Analysis of cartilage oligomeric matrix protein (COMP) degradation and synthesis in equine joint disease

Reasons for performing study: Cartilage oligomeric matrix protein (COMP) is abundant within cartilage; its turnover and/or degradation have been investigated in various equine joint diseases and it has been suggested that COMP fragmentation might be useful for monitoring such conditions. Objectives: To determine whether COMP metabolism is compromised in equine osteoarthritis (OA) and whether COMP degradation is a useful joint marker representing cartilage destruction. Hypothesis: A monoclonal antibody (mAb) with a higher affinity for degraded COMP allows discrimination of diseased joints by quantifying COMP levels and fragmentation. Methods: A mAb (clone14G4) was generated against equine cartilage COMP. The NH2-terminal sequence of enzyme-cut COMP fragments recognised by 14G4 was determined, as was the efficiency of binding to COMP (using a generated COMP peptide). COMP concentration and fragmentation were analysed in synovial fluid (SF) from normal horses and those with OA. Results: The mAb 14G4 had a higher affinity for the smaller fragments of equine COMP, compared with a mAb (clone 12C4) generated against human COMP. The 14G4 epitope was identified as between C-134 and F-147. The COMP values in OA (mean s.d. 205.8 +/- 90.9 mug/ml) were significantly higher than in the normal (133.1 +/- 31.5 mug/ml) SF. On the immunoblots of OA sample, the proportions of intact COMP were significantly lower, while smaller fragments ranging from 75 to 290 kDa were higher compared with the normal SF. Conclusions and potential relevance: The mAb 14G4 reliably detects COMP degradation as well as synthesis, and fragmentation analysis combined with quantification in SF could be useful to study equine OA.