Elevated <em>MMP9</em> Expression – a Potential <em>In Vitro</em> Biomarker for COMPopathies

The intracellular retention of misfolded extracellular matrix proteins is a common disease mechanism in various individually rare skeletal diseases. This discovery has driven the study of ER-stress and the unfolded protein response (UPR) as a promising therapeutic target in several skeletal dysplasias. In the case of COL10A1 mutations, targeting the UPR resulted in a clinical trial of the repurposed drug carbamazepine; however, for other closely related skeletal disorders treatment with carbamazepine was ineffective, indicating the need for suitable markers for in vitro screenings of potential drug treatments. Mutations in cartilage oligomeric matrix protein (COMP), a cartilage structural protein, causes both multiple epiphyseal dysplasia (MED) and pseudoachondroplasia (PSACH); together referred to as COMPopathies, and result in intracellular retention of mutant COMP protein to varying degrees. In contrast to other closely related skeletal disorders, caused by mutations in cartilage structural proteins, the involvement of the UPR is less clear, and so far, no common COMPopathy marker has been identified. Here, using cell models of COMPopathies we identified MMP9 upregulation as a common feature of six pathogenic COMP variants that do not induce a prominent UPR. We further show that the archetypal p.V194D matrilin-3 MED variant (which causes MED), does not induce MMP9 expression, suggesting that MMP9 upregulation could serve as a specific marker of COMPopathies in vitro.