Reduced cell proliferation and increased apoptosis are significant pathological mechanisms in a murine model of mild pseudoachondroplasia resulting from a mutation in the C-terminal domain of COMP

Abstract

Pseudoachondroplasia (PSACH) is one of the more common skeletal dysplasias and results from mutations in cartilage oligomeric matrix protein (COMP). Most COMP mutations identified to date cluster in the TSP3 repeat region of COMP and the mutant protein is retained in the rough endoplasmic reticulum (rER) of chondrocytes and may result in increased cell death. In contrast, the pathomolecular mechanism of PSACH resulting from C-terminal domain COMP mutations remain largely unknown. This study describes the generation and analysis of a murine model of mild PSACH resulting from a p.Thr583Met mutation in the C-terminal globular domain (CTD) of COMP. Mutant animals are normal at birth, but grow slower than their wild-type littermates and by 9 weeks of age they have mild short-limb dwarfism. Furthermore, by 16 months of age mutant animals exhibit severe degeneration of articular cartilage, which is consistent with early onset osteoarthritis seen in PSACH patients. In the growth plates of mutant mice the chondrocyte columns are sparser and poorly organized. Mutant COMP is secreted into the extracellular matrix, but its localization is disrupted along with the distribution of several COMP-binding proteins. Although mutant COMP is not retained within the rER there is an unfolded protein/cell stress response and chondrocyte proliferation is significantly reduced, while apoptosis is both increased and spatially dysregulated. Overall, these data suggests a mutation in the CTD of COMP exerts a dominant-negative effect on both intra- and extracellular processes. This ultimately affects the morphology and proliferation of growth plate chondrocytes, eventually leading to chondrodysplasia and reduced long bone growth.

Antisense Reduction of Mutant COMP Reduces Growth Plate Chondrocyte Pathology

“…This is further supported by the minimal chondrocyte pathology observed in two knockin COMP mutation mouse models that do not completely recapitulate PSACH chondrocyte pathology and clinical manifestations. [50][51][52][53] Altogether, this suggests that the total amount of COMP is an important factor in MT-COMP pathology and that reduction of total COMP with ASOs ameliorates the disease process.…”

“…This is further supported by the minimal chondrocyte pathology observed in two knockin COMP mutation mouse models that do not completely recapitulate PSACH chondrocyte pathology and clinical manifestations. [50][51][52][53] Altogether, this suggests that the total amount of COMP is an important factor in MT-COMP pathology and that reduction of total COMP with ASOs ameliorates the disease process.We show that mitigation of the chondrocyte pathology was achieved by administering ASO1 for 3 weeks at 180 mg/kg/week. This level of COMP knockdown (38% hCOMP and 60% mCOMP) reduced intracellular retention of MT-COMP, chondrocyte death, and inflammatory markers and partially restored normal levels of chondrocyte proliferation ( Figures 5B-5J and 6).…”

Employing molecular genetics of chondrodysplasias to inform the study of osteoarthritis

“…In addition, recent studies have shown that mutations that cause misfolding of matrix proteins can activate a cellular stress response: the unfolded protein response. This has been shown with COMP and matrilin 3 mutations in multiple epiphyseal dysplasia and type X collagen in Schmid metaphyseal chondrodysplasia (48)(49)(50). This unfolded protein response unleashes a cascade of downstream gene expression changes that can include changes in cellular differentiation, increased protein breakdown, and apoptosis.…”

Changes in the Chondrocyte and Extracellular Matrix Proteome during Post-natal Mouse Cartilage Development

“…The chaperone activity of Pdia4 (ERp72) is of particular interest because ERp72 forms stable complexes with disease-causing COMP and matrilin-3 mutants. Accumulation of these complexes in dilated ER cisternae is a hallmark of multiple epiphyseal dysplasia and pseudoachondroplasia (60 -62), and the resulting ER stress affects normal chondrocyte differentiation and organization in the growth plate (63)(64)(65). ER stress induced by misfolding of collagen X in hypertrophic chondrocytes also contributes to the pathology of Schmid metaphyseal chondrodysplasia, type Schmid (66,67), but the role of specific ER chaperones is not known.…”