Shox2 regulates progression through chondrogenesis in the mouse proximal limb

In humans, loss of SHOX gene function is responsible for the mesomelic short stature characteristic of Turner syndrome, Leri-Weill dyschondrosteosis, and Langer dysplasia. In a mouse model of SHOX deficiency, Prrx1-Cre-driven limb-specific deletion of the paralogous gene Shox2 results in severe rhizomelia. In this study, we show that Col2a1-Cre-driven deletion of Shox2 in developing chondrocytes also results in shortening of the stylopodial skeleton (i.e. humerus, femur) and that this rhizomelia is due to precocious chondrocyte maturation and hypertrophy. We demonstrate, using the micromass culture model system, that increased BMP activity triggers accelerated maturation and hypertrophy in Col2a1-Cre Shox2 mutant chondrocytes and we confirm in vivo that elevated transcript levels and expanded expression domains of Bmp2 and 4 are associated with premature formation of the hypertrophic zone in mutant humeri. In micromass cultures of Prrx1-Cre Shox2 mutant limb cells, we find that Shox2 deletion in undifferentiated mesenchymal cells results in increased BMP activity that enhances early chondrogenesis, but is insufficient to provoke chondrocyte maturation and hypertrophy. Similarly, shRNA-mediated Shox2 knockdown in multipotent C3H10T1/2 cells and primary mouse bone marrow mesenchymal stem cells results in spontaneous chondrogenesis in the absence of chondrostimulation, but again fails to induce progression through the later stages of chondrogenic differentiation. Importantly, exogenous BMP supplementation can overcome the block to maturation and hypertrophy caused by Shox2 depletion prior to overt chondrogenesis. Thus, we provide evidence that Shox2 regulates progression through chondrogenesis at two distinct stages - the onset of early differentiation and the transition to maturation and hypertrophy.