The fragile X premutation: into the phenotypic fold

Rapamycin for treating Tuberous sclerosis and Autism spectrum disorders

Tuberous sclerosis (TSC) is a genetic disorder caused by heterozygous mutations in the TSC1 or TSC2 genes and is associated with autism spectrum disorders (ASD) in 20-60% of cases. In addition, altered TSC/mTOR signaling is emerging as a feature common to a subset of ASD. Recent findings, in animal models, show that restoration of the underlying molecular defect can improve neurological dysfunction in several of these models, even if treatment is initiated in adult animals, suggesting that pathophysiological processes in the mature brain contribute significantly to the overall neurological phenotype in these models. These findings suggest that windows for therapeutic intervention in ASD could be wider than thought previously.

The balance between excitation and inhibition and functional sensory processing in the somatosensory cortex

The balance between excitation and inhibition (E/I balance) is tightly regulated in adult cortices to maintain proper nervous system function. Disturbed E/I balance is associated with numerous neuropsychological disorders, such as autism, epilepsy and schizophrenia. The present review will discuss aspects of Hebbian and homeostatic mechanisms regulating excitatory and inhibitory balance related to sensory processing in somatosensory cortex of rodents. Additionally, changes in the E/I balance during sensory manipulation will be discussed.

Exploring the adult life of men and women with fragile X syndrome: results from a national survey

Using data from a national family survey, the authors describe the adult lives (i.e., residence, employment, level of assistance needed with everyday life, friendships, and leisure activities) of 328 adults with the full mutation of the FMR1 gene and identify characteristics related to independence in these domains. Level of functional skills was the strongest predictor of independence in adult life for men, whereas ability to interact appropriately was the strongest predictor for women. Co-occurring mental health conditions influenced independence in adult life for men and women, in particular, autism spectrum disorders for men and affect problems for women. Services for adults with fragile X syndrome should not only target functional skills but interpersonal skills and co-occurring mental health conditions.

An information-rich CGG repeat primed PCR that detects the full range of fragile X expanded alleles and minimizes the need for southern blot analysis

(CGG)(n) repeat expansion in the FMR1 gene is associated with fragile X syndrome and other disorders. Current methods for FMR1 molecular testing rely on Southern blot analysis to detect expanded alleles too large to be PCR-amplified and to identify female homozygous alleles that often confound interpretations of PCR data. A novel, single-tube CGG repeat primed FMR1 PCR technology was designed with two gene-specific primers that flank the triplet repeat region, as well as a third primer that is complementary to the (CGG)(n) repeat. This PCR was evaluated with 171 unique DNA samples, including a blinded set of 146 clinical specimens. The method detected all alleles reported by Southern blot analysis, including full mutations in 66 clinical samples and comprised up to 1300 CGG. Furthermore, a blinded cohort of 42 female homozygous and heterozygous specimens, including 21 with full mutation alleles, was resolved with 100% accuracy. Last, AGG interrupter sequences, which may influence the risk of (CGG)(n) expansion in the children of some carriers, were each correctly identified in 14 male and female clinical samples as referenced to DNA sequencing. As a result, this PCR provides robust detection of expanded alleles and resolves allele zygosity, thus minimizing the number of samples that require Southern blot analysis and producing more comprehensive FMR1 genotyping data than other methods.

Neuro-endocrine basis for altered plasma glucose homeostasis in the Fragile X mouse

BACKGROUND

The fragile X mouse model shows an increase in seizure susceptibility, indicating an involvement of the GABAergic system via an alteration in cellular excitability. In the brain, we have previously described a reduction in GABAA receptor expression as a likely basis for this susceptibility. In the brains of fragile X mice, this reduction in receptor expression culminates with a concomitant increase in the expression of glutamic acid decarboxylase (GAD), the enzyme responsible for GABA synthesis. Further, voltage-sensitive calcium channel expression is reduced in the pancreas of the fragile X mouse. Since there are considerable similarities in the GABAergic system in the brain and pancreas, we evaluated the protective role of taurine in pancreatic islet development in both wild type (WT) and fragile X mice (KO).

METHODS

One-month-old FVB/NJ males or age-matched fmr1-knockout (KO) mice were supplemented with taurine in drinking water (0.05% w/v) for four weeks. Age-matched controls were fed water only for the same duration. At four weeks, mice were sacrificed and pancreases processed for histology and immunohistochemical studies on changes of insulin, glucagon and somatostatin expression. Additional mice were subjected to a glucose tolerance test.

RESULTS

Taurine treatment resulted in a significant increase in the number and size of islets. WT taurine-fed mice, slightly hypoglycemic prior to glucose injection, showed significantly reduced plasma glucose at 30 min post-injection when compared to control mice. KO mice had normal baseline plasma glucose concentration; however, following glucose injection they had higher plasma glucose levels at 30 min when compared to controls. Supplementation of taurine to KO mice resulted in reduced baseline levels of plasma glucose. After glucose injection, the taurine-fed KO mice had reduced plasma glucose at 30 min compared to KO. Concomitant with the increased islets size and glucose tolerance observed in taurine-fed mice there was an increase in insulin, glucagon and somatostatin immunoreactivity in the islets of WT mice. In the KO mice however, insulin levels were not affected whereas glucagon and somatostatin levels were reduced. Exocytosis of these hormones is calcium-dependent, therefore any exacerbation of calcium homeostasis could affect hormone release. We found the expression of the voltage sensitive calcium channels (VSCC) is drastically reduced in the pancreas of fragile X mice.

CONCLUSIONS

During early development, the VSCC play an important role in calcium-dependent gene expression. Since these channels are also involved in depolarization and calcium-mediated vesicular release of neurotransmitters and pancreatic hormones, alterations in the expression of VSCC not only will affect calcium-mediated gene expression but also hormonal and neurotransmitter release creating therefore a neuroendocrine perturbation in the fragile X that may potentially affect other organ systems. We find that in the fragile X mouse, taurine treatment may partially restore functionality of the neuro-endocrine pancreas.

Transcranial magnetic stimulation provides means to assess cortical plasticity and excitability in humans with fragile x syndrome and autism spectrum disorder

Fragile X Syndrome (FXS) is the most common heritable cause of intellectual disability. In vitro electrophysiologic data from mouse models of FXS suggest that loss of fragile X mental retardation protein affects intracortical excitability and synaptic plasticity. Specifically, the cortex appears hyperexcitable, and use-dependent long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength are abnormal. Though animal models provide important information, FXS and other neurodevelopmental disorders are human diseases and as such translational research to evaluate cortical excitability and plasticity must be applied in the human. Transcranial magnetic stimulation paradigms have recently been developed to non-invasively investigate cortical excitability using paired pulse stimulation, as well as LTP- and LTD-like synaptic plasticity in response to theta burst stimulation (TBS) in vivo in the human. TBS applied on consecutive days can be used to measure metaplasticity (the ability of the synapse to undergo a second plastic change following a recent induction of plasticity). The current study investigated intracortical inhibition, plasticity and metaplasticity in full mutation females with FXS, participants with autism spectrum disorders (ASD), and neurotypical controls. Results suggest that intracortical inhibition is normal in participants with FXS, while plasticity and metaplasticity appear abnormal. ASD participants showed abnormalities in plasticity and metaplasticity, as well as heterogeneity in intracortical inhibition. Our findings highlight the utility of non-invasive neurophysiological measures to translate insights from animal models to humans with neurodevelopmental disorders, and thus provide direct confirmation of cortical dysfunction in patients with FXS and ASD.

Psychiatric features in children with genetic syndromes: toward functional phenotypes

Neurodevelopmental disorders with identified genetic etiologies present a unique opportunity to study gene-brain-behavior connections in child psychiatry. Parsing complex human behavior into dissociable components is facilitated by examining a relatively homogenous genetic population. As children with developmental delay carry a greater burden of mental illness than the general population, familiarity with the most common genetic disorders will serve practitioners seeing a general child population. In this article basic genetic testing and 11 of the most common genetic disorders are reviewed, including the evidence base for treatment. Based on their training in child development, family systems, and multimodal treatment, child psychiatrists are well positioned to integrate cognitive, behavioral, social, psychiatric, and physical phenotypes, with a focus on functional impairment.

Dysregulation of mTOR signaling in fragile X syndrome

Fragile X syndrome, the most common form of inherited mental retardation and leading genetic cause of autism, is caused by transcriptional silencing of the Fmr1 gene. The fragile X mental retardation protein (FMRP), the gene product of Fmr1, is an RNA binding protein that negatively regulates translation in neurons. The Fmr1 knock-out mouse, a model of fragile X syndrome, exhibits cognitive deficits and exaggerated metabotropic glutamate receptor (mGluR)-dependent long-term depression at CA1 synapses. However, the molecular mechanisms that link loss of function of FMRP to aberrant synaptic plasticity remain unclear. The mammalian target of rapamycin (mTOR) signaling cascade controls initiation of cap-dependent translation and is under control of mGluRs. Here we show that mTOR phosphorylation and activity are elevated in hippocampus of juvenile Fmr1 knock-out mice by four functional readouts: (1) association of mTOR with regulatory associated protein of mTOR; (2) mTOR kinase activity; (3) phosphorylation of mTOR downstream targets S6 kinase and 4E-binding protein; and (4) formation of eukaryotic initiation factor complex 4F, a critical first step in cap-dependent translation. Consistent with this, mGluR long-term depression at CA1 synapses of FMRP-deficient mice is exaggerated and rapamycin insensitive. We further show that the p110 subunit of the upstream kinase phosphatidylinositol 3-kinase (PI3K) and its upstream activator PI3K enhancer PIKE, predicted targets of FMRP, are upregulated in knock-out mice. Elevated mTOR signaling may provide a functional link between overactivation of group I mGluRs and aberrant synaptic plasticity in the fragile X mouse, mechanisms relevant to impaired cognition in fragile X syndrome.

A novel FMR1 PCR method for the routine detection of low abundance expanded alleles and full mutations in fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is a trinucleotide-repeat disease caused by the expansion of CGG sequences in the 5' untranslated region of the FMR1 (fragile X mental retardation 1) gene. Molecular diagnoses of FXS and other emerging FMR1 disorders typically rely on 2 tests, PCR and Southern blotting; however, performance or throughput limitations of these methods currently constrain routine testing.

METHODS

We evaluated a novel FMR1 gene-specific PCR technology with DNA templates from 20 cell lines and 146 blinded clinical samples. The CGG repeat number was determined by fragment sizing of PCR amplicons with capillary electrophoresis, and results were compared with those for FMR1 Southern blotting analyses with the same samples.

RESULTS

The FMR1 PCR accurately detected full-mutation alleles up to at least 1300 CGG repeats and consisting of >99% GC character. All categories of alleles detected by Southern blotting, including 66 samples with full mutations, were also identified by the FMR1 PCR for each of the 146 clinical samples. Because all full mutation alleles in samples from heterozygous females were detected by the PCR, allele zygosity was reconciled in every case. The PCR reagents also detected a 1% mass fraction of a 940-CGG allele in a background of 99% 23-CGG allele-a roughly 5- fold greater sensitivity than obtained with Southern blotting.

CONCLUSIONS

The novel PCR technology can accurately categorize the spectrum of FMR1 alleles, including alleles previously considered too large to amplify; reproducibly detect low abundance full mutation alleles; and correctly infer homozygosity in female samples, thus greatly reducing the need for sample reflexing to Southern blotting.

Autism spectrum and obsessive-compulsive disorders: OC behaviors, phenotypes and genetics

Autism spectrum disorders (ASDs) are a phenotypically and etiologically heterogeneous set of disorders that include obsessive-compulsive behaviors (OCB) that partially overlap with symptoms associated with obsessive-compulsive disorder (OCD). The OCB seen in ASD vary depending on the individual's mental and chronological age as well as the etiology of their ASD. Although progress has been made in the measurement of the OCB associated with ASD, more work is needed including the potential identification of heritable endophenotypes. Likewise, important progress toward the understanding of genetic influences in ASD has been made by greater refinement of relevant phenotypes using a broad range of study designs, including twin and family-genetic studies, parametric and nonparametric linkage analyses, as well as candidate gene studies and the study of rare genetic variants. These genetic analyses could lead to the refinement of the OCB phenotypes as larger samples are studied and specific associations are replicated. Like ASD, OCB are likely to prove to be multidimensional and polygenic. Some of the vulnerability genes may prove to be generalist genes influencing the phenotypic expression of both ASD and OCD while others will be specific to subcomponents of the ASD phenotype. In order to discover molecular and genetic mechanisms, collaborative approaches need to generate shared samples, resources, novel genomic technologies, as well as more refined phenotypes and innovative statistical approaches. There is a growing need to identify the range of molecular pathways involved in OCB related to ASD in order to develop novel treatment interventions.

Signals, synapses, and synthesis: how new proteins control plasticity

Localization of mRNAs to dendrites and local protein synthesis afford spatial and temporal regulation of gene expression and endow synapses with the capacity to autonomously alter their structure and function. Emerging evidence indicates that RNA binding proteins, ribosomes, translation factors and mRNAs encoding proteins critical to synaptic structure and function localize to neuronal processes. RNAs are transported into dendrites in a translationally quiescent state where they are activated by synaptic stimuli. Two RNA binding proteins that regulate dendritic RNA delivery and translational repression are cytoplasmic polyadenylation element binding protein and fragile X mental retardation protein (FMRP). The fragile X syndrome (FXS) is the most common known genetic cause of autism and is characterized by the loss of FMRP. Hallmark features of the FXS include dysregulation of spine morphogenesis and exaggerated metabotropic glutamate receptor-dependent long term depression, a cellular substrate of learning and memory. Current research focuses on mechanisms whereby mRNAs are transported in a translationally repressed state from soma to distal process and are activated at synaptic sites in response to synaptic signals.

The FMR1 gene and fragile X-associated tremor/ataxia syndrome

The CGG-repeat present in the 5'UTR of the FMR1 gene is unstable upon transmission to the next generation. The repeat is up to 55 CGGs long in the normal population. In fragile X patients, a repeat length exceeding 200 CGGs (full mutation: FM) generally leads to methylation of the repeat and the promoter region, which is accompanied by silencing of the FMR1 gene. The gene product FMRP is involved in regulation of transport and translation of certain mRNA in the dendrite, thereby affecting synaptic plasticity. This is central to learning and memory processes. The absence of FMRP seen in FM is the cause of the mental retardation seen in fragile X patients. The premutation (PM) is defined as 55-200 CGGs. Female PM carriers are at risk of developing primary ovarian insufficiency. Recently it was discovered that elderly PM carriers might develop a progressive neurodegenerative disorder called fragile X-associated tremor/ataxia syndrome. Although arising from the mutations in the same gene, distinct mechanisms lead to fragile X syndrome (absence of FMRP) and FXTAS (toxic RNA gain of function). The pathogenic mechanisms thought to underlie these disorders are discussed, with a specific emphasis on FXTAS. This review gives insight on the implications of all possible repeat length categories seen in fragile X families.

Adult reversal of cognitive phenotypes in neurodevelopmental disorders

Recent findings in mice suggest that it is possible to reverse certain neurodevelopmental disorders in adults. Changes in development, previously thought to be irreparable in adults, were believed to underlie the neurological and psychiatric phenotypes of a range of common mental health problems with a clear developmental component. As a consequence, most researchers have focused their efforts on understanding the molecular and cellular processes that alter development with the hope that early intervention could prevent the emergent pathology. Unexpectedly, several different animal model studies published recently, including animal models of autism, suggest that it may be possible to reverse neurodevelopmental disorders in adults: Addressing the underlying molecular and cellular deficits in adults could in several cases dramatically improve the neurocognitive phenotypes in these animal models. The findings reviewed here provide hope to millions of individuals afflicted with a wide range of neurodevelopmental disorders, including autism, since they suggest that it may be possible to treat or even cure them in adults.

Expansion of an FMR1 grey-zone allele to a full mutation in two generations

Fragile X Syndrome is caused by the expansion of an unstable CGG-repeat tract in the 5'-UTR of the FMR1 gene, which generally results in transcriptional silencing and consequent absence of the FMR1 protein. To date, the smallest premutation allele reported to expand to a full mutation allele in a single generation is 59 CGG repeats. Here, we report a single-generation expansion to a full mutation allele (male with approximately 538 CCG repeats) from a mother who is a carrier of a premutation allele of 56 CGG repeats. Furthermore, the maternal grandfather was a carrier of a gray (or intermediate)-zone allele (45 to 54 repeats) of 52 CGG repeats. Thus, in this family, a gray-zone allele expanded to the full mutation range in two generations. Interestingly, the two AGG interruptions present in the grandfather's allele were absent in the mother's premutation allele. These observations underscore the need to consider carriers of alleles of greater than 55 CGG repeats as being at risk for transmission of a full mutation allele in a single generation, and those with even smaller alleles in the gray zone as being at risk of having grandchildren with full mutation alleles.

A review of fragile X premutation disorders: expanding the psychiatric perspective

CONTEXT

Fragile X premutation conditions are associated with a significant degree of psychopathology and thus are of interest to the psychiatrist. Remarkable advances at the molecular level have enhanced our understanding of fragile X premutation disorders.

OBJECTIVE

The authors review the genetic, molecular, neuroimaging, and clinical (systemic, neurologic, and psychiatric) manifestations of the premutation carrier state (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene.

DATA SOURCES

The search for the psychiatric clinical manifestations of fragile X-associated conditions was accomplished by PubMed for clinical papers published between 1970 and 2008 with the following search terms: Fragile X syndrome, depression, psychosis, anxiety, and dementia.

STUDY SELECTION

Articles addressing psychiatric symptoms in premutation carriers based on review of the abstracts were reviewed. As the majority of the literature on this topic is based on case reports and small case series, these were included in the database.

RESULTS

Reported clinical manifestations of psychiatric illness in premutation carriers include an apparently significant rate of cognitive, mood, anxiety, and other psychiatric disorders. Fragile X premutation-associated conditions are part of the clinical differential diagnosis of several psychiatric syndromes, particularly in pedigrees with known fragile X syndrome cases.

CONCLUSIONS

Fragile X-associated psychiatric manifestations serve as a useful model for a molecular genesis of neuropsychiatric illness. Because of the multigenerational expression of fragile X-associated neuropsychiatric illness, there is a prominent role for genetic testing and genetic counseling of patients and their relatives. Genetic testing is confirmatory of the FMR1 premutation and is an essential component of the clinical evaluation. Psychopharmacologic and psychotherapeutic treatment of fragile X-associated psychiatric illnesses may improve patient function and assist in adaptation to the burden of a genetic neuropsychiatric illness.

Screening for expanded alleles of the FMR1 gene in blood spots from newborn males in a Spanish population

Fragile X syndrome, which is caused by expanded CGG repeats of the FMR1 gene, is associated with a broad spectrum of clinical involvement and is the most common inherited form of intellectual disability. Early diagnosis and intervention are likely to lead to improved outcome for children with fragile X syndrome, but such strategies require better estimates of the frequencies of expanded alleles of the FMR1 gene. In this study, we report the results of a newborn screening study of 5267 male blood spots collected from the Northwest region of Spain as part of the national newborn screening program. The blood spots were screened using a rapid polymerase chain reaction-based method that is capable of identifying the presence of all expanded alleles for both males and females. The screened samples included 199 gray zone alleles, 21 premutation alleles, and two full mutation alleles (1 in 2633). The frequency of premutation alleles was three times higher (1 in 251) than the quoted value of 1 in 813 from a Canadian population and is fully consistent with the results of large-scale Israeli screening studies. Our results demonstrate that newborn screening for the presence of expanded FMR1 alleles is an effective means for defining the distribution of expanded FMR1 alleles in newborn populations; as such, this method is suitable for large-scale newborn screening.

Lifespan changes in working memory in fragile X premutation males

Fragile X syndrome is the world's most common hereditary cause of developmental delay in males and is now well characterized at the biological, brain and cognitive levels. The disorder is caused by the silencing of a single gene on the X chromosome, the FMR1 gene. The premutation (carrier) status, however, is less well documented but has an emerging literature that highlights a more subtle profile of executive cognitive deficiencies that mirror those reported in fully affected males. Rarely, however, has the issue of age-related declines in cognitive performance in premutation males been addressed. In the present study, we focus specifically on the cognitive domain of working memory and its subcomponents (verbal, spatial and central executive memory) and explore performance across a broad sample of premutation males aged 18-69 years matched on age and IQ to unaffected comparison males. We further tease apart the premutation status into those males with symptoms of the newly identified neurodegenerative disorder, the fragile X-associated tremor/ataxia syndrome (FXTAS) and those males currently symptom-free. Our findings indicate a specific vulnerability in premutation males on tasks that require simultaneous manipulation and storage of new information, so-called executive control of memory. Furthermore, this vulnerability appears to exist regardless of the presence of FXTAS symptoms. Males with FXTAS symptoms demonstrated a more general impairment encompassing phonological working memory in addition to central executive working memory. Among asymptomatic premutation males, we observed the novel finding of a relationship between increased CGG repeat size and impairment to central executive working memory.

The quadruplex r(CGG)n destabilizing cationic porphyrin TMPyP4 cooperates with hnRNPs to increase the translation efficiency of fragile X premutation mRNA

The 5' untranslated region of the FMR1 gene which normally includes 4-55 d(CGG) repeats expands to > 55-200 repeats in carriers of fragile X syndrome premutation. Although the levels of premutation FMR1 mRNA in carrier cells are 5-10-fold higher than normal, the amount of the product FMR protein is unchanged or reduced. We demonstrated previously that premutation r(CGG)(n) tracts formed quadruplex structures that impeded translation and lowered the efficiency of protein synthesis. Normal translation could be restored in vivo by the quadruplex r(CGG)(n) destabilizing action of CBF-A and hnRNP A2 proteins. Here we report that the quadruplex-interacting cationic porphyrin TMPyP4 by itself and in cooperation with CBF-A or hnRNP A2 also unfolded quadruplex r(CGG)(n) and increased the efficiency of translation of 5'-(CGG)(99) containing reporter firefly (FL) mRNA. TMPyP4 destabilized in vitro a (CGG)(33) intramolecular quadruplex structure and enhanced the translation of 5'-(CGG)(99)-FL mRNA in a rabbit reticulocyte lysate and in HEK293 cells. The efficiency of translation of (CGG)(99)-FL mRNA was additively increased in cells exposed to TMPyP4 together with CBF-A. Whereas low doses of TMPyP4, CBF-A or hnRNP A2 by themselves did not affect the in vivo utilization of (CGG)(99)-FL mRNA, introduction of TMPyP4 together with either protein synergistically augmented its translation efficiency.

Reversing neurodevelopmental disorders in adults

Abnormalities in brain development, thought to be irreversible in adults, have long been assumed to underlie the neurological and psychiatric symptoms associated with neurodevelopmental disorders. Surprisingly, a number of recent animal model studies of neurodevelopmental disorders demonstrate that reversing the underlying molecular deficits can result in substantial improvements in function even if treatments are started in adulthood. These findings mark a paradigmatic change in the way we understand and envision treating neurodevelopmental disorders.

CGG-repeat length and neuropathological and molecular correlates in a mouse model for fragile X-associated tremor/ataxia syndrome

The 5'untranslated region (UTR) of the FMR1 gene contains a CGG-repeat, which may become unstable upon transmission to the next generation. When repeat length exceeds 200, the FMR1 gene generally undergoes methylation-mediated transcriptional silencing. The subsequent absence of the gene product Fragile X Mental Retardation Protein (FMRP)causes the mental retardation seen in fragile X patients. A CGG-repeat length between 55 and 200 trinucleotides has been termed the premutation (PM). Predominantly elderly male PM carriers are at risk of developing a progressive neurodegenerative disorder: fragile X-associated tremor/ataxia syndrome (FXTAS). All PM carriers have elevated FMR1 mRNA levels, in spite of slightly decreased FMRP levels. The presence of intranuclear ubiquitin-positive inclusions in many brain regions is a neuropathological hallmark of FXTAS. Studies in humans attempting to correlate neuropathological outcomes with molecular measures are difficult because of the limited availability of tissue. Therefore, we have used the expanded CGG-repeat knock-in mouse model of FXTAS to examine the relationship between the molecular and neuropathological parameters in brain. We present Fmr1 mRNA and Fmrp levels and the presence of intranuclear inclusions at different repeat lengths. Contrary to existing hypotheses, our results suggest that inclusion formation may not depend on the elevation per se of Fmr1 transcript levels in aged CGG mice.

Long intronic GAA*TTC repeats induce epigenetic changes and reporter gene silencing in a molecular model of Friedreich ataxia

Friedreich ataxia (FRDA) is caused by hyperexpansion of GAA*TTC repeats located in the first intron of the FXN gene, which inhibits transcription leading to the deficiency of frataxin. The FXN gene is an excellent target for therapeutic intervention since (i) 98% of patients carry the same type of mutation, (ii) the mutation is intronic, thus leaving the FXN coding sequence unaffected and (iii) heterozygous GAA*TTC expansion carriers with approximately 50% decrease of the frataxin are asymptomatic. The discovery of therapeutic strategies for FRDA is hampered by a lack of appropriate molecular models of the disease. Herein, we present the development of a new cell line as a molecular model of FRDA by inserting 560 GAA*TTC repeats into an intron of a GFP reporter minigene. The GFP_(GAA*TTC)(560) minigene recapitulates the molecular hallmarks of the mutated FXN gene, i.e. inhibition of transcription of the reporter gene, decreased levels of the reporter protein and hypoacetylation and hypermethylation of histones in the vicinity of the repeats. Additionally, selected histone deacetylase inhibitors, known to stimulate the FXN gene expression, increase the expression of the GFP_(GAA*TTC)(560) reporter. This FRDA model can be adapted to high-throughput analyses in a search for new therapeutics for the disease.

Treatment of fragile X-associated tremor ataxia syndrome (FXTAS) and related neurological problems

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a progressive neurological disorder that affects older adult carriers, predominantly males, of premutation alleles (55 to 200 CGG repeats) of the fragile X (FMR1) gene. Principal features of FXTAS are intention tremor, ataxia, parkinsonism, cognitive decline, and peripheral neuropathy; ancillary features include, autonomic dysfunction, and psychiatric symptoms of anxiety, depression, and disinhibition. Although controlled trials have not been carried out in individuals with FXTAS, there is a significant amount of anecdotal information regarding various treatment modalities. Moreover, there exists a great deal of evidence regarding the efficacy of various medications for treatment of other disorders (eg, Alzheimer disease) that have substantial phenotypic overlap with FXTAS. The current review summarizes what is currently known regarding the symptomatic treatment, or potential for treatment, of FXTAS.

Altered hypothalamus-pituitary-adrenal gland axis regulation in the expanded CGG-repeat mouse model for fragile X-associated tremor/ataxia syndrome

The human FMR1 gene contains an unstable CGG-repeat in its 5' untranslated region. The repeat length in the normal population is polymorphic (5-54 CGG-repeats). Individuals carrying lengths beyond 200 CGGs (i.e. the full mutation) show hypermethylation and as a consequence gene silencing of the FMR1 gene. The absence of the gene product FMRP causes the fragile X syndrome, the most common inherited form of mental retardation. Elderly carriers of the premutation (PM), which is defined as a repeat length between 55 and 200 CGGs, can develop a progressive neurodegenerative syndrome: fragile X-associated tremor/ataxia syndrome (FXTAS). The high FMR1 mRNA levels observed in cells from PM carriers have led to the hypothesis that FXTAS is caused by a pathogenic RNA gain-of-function mechanism. Apart from tremor/ataxia, specific psychiatric symptoms have been described in PM carriers with or without FXTAS. Since these symptoms could arise from elevated stress hormone levels, we investigated hypothalamic-pituitary-adrenal (HPA) axis regulation using a knock-in mouse model with an expanded CGG-repeat in the PM range (>98 repeats) in the Fmr1 gene, which shows repeat instability, and displays biochemical, phenotypic and neuropathological characteristics of FXTAS. We show elevated levels of corticosterone in serum and ubiquitin-positive inclusions in both the pituitary and adrenal gland of 100-week-old animals. In addition, we demonstrate ubiquitin-positive inclusions in the amygdala from aged expanded CGG-repeat mice. We hypothesize that altered regulation of the HPA axis and the amygdala and higher stress hormone levels in the mouse model for FXTAS may explain associated psychological symptoms in humans.

Age-dependent cognitive changes in carriers of the fragile X syndrome

Fragile X syndrome is a neurodevelopmental disorder that is caused by the silencing of a single gene on the X chromosome, the fragile X mental retardation 1 (FMR1) gene. Affected individuals display a unique neurocognitive phenotype that includes significant impairment in inhibitory control, selective attention, working memory, and visual-spatial cognition. In contrast, little is known about the trajectory and specificity of any cognitive impairment associated with the fragile X premutation (i.e., "carrier status") or its relationship with the recently identified neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). In the present study, we evaluated a broad sample of 40 premutation males (PM) aged 18-69 years matched on age and IQ to 67 unaffected comparison males (NC). Performance was compared across a range of cognitive domains known to be impaired in fragile X syndrome (i.e., "full mutation"). Tremor was also assessed using a self-report neurological questionnaire. PM displayed statistically significant deficits in their ability to inhibit prepotent responses, differentiating them from NC from age 30 onwards. With increasing age, the two groups follow different trajectories, with PM developing progressively more severe problems in inhibitory control. This deficit also has a strong co-occurrence in males displaying FXTAS-related symptomatology (p<.001). Selective attention was also impaired in PM but did not show any disproportionate aging effect. No other cognitive deficits were observed. We conclude that an inhibitory deficit and its impact across the lifespan are specifically associated with the fragile X premutation status, and may be a precursor for development of a more severe form of cognitive impairment or dementia, which has been reported in patients with the diagnosis of FXTAS.

Reduced Hippocampal Activation During Recall is Associated with Elevated FMR1 mRNA and Psychiatric Symptoms in Men with the Fragile X Premutation

Recent studies reveal that young carriers of the fragile X premutation are at increased risk for psychiatric conditions, memory problems and executive deficits. Post mortem and structural MRI studies suggest the hippocampus is preferentially affected by the premutation. The current study utilized magnetic resonance imaging (MRI) to explore the relationship between hippocampal structure and function as well as molecular/genetic and psychiatric measures in men with the fragile X premutation. Although the groups did not differ in hippocampal volume, the premutation group showed reduced left hippocampal activation and increased right parietal activation during a recall task relative to controls. These results suggest that brain function underlying memory recall is affected by premutation status. Left hippocampal activation was negatively correlated with both FMR1 mRNA level and psychiatric symptomology in the premutation group. These associations support the theory that increased levels of FMR1 mRNA affect brain function and contribute to psychiatric symptoms.

Consensus characterization of 16 FMR1 reference materials: a consortium study

Fragile X syndrome, which is caused by expansion of a (CGG)(n) repeat in the FMR1 gene, occurs in approximately 1:3500 males and causes mental retardation/behavioral problems. Smaller (CGG)(n) repeat expansions in FMR1, premutations, are associated with premature ovarian failure and fragile X-associated tremor/ataxia syndrome. An FMR1-sizing assay is technically challenging because of high GC content of the (CGG)(n) repeat, the size limitations of conventional PCR, and a lack of reference materials available for test development/validation and routine quality control. The Centers for Disease Control and Prevention and the Association for Molecular Pathology, together with the genetic testing community, have addressed the need for characterized fragile X mutation reference materials by developing characterized DNA samples from 16 cell lines with repeat lengths representing important phenotypic classes and diagnostic cutoffs. The alleles in these materials were characterized by consensus analysis in nine clinical laboratories. The information generated from this study is available on the Centers for Disease Control and Prevention and Coriell Cell Repositories websites. DNA purified from these cell lines is available to the genetics community through the Coriell Cell Repositories. The public availability of these reference materials should help support accurate clinical fragile X syndrome testing.

Pur alpha binds to rCGG repeats and modulates repeat-mediated neurodegeneration in a Drosophila model of fragile X tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a recently recognized neurodegenerative disorder in fragile X premutation carriers with FMR1 alleles containing 55-200 CGG repeats. Previously, we developed a Drosophila model of FXTAS and demonstrated that transcribed premutation repeats alone are sufficient to cause neurodegeneration, suggesting that rCGG-repeat-binding proteins (RBPs) may be sequestered from their normal function by rCGG binding. Here, we identify Pur alpha and hnRNP A2/B1 as RBPs. We show that Pur alpha and rCGG repeats interact in a sequence-specific fashion that is conserved between mammals and Drosophila. Overexpression of Pur alpha in Drosophila could suppress rCGG-mediated neurodegeneration in a dose-dependent manner. Furthermore, Pur alpha is also present in the inclusions of FXTAS patient brains. These findings support the disease mechanism of FXTAS of rCGG repeat sequestration of specific RBPs, leading to neuronal cell death, and implicate that Pur alpha plays an important role in the pathogenesis of FXTAS.

RNA-binding proteins hnRNP A2/B1 and CUGBP1 suppress fragile X CGG premutation repeat-induced neurodegeneration in a Drosophila model of FXTAS

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a recently described neurodegenerative disorder of older adult carriers of premutation alleles (60-200 CGG repeats) in the fragile X mental retardation gene (FMR1). It has been proposed that FXTAS is an RNA-mediated neurodegenerative disease caused by the titration of RNA-binding proteins by the CGG repeats. To test this hypothesis, we utilize a transgenic Drosophila model of FXTAS that expresses a premutation-length repeat (90 CGG repeats) from the 5' UTR of the human FMR1 gene and displays neuronal degeneration. Here, we show that overexpression of RNA-binding proteins hnRNP A2/B1 and CUGBP1 suppresses the phenotype of the CGG transgenic fly. Furthermore, we show that hnRNP A2/B1 directly interacts with riboCGG repeats and that the CUGBP1 protein interacts with the riboCGG repeats via hnRNP A2/B1.

The tetraplex (CGG)n destabilizing proteins hnRNP A2 and CBF-A enhance the in vivo translation of fragile X premutation mRNA

Expansion of a (CGG)n sequence in the 5'-UTR of the FMR1 gene to >200-2000 repeats abolishes its transcription and initiates fragile X syndrome (FXS). By contrast, levels of FMR1 mRNA are 5-10-fold higher in FXS premutation carriers of >55-200 repeats than in normal subjects. Lack of a corresponding increase in the amount of the product FMRP protein in carrier cells suggest that (CGG)>55-200 tracts thwart translation. Here we report that a (CGG)99 sequence positioned upstream to reporter firefly (FL) gene selectively diminished mRNA translation in coupled and separate T7 promoter-driven in vitro transcription and translation systems. The (CGG)99 tract similarly depressed mRNA utilization in HEK293 human cells transfected with plasmids bearing FMR1 promoter-driven FL gene. A (CGG)33 RNA tract formed a largely RNase T1-resistant intramolecular secondary structure in the presence of K+ ions. Expression of the quadruplex (CGG)n disrupting proteins hnRNP A2 or CBF-A in HEK293 cells significantly elevated the efficacy of (CGG)99 FL mRNA translation whereas hnRNP A2 or CBF-A mutants lacking quadruplex (CGG)n disrupting activity did not. Taken together, our results suggest that secondary structures of (CGG)n in mRNA obstruct its translation and that quadruplex-disrupting proteins alleviate the translational block.

Argonaute-2-dependent rescue of a Drosophila model of FXTAS by FRAXE premutation repeat

Fragile X Syndrome is the most common form of hereditary mental retardation. It is caused by a large expansion of the CGG trinucleotide repeat (>200 repeats) in the 5'-untranslated region (UTR) of the FMR1 gene that leads to silencing of its transcript. Individuals with CGG repeat expansions approximately between 60 and 200 are referred to as premutation carriers. Fragile X-associated tremor and ataxia syndrome (FXTAS), an RNA-mediated neurodegenerative disease has been described in up to 50% of males carrying premutation alleles. FRAXE, the most common form of non-syndromic X-linked mental retardation, is caused by expansion of a CCG trinucleotide repeat (>200) in the 5'-UTR of the FMR2 gene. While the FRAXE premutation length repeat is observed in the general population, there has not yet been a report of a neurodegenerative phenotype associated with these alleles. In this study, we show that the CCG premutation length repeat leads to an RNA-mediated neurodegenerative phenotype in a Drosophila model. Furthermore, we show that co-expression of both the CCG and CGG-containing RNAs suppresses their independent toxicity and is dependent on the RNAi pathway. These data support the concept that RNA toxicity is the mechanism of neuronal toxicity and suggests potential reversal of RNA-mediated phenotypes with complementary RNA molecules.

Fragile X-associated tremor/ataxia syndrome: intrafamilial variability and the size of the FMR1 premutation CGG repeat

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurological progressive disorder associated with the FMR1 gene premutation. We report on variable presentation of findings associated with FXTAS in 3 brothers aged 68, 74, and 73 years, carrying premutation alleles of (CGG)(123,) (CGG)(109), and (CGG)(91) triplets, respectively. Based on previously proposed diagnostic criteria for the syndrome, clinical and radiological data allowed establishing a "definite" diagnosis of FXTAS in the two carriers of the longest (CGG)(n). The carrier of the (CGG)(91) allele, although presenting a major radiological sign of the syndrome (symmetrical white-matter lesions in the middle cerebellar peduncles), did not have any significant neurological manifestation at 73 years of age.

Examination of reproductive aging milestones among women who carry the FMR1 premutation

BACKGROUND

The fragile X premutation is characterized by a large CGG repeat track (55-199 repeats) in the 5' UTR of the FMR1 gene. This X-linked mutation leads to an increased risk for premature ovarian failure; interestingly, the association of repeat size with risk is non-linear. We hypothesize that the premutation-associated ovarian insufficiency is due to a diminished oocyte pool and examined reproductive aging milestones by repeat size group to determine if the same non-linear association is observed.

METHODS

We analyzed cross-sectional reproductive history questionnaire data from 948 women with a wide range of repeat sizes.

RESULTS

We have confirmed the non-linear relationship among premutation carriers for ovarian insufficiency. The mid-range repeat size group (80-100 repeats), not the highest group, had an increased risk for: altered cycle traits (shortened cycle length, irregular cycles and skipped cycles), subfertility and dizygotic twinning. Smoking, a modifiable risk, decreased the reproductive lifespan of women with the premutation by about 1 year, similar to its effect on non-carriers. As expected, premutation carriers were found to be at an increased risk for osteoporosis.

CONCLUSIONS

Possible molecular mechanisms to explain the non-linear repeat size risk for ovarian insufficiency are discussed.

A place for genetic uncertainty: parents valuing an unknown in the meaning of disease

Klinefelter, Turner, and fragile X syndromes are conditions defined by a genetic or chromosomal variant. The timing of diagnosis, tests employed, specialists involved, symptoms evident, and prognoses available vary considerably within and across these syndromes, but all three share in common a diagnosis verified through a molecular or cytogenetic test. The genetic or chromosomal variant identified designates a syndrome, even when symptoms associated with the particular syndrome are absent. This article analyzes interviews conducted with parents and grandparents of children with these syndromes from across the USA to explore how they interpret a confirmed genetic diagnosis that is associated with a range of possible symptoms that may never be exhibited. Parents' responses indicate that they see the genetic aspects of the syndrome as stable, permanent, and authoritative. But they allow, and even embrace, uncertainty about the condition by focusing on variation between diagnosed siblings, the individuality of their diagnosed child, his or her accomplishments, and other positive aspects that go beyond the genetic diagnosis. Some families counter the genetic diagnosis by arguing that in the absence of symptoms, the syndrome does not exist. They use their own expertise to question the perceived certainty of the genetic diagnosis and to employ the diagnosis strategically. These multiple and often conflicting evaluations of the diagnostic label reveal the rich ways families make meaning of the authority attributed to genetic diagnosis.

Language development and fragile X syndrome: profiles, syndrome-specificity, and within-syndrome differences

Fragile X syndrome (FXS) is the leading inherited cause of mental retardation. In this article, we review what is known about the language and related problems of individuals with FXS. In doing so, we focus on the syndrome-specific features of the language phenotype and on the organismic (i.e., genetic and individual neurocognitive and behavioral) and environmental factors associated with within-syndrome variation in the phenotype. We also briefly review those aspects of the behavioral phenotype of FXS that are relevant for understanding syndrome-specific features of, and within-syndrome variability in, language. The review includes summaries of research on the prelinguistic foundations for language development and on each of the major components of language (i.e., vocabulary, morphosyntax, and pragmatics). Throughout the review, we point out implications of existing research for intervention as well as directions for future research.

The FMR1 premutation and reproduction

OBJECTIVE

To update clinicians on the reproductive implications of premutations in FMR1 (fragile X mental retardation 1). Fragile X syndrome, a cause of mental retardation and autism, is due to a full mutation (>200 CGG repeats). Initially, individuals who carried the premutation (defined as more than 55 but less than 200 CGG repeats) were not considered at risk for any clinical disorders. It is now recognized that this was incorrect, specifically with respect to female reproduction.

DESIGN AND SETTING

Literature review and consensus building at two multidisciplinary scientific workshops.

CONCLUSION(S)

Convincing evidence now relates the FMR1 premutation to altered ovarian function and loss of fertility. An FMR1 mRNA gain-of-function toxicity may underlie this altered ovarian function. There are major gaps in knowledge regarding the natural history of the altered ovarian function in women who carry the FMR1 premutation, making counseling about reproductive plans a challenge. Women with premature ovarian failure are at increased risk of having an FMR1 premutation and should be informed of the availability of fragile X testing. Specialists in reproductive medicine can provide a supportive environment in which to explain the implications of FMR1 premutation testing, facilitate access to testing, and make appropriate referral to genetic counselors.

Cognitive Impairment in a 65-year-old Male With the Fragile X-associated Tremor-Ataxia Syndrome (FXTAS)

OBJECTIVE

This is the first case report of a comprehensive neuropsychologic examination of an older man with the fragile X-associated tremor-ataxia syndrome (FXTAS).

BACKGROUND

FXTAS, a newly identified phenotype affecting older male carriers of the fragile X premutation allele, is a progressive disorder marked by gait ataxia, action tremor, peripheral neuropathy, executive cognitive deficits, generalized brain atrophy, and neuronal and astrocytic intranuclear inclusion bodies throughout the brain. The patient previously had undergone neurologic evaluation, molecular analysis, and magnetic resonance imaging.

METHOD

The patient was administered a neuropsychologic examination, assessing motor and somatosensory functioning, visual and spatial functioning, speech and language, attention, executive abilities, learning and memory, and reasoning.

RESULTS

The patient showed a pattern of cognitive impairment characterized by essentially normal speech and language, moderately impaired control of attention, and moderate to severe deficits in working memory, executive functioning, and both declarative and procedural learning. Visual and spatial abilities were relatively unimpaired, and verbal reasoning was only mildly deficient.

CONCLUSIONS

The findings suggest that a cognitive disorder, with especially marked executive cognitive function and memory deficits, accompanies FXTAS. The findings in FXTAS are compared with those in several other neurodegenerative disorders.

A woman with spontaneous premature ovarian failure gives birth to a child with fragile X syndrome

OBJECTIVE

To inform clinicians about a reproductive risk associated with spontaneous premature ovarian failure and the fragile X mental retardation 1 gene (FMR1).

DESIGN

Case report.

SETTING

National Institutes of Health Clinical Center.

PATIENT(S)

A 35-year-old woman with confirmed spontaneous premature ovarian failure.

INTERVENTION(S)

FMR1 genetic testing.

MAIN OUTCOME MEASURE(S)

Number of CGG trinucleotide repeats in the 5' untranslated region of FMR1.

RESULT(S)

Despite having ovarian failure the woman subsequently conceived and delivered a son with fragile X syndrome (>200 CGG repeats). She was then found to carry an FMR1 premutation (85 CGG repeats).

CONCLUSION(S)

This is a real-life manifestation of a theoretical risk; a woman conceived subsequent to the diagnosis of spontaneous premature ovarian failure and has a child who manifests mental retardation due to fragile X syndrome. Women with spontaneous premature ovarian failure are at increased risk of having an FMR1 premutation and should be informed of the availability of fragile X testing. Should an FMR1 premutation be uncovered, this will allow patients to make informed reproductive decisions and help clinicians to properly diagnose family members who may have menstrual irregularity, developmental delay, or neurologic symptoms.

An update: spontaneous premature ovarian failure is not an early menopause

OBJECTIVE

To update clinicians regarding the management of women with spontaneous premature ovarian failure (POF).

DESIGN

Literature review and consensus building among three clinicians with experience in caring for women with spontaneous POF.

CONCLUSION(S)

Clearly the ovarian "failure" in this disorder is not permanent in all women. Approximately 5%-10% may conceive spontaneously and unexpectedly after the diagnosis. An integrated approach to management is best, and there is a need to first address physical and mental health issues before addressing plans for family building. Women with spontaneous POF are at increased risk of adrenal insufficiency, which should be detected and managed appropriately, especially before proceeding to ovum or embryo donation procedures. Young women with POF experience pathologically low serum E2 levels at least intermittently. Despite the absence of controlled evidence for this specific population, physiologic replacement of ovarian steroid hormones seems rational until the age of normal menopause. The disorder may be associated with other conditions that require evaluation and management, including hypothyroidism, dry eye syndrome, abnormal karyotype, or a premutation of the FMR1 gene. Finally, clinicians need to be sensitive to the emotional aspects of this disorder when delivering the diagnosis and during subsequent management.

Abnormal elevation of FMR1 mRNA is associated with psychological symptoms in individuals with the fragile X premutation

Until recently, individuals with premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene were believed to be psychologically unaffected. However, the recent documentation of abnormal elevation of FMR1 mRNA, discovery of fragile X-associated tremor/ataxia syndrome (FXTAS), and reports of psychiatric disorders in children and adults with the premutation have suggested a pathogenic gene-brain-behavior mechanism. In a large collaborative study, 68 men and 144 women with the FMR1 premutation completed a psychological symptoms checklist and FMR1 genetic testing, including determination of CGG repeat size, percentage of FMR1 protein (FMRP)-positive lymphocytes, and FMR1 mRNA levels. Relative to published norms, men and women with FXTAS symptoms reported higher levels of several types of psychological symptoms. In addition, men and women with the premutation and no overt evidence of FXTAS reported higher levels of obsessive-compulsive symptoms. Elevated FMR1 mRNA, but not CGG repeat size or reduced FMRP (as measured by immunocytochemistry), was significantly associated with increased psychological symptoms, predominantly obsessive-compulsive symptoms and psychoticism, in premutation men with and without FXTAS symptoms. There was no relationship between CGG repeat size, FMR1 mRNA or FMRP and psychological symptoms in premutation women unless the sample was restricted to those with skewed X-activation ratio toward >50% active premutation alleles. The results of this study support the hypothesis that FMR1 function is associated with psychological difficulties in individuals with the premutation, and provide evidence concordant with an RNA toxic gain-of-function model in a neuropsychiatric phenotype.

Fragile X syndrome

Fragile X syndrome, the most common genetic disorder associated with mental retardation is caused by an expansion of the unstable CGG repeat within the FMR1 gene. Although overgrowth is not the main hallmark of this condition, the fragile X syndrome is usually included in the differential diagnosis of children with mental retardation and excess growth. This review highlights the most recent advances in the field of fragile X research.

GRAND ROUNDS: An Atypical Progressive Dementia in a Male Carrier of the Fragile X Premutation: An Example of Fragile X-Associated Tremor/Ataxia Syndrome

This case study describes a 65-year-old man initially diagnosed with an atypical rapidly progressive dementia who subsequently participated in a research project at the MIND Institute at the University of California-Davis, where he was diagnosed with a recently identified neurodegenerative syndrome, fragile X-associated tremor/ataxia syndrome (FXTAS). He was a carrier of the fragile X premutation and in later life developed tremor, gait ataxia, parkinsonism, and cognitive deficits that progressed very rapidly. This case study provides a detailed description of the individual's history, presenting symptoms, neuropsychological test results, and postmortem neuropathological analysis. Pathological findings showed diagnostic features of both FXTAS and Alzheimer's disease, which might help to explain the rapid progression of his dementia.