Over 70 mutations in the
cartilage oligomeric matrix protein (COMP), a large extracellular pentameric
glycoprotein synthesized by chondrocytes, have been identified as causing two skeletal dysplasias:
multiple epiphyseal dysplasia (MED/EDM1), and a dwarfing condition,
pseudoachondroplasia (
PSACH). These mutations induce misfolding of intracellular COMP, resulting in retention of the
protein in the rough endoplasmic reticulum (rER) of chondrocytes. This accumulation of COMP in the rER creates the phenotypic enlarged rER cisternae in the cells, which is believed to compromise chondrocyte function and eventually cause cell death. To study the molecular mechanisms involved with the disease, we sought to develop an in vitro model that recapitulates the
PSACH phenotype. Normal human chondrocytes were transfected with wildtype (wt-) COMP or with mutant COMP (D469del; mt-) recombinant adenoviruses and grown in a nonattachment redifferentiating culture system that provides an environment allowing formation of a differentiated chondrocyte nodule. Visualization of normal cells expressing COMP suggested the hallmarks of the
PSACH phenotype. Mutant COMP expressed in normal cells was retained in enlarged rER cisternae, which also retained IX
collagen (COL9) and
matrilin-3 (MATN3). Although these
proteins were secreted normally into the ECM of the wt-COMP nodules, reduced secretion of these
proteins was observed in nodules composed of cells transfected with mt-COMP. The findings
complement those found in chondrocytes from
PSACH patient growth plates. This new model system allows for production of
PSACH chondrocyte pathology in normal costochondral chondrocytes and can be used for future mechanistic and potential gene therapy studies.