Characterizing cardiac phenotype in Friedreich's ataxia: The CARFA study

Frataxin deficiency and the pathology of Friedreich's Ataxia across tissues

Friedreich's Ataxia (FRDA) is a neurodegenerative disease that affects a variety of different organ systems. The disease is caused by GAA repeat expansions in intron 1 of the Frataxin gene (FXN), which results in a decrease in the expression of the FXN protein. FXN is needed for the biogenesis of iron-sulfur clusters (ISC) which are required by key metabolic processes in the mitochondria. Without ISCs those processes do not occur properly. As a result, reactive oxygen species accumulate, and the mitochondria cease to function. Iron is also thought to accumulate in the cells of certain tissue types. These processes are thought to be intimately related to the pathologies affecting a myriad of tissues in FRDA. Most FRDA patients suffer from loss of motor control, cardiomyopathy, scoliosis, foot deformities, and diabetes. In this review, we discuss the known features of FRDA pathology and the current understanding about the basis of these alterations.

Poincaré plot analysis of electrocardiogram uncovers beneficial effects of omaveloxolone in a mouse model of Friedreich's ataxia

BACKGROUND

Friedreich's ataxia (FA) is a rare inherited neuromuscular disorder whereby most patients die of lethal cardiomyopathy and arrhythmias. Mechanisms leading to arrhythmic events in patients with FA are poorly understood.

OBJECTIVE

This study aimed to examine cardiac electrical signal propagation in a mouse model of FA with severe cardiomyopathy and to evaluate effects of omaveloxolone (OMAV), the first Food and Drug Administration-approved therapy.

METHODS

Cardiac-specific MCK-Cre frataxin knockout (FXN-cKO) mice were used to mimic FA cardiomyopathy. In vivo surface electrocardiogram (ECG) recordings, Western blotting, quantitative real-time polymerase chain reaction analysis, and histochemistry were performed.

RESULTS

Characteristics like long QT syndrome, interatrial block, and ST-segment abnormalities in patients with FA were identified in FXN-cKO mice. FXN-cKO mice exhibited sexual dimorphism in electrical signal propagation and cardiac structural integrity. Untreated FA males showed increased ventricular propagation intervals, whereas females exhibited delayed atrial propagation. OMAV showed no significant therapeutic effect on average ECG time intervals but improved chamber-specific waveforms when aggregated frequency distributions were analyzed. The J wave was absent in FXN-cKO male mice but reappeared with OMAV treatment. Poincaré plots revealed disparate idiopathic arrhythmias with multi-clustering events in individual mice with high incidence in FXN-cKO males. OMAV treatment reduced multi-clustering events to a single cluster; however, autonomic nervous system dysfunction still remained.

CONCLUSION

Our study revealed significant electrical propagation disturbances and sexual dimorphism in FXN-cKO mice with severe cardiomyopathy. Poincaré plots identified irregularities in heart rhythm and autonomic nervous system dysfunction. OMAV improved heart function by stabilizing early repolarization and reducing disparate arrhythmias. This work stresses sex-specific ECG interpretations and alternative mathematical approaches for drug testing in FA models.

Ferroptosis in diabetic cardiomyopathy: from its mechanisms to therapeutic strategies

Diabetic cardiomyopathy (DCM) is defined as structural and functional cardiac abnormalities in diabetes, and cardiomyocyte death is the terminal event of DCM. Ferroptosis is iron-dependent oxidative cell death. Evidence has indicated that iron overload and ferroptosis play important roles in the pathogenesis of DCM. Mitochondria, an important organelle in iron homeostasis and ROS production, play a crucial role in cardiomyocyte ferroptosis in diabetes. Studies have shown some anti-diabetic medicines, plant extracts, and ferroptosis inhibitors might improve DCM by alleviating ferroptosis. In this review, we systematically reviewed the evidence of ferroptosis in DCM. Anti-ferroptosis might be a promising therapeutic strategy for the treatment of DCM.

Sexual dimorphism in a mouse model of Friedreich's ataxia with severe cardiomyopathy

Friedreich's ataxia (FA) is an autosomal recessive disorder caused by reduced frataxin (FXN) expression in mitochondria, where the lethal component is cardiomyopathy. Using the conditional Fxnflox/null::MCK-Cre knock-out (Fxn-cKO) mouse model, we discovered significant sex differences in the progression towards heart failure, with Fxn-cKO males exhibiting a worse cardiac phenotype, low survival rate, kidney and reproductive organ deficiencies. These differences are likely due to a decline in testosterone in Fxn-cKO males. The decrease in testosterone was related to decreased expression of proteins involved in cholesterol transfer into the mitochondria: StAR and TSPO on the outer mitochondrial membrane, and the cholesterol side-chain cleavage enzyme P450scc and ferredoxin on the inner mitochondrial membrane. Expression of excitation-contraction coupling proteins (L-type calcium channel, RyR2, SERCA2, phospholamban and CaMKIIδ) was decreased significantly more in Fxn-cKO males. This is the first study that extensively investigates the sexual dimorphism in FA mouse model with cardiac calcium signaling impairment.

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.

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.

Management of Friedreich Ataxia-Associated Cardiomyopathy in Pregnancy: A Review of the Literature

A major manifestation of Friedreich ataxia (FRDA) is cardiomyopathy, caused by mitochondrial proliferation in myocytes. Because the lifespan for patients with FRDA improves with better treatment modalities, more patients are becoming pregnant, meaning that more medical providers must know how to care for this population. This report provides a review of the literature on multidisciplinary management of pregnant patients with FRDA and cardiomyopathy from preconception through lactation. A cardio-obstetrics team, including cardiology, anesthesiology, and obstetrics, should be involved for this entire period. All patients should be counseled on pregnancy risk using elements of existing stratification systems, and contraception should be discussed, highlighting the safety of intrauterine devices. Electrocardiogram should be obtained at baseline and each trimester, looking for atrial arrhythmias and ST-segment changes, as should transthoracic echocardiogram, with a focus on left ventricular ejection fraction-which is typically normal in FRDA cardiomyopathy-and relative wall thickness and global longitudinal strain-which tend to decrease as cardiomyopathy progresses. Brain natriuretic peptide is also a helpful marker to detect adverse events. If heart failure develops, it should be treated like any other etiology of heart failure during pregnancy. Atrial arrhythmias should be treated with β blockers or electrical cardioversion and anticoagulation, as necessary. Most patients with FRDA can deliver vaginally, and neuraxial analgesia is recommended during labor because of the risks associated with general anesthesia. Breastfeeding is encouraged, even for those taking cardiac medications.

Friedreich ataxia: clinical features and new developments

Friedreich's ataxia (FRDA), a neurodegenerative disease characterized by ataxia and other neurological features, affects 1 in 50,000-100,000 individuals in the USA. However, FRDA also includes cardiac, orthopedic and endocrine dysfunction, giving rise to many secondary disease characteristics. The multifaceted approach for clinical care has necessitated the development of disease-specific clinical care guidelines. New developments in FRDA include the advancement of clinical drug trials targeting the NRF2 pathway and frataxin restoration. Additionally, a novel understanding of gene silencing in FRDA, reflecting a variegated silencing pattern, will have applications to current and future therapeutic interventions. Finally, new perspectives on the neuroanatomy of FRDA and its developmental features will refine the time course and anatomical targeting of novel approaches.