Transcriptional Upregulation of the Epigenetically Repressed FXN Gene as a Therapeutic Approach for Friedreich Ataxia

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Friedreich ataxia (FA) is an autosomal recessive disease characterized by progressive damage to the nervous system and severe cardiac abnormalities. The disease is caused by a GAA•TTC triplet repeat expansion in the first intron of the FXN gene (hereafter called the triplet repeat expansion (TRE)-FXN gene), which represses FXN transcription. The FXN gene encodes a protein called frataxin, a ubiquitous, nuclear-encoded mitochondrial protein that plays a key role in iron metabolism. Transcriptional repression of TRE-FXN results in reduced levels of frataxin, leading to mitochondrial dysfunction, which is the underlying basis of the disease. Currently there are no effective treatments for FA. Transcriptional upregulation of the repressed TRE-FXN gene is a potential therapeutic approach for FA that would correct the root cause of the disease rather than a secondary, downstream consequence of the frataxin deficiency. In preliminary experiments, we have identified 10 epigenetic regulators and 9 protein kinases that mediate repression of the TRE-FXN gene, which we refer to as FXN Repressing Factors (FXN-RFs). Inhibition of FXN-RFs by short hairpin RNAs (shRNAs) or small molecules can restore normal levels of FXN mRNA and frataxin in FA induced pluripotent stem cells as well as FA neurons and cardiomyocytes, which are the cell types most relevant to the disease. In addition, we find that upregulating TRE-FXN transcription by FXN-RF inhibition mitigates characteristic mitochondrial defects of FA neurons and cardiomyocytes. These preliminary results provide important proof-of-concept regarding the feasibility of upregulating TRE-FXN transcription as a therapeutic approach for FA. We hypothesize that there are other, yet-to-be-identified FXN-RFs, which may provide more desirable targets for the development of drugs that function by upregulating TRE-FXN transcription. Toward this end, we will screen a large-scale shRNA library and a series of chemical libraries directed against epigenetic regulators to identify new FXN-RFs and small molecule FXN-RF inhibitors, and analyze their ability to upregulate TRE-FXN transcription in FA neurons and cardiomyocytes. The most promising small molecule FXN-RF inhibitors will be analyzed in a humanized mouse model of FA for upregulation of TRE-FXN and amelioration of disease symptoms. Among the FXN-RF inhibitors that will be tested in FA mice are two drugs we identified in preliminary experiments with established safety in human clinical trials. The results of the proposed experiments will provide: (1) a collection of validated and characterized protein targets (FXN-RFs) whose inhibition upregulates TRE-FXN transcription, (2) a set of validated and characterized small molecule FXN-RF inhibitors that may provide lead candidates for pre-clinical development, and (3) a determination of whether the most promising small molecule FXN-RF inhibitors can upregulate TRE-FXN and ameliorate disease symptoms in a humanized mouse model of FA. The results of the proposed experiments are expected to have a major impact on the field of FA therapeutics and have the potential to lead to development of a new class of drugs to treat this devastating disease.