Frataxin knockin mouse

Dyclonine rescues frataxin deficiency in animal models and buccal cells of patients with Friedreich's ataxia

Inherited deficiency in the mitochondrial protein frataxin (FXN) causes the rare disease Friedreich's ataxia (FA), for which there is no successful treatment. We identified a redox deficiency in FA cells and used this to model the disease. We screened a 1600-compound library to identify existing drugs, which could be of therapeutic benefit. We identified the topical anesthetic dyclonine as protective. Dyclonine increased FXN transcript and FXN protein dose-dependently in FA cells and brains of animal models. Dyclonine also rescued FXN-dependent enzyme deficiencies in the iron-sulfur enzymes, aconitase and succinate dehydrogenase. Dyclonine induces the Nrf2 [nuclear factor (erythroid-derived 2)-like 2] transcription factor, which we show binds an upstream response element in the FXN locus. Additionally, dyclonine also inhibited the activity of histone methyltransferase G9a, known to methylate histone H3K9 to silence FA chromatin. Chronic dosing in a FA mouse model prevented a performance decline in balance beam studies. A human clinical proof-of-concept study was completed in eight FA patients dosed twice daily using a 1% dyclonine rinse for 1 week. Six of the eight patients showed an increase in buccal cell FXN levels, and fold induction was significantly correlated with disease severity. Dyclonine represents a novel therapeutic strategy that can potentially be repurposed for the treatment of FA.

Frataxin deficiency increases cyclooxygenase 2 and prostaglandins in cell and animal models of Friedreich's ataxia

An inherited deficiency of the mitochondrial protein frataxin causes Friedreich's ataxia (FRDA); the mechanism by which this deficiency triggers neuro- and cardio-degeneration is unclear. Microarrays of neural tissue of animal models of the disease showed decreases in antioxidant genes, and increases in inflammatory genes. Cyclooxygenase (COX)-derived oxylipins are important mediators of inflammation. We measured oxylipin levels using tandem mass spectrometry and ELISAs in multiple cell and animal models of FRDA. Mass spectrometry revealed increases in concentrations of prostaglandins, thromboxane B2, 15-HETE and 11-HETE in cerebellar samples of knockin knockout mice. One possible explanation for the elevated oxylipins is that frataxin deficiency results in increased COX activity. While constitutive COX1 was unchanged, inducible COX2 expression was elevated over 1.35-fold (P < 0.05) in two Friedreich's mouse models and Friedreich's lymphocytes. Consistent with higher COX2 expression, its activity was also increased by 58% over controls. COX2 expression is driven by multiple transcription factors, including activator protein 1 and cAMP response element-binding protein, both of which were elevated over 1.52-fold in cerebella. Taken together, the results support the hypothesis that reduced expression of frataxin leads to elevation of COX2-mediated oxylipin synthesis stimulated by increases in transcription factors that respond to increased reactive oxygen species. These findings support a neuroinflammatory mechanism in FRDA, which has both pathomechanistic and therapeutic implications.

The role of frataxin in fission yeast iron metabolism: implications for Friedreich's ataxia

BACKGROUND

The neurodegenerative disease Friedreich's ataxia is the result of frataxin deficiency. Frataxin is a mitochondrial protein involved in iron-sulfur cluster (Fe-S) cofactor biogenesis, but its functional role in this pathway is debated. This is due to the interconnectivity of iron metabolic and oxidative stress response pathways that make distinguishing primary effects of frataxin deficiency challenging. Since Fe-S cluster assembly is conserved, frataxin overexpression phenotypes in a simple eukaryotic organism will provide additional insight into frataxin function.

METHODS

The Schizosaccharomyces pombe frataxin homologue (fxn1) was overexpressed from a plasmid under a thiamine repressible promoter. The S. pombe transformants were characterized at several expression strengths for cellular growth, mitochondrial organization, iron levels, oxidative stress, and activities of Fe-S cluster containing enzymes.

RESULTS

Observed phenotypes were dependent on the amount of Fxn1 overexpression. High Fxn1 overexpression severely inhibited S. pombe growth, impaired mitochondrial membrane integrity and cellular respiration, and led to Fxn1 aggregation. Cellular iron accumulation was observed at moderate Fxn1 overexpression but was most pronounced at high levels of Fxn1. All levels of Fxn1 overexpression up-regulated oxidative stress defense and mitochondrial Fe-S cluster containing enzyme activities.

CONCLUSIONS

Despite the presence of oxidative stress and accumulated iron, activation of Fe-S cluster enzymes was common to all levels of Fxn1 overexpression; therefore, Fxn1 may regulate the efficiency of Fe-S cluster biogenesis in S. pombe.

GENERAL SIGNIFICANCE

We provide evidence that suggests that dysregulated Fe-S cluster biogenesis is a primary effect of both frataxin overexpression and deficiency as in Friedreich's ataxia.

Iron-sulfur cluster biogenesis in mammalian cells: New insights into the molecular mechanisms of cluster delivery

Iron-sulfur (Fe-S) clusters are ancient, ubiquitous cofactors composed of iron and inorganic sulfur. The combination of the chemical reactivity of iron and sulfur, together with many variations of cluster composition, oxidation states and protein environments, enables Fe-S clusters to participate in numerous biological processes. Fe-S clusters are essential to redox catalysis in nitrogen fixation, mitochondrial respiration and photosynthesis, to regulatory sensing in key metabolic pathways (i.e. cellular iron homeostasis and oxidative stress response), and to the replication and maintenance of the nuclear genome. Fe-S cluster biogenesis is a multistep process that involves a complex sequence of catalyzed protein-protein interactions and coupled conformational changes between the components of several dedicated multimeric complexes. Intensive studies of the assembly process have clarified key points in the biogenesis of Fe-S proteins. However several critical questions still remain, such as: what is the role of frataxin? Why do some defects of Fe-S cluster biogenesis cause mitochondrial iron overload? How are specific Fe-S recipient proteins recognized in the process of Fe-S transfer? This review focuses on the basic steps of Fe-S cluster biogenesis, drawing attention to recent advances achieved on the identification of molecular features that guide selection of specific subsets of nascent Fe-S recipients by the cochaperone HSC20. Additionally, it outlines the distinctive phenotypes of human diseases due to mutations in the components of the basic pathway. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Cellular, molecular and functional characterisation of YAC transgenic mouse models of Friedreich ataxia

BACKGROUND

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder, caused by a GAA repeat expansion mutation within intron 1 of the FXN gene. We have previously established and performed preliminary characterisation of several human FXN yeast artificial chromosome (YAC) transgenic FRDA mouse models containing GAA repeat expansions, Y47R (9 GAA repeats), YG8R (90 and 190 GAA repeats) and YG22R (190 GAA repeats).

METHODOLOGY/PRINCIPAL FINDINGS

We now report extended cellular, molecular and functional characterisation of these FXN YAC transgenic mouse models. FXN transgene copy number analysis of the FRDA mice demonstrated that the YG22R and Y47R lines each have a single copy of the FXN transgene while the YG8R line has two copies. Single integration sites of all transgenes were confirmed by fluorescence in situ hybridisation (FISH) analysis of metaphase and interphase chromosomes. We identified significant functional deficits, together with a degree of glucose intolerance and insulin hypersensitivity, in YG8R and YG22R FRDA mice compared to Y47R and wild-type control mice. We also confirmed increased somatic GAA repeat instability in the cerebellum and brain of YG22R and YG8R mice, together with significantly reduced levels of FXN mRNA and protein in the brain and liver of YG8R and YG22R compared to Y47R.

CONCLUSIONS/SIGNIFICANCE

Together these studies provide a detailed characterisation of our GAA repeat expansion-based YAC transgenic FRDA mouse models that will help investigations of FRDA disease mechanisms and therapy.

From mice to men: lessons from mutant ataxic mice

Ataxic mutant mice can be used to represent models of cerebellar degenerative disorders. They serve for investigation of cerebellar function, pathogenesis of degenerative processes as well as of therapeutic approaches. Lurcher, Hot-foot, Purkinje cell degeneration, Nervous, Staggerer, Weaver, Reeler, and Scrambler mouse models and mouse models of SCA1, SCA2, SCA3, SCA6, SCA7, SCA23, DRPLA, Niemann-Pick disease and Friedreich ataxia are reviewed with special regard to cerebellar pathology, pathogenesis, functional changes and possible therapeutic influences, if any. Finally, benefits and limitations of mouse models are discussed.

Dysregulation of cellular iron metabolism in Friedreich ataxia: from primary iron-sulfur cluster deficit to mitochondrial iron accumulation

Friedreich ataxia (FRDA) is the most common recessive ataxia in the Caucasian population and is characterized by a mixed spinocerebellar and sensory ataxia frequently associating cardiomyopathy. The disease results from decreased expression of the FXN gene coding for the mitochondrial protein frataxin. Early histological and biochemical study of the pathophysiology in patient's samples revealed that dysregulation of iron metabolism is a key feature of the disease, mainly characterized by mitochondrial iron accumulation and by decreased activity of iron-sulfur cluster enzymes. In the recent past years, considerable progress in understanding the function of frataxin has been provided through cellular and biochemical approaches, pointing to the primary role of frataxin in iron-sulfur cluster biogenesis. However, why and how the impact of frataxin deficiency on this essential biosynthetic pathway leads to mitochondrial iron accumulation is still poorly understood. Herein, we review data on both the primary function of frataxin and the nature of the iron metabolism dysregulation in FRDA. To date, the pathophysiological implication of the mitochondrial iron overload in FRDA remains to be clarified.

Pathophysiogical and therapeutic progress in Friedreich ataxia

Friedreich ataxia (FRDA) is the most common hereditary autosomal recessive ataxia, but is also a multisystemic condition with frequent presence of cardiomyopathy or diabetes. It has been linked to expansion of a GAA-triplet repeat in the first intron of the FXN gene, leading to a reduced level of frataxin, a mitochondrial protein which, by controlling both iron entry and/or sulfide production, is essential to properly assemble and protect the Fe-S cluster during the initial stage of biogenesis. Several data emphasize the role of oxidative damage in FRDA, but better understanding of pathophysiological consequences of FXN mutations has led to develop animal models. Conditional knockout models recapitulate important features of the human disease but lack the genetic context, GAA repeat expansion-based knock-in and transgenic models carry a GAA repeat expansion but they only show a very mild phenotype. Cells derived from FRDA patients constitute the most relevant frataxin-deficient cell model as they carry the complete frataxin locus together with GAA repeat expansions and regulatory sequences. Induced pluripotent stem cell (iPSC)-derived neurons present a maturation delay and lower mitochondrial membrane potential, while cardiomyocytes exhibit progressive mitochondrial degeneration, with frequent dark mitochondria and proliferation/accumulation of normal mitochondria. Efforts in developing therapeutic strategies can be divided into three categories: iron chelators, antioxidants and/or stimulants of mitochondrial biogenesis, and frataxin level modifiers. A promising therapeutic strategy that is currently the subject of intense research is to directly target the heterochromatin state of the GAA repeat expansion with histone deacytelase inhibitors (HDACi) to restore frataxin levels.

Methylene blue rescues heart defects in a Drosophila model of Friedreich's ataxia

Friedreich's ataxia (FRDA), the most common hereditary ataxia, is characterized by progressive degeneration of the central and peripheral nervous system, hypertrophic cardiomyopathy and a high risk of diabetes. FRDA is caused by abnormally low levels of frataxin, a highly conserved mitochondrial protein. Drosophila has been previously successfully used to model FRDA in various cell types, including neurons and glial cells. Here, we report the development of a Drosophila cardiac model of FRDA. In vivo heart imaging revealed profound impairments in heart function in frataxin-depleted Drosophila, including a strong increase in end-systolic and end-diastolic diameters and a decrease in fractional shortening (FS). These features, reminiscent of pathological phenotypes in humans, are fully rescued by complementation with human frataxin, suggesting conserved cardiac functions of frataxin between the two organisms. Oxidative stress is not a major factor of heart impairment in frataxin-depleted flies, suggesting the involvement of other pathological mechanisms notably mitochondrial respiratory chain (MRC) dysfunction. Accordingly, we report that methylene blue (MB), a compound known to act as an alternative electron carrier that bypasses mitochondrial complexes I-III, was able to prevent heart dysfunction. MB also partially rescued the phenotype when administered post-symptomatically. Analysis of MB derivatives demonstrates that only compounds with electron carrier properties are able to prevent the heart phenotype. Thus MB, a compound already used for several clinical applications, appears promising for the treatment of the heart dysfunctions that are a major cause of death of FRDA patients. This work provides the grounds for further evaluation of MB action in mammals.

Optimizing mouse models of neurodegenerative disorders: are therapeutics in sight?

The genomic and biologic conservation between mice and humans, along with our increasing ability to manipulate the mouse genome, places the mouse as a premier model for deciphering disease mechanisms and testing potential new therapies. Despite these advantages, mouse models of neurodegenerative disease are sometimes difficult to generate and can present challenges that must be carefully addressed when used for preclinical studies. For those models that do exist, the standardization and optimization of the models is a critical step in ensuring success in both basic research and preclinical use. This review looks back on the history of model development for neurodegenerative diseases and highlights the key strategies that have been learned in order to improve the design, development and use of mouse models in the study of neurodegenerative disease.

Clinical neurogenetics: friedreich ataxia

Friedreich ataxia is the most common autosomal recessive ataxia. It is a progressive neurodegenerative disorder, typically with onset before 20 years of age. Signs and symptoms include progressive ataxia, ascending weakness and ascending loss of vibration and joint position senses, pes cavus, scoliosis, cardiomyopathy, and arrhythmias. There are no disease-modifying medications to either slow or halt the progression of the disease, but research investigating therapies to increase endogenous frataxin production and decrease the downstream consequences of disrupted iron homeostasis is ongoing. Clinical trials of promising medications are underway, and the treatment era of Friedreich ataxia is beginning.

Modelling biochemical features of mitochondrial neuropathology

BACKGROUND

The neuropathology of mitochondrial disease is well characterised. However, pathophysiological mechanisms at the level of biochemistry and cell biology are less clear. Progress in this area has been hampered by the limited accessibility of neurologically relevant material for analysis.

SCOPE OF REVIEW

Here we discuss the recent development of a variety of model systems that have greatly extended our capacity to understand the biochemical features associated with mitochondrial neuropathology. These include animal and cell based models, with mutations in both nuclear and mitochondrial DNA encoded genes, which aim to recapitulate the neuropathology and cellular biochemistry of mitochondrial diseases.

MAJOR CONCLUSIONS

Analysis of neurological tissue and cells from these models suggests that although there is no unifying mode of pathogenesis, dysfunction of the oxidative phosphorylation (OXPHOS) system is often central. This can be associated with altered reactive oxygen species (ROS) generation, disruption of the mitochondrial membrane potential (ΔΨm) and inadequate ATP synthesis. Thus, other cellular processes such as calcium (Ca(2+)) homeostasis, cellular signaling and mitochondrial morphology could be altered, ultimately compromising viability of neuronal cells.

GENERAL SIGNIFICANCE

Mechanisms of neuronal dysfunction in mitochondrial disease are only just beginning to be characterised, are system dependent and complex, and not merely driven by energy deficiency. The diversity of pathogenic mechanisms emphasises the need for characterisation in a wide range of models, as different therapeutic strategies are likely to be needed for different diseases. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Molecular and functional alterations in a mouse cardiac model of Friedreich ataxia: activation of the integrated stress response, eIF2α phosphorylation, and the induction of downstream targets

Friedreich ataxia (FA) is a neurodegenerative and cardiodegenerative disease resulting from marked frataxin deficiency. The condition is characterized by ataxia with fatal cardiomyopathy, but the pathogenic mechanisms are unclear. We investigated the association between gene expression and progressive histopathological and functional changes using the muscle creatine kinase conditional frataxin knockout (KO) mouse; this mouse develops a severe cardiac phenotype that resembles that of FA patients. We examined KO mice from 3 weeks of age, when they are asymptomatic, to 10 weeks of age, when they die of the disease. Positive iron staining was identified in KO mice from 5 weeks of age, with markedly reduced cardiac function from 6 weeks. We identified an early and marked up-regulation of a gene cohort responsible for stress-induced amino acid biosynthesis and observed markedly increased phosphorylation of eukaryotic translation initiation factor 2α (p-eIF2α), an activator of the integrated stress response, in KO mice at 3 weeks of age, relative to wild-type mice. Importantly, the eIF2α-mediated integrated stress response has been previously implicated in heart failure via downstream processes such as autophagy and apoptosis. Indeed, expression of a panel of autophagy and apoptosis markers was enhanced in KO mice. Thus, the pathogenesis of cardiomyopathy in FA correlates with the early and persistent eIF2α phosphorylation, which precedes activation of autophagy and apoptosis.

Animal and cellular models of Friedreich ataxia

The development and use of animal and cellular models of Friedreich ataxia (FRDA) are essential requirements for the understanding of FRDA disease mechanisms and the investigation of potential FRDA therapeutic strategies. Although animal and cellular models of lower organisms have provided valuable information on certain aspects of FRDA disease and therapy, it is intuitive that the most useful models are those of mammals and mammalian cells, which are the closest in physiological terms to FRDA patients. To date, there have been considerable efforts put into the development of several different FRDA mouse models and relevant FRDA mouse and human cell line systems. We summarize the principal mammalian FRDA models, discuss the pros and cons of each system, and describe the ways in which such models have been used to address two of the fundamental, as yet unanswered, questions regarding FRDA. Namely, what is the exact pathophysiology of FRDA and what is the detailed genetic and epigenetic basis of FRDA?

Increasing frataxin gene expression with histone deacetylase inhibitors as a therapeutic approach for Friedreich's ataxia

The genetic defect in Friedreich's ataxia (FRDA) is the expansion of a GAA·TCC triplet in the first intron of the FXN gene, which encodes the mitochondrial protein frataxin. Previous studies have established that the repeats reduce transcription of this essential gene, with a concomitant decrease in frataxin protein in affected individuals. As the repeats do not alter the FXN protein coding sequence, one therapeutic approach would be to increase transcription of pathogenic FXN genes. Histone posttranslational modifications near the expanded repeats are consistent with heterochromatin formation and FXN gene silencing. In an effort to find small molecules that would reactivate this silent gene, histone deacetylase inhibitors were screened for their ability to up-regulate FXN gene expression in patient cells and members of the pimelic 2-aminobenzamide family of class I histone deacetylase inhibitors were identified as potent inducers of FXN gene expression and frataxin protein. Importantly, these molecules up-regulate FXN expression in human neuronal cells derived from patient-induced pluripotent stem cells and in two mouse models for the disease. Preclinical studies of safety and toxicity have been completed for one such compound and a phase I clinical trial in FRDA patients has been initiated. Furthermore, medicinal chemistry efforts have identified improved compounds with superior pharmacological properties.

Diabetes in Friedreich ataxia

Diabetes is a common metabolic disorder in patients with Friedreich ataxia. In this Supplement article, we review the clinical data on diabetes in Friedreich ataxia, and the experimental data from rodent and in vitro models of the disease. Increased body adiposity and insulin resistance are frequently present in Friedreich ataxia, but pancreatic β cell dysfunction and death are a conditio sine qua non for the loss of glucose tolerance and development of diabetes. The loss of frataxin function in mitochondria accounts for these pathogenic processes in Friedreich ataxia. Mitochondria are essential for the sensing of nutrients by the β cell and for the generation of signals that trigger and amplify insulin secretion, known as stimulus-secretion coupling. Moreover, in the intrinsic pathway of apoptosis, pro-apoptotic signals converge on mitochondria, resulting in mitochondrial Bax translocation, membrane permeabilization, cytochrome c release and caspase cleavage. How and at which level frataxin deficiency impacts on these processes in β cells is only partially understood. A better understanding of the molecular mechanisms mediating β cell demise in Friedreich ataxia will pave the way for new therapeutic approaches.

Central role and mechanisms of β-cell dysfunction and death in friedreich ataxia-associated diabetes

OBJECTIVE

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused in almost all cases by homozygosity for a GAA trinucleotide repeat expansion in the frataxin gene. Frataxin is a mitochondrial protein involved in iron homeostasis. FRDA patients have a high prevalence of diabetes, the pathogenesis of which is not known. We aimed to evaluate the relative contribution of insulin resistance and β-cell failure and the pathogenic mechanisms involved in FRDA diabetes.

METHODS

Forty-one FRDA patients, 26 heterozygous carriers of a GAA expansion, and 53 controls underwent oral and intravenous glucose tolerance tests. β-Cell proportion was quantified in postmortem pancreas sections from 9 unrelated FRDA patients. Using an in vitro disease model, we studied how frataxin deficiency affects β-cell function and survival.

RESULTS

FRDA patients had increased abdominal fat and were insulin resistant. This was not compensated for by increased insulin secretion, resulting in a markedly reduced disposition index, indicative of pancreatic β-cell failure. Loss of glucose tolerance was driven by β-cell dysfunction, which correlated with abdominal fatness. In postmortem pancreas sections, pancreatic islets of FRDA patients had a lower β-cell content. RNA interference-mediated frataxin knockdown impaired glucose-stimulated insulin secretion and induced apoptosis in rat β cells and human islets. Frataxin deficiency sensitized β cells to oleate-induced and endoplasmic reticulum stress-induced apoptosis, which could be prevented by the incretins glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide.

INTERPRETATION

Pancreatic β-cell dysfunction is central to diabetes development in FRDA as a result of mitochondrial dysfunction and higher sensitivity to metabolic and endoplasmic reticulum stress-induced β-cell death.

Neurons and cardiomyocytes derived from induced pluripotent stem cells as a model for mitochondrial defects in Friedreich's ataxia

Friedreich's ataxia (FRDA) is a recessive neurodegenerative disorder commonly associated with hypertrophic cardiomyopathy. FRDA is due to expanded GAA repeats within the first intron of the gene encoding frataxin, a conserved mitochondrial protein involved in iron-sulphur cluster biosynthesis. This mutation leads to partial gene silencing and substantial reduction of the frataxin level. To overcome limitations of current cellular models of FRDA, we derived induced pluripotent stem cells (iPSCs) from two FRDA patients and successfully differentiated them into neurons and cardiomyocytes, two affected cell types in FRDA. All FRDA iPSC lines displayed expanded GAA alleles prone to high instability and decreased levels of frataxin, but no biochemical phenotype was observed. Interestingly, both FRDA iPSC-derived neurons and cardiomyocytes exhibited signs of impaired mitochondrial function, with decreased mitochondrial membrane potential and progressive mitochondrial degeneration, respectively. Our data show for the first time that FRDA iPSCs and their neuronal and cardiac derivatives represent promising models for the study of mitochondrial damage and GAA expansion instability in FRDA.

Clinical data and characterization of the liver conditional mouse model exclude neoplasia as a non-neurological manifestation associated with Friedreich's ataxia

Friedreich's ataxia (FRDA) is the most common hereditary ataxia in the caucasian population and is characterized by a mixed spinocerebellar and sensory ataxia, hypertrophic cardiomyopathy and increased incidence of diabetes. FRDA is caused by impaired expression of the FXN gene coding for the mitochondrial protein frataxin. During the past ten years, the development of mouse models of FRDA has allowed better understanding of the pathophysiology of the disease. Among the mouse models of FRDA, the liver conditional mouse model pointed to a tumor suppressor activity of frataxin leading to the hypothesis that individuals with FRDA might be predisposed to cancer. In the present work, we investigated the presence and the incidence of neoplasia in the largest FRDA patient cohorts from the USA, Australia and Europe. As no predisposition to cancer could be observed in both cohorts, we revisited the phenotype of the liver conditional mouse model. Our results show that frataxin-deficient livers developed early mitochondriopathy, iron-sulfur cluster deficits and intramitochondrial dense deposits, classical hallmarks observed in frataxin-deficient tissues and cells. With age, a minority of mice developed structures similar to the ones previously associated with tumor formation. However, these peripheral structures contained dying, frataxin-deficient hepatocytes, whereas the inner liver structure was composed of a pool of frataxin-positive cells, due to inefficient Cre-mediated recombination of the Fxn gene, that contributed to regeneration of a functional liver. Together, our data demonstrate that frataxin deficiency and tumorigenesis are not associated.

Pms2 suppresses large expansions of the (GAA·TTC)n sequence in neuronal tissues

Expanded trinucleotide repeat sequences are the cause of several inherited neurodegenerative diseases. Disease pathogenesis is correlated with several features of somatic instability of these sequences, including further large expansions in postmitotic tissues. The presence of somatic expansions in postmitotic tissues is consistent with DNA repair being a major determinant of somatic instability. Indeed, proteins in the mismatch repair (MMR) pathway are required for instability of the expanded (CAG·CTG)(n) sequence, likely via recognition of intrastrand hairpins by MutSβ. It is not clear if or how MMR would affect instability of disease-causing expanded trinucleotide repeat sequences that adopt secondary structures other than hairpins, such as the triplex/R-loop forming (GAA·TTC)(n) sequence that causes Friedreich ataxia. We analyzed somatic instability in transgenic mice that carry an expanded (GAA·TTC)(n) sequence in the context of the human FXN locus and lack the individual MMR proteins Msh2, Msh6 or Pms2. The absence of Msh2 or Msh6 resulted in a dramatic reduction in somatic mutations, indicating that mammalian MMR promotes instability of the (GAA·TTC)(n) sequence via MutSα. The absence of Pms2 resulted in increased accumulation of large expansions in the nervous system (cerebellum, cerebrum, and dorsal root ganglia) but not in non-neuronal tissues (heart and kidney), without affecting the prevalence of contractions. Pms2 suppressed large expansions specifically in tissues showing MutSα-dependent somatic instability, suggesting that they may act on the same lesion or structure associated with the expanded (GAA·TTC)(n) sequence. We conclude that Pms2 specifically suppresses large expansions of a pathogenic trinucleotide repeat sequence in neuronal tissues, possibly acting independently of the canonical MMR pathway.

Oral administration of the pimelic diphenylamide HDAC inhibitor HDACi 4b is unsuitable for chronic inhibition of HDAC activity in the CNS in vivo

Histone deacetylase (HDAC) inhibitors have received considerable attention as potential therapeutics for a variety of cancers and neurological disorders. Recent publications on a class of pimelic diphenylamide HDAC inhibitors have highlighted their promise in the treatment of the neurodegenerative diseases Friedreich’s ataxia and Huntington’s disease, based on efficacy in cell and mouse models. These studies’ authors have proposed that the unique action of these compounds compared to hydroxamic acid-based HDAC inhibitors results from their unusual slow-on/slow-off kinetics of binding, preferentially to HDAC3, resulting in a distinctive pharmacological profile and reduced toxicity. Here, we evaluate the HDAC subtype selectivity, cellular activity, absorption, distribution, metabolism and excretion (ADME) properties, as well as the central pharmacodynamic profile of one such compound, HDACi 4b, previously described to show efficacy in vivo in the R6/2 mouse model of Huntington’s disease. Based on our data reported here, we conclude that while the in vitro selectivity and binding mode are largely in agreement with previous reports, the physicochemical properties, metabolic and p-glycoprotein (Pgp) substrate liability of HDACi 4b render this compound suboptimal to investigate central Class I HDAC inhibition in vivo in mouse per oral administration. A drug administration regimen using HDACi 4b dissolved in drinking water was used in the previous proof of concept study, casting doubt on the validation of CNS HDAC3 inhibition as a target for the treatment of Huntington’s disease. We highlight physicochemical stability and metabolic issues with 4b that are likely intrinsic liabilities of the benzamide chemotype in general.

Rationale for the development of 2-aminobenzamide histone deacetylase inhibitors as therapeutics for Friedreich ataxia

Numerous studies have pointed to histone deacetylase inhibitors as potential therapeutics for various neurodegenerative diseases, and clinical trials with several histone deacetylase inhibitors have been performed or are under way. However, histone deacetylase inhibitors tested to date either are highly cytotoxic or have very low specificities for different histone deacetylase enzymes. The authors' laboratories have identified a novel class of histone deacetylase inhibitors (2-aminobenzamides) that reverses heterochromatin-mediated silencing of the frataxin (FXN) gene in Friedreich ataxia. The authors have identified the histone deacetylase enzyme isotype target of these compounds and present evidence that compounds that target this enzyme selectively increase FXN expression from pathogenic alleles. Studies with model compounds show that these histone deacetylase inhibitors increase FXN messenger RNA levels in the brain in mouse models for Friedreich ataxia and relieve neurological symptoms observed in mouse models and support the notion that this class of molecules may serve as therapeutics for the human disease.

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.

Understanding the genetic and molecular pathogenesis of Friedreich's ataxia through animal and cellular models

In 1996, a link was identified between Friedreich's ataxia (FRDA), the most common inherited ataxia in men, and alterations in the gene encoding frataxin (FXN). Initial studies revealed that the disease is caused by a unique, most frequently biallelic, expansion of the GAA sequence in intron 1 of FXN. Since the identification of this link, there has been tremendous progress in understanding frataxin function and the mechanism of FRDA pathology, as well as in developing diagnostics and therapeutic approaches for the disease. These advances were the subject of the 4th International Friedreich's Ataxia Conference held on 5th-7th May in the Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch, France. More than 200 scientists gathered from all over the world to present the results of research spanning all areas of investigation into FRDA (including clinical aspects, FRDA pathogenesis, genetics and epigenetics of the disease, development of new models of FRDA, and drug discovery). This review provides an update on the understanding of frataxin function, developments of animal and cellular models of the disease, and recent advances in trying to uncover potential molecules for therapy.

Knockout mouse models of iron homeostasis

Murine models have made valuable contributions to our understanding of iron metabolism. Investigation of mice with inherited forms of anemia has led to the discovery of novel proteins involved in iron homeostasis. A growing number of murine models are being developed to investigate mitochondrial iron metabolism. Mouse strains are available for the major forms of hereditary hemochromatosis. Findings in murine models support the concept that the pathogenesis of nearly all forms of hereditary hemochromatosis involves inappropriately low expression of hepcidin. The availability of mice with floxed iron-related genes allows the study of the in vivo consequences of cell-selective deletion of these genes.

The Heart in Friedreich's Ataxia: Basic Findings and Clinical Implications

Friedreich's Ataxia is the most common inherited ataxia in man. It is a mitochondrial disease caused by severely reduced expression of the iron binding protein, frataxin. A large GAA triplet expansion in the human FRDA gene encoding this protein inhibits expression of this gene. It is inherited in an autosomal recessive pattern and typically diagnosed in childhood. The primary symptoms include severe and progressive neuropathy, and a hypertrophic cardiomyopathy that may cause death. The cardiomyopathy is difficult to treat and is frequently associated with arrhythmias, heart failure, and intolerance of cardiovascular stress, such as surgeries. Innovative approaches to therapy, such as histone deacetylase inhibitors, and enzyme replacement with cell penetrant peptide fusion proteins, hold promise for this and other similar mitochondrial disorders. This review will focus on the basic findings of this disease, and the cardiomyopathy associated with its diagnosis.

Friedreich's ataxia: pathology, pathogenesis, and molecular genetics

The pathogenic mutation in Friedreich's ataxia (FRDA) is a homozygous guanine-adenine-adenine (GAA) trinucleotide repeat expansion on chromosome 9q13 that causes a transcriptional defect of the frataxin gene. Deficiency of frataxin, a small mitochondrial protein, is responsible for all clinical and morphological manifestations of FRDA. This autosomal recessive disease affects central and peripheral nervous systems, heart, skeleton, and endocrine pancreas. Long expansions lead to early onset, severe clinical illness, and death in young adult life. Patients with short expansions have a later onset and a more benign course. Some are not diagnosed during life. The neurological phenotype reflects lesions in dorsal root ganglia (DRG), sensory peripheral nerves, corticospinal tracts, and dentate nuclei (DN). Most patients succumb to cardiomyopathy, and many become diabetic during the course of their disease. This review seeks to reconcile the diverse clinical features with pathological and molecular data. In the pathogenesis of the lesion in DRG, dorsal spinal roots, and sensory peripheral nerves, developmental defects and atrophy occur in combination. The progressive lesion of the DN lacks a known developmental component. Destruction of the DN, optic atrophy, and degeneration of the corticospinal tracts are intrinsic central nervous system lesions. Fiber loss in dorsal columns and spinocerebellar tracts, and atrophy of the neurons in the dorsal nuclei of Clarke are secondary to the lesion in DRG. The role of frataxin deficiency in the pathogenesis of FRDA is still unclear because the protein has multiple functions in the normal state, including biogenesis of iron-sulfur clusters; iron chaperoning; iron storage; and control of iron-mediated oxidative tissue damage.

Cardiomyopathy of Friedreich's ataxia: use of mouse models to understand human disease and guide therapeutic development

Friedreich's ataxia is a multisystem disorder of mitochondrial function affecting primarily the heart and brain. Patients experience a severe cardiomyopathy that can progress to heart failure and death. Although the gene defect is known, the precise function of the deficient mitochondrial protein, frataxin, is not known and limits therapeutic development. Animal models have been valuable for understanding the basic events of this disease. A significant need exists to focus greater attention on the heart disease in Friedreich's ataxia, to understand its long-term outcome, and to develop new therapeutic strategies using existing medications and approaches. This review discusses some key features of the cardiomyopathy in Friedreich's ataxia and potential therapeutic developments.

Friedreich's ataxia induced pluripotent stem cells model intergenerational GAA⋅TTC triplet repeat instability

The inherited neurodegenerative disease Friedreich's ataxia (FRDA) is caused by GAA⋅TTC triplet repeat hyperexpansions within the first intron of the FXN gene, encoding the mitochondrial protein frataxin. Long GAA⋅TTC repeats cause heterochromatin-mediated gene silencing and loss of frataxin in affected individuals. We report the derivation of induced pluripotent stem cells (iPSCs) from FRDA patient fibroblasts by transcription factor reprogramming. FXN gene repression is maintained in the iPSCs, as are the global gene expression signatures reflecting the human disease. GAA⋅TTC repeats uniquely in FXN in the iPSCs exhibit repeat instability similar to patient families, where they expand and/or contract with discrete changes in length between generations. The mismatch repair enzyme MSH2, implicated in repeat instability in other triplet repeat diseases, is highly expressed in pluripotent cells and occupies FXN intron 1, and shRNA silencing of MSH2 impedes repeat expansion, providing a possible molecular explanation for repeat expansion in FRDA.

Role of transcript and interplay between transcription and replication in triplet-repeat instability in mammalian cells

Triplet-repeat expansions cause several inherited human diseases. Expanded triplet-repeats are unstable in somatic cells, and tissue-specific somatic instability contributes to disease pathogenesis. In mammalian cells instability of triplet-repeats is dependent on the location of the origin of replication relative to the repeat tract, supporting the ‘fork-shift’ model of repeat instability. Disease-causing triplet-repeats are transcribed, but how this influences instability remains unclear. We examined instability of the expanded (GAA•TTC)_n sequence in mammalian cells by analyzing individual replication events directed by the SV40 origin from five different locations, in the presence and absence of doxycycline-induced transcription. Depending on the location of the SV40 origin, either no instability was observed, instability was caused by replication with no further increase due to transcription, or instability required transcription. Whereas contractions accounted for most of the observed instability, one construct showed expansions upon induction of transcription. These expansions disappeared when transcript stability was reduced via removal or mutation of a spliceable intron. These results reveal a complex interrelationship of transcription and replication in the etiology of repeat instability. While both processes may not be sufficient for the initiation of instability, transcription and/or transcript stability seem to further modulate the fork-shift model of triplet-repeat instability.

Friedreich ataxia: molecular mechanisms, redox considerations, and therapeutic opportunities

Mitochondrial dysfunction and oxidative damage are at the origin of numerous neurodegenerative diseases like Friedreich ataxia and Alzheimer and Parkinson diseases. Friedreich ataxia (FRDA) is the most common hereditary ataxia, with one individual affected in 50,000. This disease is characterized by progressive degeneration of the central and peripheral nervous systems, cardiomyopathy, and increased incidence of diabetes mellitus. FRDA is caused by a dynamic mutation, a GAA trinucleotide repeat expansion, in the first intron of the FXN gene. Fewer than 5% of the patients are heterozygous and carry point mutations in the other allele. The molecular consequences of the GAA triplet expansion is transcription silencing and reduced expression of the encoded mitochondrial protein, frataxin. The precise cellular role of frataxin is not known; however, it is clear now that several mitochondrial functions are not performed correctly in patient cells. The affected functions include respiration, iron-sulfur cluster assembly, iron homeostasis, and maintenance of the redox status. This review highlights the molecular mechanisms that underlie the disease phenotypes and the different hypothesis about the function of frataxin. In addition, we present an overview of the most recent therapeutic approaches for this severe disease that actually has no efficient treatment.

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

Two new pimelic diphenylamide HDAC inhibitors induce sustained frataxin upregulation in cells from Friedreich's ataxia patients and in a mouse model

BACKGROUND

Friedreich's ataxia (FRDA), the most common recessive ataxia in Caucasians, is due to severely reduced levels of frataxin, a highly conserved protein, that result from a large GAA triplet repeat expansion within the first intron of the frataxin gene (FXN). Typical marks of heterochromatin are found near the expanded GAA repeat in FRDA patient cells and mouse models. Histone deacetylase inhibitors (HDACIs) with a pimelic diphenylamide structure and HDAC3 specificity can decondense the chromatin structure at the FXN gene and restore frataxin levels in cells from FRDA patients and in a GAA repeat based FRDA mouse model, KIKI, providing an appealing approach for FRDA therapeutics.

METHODOLOGY/PRINCIPAL FINDINGS

In an effort to further improve the pharmacological profile of pimelic diphenylamide HDACIs as potential therapeutics for FRDA, we synthesized additional compounds with this basic structure and screened them for HDAC3 specificity. We characterized two of these compounds, 136 and 109, in FRDA patients' peripheral blood lymphocytes and in the KIKI mouse model. We tested their ability to upregulate frataxin at a range of concentrations in order to determine a minimal effective dose. We then determined in both systems the duration of effect of these drugs on frataxin mRNA and protein, and on total and local histone acetylation. The effects of these compounds exceeded the time of direct exposure in both systems.

CONCLUSIONS/SIGNIFICANCE

Our results support the pre-clinical development of a therapeutic approach based on pimelic diphenylamide HDACIs for FRDA and provide information for the design of future human trials of these drugs, suggesting an intermittent administration of the drug.

Friedreich's Ataxia: from the (GAA)n repeat mediated silencing to new promising molecules for therapy

Friedreich's ataxia (FRDA) is a neurodegenerative disease due to a pathological expansion of a GAA triplet repeat in the first intron of the FXN gene encoding for the mitochondrial protein frataxin. The expansion is responsible for most cases of FRDA through the formation of a nonusual B-DNA structure and heterochromatin conformation that determine a direct transcriptional silencing and the subsequent reduction in frataxin expression. Among other functions, frataxin is an iron chaperone central for the assembly of iron-sulfur clusters in mitochondria; its reduction is associated with iron accumulation in mitochondria, increased cellular sensitivity to oxidative stress and cell damage. There is, nowadays, no effective therapy for FRDA and current therapeutic strategies mainly act to slow down the consequences of frataxin deficiency. Therefore, drugs that are able to increase the amount of frataxin are excellent candidates for a rational approach to FRDA therapy. Recently, several drugs have been assessed for their ability to increase the amount of cellular frataxin, including human recombinant erythropoietin, histone deacetylase inhibitors, and the PPAR-gamma agonists.

Functional genomic analysis of frataxin deficiency reveals tissue-specific alterations and identifies the PPARgamma pathway as a therapeutic target in Friedreich's ataxia

Friedreich's ataxia (FRDA), the most common inherited ataxia, is characterized by focal neurodegeneration, diabetes mellitus and life-threatening cardiomyopathy. Frataxin, which is significantly reduced in patients with this recessive disorder, is a mitochondrial iron-binding protein, but how its deficiency leads to neurodegeneration and metabolic derangements is not known. We performed microarray analysis of heart and skeletal muscle in a mouse model of frataxin deficiency, and found molecular evidence of increased lipogenesis in skeletal muscle, and alteration of fiber-type composition in heart, consistent with insulin resistance and cardiomyopathy, respectively. Since the peroxisome proliferator-activated receptor gamma (PPARgamma) pathway is known to regulate both processes, we hypothesized that dysregulation of this pathway could play a key role in frataxin deficiency. We confirmed this by showing a coordinate dysregulation of the PPARgamma coactivator Pgc1a and transcription factor Srebp1 in cellular and animal models of frataxin deficiency, and in cells from FRDA patients, who have marked insulin resistance. Finally, we show that genetic modulation of the PPARgamma pathway affects frataxin levels in vitro, supporting PPARgamma as a novel therapeutic target in FRDA.

Multicellular models of Friedreich ataxia

Patients with Friedreich ataxia (FRDA) have severely reduced levels of the mitochondrial protein frataxin, which results from a large GAA triplet-repeat expansion within the frataxin gene (FXN). High evolutionary conservation of frataxin across species has enabled the development of disease models of FRDA in various unicellular and multicellular organisms. Mouse models include classical knockout models, in which the Fxn gene is constitutively inactivated, and knock-in models, in which a GAA repeat mutation or the conditional allele is inserted into the genome. Recently, "humanised" GAA repeat expansion mouse models were obtained by combining the constitutive knockout with the transgenic expression of a yeast artificial chromosome carrying the human FRDA locus. In lower organisms such as Caenorhabditis elegans and Drosophila, straight-forward and conditional RNA interference technology has provided an easy way to knock down frataxin expression. Conditional mouse models have been used for pre-clinical trials of potential therapeutic agents, including idebenone, MnTBAP (a superoxide dismutase mimetic), and iron chelators. Various models of FRDA have shown that different, even opposite, phenotypes can be observed, depending on the level of frataxin expression. Additional studies with animal models will be essential for an enhanced understanding of the disease pathophysiology and for the development of better therapies.

Friedreich ataxia: an update on animal models, frataxin function and therapies

Friedreich ataxia (FRDA) is an autosomal recessive progressively debilitating degenerative disease that principally affects the nervous system and the heart. Although FRDA is considered a rare disease, is the most common inherited ataxia. It is caused by loss-of-function mutations in the FXN gene, mainly an expanded GAA triplet repeat in the intron 1. The genetic defect results in the reduction of frataxin levels, a protein targeted to the mitochondria. Frataxin deficiency leads to mitochondrial dysfunction, oxidative damage and iron accumulation. Studies of the yeast and animal models of the disease have led to propose several different roles for frataxin. Animal models have also been important for dissecting the steps of pathogenesis in FRDA and they are essential for the development of effective therapies. Currently, antioxidant and iron chelation therapies are under evaluation in clinical trials. Gene reactivation, gene therapy and protein replacement strategies for FRDA are promising approaches. This review focuses on the current models developed for FRDA, the different roles proposed for frataxin and the progress of potential treatment strategies for the disease.

Association between trinucleotide CAG repeats of the DNA polymerase gene (POLG) with age of onset of Iranian Friedreich's ataxia patients

Friedreich's ataxia (FRDA), an autosomal recessive neurodegenerative disorder, is in most cases due to a homozygous intronic expansion resulting in the loss of function of frataxin. As mitochondrial DNA (mtDNA) copy number has been decreased in FRDA cells and mtDNA polymerase (POLG) is involved in the replication of mtDNA, we searched a trinucleotide CAG repeat length of this enzyme. The POLG CAG repeat length was determined in DNA samples extracted from 20 FRDA patients and 49 control subjects. Our findings showed that the distribution of the POLG CAG repeat length in the patients' samples matched the distribution for control samples, but we found a statistically significant inverse correlation (r=-0.81) between the POLG CAG repeats and age of onset in FRDA patients. Our results suggest POLG CAG repeat instability would constitute a predisposing factor that, in combination with environmental risk factors, affect age of onset and disease progression.

Long intronic GAA*TTC repeats induce epigenetic changes and reporter gene silencing in a molecular model of Friedreich ataxia

Friedreich ataxia (FRDA) is caused by hyperexpansion of GAA*TTC repeats located in the first intron of the FXN gene, which inhibits transcription leading to the deficiency of frataxin. The FXN gene is an excellent target for therapeutic intervention since (i) 98% of patients carry the same type of mutation, (ii) the mutation is intronic, thus leaving the FXN coding sequence unaffected and (iii) heterozygous GAA*TTC expansion carriers with approximately 50% decrease of the frataxin are asymptomatic. The discovery of therapeutic strategies for FRDA is hampered by a lack of appropriate molecular models of the disease. Herein, we present the development of a new cell line as a molecular model of FRDA by inserting 560 GAA*TTC repeats into an intron of a GFP reporter minigene. The GFP_(GAA*TTC)(560) minigene recapitulates the molecular hallmarks of the mutated FXN gene, i.e. inhibition of transcription of the reporter gene, decreased levels of the reporter protein and hypoacetylation and hypermethylation of histones in the vicinity of the repeats. Additionally, selected histone deacetylase inhibitors, known to stimulate the FXN gene expression, increase the expression of the GFP_(GAA*TTC)(560) reporter. This FRDA model can be adapted to high-throughput analyses in a search for new therapeutics for the disease.

Targeting the gene in Friedreich ataxia

Pathological expansions of GAA repeats in the first intron of the frataxin gene cause most cases of Friedreich ataxia, a progressively debilitating neurodegenerative disease. The disease is inherited in an autosomal recessive manner and the GAA repeats are suspected to form unusual non B-DNA conformations that decrease transcription and subsequently reduce levels of the encoded protein, frataxin. Recent work has shown that GAA repeats induce heterochromatin formation and silencing of the frataxin gene locus. Frataxin plays a crucial role in iron metabolism and detoxification and interacts with electron transport chain proteins. Clinical trials are currently underway to examine the efficacy of antioxidants in the treatment of Friedreich ataxia, but therapeutics designed to increase frataxin message levels are still in the developmental stages. This review will focus on the progress of potential treatment strategies for Friedreich ataxia that target the GAA expanded gene and seek to increase the level of frataxin message and protein.

Influence of Friedreich ataxia GAA noncoding repeat expansions on pre-mRNA processing

The intronic GAA repeat expansion in the frataxin (FXN) gene causes the hereditary neurodegenerative disorder Friedreich ataxia. Although it is generally believed that GAA repeats block transcription elongation, direct proof in eukaryotic systems is lacking. We tested in hybrid minigenes the effect of GAA and TTC repeats on nascent transcription and pre-mRNA processing. Unexpectedly, disease-causing GAA(100) repeats did not affect transcriptional elongation in a nuclear HeLa Run On assay, nor did they affect pre-mRNA transcript abundance. However, they did result in a complex defect in pre-mRNA processing. The insertion of GAA but not TTC repeats downstream of reporter exons resulted in their partial or complete exclusion from the mature mRNAs and in the generation of a variety of aberrant splicing products. This effect of GAA repeats was observed to be position and context dependent; their insertion at different distances from the reporter exons had a variable effect on splice-site selection. In addition, GAA repeats bind to a multitude of different splicing factors and induced the accumulation of an upstream pre-mRNA splicing intermediate, which is not turned over into mature mRNA. When embedded in the homologous frataxin minigene system, the GAA repeats did not affect the pre-mRNA transcript abundance but did significantly reduce the splicing efficiency of the first intron. These data indicate an association between GAA noncoding repeats and aberrant pre-mRNA processing because binding of transcribed GAA repeats to a multitude of trans-acting splicing factors can interfere with normal turnover of intronic RNA and thus lead to its degradation and a lower abundance of mature mRNA.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

HDAC inhibitors correct frataxin deficiency in a Friedreich ataxia mouse model

BACKGROUND

Friedreich ataxia, an autosomal recessive neurodegenerative and cardiac disease, is caused by abnormally low levels of frataxin, an essential mitochondrial protein. All Friedreich ataxia patients carry a GAATTC repeat expansion in the first intron of the frataxin gene, either in the homozygous state or in compound heterozygosity with other loss-of-function mutations. The GAA expansion inhibits frataxin expression through a heterochromatin-mediated repression mechanism. Histone modifications that are characteristic of silenced genes in heterochromatic regions occur at expanded alleles in cells from Friedreich ataxia patients, including increased trimethylation of histone H3 at lysine 9 and hypoacetylation of histones H3 and H4.

METHODOLOGY/PRINCIPAL FINDINGS

By chromatin immunoprecipitation, we detected the same heterochromatin marks in homozygous mice carrying a (GAA)(230) repeat in the first intron of the mouse frataxin gene (KIKI mice). These animals have decreased frataxin levels and, by microarray analysis, show significant gene expression changes in several tissues. We treated KIKI mice with a novel histone deacetylase inhibitor, compound 106, which substantially increases frataxin mRNA levels in cells from Friedreich ataxia individuals. Treatment increased histone H3 and H4 acetylation in chromatin near the GAA repeat and restored wild-type frataxin levels in the nervous system and heart, as determined by quantitative RT-PCR and semiquantitative western blot analysis. No toxicity was observed. Furthermore, most of the differentially expressed genes in KIKI mice reverted towards wild-type levels.

CONCLUSIONS/SIGNIFICANCE

Lack of acute toxicity, normalization of frataxin levels and of the transcription profile changes resulting from frataxin deficiency provide strong support to a possible efficacy of this or related compounds in reverting the pathological process in Friedreich ataxia, a so far incurable neurodegenerative disease.

Small molecules affecting transcription in Friedreich ataxia

This review concerns the development of small molecule therapeutics for the inherited neurodegenerative disease Friedreich ataxia (FRDA). FRDA is caused by transcriptional repression of the nuclear FXN gene, encoding the essential mitochondrial protein frataxin and accompanying loss of frataxin protein. Frataxin insufficiency leads to mitochrondrial dysfunction and progressive neurodegeneration, along with scoliosis, diabetes and cardiomyopathy. Individuals with FRDA generally die in early adulthood from the associated heart disease, the most common cause of death in FRDA. While antioxidants and iron chelators have shown promise in ameliorating the symptoms of the disease, there is no effective therapy for FRDA that addresses the cause of the disease, the loss of frataxin protein. Gene therapy and protein replacement strategies for FRDA are promising approaches; however, current technology is not sufficiently advanced to envisage treatments for FRDA coming from these approaches in the near future. Since the FXN mutation in FRDA, expanded GAA.TTC triplets in an intron, does not alter the amino acid sequence of frataxin protein, gene reactivation would be of therapeutic benefit. Thus, a number of laboratories have focused on small molecule activators of FXN gene expression as potential therapeutics, and this review summarizes the current status of these efforts, as well as the molecular basis for gene silencing in FRDA.

Advancements in the pathophysiology of Friedreich's Ataxia and new prospects for treatments

On November 9-12, 2006, the Friedreich's Ataxia Research Alliance (FARA) and the National Institutes of Health (NIH) hosted the Third International Friedreich's Ataxia (FRDA) Scientific Conference at the NIH in Bethesda, Maryland, highlighting the exciting research leading now to a variety of clinical trials that show promise of effective treatments for this devastating disorder. Nearly 150 leading FRDA scientists from around the world discussed their new insights and findings. The presence of six pharmaceutical and biotechnology companies underscored the importance of the public-private partnership that has grown in the past years. Some of these companies are already involved in advancing promising drug compounds into clinical trials, while others are eager to help take newer discoveries through drug development and into subsequent clinical trials. National Institute of Neurological Disorders and Stroke (NINDS) Director Dr. Story Landis noted in her opening remarks for the conference that there was a "palpable sense of energy, excitement, and enthusiasm" over the scientific progress made since the FRDA gene was discovered over 10 years ago.

Conditional mouse models for Friedreich ataxia, a neurodegenerative disorder associating cardiomyopathy

Friedreich ataxia (FRDA), the most common recessive ataxia, is characterized by degeneration of the large sensory neurons and spinocerebellar tracts and cardiomyopathy. It is caused by severely reduced levels of frataxin, a mitochondrial protein involved in iron-sulfur cluster (ISC) biosynthesis. Mouse models have been important tools in dissecting the steps of pathogenesis in FRDA. Furthermore, animal models that reproduce some of the key events in a pathology are essential for the development of effective therapies, both pharmacological and gene therapy approaches. This chapter presents an overview of the current mouse models that have been developed for FRDA.

The GAA triplet-repeat is unstable in the context of the human FXN locus and displays age-dependent expansions in cerebellum and DRG in a transgenic mouse model

Friedreich ataxia (FRDA) is caused by homozygosity for FXN alleles containing an expanded GAA triplet-repeat (GAA-TR) sequence. Patients have progressive neurodegeneration of the dorsal root ganglia (DRG) and in later stages the cerebellum may be involved. The expanded GAA-TR sequence is unstable in somatic cells in vivo, and although the mechanism of instability remains unknown, we hypothesized that age-dependent and tissue-specific somatic instability may be a determinant of the progressive pathology involving DRG and cerebellum. We show that transgenic mice containing the expanded GAA-TR sequence (190 or 82 triplets) in the context of the human FXN locus show tissue-specific and age-dependent somatic instability that is compatible with this hypothesis. Small pool PCR analysis, which allows quantitative analysis of repeat instability by assaying individual transgenes in vivo, showed age-dependent expansions specifically in the cerebellum and DRG. The (GAA)(190) allele showed some instability by 2 months, progressed at about 0.3-0.4 triplets per week, resulting in a significant number of expansions by 12 months. Repeat length was found to determine the age of onset of somatic instability, and the rate and magnitude of mutation. Given the low level of cerebellar instability seen by others in multiple transgenic mice with expanded CAG/CTG repeats, our data indicate that somatic instability of the GAA-TR sequence is likely mediated by unique tissue-specific factors. This mouse model will serve as a useful tool to delineate the mechanism(s) of disease-specific somatic instability in FRDA.