Gene editing has long been pitched as a search-and-replace function for the genome. This month, one of its most refined versions crossed a line researchers had been approaching for years. A prime editor, a tool built to rewrite DNA letter by letter without cutting both strands of the double helix, has been used to change genes inside a living person for the first time.
The patient is taking part in a clinical trial run by Prime Medicine, a biotechnology company based in Cambridge, Massachusetts. The company announced on 5 October that it had dosed the first participant in a Phase 1/2 study of a therapy called PM577a, and the journal Nature described the treatment on 22 October as the first known case of prime editing carried out directly in the human body. The target is Wilson disease, a rare inherited disorder in which the liver cannot get rid of excess copper.
Copper is essential in small amounts, but any surplus normally is packed up in the liver and sent out with bile. The job of moving it falls largely to a protein encoded by a gene called ATP7B. When both copies of that gene are faulty, copper slowly accumulates, first in the liver and later in the brain, the eyes and other organs.
The result can be liver scarring, tremors, speech problems and mood changes. Wilson disease is treatable today, but the treatment is a lifelong commitment. Patients take copper-binding drugs or zinc and watch their diets. Missing doses can let the copper creep back up. Patient advocates have long said that what people really want is a treatment they would only need once.
Correcting the underlying gene could, in principle, deliver exactly that. The trouble is that the most common faulty version of ATP7B in North America and Europe, known as H1069Q, is a single-letter misspelling. Older editing tools are not well suited to fixing it.
Why prime editing, and why the liver
The classic CRISPR-Cas9 system works by cutting both strands of DNA at a chosen spot. The cell then repairs the break, often imperfectly, which is useful for switching a gene off but less useful for writing in a precise correction. That is the approach behind Casgevy, the only gene-editing therapy approved so far by the US Food and Drug Administration, which treats sickle cell disease and beta thalassaemia by disabling a gene in blood stem cells that have been removed from the patient’s body.
Base editing, a later refinement, can swap one DNA letter for another without a full cut, but only certain kinds of swaps are possible, and the common Wilson mutations are not among them. Prime editing goes further. It combines a modified Cas protein, which nicks only one strand, with an enzyme that copies a new stretch of genetic code from a built-in template. The result is a system that can insert, delete or replace short pieces of DNA with high precision.
The liver is the obvious place to try it in the body. Many of the lipid nanoparticles used to carry genetic cargo, the same family of tiny fat bubbles used in mRNA vaccines, naturally end up in liver cells after an infusion into the bloodstream. PM577a is designed to be given as a single intravenous drip. The particles carry the instructions for the prime editor into hepatocytes, the main working cells of the liver, where the editor is supposed to find the H1069Q error in ATP7B and rewrite it to the healthy sequence.
If enough liver cells are corrected, they should start pumping copper out again. The trial will measure that directly. Researchers plan to track copper movement using a radioactive form of copper visible on PET scans, alongside blood markers such as ceruloplasmin and non-ceruloplasmin-bound copper, and the amount of copper patients excrete in their urine over 24 hours.
The study is open-label and uses ascending doses, which means early participants receive lower amounts while the team watches for side effects before moving higher. Each participant is followed closely for about 48 weeks and then invited into a long-term follow-up study. Prime Medicine has said it expects to report initial data in 2027, and the official trial record lists a completion date at the end of 2028.
There are good reasons for caution. Editing cells inside the body is fundamentally different from editing them in a laboratory dish. Once the nanoparticles are infused, there is no way to check each edited cell before it goes back into the patient, because it never left. Scientists will be looking for any unintended changes elsewhere in the genome, for immune reactions to the delivery particles or the editing proteins, and for signs that the corrections fade over time. Experts in the field have pointed out that every in vivo approach involves a trade-off between convenience and the level of risk patients and regulators are willing to accept.
Prime Medicine is not alone in betting on this route. Several companies are racing to bring in vivo editing therapies to market, and the FDA is expected to decide early next year on a CRISPR-Cas9 treatment for hereditary angioedema, a condition that causes sudden and dangerous swelling, which would be the first approved therapy that edits genes directly in the body. Prime Medicine itself has already used prime editing outside the body, correcting blood stem cells of two people with a rare immune disorder. A follow-on candidate aimed at R778L, the most common faulty ATP7B variant in East Asia, is in preclinical development.
For now, everything rests on a very small number of people and on data that will not appear for many months. But the symbolic weight of this moment is real. Prime editing was first described in a scientific paper only seven years ago. Moving from that paper to a correction made inside a human liver in such a short time is a striking pace for medicine.
For families living with Wilson disease, the hope is simple and very practical: fewer pills, fewer blood tests, less worry about a missed dose. Whether a single infusion can deliver that will be decided by the numbers in 2027. What has changed this month is that the question is finally being asked in a real patient, not just in a dish.
