| Imagine knowing that you carry a potentially deadly genetic disease and having to choose between never having children or risking passing it on to them. Doctors in the UK have now managed to spare some women that agonizing choice. Eight babies at risk of inheriting serious disorders caused by DNA mutations have been born with no sign of illness, doctors at the Newcastle Fertility Centre have announced, after the parents’ genes were transferred into healthy donor embryos. Conceived by IVF, the children are biologically related to their parents without having inherited the disease-causing DNA. The pioneering procedure, known as pronuclear transfer, was documented in a research paper published in the New England Journal of Medicine this month. One in every 5,000 babies is born with pathogenic variants in the DNA stored in their mitochondria — the energy-producing part of their cells. The altered DNA, which is inherited maternally, provides the body with less energy. That’s particularly problematic for high-energy systems such as the heart, muscles and brain and can lead to conditions such as blindness or heart disease. In five of the babies born by pronuclear transfer since 2023, the level of defective mitochondria was undetectable. In the other three, up to 16% of mitochondria were pathogenic — well below the 80% threshold thought to cause disease. The children are all healthy and will continue to be monitored. Pronuclear transfer is sometimes described as creating “three-parent babies.” But Doug Turnbull, emeritus professor of neurology at Newcastle University and one of the paper’s authors, is quick to point out that’s not quite accurate. The donor egg contains nuclei from the patient and their partner (or sperm donor), which means they pass on all crucial characteristics to the child, he told reporters in London. The technique was legalized in the UK in 2015, but barriers such as the Covid-19 pandemic and the need for a stockpile of eggs meant the research took longer than originally expected. Adding to the challenges, the donor embryos cannot be frozen, so the transfer has to take place less than a day after fertilization. So far, the procedure is only legal in two places — the UK and Australia, where a clinical trial is due to start next year. The lead researcher on the Newcastle study, Mary Herbert, is also a professor at Monash University in Melbourne. Getting the science to this point has been an “emotional roller-coaster,” said Louise Hyslop, consultant embryologist at Newcastle Fertility Centre and another of the research paper’s authors. And that’s just for the scientists. In a statement, some of the parents expressed relief that the procedure is now proved to work and is available via the National Health Service. “After years of uncertainty, this treatment gave us hope — and then it gave us our baby,” said the mother of one girl. “We look at them now, full of life and possibility, and we’re overwhelmed with gratitude.” — Elliot Burrin | 
