A novel missense mutation in SLC40A1 results in resistance to hepcidin and confirms the existence of two ferroportin-associated iron overload diseases

Insights into the role of glycerophospholipids on the iron export function of SLC40A1 and the molecular mechanisms of ferroportin disease

SLC40A1 is the sole iron export protein reported in mammals. In humans, its dysfunction is responsible for ferroportin disease, an inborn error of iron metabolism transmitted as an autosomal dominant trait and observed in different ethnic groups. As a member of the major facilitator superfamily, SLC40A1 requires a series of conformational changes to enable iron translocation across the plasma membrane. The influence of lipids on protein stability and its conformational changes has been little investigated to date. Here, we combine molecular dynamics simulations of SLC40A1 embedded in membrane bilayers with experimental alanine scanning mutagenesis to analyze the specific role of glycerophospholipids. We identify four basic residues (Lys90, Arg365, Lys366, and Arg371) that are located at the membrane-cytosol interface and consistently interact with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) molecules. These residues surround a network of salt bridges and hydrogens bonds that play a critical role in stabilizing SLC40A1 in its basal outward-facing conformation. More deeply embedded in the plasma membrane, we identify Arg179 as a charged amino acid residue also tightly interacting with lipid polar heads. This results in a local deformation of the lipid bilayer. Interestingly, Arg179 is adjacent to Arg178, which forms a functionally important salt-bridge with Asp473 and is a recurrently associated with ferroportin disease when mutated to glutamine. We demonstrate that the two p.Arg178Gln and p.Arg179Thr missense variants have similar functional behaviors. These observations provide insights into the role of phospholipids in the formation/disruption of the SLC40A1 inner gate, and give a better understanding of the diversity of molecular mechanisms of ferroportin disease.

A Revised Classification of Primary Iron Overload Syndromes

Background and Aims

The clinical introduction of hepcidin25 (Hep25) has led to a more detailed understanding of its relationship with ferroportin (FP) and divalent metal transporter1 in primary iron overload syndromes (PIOSs). In 2012, we proposed a classification of PIOSs based on the Hep25/FP system, which consists of prehepatic aceruloplasminemia, hepatic hemochromatosis (HC), and posthepatic FP disease (FP-D). However, in consideration of accumulated evidence on PIOSs, we aimed to renew the classification.

Methods

We reviewed the 2012 classification and retrospectively renewed it according to new information on PIOSs.

Results

Iron-loading anemia was included in PIOSs as a prehepatic form because of the newly discovered erythroferrone-induced suppression of Hep25, and the state of traditional FP-D was remodeled as the BIOIRON proposal. The key molecules responsible for prehepatic PIOSs are low transferrin saturation in aceruloplasminemia and increased erythroferrone production by erythroblasts in iron-loading anemia. Hepatic PIOSs comprise four genotypes of HC, in each of which the synthesis of Hep25 is inappropriately reduced in the liver. Hepatic Hep25 synthesis is adequate in posthepatic PIOSs; however, two mutant FP molecules may resist Hep25 differently, resulting in SLC40A1-HC and FP-D, respectively. PIOS phenotypes are diagnosed using laboratory tests, including circulating Hep25, followed by suitable treatments. Direct sequencing of the candidate genes may be outsourced to gene centers when needed. Laboratory kits for the prevalent mutations, such as C282Y, may be the first choice for a genetic analysis of HC in Caucasians.

Conclusions

The revised classification may be useful worldwide.

20 years of Hepcidin: How far we have come

Twenty years ago the discovery of hepcidin deeply changed our understanding of the regulation of systemic iron homeostasis. It is now clear that hepcidin orchestrates systemic iron levels by controlling the amount of iron exported into the bloodstream through ferroportin. Hepcidin expression is increased in situations where systemic iron levels should be reduced, such as in iron overload and infection. Conversely, hepcidin is repressed during iron deficiency, hypoxia or expanded erythropoiesis, to increase systemic iron availability and sustain erythropoiesis. In this review, we will focus on molecular mechanisms of hepcidin regulation and on the pathological consequences of their disruption.

Evidence for dimerization of ferroportin in a human hepatic cell line using proximity ligation assays

Mutations in the only known iron exporter ferroportin (FPN) in humans are associated with the autosomal dominantly inherited iron overload disorder ferroportin disease or type IV hereditary hemochromatosis (HH). While our knowledge of the central role of FPN in iron homeostasis has grown in the last 20 years, there exist some questions surrounding the structure and membrane topology of FPN with conflicting data on whether this receptor acts as a monomer or a multimer. To investigate and determine if FPN dimerization occurs in cells, we used novel tools including a variety of different FPN constructs expressing different tagged versions of the protein, a novel antibody that only detects cell surface FPN and proximity ligation assays. The results of the present study suggest that both the carboxy- and amino-termini of the FPN protein are intracellular. We also show that exogenously transfected FPN forms dimers; these dimers can be formed between the wild-type and mutant FPN proteins. This is the first study to examine the intracellular dimerization of FPN protein. Using proximity ligation assays, we show intracellular localization of FPN dimers and the interaction between FPN and hepcidin proteins as well. These results have important implications in the field of iron metabolism and add to our knowledge about FPN membrane topology and physiology of iron transport. This will be of importance in understanding the clinical implications of FPN mutations and of interest to future research aimed at targeting FPN expression to modulate iron homeostasis.

TAM-ing the CIA-Tumor-Associated Macrophages and Their Potential Role in Unintended Side Effects of Therapeutics for Cancer-Induced Anemia

Cancer-induced anemia (CIA) is a common consequence of neoplasia and has a multifactorial pathophysiology. The immune response and tumor treatment, both intended to primarily target malignant cells, also affect erythropoiesis in the bone marrow. In parallel, immune activation inevitably induces the iron-regulatory hormone hepcidin to direct iron fluxes away from erythroid progenitors and into compartments of the mononuclear phagocyte system. Moreover, many inflammatory mediators inhibit the synthesis of erythropoietin, which is essential for stimulation and differentiation of erythroid progenitor cells to mature cells ready for release into the blood stream. These pathophysiological hallmarks of CIA imply that the bone marrow is not only deprived of iron as nutrient but also of erythropoietin as central growth factor for erythropoiesis. Tumor-associated macrophages (TAM) are present in the tumor microenvironment and display altered immune and iron phenotypes. On the one hand, their functions are altered by adjacent tumor cells so that they promote rather than inhibit the growth of malignant cells. As consequences, TAM may deliver iron to tumor cells and produce reduced amounts of cytotoxic mediators. Furthermore, their ability to stimulate adaptive anti-tumor immune responses is severely compromised. On the other hand, TAM are potential off-targets of therapeutic interventions against CIA. Red blood cell transfusions, intravenous iron preparations, erythropoiesis-stimulating agents and novel treatment options for CIA may interfere with TAM function and thus exhibit secondary effects on the underlying malignancy. In this Hypothesis and Theory, we summarize the pathophysiological hallmarks, clinical implications and treatment strategies for CIA. Focusing on TAM, we speculate on the potential intended and unintended effects that therapeutic options for CIA may have on the innate immune response and, consequently, on the course of the underlying malignancy.

Splicing analysis of SLC40A1 missense variations and contribution to hemochromatosis type 4 phenotypes

Hemochromatosis type 4, or ferroportin disease, is considered as the second leading cause of primary iron overload after HFE-related hemochromatosis. The disease, which is predominantly associated with missense variations in the SLC40A1 gene, is characterized by wide clinical heterogeneity. We tested the possibility that some of the reported missense mutations, despite their positions within exons, cause splicing defects. Fifty-eight genetic variants were selected from the literature based on two criteria: a precise description of the nucleotide change and individual evidence of iron overload. The selected variants were investigated by different in silico prediction tools and prioritized for midigene splicing assays. Of the 15 variations tested in vitro, only two were associated with splicing changes. We confirm that the c.1402G>A transition (p.Gly468Ser) disrupts the exon 7 donor site, leading to the use of an exonic cryptic splicing site and the generation of a truncated reading frame. We observed, for the first time, that the p.Gly468Ser substitution has no effect on the ferroportin iron export function. We demonstrate alternative splicing of exon 5 in different cell lines and show that the c.430A>G (p.Asn144Asp) variant promotes exon 5 inclusion. This could be part of a gain-of-function mechanism. We conclude that splicing mutations rarely contribute to hemochromatosis type 4 phenotypes. An in-depth investigation of exon 5 auxiliary splicing sequences may help to elucidate the mechanism by which splicing regulatory proteins regulate the production of the full length SLC40A1 transcript and to clarify its physiological importance.

Structure of hepcidin-bound ferroportin reveals iron homeostatic mechanisms

The serum iron level in humans is tightly controlled by the action of the hormone hepcidin on the iron efflux transporter ferroportin. Hepcidin regulates iron absorption and recycling by inducing ferroportin internalization and degradation¹. Aberrant ferroportin activity can lead to diseases of iron overload, like hemochromatosis, or iron limitation anemias². Here, we determined cryogenic electron microscopy (cryo-EM) structures of ferroportin in lipid nanodiscs, both in the apo state and in complex with cobalt, an iron mimetic, and hepcidin. These structures and accompanying molecular dynamics simulations identify two metal binding sites within the N- and C-domains of ferroportin. Hepcidin binds ferroportin in an outward-open conformation and completely occludes the iron efflux pathway to inhibit transport. The carboxy-terminus of hepcidin directly contacts the divalent metal in the ferroportin C-domain. We further show that hepcidin binding to ferroportin is coupled to iron binding, with an 80-fold increase in hepcidin affinity in the presence of iron. These results suggest a model for hepcidin regulation of ferroportin, where only iron loaded ferroportin molecules are targeted for degradation. More broadly, our structural and functional insights are likely to enable more targeted manipulation of the hepcidin-ferroportin axis in disorders of iron homeostasis.

Twenty Years of Ferroportin Disease: A Review or An Update of Published Clinical, Biochemical, Molecular, and Functional Features

Iron overloading disorders linked to mutations in ferroportin have diverse phenotypes in vivo, and the effects of mutations on ferroportin in vitro range from loss of function (LOF) to gain of function (GOF) with hepcidin resistance. We reviewed 359 patients with 60 ferroportin variants. Overall, macrophage iron overload and low/normal transferrin saturation (TSAT) segregated with mutations that caused LOF, while GOF mutations were linked to high TSAT and parenchymal iron accumulation. However, the pathogenicity of individual variants is difficult to establish due to the lack of sufficiently reported data, large inter-assay variability of functional studies, and the uncertainty associated with the performance of available in silico prediction models. Since the phenotypes of hepcidin-resistant GOF variants are indistinguishable from the other types of hereditary hemochromatosis (HH), these variants may be categorized as ferroportin-associated HH, while the entity ferroportin disease may be confined to patients with LOF variants. To further improve the management of ferroportin disease, we advocate for a global registry, with standardized clinical analysis and validation of the functional tests preferably performed in human-derived enterocytic and macrophagic cell lines. Moreover, studies are warranted to unravel the definite structure of ferroportin and the indispensable residues that are essential for functionality.

A 10-year Follow-up Study of a Japanese Family with Ferroportin Disease A: Mild Iron Overload with Mild Hyperferritinemia Co-occurring with Hyperhepcidinemia May Be Benign

This is a 10-year follow-up study of a family with ferroportin disease A. The proband, a 59-year-old man showed no noteworthy findings with the exception of an abnormal iron level. The proband's 90-year-old father showed reduced abilities in gait and cognition; however, with the exception of his iron level, his biochemistry results were almost normal. Brain imaging showed age-matched atrophy and iron deposition. In both patients, the serum levels of ferritin and hepcidin25, and liver computed tomography scores declined over a 10-year period. These changes were mainly due to a habitual change to a low-iron diet. The iron disorder in this family was not associated with major organ damage.

Ferroportin deficiency in erythroid cells causes serum iron deficiency and promotes hemolysis due to oxidative stress

Ferroportin (FPN), the only known vertebrate iron exporter, transports iron from intestinal, splenic, and hepatic cells into the blood to provide iron to other tissues and cells in vivo. Most of the circulating iron is consumed by erythroid cells to synthesize hemoglobin. Here we found that erythroid cells not only consumed large amounts of iron, but also returned significant amounts of iron to the blood. Erythroblast-specific Fpn knockout (Fpn KO) mice developed lower serum iron levels in conjunction with tissue iron overload and increased FPN expression in spleen and liver without changing hepcidin levels. Our results also showed that Fpn KO mice, which suffer from mild hemolytic anemia, were sensitive to phenylhydrazine-induced oxidative stress but were able to tolerate iron deficiency upon exposure to a low-iron diet and phlebotomy, supporting that the anemia of Fpn KO mice resulted from erythrocytic iron overload and resulting oxidative injury rather than a red blood cell (RBC) production defect. Moreover, we found that the mean corpuscular volume (MCV) values of gain-of-function FPN mutation patients were positively associated with serum transferrin saturations, whereas MCVs of loss-of-function FPN mutation patients were not, supporting that erythroblasts donate iron to blood through FPN in response to serum iron levels. Our results indicate that FPN of erythroid cells plays an unexpectedly essential role in maintaining systemic iron homeostasis and protecting RBCs from oxidative stress, providing insight into the pathophysiology of FPN diseases.

The interactions between iron and copper in genetic iron overload syndromes and primary copper toxicoses in Japan

Iron and copper are trace elements essential for health, and iron metabolism is tightly regulated by cuproproteins. Clarification of the interactions between iron and copper may provide a better understanding of the pathophysiology and treatment strategy for hemochromatosis, Wilson disease, and related disorders. The hepcidin/ferroportin system was used to classify genetic iron overload syndromes in Japan, and ceruloplasmin and ATP7B were introduced for subtyping Wilson disease into the severe hepatic and classical forms. Interactions between iron and copper were reviewed in these genetic diseases. Iron overload syndromes were classified into pre-hepatic iron loading anemia and aceruloplasminemia, hepatic hemochromatosis, and post-hepatic ferroportin disease. The ATP7B-classical form with hypoceruloplasminemia has primary hepatopathy and late extra-hepatic complications, while the severe hepatic form is free from ATP7B mutation and hypoceruloplasminemia, and silently progresses to liver failure. A large amount of iron and trace copper co-exist in hepatocellular dense bodies of all iron overload syndromes. Cuproprotein induction to stabilize excess iron should be differentiated from copper retention in Wilson disease. The classical form of Wilson disease associated with suppressed hepacidin25 secretion may be double-loaded with copper and iron, and transformed to an iron disease after long-term copper chelation. Iron disease may not be complicated with the severe hepatic form with normal ferroxidase activity. Hepatocellular dense bodies of iron overload syndromes may be loaded with a large amount of iron and trace copper, while the classical Wilson disease may be double-loaded with copper and iron.

Structure-function analysis of ferroportin defines the binding site and an alternative mechanism of action of hepcidin

Nonclassical ferroportin disease (FD) is a form of hereditary hemochromatosis caused by mutations in the iron transporter ferroportin (Fpn), resulting in parenchymal iron overload. Fpn is regulated by the hormone hepcidin, which induces Fpn endocytosis and cellular iron retention. We characterized 11 clinically relevant and 5 nonclinical Fpn mutations using stably transfected, inducible isogenic cell lines. All clinical mutants were functionally resistant to hepcidin as a consequence of either impaired hepcidin binding or impaired hepcidin-dependent ubiquitination despite intact hepcidin binding. Mapping the residues onto 2 computational models of the human Fpn structure indicated that (1) mutations that caused ubiquitination-resistance were positioned at helix-helix interfaces, likely preventing the hepcidin-induced conformational change, (2) hepcidin binding occurred within the central cavity of Fpn, (3) hepcidin interacted with up to 4 helices, and (4) hepcidin binding should occlude Fpn and interfere with iron export independently of endocytosis. We experimentally confirmed hepcidin-mediated occlusion of Fpn in the absence of endocytosis in multiple cellular systems: HEK293 cells expressing an endocytosis-defective Fpn mutant (K8R), Xenopus oocytes expressing wild-type or K8R Fpn, and mature human red blood cells. We conclude that nonclassical FD is caused by Fpn mutations that decrease hepcidin binding or hinder conformational changes required for ubiquitination and endocytosis of Fpn. The newly documented ability of hepcidin and its agonists to occlude iron transport may facilitate the development of broadly effective treatments for hereditary iron overload disorders.

Ferroportin disease: pathogenesis, diagnosis and treatment

Ferroportin Disease (FD) is an autosomal dominant hereditary iron loading disorder associated with heterozygote mutations of the ferroportin-1 (FPN) gene. It represents one of the commonest causes of genetic hyperferritinemia, regardless of ethnicity. FPN1 transfers iron from the intestine, macrophages and placenta into the bloodstream. In FD, loss-of-function mutations of FPN1 limit but do not impair iron export in enterocytes, but they do severely affect iron transfer in macrophages. This leads to progressive and preferential iron trapping in tissue macrophages, reduced iron release to serum transferrin (i.e. inappropriately low transferrin saturation) and a tendency towards anemia at menarche or after intense bloodletting. The hallmark of FD is marked iron accumulation in hepatic Kupffer cells. Numerous FD-associated mutations have been reported worldwide, with a few occurring in different populations and some more commonly reported (e.g. Val192del, A77D, and G80S). FPN1 polymorphisms also represent the gene variants most commonly responsible for hyperferritinemia in Africans. Differential diagnosis includes mainly hereditary hemochromatosis, the syndrome commonly due to either HFE or TfR2, HJV, HAMP, and, in rare instances, FPN1 itself. Here, unlike FD, hyperferritinemia associates with high transferrin saturation, iron-spared macrophages, and progressive parenchymal cell iron load. Abdominal magnetic resonance imaging (MRI), the key non-invasive diagnostic tool for the diagnosis of FD, shows the characteristic iron loading SSL triad (spleen, spine and liver). A non-aggressive phlebotomy regimen is recommended, with careful monitoring of transferrin saturation and hemoglobin due to the risk of anemia. Family screening is mandatory since siblings and offspring have a 50% chance of carrying the pathogenic mutation.

Molecular pathogenesis and clinical consequences of iron overload in liver cirrhosis

BACKGROUND

The liver, as the main iron storage compartment and the place of hepcidin synthesis, is the central organ involved in maintaining iron homeostasis in the body. Excessive accumulation of iron is an important risk factor in liver disease progression to cirrhosis and hepatocellular carcinoma. Here, we review the literature on the molecular pathogenesis of iron overload and its clinical consequences in chronic liver diseases.

DATA SOURCES

PubMed was searched for English-language articles on molecular genesis of primary and secondary iron overload, as well as on their association with liver disease progression. We have also included literature on adjuvant therapeutic interventions aiming to alleviate detrimental effects of excessive body iron load in liver cirrhosis.

RESULTS

Excess of free, unbound iron induces oxidative stress, increases cell sensitivity to other detrimental factors, and can directly affect cellular signaling pathways, resulting in accelerated liver disease progression. Diagnosis of liver cirrhosis is, in turn, often associated with the identification of a pathological accumulation of iron, even in the absence of genetic background of hereditary hemochromatosis. Iron depletion and adjuvant therapy with antioxidants are shown to cause significant improvement of liver functions in patients with iron overload. Phlebotomy can have beneficial effects on liver histology in patients with excessive iron accumulation combined with compensated liver cirrhosis of different etiology.

CONCLUSION

Excessive accumulation of body iron in liver cirrhosis is an important predictor of liver failure and available data suggest that it can be considered as target for adjuvant therapy in this condition.

A novel mutation in the SLC40A1 gene associated with reduced iron export in vitro

Ferroportin disease is an inherited disorder of iron metabolism and is caused by mutations in the ferroportin gene (SLC40A1). We present a patient with hyperferritinemia, iron overload in the liver with reticuloendothelial distribution and also in the spleen, and under treatment with erythropheresis. A molecular study of the genes involved in iron metabolism (HFE, HJV, HAMP, TFR2, SLC40A1) was undertaken. In vitro functional studies of the novel mutation found in the SLC40A1 gene was performed. The patient was heterozygous for a novel mutation, c.386T>C (p.L129P) in the SLC40A1 gene; some of his relatives were also heterozygous for this mutation. In vitro functional studies of the L129P mutation on ferroportin showed it impairs its capacity to export iron from cells but does not alter its sensitivity to hepcidin. These findings and the iron overload phenotype of the patient suggest that the novel mutation c.386T>C (p.L129P) in the SLC40A1 gene has incomplete penetrance and causes the classical form of ferroportin disease.

Ceruloplasmin-ferroportin system of iron traffic in vertebrates

Safe trafficking of iron across the cell membrane is a delicate process that requires specific protein carriers. While many proteins involved in iron uptake by cells are known, only one cellular iron export protein has been identified in mammals: ferroportin (SLC40A1). Ceruloplasmin is a multicopper enzyme endowed with ferroxidase activity that is found as a soluble isoform in plasma or as a membrane-associated isoform in specific cell types. According to the currently accepted view, ferrous iron transported out of the cell by ferroportin would be safely oxidized by ceruloplasmin to facilitate loading on transferrin. Therefore, the ceruloplasmin-ferroportin system represents the main pathway for cellular iron egress and it is responsible for physiological regulation of cellular iron levels. The most recent findings regarding the structural and functional features of ceruloplasmin and ferroportin and their relationship will be described in this review.

A structural model of human ferroportin and of its iron binding site

A structural model of human ferroportin has been built using two Escherichia coli proteins belonging to the major facilitator superfamily of transporters. A potential iron binding site was identified in the inward-open conformation of the model, and its relevance was tested through measurement of iron export of HEK293T cells expressing wild-type or mutated ferroportin. Aspartates 39 and 181 were found to be essential for the transport ability of the protein. Noteworthy, the D181V mutation is naturally found in type 4 hemochromatosis with reticuloendothelial system iron retention phenotype. The outward-open conformation of ferroportin was also predicted, and showed that significant conformational changes must occur in the inward- to outward-open transition of ferroportin. In particular, putative iron ligands move several ångströms away from each other, leading to the logical conclusion that the iron binding site is not occupied by the metal in the outward-open conformation of ferroportin.

Structure-function analysis of the human ferroportin iron exporter (SLC40A1): effect of hemochromatosis type 4 disease mutations and identification of critical residues

Ferroportin (SLC40A1) is the only known iron exporter in mammals and is considered a key coordinator of the iron balance between intracellular and systemic iron homeostasis. However, the structural organization of ferroportin in the lipid bilayer remains controversial and very little is known about the mechanism underlying iron egress. In the present study, we have developed an approach based on comparative modeling, which has led to the construction of a model of the three-dimensional (3D) structure of ferroportin by homology to the crystal structure of a Major Facilitator Superfamily member (EmrD). This model predicts atomic details for the organization of ferroportin transmembrane helices and is in agreement with our current understanding of the ferroportin function and its interaction with hepcidin. Using in vitro experiments, we demonstrate that this model can be used to identify novel critical amino acids. In particular, we show that the tryptophan residue 42 (p.Trp42), which is localized within the extracellular end of the ferroportin pore, is likely involved in both the iron export function and in the mechanism of inhibition by hepcidin. Thus, our 3D model provides a new perspective for understanding the molecular basis of ferroportin functions and dysfunctions.

Diagnosis and treatment of hereditary hemochromatosis: an update

Hereditary hemochromatosis is an inherited iron overload disorder caused by inappropriately low hepcidin secretion leading to increased duodenal absorption of dietary iron, most commonly in C282Y homozygous individuals. This can result in elevated serum ferritin, iron deposition in various organs and ultimately end-organ damage, although there is incomplete biochemical and clinical penetrance and variable phenotypic expression of the HFE mutation in hereditary hemochromatosis. An elevated SF >1000 mg/l [corrected] is associated with an increased risk of cirrhosis and mortality in C282Y homozygotes.Conversely, a SF <1000 µg/l is associated with a very low likelihood of cirrhosis, making liver biopsy unnecessary among C282Y homozygotes in the absence of concomitant risk factors for liver disease. Phlebotomy remains the mainstay of treatment and new treatments being studied include erythrocytapheresis and 'mini-hepcidins'. Iron overload is being recognized to play a carcinogenic role in hepatocellular carcinoma and other cancers, possibly supporting iron depletion in these patients.

Clinicopathological study of Japanese patients with genetic iron overload syndromes

In addition to hemochromatosis, aceruloplasminemia and ferroportin disease may be complicated by iron-induced multiple organ damage. Therefore, clinicopathological features should be evaluated in a wider range of genetic iron disorders. This study included 16 Japanese patients with genetic iron overload syndromes. The responsible genes were CP in four, HAMP in one, HJV in three, TFR2 in five, and SLC40A1 in three patients. No phenotype dissociation was observed in patients with the CP, TFR2, or HAMP genotypes. Two of the three patients with the HJV genotype displayed classic hemochromatosis instead of the juvenile type. Patients with the SLC40A1 genotype were affected by mild iron overload (ferroportin A) or severe iron overload (ferroportin B). Transferrin saturation was unusually low in aceruloplasminemia patients. All patients, except those with ferroportin disease, displayed low serum hepcidin-25 levels. Liver pathology showed phenotype-specific changes; isolated parenchymal iron loading in aceruloplasminemia, periportal fibrosis associated with heavy iron overload in both parenchymal and Kupffer cells of ferroportin B, and parenchyma-dominant iron-loading cirrhosis in hemochromatosis. In contrast, diabetes occurred in all phenotypes of aceruloplasminemia, hemochromatosis, and ferroportin disease B. In conclusion, clinicopathological features were partially characterized in Japanese patients with genetic iron overload syndromes.

Effect of ferroportin polymorphism on iron homeostasis and infection

Ferroportin (FPN) is the sole iron export membrane protein identified in mammals that is abundantly expressed on absorptive enterocytes and macrophages, and is essential for physiological regulation of cellular iron. The expression of FPN is positively induced by cellular iron and is suppressed by liver hepcidin in response to either increased systemic iron or inflammatory stimuli. Hepcidin binds to cell surface FPN inducing FPN internalization followed by lysosomal degradation of the protein and consequently iron efflux from macrophages is blocked and there is suboptimal iron absorption by duodenal enterocytes. Dozens of FPN gene mutations have been identified in different ethnic populations and some of the mutations are associated with autosomal dominant iron overload disorder described as FPN disease or hemochromatosis type 4 that is distinct from hereditary hemochromatosis due to HFE mutations. Clinical manifestations of iron overload FPN disease can be classified into two groups according to whether there is selective macrophage iron loading or parenchymal and reticuloendothelial iron accumulation. There is evidence suggesting that altered hepcidin-FPN interaction can modulate host's response to infection. Resistance to hepcidin promotes iron egress from cells and this inhibits growth of intracellular pathogens. Conversely, iron retention due to loss of iron export activity by mutated FPN results in intracellular iron accumulation and a permissive environment for intracellular pathogens.

Review article: the iron overload syndromes

BACKGROUND

Iron overload syndromes encompass a wide range of hereditary and acquired conditions. Major developments in the field of genetics and the discovery of hepcidin as a central regulator of iron homeostasis have greatly increased our understanding of the pathophysiology of iron overload syndromes.

AIM

To review advances in iron regulation and iron overload syndrome with special emphasis on hereditary haemochromatosis, the prototype iron overload syndrome.

METHODS

A PubMed search using words such as 'iron overload', 'hemochromatosis', 'HFE', 'Non-HFE', 'secondary iron overload' was undertaken.

RESULTS

Iron overload is associated with significant morbidity and mortality. Sensitive diagnostic tests and effective therapy are widely available and can prevent complications associated with iron accumulation in end- organs. Therapeutic phlebotomy remains the cornerstone of therapy for removal of excess body iron, but novel therapeutic agents including oral iron chelators have been developed for iron overload associated with anaemia.

CONCLUSIONS

Iron overload disorders are common. Inexpensive screening tests as well as confirmatory diagnostic tests are widely available. Increased awareness of the causes and importance of early diagnosis and knowledge of the appropriate use of genetic testing are encouraged. The availability of novel treatments should increase therapeutic options for patients with iron overload disorders.

Sex and acquired cofactors determine phenotypes of ferroportin disease

BACKGROUND & AIMS

Ferroportin disease is characterized by iron overload. It has an autosomal-dominant pattern of inheritance and has been associated with mutations in the SLC40A1 gene, which encodes the cellular iron exporter ferroportin. Since the first description in 2001, about 30 mutations have been reported; the heterogeneity of ferroportin disease phenotypes has led to the hypothesis that the nature of the mutation affects the function of the protein in different ways. We studied genotypes and phenotypes of a large cohort of patients with ferroportin disease.

METHODS

We studied clinical, biochemical, imaging, histologic, and genetic data from 70 affected subjects from 33 families with 19 mutations.

RESULTS

We found that ferroportin disease, at the time of diagnosis, has limited consequences in the absence of cofactors. Data indicated that transferrin saturation, which correlated with fibrosis and levels of alanine aminotransferase, might be a marker of disease severity. Although the study was performed in a large number of families, we observed incomplete penetrance and no correlation between genotypes and phenotypes.

CONCLUSIONS

Members of families with ferroportin disease should be screened for biochemical parameters of iron metabolism as well as genotype to detect silent mutations that might cause disease with acquired or genetic cofactors. Patients should be followed up long term to identify potential complications of the disease.

Two novel mutations in the SLC40A1 and HFE genes implicated in iron overload in a Spanish man

The most common form of hemochromatosis is caused by mutations in the HFE gene. Rare forms of the disease are caused by mutations in other genes. We present a patient with hyperferritinemia and iron overload, and facial flushing. Magnetic resonance imaging was performed to measure hepatic iron overload, and a molecular study of the genes involved in iron metabolism was undertaken. The iron overload was similar to that observed in HFE hemochromatosis, and the patient was double heterozygous for two novel mutations, c.-20G>A and c.718A>G (p.K240E), in the HFE and ferroportin (FPN1 or SLC40A1) genes, respectively. Hyperferritinemia and facial flushing improved after phlebotomy. Two of the patient's children were also studied, and the daughter was heterozygous for the mutation in the SLC40A1 gene, although she did not have hyperferritinemia. The patient presented a mild iron overload phenotype probably because of the two novel mutations in the HFE and SLC40A1 genes.

Ferroportin disease: a systematic meta-analysis of clinical and molecular findings

BACKGROUND & AIMS

Classical ferroportin disease is characterized by hyperferritinemia, normal transferrin saturation, and iron overload in macrophages. A non-classical form is characterized by additional hepatocellular iron deposits and a high transferrin saturation. Both forms demonstrate autosomal dominant transmission and are associated with ferroportin gene (SLC40A1) mutations. SLC40A1 encodes a cellular iron exporter expressed in macrophages, enterocytes, and hepatocytes. The aim of the analysis is to determine the penetrance of SLC40A1 mutations and to evaluate in silico tools to predict the functional impairment of ferroportin mutations as an alternative to in vitro studies.

METHODS

We conducted a systematic review of the literature and meta-analysis of the biochemical presentation, genetics, and pathology of ferroportin disease.

RESULTS

Of the 176 individuals reported with SLC40A1 mutations, 80 were classified as classical phenotype with hyperferritinemia and normal transferrin saturation. The non-classical phenotype with hyperferritinemia and elevated transferrin saturation was present in 53 patients. The remaining patients had normal serum ferritin or the data were reported incompletely. Despite an increased hepatic iron concentration in all biopsied patients, significant fibrosis or cirrhosis was present in only 11%. Hyperferritinemia was present in 86% of individuals with ferroportin mutations. Bio-informatic analysis of ferroportin mutations showed that the PolyPhen score has a sensitivity of 99% and a specificity of 67% for the discrimination between ferroportin mutations and polymorphisms.

CONCLUSIONS

In contrast to HFE hemochromatosis, ferroportin disease has a high penetrance, is genetically heterogeneous and is rarely associated with fibrosis. Non-classical ferroportin disease is associated with a higher risk of fibrosis and a more severe overload of hepatic iron.