In this video lecture, Dr. Janet Kwiatkowski discusses:
- How gene therapy addresses the globin-chain imbalance underlying transfusion-dependent beta-thalassemia.
- The mechanisms and clinical outcomes of approved gene addition and CRISPR-Cas9 gene editing approaches.
- The treatment process, adverse effects, iron management, and practical considerations surrounding gene therapy.
Dr. Kwiatkowski is a Professor of Pediatrics at the Perelman School of Medicine of the University of Pennsylvania and the Director of the Thalassemia Program at the Children’s Hospital of Philadelphia (CHOP). She also serves as the co-director of the multidisciplinary Sickle Cell and Red Cell Disorders Curative Therapy Center at CHOP. She has participated in multiple collaborative research trials in thalassemia, sickle cell disease, and iron chelation. She has helped design and served as the principal investigator for several gene addition and gene editing trials for patients with hemoglobinopathies
(Video Lecture Summary)
Introduction
Dr. Janet Kwiatkowski reviews gene therapy for transfusion-dependent beta-thalassemia using a clinical case to illustrate its application. She discusses the pathophysiology that gene therapy seeks to correct, the two currently approved approaches, clinical outcomes, and important challenges associated with treatment.
The case involves a 24-year-old woman with transfusion-dependent beta-thalassemia and a beta-zero/beta-zero genotype who receives regular red blood cell transfusions and iron chelation. With well-controlled iron burden, adequate cardiac function, and no matched related donor, she is presented as a candidate for gene therapy.
Targeting the Globin-Chain Imbalance
Adult hemoglobin consists of two alpha-globin and two beta-globin chains. In beta-thalassemia, mutations reduce or eliminate beta-globin production, creating an excess of unmatched alpha-globin chains. These chains form aggregates that damage erythroid precursors and contribute to apoptosis and ineffective erythropoiesis.
The resulting clinical manifestations include severe anemia, bone changes, extramedullary hematopoiesis, increased iron absorption, and other complications. Conventional treatment with regular red blood cell transfusions and iron chelation is effective but creates a substantial treatment burden.
Gene therapy seeks to correct the globin-chain imbalance by increasing the production of beta-like globin chains.
Two Gene Therapy Approaches
Dr. Kwiatkowski describes two approved gene therapy strategies. Betibeglogene autotemcel, or beti-cel, uses gene addition to introduce a functional beta-globin gene. Exagamglogene autotemcel, or exa-cel, uses gene editing to increase fetal hemoglobin production by disrupting regulation of BCL11A.
Although their mechanisms differ, both approaches increase beta-like globin production and improve the imbalance between alpha- and non-alpha-globin chains.
The Ex Vivo Gene Therapy Process
The overall treatment process is similar for both products. Patients first undergo evaluation to confirm adequate organ function and suitability for transplantation. Informed consent includes repeated discussions of the procedure, risks, and alternative treatments.
Before stem cell collection, patients generally undergo a period of intensified transfusion. Hematopoietic stem and progenitor cells are then mobilized and collected by apheresis. The cells undergo either lentiviral gene addition or CRISPR-Cas9 gene editing, depending on the product, while unmanipulated backup cells are also stored.
After the manufactured product is returned, the patient is hospitalized and receives myeloablative busulfan conditioning followed by infusion of the gene therapy product. Hospitalization typically lasts approximately six to seven weeks while blood counts recover. Long-term follow-up continues for 15 years. Dr. Kwiatkowski emphasizes that the entire process commonly takes at least seven months and often approximately a year.
Gene Addition With Beti-cel
Beti-cel uses a replication-defective, self-inactivating lentiviral vector containing the beta-globin gene and beta locus control region. The introduced gene produces beta-globin with a T87Q amino acid substitution. This vector-derived beta-globin can pair with the patient’s native alpha-globin chains to produce functional hemoglobin.
Improvements to the manufacturing process following early clinical trials increased vector copy numbers and the percentage of transduced cells, contributing to improved outcomes in subsequent studies.
Clinical Outcomes With Beti-cel
Two phase 3 trials evaluated beti-cel in patients with transfusion-dependent beta-thalassemia, including patients with severe beta-zero/beta-zero and related genotypes. Approximately 90% of treated patients achieved transfusion independence, defined as discontinuation of regular transfusions while maintaining a hemoglobin level of at least 9 g/dL.
Transfusion independence was often achieved within one or two months after infusion, while hemoglobin continued to increase and generally stabilized around six months. Average hemoglobin during transfusion independence was approximately 11.7 g/dL in patients with less severe genotypes and 10.5 g/dL in those with more severe genotypes.
Response was not associated with clinical factors such as age, genotype, splenectomy status, or treatment center. The percentage of lentiviral-positive cells did predict efficacy, with all patients having greater than 60% lentiviral-positive cells achieving transfusion independence.
Gene Editing With Exa-cel
Exa-cel uses CRISPR-Cas9 gene editing to target the erythroid-specific enhancer of BCL11A. BCL11A normally represses fetal hemoglobin production. Disrupting its enhancer reduces BCL11A expression and therefore reduces repression of fetal hemoglobin.
The resulting increase in gamma-globin production improves the globin-chain imbalance that characterizes beta-thalassemia.
Clinical Outcomes With Exa-cel
Clinical trial results with exa-cel were similarly favorable, with approximately 94% of patients achieving transfusion independence. Total hemoglobin rises relatively quickly following treatment and is largely composed of fetal hemoglobin. Hemoglobin levels generally stabilize approximately six to nine months after infusion, with mean levels around 13 g/dL in the trial discussed.
Dr. Kwiatkowski notes that adolescents were included in the trial, while children younger than 12 years were not. At the time of the presentation, exa-cel is approved for patients with transfusion-dependent beta-thalassemia aged 12 years and older, with studies underway in younger children.
Improving Ineffective Erythropoiesis
Gene therapy addresses more than transfusion dependence. Markers of ineffective erythropoiesis also improve after treatment. Dr. Kwiatkowski presents reductions in soluble transferrin receptor, reticulocyte count, and erythropoietin levels following beti-cel treatment.
Bone marrow morphology also improves, including the myeloid-to-erythroid ratio and evidence of dyserythropoiesis. Similar improvements have been observed following exa-cel treatment.
Quality of Life
Health-related quality of life improves following gene therapy. Dr. Kwiatkowski presents adult and adolescent data demonstrating clinically meaningful improvements that are sustained during follow-up.
She also describes how patients report greater freedom to pursue experiences that were previously difficult because of dependence on a transfusion center, such as studying abroad. Improvements in health-related quality of life have been observed with both gene therapy approaches.
Iron Overload After Gene Therapy
Iron burden can initially worsen following gene therapy. Patients enter treatment with preexisting iron accumulation, receive intensified transfusions before stem cell collection and conditioning, and temporarily stop iron chelation around transplantation. Decreased iron utilization after myeloablation may also contribute.
Iron reduction therapy typically begins approximately six months after infusion. Phlebotomy is preferred when hemoglobin is above approximately 10 to 11 g/dL and venous access is adequate. Iron chelation can be used in patients with lower hemoglobin or difficult venous access, generally after acute transplant-related organ toxicity has resolved.
Long-Term Iron Reduction
Iron reduction continues until liver iron concentration and cardiac iron measurements reach the desired range. Although liver iron and ferritin may initially rise following treatment, they subsequently decline with iron reduction therapy.
With longer follow-up, many patients who achieve transfusion independence can eventually discontinue iron reduction therapy. Importantly, Dr. Kwiatkowski notes that recurrent iron accumulation has not been observed after iron reduction is discontinued in the available long-term follow-up.
Adverse Effects
The adverse effects of gene therapy largely resemble those associated with myeloablative conditioning and autologous transplantation. Common complications include mucositis, vomiting, poor appetite, febrile neutropenia, and the need for pain medication and nutritional support.
Hepatic veno-occlusive disease associated with busulfan occurs in approximately 10% of patients with beta-thalassemia. Infertility is another important risk. Serious adverse events have occurred in small numbers of patients, including hemophagocytosis, acute respiratory distress syndrome, idiopathic pneumonia syndrome, hemorrhage, and congestive heart failure.
Dr. Kwiatkowski emphasizes that no insertional mutagenesis, off-target effects, or cancers had been observed in the thalassemia gene therapy studies discussed.
Delayed Engraftment
Slower engraftment, particularly platelet engraftment, has been a consistent feature of gene therapy. Median platelet engraftment occurred at approximately 44 to 45 days with both products, although some patients required several months.
Splenectomy is associated with faster platelet engraftment. HLA class I alloimmunization appears to be associated with prolonged platelet engraftment and greater transfusion requirements. Optimal management of patients with high panel-reactive antibodies remains under investigation, but Dr. Kwiatkowski emphasizes counseling these patients about bleeding risk during transplantation.
Current Limitations of Gene Therapy
Current gene therapy requires myeloablative conditioning, which carries risks of infertility, organ dysfunction, and malignancy. Fertility preservation is therefore offered before treatment. Alternative approaches to cytoreduction are under investigation, including antibodies, antibody-drug conjugates, and lipid nanoparticles targeting stem cells or CD117.
Product availability is another limitation because manufacturing is specialized and production capacity is limited. Gene therapy is also extremely expensive. Dr. Kwiatkowski discusses in vivo gene therapy and more decentralized manufacturing as potential approaches to reducing some of these barriers.
Gene Therapy and Allogeneic Transplantation
Both gene therapy and allogeneic hematopoietic stem cell transplantation require chemotherapy. Gene therapy uses the patient’s own cells, eliminating the need for the immunosuppression required with an allogeneic transplant and allowing faster immune recovery.
Gene therapy also eliminates the requirement for a compatible donor and does not carry a risk of graft-versus-host disease. However, engraftment is generally slower, costs are substantially higher, and there are theoretical risks of insertional mutagenesis or off-target effects. Allogeneic transplantation remains an effective treatment, particularly for younger children with thalassemia.
Conclusion
Dr. Kwiatkowski concludes that two gene therapy products are commercially available for transfusion-dependent beta-thalassemia, with more than 90% of treated patients achieving transfusion independence in the trials discussed. Both approaches also improve ineffective erythropoiesis and health-related quality of life.
The major risks are largely those associated with myeloablative autologous transplantation. Iron overload still requires active treatment after gene therapy, although iron burden can normalize over time and has not been shown to reaccumulate after iron reduction therapy is discontinued within current follow-up. Choosing whether and how to pursue curative therapy ultimately requires consideration of patient preferences alongside clinical and programmatic factors.