A helper-dependent capsid-modified adenovirus vector expressing adeno-associated virus rep78 mediates site-specific integration of a 27-kilobase transgene cassette.

Random integration of viral gene therapy vectors and subsequent activation or disruption of cellular genes poses safety risks. Major efforts in the field are aimed toward targeting vector integration to specific sites in the host genome. The adeno-associated virus (AAV) Rep78 protein is able to target AAV integration to a specific site on human chromosome 19, called AAVS1. We studied whether this ability could be harnessed to achieve site-specific integration of a 27-kb transgene cassette into a model cell line for human hematopoietic cells (Mo7e). To deliver rep78 and the transgene to Mo7e cells, we used helper-dependent adenovirus (Ad) vectors containing Ad serotype 35 fiber knob domains (HD-Ad). An HD-Ad vector containing the rep78 gene under the control of the globin locus control region (LCR) (Ad.LCR-rep78) conferred Rep78 expression on Mo7e cells. Upon coinfection of Ad.LCR-rep78 with an HD-Ad vector containing a 27-kb globin-LCR-green fluorescent protein (GFP) transgene cassette flanked by AAV inverted terminal repeats (ITRs) (Ad.AAV-LCR-GFP), transduced cells were cloned and expanded (without selection pressure), and vector integration was analyzed in clones with more than 30% GFP-positive cells. Vector integration into the AAVS1 region was seen in 30% of analyzed integration sites, and GFP expression from these integrants was stable over time. Of the remaining integration sites, 25% were within the genomic globin LCR. In almost 90% of sites, transgene integration occurred via the Ad ITR. This indicates that rescue of the AAV ITR-flanked transgene cassette from Ad.AAV-LCR-GFP is not required for Rep78-mediated integration into AAVS1 and that free ends within the vector genome can be created by breaks within the Ad ITRs, whose structure is apparently recognized by cellular "nicking" enzymes. The finding that 55% of all analyzed integration sites were either within the AAVS1 or globin LCR region demonstrates that a high frequency of targeted integration of a large transgene cassette can be achieved in human hematopoietic stem cell lines.