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FMR1 and the Fragile X Syndrome 

FMR1 and the Fragile X Syndrome
Chapter:
FMR1 and the Fragile X Syndrome
Author(s):

Kathryn B. Garber

, Jeannie Visootsak

, and Stephen T. Warren

DOI:
10.1093/med/9780199934522.003.0158
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date: 28 February 2021

Fragile X syndrome is an X-linked dominant disorder associated with mild to severe intellectual disability and behavioral problems that are caused by defects in synaptic development. These defects result from a lack of the Fragile X Mental Retardation Protein, FMRP, which is a selective RNA-binding protein that regulates translation of neuronal mRNAs. FMRP is regulated by group 1 metabotropic glutamate receptor activation, which stimulates the translation of mRNAs to which FMRP is bound. In its absence, these transcripts are translated in excess, leading to changes in synaptic plasticity that play a large part in the fragile X syndrome phenotype. The absence of FMRP is most often caused by the transcriptional silencing of the FMR1 gene due to a CGG-repeat expansion in the 5’-untranslated region of the gene, an allele class known as a full mutation. Besides fragile X syndrome, members of affected families may also exhibit premature ovarian insufficiency or a late-onset neurodegenerative disorder, both of which are caused by smaller expansions of the FMR1 CGG repeat known as premutations. Meiotic instability is the defining feature of a premutation, and the risk of having a child with fragile X syndrome is directly correlated to the number of CGG repeat units on a premutation allele.

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