ShRNA

A short hairpin RNA or small hairpin RNA (shRNA/Hairpin Vector) is an artificial RNA molecule with 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. shRNA is an advantageous mediator of RNAi in that it has a relatively low rate of degradation and turnover. However, it requires use of an expression vector, which can pose safety concerns.

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 shRNA expression.

Expression of shRNA in cells can be obtained by delivery of plasmids or through viral or bacterial vectors.

Delivery of plasmids to cells through transfection to obtain shRNA expression can be accomplished using commercially available reagents in vitro. However, this method is not applicable in vivo and thus has limited utility.

Use of a bacterial vector to obtain shRNA expression in cells is a relatively recent approach. It builds off research showing that recombinant Escherichia coli, containing a plasmid with shRNA, fed to mice can knock-down target gene expression in the intestinal epithelium. This approach was used in 2012 in clinical trials to try to treat patients with Familial Adenomatous Polyposis.

A variety of viral vectors can be used to obtain shRNA expression in cells including adeno-associated viruses (AAVs), adenoviruses, and lentiviruses. With adeno-associated viruses and adenoviruses, the genomes remain episomal. This is advantageous as insertional mutagenesis is avoided. It is disadvantageous in that the progeny of the cell will lose the virus quickly through cell division unless the cell divides very slowly. AAVs differ from adenoviruses in that the viral genes have been removed and they have diminished packing capacity. Lentiviruses integrate into sections of transcriptionally active chromatin and are thus passed on to progeny cells. With this approach there is increased risk of insertional mutagenesis; however, the risk can be reduced by using an integrase-deficient lentivirus.

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