A human herpesvirus 6A-encoded microRNA: role in viral lytic replication

Human herpesvirus 6A (HHV-6A), a member of the betaherpesvirus family, is associated with several human diseases. Like all herpesviruses, HHV-6A establishes a lifelong, latent infection in its host. Reactivation of HHV-6A is frequent within the immunosuppressed and immunocompromised populations and results in lytic viral replication within multiple organs, often leading to severe disease. MicroRNAs (miRNAs) are key regulators of multiple cellular processes that regulate the translation of specific transcripts. miRNAs carried by herpesviruses play important roles in modulating the host cell, thereby facilitating a suitable environment for productive viral infection and/or latency. Currently, there are approximately 150 known human herpesvirus-encoded miRNAs, although an miRNA(s) encoded by HHV-6A has yet to be reported. We hypothesized that HHV-6A, like other members of the human herpesvirus family, encodes miRNAs, which function to promote viral infection. We utilized deep sequencing of small RNA species isolated from cells harboring HHV-6A to identify five novel small noncoding RNA species that originate from the viral genome, one of which has the characteristics of a viral miRNA. These RNAs are expressed during productive infection by either bacterial artificial chromosome (BAC)-derived virus in Jjhan cells or wild-type HHV-6A strain U1102 virus in HSB2 cells and are associated with the RNA induced silencing complex (RISC) machinery. Growth analyses of mutant viruses that lack each individual miRNA revealed that a viral miRNA candidate (miR-U86) targets the HHV-6A IE gene U86, thereby regulating lytic replication. The identification and biological characterization of this HHV-6A-specific miRNA is the first step to defining how the virus regulates its life cycle.

Importance: A majority of the human population is infected with human herpesvirus 6A (HHV-6A), a betaherpesvirus family member. Infections usually occur in young children, and upon resolution, the virus remains in a latent state within the host. Importantly, during times of weakened immune responses, the virus can reactivate and is correlated with significant disease states. Viruses encode many different types of factors that both undermine the host antiviral response and regulate viral replication, including small RNA species called microRNAs (miRNAs). Here we report that HHV-6A encodes at least one miRNA, which we named miR-U86. We have characterized the requirement of this viral miRNA and its impact on the viral life cycle and found that it functions to regulate a viral protein important for efficient viral replication. Our data suggest that viral miRNAs are important for HHV-6A and that they may serve as an important therapeutic target to inhibit the virus.