HMTase Inhibitors as a Potential Epigenetic-Based Therapeutic Approach for Friedreich’s Ataxia

Abstract

Friedreich's ataxia (FRDA) is a progressive neurodegenerative disorder caused by a homozygous GAA repeat expansion mutation in intron 1 of the frataxin gene (FXN), which instigates reduced transcription. As a consequence, reduced levels of frataxin protein lead to mitochondrial iron accumulation, oxidative stress, and ultimately cell death; particularly in dorsal root ganglia (DRG) sensory neurons and the dentate nucleus of the cerebellum. In addition to neurological disability, FRDA is associated with cardiomyopathy, diabetes mellitus, and skeletal deformities. Currently there is no effective treatment for FRDA and patients die prematurely. Recent findings suggest that abnormal GAA expansion plays a role in histone modification, subjecting the FXN gene to heterochromatin silencing. Therefore, as an epigenetic-based therapy, we investigated the efficacy and tolerability of two histone methyltransferase (HMTase) inhibitor compounds, BIX0194 (G9a-inhibitor) and GSK126 (EZH2-inhibitor), to specifically target and reduce H3K9me2/3 and H3K27me3 levels, respectively, in FRDA fibroblasts. We show that a combination treatment of BIX0194 and GSK126, significantly increased FXN gene expression levels and reduced the repressive histone marks. However, no increase in frataxin protein levels was observed. Nevertheless, our results are still promising and may encourage to investigate HMTase inhibitors with other synergistic epigenetic-based therapies for further preliminary studies.

Skin fibroblast metabolomic profiling reveals that lipid dysfunction predicts the severity of Friedreich’s ataxia

“…Because fibroblasts are not directly involved in disease pathology, but recapitulate so well the increased fatty acid oxidation, this compensatory mechanism could be further investigated in combination with histone deacetylation inhibitors that increase frataxin gene expression ( 53 ) where we could monitor how acetyl-CoA levels are affected. Recent advances in FRDA research have revealed the presence of several epigenetic modifications, including hypoacetylation and hypermethylation that are involved in this FXN gene silencing ( 54 , 55 ). The decreased ratio of PC to PE resulting from the lack of PE methylation could lead to the accumulation of S-adenosylmethionine (methyl donor) involved in the histone hypermethylation ( 32 ).…”

“…The decreased ratio of PC to PE resulting from the lack of PE methylation could lead to the accumulation of S-adenosylmethionine (methyl donor) involved in the histone hypermethylation (32). Increased methylation of histone residues (H3K9me2/3 and H3K27me3 ) had been observed in other FRDA fibroblasts cell lines (55). Therefore, the ratio of PC to PE in fibroblasts may reflect the hypermethylation status in FRDA patients.…”

Selected Histone Deacetylase Inhibitors Reverse the Frataxin Transcriptional Defect in a Novel Friedreich’s Ataxia Induced Pluripotent Stem Cell-Derived Neuronal Reporter System

“…A clear efficacy of specific HDACi (e.g., RG109) has been recurrently demonstrated in various model systems (Sandi et al, 2014;Soragni et al, 2014;Chutake et al, 2016;Codazzi et al, 2016). Targeting the HMT G9a affected histone methylation at FXN but did not increase its expression in FRDA lymphoblasts and fibroblasts (Punga and Bühler, 2010;Sherzai et al, 2020). On the other hand, a recent study targeting G9a and EZH2 HMTs reported, in addition to changes in H3K9me2/me3 and H3K27me3 levels, an increase of FXN mRNA expression in FRDA fibroblasts but without noticeable effects on FXN protein levels (Sherzai et al, 2020).…”

“…Targeting the HMT G9a affected histone methylation at FXN but did not increase its expression in FRDA lymphoblasts and fibroblasts (Punga and Bühler, 2010;Sherzai et al, 2020). On the other hand, a recent study targeting G9a and EZH2 HMTs reported, in addition to changes in H3K9me2/me3 and H3K27me3 levels, an increase of FXN mRNA expression in FRDA fibroblasts but without noticeable effects on FXN protein levels (Sherzai et al, 2020). This may be a result of an FRDA-specific patient cell response, as the length of the GAAs may influence the efficacy of some small molecules (Punga and Bühler, 2010).…”

“…FXN expression is upregulated by treatment with histone deacetylase inhibitors (HDACi) ( Bürk, 2017 ), especially RG109 (also known as RG2833) ( Soragni et al, 2014 ; Chutake et al, 2016 ; Codazzi et al, 2016 ), nicotinamide, and resveratrol ( Chan et al, 2013 ; Li et al, 2013 ; Georges et al, 2019 ). In addition, treating FRDA cells with histone methyltransferase (HMT) inhibitors that reduce H3K9me2/3, H3K27me3 ( Sherzai et al, 2020 ), and H4K20me2/3 ( Vilema-Enríquez et al, 2020 ) demonstrated modest increases in FXN expression, indicating a role for histone methylation also in repressing FXN transcription.…”