Bovine leukemia virus antisense transcription is essential for viral replication and oncogenesis

Bovine Leukemia Virus (BLV) is a retrovirus that naturally infects the B lymphocytes of different species of the Bovidae family. The virus integrates into the host genome as a provirus, resulting in a lifelong infection. BLV induces a fatal disease called Enzootic Bovine Leukosis (EBL). Shortly after infection, viral sense transcription (from the 5’LTR to the 3’LTR) is strongly repressed by epigenetic mechanisms. However, the provirus still constitutively transcribes non-immunogenic microRNAs and RNAs via antisense transcription (from the 3’LTR to the 5’LTR). The biological function of antisense transcription is currently unknown.
In this context, the objective of this thesis is to evaluate the relationship between antisense transcription, viral persistence, and oncogenesis. A reverse genetics approach was implemented, exploiting a mechanistic difference during the initiation of sense and antisense transcription. A mutation of only two base pairs in the provirus (A1) impaired antisense transcription in cell culture and sheep. RNA sequencing, confocal microscopy, and immunochemistry data revealed that expression of the HMCN-1 gene encoding the Hemicentin-1 protein was upregulated in B lymphocytes infected by the provirus carrying the A1 mutation. RNA interference showed that HMCN-1 was involved in cell adherence, migration, and death. In contrast to the wild-type, the A1 provirus did not induce leukemia/lymphoma.
In conclusion, this thesis highlights the role of antisense transcription in viral persistence and oncogenesis induced by BLV. These observations may be helpful to understand better the pathogenesis of the Human T-cell Leukemia Virus type 1 (HTLV-1).