Missense variants weakening a SOX9 phosphodegron linked to odontogenesis defects, scoliosis, and other skeletal features

SOX9 encodes an SRY-related transcription factor critical for chondrogenesis and sex determination among other processes. Loss-of-function variants cause campomelic dysplasia and Pierre Robin sequence, while both gain- and loss-of-function variants cause disorders of sex development. SOX9 has also been linked to scoliosis and cancers, but variants are undetermined. It is highly expressed in tooth progenitor cells, but its odontogenic roles remain elusive, and tooth defects are unreported in SOX9-related conditions. Here, we performed whole-exome sequencing for nine unrelated children with tooth eruption delay and no known syndromes and identified a 7-year-old girl heterozygous for a SOX9 p.Thr239Pro variant and a 10-year-old boy heterozygous for presumably adjacent p.Thr239Pro and p.Thr240Pro variants. These variants were de novo and rare in control populations. Both cases had primary tooth eruption delay. Additionally, the boy had mesiodens blocking permanent central upper incisor eruption, severe scoliosis, and mild craniofacial and appendicular skeleton abnormalities. p.Thr239 and p.Thr240 occupy variable and obligatory positions, respectively, in a cell division control protein 4 (Cdc4)/FBXW7-targeted phosphodegron motif (CPD) fully conserved in SOX9 vertebrate orthologs and SOX8 and SOX10 paralogs, but functionally uncharacterized in vivo. Structural modeling predicted p.Thr240Pro and p.Thr239Pro/p.Thr240Pro but not p.Thr239Pro to strongly reduce SOX9/FBXW7 interaction. Accordingly, p.Thr240Pro and p.Thr239Pro/p.Thr240Pro but not p.Thr239Pro blocked FBXW7-induced SOX9 degradation in cultured cells. All variants increased SOX9-mediated reporter activation independently of protein stabilization, suggesting that CPD may also modulate the transactivation function of SOX9. Altogether, these findings concur that CPD has critical functions, that SOX9 decisively controls odontogenesis, and that gain-of-function variants may markedly perturb both this process and skeletogenesis.