It's a world first: an American baby suffering from a rare and incurable disease is treated with tailor-made gene therapy.

With his chubby face and large blue marbles, KJ Muldoon, now 9 months old, was diagnosed shortly after birth with a very rare and serious metabolic disorder called "carbamoyl phosphate synthetase deficiency" or CPS1.

This disease is caused by mutations in a gene that codes for a key enzyme in liver function, preventing those affected from eliminating certain toxic waste products produced by their metabolism.

"When you Google what CPS1 is, you see it's either death or a liver transplant," Nicole Muldoon, the child's mother, said in a video from Children's Hospital of Philadelphia in the northeastern United States, where the infant was treated.

Faced with this serious diagnosis, the medical team suggested they try something new: a personalized treatment based on the use of molecular scissors (Crispr-Cas9), a revolutionary technology awarded the Nobel Prize in Chemistry in 2020.

"Our child was sick. We had to either do a liver transplant or give him this treatment that had never been given to anyone before (...) it was an impossible decision," recalls Kyle, the little boy's father.

"A tailor-made medicine"

Despite their fears, the couple agreed, and their baby received his first infusion in February, followed by two more. "The treatment is really designed just for KJ, so that the genetic variants he has are tailored to him. It's a tailor-made medication," explains pediatric genetics specialist Rebecca Ahrens-Nicklas.

Once in the liver, the microscopic scissors contained in the administered solution penetrate the nucleus of the cells and attempt to modify the gene in question. This therapy has shown promising results, according to the medical team, which published a study on the subject on Thursday in the New England Journal of Medicine.

KJ is now tolerating a higher protein diet and requires less medication, she said, adding that "long-term follow-up is needed to assess the safety and effectiveness of the treatment."

Physician Rebecca Ahrens-Nicklas hopes this achievement will allow the child to live "with very little or no medication" and could be of use to others. "We hope he will be the first of many to benefit from a methodology that can be adapted to the needs of each patient."