Interregulation between fragile X mental retardation protein and methyl CpG binding protein 2 in the mouse posterior cerebral cortex.

Several X-linked neurodevelopmental disorders including Rett syndrome, induced by mutations in the MECP2 gene, and fragile X syndrome (FXS), caused by mutations in the FMR1 gene, share autism-related features. The mRNA coding for methyl CpG binding protein 2 (MeCP2) has previously been identified as a substrate for the mRNA-binding protein, fragile X mental retardation protein (FMRP), which is silenced in FXS. Here, we report a homeostatic relationship between these two key regulators of gene expression in mouse models of FXS (Fmr1 Knockout (KO)) and Rett syndrome (MeCP2 KO). We found that the level of MeCP2 protein in the cerebral cortex was elevated in Fmr1 KO mice, whereas MeCP2 KO mice displayed reduced levels of FMRP, implicating interplay between the activities of MeCP2 and FMRP. Indeed, knockdown of MeCP2 with short hairpin RNAs led to a reduction of FMRP in mouse Neuro2A and in human HEK-293 cells, suggesting a reciprocal coupling in the expression level of these two regulatory proteins. Intra-cerebroventricular injection of an adeno-associated viral vector coding for FMRP led to a concomitant reduction in MeCP2 expression in vivo and partially corrected locomotor hyperactivity. Additionally, the level of MeCP2 in the posterior cortex correlated with the severity of the hyperactive phenotype in Fmr1 KO mice. These results demonstrate that MeCP2 and FMRP operate within a previously undefined homeostatic relationship. Our findings also suggest that MeCP2 overexpression in Fmr1 KO mouse posterior cerebral cortex may contribute to the fragile X locomotor hyperactivity phenotype.

FMRP links optimal codons to mRNA stability in neurons.

Fragile X syndrome (FXS) is caused by inactivation of the FMR1 gene and loss of encoded FMRP, an RNA binding protein that represses translation of some of its target transcripts. Here we use ribosome profiling and RNA sequencing to investigate the dysregulation of translation in the mouse brain cortex. We find that most changes in ribosome occupancy on hundreds of mRNAs are largely driven by dysregulation in transcript abundance. Many down-regulated mRNAs, which are mostly responsible for neuronal and synaptic functions, are highly enriched for FMRP binding targets. RNA metabolic labeling demonstrates that, in FMRP-deficient cortical neurons, mRNA down-regulation is caused by elevated degradation and is correlated with codon optimality. Moreover, FMRP preferentially binds mRNAs with optimal codons, suggesting that it stabilizes such transcripts through direct interactions via the translational machinery. Finally, we show that the paradigm of genetic rescue of FXS-like phenotypes in FMRP-deficient mice by deletion of the Cpeb1 gene is mediated by restoration of steady-state RNA levels and consequent rebalancing of translational homeostasis. Our data establish an essential role of FMRP in codon optimality-dependent mRNA stability as an important factor in FXS.

Metabolic pathways modulate the neuronal toxicity associated with fragile X-associated tremor/ataxia syndrome.

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder that affects premutation carriers (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. Much remains unknown regarding the metabolic alterations associated with FXTAS, especially in the brain, and the most affected region, the cerebellum. Investigating the metabolic changes in FXTAS will aid in the identification of biomarkers as well as in understanding the pathogenesis of disease. To identify the metabolic alterations associated with FXTAS, we took advantage of our FXTAS mouse model that expresses 90 CGG repeats in cerebellar Purkinje neurons and exhibits the key phenotypic features of FXTAS. We performed untargeted global metabolic profiling of age-matched control and FXTAS mice cerebella at 16-20 weeks and 55 weeks. Out of 506 metabolites measured in cerebellum, we identified 186 metabolites that demonstrate significant perturbations due to the (CGG)90 repeat (P<0.05) and found that these differences increase dramatically with age. To identify key metabolic changes in FXTAS pathogenesis, we performed a genetic screen using a Drosophila model of FXTAS. Out of 28 genes that we tested in the fly, 8 genes showed significant enhanced neuronal toxicity associated with CGG repeats, such as Schlank (ceramide synthase), Sk2 (sphingosine kinase) and Ras (IMP dehydrogenase). By combining metabolic profiling with a Drosophila genetic screen to identify genetic modifiers of FXTAS, we demonstrate an effective method for functional validation of high-throughput metabolic data and show that sphingolipid and purine metabolism are significantly perturbed in FXTAS pathogenesis.

Fragile X syndrome and connective tissue dysregulation.

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disabilities and autism spectrum disorders, and it is an X-linked disorder in which there is a deficiency of the fragile X mental retardation 1 protein. This protein is crucial in regulating translation of mRNAs related to dendritic maturation and cognitive development. The phenotype of FXS is characterized by neurobehavioral alterations, social deficits, communication difficulties, and findings which suggest an alteration of connective tissue, especially in the ligaments and muscles, cardiovascular system and genitourinary system. Connective tissue connects and supports all other tissues of the body and is composed of cells and extracellular matrix (ECM). Several proteins have been involved in the connective tissue abnormalities associated with the FXS, such as matrix metalloproteinase 9, which plays an important role in the homeostasis of the ECM, being a potential therapeutic target for certain tetracycline antibiotics that have shown beneficial effects in FXS. Here, we review connective tissue problems described in FXS.

Hippocampal deficits in neurodevelopmental disorders.

Neurodevelopmental disorders result from impaired development or maturation of the central nervous system. Both genetic and environmental factors can contribute to the pathogenesis of these disorders; however, the exact causes are frequently complex and unclear. Individuals with neurodevelopmental disorders may have deficits with diverse manifestations, including challenges with sensory function, motor function, learning, memory, executive function, emotion, anxiety, and social ability. Although these functions are mediated by multiple brain regions, many of them are dependent on the hippocampus. Extensive research supports important roles of the mammalian hippocampus in learning and cognition. In addition, with its high levels of activity-dependent synaptic plasticity and lifelong neurogenesis, the hippocampus is sensitive to experience and exposure and susceptible to disease and injury. In this review, we first summarize hippocampal deficits seen in several human neurodevelopmental disorders, and then discuss hippocampal impairment including hippocampus-dependent behavioral deficits found in animal models of these neurodevelopmental disorders.

CGG allele size somatic mosaicism and methylation in FMR1 premutation alleles.

BACKGROUND: Greater than 200 CGG repeats in the 5'UTR of the FMR1 gene lead to epigenetic silencing and lack of the FMR1 protein, causing fragile X Syndrome. Individual carriers of a premutation (PM) allele with 55-200 CGG repeats are typically unmethylated and can present with clinical features defined as FMR1-associated conditions. METHODS: Blood samples from 17 male PM carriers were assessed clinically and molecularly by Southern blot, western blot, PCR and QRT-PCR. Blood and brain tissue from an additional 18 PM males were also similarly examined. Continuous outcomes were modelled using linear regression and binary outcomes were modelled using logistic regression. RESULTS: Methylated alleles were detected in different fractions of blood cells in all PM cases (n=17). CGG repeat numbers correlated with percent of methylation and mRNA levels and, especially in the upper PM range, with greater number of clinical involvements. Inter-tissue/intra-tissue somatic instability and differences in percent methylation were observed between blood and fibroblasts (n=4) and also observed between blood and different brain regions in three of the 18 PM cases examined. CGG repeat lengths in lymphocytes remained unchanged over a period of time ranging from 2 to 6 years, three cases for whom multiple samples were available. CONCLUSIONS: In addition to CGG size instability, individuals with a PM expanded allele can exhibit methylation and display more clinical features likely due to RNA toxicity and/or FMR1 silencing. The observed association between CGG repeat length and percent of methylation with the severity of the clinical phenotypes underscores the potential value of methylation in affected PM to further understand penetrance, inform diagnosis and expand treatment options.

Anti-neuronal antibodies in patients with fragile X syndrome: is there a role of autoimmunity in its pathogenesis?

BACKGROUND: Fragile X syndrome (FXS) is a single-gene disorder with a broad spectrum of involvement, including cognitive and behavioural impairments of varying degrees with specific physical features and a strong association with autism. OBJECTIVES: In this study, the frequency of serum anti-neural antibodies was investigated in FXS patients who did and those who did not manifest autism spectrum disorders (ASD) in comparison to typically developing controls. METHODS: The study involved 23 males (mean age, 19.78 ± 6.56 years) who harboured a full mutation in the FMR1 gene. The control group comprised 19 healthy students (mean age 24.63 ± 1.89 years). Serum anti-neuronal antibodies were analyzed using Western blotting. RESULTS: Serum anti-neuronal antibodies were present in 10/23 (43.48%) FXS males. CONCLUSION: Serum anti-neuronal antibodies were found in a subgroup of FXS patients. Autistic symptoms in FXS may, in part, be caused by auto-immune factors. Further studies in larger patient and control groups are necessary to elucidate the aetiopathogenic role of anti-neuronal antibodies in FXS patients.

GPCR interacting proteins (GIPs) in the nervous system: Roles in physiology and pathologies.

G protein-coupled receptors (GPCRs) are key transmembrane recognition molecules for regulatory signals such as light, odors, taste hormones, and neurotransmitters. In addition to activating guanine nucleotide binding proteins (G proteins), GPCRs associate with a variety of GPCR-interacting proteins (GIPs). GIPs contain structural interacting domains that allow the formation of large functional complexes involved in G protein-dependent and -independent signaling. At the cellular level, other functions of GIPs include targeting of GPCRs to subcellular compartments and their trafficking to and from the plasma membrane. Recently, roles of GPCR-GIP interactions in central nervous system physiology and pathologies have been revealed. Here, we highlight the role of GIPs in some important neurological and psychiatric disorders, as well as their potential for the future development of therapeutic drugs.

ACMG Standards and Guidelines for fragile X testing: a revision to the disease-specific supplements to the Standards and Guidelines for Clinical Genetics Laboratories of the American College of Medical Genetics and Genomics.

Molecular genetic testing of the FMR1 gene is commonly performed in clinical laboratories. Mutations in the FMR1 gene are associated with fragile X syndrome, fragile X tremor ataxia syndrome, and premature ovarian insufficiency. This document provides updated information regarding FMR1 gene mutations, including prevalence, genotype-phenotype correlation, and mutation nomenclature. Methodological considerations are provided for Southern blot analysis and polymerase chain reaction amplification of the FMR1 gene, including triplet repeat-primed and methylation-specific polymerase chain reaction. In addition to report elements, examples of laboratory reports for various genotypes are also included.

Fragile X-associated tremor/ataxia phenotype in a male carrier of unmethylated full mutation in the FMR1 gene.

The Fragile X syndrome is caused by a CGG repeat expansion >200 in the promoter of the Fragile X mental retardation 1 (FMR1) gene termed full mutation (FM). These alleles are silenced through methylation of the FMR1 promoter, leading to deficit of the FMR1 protein (FMRP), and neurodevelopmental changes. However, occasional FM individuals have a complete lack of methylation, and those typically have only minor deficit of FMRP levels compared with normal controls and their intelligence may be in the normal range. FM alleles are generated through expansion of the CGG repeat from the premutation (PM) range of 55-200 repeats, linked to the late onset Fragile X-associated tremor/ataxia syndrome (FXTAS). This disorder has been attributed to a 'toxicity' of the FMR1 mRNA, which is significantly elevated in male carriers of PM alleles and of unmethylated FM alleles. This is the first report of a 65-year-old male with an unmethylated FM allele and history of alcohol abuse, who developed symptoms of FXTAS. We postulate that, although the elevation of FMR1 transcripts associated with unmethylated FM alleles have a potential to cause FXTAS, in some cases this disorder may occur through an additional effect of exposure to neurotoxicants including alcohol.

Association of lipid rafts cholesterol with clinical profile in fragile X syndrome.

Fragile X syndrome (FXS) is the most prevalent monogenic cause of intellectual disability and autism spectrum disorder (ASD). Affected individuals have a high prevalence of hypocholesterolemia, however, the underlying mechanisms and the clinical significance remains unknown. We hypothesized that decrease in the plasma cholesterol levels is associated with an alteration of cholesterol content within the lipid rafts (LRs) which ultimately affects the clinical profile of FXS individuals. The platelets LRs were isolated by ultracentrifugation on sucrose gradient from 27 FXS and 25 healthy controls, followed by measurements of proteins, cholesterol, and gangliosides content. Autistic and adaptive behaviour of affected individuals were respectively assessed by the Social Communication Questionnaire and Adaptive Behavior Assessment System. Our results suggest a decrease in the cholesterol content of LRs in FXS individuals as compared to controls. As opposed to controls, LR cholesterol was significantly associated with plasma total cholesterol (r = 0.47; p = 0.042) in the FXS group. Furthermore, the correlation between LRs cholesterol and the clinical profile showed a significant association with autistic traits (r = - 0.67; p < 0.001) and adaptative behavior (r = 0.70; p < 0.001). These results support the clinical significance of LR cholesterol alterations in FXS. Further studies are warranted to investigate the implication of LRs in FXS pathophysiology and ASD.

Autism spectrum disorders--a genetics review.

Autism is an etiologically and clinically heterogeneous group of disorders, diagnosed solely by the complex behavioral phenotype. On the basis of the high-heritability index, geneticists are confident that autism will be the first behavioral disorder for which the genetic basis can be well established. Although it was initially assumed that major genome-wide and candidate gene association studies would lead most directly to common autism genes, progress has been slow. Rather, most discoveries have come from studies of known genetic disorders associated with the behavioral phenotype. New technology, especially array chromosomal genomic hybridization, has both increased the identification of putative autism genes and raised to approximately 25%, the percentage of children for whom an autism-related genetic change can be identified. Incorporating clinical geneticists into the diagnostic and autism research arenas is vital to the field. Interpreting this new technology and deciphering autism's genetic montage require the skill set of the clinical geneticist including knowing how to acquire and interpret family pedigrees, how to analyze complex morphologic, neurologic, and medical phenotypes, sorting out heterogeneity, developing rational genetic models, and designing studies. The current emphasis on deciphering autism spectrum disorders has accelerated the field of neuroscience and demonstrated the necessity of multidisciplinary research that must include clinical geneticists both in the clinics and in the design and implementation of basic, clinical, and translational research.

Overlapping Molecular Pathways Leading to Autism Spectrum Disorders, Fragile X Syndrome, and Targeted Treatments.

Autism spectrum disorders (ASD) are subdivided into idiopathic (unknown) etiology and secondary, based on known etiology. There are hundreds of causes of ASD and most of them are genetic in origin or related to the interplay of genetic etiology and environmental toxicology. Approximately 30 to 50% of the etiologies can be identified when using a combination of available genetic testing. Many of these gene mutations are either core components of the Wnt signaling pathway or their modulators. The full mutation of the fragile X mental retardation 1 (FMR1) gene leads to fragile X syndrome (FXS), the most common cause of monogenic origin of ASD, accounting for ~ 2% of the cases. There is an overlap of molecular mechanisms in those with idiopathic ASD and those with FXS, an interaction between various signaling pathways is suggested during the development of the autistic brain. This review summarizes the cross talk between neurobiological pathways found in ASD and FXS. These signaling pathways are currently under evaluation to target specific treatments in search of the reversal of the molecular abnormalities found in both idiopathic ASD and FXS.

Tdrd3 is a novel stress granule-associated protein interacting with the Fragile-X syndrome protein FMRP.

Tudor domains are widespread among proteins involved in RNA metabolism, but only in a few cases their cellular function has been analyzed in detail. Here, we report on the characterization of the ubiquitously expressed Tudor domain containing protein Tdrd3. Apart from its Tudor domain, we show that Tdrd3 possesses an oligosaccharide/nucleotide binding fold (OB-fold) and an ubiquitin associated domain capable of binding tetra-ubiquitin. A set of biochemical experiments revealed an interaction of Tdrd3 with FMRP, the product of the gene affected in Fragile X syndrome, and its autosomal homologs FXR1 and FXR2. FMRP has been implicated in the translational regulation of target mRNAs and shown to be a component of stress granules (SG). We demonstrate that overexpression of Tdrd3 in cells induces the formation of SGs and as a result leads to its co-localization with endogenous FMRP in these structures. Interestingly, the disease-associated FMRP missense mutation I304N identified in a Fragile X patient severely impairs the interaction with Tdrd3 in biochemical experiments. We propose a contribution of Tdrd3 to FMRP-mediated translational repression and suggest that the loss of the FMRP-Tdrd3 interaction caused by the I304N mutation might contribute to the pathogenesis of Fragile X syndrome.

Elevated de novo protein synthesis in FMRP-deficient human neurons and its correction by metformin treatment.

FXS is the most common genetic cause of intellectual (ID) and autism spectrum disorders (ASD). FXS is caused by loss of FMRP, an RNA-binding protein involved in the translational regulation of a large number of neuronal mRNAs. Absence of FMRP has been shown to lead to elevated protein synthesis and is thought to be a major cause of the synaptic plasticity and behavioural deficits in FXS. The increase in protein synthesis results in part from abnormal activation of key protein translation pathways downstream of ERK1/2 and mTOR signalling. Pharmacological and genetic interventions that attenuate hyperactivation of these pathways can normalize levels of protein synthesis and improve phenotypic outcomes in animal models of FXS. Several efforts are currently underway to trial this strategy in patients with FXS. To date, elevated global protein synthesis as a result of FMRP loss has not been validated in the context of human neurons. Here, using an isogenic human stem cell-based model, we show that de novo protein synthesis is elevated in FMRP-deficient neural cells. We further show that this increase is associated with elevated ERK1/2 and Akt signalling and can be rescued by metformin treatment. Finally, we examined the effect of normalizing protein synthesis on phenotypic abnormalities in FMRP-deficient neural cells. We find that treatment with metformin attenuates the increase in proliferation of FMRP-deficient neural progenitor cells but not the neuronal deficits in neurite outgrowth. The elevated level of protein synthesis and the normalization of neural progenitor proliferation by metformin treatment were validated in additional control and FXS patient-derived hiPSC lines. Overall, our results validate that loss of FMRP results in elevated de novo protein synthesis in human neurons and suggest that approaches targeting this abnormality are likely to be of partial therapeutic benefit in FXS.

Translational roots for mental retardation?

A flurry of findings points to protein translation in the dendrites of neurons as a key feature leading to the changes at synapses that are vital to learning (see main text). And one recent discovery suggests that when this translation goes awry, it can lead to mental retardation.

Looping Genomes: Diagnostic Change and the Genetic Makeup of the Autism Population.

This article builds on Hacking's framework of "dynamic nominalism" to show how knowledge about biological etiology can interact with the "kinds of people" delineated by diagnostic categories in ways that "loop" or modify both over time. The authors use historical materials to show how "geneticization" played a crucial role in binding together autism as a biosocial community and how evidence from genetics research later made an important contribution to the diagnostic expansion of autism. In the second part of the article, the authors draw on quantitative and qualitative analyses of autism rates over time in several rare conditions that are delineated strictly according to genomic mutations in order to demonstrate that these changes in diagnostic practice helped to both increase autism's prevalence and create its enormous genetic heterogeneity. Thus, a looping process that began with geneticization and involved the social effects of genetics research itself transformed the autism population and its genetic makeup.

Características físicas e bucais na síndrome do X frágil: revisão de literatura / Physical and oral characteristics of fragile X syndrome: literature review

Introdução: A síndrome do cromossomo X frágil é uma síndrome genética que acomete principalmente indivíduos do sexo masculino. O nome desta síndrome ocorre como consequência de um estreitamento da extremidade distal do braço longo do cromossomo X, local chamado de sítio frágil. O presente trabalho apresenta uma revisão de literatura, apresentando etiologia, prevalência, métodos de diagnósti-co, características comportamentais, características físicas gerais e de interesse odontológico, além das considerações acerca do atendimento, realizado pelo cirurgião-dentista, em portadores da síndrome do X frágil. Revisão de literatura: As principais características comportamentais são o déficit de atenção, a dificuldade na interação social, a timidez, a ansiedade, a labilidade emocional e os movimentos este-reotipados de mãos. Os achados de interesse odontológico mais prevalentes na literatura foram palato ogival, prog-natismo mandibular, macroglossia, hipoplasia de esmalte e má oclusão. Discussão: Não foram encontrados muitos artigos voltados para a análise facial e odontológica destes pacientes. O atendimento deste público é um desafio para o cirurgião-dentista devido às características comportamentais e fisiológicas apresentadas. Conclusão: o conhecimento das características desta síndrome pelo profissional é impor-tante, pois a síndrome comumente se associa à doenças sistêmicas que podem influenciar no plano de tratamento, além de alterações orofaciais importantes.

Introduction: The fragile X syndrome is a genetic syn-drome that mainly affects males. The name of this syn-drome occurs as a consequence of a narrowing of the distal end of the long arm of the X chromosome, a site called the fragile site. This paper presents a review of the literature, presenting etiology, prevalence, diagnostic methods, behavioral characteristics, general physical characteristics and dental interest, as well as considera-tions about the care provided by the dentist in patients with fragile X syndrome. Literature review: The main behavioral characteristics are attention deficit, difficulty in social interaction, shyness, anxiety, emotional lability and stereotyped hand movements. The most prevalent findings of dental interest in the literature were the ogival palate, mandibular prognathism, macroglossia, enamel hypoplasia and malocclusion. Discussion: There were not many articles focused on facial and dental analysis of these patients. The care of this public is a challenge for the dentist due to the behavioral and physiological characteristics presented. Conclusion: professional know-ledge of the characteristics of this syndrome is important, as the syndrome is commonly associated with systemic diseases that may influence the treatment plan, as well as major orofacial changes.

Mechanisms of teratogenesis: folic acid and antiepileptic therapy.

Gestational folate deficiency has been associated with abnormal growth and development in both experimental animal and human studies and has been postulated as a putative mechanism for the teratogenic effects of antiepileptic drugs (AEDs). Animal studies have shown that the administration of AEDs results in folate depletion and teratogenic effects. Attempts to prevent the teratogenic effects of AEDs by coadministration of folate have shown variable results, perhaps because of a lack of understanding about the specific effects of AEDs on folate metabolism. Our prospective study of women with epilepsy showed that blood folate levels decreased with increasing plasma AED levels and with the number of AEDs. Low blood folate levels before and/or early in pregnancy were significantly associated with spontaneous abortion and the occurrence of developmental anomalies in the offspring. These findings suggest that folate supplementation might be one means of preventing the occurrence of abnormal pregnancy outcome in women with epilepsy, including neural-tube defects in the offspring.

Partial fra(X) phenotype with megalotestes in fra (X)-negative patients with acquired lesions of the central nervous system.

We describe 3 men with acquired lesions of the central nervous system (one male with tumor of the third ventricle, two males with peri- or postnatal accidents). In all three patients macroorchidism was associated with facial characteristics similar to those found in fra(X) males, but they were fra(X)-negative. The finding of megalotestes associated with a partial fra(X) phenotype in the present patients and, more particularly, the documentation of a hypothalamic tumor in the first patient, suggests that a specific hypothalamic lesion in fra(X) males is responsible for some of their manifestations.