Pathogenic variants in the cellular iron exporter ferroportin (SLC40A1) cause hepatic and splenic iron overload. Low to normal transferrin saturation (TSAT) and iron accumulation in Kupffer cells with high splenic iron distinguish ferroportin disease (FD) from SLC40A1-related hemochromatosis (SLC40A1-HC), which are both caused by variants in SLC40A1. The aim of our study was to describe pathogenic mutations in SLC40A1, phenotypic variability in affected patients and compare outcomes with HFE-related hemochromatosis (HFE-HC). The international EASL non-HFE hemochromatosis patient registry prospectively collected clinical, radiological, biochemical, and genetic data for 95 patients with SLC40A1 variants from six centers. Additionally, 363 patients were identified by a systematic literature review. As a comparator, 603 patients diagnosed with HFE-HC were included. The FD phenotype presented in 65.5% of affected individuals. Patients with FD were younger at diagnosis and more often female than those with SLC40A1-HC. SLC40A1 variants were associated with higher hepatic and splenic iron concentrations compared to the HFE-HC group. Variability in phenotypic presentation was high among patients with SLC40A1 variants, and a genotype-to-phenotype correlation could only explain a small proportion of this variation. Variants that directly affect the metal binding site in ferroportin more likely presented with high TSAT. Patients with the SLC40A1-HC phenotype (TSAT >45%) had a higher risk of fibrosis. Life expectancy was similar between patients with SLC40A1 variants and matched patients with HFE-HC. Most individuals with SLC40A1 variants (73.2%) received regular phlebotomies, which were not associated with differences in life expectancy. Mutations in SLC40A1 cause a highly variable disease spectrum with hepatic and splenic iron overload. Fibrosis risk is higher in patients with elevated TSAT. Clinical management of individuals with SLC40A1 variants has largely been extrapolated from HFE-related hemochromatosis despite fundamental pathophysiological differences. Our study provides detailed phenotypic characterization that supports diagnosis and distinction of these rare iron overload disorders. Long-term follow-up shows preserved life expectancy, unaffected by phlebotomy, underscoring the need to critically assess phlebotomy on an individualized basis. Patients with SLC40A1-related hemochromatosis (transferrin saturation >45%) had a higher prevalence of chronic liver disease than those with ferroportin disease, suggesting that elevated transferrin saturation and hepatic iron drive disease progression, which can guide risk stratification and clinical decision making. Not applicable.

Iron overload disorders pose complex clinical challenges. Hinokitiol (HK) is a promising therapeutic candidate as it can shuttle iron across cell membranes and release sequestered intracellular iron, especially for conditions like Ferroportin Disease (FD), where phlebotomy and iron chelation are often inadequate or burdensome. However, HK's clinical utility is limited by its physical-chemical properties and biodistribution. In this study, we report the first evaluation of biodegradable and biocompatible Nanomedicines (NMeds) developed for the parenteral administration of HK. Polymeric and lipidic NMeds were evaluated, namely poly lactic-co-glycolide (PLGA), Cholesterol (Chol) NMeds, and Nanostructured Lipid Carriers (NLC). The optimization led to homogeneous NMeds with size < 300 nm and encapsulation efficiency up to 40 %, despite the small molecular weight and volatile nature of HK. Drug retention ability was assessed, allowing for the selection of 3 NMeds to be tested on iron-loaded macrophage model (J774 cell line) and human primary macrophages obtained from healthy blood donors and FD patient-bearing two different mutations. Data revealed that all HK-loaded NMeds are non-toxic and can accumulate in the cells, but most importantly they are more efficient than the free HK in reducing the intracellular iron pool. NLCs in particular showed the most promising behavior in terms of high efficacy and low toxicity. These results demonstrate that delivering HK via NMeds is preferable to administering free HK, representing the first step towards the development of a more efficient treatment of this currently challenging disease.

The Major Facilitator Superfamily (MFS) is the largest known family of secondary transporters. These proteins share a common architecture comprising two lobes, each including 6 transmembrane (TM) helices, related by twofold pseudosymmetry. They transport a wide range of substrates through large conformational changes relying on the opening and closing of gates located on either side of biological membranes. Human ferroportin 1 (HsFPN1), the sole characterized mammalian iron exporter, follows this pattern. It is, however, characterized by an unusual intracellular gate, formed by two asymmetric networks of non-covalent bonds linking the two lobes. We studied the behavior of these networks in all-atom molecular dynamics simulations and functionally assessed the effect of alanine substitutions on HsFPN1 plasma membrane expression and iron export activity. We identified two new critical residues, Arg156 and Tyr318, connecting the networks to each other and to one of two metal-coordinating sites, located in an unwound region of TM7. We extended the analysis to a previously unreported missense variation, p.Gln478Arg, which was found to have a very strong impact on one of the two inter-lobe connection networks, and to result in a significant HsFPN1 loss-of-function. This led us to present the p.Gln478Arg substitution as a new pathogenic variation causing ferroportin disease. Together, our results provide new insights into the structure and dynamics of the human FPN1 inner gate and its asymmetry, shedding light on its potential role in the mechanism of iron export while offering a framework to better understand previously unexplained clinical observations.

Beyond the classic HFE-hemochromatosis, several genetic iron-loading disorders arise from mutations in genes regulating iron homeostasis, such as TFR2, HAMP, HJV, and the SLC40A1 (ferroportin) gene, as well as those involved in iron transport and mitochondrial function. These disorders lead to systemic or localized iron overload, resulting in complications such as liver disease, cardiomyopathy, endocrine dysfunction, and neurodegeneration. This chapter reviews the genetic basis, clinical presentations, and therapeutic approaches, emphasizing the importance of early diagnosis and intervention to mitigate organ damage and improve outcomes.

Heterozygous mutations in SLC40A1, encoding a multi-pass membrane protein of the major facilitator superfamily known as ferroportin 1 (FPN1), are responsible for two distinct hereditary iron-overload diseases: ferroportin disease, which is associated with reduced FPN1 activity (i.e., decrease in cellular iron export), and SLC40A1-related hemochromatosis, which is associated with abnormally high FPN1 activity (i.e., resistance to hepcidin). Here, we report three SLC40A1 missense variants with opposite functional consequences. In cultured cells, the p.Arg40Gln and p.Ser47Phe substitutions partially reduced the ability of FPN1 to export iron and also partially reduced its sensitivity to hepcidin. The p.Ala350Val substitution had more profound effects, resulting in low FPN1 iron egress and weak FPN1/hepcidin interaction. Structural analyses helped to differentiate the first two substitutions, which are predicted to cause local instabilities, and the third, which is thought to prevent critical rigid-body movements that are essential to the iron transport cycle. The phenotypic traits observed in a total of 12 affected individuals are highly suggestive of ferroportin disease. Our findings dismantle the classical dualism of FPN1 loss versus gain of function, highlight some specific and unexpected functions of FPN1 transmembrane helices in the molecular mechanism of iron export and its regulation by hepcidin, and extend the spectrum of rare genetic variants that may cause ferroportin disease.