title: Enhancers are major targets for murine leukemia virus vector integration.
abstract: Retroviral vectors have been used in successful gene therapies. However, in some patients, insertional mutagenesis led to leukemia or myelodysplasia. Both the strong promoter/enhancer elements in the long terminal repeats (LTRs) of murine leukemia virus (MLV)-based vectors and the vector-specific integration site preferences played an important role in these adverse clinical events. MLV integration is known to prefer regions in or near transcription start sites (TSS). Recently, BET family proteins were shown to be the major cellular proteins responsible for targeting MLV integration. Although MLV integration sites are significantly enriched at TSS, only a small fraction of the MLV integration sites (<15%) occur in this region. To resolve this apparent discrepancy, we created a high-resolution genome-wide integration map of more than one million integration sites from CD34(+) hematopoietic stem cells transduced with a clinically relevant MLV-based vector. The integration sites form ∼60,000 tight clusters. These clusters comprise ∼1.9% of the genome. The vast majority (87%) of the integration sites are located within histone H3K4me1 islands, a hallmark of enhancers. The majority of these clusters also have H3K27ac histone modifications, which mark active enhancers. The enhancers of some oncogenes, including LMO2, are highly preferred targets for integration without in vivo selection.
  
  
  OBJECTIVE
  We show that active enhancer regions are the major targets for MLV integration; this means that MLV preferentially integrates in regions that are favorable for viral gene expression in a variety of cell types. The results provide insights for MLV integration target site selection and also explain the high risk of insertional mutagenesis that is associated with gene therapy trials using MLV vectors.
authors: Laboratory of Host Defenses, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland, USA.
authors: Suk See
authors: SS
authors: De Ravin
authors: Ling
authors: L
authors: Su
authors: Narda
authors: N
authors: Theobald
authors: Uimook
authors: U
authors: Choi
authors: Janet L
authors: JL
authors: Macpherson
authors: Michael
authors: M
authors: Poidinger
authors: Geoff
authors: G
authors: Symonds
authors: Susan M
authors: SM
authors: Pond
authors: Andrea L
authors: AL
authors: Ferris
authors: Stephen H
authors: SH
authors: Hughes
authors: Harry L
authors: HL
authors: Malech
authors: Xiaolin
authors: X
authors: Wu
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004742:Enhancer Elements, Genetic; D015316:Genetic Therapy; D005822:Genetic Vectors / Q000235:genetics / Q000502:physiology*; D015894:Genome, Human; D006412:Hematopoietic Stem Cells / Q000821:virology; D006657:Histones / Q000235:genetics / Q000378:metabolism; D006801:Humans; D009052:Leukemia Virus, Murine / Q000235:genetics / Q000502:physiology*; D051379:Mice; D016254:Mutagenesis, Insertional; D016662:Virus Integration
chemical_list: D006657:Histones