Genetic medicine

A new technique to help cure mitochondrial diseases should be permitted by the law

In September Britain’s Human Fertilisation and Embryology Authority launched a public consultation on what sounds like a crackpot idea: to create children with three genetic parents. Yet this could be a way to eliminate a set of rare but nasty diseases caused by problems with pieces of cellular machinery called mitochondria. According to research published this week (see article), the basic technique of substituting problem-free mitochondria has now been tested in a laboratory and the researchers seem confident that, given the green light, they could bring a healthy child into the world.

Most of a child’s genes would come from the couple it would learn to call mum and dad. A tiny fraction of the DNA, however, would come from a female donor who would provide part of the egg from which the child grew. At present the law in Britain, like that in most other places, prohibits any genetic modification of embryos. It should be changed.

An in-gene-ious idea

Mitochondria turn the energy in sugar into a form a cell can use, so if they go wrong the consequences are dire. The brain, the nerves and the muscles, all huge consumers of energy, are the organs that suffer most. Mitochondria are also special, because they contain their own genes, completely separate from those in the cell nucleus, which are thus transmitted from mother to child in the egg.

Some mitochondrial disease is caused by mutations in these genes and is thus also inherited solely from the mother. Such diseases affect one person in 5,000 during his or her lifetime. But the separateness of mitochondrial genes means that by moving the nucleus from an afflicted egg into a healthy one, the mutated, disease-causing genes can be left behind. In effect, the nucleus would receive a mitochondrial transplant.

This incorporation of the DNA of two women might be seen by the nervous as a step down the slippery slope towards the genetic engineering of people. But that is unlikely.

The principal worry about genetic engineering is that it will lead to “designer babies” with customised DNA. But mitochondrial transplants involve no tinkering with the DNA itself. Though on a microscopic scale, the process is quite like a heart, liver or kidney transplant, with the caveat that the transplant will be passed on to the recipient’s children, if she is female. Any organ transplant introduces new genes into the body. Mitochondrial genes are ubiquitous, it is true, but this difference is one of degree, not kind.

Another reason not to worry is that the mitochondria carry only 37 genes, compared with about 20,000 in the cell nucleus, and these genes are exclusively concerned with energy metabolism. Pushy parents will not be picking mitochondrial donors on the basis of looks, personality or intelligence.

Non-biological objections are sometimes raised as well. Some worry, for instance, that a person with three genetic parents might suffer an identity crisis. But that seems less likely than in the case of people conceived by in vitro fertilisation using sperm donated by strangers who have contributed half of their offspring’s genes, not a paltry three dozen. And for that reason mitochondrial donors are even less likely than sperm donors to want to be involved with bringing up children in whom they have but a fractional genetic interest.

But is the process safe? Doing the experiment is the only way of finding out. It should be preceded by a lot of tests in Petri dishes and laboratory animals. But in the end, you just have to try it and see.

First published as a Leader in The Economist. Also available in audio hereThis editorial also had an accompanying article with it.

Free image from here.