ROUGHLY one baby in 6,500 is born with misfiring mitochondria, the tiny power plants found in virtually every cell that release energy from food and oxygen. That can cause a long list of problems, all of which are unpleasant, and many of which are fatal. They include diabetes, deafness, debilitating muscle weakness and progressive blindness, as well as epilepsy, liver failure and dementia. Some babies die shortly after birth. The (relatively) lucky face a life of permanent ill-health. At the moment, such diseases are simply a tragedy that must be lived with. But doctors in Britain and America are working on a cure. If they can perfect a new technique, and if they can persuade the world's governments to legalise it, it will mark a significant moment in medical history, and not just for the benefits it will bring. For one thing, babies born via this technique would possess DNA from three people—the mother, the father and an unrelated egg donor—rather than the usual two. And it would be the first time that a genetic treatment has been licensed that affects not just the individual in question, but his descendants too. How does the procedure work?
The treatment relies on the fact that mitochondria are not just another part of a living cell. They are the distant descendants of bacteria that, a billion years ago, gave up their free-living lifestyle to form symbiotic partnerships with other cells. As a result, mitochondria possess their own tiny genomes, entirely separate from the much bigger hunk of DNA that sits inside the cell nucleus. A baby inherits its "nuclear DNA" almost equally from its mother and father. But it inherits its mitochondria only from its mother: every single one is a descendant of the original set that lived inside the egg. Although the British and American researchers are using different techniques, the basic idea is the same: to give the baby a working set of mitochondria donated by another woman. The scientists take an egg with damaged mitochondria, remove the nucleus (and the DNA it contains) and transplant it into a second, donor egg, whose nucleus has been removed but whose mitochondria are working normally. The result is a baby that will have nuclear DNA inherited from its mother and father in the usual way, but mitochondria inherited from the egg donor.
For some people the idea of a baby that is genetically related to three different people is viscerally unsettling (something that ethicists refer to as the "yuck factor"). Yet the Human Fertilisation and Embryology Authority (HFEA), which regulates fertility treatments in Britain, found that when it explained the procedure to a sample of the British public, most were in favour. The details of the biology may help. The amount of DNA contained within a mitochondrion, and therefore inherited from the egg donor, is miniscule: human mitochondrial DNA encodes just 37 genes, compared with more than 20,000 for the DNA in the cell nucleus. And the mitochondrial stuff is involved with only the basic, low-level functioning of the cell. So there is no chance of the children in question ending up with the egg donor's eyes, hair or personality.
That is not to say that scientists are entirely unconcerned. A report published by the HFEA on June 3rd noted that a few worries remain about technical issues, like possible incompatibilities between the donor's mitochondrial DNA and the foreign nuclear DNA with which it must interact. The fact that the modifications caused by the treatment would also be passed to the descendants of any woman born through the technique is of some concern, too. If mitochondrial replacement is legalised in humans, it would thus be the first genetic treatment whose effects would pass down the generations. The HFEA recommended a few more tests to investigate such loose ends, some of which are already under way. But the scientists' main conclusion was that there is no evidence so far, including from animal trials, to suggest that the treatment would be unsafe. That is not a guarantee, of course. But, as the panel pointed out, those unknowable (and possibly non-existent) risks must be weighed against the very real suffering that would be caused by doing nothing. Britain's government, for one, seems convinced: it is consulting on new rules that would make the procedure legal. Three-parent, disease-free babies could be a reality within a couple of years.
- See more at: http://www.economist.com/blogs/economist-explains/2014/06/economist-explains-1#sthash.R7V6gpaj.dpuf
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