Sex-determining region Y-related protein SOX13 is a diabetes autoantigen expressed in pancreatic islets.

The SOX (sex-determining region [SRY]-type high mobility group [HMG] box) family of transcription factors play key roles in determining cell fate during organ development. In this study, we have identified a new human SOX gene, SOX13, as encoding the type 1 diabetes autoantigen, islet cell antigen 12 (ICA12). Sequence analysis showed that SOX13 belongs to the class D subgroup of SOX transcription factors, which contain a leucine zipper motif and a region rich in glutamine. SOX13 autoantibodies occurred at a significantly higher frequency among 188 people with type 1 diabetes (18%) than among 88 with type 2 diabetes (6%) or 175 healthy control subjects (4%). Deletion mapping of the antibody epitopes showed that the autoantibodies were primarily directed against an epitope requiring the majority of the protein. SOX13 RNA was detected in most human tissues, with the highest levels in the pancreas, placenta, and kidney. Immunohistochemistry on sections of human pancreas identified SOX13 in the islets of Langerhans, where staining was mostly cytoplasmic. In mouse pancreas, Sox13 was present in the nucleus and cytoplasm of beta-cells as well as other islet cell types. Recombinant SOX13 protein bound to the SOX consensus DNA motif AACAAT, and binding was inhibited by homodimer formation. These observations-along with the known molecular interactions of the closely related protein, rainbow trout Sox23-suggest that SOX13 may be activated for nuclear import and DNA binding through heterodimer formation. In conclusion, we have identified ICA12 as the putative transcription factor SOX13 and demonstrated an increased frequency of autoantibody reactivity in sera from type 1 diabetic subjects compared with type 2 diabetic and healthy control subjects.

Dexamethasone enhances SOX9 expression in chondrocytes.

SOX9 is a transcription factor that activates type II procollagen (Col2a1) gene expression during chondrocyte differentiation. Glucocorticoids are also known to promote chondrocyte differentiation via unknown molecular mechanisms. We therefore investigated the effects of a synthetic glucocorticoid, dexamethasone (DEX), on Sox9 gene expression in chondrocytes prepared from rib cartilage of newborn mice. Sox9 mRNA was expressed at high levels in these chondrocytes. Treatment with DEX enhanced Sox9 mRNA expression within 24 h and this effect was observed at least up to 48 h. The effect of DEX was dose dependent, starting at 0.1 nM and maximal at 10 nM. The half life of Sox9 mRNA was approximately 45 min in the presence or absence of DEX. Western blot analysis revealed that DEX also enhanced the levels of SOX9 protein expression. Treatment with DEX enhanced Col2a1 mRNA expression in these chondrocytes and furthermore, DEX enhanced the activity of Col2-CAT (chloramphenicol acetyltransferase) construct containing a 1.6 kb intron fragment where chondrocyte-specific Sry/Sox- consensus sequence is located. The enhancing effect of DEX was specific to SOX9, as DEX did not alter the levels of Sox6 mRNA expression. These data suggest that DEX promotes chondrocyte differentiation through enhancement of SOX9.

The DNA-binding specificity of SOX9 and other SOX proteins.

SOX (SRY-related HMG box) proteins are transcription factors that have critical roles in the regulation of numerous developmental processes. They share at least 50% homology in their HMG domains, which bind the DNA element AACAAT. How different SOX proteins achieve specific regulation of target genes is not known. We determined the DNA-binding specificity of SOX9 using a random oligonucleotide selection assay. The optimal SOX9 binding sequence, AGAACAATGG, contained a core DNA-binding element AACAAT, flanked by 5' AG and 3' GG nucleotides. The specific interaction between SOX9 and AGAACAATGG was confirmed by mobility shift assays, DNA competition and dissociation studies. The 5' AG and 3' GG flanking nucleotides enhance binding by SOX9 HMG domain, but not by the HMG domain of another SOX factor, SRY. For SRY, different 5' and 3' flanking nucleotides are preferred. Our studies support the notion that SOX proteins achieve DNA sequence specificity through subtle preferences for flanking nucleotides and that this is likely to be dictated by signature amino acids in their HMG domains. Furthermore, the related HMG domains of SOX9 and Sox17 have similar optimal binding sites that differ from those of SRY and Sox5, suggesting that SOX factors may co-evolve with their DNA targets to achieve specificity.

Transcriptional suppression of Sox9 expression in chondrocytes by retinoic acid.

SOX9 is a transcription factor that is expressed in chondrocytes and regulates expression of chondrocyte phenotype related genes. Expression of these genes is known to be suppressed by retinoic acid (RA). We, therefore, examined whether the Sox9 gene expression is regulated by RA in chondrocytes. RA treatment suppressed Sox9 mRNA expression in primary chondrocytes prepared from newborn mouse rib cartilage within 12 h and this suppression lasted at least up to 24 h. The RA suppression of Sox9 mRNA levels was dose-dependent starting at 0.5 microM with a maximum at 1 microM. Nuclear run-on assays revealed that RA reduced the rate of transcription of Sox9 gene. Finally, Western blot analysis indicated that RA suppressed SOX9 protein levels in these chondrocytes. Furthermore, overexpression of SOX9 reversed RA suppression of Col2a1 enhancer activity. These observations indicate that RA suppresses Sox9 gene expression in chondrocytes at least in part through transcriptional events. J. Cell. Biochem. Suppl. 36: 71-78, 2001.

Functional and structural studies of wild type SOX9 and mutations causing campomelic dysplasia.

In humans, mutations in SOX9 result in a skeletal malformation syndrome, campomelic dysplasia (CD). The present study investigated two major classes of CD mutations: 1) point mutations in the high mobility group (HMG) domain and 2) truncations and frameshifts that alter the C terminus of the protein. We analyzed the effect of one novel mutation and three other point mutations in the HMG domain of SOX9 on the DNA binding and DNA bending properties of the protein. The F12L mutant HMG domain shows negligible DNA binding, the H65Y mutant shows minimal DNA binding, whereas the A19V mutant shows near wild type DNA binding and bends DNA normally. Interestingly, the P70R mutant has altered DNA binding specificity, but also bends DNA normally. The effects of the point mutations were interpreted using a molecular model of the SOX9 HMG domain. We analyzed the effects upon transcription of mutations resembling the truncation and frameshift mutations in CD patients, and found that progressive deletion of the C terminus causes progressive loss of transactivation. Maximal transactivation by SOX9 requires both the C-terminal domain rich in proline, glutamine, and serine and the adjacent domain composed entirely of proline, glutamine, and alanine. Thus, CD arises by mutations that interfere with DNA binding by SOX9 or truncate the C-terminal transactivation domain and thereby impede the ability of SOX9 to activate target genes during organ development.