The Clinical and Genetic Spectrum of TMPRSS6 Mutations Leading to Inappropriate Hepcidin Expression and Iron Refractory Iron Deficiency Anemia (IRIDA).

Introduction: The peptide hormone hepcidin is a negative regulator of iron efflux from cells. In iron deficiency anemia (IDA) hepcidin levels are ordinarily undetectable, thereby stimulating the release of iron from macrophages, hepatocytes and duodenal enterocytes, promoting iron availability for erythropoieisis and increasing iron stores. Rare individuals and kindreds with lifelong hypoferremia, microcytic red blood cells and IDA unresponsive to enteral iron therapy and only an incomplete, transient response to parenteral iron supplementation—so-called iron refractory iron deficiency anemia—have been described. We previously reported that mutations in the hepatocyte-specific transmembrane serine protease TMPRSS6 cause IRIDA and that the physiology of the phenotype can be attributed to inappropriately elevated hepcidin levels (Nat Genet, 2008, 40:561). Here, we extend our description of the homozygous TMPRSS6-mutated IRIDA phenotype, identify disease-causing mutations in an additional probands, including the original family reported by Buchanan and Sheehan (J Pediatr, 1981, 98:72) and describe the phenotype of heterozygous TMPRSS6 deficiency. Methods: Individuals and kindreds with IRIDA were recruited in an IRB approved protocol and assessed for hematologic and biochemical markers of iron status, inflammation, urinary and plasma hepcidin and TMPRSS6 sequencing by previously described methods. Given that the transferrin saturation (TfSat) appears to have a central role in hepcidin regulation, an index relating the hepcidin to transferrin saturation, TfSat/log₁₀hepcidin, was evaluated to attempt to distinguish inappropriate from appropriate hepcidin expression in probands and their family members. Results: Seven additional IRIDA probands had biallelic TMPRSS6 mutations identified including 9 novel mutations (L62fs, T182fs, Y191X, C245F, L281fs, Q238fs, C557S, L671fs, G706C, W775X) and 3 previously described mutations (G422R, E522K, R599X). Five additional probands each had only single mutations identified including 3 novel mutations (A80V, R93C, IVS4+1G>T) and 2 previously described mutations (E522K, IVS7+1G>A). All disease-associated missense variants were not present in the NCBI and Ensembl SNP databases as well as 100 control chromosomes. Each of these patients had elevated absolute urinary and/or plasma hepcidin concentrations. When hepcidin levels were normalized for serum transferrin saturation (‘Hepcidin Index’ = TfSat/log₁₀hepcidin) probands with bialleleic mutations were easily distinguished from wild-type family members. Heterozygous family members had normal hematological and biochemical iron studies, but had an intermediate Hepcidin Index, indicative of a codominant phenotype. Several other individuals referred for a clinically milder IRIDA phenotype were found to have heterozygous mutations, suggesting that some heterozygotes may manifest clinically apparent disease. In addition, several individuals with a severe IRIDA phenotype were found to have high hepcidin levels in the absence of evidence of inflammation and TMPRSS6 mutations, raising the possibility that IRIDA is a genetically heterogeneous disorder. Discussion/Conclusions:TMPRSS6 mutations leading to inappropriately elevated hepcidin levels explain many, but not all, patients with the IRIDA phenotype. We add nine new mutations to previously described mutations in TMPRSS6. The Hepcidin Index (TfSat/log₁₀hepcidin) can assist in distinguishing those with iron deficiency from those likely to haveTMPRSS6 mutations.