Mini-review: toward understanding mechanisms of genetic neural tube defects in mice

Harris MJ, Juriloff DM. Mini-review: toward understanding mechanisms of genetic neural tube defects in mice. Teratology. 1999 Nov;60(5):292-305. Review.

We review the data from studies of mouse mutants that lend insight to the mechanisms that lead to neural tube defects (NTDs). Most of the 50 single-gene mutations that cause neural tube defects (NTDs) in mice also cause severe embryonic-lethal syndromes, in which exencephaly is a nonspecific feature. In a few mutants (e.g., Trp53, Macs, Mlp or Sp), other defects may be present, but affected fetuses can survive to birth. Multifactorial genetic causes, as are present in the curly tail stock (15-20% spina bifida), or the SELH/Bc strain (15-20% exencephaly), lead to nonsyndromic NTDs. The mutations indicate that "spina bifida occulta," a dorsal gap in the vertebral arches over an intact neural tube, is usually genetically and developmentally unrelated to exencephaly or "spina bifida" (aperta). Almost all exencephaly or spina bifida aperta of genetic origin is caused by failure of neural fold elevation. The developmental mechanisms in genetic NTDs are considered in terms of distinct rostro-caudal zones along the neural folds that likely differ in mechanism of elevation. Failure of elevation leads to: split face (zone A), exencephaly (zone B), rachischisis (all of zone D), or spina bifida (caudal zone D). The developmental mechanisms leading to these genetic NTDs are heterogeneous, even within one zone. At the tissue level, the mutants show that the mechanism of failure of elevation can involve, e.g., (1) slow growth of adjacent tethered tissue (curly tail), (2) defective forebrain mesenchyme (Cart1 or twist), (3) defective basal lamina in surface ectoderm (Lama5), (4) excessive breadth of floorplate and notochord (Lp), (5) abnormal neuroepithelium (Apob, Sp, Tcfap2a), (6) morphological deformation of neural folds (jmj), (7) abnormal neuroepithelial and neural crest cell gap-junction communication (Gja1), or (8) incomplete compensation for a defective step in the elevation sequence (SELH/Bc). At the biochemical level, mutants suggest involvement of: (1) faulty regulation of apoptosis (Trp53 or p300), (2) premature differentiation (Hes1), (3) disruption of actin function (Macs or Mlp), (4) abnormal telomerase complex (Terc), or (5) faulty pyrimidine synthesis (Sp). The NTD preventative effect of maternal dietary supplementation is also heterogeneous, as demonstrated by: (1) methionine (Axd), (2) folic acid or thymidine (Sp), or (3) inositol (curly tail). The heterogeneity of mechanism of mouse NTDs suggests that human NTDs, including the common nonsyndromic anencephaly or spina bifida, may also reflect a variety of genetically caused defects in developmental mechanisms normally responsible for elevation of the neural folds.

PMID: 10525207

Disruption of PDGFR{alpha}-initiated PI3K activation and migration of somite derivatives leads to spina bifida

Pickett EA, Olsen GS, Tallquist MD. Disruption of PDGFR{alpha}-initiated PI3K activation and migration of somite derivatives leads to spina bifida. Development. 2008 Feb;135(3):589-98.

Spina bifida, or failure of the vertebrae to close at the midline, is a common congenital malformation in humans that is often synonymous with neural tube defects (NTDs). However, it is likely that other etiologies exist. Genetic disruption of platelet-derived growth factor receptor (PDGFR) alpha results in spina bifida, but the underlying mechanism has not been identified. To elucidate the cause of this birth defect in PDGFRalpha mutant embryos, we examined the developmental processes involved in vertebrae formation. Exposure of chick embryos to the PDGFR inhibitor imatinib mesylate resulted in spina bifida in the absence of NTDs. We next examined embryos with a tissue-specific deletion of the receptor. We found that loss of the receptor from chondrocytes did not recapitulate the spina bifida phenotype. By contrast, loss of the receptor from all sclerotome and dermatome derivatives or disruption of PDGFRalpha-driven phosphatidyl-inositol 3' kinase (PI3K) activity resulted in spina bifida. Furthermore, we identified a migration defect in the sclerotome as the cause of the abnormal vertebral development. We found that primary cells from these mice exhibited defects in PAK1 activation and paxillin localization. Taken together, these results indicate that PDGFRalpha downstream effectors, especially PI3K, are essential for cell migration of a somite-derived dorsal mesenchyme and disruption of receptor signaling in these cells leads to spina bifida.

PMID: 18192285

Identification of minimal enhancer elements sufficient for Pax3 expression in neural crest and implication of Tead2 as a regulator of Pax3

Milewski RC, Chi NC, Li J, Brown C, Lu MM, Epstein JA. Identification of minimal enhancer elements sufficient for Pax3 expression in neural crest and implication of Tead2 as a regulator of Pax3. Development. 2004 Feb;131(4):829-37. Epub 2004 Jan 21.

Pax3 is a transcription factor that is required by pre-migratory neural crest cells, which give rise to the peripheral nervous system, melanocytes, some vascular smooth muscle, and numerous other derivatives [corrected]. Both mice and humans with Pax3 deficiency exhibit neural crest-related developmental defects [corrected]. Pax3 is also expressed in the dorsal neural tube, and by myogenic progenitors in the presomitic mesoderm and the hypaxial somites. Molecular pathways that regulate Pax3 expression in the roof plate probably represent early upstream signals in neural crest induction. We have identified an enhancer region in the Pax3 genomic locus that is sufficient to recapitulate expression in neural crest precursors in transgenic mice. We show that Tead2, a member of the Tead box family of transcription factors, binds to a neural crest enhancer and activates Pax3 expression. Tead2, and its co-activator YAP65, are co-expressed with Pax3 in the dorsal neural tube, and mutation of the Tead2 binding site in the context of Pax3 transgenic constructs abolishes neural expression. In addition, a Tead2-Engrailed fusion protein is able to repress retinoic acid-induced Pax3 expression in P19 cells and in vivo. These results suggest that Tead2 is an endogenous activator of Pax3 in neural crest.

PMID: 14736747

Hyperglycemic condition disturbs the proliferation and cell death of neural progenitors in mouse embryonic spinal cord

Gao Q, Gao YM. Hyperglycemic condition disturbs the proliferation and cell death of neural progenitors in mouse embryonic spinal cord. International Journal of Developmental Neuroscience. 2007 Aug 19;

Spina bifida, which results from failure of fusion in the spinal region of neural tube, is among the most common birth defects associated with diabetic pregnancy. However, the mechanism underlying maternal diabetes-induced congenital malformations including spina bifida is not fully understood. It was hypothesized that hyperglycemic conditions affect the proliferation and apoptosis of neural progenitor cells in the developing spinal neural tube, leading to abnormal neurodevelopment. In the present study, biological processes such as proliferation and apoptosis were investigated in the neuroepithelial cells of the developing spinal neural tube of embryos from diabetic mice, and in embryonic spinal neural tube derived neural progenitor cell cultures exposed to high glucose in vitro. Maternal diabetes caused decreased proliferation and increased apoptosis of the neuroepithelial cells in the developing spinal cord of embryos from diabetic mouse. Decreased proliferation and increased apoptosis were also found in neural progenitor cells exposed to high glucose. In addition, high glucose-induced apoptosis in neural progenitor cells was associated with activation of caspase-3. Thus, high glucose disturbs both proliferation and cell death of neural progenitors in the developing spinal neural tube. This could provide a cellular mechanism by which maternal hyperglycemia induces spina bifida in embryos from diabetic pregnancy.

PMID: 17888615

Neural plate morphogenesis during mouse neurulation is regulated by antagonism of Bmp signalling

Ybot-Gonzalez P, Gaston-Massuet C, Girdler G, Klingensmith J, Arkell R, Greene ND, Copp AJ. Neural plate morphogenesis during mouse neurulation is regulated by antagonism of Bmp signalling. Development. 2007 Sep;134(17):3203-11.

Dorsolateral bending of the neural plate, an undifferentiated pseudostratified epithelium, is essential for neural tube closure in the mouse spinal region. If dorsolateral bending fails, spina bifida results. In the present study, we investigated the molecular signals that regulate the formation of dorsolateral hinge points (DLHPs). We show that Bmp2 expression correlates with upper spinal neurulation (in which DLHPs are absent); that Bmp2-null embryos exhibit premature, exaggerated DLHPs; and that the local release of Bmp2 inhibits neural fold bending. Therefore, Bmp signalling is necessary and sufficient to inhibit DLHPs. By contrast, the Bmp antagonist noggin is expressed dorsally in neural folds containing DLHPs, noggin-null embryos show markedly reduced dorsolateral bending and local release of noggin stimulates bending. Hence, Bmp antagonism is both necessary and sufficient to induce dorsolateral bending. The local release of Shh suppresses dorsal noggin expression, explaining the absence of DLHPs at high spinal levels, where notochordal expression of Shh is strong. DLHPs ;break through' at low spinal levels, where Shh expression is weaker. Zic2 mutant embryos fail to express Bmp antagonists dorsally and lack DLHPs, developing severe spina bifida. Our findings reveal a molecular mechanism based on antagonism of Bmp signalling that underlies the regulation of DLHP formation during mouse spinal neural tube closure.

PMID: 17693602

The human T locus and spina bifida risk

Jensen LE, Barbaux S, Hoess K, Fraterman S, Whitehead AS, Mitchell LE. The human T locus and spina bifida risk. Human Genetics. 2004 Nov;115(6):475-82. Epub 2004 Sep 24.

The transcription factor T is essential for mesoderm formation and axial development during embryogenesis. Embryonic genotype for a single-nucleotide polymorphism in intron 7 of T ( TIVS7 T/C) has been associated with the risk of spina bifida in some but not all studies. We developed a novel genotyping assay for the TIVS7 polymorphism using heteroduplex generator methodology. This assay was used to genotype spina bifida case-parent trios and the resulting data were analyzed using the transmission disequilibrium test and log-linear analyses. Analyses of these data demonstrated that heterozygous parents transmit the TIVS7-C allele to their offspring with spina bifida significantly more frequently than expected under the assumption of Mendelian inheritance (63 vs 50%, P=0.02). Moreover, these analyses suggest that the TIVS7-C allele acts in a dominant fashion, such that individuals carrying one or more copies of this allele have a 1.6-fold increased risk of spina bifida compared with individuals with zero copies. In silico analysis of the sequence surrounding this polymorphism revealed a potential target site for olfactory neuron-specific factor-1, a transcription factor expressed in the neural tube during development, spanning the polymorphic site. Several other putative, developmentally important and/or environmentally responsive transcription factor-binding sites were also identified close to the TIVS7 polymorphism. The TIVS7 polymorphism or a variant that is in linkage disequilibrium with the TIVS7 polymorphism may, therefore, play a role in T gene expression and influence the risk of spina bifida.

PMID: 15449172

Human neural tube defects: developmental biology, epidemiology, and genetics

Detrait ER, George TM, Etchevers HC, Gilbert JR, Vekemans M, Speer MC. Human neural tube defects: developmental biology, epidemiology, and genetics.
Neurotoxicology & Teratology. 2005 May-Jun;27(3):515-24. Epub 2005 Mar 5. Review.

Birth defects (congenital anomalies) are the leading cause of death in babies under 1 year of age. Neural tube defects (NTD), with a birth incidence of approximately 1/1000 in American Caucasians, are the second most common type of birth defect after congenital heart defects. The most common presentations of NTD are spina bifida and anencephaly. The etiologies of NTDs are complex, with both genetic and environmental factors implicated. In this manuscript, we review the evidence for genetic etiology and for environmental influences, and we present current views on the developmental processes involved in human neural tube closure.

PMID: 15939212

Fetal spina bifida in a mouse model: loss of neural function in utero.

Stiefel D, Copp AJ, Meuli M. Fetal spina bifida in a mouse model: loss of neural function in utero. Journal of Neurosurgery. 2007 Mar;106(3 Suppl):213-21.

OBJECT: The devastating neurological deficit associated with myelomeningocele has previously been assumed to be a direct and inevitable consequence of the primary malformation-failure of neural tube closure. An alternative view is that secondary damage to the pathologically exposed spinal cord tissue in utero is responsible for the neurological deficiency. If the latter mechanism were shown to be correct, it would provide an objective rationale for the performance of in utero surgery for myelomeningocele, because coverage of the exposed spinal cord could be expected to alleviate or perhaps prevent neurodegeneration. To examine this question, the authors studied the development of neuronal connections and neurological function of mice during fetal and neonatal stages in a genetic model of exposed lumbosacral spina bifida.

METHODS: The persistently exposed spinal cord of mouse fetuses carrying both curly tail and loop-tail mutations exhibited essentially normal anatomical and functional hallmarks of development during early gestation (embryonic Days 13.5-16.5), including sensory and motor projections to and from the cord. A significant proportion of fetuses with spina bifida at early gestation exhibited sensorimotor function identical to that seen in age-matched healthy controls. However, at later gestational stages, increasing neurodegeneration within the spina bifida lesion was detected, which was paralleled by a progressive loss of neurological function.

CONCLUSIONS: These findings provide support for the hypothesis that neurological deficit in human myelomeningocele arises following secondary neural tissue destruction and loss of function during pregnancy.

PMID: 17465388

Double spinal dysraphism. Report of three cases

Keshavarzi S, Nejat F, Kazemi H. Double spinal dysraphism. Report of three cases. Journal of Neurosurgery. 2007 Apr;106(4 Suppl):316-8.

The simultaneous presence of multiple spinal neural tube defects is unusual. There have been only a few of these cases reported in the literature. The authors report on three cases of double spina bifida cystica. One patient had two myelomeningoceles (MMCs) at the cervical and lumbosacral regions, one was noted to have both thoracolumbar and sacral defects, and the third presented with double MMCs at lumbar and lumbosacral levels. All three neonates in these cases underwent surgical treatment and ventriculoperitoneal (VP) shunt insertion for associated hydrocephalus. One child died at the age of 2 months despite a well-functioning VP shunt. The other two patients had no complications. Current models of neural tube closure do not thoroughly explain the mechanisms of multiple spinal dysraphism, but the multisite closure model provides a better understanding of caudal neural tube closure than other closure-site models.

PMID: 17465369

Embryology of myelomeningocele and anencephaly

Dias MS, Partington M. Embryology of myelomeningocele and anencephaly. Neurosurgical Focus. 2004 Feb 15;16(2):E1. Review.

The authors review current views on of the embryogenesis of the neural tube defects (NTDs) myelomeningocele and anencephaly. In this context, the following four approaches to the study of NTDs are discussed: normal morphogenesis and timing of early human neural development from conception to the ascent of the conus medullaris; mechanical and molecular biology of neural tube closure derived from experimental and animal models; morphological and biomechanical features of the NTDs myelomeningocele and anencephaly; and the experimental evidence for the importance of both genetic and environmental influences on human NTDs. Although considerable insight into both normal neural tube closure and the factor(s) by which this process may be disrupted has been reported in recent years, the exact mechanism(s) by which human myelomeningoceles and anencephaly arise remain elusive.

PMID: 15209484