The molecular genetics of haemochromatosis

Hereditary hemochromatosis caused by a C282Y/H63D mutation in the HFE gene: A case report

Hereditary hemochromatosis (HH) is a disease characterized by disordered iron metabolism. It often involves mutations of the HFE gene, which encodes the homeostatic iron regulator protein (HFE), as well as mutations affecting hepcidin antimicrobial peptide, hemojuvelin, or transferrin receptor 2. Historically, HH has been observed primarily in European and European diaspora populations, while classical HH is rare in Asian populations, including in China. In this article, we report a rare case of HH in a Chinese man that could be attributed to a heterozygous C282Y/H63D HFE mutation. Based on clinical examination, liver biopsy, and genetic testing results, the patient was diagnosed with HH. Clinical signs and symptoms and serum iron-related test results were recorded for a period of two years after the patient began treatment. Over this observation period, the patient was subjected to 25 phlebotomies (accounting for a total blood loss of 10.2 L). His serum ferritin levels decreased from 1550 μg/L to 454 μg/L, his serum iron concentration decreased from 40 μmol/L to 24.6 μmol/L, and his transferrin saturation decreased from 97.5% to 55.1%. Early diagnosis is essential for patients with HH to obtain good outcomes. Regular phlebotomy after diagnosis can improve HH symptoms and delay HH disease progression.

Differences in DNA methylation of HAMP in blood cells predicts the development of type 2 diabetes

OBJECTIVES

Better disease management can be achieved with earlier detection through robust, sensitive, and easily accessible biomarkers. The aim of the current study was to identify novel epigenetic biomarkers determining the risk of type 2 diabetes (T2D).

METHODS

Livers of 10-week-old female New Zealand Obese (NZO) mice, slightly differing in their degree of hyperglycemia and liver fat content and thereby in their diabetes susceptibility were used for expression and methylation profiling. We screened for differences in hepatic expression and DNA methylation in diabetes-prone and -resistant mice, and verified a candidate (HAMP) in human livers and blood cells. Hamp expression was manipulated in primary hepatocytes and insulin-stimulated pAKT was detected. Luciferase reporter assays were conducted in a murine liver cell line to test the impact of DNA methylation on promoter activity.

RESULTS

In livers of NZO mice, the overlap of methylome and transcriptome analyses revealed a potential transcriptional dysregulation of 12 hepatokines. The strongest effect with a 52% decreased expression in livers of diabetes-prone mice was detected for the Hamp gene, mediated by elevated DNA methylation of two CpG sites located in the promoter. Hamp encodes the iron-regulatory hormone hepcidin, which had a lower abundance in the livers of mice prone to developing diabetes. Suppression of Hamp reduces the levels of pAKT in insulin-treated hepatocytes. In liver biopsies of obese insulin-resistant women, HAMP expression was significantly downregulated along with increased DNA methylation of a homologous CpG site. In blood cells of incident T2D cases from the prospective EPIC-Potsdam cohort, higher DNA methylation of two CpG sites was related to increased risk of incident diabetes.

CONCLUSIONS

We identified epigenetic changes in the HAMP gene which may be used as an early marker preceding T2D.

Iron, glucose and fat metabolism and obesity: an intertwined relationship

A bidirectional relationship exists between adipose tissue metabolism and iron regulation. Total body fat, fat distribution and exercise influence iron status and components of the iron-regulatory pathway, including hepcidin and erythroferrone. Conversely, whole body and tissue iron stores associate with fat mass and distribution and glucose and lipid metabolism in adipose tissue, liver, and muscle. Manipulation of the iron-regulatory proteins erythroferrone and erythropoietin affects glucose and lipid metabolism. Several lines of evidence suggest that iron accumulation and metabolism may play a role in the development of metabolic diseases including obesity, type 2 diabetes, hyperlipidaemia and non-alcoholic fatty liver disease. In this review we summarise the current understanding of the relationship between iron homoeostasis and metabolic disease.

Thermal stabilisation of the short DNA duplexes by acridine-4-carboxamide derivatives

The short oligodeoxynucleotide (ODN) probes are suitable for good discrimination of point mutations. However, the probes suffer from low melting temperatures. In this work, the strategy of using acridine-4-carboxamide intercalators to improve thermal stabilisation is investigated. The study of large series of acridines revealed that optimal stabilisation is achieved upon decoration of acridine by secondary carboxamide carrying sterically not demanding basic function bound through a two-carbon linker. Two highly active intercalators were attached to short probes (13 or 18 bases; designed as a part of HFE gene) by click chemistry into positions 7 and/or 13 and proved to increase the melting temperate (T_m) of the duplex by almost 8°C for the best combination. The acridines interact with both single- and double-stranded DNAs with substantially preferred interaction for the latter. The study of interaction suggested higher affinity of the acridines toward the GC- than AT-rich sequences. Good discrimination of two point mutations was shown in practical application with HFE gene (wild type, H63D C > G and S65C A > C mutations). Acridine itself can also serve as a fluorophore and also allows discrimination of the fully matched sequences from those with point mutations in probes labelled only with acridine.

Twenty-Five Years of Contemplating Genotype-Based Hereditary Hemochromatosis Population Screening

Hereditary hemochromatosis (HH) is a rather frequent, preventable disease because the progressive iron overload affecting many organs can be effectively reduced by phlebotomy. Even before the discovery of the major gene, HFE, in 1996, hemochromatosis was seen as a candidate for population-wide screening programmes. A US Centers of Disease Control and the National Human Genome Research Institute expert panel convened in 1997 to consider genotype-based HH population-wide screening and decided that the scientific evidence available at that time was insufficient and advised against. In spite of a large number of studies performed within the last 25 years, addressing all aspects of HH natural history, health economics, and social acceptability, no professional body worldwide has reverted this decision, and HH remains a life-threatening condition that often goes undetected at a curable stage.

Prevalence of HFE-related haemochromatosis and secondary causes of hyperferritinaemia and their association with iron overload in 1059 French patients treated by venesection

BACKGROUND

Venesection is the key therapy in haemochromatosis, but it remains controversial in hyperferritinaemia with moderate iron accumulation. There is substantial evidence that the results of HFE genotyping are routinely misinterpreted, while elevated serum ferritin has become more frequent in recent years in white adult populations following the increase of obesity and metabolic traits.

AIMS

To examine the reasons for prescribing venesection in 1,059 French patients during the period 2012-2015, determine the true prevalence of HFE-related haemochromatosis, and compare iron overload profiles between haemochromatosis and non-haemochromatosis patients.

RESULTS

Only 258 of the 488 patients referred for haemochromatosis had the p.[Cys282Tyr];[Cys282Tyr] disease causative genotype (adjusted prevalence: 24.4%). Of the 801 remaining patients, 112 (14.0%) had the debated p.[Cys282Tyr];[His63Asp] compound heterozygote genotype, 643 (80.3%) had central obesity, 475 (59.3%) had metabolic syndrome (MetS) and 93 (11.6%) were heavy drinkers. The non-haemochromatosis patients started therapeutic venesection 9 years later than haemochromatosis patients (P < 0.001). Despite similar serum ferritin values, they had lower transferrin saturation (41.1% vs 74.3%; P < 0.001), lower amounts of iron removed by venesection (1.7 vs 3.2 g; P < 0.001) and lower hepatic iron concentrations (107 vs 237 µmol/g; P < 0.001).

CONCLUSIONS

Haemochromatosis is over-diagnosed and is no longer the main reason for therapeutic venesection in France. Obesity and other metabolic abnormalities are frequently associated with mild elevation of serum ferritin, the MetS is confirmed in ~50% of treated patients. There is a minimal relationship between serum ferritin and iron overload in non-p.Cys282Tyr homozygotes. Our observations raise questions about venesection indications in non-haemochromatosis patients.

Diagnosis and management of non-clonal erythrocytosis remains challenging: a single centre clinical experience

Erythrocytosis has a diverse background. While polycythaemia vera has well defined criteria, the diagnostic approach and management of other types of erythrocytosis are more challenging. The aim of study was to retrospectively analyse the aetiology and management of non-clonal erythrocytosis patients referred to a haematology outpatient clinic in an 8-year period using a 3-step algorithm. The first step was inclusion of patients with Hb > 185 g/L and/or Hct > 0.52 in men and Hb > 165 g/L and/or Hct > 0.48 in women on two visits ≥ two months apart, thus confirming true erythrocytosis. Secondly, polycythaemia vera was excluded and secondary causes of erythrocytosis (SE) identified. Thirdly, idiopathic erythrocytosis patients (IE) were referred to next-generation sequencing for possible genetic background evaluation. Of the 116 patients, 75 (65%) are men and 41 (35%) women, with non-clonal erythrocytosis 34/116 (29%) had SE, 15/116 (13%) IE and 67/116 (58%) stayed incompletely characterized (ICE). Patients with SE were significantly older and had significantly higher Hb and Hct compared to patients with IE. Most frequently, SE was attributed to obstructive sleep apnoea and smoking. Phlebotomies were performed in 56, 53 and 40% of patients in the SE, IE, and ICE group, respectively. Approx. 70% of patients in each group received aspirin. Thrombotic events were registered in 12, 20 and 15% of SE, IE and ICE patients, respectively. Congenital erythrocytosis type 4 (ECYT4) was diagnosed in one patient. The study demonstrates real-life management of non-clonal erythrocytosis which could be optimized using a 3-step diagnostic algorithm.

Low iron mitigates viral survival: insights from evolution, genetics, and pandemics-a review of current hypothesis

Background

Upon re-examination of our human history, evolutionary perspectives, and genetics, a prevailing iron deficiency phenotype appears to have evolved to protect the human race from extinction.

Body

In this review, we summarize the evolutionary and genetic perspectives pointing towards the hypothesis that low iron mitigates infection. The presence of infection promotes the generation of resistance alleles, and there are some evolutionary and genetic clues that suggest the presence of an iron deficiency phenotype that may have developed to protect against infection. Examples include the relative paucity of iron overload genes given the essential role of iron, as well as the persistence of iron deficiency among populations in spite of public health efforts to treat it. Additional examination of geographic areas with severe iron deficiency in the setting of pandemics including H1N1, SARS, and COVID-19 reveals that areas with higher prevalence of iron deficiency are less affected. RNA viruses have several evolutionary adaptations which suggest their absolute need for iron, and this dependency may be exploited during treatment.

Conclusion

RNA viruses pose a unique challenge to modern healthcare, with an average of 2-3 new pathogens being discovered yearly. Their overarching requirements for iron, along with human evolutionary and genetic adaptations which favored an iron deficiency phenotype, ultimately suggest the potential need for iron control in these infections.

Determination of mutations in iron regulating genes of beta thalassemia major patients of Khyber Pakhtunkhwa, Pakistan

Background

Hepcidin and hemochromatosis (HFE) are iron regulatory proteins that are encoded by HAMP and HFE genes. Mutation in either HAMP gene or HFE gene causes Hepcidin protein deficiency that can lead to iron overload in beta thalassemia patients. The aim of this research work was to study the presence of G71D mutation of HAMP gene and H63D mutation of HFE gene in beta thalassemia major and minor group to check the association of these mutations with serum ferritin level of beta thalassemia patients.

Methods

The study was conducted on 42 beta thalassemia major and 20 beta thalassemia minor samples along with 20 control samples. The genotyping of both mutations has done by ARM‐PCR technique with specific set of primers.

Results

Significant effect of G71D and H63D mutations was observed on serum ferritin level of thalassemia major group. The risk allele of HAMP G71D and HFE H63D was found with high frequency (48% and 49%, respectively) in beta thalassemia major than in control group. High genotypic frequency of HAMP and HFE gene mutation gene mutation was observed in beta thalassemia major than beta thalassemia minor and control group (7% and 9%, respectively).

Conclusion

It can be concluded that both HAMP and HFE gene mutations show high frequency in beta thalassemia major patients and mean significant association between mutations and high serum ferritin level of beta thalassemia major patients but the nonsignificant results of Odd ratios showed that both mutations do not act as major risk factor in beta thalassemia major.

How to use… iron studies

Iron studies are frequently requested in paediatric practice. They are useful both as a diagnostic tool and as a way of monitoring certain conditions, particularly those causing iron overload. This article outlines the physiology of iron metabolism and discusses laboratory aspects of performing iron studies, including factors influencing interpretation. Clinical scenarios are used to highlight how the tests can be used in different clinical situations.

Identification of Genes for Hereditary Hemochromatosis

Hereditary hemochromatosis (HH) is one of the most common genetically transmitted conditions in individuals of Northern European ancestry. The disease is characterized by excessive intestinal absorption of dietary iron, resulting in pathologically high iron storage in tissues and organs. If left untreated, HH can damage joints and organs, and eventually lead to death. There are four main classes of HH, as well as five individual molecular subtypes, caused by mutations in five genes, and the approaches implemented in the discovery of each HH type have specific histories and unique aspects. In this chapter, we review the genetics of the different HH types, including the strategies used to detect the causal variants in each case and the manner in which genetic variants were found to affect iron metabolism.

HFE Gene Mutations and Iron Status in 100 Healthy Polish Children

Iron participates in oxygen transport, energetic, metabolic, and immunologic processes. There are 2 main causes of iron overload: hereditary hemochromatosis which is a primary cause, is a metabolic disorder caused by mutations of genes that control iron metabolism and secondary hemochromatosis caused by multitransfusions, chronic hemolysis, and intake of iron rich food. The most common type of hereditary hemochromatosis is caused by HFE gene mutation. In this study, we analyzed iron metabolism in 100 healthy Polish children in relation to their HFE gene status. The wild-type HFE gene was predominant being observed in 60 children (60%). Twenty-five children (25%), presented with heterozygotic H63D mutation, and 15 children (15%), presented with other mutations (heterozygotic C282Y and S65C mutation, compound heterozygotes C282Y/S65C, C282Y/H63D, H63D homozygote). The mean concentration of iron, the level of ferritin, and transferrin saturation were statistically higher in the group of HFE variants compared with the wild-type group. H63D carriers presented with higher mean concentration of iron, ferritin levels, and transferrin saturation compared with the wild-type group. Male HFE carriers presented with higher iron concentration, transferrin saturation, and ferritin levels than females. This preliminary investigation demonstrates allelic impact on potential disease progression from childhood.

The impact of H63D HFE gene carriage on hemoglobin and iron status in children

The molecular mechanism that regulates iron homeostasis is based on a network of signals, which reflect on the iron requirements of the body. Hereditary hemochromatosis is a heterogenic metabolic syndrome which is due to unchecked transfer of iron into the bloodstream and its toxic effects on parenchymatous organs. It is caused by the mutation of genes that encode proteins that help hepcidin to monitor serum iron. These proteins include the human hemochromatosis protein -HFE, transferrin-receptor 2, hemojuvelin in rare instances, and ferroportin. HFE-related hemochromatosis is the most frequent form of the disease. Interestingly, the low penetrance of polymorphic HFE genes results in rare clinical presentation of the disease, predominantly in middle-aged males. Taking into account the wide dispersion of HFE mutation in our population and also its unknown role in heterozygotes, we analyzed the impact of H63D HFE carriage in the developmental age, with respect to gender, on the iron status and hemoglobin concentration of carriers in comparison to those of wild-type HFE gene (12.7 ± 3.07 years, 42 boys and 41 girls). H63D carriers presented higher blood iron, transferrin saturation, and ferritin concentration than wild-type probands (p < 0.05.) Interestingly, male H63D carriers showed higher hemoglobin concentration than the unburdened children. Moreover, in the H63D carrier group, a positive correlation between iron and hemoglobin was noted. In conclusion, this study demonstrates that changes in iron metabolism occur at a young age in HFE heterozygotes.

Next-generation sequencing of hereditary hemochromatosis-related genes: Novel likely pathogenic variants found in the Portuguese population

Hereditary hemochromatosis (HH) is an autosomal recessive disorder characterized by excessive iron absorption resulting in pathologically increased body iron stores. It is typically associated with common HFE gene mutation (p.Cys282Tyr and p.His63Asp). However, in Southern European populations up to one third of HH patients do not carry the risk genotypes. This study aimed to explore the use of next-generation sequencing (NGS) technology to analyse a panel of iron metabolism-related genes (HFE, TFR2, HJV, HAMP, SLC40A1, and FTL) in 87 non-classic HH Portuguese patients. A total of 1241 genetic alterations were detected corresponding to 53 different variants, 13 of which were not described in the available public databases. Among them, five were predicted to be potentially pathogenic: three novel mutations in TFR2 [two missense (p.Leu750Pro and p.Ala777Val) and one intronic splicing mutation (c.967-1G>C)], one missense mutation in HFE (p.Tyr230Cys), and one mutation in the 5'-UTR of HAMP gene (c.-25G>A). The results reported here illustrate the usefulness of NGS for targeted iron metabolism-related gene panels, as a likely cost-effective approach for molecular genetics diagnosis of non-classic HH patients. Simultaneously, it has contributed to the knowledge of the pathophysiology of those rare iron metabolism-related disorders.

Haemochromatosis Gene (HFE) Polymorphisms and Migraine: An Association Study

Several studies have suggested that iron metabolism may be involved in the pathogenesis of migraine. Using a case-control design, we performed an association study in a cohort of Italian migraine patients to evaluate whether a particular allele or genotype of the haemochromatosis gene ( HFE) would modify the occurrence and clinical features of the disease. We genotyped 256 migraine patients and 237 healthy age-, sex- and ethnicity-matched controls for the C282Y and H63D polymorphisms of the HFE gene. Phenotype and allele frequencies of both polymorphisms were similarly distributed in migraine patients and controls. The patients carrying the DD genotype of the H63D polymorphism showed a later age at onset of the disease and an increased number of migraine attacks. Our data suggest that the HFE gene is not a major disease gene for migraine. However, the H63D polymorphism of the HFE gene may be considered a modifying genetic factor in migraine.

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.

Comprehensive functional annotation of 18 missense mutations found in suspected hemochromatosis type 4 patients

Hemochromatosis type 4 is a rare form of primary iron overload transmitted as an autosomal dominant trait caused by mutations in the gene encoding the iron transport protein ferroportin 1 (SLC40A1). SLC40A1 mutations fall into two functional categories (loss- versus gain-of-function) underlying two distinct clinical entities (hemochromatosis type 4A versus type 4B). However, the vast majority of SLC40A1 mutations are rare missense variations, with only a few showing strong evidence of causality. The present study reports the results of an integrated approach collecting genetic and phenotypic data from 44 suspected hemochromatosis type 4 patients, with comprehensive structural and functional annotations. Causality was demonstrated for 10 missense variants, showing a clear dichotomy between the two hemochromatosis type 4 subtypes. Two subgroups of loss-of-function mutations were distinguished: one impairing cell-surface expression and one altering only iron egress. Additionally, a new gain-of-function mutation was identified, and the degradation of ferroportin on hepcidin binding was shown to probably depend on the integrity of a large extracellular loop outside of the hepcidin-binding domain. Eight further missense variations, on the other hand, were shown to have no discernible effects at either protein or RNA level; these were found in apparently isolated patients and were associated with a less severe phenotype. The present findings illustrate the importance of combining in silico and biochemical approaches to fully distinguish pathogenic SLC40A1 mutations from benign variants. This has profound implications for patient management.

Evidence for the high importance of co-morbid factors in HFE C282Y/H63D patients cared by phlebotomies: results from an observational prospective study

Despite type I haemochromatosis (HC) is mainly associated with the HFE C282Y/C282Y genotype, a second genotype -C282Y/H63D- has mostly been described in other patients. Its association with HC, apart from any associated co-morbid factors, remains unclear and complex to interpret for physicians. This study assesses the weight of this genotype and the role of co-morbid factors in the occurrence of iron overload. This prospective study included the C282Y/C282Y (n = 172) and C282Y/H63D (n = 58) patients enrolled in a phlebotomy program between 2004 and 2007 in a blood centre of western Brittany (Brest, France), where HC is frequent. We compared prevalence of these two genotypes, as well as patients' profile regarding degree of iron overload and prevalence of co-morbid factors. First, we confirmed the obvious deficit of C282Y/H63D compound heterozygotes among patients cared by phlebotomies. This genotype was 3.0 times less frequent than the C282Y/C282Y genotype among those patients (18.9% vs. 56.0%) whereas it was 4.9 times more frequent in the general population (4.3% vs. 0.9%; p<0.0001). Despite a similar level of hyperferritinaemia, the C282Y/H63D patients who came to medical attention had a milder plasma iron overload, reflected by a lower transferrin saturation median (52.0% vs. 84.0%; p<0.0001). They also exhibited more frequently co-morbid factors, as heavy drinking (26.0% vs. 13.9%; p = 0.0454), overweight (66.7% vs. 39.4%; p = 0.0005) or both (21.3% vs. 2.6%; p<0.0001). Ultimately, they required a lower amount of iron removed to reach depletion (2.1 vs. 3.4 g; p<0.0001), clearly reflecting their lower tissue iron. This study confirms that H63D is a discrete genetic susceptibility factor whose expression is most visible in association with other co-factors. It highlights the importance of searching for co-morbidities in these diagnostic situations and of providing lifestyle and dietary advice.

Mammalian iron metabolism

ABSTRACT Iron is an essential transition metal for mammalian cellular and tissue viability. It is critical to supplying oxygen through heme, the mitochondrial respiratory chain, and enzymes such as ribonucleotide reductase. Mammalian organisms have evolved with the means of regulating the metabolism of iron, because if left unregulated, the resulting excess amounts of iron may induce chronic toxicities affecting multiple organ systems. Several homeostatic mechanisms exist to control the amount of intestinal dietary iron uptake, cellular iron uptake, distribution, and export. Within these processes, numerous molecular participants have been identified because of advancements in basic cell biology and efforts in disease-based research of iron storage abnormalities. For example, dietary iron uptake across the intestinal duodenal mucosa is mediated by an intramembrane divalent metal transporter 1 (DMT1), and cellular iron efflux involves ferroportin, the only known iron exporter. In addition to duodenal enterocytes, ferroportin is present in other cell types, and exports iron into plasma. Ferroportin was recently discovered to be regulated by the expression of the circulating hormone hepcidin, a small peptide synthesized in hepatocytes. These recent studies on the role of hepcidin in the regulation of dietary, cellular, and extracellular iron have led to a better understanding of the pathways by which iron balance in humans is influenced, especially its involvement in human genetic diseases of iron overload. Other important molecular pathways include iron binding to transferrin in the bloodstream for cellular delivery through the plasma membrane transferrin receptor (TfR1). In the cytosol, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a prominent role in sensing the presence of iron in order to posttranscriptionally regulate the expression of TfR1 and ferritin, two important participants in iron metabolism. From a toxicological standpoint, posttranscriptional regulation of these genes aids in the sequestration, control, and hence prevention of cytotoxic effects from free-floating nontransferrin-bound iron. Given the importance of dietary iron in normal physiology, its potential to induce chronic toxicity, and recent discoveries in the regulation of human iron metabolism by hepcidin, this review will address the regulatory mechanisms of normal iron metabolism in mammals with emphasis on dietary exposure. It is the goal of this review that this information may provide in a concise format our current understanding of major pathways and mechanisms involved in mammalian iron metabolism, which is a basis for control of iron toxicity. Such a discussion is intended to facilitate the identification of deficiencies so that future metabolic or toxicological studies may be appropriately focused. A better knowledge of iron metabolism from normal to pathophysiological conditions will ultimately broaden the spectrum of the usefulness of this information in biomedical and toxicological sciences for improving and protecting human health.

Genetic polymorphisms in bovine transferrin receptor 2 (TFR2) and solute carrier family 40 (iron-regulated transporter), member 1 (SLC40A1) genes and their association with beef iron content

Beef is considered to be an excellent source of dietary iron. However, little is known about the genetic control of beef iron content. We hypothesized that genetic polymorphisms in transferrin receptor 2 (TFR2) and solute carrier family 40 (iron-regulated transporter), member 1 (SLC40A1) could influence skeletal muscle iron content. The objective of this study was to use Angus cattle to identify single-nucleotide polymorphisms (SNPs) in the exons and flanking regions of the bovine TFR2 and SLC40A1 genes and to evaluate the extent to which genetic variation in them was associated with bovine longissimus dorsi muscle iron content. Ten novel SNPs were identified in TFR2, of which one SNP tended to be associated (P < 0.013) with skeletal muscle iron content. Nine novel SNPs in SLC40A1, NC007300: rs133108154, rs137140497, rs135205621, rs136600836, rs134388440, rs136347850, rs134186279, rs134621419 and rs137555693, were identified, of which SNPs rs134388440, rs136347850 and rs137555693 were significantly associated (P < 0.007) with skeletal muscle iron content. High linkage disequilibrium was observed among SLC40A1 SNPs rs134388440, rs136347850 and rs137555693 (R(2)  > 0.99), from which two haplotypes, TGC and CAT, were defined. Beef from individuals that were homozygous for the TGC haplotype had significantly (P < 0.001) higher iron content than did beef from CAT homozygous or heterozygous individuals. The estimated size of effect of the identified haplotypes was 0.3% of the phenotypic variance. In conclusion, our study provides evidence for genetic control of beef iron concentration. Moreover, SNPs identified in SLC40A1, rs134388440, rs136347850 and rs137555693 might be useful markers for the selection of Angus cattle for altered iron content.

Hemochromatosis gene mutations in the general population of Slovakia

This is an epidemiologic study of the Slovak population with the aim of determining the frequencies of three hemochromatosis gene (HFE) variants C282Y, H63D and S65C known to be associated with manifestation of hereditary hemochromatosis and to assess deviations of these frequencies from those reported elsewhere. Mutations were detected in 359 ethnic Slovaks by real-time PCR assay based on TaqMan technology. The allelic frequencies were 4.03% for C282Y, 12.67% for H63D and 1.25% for S65C mutation. We observed 0.28% of C282Y/C282Y homozygotes, 3.34% H63D/H63D homozygotes, 0.84% of C282Y/H63D compound heterozygotes and 0.56% of H63D/S65C compound heterozygotes. This is the first time the frequencies of H63D and S65C mutations have been reported in the general population in Slovakia. The observed allelic frequencies are consistent with the previous studies of Slavic and Central European populations.

The distribution of the parasitic fauna dictates the distribution of the haemochromatosis genes

No satisfactory explanation has been offered, to date, to account for the prevalence of the haemochromatosis genes in the European population and yet relative paucity of the gene in the tropics. Traditional wisdom suggests that, in antiquity, the haemochromatosis gene, which promotes iron absorption, would have protected ancient man from iron loss resulting from injury either during hunting or through war. However, such an advantage would be equally desirable for other populations where the incidence of the alleles is negligible. Others have tackled the polemic from the another view, postulating that the paucity of the haemochromatosis alleles in populations outside of Europe may be explained by the fact that iron load predisposes to infection and that iron deficiency anaemia is protective against this by limiting parasitic access to host stores of iron. This explanation alone is equally unsatisfactory as European populations are exposed to pathogens and would benefit from any protection afforded by mild anaemia. Others have mooted genetic drift as another alternative explanation. Yet this would be unexpected for a gene which is deleterious. We propose here that the driving force for the propagation of the haemochromatosis alleles was not infection per se but the nature of the parasitic fauna which sojourned with mankind. The tropics are inhabited with multicellular parasitic and highly pathogenic organisms, which consequently have a high demand for iron. The organisms have developed aggressive means of iron extraction from their hosts. Where there is iron in abundance such organisms would have a licence to multiply in an unbridled fashion at the expense of the host. Such a host, due to their increased iron load, would be able to harbour a high parasitic load which would be harmful to the population as a whole, not just the individual with the haemochromatosis allele. As man migrated from the tropics many of the larger pathogens disappeared and man had only to contend with traditional unicellular adversaries. Iron is a critical micronutrient that the host attempts to withhold for invading pathogens. We also advance the theory that the tropical anaemias including sickle cell trait, thalassaemia, glucose-6-phosphate dehydrogenase deficiency, and pyruvate kinase deficiency are an ingenious evolutionary means by the host of withholding iron from tropical pathogens while simultaneously avoiding the deleterious effects of frank iron deficiency and/or iron deficiency anaemia. The mechanism is essentially an immunological passive aggressive orchestrated by man kind.

Association of hepcidin promoter c.-582 A>G variant and iron overload in thalassemia major

Hepcidin is a 25-amino acid peptide, derived from cleavage of an 84 amino acid pro-peptide produced predominantly by hepatocytes. This molecule, encoded by the hepcidin antimicrobial peptide (HAMP) gene shows structural and functional properties consistent with a role in innate immunity. Moreover, as demonstrated in mice and humans, hepcidin is a major regulator of iron metabolism, and acts by binding to ferroportin and controlling its concentration and trafficking. In this study we investigated the influence that mutations in HAMP and/or hemocromatosis (HFE) genes might exert on iron metabolism in a group of poly-transfused thalassemic patients in preparation for bone marrow transplantation. Our results showed that the presence of the c.-582 A>G polymorphism (rs10421768) placed in HAMP promoter (HAMP-P) might play a role in iron metabolism, perhaps varying the transcriptional activation that occurs through E-boxes located within the promoter.

HFE-Related Hemochromatosis: The Haptoglobin 2-2 Type Has a Significant but Limited Influence on Phenotypic Expression of the Predominant p.C282Y Homozygous Genotype

Phenotypic expression of the common p.C282Y/p.C282Y HFE-related hemochromatosis genotype is heterogeneous and depends on a complex interplay of genetic and non-genetic factors. Haptoglobin has a crucial role in free hemoglobin iron recovery, and exists as three major types: Hp1-1, Hp2-1 and Hp2-2. Hp2-2 favors endocytosis of hemoglobin iron in monocytes/macrophages, resulting in partial iron retention and increased intracellular ferritin levels. This situation is generally not expected to severely affect iron homeostasis, but was found to correlate with elevated serum iron indices in healthy men. Whether the Hp2-2 genotype acts as a modifier in HFE-related hemochromatosis is unclear. In this study we investigated influence of Hp2-2 and of potential confounders on the iron indices of 351 p.C282Y homozygous patients. We conclude that there is a cause-and-effect relationship between the Hp2-2 genotype and increased iron indices in p.C282Y homozygous patients. The Hp2-2 effect is, however, limited and only apparent in males.

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

Ferroportin-related iron overload disease differs from haemochromatosis in that it has a dominant mode of inheritance and is usually associated with macrophage iron sequestration. However, it is thought that mutations with opposite effects on protein functions, i.e. loss-of-function versus gain-of-function mutations, are responsible for variable phenotype presentations. The present study investigated the functional relevance of a novel ferroportin variant: the c.1502 A>G transition, which changes amino acid 501 from tyrosine to cysteine (p.Y501C). This novel variant was identified in a pedigree originating from Central Italy and, although an intra-familial phenotype heterogeneity was observed, it co-segregated with an iron overload picture similar to that of the HFE-related typical haemochromatosis. In cultured cells, the p.Y501C mutant protein reached the plasma membrane and retained a full iron export ability. By contrast, it was resistant to inhibition by hepcidin. These findings confirm that certain ferroportin mutations compromise the activity of hepcidin in iron homeostasis, mimicking hepcidin deficiency as described in all types of hemochromatosis.

Regulation of hepcidin and iron-overload disease

Hepcidin, a 25-amino-acid antimicrobial peptide, is the central regulator of iron homeostasis. Hepcidin transcription is upregulated by inflammatory cytokines, iron, and bone morphogenetic proteins and is downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. The iron transporter ferroportin is the cognate receptor of hepcidin and is destroyed as a result of interaction with the peptide. Except for inherited defects of ferroportin and hepcidin itself, all forms of iron-storage disease appear to arise from hepcidin dysregulation. Studies using multiple approaches have begun to delineate the molecular mechanisms that regulate hepcidin expression, particularly at the transcriptional level. Knowledge of the regulation of hepcidin by inflammation, iron, erythropoiesis, and hypoxia will lead to an understanding of the pathogenesis of primary hemochromatosis, secondary iron overload, and anemia of inflammatory disease.

Homozygous deletion of HFE produces a phenotype similar to the HFE p.C282Y/p.C282Y genotype

Hemochromatosis is predominantly associated with the HFE p.C282Y homozygous genotype, which is carried by approximately 1 person in 200 in Northern European populations. However, p.C282Y homozygosity is often characterized by incomplete penetrance. Here, we describe the case of a woman who had a major structural alteration in the HFE gene. Molecular characterization revealed an Alu-mediated recombination leading to the loss of the entire HFE gene sequence. Although homozygous for the HFE deleted allele, the woman had a phenotype similar to that seen in most women homozygous for the common p.C282Y mutation. Contrasting with previously reported results in Hfe knockout and Hfe knockin mice, our report gives further evidence that progression of the disease depends on modifying factors.

A novel (Leu183Pro-)mutation in the HFE-gene co-inherited with the Cys282Tyr mutation in two unrelated Dutch hemochromatosis patients

We describe a novel heterozygous mutation in exon 3 of the HFE-gene that was co-inherited with Cys282Tyr in two unrelated Dutch men both presenting a classical form of hereditary hemochromatosis. Heterozygosity for this mutation was also found in one out of 100 healthy controls of Dutch descent. This c.548T>C mutation converts a leucine to a proline residue at position 183 in the alpha2-helix of the HFE-protein (Leu183Pro). Standard bioinformatics analysis shows that the mutation is likely to disturb the HFE interaction with TfR1. This disrupting role of the mutation in the iron regulatory pathway is further corroborated by the familial co-occurrence of the observed compound heterozygosity with increased serum iron parameters. Haplotype analysis strongly suggests that this novel mutation arose from a common ancestor in the distant past. These findings may have implications for HFE-testing of iron overloaded heterozygous Cys282Tyr-patients of Northern European origin and their relatives.

A novel mutation of transferrin receptor 2 in a Taiwanese woman with type 3 hemochromatosis

Hereditary hemochromatosis (HH) is very rare in Asia. Here, we describe a Taiwanese woman presenting with fully developed characteristics of HH including bronze skin, DM, decreased MRI T2 signal intensity over liver and pituitary gland. Biochemistry of iron profile indicated a severe status of iron overload by serum iron: 194 microg/dL, serum ferritin: 6640 microg/L, transferrin saturation: 92.8%. By measuring the hepatic iron index 8.48 (>1.9) of her liver biopsy tissue, the diagnosis of HH was established. Diagnosis of non-HFE HH was carried out since the whole HFE genome was sequenced but failed to localize any genetic alterations. The whole genome of transferrin receptor 2 (TfR2) was sequenced and a novel mutation of 13528 G-->A (Arg 481 His) in exon 11 was detected. Therefore, type 3 hemochromatosis was confirmed. The distinct clinical features, extremely high iron index and impressive iron staining in her liver biopsy tissue may represent an aggravated iron deposition in the liver caused by this novel mutation. Our finding implicates functional importance of histidine in exchange of arginine at amino acid 481 of transferrin receptor 2 in iron homeostasis. This case reminds physicians in Asia to keep in mind that hemochromatosis could be a rare cause of DM.

Nutritional iron deficiency: an evolutionary perspective

Iron deficiency, with or without iron-deficiency anemia, is so ubiquitous that it affects all populations of the world irrespective of race, culture, or ethnic background. Despite all the latest advances in modern medicine, improved nutrition, and the ready availability of cheap oral iron, there is still no good explanation for the widespread persistence of iron deficiency. It is possible that the iron deficiency phenotype is very prevalent because of many factors other than the commonly cited causes such as a decreased availability or an increased utilization of iron. Several thousand years ago, human culture changed profoundly with the agrarian revolution, when humans turned to agriculture. Their diet became iron deficient and new epidemic infections emerged due to crowding and lifestyle changes. There is convincing evidence that iron deficiency protects against many infectious diseases such as malaria, plague, and tuberculosis as shown by diverse medical, historical, and anthropologic studies. Thus, this change of diet increased the frequency of iron deficiency, and epidemic infections exerted a selection pressure under which the iron deficiency phenotype survived better. Multiple evolutionary factors have contributed in making iron deficiency a successful phenotype. We analyze some of the recent findings of iron metabolism, the theories explaining excessive menstruation in human primates, the unexplained relative paucity of hemochromatosis genes, the former medical practice of "blood-letting," and other relevant historical data to fully understand the phenomenon of iron deficiency. We suggest that, due to a long evolutionary persistence of iron deficiency, efforts at its prevention will take a long time to be effective.

Mapping genes responsible for strain-specific iron phenotypes in murine chromosome substitution strains

The highly variable clinical phenotype observed in patients homozygous for the C282Y mutation of the hereditary hemochromatosis gene (HFE) is likely due to the influence of non-HFE modifier genes. The primary functional abnormality causing iron overload in hemochromatosis is hyper-absorption of dietary iron. We found that iron absorption in inbred mice varies in a strain-specific manner, as does the pattern of iron distribution to the liver and spleen. A/J mice absorbed approximately twice the amount of 59Fe delivered by gavage compared to the C57BL/6 strain. Genetic comparisons between A/J and C57BL/6 were facilitated by the availability of consomic chromosome substitution strains (CSS). Each CSS has an individual chromosome pair from A/J on an otherwise C57BL/6J background. We found that iron absorption and iron content in liver and in spleen were continuous variables suggesting that each trait is under multigenic control. No trait co-segregated among the CSS. Chromosome 5 from A/J, however, imparted the highest iron absorption phenotype and multiple CSS had absorption levels equivalent to A/J. Chromosomes 9 and X were associated with high spleen iron content. These data suggest that multiple genes contribute to the regulation of iron absorption and that individual organ iron phenotypes are independently regulated.

Transferrin receptor 2 is emerging as a major player in the control of iron metabolism

Our knowledge of mammalian iron metabolism has advanced dramatically over recent years. Iron is an essential element for virtually all living organisms. Its intestinal absorption and accurate cellular regulation is strictly required to ensure the coordinated synthesis of the numerous iron-containing proteins involved in key metabolic processes, while avoiding the uptake of excess iron that can lead to organ damage. A range of different proteins exist to ensure this fine control within the various tissues of the body. Among these proteins, transferrin receptor (TFR2) seems to play a key role in the regulation of iron homeostasis. Disabling mutations in TFR2 are responsible for type 3 hereditary hemochromatosis (Type 3 HH). This review describes the biological properties of this membrane receptor, with a particular emphasis paid to the structure, function and cellular localization. Although much information has been garnered on TFR2, further efforts are needed to elucidate its function in the context of the iron regulatory network.

Hemochromatosis (HFE) gene splice site mutation IVS5+1 G/A in North American Vietnamese with and without phenotypic evidence of iron overload

Homozygosity for a novel hemochromatosis (HFE) gene splice site mutation (IVS5+1 G/A) was previously reported in a 48-year-old Vietnamese man residing in Germany who had an elevated serum ferritin (SF) and transferrin saturation (TS) and severe iron overload on liver biopsy. This mutation was not found in 222 controls of central European origin but has been found in Southeast Asians living in Vietnam without evidence of iron overload. Hemochromatosis and iron overload screening (HEIRS) Study is an ongoing, multiethnic, primary care-based study of 101,168 North American adults, including 12,772 Asians, a group that the HEIRS Study found has a significantly higher than expected prevalence of elevated serum TS and SF but very low prevalence of the common C282Y and H63D HFE alleles usually associated with hereditary hemochromatosis. It was hypothesized that the IVS5+1 G/A splice site mutation might explain some elevated biochemical iron measures in North American Asians. Overall, 200 Vietnamese subjects from the Los Angeles Field Center who had TS and SF values greater than the 75th percentile of all HEIRS Study participants after adjusting for covariates and 149 controls randomly selected to represent this Vietnamese population were genotyped. Among cases, 1 homozygous mutant and 7 heterozygotes were found; among controls, 1 homozygous mutant and 4 heterozygotes were found yielding an allele frequency of 2.32% for cases and 2.04% for controls (P>0.5). This finding suggests that the HFE IVS5+1 G/A splice site mutation is not the major explanation for unexpectedly high prevalence of TS and SF in North American Asians.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

Mixture models of serum iron measures in population screening for hemochromatosis and iron overload

Homozygosity for the C282Y mutation of the hemochromatosis gene on chromosome 6p (HFE) is a common genetic trait that increases susceptibility to iron overload. The authors describe and apply methodology developed for the analysis of phenotypic and genotypic data from 46,136 non-Hispanic Caucasians, a subset of the multi-ethnic cohort enrolled in the Hemochromatosis and Iron Overload Screening (HEIRS) Study. For analysis of the distribution of transferrin saturation (TS), mixtures of normal distributions were considered and the expectation-maximization (EM) algorithm was applied for parameter estimation. Maximized log-likelihoods were compared, and significance was assessed by resampling. Sensitivity, specificity, and predictive values from the modeled subpopulations were compared with the actual observed genotypes for C282Y and H63D mutations in the HFE gene. A strong association between HFE genotype and TS subpopulations was found in these data collected from different geographic regions, confirming the external validity of the statistical approach when applied to population-based data. It was concluded that mixture modeling of phenotypic data may provide a clinical guide for screening with gender-specific thresholds to identify potential samples for genetic testing.

Expression studies of neogenin and its ligand hemojuvelin in mouse tissues

Juvenile hemochromatosis is a severe hereditary iron overload disease caused by mutations in the HJV (hemojuvelin) and HAMP (hepcidin) genes. Hepcidin is an important iron regulatory hormone, and hemojuvelin may regulate hepcidin synthesis via the multifunctional membrane receptor neogenin. We explored the expression of murine hemojuvelin and neogenin mRNAs and protein. Real-time RT-PCR analysis of 18 tissues from male and female mice was performed to examine the mRNA expression profiles. To further study protein expression and localization we used immunohistochemistry on several tissues from three mouse strains. Mouse Neo1 mRNA was detectable in the 18 tissues tested, the highest signals being evident in the ovary, uterus, and testis. Neogenin protein was observed in the brain, skeletal muscle, heart, liver, stomach, duodenum, ileum, colon, renal cortex, lung, testis, ovary, oviduct, and uterus. The spleen, thymus, and pancreas were negative for neogenin. The highest signals for Hjv mRNA were detectable in the skeletal muscle, heart, esophagus, and liver. The results indicate that Neo1 mRNA is widely expressed in both male and female mouse tissues with the highest signals detected in the reproductive system. Moreover, Hjv and Neo1 mRNAs are simultaneously expressed in skeletal muscle, heart, esophagus, and liver.

Hereditary Hemochromatosis Protein, HFE, Interaction with Transferrin Receptor 2 Suggests a Molecular Mechanism for Mammalian Iron Sensing

HFE and transferrin receptor 2 (TFR2) are membrane proteins integral to mammalian iron homeostasis and associated with human hereditary hemochromatosis. Here we demonstrate that HFE and TFR2 interact in cells, that this interaction is not abrogated by disease-associated mutations of HFE and TFR2, and that TFR2 competes with TFR1 for binding to HFE. We propose a new model for the mechanism of iron status sensing that results in the regulation of iron homeostasis.

Association of HFE common mutations with Parkinson's disease, Alzheimer's disease and mild cognitive impairment in a Portuguese cohort

Background

Pathological brain iron deposition has been implicated as a source of neurotoxic reactive oxygen species in Alzheimer (AD) and Parkinson diseases (PD). Iron metabolism is associated with the gene hemochromatosis (HFE Human genome nomenclature committee ID:4886), and mutations in HFE are a cause of the iron mismetabolism disease, hemochromatosis. Several reports have tested the association of HFE variants with neurodegenerative diseases, such as AD and PD with conflicting results.

Methods

Genotypes were analysed for the two most common variants of HFE in a series of 130 AD, 55 Mild Cognitive Impairment (MCI) and 132 PD patients. Additionally, a series of 115 healthy age-matched controls was also screened.

Results

A statistically significant association was found in the PD group when compared to controls, showing that the presence of the C282Y variant allele may confer higher risk for developing the disease.

Conclusion

Taken together these results suggest that the common variants in HFE may be a risk factor for PD, but not for AD in the Portuguese population.

Frequency of common HFE variants in the Saudi population: a high throughput molecular beacon-based study

Background

Hereditary Hemochromatosis (HH) is an autosomal recessive disorder highlighted byiron-overload. Two popular mutations in HFE, p.C282Y and p.H63D, have been discovered and found to associate with HH in different ethnic backgrounds. p.C282Y and p.H63D diagnosis is usually made byrestriction enzyme analysis. However, the use of this technique is largelylimited to research laboratories because they are relativelyexpensive, time-consuming, and difficult to transform into a high throughput format.

Methods

Single nucleotide variations in target DNA sequences can be readily identified using molecular beacon fluorescent probes. These are quenched probes with loop and hairpin structure, and they become fluorescent upon specific target recognition. We developed high throughput homogeneous real-time PCR assays using molecular beacon technology, to genotype p.C282Y and p.H63D variants. Representative samples of different genotypes for these variants were assayed by restriction enzyme analysis and direct sequencing as bench mark methods for comparison with the newly developed molecular beacon-based real-time PCR assay.

Results

Complete concordance was achieved by all three assay formats. Homozygotes (mutant and wildtype) and heterozygotes were readily differentiated by the allele specific molecular beacons as reported by the associated fluorophore in the real-time assay developed in this study. Additionally, these assays were used in a high throughput format to establish the allele frequency of C282Y and H63D in Saudis for the first time.

Conclusion

These assays may be reliably applied as a diagnostic test or large scale method for population screening.

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.