Transfusion independence and HMGA2 activation after gene therapy of human β-thalassaemia

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Abstract
Blood disorders caused by abnormal β-globin — β-thalassaemia and sickle cell disease — are the most prevalent inherited disorders worldwide, with patients often remaining dependent on blood transfusions throughout their lives. So a report of the successful use of gene therapy in a case of severe β-thalassaemia — using a lentiviral vector expressing the β-globin gene — is an eagerly awaited event. More than two years after gene transfer, the adult male patient has been transfusion-independent for 21 months. The therapeutic benefit seems to result from a dominant, myeloid-biased cell clone that may remain benign, although it could yet develop into leukaemia — a reminder that gene therapy is still at an early stage. Disorders caused by abnormal β-globin, such as β-thalassaemia, are the most prevalent inherited disorders worldwide. For treatment, many patients are dependent on blood transfusions; thus far the only cure has involved matched transplantation of haematopoietic stem cells. Here it is shown that lentiviral β-globin gene transfer can be an effective substitute for regular transfusions in a patient with severe β-thalassaemia. The β-haemoglobinopathies are the most prevalent inherited disorders worldwide. Gene therapy of β-thalassaemia is particularly challenging given the requirement for massive haemoglobin production in a lineage-specific manner and the lack of selective advantage for corrected haematopoietic stem cells. Compound βE/β0-thalassaemia is the most common form of severe thalassaemia in southeast Asian countries and their diasporas1,2. The βE-globin allele bears a point mutation that causes alternative splicing. The abnormally spliced form is non-coding, whereas the correctly spliced messenger RNA expresses a mutated βE-globin with partial instability1,2. When this is compounded with a non-functional β0 allele, a profound decrease in β-globin synthesis results, and approximately half of βE/β0-thalassaemia patients are transfusion-dependent1,2. The only available curative therapy is allogeneic haematopoietic stem cell transplantation, although most patients do not have a human-leukocyte-antigen-matched, geno-identical donor, and those who do still risk rejection or graft-versus-host disease. Here we show that, 33 months after lentiviral β-globin gene transfer, an adult patient with severe βE/β0-thalassaemia dependent on monthly transfusions since early childhood has become transfusion independent for the past 21 months. Blood haemoglobin is maintained between 9 and 10 g dl−1, of which one-third contains vector-encoded β-globin. Most of the therapeutic benefit results from a dominant, myeloid-biased cell clone, in which the integrated vector causes transcriptional activation of HMGA2 in erythroid cells with further increased expression of a truncated HMGA2 mRNA insensitive to degradation by let-7 microRNAs. The clonal dominance that accompanies therapeutic efficacy may be coincidental and stochastic or result from a hitherto benign cell expansion caused by dysregulation of the HMGA2 gene in stem/progenitor cells.