Abstract
In this issue of Human Gene Therapy, the article by Ghosh et al. (2009) describes a new innovation in AAV vector design, which opens the door to delivery of transgenes that would previously not fit in traditional AAV vectors. A prime example of the limitations in AAV packaging capacity is found with gene transfer approaches for muscular dystrophy. Trans-splicing breaks down a barrier in AAV biology that has been a limitation of the vector system since the small parvovirus was first co-opted for use as a gene therapy vector (Muzyczka, 1992, 1994). AAV as a vector is well suited to terminally differentiated cells such as neurons (Flotte and Carter, 1995) and muscle (Kessler et al., 1996; Xiao et al., 1996). The features of AAV as a small and relatively simple capsid structure certainly facilitate the delivery of vector to muscle tissue either by direct injection or a vascular perfusion approach. Smaller transgenes have been successfully delivered in conventional AAV vectors (Blankinship et al., 2006) and substantial alpha-sarcoglycan gene transfer in has been observed in patients (Mendell et al., 2009). However, the advent of efficient trans-splicing provides an opportunity to overcome one of the final limitations to full utility of AAV vectors in neuromuscular disease, especially Duchenne muscular dystrophy (DMD).
Over the last 10 years, several barriers to successful application of AAV vectors in the treatment of human disease have been significantly reduced or eliminated. The elucidation of a wide range of AAV capsids, each with distinct tissue tropism and unique cellular receptors has been a critical basic observation with profound implication for clinical programs (Gao et al., 2002, 2003). The increased efficiency of these novel capsids in achieving a clinically relevant result following in vivo delivery is a critical component of successful application in a disease like DMD where the target tissue represents 30–40% of the body mass. Of particular interest for muscular dystrophy is the efficient delivery of dystrophin to the heart. Both AAV8 (Wang et al., 2005) and AAV9 (Pacak et al., 2006; Inagaki et al., 2006; Bish et al., 2008) vectors lead to high levels of cardiac gene transfer. The potential to deliver a cardiotropic vector via the systemic circulation can be enhanced with tissue-specific gene expression (Pacak et al., 2008) and this route of delivery has been shown to be of utility in neonatal mdx mice (Bostick et al., 2008). The observation of systemic delivery in neonatal mice has now been extended to successful transplicing of the AAV-9 tsAAV alkaline phosphate reporter in adult mdx mice. In adult animals there are additional challenges to global gene transfer, such as greater tissue mass, inflammation, and other dystrophic changes which may affect blood supply (Ghosh et al., 2009).
The collective challenges to providing clinically relevant gene transfer in muscular dystrophy has led the community of gene therapy investigators to a series of innovative discoveries. Certainly, trans-splicing AAV vectors will be a valuable additional tool to use in the treatment of DMD and a number of related neuromuscular disorders.