Dual-regulated gene expression in mammalian cells, a novel approach to gene therapy

A variety of heterologous mammalian gene regulation systems are currently available for use in human gene therapy and tissue engineering. While individual gene regulation concepts vary significantly in key characteristics (regulation profiles, basal expression levels, human compatibility, immunogenicity, pharmacokinetics and bioavailability of the inducing agent), most of them are sufficiently elaborated to be considered for preclinical research and clinical gene therapy trials. As forefront basic research describes the cell-cycle, differentiation and apoptosis regulatory networks in mammalian cells as an increasingly complex highly interconnected globular regulon, it becomes apparent that many human diseases may originate from the slightest expression imbalances of key regulatory genes. We believe that successful gene therapy of such diseases will involve complex multi-(level) regulated multigene interventions based on a combination of several human-compatible heterologous gene regulation systems which enable optimal integration of therapeutic interventions into the cellular regulon as well as well-balanced expression of several therapeutic transgenes. Although such multiregulated multigene expression scenarios are still a vision, the following are already a scientific reality with cultured cells: (i) combination of two human compatible gene regulation systems to enable dual-regulated expression technology for independent adjustment of two different gene activities, (ii) regulatory cascades for muli-level regulated multigene interventions, and (iii) dual-autoregulated expression configurations for one-step installation of multi-level or reciprocal regulation of heterologous gene expression. This review summarizes pioneering efforts to establish dual-regulated expression technology, a strategy to combine more than one heterologous gene regulation system, to achieve difficult-to-attain cell phenotypes relevant for gene therapy, tissue engineering and basic research.