To its supporters, mitochondrial donation is an exciting new therapy that can prevent a life-limiting condition for which there is no cure. But critics say the technique, which would allow the creation of IVF babies with DNA from three different people, is dangerous and unethical. Ray Crisp reports

BRITAIN is set to become the first country in the world to permit mitochondrial donation after MPs backed an historic amendment to the Human Fertilisation and Embryology Act last night.

The arguments over the controversial technique for making babies goes to the heart of the on-going debate over the benefits and dangers of genetic advances.

Mitochondrial donation could potentially help more than 2,400 women in the UK, according to new research.

All are said to be at risk of transmitting harmful DNA mutations in the mitochondria, tiny rod-like power plants in cells, onto their children and future generations. The new treatments would allow a child to receive normal "nuclear" DNA from its mother and father, but also a small amount of healthy mitochondrial DNA (mDNA) from a woman donor.

WHAT ARE MITOCHONDRIA?

Mitochondria are tiny rod-like structures in cells which act as power houses, generating the energy that allows our bodies to function. Unusually, they have their own DNA, distinct from the genetic material within the cell nucleus. Mitochondrial DNA (mDNA) makes up about 0.1 per cent of a cell's total DNA and does not affect individual characteristics such as appearance and personality.

WHAT CAUSES MITOCHONDRIAL DISEASE?

Harmful mutations in mitochondrial DNA can prevent the mitochondria working properly, resulting in a number of diseases some of which can be serious and life threatening. They may affect major organs and cause conditions ranging from poor vision to diabetes and muscle wasting.

HOW ARE MITOCHONDRIAL DISEASES PASSED ON?

Children may inherit mitochondrial DNA defects from their mothers, but not their fathers. People with faulty mDNA can develop symptoms or be carriers of the condition without experiencing ill-effects themselves.

WHAT IS MITOCHONDRIAL DONATION?

Defective mDNA from a mother's egg can be replaced with healthy mDNA from a donor. This will then prevent the harmful mutations being inherited and passed onto future generations.

WHAT ARE THE TECHNIQUES INVOLVED?

There are two different procedures, one carried out before fertilisation and the other after.

Maternal Spindle Transfer (MST) involves first removing the nuclear DNA from a donor egg whose mitochondria are healthy. The "spindle" of chromosomes containing the mother's nuclear DNA is then taken from her egg and inserted into the donor egg. As a result, the donor egg is left with nuclear DNA from the mother and mDNA from the donor. This healthy egg is then fertilised and implanted into the mother's womb.

Pronuclear Transfer (PT) is similar but in this case the mother's egg is fertilised first. Its nuclear DNA is then transferred to a fertilised donor egg, containing healthy mitochondria, whose own nuclear DNA has been removed. This healthy fertilised egg is then implanted.

HOW SAFE IS MITOCHONDRIAL DONATION?

Animal and laboratory experiments suggest that the procedures are safe, but no-one can say that the risk is zero. Three separate reviews by an expert panel convened by the Human Fertilisation and Embryology's Authority have found no evidence that the techniques are unsafe for clinical use.

WHAT ARE THE ARGUMENTS IN FAVOUR?

Supporters argue that it would be immoral not to take advantage of a technique that could prevent devastating and potentially lethal diseases.

They stress iIwill be up to the fertility regulator, the Human Fertilisation and Embryology Authority (HFEA) to judge each application on its merits and decide whether or not it should be approved. Licences permitting mitochondrial donation will only be granted if the authority is satisfied that women or their babies will not be put in harm’s way.

Changing the law to allow mitochondrial donation in no way affects the firm ban on altering nuclear DNA or reproductive human cloning.

Critics, such as the group Human Genetics Alert and Roman Catholic Church in England and Wales, maintain that despite the scientific reviews, the decision to legalise mitochondrial donation is being taken too hastily.

No clinical trial has taken place to show conclusively that the treatments are safe in humans. The first women to be treated will therefore be human guinea pigs.

Opponents point out that potentially serious problems might only arise once the procedures are carried out for real. For instance, it might turn out that mDNA affects personal traits as well as metabolism in unforeseen ways.

More generally, they say mitochondrial donation is a step too far, and crosses an ethical boundary. It could mark the start of a slippery slope that leads to the creation of "designer babies" whose genes are tweaked to provide desirable characteristics.