FACIT Collagens and Their Biological Roles

The FACIT family of collagen genes includes genes encoding the polypeptide subunits of types IX, XII, XIV, XVI, and XIX collagen. Studies of collagens IX, XII, and XIV indicate that they are associated with collagen fibrils and suggest that they modulate the biomechanical properties of tissues. For type IX collagen a direct linkage to type II-containing fibrils has been demonstrated, and studies of transgenic mice demonstrate that mutations in type IX collagen lead to degenerative changes in articular cartilage resembling osteoarthritis in humans. Type IX collagen genes are therefore candidates for genetic skeletal disorders that include early-onset osteoarthritis. Interestingly, one locus for multiple epiphyseal dysplasia has been linked to the α2(IX) collagen gene locus. The alternative use of promoters and alternative splicing provides a basis for considerable diversity among many members of the FACIT family. Some of these variants contain glycosaminoglycan side-chains and are therefore part-time proteoglycans.