Surgery at the Molecular Level: How In Vivo Gene Editing Is Saving Lives

A new gene-editing method has already helped save a child's life. His body was slowly poisoning itself.

Kay Jay Muldune, who was born in the summer of 2024 in the United States, came into the world with a congenital mutation. The child lacked a vital enzyme that removes ammonia — a byproduct of protein digestion. Ammonia builds up in the blood, causing lethargy, coma, and irreversible brain damage. About half of children diagnosed with this condition do not survive their first few years. Not long ago, the only option was a risky liver transplant.

But in 2024, doctors did something that would have seemed like magic just a few years earlier: they did not replace an organ or transplant cells — they literally edited the child's DNA right inside his body. You could say they fixed a single "typo" in the three-billion-letter sequence of the genetic code.

What the Scientists Did

To grasp the scale of this breakthrough, it helps to recall high school biology. Our DNA consists of four "letters" — nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). Their sequence forms the instructions for assembling all the proteins in the body. Sometimes one letter gets swapped for another. It seems like a small thing. But because of that typo, a protein either fails to be made or works incorrectly. In Kay Jay's case, in the CPS1 gene, adenine had accidentally taken the place of guanine. The enzyme responsible for processing ammonia was essentially never produced.

A team of scientists from the Children's Hospital of Philadelphia and the University of Pennsylvania used a technology called base editing.

If conventional CRISPR — a precise genome-editing technology that allows "cutting" and "pasting" of DNA segments in living organisms — works like scissors, this new revolutionary approach works like a hyperfine pencil eraser. The scientists modified the CRISPR molecule so that it does not cut but instead gently pries open one strand of DNA. Then a second enzyme attached to it chemically converts adenine into guanine. No breaks, no risk of things going wrong.

The entire construction — the guide RNA (which shows exactly where to go) and the editing protein — was packaged into lipid nanoparticles. These are the same "fat bubbles" used in mRNA vaccines for COVID-19. Their special feature is that almost all of them end up settling in the liver — precisely where the CPS1 enzyme is needed. The system was tested on cell cultures, mice, and monkeys. Only after confirming its safety did they administer the treatment to seven-month-old Kay Jay.

The result: his blood ammonia levels dropped sharply. The baby, who until then could only eat a special formula with minimal protein content, began to tolerate more regular food. Two viral infections that would previously have inevitably triggered an ammonia crisis passed without complications. His father, Kyle Muldune, said briefly at a press conference: "We are very, very pleased with the results."

Not Alone in the Field

Kay Jay's story is not the only miracle. The entire year of 2025 has been marked by a series of breakthroughs in the field of in vivo gene editing — that is, inside a living organism.

In March 2025, the biotech company Beam Therapeutics reported successful treatment of nine patients with alpha-1 antitrypsin deficiency (AATD). This is a genetic disorder in which a single mutation in the SERPINA1 gene causes the liver to produce a toxic form of a protein that damages the lungs (causing emphysema) and clogs up the liver itself. Patients with the severe form — the so-called PiZZ genotype — live under a constant threat of needing a lung or liver transplant.

Scientists administered the same lipid nanoparticles with the base editor intravenously. The level of normal AAT protein in the blood increased 2.8-fold, while the amount of the mutant form decreased by 78%. In the group of patients receiving the highest dose, the concentration of the beneficial protein reached 12.4 micromoles. One infusion — and the disease retreated. The researchers hope that the edited liver cells will remain with the patient for life.

And in November 2023, the company Verve Therapeutics demonstrated that such a technology could tackle even a common problem like high cholesterol. Patients with hereditary hypercholesterolemia — a condition where levels of "bad" cholesterol (LDL) are sky-high and the risk of a heart attack by age 50 approaches 100% — had their PCSK9 gene edited. This gene encodes a protein that destroys the receptors responsible for clearing cholesterol from the blood. By disabling it, scientists achieved a 39–55% reduction in LDL levels — and this effect lasted for at least six months.

The Race for Rare Mutations

But perhaps the most important aspect of Kay Jay's story is not even the fact of the successful treatment itself, but the speed.

Developing a gene therapy tailored to a specific mutation usually takes years. The Pennsylvania team did it in six months. They set up a pipeline: they introduced the patient's mutation into cell cultures, tested dozens of guide RNA and enzyme variants until they found the ideal combination. And they acted so quickly that they saved the child at the very moment when it was critically important.

This opens the era of personalized genomic medicine. Imagine: a child is born with an ultra-rare mutation that occurs in one in a million. In the past, doctors would throw up their hands — it was commercially unviable to develop a drug for a single patient. Now, theoretically, a team can be assembled, create a customized "molecular patch" in six months, and administer it to the baby before the disease causes irreversible damage.

Safety Questions

Of course, like any revolutionary technology, in vivo gene editing raises questions. The main one is safety.

In Verve Therapeutics' trial, one out of ten patients experienced a serious cardiac complication — he suffered a heart attack. Another patient died. The safety committee concluded that the death was not related to the therapy, and the second patient had symptoms before the trial began. But still, proving this with 100% certainty is difficult.

Another issue is so-called off-target effects. What if the editor changes a letter in the wrong place? In the tests of the system for Kay Jay, they found one such change, but fortunately, it did not pose any danger. Scientists are constantly refining the enzymes to minimize risks.

And finally, the question of delivery. Current systems are delivered via lipid nanoparticles, which effectively reach only the liver. Treating diseases of the lungs, brain, or blood will require different delivery methods. But work on that is already underway.

What Comes Next

The year 2025 will go down in history as the time when gene editing ceased to be a laboratory curiosity and became a real clinical tool. Base editing has already been tested for treating:

• hereditary liver diseases (AATD, CPS1 deficiency);

• cardiovascular risks (hypercholesterolemia);

• immunodeficiencies (X-linked chronic granulomatous disease).

The researchers from Pennsylvania plan to scale their approach to other rare metabolic disorders. Beam Therapeutics is expanding its clinical trials. Verve is preparing for a placebo-controlled trial in a larger study.

We stand on the threshold of a new era. An era in which doctors do not merely treat symptoms but correct the very "firmware" of the human body.