In this study, we were interested in the overall methylation level in aged and degenerated cartilage. Also, we looked at one gene which might be involved in the re-initiation of replicative activity in osteoarthritis (OA) chondrocytes, p21^WAF1/CIP1. p21^WAF1/CIP1 was previously suggested to be down-regulated in OA chondrocytes and is known to be regulated by epigenetic modulation.

Total methylation levels were analyzed by high pressure liquid chromatography (HPLC), mRNA expression of p21^WAF1/CIP1 and DNMT enzymes by real-time polymerase chain reaction. The methylation status of the p21^WAF1/CIP1- promotor using bisulfite genomic sequencing was evaluated.

General methylation analysis of genomic DNA showed no difference in between normal and aged/OA chondrocytes. Also no difference in methylation of the promotor of the p21^WAF1/CIP1 gene was detectable, which was significantly down-regulated in OA chondrocytes. DNMT1 and DNMT3a were expressed with no significant changes of expression levels found in OA chondrocytes.

Cell cycle progression inhibitor p21^WAF1/CIP1 is expressed in normal and significantly down-regulated in OA articular chondrocytes, which may mediate the re-initiation of cell proliferation in OA cartilage. However, the suppression of p21^WAF1/CIP1 mRNA expression is not due to hypermethylation of its promotor. No overall changes in genome methylation levels were found in aged or OA cartilage. Interestingly, significant expression of DNA methyltransferases was found in articular chondrocytes, which supports that DNA methylation could still be a relevant mechanism of gene regulation in (osteoarthritic) chondrocytes, though not on an overall genomic level nor specifically for the regulation of the p21^WAF1/CIP1 gene.