Review
Using human pluripotent stem cells to study Friedreich ataxia cardiomyopathy

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Highlights

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    We discuss Friedreich Ataxia cardiomyopathy.

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    We summarise experimental models to study Friedreich Ataxia cardiomyopathy.

  • •

    We focus on the use of human pluripotent stem cells as a disease model.

Abstract

Friedreich ataxia (FRDA) is the most common of the inherited ataxias. It is an autosomal recessive disease characterised by degeneration of peripheral sensory neurons, regions of the central nervous system and cardiomyopathy. FRDA is usually due to homozygosity for trinucleotide GAA repeat expansions found within first intron of the FRATAXIN (FXN) gene, which results in reduced levels of the mitochondrial protein FXN. Reduced FXN protein results in mitochondrial dysfunction and iron accumulation leading to increased oxidative stress and cell death in the nervous system and heart. Yet the precise functions of FXN and the underlying mechanisms leading to disease pathology remain elusive. This is particularly true of the cardiac aspect of FRDA, which remains largely uncharacterized at the cellular level. Here, we summarise current knowledge on experimental models in which to study FRDA cardiomyopathy, with a particular focus on the use of human pluripotent stem cells as a disease model.

Introduction

Friedreich ataxia (FRDA) was first described over 150 years ago in a series of papers by physician Nikolaus Friedreich. It is a hereditary degenerative condition that includes neurological and non-neurological symptoms (reviewed in [1]). The predominant neuronal manifestations occur through degeneration of dorsal root ganglia, cerebellar neurons and long tracts of the spinal cord leading to development of ataxia, dysarthia and areflexia of the lower limbs. The main cause of death in FRDA is cardiomyopathy. Approximately 96% of individuals with FRDA are homozygous for an unstable expanded GAA repeat mutation within the first intron of FXN [2]. The remaining 4% are compound heterozygous for a GAA expansion in one allele and point mutation/deletion in the other. FXN is a nuclear-encoded mitochondrial protein [3]. The intron 1 GAA expansion interferes with transcription and results in reduced amounts of structurally normal FXN [4]. This interference is thought to be caused by abnormal DNA structures at the site of the GAA repeats as well as aberrant methylation and altered chromatin formation [5]. Furthermore, studies in patient cohorts demonstrated that the length of the GAA repeats, and in particular the shorter of the two alleles, correlate with disease severity whilst being inversely correlated with the age of onset and FXN protein levels [3], [6].

Section snippets

Cardiac manifestations of FRDA

Cardiomyopathy in individuals with FRDA is usually hypertrophic, with dilated cardiomyopathy being an inconsistent and generally late manifestation. Arrhythmias are also common [7]. In cross-sectional studies, about two-thirds of individuals with FRDA have cardiac hypertrophy on echocardiogram [6], [8], [9]. The most common findings on echocardiography are increased relative wall thickness with left ventricular wall thickness and left ventricular mass index also commonly seen [9]. Diastolic

Functions of FXN

The precise functions of FXN are not clearly understood. Nuclear-encoded FXN protein is synthesised as an immature form that is transported to the mitochondria where it is cleaved by mitochondrial processing peptidase to become a mature protein [19]. Immunoprecipitation and yeast 2-hybrid systems have provided evidence that FXN interacts with the ISCU/NFS1/ISD11 iron–sulphur cluster assembly, which forms a complex that synthesises iron–sulphur clusters [20], [21] (Fig. 1). Iron–sulphur cluster

Models of FRDA

Studies of oxidative stress in Saccharomyces cerevisiae null mutants, lacking the yeast fxn homologue, demonstrate mitochondrial iron accumulation, oxidative stress and deficiencies of iron–sulphur-cluster-containing complexes I–III due to a reduction of iron–sulphur clusters [25], [26]. Models generated by RNA interference (RNAi) knock-down of the fxn homologue in Caenorhabditis elegans are contradictory, with reports of both increased and reduced motility and lifespan [27], [28]. RNAi

Human cell lines

Animal models of FRDA do not recapitulate many aspects of the onset, severity and progression of the human condition [29], [30], [40], [41], [42]. The establishment of FRDA cell lines from a range of primary cell types — such as fibroblasts, keratinocytes and lymphoblasts — to genetically modify human cells either lacking or with reduced FXN expression either by knockdown or with GAA repeats have allowed for study of FXN in human systems [5], [43], [44], [45], [46], [47]. In vitro study of

Human embryonic stem cells

Human embryonic stem cells (hESCs) are isolated from the inner cell mass of pre-implantation blastocysts and are pluripotent, i.e. have the ability to differentiate into any cell type of the body, bringing great potential to model human development and diseases as well as a potential source of differentiated cells for replacement therapy [53], [54], [55], [56], [57]. However, in order to study genetic conditions, one would need to introduce genetic mutations to established hESCs or derive novel

Modelling FRDA using iPSCs

Modelling of genetic cardiac disorders using iPSCs has already been reported for long QT syndrome and catecholaminergic polymorphic ventricular tachycardia [81], [82], [83], which involve ion channel mutations. More recently, a report of iPSCs modelling arrhythmogenic right ventricular dysplasia/cardiomyopathy, which has a mean disease onset of 26 years [84], provides evidence that the relatively immature cardiomyocytes derived from iPSCs [71] are capable of modelling disease of mature

Conclusion

FRDA is the most common of the inherited ataxias and affects many organs and systems leading to disability. Most individuals with FRDA have a reduced lifespan usually due to complications of cardiomyopathy. Currently there is no treatment to effectively cure, halt or even slow disease progression. In order to identify more effective treatments, disease models that effectively recapitulate FRDA pathology must be developed to allow for meaningful drug screening. There are a number of FRDA animal

Conflict of interest

The authors report no relationships that could be construed as a conflict of interest.

Acknowledgements

This work was supported by grants from the Friedrich Ataxia Research Alliance, a National Health and Medical Research Council (NHMRC) — CSL Gustav Nossal postgraduate research scholarship (DEC), an Australian Research Council (ARC) Future Fellowship (AP, FT140100047), an ARC special Initiative Stem Cells Australia grant (MFP, AP) and Operational Infrastructure Support from the Victorian Government.

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