Developing gene therapy strategies to treat alpha thalassemia

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ABSTRACT Alpha-thalassemia is one of the most common monogenic diseases in the world; while the carrier frequency is highest in those with South East Asian heritage, there is an expanding health burden in the US due to immigration patterns. Although the most severe form of disease, α-thalassemia major (ATM, in which all four alpha-globin genes are deleted), was formerly often lethal in utero, numerous patients are now surviving to birth after intrauterine blood transfusions, often with excellent neurologic outcomes. However, these patients have a severe chronic disease that requires monthly transfusions or a stem cell transplantation after birth. Patients with a three-gene mutation (such as those with Hemoglobin H-Constant Spring, HbH-CS) can also have severe disease requiring chronic transfusions. While several gene therapy treatments have been developed for patients with β-thalassemia, there are no such therapies for patients with the most severe forms of α-thalassemia, indicating a major unmet medical need. Due to the similarity to β-thalassemia—lack of functional hemoglobin tetramers and formation of toxic globin aggregates in absence of the corresponding binding partner—we believe we can adapt gene therapy strategies that have successfully corrected β-thalassemia in the clinic into analogous approaches for correction of α-thalassemia. These strategies include: 1) CRISPR/AAV- mediated genome editing to replace a copy of β-globin with an α-globin transgene (Aim 1, conducted by Drs. Matthew Porteus and Kyle Cromer at Stanford); 2) Lentiviral delivery of an α-globin cassette with erythroid-specific expression (Aim 2, conducted by Dr. Donald Kohn at UCLA); and 3) CRISPR- mediated de-repression of ζ-globin, the embryonic precursor to α-globin (Aim 3, conducted by Drs. Tippi MacKenzie and Bruce Conklin at UCSF). Our multi-institutional team has been actively collaborating to develop these strategies and the preliminary data presented in this grant. All three independent strategies will be developed in vitro and assessed based on their ability to normalize the globin chain imbalance and restore functional hemoglobin tetramers to α-thalassemia-derived HSCs (obtained from patients with ATM and HbH-CS cared for at UCSF). Furthermore, primary and secondary mouse transplantation experiments will be performed to ensure that edited HSCs retain their ability to engraft and reconstitute hematopoietic lineages in vivo. The expected outcome of the proposed work is a significant advancement toward a universal cure for α-thalassemia by generating substantial pre-clinical data (for one or more approaches) that may be developed into an IND with the FDA for an innovative first-in-human phase I/II clinical trial for ex vivo correction of this disease.