Highly efficient base editing at PCSK9 and normal human embryo development

Cas9-based tools enable programmable DNA lesions for studying repair outcomes, gene function and genome correction. In human embryos, Cas9-induced DNA double-stranded breaks are genotoxic, causing frequent aneuploidy and large deletions^1,2. 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 breakages and chromosomal abnormalities occurred. Furthermore, 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, in contrast to Cas9-induced DNA breaks, base-editor-induced lesions are efficiently repaired. However, undesirable consequences for the genome and development can occur, currently precluding the clinical use of base editing in reproduction.