Wild-type HFE protein normalizes transferrin iron accumulation in macrophages from subjects with hereditary hemochromatosis

Hereditary hemochromatosis (HC) is one of the most common single-gene hereditary diseases. A phenotypic hallmark of HC is low iron in reticuloendothelial cells in spite of body iron overload. Most patients with HC have the same mutation, a change of cysteine at position 282 to tyrosine (C282Y) in the HFE protein. The role of HFE in iron metabolism and the basis for the phenotypic abnormalities of HC are not understood. To clarify the role of HFE in the phenotypic expression of HC, we studied monocytes-macrophages from subjects carrying the C282Y mutation in the HFE protein and clinically expressing HC and transfected them with wild-type HFE by using an attenuated Salmonella typhimurium strain as a gene carrier. The Salmonella system allowed us to deliver genes of interest specifically to monocytes-macrophages with high transduction efficiency. The accumulation of (55)Fe delivered by (55)Fe-Tf was significantly lower in macrophages from patients with HC than from controls expressing wild-type HFE. Transfection of HC macrophages with the HFE gene resulted in a high level of expression of HFE protein at the cell surface. The accumulation of (55)Fe delivered by (55)Fe-Tf was raised by 40% to 60%, and this was reflected by an increase in the (55)Fe-ferritin pool within the HFE-transfected cells. These results suggest that the iron-deficient phenotype of HC macrophages is a direct effect of the HFE mutation, and they demonstrate a role for HFE in the accumulation of iron in these cells.     

Human platelets express hemochromatosis protein (HFE) and transferrin receptor 2

OBJECTIVES: While body iron status may influence platelets, little information is available about platelet expression of proteins regulating iron homeostasis. HFE, the protein defective in hereditary hemochromatosis, and transferrin receptor 2 (TfR2) are two novel protein candidates that could be involved in mechanisms of iron transport across the platelet plasma membrane. METHODS: The expression and localization of HFE, TfR1 and TfR2 proteins in human platelets were examined using Western blotting and immunocytochemistry. RESULTS: Human platelets expressed HFE and TfR2, whereas no signal for TfR1 was found. The positive reactions for HFE and TfR2 were mainly confined to the platelet plasma membrane. CONCLUSIONS: Expression of HFE and TfR2 proteins in human platelets may indicate that the mutations in the corresponding genes could influence platelet count, size and/or activation. The presence of TfR2 and absence of TfR1 suggests that HFE may serve a different function in platelets compared with the other HFE-positive cell types, e.g. enterocytes, macrophages and syncytiotrophoblasts.     

Relationship of body iron stores to levels of serum ferritin, serum iron, unsaturated iron binding capacity and transferrin saturation in patients with iron storage disease

None of the methods for assessing total body iron burden in patients with hemochromatosis is satisfactory. Although it is commonly believed that a relationship exists between serum ferritin levels and total iron burden, the extent of this relationship has not previously been documented. In the present investigation we measured the total body iron burden of 88 patients with putative hemochromatosis, 54 of whom were homozygotes for the 845G-->A (C282Y) mutation. The total body iron stores were estimated from the volume of red cells removed during therapeutic phlebotomy corrected for an estimated 2 mg/day dietary iron absorbed during the phlebotomy period; the amount of storage iron was compared to the serum ferritin, serum iron, unsaturated iron binding capacity, and transferrin saturation before the beginning of phlebotomy. The serum ferritin proved to be the best predictor of body iron stores. The correlation between all of the analytes and the body iron burden was greater in patients homozygous for the C282Y mutation than in those who were not, including the compound heterozygotes for C282Y and H63D. The body iron burden tended to be greater in patients homozygous for the C282Y mutation than the other patients at any other given ferritin level. We conclude that the serum ferritin level does provide some information regarding total iron burden but even in the case of C282Y homozygotes, the correlation is not very strong.     

The hereditary hemochromatosis protein, HFE, lowers intracellular iron levels independently of transferrin receptor 1 in TRVb cells

Hereditary hemochromatosis (HH) is an autosomal recessive disease that leads to parenchymal iron accumulation. The most common form of HH is caused by a single amino acid substitution in the HH protein, HFE, but the mechanism by which HFE regulates iron homeostasis is not known. In the absence of transferrin (Tf), HFE interacts with transferrin receptor 1 (TfR1) and the 2 proteins co-internalize, and in vitro studies have shown that HFE and Tf compete for TfR1 binding. Using a cell line lacking endogenous transferrin receptors (TRVb cells) transfected with different forms of HFE and TfR1, we demonstrate that even at low concentrations Tf competes effectively with HFE for binding to TfR1 on living cells. Transfection of TRVb cells or the derivative line TRVb1 (which stably expresses human TfR1) with HFE resulted in lower ferritin levels and decreased Fe2+ uptake. These data indicate that HFE can regulate intracellular iron storage independently of its interaction with TfR1. Earlier studies found that in HeLa cells, HFE expression lowers Tf-mediated iron uptake; here we show that HFE lowers non-Tf-bound iron in TRVb cells and add to a growing body of evidence that HFE may play different roles in different cell types.     

HFE hemochromatosis in African Americans: Prevalence estimates of iron overload and iron overload-related disease

BackgroundLittle is known about the prevalence of HFE (homeostatic iron regulator) hemochromatosis in African Americans (AA).MethodsWe defined AA as self-identified AA, blacks, or non-Hispanic blacks. We defined hemochromatosis-associated HFE genotypes as p.C282Y/p.C282Y and p.C282Y/p.H63D. We compiled prevalences of these genotypes in AA using published population and cohort data and numbers of men and women ≥18 y? in 2018 U.S. Census estimates. We defined iron overload (IO) and IO-related disease by genotype as previously reported in population and cohort studies of hemochromatosis in whites of European ancestry. We used these definitions to estimate prevalences and numbers of AA with IO and IO-related disease associated with hemochromatosis-associated HFE genotypes.ResultsThere were ～16,287,599 men and ～17,644,898 women. HFE genotypes and their respective prevalences were: p.C282Y/p.C282Y, 0.00017 (6/34,905) [95% confidence interval 0.000034, 0.00031] and p.C282Y/p.H63D, 0.0012 (41/33,596) [0.000084, 0.0016]. IO prevalences were: men 0.000076 [0.000072, 0.000081] and women 0.0000061 [0.0000050, 0.0000073]. IO-related disease prevalences were: men 0.000063 [0.000059, 0.000067] and women 0.0000021 [0.0000014, 0.0000027]. There were ～1021 [961, 1091] men and ～36 [25, 48] women with IO-related disease.ConclusionsWe conclude that ～1/25,061 AA >18 y have a hemochromatosis-associated HFE genotype and IO and that ～1/32,103 AA >18 y have a hemochromatosis-associated HFE genotype and IO-related disease.     

Hepatic iron metabolism gene expression profiles in HFE associated hereditary hemochromatosis

BACKGROUND: Individuals with pathogenic mutations in HFE, hemojuvelin (HJV) and transferrin receptor 2 (TfR2) have low levels of hepcidin, but little is known about the hepatic expression of these molecules in patients with physiological iron overload or HFE associated Hemochromatosis (HH). AIMS: To examine the hepatic mRNA expression of iron homeostasis genes in patients with HH, physiological iron overload and healthy controls. PATIENTS: Untreated C282Y homozygous HH patients (n=20) with elevated serum ferritin (SF) and patients with physiological iron overload (n=12) with positive hepatocellular iron staining and negative HFE mutation analysis were evaluated. The control cohort (n=10) had normal iron parameters, negative HFE mutation analysis and negative hepatocellular iron staining. METHODS: Hepcidin, HJV (hemojuvelin), TfR2 (transferrin receptor 2), HFE, IL6 (interleukin 6) and ferroportin mRNA expression patterns were evaluated using quantitative real-time PCR. RESULTS: Physiological iron overload led to significantly upregulated hepcidin, HJV and ferroportin mRNA expression while TfR2 expression was not significantly different to controls. In contrast, HFE associated iron overload failed to induce hepcidin or HJV. TfR2 mRNA expression was significantly reduced when compared to controls. Ferroportin expression in HH was comparable to that found in physiological iron overload. Neither HFE nor IL6 expression was altered by variation in iron status. CONCLUSIONS: These findings suggest that patients with HH, in contrast to those with physiological iron overload, have a weakened TfR2 sensing mechanism that leads to the lack of induction of hepcidin and HJV. The C282Y HFE mutation does not appear to impede the hepatocellular iron export function of ferroportin.     

Frequency of HFE mutations among Turkish blood donors according to transferrin saturation: genotype screening for hereditary hemochromatosis among voluntary blood donors in Turkey

BACKGROUND AND GOALS: The C282Y and H63D mutations of HFE gene are associated with hereditary hemochromatosis (HH), the most common autosomal recessive disorder in European population. This is the first Turkish population study of, the prevalence of these mutations. STUDY: 2677 healthy volunteer blood donors were screened by means of transferrin saturation (TS) with the cutoff value of 45. As study group, 86 donors with a TS 45 or higher and as control group 57 donors with TS less than 45 were tested for these mutations, ferritin, and alanin aminotransferase (ALT) levels. RESULTS: The mean age of donors were 33+/-9 and 94.1% of them were male. The number of donors with TS 45 or higher was 265 (9.9%). C282Y mutation was not detected. The frequency of H63D mutation in the study, control and general groups were 27.32%, 21.05%, and 24.83%, respectively. As a result, the H63D mutation was present in 60 out of 143 participants in whom 49 were heterozygote (frequency of heterozygote allele 49/286 = 17.13%), 11 were homozygote (frequency of homozygote allele 22/286 = 7.69%). Serum ALT and TS were not affected from the type of H63D mutation. There was no difference in ferritin levels according to type of H63D mutations among 143 blood donors. CONCLUSION: This study revealed the absence of C282Y mutation in our population. Although the frequency of H63D heterozygosity seems to be higher than the other population, the genetic screening for the HFE gene mutation is inadequate and the phenotypic screening with TS and ferritin seems to be preferable in Turkish population.     

Biological variability of transferrin saturation and unsaturated iron-binding capacity

Background

Transferrin saturation is widely considered the preferred screening test for hemochromatosis. Unsaturated iron-binding capacity has similar performance at lower cost. However, the within-person biological variability of both these tests may limit their ability at commonly used cut points to detect HFE C282Y homozygous patients.

Methods

The Hemochromatosis and Iron Overload Screening Study screened 101,168 primary care participants for iron overload using transferrin saturation, unsaturated iron-binding capacity, ferritin, and HFE C282Y and H63D genotyping. Transferrin saturation and unsaturated iron-binding capacity were performed at initial screening and again when selected participants and controls returned for a clinical examination several months later. A missed case was defined as a C282Y homozygote who had transferrin saturation below the cut point (45% for women, 50% for men) or unsaturated iron-binding capacity above the cut point (150 μmol/L for women, 125 μmol/L for men) at the initial screening or the clinical examination, or both, regardless of serum ferritin.

Results

There were 209 C282Y previously undiagnosed homozygotes with transferrin saturation and unsaturated iron-binding capacity testing performed at the initial screening and clinical examination. Sixty-eight C282Y homozygotes (33%) would have been missed at these transferrin saturation cut points (19 men, 49 women; median serum ferritin level of 170 μg/L; first and third quartiles, 50 and 474 μg/L), and 58 homozygotes (28%) would have been missed at the unsaturated iron-binding capacity cut points (20 men, 38 women; median serum ferritin level of 168 μg/L; first and third quartiles, 38 and 454 μg/L). There was no advantage to using fasting samples.

Conclusions

The within-person biological variability of transferrin saturation and unsaturated iron-binding capacity limits their usefulness as an initial screening test for expressing C282Y homozygotes.     

The effect of HFE mutations on serum ferritin and transferrin saturation in the Jersey population

High frequencies of the haemochromatosis-related HFE C282Y mutation have been reported in North European populations, in which a high proportion of patients with the disease are homozygotes. However, the degree of penetrance of this genotype is unknown. We determined the HFE C282Y and H63D genotypes of 411 consenting volunteer blood donors on Jersey, and the serum ferritin and transferrin saturation levels of 204 of these volunteers. The C282Y allele frequency was found to be 8.3% in 822 chromosomes, indicating a homozygote frequency of 1/145. Consistent with this, four C282Y homozygotes were detected in 411 volunteers. As there are only 18 patients presently receiving treatment for haemochromatosis on Jersey, out of a total population of about 85 000, there is a large discrepancy between the number of haemochromatosis patients and the number of C282Y homozygotes in this population. In a preliminary study of 204 consenting volunteers we found a correlation between transferrin saturation and HFE H63D/C282Y genotype ( P  = 0.017) and between serum ferritin and genotype ( P  = 0.056). We also observed elevated values of transferrin saturation in the two C282Y homozygotes assayed. These results suggest that a large proportion of the many undetected C282Y homozygotes on Jersey and in similar populations could be in the preclinical stages of haemochromatosis, and warrant investigation. However, there may be a wide variation in the expression of the condition, and a more extensive study of the level of disease penetrance encompassing a large number of hitherto undetected C282Y homozygotes is therefore imperative.     

Mutations of the hemochromatosis gene in Italian candidate blood donors with increased transferrin saturation

The aim of this study was to analyze the role of HFE mutations in blood donors with iron parameters suggesting iron overload, taking into account the regional distribution of HFE mutations in Italy. We studied 5880 subjects undergoing evaluation for blood donation eligibility, from different areas of Italy. Abnormal iron parameters were defined as transferrin saturation (TS) >50% or >45% and serum ferritin (SF) >300 or >250 microg/ml in males and females, respectively. Subjects with increased TS and/or SF were re-tested and typed for HFE mutations C282Y and H63D. A total of 548 individuals had increased iron parameters at first testing. In total, 179/548 were available for retesting, and in 109 increased TS and/or SF were confirmed. Increased TS was confirmed in 25 individuals, among whom three were C282Y homozygotes and six were compound heterozygotes for C282Y and H63D. Increased TS was more frequent in northern Italy than in southern regions. In individuals with increased TS and/or SF, the frequency of C282Y and H63D was 0.13 and 0.21 in northern-Italy versus 0.05 and 0.45 in southern Italy (P=0.004 for H63D). Nine out of 10 individuals carrying hemochromatosis-associated genotypes (including compound heterozygosity for C282Y and H63D) originated from northern regions. Among controls, the allelic frequencies of C282Y and H63D were 0.037 and 0.16 in the northern regions and 0.015 and 0.16 in the southern regions. In conclusion, over one-third of individuals with persistently altered TS carried hemochromatosis-associated genotypes, confirming that a diagnostic approach based on TS and genotyping of selected cases may represent a viable screening procedure.     

HFE genotype and parameters of iron metabolism in German first-time blood donors - evidence for an increased transferrin saturation in C282Y heterozygotes

BACKGROUND: Hemochromatosis is usually inherited in an autosomal recessive mode and associated with missense mutations in the hemochromatosis gene (HFE), an HLA class 1 related gene. However the degree of penetrance is presently matter of debate. METHODS: To elucidate the frequency of HFE mutations in a German population and the relationship between genotype and phenotype, we determined the HFE C282Y and H63D genotypes in 500 first-time blood donors using an allele-specific ligase chain reaction (LCR). Ferritin and transferrin saturation (TS) of all donors found to have at least one mutation were compared to gender- and age-matched controls. RESULTS: The C282Y allele frequency was 46 in 1000 chromosomes (4.6 %). The allele frequency of H63D was 108 in 1000 (10.8 %) chromosomes. We found three persons homozygous for H63D, nine compound heterozygotes and none homozygous for C282Y. TS was elevated in C282Y heterozygotes (p = 0.002) and C282Y/H63D compound heterozygotes (p = 0.04) compared to wild-type controls. Serum ferritin tended to be elevated in compound heterozygotes (p = 0.053). Mean corpuscular volume (MCV) and hemoglobin (MCH) were not different from controls. CONCLUSION: The frequency of HFE mutations in the tested population was comparable to those of other northern European populations. The elevated TS in subjects carrying a single copy of the C282Y mutation suggests that C282Y heterozygosity is associated with an increased intestinal iron absorption and might therefore offer a selection advantage in conditions of iron depletion.     

Molecular aspects of iron absorption: Insights into the role of HFE in hemochromatosis

Hereditary hemochromatosis is the most common genetic disorder occurring in persons of northern European descent, and the clinical hallmark of the disease is the gradual accumulation of iron in internal organs, especially the liver, heart, and pancreas, which ultimately leads to organ failure. HFE, the gene that is defective in the majority of cases, was identified in 1996 and, although the exact role that HFE plays in the uptake and utilization of iron is not yet clear, important aspects of HFE function are emerging. Identification and studies of new proteins involved in the absorption of iron in the gut and in somatic cells has led to a clearer picture of how humans absorb iron from the diet and regulate this absorption to meet metabolic needs and to balance body iron stores. This review focuses on the molecular aspects of iron absorption and the role that HFE may play in these processes.     

Regulation of transferrin-mediated iron uptake by HFE, the protein defective in hereditary hemochromatosis

The protein defective in hereditary hemochromatosis, called HFE, is similar to MHC class I-type proteins and associates with beta2-microglobulin (beta2M). Its association with beta2M was previously shown to be necessary for its stability, normal intracellular processing, and cell surface expression in transfected COS cells. Here we use stably transfected Chinese hamster ovary cell lines expressing both HFE and beta2M or HFE alone to study the effects of beta2M on the stability and maturation of the HFE protein and on the role of HFE in transferrin receptor 1 (TfR1)-mediated iron uptake. In agreement with prior studies on other cell lines, we found that overexpression of HFE, without overexpressing beta2M, resulted in a decrease in TfR1dependent iron uptake and in lower iron levels in the cells, as evidenced by ferritin and TfR1 levels measured at steady state. However, overexpression of both HFE and beta2M had the reverse effect and resulted in an increase in TfR1-dependent iron uptake and increased iron levels in the cells. The HFE-beta2M complex did not affect the affinity of TfR1 for transferrin or the internalization rate of transferrin-bound TfR1. Instead, HFE-beta2M enhanced the rate of recycling of TfR1 and resulted in an increase in the steady-state level of TfR1 at the cell surface of stably transfected cells. We propose that Chinese hamster ovary cells provide a model to explain the effect of the HFE-beta2M complex in duodenal crypt cells, where the HFE-beta2M complex appears to facilitate the uptake of transferrin-bound iron to sense the level of body iron stores. Impairment of this process in duodenal crypt cells leads them to be iron poor and to signal the differentiating enterocytes to take up iron excessively after they mature into villus cells in the duodenum of hereditary hemochromatosis patients.