Deletion of TMPRSS6 attenuates the phenotype in a mouse model of β-thalassemia

Abstract

Inappropriately low expression of the key iron regulator hepcidin (HAMP) causes iron overload in untransfused patients affected by ␤-thalassemia intermedia and Hamp modulation provides improvement of the thalassemic phenotype of the Hbb th3/؉ mouse. HAMP expression is activated by iron through the bone morphogenetic protein (BMP)-son of mothers against decapentaplegic signaling pathway and inhibited by ineffective erythropoiesis through an unknown "erythroid regulator." The BMP pathway is inactivated by the serine protease TMPRSS6 that cleaves the BMP coreceptor hemojuvelin. Here, we show that homozygous loss of Tmprss6 in Hbb th3/؉ mice improves anemia and reduces ineffective erythropoiesis, splenomegaly, and iron loading. All these effects are mediated by Hamp up-regulation, which inhibits iron absorption and recycling. Because Hbb th3/؉ mice lacking Tmprss6 show residual ineffective erythropoiesis, our results indicate that Tmprss6 is essential for Hamp inhibition by the erythroid regulator. We also obtained partial correction of the phenotype in Tmprss6 haploinsufficient Hbb th3/؉ male but not female mice and showed that the observed sex difference reflects an unequal balance between iron and erythropoiesis-mediated Hamp regulation. Our study indicates that preventing iron overload improves ␤-thalassemia and strengthens the essential role of Tmprss6 for Hamp suppression, providing a proof of concept that Tmprss6 manipulation can offer a novel therapeutic option in this condition. IntroductionThe liver antimicrobial peptide hepcidin (HAMP) is the central regulator of systemic iron homeostasis. HAMP controls the surface expression of the iron exporter ferroportin on duodenal enterocytes and macrophages, modulating iron absorption and recycling. HAMP is activated by the bone morphogenetic proteins (BMP)-son of mothers against decapentaplegic (SMAD) signaling pathway, in response to increased body iron and by the IL-6-signal transducer and activator of transcription (STAT)3 pathway in inflammation. 1 The glycosylphosphatidylinositol (GPI)-anchored protein hemojuvelin (HJV) is a BMP coreceptor and homozygous mutations of HAMP or HJV cause juvenile hemochromatosis in humans 2,3 and severe iron overload in mice. 4,5 The BMP pathway is inhibited by matriptase-2 (MT-2), a type II transmembrane serine protease encoded by the transmembrane proteaSe serine 6 (TMPRSS6) gene and mainly expressed in the liver. 6,7 MT-2, by cleaving HJV from the hepatocyte surface, attenuates the BMP-SMAD signaling and down-regulates HAMP expression. 8 Mice deficient for both Tmprss6 and Hjv show markedly decreased Hamp mRNA levels and systemic iron overload, as do Hjv deficient mice, 9 in agreement with Hjv being the serine protease substrate.TMPRSS6 plays an essential role for erythropoiesis: homozygous inactivation of the Tmprss6 gene leads to excessive Hamp production, impaired dietary iron absorption and microcytic anemia in mice, 10,11 and iron-refractory iron deficiency anemia (IRIDA) in humans. [12][13][14][15][16][17] The importan...

Erythroferrone contributes to hepcidin suppression and iron overload in a mouse model of β-thalassemia

“…Amelioration of anemia in this mouse model probably requires higher hepcidin levels and mild iron restriction, which may act to limit a-globin synthesis and reduce the imbalance with b-globin synthesis. 8,18 Extrapolation of these findings to humans with b-thalassemia must take into account important differences in their iron metabolism , Th3/1 mice compared with Th3/1 mice at the age of 6 and 12 weeks. No difference was observed at 3 weeks of age.…”

“…15 Infusions of apotransferrin were shown to ameliorate anemia in a model of mild thalassemia intermedia (th1/th1). 16,17 In the Th3/1 model, moderate increase in hepcidin, caused by transgenic hepcidin overexpression, 8 knockdown of the negative hepcidin regulator matriptase 2, 18,19 or administration of hepcidin agonists, 20 not only decreased iron loading but also improved anemia. 21 The hematological benefits of mild iron restriction seem to be the result of decreased a-globin precipitation, lower reactive oxygen species levels, and decreased apoptosis.…”

-thalassemias: paradigmatic diseases for scientific discoveries and development of innovative therapies

“…154,157 Interestingly, lack of Tmprss6 in Hbb th3/+ mice significantly improved iron overload and anemia, corroborating the notion that increased hepcidin activity could be beneficial in this disorder. 158 In fact, in Hbb th3/+ mice, use of both antisense oligonucleotide (Tmprss6-ASO) and RNA interference (Tmprss6-siRNA) can reduce the synthesis of transmembrane serine protease Tmprss6 by degrading the corresponding mRNA. This led to increased hepcidin expression, decreased Tf-sat and reduction of hemichrome formation and apoptosis in erythroid cells.…”

Manipulation of the hepcidin pathway for therapeutic purposes

“…Targeting TMPRSS6: Nai et al 59 demonstrated that homozygous inactivation of TMPRSS6 in thalassemic th3/+ mice increased hepcidin levels, ameliorated iron overload, and improved ineffective erythropoiesis. The study provided the proof of principle that targeting of Tmprss6, a negative regulator of hepcidin production, may be a useful approach for the therapy of iron overload disorders.…”