Genetic Loss of Tmprss6 Increases Effective Erythropoiesis in a Mouse Model of β-Thalassemia

Abstract 482

β-thalassemia is a disorder of ineffective erythropoiesis in which oxidative damage caused by unpaired α-globin chains leads to erythroid apoptosis, increased proliferation of erythroid precursors, and impaired erythroid differentiation. Patients develop systemic iron overload that is caused by red blood cell transfusions and by insufficient inhibition of gastrointestinal iron absorption by the iron regulatory hormone hepcidin. Previously we reported that homozygous genetic loss of Tmprss6, a hepatic transmembrane serine protease that inhibits hepcidin expression by the liver, led to hepcidin elevation and systemic iron deficiency in Hbb^th3/+ mice, a model of β-thalassemia intermedia. Interestingly, we also found that while maintaining similar hemoglobin levels, Hbb^th3/+mice with homozygous loss of Tmprss6 showed a significant reduction in splenomegaly and marked improvement in peripheral red blood cell (RBC) morphology.

Here, we investigated the effects of genetic loss of Tmprss6 on erythropoiesis in Hbb^th3/+ mice. In mice of different Tmprss6-Hbb genotypes, we used flow cytometry to quantify the proportion of total bone marrow cells of the erythroid lineage by measuring expression of TER119, an antigen expressed from the pro-erythroblast through the mature erythrocyte stage. Additionally, within the TER119⁺ population, we quantified the different erythroblast subpopulations by analyzing the intensity of forward scatter and CD44 expression. Compared to wild type (Tmprss6^+/+Hbb^+/+) controls, Hbb^th3/+ mice with 2 wild-type Tmprss6 alleles (Tmprss6^+/+Hbb^th3/+) showed a significant increase in the proportion of total erythroid cells in the bone marrow, significant increases in the proportion of immature erythroid precursors (basophilic and polychromatic erythroblasts) within the erythroid population, and a significant decrease in the proportion of mature RBCs, resulting in a marrow profile consistent with ineffective erythropoiesis. In Hbb^th3/+ mice with homozygous Tmprss6 disruption (Tmprss6^−/−Hbb^th3/+), the proportion of immature erythroid precursors (basophilic and polychromatic erythroblasts) within the erythroid population remained significantly elevated; however, the proportion of total erythroid cells in the bone marrow was no longer increased. Compared to Tmprss6^+/+Hbb^th3/+ mice, Tmprss6^−/−Hbb^th3/+ mice showed a significant increase in the proportion of mature RBCs; this was accompanied by a reduction in reactive oxygen species (ROS) production (as assessed by the indicator CM-H₂DCFDA) and apoptotic cells (as assessed by annexin V binding) within both the orthochromatic erythroblast/reticulocyte and mature red cell subpopulations. Additionally, compared to Tmprss6^+/+Hbb^th3/+ mice, Tmprss6^−/−Hbb^th3/+ mice showed a marked reduction in α-globin precipitates in membrane fractions prepared from peripheral RBCs. Interestingly, when normalized to α-globin mRNA expression, bone marrow mRNA encoding α-hemoglobin stabilizing protein (AHSP), an α-globin chaperone, was significantly higher in Tmprss6^−/−Hbb^th3/+ mice compared to Tmprss6^+/+Hbb^th3/+ mice, compatible with the known stabilization of AHSP mRNA under low iron conditions. Together, these findings suggest a model in which genetic loss of Tmprss6 in Hbb^th3/+ mice leads to a systemically iron-deficient state in which reduced iron availability to erythroid precursors leads to stabilization of free α-globin chains, a reduction in both ROS formation and erythroid apoptosis, and ultimately more effective erythropoiesis. In the context of previous findings, these results indicate that hepcidin-elevating strategies based on pharmacological inhibition of Tmprss6 might alter the clinical phenotype of β-thalassemia not only by reducing systemic iron loading but also by altering erythroid maturation.