Swapping chromosomes among eggs could keep
embryos from inheriting genetic diseases.
What’s the News: Babies with three parents and fewer genetic diseases might soon be possible: A UK national health panel has found that techniques for swapping chromosomes between eggs so offspring don’t inherit disease-causing mutations from their mother’s mitochondria are not dangerous. The techniques, which have been tested in mice, monkeys, and human cells, still need to be studied more before making the transfer to the clinic, though, and as with all genetic engineering techniques, there’s a complex ethical maze ahead of researchers.
What’s the Context:
- In addition to the DNA you inherit from your mother and father’s egg and sperm, you also inherit a small amount of DNA that’s contained in the mitochondria of the egg. Mitochondria are cellular structures that produce energy for the cell, thought to be descended from bacteria that moved into cells millions of years ago, and have their own mini-genome. The mitochondria in sperm are destroyed during reproduction, so the only ones you inherit are your mother’s.
- One child in 6,500 develops a disease linked to mutations in mitochondria, including type 2 diabetes, deafness, blindness, and neurological problems; as many as 1 in 250 people carry mutations that might cause disease when passed on.
- Transferring chromosomes (genetic structures found in the nucleus) from one cell to another is old hat in biology, so it makes sense that researchers would try the same trick to avoid diseases that come from DNA mutations in mitochondria (which are outside the nucleus). If chromosomes removed from an egg carrying mitochondrial mutations are inserted in a healthy egg whose own chromosomes have been discarded, the mitochondria are left behind, and the offspring don’t have the disease. The offspring instead inherits the mitochondrial DNA of the donor egg—hence the “three parent” idea.
How the Heck:
- The panel’s report, submitted to the UK’s Human Fertilisation and Embryology Authority, reviewed two techniques.
- In the first technique, the chromosomes of an unfertilized egg are inserted into a donor egg whose nucleus has been removed; the egg is fertilized using in vitro fertilization (IVF) and implanted into the mother. Rhesus macaques born this way are healthy and develop normally, scientists reported in Nature in 2009; the team is now testing it in human cells (via The Great Beyond).
- The other technique transfers the nucleus of a fertilized egg to a fertilized donor egg whose nucleus has been removed. It’s been tested in fertilized human eggs that could not be used for IVF and the eggs developed normally to the 100-cell stage. But there’s an ethical quandary here: it involves the destruction of the donor egg after it’s been fertilized. (More details on the ethics of these techniques in general in Wired Science’s coverage of that 2010 study.)
The Future Holds: For starters, the technique tested in macaques should be tested in human cells and the technique tested in human cells should be tested in monkeys, says the panel. If the techiques continue to prove safe, it’s likely they’ll begin to be considered for clinical use. It’s difficult to say how that will proceed–the issue of destroying potentially viable fertilized eggs may be deemed unacceptable in some countries. There’s the issue, too, that the mitochondria of the third “parent” will be passed on to the child, the child’s children, children’s children, and so on, meaning this genetic engineering is heritable. How will nations (and individuals) react to having a third person’s genes permanently added to their own?
Image credit: Wikimedia Commons, 8-cell embryo.