Genetic evidence in planar cell polarity signaling pathway in human neural tube defects

Abstract

Neural tube defects (NTDs) are a group of birth anomalies having a profound physical, emotional, and financial effects on families and communities. Their etiology is complex, involving environmental and genetic factors that interact to modulate the incidence and severity of the developing phenotype. The planar cell polarity (PCP) pathway controls the process of convergent extension (CE) during gastrulation and neural tube closure and has been implicated in the pathogenesis of NTDs in animal models and human cohorts. This review summarizes the cumulative results of recent studies on PCP signaling pathway and human NTDs. These results demonstrate that PCP gene alterations contribute to the etiology of human NTDs.

Keywords

planar cell polarity / neural tube defects / rare mutations

Introduction

Neural tube defects (NTDs) are the most common and severe malformations of the central nervous system. They result from embryonic failure of neural tube closure that can occur at any level of the embryonic axis. Despite a long history of etiologic studies, the molecular and cellular pathogenic mechanisms underlying NTDs remain poorly understood. Genetic studies in NTDs have focused mainly on folate-related genes and identified a few significant associations between variants in these genes and an increased risk for NTDs [1-5]. The most significant epidemiological finding relevant to NTDs is the protective effect of periconceptional folic acid supplementation, which reduces their incidence by 50%-70% [6]. Despite this, thousands of families are still affected by these devastating conditions each year.

There are few reported associations of NTDs with chromosomal abnormalities and distinct syndromes. Most cases of NTDs are sporadic with a complex etiology involving both environmental and genetic factors [7]. Of the genes that have been associated with NTDs, planar cell polarity (PCP) genes have been implicated as predisposing factors in a fraction of isolated nonsyndromic NTDs [8]. Mutations in PCP genes lead to a wide range of developmental defects, mainly a shortened body axis, a widened neural plate, and NTDs. The PCP signaling pathway, essential for the orientation and coordinated movement of cells during embryonic morphogenesis, is a compelling candidate for investigation in studies of the etiology of human NTDs.

This review will summarize the neural tube formation, human NTDs classification and discuss the major findings of the recent studies of human PCP genes in the variety of NTDs.

Neural tube defects in humans

Neurulation is a fundamental embryonic process that leads to the development of the neural tube, which is the precursor of the brain and spinal cord. Neurulation begins with formation of the neural plate, a thickening of the ectoderm on the dorsal surface of the post-gastrulation embryo. In human beings, neurulation occurs through 2 distinct phases that occur at distinct sites along the rostrocaudal axis of the embryo: primary neurulation and secondary neurulation [9,10]. During the primary neurulation, the neural plate is subject to shaping and folding, with fusion along the midline to form the tube. The secondary neural tube is derived from a population of mesenchymal cells, the tail bud, which undergo proliferation and condensation followed by cavitation and fusion with the primary neural tube [11]. Neurulation is driven by redundant mechanisms both at the tissue and cellular level [12]. Active processes required for neural tube closure include convergent extension (CE) cell movements, expansion of the cranial mesenchyme, contraction of actin filaments, bending of the neural plate, and adhesion of the neural folds.

In the mouse, closure is initiated at the hindbrain/cervical boundary (closure 1) and then spreads bidirectionally into the hindbrain and along the spinal region. Separate closure initiation sites occur at the midbrain-forebrain boundary (closure 2) and at the rostral extremity of the forebrain (closure 3) [13]. However, Closure 2 found in mice may be absent from human neurulation [14].

Based on a study of the type and frequency of human NTDs, Van Allen et al. [15] proposed a model in which five closure sites exist in human embryos. Although this model was attractive to explain human defects, examination of histological sections of human embryos leads to different models of neural tube closure. In fact, a study by Nakatsu et al. [16] described three sites of apposition, while O’Rahilly and Muller [17] found only two regions of fusion in humans, the first one extending bidirectionally from the rhombencephalic region and the second one that proceeding caudally from the prosencephalic region. The human closure events found by O’Rahilly and Muller have striking resemblance to mouse closures 1 and 3. Therefore, multisite neural tube closure may be a universal phenomenon, although the process appears to be not the same in the human and the other species.

The most common form of NTDs is anencephaly, which results from failure of fusion of the upper and rostral end of the neural tube, and myelomeningocele (commonly called spina bifida), which results from the failure of fusion in the spinal region of the neural tube. All infants with anencephaly are stillborn or die shortly after birth, whereas many infants with myelomeningocele survive, usually as a result of extensive medical and surgical care. Anencephaly and myelomeningocele are referred to as open NTDs where the nervous system and/or meninges are exposed to the environment without normal skin covering. Other open dysraphisms include myeloschisis, hemimyelomeningocele, hemimyelocele, and are sometimes associated with a Chiari II malformation. Another rare form of open NTDs is craniorachischisis that results from failure of neural tube closure over the entire body axis. There are also a number of closed or skin-covered conditions that involve the neural tube. Closed NTDs are further categorized clinically, depending on the presence or absence of a lower back subcutaneous mass. Closed NTDs with a mass are represented by lipomyeloschisis, lipomyelomeningocele, meningocele, and myelocystocele. Closed NTDs without a mass include simple dysraphic states (intradural lipomas, diastematomyelia, teratoma, dermoid, epidermoid, tight filum terminale, persistent terminal ventricle, and dermal sinus) and complex dysraphic states (dorsal enteric fistula, neurenteric cysts, split cord malformations, caudal regression syndrome, and spinal segmental dysgenesis) [18].

Both genetic and non-genetic factors are involved in the etiology of NTDs. Many non-genetic factors include: parental age [19], parental race [20], parental socioeconomic status [21], hyperthermia during early pregnancy [22], maternal diabetes [23], maternal obesity [24], dietary agents or maternal nutrition (such as the uptake of folate [25], inositol [26]), chemical teratogenic agents (such as valproic acid [27], trichostatin A [28], exposure to pesticides [29] and selective serotonin-reuptake inhibitors [30]and so on).

As for genetic factors, a large repertoire of mouse gene mutations has flagged over 200 genes whose function is required for neural tube closure [31]. These genes are involved in a wide variety of cellular functions, and PCP genes comprise a small subset. The phenotypes of PCP mutants in mice offer a context for interpretation of the findings of studies of PCP gene variants in human NTD cases.

Planar cell polarity signaling pathway

PCP, a noncanonical Wnt signaling pathway, is a molecular mechanism that gives cells a coordinated polarized orientation necessary for numerous developmental processes, including their directional movements during vertebrate gastrulation and neurulation, orientation of stereocilia within the hair cells of the inner ear, wound repair, orientation of motile cilia and initiation of left-right asymmetry, and other steps in the development of the kidneys, lungs, and other tissue [32-35].

A role of the PCP pathway in vertebrate cell movements during morphogenesis was first shown in Xenopus and zebrafish [36,37]. In the mouse, Loop-tail (Lp) was the first mutant to implicate a role of PCP pathway and NTDs in mammals [38,39]. Lp heterozygotes are characterized by a “looped” tail appearance, while homozygotes develop a severe NTD resembling human craniorachischisis. It is caused by independent missense mutations S464N and D255E, localized in the proposed C-terminal cytoplasmic domain of a gene, now referred to as Vangl2.

Genetic and molecular analyses in Drosophila have identified components of the PCP signaling mechanism, and have suggested that they may be divided into three modules [40,41] including a global directional cue that links the direction of polarization to the tissue axes, a core module that amplifies and stabilizes subcellular asymmetry through the activity of a bistable feedback mechanism, and one of several distinct tissue specific effector modules that respond to the upstream modules to produce morphological asymmetry in individual tissue. The global module is comprised of the atypical cadherins Fat (Ft), Dachsous (Ds), and the Golgi resident protein Four-jointed (Fj) [42], whose functions are to translate tissue-wide transcription gradients of two or more components into subcellular gradients. This module is characterized by mutant phenotypes in which cells still polarize and coordinate their polarity with neighboring cells, but often fail to align with the tissue axes. The core module consists of proteins that communicate at cell boundaries, recruiting one group to the distal side of cells, and the other to the proximal side, through the function of a poorly understood feedback mechanism. The result is the molecular polarization of individual cells, as well as the coordinated polarization of neighboring cells, like dominoes, thus propagating polarity locally from cell to cell. The tissue specific effector modules respond to the upstream modules to execute morphological polarization [43].

Proteins in the “core” signaling module include the transmembrane proteins such as Frizzled (Fz), Strabismus (Stbm)/Van Gogh (Vang)/Vangl (Stbm/Vang in Drosophila and Vangl in vertebrates) and Flamingo (Fmi)/Starry night (Stan)/Celsr (Fmi or Stan in Drosophila and Celsr in vertebrates), as well as cytoplasmic proteins, including Dishevelled (Dsh/Dvl) (Dsh in Drosophila and Dvl in vertebrates), Prickle (Pk), and Diego (Dgo) [44,45](Fig. 1). These proteins are highly conserved in vertebrates.

Though the molecular mechanisms underlying local PCP signaling are incompletely understood [46,47], several specific features bear some discussion here. First, the asymmetrically localized subcellular complexes, with Fz on the distal side and Vang on the proximal side of adjacent cells, communicate information bidirectionally between those cells [48,49]. This is perhaps most simply illustrated by the observation that cells on either side of the border between adjacent Vang and Fz mutant clones both strongly polarize, indicating that cells with only the Fz complex and cells with only the Vang complex can strongly polarize and be polarized by a neighboring cell. Fmi homodimers are essential for this communication [50]. Second, it is evident that although in wild type, all of the core PCP components are required to achieve a fully asymmetric subcellular localization, they must be viewed as having distinct molecular functions, and disruption of individual functions may leave other activities intact. For example, Fz, Vang and Fmi are sufficient to mediate intercellular communication, while Dsh, Pk and Dgo functions are required for the feedback-mediated amplification of the asymmetry that develops at proximal-distal intercellular boundaries. Furthermore, residual morphological polarization can be observed in tissue mutant for any component except for dsh^null [51,52], suggesting residual function in the absence of most components.

Genetic studies of PCP genes in human NTDs

When the correct expressivity of proteins in PCP signaling is disturbed, caused either by environmental factors or by genetic factors, some NTDs can occur. The findings as follows suggest that rare mutations in the PCP pathway may play critical roles in human NTDs. Table 1 is comprised of these reported mutations.

VANGL1/VANGL2 (Strabismus/Van Gogh/Vangl)

VANGL1 and VANGL2 are mammalian homologs of Drosophila gene Van Gogh (Vang), also known as Strabismus in which mutations disrupt the organization of various epithelial structures, causing characteristic swirled patterns of hairs on wing cells and misorientation of eye ommatidia. VANGL1 and VANGL2 proteins are highly similar. Each has four predicted transmembrane domains and a cytoplasmic domain that includes a PDZ binding motif that mediates protein-protein interaction [53].

The first human mutations in developmental genes implicated in the causation of NTDs were detected in the VANGL1 gene. The VANGL1 gene was sequenced in a cohort of 810 NTD patients with various ethnic origin and 8 missense mutations were identified both in familial (V239I, R274Q, S83L, and R181Q) and sporadic (M328T, F153S, L202F, and A404S) cases [54,55] (Table 1). These mutations affected highly conserved residues and were not found in 1200 controls. The V239I mutation was demonstrated to abrogate in vitro the interaction between VANGL protein and DVL, strongly suggesting a pathogenic effect on the protein function. Finally, the V239I mutation along with another mutation, M328T, affected convergent extension in zebrafish and it was hypothesized that they most likely affect a similar process in humans [56]. These results support a role for VANGL1 as genetic risk factor for NTDs. And a study recently reported the identification of three heterozygous missense mutations in VANGL1, G205R, R186H and R173H, in 144 unrelated individuals with NTDs from Slovakia, Romania and Germany [57].

A Chinese study reported the identification of three missense mutations in VANGL2, R353C and F437S, in two fetuses with cranial NTDs and S84F in a fetus affected by holoprosencephaly [58] (Table 1). F437S completely abrogated interaction with DVL, whereas R353C diminished but did not abolish this interaction. Another study sequencing of VANGL2 in a large multi-ethnic cohort of 673 familial and sporadic NTD patients, including 453 open spina bifida and 202 closed spinal NTD cases, led to the identification of six novel heterozygous missense mutations, which could be pathogenic based on genetic and initial validation data [59]. Four of these mutations, R135W, R177H, L242V, and R270H, were predicted to be damaging to protein function using bioinformatics’ tools, and two others, T247M and R482H, affected highly conserved residues across evolution. Five mutations were identified in patients affected with closed spinal NTDs, suggesting that VANGL2 mutations may predispose to NTDs is approximately 2.5% of closed spinal NTDs (5 in 202), at a frequency that is significantly different from that of 0.4% (2 in 453) detected in open spina bifida patients.

FZD3/FZD6 (Frizzled)

The Frizzled (Fzd) family contains ten genes in humans (FZD1–FZD10). Frizzleds are seven-pass transmembrane (7TM) receptors that transduce critical cellular signals during development. Frizzleds share a cysteine-rich domain (CRD) in the N-terminal extracellular region, which binds several secreted proteins, and among them, proteins of the Wnt family [60]. Three main signaling pathways are activated by Frizzleds: the PCP pathway, the canonical Wnt/β-catenin pathway, and the Wnt/calcium pathway. In the mouse, Fzd3 and Fzd6 play a role in neural tube closure. Double mutants Fzd3^‒/‒/Fzd6 ^‒/‒ embryos exhibit craniorachischisis with nearly 100% penetrance and these mice die within minutes after birth [61]. Fzd3 is required for axonal outgrowth and guidance in the central nervous system [62]. Fzd1 and/or Fzd2 mutations can cause defects in neural tube closure [63]. Expression studies in humans have shown that FZD3 and FZD6 are widely expressed in both embryonic and adult tissue, including brain and central nervous system [64,65].

A recent study was the first systematically screened all FZD3 and FZD6 coding regions and splice sites for small alterations in NTD patients. Resequencing analysis in 473 NTD patients and 639 ethnically matched controls. While they could not demonstrate a significant contribution of FZD3 gene, they identified five rare FZD6 variants that were absent in all controls and predicted to have a functional effect by computational analysis: one de novo frameshift mutation (C615X), three missense changes (R405Q, R511C, and R511H), and one substitution (c.*20C>T) affecting the 3′-untranslated region (UTR) of the gene [66] (Table 1).

CELSR1 (Flamingo/Starry night/Celsr)

The human CELSR genes (CELSR1, CELSR2, CELSR3), homologs of the PCP core gene Fmi, are evolutionarily conserved seven-pass transmembrane receptors that belong to the cadherin superfamily. Cadherins are calcium-dependent cell adhesion molecules that are implicated in many biological processes, including cell signaling during embryogenesis, and the formation of neural circuits. CELSR proteins contain nine N-terminal cadherin repeats, followed by eight EGF-like domains, seven putative transmembrane segments, and an anonymous intracellular C terminus [67]. To date, only CELSR1 was shown to cause NTDs when mutated in human.

A study reported potentially causative mutations in CELSR1 (A773V, R2312P, R2438Q, N2739T, S2964L, P2983A) in 36 fetuses affected with the severe NTDs craniorachischisis [68] (Table 1). Importantly, three CELSR1 mutations (S2964L, R2438Q, and P2983A) detected in the study exhibited a profound alteration in subcellular protein localization in polarized Madin Darby canine kidney epithelial cells (strain II), suggesting a defective PCP protein trafficking to the plasma membrane as a likely pathogenic mechanism for these mutations in NTDs.

A recent research reported that they sequenced the coding region and the exon-intron junctions of CELSR1 in a cohort of 473 patients affected with various forms of open and closed NTDs (412) or caudal agenesis (61) [69]. Finally, they identified one nonsense mutation in exon 1 of CELSR1 (Q834X) that truncated the majority of the protein in one patient with NTDs and one in-frame 12 bp deletion (S2963_T2966del) that removed a putative PKC phosphorylation “SSR” motif in one caudal agenesis patient. The variant Q834X was detected in a sporadic case affected with myelomeningocele localized in D12-L1, Chiari II malformation, hydrocephalus, and hydromyelia. They also detected a total of 13 novel missense variants in 12 patients (11 NTDs and 1 caudal agenesis) that were predicted to be pathogenic in silico.

DVL2/DVL3 (Dishevelled)

The multifunctional protein Dishevelled (Dsh/Dvl) is involved in both the canonical Wnt signaling pathway as well as the PCP pathway and regulates many biological processes, ranging from cell-fate specification and cell polarity to social behavior. Homologs of Dishevelled are DVL1, DVl2 and DVL3 in humans. They function as essential scaffold proteins that interact with diverse proteins, including kinases, phosphatases, and adaptor proteins. They contain three highly conserved domains: the DIX domain that is largely α-helical structure [70], the PDZ domain, which consists of six β-sheets that enfold two α-helices [71], and the DEP domain, consisting of a bundle of three α-helices domain [72].

In the mouse, Dvl1^‒/‒, Dvl3^‒/‒ single mutants and Dvl1^‒/‒; Dvl3^‒/‒ double mutants did not display neural tube defects [73]. Dvl2^‒/‒ embryos displayed thoracic spina bifida, while virtually all Dvl1^‒/‒; Dvl2^‒/‒ double mutant embryos displayed craniorachischisis, a completely open neural tube from the midbrain to the tail. For Dvl3, neurulation appeared normal both in Dvl3^‒/‒ and Ltap^{Lp/ +} (Vangl2/Ltap) mutants, while defects were seen in both Dvl3^+/‒; Ltap^{Lp/ +} and Dvl3^‒/‒; Ltap^{Lp/ +} mutants, indicating genetic interaction between the Dvl genes and the other PCP gene Vangl2 [74]. These findings indicate that Dvl2 is the most important mammalian Dvl gene for neural tube closure and is sufficient by itself for the correct process. By contrast, Dvl1 and Dvl3 are not sufficient by themselves but contribute significantly when Dvl2 is completely missing.

A recent study investigated if the human orthologs DVL2 and DVL3 genes could play a role in NTDs pathogenesis [75]. In this study, they analyzed the role of the human orthologs DVL2 and DVL3 in a cohort of 473 NTD patients. Rare variants were genotyped in 639 ethnically matched controls. Two mutations, Y667C and A53V, identified in DVL2 were predicted to be detrimental in silico (Table 1). Significantly, a 1-bp insertion (c.1801_1802insG, p.E620X) in exon 15 of DVL2 predicted to lead to the truncation of the protein was identified in a patient with a complex form of caudal agenesis. In summary, this study demonstrates a possible role for rare variants in DVL2 gene as risk factors for NTDs. But they could not demonstrate a significant contribution of DVL3 gene in the pathogenesis of NTDs because none of the three DVL3 missense variants(Table 1) detected in patients and absent in controls caused significant changes in the protein function since they were scored as neutral by the two algorithms(PolyPhen and SIFT). Moreover, these mutations did not create cryptic splice sites, suggesting that they did not affect the gene splicing.

Interestingly, in this study, they identified two patients that were double heterozygote for missense variants in DVL2 and VANGL2 (DVL2 T535I/VANGL2 R135W; DVL2 E620X/VANGL2 R482H), one patient in DVL2 and CELSR1 (DVL2 A111V/CELSR1 T346S), and last one in DVL3 and VANGL1 (DVL3 I353V/VANGL1 A404S). Genetic interactions between core PCP genes have been demonstrated in double mutant mice developing alteration of neural tissue closure processes, including both open rostral and caudal neural tube defects [63]. According to the multifactorial model of human NTDs etiology, here they reported that human DVL mutations may genetically interact with VANGL and CELSR1 alleles to affect neural tube closure. This study was the first describing a comprehensive resequencing analysis of the DVL genes in human NTDs.

PRICKLE1/ PRICKLE2 (Prickle)

Prickle encodes a cytoplasmic protein with an N-terminal PET domain, three LIM domains and a C-terminal PKH domain. The LIM domain is a cysteine-rich sequence with two zinc-finger motifs that mediates protein-protein interactions. The N-terminal PET domain combines with the three LIM domains during interactions with other proteins. The C-terminal domain contains a CaaX-motif prenylation site that determines protein-protein and protein-membrane interactions. Both gain-of-function and loss-of-function of Prickle1 in Xenopus and zebrafish lead to defective convergent extension movements, manifested mainly by a shortened body axis (in both organisms) and spina bifida (in frog embryos) [76-78].

Homologs of Prickle are PRICKLE1, PRICKLE2, PRICKLE3 and PRICKLE4 in humans. A study suggested that the PRICKLE2 gene may potentially modify the risk of spina bifida and deserved further investigation [79]. A recent study showed that mutations in PRICKLE genes were associated with seizures in humans, mice, and flies [80]. Another recent study identified novel rare mutations in PRICKLE1 in human NTDs [81]. In this study, they screened this gene in 810 unrelated NTDs and identified 7 rare missense heterozygous mutations that were absent in all controls analyzed and predicted to be functionally deleterious using bioinformatics. Two variants, I69T and N81H, were identified in the PET domain of PRICKLE1. One variant, T275M, was identified in the 3rd LIM domain. Four variants (V550M, R682C, S739F, and D771N) were identified in the last 284 amino acids at the carboxy terminus of PRICKLE1 (Table 1).

FUZ (Fuzzy)

Fuzzy(Fuz), a PCP effector gene, is involved in ciliogenesis and directional cell movement. Knockout studies in mice exhibit severe cranial NTD, exencephaly, severely dilated brain ventricles and defective cilia [82,83].

Recently, five nonsynonymous human FUZ mutations in 234 patients with NTDs have been identified (P39S, G140E, S142T, D354Y, and R404Q) [84] (Table 1). Further analyses predicted that the D354Y and R404Q mutations might disrupt protein function, whereas the others were more likely benign polymorphic variants.

SCRIB (Scribble)

Scribble (Scrib), non-core PCP-related gene, is a large cytoplasmic protein containing multiple domains including 4 PDZ domains. In the mouse, a point mutation in Scrib causes severe defects in neural tube development as do mutations in the protein Vangl2, which interacts genetically with Scrib [85].

Two putative mutations in the human SCRIB gene, P454S and R1535Q, were identified in 36 fetuses with craniorachischisis [68](Table 1). The proline residue at P454 was not conserved between species. No digenic mutant combinations with SCRIB were found.

DACT1 (Dapper)

Three Dact family members (Dact1-3), non-core PCP-related genes, have been identified in vertebrates. In Xenopus embryos, Dact1 is required for notochord and head structures formation and for neural development [86]. In mice, Dact1 mutants exhibit NTD phenotypes and deregulate PCP signaling [87,88]. Sestd1 cooperates with Dact1 in Vangl2 regulation and in the PCP pathway during mammalian embryonic development [89].

Five missense mutations in human DACT1 (R45W, D142G, N356K, V702G, T808K) were identified in 167 fetuses with NTDs [90] (Table 1), which were absent in 480 controls. Among these mutations, three occurred in the residues highly conserved in vertebrates (R45W, D142G, N356K), and the other two in the less conserved residues (V702G, T808K). Further biochemical analyses revealed that among the five mutations, N356K and R45W show loss-of-function or reduced activities in inducing Dishevelled2 (DVL2) degradation and inhibiting jun-N-terminal kinase (JNK) phosphorylation, implicating mutated DACT1 as a risk factor for human NTDs.

Conclusions

The identification of genetic risk factors for human NTDs is complicated by the multiplicity of genes participating in neurulation, and the importance of gene-environment interactions. Elucidation of genetic causes predisposing to NTDs is important challenge in light of preconception care program that aims to reduce reproductive risks before conception and to improve the chance for a healthy birth outcome.

In this paper, we have reviewed recent studies and highlighted the relationship between PCP signaling pathway and the development of NTDs. Thus, the evidence is accumulating for an important contribution of PCP genes to the pathogenesis of human NTDs, necessitating a detailed analysis of other not yet explored PCP genes in large cohorts of patients. But the reliability of the search in NTD cases for rare mutations and predictions of deleterious effects on protein is unknown. Other rare deleterious variants may have been overlooked because of the strategy of excluding variants found in as few as one controls. Furthermore, the sequencing approach could detect only variants in coding sequence and splice sites, a subset of the possible types of mutation. Emerging technologies for high throughput sequencing and analysis of genomic deletions and copy number variations offer the prospect, in the coming years, of progress in identification of candidate genes and screening for novel mutations in human NTDs.

References

[1] Beaudin AE, Stover PJ. Insights into metabolic mechanisms underlying folate-responsive neural tube defects: a minireview. Birth Defects Res A Clin Mol Teratol2009; 85(4): 274-284

[2] De Marco P, Calevo MG, Moroni A, Merello E, Raso A, Finnell RH, Zhu H, Andreussi L, Cama A, Capra V. Reduced folate carrier polymorphism (80A—>G) and neural tube defects. Eur J Hum Genet2003; 11(3): 245-252

[3] O’Leary VB, Pangilinan F, Cox C, Parle-McDermott A, Conley M, Molloy AM, Kirke PN, Mills JL, Brody LC, Scott JM; Members of the Birth Defects Research Group. Reduced folate carrier polymorphisms and neural tube defect risk. Mol Genet Metab2006; 87(4): 364-369PMID:16343969

[4] De Marco P, Merello E, Calevo MG, Mascelli S, Raso A, Cama A, Capra V. Evaluation of a methylenetetrahydrofolate-dehydrogenase 1958G>A polymorphism for neural tube defect risk. J Hum Genet2006; 51(2): 98-103

[5] Kibar Z, Capra V, Gros P. Toward understanding the genetic basis of neural tube defects. Clin Genet2007; 71(4): 295-310

[6] van der Linden IJ, Afman LA, Heil SG, Blom HJ. Genetic variation in genes of folate metabolism and neural-tube defect risk. Proc Nutr Soc2006; 65(2): 204-215

[7] Bassuk AG, Kibar Z. Genetic basis of neural tube defects. Semin Pediatr Neurol2009; 16(3): 101-110

[8] De Marco P, Merello E, Cama A, Kibar Z, Capra V. Human neural tube defects: genetic causes and prevention. Biofactors2011; 37(4): 261-268

[9] Rufener S, Ibrahim M, Parmar HA. Imaging of congenital spine and spinal cord malformations. Neuroimaging Clin N Am2011; 21(3): 659-676, viii

[10] Greene ND, Copp AJ. Development of the vertebrate central nervous system: formation of the neural tube. Prenat Diagn2009; 29(4): 303-311

[11] Catala M. Genetic control of caudal development. Clin Genet2002; 61(2): 89-96

[12] Colas JF, Schoenwolf GC. Towards a cellular and molecular understanding of neurulation. Dev Dyn2001; 221(2): 117-145

[13] Copp AJ, Greene ND, Murdoch JN. The genetic basis of mammalian neurulation. Nat Rev Genet2003; 4(10): 784-793

[14] Juriloff DM, Harris MJ, Tom C, MacDonald KB. Normal mouse strains differ in the site of initiation of closure of the cranial neural tube. Teratology1991; 44(2): 225-233

[15] Van Allen MI, Kalousek DK, Chernoff GF, Juriloff D, Harris M, McGillivray BC, Yong SL, Langlois S, MacLeod PM, Chitayat D, Friedman JM, Wilson RD, McFadden D, Pantzar J, Ritchie S, Hall JG. Evidence for multi-site closure of the neural tube in humans. Am J Med Genet1993; 47(5): 723-743

[16] Nakatsu T, Uwabe C, Shiota K. Neural tube closure in humans initiates at multiple sites: evidence from human embryos and implications for the pathogenesis of neural tube defects. Anat Embryol (Berl)2000; 201(6): 455-466

[17] O’Rahilly R, Müller F. The two sites of fusion of the neural folds and the two neuropores in the human embryo. Teratology2002; 65(4): 162-170

[18] Rossi A, Cama A, Piatelli G, Ravegnani M, Biancheri R, Tortori-Donati P. Spinal dysraphism: MR imaging rationale. J Neuroradiol2004; 31(1): 3-24

[19] Vieira AR, Castillo Taucher S. Maternal age and neural tube defects: evidence for a greater effect in spina bifida than in anencephaly. Rev Med Chil2005; 133(1): 62-70(in Spanish)

[20] Njamnshi AK, Djientcheu VP, Lekoubou A, Guemse M, Obama MT, Mbu R, Takongmo S, Kago I. Neural tube defects are rare among black Americans but not in sub-Saharan black Africans: the case of Yaounde-Cameroon. J Neurol Sci2008; 270(1-2): 13-17

[21] Grewal J, Carmichael SL, Song J, Shaw GM. Neural tube defects: an analysis of neighbourhood- and individual-level socio-economic characteristics. Paediatr Perinat Epidemiol2009; 23(2): 116-124

[22] Moretti ME, Bar-Oz B, Fried S, Koren G. Maternal hyperthermia and the risk for neural tube defects in offspring: systematic review and meta-analysis. Epidemiology2005; 16(2): 216-219

[23] Loeken MR. Current perspectives on the causes of neural tube defects resulting from diabetic pregnancy. Am J Med Genet C Semin Med Genet2005; 135C(1): 77-87

[24] Ray JG, Wyatt PR, Vermeulen MJ, Meier C, Cole DE. Greater maternal weight and the ongoing risk of neural tube defects after folic acid flour fortification. Obstet Gynecol2005; 105(2): 261-265

[25] De Wals P, Tairou F, Van Allen MI, Uh SH, Lowry RB, Sibbald B, Evans JA, Van den Hof MC, Zimmer P, Crowley M, Fernandez B, Lee NS, Niyonsenga T. Reduction in neural-tube defects after folic acid fortification in Canada. N Engl J Med2007; 357(2): 135-142

[26] Cogram P, Hynes A, Dunlevy LPE, Greene NDE, Copp AJ. Specific isoforms of protein kinase C are essential for prevention of folate-resistant neural tube defects by inositol. Hum Mol Genet2004; 13(1): 7-14

[27] Gurvich N, Berman MG, Wittner BS, Gentleman RC, Klein PS, Green JB. Association of valproate-induced teratogenesis with histone deacetylase inhibition in vivo. FASEB J2005; 19(9): 1166-1168

[28] Menegola E, Di Renzo F, Broccia ML, Prudenziati M, Minucci S, Massa V, Giavini E. Inhibition of histone deacetylase activity on specific embryonic tissues as a new mechanism for teratogenicity. Birth Defects Res B Dev Reprod Toxicol2005; 74(5): 392-398

[29] Brender JD, Felkner M, Suarez L, Canfield MA, Henry JP. Maternal pesticide exposure and neural tube defects in Mexican Americans. Ann Epidemiol2010; 20(1): 16-22

[30] Alwan S, Reefhuis J, Rasmussen SA, Olney RS, Friedman JM; National Birth Defects Prevention Study. Use of selective serotonin-reuptake inhibitors in pregnancy and the risk of birth defects. N Engl J Med2007; 356(26): 2684-2692

[31] Harris MJ, Juriloff DM. An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure. Birth Defects Res A Clin Mol Teratol2010; 88(8): 653-669

[32] Bayly R, Axelrod JD. Pointing in the right direction: new developments in the field of planar cell polarity. Nat Rev Genet2011; 12(6): 385-391

[33] Goodrich LV, Strutt D. Principles of planar polarity in animal development. Development2011; 138(10): 1877-1892

[34] Gray RS, Roszko I, Solnica-Krezel L. Planar cell polarity: coordinating morphogenetic cell behaviors with embryonic polarity. Dev Cell2011; 21(1): 120-133

[35] Henderson DJ, Chaudhry B. Getting to the heart of planar cell polarity signaling. Birth Defects Res A Clin Mol Teratol2011; 91(6): 460-467

[36] Heisenberg CP, Tada M, Rauch GJ, Saúde L, Concha ML, Geisler R, Stemple DL, Smith JC, Wilson SW. Silberblick/Wnt11 mediates convergent extension movements during zebrafish gastrulation. Nature2000; 405(6782): 76-81

[37] Wallingford JB, Rowning BA, Vogeli KM, Rothbächer U, Fraser SE, Harland RM. Dishevelled controls cell polarity during Xenopus gastrulation. Nature2000; 405(6782): 81-85

[38] Kibar Z, Vogan KJ, Groulx N, Justice MJ, Underhill DA, Gros P, Gros P. Ltap, a mammalian homolog of Drosophila Strabismus/Van Gogh, is altered in the mouse neural tube mutant Loop-tail. Nat Genet2001; 28(3): 251-255

[39] Murdoch JN, Doudney K, Paternotte C, Copp AJ, Stanier P. Severe neural tube defects in the loop-tail mouse result from mutation of Lpp1, a novel gene involved in floor plate specification. Hum Mol Genet2001; 10(22): 2593-2601

[40] Tree DR, Ma D, Axelrod JD. A three-tiered mechanism for regulation of planar cell polarity. Semin Cell Dev Biol2002; 13(3): 217-224

[41] Axelrod JD. Progress and challenges in understanding planar cell polarity signaling. Semin Cell Dev Biol2009; 20(8): 964-971

[42] Ma D, Yang CH, McNeill H, Simon MA, Axelrod JD. Fidelity in planar cell polarity signalling. Nature2003; 421(6922): 543-547

[43] Zallen JA. Planar polarity and tissue morphogenesis. Cell2007; 129(6): 1051-1063PMID:17574020

[44] Jones C, Chen P. Planar cell polarity signaling in vertebrates. Bioessays2007; 29(2): 120-132

[45] Greene ND, Stanier P, Copp AJ. Genetics of human neural tube defects. Hum Mol Genet2009; 18(R2): R113-R129

[46] Lawrence PA, Struhl G, Casal J. Planar cell polarity: one or two pathways? Nat Rev Genet2007; 8(7): 555-563

[47] Lapébie P, Borchiellini C, Houliston E. Dissecting the PCP pathway: one or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis? Bioessays2011; 33(10): 759-768PMID:21919026

[48] Chen WS, Antic D, Matis M, Logan CY, Povelones M, Anderson GA, Nusse R, Axelrod JD. Asymmetric homotypic interactions of the atypical cadherin flamingo mediate intercellular polarity signaling. Cell2008; 133(6): 1093-1105

[49] Strutt D, Strutt H. Differential activities of the core planar polarity proteins during Drosophila wing patterning. Dev Biol2007; 302(1): 181-194

[50] Lawrence PA, Casal J, Struhl G. Cell interactions and planar polarity in the abdominal epidermis of Drosophila. Development2004; 131(19): 4651-4664

[51] Amonlirdviman K, Khare NA, Tree DR, Chen WS, Axelrod JD, Tomlin CJ. Mathematical modeling of planar cell polarity to understand domineering nonautonomy. Science2005; 307(5708): 423-426

[52] Casal J, Lawrence PA, Struhl G. Two separate molecular systems, Dachsous/Fat and Starry night/Frizzled, act independently to confer planar cell polarity. Development2006; 133(22): 4561-4572

[53] Katoh Y, Katoh M. Comparative genomics on Vangl1 and Vangl2 genes. Int J Oncol2005; 26(5): 1435-1440

[54] Kibar Z, Torban E, McDearmid JR, Reynolds A, Berghout J, Mathieu M, Kirillova I, De Marco P, Merello E, Hayes JM, Wallingford JB, Drapeau P, Capra V, Gros P. Mutations in VANGL1 associated with neural-tube defects. N Engl J Med2007; 356(14): 1432-1437

[55] Kibar Z, Bosoi CM, Kooistra M, Salem S, Finnell RH, De Marco P, Merello E, Bassuk AG, Capra V, Gros P. Novel mutations in VANGL1 in neural tube defects. Hum Mutat2009; 30(7): E706-E715

[56] Reynolds A, McDearmid JR, Lachance S, De Marco P, Merello E, Capra V, Gros P, Drapeau P, Kibar Z. VANGL1 rare variants associated with neural tube defects affect convergent extension in zebrafish. Mech Dev2010; 127(7-8): 385-392

[57] Bartsch O, Kirmes I, Thiede A, Lechno S, Gocan H, Florian IS, Haaf T, Zechner U, Sabova L, Horn F. Novel VANGL1 Gene Mutations in 144 Slovakian, Romanian and German Patients with Neural Tube Defects. Mol Syndromol2012; 3(2): 76-81

[58] Lei YP, Zhang T, Li H, Wu BL, Jin L, Wang HY. VANGL2 mutations in human cranial neural-tube defects. N Engl J Med2010; 362(23): 2232-2235

[59] Kibar Z, Salem S, Bosoi CM, Pauwels E, De Marco P, Merello E, Bassuk AG, Capra V, Gros P. Contribution of VANGL2 mutations to isolated neural tube defects. Clin Genet2011; 80(1): 76-82

[60] Schulte G, Bryja V. The Frizzled family of unconventional G-protein-coupled receptors. Trends Pharmacol Sci2007; 28(10): 518-525

[61] Wang Y, Guo N, Nathans J. The role of Frizzled3 and Frizzled6 in neural tube closure and in the planar polarity of inner-ear sensory hair cells. J Neurosci2006; 26(8): 2147-2156

[62] Wang Y, Zhang J, Mori S, Nathans J. Axonal growth and guidance defects in Frizzled3 knock-out mice: a comparison of diffusion tensor magnetic resonance imaging, neurofilament staining, and genetically directed cell labeling. J Neurosci2006; 26(2): 355-364

[63] Yu H, Smallwood PM, Wang Y, Vidaltamayo R, Reed R, Nathans J. Frizzled 1 and frizzled 2 genes function in palate, ventricular septum and neural tube closure: general implications for tissue fusion processes. Development2010; 137(21): 3707-3717

[64] Sala CF, Formenti E, Terstappen GC, Caricasole A. Identification, gene structure, and expression of human frizzled-3 (FZD3). Biochem Biophys Res Commun2000; 273(1): 27-34

[65] Tokuhara M, Hirai M, Atomi Y, Terada M, Katoh M. Molecular cloning of human Frizzled-6. Biochem Biophys Res Commun1998; 243(2): 622-627

[66] De Marco P, Merello E, Rossi A, Piatelli G, Cama A, Kibar Z, Capra V. FZD6 is a novel gene for human neural tube defects. Hum Mutat2012; 33(2): 384-390

[67] Takeichi M. The cadherin superfamily in neuronal connections and interactions. Nat Rev Neurosci2007; 8(1): 11-20

[68] Robinson A, Escuin S, Doudney K, Vekemans M, Stevenson RE, Greene ND, Copp AJ, Stanier P. Mutations in the planar cell polarity genes CELSR1 and SCRIB are associated with the severe neural tube defect craniorachischisis. Hum Mutat2012; 33(2): 440-447

[69] Allache R, De Marco P, Merello E, Capra V, Kibar Z. Role of the planar cell polarity gene CELSR1 in neural tube defects and caudal agenesis. Birth Defects Res A Clin Mol Teratol2012; 94(3): 176-181

[70] Capelluto DG, Kutateladze TG, Habas R, Finkielstein CV, He X, Overduin M. The DIX domain targets dishevelled to actin stress fibres and vesicular membranes. Nature2002; 419(6908): 726-729

[71] Wong HC, Bourdelas A, Krauss A, Lee HJ, Shao Y, Wu D, Mlodzik M, Shi DL, Zheng J. Direct binding of the PDZ domain of Dishevelled to a conserved internal sequence in the C-terminal region of Frizzled. Mol Cell2003; 12(5): 1251-1260

[72] Wong HC, Mao J, Nguyen JT, Srinivas S, Zhang W, Liu B, Li L, Wu D, Zheng J. Structural basis of the recognition of the dishevelled DEP domain in the Wnt signaling pathway. Nat Struct Biol2000; 7(12): 1178-1184

[73] Wang J, Hamblet NS, Mark S, Dickinson ME, Brinkman BC, Segil N, Fraser SE, Chen P, Wallingford JB, Wynshaw-Boris A. Dishevelled genes mediate a conserved mammalian PCP pathway to regulate convergent extension during neurulation. Development2006; 133(9): 1767-1778

[74] Etheridge SL, Ray S, Li S, Hamblet NS, Lijam N, Tsang M, Greer J, Kardos N, Wang J, Sussman DJ, Chen P, Wynshaw-Boris A. Murine dishevelled 3 functions in redundant pathways with dishevelled 1 and 2 in normal cardiac outflow tract, cochlea, and neural tube development. PLoS Genet2008; 4(11): e1000259

[75] De Marco P, Merello E, Consales A, Piatelli G, Cama A, Kibar Z, Capra V. Genetic analysis of disheveled 2 and disheveled 3 in human neural tube defects. J Mol Neurosci2013; 49(3): 582-588

[76] Carreira-Barbosa F, Concha ML, Takeuchi M, Ueno N, Wilson SW, Tada M. Prickle 1 regulates cell movements during gastrulation and neuronal migration in zebrafish. Development2003; 130(17): 4037-4046

[77] Takeuchi M, Nakabayashi J, Sakaguchi T, Yamamoto TS, Takahashi H, Takeda H, Ueno N. The prickle-related gene in vertebrates is essential for gastrulation cell movements. Curr Biol2003; 13(8): 674-679

[78] Veeman MT, Slusarski DC, Kaykas A, Louie SH, Moon RT. Zebrafish prickle, a modulator of noncanonical Wnt/Fz signaling, regulates gastrulation movements. Curr Biol2003; 13(8): 680-685

[79] Wen S, Zhu H, Lu W, Mitchell LE, Shaw GM, Lammer EJ, Finnell RH. Planar cell polarity pathway genes and risk for spina bifida. Am J Med Genet A2010; 152A(2): 299-304

[80] Tao H, Manak JR, Sowers L, Mei X, Kiyonari H, Abe T, Dahdaleh NS, Yang T, Wu S, Chen S, Fox MH, Gurnett C, Montine T, Bird T, Shaffer LG, Rosenfeld JA, McConnell J, Madan-Khetarpal S, Berry-Kravis E, Griesbach H, Saneto RP, Scott MP, Antic D, Reed J, Boland R, Ehaideb SN, El-Shanti H, Mahajan VB, Ferguson PJ, Axelrod JD, Lehesjoki AE, Fritzsch B, Slusarski DC, Wemmie J, Ueno N, Bassuk AG. Mutations in prickle orthologs cause seizures in flies, mice, and humans. Am J Hum Genet2011; 88(2): 138-149

[81] Bosoi CM, Capra V, Allache R, Trinh VQ, De Marco P, Merello E, Drapeau P, Bassuk AG, Kibar Z. Identification and characterization of novel rare mutations in the planar cell polarity gene PRICKLE1 in human neural tube defects. Hum Mutat2011; 32(12): 1371-1375

[82] Gray RS, Abitua PB, Wlodarczyk BJ, Szabo-Rogers HL, Blanchard O, Lee I, Weiss GS, Liu KJ, Marcotte EM, Wallingford JB, Finnell RH. The planar cell polarity effector Fuz is essential for targeted membrane trafficking, ciliogenesis and mouse embryonic development. Nat Cell Biol2009; 11(10): 1225-1232

[83] Heydeck W, Zeng H, Liu A. Planar cell polarity effector gene Fuzzy regulates cilia formation and Hedgehog signal transduction in mouse. Dev Dyn2009; 238(12): 3035-3042

[84] Seo JH, Zilber Y, Babayeva S, Liu J, Kyriakopoulos P, De Marco P, Merello E, Capra V, Gros P, Torban E. Mutations in the planar cell polarity gene, Fuzzy, are associated with neural tube defects in humans. Hum Mol Genet2011; 20(22): 4324-4333

[85] Murdoch JN, Henderson DJ, Doudney K, Gaston-Massuet C, Phillips HM, Paternotte C, Arkell R, Stanier P, Copp AJ. Disruption of scribble (Scrb1) causes severe neural tube defects in the circletail mouse. Hum Mol Genet2003; 12(2): 87-98

[86] Cheyette BN, Waxman JS, Miller JR, Takemaru K, Sheldahl LC, Khlebtsova N, Fox EP, Earnest T, Moon RT. Dapper, a Dishevelled-associated antagonist of beta-catenin and JNK signaling, is required for notochord formation. Dev Cell2002; 2(4): 449-461

[87] Suriben R, Kivimäe S, Fisher DA, Moon RT, Cheyette BN. Posterior malformations in Dact1 mutant mice arise through misregulated Vangl2 at the primitive streak. Nat Genet2009; 41(9): 977-985

[88] Wen J, Chiang YJ, Gao C, Xue H, Xu J, Ning Y, Hodes RJ, Gao X, Chen YG. Loss of Dact1 disrupts planar cell polarity signaling by altering dishevelled activity and leads to posterior malformation in mice. J Biol Chem2010; 285(14): 11023-11030

[89] Yang X, Cheyette BN. SEC14 and spectrin domains 1 (Sestd1) and Dapper antagonist of catenin 1 (Dact1) scaffold proteins cooperatively regulate the Van Gogh-like 2 (Vangl2) four-pass transmembrane protein and planar cell polarity (PCP) pathway during embryonic development in mice. J Biol Chem2013; 288(28): 20111-20120

[90] Shi Y, Ding Y, Lei YP, Yang XY, Xie GM, Wen J, Cai CQ, Li H, Chen Y, Zhang T, Wu BL, Jin L, Chen YG, Wang HY. Identification of novel rare mutations of DACT1 in human neural tube defects. Hum Mutat2012; 33(10): 1450-1455

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