Bench to Bedside-Gene Therapy for Thalassemia and Sickle Cell Disease
Authors: Leboulch Philippe 
Published: October 2019

Abstract
The beta-hemoglobinopathies (beta-thalassemia and sickle cell disease) are the most prevalent inherited disorders worldwide and affect millions. Patients with beta-thalassemia major cannot survive without monthly, lifelong transfusions together with iron chelation therapy, and severe cases of sickle cell disease suffer from multiple life-threatening complications. Both categories of patients often have a shortened life expectancy in spite of supportive therapies, which impose an enormous financial burden on affected countries. The only available curative therapy is allogeneic hematopoietic stem cell transplantation, although most patients do not have an HLA-matched sibling donor, and those who do still risk graft rejection or graft-versus-host disease with associated morbidity and early mortality. This is why autologous gene therapy, by ex vivo transfer into the patient's own hematopoietic stem cells of a derivative of the normal beta-globin gene whose expression is appropriately regulated, is an attractive novel therapeutic modality. In addition, the very large number of known mutations causing beta-thalassemia makes gene therapy by gene addition ideally suited for regulatory product approval rather than the many mutation-specific products that would be required for site-specific gene correction. However, gene therapy of these disorders is especially challenging given the requirement for massive hemoglobin production in a lineage specific manner and the lack of selective advantage for corrected hematopoietic stem cells. During the past two decades, we and others have devised lentiviral vectors and applicable protocols to achieve the permanent correction of mouse models of the beta-hemoglobinopathies. The first approved human clinical trial worldwide resulted in the conversion to transfusion-independence of a patient with severe betaE/beta0-thalassemia, who required monthly transfusions since early childhood. This patient demonstrated prolonged transfusion-independent for over 8 years after gene therapy, and the initially identified partially dominant integration site (HMGA2) is no longer preeminent. A further optimized vector with high-grade purification is now being used in subsequent multi-center clinical trials in the USA, France, Australia, and Thailand both for beta-thalassemia major and severe sickle cell disease. As of the last public disclosure in June 2015, 34 subjects have been enrolled, of whom 12 and 2 patients with beta-thalassemia major and severe sickle cell disease, respectively, have undergone the gene therapy procedure. Currently analysable patients with beta-thalassemia major have rapidly decreased their transfusional needs or become completely transfusion-independent, often with near normal hemoglobin values and polyclonal distribution of vector bearing progenitors. Interestingly, conversion to transfusion-independence also occurred in a Cooley's anemia patient of the beta0/beta0 genotype. With regard to the first analysable patient with sickle cell disease, the anti-sickling globin variant utilized in the vector (βT87Q) is expressed at 40% level of all hemoglobin chains - a level well above the expected sickling inhibitory threshold - resulting in improvements in disease-specific biological markers and no hospitalization for sickle cell complications or acute episode despite weaning off transfusions, which this patient was receiving regularly for the prevention of stroke relapse. Prospects for bringing this novel therapeutic approach to medical practice and for complementary approaches to increase safety and efficacy will be discussed.

Disclosures
Leboulch: bluebird bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding.