Abnormal Chondrocyte Apoptosis in the Cartilage Growth Plate is Influenced by Genetic Background and Deletion of CHOP in a Targeted Mouse Model of Pseudoachondroplasia

Abstract

Pseudoachondroplasia (PSACH) is an autosomal dominant skeletal dysplasia caused by mutations in cartilage oligomeric matrix protein (COMP) and characterised by short limbed dwarfism and early onset osteoarthritis. Mouse models of PSACH show variable retention of mutant COMP in the ER of chondrocytes, however, in each case a different stress pathway is activated and the underlying disease mechanisms remain largely unknown. T585M COMP mutant mice are a model of moderate PSACH and demonstrate a mild ER stress response. Although mutant COMP is not retained in significant quantities within the ER of chondrocytes, both BiP and the pro-apoptotic ER stress-related transcription factor CHOP are mildly elevated, whilst bcl-2 levels are decreased, resulting in increased and spatially dysregulated chondrocyte apoptosis. To determine whether the abnormal chondrocyte apoptosis observed in the growth plate of mutant mice is CHOP-mediated, we bred T585M COMP mutant mice with CHOP-null mice to homozygosity, and analysed the resulting phenotype. Although abnormal apoptosis was alleviated in the resting zone following CHOP deletion, the mutant growth plates were generally more disorganised. Furthermore, the bone lengths of COMP mutant CHOP null mice were significantly shorter at 9 weeks of age when compared to the COMP mutant mice, including a significant difference in the skull length. Overall, these data demonstrate that CHOP-mediated apoptosis is an early event in the pathobiology of PSACH and suggest that the lack of CHOP, in conjunction with a COMP mutation, may lead to aggravation of the skeletal phenotype via a potentially synergistic effect on endochondral ossification.

ER Stress During the Pubertal Growth Spurt Results in Impaired Long-Bone Growth in Chondrocyte-Specific ERp57 Knockout Mice

“…In addition, other skeletal abnormalities, such as pseudoachondrodysplasia (PSACH) or multiple epiphyseal dysplasia (MED) are caused by mutations in cartilage ECM proteins like collagens II, IX, X, and XI, or aggrecan, COMP, and matrilin-3. (26)(27)(28)(29)34,(37)(38)(39)(40)(41)(42)(43) ER stress is thought to constitute the underlying common disease mechanism due to retention of abnormal, improperly folded proteins within the ER. The cells can be rescued after the ER stress-induced unfolded protein response, which arrests the cell cycle, reduces the general protein synthesis, and enhances the production of new chaperones facilitating additional folding processes.…”

Emerging targets in osteoarthritis therapy

“…The UPR is important for the normal differentiation program in chondrocytes proceeding to hypertrophy and apoptosis during growth plate development. [88–90]. Inflammatory, mechanical, and oxidative stress can induce ER stress and the UPR in cartilage via C/EBP homologous protein (CHOP) and X-box protein 1 (XBP1) [••47], both of which potentiate IL-1β-induced oxidative stress and pro-catabolic responses [91]; however, XBP1 can promote chondrocyte survival under certain conditions [92].…”

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).…”