Chromatin disruption in the promoter of Bovine Leukemia Virus during transcriptional activation

Abstract

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. The defective YR2 provirus was characterized by the presence of nuclease hypersensitive sites at the U3/R junction and in the R/U5 region of the 5′-long terminal repeat (5′-LTR), whereas the L267 provirus displayed a closed chromatin configuration at the U3/R junction. Reactivation of viral expression in YR2 cells by the phorbol 12-myristate 13-acetate (PMA) plus ionomycin combination was accompanied by a rapid but transient chromatin remodeling in the 5′-LTR, leading to an increased PU.1 and USF-1/USF-2 recruitment in vivo sustained by PMA/ionomycin-mediated USF phosphorylation. In contrast, viral expression was not reactivated by PMA/ionomycin in L267 cells, because the 5′-LTR U3/R region remained inaccessible to nucleases and hypermethylated at CpG dinucleotides. Remarkably, we elucidated the BLV 5′-LTR chromatin organization in PBMCs isolated from BLV-infected cows, thereby depicting the virus hiding in vivo in its natural host.

Bovine leukemia virus: A major silent threat to proper immune responses in cattle

“…A wide range of host cell proteins can bind to the BLV LTR, including CREB, ATF1, ATF2 (Adam et al, 1996) and CREM (Nguyen et al, 2007). Phosphorylated USF1 and USF2 will both bind to the enhancer sequence Ebox4 motif in the LTR and this binding is increased in response to P/I stimulation and can stimulate BLV transcription in the absence of Tax (Calomme et al, 2002;Colin et al, 2011). Both IRF1 and IRF2 will bind to an IRSE-like motif in the LTR, but the LTR is not responsive to IFN␣ (Kiermer et al, 1998).…”

Prdm12 specifies V1 interneurons through cross-repressive interactions with Dbx1 and Nkx6 genes in Xenopus

“…ChIP was essentially conducted in mouse (Borromeo et al, 2014) and in Xenopus as described (Ma et al, 2014;Colin et al, 2011) (see supplementary material methods). RNAseq and ChIP data have been deposited at GEO under accession GSE64551.…”

Deep sequencing reveals abundant noncanonical retroviral microRNAs in B-cell leukemia/lymphoma

“…In conclusion, we show that in tumors BLV miRNAs are processed from five independent transcriptionally self-sufficient Pol III units, which each possess intragenic control elements, in a Drosha-independent, Dicer-dependent manner. (10)(11)(12)(13)(14). Thus, in B-cell tumors, abundant Pol III-driven viral miRNA expression is accompanied by undetectable 5′-LTR Pol II transcripts.…”

“…Evidence from several studies indicates that both genetic and epigenetic changes account for the absence of 5′-LTR-driven RNA polymerase II (Pol II)-dependent transcription of the BLV genome in primary B-cell tumors, resulting in undetectable viral mRNA and protein (11)(12)(13)(14).…”