Epigenetic silencing in Friedreich ataxia is associated with depletion of CTCF (CCCTC-binding factor) and antisense transcription.
BACKGROUND
Over 15 inherited diseases are caused by expansion of triplet-repeats. Friedreich ataxia (FRDA) patients are homozygous for an expanded GAA triplet-repeat sequence in intron 1 of the FXN gene. The expanded GAA triplet-repeat results in deficiency of FXN gene transcription, which is reversed via administration of histone deacetylase inhibitors indicating that transcriptional silencing is at least partially due to an epigenetic abnormality.


RESULTS
We found a severe depletion of the chromatin insulator protein CTCF (CCCTC-binding factor) in the 5'UTR of the FXN gene in FRDA, and coincident heterochromatin formation involving the +1 nucleosome via enrichment of H3K9me3 and recruitment of heterochromatin protein 1. We identified FAST-1 (FXNAntisense Transcript - 1), a novel antisense transcript that overlaps the CTCF binding site in the 5'UTR, which was expressed at higher levels in FRDA. The reciprocal relationship of deficient FXN transcript and higher levels of FAST-1 seen in FRDA was reproduced in normal cells via knockdown of CTCF.


CONCLUSIONS
CTCF depletion constitutes an epigenetic switch that results in increased antisense transcription, heterochromatin formation and transcriptional deficiency in FRDA. These findings provide a mechanistic basis for the transcriptional silencing of the FXN gene in FRDA, and broaden our understanding of disease pathogenesis in triplet-repeat diseases.
De Biase
Irene
I
Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States of America.
Chutake
Yogesh K
YK
Rindler
Paul M
PM
Bidichandani
Sanjay I
SI
D020121:5' Untranslated Regions; D000483:Alleles; D000076246:CCCTC-Binding Factor; D044127:Epigenesis, Genetic; D005347:Fibroblasts / Q000378:metabolism; D051858:Forkhead Transcription Factors / Q000378:metabolism; D005621:Friedreich Ataxia / Q000235:genetics*; D020869:Gene Expression Profiling; D020868:Gene Silencing; D006570:Heterochromatin / Q000378:metabolism; D006801:Humans; D033862:Iron-Binding Proteins / Q000502:physiology*; D016376:Oligonucleotides, Antisense / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics* / Q000502:physiology*; D014158:Transcription, Genetic; D016335:Zinc Fingers; D000096923:Frataxin
D020121:5' Untranslated Regions; D000076246:CCCTC-Binding Factor; D051858:Forkhead Transcription Factors; D006570:Heterochromatin; D033862:Iron-Binding Proteins; D016376:Oligonucleotides, Antisense; D012097:Repressor Proteins; D000096923:Frataxin; C000619697:CTCF protein, human; C510350:FOXH1 protein, human