Juvenile hemochromatosis associated with pathogenic mutations of adult hemochromatosis genes

Role of Iron in Aging Related Diseases

Iron progressively accumulates with age and can be further exacerbated by dietary iron intake, genetic factors, and repeated blood transfusions. While iron plays a vital role in various physiological processes within the human body, its accumulation contributes to cellular aging in several species. In its free form, iron can initiate the formation of free radicals at a cellular level and contribute to systemic disorders. This is most evident in high iron conditions such as hereditary hemochromatosis, when accumulation of iron contributes to the development of arthritis, cirrhosis, or cardiomyopathy. A growing body of research has further identified iron’s contributory effects in neurodegenerative diseases, ocular disorders, cancer, diabetes, endocrine dysfunction, and cardiovascular diseases. Reducing iron levels by repeated phlebotomy, iron chelation, and dietary restriction are the common therapeutic considerations to prevent iron toxicity. Chelators such as deferoxamine, deferiprone, and deferasirox have become the standard of care in managing iron overload conditions with other potential applications in cancer and cardiotoxicity. In certain animal models, drugs with iron chelating ability have been found to promote health and even extend lifespan. As we further explore the role of iron in the aging process, iron chelators will likely play an increasingly important role in our health.

Effects of Excess Iron on the Retina: Insights From Clinical Cases and Animal Models of Iron Disorders

Iron plays an important role in a wide range of metabolic pathways that are important for neuronal health. Excessive levels of iron, however, can promote toxicity and cell death. An example of an iron overload disorder is hemochromatosis (HH) which is a genetic disorder of iron metabolism in which the body’s ability to regulate iron absorption is altered, resulting in iron build-up and injury in several organs. The retina was traditionally assumed to be protected from high levels of systemic iron overload by the blood-retina barrier. However, recent data shows that expression of genes that are associated with HH can disrupt retinal iron metabolism. Thus, the effects of iron overload on the retina have become an area of research interest, as excessively high levels of iron are implicated in several retinal disorders, most notably age–related macular degeneration. This review is an effort to highlight risk factors for excessive levels of systemic iron build-up in the retina and its potential impact on the eye health. Information is integrated across clinical and preclinical animal studies to provide insights into the effects of systemic iron loading on the retina.

New Mutations in HFE2 and TFR2 Genes Causing Non HFE-Related Hereditary Hemochromatosis

Hereditary hemochromatosis (HH) is an iron metabolism disease clinically characterized by excessive iron deposition in parenchymal organs such as liver, heart, pancreas, and joints. It is caused by mutations in at least five different genes. HFE hemochromatosis is the most common type of hemochromatosis, while non-HFE related hemochromatosis are rare cases. Here, we describe six new patients of non-HFE related HH from five different families. Two families (Family 1 and 2) have novel nonsense mutations in the HFE2 gene have novel nonsense mutations (p.Arg63Ter and Asp36ThrfsTer96). Three families have mutations in the TFR2 gene, one case has one previously unreported mutation (Family A-p.Asp680Tyr) and two cases have known pathogenic mutations (Family B and D-p.Trp781Ter and p.Gln672Ter respectively). Clinical, biochemical, and genetic data are discussed in all these cases. These rare cases of non-HFE related hereditary hemochromatosis highlight the importance of an earlier molecular diagnosis in a specialized center to prevent serious clinical complications.

Genetic Diagnosis in Hereditary Hemochromatosis: Discovering and Understanding the Biological Relevance of Variants

BACKGROUND

Hereditary hemochromatosis (HH) is a genetic disease, leading to iron accumulation and possible organ damage. Patients are usually homozygous for p. Cys282Tyr in the homeostatic iron regulator gene but may have mutations in other genes involved in the regulation of iron. Next-generation sequencing is increasingly being utilized for the diagnosis of patients, leading to the discovery of novel genetic variants. The clinical significance of these variants is often unknown.

CONTENT

Determining the pathogenicity of such variants of unknown significance is important for diagnostics and genetic counseling. Predictions can be made using in silico computational tools and population data, but additional evidence is required for a conclusive pathogenicity classification. Genetic disease models, such as in vitro models using cellular overexpression, induced pluripotent stem cells or organoids, and in vivo models using mice or zebrafish all have their own challenges and opportunities when used to model HH and other iron disorders. Recent developments in gene-editing technologies are transforming the field of genetic disease modeling.

SUMMARY

In summary, this review addresses methods and developments regarding the discovery and classification of genetic variants, from in silico tools to in vitro and in vivo models, and presents them in the context of HH. It also explores recent gene-editing developments and how they can be applied to the discussed models of genetic disease.

Appropriate Clinical Genetic Testing of Hemochromatosis Type 2-4, Including Ferroportin Disease

Hereditary hemochromatosis (HH) is an inherited iron overload disorder due to a deficiency of hepcidin, or a failure of hepcidin to degrade ferroportin. The most common form of HH, Type 1 HH, is most commonly due to a homozygous C282Y mutation in HFE and is relatively well understood in significance and action; however, other rare forms of HH (Types 2–4) exist and are more difficult to identify and diagnose in clinical practice. In this review, we describe the clinical characteristics of HH Type 2–4 and the mutation patterns that have been described in these conditions. We also review the different methods for genetic testing available in clinical practice and a pragmatic approach to the patient with suspected non-HFE HH.

Hepcidin Signaling in Health and Disease: Ironing Out the Details

Hepcidin, a peptide hormone produced by hepatocytes, is the central regulator of systemic iron homeostasis through its interaction with ferroportin, the major cellular iron export protein. Hepcidin binding to ferroportin results in reduced iron export from macrophages and intestinal absorptive cells, leading to decreased serum iron levels. Hepcidin expression is influenced by several factors that include serum and liver iron stores, erythropoiesis, hypoxia, inflammation, and infection. Erythropoietic drive and hypoxia suppress hepcidin expression and promote red cell production. In contrast, inflammation and infection are associated with increased hepcidin production to sequester iron intracellularly as a means of depriving microorganisms of iron. Chronic inflammation may up-regulate hepcidin expression through the interleukin-6 (IL-6)-Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) pathway. The bone morphogenetic protein (BMP)-mothers against decapentaplegic homolog (SMAD) pathway is a major positive driver of hepcidin expression in response to either increased circulating iron in the form of transferrin or iron loading in organs. Hereditary hemochromatosis (HH) consists of several inherited disorders that cause inappropriately reduced hepcidin expression in response to body iron stores, leading to increased iron absorption from a normal diet. The most common form of HH is due to a mutation in the HFE gene, which causes a failure in the hepatocyte iron-sensing mechanism, leading to reduced hepcidin expression; the clinical manifestations of HFE-HH include increased serum transferrin-iron saturation and progressive iron loading in the liver and other tissues over time among patients who express the disease phenotype. In this article, we review the physiologic mechanisms and cellular pathways by which hepcidin expression is regulated, and the different forms of HH resulting from various mutations that cause hepcidin deficiency. We also review other drivers of hepcidin expression and the associated pathophysiologic consequences.

Revisiting hemochromatosis: genetic vs. phenotypic manifestations

Iron overload disorders represent an important class of human diseases. Of the primary iron overload conditions, by far the most common and best studied is HFE-related hemochromatosis, which results from homozygosity for a mutation leading to the C282Y substitution in the HFE protein. This disease is characterized by reduced expression of the iron-regulatory hormone hepcidin, leading to increased dietary iron absorption and iron deposition in multiple tissues including the liver, pancreas, joints, heart and pituitary. The phenotype of HFE-related hemochromatosis is quite variable, with some individuals showing little or no evidence of increased body iron, yet others showing severe iron loading, tissue damage and clinical sequelae. The majority of genetically predisposed individuals show at least some evidence of iron loading (increased transferrin saturation and serum ferritin), but a minority show clinical symptoms and severe consequences are rare. Thus, the disorder has a high biochemical penetrance, but a low clinical prevalence. Nevertheless, it is such a common condition in Caucasian populations (1:100–200) that it remains an important clinical entity. The phenotypic variability can largely be explained by a range of environmental, genetic and physiological factors. Men are far more likely to manifest significant disease than women, with the latter losing iron through menstrual blood loss and childbirth. Other forms of blood loss, immune system influences, the amount of bioavailable iron in the diet and lifestyle factors such as high alcohol intake can also contribute to iron loading and disease expression. Polymorphisms in a range of genes have been linked to variations in body iron levels, both in the general population and in hemochromatosis. Some of the genes identified play well known roles in iron homeostasis, yet others are novel. Other factors, including both co-morbidities and genetic polymorphisms, do not affect iron levels per se, but determine the propensity for tissue pathology.

Genetic iron overload disorders

Due to its pivotal role in orchestrating vital cellular functions and metabolic processes, iron is an essential component of the human body and a main micronutrient in the human diet. However, excess iron causes an increased production of reactive oxygen species leading to cell dysfunction or death, tissue damage and organ disease. Iron overload disorders encompass a wide spectrum of pathological conditions of hereditary or acquired origin. A number of 'iron genes' have been identified as being associated with hereditary iron overload syndromes, the most common of which is hemochromatosis. Although linked to at least five different genes, hemochromatosis is recognized as a unique syndromic entity based on a common pathogenetic mechanism leading to excessive entry of unneeded iron into the bloodstream. In this review, we focus on the pathophysiologic basis and clinical aspects of the most common genetic iron overload syndromes in humans.

Bone morphogenic proteins in iron homeostasis

The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.

Regulation of Iron Homeostasis and Related Diseases

The liver is the organ for iron storage and regulation; it senses circulating iron concentrations in the body through the BMP-SMAD pathway and regulates the iron intake from food and erythrocyte recovery into the bloodstream by secreting hepcidin. Under iron deficiency, hypoxia, and hemorrhage, the liver reduces the expression of hepcidin to ensure the erythropoiesis but increases the excretion of hepcidin during infection and inflammation to reduce the usage of iron by pathogens. Excessive iron causes system iron overload; it accumulates in never system and damages neurocyte leading to neurodegenerative diseases such as Parkinson's syndrome. When some gene mutations affect the perception of iron and iron regulation ability in the liver, then they decrease the expression of hepcidin, causing hereditary diseases such as hereditary hemochromatosis. This review summarizes the source and utilization of iron in the body, the liver regulates systemic iron homeostasis by sensing the circulating iron concentration, and the expression of hepcidin regulated by various signaling pathways, thereby understanding the pathogenesis of iron-related diseases.

Liver iron sensing and body iron homeostasis

The liver orchestrates systemic iron balance by producing and secreting hepcidin. Known as the iron hormone, hepcidin induces degradation of the iron exporter ferroportin to control iron entry into the bloodstream from dietary sources, iron recycling macrophages, and body stores. Under physiologic conditions, hepcidin production is reduced by iron deficiency and erythropoietic drive to increase the iron supply when needed to support red blood cell production and other essential functions. Conversely, hepcidin production is induced by iron loading and inflammation to prevent the toxicity of iron excess and limit its availability to pathogens. The inability to appropriately regulate hepcidin production in response to these physiologic cues underlies genetic disorders of iron overload and deficiency, including hereditary hemochromatosis and iron-refractory iron deficiency anemia. Moreover, excess hepcidin suppression in the setting of ineffective erythropoiesis contributes to iron-loading anemias such as β-thalassemia, whereas excess hepcidin induction contributes to iron-restricted erythropoiesis and anemia in chronic inflammatory diseases. These diseases have provided key insights into understanding the mechanisms by which the liver senses plasma and tissue iron levels, the iron demand of erythrocyte precursors, and the presence of potential pathogens and, importantly, how these various signals are integrated to appropriately regulate hepcidin production. This review will focus on recent insights into how the liver senses body iron levels and coordinates this with other signals to regulate hepcidin production and systemic iron homeostasis.

Inherited Disorders of Iron Overload

Dietary iron absorption and systemic iron traffic are tightly controlled by hepcidin, a liver-derived peptide hormone. Hepcidin inhibits iron entry into plasma by binding to and inactivating the iron exporter ferroportin in target cells, such as duodenal enterocytes and tissue macrophages. Hepcidin is induced in response to increased body iron stores to inhibit further iron absorption and prevent iron overload. The mechanism involves the BMP/SMAD signaling pathway, which triggers transcriptional hepcidin induction. Inactivating mutations in components of this pathway cause hepcidin deficiency, which allows inappropriately increased iron absorption and efflux into the bloodstream. This leads to hereditary hemochromatosis (HH), a genetically heterogenous autosomal recessive disorder of iron metabolism characterized by gradual buildup of unshielded non-transferrin bound iron (NTBI) in plasma and excessive iron deposition in tissue parenchymal cells. The predominant HH form is linked to mutations in the HFE gene and constitutes the most frequent genetic disorder in Caucasians. Other, more severe and rare variants are caused by inactivating mutations in HJV (hemojuvelin), HAMP (hepcidin) or TFR2 (transferrin receptor 2). Mutations in SLC40A1 (ferroportin) that cause hepcidin resistance recapitulate the biochemical phenotype of HH. However, ferroportin-related hemochromatosis is transmitted in an autosomal dominant manner. Loss-of-function ferroportin mutations lead to ferroportin disease, characterized by iron overload in macrophages and low transferrin saturation. Aceruloplasminemia and atransferrinemia are further inherited disorders of iron overload caused by deficiency in ceruloplasmin or transferrin, the plasma ferroxidase and iron carrier, respectively.

Small Molecule Inhibitors of NFkB Reverse Iron Overload and Hepcidin Deregulation in a Zebrafish Model for Hereditary Hemochromatosis Type 3

Hereditary hemochromatosis (HH) is one of the most common genetic disorders in Caucasian populations, with no viable therapeutic options except phlebotomy. We describe a zebrafish model of human HH (HH) created by targeted mutagenesis of the gene encoding transferrin receptor 2 ( tfr2). TFR2 mutations in humans lead to HH Type 3, a rare but severe form of the disease. The tfr2 mutant model in zebrafish recapitulates the defining features of HH3: iron overload and suppression of hepcidin, the iron regulatory hormone. Using in vivo chemical screens in zebrafish embryos, we identify a new small molecule inducer of hepcidin: SC-514, a specific chemical inhibitor of NFkB signaling. Using independent small molecule inhibitors of the NFkB pathway, we demonstrate that inhibition of NFkB signaling causes induction of hepcidin transcription and reduction of iron overload in the HH3 model. This first successful chemical intervention for hereditary hemochromatosis may also have relevance in treatment of other very prevalent iron regulatory iron overload disorders such as thalassemia.

Phenotypic analysis of hemochromatosis subtypes reveals variations in severity of iron overload and clinical disease

The clinical progression of HFE-related hereditary hemochromatosis (HH) and its phenotypic variability has been well studied. Less is known about the natural history of non-HFE HH caused by mutations in the HJV, HAMP, or TFR2 genes. The purpose of this study was to compare the phenotypic and clinical presentations of hepcidin-deficient forms of HH. A literature review of all published cases of genetically confirmed HJV, HAMP, and TFR2 HH was performed. Phenotypic and clinical data from a total of 156 patients with non-HFE HH was extracted from 53 publications and compared with data from 984 patients with HFE-p.C282Y homozygous HH from the QIMR Berghofer Hemochromatosis Database. Analyses confirmed that non-HFE forms of HH have an earlier age of onset and a more severe clinical course than HFE HH. HJV and HAMP HH are phenotypically and clinically very similar and have the most severe presentation, with cardiomyopathy and hypogonadism being particularly prevalent findings. TFR2 HH is more intermediate in its age of onset and severity. All clinical outcomes analyzed were more prevalent in the juvenile forms of HH, with the exception of arthritis and arthropathy, which were more commonly seen in HFE HH. This is the first comprehensive analysis comparing the different phenotypic and clinical aspects of the genetic forms of HH, and the results will be valuable for the differential diagnosis and management of these conditions. Importantly, our analyses indicate that factors other than iron overload may be contributing to joint pathology in patients with HFE HH.

Hepcidin: Homeostasis and Diseases Related to Iron Metabolism

Iron is an essential metal for cell survival that is regulated by the peptide hormone hepcidin. However, its influence on certain diseases is directly related to iron metabolism or secondary to underlying diseases. Genetic alterations influence the serum hepcidin concentration, which can lead to an iron overload in tissues, as observed in haemochromatosis, in which serum hepcidin or defective hepcidin synthesis is observed. Another genetic imbalance of iron is iron-refractory anaemia, in which serum concentrations of hepcidin are increased, precluding the flow and efflux of extra- and intracellular iron. During the pathogenesis of certain diseases, the resulting oxidative stress, as well as the increase in inflammatory cytokines, influences the transcription of the HAMP gene to generate a secondary anaemia due to the increase in the serum concentration of hepcidin. To date, there is no available drug to inhibit or enhance hepcidin transcription, mostly due to the cytotoxicity described in the in vitro models. The proposed therapeutic targets are still in the early stages of clinical trials. Some candidates are promising, such as heparin derivatives and minihepcidins. This review describes the main pathways of systemic and genetic regulation of hepcidin, as well as its influence on the disorders related to iron metabolism.

Pathophysiological consequences and benefits of HFE mutations: 20 years of research

Mutations in the HFE (hemochromatosis) gene cause hereditary hemochromatosis, an iron overload disorder that is hallmarked by excessive accumulation of iron in parenchymal organs. The HFE mutation p.Cys282Tyr is pathologically most relevant and occurs in the Caucasian population with a carrier frequency of up to 1 in 8 in specific European regions. Despite this high prevalence, the mutation causes a clinically relevant phenotype only in a minority of cases. In this review, we summarize historical facts and recent research findings about hereditary hemochromatosis, and outline the pathological consequences of the associated gene defects. In addition, we discuss potential advantages of HFE mutations in asymptomatic carriers.

Brain iron accumulation affects myelin-related molecular systems implicated in a rare neurogenetic disease family with neuropsychiatric features

The 'neurodegeneration with brain iron accumulation' (NBIA) disease family entails movement or cognitive impairment, often with psychiatric features. To understand how iron loading affects the brain, we studied mice with disruption of two iron regulatory genes, hemochromatosis (Hfe) and transferrin receptor 2 (Tfr2). Inductively coupled plasma atomic emission spectroscopy demonstrated increased iron in the Hfe^-/- × Tfr2^mut brain (P=0.002, n ≥5/group), primarily localized by Perls' staining to myelinated structures. Western immunoblotting showed increases of the iron storage protein ferritin light polypeptide and microarray and real-time reverse transcription-PCR revealed decreased transcript levels (P<0.04, n ≥5/group) for five other NBIA genes, phospholipase A2 group VI, fatty acid 2-hydroxylase, ceruloplasmin, chromosome 19 open reading frame 12 and ATPase type 13A2. Apart from the ferroxidase ceruloplasmin, all are involved in myelin homeostasis; 16 other myelin-related genes also showed reduced expression (P<0.05), although gross myelin structure and integrity appear unaffected (P>0.05). Overlap (P<0.0001) of differentially expressed genes in Hfe^-/- × Tfr2^mut brain with human gene co-expression networks suggests iron loading influences expression of NBIA-related and myelin-related genes co-expressed in normal human basal ganglia. There was overlap (P<0.0001) of genes differentially expressed in Hfe^-/- × Tfr2^mut brain and post-mortem NBIA basal ganglia. Hfe^-/- × Tfr2^mut mice were hyperactive (P<0.0112) without apparent cognitive impairment by IntelliCage testing (P>0.05). These results implicate myelin-related systems involved in NBIA neuropathogenesis in early responses to iron loading. This may contribute to behavioral symptoms in NBIA and hemochromatosis and is relevant to patients with abnormal iron status and psychiatric disorders involving myelin abnormalities or resistant to conventional treatments.

Identification of novel mutations in hemochromatosis genes by targeted next generation sequencing in Italian patients with unexplained iron overload

Hereditary hemochromatosis, one of the commonest genetic disorder in Caucasians, is mainly associated to homozygosity for the C282Y mutation in the HFE gene, which is highly prevalent (allele frequency up to near 10% in Northern Europe) and easily detectable through a widely available "first level" molecular test. However, in certain geographical regions like the Mediterranean area, up to 30% of patients with a HH phenotype has a negative or non-diagnostic (i.e. simple heterozygosity) test, because of a known heterogeneity involving at least four other genes (HAMP, HJV, TFR2, and SLC40A1). Mutations in such genes are generally rare/private, making the diagnosis of atypical HH essentially a matter of exclusion in clinical practice (from here the term of "non-HFE" HH), unless cumbersome traditional sequencing is applied. We developed a Next Generation Sequencing (NGS)-based test targeting the five HH genes, and applied it to patients with clinically relevant iron overload (IO) and a non-diagnostic first level genetic test. We identified several mutations, some of which were novel (i.e. HFE W163X, HAMP R59X, and TFR2 D555N) and allowed molecular reclassification of "non-HFE" HH clinical diagnosis, particularly in some highly selected IO patients without concurring acquired risk factors. This NGS-based "second level" genetic test may represent a useful tool for molecular diagnosis of HH in patients in whom HH phenotype remains unexplained after the search of common HFE mutations.

HFE gene: Structure, function, mutations, and associated iron abnormalities

The hemochromatosis gene HFE was discovered in 1996, more than a century after clinical and pathologic manifestations of hemochromatosis were reported. Linked to the major histocompatibility complex (MHC) on chromosome 6p, HFE encodes the MHC class I-like protein HFE that binds beta-2 microglobulin. HFE influences iron absorption by modulating the expression of hepcidin, the main controller of iron metabolism. Common HFE mutations account for ~90% of hemochromatosis phenotypes in whites of western European descent. We review HFE mapping and cloning, structure, promoters and controllers, and coding region mutations, HFE protein structure, cell and tissue expression and function, mouse Hfe knockouts and knockins, and HFE mutations in other mammals with iron overload. We describe the pertinence of HFE and HFE to mechanisms of iron homeostasis, the origin and fixation of HFE polymorphisms in European and other populations, and the genetic and biochemical basis of HFE hemochromatosis and iron overload.

Genetics, Genetic Testing, and Management of Hemochromatosis: 15 Years Since Hepcidin

The discovery of hepcidin in 2000 and the subsequent unprecedented explosion of research and discoveries in the iron field have dramatically changed our understanding of human disorders of iron metabolism. Today, hereditary hemochromatosis, the paradigmatic iron-loading disorder, is recognized as an endocrine disease due to the genetic loss of hepcidin, the iron hormone produced by the liver. This syndrome is due to unchecked transfer of iron into the bloodstream in the absence of increased erythropoietic needs and its toxic effects in parenchymatous organs. It is caused by mutations that affect any of the proteins that help hepcidin to monitor serum iron, including HFE and, in rarer instances, transferrin-receptor 2 and hemojuvelin, or make its receptor ferroportin, resistant to the hormone. In Caucasians, C282Y HFE homozygotes are numerous, but they are only predisposed to hemochromatosis; complete organ disease develops in a minority, due to alcohol abuse or concurrent genetic modifiers that are now being identified. HFE gene testing can be used to diagnose hemochromatosis in symptomatic patients, but analyses of liver histology and full gene sequencing are required to identify patients with rare, non-HFE forms of the disease. Due to the central pathogenic role of hepcidin, it is anticipated that nongenetic causes of hepcidin loss (eg, end-stage liver disease) can cause acquired forms of hemochromatosis. The mainstay of hemochromatosis management is still removal of iron by phlebotomy, first introduced in 1950s, but identification of hepcidin has not only shed new light on the pathogenesis of the disease and the approach to diagnosis, but etiologic therapeutic applications from these advances are now foreseen.

Impaired hepcidin expression in alpha-1-antitrypsin deficiency associated with iron overload and progressive liver disease

Liver disease due to alpha-1-antitrypsin deficiency (A1ATD) is associated with hepatic iron overload in a subgroup of patients. The underlying cause for this association is unknown. The aim of the present study was to define the genetics of this correlation and the effect of alpha-1-antitrypsin (A1AT) on the expression of the iron hormone hepcidin. Full exome and candidate gene sequencing were carried out in a family with A1ATD and hepatic iron overload. Regulation of hepcidin expression by A1AT was studied in primary murine hepatocytes. Cells co-transfected with hemojuvelin (HJV) and matriptase-2 (MT-2) were used as a model to investigate the molecular mechanism of this regulation. Observed familial clustering of hepatic iron overload with A1ATD suggests a genetic cause, but genotypes known to be associated with hemochromatosis were absent. Individuals homozygous for the A1AT Z-allele with environmental or genetic risk factors such as steatosis or heterozygosity for the HAMP non-sense mutation p.Arg59* presented with severe hepatic siderosis. In hepatocytes, A1AT induced hepcidin mRNA expression in a dose-dependent manner. Experiments in overexpressing cells show that A1AT reduces cleavage of the hepcidin inducing bone morphogenetic protein co-receptor HJV via inhibition of the membrane-bound serine protease MT-2. The acute-phase protein A1AT is an inducer of hepcidin expression. Through this mechanism, A1ATD could be a trigger of hepatic iron overload in genetically predisposed individuals or patients with environmental risk factors for hepatic siderosis.

Exome sequencing in HFE C282Y homozygous men with extreme phenotypes identifies a GNPAT variant associated with severe iron overload

To identify polymorphisms associated with variability of iron overload severity in HFE-associated hemochromatosis, we performed exome sequencing of DNA from 35 male HFE C282Y homozygotes with either markedly increased iron stores (n = 22; cases) or with normal or mildly increased iron stores (n = 13; controls). The 35 participants, residents of the United States, Canada, and Australia, reported no or light alcohol consumption. Sequencing data included 82,068 single-nucleotide variants, and 10,337 genes were tested for a difference between cases and controls. A variant in the GNPAT gene showed the most significant association with severe iron overload (P = 3 × 10(-6) ; P = 0.033 by the likelihood ratio test after correction for multiple comparisons). Sixteen of twenty-two participants with severe iron overload had glyceronephosphate O-acyltransferase (GNPAT) polymorphism p.D519G (rs11558492; 15 heterozygotes, one homozygote). No control participant had this polymorphism. To examine functional consequences of GNPAT deficiency, we performed small interfering RNA-based knockdown of GNPAT in the human liver-derived cell line, HepG2/C3A. This knockdown resulted in a >17-fold decrease in expression of the messenger RNA encoding the iron-regulatory hormone, hepcidin.

CONCLUSION

GNPAT p.D519G is associated with a high-iron phenotype in HFE C282Y homozygotes and may participate in hepcidin regulation.

Functional consequences of transferrin receptor-2 mutations causing hereditary hemochromatosis type 3

Hereditary hemochromatosis (HH) type 3 is an autosomal recessive disorder of iron metabolism characterized by excessive iron deposition in the liver and caused by mutations in the transferrin receptor 2 (TFR2) gene. Here, we describe three new HH type 3 Spanish families with four TFR2 mutations (p.Gly792Arg, c.1606-8A>G, Gln306*, and Gln672*). The missense variation p.Gly792Arg was found in homozygosity in two adult patients of the same family, and in compound heterozygosity in an adult proband that also carries a novel intronic change (c.1606-8A>G). Two new nonsense TFR2 mutations (Gln306* and Gln672*) were detected in a pediatric case. We examine the functional consequences of two TFR2 variants (p.Gly792Arg and c.1606-8A>G) using molecular and computational methods. Cellular protein localization studies using immunofluorescence demonstrated that the plasma membrane localization of p.Gly792Arg TFR2 is impaired. Splicing studies in vitro and in vivo reveal that the c.1606-8A>G mutation leads to the creation of a new acceptor splice site and an aberrant TFR2 mRNA. The reported mutations caused HH type 3 by protein truncation, altering TFR2 membrane localization or by mRNA splicing defect, producing a nonfunctional TFR2 protein and a defective signaling transduction for hepcidin regulation. TFR2 genotyping should be considered in adult but also in pediatric cases with early-onset of iron overload.

Novel loci affecting iron homeostasis and their effects in individuals at risk for hemochromatosis

Variation in body iron is associated with or causes diseases, including anaemia and iron overload. Here, we analyse genetic association data on biochemical markers of iron status from 11 European-population studies, with replication in eight additional cohorts (total up to 48,972 subjects). We find 11 genome-wide-significant (P<5 × 10(-8)) loci, some including known iron-related genes (HFE, SLC40A1, TF, TFR2, TFRC, TMPRSS6) and others novel (ABO, ARNTL, FADS2, NAT2, TEX14). SNPs at ARNTL, TF, and TFR2 affect iron markers in HFE C282Y homozygotes at risk for hemochromatosis. There is substantial overlap between our iron loci and loci affecting erythrocyte and lipid phenotypes. These results will facilitate investigation of the roles of iron in disease.

Hemojuvelin and bone morphogenetic protein (BMP) signaling in iron homeostasis

Mutations in hemojuvelin (HJV) are the most common cause of the juvenile-onset form of the iron overload disorder hereditary hemochromatosis. The discovery that HJV functions as a co-receptor for the bone morphogenetic protein (BMP) family of signaling molecules helped to identify this signaling pathway as a central regulator of the key iron hormone hepcidin in the control of systemic iron homeostasis. This review highlights recent work uncovering the mechanism of action of HJV and the BMP-SMAD signaling pathway in regulating hepcidin expression in the liver, as well as additional studies investigating possible extra-hepatic functions of HJV. This review also explores the interaction between HJV, the BMP-SMAD signaling pathway and other regulators of hepcidin expression in systemic iron balance.

The extrahepatic role of TFR2 in iron homeostasis

Transferrin receptor 2 (TFR2), a protein homologous to the cell iron importer TFR1, is expressed in the liver and erythroid cells and is reported to bind diferric transferrin, although at lower affinity than TFR1. TFR2 gene is mutated in type 3 hemochromatosis, a disorder characterized by iron overload and inability to upregulate hepcidin in response to iron. Liver TFR2 is considered a sensor of diferric transferrin, possibly in a complex with hemochromatosis protein. In erythroid cells TFR2 is a partner of erythropoietin receptor (EPOR) and stabilizes the receptor on the cell surface. However, Tfr2 null mice as well as TFR2 hemochromatosis patients do not show defective erythropoiesis and tolerate repeated phlebotomy. The iron deficient Tfr2-Tmprss6 double knock out mice have higher red cells count and more severe microcytosis than the liver-specific Tfr2 and Tmprss6 double knock out mice. TFR2 in the bone marrow might be a sensor of iron deficiency that protects against excessive microcytosis in a way that involves EPOR, although the mechanisms remain to be worked out.

The role of hepatic transferrin receptor 2 in the regulation of iron homeostasis in the body

Fine-tuning of body iron is required to prevent diseases such as iron-overload and anemia. The putative iron sensor, transferrin receptor 2 (TfR2), is expressed in the liver and mutations in this protein result in the iron-overload disease Type III hereditary hemochromatosis (HH). With the loss of functional TfR2, the liver produces about 2-fold less of the peptide hormone hepcidin, which is responsible for negatively regulating iron uptake from the diet. This reduction in hepcidin expression leads to the slow accumulation of iron in the liver, heart, joints, and pancreas and subsequent cirrhosis, heart disease, arthritis, and diabetes. TfR2 can bind iron-loaded transferrin (Tf) in the bloodstream, and hepatocytes treated with Tf respond with a 2-fold increase in hepcidin expression through stimulation of the bone morphogenetic protein (BMP)-signaling pathway. Loss of functional TfR2 or its binding partner, the original HH protein, results in a loss of this transferrin-sensitivity. While much is known about the trafficking and regulation of TfR2, the mechanism of its transferrin-sensitivity through the BMP-signaling pathway is still not known.

Parenteral vs. oral iron: influence on hepcidin signaling pathways through analysis of Hfe/Tfr2-null mice

Treatment for iron deficiency anemia can involve iron supplementation via dietary or parenteral routes that result in different cellular iron distributions. The effect of the administered iron on the iron regulatory system and hepcidin in the liver has not been well studied. Hepcidin, the liver-expressed central iron-regulatory peptide, is itself regulated through the bone morphogenetic protein (BMP)/SMAD signaling pathway. Specifically, Bmp6 expression is upregulated in response to iron and induces hepcidin through phosphorylation of Smad1/5/8. The hemochromatosis-associated proteins Hfe and transferrin receptor 2 (Tfr2) are known upstream regulators of hepcidin, although their precise roles are still unclear. To investigate the mechanisms of this regulation and the roles of the Hfe and Tfr2, we subjected wild-type, Hfe(-/-), Tfr2(-/-), and Hfe(-/-)/Tfr2(-/-) mice to iron loading via dietary or parenteral routes. Systematic analysis demonstrated that Tfr2 is required for effective upregulation of Bmp6 in response to hepatocyte iron, but not nonparenchymal iron. Hfe is not required for Bmp6 upregulation, regardless of iron localization, but rather, is required for efficient downstream transmission of the regulatory signal. Our results demonstrate that Hfe and Tfr2 play separate roles in the regulatory responses to iron compartmentalized in different cell types and further elucidates the regulatory mechanisms controlling iron homeostasis.

Systemic iron homeostasis

The iron hormone hepcidin and its receptor and cellular iron exporter ferroportin control the major fluxes of iron into blood plasma: intestinal iron absorption, the delivery of recycled iron from macrophages, and the release of stored iron from hepatocytes. Because iron losses are comparatively very small, iron absorption and its regulation by hepcidin and ferroportin determine total body iron content. Hepcidin is in turn feedback-regulated by plasma iron concentration and iron stores, and negatively regulated by the activity of erythrocyte precursors, the dominant consumers of iron. Hepcidin and ferroportin also play a role in host defense and inflammation, and hepcidin synthesis is induced by inflammatory signals including interleukin-6 and activin B. This review summarizes and discusses recent progress in molecular characterization of systemic iron homeostasis and its disorders, and identifies areas for further investigation.

The iron cycle in chronic kidney disease (CKD): from genetics and experimental models to CKD patients

Iron is essential for most living organisms but iron excess can be toxic. Cellular and systemic iron balance is therefore tightly controlled. Iron homeostasis is dysregulated in chronic kidney disease (CKD) and contributes to the anemia that is prevalent in this patient population. Iron supplementation is one cornerstone of anemia management in CKD patients, but has not been rigorously studied in large prospective randomized controlled trials. This review highlights important advances from genetic studies and animal models that have provided key insights into the molecular mechanisms governing iron homeostasis and its disturbance in CKD, and summarizes how these findings may yield advances in the care of this patient population.

In situ proximity ligation assays indicate that hemochromatosis proteins Hfe and transferrin receptor 2 (Tfr2) do not interact

The hemochromatosis associated proteins HFE and Transferrin Receptor 2 (TFR2) have been shown to be important for the proper regulation of hepcidin. A number of in vitro studies using transient overexpression systems have suggested that an interaction between HFE and TFR2 is required for the regulation of hepcidin. This model of iron sensing which centers upon the requirement for an interaction between HFE and TFR2 has recently been questioned with in vivo studies in mice from our laboratory and others which suggest that Hfe and Tfr2 can regulate hepcidin independently of each other. To re-examine the postulated interaction between Hfe and Tfr2 we developed a novel expression system in which both proteins are stably co-expressed and used the proximity ligation assay to examine the interactions between Hfe, Tfr1 and Tfr2 at a cellular level. We were able to detect the previously described interaction between Hfe and Tfr1, and heterodimers between Tfr1 and Tfr2; however no interaction between Hfe and Tfr2 was observed in our system. The results from this study indicate that Hfe and Tfr2 do not interact with each other when they are stably expressed at similar levels. Furthermore, these results support in vivo studies which suggest that Hfe and Tfr2 can independently regulate hepcidin.

Non-HFE hemochromatosis

Hereditary hemochromatosis (HH) is an autosomal recessive disorder classically related to HFE mutations. However, since 1996, it is known that HFE mutations explain about 80% of HH cases, with the remaining around 20% denominated non-HFE hemochromatosis. Nowadays, four main genes are implicated in the pathophysiology of clinical syndromes classified as non-HFE hemochromatosis: hemojuvelin (HJV, type 2Ajuvenile HH), hepcidin (HAMP, type 2B juvenile HH), transferrin receptor 2 (TFR2, type 3 HH) and ferroportin (SLC40A1, type 4 HH). The aim of this review is to explore molecular, clinical and management aspects of non-HFE hemochromatosis.

A precious metal: Iron, an essential nutrient for all cells

Iron is an important cofactor required for a number of essential cell functions and hence is a vital nutrient. However, iron can also be dangerous as a catalyst of free radical reactions. Accordingly, intracellular iron homeostasis and body iron balance are tightly regulated. In this review, we presented an overview of the remarkable new insights that over the last years have been gained into the multifaceted and sophisticated molecular mechanisms controlling iron acquisition, storage and release. We also reviewed the data about nutrition-related abnormalities of iron metabolism, such as iron overload and deficiency. Finally, we discussed how pathogenic microorganisms and host cells compete for iron, a battle whose outcome has a relevant role in infectious disease.

Hepatic iron deposition does not predict extrahepatic iron loading in mouse models of hereditary hemochromatosis

Hereditary hemochromatosis is characterized by tissue iron loading and associated organ damage. However, the phenotype can be highly variable. The relationship between iron loading of different organs and the temporal nature of its deposition is still not well understood. We examined the progression of tissue iron loading in three mouse models to advance our understanding of the natural history of iron deposition in hereditary hemochromatosis. Wild-type, Hfe(-/-), Tfr2(-/-), and Hfe(-/-)/Tfr2(-/-) mice were analyzed at 3, 5, 10, 26, and 52 weeks, respectively. Hepatic, splenic, cardiac, and pancreatic iron concentrations were determined. Expression of both iron-regulatory and fibrosis genes was determined by quantitative real-time PCR in livers and hearts of 52-week-old mice. In all models, hepatic iron increased rapidly, plateauing before 10 weeks at different levels, depending on the genotype. Iron deposition in the pancreas and heart occurred after maximal iron loading of the liver was reached and was most marked in the Hfe(-/-)/Tfr2(-/-) mice. Although a significant positive correlation was identified between pancreatic and cardiac iron in all models at 52 weeks, there was no correlation between hepatic and either pancreatic or cardiac iron. There is variability in the timing and extent of tissue iron loading within a genotype, suggesting that hepatic iron levels in hereditary hemochromatosis may not accurately predict the iron content of other organs.

Mechanisms of mammalian iron homeostasis

Iron is vital for almost all organisms because of its ability to donate and accept electrons with relative ease. It serves as a cofactor for many proteins and enzymes necessary for oxygen and energy metabolism, as well as for several other essential processes. Mammalian cells utilize multiple mechanisms to acquire iron. Disruption of iron homeostasis is associated with various human diseases: iron deficiency resulting from defects in the acquisition or distribution of the metal causes anemia, whereas iron surfeit resulting from excessive iron absorption or defective utilization causes abnormal tissue iron deposition, leading to oxidative damage. Mammals utilize distinct mechanisms to regulate iron homeostasis at the systemic and cellular levels. These involve the hormone hepcidin and iron regulatory proteins, which collectively ensure iron balance. This review outlines recent advances in iron regulatory pathways as well as in mechanisms underlying intracellular iron trafficking, an important but less studied area of mammalian iron homeostasis.

Transgenic HFE-dependent induction of hepcidin in mice does not require transferrin receptor-2

Hereditary hemochomatosis (HH) is caused by mutations in several genes, including HFE and transferrin receptor-2 (TFR2). Loss of either protein decreases expression of the iron regulatory hormone hepcidin by the liver, leading to inappropriately high iron uptake from the diet, and resulting in systemic iron overload. In tissue culture, overexpressed HFE and TFR2 physically interact. Hepatocellular overexpression of Hfe in vivo increases hepcidin expression, despite an associated decrease in Tfr2. On this basis, we hypothesized that Tfr2 would not be required for Hfe-dependent up-regulation of hepcidin. We show that hepatocellular overexpression of Hfe in Tfr2(Y245X/Y245X) mice leads to hepcidin induction eventuating in iron deficiency and a hypochromic, microcytic anemia. Furthermore, coimmunoprecipitation studies using liver lysates did not provide evidence for physical interaction between Hfe and Tfr2 in vivo. In conclusion, we demonstrate that Tfr2 is not essential for Hfe-mediated induction of hepcidin expression, supporting the possibility that TFR2 may regulate iron metabolism in an HFE-independent manner.

Iron regulation of hepcidin despite attenuated Smad1,5,8 signaling in mice without transferrin receptor 2 or Hfe

BACKGROUND & AIMS

HFE and transferrin receptor 2 (TFR2) are each necessary for the normal relationship between body iron status and liver hepcidin expression. In murine Hfe and Tfr2 knockout models of hereditary hemochromatosis (HH), signal transduction to hepcidin via the bone morphogenetic protein 6 (Bmp6)/Smad1,5,8 pathway is attenuated. We examined the effect of dietary iron on regulation of hepcidin expression via the Bmp6/Smad1,5,8 pathway using mice with targeted disruption of Tfr2, Hfe, or both genes.

METHODS

Hepatic iron concentrations and messenger RNA expression of Bmp6 and hepcidin were compared with wild-type mice in each of the HH models on standard or iron-loading diets. Liver phospho-Smad (P-Smad)1,5,8 and Id1 messenger RNA levels were measured as markers of Bmp/Smad signaling.

RESULTS

Whereas Bmp6 expression was increased, liver hepcidin and Id1 expression were decreased in each of the HH models compared with wild-type mice. Each of the HH models also showed attenuated P-Smad1,5,8 levels relative to liver iron status. Mice with combined Hfe/Tfr2 disruption were most affected. Dietary iron loading increased hepcidin and Id1 expression in each of the HH models. Compared with wild-type mice, HH mice demonstrated attenuated (Hfe knockout) or no increases in P-Smad1,5,8 levels in response to dietary iron loading.

CONCLUSIONS

These observations show that Tfr2 and Hfe are each required for normal signaling of iron status to hepcidin via the Bmp6/Smad1,5,8 pathway. Mice with combined loss of Hfe and Tfr2 up-regulate hepcidin in response to dietary iron loading without increases in liver Bmp6 messenger RNA or steady-state P-Smad1,5,8 levels.

Hereditary hemochromatosis and transferrin receptor 2

BACKGROUND

Multicellular organisms regulate the uptake of calories, trace elements, and other nutrients by complex feedback mechanisms. In the case of iron, the body senses internal iron stores, iron requirements for hematopoiesis, and inflammatory status, and regulates iron uptake by modulating the uptake of dietary iron from the intestine. Both the liver and the intestine participate in the coordination of iron uptake and distribution in the body. The liver senses inflammatory signals and iron status of the organism and secretes a peptide hormone, hepcidin. Under high iron or inflammatory conditions hepcidin levels increase. Hepcidin binds to the iron transport protein, ferroportin (FPN), promoting FPN internalization and degradation. Decreased FPN levels reduce iron efflux out of intestinal epithelial cells and macrophages into the circulation. Derangements in iron metabolism result in either the abnormal accumulation of iron in the body, or in anemias. The identification of the mutations that cause the iron overload disease, hereditary hemochromatosis (HH), or iron-refractory iron-deficiency anemia has revealed many of the proteins used to regulate iron uptake.

SCOPE OF THE REVIEW

In this review we discuss recent data concerning the regulation of iron homeostasis in the body by the liver and how transferrin receptor 2 (TfR2) affects this process.

MAJOR CONCLUSIONS

TfR2 plays a key role in regulating iron homeostasis in the body.

GENERAL SIGNIFICANCE

The regulation of iron homeostasis is important. One third of the people in the world are anemic. HH is the most common inherited disease in people of Northern European origin and can lead to severe health complications if left untreated. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Evidence for distinct pathways of hepcidin regulation by acute and chronic iron loading in mice

In response to iron loading, hepcidin synthesis is homeostatically increased to limit further absorption of dietary iron and its release from stores. Mutations in HFE, transferrin receptor 2 (Tfr2), hemojuvelin (HJV), or bone morphogenetic protein 6 (BMP6) prevent appropriate hepcidin response to iron, allowing increased absorption of dietary iron, and eventually iron overload. To understand the role each of these proteins plays in hepcidin regulation by iron, we analyzed hepcidin messenger RNA (mRNA) responsiveness to short and long-term iron challenge in iron-depleted Hfe, Tfr2, Hjv, and Bmp6 mutant mice. After 1-day (acute) iron challenge, Hfe(-/-) mice showed a smaller hepcidin increase than their wild-type strain-matched controls, Bmp6(-/-) mice showed nearly no increase, and Tfr2 and Hjv mutant mice showed no increase in hepcidin expression, indicating that all four proteins participate in hepcidin regulation by acute iron changes. After a 21-day (chronic) iron challenge, Hfe and Tfr2 mutant mice increased hepcidin expression to nearly wild-type levels, but a blunted increase of hepcidin was seen in Bmp6(-/-) and Hjv(-/-) mice. BMP6, whose expression is also regulated by iron, may mediate hepcidin regulation by iron stores. None of the mutant strains (except Bmp6(-/-) mice) had impaired BMP6 mRNA response to chronic iron loading.

CONCLUSION

TfR2, HJV, BMP6, and, to a lesser extent, HFE are required for the hepcidin response to acute iron loading, but are partially redundant for hepcidin regulation during chronic iron loading and are not involved in the regulation of BMP6 expression. Our findings support a model in which acute increases in holotransferrin concentrations transmitted through HFE, TfR2, and HJV augment BMP receptor sensitivity to BMPs. A distinct regulatory mechanism that senses hepatic iron may modulate hepcidin response to chronic iron loading.

Mass spectrometry analysis of hepcidin peptides in experimental mouse models

The mouse is a valuable model for unravelling the role of hepcidin in iron homeostasis, however, such studies still report hepcidin mRNA levels as a surrogate marker for bioactive hepcidin in its pivotal function to block ferroportin-mediated iron transport. Here, we aimed to assess bioactive mouse Hepcidin-1 (Hep-1) and its paralogue Hepcidin-2 (Hep-2) at the peptide level. To this purpose, fourier transform ion cyclotron resonance (FTICR) and tandem-MS was used for hepcidin identification, after which a time-of-flight (TOF) MS-based methodology was exploited to routinely determine Hep-1 and -2 levels in mouse serum and urine. This method was biologically validated by hepcidin assessment in: i) 3 mouse strains (C57Bl/6; DBA/2 and BABL/c) upon stimulation with intravenous iron and LPS, ii) homozygous Hfe knock out, homozygous transferrin receptor 2 (Y245X) mutated mice and double affected mice, and iii) mice treated with a sublethal hepatotoxic dose of paracetamol. The results showed that detection of Hep-1 was restricted to serum, whereas Hep-2 and its presumed isoforms were predominantly present in urine. Elevations in serum Hep-1 and urine Hep-2 upon intravenous iron or LPS were only moderate and varied considerably between mouse strains. Serum Hep-1 was decreased in all three hemochromatosis models, being lowest in the double affected mice. Serum Hep-1 levels correlated with liver hepcidin-1 gene expression, while acute liver damage by paracetamol depleted Hep-1 from serum. Furthermore, serum Hep-1 appeared to be an excellent indicator of splenic iron accumulation. In conclusion, Hep-1 and Hep-2 peptide responses in experimental mouse agree with the known biology of hepcidin mRNA regulators, and their measurement can now be implemented in experimental mouse models to provide novel insights in post-transcriptional regulation, hepcidin function, and kinetics.

Hepcidin and iron regulation, 10 years later

Under evolutionary pressure to counter the toxicity of iron and to maintain adequate iron supply for hemoglobin synthesis and essential metabolic functions, humans and other vertebrates have effective mechanisms to conserve iron and to regulate its concentration, storage, and distribution in tissues. The iron-regulatory hormone hepcidin, first described 10 years ago, and its receptor and iron channel ferroportin control the dietary absorption, storage, and tissue distribution of iron. Hepcidin causes ferroportin internalization and degradation, thereby decreasing iron transfer into blood plasma from the duodenum, from macrophages involved in recycling senescent erythrocytes, and from iron-storing hepatocytes. Hepcidin is feedback regulated by iron concentrations in plasma and the liver and by erythropoietic demand for iron. Genetic malfunctions affecting the hepcidin-ferroportin axis are a main cause of iron overload disorders but can also cause iron-restricted anemias. Modulation of hepcidin and ferroportin expression during infection and inflammation couples iron metabolism to host defense and decreases iron availability to invading pathogens. This response also restricts the iron supply to erythropoietic precursors and may cause or contribute to the anemia associated with infections and inflammatory disorders.

A time course of hepcidin response to iron challenge in patients with HFE and TFR2 hemochromatosis

BACKGROUND

Inadequate hepcidin production leads to iron overload in nearly all types of hemochromatosis. We explored the acute response of hepcidin to iron challenge in 25 patients with HFE-hemochromatosis, in two with TFR2-hemochromatosis and in 13 controls. Sixteen patients (10 C282Y/C282Y homozygotes, 6 C282Y/H63D compound heterozygotes) had increased iron stores, while nine (6 C282Y/C282Y homozygotes, 3 C282Y/H63D compound heterozygotes) were studied after phlebotomy-induced normalization of iron stores.

DESIGN AND METHODS

We analyzed serum iron, transferrin saturation, and serum hepcidin by both enzyme-linked immunosorbent assay and mass-spectrometry at baseline, and 4, 8, 12 and 24 hours after a single 65-mg dose of oral iron.

RESULTS

Serum iron and transferrin saturation significantly increased at 4 hours and returned to baseline values at 8-12 hours in all groups, except in the iron-normalized patients who showed the highest and longest increase of both parameters. The level of hepcidin increased significantly at 4 hours and returned to baseline at 24 hours in controls and in the C282Y/H63D compound heterozygotes at diagnosis. The hepcidin response was smaller in C282Y-homozygotes than in controls, barely detectable in the patients with iron-depleted HFE-hemochromatosis and absent in those with TFR2-hemochromatosis. Conclusions Our results are consistent with a scenario in which TFR2 plays a prominent and HFE a contributory role in the hepcidin response to a dose of oral iron. In iron-normalized patients with HFE hemochromatosis, both the low baseline hepcidin level and the weak response to iron contribute to hyperabsorption of iron.

Conserved proximal promoter elements control repulsive guidance molecule c/hemojuvelin (Hfe2) gene transcription in skeletal muscle

Repulsive guidance molecule c (RGMc; gene symbol: Hfe2) plays a critical role in iron metabolism. Inactivating mutations cause juvenile hemochromatosis, a severe iron overload disorder. Understanding mechanisms controlling RGMc biosynthesis has been hampered by minimal information about the RGMc gene. Here we define the structure, examine the evolution, and establish mechanisms of regulation of the mouse RGMc gene. RGMc is a 4-exon gene that undergoes alternative RNA splicing to yield 3 mRNAs with 5' different untranslated regions. Gene transcription is induced during myoblast differentiation, producing all 3 mRNAs. We identify 3 critical promoter elements responsible for transcriptional activation in skeletal muscle, comprising paired E-boxes, a putative Stat and/or Ets element, and a MEF2 site, and muscle transcription factors myogenin and MEF2C stimulate RGMc promoter function in non-muscle cells. As these elements are conserved in RGMc genes from multiple species, our results suggest that RGMc has been a muscle-enriched gene throughout its evolutionary history.

Hepcidin induction by transgenic overexpression of Hfe does not require the Hfe cytoplasmic tail, but does require hemojuvelin

Mutations in HFE cause the most common form of hereditary hemochromatosis (HH). We previously showed that liver-specific, transgenic overexpression of murine Hfe stimulates production of the iron regulatory hormone hepcidin. Here, we developed several additional transgenic mouse strains to further interrogate the structural basis of HFE function in the pathophysiology of HH. We hypothesized that the small, cytoplasmic domain of HFE might be necessary for HFE-mediated induction of hepcidin. We demonstrate that, like the full-length protein, overexpression of Hfe proteins lacking the cytoplasmic domain leads to hepcidin induction, iron deficiency and a hypochromic, microcytic anemia. However, high-level expression of a liver-specific Hfe transgene carrying the mouse equivalent of the common HFE C282Y human disease-causing mutation (murine C294Y) did not cause iron deficiency. Furthermore, hepcidin induction by transgenes encoding both WT Hfe and Hfe lacking its cytoplasmic domain is greatly attenuated in the absence of hemojuvelin (Hjv). Our observations indicate that the extracellular and transmembrane domains of Hfe are sufficient, and Hjv is essential, for Hfe-mediated induction of hepcidin expression.