S1631 UNEXPECTED DIFFERENCES IN CARDIAC IRON METABOLISM AND SURVIVAL IN HAMP‐KO AND FPN(C326S) MICE
Authors: Altamura S. , Müdder K. , Dewenter M. , Garbowski M. , Galy B. , Vaulont S. , Backs J. , Gröne H.‐J. , Hentze M. , Muckenthaler M. 
Published: June 2019

Background:

The hepcidin‐ferroportin axis is crucial to maintain physiological systemic iron levels. Ferroportin is the only known iron exporter and mediates the release of iron from reticuloendothelial macrophages and from duodenal enterocytes. Hepcidin is a small hormone produced by the liver in response to iron levels. Hepcidin binds to and degrades ferroportin thereby reducing the amount of iron released into the blood stream. Mutations that cause an impairment of this regulatory system cause hereditary hemochromatosis (HH), a genetically inherited primary iron overload disorder.

The Hepcidin knock‐out (Hamp‐ko) mouse is a disease model for HH type 2A. Fpn(C326S) knock‐in mice model HH type 4 due to a point mutation in ferroportin that prevents hepcidin binding.Considering that in both mouse models the hepcidin/ferroportin regulatory circuitry is disrupted, it is expected that they would largely phenocopy each other.

Aims:

By comparing Hamp‐ko and FpnC326S mice we aim to identify new mechanisms responsible for the regulation of iron homeostasis.

Methods:

Hamp‐ko and Fpn(C326S) mice, both in C57BL/6J genetic background, were bred and maintained back to back in the same animal facility under a constant light/dark cycle and with ad libitum access to water and food (Teklad 2018S containing 200 ppm iron). Mice were analyzed at different ages for the major iron parameters (tissue and serum iron content, transferrin saturation, NTBI). Gene expression analysis of the iron‐related genes has been made in SYBR‐green qPCR. Cardiac transcriptome profiling was performed in RNAseq using the Illumina NextSeq 500 platform.

Results:

The first striking difference we obtained from the analysis of the two mouse lines is the survival time. While Hamp‐ko mice reach an age comparable to wild‐type mice, homozygous Fpn(C326S) mice show wasting from 24 weeks onwards and succumb to death around 40 weeks of age. We next analyzed iron‐related parameters and show that tissue iron content is identical for liver, spleen, lung and pancreas at all ages studied. Both disease models show a similar degree of transferrin saturation, which reaches 80% already at 10 weeks of age. Interestingly, toxic NTBI levels are higher in Fpn(C326S) mice, suggesting that this parameter may play a causative role for the reduced life‐span. Strikingly, we detected different iron accumulation in the heart. Already at 10 weeks of age Hamp‐ko mice accumulate more iron in the heart and in particular in cardiomyocytes, a difference that persists with ageing. In both disease models iron is localized to similar degrees in the cardiac smooth muscular cells. Despite the iron overload in cardiomyocytes in Hamp‐ko mice, ultrastructural analysis revealed no alterations in sarcomere and mitochondrial organization, and cardiac function was normal as assessed via transthoracic echocardiography. By contrast, the transcriptome shows major alterations in Hamp‐ko mice, whereby processes such as oxidative stress and expression of solute transporters are affected.

Summary/Conclusion:

This work shows for the first time that the lack of hepcidin and the ferroportin resistance to hepcidin binding do not fully phenocopy each other. Work is ongoing to understand the molecular mechanisms underlying the differences observed on survival and cardiac iron accumulation. Such mechanisms will be important to understand iron overload cardiomyopathy, which contributes to morbidity and mortality in patients with HH.