HIV-1-based lentiviral vectors are a promising tool for gene therapy. However, integration of a lentiviral vector into host cell genes may lead to the development of cancer. Therefore, control of integration site selection is critical to the successful outcome of gene therapy approaches that use these vectors. The discovery that integration site selection by HIV-1 and HIV-1-based vectors is controlled by the LEDGF=p75 protein has presented new opportunities to control integration site selection. In this study, we tested the hypothesis that a fusion protein containing the C-terminal HIV integrase-binding portion of LEDGF=p75, and the N-terminal chromodomain of heterochromatin protein-1a (HP1a), can target HIV-1 vector DNA outside of genes. We show that this fusion protein, termed TIHPLE, associates with the heterochromatin hallmark trimethylated Lys-9 of histone H3 (H3K9me3). Transient overexpression of TIHPLE alters integration site selection by an HIV-1-based vector and decreases the number of integration events that occur in genes. This change in integration site selection was achieved without a reduction in overall integration efficiency. Furthermore, we show that TIHPLE increases integration in the vicinity of H3K9me3 and in repetitive DNA sequences. These data provide a novel approach to address the problem of the tendency of retroviral vectors to integrate at undesirable sites of the human genome.

HIV-1-based vectors are widely used in gene therapy. In somatic cells, these vectors mainly integrate within genes. However, no distinct integration site preferences have been observed with regard to large chromosomal regions. The recent emergence of induced pluripotent stem (iPS) cells, similar to embryonic stem (ES) cells, has raised questions about where integration occurs in these cells. In this work we investigated the integration site preferences of HIV-1-based vectors in a pluripotent, ES-like cell line. We show that approximately 30% of the integrations occur in the vicinity of telomeres. We have analyzed integration sites in various somatic cells, as reported by us and other groups, and observed that this integration pattern is unique to the analyzed pluripotent cell line. We conclude that pluripotent cells may contain distinct cellular cofactors that participate in integration targeting and that are not present in somatic cells.

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