Hematopoietic deletion of transferrin receptor 2 in mice leads to a block in erythroid differentiation during iron-deficient anemia

Free to read\ud \ud Iron metabolism and erythropoiesis are inherently interlinked physiological processes. Regulation of iron metabolism is mediated by the iron-regulatory hormone hepcidin. Hepcidin limits the amount of iron released into the blood by binding to and causing the internalization of the iron exporter, ferroportin. A number of molecules and physiological stimuli, including erythropoiesis, are known to regulate hepcidin. An increase in erythropoietic demand decreases hepcidin, resulting in increased bioavailable iron in the blood. <i>Transferrin receptor 2 (TFR2)</i> is involved in the systemic regulation of iron metabolism. Patients and mice with mutations in <i>TFR2</i> develop hemochromatosis due to inappropriate hepcidin levels relative to body iron. Recent studies from our laboratory and others have suggested an additional role for <i>TFR2</i> in response to iron-restricted erythropoiesis. These studies used mouse models with perturbed systemic iron metabolism: anemic mice lacking matriptase-2 and <i>TFR2</i>, or bone marrow transplants from iron-loaded <i>TFR2</i> null mice. We developed a novel transgenic mouse model which lacks <i>TFR2</i> in the hematopoietic compartment, enabling the delineation of the role of <i>TFR2</i> in erythroid development without interfering with its role in systemic iron metabolism. We show that in the absence of hematopoietic <i>TFR2</i> immature polychromatic erythroblasts accumulate with a concordant reduction in the percentage of mature erythroid cells in the spleen and bone marrow of anemic mice. These results demonstrate that erythroid <i>TFR2</i> is essential for an appropriate erythropoietic response in iron-deficient anemia. These findings may be of relevance in clinical situations in which an immediate and efficient erythropoietic response is required

Hepatocyte-specific deletion of peroxisomal protein PEX13 results in disrupted iron homeostasis

Peroxisomes are organelles, abundant in the liver, involved in a variety of cellular functions, including fatty acid metabolism, plasmalogen synthesis and metabolism of reactive oxygen species. Several inherited disorders are associated with peroxisomal dysfunction; increasingly many are associated with hepatic pathologies. The liver plays a principal role in regulation of iron metabolism. In this study we examined the possibility of a relationship between iron homeostasis and peroxisomal integrity. We examined the effect of deleting Pex13 in mouse liver on systemic iron homeostasis. We also used siRNA-mediated knock-down of PEX13 in a human hepatoma cell line (HepG2/C3A) to elucidate the mechanisms of PEX13-mediated regulation of hepcidin. We demonstrate that transgenic mice lacking hepatocyte Pex13 have defects in systemic iron homeostasis. The ablation of Pex13 expression in hepatocytes leads to a significant reduction in hepatic hepcidin levels. Our results also demonstrate that a deficiency of PEX13 gene expression in HepG2/C3A cells leads to decreased hepcidin expression, which is mediated through an increase in the signalling protein SMAD7, and endoplasmic reticulum (ER) stress. This study identifies a novel role for a protein involved in maintaining peroxisomal integrity and function in iron homeostasis. Loss of Pex13, a protein important for peroxisomal function, in hepatocytes leads to a significant increase in ER stress, which if unresolved, can affect liver function. The results from this study have implications for the management of patients with peroxisomal disorders and the liver-related complications they may develop.</p

Evaluation of a bone morphogenetic protein 6 variant as a cause of iron loading

Abstract Background Atypical iron overload without variation in the five clinically associated hereditary hemochromatosis genes is now recognized; however, their etiology remains unknown. Since the identification of iron overload in the bone morphogenetic protein 6 (Bmp6) knockout mouse, the search has been on for clinically pathogenic variants in the BMP6 gene. A recent report proposes that variants in the pro-peptide region of BMP6 are the underlying cause of several cases of iron overload. We performed targeted next-generation sequencing on three cases of atypical iron overload with Asian ethnicity and identified a p.Q118dup (aka p.E112indelEQ, p.Q115dup, p.Q118_L119insQ) variant in BMP6. The purpose of this study was to characterize the molecular function of the identified BMP6 variant. Molecular characterization by immunofluorescence microscopy and Western blotting of transfected cells, bioinformatics, and population analyses was performed. Results In contrast to reports for other BMP6 pro-peptide variants in this region, our data indicates that this variant does not affect the function of the mature BMP6 protein. Conclusions Our data suggest that assignment of disease causation in clinical cases of iron overload to pro-peptide variants in BMP6 should thus be treated with caution and requires biological characterization

Increased frequency of GNPAT p.D519G in compound HFE p.C282Y/p.H63D heterozygotes with elevated serum ferritin levels

Glyceronephosphate O-acyltransferase (GNPAT) p.D519G (rs11558492) was identified as a genetic modifier correlated with more severe iron overload in hemochromatosis through whole-exome sequencing of HFE p.C282Y homozygotes with extreme iron phenotypes. We studied the prevalence of p.D519G in HFE p.C282Y/p.H63D compound heterozygotes, a genotype associated with iron overload in some patients. Cases were Australian participants with elevated serum ferritin (SF) levels ≥300μg/L (males) and ≥200μg/L (females); subjects whose SF levels were below these cut-offs were designated as controls. Samples were genotyped for GNPAT p.D519G. We compared the allele frequency of the present subjects, with/without elevated SF, to p.D519G frequency in public datasets. GNPAT p.D519G was more prevalent in our cohort of p.C282Y/p.H63D compound heterozygotes with elevated SF (37%) than European public datasets: 1000G 21%, gnomAD 20% and ESP 21%. We conclude that GNPAT p.D519G is associated with elevated SF in Australian HFE p.C282Y/p.H63D compound heterozygotes.</p