Iron is highly toxic because it generates tissue-damaging reactive oxygen species by the Fenton reaction. Thus, although necessary for life, careful regulation of all aspects of iron metabolism is critical. A key regulator of iron homeostasis is hepcidin, a peptide hormone produced by the liver. Hepcidin negatively regulates cellular iron export from macrophages, duodenal enterocytes, and hepatocytes by promoting degradation of ferroportin, a transmembrane iron exporter. Human diseases that involve primary or secondary dysregulation of hepcidin include hereditary hemochromatosis (HH) and β-thalassemia intermedia, respectively. HH is an autosomal recessive disorder caused by mutation (C282Y homozygosity) in HFE, a key regulator of hepcidin expression. The iron overload of HH is the result of failed upregulation of hepcidin despite ongoing dietary iron loading, while hepcidin synthesis is suppressed due to ineffective erythropoiesis in β-thalassemia intermedia. The mechanism whereby ineffective erythropoiesis suppresses hepcidin expression is largely speculative although trans acting factors produced in the bone marrow (e.g., GDF-15 and TWSG1) are candidate signaling molecules. The result of dysregulated suppression of hepcidin is iron overload. Hypothetically, patients with such seemingly disparate diseases as HH and β-thalassemia intermedia could be treated by pharmacologic manipulation of hepcidin expression. Indeed, genetic studies using animal models of HH showed that constitutive expression of hepcidin or deletion of Tmprss6, a negative modulator of hepcidin expression, could reverse iron overload. In models of β-thalassemia intermedia, targeted deletion of Tmprss6 decreased iron loading and also reduced ineffective erythropoiesis.
Based upon these and other observations, Schmidt et al. sought to demonstrate that systemic administration of lipid nanoparticle (LNP)-formulated siRNAs designed to silence Tmprss6 (LNP-Tmprss6) could increase hepcidin expression and diminish iron uptake in murine models of HH and b-thalassemia intermedia, while also reducing ineffective erythropoiesis in b-thalassemia intermedia. The first set of experiments showed that they could, in fact, silence Tmprss6 using LNP-Tmprss6, thereby upregulating hepcidin mRNA expression in a dose-dependent fashion. A single infusion of LNP-Tmprss6 decreased Tmprss6 for 14 days, increased hepcidin levels for seven days, and decreased transferrin saturation for nearly a month. Next, these investigators administered LNP-Tmprss6 to Hfe-/- mice to determine whether the HH phenotype could be ameliorated. They found sustained decreases in serum iron, transferrin saturation, and non-heme hepatic iron. There was also an increase in splenic iron, attributed to hepcidininduced sequestration of iron in splenic macrophages, and all of the mice developed a hypochromic, microcytic, iron-deficient anemia by six weeks after onset of therapy. Notably, patients with Tmprss6 deficiency due to inherited mutations of the gene have a similar phenotype as the treated mice (i.e., high serum hepcidin and iron refractory, iron deficiency anemia). The final set of experiments tested the effects of LNP-Tmprss6 in a murine model of β-thalassemia intermedia (Hbbth3/+). These mice have iron overload, similar to humans with β-thalassemia intermedia, due to suppression of hepcidin by ineffective erythropoiesis. As anticipated, LNP-Tmprss6 silenced Tmprss6, increased hepcidin, and decreased serum iron and transferrin saturation. Most interestingly, however, LNP-Tmprss6 decreased ineffective erythropoiesis, as evidenced by higher hemoglobin concentration; prolonged red blood cell (RBC) lifespan; lowered the reticulocyte count; decreased erythropoietin concentration; and reduced splenic volume. Splenic iron was lower, unlike the Hfe-/- mice, likely because of the reduction in splenic volume. RBC membrane-bound α-globin, a pathophysiologic consequence of β-thalassemia, was also markedly decreased, and peripheral blood morphology nearly normalized except for a modest increase in central pallor. Together, these results suggest that iron plays an important but as yet incompletely understood role in the pathobiology of thalassemia.
In Brief
These experiments show the potential of novel therapeutics that manipulate hepcidin expression for a number of disorders characterized by iron overload, both primary (e.g., HH) and secondary (e.g., β-thalassemia intermedia). Here, Schmidt et al. explored RNAi therapeutics, but others have investigated biomimetic “mini-hepcidins”1 and exogenous transferrin in similar animal models.2 Ultimately, such therapies will need to meet a very high standard to supplant phlebotomy, which is both inexpensive and effective, for the resolution of iron overload in typical HH patients. Most intriguing is the therapeutic potential of reducing hepcidin expression in β-thalassemia intermedia, where amelioration of both iron loading and ineffective erythropoiesis is observed.
References
Competing Interests
Dr. Quinn indicated no relevant conflicts of interest.
