Friedreich's ataxia: past, present and future

Friedreich's ataxia (FRDA) is an autosomal recessive inherited disorder characterized by progressive gait and limb ataxia, dysarthria, areflexia, loss of vibratory and position sense, and a progressive motor weakness of central origin. Additional features include hypertrophic cardiomyopathy and diabetes. Large GAA repeat expansions in the first intron of the FXN gene are the most common mutation underlying FRDA. Patients show severely reduced levels of a FXN-encoded mitochondrial protein called frataxin. Frataxin deficiency is associated with abnormalities of iron metabolism: decreased iron-sulfur cluster (ISC) biogenesis, accumulation of iron in mitochondria and depletion in the cytosol, enhanced cellular iron uptake. Some models have also shown reduced heme synthesis. Evidence for oxidative stress has been reported. Respiratory chain dysfunction aggravates oxidative stress by increasing leakage of electrons and the formation of superoxide. In vitro studies have demonstrated that Frataxin deficient cells not only generate more free radicals, but also show a reduced capacity to mobilize antioxidant defenses. The search for experimental drugs increasing the amount of frataxin is a very active and timely area of investigation. In cellular and in animal model systems, the replacement of frataxin function seems to alleviate the symptoms or even completely reverse the phenotype. Therefore, drugs increasing the amount of frataxin are attractive candidates for novel therapies. This review will discuss recent findings on FRDA pathogenesis, frataxin function, new treatments, as well as recent animal and cellular models. Controversial aspects are also discussed.     

Iron dysregulation in Friedreich ataxia

Friedreich ataxia is the most common hereditary ataxia. The signs and symptoms of the disorder derive from decreased expression of the protein frataxin, which is involved in iron metabolism. Frataxin chaperones iron for iron-sulfur cluster biogenesis and detoxifies iron in the mitochondrial matrix. Decreased expression of frataxin is associated with impairments of iron-sulfur cluster biogenesis and heme synthesis, as well as with mitochondrial dysfunction and oxidative stress. Compounds currently in clinical trials are directed toward improving mitochondrial function and lessening oxidative stress. Iron chelators and compounds that increase frataxin expression are under evaluation. Further elucidation of frataxin's function should lead to additional therapeutic approaches.     

Review: Iron metabolism and the role of iron in neurodegenerative disorders

Iron plays a role for the biogenesis of two important redox‐reactive prosthetic groups of enzymes, iron sulphur clusters (ISC) and heme. A part of these biosynthetic pathways takes plays in the mitochondria. While several important proteins of cellular iron uptake and storage and of mitochondrial iron metabolism are well‐characterized, limited knowledge exists regarding the mitochondrial iron importers (mitoferrins). A disturbed distribution of iron, hampered Fe‐dependent biosynthetic pathways and eventually oxidative stress resulting from an increased labile iron pool are suggested to play a role in several neurodegenerative diseases. Friedreich's ataxia is associated with mitochondrial iron accumulation and hampered ISC/heme biogenesis due to reduced frataxin expression, thus representing a monogenic mitochondrial disorder, which is clearly elicited solely by a disturbed iron metabolism. Less clear are the controversially discussed impacts of iron dysregulation and iron‐dependent oxidative stress in the most common neurodegenerative disorders, i.e. Alzheimer's disease (AD) and Parkinson's disease (PD). Amyotrophic lateral sclerosis (ALS) may be viewed as a disease offering a better support for a direct link between iron, oxidative stress and regional neurodegeneration. Altogether, despite significant progress in molecular knowledge, the true impact of iron on the sporadic forms of AD, PD and ALS is still uncertain. Here we summarize the current knowledge of iron metabolism disturbances in neurodegenerative disorders.     

Iron–sulfur cluster synthesis,iron homeostasis and oxidative stress in Friedreich ataxia

Friedreich ataxia (FRDA) is an autosomal recessive, multi-systemic degenerative disease that results from reduced synthesis of the mitochondrial protein frataxin. Frataxin has been intensely studied since its deficiency was linked to FRDA in 1996. The defining properties of frataxin – (i) the ability to bind iron, (ii) the ability to interact with, and donate iron to, other iron-binding proteins, and (iii) the ability to oligomerize, store iron and control iron redox chemistry – have been extensively characterized with different frataxin orthologs and their interacting protein partners. This very large body of biochemical and structural data [reviewed in (Bencze et al., 2006)] supports equally extensive biological evidence that frataxin is critical for mitochondrial iron metabolism and overall cellular iron homeostasis and antioxidant protection [reviewed in (Wilson, 2006)]. However, the precise biological role of frataxin remains a matter of debate. Here, we review seminal and recent data that strongly link frataxin to the synthesis of iron–sulfur cluster cofactors (ISC), as well as controversial data that nevertheless link frataxin to additional iron-related processes. Finally, we discuss how defects in ISC synthesis could be a major (although likely not unique) contributor to the pathophysiology of FRDA via (i) loss of ISC-dependent enzymes, (ii) mitochondrial and cellular iron dysregulation, and (iii) enhanced iron-mediated oxidative stress. This article is part of a Special Issue entitled ‘Mitochondrial function and dysfunction in neurodegeneration’.     

The pathogenesis of Friedreich ataxia and the structure and function of frataxin

Journal of Neurology - Understanding the role of frataxin in mitochondria is key to an understanding of the pathogenesis of Friedreich ataxia. Frataxins are small essential proteins whose...     

Oxidative stress, mitochondrial dysfunction and cellular stress response in Friedreich's ataxia

There is significant evidence that the pathogenesis of several neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease, Friedreich's ataxia (FRDA), multiple sclerosis and amyotrophic lateral sclerosis, may involve the generation of reactive oxygen species (ROS) and/or reactive nitrogen species (RNS) associated with mitochondrial dysfunction. The mitochondrial genome may play an essential role in the pathogenesis of these diseases, and evidence for mitochondria being a site of damage in neurodegenerative disorders is based in part on observed decreases in the respiratory chain complex activities in Parkinson's, Alzheimer's, and Huntington's disease. Such defects in respiratory complex activities, possibly associated with oxidant/antioxidant imbalance, are thought to underlie defects in energy metabolism and induce cellular degeneration. The precise sequence of events in FRDA pathogenesis is uncertain. The impaired intramitochondrial metabolism with increased free iron levels and a defective mitochondrial respiratory chain, associated with increased free radical generation and oxidative damage, may be considered possible mechanisms that compromise cell viability. Recent evidence suggests that frataxin might detoxify ROS via activation of glutathione peroxidase and elevation of thiols, and in addition, that decreased expression of frataxin protein is associated with FRDA. Many approaches have been undertaken to understand FRDA, but the heterogeneity of the etiologic factors makes it difficult to define the clinically most important factor determining the onset and progression of the disease. However, increasing evidence indicates that factors such as oxidative stress and disturbed protein metabolism and their interaction in a vicious cycle are central to FRDA pathogenesis. Brains of FRDA patients undergo many changes, such as disruption of protein synthesis and degradation, classically associated with the heat shock response, which is one form of stress response. Heat shock proteins are proteins serving as molecular chaperones involved in the protection of cells from various forms of stress. In the central nervous system, heat shock protein (HSP) synthesis is induced not only after hyperthermia, but also following alterations in the intracellular redox environment. The major neurodegenerative diseases, Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), Huntington's disease (HD) and FRDA are all associated with the presence of abnormal proteins. Among the various HSPs, HSP32, also known as heme oxygenase I (HO-1), has received considerable attention, as it has been recently demonstrated that HO-1 induction, by generating the vasoactive molecule carbon monoxide and the potent antioxidant bilirubin, could represent a protective system potentially active against brain oxidative injury. Given the broad cytoprotective properties of the heat shock response there is now strong interest in discovering and developing pharmacological agents capable of inducing the heat shock response. This may open up new perspectives in medicine, as molecules inducing this defense mechanism appear to be possible candidates for novel cytoprotective strategies. In particular, manipulation of endogenous cellular defense mechanisms, such as the heat shock response, through nutritional antioxidants, pharmacological compounds or gene transduction, may represent an innovative approach to therapeutic intervention in diseases causing tissue damage, such as neurodegeneration.     

Therapeutic developments in friedreich ataxia

Friedreich ataxia is an inherited, severe, progressive neuro- and cardiodegenerative disorder for which there currently is no approved therapy. Friedreich ataxia is caused by the decreased expression and/or function of frataxin, a mitochondrial matrix protein that binds iron and is involved in the formation of iron-sulfur clusters. Decreased frataxin function leads to decreased iron-sulfur cluster formation, mitochondrial iron accumulation, cytosolic iron depletion, oxidative stress, and mitochondrial dysfunction. Cloning of the disease gene for Friedreich ataxia and elucidation of many aspects of the biochemical defects underlying the disorder have led to several major therapeutic initiatives aimed at increasing frataxin expression, reversing mitochondrial iron accumulation, and alleviating oxidative stress. These initiatives are in preclinical and clinical development and are reviewed herein.     

The molecular pathogenesis of Friedreich ataxia

Friedreich ataxia, the most frequent cause of recessive ataxia is due in most cases to a homozygous intronic expansion resulting in the loss of function of frataxin. Frataxin is a mitochondrial protein conserved through evolution. Yeast knock-out models and histological data from patients heart autopsies have shown that frataxin defect causes mitochondrial iron accumulation. Biochemical data from patients heart biopsies or autopsies have revealed a specific deficiency in the activities of aconitases and of mitochondrial iron–sulfur proteins. These results suggest that frataxin may play a role either in mitochondrial iron transport or in iron–sulfur cluster assembly or transport. Iron abnormalities suggest a pathogenic mechanism involving free radicals production and oxidative stress, a process that might be sensitive to anti-oxidant therapies.     

Friedreich's ataxia

Friedreich’s ataxia, the most common hereditary ataxia, is caused by expansion of a GAA triplet located within the first intron of the frataxin gene on chromosome 9q13. There is a clear correlation between size of the expanded repeat and severity of the phenotype. Frataxin is a mitochondrial protein that plays a role in iron homeostasis. Deficiency of frataxin results in mitochondrial iron accumulation, defects in specific mitochondrial enzymes, enhanced sensitivity to oxidative stress, and eventually free-radical mediated cell death. Friedreich’s ataxia is considered a nuclear encoded mitochondrial disease.

This review discusses the major and rapid progress made in Friedreich’s ataxia from gene mapping and identification of the gene to pathogenesis and encouraging therapeutic implications.     

Mesenchymal Stem Cell-Derived Factors Restore Function to Human Frataxin-Deficient Cells

Friedreich’s ataxia is an inherited neurological disorder characterised by mitochondrial dysfunction and increased susceptibility to oxidative stress. At present, no therapy has been shown to reduce disease progression. Strategies being trialled to treat Friedreich’s ataxia include drugs that improve mitochondrial function and reduce oxidative injury. In addition, stem cells have been investigated as a potential therapeutic approach. We have used siRNA-induced knockdown of frataxin in SH-SY5Y cells as an in vitro cellular model for Friedreich’s ataxia. Knockdown of frataxin protein expression to levels detected in patients with the disorder was achieved, leading to decreased cellular viability, increased susceptibility to hydrogen peroxide-induced oxidative stress, dysregulation of key anti-oxidant molecules and deficiencies in both cell proliferation and differentiation. Bone marrow stem cells are being investigated extensively as potential treatments for a wide range of neurological disorders, including Friedreich’s ataxia. The potential neuroprotective effects of bone marrow-derived mesenchymal stem cells were therefore studied using our frataxin-deficient cell model. Soluble factors secreted by mesenchymal stem cells protected against cellular changes induced by frataxin deficiency, leading to restoration in frataxin levels and anti-oxidant defences, improved survival against oxidative stress and stimulated both cell proliferation and differentiation down the Schwann cell lineage. The demonstration that mesenchymal stem cell-derived factors can restore cellular homeostasis and function to frataxin-deficient cells further suggests that they may have potential therapeutic benefits for patients with Friedreich’s ataxia.     

Iron metabolism in mice with partial frataxin deficiency

Friedreich ataxia (FRDA), the most common autosomal recessive inherited ataxic disorder, is the consequence of deficiency of the mitochondrial protein frataxin, typically caused by homozygous intronic GAA expansions in the corresponding gene. The yeast frataxin homologue (yfh1p) is required for cellular respiration. Yfh1p appears to regulate mitochondrial iron homeostasis and protect from free radical toxicity. Complete loss of frataxin in knockout mice leads to early embryonic lethality, indicating an important role for frataxin during development. Heterozygous littermates with partial frataxin deficiency are apparently healthy and have no obvious phenotype. Here we evaluate iron metabolism and sensitivity to dietary and parenteral iron loading in heterozygote frataxin knockout mice (Fx(+/-)). Iron concentrations in the liver, heart, pancreas and spleen, and cellular iron distribution patterns were compared between wild type and Fx(+/-) mice. Response to parenteral iron challenge was not different between Fx(+/-) mice and wild type littermates, while sporadic iron deposits were observed in the hearts of dietary iron-loaded Fx(+/-) mice. Finally, we evaluated the effect of partial frataxin deficiency on susceptibility to cardiac damage in the mouse model of hereditary hemochromatosis (HH), the Hfe knockout mice. HH, an iron overload disease, is one of the most frequent genetic diseases in populations of European origin. By breeding Hfe(-/-) with Fx(+/-) mice, we obtained compound mutant mice lacking both Hfe and one frataxin allele. Sparse iron deposits in areas of mild to moderate cardiac fibrosis were found in the majority of these mice. However, they did not develop any neurological symptoms. Our studies indicate an association between frataxin deficiency, iron deposits and cardiac fibrosis, but no obvious association between iron accumulation and neurodegeneration similar to FRDA could be detected in our model. In addition, these results suggest that frataxin mutations may have a modifier role in HH, that predisposes to cardiomyopathy.     

Friedreich ataxia: new pathways

Friedreich ataxia is a rare disorder characterized by an autosomal recessive pattern of inheritance. The disease is noted for a constellation of clinical symptoms, notably loss of coordination and a variety of neurologic and cardiac complications. More recently, scientists have focused their research on an array of general investigations of the underlying cellular basis for the disease, including mitochondrial biogenesis, iron-sulfur cluster synthesis, iron metabolism, antioxidant responses, and mitophagy. Combined with investigations that have explored the pathogenesis of the disease and the function of the protein frataxin, these studies have led to insights that will be key to identifying new therapeutic strategies for treating the disease.     

Iron-dependent functions of mitochondria—relation to neurodegeneration

A number of neurodegenerative diseases are associated with iron dyshomeostasis and mitochondrial dysfunction. However, the pathomechanistic interplay between iron and mitochondria varies. This review summarises the physiological role of iron in mitochondria and subsequently exemplifies two neurodegenerative diseases with disturbed iron function in mitochondria: inherited Friedreich ataxia (FRDA) and idiopathic Parkinson disease (PD). In eukaryotes, mitochondria are main consumers of iron. The respiratory chain relies on iron-containing redox systems in the form of complexes I–III with iron–sulphur clusters and cytochromes with haem as prosthetic groups. The bifunctional enzyme aconitase is not only important in the citric acid cycle, but also functions as a key regulator of cell iron metabolism. Haem biosynthesis occurs partially in mitochondria as well as the biogenesis of iron–sulphur clusters that are co-factors in numerous iron–sulphur proteins. FRDA is characterised by a mutation of the frataxin gene, the protein of which serves as an iron chaperone in iron–sulphur cluster assembly. The lack of frataxin expression leads to defective iron–sulphur cluster biogenesis with decreased respiratory and aconitase activity. The resulting mitochondrial iron overload might fuel reactive oxygen species formation and contribute to clinical signs of oxidative stress. PD is typically associated with an increased iron content of the substantia nigra, the causes of which are largely unknown. Recent research demonstrated raised iron levels in individual dopaminergic neurons of the substantia nigra. Moreover, transferrin/transferrin receptor 2 mediated transport of iron into the mitochondria of these neurons was identified together with increased transferrin immunoreactivity. Resulting accumulation of iron into mitochondria might lead to oxidative stress damaging iron–sulphur cluster-containing proteins.     

Interaction of metals with prion protein: possible role of divalent cations in the pathogenesis of prion diseases

Prion diseases are fatal neurodegenerative disorders that affect both humans and animals. The rapid clinical progression, change in protein conformation, cross-species transmission and massive neuronal degeneration are some key features of this devastating degenerative condition. Although the etiology is unknown, aberrant processing of cellular prion proteins is well established in the pathogenesis of prion diseases. Normal cellular prion protein (PrP(c)) is highly conserved in mammals and expressed predominantly in the brain. Nevertheless, the exact function of the normal prion protein in the CNS has not been fully elucidated. Prion proteins may function as a metal binding protein because divalent cations such as copper, zinc and manganese can bind to octapeptide repeat sequences in the N-terminus of PrP(c). Since the binding of these metals to the octapeptide has been proposed to influence both structural and functional properties of prion proteins, alterations in transition metal levels can alter the course of the disease. Furthermore, cellular antioxidant capacity is significantly compromised due to conversion of the normal prion protein (PrP(c)) to an abnormal scrapie prion (PrP(sc)) protein, suggesting that oxidative stress may play a role in the neurodegenerative process of prion diseases. The combination of imbalances in cellular transition metals and increased oxidative stress could further exacerbate the neurotoxic effect of PrP(sc). This review includes an overview of the structure and function of prion proteins, followed by the role of metals such as copper, manganese and iron in the physiological function of the PrP(c), and the possible role of transition metals in the pathogenesis of the prion disease.     

Therapeutic strategies in Friedreich's ataxia

Friedreich's ataxia is caused by a pronounced lack of frataxin, a mitochondrial protein of not fully understood function. Lack of frataxin homologues in yeast and mice leads to increased sensitivity to oxidative stress, depletion of proteins with iron-sulfur clusters like respiratory chain complexes I-III and aconitase, and to iron accumulation in mitochondria. Similar effects have been demonstrated in human disease with increased markers of oxidative DNA damage in urine and impaired oxidative phosphorylation in in vivo exercise studies using 31 Phosphorus magnetic resonance spectroscopy (31P-MRS). Therapeutical trials mainly focus on antioxidative treatment with coenzyme Q10 or its short-chain variant idebenone. Promising effects on cardiac hypertrophy in uncontrolled preliminary studies contrast with minor effects in controlled trials and no effect of antioxidants on neurological deficits has been established. Preliminary encouraging 31P-MRS data exist for the treatment with L-carnitine but not with creatine. However, all these interventions may take effect too late in the pathogenic process. Alternative strategies aiming at an enhancement of frataxin by stem cell transplantation, gene transfer or frataxin supplementation are desirable. Additionally, more efficient biomarkers are needed to monitor treatment effects.     

Cardiomyopathy in Friedreich's ataxia

Friedreich's ataxia (FRDA), an autosomal recessive disorder, is characterized by spinocerebellar degeneration and cardiomyopathy. Here we explore some of the putative mechanisms underlying the cardiomyopathy in FRDA that have been elucidated using different experimental models. FRDA is characterized by a deficiency in frataxin, a protein vital in iron handling. Iron accumulation, lack of functional iron-sulphur clusters, and oxidative stress seem to be among the most important consequences of frataxin deficiency explaining the cardiac abnormalities in FRDA.     

Ferric cycle activity and Alzheimer disease

Elevated plasma homocysteine is an independent risk factor for the development of Alzheimer disease, however, the precise mechanisms underlying this are unclear. In this article, we expound on a novel hypothesis depicting the involvement of homocysteine in a vicious circle involving iron dysregulation and oxidative stress designated as the ferric cycle (Dwyer et al., 2004). Moreover, we suspect that the development of a critical heme deficiency in vulnerable neurons is an additional consequence of ferric cycle activity. Oxidative stress and heme deficiency are consistent with many pathological changes found in Alzheimer disease including mitochondrial abnormalities and impaired energy metabolism, cell cycle and cell signaling abnormalities, neuritic pathology, and other features of the disease involving alterations in iron homeostasis such as the abnormal expression of heme oxygenase-1 and iron response protein 2. Based on the ferric cycle concept, we have developed a model of Alzheimer disease development and progression, which offers an explanation for why sporadic Alzheimer disease is different than normal aging and why familial Alzheimer disease and sporadic Alzheimer disease could have different etiologies but a common end-stage.     

Friedreich's ataxia and iron metabolism

The possible causes of abnormal iron metabolism in patients with Friedreich's ataxia are considered. Reduced expression of a frataxin homologue in yeast is associated with mitochondrial iron accumulation at the expense of cytosolic iron, and the same phenomenon can be demonstrated in these patients. A decrease in cytosolic iron causes the expression of a high-affinity iron-uptake protein, and therefore Friedreich's ataxia can be considered to be a disease of abnormal intracellular iron distribution. Friedreich's ataxia is of autosomal recessive inheritance, and the gene associated with it has been mapped to chromosome 9. This encodes the protein frataxin which regulates mitochondrial iron transport. The commonest mutation causing this disorder is an expanded GAA repeat in the gene for this protein. Different point mutations may account for some of the variations in the phenotypic features that are often found, and these variations are discussed. These findings have raised therapeutic possibilities in a condition for which previously there was no specific treatment. There are intracellular enzymes which are very sensitive to injury by oxygen-free radicals. Treatment has therefore been tried with ibebenone which acts as a free-radical scavenger, with some evidence of improvement. Iron chelating agents, such as deferoxamine, have also been given, but the finding of normal serum iron and ferritin casts doubt on the rationale of this. However the finding that the accumulation of iron in the mitochondria of the cells in patients with this form of ataxia will cause oxidative stress and cell death, gives hope for more effective treatment in the future, possibly with gene therapy.     

Oxidative stress, antioxidants, and Alzheimer disease

Recent evidence in the field of Alzheimer disease research has highlighted the importance of oxidative processes in its pathogenesis. Examination of cellular changes shows that oxidative stress is an event that precedes the appearance of neurofibrillary tangles, one of the hallmark pathologies of the disease. Although it is still unclear what the initial source of the oxidative stress is in Alzheimer disease, it is likely that the process is highly dependent on the presence of redox-active transition metals, such as iron and copper. Because of the proximal role that oxidative stress mechanisms seem to play in the pathogenesis of Alzheimer disease, further investigation in this realm may lead to novel therapeutic strategies.     

Molecular pathogenesis of Friedreich ataxia.

Friedreich ataxia, the most common type of inherited ataxia, is itself caused in most cases by a large expansion of an intronic GAA repeat, resulting in decreased expression of the target frataxin gene. The autosomal recessive inheritance of the disease gives this triplet repeat mutation some unique features of natural history and evolution. Frataxin is a mitochondrial protein that has homologues in yeast and even in gram-negative bacteria. Yeast organisms deficient in the frataxin homologue accumulate iron in mitochondria and show increased sensitivity to oxidative stress. This suggests that Friedreich ataxia is caused by mitochondrial dysfunction and free radical toxicity.