Scientists have for the first time transferred genes between unfertilised human eggs to produce early-stage embryos in an attempt to prove that it will be safe to go ahead with a radical new technique to rid families of certain inherited diseases.
The researchers transferred the nucleus of an unfertilised egg cell into a human donor egg that had its own nucleus removed to show that it is possible to use this approach for eliminating mitochondrial diseases in all subsequent generations of an affected family.
Mitochondrial diseases are inherited solely through the mother and affect up to one in 200 babies, but only about one in 6,000 suffer serious metabolic problems such as muscular weakness, blindness, fatal heart or liver failure, learning disabilities and diabetes.
Embryos resulting from the process should carry only the healthy mitochondria – the tiny “power packs” of the cells – of the donor’s egg rather than the defective mitochondria of the affected mother.
IVF babies born by this approach will therefore be free of the mitochondrial mutations carried by their biological mothers and so will not pass on the mutations to their own children and grandchildren.
The technique is controversial because for the first time it would result in “germline” gene therapy, where the genetic material of subsequent generations is deliberately altered. It would also result in children who technically have three genetic parents, a father and two mothers.
Scientists, however, have criticised the notion of the “three parent child” on the grounds that there are 23,000 genes in the DNA of the nucleus but only 13 genes in the DNA of the mitochondria – so the genetic contribution of the second “mother” would be minimal, they argue.
The latest study, published in the journal Nature, was carried out by a team led by Shoukhrat Mitalipov of Oregon Health and Science University in Portland, Oregon, who three years ago demonstrated the same technique on laboratory monkeys that went on to give birth to healthy offspring.
“Using this process, we have shown that mutated DNA from the mitochondria can be replaced with healthy copies in human cells….this research shows that this gene therapy method may well be a viable alternative to preventing devastating diseases passed from mother to infant,” Dr Mitalipov said.
“I would say that it’s safe enough at this stage to proceed to clinical trials. The idea is to remove mitochondrial defects so that subsequent generations don’t have them. The whole idea of germline gene therapy is that you don’t just treat the one patient,” said Dr Mitalipov, who has applied for a patent on his technique.
The UK’s Human Fertilisation and Embryology Authority (HFEA) is currently conducting a public consultation on the use of mitochondria replacement. Its expert group has asked for further studies to be carried out to decide whether it is safe to lift the current ban on germline gene therapy for mitochondrial diseases.
A team from Newcastle University showed in 2010 that it is possible to use a similar approach for transferring mitochondria but they did it at the slightly later stage when the eggs were fertilised, rather than between unfertilised eggs.
Dr Mitalipov said that further work needs to be done to work out which approach will be the most effective. One problem with his technique is that about half of the eggs resulting from mitochondria transfer failed to fertilise, he said.
Professor Peter Braude of King’s College London, a member of the HFEA’s expert committee, said that more research is needed before this approach could be used in clinical trials on women affected by mitochondrial disorders.
“It is exactly the sort of science that the HFEA expert committee recommended needed doing, and demonstrates further the feasibility of this technique. However it is still a long way off ready for human use,” Professor Braude said.