A role for non-coding RNA and microRNA is being increasingly recognized in diverse cellular processes including oncogenesis. Bovine Leukemia Virus (BLV) is associated with natural B-cell tumor development in cattle and experimentally induced tumors in sheep. A complete understanding of how BLV induces tumorigenesis remains largely enigmatic, particularly as viral mRNA and proteins are undetectable in tumor cells. Analysis of the BLV tumor miRnome using deep sequencing of small RNAs isolated from primary ovine B-cell tumors revealed ten BLV-mapping microRNAs processed from five hairpin precursor structures located in a region of the proviral genome that is not known to encode proteins. Overall, the BLV microRNA cluster accounts for ∼ 40 % of the total microRNA pool in tumor cells, supporting its biological relevance in vivo. Analysis of broader windows of small RNA sizes allowed us to capture all products of BLV microRNA biogenesis in malignant B-cells. BLV microRNAs are transcribed by Pol III, in the absence of Pol II transcripts and viral protein expression. They are processed in a Drosha-independent, Dicer-dependent manner, and associated with Argonaute, suggesting a function in post-transcriptional silencing. The remarkable conservation across tumor isolates suggests that BLV microRNAs have acquired important biological targets. This unexpected observation opens a novel area in the biology of BLV, as so far, the proviral genome was believed to be completely silent and the dogma assumes that retroviruses cannot harbour microRNAs. Understanding how Pol III-dependent microRNAs subvert cellular and viral pathways will contribute in deciphering the network perturbations that underlie viral and non-viral cancer progression.
© 2013 Lippincott Williams & Wilkins, Inc.