CRISPR-based genome editing in human embryos: a review of efficiency, safety, and ethical implications

Programmable gene editing tools, particularly CRISPR/Cas9 and its advanced derivatives (base and prime editors), have revolutionized biomedical research and offer unprecedented potential for studying human embryogenesis and correcting monogenic diseases. This review systematically examines the evolution and challenges of these technologies in human embryos and mammalian models. We trace key methodological advancements, from initial studies hampered by low efficiency and mosaicism to refined strategies like RNP delivery and base editing that improved precision. A critical shift occurred with the discovery that CRISPR/Cas9 can cause severe on-target damage, such as large deletions and chromosomal loss, redirecting the field's focus toward safety. We present a comparative analysis of editing efficiencies across species (human, mouse, primate, pig, cow, and rabbit) and tools (Cas9, BEs, and PEs), consistently demonstrating the superiority of RNP for precise editing. Fundamental barriers to clinical translation are discussed, including the trade-off between efficiency and mosaicism, persistent off-target effects, and profound ethical concerns. The review concludes that while somatic gene therapy advances rapidly, heritable genome editing remains premature due to unresolved risks. Future progress depends on developing safer editors, understanding on-target consequences, and adhering to rigorous ethical standards.