The researchers also discovered something unexpected: a previously unknown way that embryos repair themselves.
In other cells in the body, the editing process is carried out by genes that copy a DNA template introduced by scientists. In these embryos, the sperm cell’s mutant gene ignored that template and instead copied the healthy DNA sequence from the egg cell.
“We were so surprised that we just couldn't get this template that we made to be used,” said Shoukhrat Mitalipov, director of the Center for Embryonic Cell and Gene Therapy at Oregon Health and Science University and senior author of the study. “It was very new and unusual.”
The research significantly improves upon previous efforts. In three sets of experiments in China since 2015, researchers seldom managed to get the intended change into embryonic genes.
And some embryos had cells that did not get repaired － a phenomenon called mosaicism that could result in the mutation being passed on － as well as unplanned mutations that could cause other health problems.
In February in the United States, Engineering and Medicine committee endorsed modifying embryos, but only to correct mutations that cause “a serious disease or condition” and when no “reasonable alternatives” exist.
Sheldon Krimsky, a biologist at Tufts University, said the main uncertainty about the new technique was whether “reasonable alternatives” to gene editing already exist.
As the authors themselves noted, many couples use pre-implantation genetic diagnosis to screen embryos at the fertility clinics, allowing only healthy ones to be implanted. For these parents, gene editing could help by repairing mutant embryos so that more disease-free embryos would be available for implantation.
Hank Greely, director of the center for Law and the Biosciences at Stanford, said creating fewer defective embryos also would reduce the number discarded by fertility clinics, which some people oppose.
The larger issue is so-called germline engineering, which refers to changes made to embryo that are inheritable.
“If you're in one camp, it's a horror to be avoided, and if you're in the other camp, it's desirable,” Dr. Greely said. “That's going to continue to be the fight, whether it's a feature or a bug.”
For now, the fight is theoretical. The United States Congress has barred the Food and Drug Administration from considering clinical trials involving germline engineering. And the National Institute of Health is prohibited from funding gene-editing research in human embryos.
The new study was founded by Oregon Health and Science University, the Institute for Basic Science in South Korea, and several foundations.
The authors say they hope that once the method is optimized and studied with other mutations, officials in the United States or another country will allow regulated clinical trials.
“I think it could be widely used, if it's proven safe,” said Dr. Paula Amato, a co-author of the study and reproductive endocrinologist at Oregon Health and Science University.
Besides creating more healthy embryos for in vitro fertilization, she said, it could be used when screening embryos is not an option or to reduce arduous IVF cycles for women.
Dr. Mitalipov has pushed the scientific envelope before, generating ethical controversy with a so-called three-parent baby procedure that would place the nucleus of egg of a woman with defective cellular mitochondria into the egg from a healthy woman. The F.D.A. has not approved trials of the method, but Britain may begin one soon.
The new study involves hypertrophic cardiomyopathy, a disease affecting about one in 500 people, which can cause sudden heart failure, often in young athletes.
It is caused by a mutation in a gene called MYBPC3. If one parent has a mutated copy, there is a 50 percent chance of passing the disease to children.
https://www.npr.org/sections/health-shots/2017/09/20/551779921/editing-embryo-dna-yields-clues-about-early-human-development
