By Doug Macron
Researchers at the University of North Carolina at Chapel Hill this week published data revealing new details on the interaction between hepatitis C and the liver-specific microRNA miR-122, showing that the virus uses the Argonaute 2 protein to stabilize its RNA and maintain infection.
"The hepatitis C virus has done two very interesting things with miR-122," senior investigator Stanley Lemon said in a statement. "First, it has evolved a unique relationship with a key regulator, since miR-122 represents about half of all microRNAs present in the liver. Second, the virus has usurped a process that usually down-regulates gene expression to up-regulate the stability of its RNA and expression of viral proteins needed for its lifecycle.
“It's a classic example of how viruses subvert normally beneficial functions of the cell to their own nefarious purposes," he said.
MiR-122 is highly abundant in the liver and has been shown to be a necessary host factor for HCV replication. Specifically, it binds to the 5' UTR of the positive-strand HCV genome, “stimulating viral protein expression and promoting viral replication,” the UNC team wrote in the Proceedings of the National Academy of Sciences this week.
Given this role, miR-122 has become a key target for a number of groups looking to develop antagonists of the small, non-coding RNA as treatments for HCV. Such firms include Regulus Therapeutics, which is working with GlaxoSmithKline (GSN 2/25/2010); and Santaris Pharma, which has a compound in phase II development (GSN 10/6/2011).
Still, the exact manner in which miR-122 aided HCV replication remained poorly understood.
While “there has been some speculation that miR-122 might promote genome amplification and viral protein expression by physically stabilizing HCV RNA, previous experimental results suggest this is not the case and that miR-122 does not enhance RNA stability,” the UNC investigators wrote in PNAS.
Working with researchers at the University of Colorado School of Medicine, UNC's Stanley Lemon and colleagues discovered that miR-122 binds to HCV RNA in association with Ago2, which slows decay of the viral genome in infected cells — a heretofore unrecognized role for miRNAs.
Notably, the stabilizing action of miR-122 “does not require the target HCV RNA to be capable of translation and thus does not result from increased ribosomal loading,” according to the PNAS paper. “Stabilization also does not require the RNA to be replication competent.”
Lemon and his team also found that a non-methylated 5' cap analog can be a functional substitute for miR-122, suggesting that the RISC-like complex recruited by the miRNA is “likely to act by protecting the RNA from 5' exonuclease.
“Whether this occurs simply as a result of physically masking the 5' end of the viral RNA from 5' exonuclease attack, or whether Ago2 plays a more complex role by influencing the association of HCV RNA with P bodies, sites of mRNA degradation and storage, remains to be determined,” they noted.
Overall, the mechanism by which miR-122 stabilizes HCV RNA is distinct from the up-regulation of translation by miRNAs targeting AU-rich elements in the 3' UTRs of certain mRNAs, according to the paper. However, it remains to be proven whether the protein composition of the RISC-like complex recruited to the 5' UTR by miR-122 is significantly different from miRNA-induced RISC complexes involved in translational repression.
Lemon and his team concluded that miR-122's stabilization of HCV RNA is likely responsible for the miR-122-induced enhancement of HCV translation and is probably a “key factor in the promotion of viral replication by miR-122.
“A recombinant HCV in which the U3 RNA sequence was inserted in lieu of the S1 binding site in the 5′ UTR was found recently to be less dependent upon miR-122 for replication, possibly because the U3 sequence stabilized the RNA, much as a 5′ cap did,” they added. “Nonetheless, it would not be surprising to find that miR-122 has other functions in the viral life cycle in addition to its role in stabilizing the viral RNA genome, perhaps in the initiation of viral RNA synthesis.”
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