FXN Promoter Silencing in the Humanized Mouse Model of Friedreich Ataxia

Abstract

BackgroundFriedreich ataxia is caused by an expanded GAA triplet-repeat sequence in intron 1 of the FXN gene that results in epigenetic silencing of the FXN promoter. This silencing mechanism is seen in patient-derived lymphoblastoid cells but it remains unknown if it is a widespread phenomenon affecting multiple cell types and tissues.Methodology / Principal FindingsThe humanized mouse model of Friedreich ataxia (YG8sR), which carries a single transgenic insert of the human FXN gene with an expanded GAA triplet-repeat in intron 1, is deficient for FXN transcript when compared to an isogenic transgenic mouse lacking the expanded repeat (Y47R). We found that in YG8sR the deficiency of FXN transcript extended both upstream and downstream of the expanded GAA triplet-repeat, suggestive of deficient transcriptional initiation. This pattern of deficiency was seen in all tissues tested, irrespective of whether they are known to be affected or spared in disease pathogenesis, in both neuronal and non-neuronal tissues, and in cultured primary fibroblasts. FXN promoter function was directly measured via metabolic labeling of newly synthesized transcripts in fibroblasts, which revealed that the YG8sR mouse was significantly deficient in transcriptional initiation compared to the Y47R mouse.Conclusions / SignificanceDeficient transcriptional initiation accounts for FXN transcriptional deficiency in the humanized mouse model of Friedreich ataxia, similar to patient-derived cells, and the mechanism underlying promoter silencing in Friedreich ataxia is widespread across multiple cell types and tissues.

Early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in the KIKO mouse model of Friedreich ataxia

“…Friedreich ataxia (FRDA), the most common autosomal recessive hereditary ataxia, is majorly caused by homozygous expanded guanine-adenine-adenine (GAA) repeats in intron 1 of the frataxin ( FXN ) gene ( Campuzano et al, 1996 ; Lynch et al, 2012 ; Lynch and Seyer, 2014 ). This expansion results in chromatin condensation and reduced expression of frataxin ( Bidichandani et al, 1998 ; Campuzano et al, 1997 ; Chutake et al, 2014 , 2015 ; Grabczyk and Usdin, 2000 ; Li et al, 2015 ). Frataxin is a highly conserved mitochondrial protein crucial for iron-sulphur (FeS) cluster formation and ATP production ( Bulteau et al, 2004 ; Fox et al, 2015 ; Isaya et al, 2004 ; Li et al, 2015 ; Lu and Cortopassi, 2007 ; Napoli et al, 2006 ; Pandolfo, 1999 ; Parent et al, 2015 ; Perdomini et al, 2014 ; Poburski et al, 2016 ; Rötig et al, 1997 ; Söderberg et al, 2016 ; Stemmler et al, 2010 ).…”

The role of oxidative stress in Friedreich's ataxia

“…Neurons derived from the YG8 mouse also showed a reduction in Complex I activity, increased oxidative stress in both mitochondria and cytosol, and lipid peroxidation [55]. These models, in addition to recapitulating the characteristic FRDA phenotype, were also suitable for studying the genetic aspects of the disease, such as GAA repeat instability [56], gene silencing induced by the expansion [57] and novel gene therapy approaches [58,59].…”

Reversal of epigenetic promoter silencing in Friedreich ataxia by a class I histone deacetylase inhibitor

“…A potential caveat of our study is that it explores the corrective role of class I HDAC inhibition in the context of lymphoblastoid cells, a cell type that is not involved in disease pathogenesis. However, class I HDAC inhibition via 2-aminobenzamides (including 109) has been shown to increase FXN transcript levels in many human and murine cell types ( 4 , 18–21 , 31 ) and, moreover, the underlying mechanism of gene silencing in FRDA does not seem to be tissue-specific ( 32 ). The analysis of lymphoblastoid cell lines therefore represents the use of a tractable and well-characterized model system to perform detailed mechanistic studies of drug action on FXN promoter structure and function.…”