Fragile X syndrome neurobiology translates into rational therapy

Mouse models of fragile X-related disorders

The fragile X-related disorders are an important group of hereditary disorders that are caused by expanded CGG repeats in the 5' untranslated region of the FMR1 gene or by mutations in the coding sequence of this gene. Two categories of pathological CGG repeats are associated with these disorders, full mutation alleles and shorter premutation alleles. Individuals with full mutation alleles develop fragile X syndrome, which causes autism and intellectual disability, whereas those with premutation alleles, which have shorter CGG expansions, can develop fragile X-associated tremor/ataxia syndrome, a progressive neurodegenerative disease. Thus, fragile X-related disorders can manifest as neurodegenerative or neurodevelopmental disorders, depending on the size of the repeat expansion. Here, we review mouse models of fragile X-related disorders and discuss how they have informed our understanding of neurodegenerative and neurodevelopmental disorders. We also assess the translational value of these models for developing rational targeted therapies for intellectual disability and autism disorders.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Overrepresentation of genetic variation in the AnkyrinG interactome is related to a range of neurodevelopmental disorders

Upon the discovery of numerous genes involved in the pathogenesis of neurodevelopmental disorders, several studies showed that a significant proportion of these genes converge on common pathways and protein networks. Here, we used a reversed approach, by screening the AnkyrinG protein-protein interaction network for genetic variation in a large cohort of 1009 cases with neurodevelopmental disorders. We identified a significant enrichment of de novo potentially disease-causing variants in this network, confirming that this protein network plays an important role in the emergence of several neurodevelopmental disorders.

GABAergic abnormalities in the fragile X syndrome

Many pathways have been involved in pathophysiology of the fragile X syndrome, one of the more frequent genetic causes of intellectual disability and autism. This review highlights the recent insights in the role the abnormalities in the GABAergic system play in the disorder. Since the initial observations showed that the expression of specific subunits of the GABA(A) receptor were underexpressed in the fragile X knockout mouse model more than a decade ago, evidence has accumulated that the expression of approximately half of the GABAergic system is compromised in multiple species, including in fragile X patients. Functional consequences of the GABAergic deficiencies could be measured using whole-cell voltage clamp recordings. Pharmalogical treatment with agonist of the receptor was been able to restore several behavioral deficits in the fragile X mouse model, including seizures, marble burying and, in part, prepulse inhibition. Trials in patients with the same agonist have demonstrated encouraging post-hoc results in the most severely affected patients, although no effect could be demonstrated in the patient group as a whole. In conclusion, there can be little doubt that the GABAergic system is compromised in the fragile X syndrome and that these abnormalities contribute to the clinical abnormalities observed. However, at the moment the difference in treatment effectiveness of agonist of the receptor in animal models as opposed to in patients remains unexplained.

Validating and Applying the CSBS-ITC in Neurogenetic Syndromes

Although social communication skills are commonly delayed in children with neurogenetic syndromes (NGS), skill profiles in very young children are largely under characterized, in part due to the lack of validated assessment measures appropriate for these populations. We addressed this gap by validating and applying a popular early social communication screening measure, the Communication and Symbolic Behavior Scales Developmental Profile - Infant-Toddler Checklist (CSBS-ITC) in three previously understudied neurogenetic groups: Angelman, Prader-Willi, and Williams syndromes. Our results suggest that when used within the appropriate scope of screening and surveillance, the CSBS-ITC detects meaningful variability in skills across ages in young children with NGS and may provide useful information about both individual- and population-level social communication profiles in these populations.

Molecular Biomarkers in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability (ID) and a known monogenic cause of autism spectrum disorder (ASD). It is a trinucleotide repeat disorder, in which more than 200 CGG repeats in the 5' untranslated region (UTR) of the fragile X mental retardation 1 (FMR1) gene causes methylation of the promoter with consequent silencing of the gene, ultimately leading to the loss of the encoded fragile X mental retardation 1 protein, FMRP. FMRP is an RNA binding protein that plays a primary role as a repressor of translation of various mRNAs, many of which are involved in the maintenance and development of neuronal synaptic function and plasticity. In addition to intellectual disability, patients with FXS face several behavioral challenges, including anxiety, hyperactivity, seizures, repetitive behavior, and problems with executive and language performance. Currently, there is no cure or approved medication for the treatment of the underlying causes of FXS, but in the past few years, our knowledge about the proteins and pathways that are dysregulated by the loss of FMRP has increased, leading to clinical trials and to the path of developing molecular biomarkers for identifying potential targets for therapies. In this paper, we review candidate molecular biomarkers that have been identified in preclinical studies in the FXS mouse animal model and are now under validation for human applications or have already made their way to clinical trials.

Fragile X Syndrome Pre-Clinical Research: Comparing Mouse- and Human-Based Models

Despite almost 30 years of biomedical research, a treatment or cure for fragile X syndrome (FXS) is not yet available. The reasons behind this are varied, and among them are discrepancies in both research methodologies and research models. For many years, the fmr1 knockout mouse model dominated the field, and was used to draw important conclusions. The establishment of FXS-human cellular models called these conclusions into question, showing conflicting evidence. Discrepancies in FXS research, between mouse and human, might arise from differences inherent to each species, and from the use of different methodologies. This chapter summarizes these discrepancies and evaluates their impact on the current status of clinical trials.

Review of Salient Investigational Drugs for the Treatment of Fragile X Syndrome

OBJECTIVES

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability, in addition to being the commonest diagnosable cause of autism. The identification of the biochemical mechanism underlying this disorder has provided amenable targets for therapy. This review aims to provide an overview of investigational drug therapies for FXS.

METHODS

The authors carried out a search of clinical and preclinical trials for FXS in PubMed and on the U.S. National Institutes of Health index of clinical trials ( www.clinicaltrials.gov ). We limited our review to Phase II trials or more preliminary and reviewed the associated publications for these studies, complemented by a review of the literature on PubMed.

RESULTS

The review of the preclinical, Phase I, and Phase II trials of agents with therapeutic potential in FXS revolves around an understanding of the putative pathways in the pathogenesis of FXS. While there is significant overlap between some of these pathways, the agents can be categorized as modulators of the metabotropic glutamate receptor system, GABAergic agents, and miscellaneous modulators affecting other pathways.

CONCLUSION

As trials involving agents targeting different aspects of the molecular biology proceed, common themes have emerged. With the great hope came great disappointment as the initial trials failed to demonstrate sufficient significance. In particular, the differences in outcome between the animal models and humans have highlighted the unique challenges of carrying out trials in these cognitively and behaviorally challenged individuals, as well as a dearth of clinically relevant outcome measures for use in medication trials. However, in reviewing and reframing the studies of the last decade, many important lessons have been learned, which will ultimately have a greater impact on therapeutic research in the field of developmental delay as a whole.

Combination Therapy in Fragile X Syndrome; Possibilities and Pitfalls Illustrated by Targeting the mGluR5 and GABA Pathway Simultaneously

Fragile X syndrome (FXS) is the most common monogenetic cause of intellectual disability and autism. The disorder is characterized by altered synaptic plasticity in the brain. Synaptic plasticity is tightly regulated by a complex balance of different synaptic pathways. In FXS, various synaptic pathways are disrupted, including the excitatory metabotropic glutamate receptor 5 (mGluR5) and the inhibitory γ-aminobutyric acid (GABA) pathways. Targeting each of these pathways individually, has demonstrated beneficial effects in animal models, but not in patients with FXS. This lack of translation might be due to oversimplification of the disease mechanisms when targeting only one affected pathway, in spite of the complexity of the many pathways implicated in FXS. In this report we outline the hypothesis that targeting more than one pathway simultaneously, a combination therapy, might improve treatment effects in FXS. In addition, we present a glance of the first results of chronic combination therapy on social behavior in Fmr1 KO mice. In contrast to what we expected, targeting both the mGluR5 and the GABAergic pathways simultaneously did not result in a synergistic effect, but in a slight worsening of the social behavior phenotype. This does implicate that both pathways are interconnected and important for social behavior. Our results underline the tremendous fine-tuning that is needed to reach the excitatory-inhibitory balance in the synapse in relation to social behavior. We believe that alternative strategies focused on combination therapy should be further explored, including targeting pathways in different cellular compartments or cell-types.

Fragile X syndrome

Fragile X syndrome (FXS) is the leading inherited form of intellectual disability and autism spectrum disorder, and patients can present with severe behavioural alterations, including hyperactivity, impulsivity and anxiety, in addition to poor language development and seizures. FXS is a trinucleotide repeat disorder, in which >200 repeats of the CGG motif in FMR1 leads to silencing of the gene and the consequent loss of its product, fragile X mental retardation 1 protein (FMRP). FMRP has a central role in gene expression and regulates the translation of potentially hundreds of mRNAs, many of which are involved in the development and maintenance of neuronal synaptic connections. Indeed, disturbances in neuroplasticity is a key finding in FXS animal models, and an imbalance in inhibitory and excitatory neuronal circuits is believed to underlie many of the clinical manifestations of this disorder. Our knowledge of the proteins that are regulated by FMRP is rapidly growing, and this has led to the identification of multiple targets for therapeutic intervention, some of which have already moved into clinical trials or clinical practice.

A randomized double-blind, placebo-controlled trial of ganaxolone in children and adolescents with fragile X syndrome

Background

Gamma-aminobutyric acid (GABA) system deficits are integral to the pathophysiologic development of fragile X syndrome (FXS). Ganaxolone, a GABA_A receptor positive allosteric modulator, is hypothesized to improve symptoms such as anxiety, hyperactivity, and attention deficits in children with FXS.

Methods

This study was a randomized, double-blind, placebo-controlled, crossover trial of ganaxolone in children with FXS, aged 6–17 years.

Results

Sixty-one participants were assessed for eligibility, and 59 were randomized to the study. Fifty-five participants completed at least the first arm and were included in the intention-to-treat analysis; 51 participants completed both treatment arms. There were no statistically significant improvements observed on the primary outcome measure (Clinical Global Impression-Improvement), the key secondary outcome measure (Pediatric Anxiety Rating Scale-R), or any other secondary outcome measures in the overall study population. However, post-hoc analyses revealed positive trends in areas of anxiety, attention, and hyperactivity in participants with higher baseline anxiety and low full-scale IQ scores. No serious adverse events (AEs) occurred, although there was a significant increase in the frequency and severity of AEs related to ganaxolone compared to placebo.

Conclusions

While ganaxolone was found to be safe, there were no significant improvements in the outcome measures in the overall study population. However, ganaxolone in subgroups of children with FXS, including those with higher anxiety or lower cognitive abilities, might have beneficial effects.

Trial registration

ClinicalTrials.gov, NCT01725152

Impaired GABAergic inhibition in the hippocampus of Fmr1 knockout mice

Many clinical and molecular features of the fragile X syndrome, a common form of intellectual disability and autism, can be modeled by deletion of the Fmr1 protein (Fmrp) in mice. Previous studies showed a decreased expression of several components of the GABAergic system in Fmr1 knockout mice. Here, we used this mouse model to investigate the functional consequences of Fmrp deletion on hippocampal GABAergic inhibition in the CA1-region of the hippocampus. Whole-cell patch-clamp recordings demonstrated a significantly reduced amplitude of evoked inhibitory postsynaptic currents (eIPSCs) and a decrease in the amplitude and frequency of spontaneous IPSCs. In addition, miniature IPSCs were reduced in amplitude and frequency and decayed significantly slower than mIPSCs in controls. Quantitative real-time PCR revealed a significantly lower expression of α2, β1 and δ GABA_A receptor subunits in the hippocampus of the juvenile mice (P22) compared to wild-type littermates. Correspondingly, we found also at the protein level reduced amounts of α2, β1 and δ subunits in Fmr1 knockout mice. Overall, these results demonstrate that the reduction in several components of the GABAergic system is already present at young age and that this reduction results in measurable abnormalities on GABA_A receptor-mediated phasic inhibition. These abnormalities might contribute to the behavioral and cognitive deficits of this fragile X mouse model.

Monogenic mouse models of autism spectrum disorders: Common mechanisms and missing links

Autism spectrum disorders (ASDs) present unique challenges in the fields of genetics and neurobiology because of the clinical and molecular heterogeneity underlying these disorders. Genetic mutations found in ASD patients provide opportunities to dissect the molecular and circuit mechanisms underlying autistic behaviors using animal models. Ongoing studies of genetically modified models have offered critical insight into possible common mechanisms arising from different mutations, but links between molecular abnormalities and behavioral phenotypes remain elusive. The challenges encountered in modeling autism in mice demand a new analytic paradigm that integrates behavioral assessment with circuit-level analysis in genetically modified models with strong construct validity.

From de novo mutations to personalized therapeutic interventions in autism

The high heritability, early age at onset, and reproductive disadvantages of autism spectrum disorders (ASDs) are consistent with an etiology composed of dominant-acting de novo (spontaneous) mutations. Mutation detection by microarray analysis and DNA sequencing has confirmed that de novo copy-number variants or point mutations in protein-coding regions of genes contribute to risk, and some of the underlying causal variants and genes have been identified. As our understanding of autism genes develops, the spectrum of autism is breaking up into quanta of many different genetic disorders. Given the diversity of etiologies and underlying biochemical pathways, personalized therapy for ASDs is logical, and clinical genetic testing is a prerequisite.

The GABAA Receptor as a Therapeutic Target for Neurodevelopmental Disorders

Intellectual disability, autism spectrum disorder, and epilepsy are prime examples of neurodevelopmental disorders that collectively affect a significant percentage of the world population. Recent technological breakthroughs allowed the elucidation of the genetic causes of many of these disorders. As neurodevelopmental disorders are genetically heterogeneous, the development of rational therapy is extremely challenging. Fortunately, many causative genes are interconnected and cluster in specific cellular pathways. Targeting a common node in such a network would allow us to interfere with a series of related neurodevelopmental disorders at once. Here, we argue that the GABAergic system is disturbed in many neurodevelopmental disorders, including fragile X syndrome, Rett syndrome, and Dravet syndrome, and is a key candidate target for therapeutic intervention. Many drugs that modulate the GABAergic system have already been tested in animal models with encouraging outcomes and are readily available for clinical trials.

The quest for targeted therapy in fragile X syndrome

Fragile X syndrome (FXS) is the most common, monogenetic cause of intellectual disability and autism-spectrum disorders. Although there is no effective therapy, greater understanding of disturbed neuronal pathways has introduced options for targeted therapy. But whereas many FXS phenotypes were improved in preclinical studies with drugs targeting these pathways in the FXS mouse model, attempts to translate these animal-model success stories into treatment of patients in clinical trials have been extremely disappointing. Complicating factors, particularly in animal studies, include mouse inbred strains, variability in functional studies between laboratories, publication bias and lack of reliable and objective primary outcome measures in both mice and patients. Possibly most important, however, is one factor that has been little explored: the complexity of the molecular imbalance in FXS and the need to simultaneously target several different disturbed pathways and different cellular compartments. New, well-conceived animal studies should generate more productive approaches in the quest for targeted therapy for FXS.

Positron Emission Tomography (PET) Quantification of GABAA Receptors in the Brain of Fragile X Patients

Over the last several years, evidence has accumulated that the GABAA receptor is compromised in animal models for fragile X syndrome (FXS), a common hereditary form of intellectual disability. In mouse and fly models, agonists of the GABAA receptor were able to rescue specific consequences of the fragile X mutation. Here, we imaged and quantified GABAA receptors in vivo in brain of fragile X patients using Positron Emission Topography (PET) and [11C]flumazenil, a known high-affinity and specific ligand for the benzodiazepine site of GABAA receptors. We measured regional GABAA receptor availability in 10 fragile X patients and 10 control subjects. We found a significant reduction of on average 10% in GABAA receptor binding potential throughout the brain in fragile X patients. In the thalamus, the brain region showing the largest difference, the GABAA receptor availability was even reduced with 17%. This is one of the first reports of a PET study of human fragile X brain and directly demonstrates that the GABAA receptor availability is reduced in fragile X patients. The study reinforces previous hypotheses that the GABAA receptor is a potential target for rational pharmacological treatment of fragile X syndrome.

GABAB receptor upregulates fragile X mental retardation protein expression in neurons

Fragile X mental retardation protein (FMRP) is an RNA-binding protein important for the control of translation and synaptic function. The mutation or silencing of FMRP causes Fragile X syndrome (FXS), which leads to intellectual disability and social impairment. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter of the mammalian central nervous system, and its metabotropic GABAB receptor has been implicated in various mental disorders. The GABAB receptor agonist baclofen has been shown to improve FXS symptoms in a mouse model and in human patients, but the signaling events linking the GABAB receptor and FMRP are unknown. In this study, we found that GABAB receptor activation upregulated cAMP response element binding protein-dependent Fmrp expression in cultured mouse cerebellar granule neurons via two distinct mechanisms: the transactivation of insulin-like growth factor-1 receptor and activation of protein kinase C. In addition, a positive allosteric modulator of the GABAB receptor, CGP7930, stimulated Fmrp expression in neurons. These results suggest a role for GABAB receptor in Fmrp regulation and a potential interest of GABAB receptor signaling in FXS improvement.

Targeted pharmacological treatment of autism spectrum disorders: fragile X and Rett syndromes

Autism spectrum disorders (ASDs) are genetically and clinically heterogeneous and lack effective medications to treat their core symptoms. Studies of syndromic ASDs caused by single gene mutations have provided insights into the pathophysiology of autism. Fragile X and Rett syndromes belong to the syndromic ASDs in which preclinical studies have identified rational targets for drug therapies focused on correcting underlying neural dysfunction. These preclinical discoveries are increasingly translating into exciting human clinical trials. Since there are significant molecular and neurobiological overlaps among ASDs, targeted treatments developed for fragile X and Rett syndromes may be helpful for autism of different etiologies. Here, we review the targeted pharmacological treatment of fragile X and Rett syndromes and discuss related issues in both preclinical studies and clinical trials of potential therapies for the diseases.

The GABAA receptor is an FMRP target with therapeutic potential in fragile X syndrome

Previous research indicates that the GABAAergic system is involved in the pathophysiology of the fragile X syndrome, a frequent form of inherited intellectual disability and associated with autism spectrum disorder. However, the molecular mechanism underlying GABAAergic deficits has remained largely unknown. Here, we demonstrate reduced mRNA expression of GABAA receptor subunits in the cortex and cerebellum of young Fmr1 knockout mice. In addition, we show that the previously reported underexpression of specific subunits of the GABAA receptor can be corrected in YAC transgenic rescue mice, containing the full-length human FMR1 gene in an Fmr1 knockout background. Moreover, we demonstrate that FMRP directly binds several GABAA receptor mRNAs. Finally, positive allosteric modulation of GABAA receptors with the neurosteroid ganaxolone can modulate specific behaviors in Fmr1 knockout mice, emphasizing the therapeutic potential of the receptor.

Clinical, molecular, and pharmacological aspects of FMR1 related disorders

BACKGROUND

Fragile X syndrome, the most common inherited cause of intellectual disability, is associated with a broad spectrum of disorders across different generations of a single family. This study reviews the clinical manifestations of fragile X-associated disorders as well as the spectrum of mutations of the fragile X mental retardation 1 gene (FMR1) and the neurobiology of the fragile X mental retardation protein (FMRP), and also provides an overview of the potential therapeutic targets and genetic counselling.

DEVELOPMENT

This disorder is caused by expansion of the CGG repeat (>200 repeats) in the 5 prime untranslated region of FMR1, resulting in a deficit or absence of FMRP. FMRP is an RNA-binding protein that regulates the translation of several genes that are important in synaptic plasticity and dendritic maturation. It is believed that CGG repeat expansions in the premutation range (55 to 200 repeats) elicit an increase in mRNA levels of FMR1, which may cause neuronal toxicity. These changes manifest clinically as developmental problems such as autism and learning disabilities as well as neurodegenerative diseases including fragile X-associated tremor/ataxia syndrome (FXTAS).

CONCLUSIONS

Advances in identifying the molecular basis of fragile X syndrome may help us understand the causes of neuropsychiatric disorders, and they will probably contribute to development of new and specific treatments.

Clinical potential, safety, and tolerability of arbaclofen in the treatment of autism spectrum disorder

Autism spectrum disorder (ASD) is a behaviorally defined disorder which has increased in prevalence over the last two decades. Despite decades of research, no effective treatment is currently available. Animal models, as well as other lines of evidence, point to abnormalities in the balance of cortical excitation to inhibition in individuals with ASD, with this imbalance resulting in an overall increase in cortical excitation. To reduce cortical excitatory glutamate pathways, arbaclofen, a selective agonist of the gamma aminobutyric acid receptor type B, has been developed. This article reviews the evidence for this treatment for ASD using a systematic review methodology. Overall, a systematic search of the literature revealed 148 relevant references with the majority of these being review papers or news items that mentioned the potential promise of arbaclofen. Five original studies were identified, four of which used STX209, a form of arbaclofen developed by Seaside Therapeutics, Inc., and one which used R-baclofen. In an animal model, treatment of Fragile X, a genetic disease with ASD features, demonstrated a reversal of behavioral, neurological, and neuropathological features associated with the disease. One double-blind, placebo-controlled study treated children and adults with Fragile X. Results from this study were promising, with signs of improvement in social function, especially in the most severely socially impaired. Two studies, one open-label and one double-blind, placebo-controlled, were conducted in children, adolescents, and young adults with ASD. These studies suggested some improvements in socialization, although the effects were limited and may have been driven by individuals with ASD that were higher-functioning. These studies and others that have used arbaclofen for the treatment of gastroesophageal reflux suggest that arbaclofen is safe and well-tolerated. Clearly, further clinical studies are needed in order to refine the symptoms and characteristics of children with ASD that are best treated with arbaclofen.

Modulation of the GABAergic pathway for the treatment of fragile X syndrome

Fragile X syndrome (FXS) is the most common genetic cause of intellectual disability and the most common single-gene cause of autism. It is caused by mutations on the fragile X mental retardation gene (FMR1) and lack of fragile X mental retardation protein, which in turn, leads to decreased inhibition of translation of many synaptic proteins. The metabotropic glutamate receptor (mGluR) hypothesis states that the neurological deficits in individuals with FXS are due mainly to downstream consequences of overstimulation of the mGluR pathway. The main efforts have focused on mGluR5 targeted treatments; however, investigation on the gamma-aminobutyric acid (GABA) system and its potential as a targeted treatment is less emphasized. The fragile X mouse models (Fmr1-knock out) show decreased GABA subunit receptors, decreased synthesis of GABA, increased catabolism of GABA, and overall decreased GABAergic input in many regions of the brain. Consequences of the reduced GABAergic input in FXS include oversensitivity to sensory stimuli, seizures, and anxiety. Deficits in the GABA receptors in different regions of the brain are associated with behavioral and attentional processing deficits linked to anxiety and autistic behaviors. The understanding of the neurobiology of FXS has led to the development of targeted treatments for the core behavioral features of FXS, which include social deficits, inattention, and anxiety. These symptoms are also observed in individuals with autism and other neurodevelopmental disorders, therefore the targeted treatments for FXS are leading the way in the treatment of other neurodevelopmental syndromes and autism. The GABAergic system in FXS represents a target for new treatments. Herein, we discuss the animal and human trials of GABAergic treatment in FXS. Arbaclofen and ganaxolone have been used in individuals with FXS. Other potential GABAergic treatments, such as riluzole, gaboxadol, tiagabine, and vigabatrin, will be also discussed. Further studies are needed to determine the safety and efficacy of GABAergic treatments for FXS.