Deep sequencing reveals abundant noncanonical 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 in vivo evidence of the production of noncanonical RNA polymerase III (Pol III)-transcribed viral microRNAs in leukemic B cells in the complete absence of Pol II 5'-LTR-driven transcriptional activity.

Chromatin disruption in the promoter of bovine leukemia virus during transcriptional activation.

Bovine leukemia virus expression relies on its chromatin organization after integration into the host cell genome. Proviral latency, which results from transcriptional repression in vivo, represents a viral strategy to escape the host immune system and likely allows for tumor progression. Here, we discriminated two types of latency: an easily reactivable latent state of the YR2 provirus and a 'locked' latent state of the L267 provirus.

Cytotoxic responses to BLV tax oncoprotein do not prevent leukemogenesis in sheep.

Delta retrovirus-mediated leukemogenesis is dependent on the oncogenic potential of Tax. It is not clear, however, whether Tax-specific immune responses play a role in leukemia onset and progression. Using the BLV-associated leukemia model in sheep, we found that Tax-specific cytotoxic responses induced by DNA immunization or viral infection of naïve animals were not predictive of disease outcome and did not prevent tumor development.

Epigenetics and leukemia: unraveling oncogenic processes in the BLV ovine model.

Bovine Leukemia Virus (BLV)-induced B-cell leukemia in sheep is a valuable large animal model for investigating oncogenic mechanisms, particularly those associated with human T-cell leukemia virus 1 (HTLV-1). Multiple factors including viral genes, genetic and epigenetic alterations, and the host immune system are likely to contribute and cooperate in the leukemogenesis of adult T-cell leukemia (ATL) in human and B-cell leukemia in sheep. While considerable effort has been made to explore the role of viral determinants in the transformation process, the participation of host-related mechanisms has been poorly addressed.

Complete suppression of viral gene expression is associated with the onset and progression of lymphoid malignancy: observations in Bovine Leukemia Virus-infected sheep.

Background: During malignant progression, tumor cells need to acquire novel characteristics that lead to uncontrolled growth and reduced immunogenicity. In the Bovine Leukemia Virus-induced ovine leukemia model, silencing of viral gene expression has been proposed as a mechanism leading to immune evasion. However, whether proviral expression in tumors is completely suppressed in vivo was not conclusively demonstrated.