Mutation of the LUNATIC FRINGE gene in humans causes spondylocostal dysostosis with a severe vertebral phenotype

Oscillatory lunatic fringe activity is crucial for segmentation of the anterior but not posterior skeleton

The Notch pathway plays multiple roles during vertebrate somitogenesis,functioning in the segmentation clock and during rostral/caudal (R/C) somite patterning. Lunatic fringe (Lfng) encodes a glycosyltransferase that modulates Notch signaling, and its expression patterns suggest roles in both of these processes. To dissect the roles played by Lfng during somitogenesis, a novel allele was established that lacks cyclic Lfngexpression within the segmentation clock, but that maintains expression during R/C somite patterning (LfngΔFCE1). In the absence of oscillatory Lfng expression, Notch activation is ubiquitous in the PSM of LfngΔFCE1 embryos. LfngΔFCE1 mice exhibit severe segmentation phenotypes in the thoracic and lumbar skeleton. However, the sacral and tail vertebrae are only minimally affected in LfngΔFCE1mice, suggesting that oscillatory Lfng expression and cyclic Notch activation are important in the segmentation of the thoracic and lumbar axial skeleton (primary body formation), but are largely dispensable for the development of sacral and tail vertebrae (secondary body formation). Furthermore, we find that the loss of cyclic Lfng has distinct effects on the expression of other clock genes during these two stages of development. Finally, we find that LfngΔFCE1 embryos undergo relatively normal R/C somite patterning, confirming that Lfngroles in the segmentation clock are distinct from its functions in somite patterning. These results suggest that the segmentation clock may employ varied regulatory mechanisms during distinct stages of anterior/posterior axis development, and uncover previously unappreciated connections between the segmentation clock, and the processes of primary and secondary body formation.

Diaphanospondylodysostosis: six new cases and exclusion of the candidate genes, PAX1 and MEOX1

We report on six cases from four families with the newly described skeletal disorder diaphanospondylodysostosis (DSD). The characteristic radiographic findings included abnormal ossification of vertebral bodies, posterior rib gaps, missing ribs, and a downward tilt of the pubic rami, but normal long bones. The typical facial features of DSD cases were ocular hypertelorism, a short nose, depressed nasal bridge, and low set ears. Other distinctive findings included a short neck with bell-shaped thorax, and nephroblastomatosis. A history of consanguinity and affected siblings with unaffected parents supports autosomal recessive inheritance. Skeletal histology showed incomplete ossification of the ribs, vertebral bodies, and sacrum as well as incomplete formation of intervertebral discs. The posterior ribs were comprised of bone with intervening cartilage interrupted by dense fibrous tissue and skeletal muscle fascicles. These findings suggest abnormal development and differentiation of the paraxial mesoderm. Because of phenotypic similarities of DSD to Pax1 and Meox1 deficient mice, we sequenced genomic DNA from three unrelated DSD cases. No mutations were identified in the PAX1 and MEOX1 exons or flanking intronic sequences, excluding them as likely causative genes.

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.

Congenital Disorders of Glycosylation: A Rapidly Expanding Disease Family

Congenital disorders of glycosylation (CDG) are a large family of genetic diseases resulting from defects in the synthesis of glycans and in the attachment of glycans to other compounds. These disorders cause a wide range of human diseases, with examples emanating from all medical subspecialties. Since our 2001 review on CDG ( 36 ), this field has seen substantial growth: The number of N-glycosylation defects has doubled (from 6 to 12), five new O-glycosylation defects have been added to the two previously known ones, three combined N- and O-glycosylation defects have been identified, the first lipid glycosylation defects have been discovered, and a new domain, that of the hyperglycosylation defects, has been introduced. A number of CDG are due to defects in enzymes with a putative glycosyltransferase function. There is also a growing group of patients with unidentified defects (CDG-x), some with typical clinical presentations and others with presentations not seen before in CDG. This review focuses on the clinical, biochemical, and genetic characteristics of CDG and on advances expected in their future study and clinical management.

Abnormal vertebral segmentation and the notch signaling pathway in man

Abnormal vertebral segmentation (AVS) in man is a relatively common congenital malformation but cannot be subjected to the scientific analysis that is applied in animal models. Nevertheless, some spectacular advances in the cell biology and molecular genetics of somitogenesis in animal models have proved to be directly relevant to human disease. Some advances in our understanding have come through DNA linkage analysis in families demonstrating a clustering of AVS cases, as well as adopting a candidate gene approach. Only rarely do AVS phenotypes follow clear Mendelian inheritance, but three genes-DLL3, MESP2, and LNFG-have now been identified for spondylocostal dysostosis (SCD). SCD is characterized by extensive hemivertebrae, trunkal shortening, and abnormally aligned ribs with points of fusion. In familial cases clearly following a Mendelian pattern, autosomal recessive inheritance is more common than autosomal dominant and the genes identified are functional within 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. Here, we deal mainly with SCD and AGS, and present a new classification system for AVS phenotypes, for which, hitherto, the terminology has been inconsistent and confusing.

Molecular clocks underlying vertebrate embryo segmentation: A 10-year-old hairy-go-round

Segmentation of the vertebrate embryo body is a fundamental developmental process that occurs with strict temporal precision. Temporal control of this process is achieved through molecular segmentation clocks, evidenced by oscillations of gene expression in the unsegmented presomitic mesoderm (PSM, precursor tissue of the axial skeleton) and in the distal limb mesenchyme (limb chondrogenic precursor cells). The first segmentation clock gene, hairy1, was identified in the chick embryo PSM in 1997. Ten years later, chick hairy2 expression unveils a molecular clock operating during limb development. This review revisits vertebrate embryo segmentation with special emphasis on the current knowledge on somitogenesis and limb molecular clocks. A compilation of human congenital disorders that may arise from deregulated embryo clock mechanisms is presented here, in an attempt to reconcile different sources of information regarding vertebrate embryo development. Challenging open questions concerning the somitogenesis clock are presented and discussed, such as When?, Where?, How?, and What for? Hopefully the next decade will be equally rich in answers.

The vertebrate segmentation clock and its role in skeletal birth defects

The segmental structure of the vertebrate body plan is most evident in the axial skeleton. The regulated generation of somites, a process called somitogenesis, underlies the vertebrate body plan and is crucial for proper skeletal development. A genetic clock regulates this process, controlling the timing of somite development. Molecular evidence for the existence of the segmentation clock was first described in the expression of Notch signaling pathway members, several of which are expressed in a cyclic fashion in the presomitic mesoderm (PSM). The Wnt and fibroblast growth factor (FGF) pathways have also recently been linked to the segmentation clock, suggesting that a complex, interconnected network of three signaling pathways regulates the timing of somitogenesis. Mutations in genes that have been linked to the clock frequently cause abnormal segmentation in model organisms. Additionally, at least two human disorders, spondylocostal dysostosis (SCDO) and Alagille syndrome (AGS), are caused by mutations in Notch pathway genes and exhibit vertebral column defects, suggesting that mutations that disrupt segmentation clock function in humans can cause congenital skeletal defects. Thus, it is clear that the correct, cyclic function of the Notch pathway within the vertebrate segmentation clock is essential for proper somitogenesis. In the future, with a large number of additional cyclic genes recently identified, the complex interactions between the various signaling pathways making up the segmentation clock will be elucidated and refined.

A case of multiple vertebral segmentation defects, unilateral renal agenesis, and an unusual 'Cooley-like' hand appearance

We report an apparently healthy 5-year-old girl with multiple vertebral segmentation defects, partial fusion of some left ribs, abnormal vertebral arches, left renal agenesis, and a 'Cooley-like' hand appearance radiologically. The costovertebral defects were extensive but not contiguous, which establishes this case as being different from the Mendelian forms of spondylocostal dysostosis. The extended skeletal involvement raises the question as to how this case is classified within this heterogeneous group of disorders and we believe this might represent a new and distinct entity.

The genetics and embryology of zebrafish metamerism

Somites are the most obvious metameric structures in the vertebrate embryo. They are mesodermal segments that form in bilateral pairs flanking the notochord and are created sequentially in an anterior to posterior sequence concomitant with the posterior growth of the trunk and tail. Zebrafish somitogenesis is regulated by a clock that causes cells in the presomitic mesoderm (PSM) to undergo cyclical activation and repression of several notch pathway genes. Coordinated oscillation among neighboring cells manifests as stripes of gene expression that pass through the cells of the PSM in a posterior to anterior direction. As axial growth continually adds new cells to the posterior tail bud, cells of the PSM become relatively less posterior. This gradual assumption of a more anterior position occurs over developmental time and constitutes part of a maturation process that governs morphological segmentation in conjunction with the clock. Segment morphogenesis involves a mesenchymal to epithelial transition as prospective border cells at the anterior end of the mesenchymal PSM adopt a polarized, columnar morphology and surround a mesenchymal core of cells. The segmental pattern influences the development of the somite derivatives such as the myotome, and the myotome reciprocates to affect the formation of segment boundaries. While somites appear to be serially homologous, there may be variation in the segmentation mechanism along the body axis. Moreover, whereas the genetic architecture of the zebrafish, mouse, and chick segmentation clocks shares many common elements, there is evidence that the gene networks have undergone independent modification during evolution.

Ripply2is essential for precise somite formation during mouse early development

The regions of expression of Ripply1 and Ripply2, presumptive transcriptional corepressors, overlap at the presomitic mesoderm during somitogenesis in mouse and zebrafish. Ripply1 is required for somite segmentation in zebrafish, but the developmental role of Ripply2 remains unclear in both species. Here, we generated Ripply2 knock-out mice to investigate the role of Ripply2. Defects in segmentation of the axial skeleton were observed in the homozygous mutant mice. Moreover, somite segmentation and expression of Notch2 and Uncx4.1 were disrupted. These findings indicate that Ripply2 is involved in somite segmentation and establishment of rostrocaudal polarity.

Identification of oscillatory genes in somitogenesis from functional genomic analysis of a human mesenchymal stem cell model

During somitogenesis, oscillatory expression of genes in the notch and wnt signaling pathways plays a key role in regulating segmentation. These oscillations in expression levels are elements of a species-specific developmental mechanism. To date, the periodicity and components of the human clock remain unstudied. Here we show that a human mesenchymal stem/stromal cell (MSC) model can be induced to display oscillatory gene expression. We observed that the known cycling gene HES1 oscillated with a 5 h period consistent with available data on the rate of somitogenesis in humans. We also observed cycling of Hes1 expression in mouse C2C12 myoblasts with a period of 2 h, consistent with previous in vitro and embryonic studies. Furthermore, we used microarray and quantitative PCR (Q-PCR) analysis to identify additional genes that display oscillatory expression both in vitro and in mouse embryos. We confirmed oscillatory expression of the notch pathway gene Maml3 and the wnt pathway gene Nkd2 by whole mount in situ hybridization analysis and Q-PCR. Expression patterns of these genes were disrupted in Wnt3a(tm1Amc) mutants but not in Dll3(pu) mutants. Our results demonstrate that human and mouse in vitro models can recapitulate oscillatory expression observed in embryo and that a number of genes in multiple developmental pathways display dynamic expression in vitro.

DLL3 as a candidate gene for vertebral malformations

Investigations have not identified a major locus for congenital vertebral malformations. Based on observations in mice, we hypothesized that mutations in DLL3, a member of the notch-signaling pathway, might contribute to human vertebral malformations. We sequenced the DLL3 gene in 50 patients with congenital vertebral malformations. A Caucasian male patient with VACTERL manifestations including a T5-T6 block vertebrae was heterozygous for a "G" to "A" missense mutation changing glycine to arginine at codon 269. This residue is conserved in mammals, including chimpanzee, mouse, dog, and rat. Additional testing in the patient did not show evidence of chromosome abnormalities. The patient's asymptomatic mother was also heterozygous for the missense mutation. Since this mutation was not observed in a control population and leads to an amino acid change, it may be clinically significant. The mutation was not found in a control population of 87 anonymous individuals. Several established mechanisms could explain the mutation in both the patient and his asymptomatic mother (susceptibility allele requiring additional environmental factors, somatic mosaicism, multigenic inheritance). Documenting the absence of the mutation in a larger control population or the presence of the mutation in additional affected patients, or documenting a functional difference in DLL3 would provide further evidence supporting its causal role.

Molecular genetics of congenital diaphragmatic defects

Congenital diaphragmatic hernia (CDH) is a severe birth defect that is accompanied by malformations of the lung, heart, testis, and other organs. Patients with CDH may have any combination of these extradiaphragmatic defects, suggesting that CDH is often a manifestation of a global embryopathy. This review highlights recent advances in human and mouse genetics that have led to the identification of genes involved in CDH. These include genes for transcription factors, molecules involved in cell migration, and extracellular matrix components. The expression patterns of these genes in the developing embryo suggest that mesenchymal cell function is compromised in the diaphragm and other affected organs in patients with CDH. We discuss potential mechanisms underlying the seemingly random combination of diaphragmatic, pulmonary, cardiovascular, and gonadal defects in these patients.

Genetic analysis of molecular oscillators in mammalian somitogenesis: Clues for studies of human vertebral disorders

The repeating pattern of the human vertebral column is shaped early in development, by a process called somitogenesis. In this embryonic process, pairs of mesodermal segments called somites are serially laid down along the developing neural tube. Somitogenesis is an iterative process, repeating at regular time intervals until the last somite is formed. This process lays down the vertebrate body axis from head to tail, making for a progression of developmental steps along the rostral-caudal axis. In this review, the roles of the Notch, Wnt, fibroblast growth factor, retinoic acid and other pathways are described during the following key steps in somitogenesis: formation of the presomitic mesoderm (PSM) and establishment of molecular gradients; prepatterning of the PSM by molecular oscillators; patterning of rostral-caudal polarity within the somite; formation of somite borders; and maturation and resegmentation of somites to form musculoskeletal tissues. Disruption of somitogenesis can lead to severe vertebral birth defects such as spondylocostal dysostosis (SCD). Genetic studies in the mouse have been instrumental in finding mutations in this disorder, and ongoing mouse studies should provide functional insights and additional candidate genes to help in efforts to identify genes causing human spinal birth defects.

Dll3 andNotch1 genetic interactions model axial segmental and craniofacial malformations of human birth defects

Mutations in the Notch1 receptor and delta-like 3 (Dll3) ligand cause global disruptions in axial segmental patterning. Genetic interactions between members of the notch pathway have previously been shown to cause patterning defects not observed in single gene disruptions. We examined Dll3-Notch1 compound mouse mutants to screen for potential gene interactions. While mice heterozygous at either locus appeared normal, 30% of Dll3-Notch1 double heterozygous animals exhibited localized, segmental anomalies similar to human congenital vertebral defects. Unexpectedly, double heterozygous mice also displayed statistically significant reduction of mandibular height and decreased length of the [corrected] maxillary hard palate. Examination of somite-stage embryos and perinatal anatomy and histology did not reveal any organ defects, so we used microarray-based analysis of Dll3 and Notch1 mutant embryos to identify gene targets that may be involved in notch-regulated segmental or craniofacial development. Thus, Dll3-Notch1 double heterozygous mice model human congenital scoliosis and craniofacial disorders.

Building the spine: the vertebrate segmentation clock

One of the most striking characteristics of many animal and plant species is their organization in a series of periodically repeated anatomical modules. In animals, this particular patterning strategy of the body axis is termed segmentation, and it is observed in both vertebrates and invertebrates. Vertebrate segmentation has been associated with a molecular oscillator-the segmentation clock-whose existence had been predicted on theoretical grounds in the clock and wave-front model. The segmentation clock is proposed to generate pulses of signaling used for the positioning of segmental boundaries. Whereas several models have proposed that simple negative autoregulatory circuits involving the transcription repressors of the hairy and enhancer of split family constitute the clock pacemaker, recent microarray studies in mouse have identified a large network of oscillating signaling genes belonging to the Notch, Wnt, and FGF (fibroblast growth factor) pathways. Thus, significant progress has been made, but the molecular nature of the clockwork underlying the oscillator remains poorly understood. Few examples of oscillators exist in developmental biology, and the segmentation clock provides a unique model of periodic regulation in patterning.

Disruption of the somitic molecular clock causes abnormal vertebral segmentation

Somites are the precursors of the vertebral column. They segment from the presomitic mesoderm (PSM) that is caudally located and newly generated from the tailbud. Somites form in synchrony on either side of the embryonic midline in a reiterative manner. A molecular clock that operates in the PSM drives this reiterative process. Genetic manipulation in mouse, chick and zebrafish has revealed that the molecular clock controls the activity of the Notch and WNT signaling pathways in the PSM. Disruption of the molecular clock impacts on somite formation causing abnormal vertebral segmentation (AVS). A number of dysmorphic syndromes manifest AVS defects. Interaction between developmental biologists and clinicians has lead to groundbreaking research in this area with the identification that spondylocostal dysostosis (SCD) is caused by mutation in Delta-like 3 (DLL3), Mesoderm posterior 2 (MESP2), and Lunatic fringe (LFNG); three genes that are components of the Notch signaling pathway. This review describes our current understanding of the somitic molecular clock and highlights how key findings in developmental biology can impact on clinical practice.

Molecular refinement of gibbon genome rearrangements

The gibbon karyotype is known to be extensively rearranged when compared to the human and to the ancestral primate karyotype. By combining a bioinformatics (paired-end sequence analysis) approach and a molecular cytogenetics approach, we have refined the synteny block arrangement of the white-cheeked gibbon (Nomascus leucogenys, NLE) with respect to the human genome. We provide the first detailed clone framework map of the gibbon genome and refine the location of 86 evolutionary breakpoints to <1 Mb resolution. An additional 12 breakpoints, mapping primarily to centromeric and telomeric regions, were mapped to approximately 5 Mb resolution. Our combined FISH and BES analysis indicates that we have effectively subcloned 49 of these breakpoints within NLE gibbon BAC clones, mapped to a median resolution of 79.7 kb. Interestingly, many of the intervals associated with translocations were gene-rich, including some genes associated with normal skeletal development. Comparisons of NLE breakpoints with those of other gibbon species reveal variability in the position, suggesting that chromosomal rearrangement has been a longstanding property of this particular ape lineage. Our data emphasize the synergistic effect of combining computational genomics and cytogenetics and provide a framework for ultimate sequence and assembly of the gibbon genome.

Glycosylation in cellular mechanisms of health and disease

Glycosylation produces an abundant, diverse, and highly regulated repertoire of cellular glycans that are frequently attached to proteins and lipids. The past decade of research on glycan function has revealed that the enzymes responsible for glycosylation-the glycosyltransferases and glycosidases-are essential in the development and physiology of living organisms. Glycans participate in many key biological processes including cell adhesion, molecular trafficking and clearance, receptor activation, signal transduction, and endocytosis. This review discusses the increasingly sophisticated molecular mechanisms being discovered by which mammalian glycosylation governs physiology and contributes to disease.

The long and short of it: somite formation in mice

A fundamental characteristic of the vertebrate body plan is its segmentation along the anterior-posterior axis. This segmental pattern is established during embryogenesis by the formation of somites, the transient epithelial blocks of cells that derive from the unsegmented presomitic mesoderm. Somite formation involves a molecular oscillator, termed the segmentation clock, in combination with gradients of signaling molecules such as fibroblast growth factor 8, WNT3A, and retinoic acid. Disruption of somitogenesis in humans can result in disorders such as spondylocostal dysostosis, which is characterized by vertebral malformations. This review summarizes recent findings concerning the role of Notch signaling in the segmentation clock, the complex regulatory network that governs somitogenesis, the genes that cause inherited spondylocostal dysostosis, and the mechanisms that regulate bilaterally symmetric somite formation.

Notch signalling in vertebrate neural development

Signals through the Notch receptors are used throughout development to control cellular fate choices. Loss- and gain-of-function studies revealed both the pleiotropic action of the Notch signalling pathway in development and the potential of Notch signals as tools to influence the developmental path of undifferentiated cells. As we review here, Notch signalling affects the development of the nervous system at many different levels. Understanding the complex genetic circuitry that allows Notch signals to affect specific cell fates in a context-specific manner defines the next challenge, especially as such an understanding might have important implications for regenerative medicine.

Roles of O-fucose glycans in notch signaling revealed by mutant mice

Notch receptor signaling is important for many developmental processes in the metazoa. Insights into how Notch receptor signaling is regulated have been obtained from the characterization of mutants of model organisms in which Notch signaling is perturbed. Here we describe the effects of mutations that alter the glycosylation of Notch receptors and Notch ligands in the mouse. The extracellular domain of Notch receptors and Notch ligands carries N-glycans and O-glycans, including O-fucose and O-glucose glycans. Mutations in several genes that inhibit the synthesis of O-fucose glycans, and one that also affects the maturation of N-glycans, cause Notch signaling defects and disrupt development.