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 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 trans-fusion 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 stochasticor resultfrom a hithertobenign cellexpansion caused by dysregulation of the HMGA2 gene in stem/progenitor cells.

Thalassemia 2016: Modern medicine battles an ancient disease

“…Unforeseen genetic effects were noted in the first successful patient treated: the integrated lentiviral vector caused transcriptional activation of HMGA2 in erythroid cells [82], thus this vector had to be altered. The modified vector has proven successful in the treatment of two adolescents with transfusion dependent thalassemia, who have been transfusion free for 11 to 14 months following autologous transplantation using stem cells corrected with the lentiviral vector [83] The subject of vectors has been expertly reviewed in great detail [84], including vectors designed for simple gene replacement, newer constructs such as vectors aimed to overcome silencing of the g-globin genes, and vectors for genome editing.…”

Engineering of PEDF-Expressing Primary Pigment Epithelial Cells by the SB Transposon System Delivered by pFAR4 Plasmids

“…Lentiviral and retroviral vectors integrate the transgene into the host cell’s genome and could possibly express the transgene for the life of the host cells. However, the preference of lentiviral and retroviral vectors to integrate into transcriptionally active genomic regions is associated with a high risk of vector-associated insertional mutagenesis, which could be harmful to the host cell and to the patient 12, 13, 14, 15, 16, 17, 18, 19…”

Comparative analysis of FV vectors with human α- or β-globin gene regulatory elements for the correction of β-thalassemia

“…However, in the murine HSCs ex vivo, the HS40.b vector expressed higher levels of b-globin and restored more effectively the impaired erythropoiesis of the thalassemic FLCs. Transduced cells were transplanted to C57Bl6J recipients conditioned with busulfan, using an adapted clinically relevant protocol, 17,18 and were monitored for expression of human b-globin by flow cytometry in the peripheral blood (PB); values are expressed as percentage of a human PB sample (Figure 3a).…”

“…DISCUSSION b-Thalassemia gene therapy has been envisioned for over 20 years, 21 but vector stability, low titers and transgene expression issues have delayed its application. 18 Human b-globin was initially expressed using only the homologous promoter without any additional regulatory elements. [22][23][24] Over the years, the importance of the LCR sequence in the regulated expression of the human b-globin gene was demonstrated, alongside the discovery and utilization of the core elements of the different hypersensitive sites of the LCR sequence.…”