Nature: Be alert! Gene-edited embryos cause a lot of DNA mutations and recombination!

, June 29, 2020 /PRNewswire/
Biovalley --Bioon/Biovalley -- Three studies show that large amounts of DNA loss and recombination have increased safety concerns about the safety ofgeneticgenome editinga series of experiments to modify human embryos using CRISPR-Cas9 gene editing tools, revealing how the process can make unnecessarily large changes to the genome at or near the target sitethe research, published this month on the preprint server bioRxiv, has not yet been peer-reviewedBut taken together, they give scientists a good idea of the risks of what some call CRISPR-Cas9 editingPrevious experiments have shown that the tool can keep "off-target" gene mutations away from targets, but changes identified in recent studies may be ignored by standard evaluation methods"Targeted effects are more important and harder to eliminate," said Gaetan Burgio, ageneticat the Australian National University in Canberra,"
these safety concerns could inform the ongoing debate about whether scientists should edit human embryos to preventgeneticdiseasesThe process is controversial because it makes a permanent change to the genome that can be passed down from generation to generation "If human embryo genetic editing for reproductive purposes or reproductive systems is compared to space flight, the new data is equivalent to a rocket exploding on a launch pad before take-off." Fyodor Urnov, a gene editor at the University of California, Berkeley, said he was not involved in any of the latest studies unexpected effects in 2015, researchers used CRISPR for the first time to edit human embryos Since then, teams around the world have explored the process with the aim of accurately editing genes But such research is still rare and often subject to strict regulation Mary Herbert, a reproductive biologist at the University of Newcastle in the United Kingdom , said the latest study highlights how human embryos can repair DNA cut by genome editing tools, a key step in CRISPR-Cas9 editing "Before we start attacking it with DNA cutting enzymes, we need a basic roadmap to understand what's going on there," she said Kathy Niakan, a developmental biologist at the Francis Crick Institute in London, , and her colleagues published the first preprinted study online On June 5 In the study, researchers used CRISPR-Cas9 to create mutations in the POU5F1 gene, which is important for embryonic development Of the 18 genome-edited embryos, about 22 percent contained unwanted mutations that affect DNA around POU5F1 They include DNA rearrangement and a large number of missing DNA bases -- far more than researchers usually expect to use this method photo source: Pascal Goetheluck Science Photo Library
another team led by Dieter Egli, a stem cell biologist at Columbia University in New York, studied embryos formed by sperm with mutations in the EYS gene that cause blindness The team used CRISPR-Cas9 to try to correct the mutation, but about half of the embryos tested lost a large number of chromosome fragments -- sometimes the entire chromosome -- where EYS was located a third group of researchers, led by Shoukhrat Mitalipov, a reproductive biologist at Oregon Health and Science University in Portland, studied embryos made from mutated sperm that causes heart disease The team also found signs that editing affects most areas of chromosomes containing mutated genes In all studies, researchers used embryos only for scientific purposes, not pregnancy The lead authors of the three preprints refused to discuss the details of their work with The Nature's news team until they were published in peer-reviewed journals unpredictable repair
these changes are the result of a process in which genome editing tools use DNA repair CRISPR-Cas9 uses a small piece of RNA to guide Cas9 enzymes to a location in the genome with a similar sequence The enzyme then cuts off two STRANDs of DNA at that location, and the cell's repair system repairs the gap editing occurs during the repair process: cells typically close wounds with an error-prone mechanism that inserts or removes small amounts of DNA letters If the researchers provide a DNA template, cells may sometimes use this sequence to repair wounds, leading to a real rewrite But fractured DNA can also lead to recombination or loss of a large area of the chromosome previous use of CRISPR in mouse embryos and other types of human cells has shown that editing chromosomes can have huge, unwanted effects But Urnov says it's also important to demonstrate this work in human embryos because different cell types can react differently to genome editing such rearrangements may be missed in many experiments, which often look for other unwanted edits, such as changes in individual DNA letters, or small insertions or deletions of only a few letters However, recent studies have specifically looked for a large number of missing and chromosomal rearrangements near the target site "All of us in the scientific community will immediately begin to take this issue more seriously," Urnov said This is not a one-off fluke "
genetic changes
the three studies provide different explanations for the production of DNA mutations Egli and Niakan's team attributed most of the changes observed in embryos to large numbers of deletions and rearrangements Mitalipov's team says 40 percent of the mutations they found were caused by a phenomenon called gene conversion, in which DNA repair processes replicate one sequence from one pair of chromosomes to repair another Mitalipov and his colleagues reported similar findings in 2017, but some researchers are skeptical that frequent genetic conversions in embryos may occur They note that the matrilineal and paternal chromosomes were not adjacent when gene conversion was assumed, and that the team's tests to identify gene conversions may have detected other chromosomal changes, including deletions Egli and his colleagues tested the genetic transformation directly in their latest preprint, but couldn't find them Burgio points out that the analytical methods used in the preprints of the Mitalipov team are similar to those used by their team in 2017 One possibility is that DNA fractures at different locations on chromosomes heal differently, says Jin-Soo Kim, a genetic scientist at Seoul National University and co-author of Mitalipov's preprints (BioValleyBioon.com) References: CRISPR Gene editing in human embryos visions chromosomal Mayhem
Alanis-Lobato, G et al P
atbioRxiv https://doi.org/0.1101/2020.06.05.135913 (2020 ) Zuccaro, M V et al.
Reading frame at the EYS locus, and allele-specific-chromosome after removal cas9 slogin in human embryos preprint at bioRxiv https://doi.org/10.1101/2020.06.17.149237 (2020.
Liang, D et al FREQUENT GENE CONVERSION IN HUMAN EMBRYOS FORED BY DOUBLE STRAND BREAKS
PREPRINT AT BIORXIV HTTPS://DOI.ORG/10.1101/2020.06.19.162214 (2020) Adikusuma, F., Piltz, S., Corbett, M.A et al Large deletions by Cas9 cleavage.
Nature 560, E8-E9 (2018) https://doi.org/10.1038/s41586-018-0380-z
Kosicki, M., Tomberg, K and Bradley, A Repair of the double-strand break sr by CRISPR-Cas9 leads to large sops and complex rears Natno Biotechl 36, 765-771 (2018) https://doi.org/10.1038/nbt.4192
Ma, H., Marti-Gutierrez, N., Park, S et al.
Reif of a o'r sion gene ei dat in human Nature 548, 413-419 (2017) https://doi.org/10.1038/nature23305
Egli, D., Zuccaro, M.V., Kosicki, M et al.
Inter-homologue repair in the med human eggs? Nature 560, E5-E7 (2018) https://doi.org/10.1038/s41586-018-0379-5
.