The House of Commons has voted to legalize the use of donor mitochondrial DNA for in-vitro fertilization (IVF) conceptions, allowing the creation of children who will have three biological parents. The process will allow mothers who carry certain genetic conditions to have children that share most of their genetic code, but without the component that could lead to horrific diseases.
The capacity to create human life in this way has existed for almost twenty years. The oldest children born from the technique are now in their teens and show no ill effects. Nevertheless, the technology has been strongly discouraged in the U.S. since 2002 and is currently unavailable everywhere in the world. The decision by the House of Commons, passed 382 to 128, may cause other countries to follow suit, with debate already starting in Australia.
Mitochondria are often described as the engines or powerhouses of cells, supplying energy to the rest of the cell, which gets stored as the molecule ATP. Unlike every other part of our cells, mitochondria replicate independently. They have their own genome that is just 37 genes long, a relic of their evolutionary origin as bacteria.
All of our mitochondrial DNA (mtDNA) is inherited from our mothers. Sperm carry mitochondria in their tails, but the small quantities present are destroyed after fertilization, even if they manage to penetrate the wall of the egg.
Unfortunately, mtDNA mutates more rapidly than other parts of the genome. While a useful feature for studying evolution, this means that many of us carry mtDNA that can lead to diseases, most often of the heart, brain and respiratory system.
For women whose mtDNA has the potential to carry disease, having children is normally a risk. Mitochondria degradation is also one reason fertilized eggs from older women are less likely to implant in the uterus wall. Mitochondria can be removed from one of the eggs of a woman with either of these problems and replaced with material from a woman with healthy mtDNA. IVF can then be performed on the egg, leading to a child with around 30,000 genes from the parents who provided the egg and sperm, and 0.1% of the child's genetic code from the mitochondrial donor.
Credit: HFEA. Mitochondrial transfer can be done in two ways, either by inserting the mother's nucleus into a donor's egg, from which the nucleus has been removed (above), or by fertilizing two eggs and transferring the pronuclei from the mother's embryo into that of the donor (below).
St. Barnabas Institute, New Jersey has produced 17 children using cytoplasmic transfer, as the technique was called in the late 1990s. One of these, a twin, was born with Turner's syndrome, while a second child subsequently showed signs of pervasive early developmental disorder. The coincidence of two serious conditions in such a small group led to the FDA asking clinics to stop the practice. However, other children conceived through the process, including Alana Saarinen (above), are healthy.
Animal studies have produced conflicting results.
An inquiry by the UK Human Fertilization and Embryology Authority, however, concluded that mitochondrial replacement is “not unsafe,” leading to a vigorous lobbying campaign by scientists and carriers of mitochondrial diseases, which has now been successful.