As far as groundbreaking developments go, there are few technologies that have captured the life science world’s imagination like CRISPR, the gene-editing tool that holds promise in everything from treating sickle cell disease to sussing out appropriate gene targets in the fight against HIV. Now, U.S. scientists have reached a revolutionary new milestone in CRISPR-Cas9 development, successfully modifying embryos to cut out defective genetic code that would have caused an inherited disease.
A study published in the journal Nature on Wednesday outlines the process used by researchers from the Oregon Health & Science University, the Salk Institute for Biological Studies in California, and Korea’s Institute for Basic Science.The faulty genetic code that would have caused hypertrophic cardiomyopathy was successfully repaired using CRISPR during the in vitro fertilization process. It wasn’t a flawless success—the rate of fixed embryos moved from the naturally expected 50% to 74%. And, to be clear, this isn’t the first time that CRISPR has been tested in a non-animal setting—last year, Chinese scientists launched the first known trials in humans.
But the new embryo experiments were striking for both their efficacy and a lack of adverse events like mutations in other parts of the embryos’ genomes. “We have demonstrated the possibility to correct mutations in a human embryo in a safe way and with a certain degree of efficiency,” said the Salk Institute’s Juan Carlos Izpisua Belmonte, who co-authored the Nature study.
The achievement is already drawing some controversy. Bioethicists have previously questioned whether or not modifying embryos—even for the purposes of preventing a disease’s spread—could foster a slippery slope. “Editing human embryos with CRISPR should be a long way off,” as J. Craig Venter, co-founder of Human Longevity, Inc and a genome expert, put it during Fortune‘s second annual Brainstorm Health conference in May. “Not something we do next week.”
But these embryos weren’t actually allowed to develop beyond a few days. And groups like the American Society of Human Genetics mostly center their opposition on modifying embryos for implantation into a human. As for the possibility of “designer babies,” CRISPR technology isn’t nearly advanced enough to make anything approaching that feasible, as Belmonte explains.
That’s part of the reason why the U.S. researchers took up the recommendations of an ethics committee which concluded that “with significant oversight and continued dialogue, the use of gene correction technologies in human embryos for the purpose of answering basic science questions needed to evaluate germline gene correction prior to the use in human models” was acceptable.
This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.
