Gene-based therapies offer enormous potential for the treatment of chronic joint conditions, presenting the capacity to directly change the biology of diseased or damaged tissues. Adeno-associated virus (AAV), a parvovirus with a single stranded DNA genome, has emerged as the most favorable viral vector for use in human clinical applications, due primarily to its safety profile. However, many challenges still face AAV-based gene therapy technology, as AAV is completely dependent on cellular mechanisms for entry, nuclear trafficking, and second strand synthesis of its genome. The recent development of self-complementary (sc) vectors, which bypass the need for second strand synthesis, and the capacity to cross-package the AAV2 vector into different capsid serotypes have expanded both the efficacy and versatility of this system.

Determining the effectiveness of scAAV was the first step of our study. To accurately compare conventional single-stranded AAV (ssAAV) and scAAV vectors, identical expression cassettes were inserted into both viral genome types, and were packaged in serotype 2 capsids for use in vitro and in vivo. These two viral genome types were used for infection of cells in culture and for intra-articular injections of rabbits. Measurements of transgene expression in vitro verified an earlier onset and higher levels of gene expression when using scAAV. The gene for the anti-inflammatory interleukin-1 receptor antagonist (IL-1Ra) was used as the transgene for animal experiments, and therapeutic results were achieved with the self-complementary vector.

Another step for optimization of AAV based gene transfer is selecting the most effective capsid serotype for the target tissue. Various joint tissues from rat, horse, and human were used to screen a battery of AAV serotypes. The outcomes of these screens showed that types 2 and 5 were most effective overall, but also that equine tissues were highly receptive to any AAV transduction with any serotype. When scAAV.IL-1Ra was injected into equine joints, the result was transgene expression levels matching the therapeutic levels seen in the rabbit study. Further enhancement of AAV transduction can be achieved through mutations of the capsid that inhibit intracellular degradation.

These data continue to support the viability of AAV as a safe and effective gene therapy vector for treatment of joint diseases.