Adeno-associated Virus (AAV)vector
The small (4.8 kb) ssDNA AAV genome consists of two open reading frames, Rep and Cap, flanked by two 145 base inverted terminal repeats (ITRs). These ITRs base pair to allow for synthesis of the complementary DNA strand. Rep and Cap are translated to produce multiple distinct proteins (Rep78, Rep68, Rep52, Rep40 – required for the AAV life cycle; VP1, VP2, VP3 – capsid proteins). When constructing an AAV transfer plasmid, the transgene is placed between the two ITRs, and Rep and Cap are supplied in trans.In addition to Rep and Cap, AAV requires a helper plasmid containing genes from adenovirus. These genes (E4, E2a and VA) mediate AAV replication. The transfer plasmid, Rep/Cap, and the helper plasmid are transfected into HEK293 cells, which contain the adenovirus gene E1+, to produce infectious AAV particles. Rep/Cap and the adenovirus helper genes may also be combined into a single plasmid; the separation of Rep and Cap shown in the figure on the right facilitates the viral pseudotyping discussed below.
Overview of AAV Plasmid System
Eleven serotypes of AAV have thus far been identified, with the best characterized and most commonly used being AAV2. These serotypes differ in their tropism, or the types of cells they infect, making AAV a very useful system for preferentially transducing specific cell types. The chart below gives a summary of the tropism of AAV serotypes, indicating the optimal serotype(s) for transduction of a given organ.