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

An In Silico Analysis of Genetic Variants and Structural Modeling of the Human Frataxin Protein in Friedreich's Ataxia

Friedreich's Ataxia (FRDA) stands out as the most prevalent form of hereditary ataxias, marked by progressive movement ataxia, loss of vibratory sensitivity, and skeletal deformities, severely affecting daily functioning. To date, the only medication available for treating FRDA is Omaveloxolone (Skyclarys®), recently approved by the FDA. Missense mutations within the human frataxin (FXN) gene, responsible for intracellular iron homeostasis regulation, are linked to FRDA development. These mutations induce FXN dysfunction, fostering mitochondrial iron accumulation and heightened oxidative stress, ultimately triggering neuronal cell death pathways. This study amalgamated 226 FXN genetic variants from the literature and database searches, with only 18 previously characterized. Predictive analyses revealed a notable prevalence of detrimental and destabilizing predictions for FXN mutations, predominantly impacting conserved residues crucial for protein function. Additionally, an accurate, comprehensive three-dimensional model of human FXN was constructed, serving as the basis for generating genetic variants I154F and W155R. These variants, selected for their severe clinical implications, underwent molecular dynamics (MD) simulations, unveiling flexibility and essential dynamic alterations in their N-terminal segments, encompassing FXN42, FXN56, and FXN78 domains pivotal for protein maturation. Thus, our findings indicate potential interaction profile disturbances in the FXN42, FXN56, and FXN78 domains induced by I154F and W155R mutations, aligning with the existing literature.

Calcitriol Treatment Is Safe and Increases Frataxin Levels in Friedreich Ataxia Patients

BACKGROUND

Calcitriol, the active form of vitamin D (also known as 1,25-dihydroxycholecalciferol), improves the phenotype and increases frataxin levels in cell models of Friedreich ataxia (FRDA).

OBJECTIVES

Based on these results, we aimed measuring the effects of a calcitriol dose of 0.25 mcg/24h in the neurological function and frataxin levels when administered to FRDA patients for a year.

METHODS

20 FRDA patients where recluted and 15 patients completed the treatment for a year. Evaluations of neurological function changes (SARA scale, 9-HPT, 8-MWT, PATA test) and quality of life (Barthel Scale and Short Form (36) Health Survey [SF-36] quality of life questionnaire) were performed. Frataxin amounts were measured in isolated platelets obtained from these FRDA patients, from heterozygous FRDA carriers (relatives of the FA patients) and from non-heterozygous sex and age matched controls.

RESULTS

Although the patients did not experience any observable neurological improvement, there was a statistically significant increase in frataxin levels from initial values, 5.5 to 7.0 pg/μg after 12 months. Differences in frataxin levels referred to total protein levels were observed among sex- and age-matched controls (18.1 pg/μg), relative controls (10.1 pg/μg), and FRDA patients (5.7 pg/μg). The treatment was well tolerated by most patients, and only some of them experienced minor adverse effects at the beginning of the trial.

CONCLUSIONS

Calcitriol dosage used (0.25 mcg/24 h) is safe for FRDA patients, and it increases frataxin levels. We cannot rule out that higher doses administered longer could yield neurological benefits. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Phenotypic variation of FXN compound heterozygotes in a Friedreich ataxia cohort

OBJECTIVE

Most individuals with Friedreich ataxia (FRDA) have homozygous GAA triplet repeat expansions in the FXN gene, correlating with a typical phenotype of ataxia and cardiomyopathy. A minority are compound heterozygotes carrying a GAA expansion on one allele and a mutation on the other. The study aim was to examine phenotypic variation among compound heterozygotes.

METHODS

Data on FXN mutations were obtained from the Friedreich Ataxia Clinical Outcome Measures Study (FA-COMS). We compared clinical features in a single-site FA-COMS cohort of 51 compound heterozygous and 358 homozygous patients, including quantitative measures of cardiac, neurologic, and visual disease progression.

RESULTS

Non-GAA repeat mutations were associated with reduced cardiac disease, and patients with minimal/no function mutations otherwise had a typical FRDA phenotype but with significantly more severe progression. The partial function mutation group was characterized by relative sparing of bulbar and upper limb function, as well as particularly low cardiac involvement. Other clinical features in this group, including optic atrophy and diabetes mellitus, varied widely depending on the specific type of partial function mutation.

INTERPRETATION

These data support that the typical FRDA phenotype is driven by frataxin deficiency, especially severe in compound heterozygotes with minimal/no function mutations, whereas the heterogeneous presentations of those with partial function mutations may indicate other contributing factors to FRDA pathogenesis.

Frataxin analysis using triple quadrupole mass spectrometry: application to a large heterogeneous clinical cohort

BACKGROUND

Friedreich ataxia is a progressive multisystem disorder caused by deficiency of the protein frataxin; a small mitochondrial protein involved in iron sulfur cluster synthesis. Two types of frataxin exist: FXN-M, found in most cells, and FXN-E, found almost exclusively in red blood cells. Treatments in clinical trials include frataxin restoration by gene therapy, protein replacement, and epigenetic therapies, all of which necessitate sensitive assays for assessing frataxin levels.

METHODS

In the present study, we have used a triple quadrupole mass spectrometry-based assay to examine the features of both types of frataxin levels in blood in a large heterogenous cohort of 106 patients with FRDA.

RESULTS

Frataxin levels (FXN-E and FXN M) were predicted by GAA repeat length in regression models (R2 values = 0.51 and 0.27, respectively), and conversely frataxin levels predicted clinical status as determined by modified Friedreich Ataxia Rating scale scores and by disability status (R2 values = 0.13-0.16). There was no significant change in frataxin levels in individual subjects over time, and apart from start codon mutations, FXN-E and FXN-M levels were roughly equal. Accounting for hemoglobin levels in a smaller sub-cohort improved prediction of both FXN-E and FXN-M levels from R2 values of (0.3-0.38 to 0.20-0.51).

CONCLUSION

The present data show that assay of FXN-M and FXN-E levels in blood provides an appropriate biofluid for assessing their repletion in particular clinical contexts.

Searching for Frataxin Function: Exploring the Analogy with Nqo15, the Frataxin-like Protein of Respiratory Complex I from Thermus thermophilus

Nqo15 is a subunit of respiratory complex I of the bacterium Thermus thermophilus, with strong structural similarity to human frataxin (FXN), a protein involved in the mitochondrial disease Friedreich's ataxia (FRDA). Recently, we showed that the expression of recombinant Nqo15 can ameliorate the respiratory phenotype of FRDA patients' cells, and this prompted us to further characterize both the Nqo15 solution's behavior and its potential functional overlap with FXN, using a combination of in silico and in vitro techniques. We studied the analogy of Nqo15 and FXN by performing extensive database searches based on sequence and structure. Nqo15's folding and flexibility were investigated by combining nuclear magnetic resonance (NMR), circular dichroism, and coarse-grained molecular dynamics simulations. Nqo15's iron-binding properties were studied using NMR, fluorescence, and specific assays and its desulfurase activation by biochemical assays. We found that the recombinant Nqo15 isolated from complex I is monomeric, stable, folded in solution, and highly dynamic. Nqo15 does not share the iron-binding properties of FXN or its desulfurase activation function.

A modified mouse model of Friedreich's ataxia with conditional Fxn allele homozygosity delays onset of cardiomyopathy

Friedreich's ataxia (FA) is an autosomal recessive disorder caused by a deficiency in frataxin (FXN), a mitochondrial protein that plays a critical role in the synthesis of iron-sulfur clusters (Fe-S), vital inorganic cofactors necessary for numerous cellular processes. FA is characterized by progressive ataxia and hypertrophic cardiomyopathy, with cardiac dysfunction as the most common cause of mortality in patients. Commonly used cardiac-specific mouse models of FA use the muscle creatine kinase (MCK) promoter to express Cre recombinase in cardiomyocytes and striated muscle cells in mice with one conditional Fxn allele and one floxed-out/null allele. These mice quickly develop cardiomyopathy that becomes fatal by 9-11 wk of age. Here, we generated a cardiac-specific model with floxed Fxn allele homozygosity (MCK-Fxnflox/flox). MCK-Fxnflox/flox mice were phenotypically normal at 9 wk of age, despite no detectable FXN protein expression. Between 13 and 15 wk of age, these mice began to display progressive cardiomyopathy, including decreased ejection fraction and fractional shortening and increased left ventricular mass. MCK-Fxnflox/flox mice began to lose weight around 16 wk of age, characteristically associated with heart failure in other cardiac-specific FA models. By 18 wk of age, MCK-Fxnflox/flox mice displayed elevated markers of Fe-S deficiency, cardiac stress and injury, and cardiac fibrosis. This modified model reproduced important pathophysiological and biochemical features of FA over a longer timescale than previous cardiac-specific mouse models, offering a larger window for studying potential therapeutics.NEW & NOTEWORTHY Previous cardiac-specific frataxin knockout models exhibit rapid and fatal cardiomyopathy by 9 wk of age. This severe phenotype poses challenges for the design and execution of intervention studies. We introduce an alternative cardiac-specific model, MCK-Fxnflox/flox, with increased longevity and delayed onset of all major phenotypes. These phenotypes develop to the same severity as previous models. Thus, this new model provides the same cardiomyopathy-associated mortality with a larger window for potential studies.

Omaveloxolone: a groundbreaking milestone as the first FDA-approved drug for Friedreich ataxia

Friedreich ataxia (FA) is an inherited autosomal recessive neurodegenerative disease (NDD) characterized primarily by progressive sensory and spinocerebellar ataxia associated with hypertrophic cardiomyopathy. FA is due to an intronic GAA repeat expansion within the frataxin gene (FXN) leading to reduced levels of frataxin (FXN) which causes mitochondrial dysfunction, production of reactive oxygen species (ROS), and altered iron metabolism. To date there is no resolutive cure for FA; however, the FDA has recently approved omaveloxolone - a potent activator of nuclear factor erythroid 2-related factor 2 (NRF2) - as the first treatment for FA. We discuss herein the urgency to find a resolutive cure for NDDs that will most probably be achieved via combinatorial therapy targeting multiple disease pathways, and how omavaloxolone serves as an example for future treatments.

Human frataxin, the Friedreich ataxia deficient protein, interacts with mitochondrial respiratory chain

Friedreich ataxia (FRDA) is a rare, inherited neurodegenerative disease caused by an expanded GAA repeat in the first intron of the FXN gene, leading to transcriptional silencing and reduced expression of frataxin. Frataxin participates in the mitochondrial assembly of FeS clusters, redox cofactors of the respiratory complexes I, II and III. To date it is still unclear how frataxin deficiency culminates in the decrease of bioenergetics efficiency in FRDA patients' cells. We previously demonstrated that in healthy cells frataxin is closely attached to the mitochondrial cristae, which contain both the FeS cluster assembly machinery and the respiratory chain complexes, whereas in FRDA patients' cells with impaired respiration the residual frataxin is largely displaced in the matrix. To gain novel insights into the function of frataxin in the mitochondrial pathophysiology, and in the upstream metabolic defects leading to FRDA disease onset and progression, here we explored the potential interaction of frataxin with the FeS cluster-containing respiratory complexes I, II and III. Using healthy cells and different FRDA cellular models we found that frataxin interacts with these three respiratory complexes. Furthermore, by EPR spectroscopy, we observed that in mitochondria from FRDA patients' cells the decreased level of frataxin specifically affects the FeS cluster content of complex I. Remarkably, we also found that the frataxin-like protein Nqo15 from T. thermophilus complex I ameliorates the mitochondrial respiratory phenotype when expressed in FRDA patient's cells. Our data point to a structural and functional interaction of frataxin with complex I and open a perspective to explore therapeutic rationales for FRDA targeted to this respiratory complex.

Decoding Neurodegeneration: A Comprehensive Review of Molecular Mechanisms, Genetic Influences, and Therapeutic Innovations

Neurodegenerative disorders often acquire due to genetic predispositions and genomic alterations after exposure to multiple risk factors. The most commonly found pathologies are variations of dementia, such as frontotemporal dementia and Lewy body dementia, as well as rare subtypes of cerebral and cerebellar atrophy-based syndromes. In an emerging era of biomedical advances, molecular-cellular studies offer an essential avenue for a thorough recognition of the underlying mechanisms and their possible implications in the patient's symptomatology. This comprehensive review is focused on deciphering molecular mechanisms and the implications regarding those pathologies' clinical advancement and provides an analytical overview of genetic mutations in the case of neurodegenerative disorders. With the help of well-developed modern genetic investigations, these clinically complex disturbances are highly understood nowadays, being an important step in establishing molecularly targeted therapies and implementing those approaches in the physician's practice.

Retinal hypoplasia and degeneration result in vision loss in Friedreich ataxia

OBJECTIVE

Friedreich ataxia (FRDA) is an inherited condition caused by a GAA triplet repeat (GAA-TR) expansion in the FXN gene. Clinical features of FRDA include ataxia, cardiomyopathy, and in some, vision loss. In this study, we characterize features of vision loss in a large cohort of adults and children with FRDA.

METHODS

Using optical coherence tomography (OCT), we measured peripapillary retinal nerve fiber layer (RNFL) thickness in 198 people with FRDA, and 77 controls. Sloan letter charts were used to determine visual acuity. RNFL thickness and visual acuity were compared to measures of disease severity obtained from the Friedreich Ataxia Clinical Outcomes Measures Study (FACOMS).

RESULTS

The majority of patients, including children, had pathologically thin RNFLs (mean = 73 ± 13 μm in FRDA; 98 ± 9 μm in controls) and low-contrast vision deficits early in the disease course. Variability in RNFL thickness in FRDA (range: 36 to 107 μm) was best predicted by disease burden (GAA-TR length X disease duration). Significant deficits in high-contrast visual acuity were apparent in patients with an RNFL thickness of ≤68 μm. RNFL thickness decreased at a rate of -1.2 ± 1.4 μm/year and reached 68 μm at a disease burden of approximately 12,000 GAA years, equivalent to disease duration of 17 years for participants with 700 GAAs.

INTERPRETATION

These data suggest that both hypoplasia and subsequent degeneration of the RNFL may be responsible for the optic nerve dysfunction in FRDA and support the development of a vision-directed treatment for selected patients early in the disease to prevent RNFL loss from reaching the critical threshold.

FXN gene methylation determines carrier status in Friedreich ataxia

BACKGROUND

Friedreich ataxia (FRDA) is typically caused by homozygosity for an expanded GAA triplet-repeat (GAA-TRE) in intron 1 of the FXN gene. Some patients are compound heterozygous for the GAA-TRE and another FXN pathogenic variant. Detection of the GAA-TRE in the heterozygous state, occasionally technically challenging, is essential for diagnosing compound heterozygotes and asymptomatic carriers.

OBJECTIVE

We explored if the FRDA differentially methylated region (FRDA-DMR) in intron 1, which is hypermethylated in cis with the GAA-TRE, effectively detects heterozygous GAA-TRE.

METHODS

FXN DNA methylation was assayed by targeted bisulfite deep sequencing using the Illumina platform.

RESULTS

FRDA-DMR methylation effectively identified a cohort of known heterozygous carriers of the GAA-TRE. In an individual with clinical features of FRDA, commercial testing showed a paternally inherited pathogenic FXN initiation codon variant but no GAA-TRE. Methylation in the FRDA-DMR effectively identified the proband, his mother and various maternal relatives as heterozygous carriers of the GAA-TRE, thus confirming the diagnosis of FRDA.

CONCLUSION

FXN DNA methylation reliably detects the GAA-TRE in the heterozygous state and offers a robust alternative strategy to diagnose FRDA due to compound heterozygosity and to identify asymptomatic heterozygous carriers of the GAA-TRE.

Neurobehavioral deficits of mice expressing a low level of G127V mutant frataxin

Friedreich's ataxia (FRDA) is a neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin (FXN). Most FRDA patients are homozygous for large expansions of GAA repeats in intron 1 of FXN, while some are compound heterozygotes with an expanded GAA tract in one allele and a missense or nonsense mutation in the other. A missense mutation, changing a glycine to valine at position 130 (G130V), is prevalent among the clinical variants. We and others have demonstrated that levels of mature FXN protein in FRDA G130V samples are reduced below those detected in samples harboring homozygous repeat expansions. Little is known regarding expression and function of endogenous FXN-G130V protein due to lack of reagents and models that can distinguish the mutant FXN protein from the wild-type FXN produced from the GAA-expanded allele. We aimed to determine the effect of the G130V (murine G127V) mutation on Fxn expression and to define its multi-system impact in vivo. We used CRISPR/Cas9 to introduce the G127V missense mutation in the Fxn coding sequence and generated homozygous mice (Fxn^G127V/G127V). We also introduced the G127V mutation into a GAA repeat expansion FRDA mouse model (Fxn^GAA230/KO; KIKO) to generate a compound heterozygous strain (Fxn^G127V/GAA230). We performed neurobehavioral tests on cohorts of WT and Fxn mutant animals at three-month intervals for one year, and collected tissue samples to analyze molecular changes during that time. The endogenous Fxn G127V protein is detected at much lower levels in all tissues analyzed from Fxn^G127V/G127V mice compared to age and sex-matched WT mice without differences in Fxn transcript levels. Fxn^G127V/G127V mice are significantly smaller than WT counterparts, but perform similarly in most neurobehavioral tasks. RNA sequencing analysis revealed reduced expression of genes in oxidative phosphorylation and protein synthesis, underscoring the metabolic consequences in our mouse model expressing extremely low levels of Fxn. Results of these studies provide insight into the unique pathogenic mechanism of the FXN G130V mechanism and the tolerable limit of Fxn/FXN expression in vivo.

Omaveloxolone: an activator of Nrf2 for the treatment of Friedreich ataxia

INTRODUCTION

Friedreich ataxia (FRDA) is a rare autosomal recessive degenerative disorder characterized by ataxia, dysarthria, diabetes, cardiomyopathy, scoliosis, and occasionally vision loss in late-stage disease. The discovery of the abnormal gene in FRDA and its product frataxin has provided insight into the pathophysiology and mechanisms of treatment.

AREAS COVERED

Although the neurologic phenotype of FRDA is well defined, there are currently no established pharmacological treatments. Omaveloxolone, a nuclear factor erythroid 2-related factor 2 (Nrf2) activator, is currently under review by the Food and Drug Administration (FDA) and has the potential to be the first approved treatment for FRDA. In the present report, we have reviewed the basic and clinical literature on Nrf2 deficiency in FRDA, and evidence for the benefit of omaveloxolone.

EXPERT OPINION

The present perspective suggests that omaveloxolone is a rational and efficacious therapy that is possibly disease modifying in treatment of FRDA.

Prediction of the disease course in Friedreich ataxia

We explored whether disease severity of Friedreich ataxia can be predicted using data from clinical examinations. From the database of the European Friedreich Ataxia Consortium for Translational Studies (EFACTS) data from up to five examinations of 602 patients with genetically confirmed FRDA was included. Clinical instruments and important symptoms of FRDA were identified as targets for prediction, while variables such as genetics, age of disease onset and first symptom of the disease were used as predictors. We used modelling techniques including generalised linear models, support-vector-machines and decision trees. The scale for rating and assessment of ataxia (SARA) and the activities of daily living (ADL) could be predicted with predictive errors quantified by root-mean-squared-errors (RMSE) of 6.49 and 5.83, respectively. Also, we were able to achieve reasonable performance for loss of ambulation (ROC-AUC score of 0.83). However, predictions for the SCA functional assessment (SCAFI) and presence of cardiological symptoms were difficult. In conclusion, we demonstrate that some clinical features of FRDA can be predicted with reasonable error; being a first step towards future clinical applications of predictive modelling. In contrast, targets where predictions were difficult raise the question whether there are yet unknown variables driving the clinical phenotype of FRDA.

Premature transcription termination at the expanded GAA repeats and aberrant alternative polyadenylation contributes to the Frataxin transcriptional deficit in Friedreich's ataxia

Frataxin deficiency in Friedreich's ataxia results from transcriptional downregulation of the FXN gene caused by expansion of the intronic trinucleotide guanine-adenine-adenine (GAA) repeats. We used multiple transcriptomic approaches to determine the molecular mechanism of transcription inhibition caused by long GAAs. We uncovered that transcription of FXN in patient cells is prematurely terminated upstream of the expanded repeats leading to the formation of a novel, truncated and stable RNA. This FXN early terminated transcript (FXN-ett) undergoes alternative, non-productive splicing and does not contribute to the synthesis of functional frataxin. The level the FXN-ett RNA directly correlates with the length of the longer of the two expanded GAA tracts. Targeting GAAs with antisense oligonucleotides or excision of the repeats eliminates the transcription impediment, diminishes expression of the aberrant FXN-ett, while increasing levels of FXN mRNA and frataxin. Non-productive transcription may represent a common phenomenon and attractive therapeutic target in diseases caused by repeat-mediated transcription aberrations.

Frataxin controls ketone body metabolism through regulation of OXCT1

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the deficiency of mitochondrial protein frataxin, which plays a crucial role in iron-sulphur cluster formation and ATP production. The cellular function of frataxin is not entirely known. Here, we demonstrate that frataxin controls ketone body metabolism through regulation of 3-Oxoacid CoA-Transferase 1 (OXCT1), a rate limiting enzyme catalyzing the conversion of ketone bodies to acetoacetyl-CoA that is then fed into the Krebs cycle. Biochemical studies show a physical interaction between frataxin and OXCT1 both in vivo and in vitro. Frataxin overexpression also increases OXCT1 protein levels in human skin fibroblasts while frataxin deficiency decreases OXCT1 in multiple cell types including cerebellum and skeletal muscle both acutely and chronically, suggesting that frataxin directly regulates OXCT1. This regulation is mediated by frataxin-dependent suppression of ubiquitin-proteasome system (UPS)-dependent OXCT1 degradation. Concomitantly, plasma ketone bodies are significantly elevated in frataxin deficient knock-in/knockout (KIKO) mice with no change in the levels of other enzymes involved in ketone body production. In addition, ketone bodies fail to be metabolized to acetyl-CoA accompanied by increased succinyl-CoA in vitro in frataxin deficient cells, suggesting that ketone body elevation is caused by frataxin-dependent reduction of OXCT1 leading to deficits in tissue utilization of ketone bodies. Considering the potential role of metabolic abnormalities and deficiency of ATP production in FRDA, our results suggest a new role for frataxin in ketone body metabolism and also suggest modulation of OXCT1 may be a potential therapeutic approach for FRDA.

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.

Bone Mineral Density and Current Bone Health Screening Practices in Friedreich's Ataxia

Introduction

Friedreich's Ataxia (FRDA) is a progressive neurological disorder caused by mutations in both alleles of the frataxin (FXN) gene. Impaired bone health is a complication of other disorders affecting mobility, but there is little information regarding bone health in FRDA.

Methods

Dual energy X-ray absorptiometry (DXA) scan-based assessments of areal bone mineral density (aBMD) in individuals with FRDA were abstracted from four studies at the Children's Hospital of Philadelphia (CHOP). Disease outcomes, including the modified FRDA Rating Scale (mFARS), were abstracted from the FRDA Clinical Outcomes Measures Study (FACOMS), a longitudinal natural history study. A survey regarding bone health and fractures was sent to individuals in FACOMS-CHOP.

Results

Adults with FRDA (n = 24) have lower mean whole body (WB) (-0.45 vs. 0.33, p = 0.008) and femoral neck (FN) (-0.71 vs. 0.004, p = 0.02) aBMD Z-scores than healthy controls (n = 24). Children with FRDA (n = 10) have a lower WB-less-head (-2.2 vs. 0.19, p < 0.0001) and FN (-1.1 vs. 0.04, p = 0.01) aBMD than a reference population (n = 30). In adults, lower FN aBMD correlated with functional disease severity, as reflected by mFARS (R = -0.56, p = 0.04). Of 137 survey respondents (median age 27 y, 50% female), 70 (51%) reported using wheelchairs as their primary ambulatory device: of these, 20 (29%) reported a history of potentially pathologic fracture and 11 (16%) had undergone DXA scans.

Conclusions

Low aBMD is prevalent in FRDA, but few of even the highest risk individuals are undergoing screening. Our findings highlight potential missed opportunities for the screening and treatment of low aBMD in FRDA.

Friedreich's Ataxia related Diabetes: Epidemiology and management practices

AIMS

Friedreich's Ataxia (FRDA) is a progressive neuromuscular disorder typically caused by GAA triplet repeat expansions in both frataxin gene alleles. FRDA can be complicated by diabetes mellitus (DM). The objective of this study was to describe the prevalence of, risk factors for, and management practices of FRDA-related DM.

METHODS

FACOMS, a prospective, multi-site natural history study, includes 1,104 individuals. Extracted data included the presence of DM and other co-morbidities, genetic diagnosis, and markers of disease severity. We performed detailed medical record review and a survey for the subset of individuals with FRDA-related DM followed at one FACOMS site, Children's Hospital of Philadelphia.

RESULTS

FRDA-related DM was reported by 8.7% of individuals. Age, severe disease, and FRDA cardiac complications were positively associated with DM risk. FRDA-related DM was generally well-controlled, as reflected by HbA1c, though diabetic ketoacidosis did occur. Insulin is the mainstay of treatment (64-74% overall); in adults, metformin use was common and newer glucose-lowering agents were used rarely.

CONCLUSIONS

Clinical factors identify individuals at increased risk for FRDA-related DM. Future studies should test strategies for FRDA-related DM screening and management, in particular the potential role for novel glucose-lowering therapies in preventing or delaying FRDA-related cardiac disease.

Recessive cerebellar and afferent ataxias - clinical challenges and future directions

Cerebellar and afferent ataxias present with a characteristic gait disorder that reflects cerebellar motor dysfunction and sensory loss. These disorders are a diagnostic challenge for clinicians because of the large number of acquired and inherited diseases that cause cerebellar and sensory neuron damage. Among such conditions that are recessively inherited, Friedreich ataxia and RFC1-associated cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) include the characteristic clinical, neuropathological and imaging features of ganglionopathies, a distinctive non-length-dependent type of sensory involvement. In this Review, we discuss the typical and atypical phenotypes of Friedreich ataxia and CANVAS, along with the features of other recessive ataxias that present with a ganglionopathy or polyneuropathy, with an emphasis on recently described clinical features, natural history and genotype-phenotype correlations. We review the main developments in understanding the complex pathology that affects the sensory neurons and cerebellum, which seem to be most vulnerable to disorders that affect mitochondrial function and DNA repair mechanisms. Finally, we discuss disease-modifying therapeutic advances in Friedreich ataxia, highlighting the most promising candidate molecules and lessons learned from previous clinical trials.

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.

Mitochondrial De Novo Assembly of Iron–Sulfur Clusters in Mammals: Complex Matters in a Complex That Matters

Iron–sulfur clusters (Fe–S or ISC) are essential cofactors that function in a wide range of biological pathways. In mammalian cells, Fe–S biosynthesis primarily relies on mitochondria and involves a concerted group of evolutionary-conserved proteins forming the ISC pathway. In the early stage of the ISC pathway, the Fe–S core complex is required for de novo assembly of Fe–S. In humans, the Fe–S core complex comprises the cysteine desulfurase NFS1, the scaffold protein ISCU2, frataxin (FXN), the ferredoxin FDX2, and regulatory/accessory proteins ISD11 and Acyl Carrier Protein (ACP). In recent years, the field has made significant advances in unraveling the structure of the Fe–S core complex and the mechanism underlying its function. Herein, we review the key recent findings related to the Fe–S core complex and its components. We highlight some of the unanswered questions and provide a model of the Fe–S assembly within the complex. In addition, we briefly touch on the genetic diseases associated with mutations in the Fe–S core complex components.

Protein Mutations and Stability, a Link with Disease: The Case Study of Frataxin

Protein mutations may lead to pathologies by causing protein misfunction or propensity to degradation. For this reason, several studies have been performed over the years to determine the capability of proteins to retain their native conformation under stress condition as well as factors to explain protein stabilization and the mechanisms behind unfolding. In this review, we explore the paradigmatic example of frataxin, an iron binding protein involved in Fe–S cluster biogenesis, and whose impairment causes a neurodegenerative disease called Friedreich’s Ataxia (FRDA). We summarize what is known about most common point mutations identified so far in heterozygous FRDA patients, their effects on frataxin structure and function and the consequences of its binding with partners.

Classification of Dystonia

Dystonia is a hyperkinetic movement disorder characterized by abnormal movement or posture caused by excessive muscle contraction. Because of its wide clinical spectrum, dystonia is often underdiagnosed or misdiagnosed. In clinical practice, dystonia could often present in association with other movement disorders. An accurate physical examination is essential to describe the correct phenomenology. To help clinicians reaching the proper diagnosis, several classifications of dystonia have been proposed. The current classification consists of axis I, clinical characteristics, and axis II, etiology. Through the application of this classification system, movement disorder specialists could attempt to correctly characterize dystonia and guide patients to the most effective treatment. The aim of this article is to describe the phenomenological spectrum of dystonia, the last approved dystonia classification, and new emerging knowledge.

Conformational stability, dynamics and function of human frataxin: Tryptophan side chain interplay

In humans, the loss of frataxin results in Friedreich's Ataxia, a neurodegenerative disease, in which a deficit in the iron-sulfur cluster assembly is observed. In this work, we analyzed three frataxin variants in which one tryptophan was replaced by a glycine: W155G, W168G and W173G. As expected, given its localization in the assembly site, W155G was not able to activate the desulfurase activity of the supercomplex for iron-sulfur cluster assembly. In turn, W168G, which was significantly more unstable than W155G, was fully active. W173G, which was highly unstable as W168G, showed a significantly decreased activity, only slightly higher than W155G. As W168G and W173G were highly sensitive to proteolysis, we investigated the protein motions by molecular dynamic simulations. We observed that W173G may display altered motions at the Trp155 site. Furthermore, we revealed a H-bond network in which Trp155 takes part, involving residues Gln148, Asn151, Gln153 and Arg165. We suggest that this motion modulation that specifically alters the population of different Trp155 rotamers can be directly transferred to the assembly site, altering the dynamics of the ISCU His137 key residue. This hypothesis was also contrasted by means of molecular dynamic simulations of frataxin in the context of the complete supercomplex. We propose that the supercomplex requires very definite motions of Trp155 to consolidate the assembly site.

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.

Longitudinal Assessment Using Optical Coherence Tomography in Patients with Friedreich's Ataxia

Ocular abnormalities occur frequently in Friedreich's ataxia (FRDA), although visual symptoms are not always reported. We evaluated a cohort of patients with FRDA to characterise the clinical phenotype and optic nerve findings as detected with optical coherence tomography (OCT). A total of 48 patients from 42 unrelated families were recruited. Mean age at onset was 13.8 years (range 4-40), mean disease duration 19.5 years (range 5-43), mean disease severity as quantified with the Scale for the Assessment and Rating of Ataxia 22/40 (range 4.5-38). All patients displayed variable ataxia and two-thirds had ocular abnormalities. Statistically significant thinning of average retinal nerve fibre layer (RNFL) and thinning in all but the temporal quadrant compared to controls was demonstrated on OCT. Significant RNFL and macular thinning was documented over time in 20 individuals. Disease severity and visual acuity were correlated with RNFL and macular thickness, but no association was found with disease duration. Our results highlight that FDRA is associated with subclinical optic neuropathy. This is the largest longitudinal study of OCT findings in FRDA to date, demonstrating progressive RNFL thickness decline, suggesting that RNFL thickness as measured by OCT has the potential to become a quantifiable biomarker for the evaluation of disease progression in FRDA.

Targeting 3' and 5' untranslated regions with antisense oligonucleotides to stabilize frataxin mRNA and increase protein expression

Friedreich's ataxia (FRDA) is a severe multisystem disease caused by transcriptional repression induced by expanded GAA repeats located in intron 1 of the Frataxin (FXN) gene encoding frataxin. FRDA results from decreased levels of frataxin; thus, stabilization of the FXN mRNA already present in patient cells represents an attractive and unexplored therapeutic avenue. In this work, we pursued a novel approach based on oligonucleotide-mediated targeting of FXN mRNA ends to extend its half-life and availability as a template for translation. We demonstrated that oligonucleotides designed to bind to FXN 5' or 3' noncoding regions can increase FXN mRNA and protein levels. Simultaneous delivery of oligonucleotides targeting both ends increases efficacy of the treatment. The approach was confirmed in several FRDA fibroblast and induced pluripotent stem cell-derived neuronal progenitor lines. RNA sequencing and single-cell expression analyses confirmed oligonucleotide-mediated FXN mRNA upregulation. Mechanistically, a significant elongation of the FXN mRNA half-life without any changes in chromatin status at the FXN gene was observed upon treatment with end-targeting oligonucleotides, indicating that transcript stabilization is responsible for frataxin upregulation. These results identify a novel approach toward upregulation of steady-state mRNA levels via oligonucleotide-mediated end targeting that may be of significance to any condition resulting from transcription downregulation.

Designing phase II clinical trials in Friedreich ataxia

INTRODUCTION

Friedreich ataxia (FRDA) is an autosomal recessive disorder caused by deficiency of frataxin, an essential mitochondrial protein involved in iron sulfur cluster biogenesis, oxidative phosphorylation and other processes. FRDA most notably affects the heart, sensory neurons, spinal cord, cerebellum, and other brain regions, and manifests clinically as ataxia, sensory loss, dysarthria, spasticity, and hypertrophic cardiomyopathy. Therapeutic approaches in FRDA have consisted of two different approaches: (1) augmenting or restoring frataxin production and (2) modulating a variety of downstream processes related to mitochondrial dysfunction, including reactive oxygen species production, ferroptosis, or Nrf2 activation.

AREAS COVERED

In this review, we summarize data from major phase II clinical trials in FRDA published between 2015 and 2020, which includes A0001/EPI743, Omaveloxolone, RT001, and Actimmune.

EXPERT OPINION

A growing number of drug candidates are being tested in phase II clinical trials for FRDA; however, most have not met their primary endpoints, and none have received FDA approval. In this review, we aim to summarize completed phase II clinical trials in FRDA, outlining critical lessons that have been learned and that should be incorporated into future trial design to ultimately optimize drug development in FRDA.

Genetic Survey of Autosomal Recessive Peripheral Neuropathy Cases Unravels High Genetic Heterogeneity in a Turkish Cohort

Background and Objectives

Inherited peripheral neuropathies (IPNs) are a group of genetic disorders of the peripheral nervous system in which neuropathy is the only or the most predominant clinical feature. The most common type of IPN is Charcot-Marie-Tooth (CMT) disease. Autosomal recessive CMT (ARCMT) is generally more severe than dominant CMT and its genetic basis is poorly understood due to high clinical and genetic diversity. Here, we report clinical and genetic findings from 56 consanguineous Turkish families initially diagnosed with CMT disease.

Methods

We initially screened the GDAP1 gene in our cohort as it is the most commonly mutated ARCMT gene. Next, whole-exome sequencing and homozygosity mapping based on whole-exome sequencing (HOMWES) analysis was performed. To understand the molecular impact of candidate causative genes, functional analyses were performed in patient primary fibroblasts.

Results

Biallelic recurrent mutations in the GDAP1 gene have been identified in 6 patients. Whole-exome sequencing and HOMWES analysis revealed 16 recurrent and 13 novel disease-causing alleles in known IPN-related genes and 2 novel candidate genes: 1 for a CMT-like disease and 1 for autosomal recessive cerebellar ataxia with axonal neuropathy. We have achieved a potential genetic diagnosis rate of 62.5% (35/56 families) in our cohort. Considering only the variants that meet the American College for Medical Genetics and Genomics (ACMG) classification as pathogenic or likely pathogenic, the definitive diagnosis rate was 55.35% (31/56 families).

Discussion

This study paints a genetic landscape of the Turkish ARCMT population and reports additional candidate genes that might help enlighten the mechanism of pathogenesis of the disease.

Down the Iron Path: Mitochondrial Iron Homeostasis and Beyond

Cellular iron homeostasis and mitochondrial iron homeostasis are interdependent. Mitochondria must import iron to form iron–sulfur clusters and heme, and to incorporate these cofactors along with iron ions into mitochondrial proteins that support essential functions, including cellular respiration. In turn, mitochondria supply the cell with heme and enable the biogenesis of cytosolic and nuclear proteins containing iron–sulfur clusters. Impairment in cellular or mitochondrial iron homeostasis is deleterious and can result in numerous human diseases. Due to its reactivity, iron is stored and trafficked through the body, intracellularly, and within mitochondria via carefully orchestrated processes. Here, we focus on describing the processes of and components involved in mitochondrial iron trafficking and storage, as well as mitochondrial iron–sulfur cluster biogenesis and heme biosynthesis. Recent findings and the most pressing topics for future research are highlighted.

Very late-onset Friedreich's ataxia with rapid course mimicking as possible multiple system atrophy cerebellar type

This 55-year-old man was admitted to the hospital with an insidious onset, progressive backward fall (due to severe truncal ataxia), dysarthria, stiffness in extremities, distal dominant muscle wasting along with behavioural changes and urinary incontinence. Clinical assessment indicated mild cognitive decline (Mini-Mental State Examination 22/27) with cerebellar, pyramidal and peripheral nerves involvement. On investigations, nerve conduction studies revealed symmetrical, sensorimotor peripheral neuropathy affecting both lower limbs. Brain and whole spine MRI revealed widespread cerebral and mild cerebellar atrophy, pons and medulla volume loss, and a normal spinal cord. Transthoracic echocardiography revealed concentric left ventricular hypertrophy. His gene analysis revealed eight GAA repeats on allele 1, and 37 GAA repeats on allele 2 in the first intron of the frataxin gene. Considering his clinical profile and genetic analysis, he was diagnosed as a case of very late-onset Friedreich's ataxia with likely compound heterozygous genotype.

Neuro-Ophthalmological Findings in Friedreich's Ataxia

Friedreich ataxia (FRDA) is a progressive neurodegenerative disease caused by a severe autosomal recessive genetic disorder of the central nervous (CNS) and peripheral nervous system (PNS), affecting children and young adults. Its onset is before 25 years of age, with mean ages of onset and death between 11 and 38 years, respectively. The incidence is 1 in 30,000–50,000 persons. It is caused, in 97% of cases, by a homozygous guanine-adenine-adenine (GAA) trinucleotide mutation in the first intron of the frataxin (FXN) gene on chromosome 9 (9q13–q1.1). The mutation of this gene causes a deficiency of frataxin, which induces an altered inflow of iron into the mitochondria, increasing the nervous system’s vulnerability to oxidative stress. The main clinical signs include spinocerebellar ataxia with sensory loss and disappearance of deep tendon reflexes, cerebellar dysarthria, cardiomyopathy, and scoliosis. Diabetes, hearing loss, and pes cavus may also occur, and although most patients with FRDA do not present with symptomatic visual impairment, 73% present with clinical neuro-ophthalmological alterations such as optic atrophy and altered eye movement, among others. This review provides a brief overview of the main aspects of FRDA and then focuses on the ocular involvement of this pathology and the possible use of retinal biomarkers.

Friedreich Ataxia: Multidisciplinary Clinical Care

Friedreich ataxia (FRDA) is a multisystem disorder affecting 1 in 50,000-100,000 person in the United States. Traditionally viewed as a neurodegenerative disease, FRDA patients also develop cardiomyopathy, scoliosis, diabetes and other manifestation. Although it usually presents in childhood, it continues throughout life, thus requiring expertise from both pediatric and adult subspecialist in order to provide optimal management. The phenotype of FRDA is unique, giving rise to specific loss of neuronal pathways, a unique form of cardiomyopathy with early hypertrophy and later fibrosis, and diabetes incorporating components of both type I and type II disease. Vision loss, hearing loss, urinary dysfunction and depression also occur in FRDA. Many agents are reaching Phase III trials; if successful, these will provide a variety of new treatments for FRDA that will require many specialists who are not familiar with FRDA to provide clinical therapy. This review provides a summary of the diverse manifestation of FRDA, existing symptomatic therapies, and approaches for integrative care for future therapy in FRDA.

Mitochondrial and metabolic dysfunction in Friedreich ataxia: update on pathophysiological relevance and clinical interventions

Friedreich ataxia (FRDA) is a recessive disorder resulting from relative deficiency of the mitochondrial protein frataxin. Frataxin functions in the process of iron–sulfur (Fe–S) cluster synthesis. In this review, we update some of the processes downstream of frataxin deficiency that may mediate the pathophysiology. Based on cellular models, in vivo models and observations of patients, ferroptosis may play a major role in the pathogenesis of FRDA along with depletion of antioxidant reserves and abnormalities of mitochondrial biogenesis. Ongoing clinical trials with ferroptosis inhibitors and nuclear factor erythroid 2-related factor 2 (Nrf2) activators are now targeting each of the processes. In addition, better understanding of the mitochondrial events in FRDA may allow the development of improved imaging methodology for assessing the disorder. Though not technologically feasible at present, metabolic imaging approaches may provide a direct methodology to understand the mitochondrial changes occurring in FRDA and provide a methodology to monitor upcoming trials of frataxin restoration.

Reverse Phase Protein Array Reveals Correlation of Retinoic Acid Metabolism With Cardiomyopathy in Friedreich's Ataxia

Identifying biomarkers is important for assessment of disease progression, prediction of symptom development, and determination of treatment effectiveness. While unbiased analyses of differential gene expression using next-generation sequencing methods are now routinely conducted, proteomics studies are more challenging because of traditional methods predominantly being low throughput and offering a limited dynamic range for simultaneous detection of hundreds of proteins that drastically differ in their intracellular abundance. We utilized a sensitive and high-throughput proteomic technique, reverse phase protein array (RPPA), to attain protein expression profiles of primary fibroblasts obtained from patients with Friedreich's ataxia (FRDA) and unaffected controls (CTRLs). The RPPA was designed to detect 217 proteins or phosphorylated proteins by individual antibody, and the specificity of each antibody was validated prior to the experiment. Among 62 fibroblast samples (44 FRDA and 18 CTRLs) analyzed, 30 proteins/phosphoproteins were significantly changed in FRDA fibroblasts compared with CTRL cells (p < 0.05), mostly representing signaling molecules and metabolic enzymes. As expected, frataxin was significantly downregulated in FRDA samples, thus serving as an internal CTRL for assay integrity. Extensive bioinformatics analyses were conducted to correlate differentially expressed proteins with critical disease parameters (e.g., selected symptoms, age of onset, guanine-adenine-adenine sizes, frataxin levels, and Functional Assessment Rating Scale scores). Members of the integrin family of proteins specifically associated with hearing loss in FRDA. Also, RPPA data, combined with results of transcriptome profiling, uncovered defects in the retinoic acid metabolism pathway in FRDA samples. Moreover, expression of aldehyde dehydrogenase family 1 member A3 differed significantly between cardiomyopathy-positive and cardiomyopathy-negative FRDA cohorts, demonstrating that metabolites such as retinol, retinal, or retinoic acid could become potential predictive biomarkers of cardiac presentation in FRDA.

Recent advances in understanding hereditary spastic paraplegias and emerging therapies

Hereditary spastic paraplegias (HSPs) are a group of rare, inherited, neurological diseases characterized by broad clinical and genetic heterogeneity. Lower-limb spasticity with first motoneuron involvement is the core symptom of all HSPs. As spasticity is a syndrome and not a disease, it develops on top of other neurological signs (ataxia, dystonia, and parkinsonism). Indeed, the definition of genes responsible for HSPs goes beyond the 79 identified SPG genes. In order to avoid making a catalog of the different genes involved in HSP in any way, we have chosen to focus on the HSP with cerebellar ataxias since this is a frequent association described for several genes. This overlap leads to an intermediary group of spastic ataxias which is actively genetically and clinically studied. The most striking example is SPG7, which is responsible for HSP or cerebellar ataxia or both. There are no specific therapies against HSPs, and there is a dearth of randomized trials in patients with HSP, especially on spasticity when it likely results from other mechanisms. Thus far, no gene-specific therapy has been developed for HSP, but emerging therapies in animal models and neurons derived from induced pluripotent stem cells are potential treatments for patients.

The displacement of frataxin from the mitochondrial cristae correlates with abnormal respiratory supercomplexes formation and bioenergetic defects in cells of Friedreich ataxia patients

Friedreich ataxia (FRDA) is a neurodegenerative disease resulting from a severe decrease of frataxin (FXN). Most patients carry a GAA repeat expansion in both alleles of the FXN gene, whereas a small fraction of them are compound heterozygous for the expansion and a point mutation in the other allele. FXN is involved in the mitochondrial biogenesis of the FeS-clusters. Distinctive feature of FRDA patient cells is an impaired cellular respiration, likely due to a deficit of key redox cofactors working as electrons shuttles through the respiratory chain. However, a definite relationship between FXN levels, FeS-clusters assembly dysregulation and bioenergetics failure has not been established. In this work, we performed a comparative analysis of the mitochondrial phenotype of cell lines from FRDA patients, either homozygous for the expansion or compound heterozygotes for the G130V mutation. We found that, in healthy cells, FXN and two key proteins of the FeS-cluster assembly machinery are enriched in mitochondrial cristae, the dynamic subcompartment housing the respiratory chain. On the contrary, FXN widely redistributes to the matrix in FRDA cells with defects in respiratory supercomplexes assembly and altered respiratory function. We propose that this could be relevant for the early mitochondrial defects afflicting FRDA cells and that perturbation of mitochondrial morphodynamics could in turn be critical in terms of disease mechanisms.

Nuclear Factor Erythroid 2-Related Factor 2 Activation Might Mitigate Clinical Symptoms in Friedreich's Ataxia: Clues of an "Out-Brain Origin" of the Disease From a Family Study

Friedreich’s ataxia (FRDA) is the most frequent autosomal recessive ataxia in western countries, with a mean age of onset at 10–15 years. Patients manifest progressive cerebellar and sensory ataxia, dysarthria, lower limb pyramidal weakness, and other systemic manifestations. Previously, we described a family displaying two expanded GAA alleles not only in the proband affected by late-onset FRDA but also in the two asymptomatic family members: the mother and the younger sister. Both of them showed a significant reduction of frataxin levels, without any disease manifestation. Here, we analyzed if a protective mechanism might contribute to modulate the phenotype in this family. We particularly focused on the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2), the first line of antioxidant defense in cells, and on the glutathione (GSH) system, an index of reactive oxygen species (ROS) detoxification ability. Our findings show a great reactivity of the GSH system to the frataxin deficiency, particularly in the asymptomatic mother, where the genes of GSH synthesis [glutamate–cysteine ligase (GCL)] and GSSG detoxification [GSH S-reductase (GSR)] were highly responsive. The GSR was activated even in the asymptomatic sister and in the proband, reflecting the need of buffering the GSSG increase. Furthermore, and contrasting the NRF2 expression documented in FRDA tissues, NRF2 was highly activated in the mother and in the younger sister, while it was constitutively low in the proband. This suggests that, also under frataxin depletion, the endogenous stimulation of NRF2 in asymptomatic FRDA subjects may contribute to protect against the progressive oxidative damage, helping to prevent the onset of neurological symptoms and highlighting an “out-brain origin” of the disease.

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.

Efficacy and Tolerability of Interferon Gamma in Treatment of Friedreich's Ataxia: Retrospective Study

Friedreich's Ataxia (FRDA) is the most common form of autosomal recessive ataxia. The disease primarily results from a GAA trinucleotide repeat expansion within the FXN gene in up to 97% of patients. The clinical presentation begins approximately between the ages of 5 and 15. The major clinical findings of FRDA are progressive extremity and gait ataxia. Although it is known that the disease is caused by low levels of functional protein in the target tissues, there is no effective treatment available for this pathology. However, significant improvements have been achieved in the research into pharmacological treatments for FRDA in recent years. Interferon-gamma (IFN-γ) has been shown to induce frataxin production in many cell types. In this study, the clinical features, tolerability, and the prognosis of individuals with FRDA to whom IFN-γ was administered in a university hospital were evaluated retrospectively and the results were discussed. To the best of our knowledge, this is the first study conducted in our country to evaluate the effect of IFN gamma on this patient group.

Onset features and time to diagnosis in Friedreich's Ataxia

Background

In rare disorders diagnosis may be delayed due to limited awareness and unspecific presenting symptoms. Herein, we address the issue of diagnostic delay in Friedreich’s Ataxia (FRDA), a genetic disorder usually caused by homozygous GAA-repeat expansions.

Methods

Six hundred eleven genetically confirmed FRDA patients were recruited within a multicentric natural history study conducted by the EFACTS (European FRDA Consortium for Translational Studies, ClinicalTrials.gov-Identifier NCT02069509). Age at first symptoms as well as age at first suspicion of FRDA by a physician were collected retrospectively at the baseline visit.

Results

In 554 of cases (90.7%), disease presented with gait or coordination disturbances. In the others (n = 57, 9.3%), non-neurological features such as scoliosis or cardiomyopathy predated ataxia. Before the discovery of the causal mutation in 1996, median time to diagnosis was 4(IQR = 2–9) years and it improved significantly after the introduction of genetic testing (2(IQR = 1–5) years, p < 0.001). Still, after 1996, time to diagnosis was longer in patients with a) non-neurological presentation (mean 6.7, 95%CI [5.5,7.9] vs 4.5, [4.2,5] years in those with neurological presentation, p = 0.001) as well as in b) patients with late-onset (3(IQR = 1–7) vs 2(IQR = 1–5) years compared to typical onset < 25 years of age, p = 0.03).

Age at onset significantly correlated with the length of the shorter GAA repeat (GAA1) in case of neurological onset (r = − 0,6; p < 0,0001), but not in patients with non-neurological presentation (r = − 0,1; p = 0,4). Across 54 siblings’ pairs, differences in age at onset did not correlate with differences in GAA-repeat length (r = − 0,14, p = 0,3).

Conclusions

In the genetic era, presentation with non-neurological features or in the adulthood still leads to a significant diagnostic delay in FRDA. Well-known correlations between GAA1 repeat length and disease milestones are not valid in case of atypical presentations or positive family history.

Mitochondrial damage and senescence phenotype of cells derived from a novel frataxin G127V point mutation mouse model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin (FXN). Most FRDA patients are homozygous for large expansions of GAA repeat sequences in intron 1 of FXN, whereas a fraction of patients are compound heterozygotes, with a missense or nonsense mutation in one FXN allele and expanded GAAs in the other. A prevalent missense mutation among FRDA patients changes a glycine at position 130 to valine (G130V). Herein, we report generation of the first mouse model harboring an Fxn point mutation. Changing the evolutionarily conserved glycine 127 in mouse Fxn to valine results in a failure-to-thrive phenotype in homozygous animals and a substantially reduced number of offspring. Like G130V in FRDA, the G127V mutation results in a dramatic decrease of Fxn protein without affecting transcript synthesis or splicing. FxnG127V mouse embryonic fibroblasts exhibit significantly reduced proliferation and increased cell senescence. These defects are evident in early passage cells and are exacerbated at later passages. Furthermore, increased frequency of mitochondrial DNA lesions and fragmentation are accompanied by marked amplification of mitochondrial DNA in FxnG127V cells. Bioenergetics analyses demonstrate higher sensitivity and reduced cellular respiration of FxnG127V cells upon alteration of fatty acid availability. Importantly, substitution of FxnWT with FxnG127V is compatible with life, and cellular proliferation defects can be rescued by mitigation of oxidative stress via hypoxia or induction of the NRF2 pathway. We propose FxnG127V cells as a simple and robust model for testing therapeutic approaches for FRDA.

Hereditary Ataxia: A Focus on Heme Metabolism and Fe-S Cluster Biogenesis

Heme and Fe-S clusters regulate a plethora of essential biological processes ranging from cellular respiration and cell metabolism to the maintenance of genome integrity. Mutations in genes involved in heme metabolism and Fe-S cluster biogenesis cause different forms of ataxia, like posterior column ataxia and retinitis pigmentosa (PCARP), Friedreich's ataxia (FRDA) and X-linked sideroblastic anemia with ataxia (XLSA/A). Despite great efforts in the elucidation of the molecular pathogenesis of these disorders several important questions still remain to be addressed. Starting with an overview of the biology of heme metabolism and Fe-S cluster biogenesis, the review discusses recent progress in the understanding of the molecular pathogenesis of PCARP, FRDA and XLSA/A, and highlights future line of research in the field. A better comprehension of the mechanisms leading to the degeneration of neural circuity responsible for balance and coordinated movement will be crucial for the therapeutic management of these patients.

Oxidative Stress in DNA Repeat Expansion Disorders: A Focus on NRF2 Signaling Involvement

DNA repeat expansion disorders are a group of neuromuscular and neurodegenerative diseases that arise from the inheritance of long tracts of nucleotide repetitions, located in the regulatory region, introns, or inside the coding sequence of a gene. Although loss of protein expression and/or the gain of function of its transcribed mRNA or translated product represent the major pathogenic effect of these pathologies, mitochondrial dysfunction and imbalance in redox homeostasis are reported as common features in these disorders, deeply affecting their severity and progression. In this review, we examine the role that the redox imbalance plays in the pathological mechanisms of DNA expansion disorders and the recent advances on antioxidant treatments, particularly focusing on the expression and the activity of the transcription factor NRF2, the main cellular regulator of the antioxidant response.

The NRF2 Signaling Network Defines Clinical Biomarkers and Therapeutic Opportunity in Friedreich's Ataxia

Friedreich’s ataxia (FA) is a trinucleotide repeats expansion neurodegenerative disorder, for which no cure or approved therapies are present. In most cases, GAA trinucleotide repetitions in the first intron of the FXN gene are the genetic trigger of FA, determining a strong reduction of frataxin, a mitochondrial protein involved in iron homeostasis. Frataxin depletion impairs iron–sulfur cluster biosynthesis and determines iron accumulation in the mitochondria. Mounting evidence suggests that these defects increase oxidative stress susceptibility and reactive oxygen species production in FA, where the pathologic picture is worsened by a defective regulation of the expression and signaling pathway modulation of the transcription factor NF-E2 p45-related factor 2 (NRF2), one of the fundamental mediators of the cellular antioxidant response. NRF2 protein downregulation and impairment of its nuclear translocation can compromise the adequate cellular response to the frataxin depletion-dependent redox imbalance. As NRF2 stability, expression, and activation can be modulated by diverse natural and synthetic compounds, efforts have been made in recent years to understand if regulating NRF2 signaling might ameliorate the pathologic defects in FA. Here we provide an analysis of the pharmaceutical interventions aimed at restoring the NRF2 signaling network in FA, elucidating specific biomarkers useful for monitoring therapeutic effectiveness, and developing new therapeutic tools.

Developing an objective evaluating system to quantify the degree of upper limb movement impairment in patients with severe Friedreich's ataxia

BACKGROUND

The use of standardized tools and objective measurements is essential to test the effectiveness of new drugs or rehabilitative protocols. Friedreich's ataxia (FRDA) patients with severe disease are often unable to perform the quantitative measurement tests currently used.

AIM

The purpose of our study was to develop an easy-to-use application, for touchscreen devices, able to quantify the degree of upper limb movement impairment in patients with severe Friedreich's ataxia. The APP, which we named "Twelve-Red-Squares App-Coo-Test" (12-RSACT), assesses the upper limb ataxia by measuring the test execution time.

METHODS

All patients were clinically evaluated using the Composite Cerebellar Functional Severity (CCFS) and the Scale for the Assessment and Rating of Ataxia (SARA). We recruited 92 healthy subjects and 36 FRDA patients with a SARA mean value of 28.8.1 ± 8.2. All participants in our study underwent upper limb movement assessment using the new 12-RSACT, the Click Test, and a well-established system, i.e., the Nine-Hole Peg Test (9HPT).

RESULTS

We observed a strong linear correlation between the measurements obtained with the 12-RSACT and those obtained with 9HPT, Click Test, CCFS, and SARA. The 12-RSACT was characterized by excellent internal consistency and intra-rater and test-retest reliability. The minimal detectable change (MDC%) was excellent too. Additionally, the 12-RSACT turned out to be faster and easier to perform compared with the 9HPT.

CONCLUSION

The 12-RSACT is an inexpensive test and is easy to use, which can be administered quickly. Therefore, 12-RSACT is a promising tool to assess the upper limb ataxia in FRDA patients and even those with severe diseases.

Ocular Involvement in Friedreich Ataxia Patients and its Relationship with Neurological Disability, a Follow-up Study

Background: This study compared functional and structural visual changes in Friedreich ataxia (FRDA) patients with healthy controls (HC) and correlated these changes with neurological disability. Methods: Eight FRDA Spanish patients and eight HC were selected from 2014 to 2018. Best corrected visual acuity (BCVA), visual field (VF), optic coherence tomography (OCT), and neurological disability measured by “scale for the assessment and rating of ataxia” (SARA) were taken in a basal exploration and repeated after 6 months. A linear mixed analysis and Bonferroni p-value correction were performed. Results: FRDA baseline and follow-up patients showed statistically significant decreases in BCVA, VF, and OCT parameters compared with the HC. Some of the VF measurements and most of the OCT parameters had an inverse mild-to-strong correlation with SARA. Moreover, the analysis of the ROC curve demonstrated that the peripapillary retinal nerve fiber layer (pRNFL) average thickness was the best parameter to discriminate between FRDA patients and HC. Conclusions: The follow-up study showed a progression in OCT parameters. Findings showed a sequential effect in pRNFL, ganglion cell complex (GCC), and macula. The VF and the OCT could be useful biomarkers in FRDA, both for their correlation with neurological disease as well as for their ability to evaluate disease progression.

The first biallelic missense mutation in the FXN gene in a consanguineous Turkish family with Charcot-Marie-Tooth-like phenotype

Charcot-Marie-Tooth (CMT) disease is the most common inherited neuropathy with a prevalence of 1 in 2500 individuals worldwide. Here, we report three Turkish siblings from consanguineous parents presenting with a CMT-like phenotype who carry a homozygous c.493C>T, p.Arg165Cys mutation in the FXN gene that is the only known causative gene for Friedreich's ataxia (FRDA). The identified missense mutation has been reported previously in two FRDA cases in compound heterozygosity with the common GAA repeat expansion in the first intron of the FXN gene. Analysis of skin biopsy samples from our family indicated that the mutation does not affect the expression levels of the frataxin, pointing to functional impairment of the corresponding protein. The CMT phenotype in the siblings was associated with visual impairment, optic nerve atrophy, and dysarthria. To the best of our knowledge, this family represents the first FXN missense mutation in homozygosity and challenges the notion that missense mutations have not been reported yet due to their embryonic lethality. Furthermore, this finding poses an interesting genetic overlap between autosomal recessive CMT and FRDA that we believe may have important implications on understanding the pathogenesis of these neurological disorders.