Homologous Recombination-Based Genome Editing by Clade F AAVs Is Inefficient in the Absence of a Targeted DNA Break.

Adeno-associated virus (AAV) vectors are frequently used as donor templates for genome editing by homologous recombination. Although modification rates are typically under 1%, they are greatly enhanced by targeted double-stranded DNA breaks (DSBs). A recent report described clade F AAVs mediating high-efficiency homologous recombination-based editing in the absence of DSBs. The clade F vectors included AAV9 and a series isolated from human hematopoietic stem and progenitor cells (HSPCs). We evaluated these vectors by packaging homology donors into AAV9 and an AAVHSC capsid and examining their ability to insert GFP at the CCR5 and AAVS1 loci in human HSPCs and cell lines. As a control, we used AAV6, which effectively edits HSPCs but only when combined with a targeted DSB. Each AAV vector promoted GFP insertion in the presence of matched CCR5 or AAVS1 zinc-finger nucleases (ZFNs), but none supported detectable editing in the absence of the nucleases. Rates of editing with ZFNs correlated with transduction efficiencies for each vector, implying no differences in the ability of donor sequences delivered by the different vectors to direct genome editing. Our results, therefore, do not support that clade F AAVs can perform high-efficiency genome editing in the absence of a DSB.