Nancy C. Andrews

p63, a p53 Homolog at 3q27–29, Encodes Multiple Products with Transactivating, Death-Inducing, and Dominant-Negative Activities

We describe the cloning of p63, a gene at chromosome 3q27-29 that bears strong homology to the tumor suppressor p53 and to the related gene, p73. p63 was detected in a variety of human and mouse tissues, including proliferating basal cells of epithelial layers in the epidermis, cervix, urothelium, and prostate. Unlike p53, the p63 gene encodes multiple isotypes with remarkably divergent abilities to transactivate p53 reporter genes and induce apoptosis. Importantly, the predominant p63 isotypes in many epithelial tissues lack an acidic N terminus corresponding to the transactivation domain of p53. We demonstrate that these truncated p63 variants can act as dominant-negative agents toward transactivation by p53 and p63, and we suggest the possibility of physiological interactions among members of the p53 family.

Balancing Acts: Molecular Control of Mammalian Iron Metabolism

Iron is ubiquitous in the environment and in biology. The study of iron biology focuses on physiology and homeostasis-understanding how cells and organisms regulate their iron content, how diverse tissues orchestrate iron allocation, and how dysregulated iron homeostasis leads to common hematological, metabolic, and neurodegenerative diseases. This has provided novel insights into gene regulation and unveiled remarkable links to the immune system.

Positional cloning of zebrafish ferroportin1 identifies a conservedvertebrate iron exporter

Defects in iron absorption and utilization lead to iron deficiency and overload disorders. Adult mammals absorb iron through the duodenum, whereas embryos obtain iron through placental transport. Iron uptake from the intestinal lumen through the apical surface of polarized duodenal enterocytes is mediated by the divalent metal transporter, DMT1 (refs 1,2,3). A second transporter has been postulated to export iron across the basolateral surface to the circulation. Here we have used positional cloning to identify the gene responsible for the hypochromic anaemia of the zebrafish mutant weissherbst. The gene, ferroportin1, encodes a multiple-transmembrane domain protein, expressed in the yolk sac, that is a candidate for the elusive iron exporter. Zebrafish ferroportin1 is required for the transport of iron from maternally derived yolk stores to the circulation and functions as an iron exporter when expressed in Xenopus oocytes. Human Ferroportin1 is found at the basal surface of placental syncytiotrophoblasts, suggesting that it also transports iron from mother to embryo. Mammalian Ferroportin1 is expressed at the basolateral surface of duodenal enterocytes and could export cellular iron into the circulation. We propose that Ferroportin1 function may be perturbed in mammalian disorders of iron deficiency or overload.

Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression

Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2−/− hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression.

Autosomal-dominant hemochrom-atosis is associated with a mutation in the ferroportin (SLC11A3) gene

Hemochromatosis is a progressive iron overload disorder that is prevalent among individuals of European descent. It is usually inherited in an autosomal-recessive pattern and associated with missense mutations in HFE, an atypical major histocompatibility class I gene. Recently, we described a large family with autosomal-dominant hemochromatosis not linked to HFE and distinguished by early iron accumulation in reticuloendothelial cells. Through analysis of a large pedigree, we have determined that this disease maps to 2q32. The gene encoding ferroportin (SLC11A3), a transmembrane iron export protein, lies within a candidate interval defined by highly significant lod scores. We show that the iron-loading phenotype in autosomal-dominant hemochromatosis is associated with a nonconservative missense mutation in the ferroportin gene. This missense mutation, converting alanine to aspartic acid at residue 77 (A77D), was not seen in samples from 100 unaffected control individuals. We propose that partial loss of ferroportin function leads to an imbalance in iron distribution and a consequent increase in tissue iron accumulation.

Mutations in TMPRSS6 cause iron-refractory iron deficiency anemia (IRIDA)

Iron deficiency is usually attributed to chronic blood loss or inadequate dietary intake. Here, we show that iron deficiency anemia refractory to oral iron therapy can be caused by germline mutations in TMPRSS6, which encodes a type II transmembrane serine protease produced by the liver that regulates the expression of the systemic iron regulatory hormone hepcidin. These findings demonstrate that TMPRSS6 is essential for normal systemic iron homeostasis in humans.

Forging a field: the golden age of iron biology

Oh what a difference a decade makes! In a landmark paper published in 1996, Feder and colleagues identified the long-sought gene mutated in patients with classical hereditary hemochromatosis. In many ways, this seemed to ignite an explosion in iron biology which, over the next 10 years, led to a

Transferrin receptor is necessary for development of erythrocytes and the nervous system

Plasma iron circulates bound to transferrin (Trf), which solubilizes the ferric ion and attenuates its reactivity. Diferric Trf interacts with cell-surface Trf receptor (Trfr) to undergo receptor-mediated endocytosis into specialized endosomes. Endosomal acidification leads to iron release, and iron is transported out of the endosome through the activity of divalent metal transporter 1 (DMT1, formerly Nramp2), a transmembrane iron transporter that functions only at low pH (ref. 1). Trf and Trfr then return to the cell surface for reuse, completing a highly efficient cycle. Although the Trf cycle is assumed to be the general mechanism for cellular iron uptake, this has not been validated experimentally. Mice with hypotransferrinaemia (hpx) have little or no plasma Trf (Refs 2,3). They have severe anaemia, indicating that the Trf cycle is essential for iron uptake by erythroid cells. Other hpx tissues, however, are generally normal, and there is a paradoxical increase in intestinal iron absorption and iron storage. To test the hypothesis that the Trf cycle has unique importance for erythropoiesis, we disrupted the Trfr gene in mice. This results in elimination of the Trf cycle, but leaves other Trf functions intact. Mice lacking Trfr have a more severe phenotype than hpx mice, affecting both erythropoiesis and neurologic development. Furthermore, haploinsufficiency for Trfr results in impaired erythroid development and abnormal iron homeostasis.

Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance

Systemic iron balance is regulated by hepcidin, a peptide hormone secreted by the liver. By decreasing cell surface expression of the iron exporter ferroportin, hepcidin decreases iron absorption from the intestine and iron release from reticuloendothelial stores. Hepcidin excess has been implicated in the pathogenesis of anemia of chronic disease, while hepcidin deficiency has a key role in the pathogenesis of the iron overload disorder hemochromatosis. We have recently shown that hemojuvelin is a coreceptor for bone morphogenetic protein (BMP) signaling and that BMP signaling positively regulates hepcidin expression in liver cells in vitro. Here we show that BMP-2 administration increases hepcidin expression and decreases serum iron levels in vivo. We also show that soluble hemojuvelin (HJV.Fc) selectively inhibits BMP induction of hepcidin expression in vitro and that administration of HJV.Fc decreases hepcidin expression, increases ferroportin expression, mobilizes splenic iron stores, and increases serum iron levels in vivo. These data support a role for modulators of the BMP signaling pathway in treating diseases of iron overload and anemia of chronic disease.

A mouse model of juvenile hemochromatosis

Hereditary hemochromatosis is an iron-overload disorder resulting from mutations in proteins presumed to be involved in the maintenance of iron homeostasis. Mutations in hemojuvelin (HJV) cause severe, early-onset juvenile hemochromatosis. The normal function of HJV is unknown. Juvenile hemochromatosis patients have decreased urinary levels of hepcidin, a peptide hormone that binds to the cellular iron exporter ferroportin, causing its internalization and degradation. We have disrupted the murine Hjv gene and shown that Hjv-/- mice have markedly increased iron deposition in liver, pancreas, and heart but decreased iron levels in tissue macrophages. Hepcidin mRNA expression was decreased in Hjv-/- mice. Accordingly, ferroportin expression detected by immunohistochemistry was markedly increased in both intestinal epithelial cells and macrophages. We propose that excess, unregulated ferroportin activity in these cell types leads to the increased intestinal iron absorption and plasma iron levels characteristic of the juvenile hemochromatosis phenotype.