Hereditary microcytic anaemia in the mouse; studies in iron distribution and metabolism

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.

Knockout mouse models of iron homeostasis

Murine models have made valuable contributions to our understanding of iron metabolism. Investigation of mice with inherited forms of anemia has led to the discovery of novel proteins involved in iron homeostasis. A growing number of murine models are being developed to investigate mitochondrial iron metabolism. Mouse strains are available for the major forms of hereditary hemochromatosis. Findings in murine models support the concept that the pathogenesis of nearly all forms of hereditary hemochromatosis involves inappropriately low expression of hepcidin. The availability of mice with floxed iron-related genes allows the study of the in vivo consequences of cell-selective deletion of these genes.

Iron-refractory iron deficiency anemia

Iron-refractory iron deficiency anemia (IRIDA) is an autosomal recessive disorder characterized by iron deficiency anemia unresponsive to oral iron treatment but partially responsive to parenteral iron therapy. IRIDA has recently been shown to be caused by mutations in the gene TMPRSS6, which encodes a transmembrane serine protease (also known as matriptase-2) expressed by the liver. IRIDA patients show inappropriately elevated levels of hepcidin, a circulating hormone produced by the liver that inhibits both iron absorption from the intestine and iron release from macrophage stores. Recent studies suggest that TMPRSS6 normally acts to downregulate hepcidin expression by cleaving hemojuvelin, a membrane-bound protein that promotes hepcidin signaling in hepatocytes. A discussion of the clinical presentation of IRIDA, the molecular genetics of this disorder, and recent studies elucidating the underlying pathophysiology are presented.

Iron metabolism: iron deficiency and iron overload

Iron is an essential cofactor in a variety of cellular processes. Except for a few unusual bacterial species, iron is indispensable for living organisms. However, free iron is toxic because of its propensity to induce the formation of dangerous free radicals. Consequently, iron balance is tightly regulated. Disorders of iron homeostasis are among the most common afflictions of humans. This review discusses inherited iron deficiency and iron overload disorders and recent insights into their pathophysiology.

Characterization of the iron transporter DMT1 (NRAMP2/DCT1) in red blood cells of normal and anemic mk/mk mice

Divalent metal transporter 1 (DMT1) is the major transferrin-independent iron uptake system at the apical pole of intestinal cells, but it may also transport iron across the membrane of acidified endosomes in peripheral tissues. Iron transport and expression of the 2 isoforms of DMT1 was studied in erythroid cells that consume large quantities of iron for biosynthesis of hemoglobin. In mk/mk mice that express a loss-of-function mutant variant of DMT1, reticulocytes have a decreased cellular iron uptake and iron incorporation into heme. Interestingly, iron release from transferrin inside the endosome is normal in mk/mk reticulocytes, suggesting a subsequent defect in Fe(++) transport across the endosomal membrane. Studies by immunoblotting using membrane fractions from peripheral blood or spleen from normal mice where reticulocytosis was induced by erythropoietin (EPO) or phenylhydrazine (PHZ) treatment suggest that DMT1 is coexpressed with transferrin receptor (TfR) in erythroid cells. Coexpression of DMT1 and TfR in reticulocytes was also detected by double immunofluorescence and confocal microscopy. Experiments with isoform-specific anti-DMT1 antiserum strongly suggest that it is the non-iron-response element containing isoform II of DMT1 that is predominantly expressed by the erythroid cells. As opposed to wild-type reticulocytes, mk/mk reticulocytes express little if any DMT1, despite robust expression of TfR, suggesting a possible effect of the mutation on stability and targeting of DMT1 isoform II in these cells. Together, these results provide further evidence that DMT1 plays a central role in iron acquisition via the transferrin cycle in erythroid cells.

Iron transport

Iron homeostasis is maintained by regulating its absorption: Under conditions of deficiency, assimilation is enhanced but iron uptake is otherwise limited to prevent toxicity due to overload. Iron deficiency remains the most important micronutrient deficiency worldwide, but increasing awareness of the genetic basis for iron-loading diseases points to iron overload as a major public health issue as well. Recent identification of mutant alleles causing iron uptake disorders in mice and humans provides new insights into the mechanisms involved in iron transport and its regulation. This article summarizes these discoveries and discusses their impact on our current understanding of iron transport and its regulation.

Inherited disorders of iron storage and transport

Diverse hereditary disorders associated with iron accumulation cause widespread organ damage. New insights into cellular pathways of iron transport have emerged from the identification of molecules implicated in heritable defects of iron metabolism. Unravelling the genetic basis of rare variants of haemochromatosis should provide vital functional information to further our mechanistic understanding of iron homeostasis.

Ferrokinetics in the syndrome of familial hypoferremic microcytic anemia with iron malabsorption

PURPOSE

In 1981, Buchanan and Sheehan described a previously unreported syndrome in three siblings who had iron malabsorption, hypoferremia, and microcytic anemia that did not respond to oral iron and responded only partly to parenteral iron dextran. Ferrokinetic studies were not done in these or subsequently reported patients with this syndrome. It has been postulated that this syndrome of abnormal iron metabolism is analogous to that observed in the mk/mk mouse, which has similar hematologic findings but also has abnormal ferrokinetics. Ferrokinetic studies were performed in one patient to determine whether the abnormality of iron metabolism in the human syndrome is analogous to the mk/mk mouse.

PATIENTS AND METHODS

Two sisters with severe microcytic anemia and iron malabsorption who have had only partial response to parenteral iron have been followed up for 15 years. Ferrokinetic studies with 59Fe were performed in one sister.

RESULTS

Ferrokinetic studies with radio iron were characteristic of iron deficient erythropoiesis (rapid 59Fe T1/2; rapid, complete incorporation of 59Fe into erythrocyte hemoglobin). These ferrokinetics differ from those of the mk/mk mouse, which has a missense mutation in Nramp2, a putative iron transporter protein. In these children, once iron enters the plasma its subsequent metabolism (including binding to transferrin), transfer into erythroid bone marrow cells, and subsequent incorporation into erythrocyte hemoglobin are all normal. The defect in these patients appears to be an undefined, novel abnormality that governs mobilization of iron into the plasma from both the intestinal mucosal and reticuloendothelial cells. Despite lifelong severe hypoferremia, the growth, development and intellectual performance of these children, who are teen-agers, are normal.

Iron transport across biologic membranes

Iron is essential for life, but is toxic in excess. Nearly all organisms have therefore developed regulated mechanisms for efficient transport of iron into cells. This paper reviews the current understanding of iron transport, focusing on valuable lessons from studies of yeast iron transport and the discovery of the first mammalian transmembrane iron transporter.

Mammalian iron transport: an unexpected link between metal homeostasis and host defense

Iron deficiency and iron overload disorders are common in clinical practice. Both can result from perturbations in the flux of iron across the absorptive intestinal enterocyte. Until recently iron transport has been poorly understood. In 1997 two independent cloning strategies identified Nramp2 (DCT1) as the first mammalian transmembrane iron transporter. In this review we discuss evidence that Nramp-related proteins play essential roles in metal homeostasis and host defense.

The G185R Mutation Disrupts Function of the Iron Transporter Nramp2

Microcytic anemia (mk) mice and Belgrade (b) rats have severe iron deficiency anemia due to defects in intestinal iron transport and erythroid iron utilization. Both animal mutants carry the same missense mutation in Nramp2, the first mammalian iron transporter to be identified. This mutation, in which glycine 185 is changed to arginine (G185R), occurs within predicted transmembrane domain 4 of the protein. We have performed site-directed mutagenesis of murine Nramp2, focusing on amino acids of transmembrane domain 4 that are highly conserved among Nramp-like proteins. We have expressed each mutant form in transfected cells and examined iron transport function, subcellular localization, and protein amounts. All tested forms of Nramp2 localize to the plasma membrane and to transferrin-containing endosomes. Most transmembrane domain 4 mutations affect the amount of protein detected and consequently show diminished iron transport. The G185R mutation, however, causes near total loss of Nramp2 function that cannot be fully explained by a decreased amount of protein, indicating that G185R disrupts iron transport through an alteration in the function of Nramp2, rather than degradation of the protein.

© 1998 by The American Society of Hematology.

The G185R Mutation Disrupts Function of the Iron Transporter Nramp2

Microcytic anemia (mk) mice and Belgrade (b) rats have severe iron deficiency anemia due to defects in intestinal iron transport and erythroid iron utilization. Both animal mutants carry the same missense mutation in Nramp2, the first mammalian iron transporter to be identified. This mutation, in which glycine 185 is changed to arginine (G185R), occurs within predicted transmembrane domain 4 of the protein. We have performed site-directed mutagenesis of murine Nramp2, focusing on amino acids of transmembrane domain 4 that are highly conserved among Nramp-like proteins. We have expressed each mutant form in transfected cells and examined iron transport function, subcellular localization, and protein amounts. All tested forms of Nramp2 localize to the plasma membrane and to transferrin-containing endosomes. Most transmembrane domain 4 mutations affect the amount of protein detected and consequently show diminished iron transport. The G185R mutation, however, causes near total loss of Nramp2 function that cannot be fully explained by a decreased amount of protein, indicating that G185R disrupts iron transport through an alteration in the function of Nramp2, rather than degradation of the protein.

© 1998 by The American Society of Hematology.

Identification of Nramp2 as an iron transport protein: another piece of the intestinal iron absorption puzzle

Although a number of iron-binding proteins have been identified, the roles for specific proteins in mediating iron absorption have not been definitively assigned. Two recent papers report the identification of an iron transport protein that may be responsible for movement of iron from the intestinal lumen into the enterocyte. Coupled with the recent identification of the protein mutated in hemochromatosis, researchers are now establishing a clearer picture of the mechanism of intestinal iron absorption.

Microcytic anemia with iron malabsorption: an inherited disorder of iron metabolism

Two siblings were identified with severe hypoproliferative microcytic anemia and iron malabsorption, in the absence of any gastrointestinal disorder or blood loss. These children had severe microcytosis (MCV 48 fl, hemoglobin 7.5 g/dl) with decreased serum iron, elevated serum TIBC, and decreased serum ferritin, despite prolonged treatment with oral iron. An iron challenge study with an oral dose of 2 mg/kg elemental iron as ferrous sulfate documented iron malabsorption. After treatment with intravenous iron dextran, there was an absence of the expected reticulocytosis and only a partial correction of the hemoglobin, hematocrit, and microcytosis. The bone marrow was hypocellular with abnormal iron incorporation into erythroid precursor cells. This appears to be a rare form of inherited anemia characterized by iron malabsorption and disordered iron metabolism that only partially corrects after the administration of parenteral iron. These features resemble those found in the microcytic mouse (mk/mk), which also has severe microcytic anemia and iron malabsorption that partially responds to parenteral iron.

Duodenal expression of NF-E2 in mouse models of altered iron metabolism

This study investigated the relationship between duodenal mucosal mRNA levels of the transcription factor, NF-E2, H-ferritin (a putative NF-E2 regulated gene) and iron absorption in mice. CD1-strain mice with normal and altered iron metabolism (hypoxic, iron-deficient, iron-loaded) and animals with genetic defects of iron metabolism (hypotransferrinaemia, beta-thalassaemia) were studied. Tissue RNA from these mouse models was subjected to reverse transcription and PCR amplification for NF-E2 and a stable ribosomal protein (S14) and the products analysed with an automated laser fluorescent sequencer. Duodenal NF-E2 mRNA levels were generally low and decreased in the hypoxic and iron-deficient groups, both of which exhibited elevated iron absorption as compared to controls. A modest increase in the NF-E2 mRNA level was seen in the iron-loaded mice, whose iron absorption was decreased. In contrast, both the genetic strains showed elevated NF-E2 mRNA levels in conjunction with raised iron absorption values. Only the iron-deficient group exhibited an alteration in the duodenal mucosal H/L ferritin ratio. Hence, no relationship was evident between the NF-E2 mRNA levels and the H/L ferritin ratio. These data indicate that NF-E2 is not the primary regulator of intestinal iron absorption.

Mouse microcytic anaemia caused by a defect in the gene encoding the globin enhancer-binding protein NF-E2

The nuclear DNA-binding protein NF-E2 is thought to mediate the powerful erythroid enhancer activity of the alpha- and beta-globin locus control regions and participates in the control of genes encoding two enzymes of haem biosynthesis (porphobilinogen deaminase and ferrochelatase). The major component of NF-E2 is a 45K polypeptide (designated p45 NF-E2) that belongs to the basic region-leucine zipper family of transcription factors. This subunit of NF-E2 is specifically expressed in haematopoietic progenitor cells and differentiated cells of the erythroid, megakaryocyte and mast cell lineages. The gene encoding p45 NF-E2 (murine gene Nfe2) has been mapped to mouse chromosome 15 near the mutation microcytosis (mk). Homozygous mk mice have severe hypochromic microcytic anaemia as a result of decreased globin synthesis and defects in intestinal and erythroid iron absorption. Here we investigate whether the mk mutation lies within Nfe2 by characterizing the p45 NF-E2 gene and determining its DNA sequence in wild-type and mk alleles. The mk allele carries a missense mutation that causes substitution of valine by alanine at amino acid 173 of the p45 NF-E2 protein. Expression of p45 NF-E2 messenger RNA was detected in erythroid tissues of normal mice and in the duodenum of normal and severely anaemic beta-thalassaemic (Hbbd-th3/Hbbd-th3) mice. We propose that the mk mutation results in an impaired form of NF-E2 which fails to regulate both globin production and iron metabolism properly.

Animal models of inherited hematologic disease

Inherited or acquired hematologic disease is the most prevalent of all human disease when we include the hematologic disorders which are secondary to disease of other systems. It follows that the study of the fundamental mechanisms of the disease processes affecting the hematopoietic system is of prime importance and much remains to be done when one considers that in only 25% of ail hemolytic anemias is the fundamental cause eventually discovered [150]. In the current climate of societal pressures on experimental animal research, animals with spontaneous inherited disease mimicking diseases of the various physiological systems assume proportionately greater importance. These animal models have been extremely valuable in the study of fundamental questions of molecular genetics, metabolic aberrations of the cell and its membrane, synthetic mechanisms of the cell as well as clinical questions of disease manifestations, pathogenetic mechanisms and management. Exploration of differences between normal animal species offer a secondary avenue of investigation into these same fundamental questions. New animal models are being uncovered constantly and this augurs well for the future of biomedical research and the ultimate benefit to humankind and to animals in their own right.

Acquired iron-deficiency anemia caused by an antibody against the transferrin receptor

We report a case of anemia due to autoantibodies to the transferrin receptor interfering with iron incorporation by erythroid progenitors. A previously healthy woman with severe acquired microcytic anemia had increased serum iron levels, electrophoretically normal transferrin concentrations, and very high levels of free protoporphyrin in red cells. The bone marrow had no stainable iron but had an excess of normal-appearing plasma cells. Erythroid precursors stained with fluorescent mouse antihuman IgM. The serum contained an antibody that reduced 59Fe incorporation by erythroleukemia K562 cells in vitro but did not inhibit iron transferrin binding. An IgM fraction of the patient's serum immunoprecipitated the human transferrin receptor obtained from solubilized [35S]methionine-labeled K562 membranes. Binding of [59Fe]transferrin or fluorescent iron transferrin was not diminished by the patient's serum at 4 degrees C, but at 37 degrees C uptake was markedly reduced, as was the binding of fluorescent monoclonal antibodies to either surface transferrin or the human transferrin receptor. A complete clinical and hematologic remission occurred with azathioprine and prednisone therapy. We conclude that the patient's autoreactive IgM down-regulated the number of transferrin receptors and diminished iron incorporation by erythroblasts, leading to an iron-deficiency anemia.

Acquired iron-deficiency anaemia due to impaired iron transport

A previously healthy 39-year-old woman presented with severe iron-deficiency anaemia, but she had lost no blood and her serum iron level was high. Her bone marrow was hypercellular with a predominance of erythroid elements and had no stainable iron deposits, but it also showed dyserythropoiesis and an excess of apparently normal plasma cells. IgM was demonstrated on her bone-marrow erythrocytes and their precursors. On azathioprine and prednisone therapy she had a complete clinical and haematological remission. The impaired iron transport and the associated dyserythropoiesis were probably due to an IgM-mediated autoimmune process. Diabetes mellitus, which first appeared during her anaemic illness, could also have been due to an autoimmune process. This is the first report of an iron-deficiency anaemia caused by a naturally acquired impairment of iron transport.

Malabsorption and defective utilization of iron in three siblings

Three siblings had iron deficiency anemia without evidence of reduced iron intake or gastrointestinal blood loss. They failed to respond to oral iron therapy, and malabsorption of oral medicinal iron was demonstrated in two of the children. All three had a partial but incomplete hematologic response to intramuscular iron dextran treatment. There was no evidence for other well-defined causes of hypochromic microcytic anemia or for a generalized disorder of intestinal absorption. These three patients appear to have a familial disorder characterized by impaired iron absorption and utilization, similar to that observed in the mk/mk mouse.