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A Non-random Mouse Model for Pharmacological Reactivation of Mecp2 on the Inactive X Chromosome
Published on: May 22, 2019
A mouse model of the human Fragile X syndrome I304N mutation
Julie B Zang1, Elena D Nosyreva, Corinne M Spencer
1Laboratory of Molecular Neuro-Oncology, The Rockefeller University, New York, New York, USA.
Abstract:
The mental retardation, autistic features, and behavioral abnormalities characteristic of the Fragile X mental retardation syndrome result from the loss of function of the RNA-binding protein FMRP. The disease is usually caused by a triplet repeat expansion in the 5'UTR of the FMR1 gene. This leads to loss of function through transcriptional gene silencing, pointing to a key function for FMRP, but precluding genetic identification of critical activities within the protein. Moreover, antisense transcripts (FMR4, ASFMR1) in the same locus have been reported to be silenced by the repeat expansion. Missense mutations offer one means of confirming a central role for FMRP in the disease, but to date, only a single such patient has been described. This patient harbors an isoleucine to asparagine mutation (I304N) in the second FMRP KH-type RNA-binding domain, however, this single case report was complicated because the patient harbored a superimposed familial liver disease. To address these issues, we have generated a new Fragile X Syndrome mouse model in which the endogenous Fmr1 gene harbors the I304N mutation. These mice phenocopy the symptoms of Fragile X Syndrome in the existing Fmr1-null mouse, as assessed by testicular size, behavioral phenotyping, and electrophysiological assays of synaptic plasticity. I304N FMRP retains some functions, but has specifically lost RNA binding and polyribosome association; moreover, levels of the mutant protein are markedly reduced in the brain specifically at a time when synapses are forming postnatally. These data suggest that loss of FMRP function, particularly in KH2-mediated RNA binding and in synaptic plasticity, play critical roles in pathogenesis of the Fragile X Syndrome and establish a new model for studying the disorder.
Insights
Fragile X Syndrome results from FMRP loss. A new mouse model with an I304N mutation shows disease symptoms, confirming FMRP
Area of Science:
- Neuroscience
- Genetics
- Molecular Biology
Background:
- Fragile X Syndrome (FXS) is caused by FMRP loss-of-function, typically due to triplet repeat expansion in the FMR1 gene.
- Previous studies identified a missense mutation (I304N) in a patient, but this case was complicated by other health issues.
- Understanding FMRP's specific functions is crucial for FXS pathogenesis, but genetic studies are limited.
Purpose of the Study:
- To create and validate a novel mouse model for Fragile X Syndrome using the I304N FMRP mutation.
- To investigate the functional consequences of the I304N mutation on FMRP's RNA-binding capabilities and its role in synaptic plasticity.
- To establish a new preclinical model for studying FXS and potential therapeutic strategies.
Main Methods:
- Generated a mouse model with the endogenous Fmr1 gene harboring the I304N mutation.
- Assessed FXS phenotypes in mice, including testicular size, behavioral tests, and electrophysiological recordings of synaptic plasticity.
- Analyzed I304N FMRP protein levels, RNA binding, and polyribosome association in the mouse brain.
Main Results:
- The I304N mouse model recapitulates key FXS symptoms seen in Fmr1-null mice.
- Mutant I304N FMRP exhibits impaired RNA binding and reduced polyribosome association.
- Levels of I304N FMRP are significantly decreased in the developing brain during critical periods of synapse formation.
- Synaptic plasticity is specifically impaired in the I304N mouse model.
Conclusions:
- Loss of FMRP function, particularly RNA binding via the KH2 domain, is critical for Fragile X Syndrome pathogenesis.
- The I304N mutation disrupts FMRP's function, leading to reduced protein levels and impaired synaptic plasticity.
- This novel I304N mouse model provides a valuable tool for further research into FXS mechanisms and therapeutic development.
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