Deep sequencing reveals abundant non-canonical onco-retroviral microRNAs in B-cell leukemia/lymphoma

Viral tumor models have significantly contributed to our understanding of oncogenic mechanisms. How transforming delta-retroviruses induce malignancy however remains poorly understood, especially as viral mRNA/protein are tightly silenced in tumors. Here, using deep sequencing of broad windows of small RNA sizes in the Bovine Leukemia Virus ovine model of leukemia/lymphoma, we provide evidence of the production of non-canonical Pol III-transcribed viral microRNAs in leukemic B-cells in the complete absence of Pol II 5' LTR-driven transcriptional activity. Processed from a cluster of five independent self-sufficient transcriptional units located in a proviral region dispensable for in vivo infectivity, BLV microRNAs represent ~ 40 % of all microRNAs in both experimental and natural malignancy. They are conserved across tumors and associate with Argonautes, consistent with a critical function in silencing of important cellular and/or viral targets. BLV microRNAs are strongly expressed at pre-leukemic stages and remain at high levels in malignant cells despite the absence of structural and regulatory gene expression, suggesting a key role in tumor onset and progression. Identification of small RNA populations in BLV-induced leukemia