mdv1-miR-M7-5p, located in the newly identified first intron of the latency-associated transcript of Marek's disease virus, targets the immediate-early genes ICP4 and ICP27.

Marek's disease virus serotype 1 (MDV-1) is an oncogenic alphaherpesvirus causing fatal T-cell lymphoma in chickens. MDV latency is characterized by the production of latency-associated transcripts (LATs), a family of non-protein-coding spliced RNAs. A cluster of four microRNAs (cluster mdv1-miR-M8-M10) was identified, but not formally mapped, at the predicted LAT 5' end. We established a LAT cDNA library from latently MDV-infected cell line MSB-1. We identified 22 highly variable LATs, which were due to the extensive alternative splicing of a total of 14 introns. RACE PCR confirmed the predicted 3' end and allowed identification of the 5' end, 400 nt upstream of the previously predicted LAT end. The LATs share their transcription start site with the microRNA-expressing transcript described previously, localizing the microRNAs to the first LAT intron and identifying the LATs as the primary transcripts of the microRNAs. We identified MDV immediate-early (IE) genes ICP4 and ICP27 as putative targets of mdv1-miR-M7-5p, the third microRNA of the cluster mdv1-miR-M8-M10. Endogenously expressed mdv1-miR-M7-5p in MSB-1 cells reduced luciferase activity significantly when microRNA-responsive elements from ICP4 or ICP27 were cloned in the 3' UTR of the firefly luciferase gene. ICP27 protein levels were decreased by 70 % when the mdv1-miR-M7-5p precursor was co-expressed with an ICP27 expression plasmid. Additionally, we showed a negative correlation between the decreased expression of mdv1-miR-M7-5p and an increase in ICP27 expression during virus reactivation. Our results suggest that, by targeting two IE genes, MDV microRNAs produced from LAT transcripts may contribute to establish and/or maintain latency.