Abstract
In the past seven years numerous genes that influence iron homeostasis have been discovered. Dr. Beutler provides a brief overview of these genes, genes that encode HFE, DMT-1, ferroportin, transferrin receptor 2, hephaestin, and hepcidin to lay the groundwork for a discussion of the various clinical forms of iron storage disease and how they differ from one another.
In Section I, Dr. Beutler also discusses the types of hemochromatosis that exist as acquired and as hereditary forms. Acquired hemochromatosis occurs in patients with marrow failure, particularly when there is active ineffective erythropoiesis. Hereditary hemochromatosis is most commonly due to mutations in the HLA-linked HFE gene, and hemochromatosis clinically indistinguishable from HFE hemochromatosis is the consequence of mutations in three transferrin receptor-2 gene. A more severe, juvenile form of iron storage disease results from mutations of the gene encoding hepcidin or of a not-yet-identified gene on chromosome 1q. Autosomal dominant iron storage disease is a consequence of ferroportin mutations, and a polymorphism in the ferroportin gene appears to be involved in the African iron overload syndrome.
Evidence regarding the biochemical and clinical penetrance of hemochromatosis due to mutations of the HFE gene is rapidly accumulating. These studies, emanating from several centers in Europe and the United States, all agree that the penetrance of hemochromatosis is much lower than had previously been thought. Probably only 1% of homozygotes develop clinical findings. The implications of these new findings for the management of hemochromatosis will be discussed.
In Section II, Dr. Victor Hoffbrand discusses the management of iron storage disease by chelation therapy, treatment that is usually reserved for patients with secondary hemochromatosis such as occurs in the thalassemias and in patients with transfusion requirements due to myelodysplasia and other marrow failure states. Tissue iron can be estimated by determining serum ferritin levels, measuring liver iron, and by measuring cardiac iron using the MRI-T2* technique. The standard form of chelation therapy is the slow intravenous or subcutaneous infusion of desferoxamine. An orally active bidentate iron chelator, deferiprone, is now licensed in 25 countries for treatment of patients with thalassemia major. Possibly because of the ability of this compound to cross membranes, it appears to have superior cardioprotective properties. Agranulocytosis is the most serious complication of deferiprone therapy and occurs in about 1% of treated patients. Deferiprone and desferoxamine can be given together or on alternating schedules. A new orally active chelating agent ICL 670 seems promising in early clinical studies.
In Section III, Dr. James Cook discusses the most common disorder of iron homeostasis, iron deficiency. He will compare some of the standard methods for identifying iron deficiency, the hemoglobin level, transferrin saturation, and mean corpuscular hemoglobin and compare these with some of the newer methods that have been introduced, specifically the percentage of hypochromic erythrocytes and reticulocyte hemoglobin content. The measurement of storage iron is achieved by measuring serum ferritin levels. The soluble transferrin receptor is a truncated form of the cellular transferrin receptor and the possible value of this measurement in the diagnosis of iron deficiency will be discussed. Until recently iron dextran was the only parental iron preparation available in the US. Sodium ferric gluconate, which has been used extensively in Europe for many years, is now available in the United States. It seems to have a distinct advantage over iron dextran in that anaphylactic reactions are much less common with the latter preparation.
Co-localization of the Mammalian Hemochromatosis Gene Product (HFE) and a Newly Identified Transferrin Receptor (TfR2) in Intestinal Tissue and Cells