Increasing the size of rAAV-mediated expression cassettes in vivo by intermolecular joining of two complementary vectors

A major shortcoming to the use of adeno-associated virus (rAAV) vectors is their limited packaging size. To overcome this hurdle, we split an expression cassette and cloned it into two separate vectors. The vectors contained either a nuclear localizing Escherichia coli lacZ transgene (nlslacZ) with a splice acceptor, or the human elongation factor 1 alpha (EF1 alpha) gene enhancer/promoter(s) (EF1 alpha EP) with a splice donor. We co-injected a promoter-less nlslacZ vector with a vector containing either a single EF1 alpha EP or a double copy of the EF1 alpha EP in a head-to-head orientation, into the portal vein of mice. Gene expression, measured by both transduction efficiency and quantitation of the recombinant protein, was as much as 60-70% of that obtained from mice that received a single vector containing a complete EF1 alpha EP/nlslacZ expression cassette. This two-vector approach may allow development of gene therapy strategies that will carry exogenous DNA sequences with large therapeutic cDNAs and/or regulatory elements.