CNS expression of murine fragile X protein (FMRP) as a function of CGG-repeat size

Hum Mol Genet. 2014 Jun 15;23(12):3228-38. doi: 10.1093/hmg/ddu032. Epub 2014 Jan 23.


Large expansions of a CGG-repeat element (>200 repeats; full mutation) in the fragile X mental retardation 1 (FMR1) gene cause fragile X syndrome (FXS), the leading single-gene form of intellectual disability and of autism spectrum disorder. Smaller expansions (55-200 CGG repeats; premutation) result in the neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). Whereas FXS is caused by gene silencing and insufficient FMR1 protein (FMRP), FXTAS is thought to be caused by 'toxicity' of expanded-CGG-repeat mRNA. However, as FMRP expression levels decrease with increasing CGG-repeat length, lowered protein may contribute to premutation-associated clinical involvement. To address this issue, we measured brain Fmr1 mRNA and FMRP levels as a function of CGG-repeat length in a congenic (CGG-repeat knock-in) mouse model using 57 wild-type and 97 expanded-CGG-repeat mice carrying up to ~250 CGG repeats. While Fmr1 message levels increased with repeat length, FMRP levels trended downward over the same range, subject to significant inter-subject variation. Human comparisons of protein levels in the frontal cortex of 7 normal and 17 FXTAS individuals revealed that the mild FMRP decrease in mice mirrored the more limited data for FMRP expression in the human samples. In addition, FMRP expression levels varied in a subset of mice across the cerebellum, frontal cortex, and hippocampus, as well as at different ages. These results provide a foundation for understanding both the CGG-repeat-dependence of FMRP expression and for interpreting clinical phenotypes in premutation carriers in terms of the balance between elevated mRNA and lowered FMRP expression levels.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Cerebellum / metabolism*
  • Disease Models, Animal
  • Female
  • Fragile X Mental Retardation Protein / genetics*
  • Fragile X Mental Retardation Protein / metabolism*
  • Fragile X Syndrome / genetics
  • Fragile X Syndrome / pathology*
  • Frontal Lobe / metabolism*
  • Gene Expression Regulation
  • Hippocampus / metabolism*
  • Humans
  • Male
  • Mice
  • Organ Specificity
  • RNA, Messenger / metabolism
  • Trinucleotide Repeat Expansion*


  • RNA, Messenger
  • Fragile X Mental Retardation Protein