Targeted disruption of the murine Hjv
gene resulted in a mouse phenotype similar to that of human patients with juvenile hemochromatosis. Hjv–/–
mice rapidly accumulated excess iron in the liver, heart, and pancreas, which confirmed that HJV
is the juvenile hemochromatosis gene and that loss of Hjv protein leads to iron overload. Interestingly, Hjv+/–
mice were indistinguishable from wild-type mice in every study, which suggests that haploinsufficiency does not produce any clinical consequences. This is consistent with the fact that parents of juvenile hemochromatosis patients have never been reported to be affected but is notably different from previous observations in Hfe+/–
mice, in which a mild heterozygote phenotype has been detected (22
By 6 to 7 weeks of age, Hjv–/–
mice had accumulated significantly more iron than 12-week-old Hfe–/–
mice of the same genetic background (23
), which is consistent with the difference in severity between juvenile hemochromatosis and HFE hemochromatosis in human patients. However, similar to other mouse models of iron overload (refs. 23
, and our unpublished observations), Hjv–/–
mice did not develop significant hepatic fibrosis, frank cardiomyopathy, overt diabetes, infertility, or other end-organ dysfunction. Mice appeared to be protected, by some unknown mechanism, from toxic effects of iron overload.
Although HJV is expressed at high levels in cardiac and skeletal myocytes, Hjv–/–
mice, similar to juvenile hemochromatosis patients, had no grossly apparent abnormalities aside from iron overload, which suggests that Hjv plays no essential role in muscle development or function within the first 2 months of life. Based on this phenotype, we conclude that the critical role of HJV is to control iron homeostasis. Our data suggest that it acts by potentiating hepcidin expression. Hepcidin is primarily produced in the liver, but smaller amounts are made in the heart and pancreas (26
), which is similar to the pattern of HJV expression (refs. 6
, and our unpublished observations). It is possible that HJV controls hepcidin expression at several of those sites. Intriguingly, HJV is expressed at the primary sites of iron loading in hemochromatosis, which suggests that it could have a second, cell-autonomous function in either sensing or directly modulating cellular iron uptake.
In the absence of appropriate hepcidin expression, we would expect impaired regulation of the iron exporter, ferroportin. Accordingly, we observed markedly elevated levels of ferroportin in 2 critical sites: the absorptive intestinal epithelium and tissue macrophages. The iron content of both enterocytes and macrophages was decreased compared with that of wild-type animals. Thus, increased intestinal ferroportin expression could account for accelerated iron absorption and increased body iron burden by maximizing the amount of enterocyte iron transferred to serum. Similarly, increased macrophage ferroportin expression could potentiate macrophage iron release, resulting in decreased macrophage storage iron and increased serum iron.
Our study also has implications for autosomal-dominant iron overload associated with mutations in the gene encoding ferroportin. There appear to be 2 classes of patients with “ferroportin disease” (27
). One group has increased macrophage iron stores with no apparent increase in total body iron, which suggests a partial loss of ferroportin function (4
). Their phenotype is similar to that observed in mice lacking 1 ferroportin allele (14
). However, other patients with a different constellation of ferroportin missense mutations present with symptoms that are more similar to those of the other hemochromatosis disorders (29
). These patients may have increased ferroportin activity similar to that of Hjv–/–
mice. This might result either from increased ferroportin transporter activity or from impaired binding of hepcidin.
In summary, our findings define a mechanism for iron overload in juvenile hemochromatosis due to mutations in HJV. Together with results of other studies showing decreased hepcidin expression in other hemochromatosis disorders, they suggest that increased body iron burden results from overactivity of ferroportin due to impaired regulation by hepcidin. We propose that increased ferroportin expression is the direct cause of increased intestinal iron absorption and increased serum iron, which leads to rapid deposition in target organs.