Biallelic Variants in CCN2 Underlie an Autosomal Recessive Kyphomelic Dysplasia

Kyphomelic dysplasia is a rare heterogenous group of skeletal dysplasia, characterized by bowing of the limbs, severely affecting femora with distinct facial features. Despite its first description nearly four decades, the precise molecular basis of this condition remained elusive until the recent discovery of de novo variants in the KIF5B-related kyphomelic dysplasia. We ascertained two unrelated consanguineous families with kyphomelic dysplasia. They had six affected offsprings and we performed a detailed clinical evaluation, skeletal survey, and exome sequencing in three probands. All the probands had short stature, cleft palate, and micro-retrognathia. Radiographs revealed kyphomelic femora, bowing of long bones, radial head dislocations and spondyloepimetaphyseal dysplasia. We noted two novel homozygous variants in CCN2 as possible candidates that segregated with the phenotype in the families: a missense variant c.443G>A; p.(Cys148Tyr) in exon 3 and a frameshift variant, c.779_786del; p.(Pro260LeufsTer7) in exon 5. CCN2 is crucial for proliferation and differentiation of chondrocytes. Earlier studies have shown that Ccn2 -deficient mice exhibit twisted limbs, short and kinked sterna, broad vertebrae, domed cranial vault, shorter mandibles, cleft palate and impaired osteoclastogenesis. We studied the impact of CCN2 knockout in zebrafish models via CRISPR-Cas9 gene editing. F0 knockouts of ccn2a in zebrafish showed altered body curvature, impaired cartilage formation in craniofacial region and either bent or missing tails recapitulating the human phenotype. Our observations in humans and zebrafish combined with previously described skeletal phenotype of Ccn2 knock out mice, confirm that biallelic loss of function variants in CCN2 result in an autosomal recessive kyphomelic dysplasia. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement We gratefully acknowledge the support provided by DBT/Wellcome Trust India Alliance for the project titled Center for Rare Disease Diagnosis, Research and Training (Grant Reference number: IA/CRC/20/1/600002) awarded to Katta M Girisha; Joint CSIR-UGC NET Junior Research Fellowship awarded by Human Resource Development Group under Council of Scientific and Industrial Research (CSIR), Government of India, to Swati Singh (08/028(0002)/2019-EMR-I). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Informed consents were obtained from the participants for the study and publication of clinical photographs. The research protocol is approved by the Institutional Ethics Committee, Kasturba Medical College and Hospital, Manipal (IEC: 363/2020). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The data that support the findings of this study are available from the corresponding author upon reasonable request.