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

Abstract : 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 β$^E$/β$^0$-thalassaemia is the most common form of severe thalassaemia in southeast Asian countries and their diasporas1, 2. The β$^E$-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 β$^E$-globin with partial instability. When this is compounded with a non-functional β$^0$ allele, a profound decrease in β-globin synthesis results, and approximately half of β$^E$/β$^0$-thalassaemia patients are transfusion-dependent. 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 β$^E$/β$^0$-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 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