BET-Independent Murine Leukemia Virus Integration Is Retargeted In Vivo and Selects Distinct Genomic Elements for Lymphomagenesis

Abstract

In this study, we have shown that the in vivo replication of murine leukemia virus happens independently of BET proteins, which are key host determinants involved in retroviral integration site selection. This finding opens a new research line in the discovery of alternative viral or host factors that may complement the dominant host factor.

Determinants of Retroviral Integration and Implications for Gene Therapeutic MLV—Based Vectors and for a Cure for HIV-1 Infection

“…Ashkar et al showed that the introduction of W390A in MLV IN resulted in an MLV vector characterized by an altered integration pattern in cell culture but produced at similar titers as the WT MLV [ 54 ]. Next, Nombela et al inserted W390A as a single point mutation in the viral IN of the MLV molecular clone p63.2 [ 55 ]. By comparing wild–type (WT) MLV with BET–independent (Bin) W390A MLV replication, the role of BET proteins in MLV replication, integration and tumorigenesis was studied in vivo.…”

“…The W390 mutation induced the retargeting of integration away from CpG islands, DNase I–hypersensitive sites, GC–enriched regions, TSSs and enhancers. Uncoupling MLV integration from BET proteins resulted in increased integration in proximity to histone marks linked with active transcription, such as H3K36me3 [ 55 ]. Next, the effect of the W390A mutation on MLV leukemogenesis in the blood, spleen and thymus was assessed.…”

“…There was no statistical difference in the proliferation of white blood cells (WBCs) from WT and W390A MLV–infected mice. In addition, there was no statistical difference in the survival rates of WT compared to W390A MLV–infected mice [ 55 ]. Furthermore, an assessment of the histopathology of the spleen and thymus of mice infected with WT or W390A mutant MLV revealed no pathological differences [ 55 ].…”

“…In addition, there was no statistical difference in the survival rates of WT compared to W390A MLV–infected mice [ 55 ]. Furthermore, an assessment of the histopathology of the spleen and thymus of mice infected with WT or W390A mutant MLV revealed no pathological differences [ 55 ]. Lastly, Nombela et al performed a second integration site sequencing analysis on spleen cells from infected mice that had already developed lymphoma due to insertional oncogenesis.…”

“…The W390A mutant MLV displayed fewer integrations near enhancers and a higher frequency of integration into oncogene bodies such as Notch1 and Ppp1r16b and promotors. Due to extensive clonal expansion, integration into these enhancers, promotors and oncogene bodies further drove the development of the lymphomas in both WT and MLV mutant–infected mice [ 55 ]. Overall these in vivo data indicate that abolishing the IN–BET interaction via a single point mutation in MLV IN can significantly alter the integration pattern by retargeting MLV integration away from enhancers, but it cannot suppress the development of lymphomas due to insertional mutagenesis [ 55 ].…”