[Type IX collagen diseases].

Type IX collagen is a structural protein which consists of the cartilage collagen II/IX/XI heteropolymer. Transgenic mice overexpressing a mutant alpha 1 (IX) collagen chain in cartilage develop a chondrodysplasia-like phenotype associated with early-onset osteoarthritis and spondylosis. Subsequent identifications of the human type IX collagen diseases proved the correctness of the work on mice. Now we know that type IX collagen gene mutations cause multiple epiphyseal dysplasia, an osteochondrodysplasia which frequently features early onset osteoarthritis. Polymorphic variants of the type IX collagen genes are associated with lumbar disc diseases. It is getting more important to understand the biological roles of collagen IX in the musculoskeletal system.