Animal models indicate that the antimicrobial peptide hepcidin (HAMP; OMIM 606464) is probably a key regulator of iron absorption in mammals. Here we report the identification of two mutations (93delG and 166C-->T) in HAMP on 19q13 in two families with a new type of juvenile hemochromatosis.

Haemochromatosis is a common recessive disorder characterized by progressive iron overload, which may lead to severe clinical complications. Most patients are homozygous for the C282Y mutation in HFE on 6p (refs 1-5). A locus for juvenile haemochromatosis (HFE2) maps to 1q (ref. 7). Here we report a new locus (HFE3) on 7q22 and show that a homozygous nonsense mutation in the gene encoding transferrin receptor-2 (TFR2) is found in people with haemochromatosis that maps to HFE3.

The hepatic peptide hepcidin is the key regulator of iron metabolism in mammals. Recent evidence indicates that certain forms of hereditary hemochromatosis are caused by hepcidin deficiency. Juvenile hemochromatosis is associated with hepcidin or hemojuvelin mutations, and these patients have low or absent urinary hepcidin. Patients with C282Y HFE hemochromatosis also have inappropriately low hepcidin levels for the degree of iron loading. The relationship between the hemochromatosis due to transferrin receptor 2 (TFR2) mutations and hepcidin was unknown. We measured urinary hepcidin levels in 10 patients homozygous for TFR2 mutations, all with increased transferrin saturation. IntroductionThe hepatic peptide hepcidin is the central regulator of iron absorption in mammals. Evidence is accumulating that the pathophysiology of hemochromatosis, a genetic disorder characterized by deregulation of iron absorption, converges on hepcidin. Total hepcidin deficiency characterizes the severe iron overload of juvenile hemochromatosis, which rarely results from hepcidin-inactivating mutations 1 and more frequently from mutations of the HJV gene encoding hemojuvelin. [2][3] Patients with hemojuvelin-related hemochromatosis have low/undetectable urinary hepcidin levels, 2 suggesting that hemojuvelin protein is an important regulator of hepcidin expression. Hepcidin mRNA is also decreased or inappropriately low for the degree of iron overload both in Hfe-deficient or Hfe (845A/ 845A) (C282Y) mice 4 and in patients with HFE hemochromatosis, 5 implying that HFE is another modulator of hepcidin production in response to iron loading.A rare form of hemochromatosis is due to mutations of transferrin receptor 2 (TFR2), 6 a member of the transferrin receptor family with an unclear function in iron metabolism. TFR2 has a capability of binding and internalizing diferric transferrin. 7 However, cellular iron uptake might not be the function of TFR2 in vivo, because mutational disruption of TFR2, both in humans 7 and in animal models, 8 leads to liver iron accumulation and not to iron restriction. In addition, iron overload that follows TFR2 inactivation occurs early in life, 9 as in juvenile hemochromatosis, although TFR2-related disease runs a milder clinical course. Based on these observations, TFR2 could be another regulator of hepcidin, but its relationship with hepcidin in humans has so far remained speculative.To ascertain the involvement of TFR2 in the hepcidin pathway, we measured urinary hepcidin levels in 10 hemochromatosis patients carrying different TFR2 mutations. Our results show low/absent hepcidin in most patients, except for 2 who had concomitant inflammatory conditions. These results confirm the proposed role of TFR2 as a regulator of hepcidin production. Patients, materials, and methodsClinical data and molecular defects of the patients studied have been previously reported. 9-11 Controls were healthy adult subjects from the laboratory staff and their children. Informed consent was obtained from all subjects involved...

IntroductionErythropoiesis is mainly regulated by the kidney-produced hormone erythropoietin (Epo), which is absolutely required for the survival and proliferation of erythroid progenitors and their terminal differentiation to red cells. 1 The Epo receptor (EpoR) is a type 1 transmembrane protein that belongs to the class 1 cytokine receptor family. Its expression level in erythroid cells is low whatever the differentiation stage and even the most Epo-sensitive cells, colony-forming units erythroid (CFU-Es), or proerythroblasts express less than 1000 EpoRs per cell at their surface. 2 In the absence of Epo, EpoR is believed to be homodimeric, in which each dimer protein is constitutively associated with a Janus kinase-2 (Jak-2) tyrosine-kinase molecule. Epo binding modifies the organization of the receptor complex leading to Jak-2 activation. Association between Jak-2 and the EpoR occurs during the receptor maturation process, most likely before EpoR leaves the endoplasmic reticulum, and is essential for expression of the receptor at the cell surface. 3 However, the mechanisms that control the maturation and the transport of the EpoR to the cell surface remain largely unknown. Indeed, the number of cell surface EpoRs is not related to the synthesis level of this protein and most of these molecules accumulate in the endoplasmic reticulum in EpoRoverexpressing cells. 4 Both functional and direct evidence also suggests that unidentified proteins are associated with EpoR on the cell surface. In particular, chemical cross-linking of radiolabeled Epo to cell surface erythroid cells has frequently revealed the presence of 2 additional proteins with apparent molecular masses of 85 and 100 kDa in the EpoR complex. 2,5-8 Unfortunately, because of the low expression level of the EpoR, these proteins could never be identified up to now. The dramatic progress that has been recently introduced by the application of mass spectrometry methods to protein analysis led us to address the question of the identity of the EpoR-associated proteins.Here, we have purified the EpoR complex from UT7 erythroleukemic cells that express endogenous EpoRs to identify EpoRassociated proteins. Mass spectrometry analysis of the purified proteins revealed the presence of the type 2 transferrin receptor (TfR2) in the EpoR complex.Mammals express 2 transferrin receptors that share similar overall structures but possess specific functions. Most cells including erythroid precursors internalize iron from plasma diferric transferrin through the ubiquitously expressed transferrin receptor 1 (TfR1). A Tfr1 gene deletion is lethal in mice with severe anemia and is not compensated by TfR2. 9 In contrast, TfR2 expression is tissue-restricted with high expression in the liver 10 where it plays a key role in iron metabolism regulation. Indeed, TfR2 contributes to the adaptation of hepcidin production to the needs of the body by sensing the circulating iron bound to transferrin. 11 Familial inactivating or non-sense mutations in the TfR2 gene are responsible for...