Mutations in cartilage oligomeric matrix protein causing pseudoachondroplasia and multiple epiphyseal dysplasia affect binding of calcium and collagen I, II, and IX

Mutations in type 3 repeats of cartilage oligomeric matrix protein (COMP) c ause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epi physeal dysplasia (MED). We expressed recombinant wildtype COMP that showed structural and functional properties identical to COMP isolated from carti lage. A fragment encompassing the eight type 3 repeats binds 14 calcium ion s with moderate affinity and high cooperativity and presumably forms one la rge disulfide bonded folding unit. A recombinant PSACH mutant COMP in which Asp-469 was deleted (D469 Delta) and a MED mutant COMP in which Asp-361 wa s substituted by Tyr (D361Y) were both secreted into the cell culture mediu m of human cells. Circular dichroism spectroscopy revealed only small chang es in the secondary structures of D469 Delta and D361Y, demonstrating that the mutations do not dramatically affect the folding and stability of COMP. However, the local conformations of the type 3 repeats were disturbed, and the number of bound calcium ions was reduced to 10 and 8, respectively. In addition to collagen I and II, collagen IX also binds to COMP with high af finity. The PSACH and MED mutations reduce the binding to collagens I, II, and IX and result in an altered zinc dependence. These interactions may con tribute to the development of the patient phenotypes and may explain why ME D can also be caused by mutations in collagen IX genes.