A global milestone in genetic medicine: A new path forward for rare diseases
Canadian researchers at CHU Sainte-Justine used “prime editing” to correct a life-threatening genetic variation

For one teenager from Vancouver, living with chronic granulomatous disease (CGD), a common cold once meant a life-threatening medical crisis. CGD is a rare genetic condition that weakens the immune system and leaves patients vulnerable to specific, life-threatening infections and severe inflammation. For years, his reality was one of repeated hospitalizations and the constant threat of illness—until a team of Canadian researchers offered a different future.

In February 2025, Dr. Élie Haddad and his team at CHU Sainte-Justine participated in an international clinical trial led by Prime Medicine to test a brand-new treatment. The trial utilized an innovative technology called “prime editing” to correct the exact genetic error causing the patient’s condition.

Unlike earlier forms of gene editing that cut through DNA strands, prime editing works like a precise “search and replace” function. The process was a coordinated effort: the team at CHU Sainte-Justine first collected the patient’s stem cells and sent them to the Prime Medicine laboratory. There, scientists repaired the genetic code before sending the corrected cells back to Montreal to be returned to the patient’s body. Ten months after treatment, the results confirm the patient is healthy, stable, and living with a functioning immune system.

“The moment we saw the corrected white blood cells beginning to function was a turning point not just for this patient, but for the field of genetics,” says Dr. Haddad. “We are no longer just compensating for a faulty gene; we are literally fixing it at its source.”

This breakthrough was made possible through the support of the Canadian Institutes of Health Research (CIHR). By funding research into rare diseases and clinical innovation, CIHR helped build the essential infrastructure and patient engagement networks required to move this therapy from the laboratory to the bedside.

“CIHR’s investment in rare disease research is what allows Canadian scientists to participate in these kinds of world-first trials,” Dr. Haddad explains. “It provides the framework for the rigorous safety standards and the multidisciplinary collaboration needed to transform a complex scientific concept into a life-saving reality.”

The impact of such advancements is felt most deeply by those who have spent years navigating the healthcare system. Marie-France Langlet, a patient partner at CHU Sainte-Justine for the last 13 years and leader of the Bureau du partenariat patients-familles-soignants, sees this as the culmination of a long-held vision.

“My participation in the global CHU Sainte-Justine precision health strategy allows me to see some of my biggest dreams come true for the children and families who are experiencing today what we personally went through with my son,” says Langlet, whose own son underwent a bone marrow transplant for leukemia in 2004. “Precision health is offering them therapies that are more precise, more safe, less toxic and so much more effective. To witness this revolution in pediatric care is a unique and exhilarating privilege!”

Despite this historic success, efforts to reach more children have encountered a significant barrier. Even though the science is proven to work, the standard process for developing new medicines often focuses on conditions that affect larger numbers of people. This highlights a common difficulty in healthcare: even when a treatment works, the path to making it available long-term is often more complex for rare diseases.

“We are entering an era where collaboration among academia and the public sector are essential to the delivery of these therapies,” Dr. Haddad explains. “Because private investment is often difficult to sustain for ultra-rare conditions, there is an opportunity for a new model—one with increased emphasis on public sector involvement to ensure these groundbreaking treatments actually reach the patients who need them.”

The next frontier for the team is the transition to in vivo gene editing. While the current in vitro process is successful, it is labour-intensive and requires patients to undergo chemotherapy to make room for corrected cells. An in vivo approach would deliver the genetic “toolkit” directly into the patient’s body via a simple injection. This innovation would significantly streamline the process, reduce the need for intensive clinical interventions, and allow the treatment to be delivered in standard clinical settings rather than only in specialized centres.

To turn this vision into reality, Dr. Haddad emphasizes the importance of sustainable funding frameworks and specialized review processes designed for the unique, innovative nature of gene therapy. By contributing to the work that showed how prime editing is successful in a clinical setting, Canadian researchers have opened the door to a future where many rare genetic conditions can be treated.

The success at CHU Sainte-Justine is a beacon of hope and shows how international partnerships can help improve health outcomes for children.

At a glance

Issue

Many rare genetic disorders are caused by single errors in DNA that traditional treatments cannot fully correct, leaving patients dependent on life-long symptom management.

Research

At CHU Sainte-Justine, Dr. Élie Haddad was involved in a world-first clinical application of “prime editing”—a high-precision gene therapy that corrects variations directly within a patient’s own stem cells.

The first patient was treated at CHU Saint-Justine in Montreal and now has a functioning immune system. This breakthrough demonstrates the potential of precision medicine to provide curative treatments for thousands of rare diseases.