A U.S. research team has reportedly edited the DNA of a human embryo just as a sperm fertilizes an egg, well before its eight-cell stage.

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First U.S. team to gene-edit human embryos revealed

Since Chinese researchers announced the first gene editing of a human embryo 2 years ago, many expected that similar work in the United States was inevitable. Last night, the MIT Technology Review broke the news that such experiments have happened. The research, led by embryologist Shoukhrat Mitalipov of Oregon Health and Science University in Portland, also reportedly sidestepped problems of incomplete and off-target editing that plagued previous attempts, though details could not be confirmed since the work is not yet published and Mitalipov has so far declined to comment.

If a peer-reviewed paper bears out the news story, “It’s one more step on the path to potential clinical application,” says bioethicist Jeffrey Kahn of Johns Hopkins University in Baltimore, Maryland, who served on a committee convened by the U.S. National Academy of Sciences (NAS) and the National Academy of Medicine in Washington, D.C., to address gene editing. The panel’s report earlier this year concluded that a clinical trial involving embryo editing would be ethically allowable under narrow circumstances.

The first published human embryo–editing work, in 2015, used nonviable embryos and targeted a gene mutated in the heritable blood disorder beta thalassemia. But it revealed major shortcomings in applying the increasingly popular CRISPR gene-editing technology. The few embryos that took up the change made by CRISPR were a patchwork of edited and unchanged cells, and they bore unintended edits outside the targeted gene.

Another Chinese team, from Guangzhou Medical University, in March became the first to report repairing disease-causing mutations in viable embryos, but some still contained a patchy mix of edited cells—a phenomenon called mosaicism. In none of the Chinese efforts did the researchers go on to implant the manipulated embryos in women.

Sources familiar with the new work from Mitalipov’s group told the MIT Technology Review that they had produced tens of successfully edited embryos, and had avoided the issue of mosaicism by injecting eggs with CRISPR right as they were fertilized with donor sperm. The Guangzhou team injected its CRISPR system into single-celled human embryos—it’s not yet clear how much their timing differed from Mitalipov’s. (The new research presumably relied on nonfederal government funding, since Congress prohibits the use of taxpayer funds on research that destroys human embryos.)

Concerns about mosaicism and off-target effects after the published work by the Chinese teams led some to conclude that CRISPR wasn’t safe as a strategy for preventing a disease in a baby, much less adding some “enhancement.” But even with the apparent advance by the Oregon team, a U.S. clinical trial probably isn’t imminent. “It’s noteworthy that … they’ve been able to make some of these claims,” offers Michael Werner, executive director of the Alliance for Regenerative Medicine in Washington, D.C., who argued in a 2015 Nature commentary that ethical and safety issues should put germline editing research off limits. “It’s still a little premature to say that we’ve resolved all these safety issues now.”

The NAS report notes that many inherited diseases can be prevented by selecting healthy embryos for in vitro fertilization, and that embryo editing might only be justified if it presents the only option for a couple to have a healthy biological child. Congress has meanwhile prohibited the U.S. Food and Drug Administration from reviewing applications for clinical trials involving embryo editing.