A small hairpin RNA or short hairpin RNA (shRNA) is a sequence of RNA that makes a tight hairpin turn that can be used to silence target gene expression via RNA interference (RNAi). Expression of shRNA in cells is typically accomplished by delivery of plasmids or through viral or bacterial vectors. The promoter choice is essential to achieve robust shRNA expression. At first, polymerase III promoters such as U6 and H1 were used; however, these promoters lack spatial and temporal control. As such, there has been a shift to using polymerase II promoters to regulate expression of shRNA. shRNA is an advantageous mediator of RNAi in that it has a relatively low rate of degradation and turnover.
Expression of shRNA in cells can be obtained by delivery of plasmids or through viral or bacterial vectors. Through transfection vectors to cells to obtain shRNA expression can be accomplished using commercially available reagents in vitro, but not in vivo and limited the utility.
Several challenges typically confront shRNA-based therapeutics. The most significant challenge is delivery. shRNA is typically delivered through use of a vector, and although they are generally efficient, they pose significant safety concerns. In particular, viral based gene therapy approaches have proved dangerous in past clinical trials. Potential oversaturation of RISC is also a problem. If the shRNA is expressed at levels that are too high the cell might not be able to correctly process the endogenous RNA which could cause significant problems. Another challenge is the possibility that the patient might mount an immune response against the therapy. Finally, there might be off-target effects and the shRNA could silence other unintended genes. In developing successful new shRNA-based therapeutics, all of these challenges must be taken into account.