Hepcidin Resistance Research Articles

Identification of Mutations in SLC40A1 That Affect Ferroportin Function and Phenotype of Human Ferroportin Iron Overload

Patients with ferroportin iron overload due to loss-of-function mutations in SLC40A1 have macrophage iron overload, hyperferritinemia, and normal transferrin saturation. In contrast, hepatocellular iron storage, hyperferritinemia, and increased saturation of transferrin are a distinct clinical presentation of ferroportin iron overload that results from SLC40A1 mutations that confer resistance of ferroportin to hepcidin-mediated inactivation. SLC40A1 was sequenced in patients from 2 independent pedigrees affected by hepatic iron overload unrelated to HFE. Functions of the ferroportin variants were tested in vitro. A patient heterozygous for the variant p.W158C in SLC40A1 presented with macrophage iron overload, hyperferritinemia, and normal transferrin saturation. A patient with hepatocellular iron storage, hyperferritinemia, and increased transferrin saturation was heterozygous for p.H507R. Expression of the p.W158C form of ferroportin in 293T cells resulted in defective trafficking to the plasma membrane and reduced iron export activity; the iron export activity of cells that expressed the p.H507R form of ferroportin did not differ from cells that expressed ferroportin without this mutation. The p.H507R of ferroportin localizes normally to the plasma membrane but is resistant to hepcidin-mediated inactivation. Addition of a synthetic peptide derived from ferroportin without these mutations (amino acids 500-518) decreased the inhibitory activity of hepcidin in cells, whereas a peptide from the same region, with p.H507R, had no effect on hepcidin activity. The variant p.W158C in SLC40A1 impairs intracellular trafficking of ferroportin, resulting in reduced iron export. The variant p.H507R does not bind hepcidin in vitro and results in apparent hepcidin resistance.

Inherited disorders of iron metabolism

To discuss inherited iron disorders, their pathophysiology and clinical implications in the light of the recent advances in our knowledge of iron metabolism and its regulation. In previous years the molecular mechanisms of cellular iron uptake and release and the cellular and systemic iron homeostasis have been substantially clarified. New proteins (hepcidin, hemojuvelin, HFE, TFR2 and ferroportin), mutated in hereditary hemochromatosis, have been identified with a crucial role in iron regulation. These advances have modified our understanding of the pathophysiology of hemochromatosis, now considered a disorder either due to hepcidin deficiency or (rarely) due to hepcidin resistance. Novel genetic forms of iron-related microcytic anemia have been identified, due to defects of iron transport/utilization or to TMPRSS6 deficiency and hepcidin hyperproduction, as occurs in iron-refractory iron deficiency anemia (IRIDA). A role for hepcidin has been identified also in acquired conditions, as in iron-loading anemias and in anemia of chronic diseases and inflammation. Advances in basic research have improved the classification and diagnosis of genetic anemias and iron overload and are paving the way towards the development of drugs that target the molecular lesions.