Optimization of genome editing strategies for targeted integration at chicken sex chromosome loci

The precise targeted integration of large exogenous genes into chicken sex chromosomes is of particular interest for sex-linked trait manipulation, sex-control breeding, and the development of avian bioreactor models. However, efficient targeted integration at sex chromosome loci remains technically challenging, and optimized editing strategies for these loci are still lacking. To improve targeted genome editing at two previously identified chicken sex chromosome safe-harbor loci, EE0.6 and NC_006127.4, this study systematically evaluated and optimized key parameter affecting editing efficiency. First, we evaluated the effects of different sgRNA combinations on targeted knockout efficiency, establishing the advantage of a dual-sgRNA/Cas9 architecture, which achieved knockout efficiencies of 86.67% and 75.00% at the EE0.6 and NC_006127.4 loci, respectively. We next introduced the Cas9 nickase (Cas9n) system, which has previously been reported to exhibit improved editing specificity, and evaluated its performance at both target loci. Quantitative analysis showed that the dual-sgRNA/Cas9 targeting system successfully mediated the precise targeted integration of a 1.1-kb SV40-mCherry reporter cassette, reaching 100% (28/28) at the EE0.6 locus and 80.00% (20/25) at the NC_006127.4 locus. Based on this result, this study further investigated the effects of donor homology arm (HA) lengths (200 bp, 600 bp, and, 1000 bp) and vector topologies (circular and linearized) on targeted knock-in efficiency. The results revealed that in the circular donor system, the optimal HA lengths for the EE0.6 and NC_006127.4 loci were 200 bp (50.4% ± 4.4%) and 600 bp (30.1% ± 1.2%), respectively. However, upon the introduction of linearized donors with free ends, the knock-in efficiency of the exogenous target fragment was significantly enhanced, and its HA length preference underwent a significant reversal. The optimal HA length for EE0.6 was extended to 600 bp (78.9% ± 1.0%), whereas that for NC_006127.4 was shortened to 200 bp (49.9% ± 0.4%). In summary, this study established an efficient targeted integration strategy for chicken sex chromosome loci. The optimized system provides a foundation for future applications in sex-linked breeding and avian bioreactor development.