was inversely related to H3K27me2 levels. These epigenetic changes were reversed in folate-rescued Sp(-/-) embryos. Thus, one of the mechanisms by which folate may rescue the Sp(-/-) phenotype is by increasing the expression of KDM6B, which in turn decreases H3K27 methylation marks on Hes1 and Neurog2 promoters thereby affecting gene transcription.

Gene-environment interactions, folate metabolism and the embryonic nervous system

Formation of brain and spinal cord requires the successful closure of neural ectoderm into an embryonic neural tube. Defects in this process result in anencephaly or spina bifida, which together constitute a leading cause of mortality and morbidity in children, affecting all ethnic and socioeconomic groups. The subject of intensive research for decades, neural tube defects (NTDs), are understood to arise from complex interactions of genes and environmental conditions, though systems-level details are still elusive. Despite the variety of underlying causes, a single intervention, folic acid supplementation given in the first gestational month, can measurably reduce the occurrence of NTDs in a population. Evidence for and the scope of gene-environment interactions in the genesis of NTDs is discussed. A systems-based approach is now possible toward studies of genetic and environmental influences underlying NTDs that will enable the assessment of individual risk and personalized optimization of prevention.