Ovine herpesvirus-2 encoded microRNAs target virus genes involved in virus latency

Herpesviruses encode miRNAs that target both virus and host genes; however their role in herpesvirus biology is poorly understood. We previously identified eight miRNAs encoded by OvHV-2; the causative agent of malignant catarrhal fever (MCF) and have now investigated the role of these miRNAs in regulating expression of OvHV-2 genes that play important roles in virus biology. ORF 20 (cell cycle inhibition), ORF 50 (reactivation) and ORF 73 (latency maintenance) each contain predicted targets for several OvHV-2 miRNAs. Co-transfection of miRNA mimics with luciferase reporter constructs containing the predicted targets showed the 5’ UTRs of ORF 20 and ORF73 contain functional targets for ovhv-miR-2 and ovhv2-miR-8 respectively, and the 3’UTR of ORF 50 contains a functional target for ovhv2-miR-5. Transfection of BJ1035 cells (an OvHV-2 infected bovine T cell line) with the relevant miRNA mimic resulted in a significant decrease in ORF 50 and a smaller but non-significant decrease in ORF 20. However, we were unable to demonstrate a decrease in ORF73. MCF is a disease of dysregulated lymphocyte proliferation, miRNA inhibition of ORF 20 expression may play a role in this aberrant lymphocyte proliferation. The proteins encoded by ORFs 50 and 73 play opposing roles in latency, it has been hypothesized that miRNA-induced inhibition of virus genes acts to ensure that fluctuations in virus mRNA levels do not result in reactivation in conditions that are unfavourable for viral replication, our data would support this hypothesis