Lack of evidence of WNT3A as a candidate gene for congenital vertebral malformations

COL11A2 as a candidate gene for vertebral malformations and congenital scoliosis

Human vertebral malformations (VMs) have an estimated incidence of 1/2000 and are associated with significant health problems including congenital scoliosis (CS) and recurrent organ system malformation syndromes such as VACTERL (vertebral anomalies; anal abnormalities; cardiac abnormalities; tracheo-esophageal fistula; renal anomalies; limb anomalies). The genetic cause for the vast majority of VMs are unknown. In a CS/VM patient cohort, three COL11A2 variants (R130W, R1407L and R1413H) were identified in two patients with cervical VM. A third patient with a T9 hemivertebra and the R130W variant was identified from a separate study. These substitutions are predicted to be damaging to protein function, and R130 and R1407 residues are conserved in zebrafish Col11a2. To determine the role for COL11A2 in vertebral development, CRISPR/Cas9 was used to create a nonsense mutation (col11a2L642*) as well as a full gene locus deletion (col11a2del) in zebrafish. Both col11a2L642*/L642* and col11a2del/del mutant zebrafish exhibit vertebral fusions in the caudal spine, which form due to mineralization across intervertebral segments. To determine the functional consequence of VM-associated variants, we assayed their ability to suppress col11a2del VM phenotypes following transgenic expression within the developing spine. While wildtype col11a2 expression suppresses fusions in col11a2del/+ and col11a2del/del backgrounds, patient missense variant-bearing col11a2 failed to rescue the loss-of-function phenotype in these animals. These results highlight an essential role for COL11A2 in vertebral development and support a pathogenic role for two missense variants in CS.

TBX6 missense variants expand the mutational spectrum in a non-Mendelian inheritance disease

Congenital scoliosis (CS) is a birth defect with variable clinical and anatomical manifestations due to spinal malformation. The genetic etiology underlying about 10% of CS cases in the Chinese population is compound inheritance by which the gene dosage is reduced below that of haploinsufficiency. In this genetic model, the trait manifests as a result of the combined effect of a rare variant and common pathogenic variant allele at a locus. From exome sequencing (ES) data of 523 patients in Asia and two patients in Texas, we identified six TBX6 gene-disruptive variants from 11 unrelated CS patients via ES and in vitro functional testing. The in trans mild hypomorphic allele was identified in 10 of the 11 subjects; as anticipated these 10 shared a similar spinal deformity of hemivertebrae. The remaining case has a homozygous variant in TBX6 (c.418C>T) and presents a more severe spinal deformity phenotype. We found decreased transcriptional activity and abnormal cellular localization as the molecular mechanisms for TBX6 missense loss-of-function alleles. Expanding the mutational spectrum of TBX6 pathogenic alleles enabled an increased molecular diagnostic detection rate, provided further evidence for the gene dosage-dependent genetic model underlying CS, and refined clinical classification.

An N-Ethyl-N-Nitrosourea (ENU) Mutagenized Mouse Model for Autosomal Dominant Nonsyndromic Kyphoscoliosis Due to Vertebral Fusion

Kyphosis and scoliosis are common spinal disorders that occur as part of complex syndromes or as nonsyndromic, idiopathic diseases. Familial and twin studies implicate genetic involvement, although the causative genes for idiopathic kyphoscoliosis remain to be identified. To facilitate these studies, we investigated progeny of mice treated with the chemical mutagen N-ethyl-N-nitrosourea (ENU) and assessed them for morphological and radiographic abnormalities. This identified a mouse with kyphoscoliosis due to fused lumbar vertebrae, which was inherited as an autosomal dominant trait; the phenotype was designated as hereditary vertebral fusion (HVF) and the locus as Hvf. Micro-computed tomography (μCT) analysis confirmed the occurrence of nonsyndromic kyphoscoliosis due to fusion of lumbar vertebrae in HVF mice, consistent with a pattern of blocked vertebrae due to failure of segmentation. μCT scans also showed the lumbar vertebral column of HVF mice to have generalized disc narrowing, displacement with compression of the neural spine, and distorted transverse processes. Histology of lumbar vertebrae revealed HVF mice to have irregularly shaped vertebral bodies and displacement of intervertebral discs and ossification centers. Genetic mapping using a panel of single nucleotide polymorphic (SNP) loci arranged in chromosome sets and DNA samples from 23 HVF (eight males and 15 females) mice, localized Hvf to chromosome 4A3 and within a 5-megabase (Mb) region containing nine protein coding genes, two processed transcripts, three microRNAs, five small nuclear RNAs, three large intergenic noncoding RNAs, and 24 pseudogenes. However, genome sequence analysis in this interval did not identify any abnormalities in the coding exons, or exon-intron boundaries of any of these genes. Thus, our studies have established a mouse model for a monogenic form of nonsyndromic kyphoscoliosis due to fusion of lumbar vertebrae, and further identification of the underlying genetic defect will help elucidate the molecular mechanisms involved in kyphoscoliosis. © 2018 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Abnormalities associated with congenital scoliosis in high-altitude geographic regions

PURPOSE

To assess the different characteristics of congenital scoliosis between low-altitude geographic regions and high-altitude geographic regions in Chinese population and discuss the role of hypoxia on those differences.

METHODS

A total of 120 patients with congenital scoliosis who underwent surgical treatment in our Hospital between January 2009 and October 2017 were identified. Complete data were reviewed, including medical records, X-ray, CT, and MRI pre-operatively. According to the patient's birthplace, they were divided into low-altitude geographic regions (low group) and high-altitude geographic regions (high group). Characteristics of vertebral deformities, rib deformities, and intra-spinal malformations in two groups were analyzed.

RESULTS

A total of 397 segments of vertebral deformities were involved in the two groups, of which 136 were involved in high group and 261 in low group. The average segments involved were 4.5 and 2.9, respectively, in two groups. 63.3% patients in high group have rib deformities, which is significantly higher than that of low group (41.1%); and the proportion of patients with complex rib deformities in high group was also higher than that in low group (57.9% VS 24.3%). The incidence of CS associated with intra-spinal malformations in low group was 38.9%, which was similar to those reported previously; however, the incidence of that in high group was 63%, significantly higher than previous reports.

CONCLUSION

Our results suggested that CS patients in high-altitude geographic regions might tend to have higher proportion and more severe of rib deformities, and also be more likely to accompany with intra-spinal malformations. So we supposed that hypoxia not only aggravated the proportion and severity of rib deformities, but also affected the development of spinal cord in humans.

Compound Heterozygosity for Null Mutations and a Common Hypomorphic Risk Haplotype in TBX6 Causes Congenital Scoliosis

Congenital scoliosis (CS) occurs as a result of vertebral malformations and has an incidence of 0.5-1/1,000 births. Recently, TBX6 on chromosome 16p11.2 was reported as a disease gene for CS; about 10% of Chinese CS patients were compound heterozygotes for rare null mutations and a common haplotype defined by three SNPs in TBX6. All patients had hemivertebrae. We recruited 94 Japanese CS patients, investigated the TBX6 locus for both mutations and the risk haplotype, examined transcriptional activities of mutant TBX6 in vitro, and evaluated clinical and radiographic features. We identified TBX6 null mutations in nine patients, including a missense mutation that had a loss of function in vitro. All had the risk haplotype in the opposite allele. One of the mutations showed dominant negative effect. Although all Chinese patients had one or more hemivertebrae, two Japanese patients did not have hemivertebra. The compound heterozygosity of null mutations and the common risk haplotype in TBX6 also causes CS in Japanese patients with similar incidence. Hemivertebra was not a specific type of spinal malformation in TBX6-associated CS (TACS). A heterozygous TBX6 loss-of-function mutation has been reported in a family with autosomal-dominant spondylocostal dysostosis, but it may represent a spectrum of the same disease with TACS.

Clinical, genetic and environmental factors associated with congenital vertebral malformations

Congenital vertebral malformations (CVM) pose a significant health problem because they can be associated with spinal deformities, such as congenital scoliosis and kyphosis, in addition to various syndromes and other congenital malformations. Additional information remains to be learned regarding the natural history of congenital scoliosis and related health problems. Although significant progress has been made in understanding the process of somite formation, which gives rise to vertebral bodies, there is a wide gap in our understanding of how genetic factors contribute to CVM development. Maternal diabetes during pregnancy most commonly contributes to the occurrence of CVM, followed by other factors such as hypoxia and anticonvulsant medications. This review highlights several emerging clinical issues related to CVM, including pulmonary and orthopedic outcome in congenital scoliosis. Recent breakthroughs in genetics related to gene and environment interactions associated with CVM development are discussed. The Klippel-Feil syndrome which is associated with cervical segmentation abnormalities is illustrated as an example in which animal models, such as the zebrafish, can be utilized to provide functional evidence of pathogenicity of identified mutations.

Molecular diagnosis of vertebral segmentation disorders in humans

BACKGROUND

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement and functional distress.

OBJECTIVE

To provide an overview of the current understanding of vertebral malformations, at both the clinical level and the molecular level, and factors that contribute to their occurrence.

METHODS

The literature related to the following was reviewed: recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformations; and complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and non-syndromic congenital vertebral malformations.

RESULTS/CONCLUSION

Expert opinions extend to discussion of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

Genetic aspects of congenital and idiopathic scoliosis

Congenital and idiopathic scoliosis represent disabling conditions of the spine. While congenital scoliosis (CS) is caused by morphogenic abnormalities in vertebral development, the cause(s) for idiopathic scoliosis is (are) likely to be varied, representing alterations in skeletal growth, neuromuscular imbalances, disturbances involving communication between the brain and spine, and others. Both conditions are characterized by phenotypic and genetic heterogeneities, which contribute to the difficulties in understanding their genetic basis that investigators face. Despite the differences between these two conditions there is observational and experimental evidence supporting common genetic mechanisms. This paper focuses on the clinical features of both CS and IS and highlights genetic and environmental factors which contribute to their occurrence. It is anticipated that emerging genetic technologies and improvements in phenotypic stratification of both conditions will facilitate improved understanding of the genetic basis for these conditions and enable targeted prevention and treatment strategies.

Scoliosis and segmentation defects of the vertebrae

The vertebral column derives from somites, which are transient paired segments of mesoderm that surround the neural tube in the early embryo. Somites are formed by a genetic mechanism that is regulated by cyclical expression of genes in the Notch, Wnt, and fibroblast growth factor (FGF) signaling pathways. These oscillators together with signaling gradients within the presomitic mesoderm help to set somitic boundaries and rostral-caudal polarity that are essential for the precise patterning of the vertebral column. Disruption of this mechanism has been identified as the cause of severe segmentation defects of the vertebrae in humans. These segmentation defects are part of a spectrum of spinal disorders affecting the skeletal elements and musculature of the spine, resulting in curvatures such as scoliosis, kyphosis, and lordosis. While the etiology of most disorders with spinal curvatures is still unknown, genetic and developmental studies of somitogenesis and patterning of the axial skeleton and musculature are yielding insights into the causes of these diseases.

Progress in the understanding of the genetic etiology of vertebral segmentation disorders in humans

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement, and functional distress. This review explores (1) recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; (2) outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformation; and (3) complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and nonsyndromic congenital vertebral malformation. Discussion includes exploration of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation, and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

Congenital scoliosis in monozygotic twins: case report and review of possible factors contributing to its development

BACKGROUND

The exact etiology of congenital scoliosis remains unknown as yet. It seems that its development may be influenced by both genetic predisposition and environmental factors, at varying degrees. International bibliography features few cases of monozygotic twins with congenital scoliosis. The aim of this study is to report a case in monozygotic twins and review the literature relating to the description of similar cases as well as the pathophysiological mechanism involved in its development.

METHODS

Clinical examination and simple X-rays revealed scoliosis of differing degrees and types in male monozygotic twins with moderate mental retardation and dyslalia.

RESULTS

Congenital scoliosis identified in both twins. In the first, this was manifested as left thoracic scoliosis, with Cobb angle of 34 degrees while in the second as left thoracolumbar scoliosis with Cobb angle of 10 degrees. Both were found to suffer from incarcerated hemivertebrae.

CONCLUSION

According to both its clinical identification and severity and to its course, not only the genetic but the environmental factors seem to play a leading role in the appearance of the condition.

A missense T (Brachyury) mutation contributes to vertebral malformations

No major susceptibility genes for sporadically occurring congenital vertebral malformations (CVM) in humans have been identified to date. Body patterning genes whose mutants cause axial skeletal anomalies in mice are candidates for human CVM susceptibility. T (also known as Brachyury) and TBX6 are critical genes needed to establish mesodermal identity. We hypothesized that mutations in T and/or TBX6 contribute to the pathogenesis of human CVMs. The complete T and TBX6 coding regions, splice junctions, and proximal 500 bp of the promoters were sequenced in 50 phenotyped patients with CVM. Three unrelated patients with sacral agenesis, Klippel-Feil syndrome, and multiple cervical and thoracic vertebral malformations were heterozygous for a c.1013C>T substitution, resulting in a predicted Ala338Val missense alteration in exon 8. A clinically unaffected parent of each patient also harbored the substitution, but the variant did not occur in an ethnically diverse, 443-person reference population. The c.1013C>T variant is significantly associated with CVM (p < 0.001). Alanine 338 shows moderate conservation across species, and valine at this position has not been reported in any species. A fourth patient harbored a c.908-8C>T variant in intron 7. This previously unreported variant was tested in 347 normal control subjects, and 11 heterozygotes and 2 T/T individuals were found. No TBX6 variants were identified. We infer that the c.1013C>T substitution is pathogenic and represents the first report of an association between a missense mutation in the T gene and the occurrence of sporadic CVMs in humans. It is uncertain whether the splice junction variant increases CVM risk. TBX6 mutations do not seem to be associated with CVM. We hypothesize that epistatic interactions between T and other developmental genes and the environment modulate the phenotypic consequences of T variants.