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

Abstract

SOX9 is a crucial transcriptional regulator of cartilage development and homeostasis. Dysregulation of <i>SOX9</i> is associated with a wide spectrum of skeletal disorders, including campomelic dysplasia, acampomelic campomelic dysplasia, and scoliosis. Yet how <i>SOX9</i> variants contribute to the spectrum of axial skeletal disorders is not well understood. Here, we report four pathogenic variants of <i>SOX9</i> identified in a cohort of patients with congenital vertebral malformations. We report a pathogenic missense variant in the transactivation middle (TAM) domain of <i>SOX9</i> associated with mild skeletal dysplasia and scoliosis. We isolated a <i>Sox9</i> mutant mouse with an in-frame microdeletion in the TAM domain (<i>Sox9^Asp272del</i>), which exhibits skeletal dysplasia including kinked tails, rib cage anomalies, and scoliosis in homozygous mutants. We find that both the human missense and the mouse microdeletion mutations resulted in reduced SOX9 protein stability in cell culture, while <i>Sox9^Asp272del</i> mutant mice show decreased SOX9 expression in the growth plate and annulus fibrosus tissues of the spine. This reduction in SOX9 expression was correlated with the reduction of extracellular matrix components, such as tenascin-X and the Adhesion G-protein coupled receptor ADGRG6. In summary, our work identified and modeled a pathologic variant of <i>SOX9</i> within the TAM domain and demonstrated its importance for SOX9 protein stability. Our work demonstrates that SOX9 stability is important for the regulation of ADGRG6 expression, which is a known regulator of postnatal spine homeostasis, underscoring the essential role of SOX9 dosage in a spectrum of axial skeleton dysplasia in humans.