Background

Transfusion-dependent beta-thalassemia is a genetic blood disorder caused by mutations in the HBB gene, which prevents the body from producing enough beta-globin chains of adult hemoglobin. Severe patients require frequent red blood cell transfusions to survive and also undergo chelation therapy to remove iron that accumulates in the body. Long-term transfusions can lead to iron overload in the liver, heart, and endocrine organs.

Existing curative treatments include allogeneic hematopoietic stem cell transplantation, but it is difficult to find a matched donor, and there is a risk of graft-versus-host disease. Gene therapy using the patient's own cells can avoid these limitations, but the method of inserting a normal HBB gene using a viral vector is complex and difficult to precisely control the insertion site.

Fetal hemoglobin (HbF), which is mainly expressed in the fetus, decreases rapidly after birth due to the action of the BCL11A protein. The observation that reactivating HbF, which uses gamma-globin chains instead of beta-globin, in adults can bypass the defective beta-globin, is the basis of this treatment strategy.

Key Findings

The researchers' approach is not to correct each HBB mutation in the patient. Instead, they used CRISPR-Cas9 to cleave the erythroid-specific enhancer of BCL11A in CD34+ hematopoietic stem and progenitor cells obtained from the patient, thereby releasing the suppression of HbF expression. The edited autologous cell therapy was developed as exagamglogene autotemcel (exa-cel).

In an initial clinical trial, beta-thalassemia patients were transfusion-free after exa-cel administration, and at 18 months after treatment, their total hemoglobin was 14.1 g/dL and HbF was 13.1 g/dL. The proportion of F cells, which are red blood cells that express HbF, was also 99.7%. This is the result of changing the expression program itself to allow HbF to be produced extensively and continuously in the erythroid lineage, rather than simply adding a normal gene to some cells.

However, the treatment is not a simple in vivo gene editing. It involves collecting hematopoietic stem cells, ex vivo editing, quality control, followed by busulfan-based myeloablative conditioning and re-infusion of the cells. A significant portion of the reported serious adverse events are related to the toxicities expected in the myeloablation and autologous transplantation process, such as neutropenia, thrombocytopenia, and infection.

The DOI provided, NEJMx260013, appears to be a one-page correction item published on August 6, 2026, in the NEJM 395, issue 6, page 624, and the input data does not include the content of the correction or new patient data. Therefore, the above figures are based on the previous NEJM clinical report that established this treatment strategy, rather than the new clinical results of the DOI.

Significance and Prospects

This study shows that it is possible to use a common physiological bypass without correcting each of the various HBB mutations in beta-thalassemia. Once the edited hematopoietic stem cells engraft in the bone marrow, they can continue to produce HbF in multiple generations of red blood cells, which could potentially replace lifelong transfusions and iron chelation with a single treatment.

Industrially, the success of the treatment depends on the treatment system that combines gene editing efficiency with cell collection, manufacturing facility transportation, myeloablation, and long-term follow-up, rather than just gene editing. The high cost of customized manufacturing and the hospitalization process lasting several weeks limit access to treatment, and infertility and the risk of infection due to myeloablation must also be considered in patient selection. Off-target editing, the possibility of leukemia, and the long-term persistence of edited cells are issues that need to be observed for years.

The focus of future development will shift to targeted preconditioning and in vivo gene editing techniques to reduce myeloablative toxicity. If these challenges can be addressed, HbF reactivation could become a platform that encompasses not only beta-thalassemia but also sickle cell disease.