Doctors used CRISPR to rewrite a baby's DNA and fix a rare liver disorder

A nine-month-old with a fatal liver condition became the first to receive a CRISPR-based DNA-editing therapy custom-built to repair his unique mutation

A nine-month-old baby in the United States became the first person to receive a personalised CRISPR-based gene-editing therapy tailored to their unique genetic mutation. 

 

How a custom CRISPR therapy saved baby KJ from fatal liver damage

The treatment, designed for an infant named KJ Muldoon, who suffers from a rare and life-threatening liver condition, was administered earlier this year by doctors at the Children’s Hospital of Philadelphia (CHOP) and Penn Medicine, marking a breakthrough in the future of precision medicine.

 

What is CPS1 deficiency and why is it life-threatening?

KJ was born with a condition called carbamoyl phosphate synthetase 1 (CPS1) deficiency, a rare and severe disorder that prevents the body from safely processing ammonia.

 

 

Ammonia is a natural by-product when the body breaks down protein, but in individuals with CPS1 deficiency, the liver lacks a crucial enzyme needed to convert ammonia into urea.

 

As a result, toxic levels of ammonia build up in the blood, risking damage to the brain and other organs. In infants, this condition can escalate quickly and become fatal or cause permanent harm within days if untreated.

 

Faced with this urgent threat, KJ’s doctors turned to an experimental approach: CRISPR.  ALSO READ | 12 common cancer myths busted: What doctors say you must stop believing

 

What is CRISPR and how does gene editing work?

Gene editing is a scientific technique that allows scientists to change an organism’s DNA by adding, removing, or altering genetic material at specific locations. Within gene-editing CRISPR, an acronym for clustered regularly interspaced short palindromic repeat, is a tool known for its precision.

 

CRISPR was first developed as a gene-editing tool around 2012, and since then, it has been used to study gene function, develop disease-resistant crops, and even treat genetic disorders like sickle cell disease and certain forms of blindness.

 

How doctors created a personalised gene therapy in six months

Using the CRISPR tool, researchers created a custom therapy that targeted the exact mutation affecting KJ’s liver enzyme. The treatment involved “base editing”—a highly specific method that alters a single DNA letter—and was delivered directly to KJ’s liver using lipid nanoparticles.

 

In 2023, doctors Ahrens-Nicklas and Musunuru began developing personalised gene-editing therapies for urea cycle disorders. After identifying KJ’s CPS1 variant at birth, they created a targeted base editing treatment.

 

Within six months, the team had designed, tested, and manufactured the therapy. KJ received the first dose in February 2025 when he was about six months old.  ALSO READ | Is this a heart attack? These are the warning signs every man must know

 

How is baby KJ responding to the treatment?

Since beginning therapy, KJ has shown encouraging signs of improvement. He can now tolerate more protein in his diet, something that was previously dangerous, and requires fewer medications to remove excess nitrogen from his body. Importantly, he has also weathered common childhood illnesses without the dangerous spikes in ammonia that his condition would typically trigger.

 

While long-term monitoring is needed, the initial response suggests the therapy is working as intended and without serious side effects.

 

Why this could change rare disease treatment forever

Traditional gene editing has focused on more common conditions like sickle cell disease. This tailored approach opens new possibilities for thousands of rare mutations that don’t fit standard models. For families affected by rare genetic disorders, KJ’s case offers a glimpse into a more hopeful future.

 

“We hope he is the first of many,” said Dr Rebecca Ahrens-Nicklas. Her team’s goal is to build a repeatable framework—one that other researchers and clinicians can adapt to treat rare conditions with no cure.

 

“We want each and every patient to have the potential to experience the same results we saw in this first patient,” Musunuru said, “and we hope that other academic investigators will replicate this method for many rare diseases and give many patients a fair shot at living a healthy life.”

 

What it meant for KJ’s family to choose experimental gene therapy

For KJ’s parents, the experience has been both daunting and transformative. With a child too young for a liver transplant and facing a precarious medical reality, they chose to trust their doctors and the experimental therapy in the hope of giving their son a chance at a normal life.

 

“We thought it was our responsibility to help our child,” said KJ’s mother, Nicole. “We put our trust in the doctors, not just for KJ but for other families like ours.”

 

Today, KJ is home with his siblings, growing stronger and healthier each day.