Mechanisms of Adeno-Associated Virus Genome Encapsidation

The defective parvovirus, adeno-associated virus (AAV), is under close scrutiny as a human gene therapy vector. AAVs non-pathogenic character, reliance on helper virus co-infection for replication and wide tissue tropism, make it an appealing vector system. The virus simplicity and ability to generate high titer vector preparations have contributed to its wide spread use in the gene therapy community. The single stranded AAV DNA genome is encased in a 20-25 nm diameter, icosahedral protein capsid. Assembly of AAV occurs in two distinct phases. First, the three capsid proteins, VP1- 3, are rapidly synthesized and assembled into an empty virion in the nucleus. In the second, rate-limiting phase, singlestrand genomic DNA is inserted into pre-formed capsids. Our rudimentary knowledge of these two phases comes from radioactive labeling pulse-chase experiments, cellular fractionation and immunocytological analysis of infected cells. Although the overall pattern of virus assembly and encapsidation is known, the biochemical mechanisms involved in these processes are not understood. Elucidation of the processes of capsid assembly and encapsidation may lead to improved vector production. While all of the parvoviruses share the characteristic icosahedral particle, differences in their surface topologies dictate different receptor binding and tissue tropism. Based on the analysis of the molecular structures of the parvoviruses and capsid mutagenesis studies, investigators have manipulated the capsid to change tissue tropism and to target different cell types, thus expanding the targeting potential of AAV vectors.