Clinical presentation and molecular pathophysiology of autosomal dominant hemochromatosis caused by a novel ferroportin mutation.

UNLABELLED

Mutations in the SLC40A1 gene, which encodes ferroportin, are associated with autosomal dominant hemochromatosis. Ferroportin is inhibited directly by hepcidin, a key iron-regulatory peptide, and functional consequences of SLC40A1 mutations account for observed phenotypic differences in patients with ferroportin disease. We describe a large pedigree with a novel SLC40A1 mutation and, through in vitro analysis, elucidate the associated molecular mechanism of iron overload. The entire coding sequence of the SLC40A1 gene was sequenced in a pedigree, presenting with autosomal dominant hyperferritinemia. The functional effects of a novel SLC40A1 mutation were studied by overexpression of wild-type and mutant ferroportin fusion proteins in human embryonic kidney cells. Iron export was studied in these cells using (59)Fe transport assays; subcellular localization of ferroportin was examined by way of confocal microscopy. A novel SLC40A1 mutation p.R489K segregated with iron overload in a family with clinical and histopathological signs of macrophage-type ferroportin disease. Human embryonic kidney cells overexpressing p.R489K ferroportin showed decreased iron export capacity when compared with wild-type ferroportin overexpressing cells. Subcellular localization studies demonstrated that p.R489K ferroportin was retained abnormally within an intracellular compartment.

CONCLUSION

We report a novel pathological SLC40A1 variant associated with abnormal cell surface expression of ferroportin due to intracellular retention of the mutant protein. These findings predict macrophage-type ferroportin disease, the phenotype observed in this kindred. Study of the molecular cell biology of ferroportin and its mutants is key to understanding the pathogenesis of this increasingly recognized form of hemochromatosis, which responds poorly to conventional therapy.