Mixed results have been obtained from ground-breaking studies of a "three parent" method of preventing inherited mitochondrial diseases.

Scientists testing the technique, which involves switching DNA in a donor egg, successfully produced human embryos, but over half contained abnormalities.

Despite this, the US researchers said they had demonstrated a viable way of preventing a mother passing devastating diseases onto her offspring. The technique, known as maternal spinal transfer (MST), is one of the procedures that could be allowed following a landmark change in the law under consideration in the UK.

A public consultation is now under way on the proposal, which would effectively permit the creation of IVF babies with three genetic parents - a mother, a father and a donor.

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Mitochondria are rod-like structures in cells that have their own DNA - separate from that in the nucleus - and act as powerhouses, pumping out energy. Only mothers pass on mitochondrial genes to sons and daughters.

Defects in mitochondrial DNA (mDNA) give rise to a range of serious and potentially life-threatening diseases, including a form of muscular dystrophy, and conditions leading to hearing and vision loss, heart problems and intestinal disorders.

The aim of mitochondrial replacement is to prevent embryos being created with abnormal mitochondria, thereby breaking the chain of inherited disease.

MST is carried out before fertilisation. A spindle-shaped structure containing a mother's nuclear DNA is first removed from one of her eggs. This is transplanted into a healthy donor egg - with normal mitochondria - which has had its own spindle removed, and then fertilised by the intended father's sperm.

The developing embryo and baby now has cells containing nuclear DNA from both its parents, and healthy mitochondrial DNA from the donor.

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'Three parent' gene therapy results

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