Transferrin receptor 2 (Tfr2) genetic deletion makes transfusion-independent a murine model of transfusion-dependent ß-thalassemia.

ß-thalassemia is a genetic disorder caused by mutations in the ß-globin gene, and characterized by anemia, ineffective erythropoiesis and iron overload. Patients affected by the most severe transfusion-dependent form of the disease (TDT) require lifelong blood transfusions and iron chelation therapy, a symptomatic treatment associated with several complications. Other therapeutic opportunities are available, but none is fully effective and/or applicable to all patients, calling for the identification of novel strategies. Transferrin receptor 2 (TFR2) balances red blood cells production according to iron availability, being an activator of the iron-regulatory hormone hepcidin in the liver and a modulator of erythropoietin signaling in erythroid cells. Selective Tfr2 deletion in the BM improves anemia and iron-overload in non-TDT mice, both as a monotherapy and, even more strikingly, in combination with iron-restricting approaches. However, whether Tfr2 targeting might represent a therapeutic option for TDT has never been investigated so far. Here, we prove that BM Tfr2 deletion improves anemia, erythrocytes morphology and ineffective erythropoiesis in the Hbbth1/th2 murine model of TDT. This effect is associated with a decrease in the expression of α-globin, which partially corrects the unbalance with ß-globin chains and limits the precipitation of misfolded hemoglobin, and with a decrease in the activation of unfolded protein response. Remarkably, BM Tfr2 deletion is also sufficient to avoid long-term blood transfusions required for survival of Hbbth1/th2 animals, preventing mortality due to chronic anemia and reducing transfusion-associated complications, such as progressive iron-loading. Altogether, TFR2 targeting might represent a promising therapeutic option also for TDT.

Hepcidin and Anemia: A Tight Relationship.

Hepcidin, the master regulator of systemic iron homeostasis, tightly influences erythrocyte production. High hepcidin levels block intestinal iron absorption and macrophage iron recycling, causing iron restricted erythropoiesis and anemia. Low hepcidin levels favor bone marrow iron supply for hemoglobin synthesis and red blood cells production. Expanded erythropoiesis, as after hemorrhage or erythropoietin treatment, blocks hepcidin through an acute reduction of transferrin saturation and the release of the erythroblast hormone and hepcidin inhibitor erythroferrone. Quantitatively reduced erythropoiesis, limiting iron consumption, increases transferrin saturation and stimulates hepcidin transcription. Deregulation of hepcidin synthesis is associated with anemia in three conditions: iron refractory iron deficiency anemia (IRIDA), the common anemia of acute and chronic inflammatory disorders, and the extremely rare hepcidin-producing adenomas that may develop in the liver of children with an inborn error of glucose metabolism. Inappropriately high levels of hepcidin cause iron-restricted or even iron-deficient erythropoiesis in all these conditions. Patients with IRIDA or anemia of inflammation do not respond to oral iron supplementation and show a delayed or partial response to intravenous iron. In hepcidin-producing adenomas, anemia is reverted by surgery. Other hepcidin-related anemias are the "iron loading anemias" characterized by ineffective erythropoiesis and hepcidin suppression. This group of anemias includes thalassemia syndromes, congenital dyserythropoietic anemias, congenital sideroblastic anemias, and some forms of hemolytic anemias as pyruvate kinase deficiency. The paradigm is non-transfusion-dependent thalassemia where the release of erythroferrone from the expanded pool of immature erythroid cells results in hepcidin suppression and secondary iron overload that in turn worsens ineffective erythropoiesis and anemia. In thalassemia murine models, approaches that induce iron restriction ameliorate both anemia and the iron phenotype. Manipulations of hepcidin might benefit all the above-described anemias. Compounds that antagonize hepcidin or its effect may be useful in inflammation and IRIDA, while hepcidin agonists may improve ineffective erythropoiesis. Correcting ineffective erythropoiesis in animal models ameliorates not only anemia but also iron homeostasis by reducing hepcidin inhibition. Some targeted approaches are now in clinical trials: hopefully they will result in novel treatments for a variety of anemias.

Defective targeting of hemojuvelin to plasma membrane is a common pathogenetic mechanism in juvenile hemochromatosis.

Hemojuvelin (HJV) positively modulates the iron regulator hepcidin, and its mutations are the major cause of juvenile hemochromatosis (JH), a recessive disease leading to iron overload. Defective HJV reduces hepcidin up-regulation both in humans and in Hjv-deficient mice. To investigate the JH pathogenesis and the functional properties of human HJV we studied the biosynthesis and maturation of 6 HJV pathogenic mutants in HeLa and HepG2 cells. We show that proteolytic processing is defective in mutants F170S, W191C, and G320V, but not in G99V and C119F. Moreover, we show that mutants G99V and C119F are targeted to the cell surface, while F170S, W191C, G320V, and R326X (lacking the glycosilphosphatidylinositol [GPI] anchor) are mainly retained in the endoplasmic reticulum, although all mutants are released as soluble forms (s-HJV) in a proportion that is modulated by iron supplementation. Membrane HJV (m-HJV) is mainly composed of the cleaved protein, and its level is increased by iron in wild-type (WT) mice but not in the mutants. Altogether, the data demonstrate that the loss of HJV membrane export is central to the pathogenesis of JH, and that HJV cleavage is essential for the export. The results support a dual function for s- and m-HJV in iron deficiency and overload, respectively.