A Campomelic Dysplasia A76E Mutation in Sox9 Destabilizes Protein and DNA Binding Dynamics

Sox9, a pivotal transcription factor belonging to the Sox family, orchestrates critical processes throughout embryonic development, maintenance and differentiation, and exerts a profound influence on organogenesis. Its regulatory versatility stems from precise binding to defined DNA regions, often in collaboration with tissue-specific partners. The dysregulation of Sox9 during chondrogenesis leads to a skeletal malformation termed campomelic dysplasia and has emerged as a significant factor in various other human diseases, including cancer. A point mutation at position 76 (alanine to glutamic acid, A76E) of Sox9 is recognized as one of the causes of campomelic dysplasia. We have used a combination of biophysical, structural and computational techniques to characterize the Sox9 A76E mutant and compare it with the wild-type (WT) Sox9. WT and A76E Sox9 assemble as homodimers, but form predominantly monomeric complexes in the presence of Sox-specific DNA. A CD analysis shows that the A76E mutant preserves the folding as well as the overall secondary structure of Sox9. Both A76E and WT Sox9 behave similarly in the presence of Sox-specific DNA. Perturbation, with increased temperature, displays a lower melting point for A76E, relative to WT Sox9, indicating decreased stability that may arise due to the long and charged side chain of glutamic acid compared to the small hydrophobic alanine, making unfavorable intra-molecular interactions. The destabilizing effect of the A76E mutant may disturb the formation of a stable higher-order complex that is a prerequisite for normal gene expression.