Modulation of hepatitis C virus RNA by a liver‐specific microRNA

MicroRNAs are small RNAs that regulate mRNA expression in a variety of organisms. MicroRNA‐122 (mir‐122) is specifically expressed and highly abundant in the human liver and in cultured liver cells that support replication of HCV. We have discovered that sequestration of miR‐122 by 2′O‐methylated antisense oligomers in cultured liver cells that express full‐length HCV RNAs resulted in dramatic loss of viral RNA. Conversely, HCV RNA levels increased when the intracellular levels of miR‐122 were elevated by ectopic expression of miR‐122. A genetic interaction between miR‐122 and the 5′ noncoding region of HCV was revealed by mutational analyses of the predicted microRNA binding site and ectopic expression of miR‐122 molecules that contained compensatory mutations. Studies with replication‐defective viral RNAs demonstrated that miR‐122 did not affect mRNA translation or mRNA stability, suggesting that miR‐122 affects mRNA abundance likely by increasing RNA replication. One of the predicted mRNA targets for miR‐122 is CAT‐1, encoding the cationic amino acid transporter. Thus, we examined whether CAT‐1 protein levels were dependent on the intracellular levels of miR‐122 and noted that CAT‐1 levels increased when miR‐122 was diminished. Overall, these data argue for roles of miR‐122 in HCV pathogenesis and proliferation of liver cells and, thus, suggest miR‐122 to be a potential target for antiviral therapy.