Decreased differentiation of erythroid cells exacerbates ineffective erythropoiesis in β-thalassemia

Abstract

In ␤-thalassemia, the mechanism driving ineffective erythropoiesis (IE) is insufficiently understood. We analyzed mice affected by ␤-thalassemia and observed, unexpectedly, a relatively small increase in apoptosis of their erythroid cells compared with healthy mice. Therefore, we sought to determine whether IE could also be characterized by limited erythroid cell differentiation. In thalassemic mice, we observed that a greater than normal percentage of erythroid cells was in Sphase, exhibiting an erythroblast-like morphology. Thalassemic cells were associated with expression of cell cycle-promoting genes such as EpoR, Jak2, Cyclin-A, Cdk2, and Ki-67 and the antiapoptotic protein Bcl-X L . The cells also differentiated less than normal erythroid ones in vitro. To investigate whether Jak2 could be responsible for the limited cell differentiation, we administered a Jak2 inhibitor, TG101209, to healthy and thalassemic mice. Exposure to TG101209 dramatically decreased the spleen size but also affected anemia. Although our data do not exclude a role for apoptosis in IE, we propose that expansion of the erythroid pool followed by limited cell differentiation exacerbates IE in thalassemia. In addition, these results suggest that use of Jak2 inhibitors has the potential to profoundly change the management of this disorder. (Blood. 2008;112:875-885) Introduction ␤-Thalassemia, one of the most common congenital anemias, arises from partial or complete lack of ␤-globin synthesis. ␤-Thalassemia major, also known as Cooley anemia, 1 is the most severe form of this disease, and is characterized by ineffective erythropoiesis (IE) and extramedullary hematopoiesis (EMH), requiring regular blood transfusions to sustain life. [1][2][3][4][5] In ␤-thalassemia intermedia, where a larger amount of ␤-globin is synthesized, the clinical picture is milder and the patients do not require frequent transfusions. The ineffective production of red blood cells in both forms of the disease has been attributed to erythroid cell death during the maturation process mediated by apoptosis or hemolysis. It was proposed that accumulation of alpha-globin chains leads to the formation of aggregates, which impair erythroid maturation triggering apoptosis. [6][7][8][9][10][11][12][13] Ferrokinetic studies done in 1970 suggested that 60% to 80% of the erythroid precursors in ␤-thalassemia major die in the marrow or extramedullary sites. 14 However, several observations call into question the view that cell death is the only cause of IE in ␤-thalassemia.First, the number of apoptotic erythroid cells in thalassemic patients is low compared with that anticipated by ferrokinetic studies. 14,15 In fact, only 15% to 20% of bone marrow (BM) erythroid precursors (CD45 Ϫ /CD71 ϩ ) present apoptotic features in aspirates from affected patients. 6,8,16 Second, hemolytic markers in young ␤-thalassemic patients are normal or only slightly increased, unless the patients suffer from splenomegaly or the liver has been damaged by iron overload or viral infections. 17 Third...

Hepcidin as a therapeutic tool to limit iron overload and improve anemia in β-thalassemic mice

“…In contrast, untreated Tg -/th3 mice increase 3 to 6 times ( Figure 8B) between fractions I and II as well as between II and III. This observation corroborates previous studies that indicate that β-thalassemic erythroid cells exhibit increased cell proliferation and reduced cell differentiation compared with those from normal mice (20). However, comparing the FACS profiles of WT and Tg -/th3 mice ( Figure 8, A and B, respectively) with those of Tg-Hamp/th3 mice ( Figure 8C), we observed that the latter group of mice exhibited an improved profile both in the spleen and BM (not shown), as indicated by a reduction in the number of cells in fractions II to IV, a more balanced ratio of the number of cells between these fractions and a relative increase in terminally differentiated cells in fraction V.…”

-thalassemias: paradigmatic diseases for scientific discoveries and development of innovative therapies

“…[119][120][121] The Hbb th1/th1 mice were generated by a homozygous deletion of the b-major mouse globin gene, whereas Hbb th3/+ mice present a heterozygous deletion of both the b-major and b-minor globin genes in cis. [119][120][121] In these two models, the animals show a phenotype very similar to that observed in patients affected by NTDT, [122][123][124] such as splenomegaly and iron overload in absence of transfusion. An additional transplantable model showing features of b-thalassemia major has been generated by transplanting fetal liver cells from Hbb th3/th3 embryos into wild-type mice.…”

Erythropoietin-driven signaling ameliorates the survival defect of DMT1-mutant erythroid progenitors and erythroblasts

“…33 In this regard; the condition of mk/mk mice treated with EPO resembles that of β-thalassemia mice with dramatically elevated endogenous EPO and increased BCL-X L expression, which is responsible for the prevention of apoptosis of thalassemic erythroid cells in this model. 19 The possible involvement of any substantial increase in iron utilization by individual erythroid cells as a contributing factor to the positive effects of EPO therapy can be ruled out by the fact that the patients' erythrocyte characteristics (MCV and MCH) remained unchanged with EPO supplementation. 7,32 Our observation of decreased MCV and MCH in the mk/mk mice following EPO treatment further supports this conclusion.…”

“…16 Ineffective erythropoiesis in different erythroid disorders is thought to be associated with increased apoptosis in the erythroid compartment. 17,18 Because a detailed study of β-thalassemia mice revealed only a low level of apoptosis, demonstrating that prevention of erythroid differentiation was causing ineffective erythropoiesis, 19 we set out to study the erythroid parameters that are rescued by a recombinant EPO in DMT1-mutant patients and mice.…”