Potential of recombinant avian adeno-associated virus as a viral vector for CRISPR/Cas9 delivery to avian cells

While genome editing has been established in chickens, where cultured primordial germ cell (PGC) systems are available, the implementation of genome editing remains a major challenge in many other birds due to the lack of robust PGC culture methods. Therefore, the development of reliable and efficient tools can significantly accelerate precision genome modification in avian species. Here, we evaluated the applicability of recombinant avian adeno-associated virus (rA3V) as a delivery vector for a CRISPR/Cas9 construct in avian cells using Staphylococcus aureus-derived Cas9 (SaCas9) and single-guide RNA (sgRNA). Infection with rA3V particles carrying an EGFP expression cassette (rA3V-EGFP) successfully induced EGFP expression in chicken fibroblasts (DF-1) cells, with approximately 80 % EGFP-positive cells at the maximum multiplicity of infection (MOI = 10,000). In plasmid-based transfection experiments, sgRNAs targeting the chicken tyrosinase locus and SaCas9 exhibited DNA cleavage activity in DF-1 cells. Furthermore, infection with rA3V particles encoding these CRISPR components successfully introduced indel mutations into the tyrosinase gene in DF-1 cells, with a calculated indel frequency of approximately 5.4 % at MOI = 40,000 without drug selection. Although EGFP expression was observed in quail fibrosarcoma cells, the percentage of EGFP-positive cells was much lower than that in DF-1 cells. In addition, in vivo infection with rA3V-EGFP of the chicken blastoderm failed to induce EGFP expression in germline cells, even at the highest applicable viral dose. In summary, rA3V can be used as a genome-editing vector in birds, although further investigation of its infectivity and tropism is necessary to expand its applicability to diverse avian species.