Epigenetic therapy for Friedreich ataxia.

Friedreich ataxia (FRDA; Online Mendelian Inheritance in Man database #229300) is an autosomal recessive inherited degenerative disorder affecting the nervous system and the heart, with a prevalence of approximately 2 to 3 in 100,000 in North America and in Europe.1 This neurological syndrome is characterized by progressive trunk and limb ataxia, dysarthria, instability of fixation, sensory neuropathy, and pyramidal weakness. Signs of hypertrophic cardiomyopathy are found in most patients,2 10% have diabetes, and almost all have systemic carbohydrate metabolism abnormalities.3 At the molecular level, >95% of FRDA patients carry a GAA•TTC trinucleotide repeat expansion in the first intron of the FXN gene,4 leading to heterochromatin-mediated transcriptional repression5–9 and reduction of the essential mitochondrial protein frataxin.4 Frataxin is a component of the protein complex that assembles iron-sulfur clusters in mitochondria.10 Its loss leads to impaired mitochondrial function and altered cellular iron homeostasis.11 One therapeutic approach for FRDA is epigenetic modulation of gene expression at the FXN locus through chromatin acetylation by histone deacetylase (HDAC) inhibition.6 A recent report has shown efficacy of the sirtuin inhibitor nicotinamide at high doses in reactivating the FXN gene in blood from patients in a phase I clinical trial, providing support for this therapeutic approach.12 It has been shown previously that HDAC inhibition leads to increased expression of FXN mRNA in patient lymphoblastoid cell lines and peripheral blood mononuclear cells (PBMCs)6,13–15 treated ex vivo. Although in vivo treatment using transgenic animal models that carry expanded GAA•TTC repeats has corroborated the findings in human blood cells, showing increased FXN mRNA and protein in target tissues13,16,17 and reduced disease-related pathology,17 the question remains whether the human target tissue in FRDA, the neuron, would demonstrate the same molecular pathology and response to treatment with a disease-modifying agent as the surrogate tissue, the PBMC. Derivation of neurons from patient-derived induced pluripotent stem cells (iPSCs) is an important new tool to address this question.18,19 Here we demonstrate that HDAC inhibition in vitro via 10913 (under the development name of RG2833 for the formulated drug product) in FRDA neurons derived from patient iPSCs reverses FXN gene silencing to a degree comparable to that found in previous studies employing human PBMCs and mouse models. 6,13,16,17 In these latter studies, brain penetration and HDAC inhibition were established in vivo. We now report reversal of the heterochromatin state and upregulation of FXN mRNA and frataxin protein in these neuronal cells. We also demonstrate HDAC inhibition and increased H3K9 acetylation in PBMCs and an increase in FXN mRNA in blood from patients treated with RG2833. Importantly, we observe that threshold exposures for FXN gene expression changes in vivo are comparable to those observed in vitro with both patient PBMCs and iPSC-derived neurons, validating these cellular systems as valuable tools for projecting effective doses in vivo.