Variants in the SOX9 transactivation middle domain induce axial skeleton dysplasia and scoliosis.

SOX9 is an essential transcriptional regulator of cartilage development and homeostasis. In humans, dysregulation of is associated with a wide spectrum of skeletal disorders, including campomelic and acampomelic dysplasia, and scoliosis. The mechanism of how variants contribute to the spectrum of axial skeletal disorders is not well understood. Here, we report four novel pathogenic variants of identified in a large cohort of patients with congenital vertebral malformations. Three of these heterozygous variants are in the HMG and DIM domains, and for the first time, we report a pathogenic variant within the transactivation middle (TAM) domain of . Probands with these variants exhibit variable skeletal dysplasia, ranging from isolated vertebral malformation to acampomelic dysplasia. We also generated a hypomorphic mutant mouse model bearing a microdeletion within the TAM domain ( ). We demonstrated that disturbance of the TAM domain with missense mutation or microdeletion results in reduced protein stability but does not affect the transcriptional activity of SOX9. Homozygous mice exhibited axial skeletal dysplasia including kinked tails, ribcage anomalies, and scoliosis, recapitulating phenotypes observed in human, while heterozygous mutants display a milder phenotype. Analysis of primary chondrocytes and the intervertebral discs in mutant mice revealed dysregulation of a panel of genes with major contributions of the extracellular matrix, angiogenesis, and ossification-related processes. In summary, our work identified the first pathologic variant of within the TAM domain and demonstrated that this variant is associated with reduced SOX9 protein stability. Our finding suggests that reduced SOX9 stability caused by variants in the TAM domain may be responsible for the milder forms of axial skeleton dysplasia in humans.