CGG Repeat-Induced FMR1 Silencing Depends on the Expansion Size in Human iPSCs and Neurons Carrying Unmethylated Full Mutations

Stem Cell Reports. 2016 Dec 13;7(6):1059-1071. doi: 10.1016/j.stemcr.2016.10.004. Epub 2016 Nov 10.


In fragile X syndrome (FXS), CGG repeat expansion greater than 200 triplets is believed to trigger FMR1 gene silencing and disease etiology. However, FXS siblings have been identified with more than 200 CGGs, termed unmethylated full mutation (UFM) carriers, without gene silencing and disease symptoms. Here, we show that hypomethylation of the FMR1 promoter is maintained in induced pluripotent stem cells (iPSCs) derived from two UFM individuals. However, a subset of iPSC clones with large CGG expansions carries silenced FMR1. Furthermore, we demonstrate de novo silencing upon expansion of the CGG repeat size. FMR1 does not undergo silencing during neuronal differentiation of UFM iPSCs, and expression of large unmethylated CGG repeats has phenotypic consequences resulting in neurodegenerative features. Our data suggest that UFM individuals do not lack the cell-intrinsic ability to silence FMR1 and that inter-individual variability in the CGG repeat size required for silencing exists in the FXS population.

Keywords: CGG repeat; DNA methylation; FMR1; de novo silencing; fragile X syndrome; fragile X tremor ataxia syndrome; neuron; triplet expansion; ubiquitin inclusion; unmethylated full mutation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cell Differentiation / genetics
  • Clone Cells
  • DNA Methylation / genetics*
  • Epigenesis, Genetic
  • Female
  • Fragile X Mental Retardation Protein / genetics*
  • Fragile X Syndrome / genetics
  • Gene Silencing*
  • Genetic Loci
  • Humans
  • Induced Pluripotent Stem Cells / cytology
  • Induced Pluripotent Stem Cells / metabolism*
  • Male
  • Mutation / genetics*
  • Neurons / metabolism*
  • Pedigree
  • Trinucleotide Repeat Expansion / genetics*


  • Fragile X Mental Retardation Protein