Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are dominantly inherited chondrodysplasias characterized by short stature and early-onset osteoarthrosis. The disease genes in families with PSACH and MED have been localized to an 800 kilobase interval on the short arm of chromosome 19. Recently the gene for cartilage oligomeric matrix protein (COMP) was localized to chromosome 19p13.1. In three patients with these diseases, we identified COMP mutations in a region of the gene that encodes a Ca++ binding motif. Our data demonstrate that PSACH and some forms of MED are allelic and suggest an essential role for Ca++ binding in COMP structure and function.

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) constitute a bone dysplasia family, which is both genetically and phenotypically heterogeneous. The disease spectrum ranges from mild MED, which manifests with pain and stiffness in the joints and delayed and irregular ossification of the epiphyses, to the more severe PSACH, which is characterized by marked short stature, deformity of the legs, and ligamentous laxity. PSACH is almost exclusively caused by mutations in cartilage oligomeric matrix protein (COMP) whereas various forms of MED are caused by mutations in the genes encoding COMP, type IX collagen (COL9A1, COL9A2, and COL9A3), matrilin-3 (MATN3), and solute carrier member 26, member 2 gene (SLC26A2). In this review we discuss specific disease-causing mutations and the clustering of these mutations in functionally and structurally important regions of the respective gene products, genotype to phenotype correlations, and the diagnostic relevance of mutation screening in these osteochondrodysplasias.

Pathologies caused by mutations in extracellular matrix proteins are generally considered to result from the synthesis of extracellular matrices that are defective. Mutations in type X collagen cause metaphyseal chondrodysplasia type Schmid (MCDS), a disorder characterised by dwarfism and an expanded growth plate hypertrophic zone. We generated a knock-in mouse model of an MCDS–causing mutation (COL10A1 p.Asn617Lys) to investigate pathogenic mechanisms linking genotype and phenotype. Mice expressing the collagen X mutation had shortened limbs and an expanded hypertrophic zone. Chondrocytes in the hypertrophic zone exhibited endoplasmic reticulum (ER) stress and a robust unfolded protein response (UPR) due to intracellular retention of mutant protein. Hypertrophic chondrocyte differentiation and osteoclast recruitment were significantly reduced indicating that the hypertrophic zone was expanded due to a decreased rate of VEGF–mediated vascular invasion of the growth plate. To test directly the role of ER stress and UPR in generating the MCDS phenotype, we produced transgenic mouse lines that used the collagen X promoter to drive expression of an ER stress–inducing protein (the cog mutant of thyroglobulin) in hypertrophic chondrocytes. The hypertrophic chondrocytes in this mouse exhibited ER stress with a characteristic UPR response. In addition, the hypertrophic zone was expanded, gene expression patterns were disrupted, osteoclast recruitment to the vascular invasion front was reduced, and long bone growth decreased. Our data demonstrate that triggering ER stress per se in hypertrophic chondrocytes is sufficient to induce the essential features of the cartilage pathology associated with MCDS and confirm that ER stress is a central pathogenic factor in the disease mechanism. These findings support the contention that ER stress may play a direct role in the pathogenesis of many connective tissue disorders associated with the expression of mutant extracellular matrix proteins.