A base editor changed all PCSK9 alleles in human embryos; a Nature paper reported chromosomal damage and mosaic edits
A base editor changed all PCSK9 alleles in human embryos; a Nature paper reported chromosomal damage and mosaic edits
A Nature paper published on September 9, 2026 describes experiments in early human embryos targeting two genes, PCSK9 and HBG. The adenine base editor ABE8e-V106W, delivered as a protein at fertilization, changed all PCSK9 alleles and allowed some embryos to reach the blastocyst stage, which precedes implantation. In the published version, the authors also report rare chromosomal damage at the target site, chromosomal abnormalities, and mosaic unintended edits: different cells within the same embryo carried different changes.
Conventional CRISPR/Cas9 cuts both strands of DNA. In early human embryos, repair of these breaks has been associated with large DNA deletions and the loss of chromosome segments. An adenine base editor nicks one strand and chemically changes an individual DNA “letter.” PCSK9 and HBG were selected as two targets already studied in somatic cell editing to examine how embryos repair this type of damage.
The team first tested guide RNAs, molecules that direct the editor to the intended DNA site, in cell models, then tested the editor in human zygotes. Delivering the editor as a ready-made protein together with a guide RNA allowed some embryos to reach the blastocyst stage and yielded embryonic stem cell lines with edited PCSK9. Delivering mRNA, the instructions a cell uses to produce the editor, often caused development to arrest during the first few cell divisions. The published version links this arrest to the activity of the deaminase, the editor’s enzymatic component, independently of the guide RNA.
The June analysis of the study’s preprint recorded the findings available at the time: no large DNA deletions or chromosomal abnormalities had been found after editing. In the September paper, the authors likewise found no DNA insertions or deletions, but reported a different pattern of damage: a rare chromosome break at the target site, chromosomal abnormalities, and mosaic unintended edits near the targets and elsewhere in the DNA.
To assess safety, researchers examine the embryo’s entire genome, including chromosome integrity and whether different cells carry the same changes. The authors state that the effects on the genome and development currently rule out clinical use of this approach in reproduction.