Intragenic integration in DLC1 sustains factor VIII expression in primary human cells without insertional oncogenicity

Techniques enabling precise genome modifications enhance the safety of gene-based therapy. DLC1 is a hot spot for phiC31 integrase-mediated transgene integration in vitro and in vivo. Here we show that integration of a coagulation factor VIII transgene into intron 7 of DLC1 supports durable expression of factor VIII in primary human umbilical cord-lining epithelial cells. Oligoclonal cells with factor VIII transgene integrated in DLC1 did not have altered expression of DLC1 or neighbouring genes within a 1-Mb interval. Only 1.9% of all expressed genes were transcriptionally altered; most were downregulated and mapped to cell cycle and DNA repair pathways. DLC1-integrated cells were not tumourigenic in vivo and were normal by high-resolution genomic DNA copy number analysis. Our data identify DLC1 as a locus for durable transgene expression that does not incur features of insertional oncogenesis, thus expanding options for developing ex vivo cell therapy mediated by site-specific integration methods.