Primary iron overload with inappropriate hepcidin expression in V162del ferroportin disease

EASL Clinical Practice Guidelines on haemochromatosis

Haemochromatosis is characterised by elevated transferrin saturation (TSAT) and progressive iron loading that mainly affects the liver. Early diagnosis and treatment by phlebotomy can prevent cirrhosis, hepatocellular carcinoma, diabetes, arthropathy and other complications. In patients homozygous for p.Cys282Tyr in HFE, provisional iron overload based on serum iron parameters (TSAT >45% and ferritin >200 μg/L in females and TSAT >50% and ferritin >300 μg/L in males and postmenopausal women) is sufficient to diagnose haemochromatosis. In patients with high TSAT and elevated ferritin but other HFE genotypes, diagnosis requires the presence of hepatic iron overload on MRI or liver biopsy. The stage of liver fibrosis and other end-organ damage should be carefully assessed at diagnosis because they determine disease management. Patients with advanced fibrosis should be included in a screening programme for hepatocellular carcinoma. Treatment targets for phlebotomy are ferritin <50 μg/L during the induction phase and <100 μg/L during the maintenance phase.

A simple clinical score to promote and enhance ferroportin disease screening

BACKGROUND & AIMS

Ferroportin disease is a rare genetic iron overload disorder which may be underdiagnosed, with recent data suggesting it occurs at a higher prevalence than suspected. Costs and the lack of defined criteria to prompt genetic testing preclude large-scale molecular screening. Hence, we aimed to develop a readily available scoring system to promote and enhance ferroportin disease screening.

METHODS

Our derivation cohort included probands tested for ferroportin disease from 2008 to 2016 in our rare disease network. Data were prospectively recorded. Univariate and multivariate logistic regression were used to determine significant criteria, and odds ratios were used to build a weighted score. A cut-off value was defined using a ROC curve with a predefined aim of 90% sensitivity. An independent cohort was used for cross validation.

RESULTS

Our derivation cohort included 1,306 patients. Mean age was 55±14 years, ferritin 1,351±1,357 μg/L, and liver iron concentration (LIC) 166±77 μmol/g. Pathogenic variants (n = 32) were identified in 71 patients. In multivariate analysis: female sex, younger age, higher ferritin, higher LIC and the absence of hypertension or diabetes were significantly associated with the diagnosis of ferroportin disease (AUROC in whole derivation cohort 0.83 [0.78-0.88]). The weighted score was based on sex, age, the presence of hypertension or diabetes, ferritin level and LIC. An AUROC of 0.83 (0.77-0.88) was obtained in the derivation cohort without missing values. Using 9.5 as a cut-off, sensitivity was 93.6 (91.7-98.3) %, specificity 49.5 (45.5-53.6) %, positive likelihood ratio 1.8 (1.6-2.0) and negative likelihood ratio 0.17 (0.04-0.37).

CONCLUSION

We describe a readily available score with simple criteria and good diagnostic performance that could be used to screen patients for ferroportin disease in routine clinical practice.

LAY SUMMARY

Increased iron burden associated with metabolic syndrome is a very common condition. Ferroportin disease is a dominant genetic iron overload disorder whose prevalence is higher than initially thought. They can be difficult to distinguish from each other, but the limited availability of genetic testing and the lack of definitive guidelines prevent adequate screening. We herein describe a simple and definitive clinical score to help clinicians decide whether to perform genetic testing.

Sex-dependent effects of long-term clozapine or haloperidol medication on red blood cells and liver iron metabolism in Sprague Dawley rats as a model of metabolic syndrome

BACKGROUND

Patients with liver diseases often have some form of anemia. Hematological dyscrasias are known side effects of antipsychotic drug medication and the occurrence of agranulocytosis under clozapine is well described. However, the sex-dependent impact of clozapine and haloperidol on erythrocytes and symptoms like anemia, and its association with hepatic iron metabolism has not yet been completely clarified. Therefore, in the present study, we investigated the effect of both antipsychotic drugs on blood parameters and iron metabolism in the liver of male and female Sprague Dawley rats.

METHODS

After puberty, rats were treated orally with haloperidol or clozapine for 12 weeks. Blood count parameters, serum ferritin, and liver transferrin bound iron were determined by automated counter. Hemosiderin (Fe³⁺) was detected in liver sections by Perl's Prussian blue staining. Liver hemoxygenase (HO-1), 5'aminolevulinate synthase (ALAS1), hepcidin, heme-regulated inhibitor (HRI), cytochrome P4501A1 (CYP1A1) and 1A2 (CYP1A2) were determined by Western blotting.

RESULTS

We found anemia with decreased erythrocyte counts, associated with lower hemoglobin and hematocrit, in females with haloperidol treatment. Males with clozapine medication showed reduced hemoglobin and increased red cell distribution width (RDW) without changes in erythrocyte numbers. High levels of hepatic hemosiderin were found in the female clozapine and haloperidol medicated groups. Liver HRI was significantly elevated in male clozapine medicated rats. CYP1A2 was significantly reduced in clozapine medicated females.

CONCLUSIONS

The characteristics of anemia under haloperidol and clozapine medication depend on the administered antipsychotic drug and on sex. We suggest that anemia in rats under antipsychotic drug medication is a sign of an underlying liver injury induced by the drugs. Changing hepatic iron metabolism under clozapine and haloperidol may help to reduce these effects of liver diseases.

Dietary Iron Overload Differentially Modulates Chemically-Induced Liver Injury in Rats

Hepatic iron overload is well known as an important risk factor for progression of liver diseases; however, it is unknown whether it can alter the susceptibility to drug-induced hepatotoxicity. Here we investigate the pathological roles of iron overload in two single-dose models of chemically-induced liver injury. Rats were fed a high-iron (Fe) or standard diet (Cont) for four weeks and were then administered with allyl alcohol (AA) or carbon tetrachloride (CCl₄). Twenty-four hours after administration mild mononuclear cell infiltration was seen in the periportal/portal area (Zone 1) in Cont-AA group, whereas extensive hepatocellular necrosis was seen in Fe-AA group. Centrilobular (Zone 3) hepatocellular necrosis was prominent in Cont-CCl₄ group, which was attenuated in Fe-CCl₄ group. Hepatic lipid peroxidation and hepatocellular DNA damage increased in Fe-AA group compared with Cont-AA group. Hepatic caspase-3 cleavage increased in Cont-CCl₄ group, which was suppressed in Fe-CCl₄ group. Our results showed that dietary iron overload exacerbates AA-induced Zone-1 liver injury via enhanced oxidative stress while it attenuates CCl₄-induced Zone-3 liver injury, partly via the suppression of apoptosis pathway. This study suggested that susceptibility to drugs or chemical compounds can be differentially altered in iron-overloaded livers.

Reduced iron export associated with hepcidin resistance can explain the iron overload spectrum in ferroportin disease

BACKGROUND & AIMS

Ferroportin disease (FD) and hemochromatosis type 4 (HH4) are associated with variants in the ferroportin-encoding gene SLC40A1. Both phenotypes are characterized by iron overload despite being caused by distinct variants that either mediate reduced cellular iron export in FD or resistance against hepcidin-induced inactivation of ferroportin in HH4. The aim of this study was to assess if reduced iron export also confers hepcidin resistance and causes iron overload in FD associated with the R178Q variant.

METHODS

The ferroportin disease variants R178Q andA77D and the HH4-variant C326Y were overexpressed in HEK-293T cells and subcellular localization was characterized by confocal microscopy and flow cytometry. Iron export and cytosolic ferritin were measured as markers of iron transport and radioligand binding studies were performed. The hepcidin-ferroportin axis was assessed by ferritin/hepcidin correlation in patients with different iron storage diseases.

RESULTS

In the absence of hepcidin, the R178Q and A77D variants exported less iron when compared to normal and C326Y ferroportin. In the presence of hepcidin, the R178Q and C326Y, but not the A77D-variant, exported more iron than cells expressing normal ferroportin. Regression analysis of serum hepcidin and ferritin in patients with iron overload are compatible with hepcidin deficiency in HFE hemochromatosis and hepcidin resistance in R178Q FD.

CONCLUSIONS

These results support a novel concept that in certain FD variants reduced iron export and hepcidin resistance could be interlinked. Evasion of mutant ferroportin from hepcidin-mediated regulation could result in uncontrolled iron absorption and iron overload despite reduced transport function.

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.

The SLC40A1 R178Q mutation is a recurrent cause of hemochromatosis and is associated with a novel pathogenic mechanism

Hemochromatosis type 4 is one of the most common causes of primary iron overload, after HFE-related hemochromatosis. It is an autosomal dominant disorder, primarily due to missense mutations in SLC40A1. This gene encodes ferroportin 1 (FPN1), which is the sole iron export protein reported in mammals. Not all heterozygous missense mutations in SLC40A1 are disease-causing. Due to phenocopies and an increased demand for genetic testing, rare SLC40A1 variations are fortuitously observed in patients with a secondary cause of hyperferritinemia. Structure/function analysis is the most effective way of establishing causality when clinical and segregation data are lacking. It can also provide important insights into the mechanism of iron egress and FPN1 regulation by hepcidin. The present study aimed to determine the pathogenicity of the previously reported p.Arg178Gln variant. We present the biological, clinical, histological and radiological findings of 22 patients from six independent families of French, Belgian or Iraqi decent. Despite phenotypic variability, all patients with p.Arg178Gln had elevated serum ferritin concentrations and normal to low transferrin saturation levels. In vitro experiments demonstrated that the p.Arg178Gln mutant reduces the ability of FPN1 to export iron without causing protein mislocalization. Based on a comparative model of the 3D structure of human FPN1 in an outward facing conformation, we argue that p.Arg178 is part of an interaction network modulating the conformational changes required for iron transport. We conclude that p.Arg178Gln represents a new category of loss-of-function mutations and that the study of “gating residues” is necessary in order to fully understand the action mechanism of FPN1.

Low hepcidin in liver fibrosis and cirrhosis; a tale of progressive disorder and a case for a new biochemical marker

Liver fibrosis is a precursor of liver cirrhosis, which is associated with increased mortality. Though liver biopsy remains the gold standard for the diagnosis of fibrosis, noninvasive biochemical methods are cost-effective, practical and are not linked with major risks of complications. In this respect, serum hepcidin, has emerged as a new marker of fibrosis and cirrhosis. In this review the discussion uncovers molecular links between hepcidin disturbance and liver fibrosis/cirrhosis. The discussion also expands on clinical studies that suggest that hepcidin can potentially be used as a biochemical parameter of fibrosis/cirrhosis and target of therapeutic strategies to treat liver diseases. The debatable issues such as the complicated nature of hepcidin disturbance in non-alcoholic liver disease, serum levels of hepcidin in acute hepatitis C virus infection, cause of hepcidin disturbance in autoimmune hepatitis and hepatic insulin resistance are discussed, with potential solutions unveiled in order to be studied by future research.

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.

Human macrophage ferroportin biology and the basis for the ferroportin disease

Ferroportin (FPN1) is the sole iron exporter in mammals, but its cell-specific function and regulation are still elusive. This study examined FPN1 expression in human macrophages, the cells that are primarily responsible on a daily basis for plasma iron turnover and are central in the pathogenesis of ferroportin disease (FD), the disease attributed to lack-of-function FPN1 mutations. We characterized FPN1 protein expression and traffic by confocal microscopy, western blotting, gel filtration, and immunoprecipitation studies in macrophages from control blood donors (donor) and patients with either FPN1 p.A77D, p.G80S, and p.Val162del lack-of-function or p.A69T gain-of-function mutations. We found that in normal macrophages, FPN1 cycles in the early endocytic compartment does not multimerize and is promptly degraded by hepcidin (Hepc), its physiological inhibitor, within 3-6 hours. In FD macrophages, endogenous FPN1 showed a similar localization, except for greater accumulation in lysosomes. However, in contrast with previous studies using overexpressed mutant protein in cell lines, FPN1 could still reach the cell surface and be normally internalized and degraded upon exposure to Hepc. However, when FD macrophages were exposed to large amounts of heme iron, in contrast to donor and p.A69T macrophages, FPN1 could no longer reach the cell surface, leading to intracellular iron retention.

CONCLUSION

FPN1 cycles as a monomer within the endocytic/plasma membrane compartment and responds to its physiological inhibitor, Hepc, in both control and FD cells. However, in FD, FPN1 fails to reach the cell surface when cells undergo high iron turnover. Our findings provide a basis for the FD characterized by a preserved iron transfer in the enterocytes (i.e., cells with low iron turnover) and iron retention in cells exposed to high iron flux, such as liver and spleen macrophages. (Hepatology 2017;65:1512-1525).

Studying disorders of vertebrate iron and heme metabolism using zebrafish

Iron is a crucial component of heme- and iron-sulfur clusters, involved in vital cellular functions such as oxygen transport, DNA synthesis, and respiration. Both excess and insufficient levels of iron and heme-precursors cause human disease, such as iron-deficiency anemia, hemochromatosis, and porphyrias. Hence, their levels must be tightly regulated, requiring a complex network of transporters and feedback mechanisms. The use of zebrafish to study these pathways and the underlying genetics offers many advantages, among others their optical transparency, ex-vivo development and high genetic and physiological conservations. This chapter first reviews well-established methods, such as large-scale mutagenesis screens that have led to the initial identification of a series of iron and heme transporters and the generation of a variety of mutant lines. Other widely used techniques are based on injection of RNA, including complementary morpholino knockdown and gene overexpression. In addition, we highlight several recently developed approaches, most notably endonuclease-based gene knockouts such as TALENs or the CRISPR/Cas9 system that have been used to study how loss of function can induce human disease phenocopies in zebrafish. Rescue by chemical complementation with iron-based compounds or small molecules can subsequently be used to confirm causality of the genetic defect for the observed phenotype. All together, zebrafish have proven to be - and will continue to serve as an ideal model to advance our understanding of the pathogenesis of human iron and heme-related diseases and to develop novel therapies to treat these conditions.

An update on iron homeostasis: make new friends, but keep the old

A classic Girl Scout song says, "Make new friends/but keep the old/One is silver/and the other gold." This review focuses on the past decade of discovery in the field of iron homeostasis, which has identified "new friends" or key modifiers of the critical systemic iron regulator, hepcidin antimicrobial peptide. The foundation for these discoveries has been the identification of mutated genes in well-characterized cohorts of patients with inherited hemochromatosis from across the globe. Transgenic mouse models of iron overload and iron-restricted anemia have also contributed to understanding molecular pathophysiology in ways that could never be accomplished in human subjects alone. The majority of these newly discovered molecules coordinate signaling through the bone morphogenetic protein pathway of ligands, receptors and coreceptors, intracellular signaling and transcription. The discovery of these proteins and their interactions with "old friends," such as the 1st known hereditary hemochromatosis gene product, HFE and transferrin receptor, has opened the field of iron homeostasis to include regulatory networks involving signal transduction pathways, in particular, the mitogen-activated protein kinase and Smad pathways. These newly discovered partnerships have also made way for opportunities to develop novel therapeutics for the treatment of iron regulatory disorders, including hemochromatosis.

An RNAi therapeutic targeting Tmprss6 decreases iron overload in Hfe(-/-) mice and ameliorates anemia and iron overload in murine β-thalassemia intermedia

Mutations in HFE lead to hereditary hemochromatosis (HH) because of inappropriately high iron uptake from the diet resulting from decreased hepatic expression of the iron-regulatory hormone hepcidin. -thalassemia is a congenital anemia caused by partial or complete loss of -globin synthesis causing ineffective erythropoiesis, anemia, decreased hepcidin production, and secondary iron overload. Tmprss6 is postulated to regulate hepcidin production by cleaving Hemojuvelin (Hjv), a key modulator of hepcidin expression, from the hepatocyte surface. On this basis, we hypothesized that treatment of mouse models of HH (Hfe(-/-)) and -thalassemia intermedia (Hbb(th3/+)) with Tmprss6 siRNA formulated in lipid nanoparticles (LNPs) that are preferentially taken up by the liver would increase hepcidin expression and lessen the iron loading in both models. In the present study, we demonstrate that LNP-Tmprss6 siRNA treatment of Hfe(-/-) and Hbb(th3/+) mice induces hepcidin and diminishes tissue and serum iron levels. Furthermore, LNP-Tmprss6 siRNA treatment of Hbb(th3/+) mice substantially improved the anemia by altering RBC survival and ineffective erythropoiesis. Our results indicate that pharmacologic manipulation of Tmprss6 with RNAi therapeutics isa practical approach to treating iron overload diseases associated with diminished hepcidin expression and may have efficacy in modifying disease-associated morbidities of -thalassemia intermedia.

Identification of mutations in SLC40A1 that affect ferroportin function and phenotype of human ferroportin iron overload

BACKGROUND & AIMS

Patients with ferroportin iron overload due to loss-of-function mutations in SLC40A1 have macrophage iron overload, hyperferritinemia, and normal transferrin saturation. In contrast, hepatocellular iron storage, hyperferritinemia, and increased saturation of transferrin are a distinct clinical presentation of ferroportin iron overload that results from SLC40A1 mutations that confer resistance of ferroportin to hepcidin-mediated inactivation.

METHODS

SLC40A1 was sequenced in patients from 2 independent pedigrees affected by hepatic iron overload unrelated to HFE. Functions of the ferroportin variants were tested in vitro.

RESULTS

A patient heterozygous for the variant p.W158C in SLC40A1 presented with macrophage iron overload, hyperferritinemia, and normal transferrin saturation. A patient with hepatocellular iron storage, hyperferritinemia, and increased transferrin saturation was heterozygous for p.H507R. Expression of the p.W158C form of ferroportin in 293T cells resulted in defective trafficking to the plasma membrane and reduced iron export activity; the iron export activity of cells that expressed the p.H507R form of ferroportin did not differ from cells that expressed ferroportin without this mutation. The p.H507R of ferroportin localizes normally to the plasma membrane but is resistant to hepcidin-mediated inactivation. Addition of a synthetic peptide derived from ferroportin without these mutations (amino acids 500-518) decreased the inhibitory activity of hepcidin in cells, whereas a peptide from the same region, with p.H507R, had no effect on hepcidin activity.

CONCLUSIONS

The variant p.W158C in SLC40A1 impairs intracellular trafficking of ferroportin, resulting in reduced iron export. The variant p.H507R does not bind hepcidin in vitro and results in apparent hepcidin resistance.

A novel monoclonal antibody immunoassay for the detection of human serum hepcidin

BACKGROUND

Hepcidin is a liver-derived peptide hormone regulating iron metabolism. Changes in the expression of hepcidin are known to be the key pathogenic factors in hereditary hemochromatosis and are associated with infection and inflammation. To better understand the hormone's function in human disease, we aimed to establish an immunoassay to determine hepcidin concentrations in serum.

METHODS

Monoclonal antibodies mHK(8) and mHK(9) were generated and characterized by dot blot, Western blot, and immunofluorescence. A competitive enzyme-linked immunosorbent assay (ELISA) was established with mHK(9).

RESULTS

Both antibodies recognized hepcidin, by dot blot and Western blot, respectively. In human liver, mHK(8)/(9) showed an immunofluorescence staining pattern in hepatocytes identical to that of established prohepcidin antibodies. The developed immunoassay with mHK(9), reliably detecting mature hepcidin in serum over a large concentration range (0.9-140 ng ml⁻¹), showed high sensitivity and precision (intra-/interassay coefficients of variation: 4-5 and 7-11%; mean linearity: 85-112%; mean recovery: 87-114%). To test the clinical functionality of the developed assay we measured hepcidin serum concentrations in healthy volunteers, hepatitis C virus (HCV) patients, and two groups of hemochromatotic patients undergoing phlebotomy. The assay distinguished low hepcidin level in HCV and homozygous hemochromatosis patients from normal-range controls and compound heterozygous hemochromatosis patients. In healthy subjects and HCV patients, hepcidin levels were correlated with iron and transferrin saturation; no correlation was observed in the hemochromatotic patients.

CONCLUSION

We developed a monoclonal antibody ELISA that quantifies serum hepcidin levels with high sensitivity, robustness, and reliability of detection. The hepcidin ELISA should help to enhance our understanding of hepcidin-related human 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.

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.

Measurement of serum hepcidin-25 levels as a potential test for diagnosing hemochromatosis and related disorders

BACKGROUND

Iron overload syndromes include a wide spectrum of genetic and acquired conditions. Recent studies suggest suppressed hepcidin synthesis in the liver to be the molecular basis of hemochromatosis. However, a liver with acquired iron overload synthesizes an adequate amount of hepcidin. Thus, hepcidin could function as a biochemical marker for differential diagnosis of iron overload syndromes.

METHODS

We measured serum iron parameters and hepcidin-25 levels followed by sequencing HFE, HJV, HAMP, TFR2, and SLC40A1 genes in 13 Japanese patients with iron overload syndromes. In addition, we performed direct measurement of serum hepcidin-25 levels using liquid chromatography-tandem mass spectrometry in 3 Japanese patients with aceruloplasminemia and 4 Italians with HFE hemochromatosis.

RESULTS

One patient with HJV hemochromatosis, 2 with TFR2 hemochromatosis, and 3 with ferroportin disease were found among the 13 Japanese patients. The remaining 7 Japanese patients showed no evidence for genetic basis of iron overload syndrome. As far as the serum hepcidin-25 was concerned, seven patients with hemochromatosis and 3 with aceruloplasminemia showed markedly decreased serum hepcidin-25 levels. In contrast, 3 patients with ferroportin disease and 7 with secondary iron overload syndromes showed serum hepcidin levels parallel to their hyperferritinemia. Patients with iron overload syndromes were divided into 2 phenotypes presenting as low and high hepcidinemia. These were then associated with their genotypes.

CONCLUSION

Determining serum hepcidin-25 levels may aid differential diagnosis of iron overload syndromes prior to genetic analysis.

Clinical presentation and molecular pathophysiology of autosomal dominant hemochromatosis caused by a novel ferroportin mutation

Mutations in the SLC40A1 gene, which encodes ferroportin, are associated with autosomal dominant hemochromatosis. Ferroportin is inhibited directly by hepcidin, a key iron-regulatory peptide, and functional consequences of SLC40A1 mutations account for observed phenotypic differences in patients with ferroportin disease. We describe a large pedigree with a novel SLC40A1 mutation and, through in vitro analysis, elucidate the associated molecular mechanism of iron overload. The entire coding sequence of the SLC40A1 gene was sequenced in a pedigree, presenting with autosomal dominant hyperferritinemia. The functional effects of a novel SLC40A1 mutation were studied by overexpression of wild-type and mutant ferroportin fusion proteins in human embryonic kidney cells. Iron export was studied in these cells using (59)Fe transport assays; subcellular localization of ferroportin was examined by way of confocal microscopy. A novel SLC40A1 mutation p.R489K segregated with iron overload in a family with clinical and histopathological signs of macrophage-type ferroportin disease. Human embryonic kidney cells overexpressing p.R489K ferroportin showed decreased iron export capacity when compared with wild-type ferroportin overexpressing cells. Subcellular localization studies demonstrated that p.R489K ferroportin was retained abnormally within an intracellular compartment.

CONCLUSION

We report a novel pathological SLC40A1 variant associated with abnormal cell surface expression of ferroportin due to intracellular retention of the mutant protein. These findings predict macrophage-type ferroportin disease, the phenotype observed in this kindred. Study of the molecular cell biology of ferroportin and its mutants is key to understanding the pathogenesis of this increasingly recognized form of hemochromatosis, which responds poorly to conventional therapy.

Functional analysis and theoretical modeling of ferroportin reveals clustering of mutations according to phenotype

Ferroportin disease is a heterogeneous iron release disorder resulting from mutations in the ferroportin gene. Ferroportin protein is a multitransmembrane domain iron transporter, responsible for iron export from cells, which, in turn, is regulated by the peptide hormone hepcidin. Mutations in the ferroportin gene may affect either regulation of the protein's transporter function or the ability of hepcidin to regulate iron efflux. We have used a combination of functional analysis of epitope-tagged ferroportin variants coupled with theoretical modeling to dissect the relationship between ferroportin mutations and their cognate phenotypes. Myc epitope-tagged human ferroportin expression constructs were transfected into Caco-2 intestinal cells and protein localization analyzed by immunofluorescence microscopy and colocalization with organelle markers. The effect of mutations on iron efflux was assessed by costaining with anti-ferritin antibodies and immunoblotting to quantitate cellular expression of ferritin and transferrin receptor 1. Wild-type ferroportin localized mainly to the cell surface and intracellular structures. All ferroportin disease-causing mutations studied had no effect on localization at the cell surface. N144H, N144T, and S338R mutant ferroportin retained the ability to transport iron. In contrast, A77D, V162Delta, and L170F mutants were iron transport defective. Surface staining experiments showed that both ends of the protein were located inside the cell. These data were used as the basis for theoretical modeling of the ferroportin molecule. The model predicted phenotypic clustering of mutations with gain-of-function variants associated with a hypothetical channel through the axis of ferroportin. Conversely, loss-of-function variants were located at the membrane/cytoplasm interface.

[Role of hepcidin in the pathogenesis of hemochromatosis]

In the last few years, biochemical and molecular study of the various types of hemochromatosis have established that the hepcidin peptide is the central regulator of iron absorption. This peptide, which is synthesized in the liver, acts through ferroportin degradation. Ferroportin is an iron exporter situated in the intestinal epithelium and in the macrophage membrane whose function is to transport iron from the intestinal cell to plasma and from the macrophage to the erythron. In hemochromatosis, there is a physical or functional hepcidin deficit that increases ferroportin, thus producing excessive iron absorption. The opposite occurs in situations of inflammation: hepcidin synthesis is stimulated while iron entry into the organism and hemoglobin synthesis are blocked.

Hepcidin modulation in human diseases: From research to clinic

By modulating hepcidin production, an organism controls intestinal iron absorption, iron uptake and mobilization from stores to meet body iron need. In recent years there has been important advancement in our knowledge of hepcidin regulation that also has implications for understanding the physiopathology of some human disorders. Since the discovery of hepcidin and the demonstration of its pivotal role in iron homeostasis, there has been a substantial interest in developing a reliable assay of the hormone in biological fluids. Measurement of hepcidin in biological fluids can improve our understanding of iron diseases and be a useful tool for diagnosis and clinical management of these disorders. We reviewed the literature and our own research on hepcidin to give an updated status of the situation in this rapidly evolving field.

Regulation of iron metabolism through GDF15 and hepcidin in pyruvate kinase deficiency

Iron absorption is inadequately increased in patients with chronic haemolytic anaemia, which is commonly complicated by iron overload. Growth differentiation factor 15 (GDF15) has been identified as a bone marrow-derived factor that abrogates hepcidin-mediated protection from iron overload under conditions of increased erythropoiesis. Increased concentrations of GDF15 have been reported in beta-thalassaemia patients and GDF15 has been found to suppress hepcidin expression in vitro. To further study the interdependencies of iron metabolism and erythropoiesis in vivo, the concentrations of hepcidin and GDF15 were determined in sera from 22 patients with pyruvate kinase deficiency (PKD) and 21 healthy control subjects. In PKD patients, serum hepcidin levels were 13-fold lower than in controls (2.0 ng/ml vs. 26.2 ng/ml) and GDF15 was significantly higher (859 pg/ml vs. 528 pg/ml). Serum hepcidin concentrations correlated positively with haemoglobin and negatively with serum GDF15. These results suggest that GDF15 contributes to low hepcidin expression and iron loading in PKD.

Iron homeostasis and erythropoiesis

Erythrocytes require iron to perform their duty as oxygen carriers. Mammals have evolved a mechanism to maintain systemic iron within an optimal range that fosters erythroid development and function while satisfying other body iron needs. This chapter reviews erythroid iron uptake and utilization as well as systemic factors that influence iron availability. One of these factors is hepcidin, a circulating peptide hormone that maintains iron homeostasis. Elevated levels of hepcidin in the bloodstream effectively shut off iron absorption by disabling the iron exporter ferroportin. Conversely, low levels of circulating hepcidin allow ferroportin to export iron into the bloodstream. Aberrations in hepcidin expression or responsiveness to hepcidin result in disorders of iron deficiency and iron overload. It is clear that erythroid precursors communicate their iron needs to the liver to influence the production of hepcidin and thus the amount of iron available for use. However, the mechanism by which erythroid cells accomplish this remains unclear and is an area of active investigation.

The Regulation of Hepcidin and Its Effects on Systemic and Cellular Iron Metabolism

Systemic iron homeostasis depends on the regulated expression of hepcidin, a peptide hormone that negatively regulates iron egress from intestinal cells and macrophages by altering the expression of the cellular iron exporter ferroportin. In doing so, hepcidin can control both the total body iron by modulating intestinal iron absorption as well as promote iron available for erythropoiesis by affecting the efficiency with which macrophages recycle iron from effete red blood cells. This review focuses on the systemic and cellular physiology of hepcidin regulation in relation to iron stores, erythropoiesis, inflammation, and hypoxia and how hepcidin regulation and dysregulation contributes to normal iron homeostasis and iron metabolism disorders.

Non-HFE haemochromatosis

Non-HFE hereditary haemochromatosis (HH) refers to a genetically heterogeneous group of iron overload disorders that are unlinked to mutations in the HFE gene. The four main types of non-HFE HH are caused by mutations in the hemojuvelin, hepcidin, transferrin receptor 2 and ferroportin genes. Juvenile haemochromatosis is an autosomal recessive disorder and can be caused by mutations in either hemojuvelin or hepcidin. An adult onset form of HH similar to HFE-HH is caused by homozygosity for mutations in transferrin receptor 2. The autosomal dominant iron overload disorder ferroportin disease is caused by mutations in the iron exporter ferroportin. The clinical characteristics and molecular basis of the various types of non-HFE haemochromatosis are reviewed. The study of these disorders and the molecules involved has been invaluable in improving our understanding of the mechanisms involved in the regulation of iron metabolism.

Review article: the genetic basis of haemochromatosis

BACKGROUND

Since the seminal discovery of the HFE gene a decade ago, considerable further progress in unravelling the genetic basis of haemochromatosis has been made. Novel genes and iron overload phenotypes have been described with potential insights into the molecular pathophysiology of human iron metabolism.

AIM

To review recent key advances in the field of inherited iron overload and assess their impact on clinical practice and on our understanding of iron regulation.

METHODS

A PubMed search was undertaken predominantly using 'haemochromatosis', 'HFE', 'hepcidin' and 'ferroportin'. Illustrative cases were sought.

RESULTS

The impact of HFE mutation analysis on the management of haemochromatosis is significant and allows early accurate diagnosis. HFE is also implicated in the siderosis associated with other liver pathologies. Non-HFE genes underpinning other forms of haemochromatosis are now recognized and genotype-phenotype interactions result in a spectrum of disease. These novel gene products interact with HFE in a common pathway for iron homeostasis.

CONCLUSIONS

Further identification of non-HFE genes associated with iron homeostasis will enhance our diagnostic certainty of primary haemochromatosis and may explain the variable expression seen in HFE-related disease. Improving our understanding of the mechanisms of iron regulation may lead to novel therapeutic strategies for the management of iron overload.

The flatiron mutation in mouse ferroportin acts as a dominant negative to cause ferroportin disease

Ferroportin disease is caused by mutation of one allele of the iron exporter ferroportin (Fpn/IREG1/Slc40a1/MTP1). All reported human mutations are missense mutations and heterozygous null mutations in mouse Fpn do not recapitulate the human disease. Here we describe the flatiron (ffe) mouse with a missense mutation (H32R) in Fpn that affects its localization and iron export activity. Similar to human patients with classic ferroportin disease, heterozygous ffe/+ mice present with iron loading of Kupffer cells, high serum ferritin, and low transferrin saturation. In macrophages isolated from ffe/+ heterozygous mice and through the use of Fpn plasmids with the ffe mutation, we show that Fpn(ffe) acts as a dominant negative, preventing wild-type Fpn from localizing on the cell surface and transporting iron. These results demonstrate that mutations in Fpn resulting in protein mislocalization act in a dominant-negative fashion to cause disease, and the Fpn(ffe) mouse represents the first mouse model of ferroportin disease.

A Toxicogenomic Approach Revealed Hepatic Gene Expression Changes Mechanistically Linked to Drug-Induced Hemolytic Anemia

A variety of pharmaceutical compounds causes hemolytic anemia as a significant adverse effect and this toxicity restricts the clinical utility of these drugs. In this study, we applied microarray technology to investigate hepatic gene expression changes associated with drug-induced hemolytic anemia and to identify potential biomarker genes for this hematotoxicity. We treated female Sprague-Dawley rats with two hemolytic anemia-inducing compounds: phenylhydrazine and phenacetin. Hepatic gene expression profiles were obtained using a whole-genome oligonucleotide microarray with pooled RNA samples from individual rats within each dose group and analyzed in comparison with hepatic histopathology, hematology, and blood chemistry data. We identified a small subset of genes that were commonly deregulated in all the severe hemolytic conditions, some of which were considered to be involved in hepatic events characteristic of hemolytic anemia, such as hemoglobin biosynthesis, heme metabolism, and phagocytosis. Among them, we selected six upregulated genes as putative biomarkers, and their expression changes from microarray measurements were confirmed by quantitative real-time PCR using RNAs from individual animals. They were Alas2, beta-glo, Eraf, Hmox1, Lgals3, and Rhced. Expression patterns of all these genes showed high negative and positive correlation against erythrocyte counts and total bilirubin levels in circulation, respectively, suggesting that these genes may be the potential biomarkers for hemolytic anemia. These findings indicate that drug-induced hemolytic anemia may be detected based on hepatic changes in the expression of a subset of genes that are mechanistically linked to the hematotoxicity.

Wild-type and mutant ferroportins do not form oligomers in transfected cells

Ferroportin [FPN; Slc40a1 (solute carrier family 40, member 1)] is a transmembrane iron export protein expressed in macrophages and duodenal enterocytes. Heterozygous mutations in the FPN gene result in an autosomal dominant form of iron overload disorder, type-4 haemochromatosis. FPN mutants either have a normal iron export activity but have lost their ability to bind hepcidin, or are defective in their iron export function. The mutant protein has been suggested to act as a dominant negative over the wt (wild-type) protein by multimer formation. Using transiently transfected human epithelial cell lines expressing mouse FPN modified by the addition of a haemagglutinin or c-Myc epitope at the C-terminus, we show that the wtFPN is found at the plasma membrane and in Rab5-containing endosomes, as are the D157G and Q182H mutants. However, the delV162 mutant is mostly intracellular in HK2 cells (human kidney-2 cells) and partially addressed at the cell surface in HEK-293 cells (human embryonic kidney 293 cells). In both cell types, it is partially associated with the endoplasmic reticulum and with Rab5-positive vesicles. However, this mutant is complex-glycosylated like the wt protein. D157G and G323V mutants have a defective iron export capacity as judged by their inability to deplete the intracellular ferritin content, whereas Q182H and delV162 have normal iron export function and probably have lost their capacity to bind hepcidin. In co-transfection experiments, the delV162 mutant does not co-localize with the wtFPN, does not prevent its normal targeting to the plasma membrane and cannot be immunoprecipitated in the same complex, arguing against the formation of FPN hetero-oligomers.

Hepatobiliary pathology

PURPOSE OF REVIEW

Publications concerning liver histopathology in fatty liver disease and chronic hepatitis C, iron and copper overload, and liver transplantation from the past year have been surveyed to highlight useful concepts and diagnostic information.

RECENT FINDINGS

Two microscopic forms of pediatric nonalcoholic steatohepatitis have been described: type 1 in which hepatocyte ballooning and/or pericellular fibrosis accompany the steatosis; and type 2 which has portal tract inflammation and/or fibrosis as the salient accompanying feature. In chronic hepatitis C, the ductular reaction appears to be a major factor associated with fibrosis. In patients transplanted for hepatitis C virus-related cirrhosis, immunostaining of post-transplant liver biopsies for alpha-smooth muscle actin (i.e. in activated hepatic stellate cells) may identify those individuals at risk for severe recurrence. Clinicopathological papers on several forms of non-HFE hemochromatosis were published and Wilson's disease was described in individuals of 60 years or more in age. Cholestasis in childhood was expertly reviewed and histopathologic precursor lesions of hepatocellular carcinoma were also examined in a comprehensive article.

SUMMARY

Recent publications with impact on liver biopsy interpretation include a morphologic classification of nonalcoholic steatohepatitis in childhood, the differential diagnosis of childhood cholestasis and pathogenetic factors involved in fibrogenesis in chronic hepatitis C.

Hereditary hemochromatosis: genetic complexity and new diagnostic approaches

Since the discovery of the hemochromatosis gene (HFE) in 1996, several novel gene defects have been detected, explaining the mechanism and diversity of iron-overload diseases. At least 4 main types of hereditary hemochromatosis (HH) have been identified. Surprisingly, genes involved in HH encode for proteins that all affect pathways centered around liver hepcidin synthesis and its interaction with ferroportin, an iron exporter in enterocytes and macrophages. Hepcidin concentrations in urine negatively correlate with the severity of HH. Cytokine-mediated increases in hepcidin appear to be an important causative factor in anemia of inflammation, which is characterized by sequestration of iron in the macrophage system. For clinicians, the challenge is now to diagnose HH before irreversible damage develops and, at the same time, to distinguish progressive iron overload from increasingly common diseases with only moderately increased body iron stores, such as the metabolic syndrome. Understanding the molecular regulation of iron homeostasis may be helpful in designing innovative and reliable DNA and protein tests for diagnosis. Subsequently, evidence-based diagnostic strategies must be developed, using both conventional and innovative laboratory tests, to differentiate between the various causes of distortions of iron metabolism. This review describes new insights in mechanisms of iron overload, which are needed to understand new developments in diagnostic medicine.

Hepcidin and Its Role in Regulating Systemic Iron Metabolism

Maintenance of stable extracellular iron concentrations requires the coordinate regulation of iron transport into plasma from dietary sources in the duodenum, from recycled senescent red cells in macrophages and from storage in hepatocytes. Moreover, during fetal development, the iron requirements of the fetus must be matched by the transport of maternal iron across the placenta. Hepcidin is a 25–amino acid disulfide-rich peptide synthesized in the liver that acts as a systemic iron-regulatory hormone by regulating iron transport from iron-exporting tissues into plasma. Hepcidin inhibits the cellular efflux of iron by binding to, and inducing the degradation of, ferroportin, the sole iron exporter in iron-transporting cells. In turn, hepcidin synthesis is increased by iron loading and decreased by anemia and hypoxia. Additionally, hepcidin synthesis is greatly increased during inflammation, trapping iron in macrophages, decreasing plasma iron concentrations and causing iron-restricted erythropoiesis characteristic of anemia of inflammation (anemia of chronic disease). Recent studies indicate that hepcidin deficiency underlies most known forms of hereditary hemochromatosis. This implies that, with the exception of very rare mutations that affect the hepcidin gene itself or modify ferroportin to make it less responsive to hepcidin, hemochromatosis genes encode molecules that regulate hepcidin synthesis. The central involvement of hepcidin in iron regulation and its pathologies should make the eventual hepcidin assay useful for the diagnosis of iron disorders and the monitoring of their treatments. The development of hepcidin agonists and antagonists may provide useful therapeutics for the treatment of iron disorders.