Three-parent babies. Now that's a headline: emotional, evocative, and brief. It's not entirely accurate but every major news publication has used this headline in the past week. Why? Well, the UK just passed legislation that will allow mitochondiral replacement.
Let's go back to cell anatomy. First, in cells we have lots of different structures, including a nucleus and mitochondria. The nucleus has 99.9% of our DNA. We got half of that DNA from our biologic father and half from our biologic mother. We also have mitochrondria. Mitochondria are the cell's energy source and they also have a few genes (about 0.1%). We get 100% of our mitochondria from our mothers.
Some women have unhealthy mitochondria. Unhealthy mitochondria cause mitochondrial disease in their children - all of them. Mitochondrial diseases are universally terrible. They cause muscle weakness, vision problems, hearing problems, gastrointestinal issues, and cognitive delay. Affected children usually die young. Mitochondrial diseases affect about 1 in 4000 children born. A group in the UK has figured out how to replace mitochondria in women who are know to carry diseased mitochondria. The replacement mitochondria comes from an egg donor. As a result, the baby that is created from mitochondrial replacement has 49.9% of its DNA from the father (in the nucleus), 49.9% of its DNA from the mother (in the nucleus), and 0.1% of its DNA from an egg donor. Hence the "three-parent baby" headline.
I don't think it can be argued how powerful this could be for women who carry a mitochondrial disease. Like any new science, however, people don't worry about how it will be applied to those who genuinely need it. People worry about how it might be applied elsewhere.
For example, when we developed the science to test embryos for terrible genetic disorders (in the nucleus), people worried we (infertility doctors) would start making "designer babies." Well, this hasn't happened yet and thousands of families have been able to prevent horrible genetic diseases from being passed on to their children. I am not dismising these worries. If I am honest, I too worry that this kind of science will be used in an unethical way. In fact, I am sure there are doctors out there doing just that. Thankfully, the unethical ones are a tiny minority. We shouldn't avoid helping the majority because we are worried about a small minority. We should just police the minority better. Anyway, I digress.
How else could mitochondrial replacement be used? One way is to help an aging egg. You see, as eggs get older, the productivity of their mitochondria fades. As a result, the cell has less energy, which translates to poor egg quality. There is a group called Ovascience (google "ovascience augment") that is studying adding donor mitochondria to women's cells during IVF treatment. It is not approved in the USA. Though there is a clinic in Toronto (TCART) involved in the study, it's unclear if it could ever be used in Canada either, because Canadian federal law prohibits creating babies with more than one source of genetic material. Time will tell if adding healthy mitochondria to less healthy eggs will help and, if it does, whether we can ever use the technology in North America. It would be powerful though for women who have poor egg quality to be able to improve the quality by adding mitochondria. Poor egg quality is usually due to increased age, genetics, or previous chemotherapy exposure. A woman with poor quality eggs who can't get pregnant with IVF using her own eggs would typically use an egg donor or adopt. If they use an egg donor, their child will have 49.9% of her male partner's DNA and 50.1% of an egg donor's DNA. Using mitochondrial replacement, the child will have 49.9% of her DNA, 49.9% from a male partner, and 0.1% from the mitochondrial donor.
This is an advancement and it's exciting. Time will tell how it is implemented. Stay tuned.