Frataxin fracas

The inherited cerebellar ataxias: an update

This narrative review aims at providing an update on the management of inherited cerebellar ataxias (ICAs), describing main clinical entities, genetic analysis strategies and recent therapeutic developments. Initial approach facing a patient with cerebellar ataxia requires family medical history, physical examination, exclusions of acquired causes and genetic analysis, including Next-Generation Sequencing (NGS). To guide diagnosis, several algorithms and a new genetic nomenclature for recessive cerebellar ataxias have been proposed. The challenge of NGS analysis is the identification of causative variant, trio analysis being usually the most appropriate option. Public genomic databases as well as pathogenicity prediction software facilitate the interpretation of NGS results. We also report on key clinical points for the diagnosis of the main ICAs, including Friedreich ataxia, CANVAS, polyglutamine spinocerebellar ataxias, Fragile X-associated tremor/ataxia syndrome. Rarer forms should not be neglected because of diagnostic biomarkers availability, disease-modifying treatments, or associated susceptibility to malignancy. Diagnostic difficulties arise from allelic and phenotypic heterogeneity as well as from the possibility for one gene to be associated with both dominant and recessive inheritance. To complicate the phenotype, cerebellar cognitive affective syndrome can be associated with some subtypes of cerebellar ataxia. Lastly, we describe new therapeutic leads: antisense oligonucleotides approach in polyglutamine SCAs and viral gene therapy in Friedreich ataxia. This review provides support for diagnosis, genetic counseling and therapeutic management of ICAs in clinical practice.

Phosphodiesterase Inhibitors Revert Axonal Dystrophy in Friedreich's Ataxia Mouse Model

Friedreich's ataxia (FRDA) is a neurodegenerative disorder caused by an unstable GAA repeat expansion within intron 1 of the FXN gene and characterized by peripheral neuropathy. A major feature of FRDA is frataxin deficiency with the loss of large sensory neurons of the dorsal root ganglia (DRG), namely proprioceptive neurons, undergoing dying-back neurodegeneration with progression to posterior columns of the spinal cord and cerebellar ataxia. We used isolated DRGs from a YG8R FRDA mouse model and C57BL/6J control mice for a proteomic study and a primary culture of sensory neurons from DRG to test novel pharmacological strategies. We found a decreased expression of electron transport chain (ETC) proteins, the oxidative phosphorylation (OXPHOS) system and antioxidant enzymes, confirming a clear impairment in mitochondrial function and an oxidative stress-prone phenotype. The proteomic profile also showed a decreased expression in Ca2+ signaling related proteins and G protein-coupled receptors (GPCRs). These receptors modulate intracellular cAMP/cGMP and Ca2+ levels. Treatment of frataxin-deficient sensory neurons with phosphodiesterase (PDE) inhibitors was able to restore improper cytosolic Ca2+ levels and revert the axonal dystrophy found in DRG neurons of YG8R mice. In conclusion, the present study shows the effectiveness of PDE inhibitors against axonal degeneration of sensory neurons in YG8R mice. Our findings indicate that PDE inhibitors may become a future FRDA pharmacological treatment.

Oxidative stress and the homeodynamics of iron metabolism

Iron and oxygen share a delicate partnership since both are indispensable for survival, but if the partnership becomes inadequate, this may rapidly terminate life. Virtually all cell components are directly or indirectly affected by cellular iron metabolism, which represents a complex, redox-based machinery that is controlled by, and essential to, metabolic requirements. Under conditions of increased oxidative stress—i.e., enhanced formation of reactive oxygen species (ROS)—however, this machinery may turn into a potential threat, the continued requirement for iron promoting adverse reactions such as the iron/H2O2-based formation of hydroxyl radicals, which exacerbate the initial pro-oxidant condition. This review will discuss the multifaceted homeodynamics of cellular iron management under normal conditions as well as in the context of oxidative stress.

IGF-1 in Friedreich's Ataxia - proof-of-concept trial

BACKGROUND

Friedreich's ataxia is an autosomal recessive, severely incapacitating disorder. There is little objective evidence regarding FRDA management. Abnormalities in the insulin/insulin-like growth factor 1 (IGF-1) system (IIS) signalling pathway were thought to play a role in the physiopathological processes of various neurodegenerative disorders, including spinocerebellar ataxias. The objective of the study was to test the safety, tolerability, and efficacy of therapy with IGF-1 in Friedreich's ataxia (FRDA) patients in a clinical pilot study.

RESULTS

A total of 4 females and 1 male were included in the study; 23 to 36 years of age (average 26.6 ± 5.4), diagnosed with FRDA with normal ventricular function. Patients were treated with IGF-1 therapy with 50 μg/kg twice a day subcutaneously for 12 months. The efficacy of this therapy was assessed by changes from baseline on the scale for the assessment and rating of ataxia, (SARA) and by changes from baseline in echocardiogram parameters. The annual worsening rate (AWR) was estimated in this series as a SARA score of -0.4 ± 0.83 (CI 95%: -1.28 to 0.48) SARA score, whereas the AWR for our FRDA cohort was estimated as a SARA score of 2.05 ± 1.23 (CI 95%: 1.58 to 2.52). Echocardiographic parameters remained normal and stable.

CONCLUSION

Our results seem to indicate a benefit of this IGF-1 therapy to neurological functions in FRDA.

Triple therapy with deferiprone, idebenone and riboflavin in Friedreich's ataxia - open-label trial

OBJECTIVES

The objective of the study was to test the efficacy, safety and tolerability of triple therapy with deferiprone, idebenone and riboflavin in Friedreich's ataxia (FRDA) patients in a clinical pilot study.

PATIENTS AND METHODS

Patients included in this study were 10 males and three females, 14-61 years of age (average 30.2 ± 12.1), diagnosed with FRDA with normal ventricular function. Patients were treated with triple therapy with deferiprone at 5-25 mg/kg/day, idebenone at 10-20 mg/kg/day and riboflavin at 10-15 mg/kg/day for 15-45 months. The efficacy of this triple therapy was assessed by change from baseline on the scale for the assessment and rating of ataxia (SARA) and by the change from baseline in echocardiogram parameters.

RESULTS

Four patients discontinued due to adverse events (AEs) related with deferiprone. The annual worsening rate (AWR) was estimated in this series as 0.96 (CI 95%: 0.462-1.608) SARA score, whereas AWR for our FRDA cohort was estimated as 2.05 ± 1.23 SARA score. LVMI only decreased by 6.5 g/m(2) (6.2%) at the end of the first year of therapy. LVEF remained stable, except in case of three patients.

CONCLUSION

Our results seem to indicate some uncertain benefit on the neurological and heart functions of this triple therapy in FRDA.

Friedreich's ataxia, frataxin, PIP5K1B: echo of a distant fracas

"Frataxin fracas" were the words used when referring to the frataxin-encoding gene (FXN) burst in as a motive to disqualify an alternative candidate gene, PIP5K1B, as an actor in Friedreich's ataxia (FRDA) (Campuzano et al., 1996; Cossee et al., 1997; Carvajal et al., 1996). The instrumental role in the disease of large triplet expansions in the first intron of FXN has been thereafter fully confirmed, and this no longer suffers any dispute (Koeppen, 2011). On the other hand, a recent study suggests that the consequences of these large expansions in FXN are wider than previously thought and that the expression of surrounding genes, including PIP5K1B, could be concurrently modulated by these large expansions (Bayot et al., 2013). This recent observation raises a number of important and yet unanswered questions for scientists and clinicians working on FRDA; these questions are the substratum of this paper.

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.

Potential therapeutic benefits of strategies directed to mitochondria

The mitochondrion is the most important organelle in determining continued cell survival and cell death. Mitochondrial dysfunction leads to many human maladies, including cardiovascular diseases, neurodegenerative disease, and cancer. These mitochondria-related pathologies range from early infancy to senescence. The central premise of this review is that if mitochondrial abnormalities contribute to the pathological state, alleviating the mitochondrial dysfunction would contribute to attenuating the severity or progression of the disease. Therefore, this review will examine the role of mitochondria in the etiology and progression of several diseases and explore potential therapeutic benefits of targeting mitochondria in mitigating the disease processes. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate and manipulate mitochondrial function and genomics for therapeutic benefit. These approaches to treat mitochondrial dysfunction rationally could lead to selective protection of cells in different tissues and various disease states. However, most of these approaches are in their infancy.

Immune evasion by gammaherpesvirus genome maintenance proteins

Viruses that establish lifelong latent infections must ensure that the viral genome is maintained within the latently infected cell throughout the life of the host, yet at the same time must also be capable of avoiding elimination by the immune surveillance system. Gammaherpesviruses, which include the human viruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, establish latent infections in lymphocytes. Infection of this dynamic host-cell population requires that the viruses have appropriate strategies for enabling the viral genome to persist while these cells go through rounds of mitosis, but at the same time must avoid detection by host CD8(+) cytotoxic T lymphocytes (CTLs). The majority of gammaherpesviruses studied have been found to encode a specific protein that is critical for maintenance of the viral genome within latently infected cells. This protein is termed the genome maintenance protein (GMP). Due to its vital role in long-term latency, this offers the immune system a crucial target for detection and elimination of virus-infected cells. GMPs from different gammaherpesviruses have evolved related strategies that allow the protein to be present within latently infected cells, but to remain effectively hidden from circulating CD8(+) CTLs. In this review, I will summarize the role of the GMPs and highlight the available data describing the immune-evasion properties of these proteins.

Limitations in a frataxin knockdown cell model for Friedreich ataxia in a high-throughput drug screen

BACKGROUND

Pharmacological high-throughput screening (HTS) represents a powerful strategy for drug discovery in genetic diseases, particularly when the full spectrum of pathological dysfunctions remains unclear, such as in Friedreich ataxia (FRDA). FRDA, the most common recessive ataxia, results from a generalized deficiency of mitochondrial and cytosolic iron-sulfur cluster (ISC) proteins activity, due to a partial loss of frataxin function, a mitochondrial protein proposed to function as an iron-chaperone for ISC biosynthesis. In the absence of measurable catalytic function for frataxin, a cell-based assay is required for HTS assay.

METHODS

Using a targeted ribozyme strategy in murine fibroblasts, we have developed a cellular model with strongly reduced levels of frataxin. We have used this model to screen the Prestwick Chemical Library, a collection of one thousand off-patent drugs, for potential molecules for FRDA.

RESULTS

The frataxin deficient cell lines exhibit a proliferation defect, associated with an ISC enzyme deficit. Using the growth defect as end-point criteria, we screened the Prestwick Chemical Library. However no molecule presented a significant and reproducible effect on the proliferation rate of frataxin deficient cells. Moreover over numerous passages, the antisense ribozyme fibroblast cell lines revealed an increase in frataxin residual level associated with the normalization of ISC enzyme activities. However, the ribozyme cell lines and FRDA patient cells presented an increase in Mthfd2 transcript, a mitochondrial enzyme that was previously shown to be upregulated at very early stages of the pathogenesis in the cardiac mouse model.

CONCLUSION

Although no active hit has been identified, the present study demonstrates the feasibility of using a cell-based approach to HTS for FRDA. Furthermore, it highlights the difficulty in the development of a stable frataxin-deficient cell model, an essential condition for productive HTS in the future.

DNA triplexes and Friedreich ataxia

Friedreich ataxia, the most common inherited ataxia, is caused by the transcriptional silencing of the FXN gene, which codes for the 210 amino acid frataxin, a mitochondrial protein involved in iron-sulfur cluster biosynthesis. The expansion of the GAA x TTC tract in intron 1 to as many as 1700 repeats elicits the transcriptional silencing by the formation of non-B DNA structures (triplexes or sticky DNA), the formation of a persistent DNA x RNA hybrid, or heterochromatin formation. The triplex (sticky DNA) adopted by the long repeat sequence also elicits profound mutagenic, genetic instability, and recombination behaviors. Early stage therapeutic investigations involving polyamides or histone deacetylase inhibitors are being pursued. Friedreich ataxia may be one of the most thoroughly studied hereditary neurological disease from a pathophysiological standpoint.

Autosomal recessive cerebellar ataxias

Autosomal recessive cerebellar ataxias (ARCA) are a heterogeneous group of rare neurological disorders involving both central and peripheral nervous system, and in some case other systems and organs, and characterized by degeneration or abnormal development of cerebellum and spinal cord, autosomal recessive inheritance and, in most cases, early onset occurring before the age of 20 years. This group encompasses a large number of rare diseases, the most frequent in Caucasian population being Friedreich ataxia (estimated prevalence 2–4/100,000), ataxia-telangiectasia (1–2.5/100,000) and early onset cerebellar ataxia with retained tendon reflexes (1/100,000). Other forms ARCA are much less common. Based on clinicogenetic criteria, five main types ARCA can be distinguished: congenital ataxias (developmental disorder), ataxias associated with metabolic disorders, ataxias with a DNA repair defect, degenerative ataxias, and ataxia associated with other features. These diseases are due to mutations in specific genes, some of which have been identified, such as frataxin in Friedreich ataxia, α-tocopherol transfer protein in ataxia with vitamin E deficiency (AVED), aprataxin in ataxia with oculomotor apraxia (AOA1), and senataxin in ataxia with oculomotor apraxia (AOA2). Clinical diagnosis is confirmed by ancillary tests such as neuroimaging (magnetic resonance imaging, scanning), electrophysiological examination, and mutation analysis when the causative gene is identified. Correct clinical and genetic diagnosis is important for appropriate genetic counseling and prognosis and, in some instances, pharmacological treatment. Due to autosomal recessive inheritance, previous familial history of affected individuals is unlikely. For most ARCA there is no specific drug treatment except for coenzyme Q10 deficiency and abetalipoproteinemia.

Iron trafficking in the mitochondrion: novel pathways revealed by disease

It is well known that iron (Fe) is transported to the mitochondrion for heme synthesis. However, only recently has the importance of this organelle for many other facets of Fe metabolism become widely appreciated. Indeed, this was stimulated by the description of human disease states that implicate mitochondrial Fe metabolism. In particular, studies assessing various diseases leading to mitochondrial Fe loading have produced intriguing findings. For instance, the disease X-linked sideroblastic anemia with ataxia (XLSA/A) is due to a mutation in the ATP-binding cassette protein B7 (ABCB7) transporter that is thought to transfer [Fe-S] clusters from the mitochondrion to the cytoplasm. This and numerous other findings suggest the mitochondrion is a dynamo of Fe metabolism, being vital not only for heme synthesis but also for playing a critical role in the genesis of [Fe-S] clusters. Studies examining the disease Friedreich ataxia have suggested that a mutation in the gene encoding frataxin leads to mitochondrial Fe loading. Apart from these findings, the recently discovered mitochondrial ferritin that may store Fe in ring sideroblasts could also regulate the level of Fe needed for heme and [Fe-S] cluster synthesis. In this review, we suggest a model of mitochondrial Fe processing that may account for the pathology observed in these disease states.

Structure-dependent Recombination Hot Spot Activity of GAA·TTC Sequences from Intron 1 of the Friedreich's Ataxia Gene

The recombinational properties of long GAA.TTC repeating sequences were analyzed in Escherichia coli to gain further insights into the molecular mechanisms of the genetic instability of this tract as possibly related to the etiology of Friedreich's ataxia. Intramolecular and intermolecular recombination studies showed that the frequency of recombination between the GAA.TTC tracts was as much as 15 times higher than the non-repeating control sequences. Homologous, intramolecular recombination between GAA.TTC tracts and GAAGGA.TCCTTC repeats also occurred with a very high frequency (approximately 0.8%). Biochemical analyses of the recombination products demonstrated the expansions and deletions of the GAA.TTC repeats. These results, together with our previous studies on the CTG.CAG sequences, suggest that the recombinational hot spot characteristics may be a common feature of all triplet repeat sequences. Unexpectedly, we found that the recombination properties of the GAA.TTC tracts were unique, compared with CTG.CAG repeats, because they depended on the DNA secondary structure polymorphism. Increasing the length of the GAA.TTC repeats decreased the intramolecular recombination frequency between these tracts. Also, a correlation was found between the propensity of the GAA.TTC tracts to adopt the sticky DNA conformation and the inhibition of intramolecular recombination. The use of novobiocin to modulate the intracellular DNA topology, i.e. the lowering of the negative superhelical density, repressed the formation of the sticky DNA structure, thereby restoring the expected positive correlation between the length of the GAA.TTC tracts and the frequency of intramolecular recombination. Hence, our results demonstrate that sticky DNA exists and functions in E. coli.

Friedreich ataxia

There has been rapid progress in the understanding of several aspects of Friedreich's ataxia (FA) since the gene mutation was identified in 1996. At the clinical level, now it is possible to confirm that the majority of patients fullfilling clinical criteria for classic FA have the FA gene mutation but some do not, indicating genetic heterogeneity. Also, the phenotype associated with the FA mutation is much wider than that defined by clinical criteria and includes ataxia with retained or brisk reflexes as well as late onset ataxia with or without retained reflexes. It is now clear that the unstable GAA expansion that underlies FA causes a deficiency of the mitochondrial protein frataxin, leading to potentially harmful oxidative injury associated with excessive iron deposits in mitochondria. In addition, pathogenesis may involve a primary defect in synthesis of iron-sulfur cluster containing enzymes. Therapeutic attempts are already using anti-oxidant strategies and such efforts are likely to be enhanced by the rapid availability of animal models of the disease.

Iron in neurodegenerative disorders

Increasing evidence implicates a role of iron in the pathogenesis of numerous neurodegenerative diseases due to its capacity to enhance production of toxic reactive radicals and to induce protein aggregation. The underlying mechanism of iron accumulation in areas of the brain specific for the respective disease, however, is still unknown. Recent molecular and biochemical studies provide new insights into the consequences of impairment of brain iron metabolism. This review summarizes our understanding of the regulation of iron in the brain and defines the current knowledge on the involvement of iron metabolism in neurodegenerative diseases with genetically determined iron accumulation in the brain.

Iron metabolism and mitochondrial abnormalities in Friedreich ataxia

Friedreich ataxia is an autosomal recessive disease causing degeneration in the central and peripheral nervous system, cardiomyopathy, skeletal abnormalities and increased risk of diabetes. It is caused by deficiency of frataxin, a highly conserved nuclear-encoded mitochondrial protein. The genetic mutation found in 98% of Friedreich ataxia chromosomes is the unstable hyperexpansion of a GAA triplet repeat in the first intron of the gene. The expanded GAA repeat, by adopting an abnormal triple helical structure, impairs frataxin transcription. Longer repeats cause a more profound frataxin deficiency and are associated with earlier onset and increased severity of the disease. Yeast cells deficient in the frataxin homologue (Deltayfh1) become unable to carry out oxidative phosphorylation, lose mitochondrial DNA, accumulate iron in mitochondria, show unregulated high expression of high affinity iron uptake, and have an increased sensitivity to oxidative stress. Loss of respiratory competence in Deltayfh1 is iron-dependent. Additional properties of these cells include a deficiency of iron-sulfur cluster containing proteins (ISPs) and impaired iron efflux out of mitochondria. Evidence of oxidative stress, mitochondrial dysfunction, deficiency of multiple ISPs and iron deposits are also found in the human disease and in mouse models. The primary function of frataxin is still unknown, however much recent evidence suggests that it enhances iron-sulfur cluster synthesis and protects iron from free radical-generating reactions. The search for frataxin function stimulated more investigations on the role of mitochondria in cellular iron homeostasis. Their results suggest that these organelles may play a central role in controlling iron homeostasis, which is not surprising considering that they are the major cellular site where this metal is utilized. I propose a model, valid in yeast as well as in higher eukaryotes, in which iron transport into mitochondria is directly coupled to its uptake at the cell membrane and iron transport out of mitochondria depends on adequate iron-sulfur cluster synthesis. Regulatory mechanisms in the cytosol would then sense a post-mitochondrial iron pool. Much circumstantial evidence from genetically manipulated yeast and from human diseases supports this model.

Sticky DNA, a long GAA.GAA.TTC triplex that is formed intramolecularly, in the sequence of intron 1 of the frataxin gene

Friedreich's ataxia is caused by the massive expansion of GAA.TTC repeats in intron 1 of the frataxin (X25) gene. Our prior investigations showed that long GAA.TTC repeats formed very stable triplex structures which caused two repeat tracts to adhere to each other (sticky DNA). This process was dependent on negative supercoiling and the presence of divalent metal ions. Herein, we have investigated the formation of sticky DNA from plasmid monomers and dimers; sticky DNA is formed only when two tracts of sufficiently long (GAA.TTC)(n) (n = 59-270) are present in a single plasmid DNA and are in the direct repeat orientation. If the inserts are in the indirect (inverted) repeat orientation, no sticky DNA was observed. Furthermore, kinetic studies support the intramolecular nature of sticky DNA formation. Electron microscopy investigations also provide strong data for sticky DNA as a single long triplex. Hence, these results give new insights into our understanding of the capacity of sticky DNA to inhibit transcription and thereby reduce the level of frataxin protein as related to the etiology of Friedreich's ataxia.

Friedreich's ataxia: treatment within reach

Friedreich's ataxia (FA) is the most prevalent cerebellar ataxia in children and adults in Europe. FA is one of a growing number of diseases known to be caused by triplet-repeat expansions. The causative mutation is a GAA trinucleotide-repeat expansion in the first intron of the frataxin gene. The mitochondrial localisation of frataxin and decreased oxidation activity in vivo and in vitro show that FA is a mitochondrial disease. Frataxin is involved in iron metabolism and may protect mitochondria from oxidative damage. The understanding of the disease has only just begun and possible treatments are within reach. In this review I discuss the clinical knowledge of FA and recent developments that have helped to elucidate the pathogenesis of the disease and made the first therapeutic attempts possible.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

Fragile X syndrome and Friedreich's ataxia: two different paradigms for repeat induced transcript insufficiency

DNA repeat expansion is the genetic basis for a growing number of neurological disorders. While the largest subset of these diseases results in an increase in the length of a polyglutamine tract in the protein encoded by the affected gene, the most common form of inherited mental retardation, fragile X syndrome, and the most common inherited ataxia, Friedreich's ataxia, are both caused by expansions that are transcribed but not translated. These expansions both decrease expression of the gene in which the expanded repeat is located, but they do so by quite different mechanisms. In fragile X syndrome, CGG. CCG expansion in the 5' untranslated region of the FMR1 gene leads to hypermethylation of the repeats and the adjacent CpG-rich promoter. Methylation prevents the binding of the transcription factor alpha-Pal/NRF-1, and may indirectly affect the binding of other factors via the formation of transcriptionally silent chromatin. In Friedreich's ataxia, GAA. TTC expansion in an intron of the FRDA gene reduces expression by interfering with transcription elongation. The model that best describes the available data is transcription-driven formation of a transient purine. purine. pyrimidine DNA triplex behind an advancing RNA polymerase. This structure lassoes the RNA polymerase that caused it, trapping the enzyme on the template.

Sticky DNA, a self-associated complex formed at long GAA*TTC repeats in intron 1 of the frataxin gene, inhibits transcription

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the expansion of GAA.TTC repeats in the first intron of the frataxin (X25) gene. FRDA patients carrying two expanded GAA.TTC repeats show very low levels of mature frataxin mRNA and protein. A novel type of unusual DNA structure, sticky DNA, was previously found in the expanded GAA.TTC repeats from FRDA patients. To evaluate the effect of sticky DNA on transcription, in vitro transcription studies of (GAA.TTC)(n) repeats (where n = 9-150) were carried out using T7 or SP6 RNA polymerase. When a gel-isolated sticky DNA template was transcribed, the amount of full-length RNA synthesized was significantly reduced compared with the transcription of the linear template. Surprisingly, transcriptional inhibition was observed not only for the sticky DNA template but also another DNA molecule used as an internal control in an orientation-independent manner. The molecular mechanism of transcriptional inhibition by sticky DNA was a sequestration of the RNA polymerases by direct binding to the complex DNA structure. Moreover, plasmids containing the (GAAGGA.TCCTTC)(65) repeat, which does not form sticky DNA, did not inhibit in vitro transcription, as expected. These results suggest that the role of sticky DNA in FRDA may be the sequestration of transcription factors.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Alleviating transcript insufficiency caused by Friedreich's ataxia triplet repeats

Expanded GAA.TTC trinucleotide repeats in intron 1 of the frataxin gene cause Friedreich's ataxia (FRDA) by reducing frataxin mRNA levels. Insufficient frataxin, a nuclear encoded mitochondrial protein, leads to the progressive neurodegeneration and cardiomyopathy characteristic of FRDA. Previously we demonstrated that long GAA.TTC tracts impede transcription elongation in vitro and provided evidence that the impediment results from an intramolecular purine.purine.pyrimidine DNA triplex formed behind an advancing RNA polymerase. Our model predicts that inhibiting formation of this triplex during transcription will increase successful elongation through GAA.TTC tracts. Here we show that this is the case. Oligodeoxyribonucleotides designed to block particular types of triplex formation provide specific and concentration-dependent increases in full-length transcript. In principle, therapeutic agents that selectively interfere with triplex formation could alleviate the frataxin transcript insufficiency caused by pathogenic FRDA alleles.

dfh is a Drosophila homolog of the Friedreich's ataxia disease gene

A putative Drosophila homolog of the Friedreich's ataxia disease gene (FRDA) has been cloned and characterized; it has been named Drosophila frataxin homolog (dfh). It is located at 8C/D position on X chromosome and is spread over 1kb, a much smaller genomic region than the human gene. Its genomic organization is simple, with a single intron dividing the coding region into two exons. The predicted encoded product has 190 amino acids, being considered a frataxin-like protein on the basis of the sequence and secondary structure conservation when compared with human frataxin and related proteins from other eukaryotes. The closest match between the Drosophila and the human proteins involved a stretch of 38 amino acids at C-terminus, encoded by dfh exon 2, and exons 4 and 5a of the FRDA gene, respectively. This highly conserved region is very likely to form a functional domain with a beta sheet structure flanked by alpha-helices where the sequence is less conserved. A signal peptide for mitochondrial import has also been predicted in the Drosophila frataxin-like protein, suggesting its mitochondrial localization, as occurs for human frataxin and other frataxin-like proteins described in eukaryotes. The Drosophila gene is expressed throughout the development of this organism, with a peak of expression in 6-12h embryos, and showing a spatial ubiquitous pattern from 4h embryos to the last embryonic stage examined. The isolation of dfh will soon make available specific dfh mutants that help in understanding the pathogenesis of FRDA.

The GAA*TTC triplet repeat expanded in Friedreich's ataxia impedes transcription elongation by T7 RNA polymerase in a length and supercoil dependent manner

Large expansions of the trinucleotide repeat GAA*TTC within the first intron of the X25 (frataxin) gene cause Friedreich's ataxia, the most common inherited ataxia. Expansion leads to reduced levels of frataxin mRNA in affected individuals. Here we show that GAA*TTC tracts, in the absence of any other frataxin gene sequences, can reduce the amount of GAA-containing transcript produced in a defined in vitro transcription system. This effect is due to an impediment to elongation that forms in the GAA*TTC tract during transcription, a phenomenon that is exacerbated by both superhelical stress and increased tract length. On supercoiled templates the major truncations of the GAA-containing transcripts occur in the distal (3') end of the GAA repeat. To account for these observations we present a model in which an RNA polymerase advancing within a long GAA*TTC tract initiates the transient formation of an R*R*Y intramolecular DNA triplex. The non-template (GAA) strand folds back creating a loop in the template strand, and the polymerase is paused at the distal triplex-duplex junction.

Mutation detection in an equivocal case of Friedreich's ataxia

Compound heterozygosity at the Friedreich's ataxia locus accounts for approximately 2% of molecularly confirmed cases. Genotype-phenotype correlation in this subgroup of patients reveals a spectrum of clinical variability. This report describes the clinical and molecular findings in a 6-year-old patient with Friedreich's ataxia who carried a pathologic GAA expansion of approximately 1,000 repeats on one allele and a novel initiation codon point mutation (3G-->A) on the other.

Functional and genomic analysis of the human mitochondrial intermediate peptidase, a putative protein partner of frataxin

We showed recently that the yeast mitochondrial intermediate peptidase (YMIP polypeptide; gene symbol, OCT1) promotes mitochondrial iron uptake by catalyzing the maturation of iron-utilizing proteins and exacerbates the mitochondrial iron accumulation that results from loss of yeast frataxin, a mitochondrial protein required for mitochondrial iron efflux. This suggests that the human MIP (HMIP polypeptide; gene symbol MIPEP) may be one of the loci predicted to influence the clinical manifestations of Friedreich's ataxia (FRDA), an autosomal recessive neurodegenerative disease caused by lack of human frataxin. To begin to test this hypothesis, we have characterized HMIP at the functional and genomic levels. We show that HMIP can complement a yeast knock-out mutant lacking YMIP, demonstrating that HMIP and YMIP are functional homologues. The MIPEP gene spans 57 kb and consists of 19 exons that correlate with the functional domains of HMIP. Primer extension analysis has identified a major transcript of the MIPEP gene expressed differentially and predominantly in tissues with high oxygen consumption, while sequence analysis of approximately 2 kb of 5'-flanking DNA has revealed putative Mt1/3/4, NF-kappaB, and AP-1 elements that may regulate MIPEP expression in these tissues. Using a new polymorphic (CA)(n) repeat in intron 4, MIPEP has been genetically mapped within a 7-cM interval between markers D13S283 and D13S217 on 13q12. This work provides the basis for molecular analysis of MIPEP in FRDA and possibly other neurodegenerative diseases.

Atypical Friedreich ataxia phenotype associated with a novel missense mutation in the X25 gene

We describe two sisters with early onset gait ataxia, rapid disease progression, absent or very mild dysarthria and upper limb dysmetria, retained knee jerks in one, slight to moderate peripheral nerve involvement, and diabetes. Molecular analysis showed that they are compound heterozygotes for GAA expansion and a novel exon 5a missense mutation (R165P). This mutation appears to be associated with an atypical but not milder Friedreich ataxia phenotype.

Somatic sequence variation at the Friedreich ataxia locus includes complete contraction of the expanded GAA triplet repeat, significant length variation in serially passaged lymphoblasts and enhanced mutagenesis in the flanking sequence

The vast majority of Friedreich ataxia patients are homozygous for large GAA triplet repeat expansions in intron 1 of the X25 gene. Instability of the expanded GAA repeat was examined in 23 chromosomes bearing 97-1250 triplets in lymphoblastoid cell lines passaged 20-39 times. Southern analyses revealed 18 events of significant changes in length ranging from 69 to 633 triplets, wherein the de novo allele gradually replaced the original over 1-6 passages. Contractions and expansions occurred with equal frequency and magnitude. This behavior is unique in comparison with other large, non-coding triplet repeat expansions [(CGG)(n)and (CTG)(n)] which remain relatively stable under similar conditions. We also report a rare patient who, having inherited two expanded alleles, showed evidence of contracted GAA repeats ranging from nine to 29 triplets in DNA from two independent peripheral blood samples. The GAA triplet repeat is known to adopt a triplex structure, and triplexes in transcribed templates cause enhanced mutagenesis. The poly(A) tract and a 135 bp sequence, both situated immediately upstream of the GAA triplet repeat, were therefore examined for somatic mutations. The poly(A) tract showed enhanced instability when in cis with the GAA expansion. The 135 bp upstream sequence was found to harbor a 3-fold excess of point mutations in DNA derived from individuals homozygous for the GAA triplet repeat expansion compared with normal controls. These data are likely to have important mechanistic and clinical implications.

Sticky DNA: self-association properties of long GAA.TTC repeats in R.R.Y triplex structures from Friedreich's ataxia

A novel DNA structure, sticky DNA, is described for lengths of (GAA.TTC)n found in intron 1 of the frataxin gene of Friedreich's ataxia patients. Sticky DNA is formed by the association of two purine.purine.pyrimidine (R.R.Y) triplexes in negatively supercoiled plasmids at neutral pH. An excellent correlation was found between the lengths of (GAA.TTC) (> 59 repeats): first, in FRDA patients, second, required to inhibit transcription in vivo and in vitro, and third, required to adopt the sticky conformation. Fourth, (GAAGGA.TCCTTC)65, also found in intron 1, does not form sticky DNA, inhibit transcription, or associate with the disease. Hence, R.R.Y triplexes and/or sticky DNA may be involved in the etiology of FRDA.

Kallmann syndrome gene (KAL-X) is not mutated in schizophrenia

Kallmann syndrome and schizophrenia share several clinical features, including dysfunctional olfactory ability, hypogonadotrophic hypogonadism, an excess of affected males, and psychiatric presentation. Because of this congruence, it has been proposed that up to 70% of male schizophrenics might have mutations affecting the function or expression of the gene mutated in Kallmann syndrome, KAL-X. We identified and studied 9 unrelated males with schizophrenia (as defined by DSM-IIIR criteria) who also have severe anosmia (first percentile of normal range) and low sex drive (seventh percentile of the normal range), and we sequenced the exons and the intron-exon junctions of the KAL-X gene for each. We found no mutations, and conclude that schizophrenia is rarely, if ever, due to a mutation in the coding sequence or splice junctions of KAL-X.

Friedreich's ataxia: point mutations and clinical presentation of compound heterozygotes

Friedreich's ataxia is the most common inherited ataxia. Ninety-six percent of patients are homozygous for GAA trinucleotide repeat expansions in the first intron of the frataxin gene. The remaining cases are compound heterozygotes for a GAA expansion and a frataxin point mutation. We report here the identification of 10 novel frataxin point mutations, and the detection of a previously described mutation (G130V) in two additional families. Most truncating mutations were in exon 1. All missense mutations were in the last three exons coding for the mature frataxin protein. The clinical features of 25 patients with identified frataxin point mutations were compared with those of 196 patients homozygous for the GAA expansion. A similar phenotype resulted from truncating mutations and from missense mutations in the carboxy-terminal half of mature frataxin, suggesting that they cause a comparable loss of function. In contrast, the only two missense mutations located in the amino-terminal half of mature frataxin (D122Y and G130V) cause an atypical and milder clinical presentation (early-onset spastic gait with slow disease progression, absence of dysarthria, retained or brisk tendon reflexes, and mild or no cerebellar ataxia), suggesting that they only partially affect frataxin function. The incidence of optic disk pallor was higher in compound heterozygotes than in expansion homozygotes, which might correlate with a very low residual level of normal frataxin produced from the expanded allele.

Molecular genetics and pathogenesis of Friedreich ataxia

Friedreich ataxia, the most frequent cause of inherited ataxia, is due in most cases to 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 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.

Inhibitory effects of expanded GAA.TTC triplet repeats from intron I of the Friedreich ataxia gene on transcription and replication in vivo

Friedreich ataxia (FRDA) is associated with the expansion of a GAA. TTC triplet repeat in the first intron of the frataxin gene, resulting in reduced levels of frataxin mRNA and protein. To investigate the mechanisms by which the intronic expansion produces its effect, GAA.TTC repeats of various lengths (9 to 270 triplets) were cloned in both orientations in the intron of a reporter gene. Plasmids containing these repeats were transiently transfected into COS-7 cells. A length- and orientation-dependent inhibition of reporter gene expression was observed. RNase protection and Northern blot analyses showed very low levels of mature mRNA when longer GAA repeats were transcribed, with no accumulation of primary transcript. Replication of plasmids carrying long GAA.TTC tracts (approximately 250 triplets) was greatly inhibited in COS-7 cells compared with plasmids carrying (GAA.TTC)9 and (GAA.TTC)90. Replication inhibition was five times greater for the plasmid whose transcript contains (GAA)230 than for the plasmid whose transcript contains (UUC)270. Our in vivo investigation revealed that expanded GAA.TTC repeats from intron I of the FRDA gene inhibit transcription rather than post-transcriptional RNA processing and also interfere with replication. The molecular basis for these effects may be the formation of non-B DNA structures.

A mild case of Friedreich ataxia: lymphocyte and sural nerve analysis for GAA repeat length reveals somatic mosaicism

Friedreich ataxia (FRDA) is an autosomal recessive, neurodegenerative disease, characterized by progressive gait and limb ataxia, dysarthria, lower-limb areflexia, Babinski sign, loss of position and vibration senses, cardiomyopathy, and carbohydrate intolerance. It is the most common inherited ataxia, and is associated with a GAA triplet repeat expansion in the first intron of the X25 gene on the long arm of chromosome 9. We present a case whose clinical diagnosis was initially confounded by the mildness of the ataxic phenotype and a family history of multiple sclerosis. Evaluation of the X25 gene revealed that the patient was homozygous for the GAA triplet repeat expansion, pathognomonic of FRDA. Investigation of her sural nerve biopsy revealed a significantly smaller expansion size, constituting the first direct demonstration of somatic mosaicism involving the nervous system in FRDA. We speculate that a similar contraction in pathologically affected tissues could be the molecular basis for the mildness of the ataxia.

The GAA triplet-repeat expansion in Friedreich ataxia interferes with transcription and may be associated with an unusual DNA structure

Friedreich ataxia (FRDA), an autosomal recessive, neurodegenerative disease is the most common inherited ataxia. The vast majority of patients are homozygous for an abnormal expansion of a polymorphic GAA triplet repeat in the first intron of the X25 gene, which encodes a mitochondrial protein, frataxin. Cellular degeneration in FRDA may be caused by mitochondrial dysfunction, possibly due to abnormal iron accumulation, as observed in yeast cells deficient for a frataxin homologue. Using RNase protection assays, we have shown that patients homozygous for the expansion have a marked deficiency of mature X25 mRNA. The mechanism(s) by which the intronic GAA triplet expansion results in this reduction of X25 mRNA is presently unknown. No evidence was found for abnormal splicing of the expanded intron 1. Using cloned repeat sequences from FRDA patients, we show that the GAA repeat per se interferes with in vitro transcription in a length-dependent manner, with both prokaryotic and eukaryotic enzymes. This interference was most pronounced in the physiological orientation of transcription, when synthesis of the GAA-rich transcript was attempted. These results are consistent with the observed negative correlation between triplet-repeat length and the age at onset of disease. Using in vitro chemical probing strategies, we also show that the GAA triplet repeat adopts an unusual DNA structure, demonstrated by hyperreactivity to osmium tetroxide, hydroxylamine, and diethyl pyrocarbonate. These results raise the possibility that the GAA triplet-repeat expansion may result in an unusual yet stable DNA structure that interferes with transcription, ultimately leading to a cellular deficiency of frataxin.

Frataxin is reduced in Friedreich ataxia patients and is associated with mitochondrial membranes

Friedreich ataxia is a progressive neurodegenerative disorder caused by loss of function mutations in the frataxin gene. In order to unravel frataxin function we developed monoclonal antibodies raised against different regions of the protein. These antibodies detect a processed 18 kDa protein in various human and mouse tissues and cell lines that is severely reduced in Friedreich ataxia patients. By immunocytofluorescence and immunocytoelectron microscopy we show that frataxin is located in mitochondria, associated with the mitochondrial membranes and crests. Analysis of cellular localization of various truncated forms of frataxin expressed in cultured cells and evidence of removal of an N-terminal epitope during protein maturation demonstrated that the mitochondrial targetting sequence is encoded by the first 20 amino acids. Given the shared clinical features between Friedreich ataxia, vitamin E deficiency and some mitochondriopathies, our data suggest that a reduction in frataxin results in oxidative damage.

Deciphering the cause of Friedreich ataxia

Friedreich ataxia (FA), the most frequent cause of recessive ataxia, is attributable, in most cases, to a large expansion of an intronic GAA repeat, resulting in decreased expression of the target frataxin gene. This gene encodes a novel mitochondrial protein that has homologues of unknown function in yeast and even in gram-negative bacteria. Yeast deficient in the frataxin homologue accumulate iron in their mitochondria and show increased sensitivity to oxidative stress. This finding suggests that FA patients suffer from a mitochondrial dysfunction that causes free-radical toxicity, reminiscent of the clinically similar ataxia caused by inherited isolated vitamin E deficiency.

Studies of human, mouse and yeast homologues indicate a mitochondrial function for frataxin

Friedreich's ataxia is due to loss of function mutations in the gene encoding frataxin (FRDA). Frataxin is a protein of unknown function. In situ hybridization analyses revealed that mouse frataxin expression correlates well with the main site of neurodegeneration, but the expression pattern is broader than expected from the pathology of the disease. Frataxin mRNA is predominantly expressed in tissues with a high metabolic rate, including liver, kidney, brown fat and heart. We found that mouse and yeast frataxin homologues contain a potential mitochondrial targeting sequence in their N-terminal domains and that disruption of the yeast gene results in mitochondrial dysfunction. Finally, tagging experiments demonstrate that human frataxin co-localizes with a mitochondrial protein. Friedreich's ataxia is therefore a mitochondrial disease caused by a mutation in the nuclear genome.

The neuropathology of CAG repeat diseases: review and update of genetic and molecular features

Classification of inherited neurodegenerative diseases is increasingly based on their genetic features, which supplement, clarify, and sometimes replace the older clinical and pathologic schemata. This change has been particularly rapid and impressive for the CAG repeat disorders. In Huntington's disease, X-linked spinobulbar muscular atrophy, dentatorubropallidoluysian atrophy, and a series of autosomal dominant cerebellar atrophies, genetic advances have resolved many nosologic issues, and opened new avenues for exploration of pathogenesis. In this review, we summarize classic and current concepts in neuropathology of these CAG repeat diseases.

Regulation of mitochondrial iron accumulation by Yfh1p, a putative homolog of frataxin

The gene responsible for Friedreich's ataxia, a disease characterized by neurodegeneration and cardiomyopathy, has recently been cloned and its product designated frataxin. A gene in Saccharomyces cerevisiae was characterized whose predicted protein product has high sequence similarity to the human frataxin protein. The yeast gene (yeast frataxin homolog, YFH1) encodes a mitochondrial protein involved in iron homeostasis and respiratory function. Human frataxin also was shown to be a mitochondrial protein. Characterizing the mechanism by which YFH1 regulates iron homeostasis in yeast may help to define the pathologic process leading to cell damage in Friedreich's ataxia.

The vibrator mutation causes neurodegeneration via reduced expression of PITP alpha: positional complementation cloning and extragenic suppression

The mouse vibrator mutation causes an early-onset progressive action tremor, degeneration of brain stem and spinal cord neurons, and juvenile death. We cloned the vibrator mutation using an in vivo positional complementation approach and complete resequencing of the resulting 76 kb critical region from vibrator and its parental chromosome. The mutation is an intracisternal A particle retroposon insertion in intron 4 of the phosphatidylinositol transfer protein alpha gene, causing a 5-fold reduction in RNA and protein levels. Expression of neurofilament light chain is also reduced in vibrator, suggesting one signaling pathway that may underlie vibrator pathology. The vibrator phenotype is suppressed in one intercross. We performed a complete genome scan and mapped a major suppressor locus (Mvb-1) to proximal chromosome 19.

Frataxin shows developmentally regulated tissue-specific expression in the mouse embryo

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disease caused either by an intronic GAA triplet repeat expansion that suppresses the expression of the frataxin gene on chromosome 9q13, or, rarely, by point mutations in the frataxin gene. We investigated the expression of the mouse frataxin homologue during embryonic development by Northern blot analysis and RNA in situ hybridization. Very faint expression could be detected from E10.5 in the neuroepithelium and more clearly from E12.5 in the developing central nervous system. At E14.5, frataxin was expressed at a much higher level that remained constant into the postnatal period. Maximum expression was observed in the spinal cord, particularly at the thoracolumbar level, and in the dorsal root ganglia. Significant levels of transcript could also be detected in the proliferating cells in the periventricular zone, in the cortical plates, in the heart, in the axial skeleton, and in some epithelial and mesenchymal tissues. Overall, the distribution of frataxin mRNA was in good accordance with previous data from Northern analysis of adult human tissues, the major discrepancy being the expression in mouse embryonic cerebral cortex which was not observed in adult human brain. The tissues expressing frataxin during development appear to be those that become dysfunctional or atrophied in FRDA. Overall, our data suggest that frataxin is a protein whose expression is cell-specific and developmentally regulated.