Background Congenital scoliosis (CS) is a common vertebral malformation. Spondylocostal dysostosis (SCD) is a rare skeletal dysplasia characterised by multiple vertebral malformations and rib anomalies. In a previous study, a compound heterozygosity for a null mutation and a risk haplotype composed by three single-nucleotide polymorphisms in TBX6 have been reported as a disease-causing model of CS. Another study identified bi-allelic missense variants in a SCD patient. The purpose of our study is to identify TBX6 variants in CS and SCD and examine their pathogenicity.
Methods We recruited 200 patients with CS or SCD and investigated TBX6 variants. We evaluated the pathogenicity of the variants by in silico prediction and in vitro experiments.
Results We identified five 16p11.2 deletions, one splice-site variant and five missense variants in 10 patients. In vitro functional assays for missense variants identified in the previous and present studies demonstrated that most of the variants caused abnormal localisation of TBX6 proteins. We confirmed mislocalisation of TBX6 proteins in presomitic mesoderm cells induced from SCD patient-derived iPS cells. In induced cells, we found decreased mRNA expressions of TBX6 and its downstream genes were involved in somite formation. All CS patients with missense variants had the risk haplotype in the opposite allele, while a SCD patient with bi-allelic missense variants did not have the haplotype.
Conclusions Our study suggests that bi-allelic loss of function variants of TBX6 cause a spectrum of phenotypes including CS and SCD, depending on the severity of the loss of TBX6 function.
- congenital scoliosis
- spondylocostal dysostosis
- bi-allelic mutation
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Contributors All authors contributed to the feedback of the manuscript and played an important role in implementing the study. NO coordinated the study. NO, KT, MN, JT, MM, KoW and SI designed the study. NO, KT, NK, TK, TS, KU, HSu, SI, HT, HSh, KeW, IY, RS, YT, SM, KK, KoW provided patient care and collected samples and data. IK, LG, NoM and NaM performed genetic analysis. NO, KT, SK, MO, AC and YY performed functional analysis. NO, JT, KoW and SI interpreted the results and wrote the manuscript. All authors critically reviewed the report. No writing assistance was provided. NO and SI had full access to all of the data in the study and take responsibility for the integrity of the data. All authors revised the manuscript critically and approved the final version for publication.
Funding This work was supported by research grants from Japan Orthopedics and Traumatology Foundation (for NO and KT), Japan Agency For Medical Research and Development (AMED) (contract Nos. 17ek0109212h0001 for SI and 17ek0109280h0001 for SI), the Japan Society for the Promotion of Science (WAKATE B, No. 17K16710 for LG), the Cooperative Research Program (Joint Usage/Research Center program) of Institute for Frontier Life and Medical Sciences, Kyoto University (for SI, LG and JT) and the Acceleration Program for Intractable Disease Research Utilizing Disease Specific iPS Cells (AMED) to JT.
Competing interests None declared.
Patient consent for publication Informed consent was obtained from the patients or their parents.
Ethics approval The Medical Ethics Committee of the Keio University Hospital, Tokyo, approved the study protocol (20080129).
Provenance and peer review Not commissioned; externally peer reviewed.