Membrane Phospholipid Asymmetry in Human Thalassemia

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Abstract
Phospholipid asymmetry in the red blood cell (RBC) lipid bilayer is well maintained during the life of the cell, with phosphatidylserine (PS) virtually exclusively located in the inner monolayer. Loss of phospholipid asymmetry, and consequently exposure of PS, is thought to play an important role in red cell pathology. The anemia in the human thalassemias is caused by a combination of ineffective erythropoiesis (intramedullary hemolysis) and a decreased survival of adult RBCs in the peripheral blood. This premature destruction of the thalassemic RBC could in part be due to a loss of phospholipid asymmetry, because cells that expose PS are recognized and removed by macrophages. In addition, PS exposure can play a role in the hypercoagulable state reported to exist in severe β-thalassemia intermedia. We describe PS exposure in RBCs of 56 comparably anemic patients with different genetic backgrounds of the α- or β-thalassemia phenotype. The use of fluorescently labeled annexin V allowed us to determine loss of phospholipid asymmetry in individual cells. Our data indicate that in a number of thalassemic patients, subpopulations of red cells circulate that expose PS on their outer surface. The number of such cells can vary dramatically from patient to patient, from as low as that found in normal controls (less than 0.2%) up to 20%. Analysis by fluorescent microscopy of β-thalassemic RBCs indicates that PS on the outer leaflet is distributed either over the entire membrane or localized in areas possibly related to regions rich in membrane-bound α-globin chains. We hypothesize that these membrane sites in which iron carrying globin chains accumulate and cause oxidative damage, could be important in the loss of membrane lipid organization. In conclusion, we report the presence of PS-exposing subpopulations of thalassemic RBC that are most likely physiologically important, because they could provide a surface for enhancing hemostasis as recently reported, and because such exposure may mediate the rapid removal of these RBCs from the circulation, thereby contributing to the anemia.