If scientists are allowed to perform a simple genetic engineering procedure, they will be able to offer a reprieve to a small group of women who are condemned to pass certain fatal genetic diseases to each and every one of their children. This relatively straightforward procedure will allow a woman with one of these diseases to have a healthy baby with whom she will pretty much share half of her DNA. Only the tiniest snippet of DNA would come from an egg donor.
This procedure isn’t for all women in this situation though; it can only help women who carry certain mitochondrial diseases. People with these genetic diseases suffer because they have mitochondria that don’t work properly. Since the mitochondria are the organelles in the cell responsible for making the energy that keeps each of us running, defective mitochondria can have pretty severe effects. In some cases, it causes death at a very early age.
As you can see in the image on the right, the procedure to deal with this swaps out the defective mitochondria in a woman’s egg for the functioning mitochondria of a donated egg. It really is just taking a perfectly healthy nucleus from the fertilized egg and putting it into an egg with perfectly healthy mitochondria. The newly combined fertilized egg is then put back into mom where it can go on to develop into a mitochondrial disease-free baby.
Unfortunately, not every woman carrying a mitochondrial disease can be helped with this procedure. Only those who carry a mitochondrial disease caused by mutation(s) in her mitochondrial DNA (mtDNA) can benefit.
Mitochondria are unique in animal cells because they have a tiny bit of their own DNA (probably an evolutionary leftover from when they were free living beasts hundreds of millions of years ago). This mtDNA only makes up about 1/300,000th of our total DNA and has only about 37 genes, but it can still cause problems when it is damaged. And it is this DNA that also makes this procedure so controversial.
Ever since scientists have been able to tinker with people’s DNA, it has been strictly taboo for them to engineer any DNA in egg or sperm cells. This made perfect sense in the past because the tweaking they were doing involved actually changing a patient’s DNA. They didn’t want any unintended side effects that resulted from their genetic engineering to be passed on to the next generation. Their genetic tinkering was meant to stop with the patient.
The genetic engineering here is done on an egg and does involve tinkering that could be passed on to the next generation and so would seem to be taboo as well. But this sort of genetic engineering is fundamentally different from procedures done in the past.
There are no real changes in a patient’s DNA with this procedure. No one is mutating a gene or inserting a new gene in some random place in a patient’s DNA. Instead they are simply exchanging a complete, bundled set of DNA for a nearly identical, bundled set of DNA.
It is like replacing your Firefox browser for the Chrome browser on your computer as opposed to tweaking the Firefox browser so it is more like Chrome. In the first case, there is very little risk of any issues but in the second, there is a real chance for some sort of fatal bug (especially since we don’t have a good grasp of the programming language we are playing with).
Given that this is such a simple and straightforward switch, it is hard to come up with why exchanging mitochondria would pose a safety issue. There is no risk of causing cancer (as has happened with adding a gene in the wrong place) or creating some never before seen trait or anything like that. It is a clean swap of a discrete bit of DNA.
And in fact, this procedure was used over a decade ago with the end result being at least twenty healthy babies being born. There have been no reports of any issues with any of these children. Thus far it appears to be safe.
All of this taken together means it is extremely unlikely that children conceived this way will have any adverse side effects to pass on to the next generation. But if the worry about passing on this mtDNA is the major hurdle to letting these women give birth to healthy babies, then maybe a good compromise might be to let these women only have sons with this procedure. Men do not pass on their mtDNA, only women do. (The egg destroys any sperm mitochondria lurking around after fertilization). The genetic engineering would stop with the engineered child.
Of course, the safety of future generations is not the only concern. Bioethicists are concerned that these children might be confused as to their identity as they technically have three parents. Yes, this might be a concern but it seems like lots of kids are dealing with very similar issues these days. For example, if the couple had used an egg donor then the child would have to deal with the fact that their mom isn’t their biological mom. The same sorts of issues arise with same sex couples, adoptions and so on.
Another objection is that these women should not take even this small risk and should instead elect to adopt. Undoubtedly many women would choose this option but they probably shouldn’t be forced to. If it is important to a woman that her child share her DNA and there is a safe way to do it, then she should probably have the option.
It will be interesting to see how this all shakes out. Replacing malfunctioning mitochondria is fundamentally different than any other genetic engineering that has been done in the past and so would seem to be exempt from the previous taboo. If any genetic engineering is ever allowed in the egg, this is probably the one that should be.
Read more about this in this NPR story.