Anemia, Shortened Erythrocyte Lifespan and Stomatocytosis In a Flippase Mutant Mouse Strain

Mammalian erythrocytes are anucleate biconcave discs with marked flexibility and deformability, features necessary for efficient function. The cell membrane consists of a lipid bilayer containing proteins and an asymmetric distribution of phospholipids (PL). Phosphatidylcholine and sphingomyelin are predominantly concentrated in the outer layer and phosphatidylserine (PS) and phosphatidylethanolamine are mainly confined to the inner layer of the erythrocyte membrane. Maintenance of PL asymmetry is essential for erythrocyte survival and function as increased externalization of PS results in adherence of erythrocytes to vascular endothelium, activation of plasma blood clotting factors and premature removal from the circulation. Flippases, floppases and scramblases are the enzymes responsible for the establishment and maintenance of PL distribution.

A mouse deficient in the flippase ATP11C was generated through ENU mutagenesis and was found to have reduced hemoglobin in comparison to wild type (WT) littermates. This study was performed to characterize the etiology of anemia in these mice.

ATP11C-deficient mice had significant reductions in erythrocyte numbers and hematocrit compared to WT but higher MCH and MCV. Reticulocyte numbers were comparable to WT as were serum iron parameters. Bone marrow and splenic erythropoiesis was normal in ATP11C mutant mice, however, erythrocyte lifespan was reduced by 35%. A marked increase in the frequency of PS+ erythrocytes was demonstrated in ATP11C mutant mice and was shown to increase with erythrocyte age. Bone marrow and splenic erythroblasts from ATP11C-deficient animals displayed a lower rate of PS translocation in vitro compared to WT confirming defective flippase activity.

Erythrocytes and late-stage splenic erythroblasts in ATP11C mutants were significantly larger than WT based on flow cytometry. SEM revealed the majority of mutant erythrocytes displayed stomatocyte-like morphology, as confirmed on peripheral blood smears. The osmotic fragility of the ATP11C-deficient erythrocytes was comparable to WT erythrocytes as was Na+ and K+ homeostasis.

These studies reveal the important role of flippases in maintaining normal erythrocyte function through the maintenance of the asymmetric distribution of PS within the membrane. Perturbation of this process, as seen in the ATP11C-deficient mice, results in (i) increased exposure of PS on the erythrocyte outer membrane, (ii) increased size of late-stage erythroblasts and erythrocytes with stomatocyte formation (iii) reduced erythrocyte lifespan and (iv) normochromic anemia. This study, therefore, reveals a new mechanism for stomatocytosis and raises the question of whether mutations in ATP11C may serve as a previously unclassified cause of anemia in humans.

No relevant conflicts of interest to declare.