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Retroviral integrase (IN) proteins catalyze the permanent integration of proviral genomes into host DNA with the help of cellular cofactors. Lens epithelium-derived growth factor (LEDGF) is a cofactor for lentiviruses, including human immunodeficiency virus type 1 (HIV-1), and targets lentiviral integration toward active transcription units in the host genome. In contrast to lentiviruses, murine leukemia virus (MLV), a gammaretrovirus, tends to integrate near transcription start sites. Here, we show that the bromodomain and extraterminal domain (BET) proteins BRD2, BRD3, and BRD4 interact with gammaretroviral INs and stimulate the catalytic activity of MLV IN in vitro. We mapped the interaction site to a characteristic structural feature within the BET protein extraterminal (ET) domain and to three amino acids in MLV IN. The ET domains of different BET proteins stimulate MLV integration in vitro and, in the case of BRD2, also in vivo. Furthermore, two small-molecule BET inhibitors, JQ1 and I-BET, decrease MLV integration and shift it away from transcription start sites. Our data suggest that BET proteins might act as chromatin-bound acceptors for the MLV preintegration complex. These results could pave a way to redirecting MLV DNA integration as a basis for creating safer retroviral vectors.

Accidental insertional activation of proto-oncogenes and potential vector mobilization pose serious challenges for human gene therapy using retroviral vectors. Comparative analyses of integration sites of different retroviral vectors have elucidated distinct target site preferences, highlighting vectors based on the alpharetrovirus Rous sarcoma virus (RSV) as those with the most neutral integration spectrum. To date, alpharetroviral vector systems are based mainly on single constructs containing viral coding sequences and intact long terminal repeats (LTR). Even though they are considered to be replication incompetent in mammalian cells, the transfer of intact viral genomes is unacceptable for clinical applications, due to the risk of vector mobilization and the potentially immunogenic expression of viral proteins, which we minimized by setting up a split-packaging system expressing the necessary viral proteins in trans. Moreover, intact LTRs containing transcriptional elements are capable of activating cellular genes. By removing most of these transcriptional elements, we were able to generate a self-inactivating (SIN) alpharetroviral vector, whose LTR transcriptional activity is strongly reduced and whose transgene expression can be driven by an internal promoter of choice. Codon optimization of the alpharetroviral Gag/Pol expression construct and further optimization steps allowed the production of high-titer self-inactivating vector particles in human cells. We demonstrate proof of principle for the versatility of alpharetroviral SIN vectors for the genetic modification of murine and human hematopoietic cells at a low multiplicity of infection.

Analyzing cellular restriction mechanisms provides insight into viral replication strategies, identifies targets for antiviral drug design, and is crucial for the development of novel tools for experimental or therapeutic delivery of genetic information. We have previously shown that retroviral vector mutants that are unable to initiate reverse transcription mediate a transient expression of any sequence which replaces the gag-pol transcription unit, a process we call retrovirus particle-mediated mRNA transfer (RMT). Here, we further examined the mechanism of RMT by testing its sensitivity to cellular restriction factors and short hairpin RNAs (shRNAs). We found that both human TRIM5alpha and, to a lesser extent, Fv1 effectively restrict RMT if the RNA is delivered by a restriction-sensitive capsid. While TRIM5alpha restriction of RMT led to reduced levels of retroviral mRNA in target cells, restriction by Fv1 did not. Treatment with the proteasome inhibitor MG132 partially relieved TRIM5alpha-mediated restriction of RMT. Finally, cells expressing shRNAs specifically targeting the retroviral mRNA inhibited RMT particles, but not reverse-transcribing particles. Retroviral mRNA may thus serve as a translation template if not used as a template for reverse transcription. Our data imply that retroviral nucleic acids become accessible to host factors, including ribosomes, as a result of particle remodeling during cytoplasmic trafficking.