Comparison of HIV-derived Lentiviral and MLV-based Gammaretroviral Vector Integration Sites in Primate Repopulating Cells

The potential for leukemia caused by retroviral vector integration has become a significant concern for hematopoietic stem cell gene therapy. We analyzed the distribution of vector integrants in pigtailed macaque and baboon repopulating cells for the two most commonly used retroviral vector systems, human immunodeficiency virus (HIV)-based lentiviral vectors and murine leukemia virus (MLV)-based gammaretroviral vectors, to help define their relative genotoxicity. All animals had polyclonal engraftment with no apparent adverse effects from transplantation with gene-modified cells. In all, 380 MLV and 235 HIV unique vector integration sites were analyzed and had distinct distribution patterns in relation to genes and CpG islands as observed in previous in vitro studies. Both vector types were found more frequently in and near proto-oncogenes in repopulating cells than in a random dataset. Analysis of functional classes of genes with integrants within 100 kilobases (kb) of their transcription start sites showed an over-representation of genes involved in growth or survival near both lentiviral and gammaretroviral integrants. Microarray analysis showed that both gammaretroviral and lentiviral vectors were found close to genes with high expression levels in primitive cells enriched for hematopoietic stem cells. These data help define the relative risk of insertional mutagenesis with MLV-, HIV-, and simian immunodeficiency virus (SIV)-based vectors in a highly relevant primate model.