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Normal phenotype in two brothers with a full FMR1 mutation

H J Smeets1, A P Smits, C E Verheij

  • 1Department of Human Genetics, University Hospital Nijmegen, The Netherlands.

Human Molecular Genetics
|November 1, 1995
PubMed
Summary

Fragile X syndrome is linked to FMR1 gene CGG repeats. In normal individuals, unmethylated FMR1 allows gene expression, showing gene inactivation, not just repeat expansion, causes fragile X.

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Area of Science:

  • Genetics
  • Molecular Biology
  • Neurodevelopmental Disorders

Background:

  • Fragile X syndrome is characterized by CGG repeat expansion in the FMR1 gene's 5' untranslated region.
  • Promoter methylation typically silences FMR1 gene expression, leading to the fragile X phenotype.

Observation:

  • Two clinically unaffected brothers presented with large CGG repeats and visible fragile sites.
  • Crucially, their FMR1 gene promoter regions remained unmethylated.
  • Both FMR1 RNA and protein were detectable in these individuals.

Findings:

  • FMR1 gene inactivation, rather than repeat expansion alone, is the key determinant of the fragile X phenotype.
  • CGG repeat expansion does not invariably lead to promoter methylation.
  • Methylation is not an absolute prerequisite for the development of fragile sites.

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Implications:

  • This research clarifies the molecular mechanisms underlying fragile X syndrome.
  • It suggests potential therapeutic targets focused on preventing or reversing FMR1 gene silencing.
  • Understanding the role of methylation provides new insights into fragile X-associated disorders.