Chondrocyte Gene Expression in Osteoarthritis: Correlation with Disease Severity

Osteoarthritis is characterized by a progressive degradation of cartilage structure and function. This study tests the hypothesis that disease severity is characterized by alterations in expression of cartilage-specific genes for aggrecan and collagen type II. Cartilage, discarded from six subjects undergoing knee replacement, was subdivided into homogeneous portions by the surgeon according to the Outerbridge classification. For four subjects, it was possible to separate the tissue into two or three fractions with different disease severity. Portions of each sample were prepared either for histological analysis and ranking according to the Mankin system or for RNA extraction. Quantitative, competitive RT-PCR assays were used for measurement of mRNA for aggrecan, collagen type II, and glyceraldehyde-3-phosphate dehydrogenase. Clinical grading was correlated with histological score (Spearman r = 0.60, p = 0.043). There was a striking decrease in expression of aggrecan and collagen II that was correlated with increase in the grade in regions of cartilage within an individual subject. In the series of 12 samples, there was an inverse correlation between aggrecan expression and osteoarthritis grade (Spearman r = -0.59, p = 0.042). In conclusion, there was an inverse relationship between regional disease severity in osteoarthritis and expression of aggrecan. Use of quantitative, competitive RT-PCR is practical for assessment of chondrocyte gene signatures.