CRISPR Gene Editing In Human Embryos Wreaks Chromosomal Mayhem

A suite of experiments that use the gene-editing tool CRISPR-Cas9 to modify human embryos have revealed how the process can make large, unwanted changes to the genome at or near the target site. Nature reports: The first preprint was posted online on June 5 by developmental biologist Kathy Niakan of the Francis Crick Institute in London and her colleagues. In that study, the researchers used CRISPR-Cas9 to create mutations in the POU5F1 gene, which is important for embryonic development. Of 18 genome-edited embryos, about 22% contained unwanted changes affecting large swathes of the DNA surrounding POU5F1. They included DNA rearrangements and large deletions of several thousand DNA letters — much greater than typically intended by researchers using this approach. Another group, led by stem-cell biologist Dieter Egli of Columbia University in New York City, studied embryos created with sperm carrying a blindness-causing mutation in a gene called EYS2. The team used CRISPR-Cas9 to try to correct that mutation, but about half of the embryos tested lost large segments of the chromosome — and sometimes the entire chromosome — on which EYS is situated. And a third group, led by reproductive biologist Shoukhrat Mitalipov of Oregon Health & Science University in Portland, studied embryos made using sperm with a mutation that causes a heart condition. This team also found signs that editing affected large regions of the chromosome containing the mutated gene. The three studies offered different explanations for how the DNA changes arose. Egli and Niakan’s teams attributed the bulk of the changes observed in their embryos to large deletions and rearrangements. Mitalipov’s group instead said that up to 40% of the changes it found were caused by a phenomenon called gene conversion, in which DNA-repair processes copy a sequence from one chromosome in a pair to heal the other. Mitalipov and his colleagues reported similar findings in 2017, but some researchers were skeptical that frequent gene conversions could occur in embryos. They noted that the maternal and paternal chromosomes are not next to each other at the time the gene conversion is postulated to occur, and that the assays the team used to identify gene conversions could have been picking up other chromosomal changes, including deletions. Egli and his colleagues directly tested for gene conversions in their latest preprint and failed to find them, and Burgio points out that the assays used in the Mitalipov preprint are similar to those the team used in 2017. One possibility is that DNA breaks are healed differently at various positions along the chromosome, says Jin-Soo Kim, a geneticist at Seoul National University and a co-author of the Mitalipov preprint.