Helper plasmids for production of HIV-1-derived vectors

Vectors derived from human immunodeficiency virus type 1 (HIV-1) appear an attractive option for many gene therapy applications. This is due to their ability to transduce noncycling cell populations and to integrate their gen ome into the host cell chromosome, resulting in the stable genetic modifica tion of the transduced cell. These properties have permitted the direct in vivo transduction of several tissues, including the central nervous system, retina, and liver. However, the pathogenic nature of HIV-1 has raised cons iderable concerns about the safety of such vector systems. To help address these concerns, we have expressed each of the primary transcriptional units encoding trans functions relevant for vector production in individual plas mid constructs. The gag-pol gene sequence was codon-optimized for expressio n in mammalian cells resulting in high level Rev/Rev-response element (RRE) -independent expression. Codon optimization of gag-pol also reduces sequen ce homology with vectors containing gag gene sequences, which results in re duced transfer of biologically active gag-pol sequences to transduced cells . Furthermore, the vif reading frame overlapping the 3' end of the pol codi ng sequence is destroyed by codon optimization. We have also shown that the Gag and Gag-Pol polyproteins can be efficiently expressed from separate tr anscriptional units. This has enabled the removal of a cis-acting viral ele ment, the gag-pol translational frameshift sequence, from the vector/packag ing system and prevents detectable transfer of biologically active sequence s equivalent to the gag-pol gene to transduced cells.