Analysis of recombinant adeno-associated virus packaging and requirements for rep and cap gene products

Adeno-associated virus (AAV) is a human parvovirus currently being developed as a vector for gene therapy applications. Because the gene transfer vector commonly retains only the AAV terminal repeats, propagation of recombinant AAV (rAAV) requires that the viral replication (Rep) and capsid (Cap) proteins be supplied in trans. In an effort to optimize the production of these vectors, a panel of helper plasmids was constructed to determine if expression of the rep and/or cap genes is a limiting factor for rAAV packaging. Expression of the Rep and Cap proteins was increased by replacing the endogenous AAV promoters, p5 and p40, with the Rous sarcoma virus (RSV) long terminal repeat (LTR) and the cytomegalovirus immediate-early promoter, respectively. Increased synthesis of the Cap proteins resulted in an approximately 10-fold increase in the yield of rAAV, indicating that production of capsid proteins is one limiting factor for rAAV packaging. Expression of the rep gene from the RSV LTR not only failed to increase the yield of rAAV but also prevented activation of p40 transcription with adenovirus infection, resulting in a reduced level of capsid protein synthesis.