HITS-CLIP and PAR-CLIP advance viral miRNA targetome analysis.

MiRNAs regulate gene expression by binding predominantly to the 3' untranslated region (UTR) of target transcripts to prevent their translation and/or induce target degradation. In addition to the more than 1200 human miRNAs, human DNA tumor viruses such as Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) encode miRNAs. Target predictions indicate that each miRNA targets hundreds of transcripts, many of which are regulated by multiple miRNAs. Thus, target identification is a big challenge for the field. Most methods used currently investigate single miRNA-target interactions and are not able to analyze complex miRNA-target networks. To overcome these challenges, cross-linking and immunoprecipitation (CLIP), a recently developed method to study direct RNA-protein interactions in living cells, has been successfully applied to miRNA target analysis. It utilizes Argonaute (Ago)-immunoprecipitation to isolate native Ago-miRNA-mRNA complexes. In four recent publications, two variants of the CLIP method (HITS-CLIP and PAR-CLIP) were utilized to determine the targetomes of human and viral miRNAs in cells infected with the gamma-herpesviruses KSHV and EBV, which are associated with a number of human cancers. Here, we briefly introduce herpesvirus-encoded miRNAs and then focus on how CLIP technology has largely impacted our understanding of viral miRNAs in viral biology and pathogenesis.