Mitochondrial DNA transfer is an IVF technique that stops serious genetic diseases passing from a mother to her baby.
The IVF technique replaces faulty mitochondrial DNA in an egg or embryo with healthy DNA from a female donor.
Next month, when the House of Lords votes to support the change - which it is expected to - the UK will be the first country in the world to permit embryos to be made with the genetic material of three people: a man and two women.
Diseases arising from faulty mitochondrial DNA usually affect several organs, and include symptoms like muscle wasting, loss of movement control, diabetes and epilepsy. They can also cause liver failure, heart problems, stroke-like episodes, and - in severe cases - death.
According to a new study published in a leading journal recently, around 2,500 women could benefit from mitochondrial donation in the UK. This would amount to an average of 150 babies born each year.
The study also estimates around 12,400 women in the US would benefit from the procedure should it be introduced, amounting to around 770 or more births a year.
Three-parent babies could be born in 2016
The team that pioneered the technique at Newcastle University in the UK - led by Doug Turnbull, professor of neurology - is ready to begin trialling the procedure as soon as the new law comes into effect in October 2015, so the first three-parent babies could be born in 2016.
Mitochondria are small "battery packs" inside the body's cells that convert food into energy for cells to function. They have their own DNA that only controls mitochondrial function and energy production.
Mitochondrial diseases particularly affect tissues that have high energy demands, such as the brain, heart, muscles, liver and kidneys.
Mitochondrial DNA is separate from the DNA in the cell nucleus that determines a person's physical appearance and personality.
In normal human reproduction, mitochondria are usually passed on almost exclusively from the mother; the small amount that is present in sperm is only just enough to fuel its passage to the egg and is normally destroyed by the egg cell after fertilization.
Thus, because babies inherit their mitochondria from their mothers, the new IVF technique only replaces genetic material in the eggs of mothers with faulty mitochondrial DNA.
Mitochondrial DNA is 'less than 0.054% of total DNA'
There are two ways to carry out mitochondrial DNA transfer. In one, the nuclear DNA is removed from the egg of the mother and is inserted into the egg of a female donor where the nuclear DNA has been stripped out. The egg is then fertilized with the father's sperm.
In the other technique, the mother's egg is first fertilized with the father's sperm, creating a fertilized egg with nuclear DNA from both parents. This nuclear DNA is then removed and inserted into an egg from a female donor that has had the nuclear DNA removed.
In both cases, the result is an embryo with nuclear DNA from the mother and the father, but with healthy mitochondrial DNA from the female donor, enabling a child to be born free of mitochondrial DNA disease.
Prof. Dame Sally Davies, chief medical officer for England - who strongly urged members of parliament (MPs) to vote in favor of changing the law - says we should remember that mitochondrial DNA is less than 0.054% of total DNA.
Concerns about mitochondrial DNA transfer
Those against legalizing mitochondrial DNA transfer are concerned that it opens the door to other types of genetic manipulation.
If it is okay to repair faulty mitochondrial DNA to prevent genetic disease passing from mother to child, then what about other genetic diseases caused by faults in nuclear DNA? Once you cross this line, then it becomes easier to move toward "designer babies," whose genes are manipulated to select preferred traits.
Campaigning watchdogs like Human Genetics Alert say their main objection to mitochondrial DNA transfer is it alters the "germline" of children. The genetic changes would be present in all cells of their bodies and would be transferred to descendants of girls using the technique.
Others object on the grounds that the new technique is not safe. Dr. Rhiannon Lloyd, from the Zoological Society of London in the UK, warns that in half of animal studies, faulty mitochondrial DNA was transferred over during the procedure, reports The Daily Telegraph. Some say this could raise the risk of cancer.
In contrast to the UK position, in the US there are no plans to change the law, as the Cellular, Tissue and Gene Therapies Advisory Committee of the Food and Drug Administration (FDA) says there is not enough evidence to support the use of mitochondrial transfer in people.
And there are also concerns that mitochondrial DNA influences personality. Dr. Ted Morrow, an evolutionary biologist at the University of Sussex in the UK, notes there are published studies that show this.
'Bold step for parliament'
The MPs who supported the change were of the opinion that the benefits outweigh the risks. During the debate in the House of Commons, ministers said the new IVF procedure was a "light at the end of a dark tunnel" for families.
Public Health Minister Jane Ellison told MPs: "This is a bold step for parliament to take, but it is a considered and informed step."
Prime Minister David Cameron - whose own son Ivan died aged only 6 in 2009 from cerebral palsy and epilepsy - says there is no danger of the procedure leading to "designer babies." He said in an interview on LBC Radio that he saw mitochondrial DNA transfer as more like a kidney donation or a lung donation than a fundamental change. "It is not like playing God," he said.
Dr. Jeremy Farrar, director of the Wellcome Trust, which funds the work of Prof. Doug Turnbull and his team at Newcastle University, welcomes the vote and hopes the House of Lords reaches a similar conclusion. He says the UK's regulatory system is admired around the world and notes:
"This is a vote of confidence in the patients, scientists, doctors and ethicists who have worked hard for a decade to explain this complex research to politicians, the public and the media, and in the exemplary process for reviewing scientific, ethical and public opinion led by the Human Fertilisation and Embryology Authority."
Robert Meadowcroft, chief executive of the Muscular Dystrophy Campaign, agrees and adds:
"There are currently no means to treat devastating mitochondrial diseases, which can cause muscle wastage, loss of vision, stroke-like episodes and a premature death. Preventing inheritance, where possible, remains our only option, and that is why we have invested in and wholly support this pioneering technique."
He says the next step is to make sure the peers in the House of Lords are fully armed with the facts about mitochondrial donation IVF before their debate next month because iIt is absolutely crucial that they fully understand what is at stake for women affected by this condition."