miR‐122 Promotion of the hepatitis C virus life cycle: sound in the silence

Advanced Review

Joyce A. Wilson, Adam Huys

Published Online: Jul 23 2013

DOI: 10.1002/wrna.1186

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The unusual role for miR‐122 in promoting the hepatitis C virus (HCV) life cycle was first identified in 2005, but its mechanism of action remains uncharacterized. The virus appears to use the microRNA (miRNA) in a way that is opposed to that of normal miRNAs. Instead of interacting with sequences in the 3′‐untranslated region (UTR), miR‐122 binds to two sites in the 5′‐UTR, and instead of silencing gene expression or promoting the degradation of the viral RNA, it stabilizes the genome and potently augments the efficiency by which HCV RNA accumulates in infected cells. This review discusses the current knowledge and models for the mechanism by which miR‐122 promotes the HCV life cycle. Annealing of miR‐122 to the HCV genome requires particular base pairing, stimulates translation, and stabilizes the viral genome by blocking degradation by host exonucleases, but these functions are unlikely to be the whole story. We will discuss other possible functions for miR‐122, the stages of the HCV life cycle at which miR‐122 may influence HCV, and other related viruses that may be similarly regulated by miR‐122. Despite our lack of detailed mechanistic information, antagonism of miR‐122 is emerging as a powerful method to inhibit HCV infections, and unique to other HCV treatment strategies does not, thus far, appear to induce emergence of escape mutants. Used alone or in combination with other antiviral drugs, miR‐122 antagonists could be useful to both inhibit the virus and provide selective pressure to inhibit the development of resistance. WIREs RNA 2013, 4:665–676. doi: 10.1002/wrna.1186 This article is categorized under: RNA Turnover and Surveillance > Regulation of RNA Stability Regulatory RNAs/RNAi/Riboswitches > RNAi: Mechanisms of Action Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA in Disease and Development > RNA in Disease

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Biogenesis of microRNA (miRNA) and mechanism miRNA‐mediated messenger RNA (mRNA) silencing. Cellular miRNAs are processed from primary miRNA (pri‐miRNA) to pre‐miRNA hairpins in the nucleus through cleavage by Drosha and DGCR8. After transit to the cytoplasm via Exportin 5, the pre‐miRNAs are processed into mature miRNAs by Dicer and its binding partner TRBP. Dicer delivers the miRNA to an Argonaute protein (Ago), which unwinds and discards the passenger strand, retaining the mature miRNA guide strand. The mature miRNA directs Ago to sites on the 3′‐untranslated region (UTR) of target mRNAs through imperfect sequence complementarity. Ago proteins associate with several proteins that mediate silencing, including GW182. GW182 associates with the poly‐A‐binding protein (PABP) and proteins in the RNA degradation pathway to inhibit translation and promote mRNA degradation. mRNA degradation can occur in cellular aggregations called P‐bodies.

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miR‐122 and the hepatitis C virus (HCV) life. The HCV life cycle can be divided into entry, uncoating, replication, assembly, and release. miR‐122 association with the HCV genome likely functions following entry, fusion, and virion uncoating to stabilize the viral genomic RNA upon release into the cytoplasm and promote primary genome translation and subsequent assembly of replication complexes. Genome translation is in association with cellular membranes and generates a polyprotein that is processed cotranslationally and posttranslationally by host and viral proteases. Viral nonstructural proteins NS3, NS4A, NS4B, NS5A, and NS5B assemble into a replication complex that resides inside membranous vesicles derived from the endoplasmic reticulum and are the sites of HCV genome replication. Following genome replication, nascent positive‐strand RNA genomes are transported to sites of virion assembly, and miR‐122 may stabilize the RNA genome during transit (a). The core protein (orange circles) resides on the surface of lipid droplets and from there, surrounds the nascent HCV genome to initiate virion assembly. The core‐RNA complex then buds through the membrane at sites of virion envelope proteins E1 and E2 (shown in blue and green). HCV virion production is linked to lipid synthesis, and infectious virions associate with very low‐density lipoproteins. However, not all newly synthesized HCV virion RNA is assembled into virions. Nascent RNA can also return to the cytoplasm to be used for additional protein expression and may be stabilized by miR‐122 during transit to cytoplasmic sites of translation (b). Alternatively, nascent viral RNA can remain at sites of replication as templates for synthesis of additional genomic RNA, and miR‐122 could regulate the fates of newly synthesized HCV genomes (c).

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Model for miR‐122 processing, annealing, and stabilizing of hepatitis C virus (HCV) genomic RNA. Pre‐miR‐122 is processed by Dicer and TRBP and loaded into Ago2. Ago2 unwinds the mature miR‐122 duplex and delivers the mature microRNA (miRNA) to the annealing sites on the HCV 5′‐untranslated region (UTR). The annealing pattern between the HCV and miR‐122 sequences involves nucleotides within the seed binding sequences (bold) and outside of the seed. The ribonucleotide/protein complex protects the 5′ terminus from degradation by Xrn‐1 and possibly from dephosphorylation by an as yet unidentified pyrophosphatase.

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