SOX9 in cartilage development and disease

Abstract

SOX9 is a pivotal transcription factor in chondrocytes, a lineage essential in skeletogenesis. Its mandatory role in transactivating many cartilage-specific genes is well established, whereas its pioneer role in lineage specification, which along with transactivation defines master transcription factors, remains to be better defined. Abundant, but yet incomplete evidence exists that intricate molecular networks control SOX9 activity during the multi-step chondrogenesis pathway. They include a highly modular genetic regulation, post-transcriptional and post-translational modifications, and varying sets of functional partners. Fully uncovering SOX9 actions and regulation is fundamental to explain mechanisms underlying many diseases that directly or indirectly affect SOX9 activities and to design effective disease treatments. We here review current knowledge, highlight recent discoveries, and propose new research directions to answer remaining questions.

Figure 1

Current knowledge and gaps in knowledge regarding SOX9’s main roles in skeletal progenitors, chondrocytes and osteoblasts. SOX9 is expressed from the multipotent skeletal progenitor stage until chondrocyte hypertrophic differentiation (blue shading of boxes) and has been shown to be involved in chondrocyte lineage specification, differentiation, and survival. Repression of SOX9 is required for osteoblast differentiation from progenitor cells and terminal chondrocytes. Confirmed and candidate co-factors of SOX9 are indicated (blue) as well as other factors involved in specific cell differentiation stages (brown).

Figure 2

Current knowledge of SOX9 protein’s modes of action and regulation. (A) Domain organization of the protein, with indication of functions validated in vivo (no parentheses), functions identified only in vitro (in parentheses), and post-translational modifications identified only in vitro (grey). The location of mutations (mut.) causing diseases as severe as campomelic dysplasia (CMPD), XY sex reversal (XYSR), acampomelic CMPD (ACMPD) and disorders of sex development (DSD) are indicated (brackets). (B) Schematic of the main mode of action of SOX9 in chondrocytes. SOX9 binds as a homodimer to pairs of inverted SOX recognition sites on multiple enhancers associated with cartilage-specific genes. It functionally interacts with SOX5/SOX6 and other co-factors that bind to nearby sites on the enhancers. SOX9 uses its transactivation domains to contact transcriptional co-activators and basal transcriptional machinery components and thereby induce gene transactivation.

Figure 3

Current knowledge of the complex modular organization of SOX9 cis-acting elements. The domain topologically associated with SOX9 spans 1.9 Mb upstream and 0.5 Mb downstream of the gene. The SOX9 proximal region is believed to be a hub for signaling pathways. The distal regions house at least a dozen and likely more enhancers driving gene expression in various tissues, chondrocyte differentiation stages, and cartilage anatomical sites. The upstream region also houses ROCR, SOX9-regulating IncRNAs. Chromosomal translocations, microdeletions and duplications in the distal regions cause chondrodysplasias (CMPD, ACMPD, and PRS) and disorders of sex development (DSD) with a severity generally proportional to their proximity to the SOX9 gene body.