Confirmation of linkage of Friedreich ataxia to chromosome 9 and identification of a new closely linked marker

Determination of Genotypic and Phenotypic Characteristics of Friedreich's Ataxia and Autosomal Dominant Spinocerebellar Ataxia Types 1, 2, 3, and 6

INTRODUCTION

This study aimed to analyze the genotypic characteristics of Friedreich's ataxia (FA) and autosomal dominant ataxias [such as spinocerebellar ataxia (SCA) types 1, 2, 3, and 6] using molecular and biological methods in hereditary cerebellar ataxia considering both clinical and electrophysiological findings.

METHODS

The study included 129 indexed cases, who applied to the neurology department and were diagnosed with hereditary cerebellar ataxia through clinical, laboratory, and electrophysiological findings, and 15 sibling patients who were diagnosed through family scanning (144 cases in total); their genetic analyses were also performed. Detailed physical and neurological examinations, pedigree analyses, electroneurography, evoked potentials, cerebral-spinal magnetic resonance imaging, and echocardiographic analyses were performed for all cases. Blood samples were collected from patients, and the genotypic characteristics of autosomal dominant SCA types 1, 2, 3, and 6 were investigated. Statistical analyses were performed with the Statistical Package for the Social Sciences (SPSS Inc; Chicago, IL, USA) 17.0.

RESULTS

Almost 50% of patients were defined as FA. Moreover, two SCA1 cases and one SCA6 case were detected.

CONCLUSION

In our study, 47.2% of patients with FA had developed hereditary cerebellar ataxia. Ground and autosomal dominant-linked SCA1 and SCA6 were each detected in one family. These data suggest that patients with cerebellar ataxia of hereditary origin should be primarily examined for FA.

Friedreich Ataxia: From the Eye of a Molecular Biologist

Friedreich ataxia (FRDA) is caused by the expansion of a GAA triplet repeat in the first intron of the FXN gene. This disease was named after Nicholaus Friedreich, Germany, who depicted the essential finding. Among ataxias, FRDA is the most common hereditary ataxia. It has the autosomal recessive pattern of inheritance. The expansion of the GAA triplet repeat hinders the transcription, thereby reducing the level of the FXN transcript and consequently reducing the level of frataxin, a 210-amino acid protein. The disease pathogenesis is fundamentally due to a lack of frataxin, which is claimed to play a role in iron-sulfur cluster synthesis. Oxidative stress builds up as a result of Fe accumulation in the mitochondria, causing degeneration of the cells, which primarily occurs in the neurons and later in the cardiac tissues, and to some extent in the pancreas. The therapeutic interventions are at infancy; however, current treatments are targeted toward the reduction of iron overload and its effects.

Genetic and phenotypic variability of optic neuropathies

Hereditary optic neuropathies comprise a group of clinically and genetically heterogeneous disorders. Two subgroups can be formed: isolated hereditary optic atrophies and optic neuropathy as part of complex disorders. In group 1 of hereditary optic neuropathies, optic nerve dysfunction is typically the only manifestation of the disease. This group comprises autosomal dominant, autosomal recessive and X-linked recessive optic atrophy and the maternally inherited Leber's hereditary optic neuropathy. Among the autosomal-dominant forms of optic atrophy, Kjer's disease is most frequently observed. In the second group of complex disorders, various neurologic and other systemic abnormalities are regularly observed. Most frequent in this group are mtDNA mutations, inherited peripheral neuropathies, Charcot-Marie-Tooth disorders (CMT2A2, CMTX5), hereditary sensory neuropathy type 3 (HSAN3), Friedreich's ataxia, leukodystrophies, sphingolipidoses, ceroid-lipofuscinoses and neurodegeneration with brain iron accumulation. We review current knowledge about the underlying genetic predispositions, the most urgent open questions and how this may affect our management of this heterogeneous group of disorders in the future.

Therapeutic strategies in Friedreich's ataxia

First established as a diagnosis by Nikolaus Friedreich in 1863, Friedreich's ataxia (FA) is an autosomal recessive progressive neurodegenerative disorder cause by a trinucleotide repeat expansion. FA begins with the functional absence of the FXN gene product frataxin, a protein whose exact function still remains unknown. This absence results in impaired intracellular antioxidant defenses, dysregulation of iron-sulfur cluster proteins, depression of aerobic electron transport chain respiration, massive mitochondrial dysfunction, and ultimately cell death in the brain, spinal cord and heart. Herein, we review the molecular and cellular pathogenesis leading to widespread organ system dysfunction, as well as current therapeutic research aimed at preventing the debilitating effects of frataxin loss and preventing the signs and symptoms associated of FA. We also discuss the ongoing treatment strategies employed by our laboratory to prevent mitochondrial damage using synergistic effects of 17β-estradiol and methylene blue, previously shown by our group and others to have protective effects in human FA fibroblasts. This article is part of a Special Issue entitled Hormone Therapy.

Estrogen prevents oxidative damage to the mitochondria in Friedreich's ataxia skin fibroblasts

Estrogen and estrogen-related compounds have been shown to have very potent cytoprotective properties in a wide range of disease models, including an in vitro model of Friedreich's ataxia (FRDA). This study describes a potential estrogen receptor (ER)-independent mechanism by which estrogens act to protect human FRDA skin fibroblasts from a BSO-induced oxidative insult resulting from inhibition of de novo glutathione (GSH) synthesis. We demonstrate that phenolic estrogens, independent of any known ER, are able to prevent lipid peroxidation and mitochondrial membrane potential (ΔΨm) collapse, maintain ATP at near control levels, increase oxidative phosphorylation and maintain activity of aconitase. Estrogens did not, however, prevent BSO from depleting GSH or induce an increased expression level of GSH. The cytoprotective effects of estrogen appear to be due to a direct overall reduction in oxidative damage to the mitochondria, enabling the FRDA fibroblast mitochondria to generate sufficient ATP for energy requirements and better survive oxidative stress. These data support the hypothesis that phenol ring containing estrogens are possible candidate drugs for the delay and/or prevention of FRDA symptoms.

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.

GAA repeat polymorphism in Turkish Friedreich's ataxia patients

Friedreich's ataxia (FRDA), the most common subtype of early onset hereditary spinocerebellar ataxia (SCA), is an autosomal recessive neurodegenerative disorder caused by unstable GAA tri-nucleotide expansions in the first intron of FRDA gene located at 9q13-q21.1 position. Results of GAA repeat polymorphism in 80 Turkish SCA patients and 38 family members of 11 typical FRDA patients were reported. GAA triplet repeat size ranged from approximately 7 to 34 in normal alleles and from approximately 66 to 1300 in mutant alleles. Twenty six patients were homozygous for GAA expansion and size of expanded alleles differed from approximately 425 to 1300 repeats. Children 2 and 6 years old (showing no ataxia symptoms) of one family had homozygous GAA expansions reaching approximately 925 repeats. All 11 families studied had at least 1 afflicted child and 9 parents and 2 siblings were carrier (heterozygous) with mutant alleles ranging from 66 to 850 repeats. Family studies confirmed the meiotic instability and stronger effect of expansion in the smaller alleles on phenotype and a negative correlation between GAA repeat expansion size and onset-age of the disease.

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.

Friedreich's ataxia

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

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: clinical and molecular study of 25 Brazilian cases

INTRODUCTION

Friedreich's ataxia is a neurodegenerative disorder whose clinical diagnostic criteria for typical cases basically include: a) early age of onset (< 20 or 25 years), b) autosomal recessive inheritance, c) progressive ataxia of limbs and gait, and d) absence of lower limb tendon reflexes.

METHODS

We studied the frequency and the size of expanded GAA and their influence on neurologic findings, age at onset, and disease progression in 25 Brazilian patients with clinical diagnosis of Friedreich's ataxia - 19 typical and 6 atypical - using a long-range PCR test.

RESULTS

Abnormalities in cerebellar signs, in electrocardiography, and pes cavus occurred more frequently in typical cases; however, plantar response and speech were more frequently normal in this group when the both typical and atypical cases were compared. Homozygous GAA expansion repeats were detected in 17 cases (68%) - all typical cases. In 8 patients (32%) (6 atypical and 2 typical), no expansion was observed, ruling out the diagnosis of Friedreich's ataxia. In cases with GAA expansions, foot deformity, cardiac abnormalities, and some neurologic findings occurred more frequently; however, abnormalities in cranial nerves and in tomographic findings were detected less frequently than in patients without GAA expansions.

DISCUSSION

Molecular analysis was imperative for the diagnosis of Friedreich's ataxia, not only for typical cases but also for atypical ones. There was no genotype-phenotype correlation. Diagnosis based only on clinical findings is limited; however, it aids in better screening for suspected cases that should be tested. Evaluation for vitamin E deficiency is recommended, especially in cases without GAA expansion.

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.

Genetic analysis of early onset cerebellar ataxia with retained tendon reflexes in four Tunisian families

Spinocerebellar ataxias comprise a poorly understood group of inherited degenerative neurological diseases. Attempts to classify hereditary ataxias on the basis of the neurological features or specific clinical signs such as tendon reflex changes have proven to be unsatisfactory. Early onset cerebellar ataxia (EOCA) is generally inherited as an autosomal-recessive trait. Thus far, we do not have accurate answers to several questions about its classification. However, significant clinical heterogeneity observed in four Tunisian families with typical EOCA clinical features reinforces the hypothesis of genetic heterogeneity underlying this phenotype. We have demonstrated that three of the four families studied were not linked to Friedreich's ataxia (FA), vitamin E deficiency ataxia (AVED), and autosomal dominant cerebellar ataxia (ADCA) loci. The fourth family showed homozygosity for a large pathological expansion of GAA repeat in all patients, the parents being heterozygous for this mutation. We have also noted, in the case of the family studied, that there was instability in the transmission of the mutation, along with a phenomenon of anticipation comparable to that observed in dominant triplet repeat diseases. EOCA is thus clinically indistinguishable from FA, yet genetically independent of all known candidate genes. Genetic mapping is required for research into the causal gene and an understanding of the disease's physiopathologic mechanisms.

Different phenotypes of Friedreich's ataxia within one 'pseudo-dominant' genealogy: relationships between trinucleotide (GAA) repeat lengths and clinical features

We examined a large Turkmen family with 'pseudo-dominant' inheritance of Friedreich's ataxia resulting from consanguineous marriage of a Friedreich's ataxia patient to a heterozygote carrying an ancestral mutated allele. Two distinct phenotypes of the disease co-segregated within this genealogy. Two brothers from the younger generation exhibited 'classical' Friedreich's ataxia with onset of symptoms before 10 years and a rapidly progressive course. In contrast, three patients (two sisters from the younger generation and their father) had a more benign phenotype of late-onset Friedreich's ataxia with the onset at 26, 45 and 48 years and slow progression over decades. The patients with 'classical' Friedreich's ataxia were homozygous for a common ancestral expanded allele of the X25 gene containing 700-800 GAA repeats, while the patients with late-onset Friedreich's ataxia had two different mutated alleles, the shorter 250-repeat expansion of paternal origin and the longer 700-repeat expansion of maternal origin. One may conclude that clinical variability of Friedreich's ataxia in our patients is accounted for predominantly by a modifying effect of one of the two (shorter or longer) expanded alleles inherited from their affected father. Our observation clearly demonstrates the significance of variable-sized alleles for the phenotypic expression of the disease.

Echocardiographic features of genetic diseases: part 1. Cardiomyopathy

Introduction To The Series Genetic disorders have characteristic cardiovascular manifestations. These cardiovascular abnormalities often are a major determinant of the morbidity and mortality in this patient population. Some characteristics are unique and can be detected with echocardiography. Drs Alizad and Seward have compiled in this series a review of genetic disorders that have recognizable morphologic and/or functional cardiovascular abnormalities. The following topics will be explored, with examples from the database of the Mayo Clinic Echocardiography Laboratory:Cardiomyopathy Storage disease Shunts Connective tissue Tumors Complex cardiovascular defects Complex genetic disorders Organ system

Friedreich ataxia: an overview

Friedreich ataxia, an autosomal recessive neurodegenerative disease, is the most common of the inherited ataxias. The recent discovery of the gene that is mutated in this condition, FRDA, has led to rapid advances in the understanding of the pathogenesis of Friedreich ataxia. About 98% of mutant alleles have an expansion of a GAA trinucleotide repeat in intron 1 of the gene. This leads to reduced levels of the protein, frataxin. There is mounting evidence to suggest that Friedreich ataxia is the result of accumulation of iron in mitochondria leading to excess production of free radicals, which then results in cellular damage and death. Currently there is no known treatment that alters the natural course of the disease. The discovery of the FRDA gene and its possible function has raised hope that rational therapeutic strategies will be developed.

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.

Electrophysiology and nerve biopsy: comparative study in Friedreich's ataxia and Friedreich's ataxia phenotype with vitamin E deficiency

The authors report a comparative study of peripheral nerve conductions and nerve biopsy and somatosensory evoked potentials between 15 patients with Friedreich's ataxia and 15 patients with Friedreich's ataxia phenotype with selective vitamin E deficiency. The patients in the two groups are of similar age, age of onset, and clinical phenotype. Peripheral motor nerve action potential amplitude, and conduction velocities are within normal ranges in the two groups. In the Friedreich's ataxia group there is an early and severe peripheral sensory axonal neuronopathy, characterised by an important reduction of the amplitude of sensory action potential, and important loss of myelinated fibres with complete disappearance of large myelinated fibres without any regenerative process. In the Friedreich's ataxia phenotype with selective vitamin E deficiency group there is slight-to-moderate axonal sensory neuropathy with normal to moderate decrease of large myelinated fibre density and important regeneration in nerve biopsy. Somatosensory evoked potentials are markedly involved in the two groups asserting a severe involvement of somatosensory pathway in lumbar, thoracic and cervical spinal cord. These findings suggest that the pathological mechanism involved in the two diseases are different: central peripheral axonopathy in Friedreich's ataxia and central distal axonopathy in Friedreich's ataxia phenotype with selective vitamin E deficiency.

Molecular genetics of the hereditary ataxias

One of us (MP) learned about the mapping of Huntington disease gene to chromosome 4 from the late Dr. Anita Harding. She got the news over the phone from her London office during a visit to Italy for a meeting on hereditary ataxias. In Britain, they receive Nature at least a week earlier than us. Dr. Harding was very excited, and she immediately said that that was the way to go if we wanted to understand the causes of hereditary ataxias, classify these diseases in a rational way, and eventually find a treatment. At that time, the challenge seemed, and indeed was, formidable. No clue was then available about the genetic basis of what Dr. Harding aptly called "hereditary ataxias of unknown cause," their classification was confused and controversial, and all attempts to find specific biochemical abnormalities had failed. Fourteen years later, the success of the molecular genetic studies is astounding. The defective genes have been identified for Friedreich ataxia, the major recessive "hereditary ataxia of unknown cause," and for five dominantly inherited "hereditary ataxias of unknown cause." Three more dominant ataxia genes have been mapped. The molecular pathogenesis of the dominant ataxias begins to be unraveled and animal models have been and are being developed. Information is also quickly accumulating about the defective protein in Friedreich ataxia. Direct molecular diagnosis is now possible. Classification has been revolutionized. Diagnostic criteria are being redefined in the light of the molecular discoveries. The goal of this review, dedicated to the memory of the late Dr. Harding, is to offer a concise summary of current knowledge about the molecular genetics of some of the hereditary ataxias that used to be classified as of "unknown cause."

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.

Phenotype correlation and intergenerational dynamics of the Friedreich ataxia GAA trinucleotide repeat

The Friedreich ataxia (FA) mutation has recently been identified as an unstable trinucleotide GAA repeat present 7-22 times in the normal population but amplified as many as > 1,000 times in FA. Since it is an autosomal recessive disease, FA does not show typical features observed in other dynamic mutation disorders, such as genetic anticipation. We have analyzed the GAA repeat in 104 FA patients and 163 carrier relatives previously defined by linkage analysis. The GAA expansion was detected in all patients, most (94%) of them being homozygous for the mutation. We have demonstrated that clinical variability in FA is related to the size of the expanded alleles: milder forms of the disease-late-onset FA and FA with retained reflexes-are associated with shorter expansions, especially with the smaller of the two expanded alleles. Absence of cardiomyopathy is also associated with shorter alleles. Dynamics of the GAA repeat has been investigated in 212 parent-offspring pairs. Meiotic instability showed a sex bias: paternally transmitted alleles tend to decrease in a linear way that depends on the paternal expansion size, whereas maternal alleles can either increase or decrease. A different pattern of intergenerational variation was also observed, depending on the genetic status of the sib: patients had shorter expansions than were seen in heterozygous carriers. This finding has been interpreted as a postzygotic event. Finally, we have observed that the size of the expansion remains constant in the population through carriers.

Friedreich ataxia in Acadian families from eastern Canada: clinical diversity with conserved haplotypes

The gene for Friedreich ataxia (FRDA), an autosomal-recessive neurodegenerative disease, remains elusive. The current candidate region of about 150 kb lies between loci FR2 and F8101 near the D9S15/D9S5 linkage group at 9q13-21.1. Linkage homogeneity between classical FRDA and a milder, slowly progressive Acadian variant (FRDA-Acad) has been demonstrated. An extended D9S15-D9S5 haplotype (C6) predominates in FRDA-Acad chromosomes from Louisiana. We studied 10 Acadian families from New Brunswick, Canada. In eight families, affected individuals conformed to the clinical description of FRDA-Acad; in one, 2 sibs presented with spastic ataxia (SPA-Acad). In the last family, 2 sibs had FRDA-Acad, and one had SPA-Acad. We found that SPA-Acad is linked to the FRDA gene region. The C6 haplotype and a second major haplotype (B7) were identified. The same ataxia-linked haplotypes segregated with both FRDA-Acad and SPA-Acad in two unrelated families. The parental origins of these haplotypes were different. Our observation of different phenotypes associated with the same combination of haplotypes may point to the influence of the parent of origin on gene expression, indicate the effect of modifier genes, or reflect the presence of different mutations on the same haplotypes. Our findings underline the need to investigate families with autosomal-recessive ataxias for linkage to the FRDA region, despite lack of key diagnostic manifestations such as cardiomyopathy or absent deep-tendon reflexes.

A family segregating a Friedreich ataxia phenotype that is not linked to the FRDA locus

Friedreich ataxia is an autosomal recessive neurodegenerative disorder. The genetic homogeneity to the FRDA locus on chromosome 9q13-21.1 has been observed in families from different ancestries. We report a Spanish family with two affected and three unaffected children. The segregated classical Friedreich ataxia did not show the expected linkage. The analysis focusses on flanking markers FR1, FR2, FR7 and FR5, excluding linkage 1 cM around the FRDA locus. The unique clinical hallmark in this family was the absence of cardiomyopathy after a long-term follow-up in the two affected children. In both patients serum vitamin E levels were normal. The present observations support the existence of a second locus in Friedreich ataxia, and we suggest that this form could be clinically characterized by the absence of muscular heart disease.

Friedreich's ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion

Friedreich's ataxia (FRDA) is an autosomal recessive, degenerative disease that involves the central and peripheral nervous systems and the heart. A gene, X25, was identified in the critical region for the FRDA locus on chromosome 9q13. This gene encodes a 210-amino acid protein, frataxin, that has homologs in distant species such as Caenorhabditis elegans and yeast. A few FRDA patients were found to have point mutations in X25, but the majority were homozygous for an unstable GAA trinucleotide expansion in the first X25 intron.

Clinical aspects of hereditary ataxias

The hereditary ataxias are a group of complex genetic disorders the understanding of which is undergoing a revolution because of advances in molecular genetics. Within the last few years, at least seven different gene loci have been found to be responsible for these syndromes, and the search is on for additional loci that undoubtedly exist. This review summarizes the clinical features of the various hereditary ataxias with known gene loci, as well as others that are now defined on a clinical basis. It also deals with some of the imaging and neuropharmacologic advances that have been made in this group of disorders.

Mapping the Friedreich ataxia locus (FRDA) by linkage disequilibrium analysis with highly polymorphic microsatellites

The Friedreich's ataxia locus (FRDA) is tightly linked to markers D9S5 and D9S15 located in 9q13-q21. Cumulated maximum lod scores between FRDA and D9S5 and between FRDA and D9S15 are above 36 and 61, respectively, at a recombination fraction of 0, indicating that recombination events needed to orient the search of the gene are very difficult to identify and ascertain. We have established a 1 Megabase PFGE map around D9S5 and D9S15 and isolated a corresponding 530 kb YAC contig. We found that the two markers are 260 kb apart. This result was surprising, since D9S5 and D9S15 were independently isolated, but in agreement with the strong linkage between the two loci (lod score > 35 at a recombination fraction of 0). Seven clusters of rare cutter enzyme sites (CpG islands), which are potential indicators of genes, were identified in the 1 Megabase region by PFGE analysis and YAC mapping. The search for genes around the CpG islands is in progress. To map the Friedreich ataxia locus in the absence of clearly identified recombination events, we chose an alternative approach based on haplotype analysis of patients from small populations with precise geographic and historical origins, such as the Louisiana-Acadians, deported from Nova-Scotia about 150 years ago and who remained isolated for historical and cultural reasons. In this population, a single mutation, associated with a specific haplotype may account for the majority of Friedreich ataxia cases. Haplotypes different from the major haplotype at one or the other extremity can indicate ancient recombinations.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of a genetic marker associated with early onset in Friedreich's ataxia

We evaluated the association between age at onset of Friedreich's ataxia and alleles of two restriction fragment length polymorphisms (RFLP) at D9S15 and D9S5 in the 9q13-9q21.1 region. We studied 65 Italian patients from 49 families. Age at onset was not normally distributed in our patients, suggesting allelic heterogeneity. Patients homozygous for allele 1 of MspI RFLP detected by probe MCT112 at D9S15 (M1) had an earlier onset (mean 9.3, SD 3.4 years) than patients homozygous for allele 2 (M2; mean 12.1, SD 4.3). Heterozygotes had an onset age similar to that of the M2 homozygotes. These findings suggest that the M1 allele might be a marker of one allelic early-onset Friedreich's ataxia mutation.

Gene in the region of the Friedreich ataxia locus encodes a putative transmembrane protein expressed in the nervous system

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disorder that affects the cerebellum, spinal cord, and peripheral nerves. The FRDA gene was localized in 9q13-q21 within 0.7 centimorgan of the D9S5 and D9S15 loci. One recently reported recombination event and haplotype analysis in a population with a founder effect suggested that the FRDA locus is on the D9S5 side. Using a conserved probe from the D9S5 locus, we have now identified an approximately 7-kilobase (kb) transcript and report cloning of its cDNA. The corresponding gene, X11, extends at least 80 kb in a direction opposite D9S15. The gene is expressed in the brain, including the cerebellum, but is not detectable in several nonneuronal tissues and cell lines. In situ hybridization of adult mouse brain sections showed prominant expression in the granular layer of the cerebellum. Expression was also found in the spinal cord. The cDNA contains an open reading frame encoding a 708-amino acid sequence that shows no significant similarity to other known proteins but contains a unique, 24-residue-long, putative transmembrane segment. On the basis of its genomic localization and its neuronal site of expression, particularly in the cerebellum, this "pioneer" gene represents a candidate for FRDA. Direct evidence of its involvement in FRDA will require a search for causative point mutations in patients.

A 530kb YAC contig tightly linked to the Friedreich ataxia locus contains five CpG clusters and a new highly polymorphic microsatellite

Friedreich ataxia (FA) is a severe autosomal recessive neurodegenerative disease. The defective gene has been previously assigned to chromosome 9q13-q21 by demonstration of tight linkage to the two independent loci D9S15 and D9S5. Linkage data indicate that FRDA is at less than 1 cM from both markers. Previous physical mapping has shown that probes defining D9S15 (MCT112) and D9S5 (26P) are less than 260 kb apart and are surrounded by at least six CpG clusters within 450 kb, which might indicate the presence of "candidate" genes for FA. We isolated and characterized a 530 kb YAC (yeast artificial chromosome) contig that contains five of the CpG clusters. The YACs were used to search for new polymorphic markers needed to map FRDA precisely with respect to the cloned segment. In particular, we found a (CA)n microsatellite polymorphism, GS4, that detects 13 alleles with a PIC value of 0.83 and allows the definition of haplotypes extending over 310 kb when used in combination with polymorphic markers at D9S5 and D9S15.

Construction and characterization of radiation hybrids for chromosome 9, and their use in mapping cosmid probes on the chromosome

Radiation hybrids were produced from a monochromosomal microcell hybrid (PK87-9) which contains only human chromosome 9 with an inserted marker on 9p. Doses of radiation ranging from 1000 to 8000 rads were used to produce a series of hybrids with different size fragments of human chromosome 9. The inserted dominant selectable marker was used to select for hybrids that preferentially maintain fragments of 9p. A panel of 53 radiation hybrids were characterized for 17 chromosome 9 markers. In addition, 17 hybrids were analyzed by fluorescent in situ hybridization (FISH). Hybrids were produced with breaks on both 9p and 9q, many of which appear to contain a single fragment of human chromosome 9. These hybrid cell lines were used to regionally localize 31 cosmids isolated from a chromosome 9 cosmid library. Six cosmids were mapped to intervals on 9p, six cosmids mapped to the centromeric region of the chromosome, and 19 mapped to 9q.

Physical mapping of two loci (D9S5 and D9S15) tightly linked to Friedreich ataxia locus (FRDA) and identification of nearby CpG islands by pulse-field gel electrophoresis

The Friedreich's ataxia locus (FRDA) has recently been mapped to 9q13-q21 by tight linkage to D9S15 and D9S5 loci. The present lack of recombination between these loci precludes further genetic mapping and suggests that the distances involved are in the megabase range. We have established a 1-Mb map around loci D9S15 (defined by probe MCT112) and D9S5 (defined by probe DR47) and found that they are at most 260 apart. Six rare cutting site clusters were found in a 450-kb segment containing both loci. Three clusters were completely unmethylated in two cell lines tested and might correspond to CpG islands flanking transcribed sequences. Cosmid mapping of a 52-kb region around D9S5 and pulse-field gel electrophoresis analysis showed the presence of three other CpG clusters that were partially or completely methylated. Two of them were present in the cosmid clones available and were associated with sequences conserved in other vertebrate species. The CpG islands and conserved sequences presented here can be used to search for genes defective in Friedreich's ataxia.

Genetic mapping of meander tail, a mouse mutation affecting cerebellar development

The meander tail mouse harbors a recessive mutation on chromosome 4 that affects the anterior lobes of the cerebellum and the caudal vertebrae. Examination of the mea/mea cerebellum reveals that the complete disorganization of all cell types seen in the anterior lobes is separated by a sharp and consistent boundary from the normal cytoarchitecture of the posterior lobes. In the absence of any biochemical information regarding the affected gene product, attempts to clone the gene must rely on the strategy of reverse genetics. As an initial step in this process we have constructed a genetic linkage map spanning 68 cM of chromosome 4 using an intersubspecific phenotypic backcross. The loci included in this analysis are Calb, Ggtb, Lv, b, Ifa, mea, D4Rp1, Glut-1, Lck, Lmyc-1, and Eno-1. This analysis positions the mea phenotypic locus in the interval between Ifa and Glut1. These results also further define regions of homology between mouse chromosome 4 and human chromosomes 8, 1, and 9. This linkage map provides the means to evaluate candidate genes, and to identify tightly linked markers useful for cloning the meander tail locus.

Identification of CpG islands in a physical map encompassing the Friedreich's ataxia locus

The Friedreich's ataxia locus has been previously assigned to chromosome 9q 13-21.1 by the demonstration of tight linkage to two anonymous DNA markers. MCT112 (Z greater than 80, theta = 0) and DR47 (Z greater than 50, theta = 0). The absence of recombination between these three loci has prevented the resolution of gene/probe order in this region, impeding strategies for gene isolation. We report physical mapping over a 4-Mb genomic interval, linking the markers MCT112 and DR47 on a common 460-kb NotI fragment and identifying 11 CpG islands in the 1.7-Mb interval most likely to contain the Friedreich's ataxia locus. Four of these islands were detected only by analysis of three YAC clones spanning a 700-kb interval including the MCT112/DR47 cluster. Without clear evidence of the precise location of the disease locus from recombination events, each of these regions must be considered as specifying a potential "candidate" sequence for the mutated gene.

Regional localization by in situ hybridization of a human chromosome 9 marker tightly linked to the Friedreich's ataxia locus

In order to determine the regional localization of the Friedreich's ataxia (FA) gene on chromosome 9, the DNA probe DR47 (D9S5), which detects a restriction fragment length polymorphism (RFLP) in tight linkage with the disease, was hybridized in situ to metaphase chromosomes. Our results enable the D9S5 locus to be assigned to the 9q12-q13 region, thus indicating that this is also the localization of the FA gene.

Additional polymorphisms at marker loci D9S5 and D9S15 generate extended haplotypes in linkage disequilibrium with Friedreich ataxia

The gene for Friedreich ataxia (FA), a severe recessive neurodegenerative disease, has previously been shown to be tightly linked to the polymorphic markers D9S15 and D9S5 on human chromosome 9. In addition, the observation of linkage disequilibrium suggested that D9S15 is within 1 centimorgan (cM) of the disease locus, FRDA. Although D9S5 did not show recombination with FRDA, its localization was less precise (0-5 cM) due to its lower informativeness. We have now identified additional polymorphisms at both marker loci. Two cosmids spanning 50 kilobases around D9S5 were isolated, and a probe derived from one of them detects an informative three-allele polymorphism. We have found a highly polymorphic microsatellite sequence at D9S15 which is rapidly typed by the DNA polymerase chain reaction. The polymorphism information contents at the D9S5 and D9S15 loci have been increased from 0.14 to 0.60 and from 0.33 to 0.74, respectively. With the additional polymorphisms the lod (log10 odds ratio) score for the D9S15-FRDA linkage is now 48.10 at recombination fraction theta = 0.005 and for D9S5-FRDA, the lod score is 27.87 at theta = 0.00. We have identified a recombinant between D9S15 and FRDA. However, due to the family structure, it will be of limited usefulness for more precise localization of FRDA. The linkage disequilibrium previously observed between D9S15 and FRDA is strengthened by analysis of the haplotypes using the microsatellite polymorphism, while weaker but significant disequilibrium is found between D9S5 and FRDA. Extended haplotypes that encompass D9S5 and D9S15 show a strikingly different distribution between chromosomes that carry the FA mutation and normal chromosomes. This suggests that both marker loci are less than 1 cM from the FRDA gene and that a small number of mutations account for the majority of FA cases in the French population studied. D9S5 and D9S15 are thus excellent start points to isolate the disease gene.

Prenatal diagnosis of Friedreich ataxia

Friedreich ataxia is a progressive neurodegenerative disorder affecting the peripheral and central nervous systems. One in 50,000 of the population are affected by this recessively inherited disorder, with onset usually before puberty. The recent localization of the disease locus to chromosome 9 has made it possible to provide genetic counselling to families with at least one affected child. Tight linkage of the disease mutation to an anonymous DNA marker MCT112 (D9S15) has been shown with a pairwise lod score of 36.1 at 0 = 0. We report here the first prenatal diagnosis in Friedreich ataxia. Using MCT112 and the confidence interval approach, we have calculated risks for a fully informative family with one affected sib.

"Acadian" and "classical" forms of Friedreich ataxia are most probably caused by mutations at the same locus

"Acadian ataxia" is a form of Friedreich ataxia found in individuals of Acadian ancestry. It was described by Barbeau (in Sobue I (ed): Spinocerebellar Degeneration; Tokyo: Univ. Tokyo Press, pp 121-142, 1980) as having a slower course of degeneration and less severe secondary symptoms than "classical" Friedreich ataxia. He suggested that these 2 forms of the disease may be distinct. The mutation causing "classical" Friedreich ataxia has recently been mapped to chromosome 9 through genetic linkage studies, and here we show that the locus causing Friedreich ataxia in Acadian families from southwestern Louisiana is tightly linked to the same DNA marker, D9S15. Thus, these 2 disorders, which may be differentiated clinically, are most probably due to mutation(s) at the same locus on chromosome 9.