Recombinant Adeno-Associated Viral Vector Types 4 and 5: Preparation and Application for CNS Gene Transfer

The adeno-associated viruses (AAVs) are dependent parvoviruses that require helper viruses for replication. They are classified based on size and structure. Because the majority of AAV isolates were first identified as contaminants of laboratory stocks of adenovirus, little is known regarding their natural tissue tropism. Of the six isolates of AAV that have been identified to date in primates, only three (AAV2, 3, and 5) have been isolated from humans. Because AAV4 can infect human and rat cells in culture, serological data suggest that its natural host is the African green monkey. All of those isolates have been cloned and their genomes appear to be organized in a similar manner, with ITRs flanking the rep and cap open reading frames (Fig. 1 ). Some elements are highly conserved within some serotypes, while others are very divergent. The amino acid sequence of each capsid is distinct. Thus, each serotype likely has unique functional properties. For example, a comparison of transduction efficiencies and binding competition experiments indicates that each serotype may utilize a distinct mechanism of uptake compared to AAV2 (1 ,2 ). Heparin sulfate proteoglycans (HSP) have a role in the binding and transduction of AAV2, however, transduction with AAV4 and AAV5 serotypes does not appear to utilize HSP (1 ,3 ). Instead, both AAV4 and AAV5 interact with α 2-3 sialic acid yet they differ in their specificity. AAV4 preferentially binds O-linked sialic acid whereas AAV5 prefers N-linked sialic acids (4 ,5 ).

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