Children conceived using mitochondria from a donor are now doing well

Eight young children appear to have been protected from serious genetic conditions after being born via a procedure that replaces defective mitochondria from their mothers with those from a female donor.

Around 1 in 5000 people have defective mitochondria, which ordinarily provide cells with energy and are only inherited from our mothers. These defects are caused by genetic mutations that can lead to issues such as blindness, seizures and, in extreme cases, death. “It’s very difficult for families to deal with these diseases; they are devastating,” says Bobby McFarland at Newcastle University, UK.

In 2015, the UK became the first country to approve a procedure called pronuclear transfer for women who are at high risk of passing on mitochondrial conditions and would not benefit from preimplantation genetic testing, which screens embryos for such mutations.

Pronuclear transfer involves eggs from both a would-be mother and a donor being fertilised with sperm from the father via IVF. About 10 hours later, the nucleus is removed from both eggs. This carries most of a cell’s genetic material, which is separate from mitochondrial DNA.

The mother’s nucleus is then inserted into the donor’s egg, resulting in an embryo that inherits most of its DNA from its biological parents, but mitochondria from the donor. A small amount of the mother’s mitochondria may still be unintentionally passed on, says Bert Smeets at Maastricht University in the Netherlands.

McFarland, who developed the approach along with his colleagues, has since used the technique on 19 women with harmful mutations in around 80 per cent or more of their mitochondria – the level that usually causes issues to arise.

Seven of the women became pregnant after the final embryos were transferred into their uteruses. They gave birth to eight healthy babies in total, with one having twins.

The researchers analysed blood samples collected from the newborns. They found no harmful mitochondrial DNA mutations in five of them, and only a very low level in the remaining three. “I would say that the results are as good as you could have hoped for,” says Mike Murphy at the University of Cambridge.

Several months or years on from these various pronuclear transfers, all of the children are hitting their developmental milestones. However, some have also experienced complications, which may or may not be related to the transfers. For instance, one developed high blood fat levels and an abnormal heartbeat, which were successfully treated, while another developed epilepsy at seven months of age, which spontaneously resolved.

The team plans to continue following the children to determine any longer-term effects of the procedure.