The Friedreich ataxia GAA repeat expansion mutation induces comparable epigenetic changes in human and transgenic mouse brain and heart tissues

Abstract

Friedreich ataxia (FRDA) is caused by a homozygous GAA repeat expansion mutation within intron 1 of the FXN gene, leading to reduced expression of frataxin protein. Evidence suggests that the mutation may induce epigenetic changes and heterochromatin formation, thereby impeding gene transcription. In particular, studies using FRDA patient blood and lymphoblastoid cell lines have detected increased DNA methylation of specific CpG sites upstream of the GAA repeat and histone modifications in regions flanking the GAA repeat. In this report we show that such epigenetic changes are also present in FRDA patient brain, cerebellum and heart tissues, the primary affected systems of the disorder. Bisulfite sequence analysis of the FXN flanking GAA regions reveals a shift in the FRDA DNA methylation profile, with upstream CpG sites becoming consistently hypermethylated and downstream CpG sites becoming consistently hypomethylated. We also identify differential DNA methylation at three specific CpG sites within the FXN promoter and one CpG site within exon 1. Furthermore, we show by chromatin immunoprecipitation analysis that there is overall decreased histone H3K9 acetylation together with increased H3K9 methylation of FRDA brain tissue. Further studies of brain, cerebellum and heart tissues from our GAA repeat expansion-containing FRDA YAC transgenic mice reveal comparable epigenetic changes to those detected in FRDA patient tissue. We have thus developed a mouse model that will be a valuable resource for future therapeutic studies targeting epigenetic modifications of the FXN gene to increase frataxin expression.

DNA Methylation and Trinucleotide Repeat Expansion Diseases

“…Furthermore, the level of DNA methylation in this region correlates with expanded GAA repeat size and inversely correlates with age of FRDA disease onset (Castaldo et al, 2008). Interestingly, DNA hypomethylation has been identified in the FXN intron 1 Alu repeat sequence (which is normally fully methylated) immediately downstream of the expanded GAA repeat (Al-Mahdawi et al, 2008) (Fig.1). This effect of demethylation may have some, as yet unknown, relevance for GAA repeat instability and frataxin expression.…”

“…There is no disease-associated change in the DNA methylation status of the CpG island that spans the FXN 5-UTR and exon 1 regions. However, disease-associated DNA hypermethylation has been identified within a region of FXN intron 1 immediately upstream of the expanded GAA repeat in FRDA cell culture, FRDA patient tissues and FRDA mouse models (Greene et al, 2007;Al-Mahdawi et al, 2008). Furthermore, the level of DNA methylation in this region correlates with expanded GAA repeat size and inversely correlates with age of FRDA disease onset (Castaldo et al, 2008).…”

“…In FRDA, expanded GAA repeats are similarly associated with depletion of CTCF binding in the 5-UTR of the FXN gene and associated hypermethylation of CpG sites just upstream of the GAA repeat. (Greene et al, 2007;Al-Mahdawi et al, 2008;De Biase et al, 2009). However, it is not currently known if depletion of CTCF actually precedes DNA methylation, or vice versa, in the context of TNR expansion diseases.…”

Epigenetics in the Human Brain

“…The 'trait' hypothesis appears very plausible in the context of monogenetic disorders associated with aberrant and excessive repressive DNA and histone methylation in cis (at the site of the mutation). Examples include the aforementioned CGG triplet expansion at the FMR1 (fragile X) gene promoter (Oberle et al, 1991), or the GAA triplet repeat expansion in the first intron of the FRATAXIN gene associated with Friedreich ataxia, an autosomal recessive neurodegenerative condition (Al-Mahdawi et al, 2008). In these cases, the epigenetic dysregulation is firmly linked to the pathophysiology of disease (resulting from silenced gene expression), and there can be little doubt that the observed changes in (postmortem) brain chromatin, like the impairments in neurological functions, most likely existed across the entire lifespan (Al-Mahdawi et al, 2008;Tassone et al, 1999).…”

Epigenetics in the Nervous System: Figure 1.

“…These mechanisms likely contribute to the neurobiology of disease. For example, dysregulation of H3-methyl-lysine 9 was reported in postmortem brain studies of subjects diagnosed with Huntington's disease (Ryu et al, 2006) or Friedreich's Ataxia (Al-Mahdawi et al, 2008), both of which are triplet repeat disorders.…”