• Gene-editing breakthrough has potential to treat array of devastating genetic diseases soon after birth

Doctors in the United States (US) have become the first to treat a baby with a customised gene-editing therapy after diagnosing the child with a severe genetic disorder that kills about half of those affected in early infancy.

The feat demonstrates the potential for treating an array of devastating genetic diseases by rewriting faulty deoxyribonucleic acid (DNA) soon after affected children are born.

Specialists at the Children’s Hospital of Philadelphia and the University of Pennsylvania started work as soon as the boy was diagnosed and completed the complex design, manufacture and safety testing of the personalised therapy within six months.

The baby, known as KJ, had the first dose of the bespoke treatment via an infusion in February of this year (2025) and two more doses in March and April. Doctors said that he was thriving, but would need careful monitoring for life.

KJ was born with severe CPS1 deficiency, a condition that affects only one in 1.3 million people. Those affected lack a liver enzyme that converts ammonia, from the natural breakdown of proteins in the body, into urea so that it can be excreted in urine. This causes a build-up of ammonia that can damage the liver and other organs, such as the brain.

While some patients receive liver transplants for CPS1 deficiency, babies with severe disease can have suffered damage by the time they are big enough to be operated on.

Writing in the New England Journal of Medicine, the doctors described the painstaking process of identifying the specific mutations behind KJ’s disorder, designing a gene-editing therapy to correct them, and testing the treatment and fatty nanoparticles needed to carry it into the liver. The therapy uses a powerful procedure called base editing which can rewrite the DNA code one letter at a time.

KJ spent his first few months of life in hospital on a restrictive diet, but, since his treatment, doctors have been able to increase the amount of protein in his food and use less medication to remove nitrogen from his body. 

A Geneticist at the Pablo de Olavide University in Seville, Spain, Dr Miguel Ángel Moreno-Mateos said: “The patient will be monitored for a long time to determine whether additional doses are needed to further improve the symptoms of the disease.”

(The Guardian)