Babies born in UK with DNA from three people to treat inherited disease takes medicine into uncharted territory

Eight babies have been born in the UK with DNA from three people following a procedure to eliminate an incurable inherited disease.

It is a major advance for the technique, called mitochondrial donation therapy, designed to prevent a life-limiting, often fatal, illness caused by genetic mutations in the structures that generate energy in all our cells.

It is also a test of the UK's permissive, but highly regulated, stance on human embryo research that allowed a technique once criticised for creating "three-parent babies" to proceed.

The babies, four girls and four boys - two of them identical twins - were all born in the last five years and are healthy, according to research published in the New England Journal of Medicine.

"It's a great success for these families," said Sir Doug Turnbull, emeritus professor at the University of Newcastle who helped pioneer the treatment.

"This is a devastating disease with no cure and without this technique, they would not feel that their families were free of mitochondrial disease. This gives them that opportunity."

Mitochondrial disease affects around one in 5,000 babies born in the UK.

Depending on the number and type of mutations in their mitochondria, the severity and type of disease can vary, but includes neurological, metabolic and developmental disorders.

Only women at high risk of passing on severe disease qualify for the procedure, provided though a specialist facility at Newcastle upon Tyne Hospitals NHS Foundation Trust.

The identities of the seven families and their babies are being withheld, but a mother of one of the baby boys speaking anonymously said: "The emotional burden of mitochondrial disease has been lifted, and in its place is hope, joy, and deep gratitude."

How does the technique work?

The procedure involves removing the genetic information from an affected mother's fertilised embryo before inserting it into one from a healthy female donor, from which the genetic information has been removed.

Crucially, the hundreds of thousands of diseased mitochondria are left behind, leaving the new embryo with healthy ones present in the donor embryo.

Mitochondria contain a tiny amount of their own unique genetic code, so the resulting babies carry DNA from three different people.

But because it represents just 0.02% of our total DNA and has no bearing on genetic traits we inherit from our parents, researchers behind the technique, have never liked the "three-parent" moniker.

However, the technique - whatever you choose to call it - isn't perfect.

A total of 22 women underwent the procedure but only seven became pregnant, resulting in eight births - a 36% success rate.

Five of the eight babies were born with no trace of disease.

But tests on the other three revealed a small percentage of mutated mitochondria had been carried over during the procedure.

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While they are at levels too low to cause mitochondrial disease, the babies will require careful follow-ups to ensure they continue to develop normally.

"We have designed a study specifically for that purpose," said Professor Bobby McFarland, who leads the service in Newcastle.

"That's what is unique about us offering this in Newcastle because there isn't anywhere else in the world that's doing this in a regulated way."

While there's good reason to expect the children will develop normally, the procedure does take medicine into new territory.

Because mitochondria contain their own genetic code, girls born via the technique - carrying those from the healthy donor - will pass that on to any children they may have in future.

Changing the "germ-line" in such a way has raised ethical concerns.

But for seven new families, and more to follow, the procedure promises to cure a disease that has affected their families for generations.