Chromatin profiling identifies chondrocyte-specific Sox9 enhancers important for skeletal development.

The transcription factor SRY-related HMG box 9 (Sox9) is essential for chondrogenesis. Mutations in and around SOX9 cause campomelic dysplasia (CD) characterized by skeletal malformations. Although the function of Sox9 in this context is well studied, the mechanisms that regulate Sox9 expression in chondrocytes remain to be elucidated. Here, we have used genome-wide profiling to identify 2 Sox9 enhancers located in a proximal breakpoint cluster responsible for CD. Enhancer activity of E308 (located 308 kb 5' upstream) and E160 (located 160 kb 5' upstream) correlated with Sox9 expression levels, and both enhancers showed a synergistic effect in vitro. While single deletions in mice had no apparent effect, simultaneous deletion of both E308 and E160 caused a dwarf phenotype, concomitant with a reduction of Sox9 expression in chondrocytes. Moreover, bone morphogenetic protein 2-dependent chondrocyte differentiation of limb bud mesenchymal cells was severely attenuated in E308/E160 deletion mice. Finally, we found that an open chromatin region upstream of the Sox9 gene was reorganized in the E308/E160 deletion mice to partially compensate for the loss of E308 and E160. In conclusion, our findings reveal a mechanism of Sox9 gene regulation in chondrocytes that might aid in our understanding of the pathophysiology of skeletal disorders.

Heterozygous deletion of Sox9 in mouse mimics the gonadal sex reversal phenotype associated with campomelic dysplasia in humans.

Heterozygous mutations in the human SOX9 gene cause the skeletal malformation syndrome campomelic dysplasia which in 75% of 46, XY individuals is associated with male-to-female sex reversal. Although studies in homozygous Sox9 knockout mouse models confirmed that SOX9 is critical for testis development, mice heterozygous for the Sox9-null allele were reported to develop normal testes. This led to the belief that the SOX9 dosage requirement for testis differentiation is different between humans, which often require both alleles, and mice, in which one allele is sufficient. However, in prior studies, gonadal phenotypes in heterozygous Sox9 XY mice were assessed only by either gross morphology, histological staining or analyzed on a mixed genetic background. In this study, we conditionally inactivated Sox9 in somatic cells of developing gonads using the Nr5a1-Cre mouse line on a pure C57BL/6 genetic background. Section and whole-mount immunofluorescence for testicular and ovarian markers showed that XY Sox9 heterozygous gonads developed as ovotestes. Quantitative droplet digital PCR confirmed a 50% reduction of Sox9 mRNA as well as partial sex reversal shown by an upregulation of ovarian genes. Our data show that haploinsufficiency of Sox9 can perturb testis development in mice, suggesting that mice may provide a more accurate model of human disorders/differences of sex development than previously thought.

Dominant-negative SOX9 mutations in campomelic dysplasia.

Campomelic dysplasia (CD) is an autosomal dominant, perinatal lethal skeletal dysplasia characterized by a small chest and short long bones with bowing of the lower extremities. CD is the result of heterozygosity for mutations in the gene encoding the chondrogenesis master regulator, SOX9. Loss-of-function mutations have been identified in most CD cases so it has been assumed that the disease results from haploinsufficiency for SOX9. Here, we identified distal truncating SOX9 mutations in four unrelated CD cases. The mutations all leave the dimerization and DNA-binding domains intact and cultured chondrocytes from three of the four cases synthesized truncated SOX9. Relative to CD resulting from haploinsufficiency, there was decreased transactivation activity toward a major transcriptional target, COL2A1, consistent with the mutations exerting a dominant-negative effect. For one of the cases, the phenotypic consequence was a very severe form of CD, with a pronounced effect on vertebral and limb development. The data identify a novel molecular mechanism of disease in CD in which the truncated protein leads to a distinct and more significant effect on SOX9 function.

SOX9 and the many facets of its regulation in the chondrocyte lineage.

SOX9 is a pivotal transcription factor in developing and adult cartilage. Its gene is expressed from the multipotent skeletal progenitor stage and is active throughout chondrocyte differentiation. While it is repressed in hypertrophic chondrocytes in cartilage growth plates, it remains expressed throughout life in permanent chondrocytes of healthy articular cartilage. SOX9 is required for chondrogenesis: it secures chondrocyte lineage commitment, promotes cell survival, and transcriptionally activates the genes for many cartilage-specific structural components and regulatory factors. Since heterozygous mutations within and around SOX9 were shown to cause the severe skeletal malformation syndrome called campomelic dysplasia, researchers around the world have worked assiduously to decipher the many facets of SOX9 actions and regulation in chondrogenesis. The more we learn, the more we realize the complexity of the molecular networks in which SOX9 fulfills its functions and is regulated at the levels of its gene, RNA, and protein, and the more we measure the many gaps remaining in knowledge. At the same time, new technologies keep giving us more means to push further the frontiers of knowledge. Research efforts must be pursued to fill these gaps and to better understand and treat many types of cartilage diseases in which SOX9 has or could have a critical role. These diseases include chondrodysplasias and cartilage degeneration diseases, namely osteoarthritis, a prevalent and still incurable joint disease. We here review the current state of knowledge of SOX9 actions and regulation in the chondrocyte lineage, and propose new directions for future fundamental and translational research projects.

Regulation of human chondrocyte function through direct inhibition of cartilage master regulator SOX9 by microRNA-145 (miRNA-145).

Articular cartilage enables weight bearing and near friction-free movement in the joints. Critical to its function is the production of a specialized, mechanocompetent extracellular matrix controlled by master regulator transcription factor SOX9. Mutations in SOX9 cause campomelic dysplasia, a haploinsufficiency disorder resulting in severe skeletal defects and dwarfism. Although much is understood about how SOX9 regulates cartilage matrix synthesis and hence joint function, how this master regulator is itself regulated remains largely unknown. Here we identify a specific microRNA, miR-145, as a direct regulator of SOX9 in normal healthy human articular chondrocytes. We show that miR-145 directly represses SOX9 expression in human cells through a unique binding site in its 3'-UTR not conserved in mice. Modulation of miR-145 induced profound changes in the human chondrocyte phenotype. Specifically, increased miR-145 levels cause greatly reduced expression of critical cartilage extracellular matrix genes (COL2A1 and aggrecan) and tissue-specific microRNAs (miR-675 and miR-140) and increased levels of the hypertrophic markers RUNX2 and MMP13, characteristic of changes occurring in osteoarthritis. We propose miR-145 as an important regulator of human chondrocyte function and a new target for cartilage repair.

Wwp2 is essential for palatogenesis mediated by the interaction between Sox9 and mediator subunit 25.

Sox9 is a direct transcriptional activator of cartilage-specific extracellular matrix genes and has essential roles in chondrogenesis. Mutations in or around the SOX9 gene cause campomelic dysplasia or Pierre Robin Sequence. However, Sox9-dependent transcriptional control in chondrogenesis remains largely unknown. Here we identify Wwp2 as a direct target of Sox9. Wwp2 interacts physically with Sox9 and is associated with Sox9 transcriptional activity via its nuclear translocation. A yeast two-hybrid screen using a cDNA library reveals that Wwp2 interacts with Med25, a component of the Mediator complex. The positive regulation of Sox9 transcriptional activity by Wwp2 is mediated by the binding between Sox9 and Med25. In zebrafish, morpholino-mediated knockdown of either wwp2 or med25 induces palatal malformation, which is comparable to that in sox9 mutants. These results provide evidence that the regulatory interaction between Sox9, Wwp2 and Med25 defines the Sox9 transcriptional mechanisms of chondrogenesis in the forming palate.

SOX9 controls epithelial branching by activating RET effector genes during kidney development.

Congenital abnormalities of the kidney and urinary tract are some of the most common defects detected in the unborn child. Kidney growth is controlled by the GDNF/RET signalling pathway, but the molecular events required for the activation of RET downstream targets are still poorly understood. Here we show that SOX9, a gene involved in campomelic dysplasia (CD) in humans, together with its close homologue SOX8, plays an essential role in RET signalling. Expression of SOX9 can be found from the earliest stages of renal development within the ureteric tip, the ureter mesenchyme and in a segment-specific manner during nephrogenesis. Using a tissue-specific knockout approach, we show that, in the ureteric tip, SOX8 and SOX9 are required for ureter branching, and double-knockout mutants exhibit severe kidney defects ranging from hypoplastic kidneys to renal agenesis. Further genetic analysis shows that SOX8/9 are required downstream of GDNF signalling for the activation of RET effector genes such as Sprouty1 and Etv5. At later stages of development, SOX9 is required to maintain ureteric tip identity and SOX9 ablation induces ectopic nephron formation. Taken together, our study shows that SOX9 acts at multiple steps during kidney organogenesis and identifies SOX8 and SOX9 as key factors within the RET signalling pathway. Our results also explain the aetiology of kidney hypoplasia found in a proportion of CD patients.