Figure 17.3 Rep protein mediated site specific recombination in Adeno-associated virus:

The use of AAV is limited by certain level of immunogenicity it induces in humans and restriction in packaging 5 kb gene fragment. Recombinant AAV can be efficiently used in human gene therapy where the rep and cap genes are replaced by therapeutic genes. Plasmid containing Rep68 and Rep78, delivered by non-viral vectors along with ITR flanked transgene can efficiently integrate the corrective gene in AASV1 site in human genome. Constitutive expression of Rep is cytotoxic and cytostatic and sometimes results in abortive rearrangements of AASV1. To overcome this problem Rep can be provided in trans , from a cotransfected plasmid that lacks the Rep binding element. Alternatively the expression of Rep in host cells can be modulated by using ligands capable of inducing Rep or the protein can be externally delivered along with the plasmid to catalyze single integration event.

Retroviruses are another major source of integrase for human gene therapy. Retroviral integrase can be fused with sequence specific DNA binding proteins, such as phage γ repressor, E.coli LexA repressor, the Ty3 tRNA binding domain or designed Zn finger protein E2C for use in gene therapy.