Cartilage proteins in serum and synovial fluid as markers for cartilage degradation

COMP (cartilage oligomeric matrix protein) is synthesized in ligament, tendon, meniscus, and articular cartilage

The presence of cartilage oligomeric matrix protein (COMP) in extracts of ligament, tendon, meniscus, and canine articular cartilage was demonstrated by Western blot analysis using anti-dog COMP antibody. When the tissues were cultured in the presence of [35-S]methionine/cysteine, metabolically labeled COMP was purified from the culture media and from tissue extracts by DEAE-cellulose gel chromatography. SDS-Polyacrylamide gel electrophoresis (SDS-PAGE) followed by autoradiography and immunoblotting under reducing and non-reducing conditions revealed that COMP is synthesized by the cells of these connective tissues. Increased levels of COMP in samples of both synovial fluid and serum of patients with various joint diseases may not only be derived from cartilage but also from ligaments and tendons. COMP is not a highly tissue-specific cartilage molecule.

Serum cartilage oligomeric matrix protein reflects osteoarthritis presence and severity: the Johnston County Osteoarthritis Project

OBJECTIVE

To characterize serum cartilage oligomeric matrix protein (COMP) levels by age and gender for a radiographically defined population free of hip and knee osteoarthritis (OA), and to examine the potential utility of COMP as a diagnostic biomarker for knee OA.

METHODS

Serum samples and knee and hip radiographs were obtained at a baseline evaluation as part of the Johnston County Osteoarthritis Project, a population-based study of OA in rural North Carolina. A total of 291 Caucasian participants were randomly selected for COMP analysis, 143 patients with radiographic knee OA (Kellgren/Lawrence [K/L] grade > or = 2) and 148 controls with neither hip nor knee OA (K/L grade 0), evenly distributed by age and gender. COMP was quantified by competitive enzyme-linked immunosorbent assay with monoclonal antibody 17-C10. The natural log-transformed COMP data were analyzed using general linear models.

RESULTS

Serum COMP levels were significantly elevated (P = 0.0001) in the age > or = 65 group (mean +/- SD 1,302.1 +/- 496.7 ng/ml) versus the age 45-54 and age 55-64 groups (1,058.1 +/- 432.4 and 1,038.6 +/- 313.3, respectively). Serum COMP levels of the OA group were significantly higher than those of the control group (1,208.57 +/- 487.47 ng/ml versus 1,061.83 +/- 370.58 ng/ml; P = 0.0093). Serum COMP levels also increased significantly with knee OA K/L grade (P = 0.0047), knee OA laterality (P = 0.0043), and number of knee and hip joints involved (P = 0.0001). There was no significant difference in serum COMP levels by gender or obesity.

CONCLUSION

We demonstrate that in a population-based sample, serum COMP levels can distinguish an OA-affected subgroup from an unaffected subgroup and can reflect disease severity and multiple joint involvement in OA.

Expression of cartilage oligomeric matrix protein by human synovium

Human synovium was analyzed for the possible expression of cartilage oligomeric matrix protein (COMP). Immunostaining with polyclonal antiserum to COMP demonstrated positive staining within the synovial cells and immediately subjacent connective tissue, with less intense staining in the deeper connective tissue. Western blot analysis using either polyclonal or monoclonal antibodies to human COMP confirmed the presence of COMP by immunoreactive bands with the same molecular mass (approximately 110 kDa) as purified articular cartilage COMP. PCR using oligonucleotides that span human COMP exons 7-13 revealed identical amplification products from cDNA prepared from either human chondrocytes or synovium. Northern blot analysis using a biotinylated-probe to human COMP, spanning exons 12-13, also reveal an identical hybridization product to either human chondrocyte or synovium total RNA. Human synovium should be considered as a potential tissue source of COMP in any investigation of biological markers of cartilage metabolism.