Introduction and Aims: Establishing pathogenic mechanisms that are important for OA progression would support development of therapies to delay arthoplasty and extend the life of the joint. The aim of this study was to define a human model system for comparing minimal and advanced OA cartilage at the tissue, cellular, and molecular level.

Method: Cartilage was isolated from femoral condyles of patients undergoing knee arthroplasty, with advanced OA cartilage obtained from areas within 1cm of overt lesions, and minimal OA cartilage taken from areas with no obvious surface defects. Representative histological sections were scored for disease severity based on four categories: fibrillation, chondrocyte cloning, matrix depletion and cellularity using Bioquant Nova v5.00.8 software. The proteoglycan and hydroxyproline content of the cartilage was determined by biochemical analysis. Following RNA isolation and reverse transcription, the cDNA was analysed for relative gene expression using real-time PCR. Gene expression patterns were compared on a patient-matched basis.

Results: Histological analysis showed that the advanced OA cartilage differed from the minimal cartilage with regard to cloning (p< 0.001), fibrillation (p< 0.001), and proteoglycan depletion (p< 0.001). There was no difference in overall cellularity. The advanced OA cartilage had significantly less proteoglycan content than the minimal tissue, with no difference found in hydroxyproline content. The following changes were observed in the relative expression level of specific genes: 1) the steady state level of osteopontin mRNA showed an overall 3.5-fold increase in advanced OA cartilage compared to minimal (p< 0.01); 2) The mRNA coding for aggre-can was down-regulated in advanced disease tissue to less than 50% the level found in minimal tissue in nine out of 11 patients; 3) the expression of mRNA coding for link protein was also significantly decreased in advanced OA cartilage compared to minimal in nine out of 11 patients; and 4) the mRNA level coding for collagen II did not show an obvious pattern of expression in the minimal versus advanced cartilage. The expression of mRNA coding for MMPs was variable with regard to disease state with the majority of patients showing decreased MMP3, MMP9, and MMP13 mRNA expression in advanced OA tissue compared to minimal.

Conclusions: This study clearly demonstrates that patient-matched minimal and advanced OA cartilage show significant differences in cell and matrix characteristics. In addition, differential patterns of gene expression are observed in this model that may relate to the pathogenic mechanism operating during progression of OA.