Expanded GAA Repeats Impair FXN Gene Expression and Reposition the FXN Locus to the Nuclear Lamina in Single Cells

Hum Mol Genet. 2015 Jun 15;24(12):3457-71. doi: 10.1093/hmg/ddv096. Epub 2015 Mar 26.

Abstract

Abnormally expanded DNA repeats are associated with several neurodegenerative diseases. In Friedreich's ataxia (FRDA), expanded GAA repeats in intron 1 of the frataxin gene (FXN) reduce FXN mRNA levels in averaged cell samples through a poorly understood mechanism. By visualizing FXN expression and nuclear localization in single cells, we show that GAA-expanded repeats decrease the number of FXN mRNA molecules, slow transcription, and increase FXN localization at the nuclear lamina (NL). Restoring histone acetylation reverses NL positioning. Expanded GAA-FXN loci in FRDA patient cells show increased NL localization with increased silencing of alleles and reduced transcription from alleles positioned peripherally. We also demonstrate inefficiencies in transcription initiation and elongation from the expanded GAA-FXN locus at single-cell resolution. We suggest that repressive epigenetic modifications at the expanded GAA-FXN locus may lead to NL relocation, where further repression may occur.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alleles
  • Cell Line
  • Gene Expression*
  • Gene Order
  • Gene Silencing
  • Genetic Loci*
  • Humans
  • Iron-Binding Proteins / genetics*
  • Iron-Binding Proteins / metabolism*
  • Nuclear Lamina / metabolism*
  • Protein Transport
  • RNA, Messenger / genetics
  • Single-Cell Analysis
  • Transcription Initiation Site
  • Transcription Initiation, Genetic
  • Transcription, Genetic
  • Trinucleotide Repeat Expansion*

Substances

  • Iron-Binding Proteins
  • RNA, Messenger
  • frataxin