Up‐regulation of cartilage oligomeric matrix protein at the onset of articular cartilage degeneration in a transgenic mouse model of osteoarthritis

Abstract Objective To investigate the suitability of cartilage oligomeric matrix protein (COMP) as a marker for articular cartilage degeneration in a transgenic mouse model of osteoarthritis (OA). Methods Northern blot analysis of total RNA extracted from the knee joints of transgenic Del1 mice, which harbor a short deletion in a type II collagen transgene, and of their nontransgenic littermates was used to monitor changes in COMP messenger RNA (mRNA) levels during cartilage degeneration. Immunohistochemistry was used to determine the distribution of COMP in articular cartilage, and serum levels of COMP were measured by immunoassay. Results Transient up‐regulation of COMP mRNA was seen in articular cartilage of transgenic Del1 mice at the onset of OA lesions at the age of 3 months. Compared with nontransgenic controls, COMP immunostaining of articular cartilage in 3–9‐month‐old transgenic mice was increased, especially at the border of uncalcified and calcified cartilage. There was also a change from predominantly interterritorial to pericellular/territorial deposition of COMP. This difference persisted until the age of 15 months, when the nontransgenic controls also demonstrated articular cartilage degeneration and increased COMP immunostaining. Increased serum levels of COMP were seen in Del1 mice at the age of 4 months, correlating temporally with the onset of cartilage degeneration. Conclusion These findings suggest that up‐regulation of COMP mRNA and redistribution of the protein are characteristic of the early stages of articular cartilage degeneration in the transgenic mouse model in which OA results from a dominant‐negative mutation in the type II collagen gene. The data provide additional support for the notion that COMP is a useful marker for altered cartilage metabolism in developing OA.