SOX9 IS A POTENT ACTIVATOR OF THE CHONDROCYTE-SPECIFIC ENHANCER OF THE PRO-ALPHA-1(II) COLLAGEN GENE

The identification of mutations in the SRY-related SOX9 gene in patien ts with campomelic dysplasia, a severe skeletal malformation syndrome, and the abundant expression of Sox9 in mouse chondroprogenitor cells and fully differentiated chondrocytes during embryonic development hav e suggested the hypothesis that SOX9 might play a role in chondrogenes is. Our previous experiments with the gene (Col2a1) for collagen II, a n early and abundant marker of chondrocyte differentiation, identified a minimal DNA element in intron 1 which directs chondrocyte-specific expression in transgenic mice, This element is also a strong chondrocy te-specific enhancer in transient transfection experiments, We show he re that Col2a1 expression is closely correlated with high levels of SO X9 RNA and protein in chondrocytes, Our experiments indicate that the minimal Col2a1 enhancer is a direct target for Sox9, Indeed, SOX9 bind s to a sequence of the minimal Col2a1 enhancer that is essential for a ctivity in chondrocytes, and SOX9 acts as a potent activator of this e nhancer in cotransfection experiments in nonchondrocytic cells, Mutati ons in the enhancer that prevent binding of SOX9 abolish enhancer acti vity in chondrocytes and suppress enhancer activation by SOX9 in nonch ondrocytic cells, Other SOX family members are ineffective, Expression of a truncated SOX9 protein lacking the transactivation domain but re taining DNA-binding activity interferes with enhancer activation by fu ll-length SOX9 in fibroblasts and inhibits enhancer activity in chondr ocytes. Our results strongly suggest a model whereby SOX9 is involved in the control of the cell-specific activation of COL2A1 in chondrocyt es, an essential component of the differentiation program of these cel ls, We speculate that in campomelic dysplasia a decrease in SOX9 activ ity would inhibit production of collagen II, and eventually other cart ilage matrix proteins, leading to major skeletal anomalies.