Cytomegalovirus microRNAs

Abstract

The discovery that animals, plants and DNA viruses encode microRNAs (miRNAs) has transformed our understanding of the regulation of gene expression. miRNAs are ubiquitous small non-coding RNAs that regulate gene expression post-transcriptionally, generally by binding to sites within the 3' untranslated regions (UTR) of messenger RNA (mRNA) transcripts. To date, over 250 viral miRNAs have been identified primarily in members of the herpesvirus family. These viral miRNAs target both viral and cellular genes in order to regulate viral replication, the establishment and maintenance of viral latency, cell survival, and innate and adaptive immunity. This review will focus on our current knowledge of the targets and functions of human cytomegalovirus (HCMV) miRNAs and their functional equivalents in other herpesviruses.

Figure 1. Known targets and functions of HCMV miRNAs

(A) miR-UL112-1 targets viral factors UL114 (uracil DNA glycosylase) and IE1. Both proteins are thought to play a role in the establishment of latency and reactivation processes in hematopoietic lineage cells such as CD34+ HPCs, monocytes and macrophages. (B) miR-US25-1 targets cellular genes involved in cell cycle control including cyclin E2, BRCC3, EID1, MAPRE2, CD147 and histones. (C) HCMV miRNAs target genes involved in immune evasion. miR-UL148D-1 directly targets RANTES, a chemokine that recruits immune cells to the site of infection, while miR-UL112 targets the NK cell ligand MICB to prevent NK cell killing of infected cells. (D) HCMV miRNAs target the secretory pathway. miR-US25-1 targets the endosomal acidification complex component ATP6V0C while miRUL112, miR-US5-1 and miR-US5-2 coordinately target multiple secretory pathway genes including VAMP3, RAB5C, RAB11A, CDC42 and SNAP23, regulating the release of inflammatory cytokines and formation of the virion assembly compartment.

Figure 2. HCMV miRNAs that target components of the secretory pathway facilitate formation of the virion assembly compartment (VAC)

Normal human dermal fibroblasts were infected with an AD169 wild type virus (WT) or a virus in which miR-US5-1, miR-US5-2, and miR-UL112-1 have been mutated (Mut) at a multiplicity of infection of 0.1. At 6 days post-infection, cells were analyzed by immunofluorescence for DNA (DAPI, blue), and the viral markers glycoprotein M (gM, red) and N (gN, green). While gM and gH co-localized in a compact perinuclear VAC during WT AD169 infection (top panel), these viral proteins localized in a diffuse pattern during infection with the triple miRNA mutant (bottom panel), suggesting that the VAC cannot form properly in the absence of these miRNAs.