The embryos were altered at the stage just after fertilization. (Bigstock)

In March, a rumor surfaced in the scientific community that was intriguing, and perhaps a bit chilling: According to those in the know, researchers in China had successfully edited the genomes of human embryos, altering their DNA in a way never accomplished in our own species. MIT Technology Review reported on the murmurings that someone had altered the germ line -- the genetic information that come together and form something new when eggs and sperm collide. Even unconfirmed, those rumors led to a lot of debate about the potential downsides of altering the germ line. Carl Zimmer has more on the controversy at his blog on National Geographic.

But it turns out the rumors are true.

On Wednesday, Nature News reported that the paper in question had been quietly published in a low-profile journal called Protein & Cell.

But no, you can't expect to see any genetically altered humans running around in the coming months, for better or for worse. From MIT Technology Review:

The team did not try to establish a pregnancy and say for ethical reasons they did their tests only in embryos that were abnormal.

“These authors did a very good job pointing out the challenges,” says Dieter Egli, a researcher at the New York Stem Cell Foundation in Manhattan. “They say themselves this type of technology is not ready for any kind of application.”

The work,  led by Junjiu Huang of Sun Yat-sen University in Guangzhou, focused on modifying the gene responsible for β-thalassaemia, a blood disorder that can be fatal. They used CRISPR, a cutting-edge gene editing tool that's already made serious waves in the genome editing of other species. By going after genes at the earliest stage of human development -- in a single-celled embryo -- theoretically one can make sure all the subsequent copies of the gene are the superior version.

But we have a long way to go before that's actually the case. From Nature News:

The team injected 86 embryos and then waited 48 hours, enough time for the CRISPR/Cas9 system and the molecules that replace the missing DNA to act — and for the embryos to grow to about eight cells each. Of the 71 embryos that survived, 54 were genetically tested. This revealed that just 28 were successfully spliced, and that only a fraction of those contained the replacement genetic material. “If you want to do it in normal embryos, you need to be close to 100%,” Huang says. “That’s why we stopped. We still think it’s too immature.”

Indeed, not all of the embryos that successfully received the new gene had it throughout all of their cells. Many were mosaics, with some good copies of the gene and some bad ones. Carl Zimmer points out that this could end really badly: If a doctor tested the potential embryos of a couple looking to edit out a deadly gene mutation, they might take a cell sample that showed the embryo as having the healthy gene. If it was a mosaic, the resulting baby might in fact have enough copies of the bad gene to cause problems.

And in some of the embryos, the gene editing caused unintended mutations in other genes.

"I believe this is the first report of CRISPR/Cas9 applied to human pre-implantation embryos and as such the study is a landmark, as well as a cautionary tale," George Daley, a stem-cell biologist at Harvard Medical School, told Nature. "Their study should be a stern warning to any practitioner who thinks the technology is ready for testing to eradicate disease genes."

So it's an exciting first step, but nothing more. Science has a long way to go before this is something we can do on embryos intended for implantation, and some are already trying to discourage researchers from taking the next steps toward that goal. Even the study authors themselves are incredibly cautious about their findings, firmly reporting that the technique is far from ready.

But you can be sure that such a caveat won't stop a lot of hoopla about the pros and cons of "designer babies."

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