Contributions of dam and conceptus to differences in sensitivity to valproic acid among C57 black and SWV mice

Embryonic Hypotaurine Levels Contribute to Strain-Dependent Susceptibility in Mouse Models of Valproate-Induced Neural Tube Defects

Valproic acid (VPA, valproate, Depakote) is a commonly used anti-seizure medication (ASM) in the treatment of epilepsy and a variety of other neurological disorders. While VPA and other ASMs are efficacious for management of seizures, they also increase the risk for adverse pregnancy outcomes, including neural tube defects (NTDs). Thus, the utility of these drugs during pregnancy and in women of childbearing potential presents a continuing public health challenge. Elucidating the underlying genetic or metabolic risk factors for VPA-affected pregnancies may lead to development of non-teratogenic ASMs, novel prevention strategies, or more targeted methods for managing epileptic pregnancies. To address this challenge, we performed unbiased, whole embryo metabolomic screening of E8.5 mouse embryos from two inbred strains with differential susceptibility to VPA-induced NTDs. We identified metabolites of differential abundance between the two strains, both in response to VPA exposure and in the vehicle controls. Notable enriched pathways included lipid metabolism, carnitine metabolism, and several amino acid pathways, especially cysteine and methionine metabolism. There also was increased abundance of ω-oxidation products of VPA in the more NTD-sensitive strain, suggesting differential metabolism of the drug. Finally, we found significantly reduced levels of hypotaurine in the susceptible strain regardless of VPA status. Based on this information, we hypothesized that maternal supplementation with L-carnitine (400 mg/kg), coenzyme A (200 mg/kg), or hypotaurine (350 mg/kg) would reduce VPA-induced NTDs in the sensitive strain and found that administration of hypotaurine prior to VPA exposure significantly reduced the occurrence of NTDs by close to one-third compared to controls. L-carnitine and coenzyme A reduced resorption rates but did not significantly reduce NTD risk in the sensitive strain. These results suggest that genetic variants or environmental exposures influencing embryonic hypotaurine status may be factors in determining risk for adverse pregnancy outcomes when managing the health care needs of pregnant women exposed to VPA or other ASMs.

Unraveling the complex genetics of neural tube defects: From biological models to human genomics and back

Neural tube defects (NTDs) are a classic example of preventable birth defects for which there is a proven-effective intervention, folic acid (FA); however, further methods of prevention remain unrealized. In the decades following implementation of FA nutritional fortification programs throughout at least 87 nations, it has become apparent that not all NTDs can be prevented by FA. In the United States, FA fortification only reduced NTD rates by 28-35% (Williams et al., 2015). As such, it is imperative that further work is performed to understand the risk factors associated with NTDs and their underlying mechanisms so that alternative prevention strategies can be developed. However, this is complicated by the sheer number of genes associated with neural tube development, the heterogeneity of observable phenotypes in human cases, the rareness of the disease, and the myriad of environmental factors associated with NTD risk. Given the complex genetic architecture underlying NTD pathology and the way in which that architecture interacts dynamically with environmental factors, further prevention initiatives will undoubtedly require precision medicine strategies that utilize the power of human genomics and modern tools for assessing genetic risk factors. Herein, we review recent advances in genomic strategies for discovering genetic variants associated with these defects, and new ways in which biological models, such as mice and cell culture-derived organoids, are leveraged to assess mechanistic functionality, the way these variants interact with other genetic or environmental factors, and their ultimate contribution to human NTD risk.

Tissue-specific effects of valproic acid on DNA repair genes and apoptosis in postimplantation mouse embryos

Exposure to the anticonvulsant drug valproic acid (VPA) is associated with an increased risk of congenital malformations. Although the mechanisms contributing to its teratogenicity are poorly understood, VPA has been shown to induce DNA double strand breaks (DSB) and to increase homologous recombination in vitro. The objective of the present study was to determine whether in utero exposure to VPA alters the frequency of intrachromosomal recombination and the expression of several genes involved in DSB repair in pKZ1 mouse embryos. Pregnant pKZ1 transgenic mice (GD 9.0) were administered VPA (500 mg/kg s.c.) and embryos were extracted and microdissected into the head, heart, and trunk regions 1, 3, 6, and 24 h after injection. Quantitative PCR was used to measure the tissue-specific expression of lacZ, a surrogate measure of recombination, Xrcc4, Rad51, Brca1, and Brca2, with Western blotting used to quantify Rad51, cleaved caspase-3 and cleaved-PARP protein. Increased recombination was only observed in the embryonic head following 6-h VPA exposure. VPA had no effect on Xrcc4 expression. Rad51, Brca1, and Brca2 expression rapidly decreased in head and trunk tissues after 1-h VPA exposure, followed by a subsequent increase in all tissues, although it was generally attenuated in the head and not due to differences in endogenous levels. Cleaved caspase-3 and cleaved-PARP expression was increased in all tissues 3 h following VPA exposure. This study indicates that the tissue-specific expression of several genes involved in DSB repair is altered following exposure to VPA and may be contributing to increased apoptosis.

Genetic basis of susceptibility to teratogen induced birth defects

Birth defects remain the leading cause of infant death in US. The field of teratology has been focused on the causes and underlying mechanisms of birth defects for decades, yet our understanding of these critical issues remain unacceptably vague. Conclusions from years of animal and human studies made it clear that the vast majority of birth defects have multifactorial origins, with contributions from environmental and genetic factors. The environment comprises not only of the physical, biological, and chemical external environment surrounding the pregnant woman, but it also includes the internal environment of the woman's body that interact with the developing embryo in a complex fashion. The importance of maternal and embryonic genetic factors consisting of countless genetic variants/mutations that exist within every individual contribute to birth defect susceptibility is only now being more fully appreciated. This great complexity of the genome and its diversity within individuals and populations seems to be the principal reason why the same teratogenic exposure can induce severe malformation in one embryo, while fail to do so to other exposed embryos. As the interaction between genetic and environmental factors has long been recognized as the first "Principle of Teratology" by Wilson and Warkany [1965. Teratology: Principles and techniques. Chicago: University of Chicago Press], it is only recently that the appropriate investigative tools have been developed with which to fully investigate this fundamental principle. The introduction of high throughput technologies like whole genome sequencing or genome-wide association studies are promising to deliver an enormous amount of new data that will shed light on the genomic factors that contribute susceptibility to environmental teratogens. In this review, we attempt to summarize the epidemiological and experimental literature concerning birth defects whose phenotypic expression can be clearly related to the interactions between several select environmental factors and those genetic pathways in which they are most likely to have significant modifying effects. © 2011 Wiley-Liss, Inc.

Maternal diet modulates the risk for neural tube defects in a mouse model of diabetic pregnancy

Pregnancies complicated by maternal diabetes have long been known to carry a higher risk for congenital malformations, such as neural tube defects. Using the FVB inbred mouse strain and the Streptozotocin-induced diabetes model, we tested whether the incidence of neural tube defects in diabetic pregnancies can be modulated by maternal diet. In a comparison of two commercial mouse diets, which are considered nutritionally replete, we found that maternal consumption of the unfavorable diet was associated with a more than 3-fold higher rate of neural tube defects. Our results demonstrate that maternal diet can act as a modifier of the risk for abnormal development in high-risk pregnancies, and provide support for the possibility that neural tube defects in human diabetic pregnancies might be preventable by optimized maternal nutrition.

Genetic and maternal effects on valproic acid teratogenesis in C57BL/6J and DBA/2J mice

Valproic acid (VPA) is used worldwide to treat epilepsy, migraine headaches, and bipolar disorder. However, VPA is teratogenic and in utero exposure can lead to congenital malformations. Using inbred C57BL/6J (B6) and DBA/2J (D2) mice, we asked whether genetic variation could play a role in susceptibility to VPA teratogenesis. Whereas B6 fetuses were more susceptible than D2 fetuses to digit and vertebral malformations, D2 fetuses were more susceptible to rib malformations. In a reciprocal cross between B6 and D2, genetically identical F1 mice carried in a B6 mother had a greater percentage of vertebral malformations following prenatal VPA exposure than F1 mice carried in a D2 mother. This reciprocal F1 difference is known as a maternal effect and shows that maternal genotype/uterine environment is an important mediator of VPA teratogenecity. VPA is a histone deacetylase inhibitor, and it is possible that the differential teratogenesis in B6 and D2 is because of strain differences in histone acetylation. We observed strain differences in acetylation of histones H3 and H4 in both embryo and placenta following in utero VPA exposure, but additional studies are needed to determine the significance of these changes in mediating teratogenesis. Our results provide additional support that genetic factors, both maternal and fetal, play a role in VPA teratogenesis. Lines of mice derived from B6 and D2 will be a useful model for elucidating the genetic architecture underlying susceptibility to VPA teratogenesis.

Importance of gene-environment interactions in the etiology of selected birth defects

It is generally understood that both genetic and environmental factors contribute to the highly complex etiology of structural birth defects, including neural tube defects, oral clefts and congenital heart defects, by disrupting highly regulated embryonic developmental processes. The intrauterine environment of the developing embryo/fetus is determined by maternal factors such as health/disease status, lifestyle, medication, exposure to environmental teratogens, as well as the maternal genotype. Certain genetic characteristics of the embryo/fetus also predispose it to developmental abnormalities. Epidemiologic and animal studies conducted over the last few decades have suggested that the interplay between genes and environmental factors underlies the etiological heterogeneity of these defects. It is now widely believed that the study of gene-environment interactions will lead to better understanding of the biological mechanisms and pathological processes that contribute to the development of complex birth defects. It is only through such an understanding that more efficient measures will be developed to prevent these severe, costly and often deadly defects. In this review, we attempt to summarize the complex clinical and experimental literature on current hypotheses of interactions between several select environmental factors and those genetic pathways in which they are most likely to have significant modifying effects. These include maternal folate nutritional status, maternal diabetes/obesity-related conditions, and maternal exposure to selected medications and environmental contaminants. Our goal is to highlight the potential gene-environment interactions affecting early embryogenesis that deserve comprehensive study.

Transcription factor TEAD2 is involved in neural tube closure

TEAD2, one of the first transcription factors expressed at the beginning of mammalian development, appears to be required during neural development. For example, Tead2 expression is greatest in the dorsal neural crest where it appears to regulate expression of Pax3, a gene essential for brain development. Consistent with this hypothesis, we found that inactivation of the Tead2 gene in mice significantly increased the risk of exencephaly (a defect in neural tube closure). However, none of the embryos exhibited spina bifida, the major phenotype of Pax3 nullizygous embryos, and expression of Pax3 in E11.5 Tead2 nullizygous embryos was normal. Thus, Tead2 plays a role in neural tube closure that is independent of its putative role in Pax3 regulation. In addition, the risk of exencephaly was greatest with Tead2 nullizygous females, and could be suppressed either by folic acid or pifithrin-alpha. These results reveal a maternal genetic contribution to neural tube closure, and suggest that Tead2-deficient mice provide a model for anencephaly, a common human birth defect that can be prevented by folic acid.

Valproic acid-induced fetal malformations are reduced by maternal immune stimulation with granulocyte-macrophage colony-stimulating factor or interferon-gamma

Valproic acid, a drug commonly used to treat seizures and other psychiatric disorders, causes neural tube defects (NTDs) in exposed fetuses at a rate 20 times higher than in the general population. Failure of the neural tube to close during development results in exencephaly or anencephaly, as well as spina bifida. In mice, nonspecific activation of the maternal immune system can reduce fetal abnormalities caused by diverse etiologies, including diabetes-induced NTDs. We hypothesized that nonspecific activation of the maternal immune system with interferon-gamma (IFN-gamma) and granulocyte-macrophage colony-stimulating factor (GM-CSF) could reduce valproic acid (VA)-induced defects as well. Female CD-1 mice were given immune stimulant prebreeding: either IFN-gamma or GM-CSF. Approximately half of the control and immune-stimulated pregnant females were then exposed to 500 mg/kg VA on the morning of gestational day 8. The incidence of developmental defects was determined on gestational day 17 from at least eight litters in each of the following treatment groups: control, VA only, IFN-gamma only, IFN-gamma+VA, GM-CSF only, and GM-CSF+VA. The incidence of NTDs was 18% in fetuses exposed to VA alone, compared to 3.7% and 2.9% in fetuses exposed to IFN-gamma+VA, or GM-CSF+VA respectively. Ocular defects were also significantly reduced from 28.0% in VA exposed groups to 9.8% in IFN-gamma+VA and 12.5% in GM-CSF+VA groups. The mechanisms by which maternal immune stimulation prevents birth defects remain unclear, but may involve maternal or fetal production of cytokines or growth factors which protect the fetus from the dysregulatory effects of teratogens.

Investigations into the etiology of neural tube defects

Neural tube defects (NTDs) are serious malformations affecting approximately 1 per 1000 births, yet the mechanisms by which they arise are unknown. There have been consistent efforts in many fields of research to elucidate the etiology of this multifactorial condition. While no single gene has been identified as a major independent risk factor for NTDs, candidate genes have been proposed that may modify the effects of maternal and/or embryonic exposures. Folate supplementation effectively reduces the occurrence of NTDs and, consequently, has focused much research on metabolism of folate-related pathways during pregnancy and development. Further understanding of normal development and how teratogens can perturb these orchestrated processes also remains at the fore of modern scientific endeavors. The composite of these factors remains fragmented; the aim of this review is to provide the reader with a summary of sentinel and current works in the body of literature addressing NTD disease etiology.

Mapping a chromosomal locus for valproic acid-induced exencephaly in mice

Human neural tube defects (NTDs) are among the most common congenital defects. They have a highly heterogeneous etiology, and, in addition to those seen in association with genetic syndromes, there are also NTDs induced by pharmaceutical compounds in utero, such as the widely used anti-epileptic drug valproic acid (VPA). Although familial studies have suggested a genetic contribution to VPA-induced NTDs, this trait has not been adequately studied, nor have the responsible genetic factors been identified. We generated a series of mouse crosses and backcrosses using the highly inbred SWV/Fnn and C57BL/6J strains, in order to identify possible chromosomal loci contributing to VPA sensitivity. When exposed to a high dose of sodium VPA (600 mg/kg) via maternal intraperitoneal injection on gestational day E8.5, the fetuses manifested exencephaly in a strain-dependent manner. Our data show an autosomal recessive trait, plus a gender-related effect or an overall X-Chromosome (Chr) effect, as being primarily responsible for determining sensitivity to VPA-induced exencephaly. Genome scanning and further linkage analysis of 131 exencephalic backcross fetuses identified a major locus linked to D7Mit285 (p < 2 x 10(-6)), exceeding the threshold for significant linkage. These results suggest a major chromosomal locus associated with the sensitivity to VPA-induced exencephaly in mice.