Hereditary hemochromatosis: update for 2003

Abstract

1. IntroductionThe term ‘hemochromatosis’ was first used by von Recklinghausen, a German pathologist in the late 1800s [[1]von Recklinghausen FD. Uber hamochromatose. Tageblatt der (62) Versammlung Deutsch. Naturforscher und Arzte in Heidelberg 1889: p. 324–5.Google Scholar]; he determined that the pigmentation seen in patients with advanced hemochromatosis was due to iron. In 1935, Joseph Sheldon, a British geriatrician, published a monograph describing over 300 patients with hemochromatosis [[2]Sheldon J.H. Haemochromatosis. Oxford University Press, London1935Google Scholar]. He concluded that the disorder was an inherited defect with all of the pathology caused by excess iron in involved tissues. In the mid-1970s, Simon and colleagues in northern France determined definitively that hereditary hemochromatosis (HH) was inherited as an autosomal recessive disorder linked to the short arm of chromosome 6 in the region of HLA-A3 [[3]Simon M. Bourel M. Genetet B. Fauchet R. Idiopathic hemochromatosis. Demonstration of recessive transmission and early detection by family HLA typing.N Engl J Med. 1977; 297: 1017-1021Crossref PubMed Scopus (306) Google Scholar]. Finally, in 1996, a team of molecular geneticists at a small biotechnology company called Mercator Genetics in California used a positional cloning technique to discover the gene responsible for HH [[4]Feder J.N. Gnirke A. Thomas W. Tsuchihashi Z. Ruddy D.A. Basava A. et al.A novel MHC class 1-like gene is mutated in patients with hereditary haemochromatosis.Nat Genet. 1996; 13: 399-409Crossref PubMed Scopus (3327) Google Scholar]. It was first called HLA-H and was later renamed HFE. Since that important discovery, tremendous advances in our understanding of the pathophysiologic mechanisms that occur in HH have taken place; further, our ability to diagnose patients, screen families, and evaluate whole populations has been enhanced.2. Classification of iron overload syndromesIron overload is a common problem found in clinical practice and can be classified as shown in Table 1. It is important to distinguish whether iron overload is primary (i.e. as a result of an inherited metabolic disorder) or whether it is secondary to another problem that results in an increase in iron absorption. The most common form of hereditary hemochromatosis is HFE-related HH that affects between one and 200 and one in 400 individuals of northern European descent [5Powell L.W. Subramaniam V.N. Yapp T.R. Haemochromatosis in the new millennium.J Hepatol. 2000; 32: 48-62Abstract Full Text PDF PubMed Scopus (100) Google Scholar, 6Bacon B.R. Hemochromatosis: diagnosis and management.Gastroenterology. 2001; 120: 718-725Abstract Full Text Full Text PDF PubMed Scopus (135) Google Scholar]. HFE-related HH is characterized by increased gastrointestinal iron absorption with subsequent tissue iron deposition in the liver, heart, pancreas, other endocrine organs, joints and skin. The HFE gene encodes a major histocompatability complex (MHC) class 1-like protein that may modulate cellular iron transport by binding to transferrin receptor-1 (TfR-1) [7Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar, 8Britton R.S. Fleming R.E. Parkkila S. Waheed A. Sly W.S. Bacon B.R. Pathogenesis of hereditary hemochromatosis: genetics and beyond.Semin Gastrointest Dis. 2002; 13: 68-79PubMed Google Scholar, 9Trinder D. Fox C. Vautier G. Olynyk J.K Molecular pathogenesis of iron overload.Gut. 2002; 51: 290-295Crossref PubMed Scopus (67) Google Scholar]. Recent advances in the genetics of HH have allowed us to identify patients who have mutations in the HFE gene: most cases of HFE-related HH are caused by the C282Y mutation. Accordingly, we are no longer confined to the old definition of HH which identified patients who had abnormal iron studies with stainable iron in hepatocytes on liver biopsy and/or patients who had symptomatic disease and who developed end organ damage including cirrhosis, heart failure, diabetes, arthritis, or skin pigmentation. Presently, if both alleles for HFE have the C282Y mutation and if the individual has direct or indirect markers of iron overload, they should be considered to have HFE-related HH.Table 1Iron overload conditionsHereditary hemochromatosis HFE-related C282Y/C282Y C282Y/H63D Other HFE mutations Non-HFE-related Juvenile hemochromatosis (HFE 2) Transferrin receptor-2 mutations (HFE 3) Ferroportin 1 mutations (HFE 4) African iron overloadSecondary iron overload Acquired iron overload Iron-loading anemias Thalassemia major Sideroblastic anemia Chronic hemolytic anemia Aplastic anemia Pyruvate kinase deficiency Pyridoxine-responsive anemia Parenteral iron overload Red blood cell transfusions Iron–dextran injections Long term hemodialysis Chronic liver disease Porphyria cutanea tarda Hepatitis C Hepatitis B Alcoholic liver disease Non-alcoholic steatohepatitis Following portocaval shunt Dysmetabolic iron overload syndrome Miscellaneous Neonatal iron overload Acerulopasminemia Congenital atransferrinemia Open table in a new tab With the advent of HFE genotyping, population studies have revealed that a substantial proportion of C282Y homozygotes do not have an increase in iron stores [[10]Adams P.C. Nonexpressing homozygotes for C282Y hemochromatosis: minority or majority of cases?.Mol Genet Metab. 2000; 71: 81-86Crossref PubMed Scopus (45) Google Scholar]. These individuals do not show evidence of phenotypic expression of the disorder and their long-term prognosis is not clear. The sub-classification of patients based on their genetics and phenotype has focused on three characteristics: (1) their genetic susceptibility, (2) their iron study results, and (3) their symptoms. Individuals with inherited iron overload syndromes can be sub-classified into four distinct groups: (1) genetic predisposition with no other abnormalities, (2) iron overload without symptoms, (3) iron overload with early symptoms, and (4) iron overload with organ damage. This classification system has become increasingly popular for patients with HFE-related HH.Over the last several years, it has become increasingly recognized that there are other inherited forms of iron overload that are not caused by HFE mutations. These include recognition of families with mutations in the genes for ferroportin 1 [11Montosi G. Donovan A. Totaro A. Garuti C. Pignatti E. Cassanelli S. et al.Autosomal-dominant hemochromatosis is associated with a mutation in the ferroportin (SLC11A3) gene.J Clin Invest. 2001; 108: 619-623Crossref PubMed Scopus (485) Google Scholar, 12Njajou O.T. Vaessen N. Joosse M. Berghuis B. van Dongen J.W. Breuning M.H. et al.A mutation in SLC11A3 is associated with autosomal dominant hemochromatosis.Nat Genet. 2001; 28: 213-214Crossref PubMed Scopus (437) Google Scholar] and transferrin receptor-2 (TfR-2) [13Camaschella C. Roetto A. Cali A. De Gobbi M. Garozzo G. Carella M. et al.The gene TFR2 is mutated in a new type of haemochromatosis mapping to 7q22.Nat Genet. 2000; 25: 14-15Crossref PubMed Scopus (726) Google Scholar, 14Roetto A. Totaro A. Piperno A. Piga A. Longo F. Garozzo G. et al.New mutations inactivating transferrin receptor 2 in hemochromatosis type 3.Blood. 2001; 97: 2555-2560Crossref PubMed Scopus (216) Google Scholar]. A mutant mouse model for TfR-2-related HH has recently been developed [[15]Fleming R.E. Ahmann J.R. Migas M.C. Waheed A. Koeffler H.P. Kawabata H. et al.Targeted mutagenesis of the murine transferrin receptor-2 gene produces hemochromatosis.Proc Natl Acad Sci USA. 2002; 99: 10653-10658Crossref PubMed Scopus (212) Google Scholar]. Juvenile hemochromatosis (linked to a locus on chromosome 1q) [[16]Roetto A. Totaro A. Cazzola M. Cicilano M. Bosio S. D'Ascola G. et al.Juvenile hemochromatosis locus maps to chromosome 1q.Am J Hum Genet. 1999; 64: 1388-1393Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar] and African iron overload [17Gordeuk V. Mukiibi J. Hasstedt S.J. Samowitz W. Edwards C.Q. West G. et al.Iron overload in Africa. Interaction between a gene and dietary iron content.N Engl J Med. 1992; 326: 95-100Crossref PubMed Scopus (263) Google Scholar, 18Kasvosve I. Gangaidzo I.T. Gomo Z.A. Gordeuk V.R. African iron overload.Acta Clin Belg. 2000; 55: 88-93PubMed Google Scholar] are also considered to be inherited disorders. African iron overload is caused by increased absorption of iron that occurs in some individuals from sub-Saharan Africa and in some African–Americans.Secondary iron overload occurs when there is a stimulus to absorb increased amounts of iron from the gastrointestinal tract that is independent from HH [[19]Bottomley S.S. Secondary iron overload disorders.Semin Hematol. 1998; 35: 77-86PubMed Google Scholar]. Examples include the various iron-loading anemias such as thalassemia major, sideroblastic anemia, and certain hemolytic anemias. Some patients with chronic liver disease such as non-alcoholic steatohepatitis, hepatitis C, and alcoholic liver disease have mild degrees of secondary iron overload [20Bacon B.R. Faravesh M.J. Janney C.G. Neuschwander-Tetri B.A. Nonalcoholic steatohepatitis: an expanded clinical entity.Gastroenterology. 1994; 107: 1102-1109Google Scholar, 21Agrawal S. Bonkovsky H.L. Management of nonalcoholic steatohepatitis: an analytic review.J Clin Gastroenterol. 2002; 35: 253-261Crossref PubMed Scopus (38) Google Scholar, 22Di Bisceglie A.M. Axiotis C.A. Hoofnagle J.H. Bacon B.R. Measurement of iron status in patients with chronic hepatitis.Gastroenterology. 1992; 102: 2108-2113Abstract PubMed Google Scholar, 23Ioannou G.N. Tung B.Y. Kowdley K.V. Iron in hepatitis C: villain or innocent bystander?.Semin Gastrointest Dis. 2002; 13: 95-108PubMed Google Scholar, 24Chapman R.W. Morgan M.Y. Laulicht M. Hoffbrand A.V. Sherlock S. Hepatic iron stores and markers of iron overload in alcoholics and patients with idiopathic hemochromatosis.Dig Dis Sci. 1982; 27: 909-916Crossref PubMed Scopus (204) Google Scholar]. These patients usually have iron deposition within sinusoidal lining cells and is not limited to hepatocytes as is seen with HFE-related HH.Parenteral iron overload is caused by transfusions of red blood cells or injections of iron–dextran given by physicians to patients who are anemic [[25]Bacon B.R. Tavill A.S. Hemochromatosis and the iron overload syndromes.in: Zakim D. Boyer T.D. Hepatology. A textbook of liver disease. 3rd ed. Philadelphia, PA, Saunders1996: 1439-1472Google Scholar]. Some patients with ineffective erythropoiesis (e.g., thalassemia) have increased iron loading from both the stimulus caused by anemia and from receipt of blood transfusions [19Bottomley S.S. Secondary iron overload disorders.Semin Hematol. 1998; 35: 77-86PubMed Google Scholar, 25Bacon B.R. Tavill A.S. Hemochromatosis and the iron overload syndromes.in: Zakim D. Boyer T.D. Hepatology. A textbook of liver disease. 3rd ed. Philadelphia, PA, Saunders1996: 1439-1472Google Scholar]. Neonatal iron overload is a rare disorder that causes serious liver injury in infants and is almost always fatal [26Silver M.M. Beverley D.W. Valberg L.S. Cutz E. Phillips M.J. Shaheed W.A. et al.Perinatal hemochromatosis. Clinical, morphologic, and quantitative iron studies.Am J Pathol. 1987; 128: 538-554PubMed Google Scholar, 27Knisely A.S. Neonatal hemochromatosis.Adv Pediatr. 1992; 39: 383-403PubMed Google Scholar].3. HFE gene and proteinIn the 1970s, Marcel Simon and colleagues identified HH as a disorder that is linked to a genetic locus on the short arm of chromosome 6 in the region of HLA-A3 and that is inherited in an autosomal recessive fashion [[3]Simon M. Bourel M. Genetet B. Fauchet R. Idiopathic hemochromatosis. Demonstration of recessive transmission and early detection by family HLA typing.N Engl J Med. 1977; 297: 1017-1021Crossref PubMed Scopus (306) Google Scholar]. About 20 years later, Feder and colleagues used a positional cloning technique to identify a novel gene called HFE in the HLA region of chromosome 6 [[4]Feder J.N. Gnirke A. Thomas W. Tsuchihashi Z. Ruddy D.A. Basava A. et al.A novel MHC class 1-like gene is mutated in patients with hereditary haemochromatosis.Nat Genet. 1996; 13: 399-409Crossref PubMed Scopus (3327) Google Scholar]. This gene encodes a MHC class 1-like protein that binds to β2-microglobulin (β2M) (like other MHC class 1 molecules) and that interacts with TfR-1 [7Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar, 8Britton R.S. Fleming R.E. Parkkila S. Waheed A. Sly W.S. Bacon B.R. Pathogenesis of hereditary hemochromatosis: genetics and beyond.Semin Gastrointest Dis. 2002; 13: 68-79PubMed Google Scholar, 9Trinder D. Fox C. Vautier G. Olynyk J.K Molecular pathogenesis of iron overload.Gut. 2002; 51: 290-295Crossref PubMed Scopus (67) Google Scholar, 28Lebron J.A. Bennett M.J. Vaughn D.E. Chirino A.J. Snow P.M. Mintier G.A. et al.Crystal structure of the hemochromatosis protein HFE and characterization of its interaction with transferrin receptor.Cell. 1998; 93: 111-123Abstract Full Text Full Text PDF PubMed Scopus (555) Google Scholar]. HFE protein has a large extracellular domain, a single transmembrane region, and a short cytoplasmic tail. In the initial study published in 1996, two missense mutations were identified in HFE [[4]Feder J.N. Gnirke A. Thomas W. Tsuchihashi Z. Ruddy D.A. Basava A. et al.A novel MHC class 1-like gene is mutated in patients with hereditary haemochromatosis.Nat Genet. 1996; 13: 399-409Crossref PubMed Scopus (3327) Google Scholar]. One results in the change of a cysteine to a tyrosine at amino acid position 282 (C282Y). This amino acid substitution prevents the binding of HFE protein to β2M by disrupting a disulfide bridge in the extracellular domain and thereby decreases the amount of HFE protein expressed on the cell surface [7Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar, 8Britton R.S. Fleming R.E. Parkkila S. Waheed A. Sly W.S. Bacon B.R. Pathogenesis of hereditary hemochromatosis: genetics and beyond.Semin Gastrointest Dis. 2002; 13: 68-79PubMed Google Scholar, 9Trinder D. Fox C. Vautier G. Olynyk J.K Molecular pathogenesis of iron overload.Gut. 2002; 51: 290-295Crossref PubMed Scopus (67) Google Scholar]. The second mutation in HFE results in a change in histidine to aspartate at amino acid position 63 (H63D) and is found in 15–20% of the general population [29Bacon B.R. Powell L.W. Adams P.C. Kresina T.F. Hoofnagle J.H. Molecular medicine and hemochromatosis: at the crossroads.Gastroenterology. 1999; 116: 193-207Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar, 30Adams P. Brissot P. Powell L.W. EASL international consensus conference on haemochromatosis.J Hepatol. 2000; 33: 485-504Abstract Full Text Full Text PDF PubMed Google Scholar]. This abnormality does not inhibit the binding of HFE protein to β2M, and individuals who are homozygous for the H63D mutation have only mild iron accumulation [[31]Gochee P.A. Powell L.W. Cullen D.J. Du Sart D. Rossi E. Olynyk J.K. A population-based study of the biochemical and clinical expression of the H63D hemochromatosis mutation.Gastroenterology. 2002; 122: 646-651Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar]. However, some individuals who carry both the C282Y and H63D mutations (compound heterozygotes) have clinically significant iron overload [29Bacon B.R. Powell L.W. Adams P.C. Kresina T.F. Hoofnagle J.H. Molecular medicine and hemochromatosis: at the crossroads.Gastroenterology. 1999; 116: 193-207Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar, 30Adams P. Brissot P. Powell L.W. EASL international consensus conference on haemochromatosis.J Hepatol. 2000; 33: 485-504Abstract Full Text Full Text PDF PubMed Google Scholar]. More recently, another mutation has been identified in HFE which results in a change of serine to cysteine at amino acid position 65 (S65C), which is very close to the H63D mutation. This mutation occurs infrequently but some C282Y/S65C individuals have mild iron loading [[32]Asberg A. Thorstensen K. Hveem K. Bjerve K.S. Hereditary hemochromatosis: the clinical significance of the S65C mutation.Genet Test. 2002; 6: 59-62Crossref PubMed Scopus (36) Google Scholar]. In the original work by Feder and colleagues, 83% of 178 phenotypic HH patients were found to be homozygous for the C282Y mutation and 4% were compound heterozygotes (C282Y/H63D) [[4]Feder J.N. Gnirke A. Thomas W. Tsuchihashi Z. Ruddy D.A. Basava A. et al.A novel MHC class 1-like gene is mutated in patients with hereditary haemochromatosis.Nat Genet. 1996; 13: 399-409Crossref PubMed Scopus (3327) Google Scholar].Numerous other genotyping studies have confirmed the findings of Feder et al. and demonstrate that approximately 85% of typical HH patients from around the world are homozygous for C282Y [29Bacon B.R. Powell L.W. Adams P.C. Kresina T.F. Hoofnagle J.H. Molecular medicine and hemochromatosis: at the crossroads.Gastroenterology. 1999; 116: 193-207Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar, 30Adams P. Brissot P. Powell L.W. EASL international consensus conference on haemochromatosis.J Hepatol. 2000; 33: 485-504Abstract Full Text Full Text PDF PubMed Google Scholar] although there is some regional variation [[33]Carella M. D'Ambrosio L. Totaro A. Grifa A. Valentino M.A. Piperno A. et al.Mutation analysis of the HLA-H gene in Italian hemochromatosis patients.Am J Hum Genet. 1997; 60: 828-832PubMed Google Scholar]. The development of mice with targeted disruption of the HFE gene (HFE knockout mice) validated the importance of HFE in the development of iron overload [34Zhou X.Y. Tomatsu S. Fleming R.E. Parkkila S. Waheed A. Jiang J. et al.HFE gene knockout produces mouse model of hereditary hemochromatosis.Proc Natl Acad Sci USA. 1998; 95: 2492-2497Crossref PubMed Scopus (489) Google Scholar, 35Levy J.E. Montross L.K. Cohen D.E. Fleming M.D. Andrews N.C. The C282Y mutation causing hereditary hemochromatosis does not produce a null allele.Blood. 1999; 94: 9-11Crossref PubMed Google Scholar, 36Bahram S. Gilfillan S. Kuhn L.C. Moret R. Schulze J.B. Lebeau A. et al.Experimental hemochromatosis due to MHC class I HFE deficiency: immune status and iron metabolism.Proc Natl Acad Sci USA. 1999; 96: 13312-13317Crossref PubMed Scopus (154) Google Scholar]. These HFE knockout mice have increased iron absorption, elevated transferrin saturation levels and increased hepatocellular iron storage similar to humans with HFE-linked HH [34Zhou X.Y. Tomatsu S. Fleming R.E. Parkkila S. Waheed A. Jiang J. et al.HFE gene knockout produces mouse model of hereditary hemochromatosis.Proc Natl Acad Sci USA. 1998; 95: 2492-2497Crossref PubMed Scopus (489) Google Scholar, 35Levy J.E. Montross L.K. Cohen D.E. Fleming M.D. Andrews N.C. The C282Y mutation causing hereditary hemochromatosis does not produce a null allele.Blood. 1999; 94: 9-11Crossref PubMed Google Scholar, 36Bahram S. Gilfillan S. Kuhn L.C. Moret R. Schulze J.B. Lebeau A. et al.Experimental hemochromatosis due to MHC class I HFE deficiency: immune status and iron metabolism.Proc Natl Acad Sci USA. 1999; 96: 13312-13317Crossref PubMed Scopus (154) Google Scholar, 37Ajioka R.S. Levy J.E. Andrews N.C. Kushner J.P. Regulation of iron absorption in Hfe mutant mice.Blood. 2002; 100: 1465-1469Crossref PubMed Scopus (72) Google Scholar].4. Pathophysiology of hemochromatosisThe primary defect in HFE-related HH is an increase in intestinal iron absorption relative to body iron stores [29Bacon B.R. Powell L.W. Adams P.C. Kresina T.F. Hoofnagle J.H. Molecular medicine and hemochromatosis: at the crossroads.Gastroenterology. 1999; 116: 193-207Abstract Full Text Full Text PDF PubMed Scopus (267) Google Scholar, 30Adams P. Brissot P. Powell L.W. EASL international consensus conference on haemochromatosis.J Hepatol. 2000; 33: 485-504Abstract Full Text Full Text PDF PubMed Google Scholar, 38Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar, 39Fleming R.E. Sly W.S. Mechanisms of iron accumulation in hereditary hemochromatosis.Annu Rev Physiol. 2002; 64: 663-680Crossref PubMed Scopus (145) Google Scholar]. Recent advances have provided a better understanding of the proteins involved in maintaining iron homeostasis and their potential involvement in the pathophysiology of HH.Normally, total body iron content ranges from 3 to 5 g, and 20–30 mg of iron are recycled daily, mainly through phagocytosis of senescent erythrocytes by macrophages [[38]Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar]. Only 1–2 mg of iron per day are typically lost from the body, and females have an increased physiologic loss of iron through menstruation. Accordingly, 1–2 mg of iron are absorbed daily from the diet to maintain normal iron homeostasis [38Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar, 39Fleming R.E. Sly W.S. Mechanisms of iron accumulation in hereditary hemochromatosis.Annu Rev Physiol. 2002; 64: 663-680Crossref PubMed Scopus (145) Google Scholar]. Homeostasis of iron is achieved through a complex array of biochemical processes within the crypt cell of the duodenum, the enterocyte on the duodenal villus, the hepatocyte, and the cells of the reticuloendothelial system. Iron absorption is regulated by two principal factors, the ‘store regulator’ and the ‘erythroid regulator’ [38Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar, 40Finch C. Regulators of iron balance in humans.Blood. 1994; 84: 1697-1702Crossref PubMed Google Scholar]. The store regulator is responsive to the amount of iron stored in the body and one component of this regulator may be circulating diferric transferrin. Another component of the store regulator may be hepcidin, a peptide expressed primarily in the liver, that may function as a humoral factor linking the level of hepatic iron stores to the rate of iron absorption in the duodenum [41Nicolas G. Bennoun M. Devaux I. Beaumont C. Grandchamp B. Kahn A. et al.Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice.Proc Natl Acad Sci USA. 2001; 98: 8780-8785Crossref PubMed Scopus (1075) Google Scholar, 42Fleming R.E. Sly W.S. Hepcidin: a putative iron-regulatory hormone relevant to hereditary hemochromatosis and the anemia of chronic disease.Proc Natl Acad Sci USA. 2001; 98: 8160-8162Crossref PubMed Scopus (257) Google Scholar]. Hepcidin expression in the liver is induced by iron, and circulating hepcidin may act to downregulate iron absorption. When erythropoiesis is stimulated, iron absorption is enhanced via the erythroid regulator, whose identity is currently unknown [38Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar, 40Finch C. Regulators of iron balance in humans.Blood. 1994; 84: 1697-1702Crossref PubMed Google Scholar].Iron is absorbed primarily by villus enterocytes of the duodenum (Fig. 1). Lumenal ferric iron is reduced to ferrous iron by the ferric reductase, Dcytb [[43]McKie A.T. Barrow D. Latunde-Dada G.O. Rolfs A. Sager G. Mudaly E. Mudaly M. et al.An iron-regulated ferric reductase associated with the absorption of dietary iron.Science. 2001; 291: 1755-1759Crossref PubMed Scopus (821) Google Scholar], and is then transported across the apical membrane by a protein called divalent metal transporter-1 (DMT-1) [44Gunshin H. Mackenzie B. Berger U.V. Gunshin Y. Romero M.F. Baron W.F. Nussberger S. et al.Cloning and characterization of a mammalian proton-coupled metal-ion transporter.Nature. 1997; 388: 482-488Crossref PubMed Scopus (2631) Google Scholar, 45Fleming M.D. Trenor 3rd, C.C. Su M.A. Foernzler D. Beier D.R. Dietrich W.F. et al.Microcytic anaemia mice have a mutation in Nramp2, a candidate iron transporter gene.Nat Genet. 1997; 16: 383-386Crossref PubMed Scopus (1015) Google Scholar]. Iron is subsequently exported from the basolateral surface of the cell by another iron transporter called ferroportin 1 [46Donovan A. Brownlie A. Zhou Y. Shepard J. Pratt S.J. Moynihan J. et al.Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter.Nature. 2000; 403: 776-781Crossref PubMed Scopus (1338) Google Scholar, 47McKie A.T. Marciani P. Rolfs A. Brennan K. Wehr K. Barrow D. et al.A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation.Mol Cell. 2000; 5: 299-309Abstract Full Text Full Text PDF PubMed Scopus (1183) Google Scholar]; during this process, iron is converted back to its ferric form by a ferroxidase called hephaestin [[48]Vulpe C.D. Kuo Y.M. Murphy T.L. Cowley L. Askwith C. Libina N. et al.Hephaestin, a ceruloplasmin homologue implicated in intestinal iron transport, is defective in the sla mouse.Nat Genet. 1999; 21: 195-199Crossref PubMed Scopus (909) Google Scholar] and is subsequently bound to apotransferrin in the portal blood stream. The levels of DMT-1 and ferroportin 1 expressed in villus enterocytes influence how much ionic iron is absorbed from the diet.Duodenal crypt cells are thought to be responsible for the sensing of body iron status, and this process influences the level of iron absorption upon the differentiation of these cells into absorptive enterocytes and migration to the villus tip [38Andrews N.C. Disorders of iron metabolism.N Engl J Med. 1999; 341: 1986-1995Crossref PubMed Scopus (1517) Google Scholar, 39Fleming R.E. Sly W.S. Mechanisms of iron accumulation in hereditary hemochromatosis.Annu Rev Physiol. 2002; 64: 663-680Crossref PubMed Scopus (145) Google Scholar]. Circulating diferric transferrin can be taken up by crypt cells after binding to TfR-1 on the basolateral surface. Crypt cells also express HFE protein, and the complex of HFE protein and β2M binds to TfR-1 [[49]Waheed A. Parkkila S. Saarnio J. Fleming R.E. Zhou X.Y. Tomatsu S. Britton R.S. et al.Association of HFE protein with transferrin receptor in crypt enterocytes of human duodenum.Proc Natl Acd Sci USA. 1999; 96: 1579-1584Crossref PubMed Scopus (212) Google Scholar]. Once diferric transferrin is bound to TfR-1, the complex undergoes endocytosis in a clatharin-coated pit that forms an endocytic vesicle within the cell [[7]Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar]. The internal milieu of this vesicle becomes acidic, and iron is released from transferrin and transported to the cytoplasm. The complex of TfR-1 and apotransferrin then recycles back to the basolateral surface where apotransferrin is released at the higher extracellular pH [[7]Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar]. The concentration of intracellular iron in duodenal crypt cells may play an important role in programming the eventual expression of DMT-1 and ferroportin 1 in daughter enterocytes [50Parkkila S. Niemela O. Britton R.S. Fleming R.E. Waheed A. Bacon B.R. et al.Molecular aspects of iron absorption and HFE expression.Gastroenterology. 2001; 121: 1489-1496Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar, 51Schumann K. Moret R. Kunzle H. Kuhn L.C. Iron regulatory protein as an endogenous sensor of iron in rat intestinal mucosa. Possible implications for the regulation of iron absorption.Eur J Biochem. 1999; 260: 362-372Crossref PubMed Scopus (71) Google Scholar]. This regulation may occur at the level of mRNA translation through the action of the iron regulatory protein/iron responsive element system, or at the level of gene transcription [[51]Schumann K. Moret R. Kunzle H. Kuhn L.C. Iron regulatory protein as an endogenous sensor of iron in rat intestinal mucosa. Possible implications for the regulation of iron absorption.Eur J Biochem. 1999; 260: 362-372Crossref PubMed Scopus (71) Google Scholar].There has been intense interest in investigating the cell biology of HFE protein. A physical association between HFE protein and TfR-1 has been demonstrated in human duodenum, cultured cells and in vitro [7Enns C.A. Pumping iron: the strange partnership of the hemochromatosis protein, a class I MHC homolog, with the transferrin receptor.Traffic. 2001; 2: 167-174Crossref PubMed Scopus (41) Google Scholar, 8Britton R.S. Fleming R.E. Parkkila S. Waheed A. Sly W.S. Bacon B.R. Pathogenesis of hereditary hemochromatosis: genetics and beyond.Semin Gastrointest Dis. 2002; 13: 68-79PubMed Google Scholar, 9Trinder D. Fox C. Vautier G. Olynyk J.K Molecular pathogenesis of iron overload.Gut. 2002; 51: 290-295Crossref PubMed Scopus (67) Google Scholar, 28Lebron J.A. Bennett M.J. Vaughn D.E. Chirino A.J. Snow P.M. Mintier G.A. et al.Crystal structure of the hemochromatosis protein HFE and characterization of its interaction with transferrin receptor.Cell. 1998; 93: 111-123Abstract Full Text Full Text PDF PubMed Scopus (555) Google Scholar, 49Waheed A. Parkkila S. Saarnio J. Fleming R.E. Zhou X.Y. Tomat