The tetracycline (Tet)-dependent regulatory system has been widely used for controlling gene expression. The Tet-on version of the system, in which the reverse Tet-responsive transcriptional activator (rtTA) is positively regulated by Tet or its analogs, such as doxycycline (Dox), is of potential utility for gene therapy applications in humans. However, rtTA may display a high basal activity, especially when delivered in vivo by using episomal vectors such as plasmids. Two novel Dox-inducible activators, called rtTA2S-S2 and rtTA2S-M2, which have a significantly lower basal activity than rtTA in stably transfected cell lines, have been described. In this study we tested the capability of these trans-activators to control expression of mouse erythropoietin (mEpo) and to modulate hematocrit (Hct) increase in vivo on delivery of plasmids into quadriceps muscles of adult mice by DNA electroinjection. Both rtTA2S-M2 and rtTA2S-S2 displayed a considerably lower background activity and higher window of induction than rtTA in vivo. Moreover, a stringent control of mEpo gene expression and Hct levels in the absence of any background activity was maintained over a 10-month period by injecting as little as 1 μg of a single plasmid containing the rtTA2S-S2 expression cassette and the Tet-responsive mEpo cDNA. This constitutes the first report of a stringent ligand-dependent control of gene expression in vivo obtained by delivering a single plasmid encoding both the trans-activator and the regulated gene. Notably, the rtTA2S-S2-based system was induced by oral doses of doxycycline comparable to those normally used in clinical practice in humans.
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