A healthy baby boy is the first person to be born with DNA from three people, according to a medical report released Tuesday.
In addition to inheriting nuclear DNA from his mother and father, the infant also has mitochondrial DNA from a second woman who served as an egg donor.
Mitochondrial DNA consists of just 37 genes, a tiny fraction of the genetic material inside a cell. Unlike nuclear DNA — which is bundled into 23 pairs of chromosomes and influences traits such as eye color, height and cancer risk — mitochondrial DNA contains instructions for the energy-producing structures inside cells. It is passed down virtually unchanged from mother to child.
That was a situation the infant's mother wanted to avoid. Her mitochondrial DNA contains mutations that cause Leigh syndrome, a neurological disorder that is usually fatal during early childhood. The 36-year-old woman had already lost an 8-month-old and a 6-year-old to the disease, according to the report in the journal Fertility and Sterility.
After seeking treatment at the New Hope Fertility Center in New York, the woman and her husband decided to find an egg donor with healthy mitochondrial DNA.
According to the study, specialists removed the nuclear DNA from five donor eggs and replaced it with nuclear DNA from the mother that was fused into place with a jolt of electricity. Then the eggs were fertilized by injecting them with the father's sperm.
Four of the fertilized eggs began dividing and developed into early-stage embryos called blastocysts. Three of the blastocysts had abnormal numbers of chromosomes, but one of them had the desired 23 pairs. After the embryo's mitochondrial DNA was screened and found to be healthy, doctors implanted the embryo into the mother's uterus.
The baby boy was born 37 weeks later after an otherwise "uneventful pregnancy" and is now 3 months old, according to the report from Dr. John Zhang, the fertility center's medical director, and colleagues.
The medical report describes the baby as the first healthy boy to be born using the technique.
Since children inherit mitochondrial DNA only from their mothers, the boy will not pass on the egg donor's DNA to his future sons or daughters.
"This work represents an important advancement in reproductive medicine," Dr. Owen K. Davis, president of the American Society for Reproductive Medicine, said in a statement.
"Mitochondrial disease has been an important and challenging problem," Davis said. If the technique proves to be safe and effective, "we look forward to it being an option for patients who risk transmitting mitochondrial diseases to their children."
The method hasn't been approved for use in the United States, so the baby's parents were treated in Mexico, according to New Scientist.
"There are no rules" in Mexico, Zhang reportedly told the magazine. "To save lives is the ethical thing to do," he added.
In February, the National Academies of Sciences, Engineering and Medicine concluded that so-called mitochondrial replacement techniques should be permissible in the U.S. as long as certain ethical conditions are met.
This technique and a similar one have been approved for couples in Britain. In the second method, both the mother's egg and a donor egg are fertilized with sperm. But before either fertilized egg can divide, the nuclear DNA is removed from the donor egg and replaced with nuclear DNA from the mother's egg.
The couple treated by Zhang opted against this method for religious reasons, according to the journal report, since it requires the creation of an embryo that is later destroyed.
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6:15 p.m.: The story was updated to include comments from the president of the American Society for Reproductive Medicine.
4:10 p.m.: The story was updated to include information about the use of mitochondrial replacement in Britain.