Understanding Idiopathic Scoliosis: A New Zebrafish School of Thought

GENETIC ASPECTS OF IDIOPATHIC ESCOLIOSIS - A LITERATURE REVIEW

ABSTRACT The idiopathic scoliosis (IS) is the most common form of spinal deformity. The pathogenesis of IS is still poorly understood. Several studies show evidence that the genetic component is determinant to the development of IS. In this setting, a crescent focus has been placed on the identification of genes, associated genetic polymorphisms, and multiple susceptibility loci. This review highlights the genes and genetic polymorphisms currently studied, identified as influential in the genesis of IS, such as MMP-3, IL-6, type I collagen, and vitamin D and estrogen receptors. We concluded that IS remains a complex disease with a polygenic background and that genetic polymorphisms are intrinsically related to this condition. Level of evidence III; Narrative Review.

Making and breaking symmetry in development, growth and disease

Consistent asymmetries between the left and right sides of animal bodies are common. For example, the internal organs of vertebrates are left-right (L-R) asymmetric in a stereotyped fashion. Other structures, such as the skeleton and muscles, are largely symmetric. This Review considers how symmetries and asymmetries form alongside each other within the embryo, and how they are then maintained during growth. I describe how asymmetric signals are generated in the embryo. Using the limbs and somites as major examples, I then address mechanisms for protecting symmetrically forming tissues from asymmetrically acting signals. These examples reveal that symmetry should not be considered as an inherent background state, but instead must be actively maintained throughout multiple phases of embryonic patterning and organismal growth.

Functional variants of hepatocyte growth factor identified in patients with adolescent idiopathic scoliosis

The genetic etiology of adolescent idiopathic scoliosis (AIS) remains obscure. Whole-genome sequencing was performed in four members of one family. Then, we performed a rigorous computational analysis to determine the deleterious effects of the identified variants. Furthermore, the structural differences between the native hepatocyte growth factor (HGF) protein and a protein encoded by an HGF variant containing one mutation (p.T596M) were analyzed using molecular dynamic stimulation. A novel heterozygous mutation (p.T596M) within the HGF gene was identified and found to cosegregate with scoliosis phenotypes in three affected family members. Subsequent modeling and structure-based analyses supported the theory that this mutation is functionally deleterious. Functional analyses demonstrated that the HGF p.T596 M mutation changed the ability of the HGF protein to be secreted and impaired migration and invasion in HEK293T cells. Furthermore, an HGF knockdown zebrafish model exhibited a curly tailed phenotype. Mutation in HGF is associated with an autosomal dominant pattern of inheritance of AIS. This finding increases our understanding of the genetic heterogeneity of AIS.

Computed tomography method for characterising the zebrafish spine

BACKGROUND

The zebrafish is widely used in research due in part to its readily manipulable genome. Zebrafish models of spinal deformities including scoliosis were developed recently. However, the methods used to assess the spine in these models vary across studies. The primary objective of this study was to investigate the feasibility and modalities of local and regional spine structure evaluation by micro-CT in the normal zebrafish. The secondary objectives were to assess the feasibility of spinal angle measurements in normal zebrafish subjected to external stresses designed to mimic spinal deformities, to determine normal angle values in the coronal and sagittal planes, and to detail the micro-CT features of the zebrafish spine.

HYPOTHESIS

Micro-CT is an effective and reproducible tool for determining orthopaedic parameters to characterise the zebrafish spine.

MATERIAL AND METHODS

Two observers conducted preliminary analyses on 15 zebrafish including 12 adults (aged 18 months) and 3 juveniles (aged 12 weeks). For the analyses, 6 of the animals were placed in an artificial position to mimic a scoliosis spinal deformity. Micro-CT (Quantum FX Caliper™) was used with 59μm resolution and a 30-mm field of view. Image processing was with RadiAnt DICOM Viewer™ software.

RESULTS

We defined several assessment planes on the 3D micro-CT reconstructions to measure orthopaedic parameters in the sagittal plane (thoracic and maximal kyphotic curves with their apices, length of the various spinal segments, and sagittal index) and coronal plane (Cobb angles, apices, end-vertebrae, coronal alignment, and side of the convexity). Mean thoracic kyphosis was 20.5°±5.0° in the adults and 8.7° in the juveniles. No curvature was apparent in the coronal plane in the zebrafish left in the neutral position. In the zebrafish with artificially induced curves, micro-CT was effective in determining the Cobb angles and apical vertebrae.

DISCUSSION

This work defines a standardised micro-CT method for assessing the zebrafish spine. In addition, spinal parameter values that can be considered normal were determined, namely, less than 30° of thoracic kyphosis in the sagittal plane and less than 10° in the coronal plane. Our method was effective in assessing induced spinal deformities on micro-CT reconstructions. We hope it will prove of value in future studies of the zebrafish model.

LEVEL OF EVIDENCE

IV.

Analysis of transgenic zebrafish expressing the Lenz-Majewski syndrome gene PTDSS1 in skeletal cell lineages

Background: Lenz-Majewski syndrome (LMS) is characterized by osteosclerosis and hyperostosis of skull, vertebrae and tubular bones as well as craniofacial, dental, cutaneous, and digit abnormalities. We previously found that LMS is caused by de novo dominant missense mutations in the  PTDSS1 gene, which encodes phosphatidylserine synthase 1 (PSS1), an enzyme that catalyses the conversion of phosphatidylcholine to phosphatidylserine. The mutations causing LMS result in a gain-of-function, leading to increased enzyme activity and blocking end-product inhibition of PSS1. Methods: Here, we have used transpose-mediated transgenesis to attempt to stably express wild-type and mutant forms of human PTDSS1 ubiquitously or specifically in chondrocytes, osteoblasts or osteoclasts in zebrafish. Results: We report multiple genomic integration sites for each of 8 different transgenes. While we confirmed that the ubiquitously driven transgene constructs were functional in terms of driving gene expression following transient transfection in HeLa cells, and that all lines exhibited expression of a heart-specific cistron within the transgene, we failed to detect PTDSS1 gene expression at either the RNA or protein levels in zebrafish. All wild-type and mutant transgenic lines of zebrafish exhibited mild scoliosis with variable incomplete penetrance which was never observed in non-transgenic animals. Conclusions: Collectively the data suggest that the transgenes are silenced, that animals with integrations that escape silencing are not viable, or that other technical factors prevent transgene expression. In conclusion, the incomplete penetrance of the phenotype and the lack of a matched transgenic control model precludes further meaningful investigations of these transgenic lines.

Impaired Annulus Fibrosus Development and Vertebral Fusion Cause Severe Scoliosis in Mice with Deficiency of c-Jun NH2-Terminal Kinases 1 and 2

Mitogen-activated protein kinases, including c-Jun NH2-terminal kinase (JNK), play an important role in the development and function of a large variety of tissues. The skeletal phenotype of JNK1 and JNK2 double-knockout (dKO) mice (JNK1^fl/flCol2-Cre/JNK2^-/-) and control genotypes were analyzed at different embryonic and postnatal stages. JNK1/2 dKO mice displayed a severe scoliotic phenotype beginning during development that was grossly apparent around weaning age. Alcian blue staining at embryonic day 17.5 showed abnormal fusion of the posterior spinal elements. In adult mice, fusion of vertebral bodies and of spinous and transverse processes was noted by micro-computed tomography, Alcian blue/Alizarin red staining, and histology. The long bones developed normally, and histologic sections of growth plate and articular cartilage revealed no significant abnormalities. Histologic sections of the vertebral column at embryonic days 15.5 and 17.5 revealed an abnormal organization of the annulus fibrosus in the dKOs, with chondrocyte-like cells and fusion of dorsal processes. Spinal sections in 10-week-old dKO mice showed replacement of intervertebral disk structures (annulus fibrosus and nucleus pulposus) by cartilage and bone tissues, with cells staining for markers of hypertrophic chondrocytes, including collagen X and runt-related transcription factor 2. These findings demonstrate a requirement for both JNK1 and JNK2 in the normal development of the axial skeleton. Loss of JNK signaling results in abnormal endochondral bone formation and subsequent severe scoliosis.

Neuroinflammatory signals drive spinal curve formation in zebrafish models of idiopathic scoliosis

The etiopathogenesis of idiopathic scoliosis (IS), a highly prevalent spinal deformity that occurs in the absence of obvious congenital or physiological abnormalities, is poorly understood. Although recent zebrafish genetic studies have linked cilia motility and cerebrospinal fluid (CSF) flow defects with scoliosis progression, underlying mechanisms were not identified. Here, we use next-generation sequencing and conditional genetic methodologies to define the spatial and biological origins of spinal curve formation in ptk7 mutant zebrafish, a faithful IS model. We demonstrate that focal activation of proinflammatory signals within the spinal cord is associated with, and sufficient for, induction of spinal curvatures. Furthermore, administration of acetylsalicylic acid (aspirin) or N-acetylcysteine (NAC) to juvenile ptk7 mutants significantly reduces the incidence and/or severity of scoliosis phenotypes. Together, our results implicate neuroinflammation, downstream of CSF defects, in spinal curve formation and provide intriguing evidence that simple immunomodulating therapies might prove effective in managing idiopathic-like spinal deformities.

Skeletal malformations of Meox1-deficient zebrafish resemble human Klippel-Feil syndrome

Klippel-Feil syndrome is a congenital vertebral anomaly, which is characterised by the fusion of at least two cervical vertebrae and a clinically broad set of symptoms, including congenital scoliosis and elevated scapula (Sprengel's deformity). Klippel-Feil syndrome is associated with mutations in MEOX1. The zebrafish mutant choker (cho) carries a mutation in its orthologue, meox1. Although zebrafish is being increasingly employed as fidelitous models of human spinal disease, the vertebral column of Meox1-deficient fish has not been assessed for defects. Here, we describe the skeletal defects of meox1^cho mutant zebrafish utilising alizarin red to stain bones and chemical maceration of soft tissue to detect fusions in an unbiased manner. Obtained data reveal that meox1^cho mutants feature aspects of a number of described symptoms of patients who suffer from Klippel-Feil syndrome and have mutations in MEOX1. These include vertebral fusion, congenital scoliosis and an asymmetry of the pectoral girdle, which resembles Sprengel's deformity. Thus, the meox1^cho mutant zebrafish may serve as a useful tool to study the pathogenesis of the symptoms associated with Klippel-Feil syndrome.

Pkd2l1 is required for mechanoception in cerebrospinal fluid-contacting neurons and maintenance of spine curvature

Defects in cerebrospinal fluid (CSF) flow may contribute to idiopathic scoliosis. However, the mechanisms underlying detection of CSF flow in the central canal of the spinal cord are unknown. Here we demonstrate that CSF flows bidirectionally along the antero-posterior axis in the central canal of zebrafish embryos. In the cfap298^tm304 mutant, reduction of cilia motility slows transport posteriorly down the central canal and abolishes spontaneous activity of CSF-contacting neurons (CSF-cNs). Loss of the sensory Pkd2l1 channel nearly abolishes CSF-cN calcium activity and single channel opening. Recording from isolated CSF-cNs in vitro, we show that CSF-cNs are mechanosensory and require Pkd2l1 to respond to pressure. Additionally, adult pkd2l1 mutant zebrafish develop an exaggerated spine curvature, reminiscent of kyphosis in humans. These results indicate that CSF-cNs are mechanosensory cells whose Pkd2l1-driven spontaneous activity reflects CSF flow in vivo. Furthermore, Pkd2l1 in CSF-cNs contributes to maintenance of natural curvature of the spine.

Alteration of cerebrospinal fluid (CSF) flow and cilia defects are clinically associated with idiopathic scoliosis. This study shows that transient receptor potential channel Pkd2l1 is required for mechanosensory function of neurons detecting CSF flow and normal spine curvature development in zebrafish.

Asymmetric expression of H19 and ADIPOQ in concave/convex paravertebral muscles is associated with severe adolescent idiopathic scoliosis

Background

Adolescent idiopathic scoliosis (AIS) is the most common paediatric spinal deformity. The etiology and pathology of AIS remain unexplained, and have been reported to involve a combination of genetic and epigenetic factors. Since paravertebral muscle imbalance plays an important role in the onset and progression of scoliosis, we aimed to investigate transcriptomic differences by RNA-seq and identify significantly differentially expressed transcripts in two sides of paravertebral muscle in AIS.

Methods

RNA-seq was performed on 5 pairs of paravertebral muscle from 5 AIS patients. Significantly differentially expressed transcripts were validated by quantitative reverse polymerase chain reaction. Gene expression difference was correlated to clinical characteristics.

Results

We demonstrated that ADIPOQ mRNA and H19 is significantly differentially expressed between two sides of paravertebral muscle, relatively specific in the context of AIS. Relatively low H19 and high ADIPOQ mRNA expression levels in concave-sided muscle are associated with larger spinal curve and earlier age at initiation. We identified miR-675-5p encoded by H19 as a mechanistic regulator of ADIPOQ expression in AIS. We demonstrated that significantly reduced CCCTC-binding factor (CCTF) occupancy in the imprinting control region (ICR) of the H19 gene in the concave-sided muscle contributes to down-regulated H19 expression.

Conclusions

RNA-seq revealed transcriptomic differences between two sides of paravertebral muscle in AIS patients. Our findings imply that transcriptomic differences caused by epigenetic factors in affected individuals may account for the structural and functional imbalance of paravertebral muscle, which can expand our etiologic understanding of this disease.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0049-y) contains supplementary material, which is available to authorized users.

The Reissner Fiber in the Cerebrospinal Fluid Controls Morphogenesis of the Body Axis

Organ development depends on the integration of coordinated long-range communication between cells. The cerebrospinal fluid composition and flow properties regulate several aspects of central nervous system development, including progenitor proliferation, neurogenesis, and migration [1-3]. One understudied component of the cerebrospinal fluid, described over a century ago in vertebrates, is the Reissner fiber. This extracellular thread forming early in development results from the assembly of the SCO-spondin protein in the third and fourth brain ventricles and central canal of the spinal cord [4]. Up to now, the function of the Reissner fiber has remained elusive, partly due to the lack of genetic invalidation models [4]. Here, by mutating the scospondin gene, we demonstrate that the Reissner fiber is critical for the morphogenesis of a straight posterior body axis. In zebrafish mutants where the Reissner fiber is lost, ciliogenesis and cerebrospinal fluid flow are intact but body axis morphogenesis is impaired. Our results also explain the frequently observed phenotype that mutant embryos with defective cilia exhibit defects in body axis curvature. Here, we reveal that these mutants systematically fail to assemble the Reissner fiber. We show that cilia promote the formation of the Reissner fiber and that the fiber is necessary for proper body axis morphogenesis. Our study sets the stage for future investigations of the mechanisms linking the Reissner fiber to the control of body axis curvature during vertebrate development.

Association of vitamin D receptor BsmI rs1544410 and ApaI rs7975232 polymorphisms with susceptibility to adolescent idiopathic scoliosis: A systematic review and meta-analysis

BACKGROUND

AIS is the most common spinal deformity disease, yet its etiology remains uncertain. Significant associations have been found between AIS risk and vitamin D receptor (VDR) gene polymorphisms; however, some of these results are controversial. The aim of this study was to determine whether VDR BsmI rs1544410 and ApaI rs7975232 polymorphisms are correlated with AIS.

METHODS

Databases, including PubMed, EMBASE, Web of Science, the Cochrane Library, the Chinese Biomedical Literature Database, and the Wanfang Database, were systematically searched, and eligible case-control studies that explored the association of VDR (BsmI and ApaI) and the susceptibility to AIS were selected. The pooled odds ratio (OR) with 95% confidence interval (95% CI) was calculated to assess the associations, and subgroup meta-analyses were performed according to the ethnicity of the study population.

RESULTS

A total of 5 studies with 717 cases and 554 controls fulfilled the inclusion criteria after assessment by 2 reviewers. Generally, significant correlations were found between the BsmI polymorphism and AIS risk in overall populations and in Asian populations (overall population: B vs b: OR = 2.12, 95% CI = 1.21-3.75, P = .009; BB vs bb: OR = 3.38, 95% CI = 1.08-10.57, P = .036; Bb vs bb: OR = 2.50, 95% CI = 1.29-4.82, P = .006; BB/Bb vs bb: OR = 2.71, 95% CI = 1.31-5.63, P = .007; Asian population: B vs b: OR = 2.42, 95% CI = 1.27-4.61, P = .007; BB vs bb: OR = 4.09, 95% CI = 1.03-16.22, P = .045; Bb vs bb: OR =  2.94, 95% CI = 1.42-6.10, P = .004; BB/Bb vs bb: OR = 3.23, 95% CI = 1.42-7.35, P = .005). There was no significant association observed in Caucasian populations (all P > .05). With regard to the ApaI polymorphism, we found that it significantly decreased the risk of AIS (Aa vs AA: OR = 0.43, 95% CI = 0.24-0.77, P = .004; Aa/aa vs AA: OR = 0.52, 95% CI = 0.30-0.91, P = .023); however, we could not draw a definitive conclusion for Caucasian populations, as no studies have been conducted in this group to determine the role of the VDR ApaI polymorphism in AIS etiology and development.

CONCLUSION

VDR BsmI was significantly associated with AIS susceptibility in the overall and Asian populations, while the VDR ApaI polymorphism only played a key role in AIS etiology and development in Asian populations.

Motile cilia defects in diseases other than primary ciliary dyskinesia: The contemporary diagnostic and research role for transmission electron microscopy

Ultrastructural studies have underpinned the cell biological and clinical investigations of the varied roles of motile cilia in health and disease, with a long history since the 1950s. Recent developments from transmission electron microscopy (TEM; cryo-electron microscopy, electron tomography) have yielded higher resolution and fresh insights into the structure and function of these complex organelles. Microscopy in ciliated organisms, disease models, and in patients with ciliopathy diseases has dramatically expanded our understanding of the ubiquity, multisystem involvement, and importance of cilia in normal human development. Here, we review the importance of motile cilia ultrastructural studies in understanding the basis of diseases other than primary ciliary dyskinesia.

A Zebrafish Model of Human Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a rare, autosomal dominant genetic disorder in humans characterized by explosive inflammatory response to injury leading to gradual ossification within fibrous tissues, including skeletal muscle, tendons, and ligaments. A variety of animal models are needed to study and understand the etiology of human FOP. To address this need, here we present characterizations of the first adult zebrafish model for FOP. In humans, activating mutations in the Type I BMP/TGFβ family member receptor, ACVR1, are associated with FOP. Zebrafish acvr1l, previously known as alk8, is the functional ortholog of human ACVR1, and has been studied extensively in the developing zebrafish embryo, where it plays a role in early dorsoventral patterning. Constitutively active and dominant negative mutations in zebrafish acvr1l cause early lethal defects. Therefore, to study roles for activating acvr1l mutations in adult zebrafish, we created transgenic animals expressing mCherry-tagged, heat-shock-inducible constitutively active Acvr1l, Acvr1l^Q204D, to investigate phenotypes in juvenile and adult zebrafish. Our studies showed that adult zebrafish expressing heat-shock-induced Acvr1l^Q204D develop a number of human FOP-like phenotypes, including heterotopic ossification lesions, spinal lordosis, vertebral fusions, and malformed pelvic fins. Together, these results suggest that transgenic zebrafish expressing heat-shock-inducible Acvr1l^Q204D can serve as a model for human FOP.