Key Protein Deficiency Leaves Mice Hypersusceptible to Viral Infection

By Eric Sauter

Dicer is an enzyme that plays a critical role in the process known as RNA interference (RNAi), a uniquely efficient way to silence gene expression in a number of different cell types. The aptly named protein—it was given the name by a Long Island laboratory graduate student—slices and dices double-stranded RNA into smaller bits called small or short interfering RNA (siRNA) and microRNA (miRNA). In plants, worms, and insects, these fragmentary bits of nucleotide in turn attach themselves to various genes and inhibit replication, in particular, the replication of viruses.

In mammals, however, such RNA interferences have not been clearly detected. Instead, double-strand RNAs are detected by specific receptors (such as Toll-like receptor 3), which leads to production of interferon. In turn, interferon exerts its own potent anti-viral influence in mammals.

Now, in a new study published in the July issue of the journal Immunity (Volume 26, Issue 7), research led by Scripps Research Institute scientists has demonstrated for the first time that Dicer can play a role in mammal immunology. The scientists showed that Dicer-deficient mice are significantly more susceptible—hypersusceptible, in fact—to the vesicular stomatitis virus (VSV), a widely studied virus with symptoms similar to hoof and mouth disease.

"Our study provides solid genetic evidence to support the hypothesis that host miRNA can influence viral growth in mammals," says Jiahuai Han, the Scripps Research scientist whose laboratory led the study, "and supports the notion that miRNA represents another layer of the complexity of virus-host interaction. We found that cellular or host miRNA could target VSV large protein and phosphoprotein genes, and that the lack of these miRNAs was responsible for increased virus replication. This suggests that host miRNA plays a major role in interactions with viruses in mammals, possibly in deciding the tissue preference of viruses and in determining the balance between host and virus ascendancy."

These micro-molecules—there are hundreds of different miRNAs—are built from a few dozen nucleotides, compared to human messenger RNAs that consist of several thousand nucleotides. There is a single Dicer gene in mammals, the study noted, and this gene presumably mediates the processing of all miRNA.

It has been reported, for example, that human miR32 limits production of the primate foamy virus type 1(PFV-1), while miR122 was found to be a positive regulator of the hepatitis C virus. Some mammalian viruses even have developed their own miRNAs which they use to evade detection by the host immune response or target cellular miRNAs to interfere with anti-viral host defense. 

However, the new study noted, in vitro studies involving Dicer-deficient macrophages demonstrated that impairment of the Dicer enzyme affected the replication of some viruses but not others, suggesting that Dicer-mediated antiviral RNA interference is not a general antiviral mechanism in mammals. 

While the targeting of specific viruses by individual miRNAs could simply be fortuitous, the study said, it might also reflect a dynamic evolutionary struggle for supremacy between virus and host. It's even possible that some miRNAs might have been favored by evolution to suppress a virus by targeting indispensable parts of the viral genome. On the other hand, viruses such as hepatitis C might have evolved their own homologies (similarities of function) to host miRNAs in order to permit the host to undertake specific biological processes.

"Since most viruses, especially RNA viruses like VSV, have a high mutation rate, they should have adapted to the host by changing their targeted sequences," says Motoyuki Otsuka, the first author of the study. "But because viral suppression by cellular miRNAs is quite modest, compared with that of IFN, the virus may be using the cellular miRNAs to control the quantity of viral production as a way to evade host immune responses."

Raising New Questions

The new study raises some interesting and as yet unanswered questions. For example, while increased virus production caused by the Dicer deficiency could have been the direct result of little or no production of virus-derived siRNAs or host miRNAs that target viral messenger RNAs, it might also have been the result of ineffective processing of viral miRNA(s) required for completing the virus life cycle. It might also have been an indirect effect of low amounts of host miRNA expression that somehow affected proteins involved in viral replication.

The possibility also exists that the relationship between host miRNAs and viruses varies so significantly from virus to virus as to virtually eliminate any single overarching mechanism. Although the study showed miRNA-related VSV replication suppression in mice, further research will be required to understand the full role of miRNAs in mammalian viral infection, Otsuka says.

The study also raises the possibility of potential clinical applications, since some miRNAs have been implicated in certain cancers. Although miR24 and 93 are common between humans and mice, human VSV infection is quite rare and very mild, so any direct human clinical application may have to wait for future studies to determine if similar phenomena occur with human pathogens.

"It may be difficult to apply our results directly to cancer therapy," Otsuka says. "But since many cancers are known to be related to chronic viral infection, if effective miRNA mimics or inhibitors, coupled with some efficient delivery methods, are developed, miRNA-related therapy may become a promising option to restrict viral infection and related oncogenesis."

Other authors of the study, Hypersusceptibility to Vesicular Stomatitis Virus Infection in Dicer1-Deficient Mice Due to Impaired miR24 and miR93 Expression include Philippe Georgel, Liguo New, Jianming Chen, Johann Mols, Young Jun Kang, Zhengfan Jiang, Xin Du, Ryan Cook, and Bruce Beutler of The Scripps Research Institute; Qing Jing of the Institute of Health Sciences, Shanghai Institutes for Biological Sciences, Shanghai, China; and Subash C. Das, and Asit K. Pattnaik, Department of Veterinary and Biomedical Sciences and Nebraska Center for Virology, University of Nebraska. See Immunity at http://www.immunity.com/content/article/abstract?uid=PIIS1074761307003251.

This study was supported by National Institutes of Health.

Send comments to: mikaono[at]scripps.edu

"Our study provides solid genetic evidence to support the hypothesis that host miRNA can influence viral growth in mammals."

—Jiahuai Han