Neural tube defects in chromosomally normal and abnormal human embryos

Fetal Pathology of Neural Tube Defects - An Overview of 68 Cases

INTRODUCTION

The prevalence of neural tube defects worldwide is 1 - 2 per 1000 neonates. Neural tube defects result from a disturbance of neurulation in the 3rd or 4th week of development and thus represent the earliest manifestation of organ malformation. Neural tube defects (NTD) are classified into cranial dysraphism leading to anencephaly or meningoencephalocele and spinal dysraphism with or without meningomyelocele. In isolated form they have multifactorial causes, and the empirical risk of recurrence in Central Europe is 2%. As associated malformations they tend to occur sporadically, and in monogenic syndromes they follow Mendelian inheritance patterns with a high risk of recurrence.

PATIENTS

Autopsies were performed on 68 fetuses following a prenatal diagnosis of NTD and induced abortion.

RESULTS

The incidence of NTDs in our autopsied fetuses was 8% and 11% in fetuses with malformations. The percentage of fetuses with anencephaly, encephalocele or spina bifida was 24, 18, and 60%*, respectively. Analysis of the sex distribution showed a female preponderance in cranial dysraphisms but the sex distribution of spina bifida cases was equal. The extent and localization of NTDs varied, with lumbosacral cases clearly predominating. The proportion of isolated, associated and syndromic neural tube defects was 56, 23.5 and 20.6% respectively. In the majority of syndromes, the neural tube defect represented a not previously observed syndromic feature.

CONCLUSION

The high proportion of NTDs with monogenic background underlines the importance of a syndrome oriented fetal pathology. At the very least it requires a thourough photographic and radiographic documentation of the fetal phenotype to enable the genetic counsellor to identify a syndromic disorder. This is necessary to determine the risk of recurrence, arrange confirming mutation analyses and offer targeted prenatal diagnosis in subsequent pregnancies.

Spina Bifida: Pathogenesis, Mechanisms, and Genes in Mice and Humans

Spina bifida is among the phenotypes of the larger condition known as neural tube defects (NTDs). It is the most common central nervous system malformation compatible with life and the second leading cause of birth defects after congenital heart defects. In this review paper, we define spina bifida and discuss the phenotypes seen in humans as described by both surgeons and embryologists in order to compare and ultimately contrast it to the leading animal model, the mouse. Our understanding of spina bifida is currently limited to the observations we make in mouse models, which reflect complete or targeted knockouts of genes, which perturb the whole gene(s) without taking into account the issue of haploinsufficiency, which is most prominent in the human spina bifida condition. We thus conclude that the need to study spina bifida in all its forms, both aperta and occulta, is more indicative of the spina bifida in surviving humans and that the measure of deterioration arising from caudal neural tube defects, more commonly known as spina bifida, must be determined by the level of the lesion both in mouse and in man.

Identification and characterization of secondary neural tube-derived embryonic neural stem cells in vitro

Secondary neurulation is an embryonic progress that gives rise to the secondary neural tube, the precursor of the lower spinal cord region. The secondary neural tube is derived from aggregated Sox2-expressing neural cells at the dorsal region of the tail bud, which eventually forms rosette or tube-like structures to give rise to neural tissues in the tail bud. We addressed whether the embryonic tail contains neural stem cells (NSCs), namely secondary NSCs (sNSCs), with the potential for self-renewal in vitro. Using in vitro neurosphere assays, neurospheres readily formed at the rosette and neural-tube levels, but less frequently at the tail bud tip level. Furthermore, we identified that sNSC-generated neurospheres were significantly smaller in size compared with cortical neurospheres. Interestingly, various cell cycle analyses revealed that this difference was not due to a reduction in the proliferation rate of NSCs, but rather the neuronal commitment of sNSCs, as sNSC-derived neurospheres contain more committed neuronal progenitor cells, even in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). These results suggest that the higher tendency for sNSCs to spontaneously differentiate into progenitor cells may explain the limited expansion of the secondary neural tube during embryonic development.

Preventing misdiagnosis in amniotic band sequence: a case report

Amniotic band sequence (ABS) is an uncommon and heterogeneous congenital disorder caused by entrapment of fetal parts by fibrous amniotic bands, causing distinctive structural abnormalities involving limbs, trunk, and craniofacial regions. The incidence ranges between 1/1200 and 1/15,000 live births, but is higher in stillbirths and previable fetuses. The intrinsic theory attributes the constriction band syndrome as an inherent development defect of embryogenesis while the extrinsic theory proposes that an early amnion rupture is responsible for the adherent bands. It is also suggested that amputations and constriction rings might be due to vascular disturbances. Anomalies resulting from amniotic bands are quite variable and sometimes may simulate chromosomal abnormalities. The authors report a case of a 36-week-gestation male neonate who lived for 29 hours after a vaginal delivery with an Apgar score of 8/9/9. The mother was primipara, and the prenatal was uneventful except for two episodes of urinary tract infections. The newborn examination depicted multiple anomalies characterized by exencephaly, bilateral labial cleft with distorted nostrils and palate cleft. There was also facial skin tag band, exophthalmos with hypoplasia of the eyelids. The limbs showed distal amputation of the fingers in both hands and feet, oligodactyly associated with syndactyly in the left foot, ring constriction in the right leg, the presence of right hyperextension, and clubfoot. The upper limbs showed length discrepancies. Karyotype analysis was normal at 46 XY. The authors conclude that the recognition of the malformations secondary to ABS is important in genetic counseling to prevent misdiagnosis between chromosomal and secondary disruption disorders.

Do assisted conception twins have an increased risk for anencephaly?

BACKGROUND

The incidence rates of anterior neural tube defects, anencephaly and encephalocele appear increased among twins compared with singletons. The current study aimed to evaluate whether the etiology of this phenomenon is related to twinning, assisted reproductive technology (ART), or both.

METHODS

The study cohort consisted of parturient women who were referred to our ultrasonography unit between January 1998 and December 2009 due to suspicion of severe fetal abnormality. The study cohort was divided into two subgroups based on mode of conception: spontaneous and ART (including IVF and ICSI). The subgroups were further subdivided into singleton and multiple pregnancies. We also compared pregnancies diagnosed with anencephaly in the study group to all live births in the Department of Obstetrics and Gynecology.

RESULTS

Anencephaly was diagnosed in 43 fetuses out of 1154 (3.7%) pregnancies diagnosed with severe fetal anomaly. Anencephaly was diagnosed in 9 out of 78 twin pregnancies (11.5%); of these, 8 of 45 (17.8%) were ART conceived and 1 of 33 (3%) spontaneously conceived. A significant correlation was found between twinning and anencephaly, with an odds ratio (OR) of 3.4 [confidence interval (CI) = 1.3-8.9, P= 0.011], while no significant correlation was found between ART and anencephaly. A significant correlation was found between anencephaly and the combination of ART conception and twinning (OR of 6.6, CI = 2.8-15.3, P< 0.01). Analyzing the distribution of pregnancies diagnosed with anencephaly in the study group compared with the total number of live births in the department revealed a significant correlation between twinning and anencephaly, with an OR of 11.4 (CI = 4.9-26.5, P< 0.01), with no significant correlation between ART and anencephaly. Among all live births, a significant correlation was found between anencephaly and the combination of ART conception and twinning (OR of 24.6, CI = 11.4-53.2, P< 0.01).

CONCLUSIONS

Our data suggest that twin pregnancies conceived by ART constitute a high-risk group for anencephaly, due to a possible synergistic effect of twinning and ART.

Spinal dysraphism in an early missed abortion: embryofetoscopic diagnosis

Embryofetoscopy is an endoscopic technique that enables direct visualization and morphologic study of embryos during the first trimester and allows a selective tissue sample to be obtained. We describe an early missed abortion in the first trimester, apparently without cause according to findings at ultrasound. Embryofetoscopy revealed a remarkable dorsal cystic formation covered by a blood clot in the thoracolumbar region of the spine, suggestive of meningocele or myelomeningocele. A selective tissue biopsy specimen of the cystic lesion demonstrated meningeal tissue such as a meningocele. Embryofetoscopy in missed abortion could provide information about the causes of severe defects in embryo development so that parents can be effectively advised insofar as future pregnancies.

Fetal spina bifida repair--current trends and prospects of intrauterine neurosurgery

Myelomeningocele is a common dysraphic defect leading to severe impairment throughout the patient's lifetime. Although surgical closure of this anomaly is usually performed in the early postnatal period, an estimated 330 cases of intrauterine repair have been performed in a few specialized centers worldwide. It was hoped prenatal intervention would improve the prognosis of affected patients, and preliminary findings suggest a reduced incidence of shunt-dependent hydrocephalus, as well as an improvement in hindbrain herniation. However, the expectations for improved neurological outcome have not been fulfilled and not all patients benefit from fetal surgery in the same way. Therefore, a multicenter randomized controlled trial was initiated in the USA to compare intrauterine with conventional postnatal care, in order to establish the procedure-related benefits and risks. The primary study endpoints include the need for shunt at 1 year of age, and fetal and infant mortality. No data from the trial will be published before the final analysis has been completed in 2008, and until then, the number of centers offering intrauterine MMC repair in the USA is limited to 3 in order to prevent the uncontrolled proliferation of new centers offering this procedure. In future, refined, risk-reduced surgical techniques and new treatment options for preterm labor and preterm rupture of the membranes are likely to reduce associated maternal and fetal risks and improve outcome, but further research will be needed.

Chromosomal abnormalities associated with neural tube defects (I): full aneuploidy

Fetuses with neural tube defects (NTDs) carry a risk of chromosomal abnormalities. The risk varies with maternal age, gestational age at diagnosis, association with other structural abnormalities, and family history of chromosome aberrations. This article provides an overview of chromosomal abnormalities associated with NTDs in embryos, fetuses, and newborn patients, and a comprehensive review of numerical chromosomal abnormalities associated with NTDs, such as trisomy 18, trisomy 13, triploidy, trisomy 9, trisomy 2, trisomy 21, trisomy 7, trisomy 8, trisomy 14, trisomy 15, trisomy 16, trisomy 5 mosaicism, trisomy 11 mosaicism, trisomy 20 mosaicism, monosomy X, and tetraploidy. NTDs may be associated with aneuploidy. Perinatal identification of NTDs should alert one to the possibility of chromosomal abnormalities and prompt a thorough cytogenetic investigation and genetic counseling.

First trimester diagnosis of iniencephaly associated with fetal malformations and trisomy 18: report of a new case and gene analysis on folate metabolism in parents

Iniencephaly is a rare congenital malformation consisting of a complex alteration of the embryonic development occurring around the third post-fertilization week and characterized by a hyper-retroflexion of the cephalic pole. We report a case of iniencephaly associated with acrania-encephalocele, spina bifida and abnormal ductus venosus in a fetus with trisomy 18 diagnosed at 12 week's gestation in a 41-year-old woman. A co-occurrence between aneuploidy and iniencephaly was documented and polymorphisms on folate metabolism-related genes were investigated in the parents to assess possible etiologic factors and recurrence risk for neural tube defects (NTD). An homozygous state for the MTRR polymorphism was diagnosed in the mother, identifying a clinical risk for NTD. Once iniencephaly or any other NTD are suspected, genetic analysis, second level ultrasound and fetal karyotype are recommended. Autopsy should also be performed in all cases of early ultrasound-based diagnosis of fetal malformations.

Myelomeningocele: a review of the epidemiology, genetics, risk factors for conception, prenatal diagnosis, and prognosis for affected individuals

Although the use of folic acid before conception decreases the chance that a fetus will have an open neural tube defect, this condition still affects 0.5-1.0/1000 pregnancies in the United States. Results of a recent survey suggest that there are gaps in obstetrician-gynecologists' knowledge of risk factors for conception, strategies for prenatal diagnosis, and prognosis for affected individuals. To address these gaps this paper reviews the epidemiology, genetics, risk factors for conception, prenatal diagnosis, and prognosis for affected individuals, presents current information, and makes suggestions for expanding obstetrician-gynecologists' knowledge of myelomeningocele.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader should be able to state that despite a large amount of professional and public education on the use of folic acid in prevention of open neural tube defects (ONTDs) the incidence still affects 0.5-1.0/1000 pregnancies and recall that a recent survey conducted by the ACOG shows substantial misunderstanding and misinformation on major categories of neural tube birth defects.

Etiology, pathogenesis and prevention of neural tube defects

Spina bifida, anencephaly, and encephalocele are commonly grouped together and termed neural tube defects (NTD). Failure of closure of the neural tube during development results in anencephaly or spina bifida aperta but encephaloceles are possibly post-closure defects. NTD are associated with a number of other central nervous system (CNS) and non-neural malformations. Racial, geographic and seasonal variations seem to affect their incidence. Etiology of NTD is unknown. Most of the non-syndromic NTD are of multifactorial origin. Recent in vitro and in vivo studies have highlighted the molecular mechanisms of neurulation in vertebrates but the morphologic development of human neural tube is poorly understood. A multisite closure theory, extrapolated directly from mouse experiments highlighted the clinical relevance of closure mechanisms to human NTD. Animal models, such as circle tail, curly tail, loop tail, shrm and numerous knockouts provide some insight into the mechanisms of NTD. Also available in the literature are a plethora of chemically induced preclosure and a few post-closure models of NTD, which highlight the fact that CNS malformations are of hetergeneitic nature. No Mendelian pattern of inheritance has been reported. Association with single gene defects, enhanced recurrence risk among siblings, and a higher frequency in twins than in singletons indicate the presence of a strong genetic contribution to the etiology of NTD. Non-availability of families with a significant number of NTD cases makes research into genetic causation of NTD difficult. Case reports and epidemiologic studies have implicated a number of chemicals, widely differing therapeutic drugs, environmental contaminants, pollutants, infectious agents, and solvents. Maternal hyperthermia, use of valproate by epileptic women during pregnancy, deficiency and excess of certain nutrients and chronic maternal diseases (e.g. diabetes mellitus) are reported to cause a manifold increase in the incidence of NTD. A host of suspected teratogens are also available in the literature. The UK and Hungarian studies showed that periconceptional supplementation of women with folate (FA) reduces significantly both the first occurrence and recurrence of NTD in the offspring. This led to mandatory periconceptional FA supplementation in a number of countries. Encouraged by the results of clinical studies, numerous laboratory investigations focused on the genes involved in the FA, vitamin B12 and homocysteine metabolism during neural tube development. As of today no clinical or experimental study has provided unequivocal evidence for a definitive role for any of these genes in the causation of NTD suggesting that a multitude of genes, growth factors and receptors interact in controlling neural tube development by yet unknown mechanisms. Future studies must address issues of gene-gene, gene-nutrient and gene-environment interactions in the pathogenesis of NTD.

Iniencephaly and chromosome mosaicism: a report of two cases

We report here two iniencephaly fetuses with chromosome mosaicism. The first fetus (22 weeks) was male with mosaic trisomy 13, and the second fetus (24 weeks) was female with mosaic monosomy X. The first fetus had anencephaly, facial clefting, left-sided rocker bottom foot, equinovarous deformity, bilateral simian crease of hands and horseshoe kidney in addition to iniencephaly. The second fetus had hydrops fetalis with cystic hygroma in addition to iniencephaly. Because the fetuses were sent in formalin, conventional chromosomal analysis was not possible in spite of strong indication. Interphase fluorescence in situ hybridization (FISH) was carried out for targeted aneuploidy analysis. This is fourth published report of chromosomal abnormality in iniencephaly.

Non-multifactorial neural tube defects

Although most neural tube defects (anencephaly, spina bifida) occur as isolated malformations, a substantial proportion are attributable to chromosome anomalies, known teratogens, or component manifestations of multiple anomaly syndromes. This review describes known chromosome alterations and the candidate genes residing in the altered region, as well as syndromes associated with neural tube defects and causative genes, if known.

Chromosomal abnormalities in fetuses with open neural tube defects: prenatal identification with ultrasound

OBJECTIVES

To determine the prevalence of chromosomal abnormalities in fetuses with open neural tube defects (NTD) undergoing prenatal chromosome analysis. The role of prenatal ultrasound in detecting those with an underlying chromosomal abnormality was also investigated.

METHODS

Over a 6-year period, 144 fetuses with open NTD underwent prenatal chromosome analysis between 12 and 37 weeks of gestation, as part of a prospective, multicenter prenatal diagnosis and counseling program in Chile. This population included 66 fetuses with spina bifida, 46 with acrania/anencephaly, 21 with cephalocele and 11 with iniencephaly. A confident prenatal diagnosis was made in 143 fetuses (99%) and confirmed postnatally in all cases.

RESULTS

An underlying chromosomal abnormality was diagnosed in 10 fetuses (7%), six with spina bifida, three with cephalocele and one with craniorachischisis. The prevalence of chromosomal abnormality varied according to the defect present in the fetus, with a 14% (3/21) prevalence among those with cephalocele, 9% (6/66) among those with spina bifida and 2% (1/57) among those with lethal defects such as acrania, anencephaly or iniencephaly. Karyotype results revealed trisomy 18 in seven cases, trisomy 13 in two and mosaicism for a marker chromosome in one. Prenatal ultrasound before the procedure showed that all chromosomally abnormal fetuses had additional findings. The prevalence of chromosomal abnormality in fetuses with spina bifida and cephalocele was higher when chromosome analysis was performed at or before 24 weeks of gestation in comparison to those performed after 24 weeks (5/31 (16%) vs. 4/56 (7%), respectively). However, this difference did not reach statistical significance, probably due to the small number of cases.

CONCLUSIONS

A significant number of fetuses with open NTD are chromosomally abnormal. Although prenatal chromosome analysis should be considered in all cases, prenatal ultrasound seems effective in identifying those fetuses with an underlying chromosomal abnormality.

Neural tube defects in missed abortions: embryoscopic and cytogenetic findings

Transcervical embryoscopy, a new technique for direct visualization of the first trimester conceptus was carried out in 116 cases of missed abortion. Embryonic neural tube defects and other localized developmental defects, were diagnosed in 10 out of 99 cases in which a complete embryoscopic evaluation was possible. All eight successfully karyotyped cases had a numerical chromosomal abnormality. A ninth unkaryotyped embryo with open spina bifida showed, on embryoscopy, a malformation pattern highly suggestive of triploidy. Our embryoscopic and cytogenetic findings provide valuable information for genetic counseling in future pregnancies.

Abnormal folate metabolism and genetic polymorphism of the folate pathway in a child with Down syndrome and neural tube defect

The association of neural tube defects (NTDs) with Down syndrome (trisomy 21) and altered folate metabolism in both mother and affected offspring provide a unique opportunity for insight into the etiologic role of folate deficiency in these congenital anomalies. We describe here the case of a male child with trisomy 21, cervical meningomyelocele, agenesis of corpus callosum, hydrocephaly, cerebellar herniation into the foramen magnum, and shallow posterior cranial fossa. Molecular analysis of the methylenetetrahydrofolate (MTHFR) gene revealed homozygosity for the mutant 677C-->T polymorphism in both the mother and child. The plasma homocysteine of the mother was highly elevated at 25.0 micromol/L and was associated with a low methionine level of 22.1 micromol/L. Her S-adenosylhomocysteine (SAH) level was three times that of reference normal women, resulting in a markedly reduced ratio of S-adenosylmethionine (SAM) to SAH and significant DNA hypomethylation in lymphocytes. The child had low plasma levels of both homocysteine and methionine and a reduced SAM/SAH ratio that was also associated with lymphocyte DNA hypomethylation. In addition, the child had a five-fold increase in cystathionine level relative to normal children, consistent with over-expression of the cystathionine beta synthase gene present on chromosome 21. We suggest that altered folate status plus homozygous mutation in the MTHFR gene in the mother could promote chromosomal instability and meiotic non-disjunction resulting in trisomy 21. Altered folate status and homozygous TT mutation in the MTHFR gene in both mother and child would be expected to increase the risk of neural tube defects. The presence of both trisomy 21 and postclosure NTD in the same child supports the need for an extended periconceptional period of maternal folate supplementation to achieve greater preventive effects for both NTD and trisomy 21.

Variability in the phenotypic expression of fryns syndrome: A report of two sibships

We report on two sibships with four fetuses of 12, 15, 17, and 20 weeks of gestation, respectively, and 1 preterm baby of 31 weeks of gestation affected by a multiple congenital disorder with manifestation suggestive of Fryns syndrome. In addition to the characteristic malformation pattern in Fryns syndrome, they presented with fetal hydrops, cystic hygroma, and multiple pterygias, allowing prenatal ultrasound diagnosis as early as in the 11th week of gestation. The two affected fetuses of family 1 showed severe craniofacial anomalies with bilateral cleft lip and palate, acral hypoplasia, postaxial oligodactyly, persistent truncus arteriosus, and interrupted aortic arch, asplenia sequence, and complex central nervous system midline malformations. In family 2 with three affected sibs, ear anomalies with atresia of the auditory canals, postaxial hexadactyly, intestinal atresias, callosal defects, and eye colobomas were the most outstanding features. On the basis of the present findings and former reports, the inter- and intrafamiliar phenotypic variability in Fryns syndrome, possible pathogenetic mechanisms, and the value of prenatal diagnosis are discussed. In the pathogenetic discussion, a special emphasis is put on the neural crest cell developmental field.

Expression of the sonic hedgehog gene in human embryos with neural tube defects

BACKGROUND

To estimate the rate of malformations observed during early human development, a series of 38,913 first-trimester abortions were studied. Neural tube defects (NTD) were found in 57 cases.

METHODS

A histological study of serial sections performed in 25 embryos revealed a spectrum of axial structure abnormalities. Expression of the SHH gene was studied by in situ hybridization in one case of CRS and in two cases of SB.

RESULTS

A cervical notochord duplication was always found in craniorachischisis (CRS, n = 8), but not in spina bifida (SB, n = 10) or diplomyelia (split cord malformation, n = 3). In the embryo with CRS, expression of SHH was found in both domains, corresponding to the duplicated part of the notochord, whereas a single signal was observed in the nonduplicated part. This expression was associated at the cervical level of the open neural tube with a broad SHH expression domain and with two or even three domains in its lumbar region, suggesting multiple functional floor plates. Similarly, in two embryos with SB, two domains of SHH expression were found in the ventral neural tube.

CONCLUSIONS

Our findings suggest that notochord splitting in the cervical region might be involved in the pathogenesis of CRS. Interestingly, similar notochord abnormality and altered expression of the shh gene are observed in Lp mice with NTD. This suggests that the Lp gene could be a candidate gene for human CRS. Further studies are needed to establish the primary event responsible for the notochord splitting and for the abnormal expression of the SHH gene in the floor plate in embryos with CRS and SB.

Genetic studies in neural tube defects. NTD Collaborative Group

Neural tube defects (NTD) are one of the most common birth defects and are caused by both environmental and genetic factors. The approach to identifying the genes predisposing to NTD, through linkage analysis and candidate gene analysis, is reviewed along with characteristics of a large, nationally ascertained cohort of families. Results from specific assessments of p53, PAX3 and MTHFR failed to suggest that these genes play a major role in NTD development in these families. Advances in genetic laboratory and statistical techniques have made this a prime opportunity for investigation into the causes of complex disorders, such as NTD. However, traditional approaches may prove to be challenging due to the difficulty of ascertaining samplable multiplex families.