Chondrocyte cell death and intracellular distribution of COMP and type IX collagen in the pseudoachondroplasia growth plate

Abstract

Cartilage oligomeric matrix protein (COMP) is a large extracellular matrix protein expressed in cartilage, ligament and tendon. Mutations in the COMP gene cause two dominantly inherited skeletal dysplasias, pseudoachondroplasia (PSACH) and Multiple Epiphyseal Dysplasia (MED/EDMI). We report on a novel point mutation D511Y in the seventh calcium-binding repeat of the COMP gene and the resulting iliac crest growth plate pathology. The PSACH iliac crest growth plate is comprised of a large region of resting chondrocytes above a narrow region composed of clusters of disorganized proliferative and hypertrophic chondrocytes. Chondrocytes in all zones show massive intracellular retention of COMP and the surrounding extracellular matrix is deficient in COMP. Moreover, the 511Y COMP mutation selectively affects type IX collagen as little is found in the growth plate matrix whereas type I1 collagen and aggrecan are abundant in the matrix. Chondrocyte remnants are observed in the chondrocyte clusters and dead cells are found throughout the growth plate. Apoptosis studies demonstrate an unusual pattern of TUNEL staining in the PSACH chondrocytes compared to the control growth plate. These in vivo findings support our previous observation that retention of COMP leads to chondrocyte death. These results also add to the increasing evidence that PSACH and E D M l are rER storage diseases and that impaired linear growth and joint erosion are caused by the disruptive effect of massive amounts of COMP within the chondrocytes.

Cartilage Oligomeric Matrix Protein Protects Cells against Death by Elevating Members of the IAP Family of Survival Proteins

“…Finally, expression of COMP in the presence of BMP2 stimulation can lead to a late increase in apoptosis (21). This latter finding is consistent with the fact that the mutant COMP proteins in multiple epiphysial dysplasia and PSACH appear to mediate apoptosis of chondrocytes in cartilage in vivo (25)(26)(27).…”

“…Given that COMP is expressed at sites other than connective tissue, such as vascular smooth muscle, synovium, skin, bone, pancreas, and liver (23,(41)(42)(43)(44)(45), this antiapoptotic effect may have broad implications for cell survival throughout the body. Interestingly, whereas the wild type COMP protein is able to block apoptosis, mutant COMP proteins found in the human diseases multiple epiphysial dysplasia and PSACH have been found to promote apoptosis (25)(26)(27). These data indicate that COMP can play opposite roles in the regulation of cell survival, depending on whether it is in a wild type or mutant form.…”

“…As mentioned above, the COMP mutants in multiple epiphysial dysplasia and PSACH are known to affect apoptosis of chondrocytes (25)(26)(27). One likely mechanism is that these COMP mutants induce apoptosis through an endoplasmic reticulum stress response because they are not efficiently secreted and are retained within the endoplasmic reticulum.…”

“…Cells-It has been previously demonstrated that expression of COMP, as either a wild type or mutant form, can affect the level of apoptosis both in vivo and in vitro (21,(25)(26)(27). To explore an effect of COMP on apoptosis, we began transient transfection experiments using a COMP expression plasmid in 293 cells.…”

Antisense Reduction of Mutant COMP Reduces Growth Plate Chondrocyte Pathology

“…[6][7][8][9][10][11][12] In contrast, mutations in COMP cause misfolding of the protein, preventing export from the chondrocyte and resulting in massive retention within the endoplasmic reticulum (ER). 10,[13][14][15] Although this finding has long been appreciated, there was little understanding of the pathologic molecular mechanisms, which are critical to develop mechanism-driven therapeutics. To circumvent this problem, we generated the mutant (MT)-COMP mouse with the common D469del PSACH mutation that expresses human MT-COMP in chondrocytes in the presence of the inducing agent doxycycline (DOX).…”