Systems providing inducible gene regulation in mammalian cells have been of great experimental value. To date most systems regulate gene expression at the level of transcription initiation. This requires a specialized promoter that is activated by an exogenous transactivator in response to addition of a small molecule. Although these systems have been quite useful, there are some drawbacks. These systems necessitate not only the introduction of the therapeutic gene but also the transactivator gene. They are also restricted to utilizing the specialized promoter obviating the use of a potentially more relevant promoter such as a tissue-specific promoter. Furthermore, high background expression is often a problem. Experimentally this can be overcome by identifying and expanding cell clones with low backgrounds and high levels of induction. However, for clinical applications, this in not typically a viable option indicating that other approaches for achieving regulated expression should be explored. There is a large body of evidence showing that cells have developed a mechanism to discriminate between premature termination codons (PTCs) and normal stop codons during translation. It was theorized that a cell's ability to make this distinction could be exploited to develop drugs to promote readthrough at a PTC but not at a normal codon. Such drugs could then be used to treat genetic disorders caused by PTCs. Already, small molecules have been developed with this ability, and one has been shown to display clinical efficacy in phase 2 clinical trials while being well tolerated. Here it is hypothesized that the use of drugs specific for PTC readthrough can be combined with gene transfer technology to develop an inducible expression system for use in vivo. The idea is to incorporate a PTC into a gene of interest so that active protein will only be made after administration of a readthrough drug. This system should: 1) afford low levels of background expression with effective levels of induction, 2) require that only the therapeutic gene be transducer, and 3) provide flexibility in selection of the promoter used to drive expression. More specifically, inducible regulation of PTC containing genes will be analyzed using a series of PTC readthrough drugs both in vitro and in vivo after transduction with HIV-2 derived vectors. Regulated expression in vivo will rely heavily upon whole-animal imaging to allow real-time monitoring of expression. Attention will focus upon controlling expression of GDNF since there is mounting evidence that GDNF expression can be very beneficial for treating Parkinson's disease. Although experiments delineated in this proposal use HIV-2 vectors in the context of a Parkinson's disease model, this approach should have broad utility for a variety of gene transfer systems and should be applicable to most disorders that can benefit from regulated gene expression. Public Health Relevance: There are diseases where being able to switch the production of a therapeutic agent on and off is very beneficial. This project aims at developing such a switch.
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