The psychiatric presentation of fragile x: evolution of the diagnosis and treatment of the psychiatric comorbidities of fragile X syndrome

EF1α-associated protein complexes affect dendritic spine plasticity by regulating microglial phagocytosis in Fmr1 knock-out mice

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. There is no specific treatment for FXS due to the lack of therapeutic targets. We report here that Elongation Factor 1α (EF1α) forms a complex with two other proteins: Tripartite motif-containing protein 3 (TRIM3) and Murine double minute (Mdm2). Both EF1α-Mdm2 and EF1α-TRIM3 protein complexes are increased in the brain of Fmr1 knockout mice as a result of FMRP deficiency, which releases the normal translational suppression of EF1α mRNA and increases EF1α protein levels. Increased EF1α-Mdm2 complex decreases PSD-95 ubiquitination (Ub-PSD-95) and Ub-PSD-95-C1q interaction. The elevated level of TRIM3-EF1α complex is associated with decreased TRIM3-Complement Component 3 (C3) complex that inhibits the activation of C3. Both protein complexes thereby contribute to a reduction in microglia-mediated phagocytosis and dendritic spine pruning. Finally, we created a peptide that disrupts both protein complexes and restores dendritic spine plasticity and behavioural deficits in Fmr1 knockout mice. The EF1α-Mdm2 and EF1α-TRIM3 complexes could thus be new therapeutic targets for FXS.

Sensory Symptoms and Signs of Hyperarousal in Individuals with Fragile X Syndrome: Findings from the FORWARD Registry and Database Multisite Study

This study was designed to increase our understanding about characteristics and the impact of sensory symptoms (SS) and signs of hyperarousal (HA) in individuals with fragile X syndrome (FXS) from childhood through early adulthood and by gender. Data derived from the Fragile X Online Registry With Accessible Research Database (FORWARD), a natural history study of FXS, were analyzed using descriptive statistics and multivariate linear and logistic regression models to examine SS and signs of HA, their impact on behavioral regulation and limitations on the subject/family. The sample (N = 933) consisted of 720 males and 213 females. More males were affected with SS (87% vs. 68%) and signs of HA (92% vs. 79%). Subjects who were endorsed as having a strong sensory response had more comorbidities, including behavioral problems. The predominant SS was difficulty with eye gaze that increased with age in both genders. As individuals age, there was less use of non-medication therapies, such as occupational therapy (OT)/physical therapy (PT), but there was more use of psychopharmacological medications and investigational drugs for behaviors. Multiple regression models suggested that endorsing SS and signs of HA was associated with statistically significantly increased ABC-C-I subscale scores and limited participation in everyday activities. This study improves our understanding of SS and signs of HA as well as their impact in FXS. It supports the need for more research regarding these clinical symptoms, especially to understand how they contribute to well-known behavioral concerns.

A review of the use of psychotropic medication to address challenging behaviour in neurodevelopmental disorders

Engagement in challenging behaviour (e.g., aggression, self-injury) is reported to occur in neurodevelopmental disorders such as intellectual disabilities (ID), autism spectrum disorder (ASD), and fragile X syndrome (FXS). Common interventions to address these behaviours include both behavioural and pharmacological approaches. Although psychotropic medications are commonly used to address challenging behaviour in ID, ASD, and FXS, demonstration of the effectiveness of treatment is limited. Furthermore, research examining interaction effects between psychotropic medication, challenging behaviour, and environmental events within specific neurodevelopmental disorders such as ID, ASD, and FXS is scarce. The purpose of this chapter is to provide an overview of challenging behaviour within ID, ASD, and FXS and of the effectiveness of psychotropic medication as an intervention for challenging behaviour within these neurodevelopmental disorders. Finally, research examining how psychotropic medication may impact the relationship between challenging behaviour and environmental events is reviewed.

Intercorrelation of Molecular Biomarkers and Clinical Phenotype Measures in Fragile X Syndrome

This study contributes to a greater understanding of the utility of molecular biomarkers to identify clinical phenotypes of fragile X syndrome (FXS). Correlations of baseline clinical trial data (molecular measures-FMR1 mRNA, CYFIP1 mRNA, MMP9 and FMRP protein expression levels, nonverbal IQ, body mass index and weight, language level, NIH Toolbox, adaptive behavior rating, autism, and other mental health correlates) of 59 participants with FXS ages of 6-32 years are reported. FMR1 mRNA expression levels correlated positively with adaptive functioning levels, expressive language, and specific NIH Toolbox measures. The findings of a positive correlation of MMP-9 levels with obesity, CYFIP1 mRNA with mood and autistic symptoms, and FMR1 mRNA expression level with better cognitive, language, and adaptive functions indicate potential biomarkers for specific FXS phenotypes. These may be potential markers for future clinical trials for targeted treatments of FXS.

Neurological, Psychiatric, and Multisystemic Involvement of Fragile X Syndrome Along With Its Pathophysiology, Methods of Screening, and Current Treatment Modalities

Fragile X syndrome (FXS) is a hereditary disease that predominantly leads to intellectual disability (ID) in boys. It is the second prominent cause of ID, which manifests as a result of the atypical development of the cytosine-guanine-guanine (CGG) region. This irregular extension of the CGG region gives rise to methylation and silencing of the fragile X mental retardation 1 (FMR1) gene, causing a loss of the fragile X mental retardation 1 protein (FMRP). This reduction or loss of FMRP is the main cause of ID. It has a multisystemic involvement showing neuropsychiatric features such as ID, speech and language delay, autism spectrum disorder, sensory hyperarousal, social anxiety, abnormal eye contact, shyness, and aggressive behaviour. It is also known to cause musculoskeletal symptoms, ocular symptoms, cardiac abnormalities, and gastrointestinal symptoms. The management is challenging, and there is no known cure for the disease; hence an early diagnosis of the condition is needed through prenatal screening offered to couples with familial history of ID before conception. The management rests on non-pharmacological modalities, including applied behaviour analysis, physical therapy, occupational therapy, speech-language therapy, and pharmacologic management through symptomatic treatment of comorbid behaviours and psychiatric problems and some forms of targeted therapy.

Role of the endocannabinoid system in fragile X syndrome: potential mechanisms for benefit from cannabidiol treatment

Multiple lines of evidence suggest a central role for the endocannabinoid system (ECS) in the neuronal development and cognitive function and in the pathogenesis of fragile X syndrome (FXS). This review describes the ECS, its role in the central nervous system, how it is dysregulated in FXS, and the potential role of cannabidiol as a treatment for FXS. FXS is caused by deficiency or absence of the fragile X messenger ribonucleoprotein 1 (FMR1) protein, FMRP, typically due to the presence of >200 cytosine, guanine, guanine sequence repeats leading to methylation of the FMR1 gene promoter. The absence of FMRP, following FMR1 gene-silencing, disrupts ECS signaling, which has been implicated in FXS pathogenesis. The ECS facilitates synaptic homeostasis and plasticity through the cannabinoid receptor 1, CB₁, on presynaptic terminals, resulting in feedback inhibition of neuronal signaling. ECS-mediated feedback inhibition and synaptic plasticity are thought to be disrupted in FXS, leading to overstimulation, desensitization, and internalization of presynaptic CB₁ receptors. Cannabidiol may help restore synaptic homeostasis by acting as a negative allosteric modulator of CB₁, thereby attenuating the receptor overstimulation, desensitization, and internalization. Moreover, cannabidiol affects DNA methylation, serotonin 5HT_1A signal transduction, gamma-aminobutyric acid receptor signaling, and dopamine D₂ and D₃ receptor signaling, which may contribute to beneficial effects in patients with FXS. Consistent with these proposed mechanisms of action of cannabidiol in FXS, in the CONNECT-FX trial the transdermal cannabidiol gel, ZYN002, was associated with improvements in measures of social avoidance, irritability, and social interaction, particularly in patients who are most affected, showing ≥90% methylation of the FMR1 gene.

A randomized, controlled trial of ZYN002 cannabidiol transdermal gel in children and adolescents with fragile X syndrome (CONNECT-FX)

BACKGROUND

Fragile X syndrome (FXS) is associated with dysregulated endocannabinoid signaling and may therefore respond to cannabidiol therapy.

DESIGN

CONNECT-FX was a double-blind, randomized phase 3 trial assessing efficacy and safety of ZYN002, transdermal cannabidiol gel, for the treatment of behavioral symptoms in children and adolescents with FXS.

METHODS

Patients were randomized to 12 weeks of ZYN002 (250 mg or 500 mg daily [weight-based]) or placebo, as add-on to standard of care. The primary endpoint assessed change in social avoidance (SA) measured by the Aberrant Behavior Checklist-Community Edition FXS (ABC-C_FXS) SA subscale in a full cohort of patients with a FXS full mutation, regardless of the FMR1 methylation status. Ad hoc analyses assessed efficacy in patients with ≥ 90% and 100% methylation of the promoter region of the FMR1 gene, in whom FMR1 gene silencing is most likely.

RESULTS

A total of 212 patients, mean age 9.7 years, 75% males, were enrolled. A total of 169 (79.7%) patients presented with ≥ 90% methylation of the FMR1 promoter and full mutation of FMR1. Although statistical significance for the primary endpoint was not achieved in the full cohort, significant improvement was demonstrated in patients with ≥ 90% methylation of FMR1 (nominal P = 0.020). This group also achieved statistically significant improvements in Caregiver Global Impression-Change in SA and isolation, irritable and disruptive behaviors, and social interactions (nominal P-values: P = 0.038, P = 0.028, and P = 0.002). Similar results were seen in patients with 100% methylation of FMR1. ZYN002 was safe and well tolerated. All treatment-emergent adverse events (TEAEs) were mild or moderate. The most common treatment-related TEAE was application site pain (ZYN002: 6.4%; placebo: 1.0%).

CONCLUSIONS

In CONNECT-FX, ZYN002 was well tolerated in patients with FXS and demonstrated evidence of efficacy with a favorable benefit risk relationship in patients with ≥ 90% methylation of the FMR1 gene, in whom gene silencing is most likely, and the impact of FXS is typically most severe.

TRIAL REGISTRATION

The CONNECT-FX trial is registered on Clinicaltrials.gov (NCT03614663).

Behavior Problems and Social Competence in Fragile X Syndrome: A Systematic Review

Fragile X syndrome (FXS) causes intellectual disability and is the known leading cause of autism. Common problems in FXS include behavior and social problems. Along with syndromic characteristics and autism comorbidity, environmental factors might influence these difficulties. This systematic review focuses on the last 20 years of studies concerning behavior and social problems in FXS, considering environmental and personal variables that might influence both problems. Three databases were reviewed, leading to fifty-one studies meeting the inclusion criteria. Attention deficit hyperactivity disorder (ADHD) problems remain the greatest behavior problems, with behavioral problems and social competence being stable during the 20 years. Some developmental trajectories might have changed due to higher methodological control, such as aggressive behavior and attention problems. The socialization trajectory from childhood to adolescence remains unclear. Comorbidity with autism in individuals with FXS increased behavior problems and worsened social competence profiles. At the same time, comparisons between individuals with comorbid FXS and autism and individuals with autism might help define the comorbid phenotype. Environmental factors and parental characteristics influenced behavior problems and social competence. Higher methodological control is needed in studies including autism symptomatology and parental characteristics. More studies comparing autism in FXS with idiopathic autism are needed to discern differences between conditions.

Functional consequences of postnatal interventions in a mouse model of Fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is a leading genetic cause of autism and intellectual disability with cortical hyperexcitability and sensory hypersensitivity attributed to loss and hypofunction of inhibitory parvalbumin-expressing (PV) cells. Our studies provide novel insights into the role of excitatory neurons in abnormal development of PV cells during a postnatal period of inhibitory circuit refinement.

METHODS

To achieve Fragile X mental retardation gene (Fmr1) deletion and re-expression in excitatory neurons during the postnatal day (P)14-P21 period, we generated Cre^CaMKIIa/Fmr1^Flox/y (cOFF) and Cre^CaMKIIa/Fmr1^FloxNeo/y (cON) mice, respectively. Cortical phenotypes were evaluated in adult mice using biochemical, cellular, clinically relevant electroencephalogram (EEG) and behavioral tests.

RESULTS

We found that similar to global Fmr1 KO mice, the density of PV-expressing cells, their activation, and sound-evoked gamma synchronization were impaired in cOFF mice, but the phenotypes were improved in cON mice. cOFF mice also showed enhanced cortical gelatinase activity and baseline EEG gamma power, which were reduced in cON mice. In addition, TrkB phosphorylation and PV levels were lower in cOFF mice, which also showed increased locomotor activity and anxiety-like behaviors. Remarkably, when FMRP levels were restored in only excitatory neurons during the P14-P21 period, TrkB phosphorylation and mouse behaviors were also improved.

CONCLUSIONS

These results indicate that postnatal deletion or re-expression of FMRP in excitatory neurons is sufficient to elicit or ameliorate structural and functional cortical deficits, and abnormal behaviors in mice, informing future studies about appropriate treatment windows and providing fundamental insights into the cellular mechanisms of cortical circuit dysfunction in FXS.

Psychotropic Drug Treatment Patterns in Persons with Fragile X Syndrome

Objective: Psychiatric comorbidity is common in fragile X syndrome (FXS) and often addressed through pharmacological management. Here we examine data in the Fragile X Online Registry With Accessible Research Database (FORWARD) to characterize specific symptoms being treated with psychotropic medication, patterns of medication use, as well as the influence of gender, intellectual disability (ID), age, and autism spectrum disorder (ASD) diagnosis. Methods: Data were drawn from the 975 participants who have a completed clinician form. We explored the frequency of psychotropic medication use for the following symptom clusters: attention, hyperactivity, anxiety, hypersensitivity, obsessive-compulsive disorder (OCD), mood swings, irritability/agitation, aggression, and self-injury (IAAS). Results: A majority of participants (617 or 63.3%) were taking a psychotropic medication, including investigational drugs. Medications were often targeting multiple symptoms. Psychotropic medication use was more common in males, adolescents, and those with comorbid ID and ASD. Anxiety was the most frequently targeted symptom, followed by attention-deficit/hyperactivity disorder symptoms and IAAS. Selective serotonin reuptake inhibitors (SSRIs) were the most frequently prescribed medication class among all patients (n = 266, 43%), followed by stimulants (n = 235, 38%), each with no gender difference. Antipsychotics were the third most frequently prescribed medication class (n = 205, 33%), and were more frequently prescribed to males and those with ID and ASD. Conclusions: Anxiety, attention and hyperactivity were the most common symptom targets for psychopharmacologic intervention in FXS. Our results support clinical knowledge that males with comorbid ASD and ID have a more severe presentation requiring more intervention including medications. These results highlight the need for examination of symptom overlap and interaction.

Deletion of Fmr1 from Forebrain Excitatory Neurons Triggers Abnormal Cellular, EEG, and Behavioral Phenotypes in the Auditory Cortex of a Mouse Model of Fragile X Syndrome

Fragile X syndrome (FXS) is a leading genetic cause of autism with symptoms that include sensory processing deficits. In both humans with FXS and a mouse model [Fmr1 knockout (KO) mouse], electroencephalographic (EEG) recordings show enhanced resting state gamma power and reduced sound-evoked gamma synchrony. We previously showed that elevated levels of matrix metalloproteinase-9 (MMP-9) may contribute to these phenotypes by affecting perineuronal nets (PNNs) around parvalbumin (PV) interneurons in the auditory cortex of Fmr1 KO mice. However, how different cell types within local cortical circuits contribute to these deficits is not known. Here, we examined whether Fmr1 deletion in forebrain excitatory neurons affects neural oscillations, MMP-9 activity, and PV/PNN expression in the auditory cortex. We found that cortical MMP-9 gelatinase activity, mTOR/Akt phosphorylation, and resting EEG gamma power were enhanced in CreNex1/Fmr1Flox/y conditional KO (cKO) mice, whereas the density of PV/PNN cells was reduced. The CreNex1/Fmr1Flox/y cKO mice also show increased locomotor activity, but not the anxiety-like behaviors. These results indicate that fragile X mental retardation protein changes in excitatory neurons in the cortex are sufficient to elicit cellular, electrophysiological, and behavioral phenotypes in Fmr1 KO mice. More broadly, these results indicate that local cortical circuit abnormalities contribute to sensory processing deficits in autism spectrum disorders.

Adenosine A2A receptor inhibition reduces synaptic and cognitive hippocampal alterations in Fmr1 KO mice

In fragile X syndrome (FXS) the lack of the fragile X mental retardation protein (FMRP) leads to exacerbated signaling through the metabotropic glutamate receptors 5 (mGlu5Rs). The adenosine A_2A receptors (A_2ARs), modulators of neuronal damage, could play a role in FXS. A synaptic colocalization and a strong permissive interaction between A_2A and mGlu5 receptors in the hippocampus have been previously reported, suggesting that blocking A_2ARs might normalize the mGlu5R-mediated effects of FXS. To study the cross-talk between A_2A and mGlu5 receptors in the absence of FMRP, we performed extracellular electrophysiology experiments in hippocampal slices of Fmr1 KO mouse. The depression of field excitatory postsynaptic potential (fEPSPs) slope induced by the mGlu5R agonist CHPG was completely blocked by the A_2AR antagonist ZM241385 and strongly potentiated by the A_2AR agonist CGS21680, suggesting that the functional synergistic coupling between the two receptors could be increased in FXS. To verify if chronic A_2AR blockade could reverse the FXS phenotypes, we treated the Fmr1 KO mice with istradefylline, an A_2AR antagonist. We found that hippocampal DHPG-induced long-term depression (LTD), which is abnormally increased in FXS mice, was restored to the WT level. Furthermore, istradefylline corrected aberrant dendritic spine density, specific behavioral alterations, and overactive mTOR, TrkB, and STEP signaling in Fmr1 KO mice. Finally, we identified A_2AR mRNA as a target of FMRP. Our results show that the pharmacological blockade of A_2ARs partially restores some of the phenotypes of Fmr1 KO mice, both by reducing mGlu5R functioning and by acting on other A_2AR-related downstream targets.

PKC epsilon as a neonatal target to correct FXS-linked AMPA receptor translocation in the hippocampus, boost PVN oxytocin expression, and normalize adult behavior in Fmr1 knockout mice

Fragile X Syndrome (FXS) is an inherited developmental disorder caused by the non-expression of the Fmr1 gene. FXS is associated with abnormal social and anxiety behavior that is more prominent among males. Given that oxytocin (OXT) regulates both social and anxiety behavior, we studied the effect of FXS in the hypothalamic paraventricular nucleus (PVN), the major central source of OXT. We observed a significant suppression of protein kinase C epsilon (PKCε) (34%) in the ventral hippocampal CA1 region of postnatal day-18 (P18) male Fmr1 knockout (KO) mice, which displayed social behavior deficits and hyper-anxiety in adulthood. These mice also displayed a 39% increase in cell surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) at P18 (measured by the surface level of the AMPAR subunit GluR2), thereby indicating excitation of the CA1 neurons. It is known that neuronal activation at CA1 is linked to an inhibition of the PVN neurons. As expected, these mice also displayed a 25% suppression of oxytocin+ (OXT+) cells in the PVN at P20. Stimulating PKCε during postnatal days 6-,14 (P6-14) mice using a selective activator, dicyclopropyl-linoleic acid (DCP-LA), corrected AMPAR externalization in CA1 and suppression of OXT+ cell number in PVN in a PKCε dependent manner. Most notably, neonatal DCP-LA treatment rescued social behavior deficits and hyper-anxiety, displayed by adult (≥P60) male but not female KO mice. Thus, neonatal stimulation of PKCε could be a strategy to correct endophenotypic anomalies during brain development and aberrant adult behavior of the FXS males to the wild-type levels.

Gamma power abnormalities in a Fmr1-targeted transgenic rat model of fragile X syndrome

Fragile X syndrome (FXS) is characteristically displayed intellectual disability, hyperactivity, anxiety, and abnormal sensory processing. Electroencephalography (EEG) abnormalities are also observed in subjects with FXS, with many researchers paying attention to these as biomarkers. Despite intensive preclinical research using Fmr1 knock out (KO) mice, an effective treatment for FXS has yet to be developed. Here, we examined Fmr1-targeted transgenic rats (Fmr1-KO rats) as an alternative preclinical model of FXS. We characterized the EEG phenotypes of Fmr1-KO rats by measuring basal EEG power and auditory steady state response (ASSR) to click trains of stimuli at a frequency of 10–80 Hz. Fmr1-KO rats exhibited reduced basal alpha power and enhanced gamma power, and these rats showed enhanced locomotor activity in novel environment. While ASSR clearly peaked at around 40 Hz, both inter-trial coherence (ITC) and event-related spectral perturbation (ERSP) were significantly reduced at the gamma frequency band in Fmr1-KO rats. Fmr1-KO rats showed gamma power abnormalities and behavioral hyperactivity that were consistent with observations reported in mouse models and subjects with FXS. These results suggest that gamma power abnormalities are a translatable biomarker among species and demonstrate the utility of Fmr1-KO rats for investigating drugs for the treatment of FXS.

ATP Synthase c-Subunit Leak Causes Aberrant Cellular Metabolism in Fragile X Syndrome

Loss of the gene (Fmr1) encoding Fragile X mental retardation protein (FMRP) causes increased mRNA translation and aberrant synaptic development. We find neurons of the Fmr1^-/y mouse have a mitochondrial inner membrane leak contributing to a "leak metabolism." In human Fragile X syndrome (FXS) fibroblasts and in Fmr1^-/y mouse neurons, closure of the ATP synthase leak channel by mild depletion of its c-subunit or pharmacological inhibition normalizes stimulus-induced and constitutive mRNA translation rate, decreases lactate and key glycolytic and tricarboxylic acid (TCA) cycle enzyme levels, and triggers synapse maturation. FMRP regulates leak closure in wild-type (WT), but not FX synapses, by stimulus-dependent ATP synthase β subunit translation; this increases the ratio of ATP synthase enzyme to its c-subunit, enhancing ATP production efficiency and synaptic growth. In contrast, in FXS, inability to close developmental c-subunit leak prevents stimulus-dependent synaptic maturation. Therefore, ATP synthase c-subunit leak closure encourages development and attenuates autistic behaviors.

Auditory EEG Biomarkers in Fragile X Syndrome: Clinical Relevance

Sensory hypersensitivities are common and distressing features of Fragile X Syndrome (FXS). While there are many drug interventions that reduce behavioral deficits in Fmr1 mice and efforts to translate these preclinical breakthroughs into clinical trials for FXS, evidence-based clinical interventions are almost non-existent potentially due to lack of valid neural biomarkers. Local circuit function in sensory networks is dependent on the dynamic balance of activity in inhibitory/excitatory synapses. Studies are needed to examine the association of electrophysiological alterations in neural systems with sensory and other clinical features of FXS to establish their clinical relevance. Adolescents and adults with FXS (n = 38, Mean age = 25.5, std = 10.1; 13 females) and age matched typically developing controls (n = 40, Mean age = 27.7, std = 12.1; 17 females) completed auditory chirp and auditory habituation tasks while undergoing dense array electroencephalography (EEG). Amplitude, latency, and percent change (habituation) in N1 and P2 event-related potential (ERP) components were characterized for the habituation task; time-frequency calculations using Morlet wavelets characterized phase-locking and single trial power for the habituation and chirp tasks. FXS patients showed increased amplitude but some evidence for reduced habituation of the N1 ERP, and reduced phase-locking in the low and high gamma frequency range and increased low gamma power to the chirp stimulus. FXS showed increased theta power in both tasks. While the habituation finding was weaker than previously found, the remaining findings replicate our previous work in a new sample of patients with FXS. Females showed less deficit in the chirp task but not the habituation task. Abnormal increases in gamma power were related to more severe behavioral and psychiatric features as well as reductions in neurocognitive abilities. Replicating electrophysiological deficits in a new group of patients using different EEG equipment at a new data collection site with differing levels of environmental noise that were robust to data processing techniques utilizing multiple researchers, indicates a potential for scalability to multi-site clinical trials. Given the robust replicability, relevance to clinical measures, and preclinical evidence for sensitivity of these EEG measures to pharmacological intervention, the observed abnormalities may provide novel translational markers of target engagement and potentially outcome measures in large-scale studies evaluating new treatments targeting neural hyperexcitability in FXS.

The zebrafish subcortical social brain as a model for studying social behavior disorders

Social behaviors are essential for the survival and reproduction of social species. Many, if not most, neuropsychiatric disorders in humans are either associated with underlying social deficits or are accompanied by social dysfunctions. Traditionally, rodent models have been used to model these behavioral impairments. However, rodent assays are often difficult to scale up and adapt to high-throughput formats, which severely limits their use for systems-level science. In recent years, an increasing number of studies have used zebrafish (Danio rerio) as a model system to study social behavior. These studies have demonstrated clear potential in overcoming some of the limitations of rodent models. In this Review, we explore the evolutionary conservation of a subcortical social brain between teleosts and mammals as the biological basis for using zebrafish to model human social behavior disorders, while summarizing relevant experimental tools and assays. We then discuss the recent advances gleaned from zebrafish social behavior assays, the applications of these assays to studying related disorders, and the opportunities and challenges that lie ahead.

Home-cage hypoactivity in mouse genetic models of autism spectrum disorder

Genome-wide association and whole exome sequencing studies from Autism Spectrum Disorder (ASD) patient populations have implicated numerous risk factor genes whose mutation or deletion results in significantly increased incidence of ASD. Behavioral studies of monogenic mutant mouse models of ASD-associated genes have been useful for identifying aberrant neural circuitry. However, behavioral results often differ from lab to lab, and studies incorporating both males and females are often not performed despite the significant sex-bias of ASD. In this study, we sought to investigate the simple, passive behavior of home-cage activity monitoring across multiple 24-h days in four different monogenic mouse models of ASD: Shank3b^-/-, Cntnap2^-/-, Pcdh10^+/-, and Fmr1 knockout mice. Relative to sex-matched wildtype (WT) littermates, we discovered significant home-cage hypoactivity, particularly in the dark (active) phase of the light/dark cycle, in male mice of all four ASD-associated transgenic models. For Cntnap2^-/- and Pcdh10^+/- mice, these activity alterations were sex-specific, as female mice did not exhibit home-cage activity differences relative to sex-matched WT controls. These home-cage hypoactivity alterations differ from activity findings previously reported using short-term activity measurements in a novel open field. Despite circadian problems reported in human ASD patients, none of the mouse models studied had alterations in free-running circadian period. Together, these findings highlight a shared phenotype across several monogenic mouse models of ASD, outline the importance of methodology on behavioral interpretation, and in some genetic lines parallel the male-enhanced phenotypic presentation observed in human ASDs.

Neuroactive Steroids Reverse Tonic Inhibitory Deficits in Fragile X Syndrome Mouse Model

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. A reduction in neuronal inhibition mediated by γ-aminobutyric acid type A receptors (GABA_ARs) has been implicated in the pathophysiology of FXS. Neuroactive steroids (NASs) are known allosteric modulators of GABA_AR channel function, but recent studies from our laboratory have revealed that NASs also exert persistent metabotropic effects on the efficacy of tonic inhibition by increasing the protein kinase C (PKC)-mediated phosphorylation of the α4 and β3 subunits which increase the membrane expression and boosts tonic inhibition. We have assessed the GABAergic signaling in the hippocampus of fragile X mental retardation protein (FMRP) knock-out (Fmr1KO) mouse. The GABAergic tonic current in dentate gyrus granule cells (DGGCs) from 3- to 5-week-old (p21–35) Fmr1KO mice was significantly reduced compared to WT mice. Additionally, spontaneous inhibitory post synaptic inhibitory current (sIPSC) amplitudes were increased in DGGCs from Fmr1 KO mice. While sIPSCs decay in both genotypes was prolonged by the prototypic benzodiazepine diazepam, those in Frm1-KO mice were selectively potentiated by RO15-4513. Consistent with this altered pharmacology, modifications in the expression levels and phosphorylation of receptor GABA_AR subtypes that mediate tonic inhibition were seen in Fmr1 KO mice. Significantly, exposure to NASs induced a sustained elevation in tonic current in Fmr1 KO mice which was prevented with PKC inhibition. Likewise, exposure reduced elevated membrane excitability seen in the mutant mice. Collectively, our results suggest that NAS act to reverse the deficits of tonic inhibition seen in FXS, and thereby reduce aberrant neuronal hyperexcitability seen in this disorder.

Paradoxical effect of baclofen on social behavior in the fragile X syndrome mouse model

INTRODUCTION

Fragile X syndrome (FXS) is a common monogenetic cause of intellectual disability, autism spectrum features, and a broad range of other psychiatric and medical problems. FXS is caused by the lack of the fragile X mental retardation protein (FMRP), a translational regulator of specific mRNAs at the postsynaptic compartment. The absence of FMRP leads to aberrant synaptic plasticity, which is believed to be caused by an imbalance in excitatory and inhibitory network functioning of the synapse. Evidence from studies in mice demonstrates that GABA, the major inhibitory neurotransmitter in the brain, and its receptors, is involved in the pathogenesis of FXS. Moreover, several FXS phenotypes, including social behavior deficits, could be corrected in Fmr1 KO mice after acute treatment with GABAB agonists.

METHODS

As FXS would probably require a lifelong treatment, we investigated the effect of chronic treatment with the GABAB agonist baclofen on social behavior in Fmr1 KO mice on two behavioral paradigms for social behavior: the automated tube test and the three-chamber sociability test.

RESULTS

Unexpectedly, chronic baclofen treatment resulted in worsening of the FXS phenotypes in these behavior tests. Strikingly, baclofen treatment also affected wild-type animals in both behavioral tests, inducing a phenotype similar to that of untreated Fmr1 KO mice.

CONCLUSION

Altogether, the disappointing results of recent clinical trials with the R-baclofen enantiomer arbaclofen and our current results indicate that baclofen should be reconsidered and further evaluated before its application in targeted treatment for FXS.

Adult ADHD and Comorbid Somatic Disease: A Systematic Literature Review

OBJECTIVE

To systematically review, synthesize, and appraise available evidence, connecting adult ADHD with somatic disease.

METHOD

Embase, Psychinfo, and Medline databases were searched for studies published from 1994 to 2015 addressing adult ADHD and somatic comorbidity. Somatic conditions were classified according to International Classification of Diseases (ICD-10) codes. Levels of evidence were graded as inconclusive, tentative, or well documented.

RESULTS

Most of the 126 studies included in the qualitative synthesis were small and of modest quality. Obesity, sleep disorders, and asthma were well-documented comorbidities in adult ADHD. Tentative evidence was found for an association between adult ADHD and migraine and celiac disease. In a large health registry study, cardiovascular disease was not associated with adult ADHD.

CONCLUSION

There are few large systematic studies using standardized diagnostic criteria evaluating adult ADHD and somatic comorbidities. Significant associations are found between adult ADHD and several somatic diseases, and these are important to consider when assessing and treating either adult ADHD or the somatic diseases.

A Screening Tool to Measure Eye Contact Avoidance in Boys with Fragile X Syndrome

We examined the reliability, validity and factor structure of the Eye Contact Avoidance Scale (ECAS), a new 15-item screening tool designed to measure eye contact avoidance in individuals with fragile X syndrome (FXS). Internal consistency of the scale was acceptable to excellent and convergent validity with the Social Responsiveness Scale, Second Edition (SRS-2) and the Anxiety, Depression, and Mood Scale (ADAMS) was good. Boys with a comorbid ASD diagnosis obtained significantly higher scores on the ECAS compared to boys without ASD, when controlling for communication ability. A confirmatory factor analysis indicated that a two-factor model (avoidance and aversion) provided an excellent fit to the data. The ECAS appears to be a promising reliable and valid tool that could be employed as an outcome measure in future pharmacological/behavioral treatment trials for FXS.

Cortisol profiles differentiated in adolescents and young adult males with fragile X syndrome versus autism spectrum disorder

BACKGROUND

Fragile X syndrome (FXS) and non-syndromic autism spectrum disorder (ASD) are distinct disorders with overlapping behavioral features. Both disorders are also highly associated with anxiety with abnormal physiological regulation implied mechanistically. Some reports suggest atypical hypothalamus-pituitary-adrenal (HPA) axis function, indexed via aberrant cortisol reactivity, in both FXS and non-syndromic ASD. However, no study has compared cortisol reactivity across these two disorders, or its relationship to ASD symptom severity.

METHODS

Cortisol reactivity (prior to and following a day of assessments) was measured in 54 adolescent/young adult males with FXS contrasted to 15 males with non-syndromic ASD who had low cognitive abilities.

RESULTS

Greater ASD symptom severity was related to increased cortisol reactivity and higher levels at the end of the day, but only in the non-syndromic ASD group. Elevated anxiety was associated with increased HPA activation in the group with FXS alone.

CONCLUSIONS

Taken together, findings suggest a unique neuroendocrine profile that distinguishes adolescent/young adult males with FXS from those with non-syndromic ASD. Severity of ASD symptoms appears to be related to cortisol reactivity in the non-syndromic ASD sample, but not in FXS; while anxiety symptoms are associated with HPA activation in the FXS sample, but not in ASD despite a high prevalence of ASD, anxiety and physiological dysregulation characteristic in both populations.

Differential Relationships of Anxiety and Autism Symptoms on Social Skills in Young Boys With Fragile X Syndrome

Social skills are critical for academic, social, and psychological success of children with both typical and atypical development. Boys with fragile X syndrome (FXS) are at high risk for social skill impairments, given intellectual impairments and secondary conditions. The present study examines the impact of adaptive behavior, autism symptoms, and anxiety symptoms to social skills at the composite and subdomain level in boys with FXS across age. This cross-sectional study included boys with FXS (3-14 years) contrasted to age-matched typical control boys. Results revealed that social skills are generally within developmental expectations, with adaptive behavior as the primary predictor. Anxiety and autism symptoms emerged as additive risk factors, particularly in the areas of responsibility and self-control.

Public Health Literature Review of Fragile X Syndrome

OBJECTIVES

The purpose of this systematic literature review is to describe what is known about fragile X syndrome (FXS) and to identify research gaps. The results can be used to help inform future public health research and provide pediatricians with up-to-date information about the implications of the condition for individuals and their families.

METHODS

An electronic literature search was conducted, guided by a variety of key words. The search focused on 4 areas of both clinical and public health importance: (1) the full mutation phenotype, (2) developmental trajectories across the life span, (3) available interventions and treatments, and (4) impact on the family. A total of 661 articles were examined and 203 were included in the review.

RESULTS

The information is presented in the following categories: developmental profile (cognition, language, functional skills, and transition to adulthood), social-emotional profile (cooccurring psychiatric conditions and behavior problems), medical profile (physical features, seizures, sleep, health problems, and physiologic features), treatment and interventions (educational/behavioral, allied health services, and pharmacologic), and impact on the family (family environment and financial impact). Research gaps also are presented.

CONCLUSIONS

The identification and treatment of FXS remains an important public health and clinical concern. The information presented in this article provides a more robust understanding of FXS and the impact of this complex condition for pediatricians. Despite a wealth of information about the condition, much work remains to fully support affected individuals and their families.

Altered Translational Control of Fragile X Mental Retardation Protein on Myelin Proteins in Neuropsychiatric Disorders

Myelin is a specialized structure of the nervous system that both enhances electrical conductance and insulates neurons from external risk factors. In the central nervous system, polarized oligodendrocytes form myelin by wrapping processes in a spiral pattern around neuronal axons through myelin-related gene regulation. Since these events occur at a distance from the cell body, post-transcriptional control of gene expression has strategic advantage to fine-tune the overall regulation of protein contents in situ. Therefore, many research interests have been focused to identify RNA binding proteins and their regulatory mechanism in myelinating compartments. Fragile X mental retardation protein (FMRP) is one such RNA binding protein, regulating its target expression by translational control. Although the majority of works on FMRP have been performed in neurons, it is also found in the developing or mature glial cells including oligodendrocytes, where its function is not well understood. Here, we will review evidences suggesting abnormal translational regulation of myelin proteins with accompanying white matter problem and neurological deficits in fragile X syndrome, which can have wider mechanistic and pathological implication in many other neurological and psychiatric disorders.

Decreased home cage movement and oromotor impairments in adult Fmr1-KO mice

Fragile X syndrome (FXS) is a common inherited disorder that significantly impacts family and patient day-to-day living across the entire life span. The childhood and adolescent behavioral consequences of FXS are well appreciated. However, there are significantly fewer studies (except those examining psychiatric comorbidities) assessing behavioral phenotypes seen in adults with FXS. Mice engineered with a genetic lesion of fragile X mental retardation 1 (Fmr1) recapitulate important molecular and neuroanatomical characteristics of FXS, and provide a means to evaluate adult behavioral phenotypes associated with FXS. We give the first description of baseline behaviors including feeding, drinking, movement and their circadian rhythms; all observed over 16 consecutive days following extensive environmental habituation in adult Fmr1-KO mutant mice. We find no genotypic changes in mouse food ingestion, feeding patterns, metabolism or circadian patterns of movement, feeding and drinking. After habituation, Fmr1-KO mice show significantly less daily movement during their active phase (the dark cycle). However, Fmr1-KO mice have more bouts of activity during the light cycle compared with wild types. In addition, Fmr1-KO mice show significantly less daily water ingestion during the circadian dark cycle, and this reduction in water intake is accompanied by a decrease in the amount of water ingested per lick. The observed water ingestion and circadian phenotypes noted in Fmr1-KO mice recapitulate known clinical aspects previously described in FXS. The finding of decreased movement in Fmr1-KO mice is novel, and suggests a dissociation between baseline and novelty-evoked activity for Fmr1-KO mice.

Intragenic FMR1 disease-causing variants: a significant mutational mechanism leading to Fragile-X syndrome

Fragile-X syndrome (FXS) is a frequent genetic form of intellectual disability (ID). The main recurrent mutagenic mechanism causing FXS is the expansion of a CGG repeat sequence in the 5'-UTR of the FMR1 gene, therefore, routinely tested in ID patients. We report here three FMR1 intragenic pathogenic variants not affecting this sequence, identified using high-throughput sequencing (HTS): a previously reported hemizygous deletion encompassing the last exon of FMR1, too small to be detected by array-CGH and inducing decreased expression of a truncated form of FMRP protein, in three brothers with ID (family 1) and two splice variants in boys with sporadic ID: a de novo variant c.990+1G>A (family 2) and a maternally inherited c.420-8A>G variant (family 3). After clinical reevaluation, the five patients presented features consistent with FXS (mean Hagerman's scores=15). We conducted a systematic review of all rare non-synonymous variants previously reported in FMR1 in ID patients and showed that six of them are convincing pathogenic variants. This study suggests that intragenic FMR1 variants, although much less frequent than CGG expansions, are a significant mutational mechanism leading to FXS and demonstrates the interest of HTS approaches to detect them in ID patients with a negative standard work-up.

APP Causes Hyperexcitability in Fragile X Mice

Amyloid-beta protein precursor (APP) and metabolite levels are altered in fragile X syndrome (FXS) patients and in the mouse model of the disorder, Fmr1KO mice. Normalization of APP levels in Fmr1KO mice (Fmr1KO /APPHET mice) rescues many disease phenotypes. Thus, APP is a potential biomarker as well as therapeutic target for FXS. Hyperexcitability is a key phenotype of FXS. Herein, we determine the effects of APP levels on hyperexcitability in Fmr1KO brain slices. Fmr1KO /APPHET slices exhibit complete rescue of UP states in a neocortical hyperexcitability model and reduced duration of ictal discharges in a CA3 hippocampal model. These data demonstrate that APP plays a pivotal role in maintaining an appropriate balance of excitation and inhibition (E/I) in neural circuits. A model is proposed whereby APP acts as a rheostat in a molecular circuit that modulates hyperexcitability through mGluR5 and FMRP. Both over- and under-expression of APP in the context of the Fmr1KO increases seizure propensity suggesting that an APP rheostat maintains appropriate E/I levels but is overloaded by mGluR5-mediated excitation in the absence of FMRP. These findings are discussed in relation to novel treatment approaches to restore APP homeostasis in FXS.

Detailed assessment of incontinence in boys with fragile-X-syndrome in a home setting

Fragile-X-syndrome (FXS) is caused by a mutation on the X chromosome (Xq27.3). Males with a full mutation have typical dysmorphic signs, moderate intellectual disability and psychological disorders. Twenty-five to fifty percent are affected by incontinence. The aim of the study was to assess subtypes of incontinence and psychological problems in children with FXS in their home environments. Twenty-two boys with FXS (mean age 11.0 years) and 22 healthy controls (mean age 11.1 years) were examined with sonography, uroflowmetry, 48-h bladder diary, physical examination, IQ test, parental psychiatric interview and questionnaires regarding incontinence and psychological symptoms in a home setting. Boys with FXS had higher rates of incontinence than controls (59.1 vs. 4.8 %). The most common subtypes in FXS boys were primary non-monosymptomatic nocturnal enuresis, urge incontinence and nonretentive faecal incontinence. 90.9 % boys with FXS had a psychological comorbidity. Incontinence and behavioural symptoms were not associated.

CONCLUSION

Boys with FXS have a higher risk for physical disabilities, psychological disorders and incontinence than healthy boys. Constipation is not a major problem in FXS. As effective treatment is available for children with ID, we recommend offering assessment and therapy to all children with FXS and incontinence.

WHAT IS KNOWN

• Boys with fragile-X-syndrome (FXS) have higher rates of incontinence, psychological disorders and somatic conditions than typically developing boys. What is New: • Constipation is a rare condition in FXS in contrast to other genetic syndromes. • Although incontinence rates are higher, urological findings (uroflowmetry, sonography) are not more pathological per se in FXS.

Somatosensory map expansion and altered processing of tactile inputs in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a common inherited form of intellectual disability caused by the absence or reduction of the fragile X mental retardation protein (FMRP) encoded by the FMR1 gene. In humans, one symptom of FXS is hypersensitivity to sensory stimuli, including touch. We used a mouse model of FXS (Fmr1 KO) to study sensory processing of tactile information conveyed via the whisker system. In vivo electrophysiological recordings in somatosensory barrel cortex showed layer-specific broadening of the receptive fields at the level of layer 2/3 but not layer 4, in response to whisker stimulation. Furthermore, the encoding of tactile stimuli at different frequencies was severely affected in layer 2/3. The behavioral effect of this broadening of the receptive fields was tested in the gap-crossing task, a whisker-dependent behavioral paradigm. In this task the Fmr1 KO mice showed differences in the number of whisker contacts with platforms, decrease in the whisker sampling duration and reduction in the whisker touch-time while performing the task. We propose that the increased excitability in the somatosensory barrel cortex upon whisker stimulation may contribute to changes in the whisking strategy as well as to other observed behavioral phenotypes related to tactile processing in Fmr1 KO mice.

Caregiver Preferences for the Treatment of Males with Fragile X Syndrome

OBJECTIVE

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability. The objective of this study was to determine the relative importance that caregivers place on improving different phenotypic traits observed in males with FXS to better understand the greatest medical needs for developing and evaluating FXS treatments.

METHOD

Fragile X syndrome caregivers (n = 614) compared hypothetical treatments in a discrete-choice experiment. The treatments varied in their effects on 6 outcomes associated with FXS: learning and applying new skills, explaining needs, controlling behavior, taking part in new social activities, caring for oneself, and paying attention. The relative importance was calculated for improving severe or moderate levels of disability and transformed to a 10-point scale. Relative importance was also quantified by patient age group (child, adolescent, and adult).

RESULTS

Most important to caregivers were controlling behavior (10.0) and caring for oneself (9.9). Least important was taking part in new social activities (4.2). A partial improvement in controlling behavior or self-care was more important than full resolution of the least important disabilities. This was consistent across age groups. Improvements from severe to moderate disability were more important than from moderate to no disability.

CONCLUSION

Caregivers expressed strong preferences for improvement in self-care and behavioral control, independent of the age of the individual with FXS. These data may be helpful when designing studies to test the efficacy of FXS treatments because small treatment effects on very important outcomes may be valued more than large treatment effects on less valued outcomes.

Burden of illness among patients with fragile X syndrome (FXS): a Medicaid perspective

BACKGROUND

Fragile X syndrome (FXS) is an inherited intellectual disability that imposes a substantial clinical and humanistic burden on patients and caregivers. This study aimed to quantify the incremental burden of illness following FXS diagnosis in Medicaid populations.

METHODS

A retrospective matched-cohort study was conducted using FL, NJ, MO, IA, and KS Medicaid claims (1997-2012). Patients with FXS were matched 1:5 to a comparison group without FXS, based on age, gender, state, and continuous Medicaid coverage. Healthcare resource utilization and costs were compared among cohorts over 1 year following first diagnosis.

RESULTS

Overall, 697 patients with FXS were matched to 3485 non-FXS patients. Median age was 12.0 years; 82% were male. Newly diagnosed FXS patients were younger (median age: 7.0 years). During the follow-up, patients with FXS had significantly higher medication use, medical procedure use, medical specialist visits, and associated costs than the non-FXS comparison group. One-fourth of FXS patients filled prescriptions for stimulants, antipsychotics, or anticonvulsants; 25% of patients with FXS had speech and language therapy and 39% had physical therapy (versus 9%, 4% and 8%, respectively, for the comparison group). At least 44% of FXS patients visited a neurologist, cardiologist, otolaryngologist, or gastroenterologist; 92% of patients with FXS had an outpatient visit, 35% had an emergency room visit, and 34% used home services (compared to 31%-32%, 64%, 27%, and 10%, respectively, for the comparison group) (all p < 0.05). Patients with FXS had an incremental annual total healthcare cost of $33,409 (2012$) per person relative to the comparison group, while newly diagnosed FXS patients had incremental total annual healthcare costs of $17,617 (2012$) per person.

CONCLUSIONS

Both established and newly diagnosed FXS were associated with significantly increased use of multiple medications and medical services, and increased healthcare costs. Treatments that could help reduce this disease burden are urgently needed.

Functional analysis and treatment of problem behavior exhibited by children with fragile X syndrome

The efficacy of function-based interventions for the treatment of severe problem behavior exhibited by individuals with intellectual and developmental disabilities (IDD) is well established. However, few studies have reported on behavioral interventions in fragile X syndrome (FXS) specifically. The present study is a consecutive case-series analysis that reports on functional analysis and treatment of problem behavior of nine children with FXS. Assessment findings were consistent with previous research indicating that among individuals with FXS, problem behavior is more commonly maintained by escape from demands and access to tangible items, relative to the broader population of individuals with IDD. Functional analysis-based behavioral interventions resulted in a mean reduction in problem behavior of 95.2% across the nine participants. Additionally, generalization of treatment effects from controlled clinical settings to home, school, and community was demonstrated. The current findings suggest that function-based behavioral interventions shown to be effective with the broader population of individuals with IDD are also effective for individuals with FXS. Our results in combination with those of previous studies describing functional analysis outcomes provide additional evidence for a unique functional behavioral phenotype for severe problem behavior in individuals with FXS. Implications of study findings for early intervention and prevention of problem behavior in children with FXS are discussed.

Molecular underpinnings of prefrontal cortex development in rodents provide insights into the etiology of neurodevelopmental disorders

The prefrontal cortex (PFC), seat of the highest-order cognitive functions, constitutes a conglomerate of highly specialized brain areas and has been implicated to have a role in the onset and installation of various neurodevelopmental disorders. The development of a properly functioning PFC is directed by transcription factors, guidance cues and other regulatory molecules and requires the intricate and temporal orchestration of a number of developmental processes. Disturbance or failure of any of these processes causing neurodevelopmental abnormalities within the PFC may contribute to several of the cognitive deficits seen in patients with neurodevelopmental disorders. In this review, we elaborate on the specific processes underlying prefrontal development, such as induction and patterning of the prefrontal area, proliferation, migration and axonal guidance of medial prefrontal progenitors, and their eventual efferent and afferent connections. We furthermore integrate for the first time the available knowledge from genome-wide studies that have revealed genes linked to neurodevelopmental disorders with experimental molecular evidence in rodents. The integrated data suggest that the pathogenic variants in the neurodevelopmental disorder-associated genes induce prefrontal cytoarchitectonical impairments. This enhances our understanding of the molecular mechanisms of prefrontal (mis)development underlying the four major neurodevelopmental disorders in humans, that is, intellectual disability, autism spectrum disorders, attention deficit hyperactivity disorder and schizophrenia, and may thus provide clues for the development of novel therapies.

Early social enrichment rescues adult behavioral and brain abnormalities in a mouse model of fragile X syndrome

Converging lines of evidence support the use of environmental stimulation to ameliorate the symptoms of a variety of neurodevelopmental disorders. Applying these interventions at very early ages is critical to achieve a marked reduction of the pathological phenotypes. Here we evaluated the impact of early social enrichment in Fmr1-KO mice, a genetic mouse model of fragile X syndrome (FXS), a major developmental disorder and the most frequent monogenic cause of autism. Enrichment was achieved by providing male KO pups and their WT littermates with enhanced social stimulation, housing them from birth until weaning with the mother and an additional nonlactating female. At adulthood they were tested for locomotor, social, and cognitive abilities; furthermore, dendritic alterations were assessed in the hippocampus and amygdala, two brain regions known to be involved in the control of the examined behaviors and affected by spine pathology in Fmr1-KOs. Enrichment rescued the behavioral FXS-like deficits displayed in adulthood by Fmr1-KO mice, that is, hyperactivity, reduced social interactions, and cognitive deficits. Early social enrichment also eliminated the abnormalities shown by adult KO mice in the morphology of hippocampal and amygdala dendritic spines, namely an enhanced density of immature vs mature types. Importantly, enrichment did not induce neurobehavioral changes in WT mice, thus supporting specific effects on FXS-like pathology. These findings show that early environmental stimulation has profound and long-term beneficial effects on the pathological FXS phenotype, thereby encouraging the use of nonpharmacological interventions for the treatment of this and perhaps other neurodevelopmental diseases.

Functional and structural deficits at accumbens synapses in a mouse model of Fragile X

Fragile X is the most common cause of inherited intellectual disability and a leading cause of autism. The disease is caused by mutation of a single X-linked gene called fmr1 that codes for the Fragile X mental retardation protein (FMRP), a 71 kDa protein, which acts mainly as a translation inhibitor. Fragile X patients suffer from cognitive and emotional deficits that coincide with abnormalities in dendritic spines. Changes in spine morphology are often associated with altered excitatory transmission and long-term plasticity, the most prominent deficit in fmr1-/y mice. The nucleus accumbens, a central part of the mesocortico-limbic reward pathway, is now considered as a core structure in the control of social behaviors. Although the socio-affective impairments observed in Fragile X suggest dysfunctions in the accumbens, the impact of the lack of FMRP on accumbal synapses has scarcely been studied. Here we report for the first time a new spike timing-dependent plasticity paradigm that reliably triggers NMDAR-dependent long-term potentiation (LTP) of excitatory afferent inputs of medium spiny neurons (MSN) in the nucleus accumbens core region. Notably, we discovered that this LTP was completely absent in fmr1-/y mice. In the fmr1-/y accumbens intrinsic membrane properties of MSNs and basal excitatory neurotransmission remained intact in the fmr1-/y accumbens but the deficit in LTP was accompanied by an increase in evoked AMPA/NMDA ratio and a concomitant reduction of spontaneous NMDAR-mediated currents. In agreement with these physiological findings, we found significantly more filopodial spines in fmr1-/y mice by using an ultrastructural electron microscopic analysis of accumbens core medium spiny neuron spines. Surprisingly, spine elongation was specifically due to the longer longitudinal axis and larger area of spine necks, whereas spine head morphology and postsynaptic density size on spine heads remained unaffected in the fmr1-/y accumbens. These findings together reveal new structural and functional synaptic deficits in Fragile X.

Annual research review: The (epi)genetics of neurodevelopmental disorders in the era of whole-genome sequencing--unveiling the dark matter

BACKGROUND AND SCOPE

Neurodevelopmental disorders (NDDs) are defined by a wide variety of behavioural phenotypes, psychopathology and clinically informed categorical classifications. Diagnostic entities include intellectual disability (ID), the autism spectrum (ASD) and attention-deficit/hyperactivity disorder (ADHD). The aetiopathogenesis of these conditions and disorders involves an interaction between both genetic and environmental risk factors on the developmental trajectory. Despite their remarkable genetic heterogeneity and complexity of pathophysiological mechanisms, NDDs display an overlap in their phenotypic features, a considerable degree of comorbidity as well as sharing of genetic and environmental risk factors. This review aims to provide an overview of the genetics and epigenetic of NDDs.

FINDINGS

Recent evidence suggests a critical role of defined and tightly regulated neurodevelopmental programs running out of control in NDDs, most notably neuronal proliferation and migration, synapse formation and remodelling, as well as neural network configuration resulting in compromised systems connectivity and function. Moreover, the machinery of epigenetic programming, interacting with genetic liability, impacts many of those processes and pathways, thus modifying vulnerability of, and resilience to, NDDs. Consequently, the categorically defined entities of ID, ADHD and ASD are increasingly viewed as disorders on a multidimensional continuum of molecular and cellular deficiencies in neurodevelopment. As such, this range of NDDs displays a broad phenotypic diversity, which may be explained by a combination and interplay of underlying loss- and potential gain-of-function traits.

CONCLUSION

In this overview, we discuss a backbone continuum concept of NDDs by summarizing pertinent findings in genetics and epigenetics. We also provide an appraisal of the genetic overlap versus differences, with a focus on genome-wide screening approaches for (epi)genetic variation. Finally, we conclude with insights from evolutionary psychobiology suggesting positive selection for discrete NDD-associated traits.

An analysis of challenging behavior, comorbid psychopathology, and Attention-Deficit/Hyperactivity Disorder in Fragile X Syndrome

The present study sought to investigate the relationship between challenging behavior, comorbid psychopathology, and Attention-Deficit/Hyperactivity Disorder (AD/HD) in Fragile X Syndrome (FRAX). Additionally, this study sought to examine how such disorders are predicted by gender, presence of autism spectrum disorder (ASD), and presence of intellectual disability (ID). A total of 47 children and adolescents with FRAX were assessed. Results revealed high levels of challenging behavior and AD/HD symptoms within the sample, with some participants exhibiting symptoms of comorbid psychopathology. Further analysis revealed that challenging behavior and comorbid psychopathology were positively correlated, with stereotypy correlating most strongly with comorbid psychopathology. In addition, ASD was found to predict challenging behavior, and gender was found to predict AD/HD symptoms. The implications of these findings are discussed.

Effects of clonidine and methylphenidate on motor activity in Fmr1 knockout mice

Fragile X syndrome (FXS), a disorder caused by a mutation in the FMR1 gene, is often associated with Attention Deficit Hyperactivity Disorder (ADHD). Common treatments for the hyperactivity often seen in ADHD involve the use of stimulants and α2-adrenergic agonists. The Fmr1 knockout (KO) mouse has been found to be a valid model for FXS both biologically and behaviorally. Of particular interest to our research, the Fmr1 KO mouse has been demonstrated to show increased locomotion in comparison to wild type (WT) littermates. In the present study, we assessed the effects of clonidine (0.05 mg/kg) and methylphenidate (5 mg/kg) on motor activity in Fmr1 KO mice and their WT littermates in the open field test. Results showed that methylphenidate increased motor activity in both genotypes. Clonidine decreased motor activity in both genotypes, but the effect was delayed in the Fmr1 KO mice.

New insights into the molecular pathophysiology of fragile X syndrome and therapeutic perspectives from the animal model

Fragile X syndrome is the most common monogenetic form of intellectual disability and is a leading cause of autism. This syndrome is produced by the reduced transcription of the fragile X mental retardation (FMR1) gene, and it is characterized by a range of symptoms heterogeneously expressed in patients such as cognitive impairment, seizure susceptibility, altered pain sensitivity and anxiety. The recent advances in the understanding of the pathophysiological mechanisms involved have opened novel potential therapeutic approaches identified in preclinical rodent models as a necessary preliminary step for the subsequent evaluation in patients. Among those possible therapeutic approaches, the modulation of the metabotropic glutamate receptor signaling or the GABA receptor signaling have focused most of the attention. New findings in the animal models open other possible therapeutic approaches such as the mammalian target of rapamycin signaling pathway or the endocannabinoid system. This review summarizes the emerging data recently obtained in preclinical models of fragile X syndrome supporting these new therapeutic perspectives.

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.

Pharmacological rescue of Ras signaling, GluA1-dependent synaptic plasticity, and learning deficits in a fragile X model

Fragile X syndrome, caused by the loss of Fmr1 gene function, is the most common form of inherited mental retardation. Lim et al. find that compounds activating serotonin (5HT) subtype 2B receptors or dopamine (DA) subtype 1-like receptors and those inhibiting 5HT_2A-Rs or D₂-Rs enhance Ras signaling, GluA1-dependent synaptic plasticity, and learning in Fmr1 knockout mice. Combining 5HT and DA compounds at low doses synergistically restored normal learning. This suggests that properly dosed and combined FDA-approved psychoactive drugs may effectively treat the cognitive impairment associated with fragile X syndrome.

Fragile X syndrome, caused by the loss of Fmr1 gene function, is the most common form of inherited mental retardation, with no effective treatment. Using a tractable animal model, we investigated mechanisms of action of a few FDA-approved psychoactive drugs that modestly benefit the cognitive performance in fragile X patients. Here we report that compounds activating serotonin (5HT) subtype 2B receptors (5HT_2B-Rs) or dopamine (DA) subtype 1-like receptors (D₁-Rs) and/or those inhibiting 5HT_2A-Rs or D₂-Rs moderately enhance Ras–PI3K/PKB signaling input, GluA1-dependent synaptic plasticity, and learning in Fmr1 knockout mice. Unexpectedly, combinations of these 5HT and DA compounds at low doses synergistically stimulate Ras–PI3K/PKB signal transduction and GluA1-dependent synaptic plasticity and remarkably restore normal learning in Fmr1 knockout mice without causing anxiety-related side effects. These findings suggest that properly dosed and combined FDA-approved psychoactive drugs may effectively treat the cognitive impairment associated with fragile X syndrome.

Fragile X syndrome: a preclinical review on metabotropic glutamate receptor 5 (mGluR5) antagonists and drug development

RATIONALE

Fragile X syndrome (FXS) is considered the leading inherited cause of intellectual disability and autism. In FXS, the fragile X mental retardation 1 (FMR1) gene is silenced and the fragile X mental retardation protein (FMRP) is not expressed, resulting in the characteristic features of the syndrome. Despite recent advances in understanding the pathophysiology of FXS, there is still no cure for this condition; current treatment is symptomatic. Preclinical research is essential in the development of potential therapeutic agents.

OBJECTIVES

This review provides an overview of the preclinical evidence supporting metabotropic glutamate receptor 5 (mGluR5) antagonists as therapeutic agents for FXS.

RESULTS

According to the mGluR theory of FXS, the absence of FMRP leads to enhanced glutamatergic signaling via mGluR5, which leads to increased protein synthesis and defects in synaptic plasticity including enhanced long-term depression. As such, efforts to develop agents that target the underlying pathophysiology of FXS have focused on mGluR5 modulation. Animal models, particularly the Fmr1 knockout mouse model, have become invaluable in exploring therapeutic approaches on an electrophysiological, behavioral, biochemical, and neuroanatomical level. Two direct approaches are currently being investigated for FXS treatment: reactivating the FMR1 gene and compensating for the lack of FMRP. The latter approach has yielded promising results, with mGluR5 antagonists showing efficacy in clinical trials.

CONCLUSIONS

Targeting mGluR5 is a valid approach for the development of therapeutic agents that target the underlying pathophysiology of FXS. Several compounds are currently in development, with encouraging results.

Psychopathology and cognition in children with 22q11.2 deletion syndrome

BACKGROUND

Children with 22q11.2 deletion syndrome (22q11.2DS) have been reported to have high rates of cognitive and psychiatric problems.

AIMS

To establish the nature and prevalence of psychiatric disorder and neurocognitive impairment in children with 22q11.2DS and test whether risk of psychopathology is mediated by the children's intellectual impairment.

METHOD

Neurocognition and psychopathology were assessed in 80 children with 22q11.2DS (mean age 10.2 years, s.d. = 2.1) and 39 sibling controls (mean age 10.9 years, s.d. = 2.0).

RESULTS

More than half (54%) of children with 22q11.2DS met diagnostic criteria for one or more DSM-IV-TR psychiatric disorder. These children had lower IQ (mean 76.8, s.d. = 13.0) than controls (mean 108.6, s.d. = 15.2) (P<0.001) and showed a range of neurocognitive impairments. Increased risk of psychopathology was not mediated by intellectual impairment.

CONCLUSIONS

22q11.2DS is not related to a specific psychiatric phenotype in children. Moreover, the deletion has largely independent effects on IQ and risk of psychopathology, indicating that psychopathology in 22q11.2DS is not a non-specific consequence of generalised cognitive impairment.

Anxiety, attention problems, hyperactivity, and the Aberrant Behavior Checklist in fragile X syndrome

Behavior problems are a common challenge for individuals with fragile X syndrome (FXS) and constitute the primary clinical outcome domain in trials testing new FXS medications. However, little is known about the relationship between caregiver-reported behavior problems and co-occurring conditions such as anxiety and attention problems. In this study, 350 caregivers, each with at least one son or daughter with full-mutation FXS, rated one of their children with FXS using the Aberrant Behavior Checklist-Community Version (ABC-C); the Anxiety subscale of the Anxiety, Depression, and Mood Scale; and the Attention/Hyperactivity Items from the Symptom Inventories. In addition to examining family consequences of these behaviors, this study also sought to replicate psychometric findings for the ABC-C in FXS, to provide greater confidence for its use in clinical trials with this population. Psychometric properties and baseline ratings of problem behavior were consistent with other recent studies, further establishing the profile of problem behavior in FXS. Cross-sectional analyses suggest that selected dimensions of problem behavior, anxiety, and hyperactivity are age related; thus, age should serve as an important control in any studies of problem behavior in FXS. Measures of anxiety, attention, and hyperactivity were highly associated with behavior problems, suggesting that these factors at least coincide with problem behavior. However, these problems generally did not add substantially to variance in caregiver burden predicted by elevated behavior problems. The results provide further evidence of the incidence of problem behaviors and co-occurring conditions in FXS and the impact of these behaviors on the family.