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Query: UMLS:C0004134 (ataxia)
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Remarkable progress is being made in understanding the molecular basis of disorders of human iron metabolism. Recent work has uncovered unanticipated relationships with the immune and nervous systems, intricate interconnections with copper metabolism, and striking homologies between yeast and human genes involved in the transport of transition metals. This review examines the clinical consequences of new insights into the pathophysiology of genetic abnormalities affecting iron metabolism. The proteins recently found to be involved in the absorption, transport, utilization, and storage of iron are briefly described, and the clinical manifestations of genetic disorders that affect these proteins are discussed. This chapter considers the most common inherited disorder in individuals of European ancestry (hereditary hemochromatosis), a widespread disease in sub-Saharan populations for which the genetic basis is still uncertain (African dietary iron overload), and several less frequent or rare disorders (juvenile hemochromatosis, atransferrinemia, aceruloplasminemia, hyperferritinemia with autosomal dominant congenital cataract, Friedreich's ataxia, and X-linked sideroblastic anemia with ataxia).
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PMID:Genetic disorders affecting proteins of iron metabolism: clinical implications. 1077 76

Iron is a vitally important element in mammalian metabolism because of its unsurpassed versatility as a biologic catalyst. However, when not appropriately shielded or when present in excess, iron plays a key role in the formation of extremely toxic oxygen radicals, which ultimately cause peroxidative damage to vital cell structures. Organisms are equipped with specific proteins designed for iron acquisition, export, transport, and storage as well as with sophisticated mechanisms that maintain the intracellular labile iron pool at an appropriate level. These systems normally tightly control iron homeostasis but their failure can lead to iron deficiency or iron overload and their clinical consequences. This review describes several rare iron loading conditions caused by genetic defects in some of the proteins involved in iron metabolism. A dramatic decrease in the synthesis of the plasma iron transport protein, transferrin, leads to a massive accumulation of iron in nonhematopoietic tissues but virtually no iron is available for erythropoiesis. Humans and mice with hypotransferrinemia have a remarkably similar phenotype. Homozygous defects in a recently identified gene encoding transferrin receptor 2 lead to iron overload (hemochromatosis type 3) with symptoms similar to those seen in patients with HFE-associated hereditary hemochromatosis (hemochromatosis type 1). Transferrin receptor 2 is primarily expressed in the liver but it is unclear how mutant forms cause iron overload. Mutations in the gene encoding the iron exporter, ferroportin 1, cause iron overload characterized by iron accumulation in macrophages yet normal plasma iron levels. Plasma iron, together with dominant inheritance, discriminates iron overload due to ferroportin mutations (hemochromatosis type 4) from hemochromatosis type 1. Heme oxygenase 1 is essential for the catabolism of heme and in the recycling of hemoglobin iron in macrophages. Homozygous heme oxygenase 1 deletion in mice leads to a paradoxical accumulation of nonheme iron in macrophages, hepatocytes, and many other cells and is associated with low plasma iron levels, anemia, endothelial cell damage, and decreased resistance to oxidative stress. A similar phenotype occurred in a child with severe heme oxygenase 1 deficiency. Recently, a mutation in the L-subunit of ferritin has been described that causes the formation of aberrant L-ferritin with an altered C-terminus. Individuals with this mutation in one allele of L-ferritin have abnormal aggregates of ferritin and iron in the brain, primarily in the globus pallidus. Patients with this dominantly inherited late-onset disease present with symptoms of extrapyramidal dysfunction. Mice with a targeted disruption of a gene for iron regulatory protein 2 (IRP2), a translational repressor of ferritin, misregulate iron metabolism in the intestinal mucosa and the central nervous system. Significant amounts of ferritin and iron accumulate in white matter tracts and nuclei, and adult IRP2-deficient mice develop a movement disorder consisting of ataxia, bradykinesia, and tremor. Mutations in the frataxin gene are responsible for Friedreich ataxia, the most common of the inherited ataxias. Frataxin appears to regulate mitochondrial iron (or iron-sulfur cluster) export and the neurologic and cardiac manifestations of Friedreich ataxia are due to iron-mediated mitochondrial toxicity. Finally, patients with Hallervorden-Spatz syndrome, an autosomal recessive, progressive neurodegenerative disorder, have mutations in a novel pantothenate kinase gene (PANK2). The cardinal feature of this extrapyramidal disease is pathologic iron accumulation in the globus pallidus. The defect in PANK2 is predicted to cause the accumulation of cysteine, which binds iron and causes oxidative stress in the iron-rich globus pallidus.
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PMID:Rare causes of hereditary iron overload. 1238

Increasing evidence suggests that iron-mediated oxidative stress might underlie the development of neurodegeneration in Friedreich's ataxia (FRDA), an autosomal recessive ataxia caused by decreased expression of frataxin, a protein implicated in iron metabolism. In this study, we demonstrate that, in fibroblasts of patients with FRDA, the cellular redox equilibrium is shifted toward more protein-bound glutathione. Furthermore, we found that actin is glutathionylated, probably as a result of the accumulation of reactive oxygen species, generated by iron overload in the disease. Indeed, high-pressure liquid chromatography analysis of control fibroblasts in vivo treated with FeSO4 showed a significant increase in the protein-bound/free GSH ratio, and Western blot analysis indicated a relevant rise in glutathionylation. Actin glutathionylation contributes to impaired microfilament organization in FRDA fibroblasts. Rhodamine phalloidin staining revealed a disarray of actin filaments and a reduced signal of F-actin fluorescence. The same hematoxylin/eosin-stained cells showed abnormalities in size and shape. When we treated FRDA fibroblasts with reduced glutathione, we obtained a complete rescue of cytoskeletal abnormalities and cell viability. Thus, we conclude that oxidative stress may induce actin glutathionylation and impairment of cytoskeletal functions in FRDA fibroblasts.
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PMID:Actin glutathionylation increases in fibroblasts of patients with Friedreich's ataxia: a potential role in the pathogenesis of the disease. 1291 1

Aceruloplasminemia is an inherited disorder of iron metabolism caused by the complete lack of ceruloplasmin ferroxidase activity caused by mutations in the ceruloplasmin gene. It is characterized by iron accumulation in the brain as well as visceral organs. Clinically, the disease consists of the triad of adult-onset neurologic disease, retinal degeneration and diabetes mellitus. The neurological symptoms, which include involuntary movements, ataxia, and dementia, reflect the sites of iron deposition. Severe iron overload and extensive neuronal loss were observed in the basal ganglia, while iron deposition and neuronal cell loss were trivial in the frontal cortices. The cerebellar cortex showed marked loss of Purkinje cells. Iron deposition was more prominent in the astrocytes than in the neurons. Excess iron functions as a potent catalyst of biologic oxidation. Astrocytic deformity and globular structures are characteristic features in aceruloplasminemia brains. The globular structures in the astrocytes were seen in proportion to the degree of iron deposition and reacted positively to anti-4-hydroxynonenal, one of the indicators of lipid peroxidation, and anti-ubiquitin antibodies, but not to anti-alpha-synuclein antibody. The lack of ceruloplasmin may primarily damage astrocytes in the aceruloplasminemia brains through lipid peroxidation. Ceruloplasmin may play an essential role in neuronal survival in the central nervous system.
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PMID:Aceruloplasminemia, an iron metabolic disorder. 1471 52

Iron is essential for oxidation-reduction catalysis and bioenergetics; however, unless appropriately shielded, this metal plays a crucial role in the formation of toxic oxygen radicals that can attack all biological molecules. Organisms are equipped with specific proteins designed for iron acquisition, export and transport, and storage, as well as with sophisticated mechanisms that maintain the intracellular labile iron pool at an appropriate level. Despite these homeostatic mechanisms, organisms often face the threat of either iron deficiency or iron overload. This review describes several hereditary iron-overloading conditions that are confined to the brain. Recently, a mutation in the L-subunit of ferritin has been described that causes the formation of aberrant L-ferritin with an altered C-terminus. Individuals with this mutation in one allele of L-ferritin have abnormal aggregates of ferritin and iron in the brain, primarily in the globus pallidus. Patients with this dominantly inherited late-onset disease present with symptoms of extrapyramidal dysfunction. Mice with a targeted disruption of a gene for iron regulatory protein 2 (IRP2), a translational repressor of ferritin, misregulate iron metabolism in the intestinal mucosa and the central nervous system. Significant amounts of ferritin and iron accumulate in white matter tracts and nuclei, and adult IRP2-deficient mice develop a movement disorder consisting of ataxia, bradykinesia, and tremor. Mutations in the frataxin gene are responsible for Friedreich's ataxia, the most common of the inherited ataxias. Frataxin appears to regulate mitochondrial iron-sulfur cluster formation, and the neurologic and cardiac manifestations of Friedreich's ataxia are due to iron-mediated mitochondrial toxicity. Patients with Hallervorden-Spatz syndrome, an autosomal recessive, progressive neurodegenerative disorder, have mutations in a novel pantothenate kinase gene (PANK2). The cardinal feature of this extrapyramidal disease is pathologic iron accumulation in the globus pallidus. The defect in PANK2 is predicted to cause the accumulation of cysteine, which binds iron and causes oxidative stress in the iron-rich globus pallidus. Finally, aceruloplasminemia is an autosomal recessive disorder of iron metabolism caused by loss-of-function mutations in ceruloplasmin gene that leads to misregulation of both systemic and central nervous system iron trafficking. Affected individuals suffer from extrapyramidal signs, cerebellar ataxia, progressive neurodegeneration of retina, and diabetes mellitus. Excessive iron depositions are found in the brain, liver, pancreas, and other parenchymal cells, but plasma iron concentrations are decreased. These conditions are not common, but awareness about them is important for differential diagnosis of various neurodegenerative disorders.
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PMID:Hereditary causes of disturbed iron homeostasis in the central nervous system. 1510 72

Aceruloplasminemia is a rare autosomal recessive disorder. The lack of ceruloplasmin ferroxidase activity leads to parenchymal and reticuloendothelial iron overload, resulting in diabetes and progressive neurodegeneration with extrapyramidal disorders, ataxia, and dementia. We describe the MR imaging findings in a 40-year-old woman with hereditary aceruloplasminemia. The abnormal T2 hypointensities were more marked than those seen in any other condition, including degenerative disorders of the basal ganglia and Wilson disease, and they may be typical of aceruloplasminemia. To our knowledge, involvement of the cortex has not been described and suggests that brain iron accumulation in aceruloplasminemia is more extensive than previously believed, even in asymptomatic patients.
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PMID:MR imaging of cerebral cortical involvement in aceruloplasminemia. 1576 Aug 83

The yeast ATM1 protein is essential for normal mitochondrial iron homeostasis. Deletion of ATM1 results in mitochondrial iron accumulation and oxidative mitochondrial damage. Mutations in ABC7, the human homolog of ATM1, result in X-linked sideroblastic anemia and ataxia. Here we show that a deletion of ATM1 also has effects on extra-mitochondrial iron metabolism. ATM1-deficient cells have an increased iron requirement for growth. When grown in iron-rich medium, mutant cells accumulate excess mitochondrial iron and have increased expression of the genes required for both high and low affinity iron uptake. Thus, ATM1 mutant cells simultaneously demonstrate features of both iron overload and iron starvation. Yfh1p is the yeast homolog of the human frataxin protein, which is deficient in Friedreich's ataxia. As in atm1 cells, a yfh1 deletion results in both mitochondrial iron accumulation and cytosolic iron starvation. In spite of their apparent roles in cellular iron homeostasis, we find that the expression of neither ATM1 nor YFH1 is responsive to cellular iron status. Based on these observations, we propose a model in which cellular iron is prioritized for use by the mitochondrion, and available to the remainder of the cell only after mitochondrial needs have been fulfilled.
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PMID:The role of the mitochondrion in cellular iron homeostasis. 1612 Feb 68

Defective iron utilization leading to either systemic or regional misdistribution of the metal has been identified as a critical feature of several different disorders. Iron concentrations can rise to toxic levels in mitochondria of excitable cells, often leaving the cytosol iron-depleted, in some forms of neurodegeneration with brain accumulation (NBIA) or following mutations in genes associated with mitochondrial functions, such as ABCB7 in X-linked sideroblastic anemia with ataxia (XLSA/A) or the genes encoding frataxin in Friedreich's ataxia (FRDA). In anemia of chronic disease (ACD), iron is withheld by macrophages, while iron levels in extracellular fluids (e.g., plasma) are drastically reduced. One possible therapeutic approach to these diseases is iron chelation, which is known to effectively reduce multiorgan iron deposition in iron-overloaded patients. However, iron chelation is probably inappropriate for disorders associated with misdistribution of iron within selected tissues or cells. One chelator in clinical use for treating iron overload, deferiprone (DFP), has been identified as a reversed siderophore, that is, an agent with iron-relocating abilities in settings of regional iron accumulation. DFP was applied to a cell model of FRDA, a paradigm of a disorder etiologically associated with cellular iron misdistribution. The treatment reduced the mitochondrial levels of labile iron pools (LIP) that were increased by frataxin deficiency. DFP also conferred upon cells protection against oxidative damage and concomitantly mediated the restoration of various metabolic parameters, including aconitase activity. Administration of DFP to FRDA patients for 6 months resulted in selective and significant reduction in foci of brain iron accumulation (assessed by T2* MRI) and initial functional improvements, with only minor changes in net body iron stores. The prospects of drug-mediated iron relocation versus those of chelation are discussed in relation to other disorders involving iron misdistribution, such as ACD and XLSA/A.
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PMID:Iron redistribution as a therapeutic strategy for treating diseases of localized iron accumulation. 2039 84

Superficial siderosis is an increasingly common diagnosis due to advances in imaging. Resulting from the presence of blood in the subarachnoid space, it leads to progressively debilitating ataxia, deafness and myelopathy. Magnetic resonance imaging has revolutionized diagnosis and management of this condition. In one-third of patients with this diagnosis, no subarachnoid bleed can be identified. A lack of sensitivity in our ability to detect subarachnoid bleed is a possible explanation in this group of patients. A novel approach would be to consider defects in the body's defense against intracranial iron overload as a possible cause of this disease. A disproportionately few patients develop superficial siderosis compared with the number who develop subarachnoid bleed from various clinical conditions. Normal physiological protective mechanisms exist in the central nervous system to protect it from damage by blood, haeme or free iron. Deficiency and defect of these protective mechanisms can provide insight into the pathogenetic mechanism of the group of patients where no subarachnoid haemorrhage is identified.
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PMID:Superficial siderosis - mechanism of disease: an alternative hypothesis. 2040 79

Iron-sulfur (Fe-S) proteins contain prosthetic groups consisting of two or more iron atoms bridged by sulfur ligands, which facilitate multiple functions, including redox activity, enzymatic function, and maintenance of structural integrity. More than 20 proteins are involved in the biosynthesis of iron-sulfur clusters in eukaryotes. Defective Fe-S cluster synthesis not only affects activities of many iron-sulfur enzymes, such as aconitase and succinate dehydrogenase, but also alters the regulation of cellular iron homeostasis, causing both mitochondrial iron overload and cytosolic iron deficiency. In this work, we review human Fe-S cluster biogenesis and human diseases that are caused by defective Fe-S cluster biogenesis. Fe-S cluster biogenesis takes place essentially in every tissue of humans, and products of human disease genes, including frataxin, GLRX5, ISCU, and ABCB7, have important roles in the process. However, the human diseases, Friedreich ataxia, glutaredoxin 5-deficient sideroblastic anemia, ISCU myopathy, and ABCB7 sideroblastic anemia/ataxia syndrome, affect specific tissues, while sparing others. Here we discuss the phenotypes caused by mutations in these different disease genes, and we compare the underlying pathophysiology and discuss the possible explanations for tissue-specific pathology in these diseases caused by defective Fe-S cluster biogenesis.
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PMID:Human iron-sulfur cluster assembly, cellular iron homeostasis, and disease. 2048 66


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