Summary A hallmark of retroviral replication is integration of the viral genome in the host cell DNA. This characteristic makes retrovirus-based vectors attractive delivery vehicles for gene therapy. However, adverse events in gene therapeutic trials, caused by activation of proto-oncogenes due to Murine Leukemia Virus (MLV)-derived vector integration, hamper their application. Here we show that bromodomain and extraterminal (BET) proteins (BRD2, BRD3 and BRD4) and MLV integrase specifically interact and co-localize within the nucleus of the cell. Inhibition of the BET proteins chromatin interaction via specific bromodomain inhibitors blocks MLV virus replication at the integration step. MLV integration site distribution parallels the chromatin binding profile of BET proteins, and expression of an artificial fusion protein of the BET integrase binding domain with the chromatin interaction domain of the lentiviral targeting factor LEDGF/p75, retargets MLV integration away from TSS and into the body of actively transcribed genes, conform to the Human Immunodeficiency Virus (HIV) integration pattern. Together these data validate BET proteins as MLV integration targeting factors.

A hallmark of retroviral replication is stable integration of the viral genome in the host cell DNA. This characteristic makes retroviral-derived vector particles attractive vehicles for gene therapy. However, retroviral integration is not a random process. Lentiviruses preferentially integrate in the body of active transcription units, while gammaretroviruses, including Moloney Murine Leukemia Virus (MLV), favour transcription start sites and CpG islands. In clinical trials using gammaretroviral vectors for gene therapy, leukemogenesis has been associated with integration of vectors near oncogene transcription start sites. We found that the bromodomain and extra-terminal (BET) proteins (BRD2, BRD3 and BRD4) interact with MLV integrase and direct integration towards transcription start regions. BET proteins specifically bind and co-localize with the gammaretrovirus integrase protein in the nucleus of the cell. The interaction is gammaretroviral-specific and mediated by the integrase C-terminal domain and the BET extraterminal (ET) domain as determined by coimmunoprecipitation assays and in an Alphascreen assay using recombinant proteins. Interfering with chromatin interaction of BET proteins via specific bromodomain inhibitors JQ1 and l-BET decreases MLV virus replication and MLV vector transduction 5-to 10-fold, while HIV vector transduction is not affected. Analysis of viral DNA intermediates by quantitative PCR revealed a block at the integration step. In addition, bromodomain inhibitors do not have an effect on the late steps of viral replication. MLV integration site distribution analysis revealed a strong correlation with the BET protein chromatin binding profile. Finally, expression of an artificial fusion protein that merges the BET integrase binding domain with the chromatin interaction domain of the lentiviral targeting factor LEDGF/p75, retargets MLV integration into the body of actively transcribed genes, paralleling the Human Immunodeficiency Virus (HIV) integration pattern. Our results explain the molecular mechanism behind gammaretroviral integration site targeting and suggest methods for engineering gammaretroviral vectors with a safer integration site profile.