Inhibition of Viruses by RNAi

Viruses are both inducers and targets of RNA silencing in plants (Vance and Vaucheret 2001). The antiviral capacity of RNA silencing has been used as a tool to generate virus resistance in plants (Lindbo and Dougherty 1992; Smith et al. 2000; Waterhouse et al. 1998). RNAi technology is currently being used to inhibit viral replication in animal cells. Promising results have been obtained with RNAi against several animal viruses both in in vitro and in vivo settings.

The first demonstration of RNAi-mediated inhibition of a human pathogenic virus was reported by Bitko and Barik in 2001 (Bitko and Barik 2001). These authors reported a tenfold inhibition of human respiratory syncytial virus (HRSV) replication in vitro using nanomolar concentrations of synthetic siR-NAs that targeted the viral polymerase subunit P and the fusion protein F. Currently, many other studies have described RNAi-mediated inhibition of a large variety of viruses. RNAi-mediated inhibition of HIV-1 has received much attention (see below, Tables 1 and 2). In addition, 17 different RNA viruses, and 10 different DNA viruses have been efficiently targeted by RNAi (Tables 3 and 4). These include important human pathogens such as hepatitis C virus (HCV), dengue (DEN) virus, severe acute respiratory syndrome (SARS) coronavirus, poliovirus, influenza A virus, hepatitis D virus (HDV), human rhinovirus-16 (HRV-16), hepatitis B virus (HBV), herpes simplex virus type-1 (HSV-1), human papillomavirus (HPV), JC virus (JCV), Epstein-Barr virus (EBV), and human cytomegalovirus (HCMV). Other viruses listed in Tables 1 and 4 are: enterovirus 71 (EV71), Semliki Forest virus (SFV), rhesus rotavirus (RRV), flock house virus (FHV), Rous sarcoma virus (RSV), porcine endogenous retrovirus (PERV), foot-and-mouth disease virus (FMDV), murine herpesvirus 68

(MHV68), Orgyia pseudotsugata M nucleopolyhedrovirus (OpMNPV), Auto-grapha californica nucleopolyhedrovirus (AcNPV), Microplitis demolitor bra-covirus (MdBV).

Initially, the standard method to induce RNAi towards viruses in mammalian cells was transfection of synthetic siRNAs corresponding to viral sequences shortly before or after a viral challenge. Currently, transient transfection of plasmids that express antiviral shRNAs is also commonly used. Both strategies can result in potent, albeit temporary inhibition of virus replication. In order to obtain long-term virus resistance, researchers have turned to a combined RNAi/gene therapy approach. In this approach, lenti-, retro-or adeno-associated virus (AAV) vectors are used to stably transduce cells with constructs expressing shRNA, resulting in viral resistance. We provide a complete overview of these antiviral studies in Tables 1-4 and discuss the possibility to develop RNAi-based antiviral therapies. The focus will be on HIV-1, and a few RNA and DNA viruses.

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