As clinical gene therapy approaches its 15th year without any obvious “home run” successes, some have questioned whether such a simple concept as replacing a single gene defect with a single gene could possibly have been fundamentally flawed. While that question will only be answered after the center-field fence has been cleared, what has been learned is that the limitations to both safety and efficacy of gene transfer lie within the vectors available for gene transfer rather than the genes themselves. Recombinant adeno-associated virus (rAAV) vectors seem to have some inherent advantages over the other systems tested so far, primarily stemming from the natural properties of this virus, which is a persistent, yet nonpathogenic, fellow traveler in human cells. As a target for rAAV-mediated gene therapy, hemophilia A is particularly appropriate, since gene augmentation of a secreted protein is only one step removed from protein replacement in a disease for which that has been the standard of care for decades. However, the original serotype 2 rAAV vectors have failed to achieve sufficient efficiency for curative gene therapy in the liver, which, given the need for posttranslational modification, is the optimal target for hemophilia A gene therapy. Sarkar and colleagues (page 1253) have now demonstrated that this limitation is primarily due to the viral capsid and can be overcome by pseudotyping the rAAV2-based gene cassette into the capsid of AAV serotype 8, one of many recently identified capsid variants.1 Using this system to deliver canine factor VIII (FVIII) to FVIII-deficient mice, these investigators have shown that a single injection of vector either to the portal or systemic venous circulation is capable of complete, long-lasting restoration of physiologic levels of FVIII and functional correction of the bleeding diathesis. In order to accomplish this, they used either a B-peptide–deleted single vector or 2 separate vectors containing heavy and light chains, thereby overcoming the one primary limitation of rAAV, its small packaging capacity.
The significance of the Sarkar et al article for hemophilia A gene therapy is obvious. If rAAV8-based vectors prove to be safe in preclinical toxicology studies, this work will provide ample proof-of-principle data to support clinical trials. The implication for the overall field of gene therapy could be even more farreaching. Consensus has arisen about the favorable profile of rAAV in terms of safety and persistence, but prior to these studies rAAV-mediated delivery to liver had encountered a “glass ceiling” of approximately 5% stable transduction of hepatocytes regardless of dose.2 The nature of this limitation has not been obvious, but it is now clear that a change of capsid can overcome this obstacle. Other recent studies have shown that AAV capsid proteins are pivotal in determining receptor-binding affinity, endosomal processing, and nuclear entry, all of which may be contributing factors.3 In any case, rAAV8 may now provide an efficient vector for the liver, the key target for gene therapy of many secreted proteins, inborn errors of metabolism, and intrinsic liver diseases. Let us hope that it proves to be the Louisville Slugger.