Tmprss6 Is a Genetic Modifier of the Hfe-Hemochromatosis Phenotype in Mice.

Abstract 625

HFE-associated hereditary hemochromatosis is an autosomal recessive disorder characterized by inappropriately elevated absorption of dietary iron by the gastrointestinal mucosa, resulting in excessive storage of iron in multiple organs. A significant proportion of individuals who are homozygous for HFE mutations fail to develop clinical symptoms, suggesting that environmental and/or genetic factors may influence the penetrance of this disorder. In vitro and animal studies have revealed that HFE promotes the expression of hepcidin, a circulating hormone produced by the liver that acts to inhibit iron absorption by the duodenum. In contrast, TMPRSS6, a transmembrane serine protease produced by the liver, acts to inhibit hepcidin expression; both humans and mice harboring TMPRSS6 mutations display impaired intestinal iron absorption, resulting in a phenotype of iron-refractory iron deficiency anemia (IRIDA). Here we asked if heterozygous or homozygous loss of Tmprss6 function could modify the iron overload phenotype of Hfe null (Hfe^-/-) mice, a mouse model of human HFE-hemochromatosis. To test this, we bred Hfe^-/- mice to Tmprss6^-/- mice; the latter harbor a targeted disruption of the Tmprss6 serine protease domain and exhibit an IRIDA phenotype. We generated Hfe^-/-Tmprss6^+/+, Hfe^-/-Tmprss6^+/-, and Hfe^-/-Tmprss6^-/- female mice (6-10 mice per genotype), in which parameters of systemic iron homeostasis were compared at eight weeks of age by Student's t test. Consistent with previous study of Hfe^-/- mice, Hfe^-/- mice harboring two wild type Tmprss6 alleles (Hfe^-/-Tmprss6^+/+ mice) showed serum iron concentration, transferrin saturation, and hepatic non-heme iron content that were significantly elevated compared to wild type mice of similar genetic background. Heterozygosity for Tmprss6 mutation, however, markedly reduced the severity of the hemochromatosis phenotype of Hfe^-/- mice. Compared to Hfe^-/- mice with two wild type Tmprss6 alleles, Hfe^-/- mice that were heterozygous for Tmprss6 mutation (Hfe^-/-Tmprss6^+/- mice) showed significant reductions in serum iron concentration (p<0.01), transferrin saturation (p<0.005), and non-heme iron content of liver (p<10^-4). Furthermore, homozygosity for Tmprss6 mutation completely ameliorated the iron overload phenotype of Hfe^-/- mice and in fact led to systemic iron deficiency. Compared to both Hfe^-/-Tmprss6^+/+ and Hfe^-/-Tmprss6^+/- mice, Hfe^-/-Tmprss6^-/- mice showed markedly reduced serum iron concentration (p<10^-7), transferrin saturation (p<10^-10), and non-heme iron content of liver (p<10^-4). Hfe^-/-Tmprss6^-/- mice also displayed iron deficiency anemia and appeared phenotypically similar to previously characterized Tmprss6^-/- mice harboring two wild type copies of Hfe. In summary, these results demonstrate that Tmprss6 is a genetic modifier of the Hfe-hemochromatosis phenotype in mice. These findings suggest that natural genetic variation in the human ortholog TMPRSS6 might modify the clinical penetrance of HFE-hemochromatosis and raise the possibility that pharmacological inhibition of TMPRSS6 activity might prove an effective therapy in this disorder.