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The precise correction of genetic mutations at the nucleotide level is an attractive permanent therapeutic strategy for human disease. However, despite significant progress, challenges to efficient and accurate genome editing persist. Here, we report a genome editing platform based upon a class of hematopoietic stem cell (HSC)-derived clade F adeno-associated virus (AAV), which does not require prior nuclease-mediated DNA breaks and functions exclusively through BRCA2-dependent homologous recombination. Genome editing is guided by complementary homology arms and is highly accurate and seamless, with no evidence of on-target mutations, including insertion/deletions or inclusion of AAV inverted terminal repeats. Efficient genome editing was demonstrated at different loci within the human genome, including a safe harbor locus, AAVS1, and the therapeutically relevant IL2RG gene, and at the murine Rosa26 locus. HSC-derived AAV vector (AAVHSC)-mediated genome editing was robust in primary human cells, including CD34 + cells, adult liver, hepatic endothelial cells, and myocytes. Importantly, high-efficiency gene editing was achieved in vivo upon a single i.v. injection of AAVHSC editing vectors in mice. Thus, clade F AAV-mediated genome editing represents a promising, highly efficient, precise, single-component approach that enables the development of therapeutic in vivo genome editing for the treatment of a multitude of human gene-based diseases., Competing Interests: Conflict of interest statement: S.C. and L.J.S. are cofounders of Homology Medicines, Inc., and S.C., L.J.S., J.W., T.S.M., H.R., J.L.E., and D.K. hold equity in Homology Medicines, Inc.

Adeno-associated virus (AAV) vectors are proving to be remarkably successful for in vivo gene delivery. Based upon reports of abundant AAV in the human marrow, we tested CD34(+) hematopoietic stem cells for the presence of natural AAV. Here, we report for the first time, the presence of novel AAV variants in healthy CD34(+) human peripheral blood stem cells. The majority of healthy peripheral blood stem cell donors were found to harbor AAV in their CD34(+) cells. Every AAV isolated from CD34(+) cells mapped to AAV Clade F. Gene transfer vectors derived from these novel AAVs efficiently underwent entry and postentry processing in human cord blood stem cells and supported stable gene transfer into long-term, in vivo engrafting human HSCs significantly better than other serotypes. AAVHSC-transduced human CD34(+) cells engrafted in vivo and gave rise to differentiated transgene-expressing progeny. Importantly, gene-marked CD34(+) stem cells persisted long term in xenograft recipients, indicating transduction of primitive progenitors. Notably, correlation of structure with function permitted identification of potential capsid components important for HSC transduction. Thus, AAVHSCs represent a new class of genetic vectors for the manipulation of HSC genomes.

Background aimsAlthough recombinant adeno-associated virus serotype 2 (AAV2) vectors have gained attention because of their safety and efficacy in numerous phase I/II clinical trials, their transduction efficiency in hematopoietic stem cells (HSCs) has been reported to be low. Only a few additional AAV serotype vectors have been evaluated, and comparative analyses of their transduction efficiency in HSCs from different species have not been performed.MethodsWe evaluated the transduction efficiency of all available AAV serotype vectors (AAV1 through AAV10) in primary mouse, cynomolgus monkey and human HSCs. The transduction efficiency of the optimized AAV vectors was also evaluated in human HSCs in a murine xenograft model in vivo.ResultsWe observed that although there are only six amino acid differences between AAV1 and AAV6, AAV1, but not AAV6, transduced mouse HSCs well, whereas AAV6, but not AAV1, transduced human HSCs well. None of the 10 serotypes transduced cynomolgus monkey HSCs in vitro. We also evaluated the transduction efficiency of AAV6 vectors containing mutations in surface-exposed tyrosine residues. We observed that tyrosine (Y) to phenylalanine (F) point mutations in residues 445, 705 and 731 led to a significant increase in transgene expression in human HSCs in vitro and in a mouse xenograft model in vivo.ConclusionsThese studies suggest that the tyrosine-mutant AAV6 serotype vectors are the most promising vectors for transducing human HSCs and that it is possible to increase further the transduction efficiency of these vectors for their potential use in HSC-based gene therapy in humans.