Gene therapy methods in bone and joint disorders. Evaluation of the adeno-associated virus vector in experimental models of articular cartilage disorders, periprosthetic osteolysis and bone healing.

BACKGROUND Gene therapy is a technique that draws on the introduction of new genes into cells for the purpose of treating disease by restoring or adding gene expression. Numerous growth factors and other proteins with the ability to promote the regeneration of tissues in the locomotive system have been identified, but their clinical use is often hindered by delivery problems. In principle, these problems can be overcome by delivering the relevant genes, as the therapeutic substances thereby can be persistently produced directly by local cells at the site of diseases. HEALING OF ARTICULAR CARTILAGE Articular chondrocytes are receptive to transduction using various gene delivery methods. Following genetic modification, they are capable of sustained expression of transgene products at biologically relevant levels. Our research has proved the AAV vector to be an effective tool for gene delivery to articular chondrocytes in vitro as well as in vivo. To this end, we have demonstrated that the AAV vector mediated TGFbeta1-overexpression stimulates cartilage anabolism. WEAR DEBRIS-INDUCED OSTEOLYSIS: The RANKL system may be a key therapeutic target in treatment of aseptic periprosthetic loosening. We investigated whether gene transfer of OPG using an AAV vector has protective effects against orthopaedic wear debris-induced bone loss. In osteoclastogenesis and in bone wafer resorption assays, the bioactivity of the transgene OPG was proven by depletion of osteoclastogenesis and reduced bone resorption. Using an in vivo model of debris-induced bone resorption, we demonstrated complete inhibition of osteolysis in animals receiving AAV-OPG gene therapy. FRACTURE HEALING IN RELATION TO OSTEOPOROSIS The success of future OPG treatment of osteoporosis is highly dependent on its effects on fracture healing and remodelling. Using an in vivo fracture healing model, our studies demonstrated that AAV-OPG gene therapy did not conflict with normal bone healing, in contrast to high-dosage intravenous treatment with OPG. However, AAV-OPG therapy depressed remodelling and integration of the genuine cortical bone at the fracture line. STRUCTURAL BONE ALLOGRAFT HEALING Structural bone allografts often fracture due to their lack of osteogenic and remodelling potiential. To overcome these limitations, we utilized allografts coated with AAV-caALK2 vector that mediated in vivo gene transfer. We showed that the AAV vector was capable of transducing adjacent inflammatory cells and osteoblasts in the fracture callus and that BMP signals delivered via AAV-caALK2 coating induced bone formation directly on the cortical surface of the allograft. CONCLUSION The presented research may be seen as initial steps towards development of gene therapeutic treatment options for complex orthopaedic diseases. As such, our studies represent proof-of-principle that the rAAV vector promotes efficient gene transfer in vitro to a spectrum of cells with orthopaedic relevance, and that in vivo targeting of somatic tissue with a single administration of a rAAV vector at the time of surgery could be sufficient for long-term expression of therapeutic proteins. Essential to the future success of transgene delivery by rAAV vectors is the absence of an immune response to either the vector or the gene product. Furthermore, development of rAAV vectors with regulatory gene expression needs further attention in future research.