Roles of planar cell polarity pathways in the development of neutral tube defects

Neural tube defects (NTDs) are the second most common birth defect in humans. Despite many advances in the understanding of NTDs and the identification of many genes related to NTDs, the fundamental etiology for the majority of cases of NTDs remains unclear. Planar cell polarity (PCP) signaling pathway, which is important for polarized cell movement (such as cell migration) and organ morphogenesis through the activation of cytoskeletal pathways, has been shown to play multiple roles during neural tube closure. The disrupted function of PCP pathway is connected with some NTDs. Here, we summarize our current understanding of how PCP factors affect the pathogenesis of NTDs.     

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.     

eNOS基因多态性与神经管畸形的研究进展

神经管畸形(neural tube defects,NTDs)是一类常见的出生缺陷,严重威胁着妇女儿童的身心健康和人口素质的提高,给社会的发展带来沉重负担,可引起孕妇流产、婴儿死亡和终生残疾。NTDs可分为无脑儿、脑膨出和脊柱裂三种类型,其病因和具体的发病机制尚不清楚。国内外多数研究认为,NTDs是一种由基因的多态性和环境因素所引起的严重的基因突变,还不能用一种单一原因来解释该病的发生。目前的研究热点是易感基因与NTDs的关系,内皮型一氧化氮合酶(endothelial nitric oxide synthase,eNOS)基因最近已被认为是导致NTDs发生的重要候选基因。eNOS基因的点突变或成串突变可以导致酶活性的变化,使eNOS的表达上调,引起NO分泌的异常,促进神经元的凋亡,进而导致大脑的发育异常。本文从eNOS基因多态性与NTDs的相关性研究进行综述。     

Does prenatal screening for 5,10-methylenetetrahydrofolate reductase (MTHFR) mutations in high-risk neural tube defect pregnancies make sense?

Despite the fact that neural tube defects (NTDs) are the most common congenital malformations of the central nervous system, investigators have yet to identify responsible gene(s). Research efforts have been productive in the identification of environmental factors, such as periconceptional folic acid supplementation, that modulate risk for the development of NTDs. Studies of the folic acid biosynthetic pathway led to the discovery of an association between elevated levels of homocysteine and NTD risk. Researchers subsequently identified single nucleotide polymorphisms in the gene coding for the enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR). Association studies suggested it was a potential risk factor for NTDs, because the thermolabile form of the enzyme led to elevated homocysteine concentrations when folic acid intake is low. Numerous studies analyzing MTHFR variants have resulted in positive associations with increased NTD risk only in certain populations, suggesting that these variants are not large contributors to the etiology of NTDs. With our limited understanding of the genes involved in regulating NTD susceptibility, the paucity of data on how folic acid protects the developing embryo, as well as the observed decrease in birth prevalence of NTDs following folic acid supplementation and food fortification, it makes little sense for prospective parents to be tested for MTHFR variants, or for variants of other known folate pathway genes.     

Genome-Wide Association Mapping in Dogs Enables Identification of the Homeobox Gene,NKX2-8, as a Genetic Component of Neural Tube Defects in Humans

Neural tube defects (NTDs) is a general term for central nervous system malformations secondary to a failure of closure or development of the neural tube. The resulting pathologies may involve the brain, spinal cord and/or vertebral column, in addition to associated structures such as soft tissue or skin. The condition is reported among the more common birth defects in humans, leading to significant infant morbidity and mortality. The etiology remains poorly understood but genetic, nutritional, environmental factors, or a combination of these, are known to play a role in the development of NTDs. The variable conditions associated with NTDs occur naturally in dogs, and have been previously reported in the Weimaraner breed. Taking advantage of the strong linkage-disequilibrium within dog breeds we performed genome-wide association analysis and mapped a genomic region for spinal dysraphism, a presumed NTD, using 4 affected and 96 unaffected Weimaraners. The associated region on canine chromosome 8 (p_genome = 3.0×10⁻⁵), after 100,000 permutations, encodes 18 genes, including NKX2-8, a homeobox gene which is expressed in the developing neural tube. Sequencing NKX2-8 in affected Weimaraners revealed a G to AA frameshift mutation within exon 2 of the gene, resulting in a premature stop codon that is predicted to produce a truncated protein. The exons of NKX2-8 were sequenced in human patients with spina bifida and rare variants (rs61755040 and rs10135525) were found to be significantly over-represented (p = 0.036). This is the first documentation of a potential role for NKX2-8 in the etiology of NTDs, made possible by investigating the molecular basis of naturally occurring mutations in dogs.     

Neural crest anomaly syndromes in children with spina bifida.

A hereditary contribution to the etiology of neural tube defects (NTDs) has been suggested by clinical studies and animal models. To evaluate the hypothesis that common genes are important for both neural tube defects and neural crest anomalies, we examined children with developmental abnormalities of the spinal cord for anomalies of neural crest-derived structures. Neural crest anomalies, particularly auditory and pigmentary disorders, were identified and classified according to inheritance and type of anomaly. Of the 515 children screened, 44 (8.5%) had neural crest anomalies, 20 (3.9%) of which were apparently familial. Another 19 (3.7%) families had neural crest anomalies in two or more close relations, but the NTD subject was unaffected. Sixteen (3.1%) children with NTDs had a recognizable syndrome, including nine (1.7%) with a subtype of the Waardenburg syndromes. The coincidence of familial neural crest anomaly syndromes in subjects with spina bifida implies that defects in genes underlying neural crest development may contribute to the etiology of neural tube defects in a fraction of cases. The rate of anomalies and familial syndromes of neural crest-derived structures must be assessed in an adequate control sample to evaluate whether or not these abnormalities constitute risk factors for NTDs.     

The involvement of cell death and survival in neural tube defects: a distinct role for apoptosis and autophagy?

Neural tube defects (NTDs), such as spina bifida (SB) or exencephaly, are common congenital malformations leading to infant mortality or severe disability. The etiology of NTDs is multifactorial with a strong genetic component. More than 70 NTD mouse models have been reported, suggesting the involvement of distinct pathogenetic mechanisms, including faulty cell death regulation. In this review, we focus on the contribution of functional genomics in elucidating the role of apoptosis and autophagy genes in neurodevelopment. On the basis of compared phenotypical analysis, here we discuss the relative importance of a tuned control of both apoptosome-mediated cell death and basal autophagy for regulating the correct morphogenesis and cell number in developing central nervous system (CNS). The pharmacological modulation of genes involved in these processes may thus represent a novel strategy for interfering with the occurrence of NTDs.     

神经管畸形的表观遗传学研究进展

神经管畸形(neural tube defects,NTDs)是胚胎在早期发育过程中,由于神经管闭合不全或障碍引起的一组以脑和(或)脊髓发育异常为主的先天畸形。目前,对于神经管畸形的发病机制和病因没有明确的定论。很多因素参与神经管畸形的发生,主要涉及遗传、环境、以及二者的相互作用。遗传因素的研究主要着重于寻找神经管畸形的致病基因并进行基因功能缺陷研究,但是神经管畸形是多因素参与的结果,基因功能缺陷研究并不能完全解释其发病机制。基于目前的研究现状,近年来,环境因素通过调控表观遗传的修饰进而参与调节NTDs发生相关基因表达的研究逐渐受到重视。     

Human neural tube defects: genetic causes and prevention

Neural tube defects (NTDs) are severe congenital malformations affecting 1-2 in 1,000 live births, whose etiology is multifactorial, involving environmental and genetic factors. NTDs arise as consequence of the failure of fusion of the neural tube early during embryogenesis. NTDs' pathogenesis has been linked to genes involved in folate metabolism, consistent with an epidemiologic evidence that 70% of NTDs can be prevented by maternal periconceptional supplementation. However, polymorphisms in such genes are not linked in all populations, suggesting that other genetic factors and environmental factors could be involved. Animal models have provided crucial mechanistic information and possible candidate genes to explain susceptibility to NTDs. A crucial role has been assigned to the planar cell polarity (PCP) pathway, a highly conserved, non-canonical Wnt-frizzled-dishevelled signaling cascade that plays a key role in establishing and maintaining polarity in the plane of the epithelium and in the process of convergent extension during gastrulation and neurulation in vertebrates. The Loop-tail (Lp) mouse that develops craniorachischisis carry missense mutations in the PCP core gene Vangl2, that is the mammalian homolog of the Drosophila Strabismus/Van gogh (Stbm/Vang). The presence of mutations in human VANGL1 and VANGL2 genes encourages us to extend the investigation to other PCP genes that, with VANGL, play an essential role in neurulation during development.     

Reviewing Dengue: Still a Neglected Tropical Disease?

Dengue is currently listed as a “neglected tropical disease” (NTD). But is dengue still an NTD or not? Classifying dengue as an NTD may carry advantages, but is it justified? This review considers the criteria for the definition of an NTD, the current diverse lists of NTDs by different stakeholders, and the commonalities and differences of dengue with other NTDs. We also review the current research gaps and research activities and the adequacy of funding for dengue research and development (R&D) (2003–2013). NTD definitions have been developed to a higher precision since the early 2000s, with the following main features: NTDs are characterised as a) poverty related, b) endemic to the tropics and subtropics, c) lacking public health attention, d) having poor research funding and shortcomings in R&D, e) usually associated with high morbidity but low mortality, and f) often having no specific treatment available. Dengue meets most of these criteria, but not all. Although dengue predominantly affects resource-limited countries, it does not necessarily only target the poor and marginalised in those countries. Dengue increasingly attracts public health attention, and in some affected countries it is now a high profile disease. Research funding for dengue has increased exponentially in the past two decades, in particular in the area of dengue vaccine development. However, despite advances in dengue research, dengue epidemics are increasing in frequency and magnitude, and dengue is expanding to new areas. Specific treatment and a highly effective vaccine remain elusive. Major research gaps exist in the area of integrated surveillance and vector control. Hence, although dengue differs from many of the NTDs, it still meets important criteria commonly used for NTDs. The current need for increased R&D spending, shared by dengue and other NTDs, is perhaps the key reason why dengue should continue to be considered an NTD.     

Incidence of neural tube defects in liveborn and stillborn infants in British Columbia over a 10-year period.

Reports of an apparent decline in the incidence of neural tube defects (NTDs) have come from various parts of the world. If these findings are consistent they would have an important impact on prenatal diagnosis and on screening programs. The incidence of NTDs over a 10-year period was examined in British Columbia, a province that has a population-based health surveillance registry through which there is virtually complete ascertainment of liveborn infants with NTDs. The results showed a significant decrease in incidence only for stillborn infants with anencephaly. The increased use of ultrasonography and the subsequent termination of pregnancies in which the fetus has been found to have anencephaly may explain this observation.     

Neglected tropical disease vaccines

The neglected tropical diseases or ‘NTDs’ represent the most common infections of the world's one billion poorest people. Unlike the better known acute or emerging infections, the NTDs are generally chronic and disabling (and often disfiguring) conditions. The long-term disability they cause has been revealed as a major reason why poor people in developing countries cannot escape the poverty trap. Because NTDs are associated with poverty, vaccines against these conditions are sometimes referred to as antipoverty vaccines. However, despite their global public health and economic importance, such vaccines have largely been ignored by industry and today are predominantly being produced through the activities of non-profit product development partnerships (PDPs). The Human Hookworm Vaccine Initiative based at the Sabin Vaccine Institute is one such PDP developing two antipoverty vaccines for hookworm and schistosomiasis, respectively. It has been proposed to combine these vaccines in order to target polyparasitic co-infections leading to severe anemia. Ultimately, to ensure global access of a multivalent anthelminthic vaccine, it may be linked to deworming programs through vaccine-linked chemotherapy. This would be an important step towards achieving the Millennium Development Goals for sustainable poverty reduction by 2015.     

Effects of folate supplementation on the risk of spontaneous and induced neural tube defects in Splotch mice

BACKGROUND: Neural tube defects (NTDs) are among the most common human congenital malformations. Although clinical investigations have reported that periconceptional folic acid supplementation can reduce the occurrence of these defects, its mechanism remains unknown. Therefore, the murine mutant Splotch, which has a high incidence of spontaneous NTDs, along with the inbred strains SWV and LM/Bc, were used to investigate the relationship between folate and NTDs. METHODS: To investigate whether folates could reduce spontaneous NTDs, heterozygous Splotch dams (+/Sp) were treated with either folate or folinic acid throughout neurulation, gestational day (GD) 6.5 to 10.5. On GD 18.5 the dams were sacrificed and the fetuses examined for any neural tube defects. Subsequently, Sp/+ dams were treated with arsenic while receiving either a folate or folinic acid supplementation. Similar experiments were performed in the LM/Bc and SWV strains. RESULTS: Neither folate nor folinic acid supplements reduced the frequency of spontaneous NTDs in the embryos from Splotch heterozygote crosses. Arsenic increased the frequency of NTDs and embryonic death in the Splotch, LM/Bc and SWV litters and folinic acid failed to ameliorate the teratogenic effect of this metal. A folate supplement given to arsenic-treated dams proved to be maternally lethal in all three strains. CONCLUSIONS: Splotch embryos were not protected from either spontaneous or arsenic-induced NTDs by folinic or folic acid supplementation. Furthermore, folinic acid supplements did not reduce the incidence of arsenic-induced NTDs in either the LM/Bc or SWV litters.     

Impact of methylenetetrahydrofolate reductase deficiency and low dietary folate on the development of neural tube defects in splotch mice

BACKGROUND: The etiology of neural tube defects (NTDs) is multifactorial, with environmental and genetic determinants. Folate supplementation prevents the majority of NTDs, and a polymorphism in methylenetetrahydrofolate reductase (MTHFR) has become recognized as a genetic risk factor. The mechanisms by which folate affects NTD development are unclear. The Splotch (Sp) mouse is a well-characterized mouse model for studying spontaneous NTDs. To assess the potential interaction between folate metabolism and the Sp mutant in NTD development, we studied mice with both Sp and Mthfr mutations, as well as the interaction between Sp and low dietary folate. METHODS: Wild-type, single Mthfr+/-mutant, single Sp/+mutant, and double mutant (Mthfr+/-, Sp/+) female mice were mated with males of the same genotype. Embryos were examined for NTDs on gestational day (GD) 13.5. To investigate the effects of folate deficiency on Sp mice, Sp/+female mice were fed a control diet (CD), a moderately folic acid-deficient diet (MFADD), or a severely folic acid-deficient diet (SFADD). They were mated with Sp/+males and the embryos were examined. RESULTS: There were no differences in the incidence or severity of NTDs in embryos from double-mutant mating pairs compared to those from single Sp mutants. Embryos from Mthfr+/-dams did not exhibit NTDs. Diets deficient in folate did not influence the incidence or severity of NTDs in embryos from Sp/+mice. CONCLUSIONS: We did not observe an interaction between Sp and Mthfr mutations, or between the Sp mutation and low dietary folate, in NTD development in Splotch mice.     

Mapping a chromosomal locus for valproic acid-inducedexencephaly 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 × 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.(Robert M. Cabrera and Kimblerly A. Greer) Both authors contributed equally to this work as second authors.     

A familial association between twinning and upper-neural tube defects.

An increased twinning rate has been observed in the near relatives (sibs, parents, and aunts and uncles) of probands with neural tube defects (NTDs) occurring at the level of the 11th thoracic vertebra and above (upper NTDs). The twin rate was more than double that of the near relatives of probands with lower NTDs and of those of probands with Mendelian disorders (the controls). The excess twinning was same sex and can therefore consist of either MZ or same-sex DZ twins. Furthermore, upper-NTD families with twins had a higher NTD-sibling occurrence rate than did families without twins. These findings, if corroborated, may imply an etiology common to twinning and NTDs and can perhaps be applied in counseling NTD families.     

Case for mandatory fortification of food with folate in Australia, for the prevention of neural tube defects

BACKGROUND: Since the publication of randomized controlled trials demonstrating the prevention of neural tube defects (NTDs) with periconceptional folate, several Australian states have promoted an increase in periconceptional use of folic acid supplements. Since 1996, voluntary fortification of food with folate has been allowed in Australia and New Zealand for the purpose of preventing NTDs. METHODS: For this report, we synthesized published and unpublished data on folic acid supplement use, voluntary fortification, and trends in NTDs. RESULTS: There has been an increase in the proportion of women (up to 30-40%) taking periconceptional folic acid supplements in Australia, and many foods (mainly breakfast cereals) are fortified. Supplement use is strongly correlated with educational and socioeconomic status; consumption of voluntarily fortified foods is not. There has been a fall in NTDs of about 30% in the non-Aboriginal population, but no change has been seen in the Aboriginal population. CONCLUSIONS: These data support mandatory fortification of food as a more equitable approach to achieving sufficient folate intake in the periconceptional period for all women in Australia and New Zealand to prevent the majority of NTDs in their offspring. In May 2004, based on these and other considerations, the Australia and New Zealand Food Regulation Ministerial Council agreed that mandatory fortification of food with folate should be considered as a priority.     

Using genomewide mutagenesis screens to identify the genes required for neural tube closure in the mouse

BACKGROUND: Neural tube closure is a critical embryological process that requires the coordination of many molecular and cellular events. Only recently has the molecular basis of the cell movements that drive neural tube closure begun to be elucidated. This has been accomplished in part due to the analysis of a growing number of genetically targeted and naturally occurring mouse mutant strains that have neural tube defects (NTDs). Currently there are more than 100 genes that when mutated result in NTDs in the mouse. Yet only approximately 10% of genes in the mouse genome have been mutated and their gross phenotype analyzed, suggesting that only a small percentage of the genes that can cause NTDs have been identified. METHODS: In order to more systematically and fully understand the genetic basis of neural tube closure and to begin to define the molecular pathways that direct this key embryonic event, our laboratories have undertaken a forward genetic screen in mice. From this we hope to gain a better understanding of the regulation of this complex morphogenic processes. CONCLUSIONS: The mouse provides a good model for human neural tube closure, and therefore the information gained from generating novel mouse models of NTDs will help to predict the genes responsible for human NTDs and provide experimental evidence for how they function.