β-thalassemia: a model for elucidating the dynamic regulation of ineffective erythropoiesis and iron metabolism

Abstract

β-thalassemia is a disease characterized by anemia and is associated with ineffective erythropoiesis and iron dysregulation resulting in iron overload. The peptide hormone hepcidin regulates iron metabolism, and insufficient hepcidin synthesis is responsible for iron overload in minimally transfused patients with this disease. Understanding the crosstalk between erythropoiesis and iron metabolism is an area of active investigation in which patients with and models of β-thalassemia have provided significant insight. The dependence of erythropoiesis on iron presupposes that iron demand for hemoglobin synthesis is involved in the regulation of iron metabolism. Major advances have been made in understanding iron availability for erythropoiesis and its dysregulation in β-thalassemia. In this review, we describe the clinical characteristics and current therapeutic standard in β-thalassemia, explore the definition of ineffective erythropoiesis, and discuss its role in hepcidin regulation. In preclinical experiments using interventions such as transferrin, hepcidin agonists, and JAK2 inhibitors, we provide evidence of potential new treatment alternatives that elucidate mechanisms by which expanded or ineffective erythropoiesis may regulate iron supply, distribution, and utilization in diseases such as β-thalassemia.

Figure 1

Cellular mechanisms by which decreased iron uptake into erythroid precursors may promote survival and differentiation. (A) In β-thalassemia, a relative excess of α-globin synthesis leads to formation of hemichromes (α-globin/heme aggregates). Hemichromes are the primary cause of cellular toxicity in β-thalassemia because they precipitate and lodge on erythrocyte membranes, altering their structure. Furthermore, excess heme leads to the formation of reactive oxygen species (ROSs), which induce oxidative stress and cellular damage. In turn, this leads to IE by increasing apoptosis of erythroid precursors and reducing the number of erythrocytes produced as well as their survival in circulation. (B) On the basis of our data, we observe that administration of apo-Tf and increased hepcidin expression lead to decreased serum iron concentration and formation of fewer holo-Tf molecules. This reduces iron delivery to erythroid precursors, reducing heme synthesis and formation of hemichromes. In contrast, decreased iron intake limits hemichrome and ROS formation, ameliorating IE by reducing apoptosis, and improving erythrocyte survival in circulation. TfR1 indicates transferrin receptor 1.

Figure 2

Efficiency of erythropoiesis and effect of transfusion in β-thalassemia. (A) Severity of disease depends on both the degree of anemia and the systemic energy expenditure for erythropoiesis. For example, persons for whom the production of 6 g of hemoglobin (Hb) requires expansion of extramedullary erythropoiesis (EMH) in the liver and spleen, resulting in splenomegaly and changes in bone architecture, and the systemic cost of erythropoiesis is high. Although such a person would probably have his or her disease classified as TI, the initiation of chronic transfusion may improve the quality of life in such cases. (B) Transfusion has a significant number of benefits in β-thalassemia, although the consequent iron overload and potential for alloimmunization are debilitating complications.

Figure 3

Schematic representation of normal and ineffective erythropoiesis. In normal conditions, erythroblasts generate erythrocytes through a homeostatic balance between proliferation, differentiation, and cell death. In ineffective erythropoiesis, formation of toxic hemichromes leads to apoptosis and cell death of many erythroid precursors, limiting production of erythrocytes. Furthermore, on the basis of several observations (as discussed in the text), we postulate that in β-thalassemia erythroid precursors increase cell proliferation concurrently with reduced cell differentiation. This leads to a net increase in the number of erythroid precursors despite higher rates of apoptosis.

Figure 4

Homeostasis of hepcidin regulation. The schema of hepcidin regulation in hepatocytes as described in the text. The involved proteins enable nuanced regulation of hepcidin expression in light of hepcidin's central role in iron homeostasis. In β-thalassemia, one or multiple mechanisms involved in hepcidin stimulation are altered, resulting in hepcidin suppression relative to the degree expected from concurrent iron overload. The erythroid regulator probably has an effect through one or multiple mechanisms involved in hepcidin regulation. BMPR I/II indicates BMP receptors type I and II; Smad-P, phosphorylated Smad complex; HFE, hemochromatosis protein; and TMPRSS6, transmembrane serine protease matriptase-2.

Figure 5

Model of regulation of erythroid iron intake, cell proliferation, and differentiation. Recent findings indicate that iron-related proteins, previously characterized in the liver, are expressed in erythroid precursors. Although their function is largely uncharacterized in these cells, here we speculate on their potential function. In normal conditions, HFE has the potential to influence TfR1-mediated iron intake, whereas TfR2, binding EPOR, may modulate erythropoiesis, iron metabolism, or both. Under conditions of low iron intake, TfR1 mRNA is stabilized by IRP2 and iron intake increases. Under conditions of high Tf saturation, erythroid iron intake is probably modulated by loss of IRP2 activity, limiting TfR1 synthesis. FPN may export iron to avoid iron toxicity or to further control cellular iron content in normal or other uncharacterized altered physiologic conditions. Furthermore, excess of intracellular heme, potentially toxic, may be prevented by the heme exporter FLVCR (feline leukemia virus C receptor), both under conditions of normal and abnormal erythroid iron intake.

Figure 6

Overview of the pathophysiology of β-thalassemia. This disease is a consequence of insufficient β-globin synthesis, leading to excess heme and α-globin, apoptosis, and anemia. Anemia results from a block of erythroid differentiation, although erythroid proliferation is increased, resulting in IE, hepcidin suppression, and iron overload. The novel therapies discussed in this review, apo-transferrin (apo-Tf), JAK2 inhibitors, and hepcidin agonists (HAMP), have the potential to affect different steps in the pathophysiology of this disease. Furthermore, studies in β-thalassemia might provide information to further understand the physiology of normal erythropoiesis.