Gene Therapy for Human Osteoarthritis

Articular cartilage has a reduced capacity for self-regeneration. Delivery of candidate genes to articular chondrocytes is an attractive strategy that has the potential to allow for a durable reestablishment of the structural integrity in osteoarthritic (OA) cartilage. Gene transfer approaches might be better suited to treat a slow and irreversible disorder such as OA over time instead of systems that are based on the application of recombinant factors with relatively short pharmacological half-lives. Current approaches that aim at re-equilibrating the metabolic balance in OA cartilage are based on the transfer of sequences coding for agents that either counteract the processes of matrix degradation or enhance the synthesis of matrix components. Importantly for the treatment of OA, the development of effective gene treatments will necessitate that the gene vehicle chosen allows for high and also sustained levels of expression of the sequence to be delivered due to the slow and irreversible progression of this disorder. The method elected to administer the therapeutic composition will be also important to achieve successful and long-term cartilage regeneration in OA patients.


Introduction
Articular cartilage has a reduced capacity for self-regeneration.Delivery of candidate genes to articular chondrocytes is an attractive strategy that has the potential to allow for a durable reestablishment of the structural integrity in osteoarthritic (OA) cartilage.Gene transfer approaches might be better suited to treat a slow and irreversible disorder such as OA over time instead of systems that are based on the application of recombinant factors with relatively short pharmacological half-lives.Current approaches that aim at re-equilibrating the metabolic balance in OA cartilage are based on the transfer of sequences coding for agents that either counteract the processes of matrix degradation or enhance the synthesis of matrix components.Importantly for the treatment of OA, the development of effective gene treatments will necessitate that the gene vehicle chosen allows for high and also sustained levels of expression of the sequence to be delivered due to the slow and irreversible progression of this disorder.The method elected to administer the therapeutic composition will be also important to achieve successful and long-term cartilage regeneration in OA patients.

Gene therapy
Gene therapy aims at treating human diseases via gene transfer techniques that introduce foreign genes or sequences in various cell types.The foreign material enters the cell where it is transferred towards the nucleus.Once there, it either integrates in the host genome or stays extrachromosomal under episomal forms that generally allows only for transient transgene expression.Gene transfer in sufficient number of cells is essential to allow for the production of therapeutically relevant concentrations of transgene products.The currently most employed vectors used in gene therapy trials include nonviral compounds (naked DNA, physical and chemical methods) and different viral gene carriers including adenoviral, herpes simplex virus-derived, retroviral, lentiviral, and recombinant adenoassociated viral vectors that utilize natural entry pathways in human cells (Cucchiarini et al., 2009a;Cucchiarini & Madry, 2005;Evans, 2004;Evans et al., 2004) (Table 1).

Gene transfer in vivo 4.2.1 Direct gene transfer
The key issue in establishing an efficient therapy against OA is to access the targets of the treatment when the cells reside in the joint cavity.The following approaches have been therefore developed: 1. systemic delivery and 2. intraarticular administration: 2.1 by injection or 2.2 using arthrotomy Systemic approaches are better suited to target diseases that are systemic in nature like rheumatoid arthritis (RA) (Evans et al., 2006b(Evans et al., , 2009;;Jorgensen & Apparailly, 2010).Local administration of components might be preferable for OA that affects only a limited number of joints without major systemic manifestations.Intraarticular injection of most vector types preferentially transduce the synovium (Ghivizzani et al., 1997;Gouze, E. et al., 2002;Nita et al., 1996;Roessler et al., 1993), being rather suited for strategies aiming at inhibiting inflammatory and catabolic pathways (and a common approach employed against experimental RA).Direct application of gene vectors has been attempted in experimental OA using sequences coding for IL-1Ra (Fernandes et al., 1999;Frisbie et al., 2002;Frisbie & McIlwraith, 2000;Zhang, X. et al., 2006), IL-10 (Zhang, X. et al., 2006), HSP70 (Grossin et al., 2006), gene silencers (Chen, L. X. et al., 2008), or kallistatin and thrombospondin-1 (Hsieh et al., 2009;Hsieh et al., 2010).Yet, even if cartilage breakdown is contained, this may not be sufficient to fully compensate for the loss of matrix elements and cells noted during the disease progression.In this regard, synthesis of cartilage matrix components might be stimulated by providing vectors carrying genes for anabolic factors (FGF-2, IGF-I) (Chen, B. et al., 2010).

Indirect gene transfer
Ex vivo gene therapy is more complex but safer as no free vector particles are introduced in the body.Also, modified cells can be controlled, tested, and selected in culture.Administration of cells is also a means to increase the cellularity like needed for cases of severe OA.Synoviocytes have been mostly employed to deliver inhibitors of inflammatory and catabolic processes (Pelletier et al., 1997;Zhang, X. et al., 2004).Such pathways were regulated by injecting cells overexpressing an IL-1Ra alone (Pelletier et al., 1997;Zhang, X. et al., 2004) or with IL-10 (Zhang, X. et al., 2004).Again, although OA was reduced in association with decreased cartilage breakdown, complete resurfacing was not reported.Nevertheless, the use of differentiated cells is impaired by the invasive methods of preparation from unaffected sites with a limited supply and by common changes in cell phenotype observed upon passaging in culture.Also, implantation of committed cells generally leads to the formation of a poorly differentiated (fibrous) cartilage.Progenitor cells might be better suited to generate a cartilage of enhanced quality in transplantation settings.
They can be easily isolated from multiple tissues (bone marrow, periosteum, perichondrium, muscle, fat, subdermis, cartilage, bone, synovial membrane, ligaments), even in OA patients, maintaining a multilineage potential with a reliability for differentiation and a capacity for expansion (Barry & Murphy, 2004;Yoo et al., 2000).Remarkably, injection of muscle-derived stem cells modified by combined gene transfer of BMP-4 with sFlt1 (an antagonist of the vascular endothelial growth factor, i.e.VEGF) allowed for cartilage repair in a rat model of OA (Matsumoto et al., 2009).

Clinical trials
Preclinical data, as those described above, have encouraged the initiation of human clinical trials mostly for RA.The first studies were based on retroviral gene transfer of human IL-1Ra in synoviocytes from RA patients and reinjection in the metacarpophalangeal joint (Evans et al., 2000a;Evans et al., 1996;Wehling et al., 2009).Transgene expression was noted locally without adverse events, leading to clinical improvements in some patients, encouraging the implementation of phase II studies (Evans, 2005;Evans et al., 2005a;Evans et al., 2000b;Evans et al., 2006aEvans et al., , 2008;;Evans et al., 2004;Evans et al., 2005b;Robbins et al., 2003).Direct intraarticular injection of rAAV carrying an sTNFR-immunoglobulin in RA patients revealed that the treatment was safe and well tolerated (Evans, 2005;Evans et al., 2005a;Evans et al., 2006aEvans et al., , 2008;;Mease et al., 2009), and a phase I/II trial was subsequently started.Regarding OA, a phase I protocol is ongoing, based on an ex vivo approach using the retroviral transfer of TGF- (Evans et al., 2008).

Conclusions
Gene therapy holds great promise, but issues that need to be addressed include the duration of transgene expression, extended analyses in clinically relevant animal models, the benefit of ex vivo genetically modified cells versus direct approaches, and the identification of optimal therapeutic factors.Future studies will also have to shed light on the safety of these approaches regarding the nonlethal nature of OA.Successful application of gene therapy for OA requires a combined effort of surgeons and basic scientists in order to improve the currently available gene transfer systems.

Declaration of conflicting interests
The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article.