REVIEW ON RNAi
Since its discovery in 1998, RNA interference (RNAi) has heralded the advent of novel tools for biological research and drug dis discovery. This exciting new technology is emerging as a powerful modality for battling s om e of the most notoriously challenging viral clinical targets such as hepatitis C virus (HCV) and hum an im immunodeficiency virus (HIV). However, several critical is s ues associated with this novel technology m us t be resolved before it can progress to testing in hum an clinical trials , and these have been the target of intensive research in recent years .
RNAi: An Antiviral Defense System in Insects:
Multicellular organisms evolved sophisticated defense system s to confer protection agains t pathogens . An important characteris tic of these im m une s ys tem s is their ability to act both locally at the s ite of infection and at dis tal uninfected locations (Baulcom be, 2004; Dorner and Radbruch, 2007; Roitt et al., 2001; Voinnet, 2005). Ins ects rely on m ultiple im m une res pons es to com bat infection; one of them is RNA interference (RNAi). Here we review the current knowledge on the general m echanis m of RNAi and s um m arize what is known about the antiviral role of RNAi in the ins ect m odel Dros ophila m elanogas ter. We als o dis cus s the s trategies evolved by virus es to s uppres s the RNAi res pons e. Finally, we briefly des cribe the RNAi m echanis m in other ins ects of econom ical and/or health relevance.
RNA Silencing in Plants and the Role of Viral Suppressors:
The term RNA s ilencing refers to s everal pathways pres ent in eukaryotic organis m s that lead to the s equence s pecific elim ination or functional blocking of RNAs with hom ology to double s tranded RNAs (ds RNAs ) that have previous ly triggered the m echanis m . Bes ides playing im portant roles in developm ental control, RNA s ilencing form s part of the defence agains t virus es in plants , acting as a potent antiviral m echanis m . To es cape from the RNA s ilencing-bas ed defence, m os t plant virus es m ake us e of different s trategies , the m os t com m on relying in the action of viral proteins with the capacity to s uppres s RNA s ilencing. The characterization of thes e viral s uppres s ors is providing us eful ins ights to unders tand how RNA s ilencing works , revealing com ponents and s teps in the s ilencing pathways .
The Properties and Roles of Virus-encoded MicroRNAs:
The dis covery that virus es could encode m icro (m i)RNAs , s im ilarly to the eukaryotic organis m s they infect, has opened new pers pectives in the s tudy of hos t-virus interactions . Thes e s m all regulatory RNAs , which are critically involved in an ever-increas ing num ber of biological proces s es , have revolutionized the way we us ed to s ee gene regulation. Som e m am m alian virus es , m ainly from the herpes virus and polyom avirus fam ilies , have hijacked this m echanis m in order to help them achieve the infection of their hos t. In this chapter, we will pres ent the divers ity of known viral m iRNAs , their s pecific properties , their viral and cellular targets and the roles they play during the cours e of infection. We will s ee that m ore and m ore it appears that virally encoded m iRNAs s eem to be critically involved in every s tep of the virus life cycle.
Virus-encoded Suppressors of RNA Silencing and the Role of Cellular miRNAs in Mammalian Antiviral Immune Responses:
Sm all RNA-directed s ilencing m echanis m s play im portant roles in the regulation of eukaryotic gene expres s ion. In plants , ins ects , nem atodes and fungi RNA s ilencing m echanis m s are als o involved in innate antiviral defence res pons es . To counter antiviral RNA s ilencing, virus es from plants , ins ects and fungi encode RNA s ilencing s uppres s ors (RSSs ). Recent s tudies s ugges t that RNA s ilencing in m am m als , or RNA interference (RNAi), is als o involved in antiviral res pons es . In particular, there is increas ing evidence that cellular regulatory m icroRNAs (m iRNAs ) have a function in res tricting virus replication in m am m alian cells . Sim ilar to plant and ins ect virus es , s everal m am m alian virus es encode RSS factors that inhibit the RNAi m echanis m . Several of thes e s uppres s ors are m ultifunctional proteins that were previous ly s hown to block innate antiviral
im m une res pons es involving the interferon (IFN) pathway. Here we s um m arize the current data on m am m alian virus -encoded RSS factors . In addition, different as pects of antiviral RNAi and the role of cellular m iRNAs in res tricting virus replication in m am m alian cells are dis cus s ed.
RNAi Gene Therapy to Control HIV-1 Infection:
RNA interference (RNAi) was dis covered as cellular gene regulation m echanis m in 1998, but s everal RNAi-bas ed applications for gene s ilencing have already m ade it into clinical trials . We will dis cus s RNAi approaches to target pathogenic hum an virus es caus ing acute or chronic infections , with a focus on pers is tent HIV-1 infection that would m os t likely require an RNAi-bas ed gene therapy. Virus es like HIV-1 are particularly difficult targets for RNAi-attack becaus e they are es cape-prone, which requires com binatorial RNAi s trategies to prevent viral es cape. The future of antiviral RNAi therapeutics is very prom is ing, but it rem ains of critical im portance to include m any controls in pre-clinical tes t m odels to unequivocally dem ons trate s equence-s pecific action of the RNAi inducers
Advances in the Use of RNAi to Treat Chronic Hepatitis B Virus Infection:
Chronic infection with the hepatitis B virus (HBV) occurs in approxim ately 6% of the world’s population and carriers of the virus are at ris k for com plicating hepatocellular carcinom a (HCC) and cirrhos is . Although effective vaccination is available, it is prophylactic and of little us e to individuals who are already infected with the virus . Furtherm ore, current treatm ent options have lim ited efficacy and chronic HBV infection is likely to be a s ignificant global m edical problem for m any years to com e. Silencing HBV gene expres s ion by harnes s ing RNA interference (RNAi) pres ents an attractive option for developm ent of novel and effective anti HBV agents . Num erous s tudies have reported highly s ucces s ful s uppres s ion of viral replication, which bodes well for em ploying this approach to counter HBV infection. However, des pite s ignificant and rapid progres s , further refinem ent of exis ting technologies is neces s ary before clinical application of RNAi-bas ed HBV therapies is realis ed. Im provem ent of delivery efficiency, dos e regulation, lim iting of off target effects and preventing reactivation of viral replication are s om e of the hurdles that need to be overcom e. Nevertheles s , the vas t potential of RNAi-bas ed therapeutics will continue to drive innovative res earch, and this prom is es to s urm ount the obs tacles that face this exciting field.
RNAi Applications to Defeat Respiratory Viral Infections:
Pathogenic respiratory virus es , exem plified by respiratory syncytial virus (RSV), influenza (Flu) and parainfluenza virus (PIV), are m ajor disease agents that kill m illions of hum ans worldwide. Other respiratory virus es that are also potential agents of bioterroris m include highly pathogenic avian flu virus , SARS coronavirus , and the henipavirus es . respiratory infection by RSV and PIV is the m o t prevalent caus e of pediatric hos pitalization in indus trialized Wes tern nations . In the US alone, it leads to about 100,000 adm is s ions per year at a cos t of about $300 M. The lack of a reliable vaccine or antiviral agains t thes e virus es , in part due to the high m utation rate of the viral RNA genom es , has led to the adoption of the novel RNA interference (RNAi) s trategy. This review s um m arizes s ignificant recent progres s in the developm ent of RNAi therapeutics to defeat thes e viral res piratory dis eas es