Biology of AAV Life Cycle

AAV is a defective parvovirus that replicates only in cells in which certain functions are provided by a coinfecting helper virus, generally an adenovirus or a herpes-virus (1). AAV has both a broad host range and wide cell and tissue specificity, and replicates in many cell lines of human, simian, or rodent origin provided an appropriate helper virus is present. There may be some limitations to AAV tissue specificity in vivo or at least some significant differences in efficiency of transduc-tion of different tissues and organs. These limitations may reflect the receptor and coreceptors apparently used by different AAV serotypes for entry into cells as well as cellular trafficking of AAV. This aspect of AAV biology is becoming of increasing importance for development of AAV vectors. A second set of parameters that may impact AAV tissue and organ specificity and its replication reflect the nature of the helper function provided by helper viruses.

An additional event required by AAV to function efficiently as a gene delivery vehicle is the need to convert the incoming single-stranded DNA genome to a double-stranded molecule to permit transcription and gene expression. This process is termed single-strand (ss) conversion or metabolic activation (33), and the rate at which it occurs may depend in part on the physiological state of the host cell. However, the process may be accelerated by treatment of the cell with genotoxic agents or by certain helper virus functions.

Infection of certain cell lines by AAV in the absence of helper functions results in its persistence as a latent provirus integrated into the host cell genome (34,35). In such cell lines, the integrated AAV genome may be rescued and replicated to yield a burst of infectious progeny AAV particles if the cells are superinfected with a helper virus such as adenovirus. Importantly, in cultured cells, AAV exhibits a high preference for integration at a specific region, the AAVS1 site, on human chromosome 19 (36,37). The efficiency and specificity of this process is mediated by the AAV rep gene (38-40). Rep-deleted AAV vectors do not retain specificity for integration into this chromosome 19 region (41) and indeed may not integrate efficiently but remain as episomes.

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