Characterization of iron metabolism and erythropoiesis in erythrocyte membrane defects and thalassemia traits.

BACKGROUND AND AIMS: Erythropoiesis is closely related to iron metabolism in a balanced homeostasis. Analyses of diverse erythroid and iron metabolism disorders have shown that disrupted erythropoiesis negatively affects iron homeostasis and vice versa. The aim of this study was to characterize the relationship between erythropoietic activity and iron homeostasis in pediatric patients with erythrocyte membrane defects and thalassemia traits. METHODS: Selected markers of erythropoietic activity (erythropoietin, soluble transferrin receptor - sTfR and growth differentiation factor 15) and iron status parameters (serum iron, ferritin and hepcidin) were evaluated in pediatric patients with erythrocyte membrane defects and thalassemia traits. RESULTS: The patients with erythrocyte membrane defects and thalassemia traits had altered iron homeostasis due to disturbed erythropoiesis. In comparison with healthy controls, they had a normal to low hepcidin/ferritin ratio and concomitantly elevated sTfR. CONCLUSION: The findings suggest that pediatric patients with erythrocyte membrane defects and thalassemia traits are more susceptible to iron overload than the general population and that the (hepcidin/ferritin)/sTfR ratio can be used to monitor any worsening of the disease.

Iron status in patients with pyruvate kinase deficiency: neonatal hyperferritinaemia associated with a novel frameshift deletion in the PKLR gene (p.Arg518fs), and low hepcidin to ferritin ratios.

Pyruvate kinase (PK) deficiency is an iron-loading anaemia characterized by chronic haemolysis, ineffective erythropoiesis and a requirement for blood transfusion in most cases. We studied 11 patients from 10 unrelated families and found nine different disease-causing PKLR mutations. Two of these mutations - the point mutation c.878A>T (p.Asp293Val) and the frameshift deletion c.1553delG (p.(Arg518Leufs*12)) - have not been previously described in the literature. This frameshift deletion was associated with an unusually severe phenotype involving neonatal hyperferritinaemia that is not typical of PK deficiency. No disease-causing mutations in genes associated with haemochromatosis could be found. Inappropriately low levels of hepcidin with respect to iron loading were detected in all PK-deficient patients with increased ferritin, confirming the predominant effect of accelerated erythropoiesis on hepcidin production. Although the levels of a putative hepcidin suppressor, growth differentiation factor-15, were increased in PK-deficient patients, no negative correlation with hepcidin was found. This result indicates the existence of another as-yet unidentified erythroid regulator of hepcidin synthesis in PK deficiency.

DMT1-mutant erythrocytes have shortened life span, accelerated glycolysis and increased oxidative stress.

BACKGROUND/AIMS: Deficiency of the divalent metal transporter 1 (DMT1) leads to hypochromic microcytic anemia. We have previously shown that DMT1 deficiency impairs erythroid differentiation and induces apoptosis of erythroid cells. Here we analyzed metabolic processes and survival of mature erythrocytes in order to address potential involvement of erythrocyte defect in the pathophysiology of the disease. METHODS: FACS analysis was used to determine the half-life of erythrocytes (CFSE fluorescence), phosphatidylserine exposure (Annexin V binding), cytosolic Ca(2+) (Fluo3/AM fluorescence) and reactive oxygen species (ROS; DCF fluorescence). Enzyme activities were determined by standard biochemical methods. The concentration of ATP and ADP was measured on HPLC-MS/MS. RESULTS: We observed an accelerated clearance of CFSE-labeled DMT1-mutant erythrocytes from circulating blood when compared to wild-type erythrocytes. In vitro, DMT1-mutant erythrocytes showed significantly increased Annexin V binding after exposure to hyperosmotic shock and glucose depletion. Despite exaggerated anti-oxidative defense, higher ROS levels were present in DMT1-mutant erythrocytes. Accelerated anaerobic glycolysis and reduced ATP/ADP ratio detected in DMT1-mutant erythrocytes indicate enhanced demand for ATP. CONCLUSIONS: We propose that DMT1 deficiency negatively affects metabolism and life span of mature erythrocytes; two other aspects of defective erythropoiesis which contribute to the pathophysiology of the disease.

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

BACKGROUND: Hypochromic microcytic anemia associated with ineffective erythropoiesis caused by recessive mutations in divalent metal transporter 1 (DMT1) can be improved with high-dose erythropoietin supplementation. The aim of this study was to characterize and compare erythropoiesis in samples from a DMT1-mutant patient before and after treatment with erythropoietin, as well as in a mouse model with a DMT1 mutation, the mk/mk mice. DESIGN AND METHODS: Colony assays were used to compare the in vitro growth of pre-treatment and post-treatment erythroid progenitors in a DMT1-mutant patient. To enable a comparison with human data, high doses of erythropoietin were administered to mk/mk mice. The apoptotic status of erythroblasts, the expression of anti-apoptotic proteins, and the key components of the bone marrow-hepcidin axis were evaluated. RESULTS: Erythropoietin therapy in vivo or the addition of a broad-spectrum caspase inhibitor in vitro significantly improved the growth of human DMT1-mutant erythroid progenitors. A decreased number of apoptotic erythroblasts was detected in the patient's bone marrow after erythropoietin treatment. In mk/mk mice, erythropoietin administration increased activation of signal transducer and activator of transcription 5 (STAT5) and reduced apoptosis in bone marrow and spleen erythroblasts. mk/mk mice propagated on the 129S6/SvEvTac background resembled DMT1-mutant patients in having increased plasma iron but differed by having functional iron deficiency after erythropoietin administration. Co-regulation of hepcidin and growth differentiation factor 15 (GDF15) levels was observed in mk/mk mice but not in the patient. CONCLUSIONS: Erythropoietin inhibits apoptosis of DMT1-mutant erythroid progenitors and differentiating erythroblasts. Ineffective erythropoiesis associated with defective erythroid iron utilization due to DMT1 mutations has specific biological and clinical features.