Friedreich's ataxia (FRDA) is caused by the epigenetic silencing of the FXN gene resulting in the deficiency of frataxin, a critical component of the iron-sulfur clusters synthesis pathway. Transcriptional repression of FXN results from large expansions of the intronic GAA repeats and can be partially reversed by modulating the epigenetic environment. The exact trigger of transcriptional inhibition and the mechanisms of epigenetic changes that occur in Friedreich's ataxia remain unknown.
The aim of this application is to define mechanisms of epigenetic silencing induced by expanded GAA repeats, which could be used as potential targets for therapy of Friedreich's ataxia. We will address three fundamental aspects of the molecular pathogenesis responsible for Friedreich's ataxia: 1) What triggers the epigenetic changes in the mutant FXN locus? 2) What is the primary chromatin modification pathway involved in silencing? 3) What factors controlling the expression of the WT FXN gene are affected by the GAA expansion in FRDA? In order to answer these questions and to define the mechanism of the epigenetic silencing in the disease-relevant cellular models we generated FRDA and control induced pluripotent stem cell lines (iPSCs) and differentiated them to neuronal cells. We will use these novel FRDA models to test our hypothesis that the expansion of GAA repeats initiates a cascade of events that begins with DNA conformational changes within the repeats, followed by epigenetic changes in the sequences flanking the GAAs, which consequently leads to deregulation of FXN expression. First, we will identify mechanisms controlling expression of the FXN gene in physiological conditions. Based on our preliminary findings, we will define the roles of GCN5 histone acetyltransferase and c-MYC transcription factor in regulating FXN expression. Next, we will identify the trigger for GAA repeats-induced transcription by defining the conformation of the expanded GAA repeats in their natural context of the FXN gene. We will also determine a link between formation of the non canonical conformations, extent of epigenetic silencing and length of the GAA repeats. Furthermore, we will employ somatic cell reprogramming to iPSCs in the presence of various epigenetic modulators to discern the contributions of chromatin modification pathways to the GAA repeats-mediated silencing. Collectively, these experiments will define the epigenetic control of FXN expression, as well as the molecular mechanisms leading to its deregulation and silencing that occur in Friedreich's ataxia. Our combined approach of genome editing and pharmacological modulation of the epigenome during somatic cell reprogramming will fuel development of new therapeutic approaches for FRDA.

Public Health Relevance

Friedreich's ataxia, the most common inherited ataxia, is a severe neurological disease caused by transcriptional silencing of the FXN gene. The proposed studies will identify the molecular mechanisms leading to the epigenetic silencing of this gene. This work will uncover new avenues for therapies of Friedreich's ataxia and other repeat expansion diseases.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Research Project (R01)
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Molecular Neurogenetics Study Section (MNG)
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Gubitz, Amelie
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University of Alabama Birmingham
Schools of Medicine
United States
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Long, Ashlee; Napierala, Jill S; Polak, Urszula et al. (2017) Somatic instability of the expanded GAA repeats in Friedreich's ataxia. PLoS One 12:e0189990
Clark, Elisia; Butler, Jill S; Isaacs, Charles J et al. (2017) Selected missense mutations impair frataxin processing in Friedreich ataxia. Ann Clin Transl Neurol 4:575-584
Napierala, Jill Sergesketter; Li, Yanjie; Lu, Yue et al. (2017) Comprehensive analysis of gene expression patterns in Friedreich's ataxia fibroblasts by RNA sequencing reveals altered levels of protein synthesis factors and solute carriers. Dis Model Mech 10:1353-1369
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Butler, Jill Sergesketter; Napierala, Marek (2016) New Reasons to Pursue the Therapeutic Potential of Synthetic Nucleic Acids for Neurological Diseases. JAMA Neurol 73:1175-1177
Li, Yanjie; Polak, Urszula; Bhalla, Angela D et al. (2015) Excision of Expanded GAA Repeats Alleviates the Molecular Phenotype of Friedreich's Ataxia. Mol Ther 23:1055-1065
Li, Yanjie; Lu, Yue; Polak, Urszula et al. (2015) Expanded GAA repeats impede transcription elongation through the FXN gene and induce transcriptional silencing that is restricted to the FXN locus. Hum Mol Genet 24:6932-43

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