Gene expression and matrix proteinase activity in osteochondrosis dessicans
Authors: Curtis C.L. , Wilson C. , Williams R. , Dent C. , Caterson B. 
Published: September 2009

Introduction  Osteochondrosis dessicans (OCD) is a disorder of unknown aetiology where often a fragment of cartilage and subchondral bone separates from the articular surface. Previous studies have shown histological changes in glycosaminoglycan content in OCD cartilage compared to normal cartilage (Koch et al. 1997). It has also been shown in equine OCD cartilage that there is excessive degradation of type‐II collagen compared to normal cartilage (Laverty et al. 2002). The present study was undertaken to examine the gene expression in human OCD cartilage compared to its normal autologous articular cartilage and human osteoarthritic (OA) cartilage.

Methods  Cartilage from five OCD patients (18–34 years) was obtained at the time of surgery. Pieces of cartilage were either snap‐frozen (in preparation for RNA isolation) or the proteoglycans extracted with 4 m GuHCl. Total RNA was isolated from the cartilage using RNeasy minicolumns and reagents (Qiagen) according to the manufacturer's protocol. RT‐PCR was performed using an RNA PCR kit (Perkin‐Elmer) using a number of oligonucleotide primers. GuHCl‐extracted proteoglycan fragments were analysed using Western blotting with a number of antibodies to aggrecan metabolites, collagen metabolites and the small leucine‐rich proteoglycans.

Results and discussion  When OCD cartilage was compared to normal and human OA cartilage, there was an increase in aggrecan, collagen type‐II and collagen type‐X RNA expression. There was no change in RNA expression of link protein or type‐I collagen. The RNA expression of the aggrecanases (ADAMTS enzymes) was also different in the three different cartilage samples. Neither ADAMTS‐1, ‐4 or ‐5 was present in the normal cartilage. In contrast, in the OCD cartilage, there was expression of both ADAMTS‐1 and ‐4, whereas in the OA cartilage, there was expression of ADAMTS‐4 and ‐5. In the case of MMP RNA expression, MMP‐3 was decreased and MMP‐13 increased in OCD cartilage compared to both normal and OA samples. In addition, the expression of all three TIMP isoforms was increased in the OCD cartilage. Although inflammatory components are not expected in OCD pathology, expressions of inflammatory mediators such as COX‐2, IL‐1‐α and TNF‐α were all increased in the OCD cartilage when compared to normal, but expression of these mRNAs in the OA cartilage was higher. Analysis of proteoglycan fragments in the OCD cartilage by Western blotting showed the presence of aggrecan fragments containing the G1 domain, interglobular domain and the C‐terminal neoepitope generated by aggrecanase cleavage. There was also immunoreactivity for biglycan and link protein.

Conclusion  These results suggest that the phenotypic expression of chondrocytes at the site of the OCD lesion are markedly different from ‘normal’ articular cartilage and also pathological OA cartilage. Interestingly, the expression patterns of matrix proteinases and their natural inhibitors were also markedly different in OCD cartilage, again suggesting that there are specific biochemical expression patterns in OCD pathology, which may potentially be biomarkers of the disease process. Further studies are necessary to elucidate how the differences in gene expression and matrix protease activity may be involved in the aetiology of OCD.