It's an interesting question that got a lot of traction in the media last week. Does the contribution of a tiny amount of DNA from a third person during in vitro fertilization really mean that the resulting child would have three genetic parents? Researchers in Oregon have proposed the technique as a way to avoid genetic diseases arising from faulty mitochondrial DNA by replacing an egg's mitochondria with one from a second, healthy woman either before or after fertilization with a man's sperm. They've shown that it works in monkeys, and the FDA met last week to consider whether the technique is safe enough to be used in humans.
Yesterday, Stanford law professor and bioethicist Hank Greely, JD, posted a great analysis of the topic on the university's Law and Biosciences blog, complete with an elegant explanation of the problem for women with mitochondrial DNA mutations:
The mitochondria (high school biology’s “energy powerhouses of the cell”) have their own very short stretch of DNA, separate from the 6.8 billion base pairs found on 46 chromosomes in the cell’s nucleus (the nuclear DNA). The 16,569 base pairs of the mitochondrial DNA (hereafter “mtDNA”) hold 37 (some say 38) genes, providing instructions for making 13 (or 14) proteins and another 24 RNA molecules. The full importance of these genes is unknown, but it is clear that some (happily rare) variations in the mtDNA cause quite severe illnesses. Unfortunately, each child gets all of its mitochondria (and hence its mtDNA) in the egg from its mother; if the mother’s mtDNA is dangerously flawed, so will be the mtDNA of all her children. With almost all other genetic diseases, no matter how inevitably the “bad” genetic variation leads to a disease (how “penetrant” the genetic variation is), a woman will have only a 50% or 25% chance of passing on the condition. With these, her genes can give rise to no healthy children.
Greely gets at the heart of the matter when he compares the statistically minute contribution from the donated mitochondria to a hypothetical child he calls Heather:
I have DNA from four people in each of my cells: my mother’s mother, my mother’s father, my father’s mother, and my father’s father. Actually, my DNA really came from all eight of my great-grandparents, and all 1024 of my great great great great great great great great grandparents, and all roughly one million of my great (18) grandparents. Yes, all that DNA passed through my (genetic) parents before coming to me, but why does that matter?
Heather gets her DNA from more than two people a bit differently from the way the rest of us do, but so what? How does getting what is, in effect, “gene therapy,” where the gene is delivered in a natural package called the mitochondrion, turn our hypothetical (and healthy) child into a powerful argument against the procedure?
It shouldn’t. Heather will not be getting superpowers, she will not be in any meaningfully way “designed” (except to avoid a nasty genetic disease), and she will not be given a newly made DNA sequence never before found in the human gene pool. She will get mitochondria with mtDNA that will allow her to have normal health, not a grave disease. That mtDNA will have been taken from a woman, who, though not a source of Heather’s nuclear DNA, is certainly a participant in the human gene pool.
“Heather has three parents” is NOT an argument. It is an irrelevant but attention-getting slogan that is uncritically put forward as, and sometimes mistaken for, a real argument. Yes, the proposed process is a way of bringing forth living and healthy babies that is somewhat new and different, but so were obstetric forceps, (safe) C-sections, and in vitro fertilization. Novelty is not, in itself, a respectable argument against it.
Previously: Medical practice, patents and "custom children": A look at the future of reproductive medicine, Five million babies and counting: Stanford expert offers conversation on reproductive medicine and Stanford researchers work to increase the odds of in vitro fertilization success
Photo by Christian Pichler