Hereditary hemochromatosis (HH) is a heterogeneous group of inherited iron overload disorders leading to iron accumulation in specific organs, including the liver, heart, thymus, and pancreas. Excess iron in affected tissues catalyzes oxidative damage, resulting in cirrhosis, hepatoma, cardiomyopathy, diabetes, hypogonadotropic hypogonadism, and arthritis. Juvenile hemochromatosis (JH) is the most severe form of HH. It is an autosomal recessive disease with high penetrance that affects young patients of both sexes and leads to severe clinical complications typically in the teens and early 20s. If untreated, JH is lethal. A recent study identified the gene, named HFE2, which is responsible for the onset of disease in the majority of JH patients. Sequence analysis of HFE2 in JH patients revealed numerous homozygous or compound heterozygous mutations, including missense, frame shift, and nonsense mutations. Although the amino acid substitution G320V was found to account for about two-thirds of the cases, the sporadic distribution of these mutations hints that JH is due to the loss of HFE2 function. The importance of HFE2 in iron homeostasis has been confirmed by studies in HFE2-disrupted (Hjv-/-) mice, showing a severe iron overload phenotype similar to JH patients. Intriguingly, a severe depression of hepatic hepcidin expression was observed both in HFE2 mutation-related JH patients and in Hjv-/- mice, indicating that HFE2 is a key upstream regulator of hepcidin, a central iron regulatory hormone. Hepcidin is mainly expressed in liver hepatocytes. The protein encoded by HFE2, hemojuvelin (HJV), shares high sequence similarity to two repulsive guidance molecule (RGM) family members in mice (RGMa and b). A recent study found that HJV is a co-receptor for bone morphogenetic protein 2 and 4 (BMP2 and BMP4) and enhances BMP-induced hepcidin expression. Our preliminary results showed that HJV is a GPI-anchored protein, undergoes a partial autocatalytic cleavage and increases iron accumulation in HEK293 cells through its interaction with neogenin, a receptor for RGMa. Our study also indicates that neogenin is required for HJV release from the cells. We propose to: 1). Determine the role of neogenin in cellular HJV processing. 2). Examine the role of BMP and neogenin signaling in HJV-mediated hepcidin expression. 3). Characterize the role of serum HJV in body iron homeostasis. Knowledge gained regarding the function of neogenin and BMP signaling in HJV-mediated hepatic hepcidin expression could lead to understanding body iron homeostasis, as well as novel treatments for iron overload disorders.
Iron is an essential nutrient required for a variety of biochemical processes such as respiration, metabolism, and DNA synthesis, but also toxic when it is in excess (Bothwell et al., 1995). Hereditary hemochromatosis (HH), the most common inherited disease in Caucasians, is a heterogeneous group of inherited iron overload disorders (Cox, 1996;Hentze et al., 2004). Elucidation of the role of HFE2, the juvenile HH-causing gene (Papanikolaou et al., 2004), will lead to a full understanding of the regulation of body iron homeostasis as well as more effective cures for iron overload disorders.
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