GoKawiilCas9-based tools enable programmable DNA lesions for studying repair outcomes, gene function, and genome correction. In human embryos, Cas9-induced DNA double-strand breaks are genotoxic, causing frequent aneuploidy and large deletions1,2. Here, we evaluate DNA repair outcomes at nicks and mismatches introduced by base editors at the PCSK9 and HBG loci in human embryos. Delivering ABE8e-V106W as a protein at fertilization achieved editing at all PCSK9 alleles, supporting development to the blastocyst stage and the derivation of homozygous edited stem cell lines. No insertions or deletions were detected, although rare on-target chromosome breakage and chromosomal abnormalities occurred. Nevertheless, editing at bystander and off-target sites was mosaic, and the introduction of the editor as mRNA caused frequent embryo arrest due to guide-independent deaminase activity. Thus, unlike Cas9-induced DNA breaks, base editor-induced lesions are efficiently repaired. However, undesirable consequences for the genome and development can occur, currently precluding clinical use in reproduction.
Highly efficient base editing at <i>PCSK9</i> and normal human embryo development
Researchers show that base editors, unlike Cas9's double-strand breaks, can edit all copies of the PCSK9 gene in human embryos without indels while still allowing development to the blastocyst stage. But mosaic off-target edits, rare chromosome breakage, and embryo arrest when delivered as mRNA mean the technique remains unsafe for reproductive use. The work sharpens the scientific and ethical debate over heritable human genome editing at a moment when gene-editing startups are courting investment.
- Protein-delivered ABE8e-V106W edited every PCSK9 allele and yielded homozygous edited stem cell lines with no detected insertions or deletions.
- Base editor lesions repair far more cleanly than Cas9 double-strand breaks, which cause frequent aneuploidy and large deletions in embryos.
- Mosaic bystander/off-target editing, rare chromosomal abnormalities, and mRNA-linked embryo arrest rule out clinical reproductive use for now.
A Crack in Creation (Doudna & Sternberg) — If this base-editing embryo study left you wanting the fuller story, Jennifer Doudna's own account of CRISPR's discovery and its thorny ethical stakes is the perfect follow-up. It walks through how programmable gene editing works and why editing human embryos remains so contested — exactly the debate this paper feeds into.
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