High spontaneous integration rates of end-modified linear DNAs upon mammalian cell transfection

In gene therapy, potential integration of therapeutic transgene into host cell genomes is a serious risk that can lead to insertional mutagenesis and tumorigenesis. Viral vectors are often used as the gene delivery vehicle, but they are prone to undergoing integration events. More recently, non-viral delivery of linear DNAs having modified geometry such as closed-end linear duplex DNA (CELiD) have shown promise as an alternative, due to prolonged transgene expression and less cytotoxicity. However, whether such modified-end linear DNAs can also provide a safe, non-integrating gene transfer remains unanswered. Herein, we provide a systematic comparison of genomic integration frequency upon transfection of cells with expression vectors in the forms of circular plasmid, unmodified linear DNA, CELiD, and Streptavidin-conjugated blocked-end linear DNA. All of these forms of linear DNA resulted in a high fraction of the cells being stably transfected - between 10% and 20% of the initially transfected cells, with CELiDs showing the highest rates of integration. These results indicate that blocking the ends of linear DNA is insufficient to prevent integration. Moreover, our analysis suggest that conventional AAV-based gene therapy may be highly susceptible to integration, which is consistent with recent findings from long-term clinical studies.