Hepcidin, the central negative regulator of iron absorption and iron release from macrophages, is upregulated by iron. Mutations in hemojuvelin, Hfe, transferrin receptor 2, and SMAD4 are known to prevent upregulation. Additionally, the bone morphogenetic proteins (BMPs), and the inflammatory cytokines IL-1 and IL-6 stimulate hepcidin gene activation. Downregulation of hepcidin is effected by anemia and hypoxia, but nothing is known of the mechanism through which this occurs. Here we describe the recessive ENU-induced phenotype Mask, so called because affected homozygotes developed regional alopecia in which truncal hair was shed while facial hair was retained. The Mask phenotype was found to be a manifestation of iron deficiency, and was eliminated by correcting the iron deficiency. When fed an iron deficient diet, mutant mice absorbed less iron than controls, as measured by total body 59Fe counting. After reaching a plateau total body counts stabilized, indicating that blood loss did not play a role in the iron deficiency. The level of liver hepcidin mRNA of iron deficient mice is normally greatly decreased; in contrast, the Mask mouse had high liver hepcidin mRNA levels. By positional cloning, we were able to ascribe the Mask phenotype to a splicing error in the Tmprss6 gene, which encodes a membrane-bound serine protease of previously unknown function. The mutation truncates the protein, eliminating the serine protease domain. Transfecting HepG2 cells to express the wildtype TMPRSS6 protein decreased baseline hepcidin reporter activity and almost entirely blunted the hepcidin inducing effect of IL-6, IL-1, hemojuvelin, and the BMPs. A construct encoding the Mask truncation mutant had diminished activity. Thus, TMPRSS6 powerfully down-regulates hepcidin gene transcription in the baseline state and prevents its upregulation by all known stimulators. TMPRSS6 is a non-redundant component of a hepcidin suppression pathway that exerts dominant effect over all known hepcidin inducing pathways, and is required for normal absorption of dietary iron.
Disclosure: Research Funding: National Institutes of Health grants DK53505 and AI054523.