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.