Hello mothers, hello father: A plan to eliminate mitochondrial diseases would result in people with three genetic parents
IS IT possible for a child to have three parents? That is the question raised by a paper just published in Nature by Shoukhrat Mitalipov and his colleagues at Oregon Health and Science University. And the answer seems to be “yes”, for this study paves the way for the birth of children who, genetically, have one father, but two mothers.
The reason this is possible is that a mother’s genetic contribution to her offspring comes in two separable pieces. By far the largest is packed into the 23 chromosomes in the nucleus of an unfertilised egg. In that, she is just like the child’s father, who provides another 23 through his sperm. But the mother also contributes what is known as mitochondrial DNA.
Mitochondria are a cell’s power-packs. They convert the energy in sugar into a form usable by the cell’s molecular machinery. And because mitochondria descend from a bacterium that, about 2 billion years ago, became symbiotic with the cell from which animals and plants are descended, they have their own, small chromosomes. In people, these chromosomes carry only 37 genes, compared with the 20,000 or so of the nucleus. But all of the mitochondria in a human body are descended from those in the egg from which it grew. The sperm contributes none. And it is that fact which has allowed doctors to conceive of the idea of people with two mothers: one providing the nuclear DNA and one the mitochondrial sort.
The reason for doing this is that mutations in mitochondrial DNA, like those in the nuclear genes, can cause disease. These diseases especially affect organs such as the brain and the muscles, which have high energy requirements. Each particular mitochondrial disease is rare. But there are lots of them. All told, there is about one chance in 5,000 that a child will develop such an inherited disease. That rate is similar, for example, to the rate of fragile-X syndrome, which is the second-most-common type of congenital learning difficulty after Down’s syndrome. Mitochondrial disease is thus not a huge problem, but it is not negligible, either.