The Serine Protease Tmprss6 Regulates Hepcidin Expression, but Its Loss Does Not Cause Systemic Iron Deficiency In the Fetal and Neonatal Periods

Abstract 4258

Mutations in TMPRSS6 (matriptase-2), a transmembrane serine protease expressed by the liver, result in the clinical phenotype of iron refractory iron deficiency anemia (IRIDA). Additionally, common polymorphisms in TMPRSS6 have been associated with variation in laboratory parameters of iron homeostasis in healthy populations. TMPRSS6 increases iron absorption by reducing expression of the hepatic hormone, hepcidin, via down-regulation of a BMP/SMAD signaling cascade. Hepcidin promotes the internalization and degradation of the duodenal iron transporter, ferroportin, thereby inhibiting iron absorption. Previous studies have demonstrated that adult mice with Tmprss6 deficiency exhibit elevated hepatic hepcidin mRNA levels that are associated with decreased hepatic iron stores. In one study, genetic loss of Tmprss6 was shown to result in significant elevation of hepatic hepcidin expression in mice at birth; however, whether this hepcidin elevation was associated with abnormalities in iron homeostasis was not reported. We therefore asked if the elevated hepcidin levels present in newborn Tmprss6^-/- pups correlate with abnormal parameters of iron homeostasis in the fetal or neonatal periods. To answer this question, we intercrossed Tmprss6^+/− mice to generate Tmprss6^+/+, Tmprss6^+/−, and Tmprss6^-/- progeny for phenotypic characterization at either gestational day 17.5 (E17.5) or postnatal day 0 (P0). Consistent with prior observations, Tmprss6^-/- pups at P0 showed a 4.6-fold increase in hepatic hepcidin mRNA compared to Tmprss6^+/+ littermates (p=.006). However, despite this elevation in hepcidin expression, Tmprss6^-/- pups were not pale, and they showed no significant differences in body mass or hepatic non-heme iron concentration compared to Tmprss6^+/+ and Tmprss6^+/− littermates. At E17.5, Tmprss6^-/- fetuses showed a 50-fold increase in hepatic hepcidin mRNA compared to Tmprss6^+/+ littermates (p=.005). However, Tmprss6^-/- fetuses also were not pale, and they showed no significant difference in body mass compared to Tmprss6^+/+ and Tmprss6^+/− littermates. Surprisingly, hepatic non-heme iron concentration at E17.5 was significantly higher in Tmprss6^-/- fetuses than in Tmprss6^+/+ fetuses (p=.003). To determine if the increased hepcidin expression of Tmprss6^-/- fetuses might affect iron homeostasis in their pregnant mothers, we measured iron parameters in Tmprss6^+/− females gestating E17.5 litters that were enriched for either Tmprss6^+/+ or Tmprss6^-/- fetuses. No significant effects of fetal genotype on maternal iron parameters were observed. In summary, our results demonstrate that Tmprss6 regulates hepcidin expression in the fetal and neonatal periods in mice. However, Tmprss6 deficiency does not appear to be associated with systemic iron deficiency at these stages of development, and fetal Tmprss6 expression does not have a significant effect on maternal iron homeostasis in late gestation. These results may have implications for understanding the maintenance of iron homeostasis in early development, and may provide insight into the evolution of IRIDA as well as other disorders of iron homeostasis.