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.