# Viroids, Viruses, and RNA Silencing



## M&M (Jan 20, 2002)

Courtesy of the Co-Cure Mailing List:


> quote:Viroids, Viruses, and RNA SilencingThe small RNA world just got bigger | <mailto:lpray###the-scientist.com>ByLeslie A. Pray PATHOGENICITY MODEL: Viroid replication generates dsRNA intermediates,which are processed by Dicer into 21- to 25-nucleotide siRNAs. These siRNAsare then incorporated into siRNA--ribonuclease complexes (RISC). If thesiRNA sequences significantly match host mRNAs, RISC may target them fordegradation leading to disease symptoms. RISC can also target the viroid,forcing it to evolve and to adopt and maintain an RNA silencing-resistantstructure. (From M.-B. Wang et al., Proc Natl Acad Sci, 101:3275-80, 2004.)There is growing evidence that small RNAs, believed to play an antiviraldefense role in many organisms, may be acting as double agents. In March2004, an international team of scientists reported that viroids, smallinfectious particles of naked RNA, may be employing RNA-silencing machineryto work their damage.1 Several weeks later, another international teamannounced the discovery of microRNAs in the Epstein-Barr virus (EBV),suggesting that miRNA-mediated gene suppression might play a role inanimal-virus pathogenicity.2 Many researchers say this is just the beginning."In the past, all of these small RNAs were totally overlooked because theyare so small in size. If they got any attention at all, they wereconsidered unspecific degradation products. Now, we are starting tounderstand that they represent a sort of cross-talk between viral orviroidal sequences and endogenous genes," says Martin Tabler, Institute ofMolecular Biology and Biotechnology, Crete.EBV, a large DNA virus responsible for mononucleosis and a number of otherdiseases, including Burkitt lymphoma, is tough to beat; it hides from thehuman immune system and infects for life. Rockefeller University'sSebastien Pfeffer, lead author on the EBV study, says that he and hiscollaborators were looking for evidence that animals defend against viralinfection using the same kind of posttranscriptional silencing pathwaysthat plants, insects, and other organisms use. But instead of antiviral RNAactivity, they discovered viral miRNA activity. According to Pfeffer, thedata suggest that miRNAs could be involved in tumor formation and mayexplain how EBV hides so well. They also give scientists reason to look formiRNAs in other viruses. Says Pfeffer: "miRNAs are found in practicallyevery eukaryotic organism. Other viruses could have them too."RNA SILENCING: A WEAPON OF CHOICE? Viroids are single RNA molecules thathave no protective protein coat and do not encode a single protein. Whilenot all viroids inflict damage, some do so severely. A potato spindle tuberviroid (PSTVd) infection can reduce yields by 50% or more. As RicardoFlores of the Valencia Polytechnical University, Spain, explains, "Virusescan induce disease by means of proteins encoded in their genomes, butviroids cannot. So the question is, how?"For years, most researchers believed that the viroids induced disease byinterrupting an unknown host factor (i.e., protein) and thereby disruptingnormal cell functioning. This hypothesis was proposed in the mid-1980's,when scientists identified a region of the viroid genome, the"virulence-modulating" region, whose sequence, when altered, affectedvirulence. But the mechanistic details of this viroid-host interactionremained a mystery. In 2001 it was suggested that, rather than interactingdirectly with the host, perhaps viroids use small regulatory RNAs toinfluence host gene expression.In March 2004, Ming-Bo Wang and colleagues from the Commonwealth Scientificand Industrial Research Organization in Canberra, Australia, provided thefirst experimental evidence suggesting that, similar to EBV, the key toviroid pathogenicity may indeed be RNA silencing, but in this case,siRNA-mediated, not miRNA-mediated. The investigators found that engineeredtomato plants expressing viroid-derived, noninfectious hairpin RNA hadsymptoms mimicking those of viroid infection. Hairpin RNA is a key playerin the RNA silencing pathway: When cleaved by a Dicer enzyme, it generatessiRNAs, which in turn guide RISC complexes to degrade target RNA. In thiscase, the target RNA hasn't been identified, and Wang says this is anecessary next step."This is one of the most exciting things in the field," says Flores. "Butmore evidence is needed before deciding which of the two alternativehypotheses is correct. If you want to prove this hypothesis, you have toidentify which host mRNA is degraded." For Tabler, the results are"surprising and actually a bit puzzling." They raise interesting questionsabout how nuclear-replicating viroids such as PSTVd exploit RNA-silencingmachinery, which is located in the cytoplasm.Future findings aside, both studies nonetheless confirm the ubiquitousnature of small RNAs and point to an additional, pathogenic function forRNA regulatory mechanisms. Says Andrew Ellington, University of Texas:"It's exciting to see the small RNA world sow its oats."Leslie A. Pray (<mailto:lpray###the-scientist.com>lpray###the-scientist.com)References1. M.-B. Wang et al., "On the role of RNA silencing in the pathogenicityand evolution of viroids and viral satellites," Proc Natl Acad Sci,101:3275-80, March 2, 2004.2. S. Pfeffer et al., "Identification of virus-encoded microRNAs," Science,304:734-6, April 30, 2004.


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