Abstract
Background: Mutations in the iron exporter Ferroportin (Fpn) lead to type IV hemochromatosis (Ferroportin Disease, FD), a dominantly inherited disorder with heterogeneous clinical and biochemical patterns. Some patients present with predominant macrophage iron overload (M), marked elevation of serum ferritin, normal-to-low transferrin saturation (TS), and, possibly, iron restricted erythropoiesis. Others present with a phenotype resembling classical HFE-related hemochromatosis, i.e. characterized by high TS and predominant hepatocyte iron overload (H). These differences are thought to reflect heterogeneity in the functional behaviour of Fpn mutant proteins.
Methods: Two unrelated probands referring to the Centre for Iron Overload Disorders in Verona because of non-HFE hemochromatosis were screened for Fpn mutations by DHPLC (Cremonesi L, Br J Haematol 2005). The functional behaviour of mutants Fpn was studied by generating Fpn-GFP constructs transfected into different cell types (HEK293T, Cos7, and mouse bone marrow macrophages), and analyzing their cellular localization, as well as their capabilities to bind hepcidin and export iron (De Domenico I, PNAS 2005). The two mutations were also expressed in zebrafish, to evaluate their impact on iron-dependent erythropoiesis.
Results: Patient 1, a 59 year old male, had clinical, biochemical (TS 74.8%, ferritin 9,000 μg/l), and pathological features (marked iron overload in either macrophages and hepatocytes, absence of overt cirrhosis) somewhat ambiguous, possibly suggesting a type M Fpn variant with late secondary hepatocyte overload. He was found to be heterozygous for the new L233P mutation. Functional studies revealed that Fpn L233P does not appropriately traffic to the cell surface, resulting in inappropriate inhibition by hepcidin. Fpn L233P expression in vivo in zebrafish resulted in iron limited erythropoiesis, consistent with a type M mutation leading to macrophage iron retention. Patient 2, a 59 year old female, had features more clearly suggesting a type M Fpn variant (TS 22.7%, ferritin 1,771 μg/l, macrophage iron load), but tolerated very well phlebotomies without developing signs of anemia. She was found to be heterozygous for the new I152F mutation. Functional studies revealed a unique pattern (never observed until now), since Fpn I152F localized appropriately on cell membrane, bound near normally to hepcidin, but showed a “primary” deficit of iron export capability. I152F expression in zebrafish resulted in a trend towards iron limited erythropoiesis, though quantitatively less clear than L223P.
Conclusions: FD is a heterogeneous disease caused by generally “private” mutations in Fpn. The clinical, biochemical, and pathological features vary depending on the different behaviour of mutant Fpn. In vitro and in vivo molecular expression studies are very useful to clarify the pathophysiogical spectrum of this disease.
Differentiation in vitro of human-mouse teratocarcinoma hybrids.
