Promoting gene therapy: expression systems for transgenes and posttranscriptional gene silencing

In this review, we have discussed expression systems employed for both transgene expression and post-transcriptional gene silencing. Although constitutive expression systems can function well for gene therapy protocols for simple Mendelian disorders, other protocols may require carefully designed systems that either ensure targeting to cells of interest or alternatively, transcription that is tightly regulated and conditionally activated. Indeed, it is of utmost importance to regulate expression of potentially immunogenic proteins, such as those encoded by suicide genes, to limit their immunogenic potential. Meanwhile, RNAi may be employed for therapeutic silencing of aberrant cellular or viral genes, but the potential for off-target effects due to activation of double-stranded RNA response pathways or undesired targeting of partial complementary sequences may necessitate carefully controlled and regulated expression of short hairpin RNAs. As we exhaust the current repertoire of simple promoter systems for transgene expression or for RNAi, we need to develop newer systems that combine the high activity of viral promoters with cell type specificity or conditionally active cis-elements. Novel combinations of promoter elements should be one approach for creating such regulated promoters.