Defective somitogenesis and abnormal vertebral segmentation in man

In recent years molecular genetics has revolutionized the study of somitogenesis in developmental biology and advances that have taken place in animal models have been applied successfully to human disease. Abnormal segmentation in man is a relatively common birth defect and advances in understanding have come through the study of cases clustered in families using DNA linkage analysis and candidate gene approaches, the latter stemming directly from knowledge gained through the study of animal models. Only a minority of abnormal segmentation phenotypes appear to follow Mendelian inheritance but three genes--DLL3, MESP2 and LNFG--have now been identified for spondylocostal dysostosis (SCD), a spinal malformation characterized by extensive hemivertebrae, trunkal shortening and abnormally aligned ribs with points of fusion. In affected families autosomal recessive inheritance is followed. These genes are all important components of the Notch signaling pathway. Other genes within the pathway cause diverse phenotypes such as Alagille syndrome (AGS) and CADASIL, conditions that may have their origin in defective vasculogenesis. This review deals mainly with SCD, with some consideration of AGS. Significant future challenges lie in identifying causes of the many abnormal segmentation phenotypes in man but it is hoped that combined approaches in collaboration with developmental biologists will reap rewards.

Expression of mutant JAGGED1 alleles in patients with Alagille syndrome

Heterozygous mutations in JAGGED1 (JAG1), encoding a ligand for Notch receptors, have been identified in patients with Alagille syndrome (AGS). These mutations map to the extracellular and transmembrane domains of JAG1, giving rise in 70% cases to a premature termination codon (PTC). Although haploinsufficiency has been hypothesised as the main mechanism of AGS, a dominant negative effect of truncated forms of Serrate/Jagged has been suggested. Only few studies of the mutant mRNAs and proteins from AGS patients have been performed to elucidate the molecular mechanisms of the disease. To gain insight into the stability of mutant mRNAs, we studied transcripts from five livers and 24 lymphoblastoid cell lines (LCLs) of AGS patients. Mutant JAG1 transcripts were recovered (albeit in different relative amounts) from RNAs with missense mutations (five) or in-frame deletions (two), and from all but two of the 21 with PTCs. In addition, results from LCL RNAs correlated well with results from liver RNAs. Mutant transcripts were also recovered from tissues of a 23-week-old AGS foetus with a PTC mutation. This suggests that most mutant transcripts with PTCs escape nonsense-mediated mRNA decay (NMD) and could lead to the synthesis of soluble forms of JAG1. Production of a truncated protein was indeed observed after transfection of COS cells with a mutant JAG1 cDNA. In conclusion, mutant JAG1 transcripts are present in LCLs, livers and tissues of AGS patients, whatever the mutation type, and mutant proteins can be produced, suggesting a dominant negative effect of some mutant proteins as another molecular mechanism of AGS.

Inhibition of Jagged-mediated Notch signaling disrupts zebrafish biliary development and generates multi-organ defects compatible with an Alagille syndrome phenocopy

The Alagille Syndrome (AGS) is a heritable disorder affecting the liver and other organs. Causative dominant mutations in human Jagged 1 have been identified in most AGS patients. Related organ defects occur in mice that carry jagged 1 and notch 2 mutations. Multiple jagged and notch genes are expressed in the developing zebrafish liver. Compound jagged and notch gene knockdowns alter zebrafish biliary, kidney, pancreatic, cardiac and craniofacial development in a manner compatible with an AGS phenocopy. These data confirm an evolutionarily conserved role for Notch signaling in vertebrate liver development, and support the zebrafish as a model system for diseases of the human biliary system.

Molecular determinants of left and right outflow tract obstruction

Congenital heart defects represent the most common group of human birth defects; they occur in 0.8-1% of live births and in 10% of spontaneously aborted fetuses. Heart defects seen in newborns typically represent specific morphogenetic defects of individual chambers or regions of the heart, with the remaining portions of the heart developing relatively normally. These developmental defects are commonly compatible with the intrauterine circulation, where the pulmonary circulation and systemic circulation work in concert, resulting in adequate embryonic growth and development. After delivery, however, significant cardiac symptoms develop. In many of these disorders, cyanosis is the earliest feature, while in others, cardiovascular collapse occurs before diagnosis. In this review, obstruction of the left and right sides of the heart are discussed. In these disorders, ventricular hypoplasia resulting in single ventricle physiologic characteristics is typical. The unaffected ventricle in these cases is usually morphologically and physiologically normal. These conditions include hypoplastic left heart syndrome and aortic coarctation on the left side, pulmonary stenosis, tetralogy of Fallot, and other complex right ventricle obstructive disorders. Many of these disorders occur in association with genetic syndromes identifiable by dysmorphic features. In some cases, the gene(s) has been identified or the genetic pathway has been defined. The purpose of this review is to discuss the molecular determinants of these obstructive disorders.

JAGGED1 gene expression during human embryogenesis elucidates the wide phenotypic spectrum of Alagille syndrome

Mutations of the JAGGED1 gene, encoding a NOTCH receptor ligand, cause Alagille syndrome (AGS), a complex malformative disorder affecting mainly the liver, heart, vertebrae, eye, and face. Minor and occasional features involving kidney, pharynx, systemic arteries, skeleton, and ear are in some cases associated with the syndrome. To describe the expression of JAGGED1 during human embryogenesis and to study its relationship with all the features of AGS, we performed in situ hybridization studies on human embryos and fetal tissue sections. JAGGED1 was mainly expressed in the cardiovascular system. In the liver, JAGGED1 transcripts were only detected in blood vessels. JAGGED1 was also expressed in other structures of mesenchymal origin (distal mesenchyme of limb buds; mesonephric and metanephric tubules of the kidney) and in epithelial structures including the ciliary margin of the retina and the posterior part of the lens, the ventral epithelium of the otic vesicle, the neurosensory epithelium of the ear vestibule, the epithelium of pharyngeal arches, and the developing central nervous system. The strong JAGGED1 expression during human embryo- and feto-genesis both in the vascular system and in other mesenchymal and epithelial tissues implicates abnormal angiogenesis in the pathogenesis of Alagille syndrome and particularly the paucity of interlobular bile ducts. However, it is probably not the only mechanism of the disease. Except for the central nervous system, there is a strong correlation between JAGGED1 expression and all the features of AGS. This implies that the features occasionally associated with the syndrome are not coincidental.