P. Motta & T. Naguro/SPL

In a historic decision, the United Kingdom's House of Commons has voted to legalize a gene-therapy technique that could help women to avoid passing genetic defects onto their children.

The vote, decided by 382 members of parliament casting in favour and 128 against, is expected to lead to the United Kingdom becoming the first country in the world to allow the transfer of DNA from diseased human eggs to healthy ones in the clinic.

This technique, known as mitochondrial replacement or three-person in vitro fertilization, aims to prevent women passing on harmful mutations in their mitochondria, the cell's energy-producing structures. An estimated 1 in 5,000 children are born with diseases caused by such mutations, which typically affect power-hungry tissues such as the brain, heart and muscles. All mitochondrial DNA is inherited from the mother, and some women carry harmful mitochondrial mutations without experiencing symptoms themselves.

"It's great news for the patients with mitochondrial disease. It gives them real hopes and that's just fantastic,” says Doug Turnbull, a neurologist at Newcastle University, UK, who has led the effort to bring mitochondrial replacement to the clinic.

The 3 February vote will not immediately allow people to seek mitochondrial-replacement procedures. The UK House of Lords is yet to debate and vote on the issue, although it is not expected to go against the Commons decision. Once approved by the Lords, a new law would not take effect until October. Moreover, clinics wishing to offer the treatment must apply for approval from United Kingdom’s fertility regulator, the Human Fertilisation and Embryology Authority (HFEA). And before giving a green light, the HFEA will probably want further evidence that the procedure is safe, and it is likely to vet applications on a case-by-case basis.

Turnbull hopes to eventually offer the procedures to patients in his clinic, but he says it will depend on HFEA licensing. “I was with some of the patients during the Commons debate. When the vote came through there was a whoop of joy," he says. "That's why people do research, isn't it.”

Transfer techniques

Researchers have said in the past that only a handful of women are likely to seek mitochondrial replacement. However, in a 28 January letter to the New England Journal of Medicine1, researchers at Newcastle University estimated that more than 2,000 UK women could benefit from the procedures, because they carry disease-causing mutations in their mitochondrial DNA.

The UK vote is the culmination of a decade-long effort on the part of researchers, advocates and funders to change the law banning mitochondrial replacement. In the early 2000s, Newcastle scientists began experiments with a technique called pronuclear transfer, in which the nuclear DNA from one woman’s fertilized egg is transplanted into another woman’s fertilized egg cell, previously emptied of its own pronucleus. Related techniques have been developed outside the UK (see 'Genome transplant'). But these procedures were only legal for research purposes; UK law had explicitly prohibited their use in human reproduction.

Ethical questions

A series of scientific, ethical and public consultations since 2011 have thrown up few major roadblocks to allowing mitochondrial replacement, and in 2014 Britain's Conservative-led coalition government said that it would seek Parliament’s approval for the procedure — giving today’s vote an air of inevitability.

Proponents of the law change — a who’s who of UK science that included Nobel prizewinners, policy-makers and directors of leading science funders — had swarmed UK media with open letters and signed statements calling on Parliament to act. The opposition was more disparate — although no less vocal — and it included clergy representatives from the Church of England and the Catholic Church, secular advocacy groups, as well as some researchers.

Proponents of allowing mitochondrial replacement in other countries say that today's vote could serve as a watershed leading to the technique's eventual approval elsewhere. The US Food and Drug Administration currently bans such procedures, but it held a meeting last year to consider the scientific data needed to lift the ban. The Institute of Medicine, which advises the US government, plans to issue a report on the ethics of mitochondrial replacement.

The UK vote is “seen as a necessary forerunner to anything happening” in Australia, says David Thorburn, a geneticist at the University of Melbourne, Australia.