Cartilage oligomeric matrix protein and hyaluronan levels in synovial fluid from horses with osteoarthritis of the tarsometatarsal joint compared to a control population

REASONS FOR PERFORMING STUDY: Quantification of cartilage oligomeric matrix protein (COMP) levels within synovial fluid from the tarsometatarsal joint has not previously been reported and an effective synovial fluid marker would allow monitoring of disease progression and treatment. OBJECTIVES: To quantify levels of COMP and hyaluronan (HA) in synovial fluid from the tarsometatarsal joint, identify differences in levels from horses with osteoarthritis (OA) of the tarsometatarsal joint compared to a control population and to correlate levels with radiographic changes in horses with OA. METHODS: Synovial fluid was collected from the tarsometatarsal joint of 25 horses without hindlimb lameness (controls) and 25 lame horses, subjected to analgesia of the joint. COMP concentrations were measured using a homologous inhibition ELISA. Immunoblots of synovial fluid from 3 lame horses and 3 controls were performed to identify fragmentation of COMP. Hyaluronan (HA) concentration in synovial fluid was determined using a competition ELISA. Radiographs of the lame horses with OA were scored and correlated with levels of COMP and HA. RESULTS: Concentrations of COMP in OA of the tarsometatarsal joint were significantly lower than in the control samples. An additional fragment band of COMP (approximately 30 kDa) was identified on the immunoblots of the horses with OA and this fragment was not identified in controls. No significant difference was identified in the HA or HA:COMP ratio between lame and control horses. There was no correlation between levels of synovial fluid COMP and HA, and radiographic changes. CONCLUSIONS AND POTENTIAL RELEVANCE: Lowered levels of COMP in synovial fluid of tarsometatarsal joints correlates with the presence of osteoarthritis. However, a single value cannot be used to stage the disease process. Levels of HA may not be a useful marker for this disease. Decreased, rather than increased COMP levels, may reflect significant loss of cartilage in established osteoarthritis. A specific assay for the COMP fragment generated with osteoarthritis may allow the earlier detection of clinical cases.