Prenatal Diagnosis and Genetic Counseling in a Case of Spina Bifida in a Family with Waardenburg Syndrome Type I

Mouse Models of Neural Tube Defects

During embryonic development, the central nervous system forms as the neural plate and then rolls into a tube in a complex morphogenetic process known as neurulation. Neural tube defects (NTDs) occur when neurulation fails and are among the most common structural birth defects in humans. The frequency of NTDs varies greatly anywhere from 0.5 to 10 in 1000 live births, depending on the genetic background of the population, as well as a variety of environmental factors. The prognosis varies depending on the size and placement of the lesion and ranges from death to severe or moderate disability, and some NTDs are asymptomatic. This chapter reviews how mouse models have contributed to the elucidation of the genetic, molecular, and cellular basis of neural tube closure, as well as to our understanding of the causes and prevention of this devastating birth defect.

Biology of human melanocyte development, Piebaldism, and Waardenburg syndrome

Melanocyte development is orchestrated by a complex interconnecting regulatory network of genes and synergistic interactions. Piebaldism and Waardenburg syndrome are neurocristopathies that arise from mutations in genes involved in this complex network. Our understanding of melanocyte development, Piebaldism, and Waardenburg syndrome has improved dramatically over the past decade. The diagnosis and classification of Waardenburg syndrome, first proposed in 1992 and based on phenotype, have expanded over the past three decades to include genotype. This review focuses on the current understanding of human melanocyte development and the evaluation and management of Piebaldism and Waardenburg syndrome. Management is often challenging and requires a multidisciplinary approach.

First report of Klein-Waardenburg Syndrome in Iran and a novel pathogenic splice site variant in PAX3 gene

OBJECTIVES

Waardenburg Syndrome (WS) as a congenital auditory-pigmentary syndrome is a clinically and genetically heterogeneous disorder. Based upon clinical manifestations, it can be classified into four types. Loss of function mutations in PAX3 gene cause WS1 and WS3 (Klein-Waardenburg syndrome). While WS2 and WS4 have locus heterogeneity with multiple causative genes. Here we report a novel splice site variant in a pedigree with multiple affected members. Based on diagnostic criteria, three of them are associated with WS3. The remained patients classified as type 1.

METHODS

PCR amplification and Sanger sequencing were performed for all exons and all exon-intron boundaries of PAX3 (NM_181,459) gene of the proband. Then available symptomatic and asymptomatic members were screened for the detected variant. Interpretation and classification of the variant were done based on the current guidelines.

RESULTS

We identified a novel heterozygous splice site variant (c.586+2T > C) in donor site of intron 4 of PAX3 gene in our proband. Moreover, this variant was co-segregated with the disease in other available five affected members. Also, the detected variant was not detected in any of the investigated asymptomatic members. This variant was classified as a pathogenic variant.

CONCLUSIONS

This study shows significant intra-familial clinical heterogeneity and absence of phenotype-genotype correlation in a pedigree with Waardenburg Syndrome. However, severity of phenotypes and additional symptoms in the patients can be related to alternative splicing and different levels of PAX3 gene expression. Detailed evaluation of more cases can shed light on this and case-reports are valuable traffic sign in the road. This article is the first report of Waardenburg syndrome type 3 in Iran.

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.

Hearing loss in Waardenburg syndrome: a systematic review

Waardenburg syndrome (WS) is a rare genetic disorder characterized by hearing loss (HL) and pigment disturbances of hair, skin and iris. Classifications exist based on phenotype and genotype. The auditory phenotype is inconsistently reported among the different Waardenburg types and causal genes, urging the need for an up-to-date literature overview on this particular topic. We performed a systematic review in search for articles describing auditory features in WS patients along with the associated genotype. Prevalences of HL were calculated and correlated with the different types and genes of WS. Seventy-three articles were included, describing 417 individual patients. HL was found in 71.0% and was predominantly bilateral and sensorineural. Prevalence of HL among the different clinical types significantly differed (WS1: 52.3%, WS2: 91.6%, WS3: 57.1%, WS4: 83.5%). Mutations in SOX10 (96.5%), MITF (89.6%) and SNAI2 (100%) are more frequently associated with hearing impairment than other mutations. Of interest, the distinct disease-causing genes are able to better predict the auditory phenotype compared with different clinical types of WS. Consequently, it is important to confirm the clinical diagnosis of WS with molecular analysis in order to optimally inform patients about the risk of HL.

De novo 2q36.1q36.3 interstitial deletion involving the PAX3 and EPHA4 genes in a fetus with spina bifida and cleft palate

BACKGROUND

Interstitial 2q36 deletion is a rare event. Only two previously published cases of 2q36 deletions were characterized using array-CGH. This is the first case diagnosed prenatally.

METHODS

We report on the prenatal diagnosis of a 2q36.1q36.3 interstitial deletion in a fetus with facial dysmorphism, spina bifida, and cleft palate.

RESULTS

Array-CGH analysis revealed a 5.6 Mb interstitial deletion of the long arm of chromosome 2q36.1q36.3, including the PAX3 and EPHA4 genes.

CONCLUSION

The present study reinforces the hypothesis that PAX3 haploinsufficiency may be associated with neural tube defects in humans and suggests that the EPHA4 gene might be implicated during palate development. This report also illustrates the added value of array-CGH to detect cryptic chromosomal imbalances in malformed fetuses and to improve genetic counseling prenatally.

Mouse as a model for multifactorial inheritance of neural tube defects

Neural tube defects (NTDs) such as spina bifida and anencephaly are some of the most common structural birth defects found in humans. These defects occur due to failures of neurulation, a process where the flat neural plate rolls into a tube. In spite of their prevalence, the causes of NTDs are poorly understood. The multifactorial threshold model best describes the pattern of inheritance of NTDs where multiple undefined gene variants interact with environmental factors to cause an NTD. To date, mouse models have implicated a multitude of genes as required for neurulation, providing a mechanistic understanding of the cellular and molecular pathways that control neurulation. However, the majority of these mouse models exhibit NTDs with a Mendelian pattern of inheritance. Still, many examples of multifactorial inheritance have been demonstrated in mouse models of NTDs. These include null and hypomorphic alleles of neurulation genes that interact in a complex fashion with other genetic mutations or environmental factors to cause NTDs. These models have implicated several genes and pathways for testing as candidates for the genetic basis of NTDs in humans, resulting in identification of putative pathogenic mutations in some patients. Mouse models also provide an experimental paradigm to gain a mechanistic understanding of the environmental factors that influence NTD occurrence, such as folic acid and maternal diabetes, and have led to the discovery of additional preventative nutritional supplements such as inositol. This review provides examples of how multifactorial inheritance of NTDs can be modeled in the mouse.

Modeling neural tube defects in the mouse

Neural tube defects (NTDs) are among the most common structural birth defects observed in humans. Mouse models provide an excellent experimental system to study the underlying causes of NTDs. These models not only allow for identification of the genes required for neurulation, they provide tractable systems for uncovering the developmental, pathological and molecular mechanisms underlying NTDs. In addition, mouse models are essential for elucidating the mechanisms of gene-environment and gene-gene interactions that contribute to the multifactorial inheritance of NTDs. In some cases these studies have led to development of approaches to prevent NTDs and provide an understanding of the underlying molecular mechanism of these therapies prevent NTDs.

Chromosomal abnormalities associated with neural tube defects (II): partial 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 a comprehensive review of structural chromosomal abnormalities associated with NTDs, such as del(13q), r(13), dup(2p), del(2q), del(1p), del(1q), dup(1q), del(3p), dup(3p), del(3q), dup(3q), del(4p), dup(4p), del(4q), dup(4q), del(5p), del(6p), dup(6q), del(6q), dup(7p), del(7q), dup(8q), del(9p), del(10q), del(11q), dup(11q), dup(12p), dup(14q), del(14q), del(15q), dup(16q), del(18q), r(18), dup(20p), +i(20p), del(22q), del(Xp), and dup(Xq). 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.

Linkage to chromosome 2q36.1 in autosomal dominant Dandy-Walker malformation with occipital cephalocele and evidence for genetic heterogeneity

We previously reported a Vietnamese-American family with isolated autosomal dominant occipital cephalocele. Upon further neuroimaging studies, we have recharacterized this condition as autosomal dominant Dandy-Walker with occipital cephalocele (ADDWOC). A similar ADDWOC family from Brazil was also recently described. To determine the genetic etiology of ADDWOC, we performed genome-wide linkage analysis on members of the Vietnamese-American and Brazilian pedigrees. Linkage analysis of the Vietnamese-American family identified the ADDWOC causative locus on chromosome 2q36.1 with a multipoint parametric LOD score of 3.3, while haplotype analysis refined the locus to 1.1 Mb. Sequencing of the five known genes in this locus did not identify any protein-altering mutations. However, a terminal deletion of chromosome 2 in a patient with an isolated case of Dandy-Walker malformation also encompassed the 2q36.1 chromosomal region. The Brazilian pedigree did not show linkage to this 2q36.1 region. Taken together, these results demonstrate a locus for ADDWOC on 2q36.1 and also suggest locus heterogeneity for ADDWOC.

Molecular prenatal diagnosis in 2 pregnancies at risk for spondyloepiphyseal dysplasia congenita

BACKGROUND

Spondyloepiphyseal dysplasia congenita (SEDC) is an autosomal dominant skeletal dysplasia characterized by short stature, abnormal epiphyses, and flattened vertebral bodies. Secondary prevention of SEDC can be achieved by prenatal diagnosis. Reports of antenatally-diagnosed SEDC fetuses have been very rare and molecular prenatal diagnosis even rarer. We previously reported a familial G504S mutation in the type II collagen (COL2A1) gene resulting in SEDC. In this study, molecular prenatal diagnosis was performed to 2 couples in this family with pregnancies at risk for SEDC.

METHODS

Amniotic fluid was sampled by amniocentesis under ultrasound guidance at 19+3 and 18+6 weeks' gestation, respectively. Karyotype and molecular genetic analysis were performed on cultured amniotic fluid cells. Maternal cell contamination was excluded by short tandem repeat (STR) analysis. Direct DNA sequencing and DHPLC were conducted to detect the potential mutation in exon 23 of COL2A1 gene. Both women underwent serial sonograms because they insisted that the molecular diagnosis should be confirmed by another method, although they had been informed that mutation analysis is predictive of the disease.

RESULTS

Karyotype of both fetuses was normal and molecular genetic analysis revealed that fetus 1 carried a G504S mutation in exon 23, while fetus 2 was normal. In case 1, femur length of the fetus was markedly below the 5th centile at 23 weeks' gestation, which confirms the accuracy of molecular diagnosis. A medical termination was carried out at 27+5 weeks' gestation and a male fetus with a relatively large head and short limbs was delivered. The fetal radiograph demonstrated a number of features, including generalised platyspondyly, absent ossification of the vertebral bodies in the cervical region and significant shortening of the long bones. The diagnosis of SEDC was thus confirmed clinically. Ultrasound monitoring of fetus 2 showed that its femur length was normal for gestational age at repeated scans, which was consistent with the molecular diagnosis.

CONCLUSIONS

Molecular analysis allows early and accurate prenatal diagnosis for SEDC once mutation is known in a family. However, considering the poor genotype/phenotype correlation in many cases of SEDC, the combination of ultrasound as well as molecular genetic approach might be needed.