TAXI/UAS: A Molecular Switch to Control Expression of GenesIn Vivo

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Abstract
Numerous therapies and biological questions could be addressed in mammals by the application of a molecular switch that would allow physicians and/or investigators to turn individual genes on or off during the lifetime of the organism. We have constructed such a switch, composed of three elements: (i) an inducible promoter that is normally absent from mammalian genomes; (ii) a receptor that, when it is bound to an inducer drug, specifically activates transcription from the inducible promoter; and (iii) inducer drugs, such as RU486, whose pharmacological properties in humans and several mammalian species including mouse have been well studied. The molecular switch is functional in transiently and stably transfected cells. Importantly, both the total output and the induction levels of the reporter gene can be finely tuned, with induction levels of over 100-fold being readily attained. Finally, we demonstrate that the molecular switch can be used to regulate a mouse transgene using a gene therapy paradigm. The specificity of the system suggests that it should be useful in the analysis of gene function in transgenic animals and in the design of strategies for human gene therapy. We describe here a molecular switch to control gene expression in vivo. The system is composed of (i) an inducible promoter (UAS) that is normally absent from mammalian genomes, (ii) a synthetic steroid receptor (TAXI) that activates transcription from the UAS promoter when it is bound to an inducer drug, and (iii) inducer drugs of the RU486 type, whose pharmacological properties are well characterized. We show that the TAXI/UAS system is very efficient in transiently as well as stably transfected cells. We also demonstrate that it can be used to regulate the expression of a therapeutic transgene in mice after transplantation of ex vivo-modified cells. This study establishes that the TAXI/UAS system is both functional and specific, and suggests that it will be useful in the design of gene therapy protocols where tight control of transgene expression is required.