Because iron export is impaired, iron is unavailable for transport by circulating transferrin. This iron unavailability potentially leads to mild anemia in type 4A hemochromatosis patients because iron is necessary for hemoglobin synthesis, and red blood cells have a relatively high turnover rate. Over time, iron stores increase, and individuals with type 4A hemochromatosis may develop hepatic fibrosis.The symptoms of type 4B hemochromatosis tend to be more severe. They resemble the symptoms of hemochromatosis types 1, 2, and 3. Plasma iron concentration is elevated, and symptoms include joint pain, diabetes, and arrhythmia. Liver iron deposition tends to be greater in type 4B than in type 4A. Liver damage occurs more frequently in this form of hemochromatosis than in type 4A, and some individuals develop cirrhosis of the liver. Genetics
Type 4 hemochromatosis is caused by mutations of the SLC40A1 gene, located on the long arm of chromosome 2, specifically at 2q32.2. The SLC40A1 gene encodes ferroportin, a protein responsible for exporting iron from cells in the intestine, liver, spleen, and kidney, as well as from reticuloendothelial macrophages and the placenta. More than 39 mutations to the SLC40A1 gene have been identified in patients with type 4 hemochromatosis. All reported SLC40A1 mutations are deletions or missense mutations, which lead to amino acid substitution.Mutations to SLC40A1 that change the amino acid sequence can result in loss of function or gain of function for the resulting ferroportin protein.
The loss of function mutation results in a phenotype that is different from that of a gain of function mutation, and these phenotypes are associated with two different forms of type 4 hemochromatosis. Loss-of-function mutations are more frequent and are associated with type 4A hemochromatosis. These mutations lead to a defect in the localization of ferroportin. Gain-of-function mutations are associated with type 4B and lead to production of ferroportin that resists negative regulation by hepcidin.Unlike other forms of hemochromatosis, which have a recessive pattern of inheritance, type 4 is an autosomal dominant disorder. The dominant inheritance pattern occurs in hemochromatosis type 4 because ferroportin is multimeric. Consequently, mutant ferroportin can associate with wild-type ferroportin in multimers and interfere with the function of normal ferroportin proteins. Pathophysiology
In normal iron regulation, iron is absorbed in the intestine, and ferroportin transports iron from the cells of the intestinal lining into the bloodstream. Iron in the bloodstream is then bound by transferrin, which carries the iron to target cells. Iron is stored in cells and blood serum in a protein called ferritin. Reticuloendothelial macrophages, which can phagocytose red blood cells, are important in the iron recycling process. Ferroportin is upregulated in the reticuloendothelial macrophages after phagocytosis occurs so that iron from the degraded red blood cells can be released into the bloodstream and transported to other types of cells as needed. Hepcidin, a protein synthesized in the liver in response to iron or inflammation, is a regulator of ferroportin expression.