A 30-base-pair element in the first intron of SOX9 acts as an enhancer in ATDC5

SOX9 is a transcription factor that is essential for chondrogenesis and testis differentiation, and haploinsufficiency of SOX9 causes campomelic dysplasia, severe skeletal malformation syndrome with variably penetrant XY sex reversal. Here we demonstrate that in several cell lines that express SOX9, 30-bp element in the first intron of human SOX9 gene act as a potential enhancer in the ATDC5 chondroprogenitor cell line, despite the apparent absence of cell-specific regulatory elements within a 5.5-kb promoter region. Deletion and site-specific mutational analyses reveal that the last 12 bp of the 30-bp element are critical for transcriptional activity, while 5'-half sequences are necessary for full transactivation. Gel retardation assays indicate the possible involvement of several binding factors along the length of this element. These results suggest that functionally interdependent elements in the 30-bp enhancer region of the first intron account for basal expression levels of Sox9 in ATDC5.