HSP70 sequestration by free α-globin promotes ineffective erythropoiesis in β-thalassaemia

beta-Thalassaemia major (beta-TM) is an inherited haemoglobinopathy caused by a quantitative defect in the synthesis of beta-globin chains of haemoglobin, leading to the accumulationof free alpha-globin chains that form toxic aggregates(1,2). Despite extensive knowledge of the molecular defects causing beta-TM, little is known of the mechanisms responsible for the ineffective erythropoiesis observed in the condition, which is characterized by accelerated erythroid differentiation, maturation arrest and apoptosis at the polychromatophilic stage(3-6). We have previously demonstrated that normal human erythroid maturation requires a transient activation of caspase-3 at the later stages of maturation(7). Although erythroid transcription factor GATA-1, themaster transcriptional factor of erythropoiesis, is a caspase-3 target, it is not cleaved during erythroid differentiation. We have shown that, in human erythroblasts, the chaperone heat shock protein70 (HSP70) is constitutively expressed and, at later stages of maturation, translocates into the nucleus and protects GATA-1 from caspase-3 cleavage(8). The primary role of this ubiquitous chaperone is to participate in the refolding of proteins denatured by cytoplasmic stress, thus preventing their aggregation(9). Here we show in vitro that during the maturation of human beta-TM erythroblasts, HSP70 interacts directly with free alpha-globin chains. As a consequence, HSP70 is sequestrated in the cytoplasm and GATA-1 is no longer protected, resulting in end-stage maturation arrest and apoptosis. Transduction of a nuclear-targeted HSP70 mutant or a caspase-3-uncleavable GATA-1 mutant restores terminal maturation of beta-TM erythroblasts, which may provide a rationale for new targeted therapies of beta-TM.