Success of gene replacement therapy depends on long-term, high level expression of the transgene. Gene therapy vectors incorporating a promoter of a constitutively active eukaryotic gene may allow long-term expression in vivo, but the expression level may be insufficient for therapeutic effects. To enhance transcription from eukaryotic promoters, a strategy with dicistronic vectors encoding the therapeutic gene of interest together with a transcription factor that binds and activates the promoter was tested. Expression vectors for the chimeric tet repressor/VP16 transcription factor (tTA) driven by the human beta-actin promoter were constructed, and tandem tet operators were inserted within the promoter. This arrangement significantly enhanced expression of G-CSF in fibroblasts to higher levels than the immediate/early CMV promoter. Stably transfected fibroblast clones produced up to 2.4 microg G-CSF per 10(6) cells x 24 h. After injection of genetically engineered cells into SCID mice, the enhanced beta-actin promoter construct resulted in marked leukocytosis, whereas the unmodified promoter had only a marginal therapeutic effect. Transcription factor-enhanced, feed-back-activated human promoters may thus achieve higher expression levels than viral control elements, and may be advantageous for gene therapy due to high constitutive activity in vivo.

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