Cartilage oligomeric matrix protein (COMP) is a large extracellular pentameric
glycoprotein found in the territorial matrix surrounding chondrocytes. More than 60 unique COMP mutations have been identified as causing two skeletal dysplasias,
pseudoachondroplasia (
PSACH) and
multiple epiphyseal dysplasia (MED/EDM1). Recent studies demonstrate that
calcium-binding and
calcium induced protein folding differ between wild type and mutant COMP
proteins and abnormal processing of the mutant COMP
protein causes the characteristic large lamellar appearing rough endoplasimic reticulum (rER) cisternae phenotype observed in
PSACH and EDMI growth plate chondrocytes. To understand the cellular events leading to this intracellular phenotype,
PSACH chondrocytes with a G427E, D469del and D511Y mutations were grown in 3-D culture to produce cartilage nodules. Each nodule was assessed for the appearance and accumulation of cartilage-specific
proteins within the rER and for matrix
protein synthesis. All three COMP mutations were associated with accumulation of COMP in the rER cisternae by 4 weeks in culture, and by 8 weeks the majority of chondrocytes had the characteristic cellular phenotype. Mutations in COMP also affect the secretion of
type IX collagen and
matrilin-3 (MATN3) but not the secretion of
aggrecan and
type II collagen. COMP,
type IX collagen and MATN3 were dramatically reduced in the
PSACH matrices, and the distribution of these
proteins in the matrix was diffuse. Ultrastructural analysis shows that the
type II collagen present in the
PSACH matrix does not form organized fibril bundles and, overall, the matrix is disorganized. The combined absence of COMP,
type IX collagen and MATN3 causes dramatic changes in the matrix and suggests that these
proteins play important roles in matrix assembly.