Distributed X chromosome inactivation in brain circuitry is associated with X-linked disease penetrance of behavior.

The precise anatomical degree of brain X chromosome inactivation (XCI) that is sufficient to alter X-linked disorders in females is unclear. Here, we quantify whole-brain XCI at single-cell resolution to discover a prevalent activation ratio of maternal to paternal X at 60:40 across all divisions of the adult brain. This modest, non-random XCI influences X-linked disease penetrance: maternal transmission of the fragile X mental retardation 1 (Fmr1)-knockout (KO) allele confers 55% of total brain cells with mutant X-active, which is sufficient for behavioral penetrance, while 40% produced from paternal transmission is tolerated. Local XCI mosaicism within affected maternal Fmr1-KO mice further specifies sensorimotor versus social anxiety phenotypes depending on which distinct brain circuitry is most affected, with only a 50%-55% mutant X-active threshold determining penetrance. Thus, our results define a model of X-linked disease penetrance in females whereby distributed XCI among single cells populating brain circuitries can regulate the behavioral penetrance of an X-linked mutation.

Clinical application value of pre-pregnancy carrier screening in Chinese Han childbearing population.

BACKGROUND: To explore the clinical application value of pre-conception expanded carrier screening (PECS) in the Chinese Han ethnicity population of childbearing age. METHODS: The results of genetic testing of infertile parents who underwent PECS in the Reproductive Medicine Center of the Second Affiliated Hospital of Zhengzhou University, China, from September 2019 to December 2021, were retrospectively analyzed. The carrier rate of single gene disease, the detection rate of high-risk parents, and the clinical outcome of high-risk parents were statistically analyzed. RESULTS: A total of 1372 Chinese Han ethnicity patients underwent PECS, among which 458 patients underwent the extended 108-gene test, their overall carrier rate was 31.7%, and the detection rate of high-risk parents was 0.3%. The highest carrier rates were SLC22A (2.4%), ATP7B (2.4%), MMACHC (2.2%), PAH (1.8%), GALC (1.8%), MLC1 (1.3%), UNC13D (1.1%), CAPN3 (1.1%), and PKHD1 (1.1%). There were 488 women with fragile X syndrome-FMR1 gene detection, and 6 patients (1.2%) had FMR1 gene mutation. A total of 426 patients were screened for spinal muscular atrophy-SMN1, and the carrier rate was 3.5%, and the detection rate of parents' co-carrier was 0.5%. CONCLUSION: Monogenic recessive hereditary diseases had a high carrier rate in the population. Pre-pregnancy screening could provide good prenatal and postnatal care guidance for patients and preimplantation genetic testing for monogenic/single gene disorders (PGT-M) and prenatal diagnosis could provide more precise reproductive choices for high-risk parents.

Mitochondrial dysfunction in Fragile X syndrome and Fragile X-associated tremor/ataxia syndrome: prospect use of antioxidants and mitochondrial nutrients.

Fragile X syndrome (FXS) is a genetic disorder characterized by mutation in the FMR1 gene, leading to the absence or reduced levels of fragile X Messenger Ribonucleoprotein 1 (FMRP). This results in neurodevelopmental deficits, including autistic spectrum conditions. On the other hand, Fragile X-associated tremor/ataxia syndrome (FXTAS) is a distinct disorder caused by the premutation in the FMR1 gene. FXTAS is associated with elevated levels of FMR1 mRNA, leading to neurodegenerative manifestations such as tremors and ataxia.Mounting evidence suggests a link between both syndromes and mitochondrial dysfunction (MDF). In this minireview, we critically examine the intricate relationship between FXS, FXTAS, and MDF, focusing on potential therapeutic avenues to counteract or mitigate their adverse effects. Specifically, we explore the role of mitochondrial cofactors and antioxidants, with a particular emphasis on alpha-lipoic acid (ALA), carnitine (CARN) and Coenzyme Q10 (CoQ10). Findings from this review will contribute to a deeper understanding of these disorders and foster novel therapeutic strategies to enhance patient outcomes.

Proteomics insights into fragile X syndrome: Unraveling molecular mechanisms and therapeutic avenues.

Fragile X Syndrome (FXS) is a neurodevelopment disorder characterized by cognitive impairment, behavioral challenges, and synaptic abnormalities, with a genetic basis linked to a mutation in the FMR1 (Fragile X Messenger Ribonucleoprotein 1) gene that results in a deficiency or absence of its protein product, Fragile X Messenger Ribonucleoprotein (FMRP). In recent years, mass spectrometry (MS) - based proteomics has emerged as a powerful tool to uncover the complex molecular landscape underlying FXS. This review provides a comprehensive overview of the proteomics studies focused on FXS, summarizing key findings with an emphasis on dysregulated proteins associated with FXS. These proteins span a wide range of cellular functions including, but not limited to, synaptic plasticity, RNA translation, and mitochondrial function. The work conducted in these proteomic studies provides a more holistic understanding to the molecular pathways involved in FXS and considerably enhances our knowledge into the synaptic dysfunction seen in FXS.

Unmethylated Mosaic Full Mutation Males without Fragile X Syndrome.

Fragile X syndrome (FXS) is the leading inherited cause of intellectual disability (ID) and single gene cause of autism. Although most patients with FXS and the full mutation (FM) have complete methylation of the fragile X messenger ribonucleoprotein 1 (FMR1) gene, some have mosaicism in methylation and/or CGG repeat size, and few have completely unmethylated FM alleles. Those with a complete lack of methylation are rare, with little literature about the cognitive and behavioral phenotypes of these individuals. A review of past literature was conducted regarding individuals with unmethylated and mosaic FMR1 FM. We report three patients with an unmethylated FM FMR1 alleles without any behavioral or cognitive deficits. This is an unusual presentation for men with FM as most patients with an unmethylated FM and no behavioral phenotypes do not receive fragile X DNA testing or a diagnosis of FXS. Our cases showed that mosaic males with unmethylated FMR1 FM alleles may lack behavioral phenotypes due to the presence of smaller alleles producing the FMR1 protein (FMRP). However, these individuals could be at a higher risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS) due to the increased expression of mRNA, similar to those who only have a premutation.

Mitochondrial Dysfunction Causes Cell Death in Patients Affected by Fragile-X-Associated Disorders.

Mitochondria are involved in multiple aspects of neurodevelopmental processes and play a major role in the pathogenetic mechanisms leading to neuro-degenerative diseases. Fragile-X-related disorders (FXDs) are genetic conditions that occur due to the dynamic expansion of CGG repeats of the FMR1 gene encoding for the RNA-binding protein FMRP, particularly expressed in the brain. This gene expansion can lead to premutation (PM, 56-200 CGGs), full mutation (FM, >200 CGGs), or unmethylated FM (UFM), resulting in neurodegeneration, neurodevelopmental disorders, or no apparent intellectual disability, respectively. To investigate the mitochondrial mechanisms that are involved in the FXD patients, we analyzed mitochondrial morphology and bioenergetics in fibroblasts derived from patients. Donut-shaped mitochondrial morphology and excessive synthesis of critical mitochondrial proteins were detected in FM, PM, and UFM cells. Analysis of mitochondrial oxidative phosphorylation in situ reveals lower respiration in PM fibroblasts. Importantly, mitochondrial permeability transition-dependent apoptosis is sensitized to reactive oxygen species in FM, PM, and UFM models. This study elucidated the mitochondrial mechanisms that are involved in the FXD phenotypes, and indicated altered mitochondrial function and morphology. Importantly, a sensitization to permeability transition and apoptosis was revealed in FXD cells. Overall, our data suggest that mitochondria are novel drug targets to relieve the FXD symptoms.

KIF5B plays important roles in dendritic spine plasticity and dendritic localization of PSD95 and FMRP in the mouse cortex in vivo.

Kinesin 1 (KIF5) is one major type of motor protein in neurons, but its members' function in the intact brain remains less studied. Using in vivo two-photon imaging, we find that conditional knockout of Kif5b (KIF5B cKO) in CaMKIIα-Cre-expressing neurons shows heightened turnover and lower stability of dendritic spines in layer 2/3 pyramidal neurons with reduced spine postsynaptic density protein 95 acquisition in the mouse cortex. Furthermore, the RNA-binding protein fragile X mental retardation protein (FMRP) is translocated to the proximity of newly formed spines several hours before the spine formation events in vivo in control mice, but this preceding transport of FMRP is abolished in KIF5B cKO mice. We further find that FMRP is localized closer to newly formed spines after fear extinction, but this learning-dependent localization is disrupted in KIF5B cKO mice. Our findings provide the crucial in vivo evidence that KIF5B is involved in the dendritic targeting of synaptic proteins that underlies dendritic spine plasticity.

Variation of FMRP Expression in Peripheral Blood Mononuclear Cells from Individuals with Fragile X Syndrome.

Fragile X syndrome (FXS) is the most common heritable cause of intellectual disability and autism spectrum disorder. The syndrome is often caused by greatly reduced or absent protein expression from the fragile X messenger ribonucleoprotein 1 (FMR1) gene due to expansion of a 5'-non-coding trinucleotide (CGG) element beyond 200 repeats (full mutation). To better understand the complex relationships among FMR1 allelotype, methylation status, mRNA expression, and FMR1 protein (FMRP) levels, FMRP was quantified in peripheral blood mononuclear cells for a large cohort of FXS (n = 154) and control (n = 139) individuals using time-resolved fluorescence resonance energy transfer. Considerable size and methylation mosaicism were observed among individuals with FXS, with FMRP detected only in the presence of such mosaicism. No sample with a minimum allele size greater than 273 CGG repeats had significant levels of FMRP. Additionally, an association was observed between FMR1 mRNA and FMRP levels in FXS samples, predominantly driven by those with the lowest FMRP values. This study underscores the complexity of FMR1 allelotypes and FMRP expression and prompts a reevaluation of FXS therapies aimed at reactivating large full mutation alleles that are likely not capable of producing sufficient FMRP to improve cognitive function.

Challenges in developing therapies in fragile X syndrome: how the FXLEARN trial can guide research.

Fragile X syndrome (FXS), the most common inherited cause of intellectual disability and the single-gene cause of autism, is caused by decreased expression of the fragile X messenger ribonucleoprotein protein (FMRP), a ribosomal-associated RNA-binding protein involved in translational repression. Extensive preclinical work in several FXS animal models supported the therapeutic potential of decreasing metabotropic glutamate receptor (mGluR) signaling to correct translation of proteins related to synaptic plasticity; however, multiple clinical trials failed to show conclusive evidence of efficacy. In this issue of the JCI, Berry-Kravis and colleagues conducted the FXLEARN clinical trial to address experimental design concerns from previous trials. Unfortunately, despite treatment of young children with combined pharmacological and learning interventions for a prolonged period, no efficacy of blocking mGluR activity was observed. Future systematic evaluation of potential therapeutic approaches should evaluate consistency between human and animal pathophysiological mechanisms, utilize innovative clinical trial design from FXLEARN, and incorporate translatable biomarkers.

Fragile X cortex is characterized by decreased parvalbumin-expressing interneurons.

Fragile X syndrome is a genetic neurodevelopmental disorder caused by a mutation of the fragile X messenger ribonucleoprotein 1 (FMR1) gene in the X chromosome. Many fragile X syndrome cases present with autism spectrum disorder and fragile X syndrome cases account for up to 5% of all autism spectrum disorder cases. The cellular composition of the fragile X syndrome cortex is not well known. We evaluated alterations in the number of Calbindin, Calretinin, and Parvalbumin expressing interneurons across 5 different cortical areas, medial prefrontal cortex (BA46), primary somatosensory cortex (BA3), primary motor cortex (BA4), superior temporal cortex (BA22), and anterior cingulate cortex (BA24) of fragile X syndrome and neurotypical brains. Compared with neurotypical cases, fragile X syndrome brains displayed a significant reduction in the number of PV+ interneurons in all areas and of CR+ interneurons in BA22 and BA3. The number of CB+ interneurons did not differ. These findings are the first to demonstrate that fragile X syndrome brains are characterized by cortical wide PV+ interneuron deficits across multiple cortical areas. These add to the idea that deficits in PV+ interneurons could disrupt the cortical balance and promote clinical deficits in fragile X syndrome patients and help to develop novel therapies for neurodevelopmental disorders like fragile X syndrome and autism spectrum disorder.

Variable expression of MECP2, CDKL5, and FMR1 in the human brain: Implications for gene restorative therapies.

MECP2, CDKL5, and FMR1 are three X-linked neurodevelopmental genes associated with Rett, CDKL5-, and fragile-X syndrome, respectively. These syndromes are characterized by distinct constellations of severe cognitive and neurobehavioral anomalies, reflecting the broad but unique expression patterns of each of the genes in the brain. As these disorders are not thought to be neurodegenerative and may be reversible, a major goal has been to restore expression of the functional proteins in the patient's brain. Strategies have included gene therapy, gene editing, and selective Xi-reactivation methodologies. However, tissue penetration and overall delivery to various regions of the brain remain challenging for each strategy. Thus, gaining insights into how much restoration would be required and what regions/cell types in the brain must be targeted for meaningful physiological improvement would be valuable. As a step toward addressing these questions, here we perform a meta-analysis of single-cell transcriptomics data from the human brain across multiple developmental stages, in various brain regions, and in multiple donors. We observe a substantial degree of expression variability for MECP2, CDKL5, and FMR1 not only across cell types but also between donors. The wide range of expression may help define a therapeutic window, with the low end delineating a minimum level required to restore physiological function and the high end informing toxicology margin. Finally, the inter-cellular and inter-individual variability enable identification of co-varying genes and will facilitate future identification of biomarkers.

mGluR7 allosteric modulator AMN082 corrects protein synthesis and pathological phenotypes in FXS.

Fragile X syndrome (FXS) is the leading cause of inherited autism and intellectual disabilities. Aberrant protein synthesis due to the loss of fragile X messenger ribonucleoprotein (FMRP) is the major defect in FXS, leading to a plethora of cellular and behavioral abnormalities. However, no treatments are available to date. In this study, we found that activation of metabotropic glutamate receptor 7 (mGluR7) using a positive allosteric modulator named AMN082 represses protein synthesis through ERK1/2 and eIF4E signaling in an FMRP-independent manner. We further demonstrated that treatment of AMN082 leads to a reduction in neuronal excitability, which in turn ameliorates audiogenic seizure susceptibility in Fmr1 KO mice, the FXS mouse model. When evaluating the animals' behavior, we showed that treatment of AMN082 reduces repetitive behavior and improves learning and memory in Fmr1 KO mice. This study uncovers novel functions of mGluR7 and AMN082 and suggests the activation of mGluR7 as a potential therapeutic approach for treating FXS.

BREACHing new grounds in fragile X syndrome: Trinucleotide expansion linked to genome-wide heterochromatin domains and genome misfolding.

In a recent study in Cell, Malachowski et al.1 show that the trinucleotide expansion in the FMR1 gene underlying fragile X syndrome triggers formation of large heterochromatin domains across the genome, resulting in the repression of synaptic genes housed within these domains.

Bisphenol F affects neurodevelopmental gene expression, mushroom body development, and behavior in Drosophila melanogaster.

Bisphenol F (BPF) is a potential neurotoxicant used as a replacement for bisphenol A (BPA) in polycarbonate plastics and epoxy resins. We investigated the neurodevelopmental impacts of BPF exposure using Drosophila melanogaster as a model. Our transcriptomic analysis indicated that developmental exposure to BPF caused the downregulation of neurodevelopmentally relevant genes, including those associated with synapse formation and neuronal projection. To investigate the functional outcome of BPF exposure, we evaluated neurodevelopmental impacts across two genetic strains of Drosophila- w1118 (control) and the Fragile X Syndrome (FXS) model-by examining both behavioral and neuronal phenotypes. We found that BPF exposure in w1118 Drosophila caused hypoactive larval locomotor activity, decreased time spent grooming by adults, reduced courtship activity, and increased the severity but not frequency of ß-lobe midline crossing defects by axons in the mushroom body. In contrast, although BPF reduced peristaltic contractions in FXS larvae, it had no impact on other larval locomotor phenotypes, grooming activity, or courtship activity. Strikingly, BPF exposure reduced both the severity and frequency of ß-lobe midline crossing defects in the mushroom body of FXS flies, a phenotype previously observed in FXS flies exposed to BPA. This data indicates that BPF can affect neurodevelopment and its impacts vary depending on genetic background. Further, BPF may elicit a gene-environment interaction with Drosophila fragile X messenger ribonucleoprotein 1 (dFmr1)-the ortholog of human FMR1, which causes fragile X syndrome and is the most common monogenetic cause of intellectual disability and autism spectrum disorder.

Pharmacological management of fragile X syndrome: a systematic review and narrative summary of the current evidence.

INTRODUCTION: Fragile X syndrome (FXS) is the most common inherited cause of Intellectual Disability. There is a broad phenotype that includes deficits in cognition and behavioral changes, alongside physical characteristics. Phenotype depends upon the level of mutation in the FMR1 (fragile X messenger ribonucleoprotein 1) gene. The molecular understanding of the impact of the FMR1 gene mutation provides an opportunity to target treatment not only at symptoms but also on a molecular level. METHODS: We conducted a systematic review to provide an up-to-date narrative summary of the current evidence for pharmacological treatment in FXS. The review was restricted to randomized, blinded, placebo-controlled trials. RESULTS: The outcomes from these studies are discussed and the level of evidence assessed against validated criteria. The initial search identified 2377 articles, of which 16 were included in the final analysis. CONCLUSION: Based on this review to date there is limited data to support any specific pharmacological treatments, although the data for cannabinoids are encouraging in those with FXS and in future developments in gene therapy may provide the answer to the search for precision medicine. Treatment must be person-centered and consider the combination of medical, genetic, cognitive, and emotional challenges.

Circuit-based intervention corrects excessive dentate gyrus output in the fragile X mouse model.

Abnormal cellular and circuit excitability is believed to drive many core phenotypes in fragile X syndrome (FXS). The dentate gyrus is a brain area performing critical computations essential for learning and memory. However, little is known about dentate circuit defects and their mechanisms in FXS. Understanding dentate circuit dysfunction in FXS has been complicated by the presence of two types of excitatory neurons, the granule cells and mossy cells. Here we report that loss of FMRP markedly decreased excitability of dentate mossy cells, a change opposite to all other known excitability defects in excitatory neurons in FXS. This mossy cell hypo-excitability is caused by increased Kv7 function in Fmr1 knockout (KO) mice. By reducing the excitatory drive onto local hilar interneurons, hypo-excitability of mossy cells results in increased excitation/inhibition ratio in granule cells and thus paradoxically leads to excessive dentate output. Circuit-wide inhibition of Kv7 channels in Fmr1 KO mice increases inhibitory drive onto granule cells and normalizes the dentate output in response to physiologically relevant theta-gamma coupling stimulation. Our study suggests that circuit-based interventions may provide a promising strategy in this disorder to bypass irreconcilable excitability defects in different cell types and restore their pathophysiological consequences at the circuit level.

Multi-level profiling of the Fmr1 KO rat unveils altered behavioral traits along with aberrant glutamatergic function.

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disabilities and the most prevalent monogenic cause of autism. Although the knockout (KO) of the Fmr1 gene homolog in mice is primarily used for elucidating the neurobiological substrate of FXS, there is limited association of the experimental data with the pathophysiological condition in humans. The use of Fmr1 KO rats offers additional translational validity in this regard. Therefore, we employed a multi-level approach to study the behavioral profile and the glutamatergic and GABAergic neurotransmission status in pathophysiology-associated brain structures of Fmr1 KO rats, including the recordings of evoked and spontaneous field potentials from hippocampal slices, paralleled with next-generation RNA sequencing (RNA-seq). We found that these rats exhibit hyperactivity and cognitive deficits, along with characteristic bidirectional glutamatergic and GABAergic alterations in the prefrontal cortex and the hippocampus. These results are coupled to affected excitability and local inhibitory processes in the hippocampus, along with a specific transcriptional profile, highlighting dysregulated hippocampal network activity in KO rats. Overall, our data provide novel insights concerning the biobehavioral profile of FmR1 KO rats and translationally upscales our understanding on pathophysiology and symptomatology of FXS syndrome.

Sex-specific modulation of early life vocalization and cognition by Fmr1 gene dosage in a mouse model of Fragile X Syndrome.

BACKGROUND: Pup-dam ultrasonic vocalizations (USVs) are essential to cognitive and socio-emotional development. In autism and Fragile X Syndrome (FXS), disruptions in pup-dam USV communication hint at a possible connection between abnormal early developmental USV communication and the later emergence of communication and social deficits. METHODS: Here, we gathered USVs from PND 10 FXS pups during a short period of separation from their mothers, encompassing animals of all possible genotypes and both sexes (i.e., Fmr1-/y vs. Fmr1+/y males and Fmr1+/+, +/-, and -/- females). This allowed comparing the influence of sex and gene dosage on pups' communication capabilities. Leveraging DeepSqueak and analyzing vocal patterns, intricate vocal behaviors such as call structure, duration, frequency modulation, and temporal patterns were examined. Furthermore, homing behavior was assessed as a sensitive indicator of early cognitive development and social discrimination. This behavior relies on the use of olfactory and thermal cues to navigate and search for the maternal or nest odor in the surrounding space. RESULTS: The results show that FMRP-deficient pups of both sexes display an increased inclination to vocalize when separated from their mothers, and this behavior is accompanied by significant sex-specific changes in the main features of their USVs as well as in body weight. Analysis of the vocal repertoire and syntactic usage revealed that Fmr1 gene silencing primarily alters the USVs' qualitative composition in males. Moreover, sex-specific effects of Fmr1 silencing on locomotor activity and homing behavior were observed. FMRP deficiency in females increased activity, reduced nest-reaching time, and extended nest time. In males, it prolonged nest-reaching time and reduced nest time without affecting locomotion. CONCLUSIONS: These findings highlight the interplay between Fmr1 gene dosage and sex in influencing communicative and cognitive skills during infancy.

In this study, we investigated ultrasonic vocalizations (USVs) and homing behavior in a mouse model of Fragile X Syndrome (FXS), a leading genetic cause of autism spectrum disorder (ASD) caused by a mutation of the X-chromosome linked Fmr1 gene. Disruptions in pup-dam USV communication and cognitive skills may be linked to the later emergence of communication and social deficits in ASD. USVs were collected from 10-day-old FXS pups of all possible genotypes and both sexes during a short period of separation from their mothers. We utilized DeepSqueak, an advanced deep learning system, to examine vocal patterns and intricate vocal behaviors, including call structure, duration, frequency modulation, and their temporal patterns. Homing, a sensitive indicator of early cognitive development and social discrimination was assessed at P13. The results showed that FXS pups of both sexes displayed an increased inclination to vocalize when separated from their mothers. Examination of the vocal repertoire and its syntactic usage revealed that the silencing of the Fmr1 gene primarily alters the qualitative composition of ultrasonic communication in males. The sex-specific changes observed in USVs were accompanied by modifications in body weight. Regarding homing behavior, the deficiency of FMRP led to opposite deficits in activity, time to reach the nest, and nesting time depending on sex. Taken together, these findings highlight the interplay between Fmr1 gene dosage and sex in shaping communication and cognition during infancy.

Fragile X syndrome in the largest world clustering. I. Genetic epidemiology and founder effect outline.

The FMR1 5' regulation gene region harbors a CGG trinucleotide repeat expansion (CGG-TRE) that causes Fragile X syndrome (FXS) when it expands to more than 200 repetitions. Ricaurte is a small village in southwestern Colombia, with an FXS prevalence of 1 in 38 men and 1 in 100 women (~100 times higher than the worldwide reported prevalence), defining Ricaurte as the largest FXS cluster in the world. In the present study, using next-generation sequencing of whole exome capture, we genotype 55 individuals from Ricaurte (49 with either full mutation or with premutation), four individuals from neighboring villages (with either the full mutation or with the premutation), and one unaffected woman, native of Ricaurte, who did not belong to any of the affected families. With advanced clustering and haplotype reconstruction, we modeled a common haplotype of 33 SNPs spanning 83,567,899 bp and harboring the FMR1 gene. This reconstructed haplotype was found in all the men from Ricaurte who carried the expansion, demonstrating that the genetic conglomerate of FXS in this population is due to a founder effect. The definition of this founder effect and its population outlining will allow a better prediction, follow-up, precise and personalized characterization of epidemiological parameters, better knowledge of the disease's natural history, and confident improvement of the clinical attention, life quality, and health interventions for this community.

The NSP3 protein of SARS-CoV-2 binds fragile X mental retardation proteins to disrupt UBAP2L interactions.

Viruses interact with numerous host factors to facilitate viral replication and to dampen antiviral defense mechanisms. We currently have a limited mechanistic understanding of how SARS-CoV-2 binds host factors and the functional role of these interactions. Here, we uncover a novel interaction between the viral NSP3 protein and the fragile X mental retardation proteins (FMRPs: FMR1, FXR1-2). SARS-CoV-2 NSP3 mutant viruses preventing FMRP binding have attenuated replication in vitro and reduced levels of viral antigen in lungs during the early stages of infection. We show that a unique peptide motif in NSP3 binds directly to the two central KH domains of FMRPs and that this interaction is disrupted by the I304N mutation found in a patient with fragile X syndrome. NSP3 binding to FMRPs disrupts their interaction with the stress granule component UBAP2L through direct competition with a peptide motif in UBAP2L to prevent FMRP incorporation into stress granules. Collectively, our results provide novel insight into how SARS-CoV-2 hijacks host cell proteins and provides molecular insight into the possible underlying molecular defects in fragile X syndrome.