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

The beta-haemoglobinopathies are the most prevalent inherited disorders worldwide. Gene therapy of beta-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 beta(E)/beta(0)-thalassaemia is the most common form of severe thalassaemia in southeast Asian countries and their diasporas(1,2). The beta(E)-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 beta(E)-globin with partial instability(1,2). When this is compounded with a non-functional beta(0) allele, a profound decrease in beta-globin synthesis results, and approximately half of beta(E)/beta(0)-thalassaemia patients are transfusion-dependent(1,2). 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 beta-globin gene transfer, an adult patient with severe beta(E)/beta(0)-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 beta-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 froma hitherto benign cell expansion caused by dysregulation of the HMGA2 gene in stem/progenitor cells.