A third form of autosomal recessive hemochromatosis (type 3) has been reported to be indistinguishable from HFE-as-sociated hemochromatosis in its clinical manifestations, but patients have no mutations in the HFE gene. Instead, their disease is caused by mutations in the transferrin receptor-2 (TFR2) gene on chromosome 7q. Transferrin receptor 2 is a protein of unknown function that is highly homologous to the transferrin receptor (also known as transferrin receptor-1), and highly expressed by hepatocytes and hematopoietic cells. TFR2 is capable of binding transferrin and bringing it into the cell, but it does so at much lower efficiency than does transferrin receptor. TFR2 does not form a complex with HFE, and its role in iron homeostasis is completely unknown. Some patients have been shown to have nonsense mutations, indicating that TFR2 is not an essential protein for survival. These patients are effectively treated by phlebotomy. It is likely that clinical tests for TFR2 mutations will be developed within the next few years.
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