Fragile X syndrome due to a missense mutation

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Fragile X syndrome (FXS) results in intellectual disability (ID) most often caused by silencing of the fragile X mental retardation 1 (FMR1) gene. The resulting absence of fragile X mental retardation protein 1 (FMRP) leads to both pre-and postsynaptic defects, yet whether the pre-and postsynaptic functions of FMRP are independent and have distinct roles in FXS neuropathology remain poorly understood. Here, we demonstrate an independent presynaptic function for FMRP through the study of an ID patient with an FMR1 missense mutation. This mutation, c.413G > A (R138Q), preserves FMRP's canonical functions in RNA binding and translational regulation, which are traditionally associated with postsynaptic compartments. However, neuronally driven expression of the mutant FMRP is unable to rescue structural defects at the neuromuscular junction in fragile x mental retardation 1 (dfmr1)-deficient Drosophila, suggesting a presynaptic-specific impairment. Furthermore, mutant FMRP loses the ability to rescue presynaptic action potential (AP) broadening in Fmr1 KO mice. The R138Q mutation also disrupts FMRP's interaction with the large-conductance calciumactivated potassium (BK) channels that modulate AP width. These results reveal a presynaptic-and translation-independent function of FMRP that is linked to a specific subset of FXS phenotypes.F ragile X syndrome (FXS) is the most common single-gene disorder responsible for intellectual disability (ID) in patients (1). Along with cognitive dysfunction, the syndrome typically presents with several other comorbidities, including behavioral and social impairments (anxiety and autism spectrum disorder), neurological defects (seizures and abnormal sleep patterns), and morphological abnormalities (dysmorphic facies and macroorchidism). Most patients inherit the syndrome through a maternal repeat expansion mutation that transcriptionally silences the FMR1 gene and results in loss of the gene product, FMRP.FMRP has complex multifaceted functions at synapses both in pre-and postsynaptic compartments. As an RNA binding protein, FMRP is best known for its function as a translation regulator in dendrites (2). Loss of FMRP has been linked to various forms of long-term synaptic plasticity defects that depend on local protein synthesis in the postsynaptic neuron (3). In addition to disrupted metabotropic glutamate receptor signaling, which has been shown across multiple brain regions (4-7), FMRP is necessary for activitydependent protein synthesis downstream of other signaling receptor pathways, including ACh, dopamine, and TrkB (8-10).Although postsynaptic control of translation is believed to be the dominant function of FMRP, it is unable to explain all of the pathophysiology observed in FXS animal models. For instance, in Drosophila, presynaptic expression of the FMR1 homolog, dfmr1, completely rescues the synaptic overgrowth phenotype at the neuromuscular junction (NMJ) in dfmr1-null mutants (11-13). In rodent brain, FMRP has been found in structures called fragile-X granules, which are ...

Section: R138q Mutation Retains Postsynaptic Functions Of Fmrp In Tramentioning

confidence: 70%

Section: Discussionmentioning

confidence: 99%

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Independent role for presynaptic FMRP revealed by an FMR1 missense mutation associated with intellectual disability and seizures

Myrick¹,

Deng²,

Hashimoto³

et al. 2015

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112

132

“…Fragile X Mental Retardation Protein (FMRP) is among the most important RBPs because of its central role in several human diseases (3). Loss of FMRP function due to CGG triplet repeat expansion-associated transcriptional silencing or missense mutations in the protein (4,5) lead to fragile X syndrome (FXS), the most common cause of inherited intellectual disability. Mutations in FMRP are also the leading monogenic cause of autism (6,7).…”

mentioning

confidence: 99%

Crystal structure reveals specific recognition of a G-quadruplex RNA by a β-turn in the RGG motif of FMRP

Vasilyev

Polonskaia

Darnell

et al. 2015

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181

Fragile X Mental Retardation Protein (FMRP) is a regulatory RNA binding protein that plays a central role in the development of several human disorders including Fragile X Syndrome (FXS) and autism. FMRP uses an arginine-glycine-rich (RGG) motif for specific interactions with guanine (G)-quadruplexes, mRNA elements implicated in the disease-associated regulation of specific mRNAs. Here we report the 2.8-Å crystal structure of the complex between the human FMRP RGG peptide bound to the in vitro selected G-rich RNA. In this model system, the RNA adopts an intramolecular K + -stabilized G-quadruplex structure composed of three G-quartets and a mixed tetrad connected to an RNA duplex. The RGG peptide specifically binds to the duplex-quadruplex junction, the mixed tetrad, and the duplex region of the RNA through shape complementarity, cation-π interactions, and multiple hydrogen bonds. Many of these interactions critically depend on a type I β-turn, a secondary structure element whose formation was not previously recognized in the RGG motif of FMRP. RNA mutagenesis and footprinting experiments indicate that interactions of the peptide with the duplex-quadruplex junction and the duplex of RNA are equally important for affinity and specificity of the RGG-RNA complex formation. These results suggest that specific binding of cellular RNAs by FMRP may involve hydrogen bonding with RNA duplexes and that RNA duplex recognition can be a characteristic RNA binding feature for RGG motifs in other proteins.R NA-binding proteins (RBPs) control all aspects of RNA metabolism and are fundamental to core cellular processes. ∼14% of identified human RBPs are implicated in a broad spectrum of human pathologies, including neurodegenerative and muscular diseases, metabolic disorders, and cancers (1, 2). Fragile X Mental Retardation Protein (FMRP) is among the most important RBPs because of its central role in several human diseases (3). Loss of FMRP function due to CGG triplet repeat expansion-associated transcriptional silencing or missense mutations in the protein (4, 5) lead to fragile X syndrome (FXS), the most common cause of inherited intellectual disability. Mutations in FMRP are also the leading monogenic cause of autism (6, 7). Intermediate length repeat expansions in the FMR1 gene are linked to fragile X-associated tremor ataxia syndrome (8) and fragile X-associated primary ovarian insufficiency (9).FMRP contains four canonical nucleic acid-binding motifs, three KH domains and one arginine-glycine-rich (RGG) box, which mediate interactions with RNAs in mRNA transport, storage, stability, and regulation of translation (Fig. 1A) (3, 10). Each KH domain has been reported to have a mutation either causing FXS or found in patients with intellectual disability (4, 5, 11). In neurons, FMRP associates with a subset of mRNAs and represses their translation both in the cell body and near synapses (12). Loss of repression of these mRNAs is associated with alterations in synaptic plasticity and dendritic spine dynamics thought to underlie...

“…Despite significant analysis of the FMR1 gene, only a small number of conventional genetic mutations, such as point mutations and insertions/deletions, have been reported to be associated with FXS or developmental delay (20)(21)(22)(23)(24)(25)(26)(27). To identify causes of developmental delay attributable to FMR1 variants other than the repeat expansion, our group sequenced the FMR1 gene in 963 developmentally delayed males, each of whom tested negative for the CGG expansion mutation, and discovered a number of previously unreported variants (28).…”

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confidence: 99%

A 3′ untranslated region variant in FMR1 eliminates neuronal activity-dependent translation of FMRP by disrupting binding of the RNA-binding protein HuR

Suhl

Muddashetty

Anderson

et al. 2015

Fragile X syndrome is a common cause of intellectual disability and autism spectrum disorder. The gene underlying the disorder, fragile X mental retardation 1 (FMR1), is silenced in most cases by a CGG-repeat expansion mutation in the 5′ untranslated region (UTR). Recently, we identified a variant located in the 3′UTR of FMR1 enriched among developmentally delayed males with normal repeat lengths. A patient-derived cell line revealed reduced levels of endogenous fragile X mental retardation protein (FMRP), and a reporter containing a patient 3′UTR caused a decrease in expression. A control reporter expressed in cultured mouse cortical neurons showed an expected increase following synaptic stimulation that was absent when expressing the patient reporter, suggesting an impaired response to neuronal activity. Mobility-shift assays using a control RNA detected an RNA–protein interaction that is lost with the patient RNA, and HuR was subsequently identified as an associated protein. Cross-linking immunoprecipitation experiments identified the locus as an in vivo target of HuR, supporting our in vitro findings. These data suggest that the disrupted interaction of HuR impairs activity-dependent translation of FMRP, which may hinder synaptic plasticity in a clinically significant fashion.