This article originally appeared on Time.com.
Scientists have successfully used CRISPR, a tool that cuts DNA with more precision than any other genome editing technology, to fix a genetic defect in human embryos that can cause serious heart problems, according to a landmark new study in the journal Nature. This is the first use of CRISPR on human embryos in the U.S.
Chinese scientists have reported using CRISPR to correct genetic defects in human embryos, but some of the embryos they used weren’t viable.
Shoukhrat Mitalipov, from Oregon Health & Science University, collaborated with researchers at the Salk Institute, as well as with scientists from China and South Korea, to improve on those results. They applied CRISPR at the earliest stage possible—when the embryo is still a single cell—to ensure that the genetic changes they introduced were propagated to every cell of the embryo as it divided and developed. Because the embryos were created for research purposes only, none were allowed to develop beyond three days.
MORE: First CRISPR Human Trial Approved in the U.S.
CRISPR, which was introduced in 2012, precisely cuts DNA but does not repair it. If combined with other techniques, however, researchers say it could both cut out disease-causing genes and replace them with healthy versions to essentially cure genetic human diseases. So in order to further the science, Mitalipov and his colleagues wanted to test what happened when CRISPR was used in a human embryo. Theoretically, once CRISPR broke the DNA in the appropriate place to cut out a mutation, the cell’s natural repair mechanisms would kick in to repair the injury, fixing the defect this time with the proper code—much like how a word processor’s autocorrect function fixes spelling mistakes.
Unfortunately, this process isn’t very efficient in adult cells in which CRISPR has been tested, so Mitalipov expected similarly low yields in the embryos.
To his surprise, however, he found that embryos were very effective at fixing breaks in DNA.
He created embryos that contained a specific defect known to cause a heart condition by fertilizing healthy donor eggs from various women with sperm from a man who carried the genetic mutation for the disease. He then introduced CRISPR to splice out the mutated gene in more than 50 embryos just after the sperm fertilized the eggs, when the embryos were still just one cell. Several days later, 72% of the embryos showed no sign of the mutated gene; the gene was essentially corrected in all of their cells.
MORE: How the Science of CRISPR Can Change Your Genes
It turns out that the embryo relies on the normal copy of the gene, in this case from the egg, to fix the break made when CRISPR cut out the mutated gene. They key was to introduce CRISPR early enough so the embryo’s own DNA repair system could fix the mutated gene. That’s encouraging for one potential use of CRISPR in the future as a way to correct inherited genetic disease, says Mitalipov, since the embryo seems to have a built-in, reliable way of repairing the injury caused by splicing out an abnormal gene…
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