Ferroportin Diseases: Functional Studies, a Link Between Genetic and Clinical Phenotype

Abstract

Ferroportin (FPN) mediates iron export from cells and this function is modulated by serum hepcidin. Mutations in the FPN gene (SLC40A1) lead to autosomal dominant iron overload diseases related either to loss or to gain of function, and usually characterized by normal or low transferrin saturation versus elevated transferrin saturation, respectively. However, for the same mutation, the phenotypic expression may vary from one patient to another. Using in vitro overexpression of wild-type or mutant FPN proteins, we characterized the functional impact of five recently identified FPN gene mutations regarding FPN localization, cell iron status, and hepcidin sensitivity. Our aim was to integrate functional results and biological findings in probands and relatives. We show that while the p.Arg371Gln (R371Q) mutation had no impact on studied parameters, the p.Trp158Leu (W158L), p.Arg88Gly (R88G), and p.Asn185Asp (N185D) mutations caused an iron export defect and were classified as loss-of-function mutations. The p.Gly204Ser (G204S) mutation induced a gain of FPN function. Functional studies are useful to determine whether or not a FPN gene mutation found in an iron overloaded patient is deleterious and to characterize its biological impact, especially when family studies are not fully informative and/or additional confounding factors may affect bio-clinical expression.

Ferroportin disease mutations influence manganese accumulation and cytotoxicity

“…The expression of Fpn-WT resulted in a reduction of ;67% in intracellular ferritin concentrations compared with the control cells. As expected from previous reports (27)(28)(29)(30), cells expressing LOF mutants, G80S, R88G, D157G, D157Y, or V162D were iron loaded, whereas the cells expressing GOF mutants N144H, N144T, C326S, or S338R were iron depleted, similar to those expressing Fpn-WT.…”

“…Studies have shown that the LOF mutations, which lead patients to exhibit the classic phenotype of Kupffer cell iron accumulation, affect the localization of Fpn on the cell surface. Consistently, several studies reported intracellular localization of the G80S, R88G, D157G, D157Y, and V162D Fpn mutants (28)(29)(30)(31)42). However, other studies found localization of G80S (43), R88G (29), D157G, and V162D (44), primarily at the cell surface.…”

“…Previous research has shown that LOF Fpn mutants failed to localize at the plasma membrane (29)(30)(31). Thus, we next validated the reported mislocalization of Fpn mutants.…”

Pathophysiological consequences and benefits of HFE mutations: 20 years of research

“…53 Adult onset non-HFE-HH forms (HH types III and IV) are either caused by mutations in TFR2 encoding transferrin receptor 2, 16 or the hepcidin receptor ferroportin ( FPN/SLC40A1 ), generally referred to as HH type IV. 55 Gain-of-function mutations within ferroportin confer resistance to hepcidin binding and thus prevent ferroportin internalization and degradation (HH type IV, classical ferroportin disease with gain-of-function mutation). As a consequence, uncontrolled iron export from cell types expressing ferroportin, such as duodenal enterocytes or macrophages, causes high levels of iron overload.…”

Functional consequences of transferrin receptor‐2 mutations causing hereditary hemochromatosis type 3

“…In HH type 4 (OMIM #606069) patients have loss‐ or gain‐of‐function mutations in the iron exporter ferroportin that is encoded by the SLC40A1 gene (OMIM *604653; Detivaud et al. ). HH type 3 (OMIM #604250) is a rare form of HH characterized by genetic alterations in the Transferrin receptor 2 ( TFR2 ) gene (OMIM *604720; Camaschella et al.…”