British scientists say they have created an embryo that contained genetic material from a man and two women in a groundbreaking step in the fight against a vicious inherited disease.
The lab-dish experiments were carried out in fertilised eggs that developed to a tiny cluster of cells between six and eight days old called a blastocyst.
The technique could one day help would-be mothers from handing down an often tragic and sometimes fatal condition called mitochondrial disease, they said.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Mitochondria, handed down in maternal DNA, are tiny bean-shaped components that provide a cell with energy.
They have their own genes -- just three dozen or so -- but are separate from the egg's nucleus, which contains almost all of the mother's DNA.
Flaws in mitochondria can cripple a cell's function and lead to sickness.
As many as one person in 250 has these mutations, but in many cases, the symptoms are mild, according to figures for the British population cited in the study, published in the journal Nature.
Around 1 in 6,500 people suffer muscular weakness, deafness, heart failure, blindness or other health problems, sometimes fatally so.
The experiments sought to demonstrate that flawed mitochondria could be swapped by healthy ones, using eggs left over from in-vitro fertilisation (IVF) treatment that could not be used for fertility.
In its just-fertilised state, the egg contains two nuclei, one from the mother and one from the father's sperm, before they fuse.
The scientists removed the two nuclei from the egg.
These nuclei were then put into another egg from which the nucleus had been removed, but which retained its properly-functioning mitochondria.
The fused egg thus contained the genes from both father and mother plus a tiny amount of DNA of mitochondrial DNA in the donor egg.
Only around two percent of residual DNA is brought over from the original egg, thus hugely reducing the risk of transferring a mutation.
"What we've done is like changing the battery on a laptop," Professor Douglass Turnbull, a neurologist at Newcastle University in northeastern England, said in a podcast news conference.
"The energy supply now works properly, but none of the information on the hard drive has been changed.
"A child born using this method would have correctly functioning mitochondria, but in every other respect would get all their genetic information from their father and mother."
The work was carried out under the close scrutiny of Britain's Human Fertilisation and Embryology Authority (HFEA).
Permission is being sought for further experiments using normally fertilised eggs, in order to see whether the technique can be used clinically.
Ultimately, the goal would be a stepped-up fertility treatment enabling a mother with a family history of mitochondrial disease to have a child that would be free of the risk of falling sick with this disorder.
Turnbull stressed that further work was needed to determine the safety and efficacy of the technique, but also acknowledged that ethical worries were a hurdle.
"Personally, I would be very disappointed if it wasn't (used on human volunteers) within the next three years," he said.
"But that would depend partly on the science and proving the science, partly on the regulation and partly on the willingness of people to see this sort of work come to fruition. Technically, I think many of the challenges have been overcome."
Britain's Muscular Dystrophy Campaign said it was excited by the work, which could have big benefits for patients with wasting muscular disease.
"We have an honest prospect of taking this disease out at the knees, of being able to say to mums and dads who have this disease in their families, 'you have a real chance of having healthy babies'," said its executive director, Phil Butcher.
"These diseases are potentially devastating, they are extremely harsh and you wouldn't wish them on your worst enemy."
