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

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. Overall design: Identification of small RNA populations in BLV-induced leukemia

病毒肿瘤模型极大地推动了我们对致癌机制的认知。然而，转化性δ逆转录病毒（transforming delta-retroviruses）如何诱导恶性肿瘤发生，目前仍知之甚少——尤其是在肿瘤中病毒mRNA/蛋白质被严密沉默的情况下。本研究通过对牛白血病病毒（Bovine Leukemia Virus, BLV）绵羊白血病/淋巴瘤模型中覆盖宽大小区间的小RNA进行深度测序，发现在完全缺乏RNA聚合酶II（Pol II）5'长末端重复序列（LTR）驱动的转录活性的白血病B细胞中，存在非经典RNA聚合酶III（Pol III）转录的病毒微小RNA（microRNA, miRNA）的生成证据。BLV微小RNA由位于对体内感染性非必需的前病毒区域内的5个独立自主转录单元构成的簇加工而来，在实验性与自然发生的恶性肿瘤中，其占所有微小RNA的比例约达40%。该类微小RNA在各类肿瘤中保守存在，并与Argonaute（AGO）蛋白相结合，这与其在沉默重要细胞和/或病毒靶标中发挥关键功能的结论相符。BLV微小RNA在白血病前期阶段即显著表达，且即便结构基因与调控基因均无表达，仍能在恶性细胞中维持高水平，这提示其在肿瘤发生与进展中发挥关键作用。实验整体设计：鉴定BLV诱导的白血病中的小RNA群体。