Base editing mediated correction of severe β0 thalassemia mutations

β-thalassemia is a highly prevalent monogenic recessive disease caused by mutations affecting the synthesis of the adult hemoglobin β-chains. Transplantation of autologous, genetically modified hematopoietic stem/progenitor cells (HSPCs) is an attractive therapeutic option. However, current gene therapy strategies based on the use of lentiviral vectors or CRISPR/Cas9 nuclease are not equally effective in all the patients and/or raise safety concerns. Base editing (BE), a new CRISPR/Cas9 derived genome editing tool, allows the effective introduction of point mutations at precise locations within the genome without generating double strand breaks. The two β0 mutations CD39 (CAG>TAG) and IVS2-1 (G>A) are among the most common and severe β-thalassemia mutations in the Mediterranean area and Middle East. We exploited the capacity of BE to correct these mutations in HSPCs from β-thalassemia patients. We demonstrated that red blood cells in vitro derived from edited HSPCs exhibited high β-globin levels and that the delayed erythroid differentiation typically observed in β-thalassemic cell cultures was corrected by our treatment. Finally, xenotransplantation experiments showed base editing in HSCs and correction of the β-thalassemic phenotype in vivo. Overall, our study provides in vitro and in vivo proof of efficacy of a BE approach to treat patients with prevalent and severe β-thalassemia mutations.