Alteration of HSC Functions in Thalassemia

Beta-thalassemia represents one of the most globally widespread monogenic disorders and is characterized by significantly reduced or absent synthesis of hemoglobin beta-chains. In its severe form the insufficient production of adult hemoglobin results in altered erythropoiesis, hemolytic anemia, bone marrow (BM) hematopoietic hyperplasia and splenomegaly often associated with extramedullary hematopoiesis, requiring regular blood transfusions and iron chelation treatment. Over the last two decades many progresses were made in the field of allogeneic bone marrow (BM) transplantation to definitively cure beta-thalassemia. In parallel, experimental autologous transplantation protocols were developed to correct the disease by gene therapy also in patients lacking a compatible donor. Both in the allogeneic and autologous setting, thalassemic hematopoietic stem cells (HSCs) and the BM niche represent central elements.

Although many aspects of the pathophysiology of thalassemia have been extensively investigated, the HSC and its niche have never been explored. In thalassemia, the BM is a stressed environment, characterized by the compensatory expansion of erythroid progenitors secondary to ineffective erythropoiesis. Whether other hematopoietic subpopulations, such as primitive progenitors and/or HSCs, might be affected by such an altered hematopoietic microenvironment is unknown.

We investigated the frequency of hematopoietic progenitors in a murine model of severe beta-thalassemia intermedia. Immunophenotypic analyses revealed no differences in MEP, GMP, CMP, LMPP and MPP committed precursor subpopulations, whereas a significantly lower frequency of HSCs (Lin^- Sca-1⁺ c-kit⁺ CD48^- CD150⁺) was observed in thalassemic mice, as compared to age-matched wild-type controls. Competitive transplantation experiments revealed a disadvantage in the engraftment capacity of thalassemic HSCs, which was substantiated by the preliminary results from in vitro and in vivo cell cycle analyses suggesting an accelerated HSC exhaustion. Analyses of other cellular components, such as BM stroma and differentiated hematopoietic cells, revealed that additional elements are altered in the thalassemic BM microenvironment. The cellular and molecular bases of HSC-niche interaction in this pathological condition are under investigation.

Our results uncover a previously ignored defect of HSCs in beta-thalassemia. The investigation of cellular and molecular players that might affect in trans HSC functions in the complexity of this altered microenvironment is ongoing.