Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND: The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains. METHODS: The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta. RESULTS: Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains. CONCLUSIONS: If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.     

Tissue-specific distribution of aberrant DNA methylation associated with maternal low-folate status in human neural tube defects

This study compares the density and tissue-specific distribution of 5-methyl cytosine (5mC) in genomic DNA from human fetuses with or without neural tube defects (NTD) and examines whether low maternal serum folate is a possible correlate and/or risk factor for NTD. The results demonstrate significant hypomethylation of brain genomic DNA in NTD fetuses relative to controls (P<.01), as well as relative hypermethylation of skin and heart in NTD fetuses. In normal fetuses, the level of 5mC in liver genomic DNA decreased from fetal week 18 to 28 and increased over the same developmental period in kidney genomic DNA, but these trends were absent in genomic DNA from NTD fetuses. Mean maternal serum folate was significantly lower in NTD fetuses than in controls (P<.01), and maternal serum folate correlated with density of 5mC in genomic brain DNA from NTD fetuses (r=0.610). The results indicate that aberrant DNA methylation in NTD may be due to maternal folate deficiency and may be involved in the pathogenesis of NTD in humans.     

Periconceptional maternal alcohol consumption and neural tube defects

BACKGROUND: Neural tube defects (NTD)s, which occur when the neural tube fails to close during early gestation, are some of the most common birth defects worldwide. Alcohol is a known teratogen and has been shown to induce NTDs in animal studies, although most human studies have failed to corroborate these results. Using data from the National Birth Defects Prevention Study, associations between maternal reports of periconceptional (1 month prior through 2 months postconception) alcohol consumption and NTDs were examined. METHODS: NTD cases and unaffected live born control infants, delivered from 1997 through 2005, were included. Interview reports of alcohol consumption (quantity, frequency, variability, and type) were obtained from 1223 case mothers and 6807 control mothers. Adjusted odds ratios (aOR)s and 95% confidence intervals were estimated using multivariable logistic regression analysis. RESULTS: For all NTDs combined, most aORs for any alcohol consumption, one or more binge episodes, and different type(s) of alcohol consumed were near unity or modestly reduced (≥0.7<aOR≤1.1) and were not statistically significant. Findings were similar for individual NTD subtypes. CONCLUSIONS: These findings suggest no elevated association between maternal periconceptional alcohol consumption and NTDs. Underreporting of alcohol consumption, due to negative social stigma associated with alcohol consumption during pregnancy, and limited reports for mothers with early pregnancy loss of a fetus with an NTD may have affected the estimated odds ratios. Future studies should aim to increase sample sizes for less prevalent subtypes, reduce exposure misclassification, and improve ascertainment offetal deaths and elective terminations. Birth Defects Research (Part A), 2013. © 2013 Wiley Periodicals, Inc.     

Maternal hair zinc concentration in neural tube defects in Turkey

Hair zinc concentration was measured in samples taken from 57 mothers who delivered infants with neural tube defects (NTD) (mainly anencephaly). Control groups consisted of 30 healthy mothers with normal offspring and 37 nonpregnant women from middle-income backgrounds. Zinc concentration was also measured in the hair of eight infants with NTD (four being anencephalic). The mean maternal hair zinc concentration in the NTD group (128.2 +/- 38.9 micrograms/g) was lower than that of the control women (p less than 0.001), whereas the mean hair zinc level of malformed babies (250.4 +/- 85.2 micrograms/g) was significantly higher than that of normal infants (193.4 +/- 39.2 micrograms/g) (p less than 0.05). Maternal nutritional zinc deficiency was thought to be one of the factors responsible for NTD in Turkey.     

Placental concentrations of manganese and the risk of fetal neural tube defects

Manganese (Mn) is an essential trace element required for normal growth, development, and cellular homeostasis, but excess Mn is toxic to the central nervous system. The present pilot study examined whether the level of Mn in the placenta was associated with the risk of fetal neural tube defects (NTDs). A case–control study was conducted. Cases were 80 fetuses or newborns with NTDs, and controls were 50 healthy, nonmalformed newborns. Placental Mn, zinc, copper, iron, and selenium were determined with inductively coupled plasma-mass spectrometry. The median Mn concentration was significantly higher in case placentas than in controls: cases, 131.60 ng/g (95% confidence interval [CI], 99.25–166.76); controls, 101.54 ng/g (95% CI, 80.14–119.79). Mn concentrations above the median were associated with a 4-fold (95% CI, 1.23–14.79) increased risk for any NTDs and a 7-fold (95% CI, 1.52–39.64) increased risk for spina bifida after other confounding factors were controlled. Elevated Mn levels were associated with an increased risk of anencephaly, although the adjusted odds ratio did not reach statistical significance. The association between higher Mn concentrations and risk of NTDs showed a clear dose–response relationship. Risk of NTDs increased to 1.51 (95% CI, 0.65–3.52) and 5.03 (95% CI, 1.89–13.33) for those whose placental Mn level was in the second and third tertiles, respectively, compared with the lowest tertile. Elevated placental concentrations of Mn may be associated with increased risks of NTDs in this population.     

Identification of early-responsive genes correlated to valproic acid-induced neural tube defects in mice

BACKGROUND: Valproic acid (VPA) causes the failure of neural tube closure in newborn mice. However, the molecular mechanism of its teratogenesis is unknown. This study was conducted to investigate the genomewide effects of VPA disruption of normal neural tube development in mice. METHODS: Microarray analysis was performed on the head part of NMRI mouse embryos treated for 1 hr with VPA on gestational day (GD) 8. Subsequently, we attempted to isolate genes that changed in correlation with the teratogenic action of VPA by employing reduced teratogenic VPA analogs, valpromide (VPD) and valnoctamide (VCD), in a real-time PCR study. RESULTS: Microarray results demonstrated that during neurulation, many genes, some of whose functions are known and some unknown, were either increased or decreased after VPA injection. Some genes were affected by VPD or VCD in the same way as VPA, but others were not changed by the analogs. In this way, our system identified 11 increased and 20 decreased genes. Annotation analysis revealed that the increased genes included gadd45b, ier5, per1, phfl3, pou3f1, and sox4, and the decreased genes included ccne2, ccnl, gas5, egr2, sirt1, and zfp105. CONCLUSIONS: These findings demonstrate that expression changes in genes having roles in the cell cycle and apoptosis pathways of neural tube cells were strongly expected to relate to the teratogenic, but not antiepileptic, activity of VPA. Our approach has allowed the expansion of the catalog of molecules immediately affected by VPA in the developing neural tube.     

A new perspective on neural tube defects: Folic acid and microRNA misexpression

Neural tube defects (NTDs) are the second most common birth defects in the United States. It is well known that folic acid supplementation decreases about 70% of all NTDs, although the mechanism by which this occurs is still relatively unknown. The current theory is that folic acid deficiency ultimately leads to depletion of the methyl pool, leaving critical genes unmethylated, and, in turn, their improper expression leads to failure of normal neural tube development. Recently, new studies in human cell lines have shown that folic acid deficiency and DNA hypomethylation can lead to misexpression of microRNAs (miRNAs). Misexpression of critical miRNAs during neural development may lead to a subtle effect on neural gene regulation, causing the sometimes mild to severely debilitating range of phenotypes exhibited in NTDs. This review seeks to cohesively integrate current information regarding folic acid deficiency, methylation cycles, neural development, and miRNAs to propose a potential model of NTD formation. In addition, we have examined the relevant gene pathways and miRNAs that are predicted to affect them, and based on our investigation, we have devised a basic template of experiments for exploring the idea that miRNA misregulation may be linked to folic acid deficiency and NTDs. genesis 48:282–294, 2010. © 2010 Wiley‐Liss, Inc.     

Proportion of neural tube defects attributable to known risk factors

BACKGROUND

Recognized risk factors for neural tube defects (NTDs) poorly predict population‐level NTD risk. However, the proportion of NTDs that can be attributed to these risk factors is uncertain.

METHODS

To determine the proportion of NTD cases that is attributable to known or suspected risk factors (i.e., female infant sex, family history of NTDs, and maternal Hispanic ethnicity, obesity, pregestational diabetes, gestational diabetes, low dietary folate intake, lack of folic acid supplementation, anticonvulsant use, and hot tub or sauna use), we estimated the adjusted population attributable fraction (aAF) for each factor, using the method of Eide and Geffler and data from the National Birth Defects Prevention Study.

RESULTS

Our analyses of these data indicate that the proportion of cases of spina bifida and anencephaly that can be attributed to known risk factors is 28% and 44%, respectively. For spina bifida, the factor with the greatest attributable fraction was maternal obesity (aAF, 10%), whereas for anencephaly it was Hispanic ethnicity (aAF, 15%).

CONCLUSION

Our analyses indicate that known risk factors account for <50% of NTD cases. Hence, the majority of NTD cases are attributable to, as yet, unidentified factors. These findings highlight the need for continued research to identify genetic and additional nongenetic risk factors for NTDs. Further, these findings suggest that strategies that aim to reduce the risk of NTDs associated with maternal Hispanic ethnicity and obesity may have the greatest impact on the population prevalence of these conditions. Birth Defects Research (Part A), 2013. © 2012 Wiley Periodicals, Inc.     

Higher serum homocysteine and lower thyroid hormone levels in pregnant women are associated with neural tube defects

BackgroundThis study tested the hypothesis that abnormal maternal metabolism of both homocysteine and thyroid hormone network in pregnant women is associated with neural tube defects (NTDs) in a part of China with high NTD prevalence.MethodsA case–control study was performed between 2007 and 2009 in Lüliang Mountains, Shanxi Province. This study included 83 pregnant women who had fetuses with NTDs (cases) and 90 pregnant women with normal fetuses (controls). In addition, a cell model to illustrate the epidemiological findings was established.ResultsFetuses of mother who had both high total homocysteine (tHcy) and inadequate free thyroxine were 3 times more at risk of developing NTDs (adjusted odds ratio = 3.5; 95 % confidence interval = 1.2–10.4; cases vs. controls) using multivariate logistic regression models. Furthermore, biological interaction between metabolisms of Hcy and thyroid hormones was demonstrated in vitro. In homocysteine thiolactone of a metabolite of Hcy-treated mouse embryonic neural stem NE4C cells, genes (Bmp7, Ctnnb1, Notch 1, Gli2, and Rxra) related to both neural tube closure and thyroid hormone network were shown to be regulated by H3K79 homocysteinylation, which increased their expression levels.ConclusionsThe effect of maternal serum high tHcy on risk of developing NTDs is depended on maternal serum level of thyroxine. Meanwhile, a higher level of tHcy might also affect both maternal metabolism of thyroid hormone and neural tube closure in embryogenesis through homocysteinylation of histones.     

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. Birth Defects Research (Part C) 96:193–205, 2012. © 2012 Wiley Periodicals, Inc.