AAV Vectors, the Future Workhorse of Human Gene Therapy

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Adeno-associated virus type 2 (AAV2) is a human parvovirus with a single-stranded DNA genome of approximately 4.68 kb. AAV2 is dependent on co-infection with a helper virus, such as adenovirus or herpes virus, to facilitate its replication. The genome of AAV consists of the rep and cap open reading frames (ORFs) flanked by inverted terminal repeats (ITRs). The replication (rep) gene encodes four overlapping proteins, Rep78, Rep68, Rep52, and Rep40. The two larger proteins, Rep78 and Rep 68, are transcribed from promoter p5 and are necessary for viral DNA replication. Rep52 and Rep 40 are transcribed from promoter p19 and function as DNA helicases facilitating the accumulation of single-stranded progeny virus. The capsid (cap) gene encodes three structural proteins, VP1, VP2, and VP3. The ratio of VP1, VP2, and VP3 in the AAV virion is 1:1:10. The ITRs are the only cis-acting elements that are required for AAV replication, packaging, integration, and rescue (Berns 1990).