A combined nucleic acid and protein analysis in Friedreich ataxia: implications for diagnosis, pathogenesis and clinical trial design

New and Emerging Drug and Gene Therapies for Friedreich Ataxia

The life shortening nature of Friedreich Ataxia (FRDA) demands the search for therapies that can delay, stop or reverse its relentless trajectory. This review provides a contemporary position of drug and gene therapies for FRDA currently in phase 1 clinical trials and beyond. Despite significant scientific advances in the specificity of both compounds and targets developed and investigated, challenges remain for the advancement of treatments in a limited recruitment population. Currently therapies focus on reducing oxidative stress and improving mitochondrial function, modulating frataxin controlled metabolic pathways and gene replacement and editing. Approval of omaveloxolone, the first treatment for individuals with FRDA aged 16 years and over, has created much excitement for both those living with FRDA and those that care for them. The process of approval of omaveloxolone by the US Food and Drug Administration highlighted the importance of sensitive outcome measures and the significant role of data from natural history studies.

Anatomical and functional analysis of the corticospinal tract in an FRDA mouse model

Friedreich's ataxia (FRDA) is one of the most common hereditary ataxias. It is caused by a GAA repeat in the first intron of the FXN gene, which encodes an essential mitochondrial protein. Patients suffer from progressive motor dysfunction due to the degeneration of mechanoreceptive and proprioceptive neurons in dorsal root ganglia (DRG) and cerebellar dentate nucleus neurons, especially at early disease stages. Postmortem analyses of FRDA patients also indicate pathological changes in motor cortex including in the projection neurons that give rise to the cortical spinal tract (CST). Yet, it remains poorly understood how early in the disease cortical spinal neurons (CSNs) show these alterations, or whether CSN/CST pathology resembles the abnormalities observed in other tissues affected by FXN loss. To address these questions, we examined CSN driven motor behaviors and pathology in the YG8JR FRDA mouse model. We find that FRDA mice show impaired motor skills, exhibit significant reductions in CSN functional output, and, among other pathological changes, show abnormal mitochondrial distributions in CSN neurons and CST axonal tracts. Moreover, some of these alterations were observed as early as two months of age, suggesting that CSN/CST pathology may be an earlier event in FRDA disease than previously appreciated. These studies warrant a detailed mechanistic understanding of how FXN loss impacts CSN health and functionality.

Loss of filamentous actin, tight junction protein expression, and paracellular barrier integrity in frataxin-deficient human brain microvascular endothelial cells-implications for blood-brain barrier physiology in Friedreich's ataxia

Introduction: Friedreich's Ataxia (FRDA) is the most prevalent inherited ataxia. FRDA results from loss of Frataxin (FXN), an essential mitochondrial iron trafficking protein. FRDA starts with an early burst of neurodegeneration of the dorsal root ganglion and cerebellar dentate nuclei, followed by progressive brain iron accumulation in the latter. End stage disease includes cardiac fibrosis that contributes to hypertrophic cardiomyopathy. The microvasculature plays an essential barrier role in both brain and heart homeostasis, thus an investigation of this tissue system in FRDA is essential to the delineation of the cellular dysfunction in this genetic disorder. Previous reports have identified cytoskeletal alterations in non-barrier forming FRDA cell models, but physiological consequences are limited. Methods: We investigated brain microvascular endothelial cell integrity in FRDA in a model of the blood-brain barrier (BBB). We have knocked down FXN in immortalized human brain microvascular endothelial cells (hBMVEC), which compose the microcapillaries of the BBB, by using shRNA. We confirmed known cellular pathophysiologies of FXN-knockdown including decreased energy metabolism, markers of oxidative stress, and increased cell size. Results: We investigated cytoskeletal architecture, identifying decreased filamentous actin and Occludin and Claudin-5 tight junction protein expression in shFXN hBMVECs. This was consistent with decreased transendothelial electrical resistance (TEER) and increased paracellular tracer flux during early barrier formation. shFXN hBMVEC start with only 67% barrier integrity of the controls, and flux a paracellular tracer at 800% of physiological levels. Discussion: We identified that insufficient FXN levels in the hBMVEC BBB model causes changes in cytoskeletal architecture and tight junction protein abundance, co-incident with increased barrier permeability. Changes in the integrity of the BBB may be related to patient brain iron accumulation, neuroinflammation, neurodegeneration, and stroke. Furthermore, our findings implicate other barrier cells, e.g., the cardiac microvasculature, loci of disease pathology in FRDA.

The C-Terminal Cross-linked Telopeptide of Type I Collagen (CTX-I) as a Potential Cardiomyopathy Biomarker in Friedreich Ataxia Patients

Friedreich's ataxia (FRDA) is the most common inherited recessive ataxia. Cardiomyopathy (CM) with myocardial hypertrophy is the predominant cause of death. The presence of CM is variable and the risk factors for cardiac involvement are not entirely clear. Markers of collagen degradation, such as C-terminal cross-linked telopeptide of type I collagen (CTX-I), seem to be associated with unfavorable cardiovascular outcomes. The aim of our study was to measure serum CTX-I as a marker of cardiac fibrosis in FRDA patients. We measured serum CTX value in twenty-five FRDA patients (mean age, 31.3 ± 14.7 years) and nineteen healthy controls (mean age, 34.0 ± 13.5 years). Patients underwent echocardiography and SARA scale evaluation. CTX values were significantly higher in the patients than in the control group (31.82 ± 2.27 vs 16.44 ± 1.6 μg/L; p = 0.006). CTX-I was inversely correlated with age (R =  - 0,535; n = 44; p < 0.001). The regression model identified disease duration and TT3 levels to be independent predictors of CTX-I (model R2 = 0.938; intercept - 64.0, p = 0.071; disease duration coefficient =  - 2.34, p = 0.005; TT3 coefficient = 127.17, p = 0.011). CTX-I, a biomarkers of collagen turnover, is elevated in FRDA and should provide complementary information to identify patients with high cardiological risk even if longitudinal studies are needed to define the role of this serologic marker of collagen metabolism in the natural history of cardiomyopathy in FRDA patients.

Patient-derived iPSC models of Friedreich ataxia: a new frontier for understanding disease mechanisms and therapeutic application

Friedreich ataxia (FRDA) is a rare genetic multisystem disorder caused by a pathological GAA trinucleotide repeat expansion in the FXN gene. The numerous drawbacks of historical cellular and rodent models of FRDA have caused difficulty in performing effective mechanistic and translational studies to investigate the disease. The recent discovery and subsequent development of induced pluripotent stem cell (iPSC) technology provides an exciting platform to enable enhanced disease modelling for studies of rare genetic diseases. Utilising iPSCs, researchers have created phenotypically relevant and previously inaccessible cellular models of FRDA. These models enable studies of the molecular mechanisms underlying GAA-induced pathology, as well as providing an exciting tool for the screening and testing of novel disease-modifying therapies. This review explores how the use of iPSCs to study FRDA has developed over the past decade, as well as discussing the enormous therapeutic potentials of iPSC-derived models, their current limitations and their future direction within the field of FRDA research.

Rationale and protocol of a double-blind, randomized, placebo-controlled trial to test the efficacy, safety, and tolerability of dimethyl fumarate in Friedreich Ataxia (DMF-FA-201)

Introduction

Friedreich Ataxia (FRDA) is an autosomal recessive neurodegenerative disorder that causes gait and limb ataxia, dysarthria, and impaired vibratory sense, with cardiomyopathy being the predominant cause of death. There is no approved therapy, which results in the use of symptomatic treatments and the chronic support of physiotherapy. Dimethyl fumarate (DMF) is a fumaric acid ester used for the treatment of psoriasis and Multiple Sclerosis (MS). It induces Nrf2 in vitro and in vivo, and it increases frataxin in FRDA patient lymphoblasts, in mouse models, and in MS treated patients.

Methods

The aim of our study is to investigate if DMF can increase the expression of the FXN gene and frataxin protein and ameliorate in-vivo detectable measures of mitochondrial dysfunction in FRDA. The study is composed of a screening visit and two sequential 12-week phases: a core phase and an extension phase. During the first phase (core), patients will be randomly assigned to either the DMF or a placebo group in a 1:1 ratio. During the first week, patients will receive a total daily dose of 240 mg of DMF or placebo; from the second week of treatment, the dose will be increased to two 120 mg tablets BID for a total daily dose of 480 mg. During the second phase (extension), all patients will be treated with DMF. EudraCT number 2021-006274-23.

Endpoints

The primary endpoint will be a change in FXN gene expression level after 12 weeks of treatment. Secondary endpoints will be frataxin protein level, cardiopulmonary exercise test outputs, echocardiographic measures, Nrf2 pathway and mitochondrial biogenesis gene expression, safety, clinical scales, and quality of life scales.

Conclusions

This is the first study aimed at exploring the ability of DMF, an already available treatment for MS and psoriasis, to correct the biological deficits of FRDA and potentially improve mitochondrial respiration in-vivo.

Frataxin-deficient human brain microvascular endothelial cells lose polymerized actin and are paracellularly permeable -implications for blood-brain barrier integrity in Friedreich's Ataxia

Background

Friedreich's Ataxia (FRDA) is the most prevalent inherited ataxia; the disease results from loss of Frataxin, an essential mitochondrial iron trafficking protein. FRDA presents as neurodegeneration of the dorsal root ganglion and cerebellar dentate nuclei, followed by brain iron accumulation in the latter. End stage disease includes cardiac fibrosis that contributes to hypertrophic cardiomyopathy. The microvasculature plays an essential barrier role in both the brain and heart, thus an investigation of this tissue system in FRDA is essential to the delineation of the cellular dysfunction in this genetic disorder. Here, we investigate brain microvascular endothelial cell integrity in FRDA in a model of the blood-brain barrier (BBB).

Methods

We used lentiviral mediated shRNA delivery to generate a novel FRDA model in immortalized human brain microvascular endothelial cells (hBMVEC) that compose the microcapillaries of the BBB. We verified known cellular pathophysiologies of FXN knockdown including increased oxidative stress, loss of energy metabolism, and increased cell size. Furthermore, we investigated cytoskeletal architecture including the abundance and organization of filamentous actin, and barrier physiology via transendothelial electrical resistance and fluorescent tracer flux.

Results

shFXN hBMVEC display the known FRDA cell morbidity including increased oxidative stress, decreased energy metabolism, and an increase in cell size. We demonstrate that shFXN hBMVEC have less overall filamentous actin, and that filamentous actin is lost at the cell membrane and cortical actin ring. Consistent with loss of cytoskeletal structure and anchorage, we found decreased barrier strength and increased paracellular tracer flux in the shFXN hBMVEC transwell model.

Conclusion

We identified that insufficient FXN levels in the hBMVEC BBB model causes changes in cytoskeletal architecture and increased barrier permeability, cell pathologies that may be related to patient brain iron accumulation, neuroinflammation, neurodegeneration, and stroke. Our findings implicate other barrier cells, e.g., the cardiac microvasculature, likely contributory also to disease pathology in FRDA.

Frataxin deficiency alters gene expression in Friedreich ataxia derived IPSC-neurons and cardiomyocytes

BACKGROUND

Friedreich's ataxia (FRDA) is an autosomal recessive disease, whereby homozygous inheritance of an expanded GAA trinucleotide repeat expansion in the first intron of the FXN gene leads to transcriptional repression of the encoded protein frataxin. FRDA is a progressive neurodegenerative disorder, but the primary cause of death is heart disease which occurs in 60% of the patients. Several functions of frataxin have been proposed, but none of them fully explain why its deficiency causes the FRDA phenotypes nor why the most affected cell types are neurons and cardiomyocytes.

METHODS

To investigate, we generated iPSC-derived neurons (iNs) and cardiomyocytes (iCMs) from an FRDA patient and upregulated FXN expression via lentivirus without altering genomic GAA repeats at the FXN locus.

RESULTS

RNA-seq and differential gene expression enrichment analyses demonstrated that frataxin deficiency affected the expression of glycolytic pathway genes in neurons and extracellular matrix pathway genes in cardiomyocytes. Genes in these pathways were differentially expressed when compared to a control and restored to control levels when FRDA cells were supplemented with frataxin.

CONCLUSIONS

These results offer novel insight into specific roles of frataxin deficiency pathogenesis in neurons and cardiomyocytes.

A Drug Combination Rescues Frataxin-Dependent Neural and Cardiac Pathophysiology in FA Models

Friedreich’s ataxia (FA) is an inherited multisystemic neuro- and cardio-degenerative disorder. Seventy-four clinical trials are listed for FA (including past and present), but none are considered FDA/EMA-approved therapy. To date, FA therapeutic strategies have focused along two main lines using a single-drug approach: a) increasing frataxin and b) enhancing downstream pathways, including antioxidant levels and mitochondrial function. Our novel strategy employed a combinatorial approach to screen approved compounds to determine if a combination of molecules provided an additive or synergistic benefit to FA cells and/or animal models. Eight single drug molecules were administered to FA fibroblast patient cells: nicotinamide riboside, hemin, betamethasone, resveratrol, epicatechin, histone deacetylase inhibitor 109, methylene blue, and dimethyl fumarate. We measured their individual ability to induce FXN transcription and mitochondrial biogenesis in patient cells. Single-drug testing highlighted that dimethyl fumarate and resveratrol increased these two parameters. In addition, the simultaneous administration of these two drugs was the most effective in terms of FXN mRNA and mitobiogenesis increase. Interestingly, this combination also improved mitochondrial functions and reduced reactive oxygen species in neurons and cardiomyocytes. Behavioral tests in an FA mouse model treated with dimethyl fumarate and resveratrol demonstrated improved rotarod performance. Our data suggest that dimethyl fumarate is effective as a single agent, and the addition of resveratrol provides further benefit in some assays without showing toxicity. Therefore, they could be a valuable combination to counteract FA pathophysiology. Further studies will help fully understand the potential of a combined therapeutic strategy in FA pathophysiology.

Simultaneous Quantification of Mitochondrial Mature Frataxin and Extra-Mitochondrial Frataxin Isoform E in Friedreich’s Ataxia Blood

Friedreich’s ataxia (FRDA) is an autosomal recessive disease caused by an intronic guanine-adenine-adenine (GAA) triplet expansion in the frataxin (FXN) gene, which leads to reduced expression of full-length frataxin (1–210) also known as isoform 1. Full-length frataxin has a mitochondrial targeting sequence, which facilitates its translocation into mitochondria where it is processed through cleavage at G⁴¹-L⁴² and K⁸⁰-S⁸¹ by mitochondrial processing (MPP) to release mitochondrial mature frataxin (81–210). Alternative splicing of FXN also leads to expression of N-terminally acetylated extra-mitochondrial frataxin (76–210) named isoform E because it was discovered in erythrocytes. Frataxin isoforms are undetectable in serum or plasma, and originally whole blood could not be used as a biomarker in brief therapeutic trials because it is present in erythrocytes, which have a half-life of 115-days and so frataxin levels would remain unaltered. Therefore, an assay was developed for analyzing frataxin in platelets, which have a half-life of only 10-days. However, our discovery that isoform E is only present in erythrocytes, whereas, mature frataxin is present primarily in short-lived peripheral blood mononuclear cells (PBMCs), granulocytes, and platelets, meant that both proteins could be quantified in whole blood samples. We now report a quantitative assay for frataxin proteoforms in whole blood from healthy controls and FRDA patients. The assay is based on stable isotope dilution coupled with immunoprecipitation (IP) and two-dimensional-nano-ultrahigh performance liquid chromatography/parallel reaction monitoring/high resolution mass spectrometry (2D-nano-UHPLC-PRM/HRMS). The lower limit of quantification was 0.5 ng/mL for each proteoform and the assays had 100% sensitivity and specificity for discriminating between healthy controls (n = 11) and FRDA cases (N = 100 in year-1, N = 22 in year-2,3). The mean levels of mature frataxin in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 7.5 ± 1.5 ng/mL and 2.1 ± 1.2 ng/mL, respectively. The mean levels of isoform E in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 26.8 ± 4.1 ng/mL and 4.7 ± 3.3 ng/mL, respectively. The mean levels of total frataxin in whole blood from healthy controls and homozygous FRDA patients were significantly different (p < 0.0001) at 34.2 ± 4.3 ng/mL and 6.8 ± 4.0 ng/mL, respectively. The assay will make it possible to rigorously monitor the natural history of the disease and explore the potential role of isoform E in etiology of the disease. It will also facilitate the assessment of therapeutic interventions (including gene therapy approaches) that attempt to increase frataxin protein expression as a treatment for this devastating disease.

DNA methylation in Friedreich ataxia silences expression of frataxin isoform E

Epigenetic silencing in Friedreich ataxia (FRDA), induced by an expanded GAA triplet-repeat in intron 1 of the FXN gene, results in deficiency of the mitochondrial protein, frataxin. A lesser known extramitochondrial isoform of frataxin detected in erythrocytes, frataxin-E, is encoded via an alternate transcript (FXN-E) originating in intron 1 that lacks a mitochondrial targeting sequence. We show that FXN-E is deficient in FRDA, including in patient-derived cell lines, iPS-derived proprioceptive neurons, and tissues from a humanized mouse model. In a series of FRDA patients, deficiency of frataxin-E protein correlated with the length of the expanded GAA triplet-repeat, and with repeat-induced DNA hypermethylation that occurs in close proximity to the intronic origin of FXN-E. CRISPR-induced epimodification to mimic DNA hypermethylation seen in FRDA reproduced FXN-E transcriptional deficiency. Deficiency of frataxin E is a consequence of FRDA-specific epigenetic silencing, and therapeutic strategies may need to address this deficiency.

Friedreich's Ataxia Frequency in a Large Cohort of Genetically Undetermined Ataxia Patients

Background: Patients with suspected genetic ataxia are often tested for Friedreich's ataxia (FRDA) and/or a variety of spinocerebellar ataxias (SCAs). FRDA can present with atypical, late-onset forms and so may be missed in the diagnostic process. We aimed to determine FRDA-positive subjects among two cohorts of patients referred to a specialist ataxia centre either for FRDA or SCA testing to determine the proportion of FRDA cases missed in the diagnostic screening process. Methods: 2000 SCA-negative ataxia patients, not previously referred for FRDA testing (group A), were tested for FRDA expansions and mutations. This group was compared with 1768 ataxia patients who had been previously referred for FRDA testing (group B) and were therefore more likely to have a typical presentation. The phenotypes of positive cases were assessed through review of the clinical case notes. Results: Three patients (0.2%) in group A had the FRDA expansion on both alleles, compared with 207 patients (11.7%) in group B. The heterozygous carrier rate across both cohorts was of 41 out of 3,768 cases (1.1%). The size of the expansions in the three FRDA-positive cases in group A was small, and their presentation atypical with late-onset. Conclusions: This study demonstrates that FRDA is very rare among patients who were referred purely for SCA testing without the clinical suspicion of FRDA. Such cases should be referred to specialist ataxia centres for more extensive testing to improve patient management and outcomes.

Age of onset modulates resting-state brain network dynamics in Friedreich Ataxia

This magnetoencephalography (MEG) study addresses (i) how Friedreich ataxia (FRDA) affects the sub‐second dynamics of resting‐state brain networks, (ii) the main determinants of their dynamic alterations, and (iii) how these alterations are linked with FRDA‐related changes in resting‐state functional brain connectivity (rsFC) over long timescales. For that purpose, 5 min of resting‐state MEG activity were recorded in 16 FRDA patients (mean age: 27 years, range: 12–51 years; 10 females) and matched healthy subjects. Transient brain network dynamics was assessed using hidden Markov modeling (HMM). Post hoc median‐split, nonparametric permutations and Spearman rank correlations were used for statistics. In FRDA patients, a positive correlation was found between the age of symptoms onset (ASO) and the temporal dynamics of two HMM states involving the posterior default mode network (DMN) and the temporo‐parietal junctions (TPJ). FRDA patients with an ASO <11 years presented altered temporal dynamics of those two HMM states compared with FRDA patients with an ASO > 11 years or healthy subjects. The temporal dynamics of the DMN state also correlated with minute‐long DMN rsFC. This study demonstrates that ASO is the main determinant of alterations in the sub‐second dynamics of posterior associative neocortices in FRDA patients and substantiates a direct link between sub‐second network activity and functional brain integration over long timescales.

Safety and feasibility of upper limb cardiopulmonary exercise test in Friedreich ataxia

Aims

To explore the feasibility of upper limbs cardiopulmonary exercise test (CPET) in Friedreich ataxia (FRDA) patients and to compare the results with sex, age, and body mass index (BMI) matched cohort of healthy controls (HC).

Methods and results

Cardiopulmonary exercise test was performed using an upper limbs cycle ergometer on fasting subjects. Peak oxygen uptake (peak VO2) was recorded as the mean value of VO2 during a 20 s period at the maximal effort of the test at an appropriate respiratory exchange rate. The ventilatory anaerobic threshold (AT) was detected by the use of the V-slope method. We performed echocardiography with an ultrasound system equipped with a 2.5 MHz multifrequency transducer for complete M-mode, two-dimensional, Doppler, and Tissue Doppler Imaging analyses. We studied 55 FRDA and 54 healthy matched controls (HC). Peak VO2 showed a significant 31% reduction in FRDA patients compared to HC (15.2 ± 5.7 vs. 22.0 ± 6.1 mL/kg/min; P &lt; 0.001). Peak workload was reduced by 41% in FRDA (42.9 ± 12.5 vs. 73.1 ± 21.2 W; P &lt; 0.001). In FRDA patients, peak VO2 is inversely correlated with the Scale for Assessment and Rating of Ataxia score, disease duration, and 9HPT performance, and directly correlated with activities of daily living. The AT occurred at 48% of peak workload time in FRDA patients and at 85% in HC (P &lt; 0.001).

Conclusions

Upper limb CPET is useful in the assessment of exercise tolerance and a possible tool to determine the functional severity of the mitochondrial oxidative defect in patients with FRDA. The cardiopulmonary exercise test is an ideal functional endpoint for Phases II and III trials through a simple, non-invasive, and safe exercise test.

Compound heterozygosity for an expanded (GAA) and a (GAAGGA) repeat at FXN locus: from a diagnostic pitfall to potential clues to the pathogenesis of Friedreich ataxia

Friedreich's ataxia (FRDA) is usually due to a homozygous GAA expansion in intron 1 of the frataxin (FXN) gene. Rarely, uncommon molecular rearrangements at the FXN locus can cause pitfalls in the molecular diagnosis of FRDA. Here we describe a family whose proband was affected by late-onset Friedreich's ataxia (LOFA); long-range PCR (LR-PCR) documented two small expanded GAA alleles both in the proband and in her unaffected younger sister, who therefore received a diagnosis of pre-symptomatic LOFA. Later studies, however, revealed that the proband's unaffected sister, as well as their healthy mother, were both carriers of an expanded GAA allele and an uncommon (GAAGGA)66-67 repeat mimicking a GAA expansion at the LR-PCR that was the cause of the wrong initial diagnosis of pre-symptomatic LOFA. Extensive studies in tissues from all the family members, including LR-PCR, assessment of methylation status of FXN locus, MboII restriction analysis and direct sequencing of LR-PCR products, analysis of FXN mRNA, and frataxin protein expression, support the virtual lack of pathogenicity of the rare (GAAGGA)66-67 repeat, also providing significant data about the modulation of epigenetic modifications at the FXN locus. Overall, this report highlights a rare but possible pitfall in FRDA molecular diagnosis, emphasizing the need of further analysis in case of discrepancy between clinical and molecular data.

Toward quantitative neuroimaging biomarkers for Friedreich's ataxia at 7 Tesla: Susceptibility mapping, diffusion imaging, R2 and R1 relaxometry

Friedreich's ataxia (FRDA) is a rare genetic disorder leading to degenerative processes. So far, no effective treatment has been found. Therefore, it is important to assist the development of medication with imaging biomarkers reflecting disease status and progress. Ten FRDA patients (mean age 37 ± 14 years; four female) and 10 age- and sex-matched controls were included. Acquisition of magnetic resonance imaging (MRI) data for quantitative susceptibility mapping, R₁ , R₂ relaxometry and diffusion imaging was performed at 7 Tesla. Results of volume of interest (VOI)-based analyses of the quantitative data were compared with a voxel-based morphometry (VBM) evaluation. Differences between patients and controls were assessed using the analysis of covariance (ANCOVA; p < 0.01) with age and sex as covariates, effect size of group differences, and correlations with disease characteristics with Spearman correlation coefficient. For the VBM analysis, a statistical threshold of 0.001 for uncorrected and 0.05 for corrected p-values was used. Statistically significant differences between FRDA patients and controls were found in five out of twelve investigated structures, and statistically significant correlations with disease characteristics were revealed. Moreover, VBM revealed significant white matter atrophy within regions of the brainstem, and the cerebellum. These regions overlapped partially with brain regions for which significant differences between healthy controls and patients were found in the VOI-based quantitative MRI evaluation. It was shown that two independent analyses provided overlapping results. Moreover, positive results on correlations with disease characteristics were found, indicating that these quantitative MRI parameters could provide more detailed information and assist the search for effective treatments.

SINEUP non-coding RNAs rescue defective frataxin expression and activity in a cellular model of Friedreich's Ataxia

Friedreich's ataxia (FRDA) is an untreatable disorder with neuro- and cardio-degenerative progression. This monogenic disease is caused by the hyper-expansion of naturally occurring GAA repeats in the first intron of the FXN gene, encoding for frataxin, a protein implicated in the biogenesis of iron-sulfur clusters. As the genetic defect interferes with FXN transcription, FRDA patients express a normal frataxin protein but at insufficient levels. Thus, current therapeutic strategies are mostly aimed to restore physiological FXN expression. We have previously described SINEUPs, natural and synthetic antisense long non-coding RNAs, which promote translation of partially overlapping mRNAs through the activity of an embedded SINEB2 domain. Here, by in vitro screening, we have identified a number of SINEUPs targeting human FXN mRNA and capable to up-regulate frataxin protein to physiological amounts acting at the post-transcriptional level. Furthermore, FXN-specific SINEUPs promote the recovery of disease-associated mitochondrial aconitase defects in FRDA-derived cells. In summary, we provide evidence that SINEUPs may be the first gene-specific therapeutic approach to activate FXN translation in FRDA and, more broadly, a novel scalable platform to develop new RNA-based therapies for haploinsufficient diseases.

Left ventricular structural and functional changes in Friedreich ataxia - Relationship with body size, sex, age and genetic severity

Introduction

Although a concentric pattern of left ventricular (LV) geometry appears to be common in Friedreich ataxia (FRDA), there is no accepted method for diagnosing LV abnormalities in FRDA, sex and body size have often not been taken into consideration, and it has not been clear whether children and adults should be classified using the same criteria. The aim of this study was to better define the LV geometric changes in FRDA with respect to sex, body size and subject age, and to investigate the relationship of LV changes with genetic severity, as assessed by GAA repeat length within the shorter allele of the FXN gene (GAA1).

Methods

Echocardiography was performed in 216 subjects (68 children, 148 adults), measurements were made at end-diastole of LV internal diameter (LVEDID), septal wall thickness (SWT), LV length (LVEDL) and LV volume (LVEDV), and calculations were made of relative wall thickness (RWT), LV mass and LV ejection fraction (LVEF).

Results

The most common LV abnormalities in both adults and children with FRDA were increases in RWT and age-normalized RWT. In adults with a normal LVEF, all LV variables other than RWT were larger in males independent of body surface area (BSA), and all LV variables other than SWT and RWT were positively correlated with BSA. After adjustment for sex and BSA, GAA1 was a positive correlate of SWT and RWT (but not of LV mass), and was an inverse correlate of LVEDID, LVEDL and LVEDV. In children with a normal LVEF, SWT, LV mass and LVEDL were larger in males than females after adjusting for BSA, and in combination with sex, BSA was a positive correlate of all the LV variables except SWT and RWT. In children there were no correlations of GAA1 with any of the LV variables.

Conclusion

In FRDA, increases in RWT and age-normalized RWT are the most frequent LV structural abnormalities, sex and body size are important determinants of most other LV structural variables in both children and adults, and increased genetic severity is associated with a smaller left ventricle and increased LV wall thickness in adults, but not associated with LV size or wall thickness in children.

Friedreich ataxia: FXN gene expression and its relationship with DNA methylation pattern

Friedreich ataxia (FRDA) is the most common autosomal recessive ataxia associated with the non-coding GAA tandem repeats expansion in the FXN gene. Transcription impairment and frataxin protein deficiency are the key features of the disease pathogenesis. Our research was aimed to study the FXN gene mRNA expression as well as to carry out the clinical, genetic and epigenetic correlation analysis in a group of patients with homozygous expansion, in a group of their relatives with heterozygous expansion and in a control group. The FXN mRNA level was determined using the real-time polymerase chain reaction. Methylation pattern of CpG sites was evaluated by direct bisulfite sequencing. As a result of the study, the threshold values were obtained between the FRDA patients group, the group of heterozygous carriers and the control group (15 and 79%, respectively). The clinical and genetic features comparison with the FXN expression level revealed no significant correlation. When comparing gene expression with an epigenetic profile, it was found that hypermethylation of a number of CpG sites upstream of the trinucleotide repeats and some non-CpG sites downstream of the region of repeats inhibited expression. Thus, the identified methylated sites may be considered as a target for epigenome editing to increase the FXN transcription and, consequently, for target therapy of the disease.

Mechanism of activation of the human cysteine desulfurase complex by frataxin

The function of frataxin (FXN) has garnered great scientific interest since its depletion was linked to the incurable neurodegenerative disease Friedreich's ataxia (FRDA). FXN has been shown to be necessary for iron-sulfur (Fe-S) cluster biosynthesis and proper mitochondrial function. The structural and functional core of the Fe-S cluster assembly complex is a low-activity pyridoxal 5'-phosphate (PLP)-dependent cysteine desulfurase enzyme that consists of catalytic (NFS1), LYRM protein (ISD11), and acyl carrier protein (ACP) subunits. Although previous studies show that FXN stimulates the activity of this assembly complex, the mechanism of FXN activation is poorly understood. Here, we develop a radiolabeling assay and use stopped-flow kinetics to establish that FXN is functionally linked to the mobile S-transfer loop cysteine of NFS1. Our results support key roles for this essential cysteine residue in substrate binding, as a general acid to advance the Cys-quinonoid PLP intermediate, as a nucleophile to form an NFS1 persulfide, and as a sulfur delivery agent to generate a persulfide species on the Fe-S scaffold protein ISCU2. FXN specifically accelerates each of these individual steps in the mechanism. Our resulting architectural switch model explains why the human Fe-S assembly system has low inherent activity and requires activation, the connection between the functional mobile S-transfer loop cysteine and FXN binding, and why the prokaryotic system does not require a similar FXN-based activation. Together, these results provide mechanistic insights into the allosteric-activator role of FXN and suggest new strategies to replace FXN function in the treatment of FRDA.

Dimethyl fumarate dosing in humans increases frataxin expression: A potential therapy for Friedreich's Ataxia

Friedreich's Ataxia (FA) is an inherited neurodegenerative disorder resulting from decreased expression of the mitochondrial protein frataxin, for which there is no approved therapy. High throughput screening of clinically used drugs identified Dimethyl fumarate (DMF) as protective in FA patient cells. Here we demonstrate that DMF significantly increases frataxin gene (FXN) expression in FA cell model, FA mouse model and in DMF treated humans. DMF also rescues mitochondrial biogenesis deficiency in FA-patient derived cell model. We further examined the mechanism of DMF's frataxin induction in FA patient cells. It has been shown that transcription-inhibitory R-loops form at GAA expansion mutations, thus decreasing FXN expression. In FA patient cells, we demonstrate that DMF significantly increases transcription initiation. As a potential consequence, we observe significant reduction in both R-loop formation and transcriptional pausing thereby significantly increasing FXN expression. Lastly, DMF dosed Multiple Sclerosis (MS) patients showed significant increase in FXN expression by ~85%. Since inherited deficiency in FXN is the primary cause of FA, and DMF is demonstrated to increase FXN expression in humans, DMF could be considered for Friedreich's therapy.

Progress in understanding Friedreich's ataxia using human induced pluripotent stem cells

Introduction

Friedreich's ataxia (FRDA) is an autosomal recessive multisystem disease mainly affecting the peripheral and central nervous systems, and heart. FRDA is caused by a GAA repeat expansion in the first intron of the frataxin (FXN) gene, that leads to reduced expression of FXN mRNA and frataxin protein. Neuronal and cardiac cells are primary targets of frataxin deficiency and generating models via differentiation of induced pluripotent stem cells (iPSCs) into these cell types is essential for progress towards developing therapies for FRDA.

Areas covered

This review is focused on modeling FRDA using human iPSCs and various iPSC-differentiated cell types. We emphasized the importance of patient and corrected isogenic cell line pairs to minimize effects caused by biological variability between individuals.

Expert opinion

The versatility of iPSC-derived cellular models of FRDA is advantageous for developing new therapeutic strategies, and rigorous testing in such models will be critical for approval of the first treatment for FRDA. Creating a well-characterized and diverse set of iPSC lines, including appropriate isogenic controls, will facilitate achieving this goal. Also, improvement of differentiation protocols, especially towards proprioceptive sensory neurons and organoid generation, is necessary to utilize the full potential of iPSC technology in the drug discovery process.

Friedreich Ataxia: current status and future prospects

Friedreich ataxia (FA) represents the most frequent type of inherited ataxia. Most patients carry homozygous GAA expansions in the first intron of the frataxin gene on chromosome 9. Due to epigenetic alterations, frataxin expression is significantly reduced. Frataxin is a mitochondrial protein. Its deficiency leads to mitochondrial iron overload, defective energy supply and generation of reactive oxygen species. This review gives an overview over clinical and genetic aspects of FA and discusses current concepts of frataxin biogenesis and function as well as new therapeutic strategies.

Gene Expression Profile in Peripheral Blood Cells of Friedreich Ataxia Patients

Friedreich ataxia (FRDA) is the most common autosomal recessive ataxia characterized by a combination of neurological involvement, cardiomyopathy, and skeletal and glucose metabolism disturbances. FRDA is caused by mutations in FXN gene that results in reduction of mRNA and protein levels of frataxin. Previous microarray and real-time quantitative PCR (qPCR) studies showed that the downregulation of FXN is associated with a complex gene expression profile. However, these studies showed a wide variability in the subset of genes with altered expression among tissues and models. Genes differentially expressed in peripheral blood cells (PBC) could potentially help in the understanding of FRDA pathophysiology and also function as reliable disease biomarkers obtained from an easily accessible tissue, which could have implications in clinical practice. This study aimed to validate by qPCR the expression of 26 genes, revealed as differentially expressed by other studies, using peripheral blood cells (PBC) of 11 FRDA patients compared to 11 healthy controls. We found a robust downregulation of FXN, but no statistically significant differences were found between FRDA and controls for the remaining genes. Except for FXN, our study did not find a differential gene expression profile in PBC of FRDA patients and a reliable gene expression profile biomarker in a clinical relevant and noninvasive tissue remains unclear.

Expanded GAA repeats impede transcription elongation through the FXN gene and induce transcriptional silencing that is restricted to the FXN locus

Friedreich's ataxia (FRDA) is a severe neurodegenerative disease caused by homozygous expansion of the guanine-adenine-adenine (GAA) repeats in intron 1 of the FXN gene leading to transcriptional repression of frataxin expression. Post-translational histone modifications that typify heterochromatin are enriched in the vicinity of the repeats, whereas active chromatin marks in this region are underrepresented in FRDA samples. Yet, the immediate effect of the expanded repeats on transcription progression through FXN and their long-range effect on the surrounding genomic context are two critical questions that remain unanswered in the molecular pathogenesis of FRDA. To address these questions, we conducted next-generation RNA sequencing of a large cohort of FRDA and control primary fibroblasts. This comprehensive analysis revealed that the GAA-induced silencing effect does not influence expression of neighboring genes upstream or downstream of FXN. Furthermore, no long-range silencing effects were detected across a large portion of chromosome 9. Additionally, results of chromatin immunoprecipitation studies confirmed that histone modifications associated with repressed transcription are confined to the FXN locus. Finally, deep sequencing of FXN pre-mRNA molecules revealed a pronounced defect in the transcription elongation rate in FRDA cells when compared with controls. These results indicate that approaches aimed to reactivate frataxin expression should simultaneously address deficits in transcription initiation and elongation at the FXN locus.

Analyzing the Effects of a G137V Mutation in the FXN Gene

Reduced levels of frataxin, an essential mitochondrial protein involved in the regulation of iron-sulfur cluster biogenesis, are responsible for the recessive neurodegenerative Friedreich Ataxia (FRDA). Expansion of a GAA triplet in the first intron of the FRDA is essential for disease development which causes partial silencing of frataxin. In the vast majority of cases, patients are homozygotes for the expansion, but a small number of FRDA patients are heterozygotes for expansion and point mutations in the frataxin coding frame. In this study, we analyze the effects of a point mutation G137V. The patient P94–2, with a history of alcohol and drug abuse, showed a FRDA onset at the border between the classic and late onset phenotype. We applied a combination of biophysical and biochemical methods to characterize its effects on the structure, folding and activity of frataxin. Our study reveals no impairment of the structure or activity of the protein but a reduced folding stability. We suggest that the mutation causes misfolding of the native chain with consequent reduction of the protein concentration in the patient and discuss the possible mechanism of disease.

Friedreich ataxia in Norway - an epidemiological, molecular and clinical study

Background

Friedreich ataxia is an autosomal recessive hereditary spinocerebellar disorder, characterized by progressive limb and gait ataxia due to proprioceptive loss, often complicated by cardiomyopathy, diabetes and skeletal deformities. Friedreich ataxia is the most common hereditary ataxia, with a reported prevalence of 1:20 000 – 1:50 000 in Central Europe. Previous reports from south Norway have found a prevalence varying from 1:100 000 – 1:1 350 000; no studies are previously done in the rest of the country.

Methods

In this cross-sectional study, Friedreich ataxia patients were identified through colleagues in neurological, pediatric and genetic departments, hospital archives searches, patients’ associations, and National Centre for Rare Disorders. All included patients, carriers and controls were investigated clinically and molecularly with genotype characterization including size determination of GAA repeat expansions and frataxin measurements. 1376 healthy blood donors were tested for GAA repeat expansion for carrier frequency analysis.

Results

Twenty-nine Friedreich ataxia patients were identified in Norway, of which 23 were ethnic Norwegian, corresponding to a prevalence of 1:176 000 and 1:191 000, respectively. The highest prevalence was seen in the north. Carrier frequency of 1:196 (95 % CI = [1:752–1:112]) was found. Homozygous GAA repeat expansions in the FXN gene were found in 27/29, while two patients were compound heterozygous with c.467 T < C, L157P and the deletion (g.120032_122808del) including exon 5a. Two additional patients were heterozygous for GAA repeat expansions only. Significant differences in the level of frataxin were found between the included patients (N = 27), carriers (N = 37) and controls (N = 27).

Conclusions

In this first thorough study of a complete national cohort of Friedreich ataxia patients, and first nation-wide study of Friedreich ataxia in Norway, the prevalence of Friedreich ataxia in Norway is lower than in Central Europe, but higher than in the last Norwegian report, and as expected from migration studies. A south–north prevalence gradient is present. Based on Hardy Weinberg’s equilibrium, the carrier frequency of 1:196 is consistent with the observed prevalence. All genotypes, and typical and atypical phenotypes were present in the Norwegian population. The patients were phenotypically similar to European cohorts. Frataxin was useful in the diagnostic work-up of heterozygous symptomatic cases.

Electronic supplementary material

The online version of this article (doi:10.1186/s13023-015-0328-4) contains supplementary material, which is available to authorized users.

Frataxin levels in peripheral tissue in Friedreich ataxia

Objective

Friedreich ataxia (FRDA) is an autosomal recessive ataxia resulting from mutations in the frataxin gene (FXN). Such mutations, usually expanded guanine–adenine–adenine (GAA) repeats, give rise to decreased levels of frataxin protein in both affected and unaffected tissues. The goal was to understand the relationship of frataxin levels in peripheral tissues to disease status.

Methods

Frataxin levels were measured in buccal cells and blood, and analyzed in relation to disease features. Site-directed mutant frataxin was also transfected into human embryonic kidney cells to model results from specific point mutations.

Results

There was no evidence for change in frataxin levels over time with repeated measures analysis, although linear regression analysis of cross-sectional data predicted a small increase over decades. GAA repeat length predicted frataxin levels in both tissues, and frataxin levels themselves predicted neurological ratings (accounting for age). Compound heterozygous patients for a GAA expansion and a point mutation in FXN generally had lower levels of frataxin than those homozygous for the presence of two GAA repeat expansions, though levels varied dramatically between tissues in some compound heterozygotes for point mutations. The G130V mutation led to decreased levels of frataxin in vitro as well as in vivo, while the R165C mutation produced normal immunoreactive levels of frataxin both in vitro and in vivo. Start codon mutations led to low levels of frataxin in buccal cells but preserved immunoreactive frataxin levels in blood.

Interpretation

The present data show that peripheral frataxin levels reflect disease features in FRDA, but emphasize the need for interpretation of such levels in the context of specific mutations.

Epigenetic and neurological effects and safety of high-dose nicotinamide in patients with Friedreich's ataxia: an exploratory, open-label, dose-escalation study

BACKGROUND

Friedreich's ataxia is a progressive degenerative disorder caused by deficiency of the frataxin protein. Expanded GAA repeats within intron 1 of the frataxin (FXN) gene lead to its heterochromatinisation and transcriptional silencing. Preclinical studies have shown that the histone deacetylase inhibitor nicotinamide (vitamin B3) can remodel the pathological heterochromatin and upregulate expression of FXN. We aimed to assess the epigenetic and neurological effects and safety of high-dose nicotinamide in patients with Friedreich's ataxia.

METHODS

In this exploratory, open-label, dose-escalation study in the UK, male and female patients (aged 18 years or older) with Friedreich's ataxia were given single doses (phase 1) and repeated daily doses of 2-8 g oral nicotinamide for 5 days (phase 2) and 8 weeks (phase 3). Doses were gradually escalated during phases 1 and 2, with individual maximum tolerated doses used in phase 3. The primary outcome was the upregulation of frataxin expression. We also assessed the safety and tolerability of nicotinamide, used chromatin immunoprecipitation to investigate changes in chromatin structure at the FXN gene locus, and assessed the effect of nicotinamide treatment on clinical scales for ataxia. This study is registered with ClinicalTrials.gov, number NCT01589809.

FINDINGS

Nicotinamide was generally well tolerated; the main adverse event was nausea, which in most cases was mild, dose-related, and resolved spontaneously or after dose reduction, use of antinausea drugs, or both. Phase 1 showed a dose-response relation for proportional change in frataxin protein concentration from baseline to 8 h post-dose, which increased with increasing dose (p=0·0004). Bayesian analysis predicted that 3·8 g would result in a 1·5-times increase and 7·5 g in a doubling of frataxin protein concentration. Phases 2 and 3 showed that daily dosing at 3·5-6 g resulted in a sustained and significant (p<0·0001) upregulation of frataxin expression, which was accompanied by a reduction in heterochromatin modifications at the FXN locus. Clinical measures showed no significant changes.

INTERPRETATION

Nicotinamide was associated with a sustained improvement in frataxin concentrations towards those seen in asymptomatic carriers during 8 weeks of daily dosing. Further investigation of the long-term clinical benefits of nicotinamide and its ability to ameliorate frataxin deficiency in Friedreich's ataxia is warranted.

FUNDING

Ataxia UK, Ataxia Ireland, Association Suisse de l'Ataxie de Friedreich, Associazione Italiana per le Sindromi Atassiche, UK National Institute for Health Research, European Friedreich's Ataxia Consortium for Translational Studies, and Imperial Biomedical Research Centre.

FXN GAA repeat expansions in amyotrophic lateral sclerosis

Homozygous trinucleotide expansions in the frataxin (FXN) gene are responsible for Friedreich's ataxia. However, heterozygous trinucleotide expansion in FXN results in a decreased expression of frataxin, a component of the mitochondrial respiratory chain, and is associated with a subclinical metabolic phenotype. In this study we thus investigated whether heterozygous FXN trinucleotide expansion is a risk factor or modifier for amyotrophic lateral sclerosis (ALS). Genomic DNA from familial and sporadic ALS patients and control individuals was tested for extended FXN trinucleotide repeats by polymerase chain reaction analysis. Screening of 652 ALS patients and 238 controls revealed a lower overall frequency of heterozygously extended FXN repeats than expected. A significant difference in the frequency of the FXN expansion or an associated modification of the disease phenotype in ALS was not detected. Our findings strengthen the view that different DNA repeat expansions are toxic on the basis of specific biological mechanisms.

Atypical Friedreich ataxia in patients with FXN p.R165P point mutation or comorbid hemochromatosis

BACKGROUND

Compound heterozygosity for a trinucleotide repeat expansion and a point mutation in the FXN gene is a rare cause of Friedreich ataxia (FRDA).

METHODS

We identified three Swedish FRDA patients with an FXN p.R165P missense mutation and compared their clinical features with six homozygote trinucleotide repeat expansion carriers. Patients were assessed clinically. Trinucleotide expansion length was determined and lymphocyte frataxin levels measured.

RESULTS

p.R165P mutation carriers became wheelchair bound early, but had retained reflexes, better arm function, milder dysarthria, and were more independent in activities of daily living. One p.R165P mutation carrier developed psychosis. Frataxin levels were higher than in homozygous trinucleotide expansion patients. One patient with homozygous trinucleotide repeat expansions and comorbid hemochromatosis had more severe FRDA symptoms than his sibling without hemochromatosis.

CONCLUSION

p.R165P patients progress to a less disabling disease state than typical FRDA. Comorbid hemochromatosis may worsen FRDA symptoms through additive effects on iron metabolism.

Safety and tolerability of carbamylated erythropoietin in Friedreich's ataxia

BACKGROUND

Erythropoietin (EPO) derivatives have been found to increase frataxin levels in Friedreich's ataxia (FRDA) in vitro. This multicenter, double-blind, placebo-controlled, phase II clinical trial aimed to evaluate the safety and tolerability of Lu AA24493 (carbamylated EPO; CEPO).

METHODS

Thirty-six ambulatory FRDA patients harboring >400 GAA repeats were 2:1 randomly assigned to either CEPO in a fixed dose (325 µg thrice-weekly) or placebo. Safety and tolerability were assessed up to 103 days after baseline. Secondary outcome measures of efficacy (exploration of biomarkers and ataxia ratings) were performed up to 43 days after baseline.

RESULTS

All patients received six doses of study medication. Adverse events were equally distributed between CEPO and placebo. There was no evidence for immunogenicity of CEPO after multiple dosing. Biomarkers, such as frataxin, or measures for oxidative stress and ataxia ratings did not differ between CEPO and placebo.

CONCLUSION

CEPO was safe and well tolerated in a 2-week treatment phase. Secondary outcome measures remained without apparent difference between CEPO and placebo.

Molecular and clinical investigation of Iranian patients with Friedreich ataxia

BACKGROUND

Friedreich ataxia (FRDA) is an autosomal recessive disorder caused by guanine-adenine-adenine (GAA) triplet expansions in the FXN gene. Its product, frataxin, which severely reduces in FRDA patients, leads to oxidative damage in mitochondria. The purpose of this study was to evaluate the triple nucleotide repeated expansions in Iranian FRDA patients and to elucidate distinguishable FRDA clinical differences in these patients.

METHODS

A number of 22 Iranian patients (8 females and 14 males) from 16 unrelated families were studied. DNA was extracted from the peripheral blood of patients. The frequency and length of (GAA)n repeats in intron 1 of the FXN gene were analyzed using long-range PCR. In this study, the clinical criteria of FRDA in our patients and the variability in their clinical signs were also demonstrated.

RESULTS

An inverse relationship was observed between GAA repeat size and the age of onset. Although some distinguishable clinical features (such as limb ataxia and lower limb areflexia) were found in our patients, 90-95% of them had extensor plantar response and dysarthria. The results showed only one positive diabetes patient and also different effects on eye movement abnormality among our patients.

CONCLUSION

The onset age of symptoms showed a significant inverse correlation with allele size in our patients (P>0.05). Based on comparisons of the clinical data of all patients, clinical presentation of FRDA in Iranian patients did not differ significantly from other FRDA patients previously reported.

Clinical neurogenetics: friedreich ataxia

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

Molecular Diagnosis of Friedreich Ataxia Using Analysis of GAA Repeats and FXN Gene Exons in Population from Western India

The diagnosis of Friedreich ataxia is based on the clinical symptoms and GAA repeats expansions. In our experience, checking FXN gene exons for mutations along with GAA repeat analysis may give better clue for its diagnosis. In the present study, total 49 suspected Friedreich ataxia patients were analyzed for GAA repeat expansion. Eleven patients have normal number of GAA repeats, thereby termed as FRDA negative patients. Thirty-eight patients showed no amplification using GAA repeat analysis. Since no conclusion was possible based on these results, these patients were designated as uninformative. We have analyzed 5 exons of the FXN gene in FRDA negative and uninformative patients to check for possible mutations. It was observed that there were no mutations found in any of FRDA negative and most uninformative patients. We further used long range PCR to check for deletion of exon 5a. It was found that 18 patients showed expression for exon 5a PCR but none in long range PCR. Five patients showed no expression for exon 5a PCR as well as long range PCR indicating that these 5 patients may be positive FRDA patients. These findings need to be correlated with clinical history of these patients for confirmation.

Clinical features of Friedreich's ataxia: classical and atypical phenotypes

One hundred and fifty years since Nikolaus Friedreich's first description of the degenerative ataxic syndrome which bears his name, his description remains at the core of the classical clinical phenotype of gait and limb ataxia, poor balance and coordination, leg weakness, sensory loss, areflexia, impaired walking, dysarthria, dysphagia, eye movement abnormalities, scoliosis, foot deformities, cardiomyopathy and diabetes. Onset is typically around puberty with slow progression and shortened life-span often related to cardiac complications. Inheritance is autosomal recessive with the vast majority of cases showing an unstable intronic GAA expansion in both alleles of the frataxin gene on chromosome 9q13. A small number of cases are caused by a compound heterozygous expansion with a point mutation or deletion. Understanding of the underlying molecular biology has enabled identification of atypical phenotypes with late onset, or atypical features such as retained reflexes. Late-onset cases tend to have slower progression and are associated with smaller GAA expansions. Early-onset cases tend to have more rapid progression and a higher frequency of non-neurological features such as diabetes, cardiomyopathy, scoliosis and pes cavus. Compound heterozygotes, including those with large deletions, often have atypical features. In this paper, we review the classical and atypical clinical phenotypes of Friedreich's ataxia.

Erythropoietin in Friedreich ataxia

In Friedreich ataxia (FRDA), several candidate substances including erythropoietin (EPO) focus on increase in the amount of frataxin and aim to counteract the consequences of frataxin deficiency. Evidence for recombinant human erythropoietin (rHuEPO) in FRDA is based on in vitro studies using mouse neuronal cell lines, human fibroblasts, cardiomyocytes, and primary lymphocytes from FRDA patients or control subjects which showed a dose-dependent increase of frataxin after incubation with different erythropoietins. The mechanism by which EPO induces frataxin increase remains to be elucidated, but may involve post-transcriptional and/or post-translational modifications of frataxin or alterations in frataxin half-life and metabolism. In vivo data on rHuEPO's ability to increase frataxin in FRDA patients is contradictory as studies on the effect of EPO derivatives in FRDA differ in treatment regimen, sample size, and duration. Open-label studies indicate for sustained frataxin increase, decrease of oxidative stress, and clinical improvement in FRDA patients after administration of rHuEPO. Two randomized controlled studies found acceptable safety and tolerability of EPO derivatives in FRDA. Secondary outcome measures, however, such as frataxin up-regulation and clinical efficacy were not met. This review will focus on (i) pre-clinical work on erythropoietins in FRDA and (ii) clinical studies in FRDA patients exposed to erythropoietins.

Gene regulation and epigenetics in Friedreich's ataxia

This is an exciting time in the study of Friedreich's ataxia. Over the last 10 years much progress has been made in uncovering the mechanisms, whereby the Frataxin gene is silenced by (GAA)n repeat expansions and several of the findings are now ripe for testing in the clinic. The discovery that the Frataxin gene is heterochromatinised and that this can be antagonised in vivo has led to the tantalizing possibility that the disease might be amenable to a more radical therapeutic approach involving epigenetic modifiers. Here, we set out to review progress in the understanding of the fundamental mechanisms whereby genes are regulated at this level and how these findings have been applied to achieve a deeper understanding of the dysregulation that occurs as the primary genetic lesion in Friedreich's ataxia.

Biochemistry of cardiomyopathy in the mitochondrial disease Friedreich's ataxia

FRDA (Friedreich's ataxia) is a debilitating mitochondrial disorder leading to neural and cardiac degeneration, which is caused by a mutation in the frataxin gene that leads to decreased frataxin expression. The most common cause of death in FRDA patients is heart failure, although it is not known how the deficiency in frataxin potentiates the observed cardiomyopathy. The major proposed biochemical mechanisms for disease pathogenesis and the origins of heart failure in FRDA involve metabolic perturbations caused by decreased frataxin expression. Additionally, recent data suggest that low frataxin expression in heart muscle of conditional frataxin knockout mice activates an integrated stress response that contributes to and/or exacerbates cardiac hypertrophy and the loss of cardiomyocytes. The elucidation of these potential mechanisms will lead to a more comprehensive understanding of the pathogenesis of FRDA, and will contribute to the development of better treatments and therapeutics.

Development of frataxin gene expression measures for the evaluation of experimental treatments in Friedreich's ataxia

BACKGROUND

Friedreich ataxia is a progressive neurodegenerative disorder caused by GAA triplet repeat expansions or point mutations in the FXN gene and, ultimately, a deficiency in the levels of functional frataxin protein. Heterozygous carriers of the expansion express approximately 50% of normal frataxin levels yet manifest no clinical symptoms, suggesting that therapeutic approaches that increase frataxin may be effective even if frataxin is raised only to carrier levels. Small molecule HDAC inhibitor compounds increase frataxin mRNA and protein levels, and have beneficial effects in animal models of FRDA.

METHODOLOGY/PRINCIPAL FINDINGS

To gather data supporting the use of frataxin as a therapeutic biomarker of drug response we characterized the intra-individual stability of frataxin over time, determined the contribution of frataxin from different components of blood, compared frataxin measures in different cell compartments, and demonstrated that frataxin increases are achieved in peripheral blood mononuclear cells. Frataxin mRNA and protein levels were stable with repeated sampling over four and 15 weeks. In the 15-week study, the average CV was 15.6% for protein and 18% for mRNA. Highest levels of frataxin in blood were in erythrocytes. As erythrocytes are not useful for frataxin assessment in many clinical trial situations, we confirmed that PBMCs and buccal swabs have frataxin levels equivalent to those of whole blood. In addition, a dose-dependent increase in frataxin was observed when PBMCs isolated from patient blood were treated with HDACi. Finally, higher frataxin levels predicted less severe neurological dysfunction and were associated with slower rates of neurological change.

CONCLUSIONS/SIGNIFICANCE

Our data support the use of frataxin as a biomarker of drug effect. Frataxin levels are stable over time and as such a 1.5 to 2-fold change would be detectable over normal biological fluctuations. Additionally, our data support buccal cells or PBMCs as sources for measuring frataxin protein in therapeutic trials.

Clinical use of frataxin measurement in a patient with a novel deletion in the FXN gene

Friedreich ataxia (FRDA) is caused by a GAA expansion in the first intron of the FXN gene, which encodes frataxin. Four percent of patients harbor a point mutation on one allele and a GAA expansion on the other. We studied an Italian patient presenting with symptoms suggestive of FRDA, and carrying a single expanded 850 GAA allele. As a second diagnostic step, frataxin was measured in peripheral blood mononuclear cells, and proved to be in the pathological range (2.95 pg/μg total protein, 12.7 % of control levels). Subsequent sequencing revealed a novel deletion in exon 5a (c.572delC) which predicted a frameshift at codon 191 and a premature truncation of the protein at codon 194 (p.T191IfsX194). FXN/mRNA expression was reduced to 69.2 % of control levels. Clinical phenotype was atypical with absent dysarthria, and rapid disease progression. L-Buthionine-sulphoximine treatment of the proband's lymphoblasts showed a severe phenotype as compared to classic FRDA.

Missense mutations linked to friedreich ataxia have different but synergistic effects on mitochondrial frataxin isoforms

Friedreich ataxia is an early-onset multisystemic disease linked to a variety of molecular defects in the nuclear gene FRDA. This gene normally encodes the iron-binding protein frataxin (FXN), which is critical for mitochondrial iron metabolism, global cellular iron homeostasis, and antioxidant protection. In most Friedreich ataxia patients, a large GAA-repeat expansion is present within the first intron of both FRDA alleles, that results in transcriptional silencing ultimately leading to insufficient levels of FXN protein in the mitochondrial matrix and probably other cellular compartments. The lack of FXN in turn impairs incorporation of iron into iron-sulfur cluster and heme cofactors, causing widespread enzymatic deficits and oxidative damage catalyzed by excess labile iron. In a minority of patients, a typical GAA expansion is present in only one FRDA allele, whereas a missense mutation is found in the other allele. Although it is known that the disease course for these patients can be as severe as for patients with two expanded FRDA alleles, the underlying pathophysiological mechanisms are not understood. Human cells normally contain two major mitochondrial isoforms of FXN (FXN(42-210) and FXN(81-210)) that have different biochemical properties and functional roles. Using cell-free systems and different cellular models, we show that two of the most clinically severe FXN point mutations, I154F and W155R, have unique direct and indirect effects on the stability, biogenesis, or catalytic activity of FXN(42-210) and FXN(81-210) under physiological conditions. Our data indicate that frataxin point mutations have complex biochemical effects that synergistically contribute to the pathophysiology of Friedreich ataxia.

Therapeutic developments in Friedreich ataxia

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

Novel diagnostic paradigms for Friedreich ataxia

Friedreich ataxia is the most common inherited ataxia, with a wide phenotypic spectrum. It is generally caused by GAA expansions on both alleles of FXN, but a small percentage of patients are compound heterozygotes for a pathogenic expansion and a point mutation. Two recent diagnostic innovations are further characterizing individuals with the phenotype but without the classic genotypes. First, lateral-flow immunoassay is able to quantify the frataxin protein, thereby further characterizing these atypical individuals as likely affected or not affected, and providing some correlation to phenotype. It also holds promise as a biomarker for clinical trials in which the investigative agent increases frataxin. Second, gene dosage analysis and the identification of affected individuals with gene deletions introduce a novel genetic mechanism of disease. Both tests are now clinically available and suggest a new diagnostic paradigm for the disorder. Genetic counseling issues and future diagnostic testing approaches are considered as well.