It is now possible for a child to have three parents: One parent, a male, contributing half of a full nuclear genome via a sperm by means of in vitro fertilization, the second parent, a female, contributing the other half of a full nuclear genome using just the cell nucleus from one of her egg cells. The third “surrogate parent,” also a female, contributes an egg cell that has had its nucleus removed but retains those vital subcellular organelles, the mitochondria, in the cytoplasm of her egg cell. These organelles have their own genome and replicate inside most of our cells. Normally, we inherit them only from our mothers via her egg cell’s cytoplasm.
Why would anyone want to do this? There are rare but devastating genetic disorders caused by mutations in the DNA of the female parent’s mitochondria which are the organelles responsible for our body’s ability to turn food into energy. Sperm from the male parent carry a couple of mitochondria which are needed to propel the sperm on its journey but which do not enter the female parent’s egg at the time of fertilization. Mito and Tracker were the first scientists to raise rhesus monkeys who had had the mitochondria in their mother’s egg cells replaced with those from another female. These three-parent monkeys will reach the breeding age of 5 later this year and they will be bred to see if their offspring suffer any unusual disorders that might have resulted from their unusual parentage.
Use of this complicated procedure in humans is currently outlawed in the United Kingdom. Nevertheless, the U. K. government has already issued draft regulations that would allow medical scientists to try this technique with parents in cases where the mother was known to have mitochondria carrying a serious genetic defect. The procedure would break a longstanding cultural and legal barrier to any genetic engineering in humans that would result in heritable genetic changes - that is genetic changes that will likely be passed on to both the male and female progeny of persons who had come from three parents. Up to now genetic engineering in humans has been limited to inducing changes in the non-reproductive organs, such as bone marrow, the blood forming organ, of individuals with genetic disorders that impair the functioning of this single organ.
Interestingly, something akin to this proposed procedure has already been done by a fertility clinic in New Jersey about 15 years ago. What was done (in an attempt to alleviate female infertility) was to inject the unfertilized eggs of these women with cytoplasm obtained from the egg cells of fertile donors. Such cytoplasm would not contain any nuclear DNA but would contain mitochondria having DNA that differed from those mitochondria already present in the egg cells from the infertile female parent. A number of the more than a dozen children that were born following this procedure had developmental disorders. However, as this was not a controlled study, we are unable to say whether or not this procedure only helped these infertile women become pregnant, or whether this procedure was also the cause of the developmental disorders that occurred in their children.
In any case this new technique will prompt much debate and discussion among bioethicists and others concerned about how we deal with humans who want children but either cannot conceive or are likely to have children born with devastating genetically caused disorders. As we already allow in vitro fertilization and surrogate parenthood it will be difficult to determine just where to draw the line. But if there is a high likelihood of a genetic engineering procedure leading to heritable changes in humans, this may well be the deciding factor in disallowing such a procedure, although we do this with our domestic plants and animals with carefree abandon.