Hepatic expression of hemochromatosis genes in two mouse strains after phlebotomy and iron overload.

BACKGROUND AND OBJECTIVES: Iron homeostasis is tightly regulated in mammals according to the needs of erythropoiesis and the iron stores present. This regulation is disrupted in hereditary hemochromatosis (HH), a genetic disorder characterized by increased intestinal iron absorption, leading to iron overload. The genes coding for HFE, transferrin receptor 2 (TFR2), ferroportin (SLC40A1 or FPN1), hepcidin (HEPC) and hemojuvelin (HJV or RGMC) are responsible for different types of genetic iron overload. All these genes are highly expressed in the liver and their protein products are likely components of a single hepcidin-related pathway. In order to gain insights into the molecular relationship among the HH proteins we evaluated the hepatic expression of HH genes in conditions of iron restriction or overload. DESIGN AND METHODS: Data were obtained after phlebotomy, to activate the erythroid regulators and following parenteral iron dextran loading, to activate the store regulators, in two mice strains (C57BL/6 and DBA/2). HH genes and proteins expression were analyzed by quantitative real time polymerase chain reaction and by Western blotting, respectively. RESULTS: Hepc RNA was reduced after phlebotomy and increased in iron overload. A statistically significant reduction of hepatic Fpn1 RNA expression was observed after phlebotomy; this effect was more evident in the DBA/2 strain. Fpn1 increased in C57BL/6 mice, but not in the DBA/2 ones in parenteral iron loading. Fpn1 protein did not change substantially in either condition. Hfe, Rgmc and Tfr2 expression was not influenced by phlebotomy. In parenteral iron overload, Tfr2 gene and protein expression decreased concomitant to the increase in Hepc, while Hfe RNA remained constant. INTERPRETATION AND CONCLUSIONS: Our results indicate that regulation of hepatic Fpn1 differs from that reported for duodenal Fpn1. Furthermore, taken the differences in gene expression in dietary overload (increased Hfe but not Tfr2), distinct roles are suggested for Hfe and Tfr2 in Hepc activation.

C29G in the iron-responsive element of L-ferritin: a new mutation associated with hyperferritinemia-cataract.

Hyperferritinemia-cataract syndrome (HHCS) is a dominant disorder characterized by high serum ferritin and early onset of bilateral cataract. The disorder is caused by mutations in the iron-responsive element (IRE) of l-ferritin, which disrupt the postranscriptional control of l-ferritin synthesis. Here, we report a new (C>G) mutation which affects base 29 in the loop (c.-169C>G), previously unrecognized as essential for the stem loop stability. The mutation was identified in two members of an Italian family. Computer modeling and electrophoretic mobility shift assay (EMSA) confirm a decreased affinity of the C29G IRE for IRPs control proteins.

Pathogenesis of hyperferritinemia cataract syndrome.

Hereditary hyperferritinemia-cataract syndrome (HHCS) is an autosomal dominant disorder characterized by bilateral cataracts and increased serum L-ferritin, in the absence of iron overload. Under physiological conditions, ferritin synthesis is finely regulated at the translational level by iron availability. This regulation is achieved by the high-affinity interaction between cytoplasmic mRNA-binding proteins (iron regulatory proteins, IRPs), and mRNA stem-loop structures, known as iron responsive elements (IREs), located in the untranslated regions (UTRs) of the mRNAs. A single IRE is located on the 5' UTR of a series of genes involved in iron metabolism, like L-ferritin, and the binding IRE-IRPs represses these genes translation. The deregulation of ferritin production responsible of HHCS is caused by heterogeneous mutations in the iron regulatory element (IRE) of L-ferritin that interfere with the binding of iron regulatory proteins, disrupting the negative control of L-ferritin synthesis and causing the constitutive up-regulation of ferritin L-chains. The HHCS families originate from different countries of Europe and North America, suggesting that HHCS may be distributed widely throughout the world and not sporadic, whereas its prevalence remains to be established. The lens seems to be particularly sensitive to the increased amount of L-ferritin and the alteration of the proteic equilibrium in this tissue can be responsible of the cataract. In spite of the elucidation of the genetic basis, the genotype phenotype correlation is not clear. Recently, a study based on the thermo-denaturation profile and dissociation constant of the IRE-IRP complex performed for several mutated IREs has provided evidence for a possible correlation between heterogeneous IRE mutations and serum ferritin levels. On the other hand, the in vivo relevance of these conclusions has not been determined completely. A clinical variability among subjects sharing the same mutation, whether they belonged to the same family or not, has also been demonstrated. These findings suggest that, besides the L-ferritin IRE genotype, additional factors are likely to modulate the lens involvement and the rate of progression to severe cataract in HHCS patients.