Introductory Review Cellscience Reviews Vol.2 No.4 ISSN 1742-8130

RNA Interference by long dsRNA: A Technical Trick
Reveals An Ancient Force in Gene Regulation

Abstract

Few discoveries have created the kind of buzz that has been generated by RNA interference (RNAi). RNAi is the process by which gene expression is silenced via double-stranded RNA (dsRNA).The phenomenon of RNA-associated gene knockdown was first observed in plants, where introduction of engineered "self-gene" (a transgene encoding for violet color pigmentation) led to unexpected color suppression rather than pigment enhancement. A decade later, the serendipitous discovery by Fire and Mello that long dsRNA was the cause of potent and specific knockdown of gene products in C. elegans, changed the face of gene expression-related studies in eukaryotes. In C. elegans and drosophila, long dsRNA works with high efficacy in knocking down protein expression; it can be introduced in a straightforward fashion and does not appear to non-specifically activate a “global panic response” by the immune system. Unfortunately, in the mammalian systems, exploration of the use of long dsRNA to silence gene expression has been delayed due to concerns about its speculated non-specific effects on innate and adaptive immune systems. Therefore, the shorter versions of long dsRNA that can be obtained by enzymatic cleavage of long dsRNA or chemically synthesized, termed small interfering RNA (siRNA), have become the cynosure of dsRNA-mediated RNAi research. Nevertheless, the increasing number of reports of the temporal transcription of antisense, as well as sense, mRNA of essential proteins suggests that long dsRNA may be generated in vivo and participate in RNAi induction. Together with its astonishing degree of conservation throughout all phyla, the growing list of RNAi’s biologic activities suggests that RNAi is an ancient regulatory mechanism, perhaps part of the original "RNA world" from which DNA and it’s regulators arose. In this review, we briefly describe the history of RNAi discovery and the current model of its mechanisms of action. We also compare the role of both siRNA and long dsRNA (abbreviated as LdsRNA) in knocking down gene expression in mammalian cells and inducing an immune response- both in vitro and in vivo. Finally, we examine the published literature for evidence whether naturally encoded antisense mRNAs in mammals can serve as substrates for RNAi and propose a new model of LdsRNA-mediated RNAi in which endogenous RNA forms non-immunogenic sense: antisense duplexes by a mechanism similar to that seen in retroviral "kissing loop" genomic dimerization.

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