The Role of the Adeno-Associated Virus Capsid in Gene Transfer

Abstract

Adeno-associated virus (AAV) is one of the most promising viral gene transfer vectors that has been shown to effect long-term gene expression and disease correction with low toxicity in animal models, and is well tolerated in human clinical trials. The surface of the AAV capsid is an essential component that is involved in cell binding, internalization, and trafficking within the targeted cell. Prior to developing a gene therapy strategy that utilizes AAV, the serotype should be carefully considered since each capsid exhibits a unique tissue tropism and transduction efficiency. Several approaches have been undertaken in an effort to target AAV vectors to specific cell types, including utilizing natural serotypes that target a desired cellular receptor, producing pseudotyped vectors, and engineering chimeric and mosaic AAV capsids. These capsid modifications are being incorporated into vector production and purification methods that provide for the ability to scale-up the manufacturing process to support human clinical trials. Protocols for small-scale and large-scale production of AAV, as well as assays to characterize the final vector product, are presented here. The structures of AAV2, AAV4, and AAV5 have been solved by X-ray crystallography or cryo-electron microscopy (cryo-EM), and provide a basis for rational vector design in developing customized capsids for specific targeting of AAV vectors. The capsid of AAV has been shown to be remarkably stable, which is a desirable characteristic for a gene therapy vector; however, recently it has been shown that the AAV serotypes exhibit differential susceptibility to proteases. The capsid fragmentation pattern when exposed to various proteases, as well as the susceptibility of the serotypes to a series of proteases, provides a unique fingerprint for each serotype that can be used for capsid identity validation. In addition to serotype identification, protease susceptibility can also be utilized to study dynamic structural changes that must occur for the AAV capsid to perform its various functions during the virus life cycle. The use of proteases for structural studies in solution complements the crystal structural studies of the virus. A generic protocol based on proteolysis for AAV serotype identification is provided here.

Preclinical Toxicology and Biodistribution Studies of Recombinant Adeno-Associated Virus 1 Human Acid α-Glucosidase

“…Gene replacement therapy with recombinant adenoassociated viral (rAAV) vectors is an attractive treatment option because of its nonpathogenic properties, specific tissue tropism of unique serotypes, and stable gene transfer caused by persistent episomal vector genomes (Van Vliet et al, 2008). Studies have demonstrated that rAAV1 vectors transduce skeletal muscle more efficiently than rAAV2 vectors, and do not elicit a humoral immune response to the transgene protein (Chao et al, 2001;Mah et al, 2007).…”

Longevity of rAAV vector and plasmid DNA in blood after intramuscular injection in nonhuman primates: implications for gene doping

“…8,22 Among the rAAV serotypes analyzed to date, rAAV1 and rAAV8 are among the most efficient for muscle transduction. 8,[23][24][25][26] After IM administration, it was demonstrated that rAAV DNA resides as episomal circles [27][28][29] in a chromatin structure; 30 and from a biosafety perspective, the inefficient integration of rAAV into the host genome is an attractive feature for the legitimate use of this gene transfer system.…”

Comparative Analysis of Adeno-Associated Virus Capsid Stability and Dynamics

“…They possess the following positive attributes: they do not cause disease, have a stable virus particle that can be purified by biomedically accepted methods used for recombinant protein products, can be produced void of viral coding genes, can transduce dividing and nondividing cells, and can induce long-term transgene expression in certain cell types (10,11). The majority of gene therapy applications to date have used AAV2, including the treatment of blindness in patients with Leber's congenital amaurosis (11,12).…”