An RNA-seq study in Friedreich ataxia patients identified hsa-miR-148a-3p as a putative prognostic biomarker of the disease.

Friedreich ataxia (FRDA) is a life-threatening hereditary ataxia; its incidence is 1:50,000 individuals in the Caucasian population. A unique therapeutic drug for FRDA, the antioxidant Omaveloxolone, has been recently approved by the US Food and Drug Administration (FDA). FRDA is a multi-systemic neurodegenerative disease; in addition to a progressive neurodegeneration, FRDA is characterized by hypertrophic cardiomyopathy, diabetes mellitus and musculoskeletal deformities. Cardiomyopathy is the predominant cause of premature death. The onset of FRDA typically occurs between the ages of 5 and 15. Given the complexity and heterogeneity of clinical features and the variability of their onset, the identification of biomarkers capable of assessing disease progression and monitoring the efficacy of treatments is essential to facilitate decision making in clinical practice. We conducted an RNA-seq analysis in peripheral blood mononuclear cells from FRDA patients and healthy donors, identifying a signature of small non-coding RNAs (sncRNAs) capable of distinguishing healthy individuals from the majority of FRDA patients. Among the differentially expressed sncRNAs, microRNAs are a class of small non-coding endogenous RNAs that regulate posttranscriptional silencing of target genes. In FRDA plasma samples, hsa-miR-148a-3p resulted significantly upregulated. The analysis of the Receiver Operating Characteristic (ROC) curve, combining the circulating expression levels of hsa-miR-148a-3p and hsa-miR-223-3p (previously identified by our group), revealed an Area Under the Curve (AUC) of 0.86 (95%, Confidence Interval 0.77-0.95; p-value < 0.0001). An in silico prediction analysis indicated that the IL6ST gene, an interesting marker of neuroinflammation in FRDA, is a common target gene of both miRNAs. Our findings support the evaluation of combined expression levels of different circulating miRNAs as potent epi-biomarkers in FRDA. Moreover, we found hsa-miR-148a-3p significantly over-expressed in Intermediate and Late-Onset Friedreich Ataxia patients' group (IOG and LOG, respectively) compared to healthy individuals, indicating it as a putative prognostic biomarker in this pathology.

Insights into the effects of Friedreich ataxia on the left ventricle using T1 mapping and late gadolinium enhancement.

BACKGROUND: The left ventricular (LV) changes which occur in Friedreich ataxia (FRDA) are incompletely understood. METHODS: Cardiac magnetic resonance (CMR) imaging was performed using a 1.5T scanner in subjects with FRDA who are homozygous for an expansion of an intron 1 GAA repeat in the FXN gene. Standard measurements were performed of LV mass (LVM), LV end-diastolic volume (LVEDV) and LV ejection fraction (LVEF). Native T1 relaxation time and the extracellular volume fraction (ECV) were utilised as markers of left ventricular (LV) diffuse myocardial fibrosis and late gadolinium enhancement (LGE) was utilised as a marker of LV replacement fibrosis. FRDA genetic severity was assessed using the shorter FXN GAA repeat length (GAA1). RESULTS: There were 93 subjects with FRDA (63 adults, 30 children, 54% males), 9 of whom had a reduced LVEF (<55%). A LVEDV below the normal range was present in 39%, a LVM above the normal range in 22%, and an increased LVM/LVEDV ratio in 89% subjects. In adults with a normal LVEF, there was an independent positive correlation of LVM with GAA1, and a negative correlation with age, but no similar relationships were seen in children. GAA1 was positively correlated with native T1 time in both adults and children, and with ECV in adults, all these associations independent of LVM and LVEDV. LGE was present in 21% of subjects, including both adults and children, and subjects with and without a reduced LVEF. None of GAA1, LVM or LVEDV were predictors of LGE. CONCLUSION: An association between diffuse interstitial LV myocardial fibrosis and genetic severity in FRDA was present independently of FRDA-related LV structural changes. Localised replacement fibrosis was found in a minority of subjects with FRDA and was not associated with LV structural change or FRDA genetic severity in subjects with a normal LVEF.

Characterization of clinical serum cardiac biomarker levels in individuals with Friedreich ataxia.

Friedreich ataxia is a progressive autosomal recessive neurodegenerative disorder characterized by ataxia, dyscoordination, and cardiomyopathy. A subset of patients with Friedreich ataxia have elevated levels of serum cardiac troponin I, but associations with disease outcomes and features of cardiomyopathy remain unclear. In this study, we characterized clinically obtained serum cardiac biomarker levels including troponin I, troponin T, and B-type natriuretic peptide in subjects with Friedreich ataxia and evaluated their association with markers of disease. While unprovoked troponin I levels were elevated in 36% of the cohort, cTnI levels associated with a cardiac event (provoked) were higher than unprovoked levels. In multivariate linear regression models, younger age predicted increased troponin I values, and in logistic regression models younger age, female sex, and marginally longer GAA repeat length predicted abnormal troponin I levels. In subjects with multiple assessments, mean unprovoked troponin I levels decreased slightly over time. The presence of abnormal troponin I values and their levels were predicted by echocardiographic measures of hypertrophy. In addition, troponin I levels predicted long-term markers of clinical cardiac dysfunction over time to a modest degree. Consequently, troponin I values provide a marker of hypertrophy but only a minimally predictive biomarker for later cardiac manifestations of disease such as systolic dysfunction or arrhythmia.

Perioperative management and outcomes for posterior spinal fusion in patients with Friedreich ataxia: A single-center, retrospective study.

BACKGROUND: Friedreich ataxia is a rare genetic disorder associated with progressive mitochondrial dysfunction leading to widespread sequelae including ataxia, muscle weakness, hypertrophic cardiomyopathy, diabetes mellitus, and neuromuscular scoliosis. Children with Friedreich ataxia are at high risk for periprocedural complications during posterior spinal fusion due to their comorbidities. AIM: To describe our single-center perioperative management of patients with Friedreich ataxia undergoing posterior spinal fusion. METHODS: Adolescent patients with Friedreich ataxia presenting for spinal deformity surgery between 2007 and 2023 were included in this retrospective case series performed at the Children's Hospital of Philadelphia. Perioperative outcomes were reviewed along with preoperative characteristics, intraoperative anesthetic management, and postoperative medical management. RESULTS: Seventeen patients were included in the final analysis. The mean age was 15 ± 2 years old and 47% were female. Preoperatively, 35% were wheelchair dependent, 100% had mild-to-moderate hypertrophic cardiomyopathy with preserved systolic function and no left ventricular outflow tract obstruction, 29% were on cardiac medications, and 29% were on pain medications. Intraoperatively, 53% had transesophageal echocardiography monitoring; 12% had changes in volume status on echo but no changes in function. Numerous combinations of total intravenous anesthetic agents were used, most commonly propofol, remifentanil, and ketamine. Baseline neuromonitoring signals were poor in four patients and one patient lost signals, resulting in 4 (24%) wake-up tests. The majority (75%) were extubated in the operating room. Postoperative complications were high (88%) and ranged from minor complications like nausea/vomiting (18%) to major complications like hypotension/tachycardia (29%) and need for extracorporeal membrane oxygenation support in one patient (6%). CONCLUSIONS: Patients with Friedreich ataxia are at high risk for perioperative complications when undergoing posterior spinal fusion and coordinated multidisciplinary care is required at each stage. Future research should focus on the utility of intraoperative echocardiography, optimal anesthetic agent selection, and targeted fluid management to reduce postoperative cardiac complications.

Pharmacotherapeutic strategies for Friedreich Ataxia: a review of the available data.

INTRODUCTION: Friedreich ataxia (FRDA) is a rare autosomal recessive disease, marked by loss of coordination as well as impaired neurological, endocrine, orthopedic, and cardiac function. There are many symptomatic medications for FRDA, and many clinical trials have been performed, but only one FDA-approved medication exists. AREAS COVERED: The relative absence of the frataxin protein (FXN) in FRDA causes mitochondrial dysfunction, resulting in clinical manifestations. Currently, the only approved treatment for FRDA is an Nrf2 activator called omaveloxolone (Skyclarys). Patients with FRDA also rely on various symptomatic medications for treatment. Because there is only one approved medication for FRDA, clinical trials continue to advance in FRDA. Although some trials have not met their endpoints, many current and upcoming clinical trials provide exciting possibilities for the treatment of FRDA. EXPERT OPINION: The approval of omaveloxolone provides a major advance in FRDA therapeutics. Although well tolerated, it is not curative. Reversal of deficient frataxin levels with gene therapy, protein replacement, or epigenetic approaches provides the most likely prospect for enduring, disease-modifying therapy in the future.

Evaluating mFARS in pediatric Friedreich's ataxia: Insights from the FACHILD study.

OBJECTIVES: Friedreich ataxia (FRDA) is a rare genetic disorder caused by mutations in the FXN gene, leading to progressive coordination loss and other symptoms. The recently approved omaveloxolone targets this condition but is limited to patients over 16 years of age, highlighting the need for pediatric treatments due to the disorder's early onset and more rapid progression in children. This population also experiences increased non-neurological complications; the FACHILD study aimed to augment and expand the knowledge about the natural history of the disease and clinical outcome assessments for trials in children in FRDA. METHODS: The study enrolled 108 individuals aged 7-18 years with a confirmed FRDA diagnosis, with visits occurring from October 2017 to November 2022 across three institutions. Several measures were introduced to minimize the impact of the COVID-19 pandemic, including virtual visits. Outcome measures centered on the mFARS score and its subscores, and data were analyzed using mixed models for repeated measures. For context and to avoid misinterpretation, the analysis was augmented with data from patients enrolled in the Friedreich's Ataxia Clinical Outcome Measures Study. RESULTS: Results confirmed the general usefulness of the mFARS score in children, but also highlighted issues, particularly with the upper limb subscore (FARS B). Increased variability, limited homogeneity across study subgroups, and potential training effects might limit mFARS application in clinical trials in pediatric populations. INTERPRETATION: The FARS E (Upright Stability) score might be a preferred outcome measure in this patient population.

Emerging therapies for childhood-onset movement disorders.

PURPOSE OF REVIEW: We highlight novel and emerging therapies in the treatment of childhood-onset movement disorders. We structured this review by therapeutic entity (small molecule drugs, RNA-targeted therapeutics, gene replacement therapy, and neuromodulation), recognizing that there are two main approaches to treatment: symptomatic (based on phenomenology) and molecular mechanism-based therapy or 'precision medicine' (which is disease-modifying). RECENT FINDINGS: We highlight reports of new small molecule drugs for Tourette syndrome, Friedreich's ataxia and Rett syndrome. We also discuss developments in gene therapy for aromatic l-amino acid decarboxylase deficiency and hereditary spastic paraplegia, as well as current work exploring optimization of deep brain stimulation and lesioning with focused ultrasound. SUMMARY: Childhood-onset movement disorders have traditionally been treated symptomatically based on phenomenology, but focus has recently shifted toward targeted molecular mechanism-based therapeutics. The development of precision therapies is driven by increasing capabilities for genetic testing and a better delineation of the underlying disease mechanisms. We highlight novel and exciting approaches to the treatment of genetic childhood-onset movement disorders while also discussing general challenges in therapy development for rare diseases. We provide a framework for molecular mechanism-based treatment approaches, a summary of specific treatments for various movement disorders, and a clinical trial readiness framework.

Expression and processing of mature human frataxin after gene therapy in mice.

Friedreich's ataxia is a degenerative and progressive multisystem disorder caused by mutations in the highly conserved frataxin (FXN) gene that results in FXN protein deficiency and mitochondrial dysfunction. While gene therapy approaches are promising, consistent induction of therapeutic FXN protein expression that is sub-toxic has proven challenging, and numerous therapeutic approaches are being tested in animal models. FXN (hFXN in humans, mFXN in mice) is proteolytically modified in mitochondria to produce mature FXN. However, unlike endogenous hFXN, endogenous mFXN is further processed into N-terminally truncated, extra-mitochondrial mFXN forms of unknown function. This study assessed mature exogenous hFXN expression levels in the heart and liver of C57Bl/6 mice 7-10 months after intravenous administration of a recombinant adeno-associated virus encoding hFXN (AAVrh.10hFXN) and examined the potential for hFXN truncation in mice. AAVrh.10hFXN induced dose-dependent expression of hFXN in the heart and liver. Interestingly, hFXN was processed into truncated forms, but found at lower levels than mature hFXN. However, the truncations were at different positions than mFXN. AAVrh.10hFXN induced mature hFXN expression in mouse heart and liver at levels that approximated endogenous mFXN levels. These results suggest that AAVrh.10hFXN can likely induce expression of therapeutic levels of mature hFXN in mice.

Frataxin deficiency shifts metabolism to promote reactive microglia via glucose catabolism.

Immunometabolism investigates the intricate relationship between the immune system and cellular metabolism. This study delves into the consequences of mitochondrial frataxin (FXN) depletion, the primary cause of Friedreich's ataxia (FRDA), a debilitating neurodegenerative condition characterized by impaired coordination and muscle control. By using single-cell RNA sequencing, we have identified distinct cellular clusters within the cerebellum of an FRDA mouse model, emphasizing a significant loss in the homeostatic response of microglial cells lacking FXN. Remarkably, these microglia deficient in FXN display heightened reactive responses to inflammatory stimuli. Furthermore, our metabolomic analyses reveal a shift towards glycolysis and itaconate production in these cells. Remarkably, treatment with butyrate counteracts these immunometabolic changes, triggering an antioxidant response via the itaconate-Nrf2-GSH pathways and suppressing the expression of inflammatory genes. Furthermore, we identify Hcar2 (GPR109A) as a mediator involved in restoring the homeostasis of microglia without FXN. Motor function tests conducted on FRDA mice underscore the neuroprotective attributes of butyrate supplementation, enhancing neuromotor performance. In conclusion, our findings elucidate the role of disrupted homeostatic function in cerebellar microglia in the pathogenesis of FRDA. Moreover, they underscore the potential of butyrate to mitigate inflammatory gene expression, correct metabolic imbalances, and improve neuromotor capabilities in FRDA.

Exploring mitochondrial biomarkers for Friedreich's ataxia: a multifaceted approach.

This study presents an in-depth analysis of mitochondrial enzyme activities in Friedreich's ataxia (FA) patients, focusing on the Electron Transport Chain complexes I, II, and IV, the Krebs Cycle enzyme Citrate Synthase, and Coenzyme Q10 levels. It examines a cohort of 34 FA patients, comparing their mitochondrial enzyme activities and clinical parameters, including disease duration and cardiac markers, with those of 17 healthy controls. The findings reveal marked reductions in complexes II and, specifically, IV, highlighting mitochondrial impairment in FA. Additionally, elevated Neurofilament Light Chain levels and cardiomarkers were observed in FA patients. This research enhances our understanding of FA pathophysiology and suggests potential biomarkers for monitoring disease progression. The study underscores the need for further clinical trials to validate these findings, emphasizing the critical role of mitochondrial dysfunction in FA assessment and treatment.

Generation and characterization of iPSC lines from Friedreich's ataxia patient (FRDA) with GAA.TTC repeat expansion in the Frataxin (FXN) gene's first intron (IGIBi016-A) and a non-FRDA healthy control individual (IGIBi017-A).

Friedreich's ataxia is a spinocerebellar degenerative disease caused by microsatellite (GAA.TTC)n repeat expansion in the first intron of FXN gene. Here, we developed iPSC lines from an FRDA patient (IGIBi016-A) and non-FRDA healthy control (IGIBi017-A). Both iPSC lines displayed typical iPSC morphology, expression of pluripotency markers, regular karyotypes (46, XY; 46, XX), capacity to grow into three germ layers, and FRDA hallmark -GAA repeat expansion and decreased FXN mRNA. Through these iPSC lines, FRDA phenotypes may be replicated in the in vitro assays, by creating neuron subtypes, cardiomyocytes and 3D organoids, for molecular and cellular biomarkers and therapeutic applications.

Cardiopulmonary exercise testing on adaptive equipment in children and adults with Friedreich ataxia.

INTRODUCTION/AIMS: Traditional exercise is often difficult for individuals with Friedreich ataxia (FRDA), and evidence is limited regarding how to measure exercise performance in this population. We evaluated the feasibility, reliability, and natural history of adaptive cardiopulmonary exercise test (CPET) performance in children and adults with FRDA. METHODS: Participants underwent CPET on either an arm cycle ergometer (ACE) or recumbent leg cycle ergometer (RLCE) at up to four visits (baseline, 2 weeks, 4 weeks, and 1 year). Maximum work, oxygen consumption (peak VO2), oxygen (O2) pulse, and anaerobic threshold (AT) were measured in those who reached maximal volition. Test-retest reliability was assessed with intraclass coefficients, and longitudinal change was assessed using regression analysis. RESULTS: In our cohort (N = 23), median age was 18 years (interquartile range [IQR], 14-23), median age of FRDA onset was 8 years (IQR 6-13), median Friedreich Ataxia Rating Scale score was 58 (IQR 54-62), and GAA repeat length on the shorter FXN allele (GAA1) was 766 (IQR, 650-900). Twenty-one (91%) completed a maximal CPET (n = 8, ACE and n = 13, RLCE). Age, sex, and GAA1 repeat length were each associated with peak VO2. Preliminary estimates demonstrated reasonable agreement between visits 2 and 3 for peak work by both ACE and RLCE, and for peak VO2, O2 pulse, and AT by RLCE. We did not detect significant performance changes over 1 year. DISCUSSION: Adaptive CPET is feasible in FRDA, a relevant clinical trial outcome for interventions that impact exercise performance and will increase access to participation as well as generalizability of findings.

The genetic landscape and phenotypic spectrum of GAA-FGF14 ataxia in China: a large cohort study.

BACKGROUND: An intronic GAA repeat expansion in FGF14 was recently identified as a cause of GAA-FGF14 ataxia. We aimed to characterise the frequency and phenotypic profile of GAA-FGF14 ataxia in a large Chinese ataxia cohort. METHODS: A total of 1216 patients that included 399 typical late-onset cerebellar ataxia (LOCA), 290 early-onset cerebellar ataxia (EOCA), and 527 multiple system atrophy with predominant cerebellar ataxia (MSA-c) were enrolled. Long-range and repeat-primed PCR were performed to screen for GAA expansions in FGF14. Targeted long-read and whole-genome sequencing were performed to determine repeat size and sequence configuration. A multi-modal study including clinical assessment, MRI, and neurofilament light chain was conducted for disease assessment. FINDINGS: 17 GAA-FGF14 positive patients with a (GAA)≥250 expansion (12 patients with a GAA-pure expansion, five patients with a (GAA)≥250-[(GAA)n (GCA)m]z expansion) and two possible patients with biallelic (GAA)202/222 alleles were identified. The clinical phenotypes of the 19 positive and possible positive cases covered LOCA phenotype, EOCA phenotype and MSA-c phenotype. Five of six patients with EOCA phenotype were found to have another genetic disorder. The NfL levels of patients with EOCA and MSA-c phenotypes were significantly higher than patients with LOCA phenotype and age-matched controls (p < 0.001). NfL levels of pre-ataxic GAA-FGF14 positive individuals were lower than pre-ataxic SCA3 (p < 0.001) and similar to controls. INTERPRETATION: The frequency of GAA-FGF14 expansion in a large Chinese LOCA cohort was low (1.3%). Biallelic (GAA)202/222 alleles and co-occurrence with other acquired or hereditary diseases may contribute to phenotypic variation and different progression. FUNDING: This study was funded by the National Key R&D Program of China (2021YFA0805200 to H.J.), the National Natural Science Foundation of China (81974176 and 82171254 to H.J.; 82371272 to Z.C.; 82301628 to L.W.; 82301438 to Z.L.; 82201411 to L.H.), the Innovation Research Group Project of Natural Science Foundation of Hunan Province (2020JJ1008 to H.J.), the Key Research and Development Program of Hunan Province (2020SK2064 to H.J.), the Innovative Research and Development Program of Development and Reform Commission of Hunan Province to H.J., the Natural Science Foundation of Hunan Province (2024JJ3050 to H.J.; 2022JJ20094 and 2021JJ40974 to Z.C.; 2022JJ40783 to L.H.; 2022JJ40703 to Z.L.), the Project Program of National Clinical Research Center for Geriatric Disorders (Xiangya Hospital, 2020LNJJ12 to H.J.), the Central South University Research Programme of Advanced Interdisciplinary Study (2023QYJC010 to H.J.) and the Science and Technology Innovation Program of Hunan Province (2022RC1027 to Z.C.). D.P. holds a Fellowship award from the Canadian Institutes of Health Research (CIHR).

Generation of two human induced pluripotent stem cell lines, IGIBi012-A and IGIBi013-A from Friedreich's ataxia (FRDA) patients with homozygous GAA repeat expansion in FXN gene.

Friedreich's ataxia is a neurodegenerative disorder caused by the hyper expansion of (GAA-TTC)n triplet repeats in the first intron of the FXN gene. Here, we generated iPSC lines from two individuals with FRDA, both of whom have homozygous GAA repeat expansion in the first intron of FXN gene. Both iPSC lines demonstrated characteristics of pluripotency, including expression of pluripotency markers, stable karyotypes and ability to develop into all three germ layers, and presence of GAA repeat expansion with reduced FXN mRNA expression. These iPSC lines will serve as invaluable tools for investigating the pathophysiology and phenotypes of FRDA.

Nerve ultrasound in Friedreich's Ataxia: enlarged nerves as a biomarker of disease severity.

OBJECTIVE: In Friedreich's ataxia research, the focus is on discovering treatments and biomarkers to assess disease severity and treatment effects. Our study examines high-resolution nerve ultrasound in these patients, seeking correlations with established clinical markers of disease severity. METHOD: Ten patients with Friedreich's Ataxia underwent a comprehensive clinical assessment with established scales (SARA, FARS, mFARS, INCAT, ADL 0-36, IADL). Additionally, they underwent nerve conduction studies and high-resolution nerve ultrasound. Quantitative evaluation of nerve cross-sectional area, conducted at 24 nerve sites using high-resolution nerve ultrasound, was compared with data obtained from 20 healthy volunteers. RESULTS: All the patients had a severe sensory axonal neuropathy. High-resolution nerve ultrasound showed significant increase, in cross sectional area, of median and ulnar nerves at the axilla and arm. The cumulative count of affected nerve sites was directly associated with clinical disability, as determined by SARA, FARS, mFARS, ADL 0-36, and INCAT score, while displaying an inverse correlation with IADL. CONCLUSIONS: Our study shows that high-resolution ultrasound reveals notable nerve abnormalities, primarily in the upper limbs of patients diagnosed with Friedreich's Ataxia. The observed correlation between these nerve abnormalities and clinical disability scales indicates the potential use of this technique as a biomarker for evaluating disease severity and treatment effects. SIGNIFICANCE: Nerve Ultrasound is a potential biomarker of disease severity in Friedreich's Ataxia.

Safety, pharmacokinetics, and pharmacodynamics of nomlabofusp (CTI-1601) in Friedreich's ataxia.

OBJECTIVE: Current treatments for Friedreich's ataxia, a neurodegenerative disorder characterized by decreased intramitochondrial frataxin, do not address low frataxin concentrations. Nomlabofusp (previously CTI-1601) is a frataxin replacement therapy with a unique mechanism of action that directly addresses this underlying frataxin deficiency. Phase 1 studies assessed the safety, pharmacokinetic, and pharmacodynamic profiles of subcutaneously administered nomlabofusp in adults with Friedreich's ataxia. METHODS: Patients were enrolled in two Phase 1, double-blind, placebo-controlled studies. The single ascending-dose (SAD) study (NCT04176991) evaluated single doses of nomlabofusp (25, 50, 75, or 100 mg) or placebo. The multiple ascending-dose (MAD) study (NCT04519567) evaluated nomlabofusp (25 mg daily for 4 days then every third day, 50 mg daily for 7 days then every 2 days, or 100 mg daily) or placebo for 13 days. RESULTS: Patients aged 19-69 years were enrolled (SAD, N = 28; MAD, N = 27). Nomlabofusp was generally well tolerated through 13 days. Most adverse events were mild and resolved quickly. No serious adverse events or deaths were reported. Peak nomlabofusp plasma concentrations occurred 15 min after subcutaneous administration. Nomlabofusp plasma exposures increased with increasing doses and daily administration and decreased with reduced dosing frequency. Increased frataxin concentrations were observed in buccal cells, skin, and platelets with higher and more frequent nomlabofusp administration. INTERPRETATION: Results from this study support a favorable safety profile for nomlabofusp. Subcutaneous nomlabofusp injections were quickly absorbed; higher doses and daily administration resulted in increased tissue frataxin concentrations. Future studies will evaluate longer-term safety and possible efficacy of nomlabofusp.

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.