Reduction of iron overload in hereditary hemochromatosis model mice by presence of iron metabolism mutant

Abstract Hereditary Hemochromatosis (HH) is an iron overloading disease for which repeated phlebotomy, followed by maintenance phlebotomy, are standard of care treatments along with the use of iron chelator drugs. Patient compliance during maintenance phlebotomy has been noted as a challenge in treating HH patients. The primary mutation in human HH patients is a C282Y mutation in the Hereditary Iron (HFE) gene. The study of an iron deficient mouse mutant, flaky skin (gene symbol fsn), has previously shown that fsn mutant mice are iron deficient due to excretion of 100X more iron in urine than normal wildtype (+/+, +/+) mice. We have undertaken a study using an Hfe gene knockout mouse to show the impact of the presence of the fsn mutation on iron overload. This study may uncover a pharmaceutical target for alternate treatment of HH. Double mutant mice (Hfe -/-, fsn/fsn) show prevention of liver iron overload in the HH mouse model, whereas Hfe knockout mice (Hfe -/-, +/+) show typical liver iron overload. Dry weight liver tissue iron levels are normal in double mutants (142 ±31 µg/g iron) whereas dry weight liver tissue iron levels in Hfe knockout mice show extensive iron overload (1,528 ±195 µg/g iron). In addition, the presence of the fsn mutation in the heterozygous state (Hfe -/-, +/fsn) also shows significant reduction of dry weight liver tissue iron (695 ±260 µg/g iron) compared to wild type mice (281 ±77 µg/g iron). The fsn mutation lies in the Ttc7 gene (Tetratricopeptide repeat domain 7) and results from the insertion of an early transposon (ETn) which generates an additional in-frame exon in Ttc7 mRNA between exons 14 and 15 of the 20-exon gene. TTC7 is a member of a protein family containing tetratricopeptide (TPR) repeats. The insertion of 61 amino acids interrupts one of the TPR repeats in the protein. The normal size of TTC7 protein in mice is 96 kDa. This mutation produces a larger protein (fsn-TTC7; 103 kDa) for which the protein partners are not known. It has been previously shown that TTC7 acts as a chaperone to traffic PI4KIIIa to the membrane, where it catalyzes the formation of PI-4-P, a precursor of PIP2 (phosphatidylinositol (4,5)-biphosphate). The mechanism by which the fsn-TTC7 protein promotes excessive iron excretion is unknown. Iron is filtered in the glomerulus and reabsorbed in the proximal convoluted tubules, where the iron transporter DMT1 is localized. We hypothesize that the fsn-TTC7 protein leads to reduced iron absorption, resulting in increasing urinary iron loss. This may be due to an impact on DMT1 activity or its associated proteins. Collectively, these studies present results which suggests a new pharmaceutical target can possibly be identified to treat HH by inducing robust urinary iron excretion.