Few studies have evaluated genetic susceptibility related to diabetes and obesity as a risk factor for neural tube defects (NTDs). The authors investigated 23 single nucleotide polymorphisms among 9 genes (ADRB3, ENPP1, FTO, LEP, PPARG, PPARGC1A, SLC2A2, TCF7L2, and UCP2) associated with type 2 diabetes or obesity. Samples were obtained from 737 NTD case-parent triads included in the National Birth Defects Prevention Study during 1999–2007. Log-linear models were used to evaluate maternal and offspring genetic effects. After application of the false discovery rate, there were 5 significant maternal genetic effects. The less common alleles at the 4 FTO single nucleotide polymorphisms showed a reduction of NTD risk (for rs1421085, relative risk (RR) = 0.73 (95% confidence interval (CI): 0.62, 0.87); for rs8050136, RR = 0.79 (95% CI: 0.67, 0.93); for rs9939609, RR = 0.79 (95% CI: 0.67, 0.94); and for rs17187449, RR = 0.80 (95% CI: 0.68, 0.95)). Additionally, maternal LEP rs2071045 (RR = 1.31, 95% CI: 1.08, 1.60) and offspring UCP2 rs660339 (RR = 1.32, 95% CI: 1.06, 1.64) were associated with NTD risk. Furthermore, the maternal genotype for TCF7L2 rs3814573 suggested an increased NTD risk among obese women. These findings indicate that maternal genetic variants associated with glucose homeostasis may modify the risk of having an NTD-affected pregnancy.
case-parent triads; diabetes; genetics; neural tube defects; obesity
Tetrahydrobiopterin (BH4) is an essential cofactor and an important cellular antioxidant. BH4 deficiency has been associated with diseases whose etiologies stem from excessive oxidative stress. GTP cyclohydrolase I (GCH1) catalyzes the first and rate-limiting step of de novo BH4 synthesis. A 3-SNP haplotype in GCH1 (rs8007267, rs3783641, and rs10483639) is known to modulate GCH1 gene expression levels and has been suggested as a major determinant of plasma BH4 bioavailability. As plasma BH4 bioavailability has been suggested as a mechanism of neural tube defect (NTD) teratogenesis, we evaluated the association between this GCH1 haplotype and the risk of NTDs. Samples were obtained from 760 NTD case-parent triads included in the National Birth Defects Prevention Study (NBDPS). The three SNPs were genotyped using TaqMan® SNP assays. An extension of the log-linear model was used to assess the association between NTDs and both offspring and maternal haplotypes. Offspring carrying two copies of haplotype C-T-C had a significantly increased NTD risk (risk ratio [RR] = 3.40, 95% confidence interval [CI]: 1.02–11.50), after adjusting for the effect of the maternal haplotype. Additionally, mothers carrying two copies of haplotype C-T-C had a significantly increased risk of having an NTD-affected offspring (RR = 3.46, 95% CI: 1.05–11.00), after adjusting for the effect of the offspring haplotype. These results suggest offspring and maternal variation in the GCH1 gene and altered BH4 biosynthesis may contribute to NTD risk.
GCH1 gene; GTP cyclohydrolase I; haplotype; neural tube defects; tetrahydrobiopterin (BH4)
Neural tube defects (NTDs) are common, serious malformations with a complex etiology that suggests involvement of both genetic and environmental factors. The authors evaluated maternal or offspring folate-related gene variants and interactions between the gene variants and maternal intake of folates on the risk of NTDs in their offspring. A case-control study was conducted on mothers and/or their fetuses and infants who were born in California from 1999–2003 with an NTD (cases n = 222, including 24 mother-infant pairs) or without a major malformation (controls n = 454, including 186 mother-infant pairs). Maternal intake of folates was assessed by food frequency questionnaire and genotyping was performed on samples from mothers and infants. For mothers in the lowest folate-intake group, risk of NTDs in offspring was significantly decreased for maternal MTHFR SNPs rs1476413, rs1801131 and rs1801133 (odds ratio (OR) = 0.55, 80% confidence interval (CI): 0.20, 1.48; OR = 0.58, 80% CI: 0.24, 1.43; OR = 0.69, 80% CI: 0.41, 1.17, respectively), and TYMS SNPs rs502396 and rs699517 (OR= 0.91, 80% CI: 0.53, 1.56; OR = 0.70, 80% CI: 0.38, 1.29). A gene-only effect was observed for maternal SHMT1 SNP rs669340 (OR = 0.69, 95% CI: 0.49, 0.96). When there was low maternal folate intake, risk of NTDs was significantly increased for infant MTHFD1 SNPs rs2236224, rs2236225 and rs11627387 (OR = 1.58, 80% CI: 0.99, 2.51; OR = 1.53, 80% CI: 0.95, 2.47; OR = 4.25, 80% CI: 2.33, 7.75, respectively) and SHMT1 SNP rs12939757 (OR = 2.01, 80% CI: 1.20, 3.37), but decreased for TYMS SNP rs2847153 (OR = 0.73, 80% CI: 0.37, 1.45). Although power to detect interaction effects was low for this birth defects association study, the gene-folate interactions observed in this study represent preliminary findings that will be useful for informing future studies on the complex etiology of NTDs.
Congenital Abnormalities; Folic Acid; Genetic Association Studies; Molecular Epidemiology; Neural Tube Defects; Maternal Nutritional Physiological Phenomena; Nervous System Malformations; Nutrigenomics
Spina bifida is one of the most common of all human structural birth defects. Despite considerable effort over several decades, the causes and mechanisms underlying this malformation remain poorly characterized. In order to better understand the pathogenesis of this abnormality, we conducted a microarray study using Mouse Whole Genome Bioarrays which have ~36,000 gene targets, to compare gene expression profiles between two mouse models; CXL-Splotch and Fkbp8Gt(neo)which express a similar spina bifida phenotype. We anticipated that there would be a collection of overlapping genes or shared genetic pathways at the molecular level indicative of a common mechanism underlying the pathogenesis of spina bifida during embryonic development.
A total of 54 genes were determined to be differentially expressed (25 down regulated, 29 upregulated) in the Fkbp8Gt(neo) mouse embryos; while 73 genes were differentially expressed (56 down regulated, 17 upregulated) in the CXL-Splotch mouse relative to their wildtype controls. Remarkably, the only two genes that showed decreased expression in both mutants were v-ski sarcoma viral oncogene homolog (Ski), and Zic1, a transcription factor member of the zinc finger family. Confirmation analysis using real time qRT-PCR indicated that only Zic1 was significantly decreased in both mutants. Gene Ontology analysis revealed striking enrichment of genes associated with mesoderm and central nervous system development in the CXL-Splotch mutant embryos, whereas in the Fkbp8Gt(neo) mutants, the genes involved in dorsal/ventral pattern formation, cell fate specification, and positive regulation of cell differentiation were most likely to be enriched. These results indicate that there are multiple pathways and gene networks perturbed in mouse embryos with shared phenotypes.
spina bifida; gene expression; microarray; mouse mutants
The complement system is involved in a range of diverse developmental processes including cell survival, growth, differentiation, and regeneration. However, little is known about the role of complement in embryogenesis. Herein we demonstrate a novel role for the canonical complement 5a receptor (C5aR) in the development of the mammalian neural tube under conditions of maternal dietary folic acid deficiency. Specifically, we found C5aR and C5 to be expressed throughout the period of neurulation in wildtype mice and localized the expression to the cephalic regions of the developing neural tube. C5aR was also found to be expressed in the neuroepithelium of early human embryos. Ablation of the C5ar1 gene or the administration of a specific C5aR peptide antagonist to folic acid-deficient pregnant mice resulted in a high prevalence of severe anterior neural tube defect-associated congenital malformations. These findings provide a new and compelling insight into the role of the complement system during mammalian embryonic development.
In this study, we investigated whether the two TYMS functional variants (28bp VNTR and 1494del6) (275 cases and 653 controls) and six selected SNPs (265 case infants, 535 control infants; 169 case mothers and 276 control mothers) were associated with risks of conotruncal heart defects. Further, we evaluated interaction effects between these gene variants and maternal folate intake for risk of CTD. Cases with diagnosis of single gene disorders or chromosomal aneusomies were excluded. Controls were randomly selected from area hospitals in proportion to their contribution to the total population of live-born infants. DNA samples were collected using buccal brushes or drawn from the repository of newborn screening blood specimens when available. Genetic variants were treated as categorical variables (homozygous referent, heterozygote, homozygous variant). Odds ratios and 95% confidence intervals (CI) were computed to estimate risks among all subjects, Hispanic and non-Hispanic whites, respectively using logistic regression. Gene-folate interactions were assessed for these variants by adding an interaction term to the logistic model. A dichotomized composite variable, “combined folate intake”, was created by combining maternal peri-conceptional use of folic acid-containing vitamin supplements with daily dietary intake of folate. In general, the results do not show strong gene-only effects on risk of CTD. We did, however, observe a 3.6-fold increase in CTD risk (95%CI:1.1 – 11.9) among infants who were homozygotes for the 6bp deletion in the 3’-untranslated region (UTR) (1694del6) and whose mothers had low folate intake during the peri-conceptional period.
thymidylate synthase; TYMS; congenital heart defects; conotruncal defects; folate
The treatment of epilepsy in women of reproductive age remains a clinical challenge. While most women with epilepsy require anticonvulsant drugs for adequate control of their seizures, the teratogenicity associated with some antiepileptic drugs is a risk that needs to be carefully addressed. Antiepileptic medications are also used to treat an ever broadening range of medical conditions such as bipolar disorder, migraine prophylaxis, cancer and neuropathic pain. Despite the fact that the majority of pregnancies of women with epilepsy who are receiving pharmacological treatment are normal, studies have demonstrated that the risk of having a pregnancy complicated by a major congenital malformation is doubled when comparing the risk of untreated pregnancies. Furthermore, when antiepileptic drugs (AEDs) are used in polytherapy regimens, the risk is tripled, especially when valproic acid (VPA) is included. However, it should be noted that the risks are specific for each anticonvulsant drug. Some investigations have suggested that the risk of teratogenicity is increased in a dose-dependent manner. More recent studies have reported that in utero exposure to AEDs can have detrimental effects on the cognitive functions and language skills in later stages of life. In fact, the FDA just issued a safety announcement on the impact of VPA on cognition (Safety Announcement 6-30-2011). The purpose of this document is to review the most commonly used compounds in the treatment of women with epilepsy, and to provide information on the latest experimental and human epidemiological studies of the effects of antiepileptic drugs in the exposed embryos.
antiepileptic drugs; teratogenicity; pregnancy; birth defects; epilepsy
Neural tube defects (NTDs) (OMIM #182940) including anencephaly, spina bifida and craniorachischisis, are severe congenital malformations that affect 0.5–1 in 1,000 live births in the United States, with varying prevalence around the world. Mutations in planar cell polarity (PCP) genes are believed to cause a variety of NTDs in both mice and humans. SCRIB is a PCP-associated gene. Mice that are homozygous for the Scrib p.I285K and circletail (Crc) mutations, present with the most severe form of NTDs, namely craniorachischisis. A recent study reported that mutations in SCRIB were associated with craniorachischisis in humans, but whether SCRIB mutations contribute to increased spina bifida risk is still unknown. We sequenced the SCRIB gene in 192 infants with spina bifida and 190 healthy controls. Among the spina bifida patients, we identified five novel missense mutations that were predicted-to-be-deleterious by the PolyPhen software. Of these five mutations, three of them (p.P1043L, p.P1332L, p.L1520R) significantly affected the subcellular localization of SCRIB. In addition, we demonstrated that the craniorachischisis mouse line-90 mutation I285K, also affected SCRIB subcellular localization. In contrast, only one novel missense mutation (p.A1257T) was detected in control samples, and it was predicted to be benign. This study demonstrated that rare deleterious mutations of SCRIB may contribute to the multifactorial risk for human spina bifida.
Ciliopathies are a broad class of human disorders with craniofacial dysmorphology as a common feature. Among these is high arched palate, a condition that affects speech and quality of life. Using the ciliopathic Fuz mutant mouse, we find that high arched palate does not, as commonly suggested, arise from midface hypoplasia. Rather, increased neural crest expands the maxillary primordia. In Fuz mutants, this phenotype stems from dysregulated Gli processing, which in turn results in excessive craniofacial Fgf8 gene expression. Accordingly, genetic reduction of Fgf8 ameliorates the maxillary phenotypes. Similar phenotypes result from mutation of oral-facial-digital syndrome 1 (Ofd1), suggesting that aberrant transcription of Fgf8 is a common feature of ciliopathies. High arched palate is also a prevalent feature of fibroblast growth factor (FGF) hyperactivation syndromes. Thus, our findings elucidate the etiology for a common craniofacial anomaly and identify links between two classes of human disease: FGF-hyperactivation syndromes and ciliopathies.
•A genetic model for high arched palate, commonly seen in human craniofacial syndromes•In ciliopathic mice, Fgf8 overexpression leads to cranial neural crest hyperplasia•Enlargement of the maxillary primordia underlies high arched palate in Fuz mutants•An etiological link between ciliopathies and FGF-hyperactivation syndromes
High arched palate is common to many human disorders, including ciliopathies and craniosynostosis syndromes. Tabler et al. develop and analyze a genetic model of high arched palate; they conclude that embryonic changes in neural crest and fibroblast growth factor signaling underlie this unusual phenotype.
The objectives of this study were to identify tissue-specific differentially methylated regions (T-DMR’s) in the folate transport genes in placental tissue compared with leukocytes, and from placental tissues obtained from normal infants or with neural tube defects (NTDs). Using pyrosequencing, we developed methylation assays for the CpG islands (CGIs) and the CGI shore regions of the folate receptor α (FOLR1), proton-coupled folate transporter (PCFT) and reduced folate carrier 1 (RFC1) genes. The T-DMRs differed in location for each gene and the difference in methylation ranged between 2 and 54%. A higher T-DMR methylated fraction was associated with a lower mRNA level of the FOLR1 and RFC1 genes. Methylation fractions differed according to RFC1 80G > A genotype in the NTD cases and in leukocytes from subjects with high total plasma homocysteine (tHcy). There were no differences in methylated fraction of folate transporter genes between NTD cases and controls. We suggest that T-DMRs participate in the regulation of expression of the FOLR1 and RFC1 genes, that the RFC1 80G > A polymorphism exerts a gene-nutrition interaction on DNA methylation in the RFC1 gene, and that this interaction appears to be most prominent in NTD-affected births and in subjects with high tHcy concentrations.
FOLR1; PCFT; RFC1 80G>A; homocysteine; tissue-specific DNA methylation; GpG island; NTD
In 2007, the International Knockout Mouse Consortium (IKMC) made the ambitious promise to generate mutations in virtually every protein-coding gene of the mouse genome in a concerted worldwide action. Now, 5 years later, the IKMC members have developed high-throughput gene trapping and, in particular, gene-targeting pipelines and generated more than 17,400 mutant murine embryonic stem (ES) cell clones and more than 1,700 mutant mouse strains, most of them conditional. A common IKMC web portal (www.knockoutmouse.org) has been established, allowing easy access to this unparalleled biological resource. The IKMC materials considerably enhance functional gene annotation of the mammalian genome and will have a major impact on future biomedical research.
Background: Exposure to polycyclic aromatic hydrocarbons (PAHs) occurs in many occupational settings. There is evidence in animal models that maternal exposure to PAHs during pregnancy is associated with gastroschisis in offspring; however, to our knowledge, no human studies examining this association have been conducted.
Objective: Our goal was to conduct a case–control study assessing the association between estimated maternal occupational exposure to PAHs and gastroschisis in offspring.
Methods: Data from gastroschisis cases and control infants were obtained from the population-based National Birth Defects Prevention Study for the period 1997–2002. Exposure to PAHs was assigned by industrial hygienist consensus, based on self-reported maternal occupational histories from 1 month before conception through the third month of pregnancy. Logistic regression was used to determine the association between estimated occupational PAH exposure and gastroschisis among children whose mothers were employed for at least 1 month during the month before conception through the third month of pregnancy.
Results: The prevalence of estimated occupational PAH exposure was 9.0% in case mothers (27 of 299) and 3.6% in control mothers (107 of 2,993). Logistic regression analyses indicated a significant association between occupational PAHs and gastroschisis among mothers ≥ 20 years of age [odds ratio (OR) = 2.53; 95% confidence interval (CI): 1.27, 5.04] after adjusting for maternal body mass index, education, gestational diabetes, and smoking. This association was not seen in mothers < 20 years (OR = 1.14; 95% CI: 0.55, 2.33), which is notable because although young maternal age is the strongest known risk factor for gastroschisis, most cases are born to mothers ≥ 20 years.
Conclusion: Our findings indicate an association between occupational exposure to PAHs among mothers who are ≥ 20 years and gastroschisis. These results contribute to a body of evidence that PAHs may be teratogenic.
birth defects; gastroschisis; maternal exposure; occupation; PAHs
Studies in avian models have demonstrated an involvement of retinoid signaling in early neural tube patterning. The roles of this signaling pathway at later stages of spinal cord development are only partly characterized. Here we use Raldh2-null mouse mutants rescued from early embryonic lethality to study the consequences of lack of endogenous retinoic acid (RA) in the differentiating spinal cord. Mid-gestation RA deficiency produces prominent structural and molecular deficiencies in dorsal regions of the spinal cord. While targets of Wnt signaling in the dorsal neuronal lineage are unaltered, reductions in Fibroblast Growth Factor (FGF) and Notch signaling are clearly observed. We further provide evidence that endogenous RA is capable of driving stem cell differentiation. Raldh2 deficiency results in a decreased number of spinal cord derived neurospheres, which exhibit a reduced differentiation potential. Raldh2-null neurospheres have a decreased number of cells expressing the neuronal marker β-III-tubulin, while the nestin-positive cell population is increased. Hence, in vivo retinoid deficiency impaired neural stem cell growth. We propose that RA has separable functions in the developing spinal cord to (i) maintain high levels of FGF and Notch signaling and (ii) drive stem cell differentiation, thus restricting both the numbers and the pluripotent character of neural stem cells.
Evidence exists for an association between use of vitamin supplements with folic acid in early pregnancy and reduced risk for offspring with cleft lip with/without cleft palate (CLP). A few observations have been made about nutrients related to one-carbon metabolism other than folate. Our prospective study attempted to extend information on nutrition and CLP by measuring nutrient analytes in mid-pregnancy sera. This study included data from a repository of women’s mid-pregnancy serum specimens collected in California from 2003–04. Each woman’s specimen was linked with delivery information to determine whether her fetus had CLP or another structural malformation, or was nonmalformed. We identified 89 CLP cases. We randomly selected 409 specimens as controls. Specimens were tested for homocysteine, methylmalonic acid, folate, vitamin B12, pyridoxal phosphate, pyridoxal, pyridoxic acid, riboflavin, choline, betaine, methionine, methionine sulfoxide, cysteine, cystathionine, arginine, and asymmetric and symmetric dimethylarginine. We observed three analytes with odds ratios unlikely to be explained by random variation, i.e., elevated CLP risks were observed for low levels and for high levels of pyridoxal phosphate (vitamin B6), higher levels of choline, and low levels of symmetric dimethylarginine. These data did not show meaningful differences between cases and controls for any other analytes.
Background & Aims
Hepatic de-differentiation, liver development, and malignant transformation are processes in which the levels of hepatic S-adenosylmethionine (SAMe) are tightly regulated by two genes, MAT1A and MAT2A. MAT1A is expressed in the adult liver, whereas MAT2A expression is primarily extra-hepatic and is strongly associated with liver proliferation. The mechanisms that regulate these expression patterns are not completely understood. In silico analysis of the 3′ untranslated region of MAT1A and MAT2A revealed putative binding sites for the RNA-binding proteins AUF1 and HuR, respectively. We investigated the post-transcriptional regulation of MAT1A and MAT2A by AUF1, HuR and methyl-HuR in the aforementioned biological processes.
During hepatic de-differentiation, the switch between MAT1A and MAT2A coincided with an increase in HuR and AUF1 expression. SAMe treatment altered this homeostasis by shifting the balance of AUF1 and methyl-HuR/HuR, which was identified for the first time as an inhibitor of MAT2A mRNA stability. We also observed a similar temporal distribution and a functional link between HuR, methyl-HuR, AUF1, and MAT1A and MAT2A during the fetal liver development. Immunofluorescent analysis revealed increased levels of HuR and AUF1, and a decrease in methyl-HuR levels in human livers with hepatocellular carcinoma (HCC).
Our data strongly support a role for AUF1 and HuR/methyl-HuR in liver de-differentiation, development and human HCC progression through the post-translational regulation of MAT1A and MAT2A mRNAs.
HuR; AUF1; MAT1A; MAT2A
Many antiepileptic drugs (AEDs) have therapeutic applications that extend beyond epilepsy to include neuropathic pain, migraine headaches and psychiatric disorders. The risk of some AEDs has been clearly established, but for newer drugs, small sample sizes and polytherapy exposures preclude a conclusive determination of their teratogenic potential. Most women with epilepsy will require AED therapy throughout their entire pregnancy to control seizures; the vast majority of pregnancies in women with epilepsy have positive outcomes. A conservative estimate suggests that AED monotherapy doubles, and polytherapy triples, the risk for major congenital malformations. Furthermore, while evidence is still accruing, recent investigations suggest that exposure to select AEDs results in altered cognitive function later in development. There is no evidence to suggest that additional folic acid supplementation ameliorates the increased risk of congenital malformations conferred by in utero AED exposure.
AED; birth defects; carbamazepine; epilepsy; lamotrigine; levetiracetam; phenobarbital; phenytoin; teratogen; topiramate; valproate
The objective of this study was to determine whether Immunoglobulin G (IgG) and M (IgM) autoantibodies to folate receptor α (FRα) in pregnant women are associated with an increased risk of oral cleft-affected offspring. A case-control study nested in the prospective Danish National Birth Cohort (100,418 pregnancies, enrolled during 1997–2003) was done. 185 children were born with an oral cleft. Maternal serum from their mothers (cases) was compared to maternal serum from 779 randomly selected mothers of non-malformed children (controls). We found that the average level of FRα IgG autoantibodies did not differ significantly among cases and controls (p=0.71). Slightly higher levels of FRα IgM autoantibodies were found among controls compared to cases. This was, however, not statistically significant (p=0.06), except for mothers of children with isolated cleft lip (p=0.04). Blocking of folate binding to folate receptor was similar among cases and controls (p=0.54). The results did not change when stratifying into the cleft subgroups, nor when only isolated oral cleft cases were considered. In conclusion, high maternal autoantibody levels and blocking of folate binding to folate receptor α in maternal serum during pregnancy are not associated with an increased risk of oral clefts in the offspring in this population based cohort.
To identify proteins with which FolBp1 may interact within lipid rafts in tissue derived from embryonic orofacial tissue.
A yeast two-hybrid screen of a cDNA library, derived from orofacial tissue from gestational day 11 to 13 mouse embryos, was conducted.
Using the full-length FolBp1 protein as bait, two proteins that bind FolBp1 were identified, Bat2d, and a fibronectin type III-containing domain protein. Results were confirmed by glutathione S-transferase pull-down assays.
As a component of membrane lipid raft protein complexes, these binding proteins may represent “helper” or chaperone proteins that associate with FolBp1 in order to facilitate the transport of folate across the plasma membrane. The protein-protein interactions detected, while limited in number, may be critical in mediating the role of FolBp1 in folate transport, particularly in the developing embryo.
two-hybrid; folate binding protein-1; orofacial; Bat2d; Fndc3; Folate binding protein; two-hybrid screen; fibronectin type III-containing domain protein; embryo
Spina bifida, a neural tube closure defect (NTD) involving the posterior portion of what will ultimately give rise to the spinal cord, is one of the most common and serious birth defects. The etiology of spina bifida is thought to be multi-factorial and involve multiple interacting genes and environmental factors. The causes of this congenital malformation remain largely unknown. However, several candidate genes for spina bifida have been identified in lower vertebrates, including the planar cell polarity (PCP) genes. We used data from a case-control study conducted in California to evaluate the association between variation within several key PCP genes and the risk of spina bifida. The PCP genes included in this study were the human homologues of the Xenopus genes Flamingo, Strabismus, Prickle, Dishevelled and Scrib, two of the homologues of Xenopus Wnt genes, WNT5A and WNT11, and two of the homologues of Xenopus Frizzled, FZD3 and FZD6. None of the 172 SNPs that were evaluated were significantly associated with spina bifida in any racial/ethnic group after correction for multiple testing. However, several SNPs in the PRICKLE2 gene had unadjusted p value<0.01. In conclusion our results, though largely negative, suggest that the PRICKLE2 gene may potentially modify the risk of spina bifida and deserves further investigation.
Neural Tube Defects (NTDs); Spina Bifida; Planar Cell Polarity (PCP); candidate gene; association study
Neural tube closure takes place during early embryogenesis and requires interactions between genetic and environmental factors. Failure of neural tube closure is a common congenital malformation that results in morbidity and mortality. A major clinical achievement has been the use of periconceptional folic acid supplements, which prevents ~50–75% of cases of neural tube defects. However, the mechanism underlying the beneficial effects of folic acid is far from clear. Biochemical, genetic and epidemiological observations have led to the development of the methylation hypothesis, which suggests that folic acid prevents neural tube defects by stimulating cellular methylation reactions. Exploring the methylation hypothesis could direct us towards additional strategies to prevent neural tube defects.
TXN2 encodes human thioredoxin 2, a small redox protein important in cellular antioxidant defenses, as well as in the regulation of apoptosis. Txn2 knockout mice fail to complete neural tube closure by E10.5 and die in utero. We hypothesized that genetic variation in human TXN2 gene may alter the function of the encoded protein in a manner associated with an increased risk for neural tube defects (NTDs). A DNA re-sequencing effort of the human TXN2 gene was taken. After a variation in the promoter was identified, the transcriptional activity of different alleles was investigated. The possible association between these variations and the risk of spina bifida was further evaluated in a subset of samples obtained from a large population-based case-control study in California in two different ethnicity groups, non-Hispanic white and Hispanic white. We identified a novel promoter insertion polymorphism located 9 base pairs upstream of the transcription start site of exon 1(−9 insertion). The GA, G and GGGA insertions were associated with a marked decrease of transcriptional activity when overexpressed in both U2-OS (an osteosarcoma cell line) and 293 cells (derived from human embryonic kidney). Further analysis revealed that the GA insertion was associated with increased spina bifida risk for Hispanic whites. Our study revealed a novel Ins/Del polymorphism in the human TXN2 gene proximal promoter region that altered the transcriptional activity and is associated with spina bifida risk. This polymorphism may be a genetic modifier of spina bifida risk in this California population.
thioredoxin 2; TXN2; re-sequencing; Ins/Del polymorphism; spina bifida
PRKACA and PRKACB are genes encoding the cAMP-dependent protein kinase A (PKA) catalytic subunits alpha and beta, respectively. PKA is known to be involved in embryonic development, as it downregulates the Hedgehog (Hh) signaling pathway, which is critical to normal pattern formation and morphogenesis. The PKA-deficient mouse model, which has only a single catalytic subunit, provided intriguing evidence demonstrating a relationship between decreased PKA activity and risk for posterior neural tube defects (NTDs) in the thoracic to sacral regions of gene-knockout mice. Unlike most other mutant mouse models of NTDs, the PKA-deficient mice develop spina bifida with 100% penetrance. We hypothesized that sequence variations in human genes encoding the catalytic subunits may alter the PKA activity and similarly increase the risk of spina bifida.
We sequenced the coding regions and the exon/intron boundaries of PRKACA and PRKACB. We also examined 3 common single-nucleotide polymorphisms (SNPs) of these 2 genes by allele discrimination.
Five sequence variants in coding region and 2 intronic sequence variants proximal to exons were detected. None of the 3 SNPs examined in the association study appeared to be associated with substantially increased risk for spina bifida.
Our results did not reveal a strong association between these PKA SNPs and spina bifida risk. Nonetheless, it is important to examine the possible gene-gene interactions between PRKACA and PRKACB when evaluating the risk for NTDs, as well as genes encoding regulatory subunits of PKA. In addition, interactions with other genes such as Sonic Hedgehog (SHH) should also be considered for future investigations.
PRKACA; PRKACB; case-control study; spina bifida
Folate binding protein 1 (Folr1) knockout mice with low maternal folate concentrations have been shown to be excellent animal models for human folate-responsive neural tube defects (NTDs). Previous studies using the Folr1 knockout mice revealed that maternal folate supplementation up-regulates the expression of the PCMT1 gene in Folr1 nullizygous neural tube tissue during neural tube closure. PCMT1 encodes the protein repair enzyme l-isoaspartate (d-aspartate) O-methyltransferase (PIMT) that converts abnormal d-aspartyl and l-isoaspartyl residues to the normal l-aspartyl form. PIMT is known to protect certain neural cells from Bax-induced apoptosis. Pcmt1-deficient mice present with abnormal AdoMet/AdoHcy homeostasis. We hypothesized that a known functional polymorphism (Ile120Val) in the human PCMT1 gene is associated with an increased risk of folate-responsive human NTDs. A case-control study was conducted to investigate a possible association between this polymorphism and risk of spina bifida. Compared to the Ile/Ile and Ile/Val genotypes, the homozygous Val/Val genotype showed decreased risk for spina bifida (adjusted odds ratio = 0.6, 95% confidence interval: 0.4–0.9). Our results showed that the Ile120Val polymorphism of PCMT1 gene is a genetic modifier for the risk of spina bifida. Val/Val genotype was associated with a reduction in risk for spina bifida.
l-Isoaspartate (d-aspartate) O-methyltransferase (PIMT); PCMT1 gene; Polymorphism; Neural tube defects; Association study