Preprint: base editing at PCSK9 and HBG in human embryos achieves efficient correction without chromosomal disruption

A preprint posted to bioRxiv describes experiments in which researchers used base editors — variants of the CRISPR-Cas9 system that introduce targeted single-nucleotide changes via DNA nicks and mismatches, rather than double-strand breaks — to edit two loci in human embryos: PCSK9, a gene involved in cholesterol metabolism, and HBG, encoding foetal haemoglobin.

The study reports efficient editing at both targets. Crucially, unlike standard Cas9-mediated editing, which the authors note causes frequent aneuploidy (gain or loss of whole chromosomes) and large deletions in early human embryos — a consequence of deficient double-strand break repair at this developmental stage — base editing did not produce detectable chromosomal abnormalities.

The findings are presented as addressing a significant safety concern that has limited the translational potential of CRISPR-based approaches in the embryo context. The authors frame the work as relevant to understanding DNA repair biology in early human development and, prospectively, to the possible correction of disease-causing variants.

As a bioRxiv preprint, this work has not yet completed peer review. The PCSK9 target is also of note given a separate recently reported Phase 1 trial of base editing in adults by Verve Therapeutics and Eli Lilly (published in Genetic Current, 27 May 2026), though the embryo-editing context here is scientifically and ethically distinct. The work will be of interest to researchers in gene editing, reproductive genomics, and developmental biology, and to genetic counsellors following advances in germline correction technologies.