A medical breakthrough was announced this week when scientists described the birth of a healthy baby boy who was born free of the mitochondrial disease that his mother carries. How did it happen?
Through a process called mitochondrial manipulation technology (MMT), a team of physicians isolated the nucleus from the mother's oocyte and transferred it to a donor oocyte that was then fertilized and implanted into the mother. The mother went on to have a normal pregnancy and delivered the boy 3 months ago, at 37 weeks gestation, according to an abstract published in Fertility and Sterility.
Here's a quick roundup of the most important things to know about this case.
1. The term "three-parent baby" is misleading.
While many news outlets ran headlines announcing that a "three-parent baby" had been born, the scientific facts make this not quite accurate. As Dr Richard J. Paulson, president-elect of the American Society for Reproductive Medicine, told the New York Times, "Mitochondria do not define who you are" because they don't carry the genes for traits.
2. Mitochondrial diseases are rare but devastating.
Mitochondrial mutations are maternally transmitted and can have fatal or debilitating consequences. The mother in this case carries a mitochondrial disease called Leigh syndrome. She previously had two children who were born with the disease and both died, one at age 6 and one at 8 months. The mitochondrial DNA inheritance pattern is unpredictable, so while a quarter of the mother's tested cells had mitochondrial mutations, the two deceased children had the mutation in over 95% of their mitochondria.
The team that performed the procedure tested several neonatal tissues, such as hair follicles and the umbilical cord, and reported that they contained about 1%-2% of the mother's mitochondrial DNA.
3. MMT is not available in the United States.
Although the ob/gyn who led the team, John Zhang, MD, PhD, is based in New York City, the nuclear transfer intervention was done in Mexico. A US Food and Drug Administration panel concluded in February 2015 that more data are needed before the procedure can be conducted in the United States.
This is in contrast to a decision by lawmakers in the United Kingdom, who voted last year to allow MMT to go forward and with a conclusion from the Institute of Medicine in February 2016 that MMT is "ethically permissible."
4. Two approaches for MMT are possible.
Medscape Medical News reporter Ricki Lewis, PhD, describes the two approaches in a recent article. In one approach, known as pronuclear transfer, the father's sperm is used to fertilize two oocytes in vitro, one from the mother and another from a donor. The nucleus from the donor's fertilized ovum is removed and replaced with the nucleus from the mother's fertilized ovum. This process creates two ova and typically means that one of them is destroyed.
The second approach, which was used in this case, is called spindle nuclear transfer. It involves transferring the nucleus of the mother's oocyte to the enucleated donor oocyte. The oocyte is then fertilized with the father's sperm.
5. Options differ as to whether the technology is necessary.
In an interview with Medscape Medical News, Marcy Darnovsky, PhD, executive director of the Center for Genetics and Society, said "mitochondrial transfer is not medically necessary."
According to Dr Darnovsky, "it doesn't cure, treat, or prevent any existing person's disease. Nor is it necessary to have a child unaffected by mitochondrial DNA disease, since all people at risk of transmitting this condition can opt to use another person's egg."
Meanwhile, in a commentary published last year, Medscape's medical ethicist, Arthur L. Caplan, PhD, said the controversies surrounding the technology seem to be unfounded, as mitochondria do not carry genes related to behaviors or traits.
"Preventing parents who want to try to have a healthy child who can live does not seem to be the right place to say no to this form of genetic engineering," he concluded.