β-Thalassemia intermedia (TI), an inherited hemoglobinopathy caused by partial loss of β-globin synthesis, is characterized by anemia, extramedullary hematopoiesis and ineffective erythropoiesis as well as secondary iron overload. Hereditary hemochromatosis (HH) is most frequently caused by mutations in HFE and is marked by excess uptake of dietary iron with concomitant tissue iron overload. In both diseases, increased iron absorption is due to inappropriately low levels of the liver hormone, hepcidin (encoded by Hamp1). The membrane serine protease Matriptase-2 (encoded by Tmprss6) attenuates BMP-mediated Hamp1 induction by cleaving the BMP co-receptor, hemojuvelin. Previously, it has been shown that elevating Hamp1 expression by genetic inactivation of Tmprss6 reduces disease severity in the Hbbth3/+ mouse model of TI and prevents iron overload in Hfe−/− mice. Therefore, a therapeutic approach comprising specific inhibition of Tmprss6 could prove efficacious in TI and HH.
Here we show that systemic administration of a potent lipid nanoparticle (LNP) formulated siRNA directed against Tmprss6 leads to >80% inhibition of Tmprss6 mRNA in the livers of Hbbth3/+ and Hfe−/− mice with concomitant >2-fold elevation in Hamp1 expression. In the TI model, Tmprss6 silencing leads to ∼30% reductions in serum iron and non-heme liver iron. In Hfe−/− mice, serum iron and non-heme liver iron are similarly reduced, and Perls staining of peri-portal iron is diminished. Remarkably, the partial iron restriction induced by Tmprss6 inhibition in Hbbth3/+ mice leads to dramatic improvements in the hematological aspects of the disease phenotype: the severity of the anemia is decreased as evidenced by an approximately 1 g/dL increase in total hemoglobin and a 50% decrease in circulating erythropoietin levels. As in the human disease, Hbbth3/+ mice exhibit the hallmarks of ineffective erythropoiesis including splenomegaly, decreased erythrocyte survival and marked reticulocytosis. Treatment with LNP formulated Tmprss6 siRNA leads to a dramatic 2–3 fold decrease in spleen size, a 3–4 fold decrease in reticulocyte counts and a >7-day increase in RBC half-life. Histological analysis of spleens from Tmprss6 siRNA treated animals demonstrates restoration of normal splenic architecture, as well as a reduction in the number of Tfr1-positive erythrocyte precursors in the spleen. Furthermore, as evidenced by the near normalization of blood smears, the overall quality of erythropoiesis in treated animals is vastly improved.
Taken together, these data demonstrate that RNAi-mediated silencing of liver Tmprss6 elevates Hamp1 expression and reduces iron overload in both TI and HH model mice. More significantly, Tmprss6 siRNA treatment ameliorates all aspects of the disease phenotype in the TI mouse model. These results support the development of an RNAi therapeutic targeting TMPRSS6 for the treatment of TI, HH and potentially other disorders characterized by excess iron absorption due to physiologically inappropriately low levels of hepcidin.
Racie:Alnylam Pharmaceuticals: Employment. Butler:Alnylam Pharmaceuticals, Inc.: Employment, Equity Ownership. Bumcrot:Alnylam Pharmaceuticals, Inc.: Employment, Equity Ownership.
Asterisk with author names denotes non-ASH members.