PCSK9 editing reached every allele; mosaicism set the clinical limit
ABE8e-V106W delivered as protein at fertilisation edited every PCSK9 allele in human embryos, which developed to the blastocyst stage. No insertions or deletions were detected, but mosaic changes at bystander and off-target sites and rare chromosomal damage occurred. The researchers say the method is useful for studying early development but is currently unsafe for clinical use in reproduction.
Science··Night
Protein delivery reached the complete target
The peer-reviewed Nature paper and Columbia University Irving Medical Center's account report that the adenine base editor ABE8e-V106W reached every PCSK9 allele in human embryos when delivered as protein at fertilisation. The embryos developed to the blastocyst stage, and homozygous edited stem-cell lines were derived. The Columbia team followed the embryos for 6-7 days; changes made before the first cell division could persist in the resulting daughter cells. The result shows that the method can be a functional tool for studying early human development.[1], [2]
Changes around the target remained mosaic
No sequence-level insertions or deletions were detected. Rare on-target chromosome breakage and chromosomal abnormalities nevertheless occurred, and Columbia's account also describes rare large chromosomal deletions. Editing at bystander and off-target sites was mosaic, meaning that not every cell in the same embryo carried the same changes. A sample drawn from a few cells therefore cannot represent the genomic state of the entire embryo with certainty. High target efficiency and unpredictable cell-to-cell variation appeared within the same experiment.[1], [2]
Delivery format changed whether development continued
When the editor was delivered as mRNA at high levels, guide-independent deaminase activity frequently stopped embryo development. Targeting success with protein delivery therefore does not transfer automatically to other delivery formats. The researchers stress that base editing reduces some damage associated with Cas9's double-strand DNA cuts but can still produce undesirable genomic and developmental outcomes. The clinical conclusion is clear: those uncertain and unpredictable changes currently preclude reproductive use, while the technique retains value as a laboratory tool for investigating early development.[1], [2]
Related columns
For more information on this topic, you can read the related columns.
