Clustered regularly interspaced short palindromic repeats‑associated protein 9 (CRISPR-Cas9) based genome editing in avian primordial germ cells: Comparative technologies, translational applications, and regulatory challenges
Primordial germ cells (PGCs) are a unique platform for heritable gene editing in avian species, because they allow easy isolation, culture, and can then be reintroduced into the host. CRISPR/Cas9 technologies have advanced avian genome editing by enabling targeted editing of genes and traits for production, health, reproduction, and welfare. This review critically evaluates genome-editing tools in avian PGCs, including CRISPR/Cas9, transcription activator-like effector nucleases (TALENs), base editing, and prime editing. Germline transmission efficiency, suitability, precision, and heritability are compared. Efficiency, cytotoxicity, and translational feasibility of delivery strategies, including viral vectors, electroporation and the use of ribonucleoproteins are assessed. Applications of these techniques in chickens are for muscle growth via myostatin (MSTN) gene disruption, viral resistance via editing of the sodium/hydrogen exchanger 1 (NHE1) gene, and control of male and female ratios through modification of sex determination genes. Additionally, applications in biopharmaceutical protein production and animal biodiversity conservation are also explored. Despite advances, some limitations remain, including low efficiency of homology-directed repair, off-target effects, mosaicism, and variability in transmission through the germ line. Current evidence demonstrates a significant lack of germline validation and scalability creating barriers to translate this potential into commercial avian production, especially, poultry breeding. Regulatory frameworks and their implications for food and commercialization are also discussed. Future research should prioritize precision editing, scalable delivery systems, and regulatory alignment to enable practical, ethical, and responsible implementation.