Article Impact Level: HIGH Data Quality: STRONG Summary of Nature https://doi.org/10.1038/s41586-026-11118-x Dr. Stepan Jerabek et al.
Points
- Columbia University and IOCB Prague researchers evaluated DNA repair responses in early human embryos by comparing CRISPR Cas9 double strand breaks with base editor single strand nicks.
- Embryos repaired single strand DNA lesions far more efficiently than double strand breaks, which frequently cause genotoxic double strand damage, chromosomal deletions, and developmental arrest.
- Delivering ABE8e-V106W base editor proteins at fertilization achieved complete editing at the PCSK9 locus while supporting development to six day old blastocyst stages.
- Stem cell lines derived from edited six day old embryos provided genetic material confirming precise homozygous editing without insertions or deletions, despite mRNA delivery causing embryo arrest.
- Researchers observed rare on target chromosome breakage, chromosomal abnormalities, and mosaic off target editing, confirming that current technology remains unready for clinical reproductive applications.
Summary
This study evaluated DNA repair outcomes in early human embryos following targeted genetic modification via standard CRISPR-Cas9 double-strand breaks versus single-strand nicks and mismatches introduced by base editors. Led by Štěpán Jeřábek under the direction of Dieter Egli at Columbia University and in collaboration with IOCB Prague scientists Iva Pichová and Michal Doležal, the investigation compared genotoxic repair fidelity across the PCSK9 and HBG loci. The research sought to determine if base editing could overcome the frequent aneuploidy, large deletions, and developmental arrest associated with Cas9-induced double-strand breaks.
To evaluate editing efficiency and embryonic viability, researchers delivered protein-based base editors, specifically ABE8e-V106W, at the fertilization stage. Protein delivery supported normal embryonic progression to the six-day-old blastocyst stage and enabled the derivation of homozygous edited embryonic stem cell lines without detectable insertion or deletion mutations. In contrast, delivery of the editor as mRNA caused frequent embryonic arrest secondary to guide-independent deaminase activity. Although base editor-induced single-strand lesions were repaired far more efficiently than double-strand breaks, rare on-target chromosome breakage and chromosomal abnormalities were still observed.
Further genomic characterization revealed that bystander and off-target editing exhibited significant mosaicism within edited embryos. The successful derivation of stem cell lines from six-day-old blastocysts provided sufficient biological material to confirm these localized structural variations across subsequent cellular generations. The authors conclude that while base editing represents a substantial technical improvement over standard Cas9-induced DNA cleavage, unresolved safety concerns—including off-target mosaicism and sporadic chromosomal damage—currently preclude the clinical application of germline gene editing in human reproduction.
Link to the article: https://www.nature.com/articles/s41586-026-11118-x
References
Jerabek, S., Jung, C., Kappy, M., Zhao, Q., Sung, J., Wang, N., Kim, E., Kim, J., Kulmann, M. I. R., King, M. B., McAndrew, M. J., Li, M., Bhatele, S., Isado, M., Jang, H.-S., Dolezal, M., Prosser, R., Xu, S., Hwang, G.-H., … Egli, D. (2026). Highly efficient base editing at PCSK9 and normal human embryo development. Nature, 1–3. https://doi.org/10.1038/s41586-026-11118-x
