CRISPR, the scientific miracle that cured a baby of a rare genetic disease

An infant is the first documented case of a child with a serious genetic disease being successfully treated using a personalized therapy based on CRISPR gene editing.

A team of researchers from the Children's Hospital of Philadelphia and the University of Pennsylvania (USA) has achieved this historic breakthrough by adapting technology and administering it to a single human patient, marking a turning point in the treatment of rare genetic diseases.

KJ, as the baby is named to maintain his anonymity, was diagnosed shortly after birth with severe carbamoyl phosphate synthase 1 (CPS1) deficiency , a rare metabolic disorder that prevents the liver from properly processing ammonia, a toxic substance generated during the breakdown of proteins. This buildup can cause severe brain and liver damage and can even be fatal.

After his diagnosis, and at just six months old, KJ received the first dose of an experimental therapy specifically designed to correct the mutation causing his disease. The therapy, developed in record time—just six months after the genetic defect was identified—uses CRISPR-based gene editing and was delivered to the baby's liver using lipid nanoparticles.

Since February 2025, KJ has received three doses of this therapy without serious adverse effects. According to the team, the baby has shown clear signs of improvement: he tolerates a higher-protein diet, has reduced his dependence on medications that remove excess nitrogen, and has overcome common illnesses such as colds and gastrointestinal issues without any elevations in his ammonia levels, which would be extremely dangerous in his condition.

The research, published in The New England Journal of Medicine and presented at the American Society for Gene and Cell Therapy Annual Meeting , demonstrates that CRISPR-based precision medicine can be rapidly tailored to individual needs.

This methodology, developed by Drs. Rebecca Ahrens-Nicklas and Kiran Musunuru , both experts in genetics and metabolism, can be replicated to treat other rare genetic diseases that, until now, had no therapeutic options.

"KJ's case represents the culmination of years of research and collaboration," Ahrens-Nicklas said. "Although he's just one patient, we hope he's the first of many who can benefit from this personalized approach ."

"The dream of gene therapy, which we've been pursuing for decades, is beginning to become a reality. This modular and reusable platform allows for rapid, targeted action when it matters most: in early childhood, before the disease causes irreversible damage," Musunuru adds.

Under normal circumstances, the standard treatment for CPS1 deficiency is a liver transplant, a procedure that requires the patient to be medically stable and old enough. KJ, like many other babies with this condition, couldn't wait that long without risking irreversible neurological damage or even death.

"We knew that if we didn't do something in time, KJ might not be a candidate for a transplant," Ahrens-Nicklas explains. Thanks to this experimental gene therapy, the need for a transplant was avoided or postponed, giving the child a chance at normal development.

The success of this intervention opens the door to a new therapeutic model for ultra-rare diseases. This personalized approach could be replicated for hundreds of genetic disorders, providing targeted, safe, and rapid treatments where there was previously no hope.

Three experts consulted by SMC highlight the results of this work.

Marc Güell of Pompeu Fabra University states that the study "reflects the great potential of gene editing for therapeutic purposes." Fernando Civeira Murillo of the University of Zaragoza describes the study as "excellent," with the only limitation being that "the follow-up is short."

Güell believes it fits perfectly with current evidence and states: "Probably in a few years, the vast majority of serious monogenic diseases will disappear thanks to gene editing."

Finally, Gemma Marfany, from the University of Barcelona and CIBERER , highlights the fact that this is "the first case of a completely customized therapy for a single baby," and describes it as " a scientific 'miracle' that has allowed a very rare severe disease to be cured ."