Effective RNAi-mediated gene silencing without interruption of the endogenous microRNA pathway

Abstract

A paper in Nature last year appeared to spell trouble for the prospects of RNA interference-mediated silencing as gene therapy. It showed that large doses of short hairpin RNA disrupted the microRNA pathway in mice, with fatal results. Now a new study suggests that it is too soon to write off RNA therapy. A different type of inhibitory RNA, small interfering RNAs (siRNAs), can be administered to mice without toxicity. The activity of liver microRNAs remains unaffected by siRNAs, despite 80% silencing of target genes in mouse and hamster liver cells. Recent work found that overexpression of short hairpin RNAs in cells could lead to mortality in mice, this was attributed to interference with the cellular machinery that makes miRNAs. This raised the issue of whether RNA interference-mediated silencing would be therapeutically possible. This paper shows that small interfering RNAs can be systemically administered and effectively suppress gene expression without compromising the activity of various liver miRNAs. Systemic administration of synthetic small interfering RNAs (siRNAs) effectively silences hepatocyte gene expression in rodents and primates1,2,3. Whether or not in vivo gene silencing by synthetic siRNA can disrupt the endogenous microRNA (miRNA) pathway remains to be addressed. Here we show that effective target-gene silencing in the mouse and hamster liver can be achieved by systemic administration of synthetic siRNA without any demonstrable effect on miRNA levels or activity. Indeed, siRNA targeting two hepatocyte-specific genes (apolipoprotein B and factor VII) that achieved efficient (∼80%) silencing of messenger RNA transcripts and a third irrelevant siRNA control were administered to mice without significant changes in the levels of three hepatocyte-expressed miRNAs (miR-122, miR-16 and let-7a) or an effect on miRNA activity. Moreover, multiple administrations of an siRNA targeting the hepatocyte-expressed gene Scap in hamsters achieved long-term mRNA silencing without significant changes in miR-122 levels. This study advances the use of siRNAs as safe and effective tools to silence gene transcripts in animal studies, and supports the continued advancement of RNA interference therapeutics using synthetic siRNA.