Cognitive, anxiety and mood disorders in the fragile X-associated tremor/ataxia syndrome

Contribution of DNA/RNA Structures Formed by Expanded CGG/CCG Repeats Within the FMR1 Locus in the Pathogenesis of Fragile X-Associated Disorders

Repeat expansion disorders (REDs) encompass over 50 inherited neurological disorders and are characterized by the expansion of short tandem nucleotide repeats beyond a specific repeat length. Particularly intriguing among these are multiple fragile X-associated disorders (FXds), which arise from an expansion of CGG repeats in the 5' untranslated region of the FMR1 gene. Despite arising from repeat expansions in the same gene, the clinical manifestations of FXds vary widely, encompassing developmental delays, parkinsonism, dementia, and an increased risk of infertility. FXds also exhibit molecular mechanisms observed in other REDs, that is, gene- and protein-loss-of-function and RNA- and protein-gain-of-function. The heterogeneity of phenotypes and pathomechanisms in FXds results from the different lengths of the CGG tract. As the number of repeats increases, the structures formed by RNA and DNA fragments containing CGG repeats change significantly, contributing to the diversity of FXd phenotypes and mechanisms. In this review, we discuss the role of RNA and DNA structures formed by expanded CGG repeats in driving FXd pathogenesis and how the genetic instability of CGG repeats is mediated by the complex interplay between transcription, DNA replication, and repair. We also discuss therapeutic strategies, including small molecules, antisense oligonucleotides, and CRISPR-Cas systems, that target toxic RNA and DNA involved in the development of FXds.

Language use predicts symptoms of fragile X-associated tremor/ataxia syndrome in men and women with the FMR1 premutation

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an age-related neurodegenerative disorder caused by a premutation of the FMR1 gene on the X chromosome. Despite the pervasive physical and cognitive effects of FXTAS, no studies have examined language in symptomatic males and females, limiting utility as an outcome measure in clinical trials of FXTAS. The goal of this work is to determine (a) the extent to which male and female FMR1 premutation carriers with FXTAS symptoms differ in their language use and (b) whether language production predicts FXTAS symptoms. Thirty-one individuals with the FMR1 premutation (21M, 10F), ages 58-85 years with some symptoms of FXTAS, were recruited from a larger cross-sectional study. Participants completed a five-minute monologic language sample. Language transcripts were assessed for rate of dysfluencies, lexical-semantics, syntax, and speech rate. Multivariable linear and ordinal regressions were used to predict FXTAS-associated symptoms, cognitive functioning, and executive functioning. Males and females did not differ in their language use. Language production predicted FXTAS symptom severity, cognitive functioning, and executive functioning. Language production difficulties may co-occur with FXTAS-associated symptoms and may be a viable outcome measure in future clinical trials, with future research needed.

Apolipoproteine and KLOTHO Gene Variants Do Not Affect the Penetrance of Fragile X-Associated Tremor/Ataxia Syndrome

In this study, the potential role and interaction of the APOε and KLOTHO genes on the penetrance of fragile X-associated tremor/ataxia syndrome (FXTAS) and on the IQ trajectory were investigated. FXTAS was diagnosed based on molecular, clinical and radiological criteria. Males with the premutation (PM) over 50 years, 165 with and 34 without an FXTAS diagnosis, were included in this study and were compared based on their APO (ε2-ε3-ε4) and KLOTHO variant (KL-VS) genotypes. The effect of APOε4 on FXTAS stage and on diagnosis did not differ significantly by KL-VS genotype with interaction effect p = 0.662 and p = 0.91, respectively. In the FXTAS individuals with an APOε2 allele, a marginal significance was observed towards a larger decline in verbal IQ (VIQ) in individuals with an APOε4 allele compared to those without an APOε4 allele (p = 0.071). In conclusion, our findings suggest that the APOε4 and KL-VS genotypes alone or through their interaction effect do not appear to predispose to either FXTAS diagnosis or stage in male carriers of the PM allele. A further study is needed to establish the trend of IQ decline in the FXTAS individuals who carry APOε4 with APOε2 compared to those without APOε4.

Mitochondrial dysfunction in brain tissues and Extracellular Vesicles Fragile X-associated tremor/ataxia syndrome

OBJECTIVE

Mitochondrial impairments have been implicated in the pathogenesis of Fragile X-associated tremor/ataxia syndrome (FXTAS) based on analysis of mitochondria in peripheral tissues and cultured cells. We sought to assess whether mitochondrial abnormalities present in postmortem brain tissues of patients with FXTAS are also present in plasma neuron-derived extracellular vesicles (NDEVs) from living carriers of fragile X messenger ribonucleoprotein1 (FMR1) gene premutations at an early asymptomatic stage of the disease continuum.

METHODS

We utilized postmortem frozen cerebellar and frontal cortex samples from a cohort of eight patients with FXTAS and nine controls and measured the quantity and activity of the mitochondrial proteins complex IV and complex V. In addition, we evaluated the same measures in isolated plasma NDEVs by selective immunoaffinity capture targeting L1CAM from a separate cohort of eight FMR1 premutation carriers and four age-matched controls.

RESULTS

Lower complex IV and V quantity and activity were observed in the cerebellum of FXTAS patients compared to controls, without any differences in total mitochondrial content. No patient-control differences were observed in the frontal cortex. In NDEVs, FMR1 premutation carriers compared to controls had lower activity of Complex IV and Complex V, but higher Complex V quantity.

INTERPRETATION

Quantitative and functional abnormalities in mitochondrial electron transport chain complexes IV and V seen in the cerebellum of patients with FXTAS are also manifest in plasma NDEVs of FMR1 premutation carriers. Plasma NDEVs may provide further insights into mitochondrial pathologies in this syndrome and could potentially lead to the development of biomarkers for predicting symptomatic FXTAS among premutation carriers and disease monitoring.

Clinical implications of somatic allele expansion in female FMR1 premutation carriers

Carriers of a premutation allele (PM) in the FMR1 gene are at risk of developing a number of Fragile X premutation asssociated disorders (FXPAC), including Fragile X-associated Tremor/Ataxia Syndrome (FXTAS), Fragile X-associated Primary Ovarian Insufficiency (FXPOI), and Fragile X-associated neuropsychiatric disorders (FXAND). We have recently reported somatic CGG allele expansion in female PM; however, its clinical significance remains unclear. The aim of this study was to examine the potential clinical association between somatic FMR1 allele instability and PM associated disorders. Participants comprised of 424 female PM carriers age 0.3- 90 years. FMR1 molecular measures and clinical information on the presence of medical conditions, were determined for all subjects for primary analysis. Two sub-groups of participants (age ≥ 25, N = 377 and age ≥ 50, N = 134) were used in the analysis related to presence of FXPOI and FXTAS, respectively. Among all participants (N = 424), the degree of instability (expansion) was significantly higher (median 2.5 vs 2.0, P = 0.026) in participants with a diagnosis of attention deficit hyperactivity disorder (ADHD) compared to those without. FMR1 mRNA expression was significantly higher in subjects with any psychiatric disorder diagnosis (P = 0.0017); specifically, in those with ADHD (P = 0.009), and with depression (P = 0.025). Somatic FMR1 expansion was associated with the presence of ADHD in female PM and FMR1 mRNA levels were associated with the presence of mental health disorders. The findings of our research are innovative as they suggest a potential role of the CGG expansion in the clinical phenotype of PM and may potentially guide clinical prognosis and management.

Artificial neural network applied to fragile X-associated tremor/ataxia syndrome stage diagnosis based on peripheral mitochondrial bioenergetics and brain imaging outcomes

No proven prognosis is available for the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Artificial neural network analyses (ANN) were used to predict FXTAS progression using data from 127 adults (noncarriers and FMR1 premutation carriers with and without FXTAS) with five outcomes from brain MRI imaging and 22 peripheral bioenergetic outcomes from two cell types. Diagnosis accuracy by ANN predictions ranged from 41.7 to 86.3% (depending on the algorithm used), and those misclassified usually presented a higher FXTAS stage. ANN prediction of FXTAS stages was based on a combination of two imaging findings (white matter hyperintensity and whole-brain volumes adjusted for intracranial volume) and four bioenergetic outcomes. Those at Stage 3 vs. 0-2 showed lower mitochondrial mass, higher oxidative stress, and an altered electron transfer consistent with mitochondrial unfolded protein response activation. Those at Stages 4-5 vs. 3 had higher oxidative stress and glycerol-3-phosphate-linked ATP production, suggesting that targeting mGPDH activity may prevent a worse prognosis. This was confirmed by the bioenergetic improvement of inhibiting mGPDH with metformin in affected fibroblasts. ANN supports the prospect of an unbiased molecular definition in diagnosing FXTAS stages while identifying potential targets for personalized medicine.

Identifying enhancer properties associated with genetic risk for complex traits using regulome-wide association studies

Genetic risk for complex traits is strongly enriched in non-coding genomic regions involved in gene regulation, especially enhancers. However, we lack adequate tools to connect the characteristics of these disruptions to genetic risk. Here, we propose RWAS (Regulome Wide Association Study), a new application of the MAGMA software package to identify the characteristics of enhancers that contribute to genetic risk for disease. RWAS involves three steps: (i) assign genotyped SNPs to cell type- or tissue-specific regulatory features (e.g., enhancers); (ii) test associations of each regulatory feature with a trait of interest for which genome-wide association study (GWAS) summary statistics are available; (iii) perform enhancer-set enrichment analyses to identify quantitative or categorical features of regulatory elements that are associated with the trait. These steps are implemented as a novel application of MAGMA, a tool originally developed for gene-based GWAS analyses. Applying RWAS to interrogate genetic risk for schizophrenia, we discovered a class of risk-associated AT-rich enhancers that are active in the developing brain and harbor binding sites for multiple transcription factors with neurodevelopmental functions. RWAS utilizes open-source software, and we provide a comprehensive collection of annotations for tissue-specific enhancer locations and features, including their evolutionary conservation, AT content, and co-localization with binding sites for hundreds of TFs. RWAS will enable researchers to characterize properties of regulatory elements associated with any trait of interest for which GWAS summary statistics are available.

Cerebral Microbleeds in Fragile X-Associated Tremor/Ataxia Syndrome

BACKGROUND

Fragile X-associated tremor/ataxia syndrome is a neurodegenerative disease of late onset developed by carriers of the premutation in the fragile x mental retardation 1 (FMR1) gene. Pathological features of neurodegeneration in fragile X-associated tremor/ataxia syndrome include toxic levels of FMR1 mRNA, ubiquitin-positive intranuclear inclusions, white matter disease, iron accumulation, and a proinflammatory state.

OBJECTIVE

The objective of this study was to analyze the presence of cerebral microbleeds in the brains of patients with fragile X-associated tremor/ataxia syndrome and investigate plausible causes for cerebral microbleeds in fragile X-associated tremor/ataxia syndrome.

METHODS

We collected cerebral and cerebellar tissue from 15 fragile X-associated tremor/ataxia syndrome cases and 15 control cases carrying FMR1 normal alleles. We performed hematoxylin and eosin, Perls and Congo red stains, ubiquitin, and amyloid β protein immunostaining. We quantified the number of cerebral microbleeds, amount of iron, presence of amyloid β within the capillaries, and number of endothelial cells containing intranuclear inclusions. We evaluated the relationships between pathological findings using correlation analysis.

RESULTS

We found intranuclear inclusions in the endothelial cells of capillaries and an increased number of cerebral microbleeds in the brains of those with fragile X-associated tremor/ataxia syndrome, both of which are indicators of cerebrovascular dysfunction. We also found a suggestive association between the amount of capillaries that contain amyloid β in the cerebral cortex and the rate of disease progression.

CONCLUSION

We propose microangiopathy as a pathologic feature of fragile X-associated tremor/ataxia syndrome. © 2021 International Parkinson and Movement Disorder Society.

Autistic traits and mental health in women with the fragile-X premutation: maternal status versus genetic risk

BACKGROUND

Research on women with the fragile-X premutation (FX-p) has been underrepresented within the field of behavioural phenotypes.

AIMS

To understand whether the FX-p confers risk for autistic traits, depression and anxiety, independent of maternal status.

METHOD

In study 1, mothers of children with fragile-X syndrome (M-FXp; n = 51, mean age 43 years (s.d. = 5.80)) were compared with mothers of autistic children (M-ASD; n = 59, mean age 42 (s.d. = 5.80)), mothers of children with Smith-Magenis syndrome (M-SMS; n = 27, mean age 39 (s.d. = 7.20)) and mothers of typically developing children (M-TD; n = 44, mean age 40 (s.d. = 4.90)). In study 2, the M-FXp group were compared with non-mothers with the FX-p (NM-FXp; n = 17, mean age 32 (s.d. = 9.20)), typically developed non-mothers (NM-TD; n = 28, mean age 31 (s.d. = 6.80)) and the M-TD group. All participants completed an online survey, including measures of IQ, autistic traits, anxiety, depression and positive affect.

RESULTS

In study 1: the M-FXp group reported more autistic traits than the M-TD group (P < 0.05, η2 = 0.046). Anxiety and parental stress were elevated in the M-FXp, M-SMS and M-ASD groups relative to the M-TD group (all P ≤ 0.003, η2 = 0.079-0.322). In study 2: a main effect of premutation status indicated that women with the FX-p report elevated autistic traits and anxiety (P ≤ 0.007, η2 = 0.055-0.060); this did not interact with maternal status.

CONCLUSIONS

The findings indicate that women with the FX-p show an increased risk for autistic traits and anxiety. This risk is specific to the presence of the FX-p and is not fully accounted for by maternal status or the stress of caring for children with neurodevelopmental disorders.

Response Inhibition Deficits in Women with the FMR1 Premutation are Associated with Age and Fall Risk

One in 113-178 females worldwide carry a premutation allele on the FMR1 gene. The FMR1 premutation is linked to neurocognitive and neuromotor impairments, although the phenotype is not fully understood, particularly with respect to age effects. This study sought to define oculomotor response inhibition skills in women with the FMR1 premutation and their association with age and fall risk. We employed an antisaccade eye-tracking paradigm to index oculomotor inhibition skills in 35 women with the FMR1 premutation and 28 control women. The FMR1 premutation group exhibited longer antisaccade latency and reduced accuracy relative to controls, indicating deficient response inhibition skills. Longer response latency was associated with older age in the FMR1 premutation and was also predictive of fall risk. Findings highlight the utility of the antisaccade paradigm for detecting early signs of age-related executive decline in the FMR1 premutation, which is related to fall risk. Findings support the need for clinical prevention efforts to decrease and delay the trajectory of age-related executive decline in women with the FMR1 premutation during midlife.

Characterization of the Metabolic, Clinical and Neuropsychological Phenotype of Female Carriers of the Premutation in the X-Linked FMR1 Gene

The X-linked FMR1 premutation (PM) is characterized by a 55-200 CGG triplet expansion in the 5'-untranslated region (UTR). Carriers of the PM were originally thought to be asymptomatic; however, they may present general neuropsychiatric manifestations including learning disabilities, depression and anxiety, among others. With age, both sexes may also develop the neurodegenerative disease fragile X-associated tremor/ataxia syndrome (FXTAS). Among carriers, females are at higher risk for developing immune disorders, hypertension, seizures, endocrine disorders and chronic pain, among others. Some female carriers younger than 40 years old may develop fragile X-associated primary ovarian insufficiency (FXPOI). To date, no studies have addressed the metabolic footprint - that includes mitochondrial metabolism - of female carriers and its link to clinical/cognitive manifestations. To this end, we performed a comprehensive biochemical assessment of 42 female carriers (24-70 years old) compared to sex-matched non-carriers. By applying a multivariable correlation matrix, a generalized bioenergetics impairment was correlated with diagnoses of the PM, FXTAS and its severity, FXPOI and anxiety. Intellectual deficits were strongly correlated with both mitochondrial dysfunction and with CGG repeat length. A combined multi-omics approach identified a down-regulation of RNA and mRNA metabolism, translation, carbon and protein metabolism, unfolded protein response, and up-regulation of glycolysis and antioxidant response. The suboptimal activation of the unfolded protein response (UPR) and endoplasmic-reticulum-associated protein degradation (ERAD) response challenges and further compromises the PM genetic background to withstand other, more severe forms of stress. Mechanistically, some of the deficits were linked to an altered protein expression due to decreased protein translation, but others seemed secondary to oxidative stress originated from the accumulation of either toxic mRNA or RAN-derived protein products or as a result of a direct toxicity of accumulated metabolites from deficiencies in critical enzymes.

Metabolic Alterations in FMR1 Premutation Carriers

FMR1 gene premutation carriers are at risk of developing Fragile X-associated tremor/ataxia syndrome (FXTAS) and Fragile X-associated primary ovarian insufficiency (FXPOI) in adulthood. Currently the development of biomarkers and effective treatments in FMR1 premutations is still in its infancy. Recent metabolic studies have shown novel findings in asymptomatic FMR1 premutation carriers and FXTAS, which provide promising insight through identification of potential biomarkers and therapeutic pathways. Here we review the latest advancements of the metabolic alterations found in asymptomatic FMR1 premutation carriers and FXTAS, along with our perspective for future studies in this emerging field.

Elevated FMR1-mRNA and lowered FMRP - A double-hit mechanism for psychiatric features in men with FMR1 premutations

Fragile X syndrome (FXS) is caused by a full mutation of the FMR1 gene (>200 CGG repeats and subsequent methylation), such that there is little or no FMR1 protein (FMRP) produced, leading to intellectual disability (ID). Individuals with the premutation allele (55-200 CGG repeats, generally unmethylated) have elevated FMR1 mRNA levels, a consequence of enhanced transcription, resulting in neuronal toxicity and a spectrum of premutation-associated disorders, including the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Here we described 14 patients who had both lowered FMRP and elevated FMR1 mRNA levels, representing dual mechanisms of clinical involvement, which may combine features of both FXS and FXTAS. In addition, the majority of these cases show psychiatric symptoms, including bipolar disorder, and/or psychotic features, which are rarely seen in those with just FXS.

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS): Pathophysiology and Clinical Implications

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder seen in older premutation (55-200 CGG repeats) carriers of FMR1. The premutation has excessive levels of FMR1 mRNA that lead to toxicity and mitochondrial dysfunction. The clinical features usually begin in the 60 s with an action or intention tremor followed by cerebellar ataxia, although 20% have only ataxia. MRI features include brain atrophy and white matter disease, especially in the middle cerebellar peduncles, periventricular areas, and splenium of the corpus callosum. Neurocognitive problems include memory and executive function deficits, although 50% of males can develop dementia. Females can be less affected by FXTAS because of a second X chromosome that does not carry the premutation. Approximately 40% of males and 16% of female carriers develop FXTAS. Since the premutation can occur in less than 1 in 200 women and 1 in 400 men, the FXTAS diagnosis should be considered in patients that present with tremor, ataxia, parkinsonian symptoms, neuropathy, and psychiatric problems. If a family history of a fragile X mutation is known, then FMR1 DNA testing is essential in patients with these symptoms.

Fragile X associated neuropsychiatric disorders in a male without FXTAS

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability and autism spectrum disorder. In most cases, it is due to an expansion of the CGG triplet to more than 200 repeats within the promoter region of the FMR1 gene. In the premutation (PM) the trinucleotide is expanded to 55-200 repeats. PM carriers can present with disorders associated with the PM including fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X-associated ovarian insufficiency (FXPOI). Recently fragile X-associated neuropsychiatric disorders (FXAND) was proposed as an umbrella term to include the neuropsychiatric disorders that are more prevalent in PM carriers compared to the general population such as anxiety, depression, chronic fatigue, alcohol abuse, and psychosis, among others. The patient in our study was evaluated by a team of clinicians from the University del Valle in Cali who traveled to Ricaurte, a Colombian town known for being a genetic geographic cluster of FXS. A detailed medical history was collected and complete physical, neurological and psychiatric evaluations were performed in addition to molecular and neuroradiological studies. We report the case of a 78-year-old man, PM carrier, without FXTAS whose main clinical presentation consists of behavioral changes and psychosis. Brain imaging revealed white matter lesions in the periventricular region and mild cerebral atrophy. Although anxiety and depression are the most common neuropsychiatric manifestations in PM carriers, it is important to perform a complete psychiatric evaluation since some patients may present with behavioral changes and psychosis.

Fragile X-Associated Neuropsychiatric Disorders (FXAND)

Fragile X syndrome (FXS) is caused by the full mutation (>200 CGG repeats) in the Fragile X Mental Retardation 1 (FMR1) gene. It is the most common inherited cause of intellectual disability (ID) and autism. This review focuses on neuropsychiatric disorders frequently experienced by premutation carriers with 55 to 200 CGG repeats and the pathophysiology involves elevated FMR1 mRNA levels, which is different from the absence or deficiency of fragile X mental retardation protein (FMRP) seen in FXS. Neuropsychiatric disorders are the most common problems associated with the premutation, and they affect approximately 50% of individuals with 55 to 200 CGG repeats in the FMR1 gene. Neuropsychiatric disorders in children with the premutation include anxiety, ADHD, social deficits, or autism spectrum disorders (ASD). In adults with the premutation, anxiety and depression are the most common problems, although obsessive compulsive disorder, ADHD, and substance abuse are also common. These problems are often exacerbated by chronic fatigue, chronic pain, fibromyalgia, autoimmune disorders and sleep problems, which are also associated with the premutation. Here we review the clinical studies, neuropathology and molecular underpinnings of RNA toxicity associated with the premutation. We also propose the name Fragile X-associated Neuropsychiatric Disorders (FXAND) in an effort to promote research and the use of fragile X DNA testing to enhance recognition and treatment for these disorders.

Age-related functional brain changes in FMR1 premutation carriers

The FMR1 premutation confers a 40–60% risk for males of developing a neurodegenerative disease called the Fragile X-associated Tremor Ataxia Syndrome (FXTAS). FXTAS is a late-onset disease that primarily involves progressive symptoms of tremor and ataxia, as well as cognitive decline that can develop into dementia in some patients. At present, it is not clear whether changes to brain function are detectable in motor regions prior to the onset of frank symptomatology. The present study therefore aimed to utilize an fMRI motor task for the first time in an asymptomatic premutation population.

Premutation carriers without a diagnosis of FXTAS (n = 17) and a group of healthy male controls (n = 17), with an age range of 24–68 years old, were recruited for this cross-sectional study. This study utilized neuroimaging, molecular and clinical measurements, employing an fMRI finger-tapping task with a block design consisting of sequential finger-tapping, random finger-tapping and rest conditions. The imaging analysis contrasted the sequential and random conditions to investigate activation changes in response to a change in task demand. Additionally, measurements were obtained of participant tremor, co-ordination and balance using the CATSYS-2000 system and measures of FMR1 mRNA were quantified from peripheral blood samples using quantitative real-time PCR methodology.

Premutation carriers demonstrated significantly less cerebellar activation than controls during sequential versus random finger tapping (FWE_corr < 0.001). In addition, there was a significant age by group interaction in the hippocampus, inferior parietal cortex and temporal cortex originating from a more negative relationship between brain activation and age in the carrier group compared to the controls (FWE_corr < 0.001).

Here, we present for the first time functional imaging-based evidence for early movement-related neurodegeneration in Fragile X premutation carriers. These changes pre-exist the diagnosis of FXTAS and are greatest in older carriers suggesting that they may be indicative of FXTAS vulnerability.

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The authors present a cross-sectional fMRI study in male carriers of the FMR1 premutation

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Carriers show decreased BOLD activation at the cerebellum in response to change in task demand in a finger-tapping task

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Carriers exhibit a group x age interaction of BOLD response in the temporoparietal area

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These changes pre-exist the diagnosis of the Fragile X-associated Tremor/Ataxia Syndrome (FXTAS)

Psychiatric disorders among women with the fragile X premutation without children affected by fragile X syndrome

Several studies have demonstrated increased rates of anxiety and depressive disorders among female carriers of the fragile X premutation. However, the majority of these studies focused on mothers of children with fragile X syndrome, who experience higher rates of parenting stress that may contribute to the emergence of these disorders. The present study compared psychiatric symptom presentation (utilizing measures of current symptoms and lifetime DSM-IV Axis I disorders) in 24 female carriers without affected children (mean age = 32.1 years) to 26 non-carrier women from the community (mean age = 30.5 years). We also examined the association between CGG repeat size (adjusted for X activation ratio) and mRNA, with severity of psychiatric symptoms. Women with the premutation reported significantly elevated symptoms of anxiety, depression, interpersonal sensitivity, obsessive-compulsiveness, and somatization relative to controls during the past week. Carriers had significantly higher rates of lifetime social phobia (42.3%) compared to controls (12.5%); however, this comparison did not remain significant after multiple comparison adjustment. Rates of other psychiatric disorders were not significantly elevated relative to controls, though it should be noted that lifetime rates among controls were much higher than previously published population estimates. Although the sample is relatively small, the study of this unique cohort suggests the premutation confers risk for mood and anxiety disorders independent of the stress of parenting children with FXS. Screening for psychiatric disorders in women with the premutation, even before they become parents, is important and highly encouraged. © 2016 Wiley Periodicals, Inc.

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS) Motor Dysfunction Modeled in Mice

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that affects some carriers of the fragile X premutation (PM). In PM carriers, there is a moderate expansion of a CGG trinucleotide sequence (55-200 repeats) in the fragile X gene (FMR1) leading to increased FMR1 mRNA and small to moderate decreases in the fragile X mental retardation protein (FMRP) expression. The key symptoms of FXTAS include cerebellar gait ataxia, kinetic tremor, sensorimotor deficits, neuropsychiatric changes, and dementia. While the specific trigger(s) that causes PM carriers to progress to FXTAS pathogenesis remains elusive, the use of animal models has shed light on the underlying neurobiology of the altered pathways involved in disease development. In this review, we examine the current use of mouse models to study PM and FXTAS, focusing on recent advances in the field. Specifically, we will discuss the construct, face, and predictive validities of these PM mouse models, the insights into the underlying disease mechanisms, and potential treatments.

Plasma metabolic profile delineates roles for neurodegeneration, pro-inflammatory damage and mitochondrial dysfunction in the FMR1 premutation

Carriers of premutation CGG expansions in the fragile X mental retardation 1 (FMR1) gene are at higher risk of developing a late-onset neurodegenerative disorder named Fragile X-associated tremor ataxia syndrome (FXTAS). Given that mitochondrial dysfunction has been identified in fibroblasts, PBMC and brain samples from carriers as well as in animal models of the premutation and that mitochondria are at the center of intermediary metabolism, the aim of the present study was to provide a complete view of the metabolic pattern by uncovering plasma metabolic perturbations in premutation carriers. To this end, metabolic profiles were evaluated in plasma from 23 premutation individuals and 16 age- and sex-matched controls. Among the affected pathways, mitochondrial dysfunction was associated with a Warburg-like shift with increases in lactate levels and altered Krebs' intermediates, neurotransmitters, markers of neurodegeneration and increases in oxidative stress-mediated damage to biomolecules. The number of CGG repeats correlated with a subset of plasma metabolites, which are implicated not only in mitochondrial disorders but also in other neurological diseases, such as Parkinson's, Alzheimer's and Huntington's diseases. For the first time, the identified pathways shed light on disease mechanisms contributing to morbidity of the premutation, with the potential of assessing metabolites in longitudinal studies as indicators of morbidity or disease progression, especially at the early preclinical stages.

Plasma Biomarkers for Monitoring Brain Pathophysiology in FMR1 Premutation Carriers

Premutation carriers have a 55–200 CGG expansion in the fragile X mental retardation 1 (FMR1) gene. Currently, 1.5 million individuals are affected in the United States, and carriers are at risk of developing the late-onset neurodegenerative disorder Fragile X-associated tremor ataxia syndrome (FXTAS). Limited efforts have been made to develop new methods for improved early patient monitoring, treatment response, and disease progression. To this end, plasma metabolomic phenotyping was obtained for 23 premutation carriers and 16 age- and sex-matched controls. Three biomarkers, phenylethylamine normalized by either aconitate or isocitrate and oleamide normalized by isocitrate, exhibited excellent model performance. The lower phenylethylamine and oleamide plasma levels in carriers may indicate, respectively, incipient nigrostriatal degeneration and higher incidence of substance abuse, anxiety and sleep disturbances. Higher levels of citrate, isocitrate, aconitate, and lactate may reflect deficits in both bioenergetics and neurotransmitter metabolism (Glu, GABA). This study lays important groundwork by defining the potential utility of plasma metabolic profiling to monitor brain pathophysiology in carriers before and during the progression of FXTAS, treatment efficacy and evaluation of side effects.

Fragile X-associated tremor/ataxia syndrome: phenotypic comparisons with other movement disorders

OBJECTIVE

The purpose of this paper is to review the typical cognitive and motor impairments seen in fragile X-associated tremor/ataxia syndrome (FXTAS), essential tremor (ET), Parkinson disease (PD), spinocerebellar ataxias (SCAs), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP) in order to enhance diagnosis of FXTAS patients.

METHODS

We compared the cognitive and motor phenotypes of FXTAS with each of these other movement disorders. Relevant neuropathological and neuroimaging findings are also reviewed. Finally, we describe the differences in age of onset, disease severity, progression rates, and average lifespan in FXTAS compared to ET, PD, SCAs, MSA, and PSP. We conclude with a flow chart algorithm to guide the clinician in the differential diagnosis of FXTAS.

RESULTS

By comparing the cognitive and motor phenotypes of FXTAS with the phenotypes of ET, PD, SCAs, MSA, and PSP we have clarified potential symptom overlap while elucidating factors that make these disorders unique from one another. In summary, the clinician should consider a FXTAS diagnosis and testing for the Fragile X mental retardation 1 (FMR1) gene premutation if a patient over the age of 50 (1) presents with cerebellar ataxia and/or intention tremor with mild parkinsonism, (2) has the middle cerebellar peduncle (MCP) sign, global cerebellar and cerebral atrophy, and/or subcortical white matter lesions on MRI, or (3) has a family history of fragile X related disorders, intellectual disability, autism, premature ovarian failure and has neurological signs consistent with FXTAS. Peripheral neuropathy, executive function deficits, anxiety, or depression are supportive of the diagnosis.

CONCLUSIONS

Distinct profiles in the cognitive and motor domains between these movement disorders may guide practitioners in the differential diagnosis process and ultimately lead to better medical management of FXTAS patients.

Clinically significant psychiatric symptoms among male carriers of the fragile X premutation, with and without FXTAS, and the mediating influence of executive functioning

OBJECTIVES

To clarify the neuropsychiatric phenotype of fragile X-associated tremor/ataxia syndrome (FXTAS), and assess the extent to which it is mediated by the dysexecutive syndrome that is a major feature of the disorder.

METHODS

We examined the prevalence of clinically meaningful psychiatric symptoms among male carriers of the fragile X premutation, with and without FXTAS, in comparison with men with a normal allele. Measures included the Neuropsychiatric Inventory (NPI), Symptom Checklist-90-R (SCL-90-R), and the Behavioral Dyscontrol Scale, a measure of executive functioning. Between-group differences were evaluated using logistic regression, followed by a mediation analysis with ordinary least squares regression to assess the contribution of dysexecutive syndrome to the observed psychiatric domains.

RESULTS

Men with FXTAS showed higher rates of clinically significant symptoms overall and in specific domains: somatization, obsessive compulsive, depression, anxiety, psychoticism, agitation/aggression, apathy/indifference, irritability, and nighttime behavior problems. Post hoc analyses suggested that findings of psychoticism among men with FXTAS may be associated with participants' accurate acknowledgment of cognitive and physical dysfunction, rather than reflecting psychosis. Asymptomatic carriers showed no evidence of clinically significant psychiatric symptoms, but when all carriers were compared with men having a normal FMR1 allele, executive function deficits were found to mediate scores in several domains on both NPI and SCL-90-R.

CONCLUSIONS

Building on prior research, the results provide evidence that the psychiatric phenotype for men includes clinically meaningful depression, hostility, and irritability, in association with behavioral and attentional disinhibition. It is likely that these problems reflect the effects of impaired executive functioning.

Neuropsychiatry of fragile X-premutation carriers with and without fragile X-associated tremor-ataxia syndrome: implications for neuropsychology

OBJECTIVES

Clinical neuropsychologists benefit from clinical currency in recently ascertained neuropsychiatric illness, such as fragile X premutation (FXPM) disorders. The author reviewed the clinical literature through 2016 for neuropsychiatric phenotypes in FXPM disorders, including patients with fragile X-associated tremor/ataxia syndrome (FXTAS).

METHODS

A PubMed search using the search terms 'Fragile X,' 'Premutation,' 'Carriers,' 'Psychiatric,' 'Dementia,' 'Mood,' and 'Anxiety' for citations in the clinical literature through 2016 was reviewed for studies specifically examining the neuropsychiatric phenotype in FXPM patients. The relevant articles were classified according to specific neuropsychiatric syndromes, including child onset, adult onset with and without FXTAS, as well as common systemic comorbidities in FXPM patients.

RESULTS

Eighty-six articles were reviewed for the neuropsychiatric and other phenotypes in FXPM patients. The neuropsychiatric phenotype in FXPM patients is distinct from that of full mutation (Fragile X Syndrome) patients. FXTAS is associated with a specific cortical-subcortical major or mild neurocognitive disorder (NCD).

CONCLUSIONS

FXPM patients are at risk for neuropsychiatric illness. In addition, FXPM patients are at risk for other systemic conditions that should raise suspicion for FXPM-associated illnesses. Clinicians should consider a diagnosis of FXPM-associated neuropsychiatric illness when patients with specific clinical scenarios are encountered; especially in patient pedigrees consistent with a typical (often multigenerational) presentation of fragile X-associated conditions, confirmatory genetic testing should be considered. Clinical management should take into account the psychological challenges of a multigenerational genetic neuropsychiatric illness with a variable CNS and systemic clinical phenotype.

What has been learned from mouse models of the Fragile X Premutation and Fragile X-associated tremor/ataxia syndrome?

OBJECTIVE

To describe in this review how research using mouse models developed to study the Fragile X premutation (PM) and Fragile X-associated tremor/ataxia syndrome (FXTAS) have contributed to understanding these disorders. PM carriers bear an expanded CGG trinucleotide repeat on the Fragile X Mental Retardation 1 (FMR1) gene, and are at risk for developing the late onset neurodegenerative disorder FXTAS.

CONCLUSIONS

Much has been learned about these genetic disorders from the development and study of mouse models. This includes new insights into the early cellular and molecular events that occur in PM carriers and in FXTAS, the presence of multiorgan pathology beyond the CNS, immunological dysregulation, unexpected synthesis of a potentially toxic peptide in FXTAS (i.e., FMRpolyG), and evidence that the disease process may be halted or reversed by appropriate molecular therapies given early in the course of disease.

Fragile X premutation carriers: A systematic review of neuroimaging findings

BACKGROUND

Expansion of the CGG repeat region of the FMR1 gene from less than 45 repeats to between 55 and 200 repeats is known as the fragile X premutation. Carriers of the fragile X premutation may develop a neurodegenerative disease called fragile X-associated tremor/ataxia syndrome (FXTAS). Recent evidence suggests that premutation carriers experience other psychiatric difficulties throughout their lifespan.

METHODS

Medline, EMBASE and PsychINFO were searched for all appropriate English language studies published between January 1990 and December 2013. 419 potentially relevant articles were identified and screened. 19 articles were included in the analysis.

RESULTS

We discuss key structural magnetic resonance imaging (MRI) findings such as the MCP sign and white matter atrophy. Additionally, we discuss how functional MRI results have progressed our knowledge of how FXTAS may manifest, including reduced brain activation during social and memory tasks in multiple regions.

LIMITATIONS

This systematic review may have been limited by the search for articles on just 3 scientific databases. Differing techniques and methods of analyses between research groups and primary research articles may have caused differences in results between studies.

CONCLUSION

Current MRI studies into the fragile X premutation have been important in the diagnosis of FXTAS and identifying potential pathophysiological mechanisms. Associations with blood based measures have also demonstrated that neurodevelopmental and neurodegenerative aspects of the fragile X premutation could be functionally and pathologically separate. Larger longitudinal studies will be required to investigate these conclusions.

Fragile X syndrome

Fragile X Syndrome (FXS) is a genetic disease due to a CGG trinucleotide expansion, named full mutation (greater than 200 CGG repeats), in the fragile X mental retardation 1 gene locus Xq27.3; which leads to an hypermethylated region in the gene promoter therefore silencing it and lowering the expression levels of the fragile X mental retardation 1, a protein involved in synaptic plasticity and maturation. Individuals with FXS present with intellectual disability, autism, hyperactivity, long face, large or prominent ears and macroorchidism at puberty and thereafter. Most of the young children with FXS will present with language delay, sensory hyper arousal and anxiety. Girls are less affected than boys, only 25% have intellectual disability. Given the genomic features of the syndrome, there are patients with a number of triplet repeats between 55 and 200, known as premutation carriers. Most carriers have a normal IQ but some have developmental problems. The diagnosis of FXS has evolved from karyotype with special culture medium, to molecular techniques that are more sensitive and specific including PCR and Southern Blot. During the last decade, the advances in the knowledge of FXS, has led to the development of investigations on pharmaceutical management or targeted treatments for FXS. Minocycline and sertraline have shown efficacy in children.

Emerging topics in FXTAS

This paper summarizes key emerging issues in fragile X-associated tremor/ataxia syndrome (FXTAS) as presented at the First International Conference on the FMR1 Premutation: Basic Mechanisms & Clinical Involvement in 2013.

Understanding the neuropsychiatric phenotype of fragile X-associated tremor ataxia syndrome: a systematic review

Fragile X-associated tremor ataxia syndrome (FXTAS) is a recently identified X-linked neurodegenerative disorder affecting a proportion of premutation carriers of the Fragile X Mental Retardation 1 (FMR1) gene. Previous research suggests that cognitive and psychiatric features of FXTAS may include primary impairments in executive function and increased vulnerability to mood and anxiety disorders. A number of these reports, however, are based on overlapping cohorts or have produced inconsistent findings. A systematic review was therefore conducted to further elucidate the neuropsychiatric features characteristic of FXTAS. Fourteen papers met inclusion criteria for the review and were considered to represent nine independent FXTAS cohorts. Findings from the review suggest that the neuropsychiatric phenotype of FXTAS is characterised primarily by poorer performance on measures of executive function, working memory, information processing speed, and fine motor control when compared to matched comparison groups. Two studies were identified in which psychiatric symptoms in FXTAS were compared with controls, and these yielded mixed results. Overall the results of this review support previous reports that the neuropsychiatric profile of FXTAS is consistent with a dysexecutive fronto-subcortical syndrome. However, additional controlled studies are required to progress our understanding of FXTAS and how the neuropsychiatric profile relates to underlying pathological mechanisms.

Eye movements reveal impaired inhibitory control in adult male fragile X premutation carriers asymptomatic for FXTAS

OBJECTIVE

Fragile X premutation carriers (fXPCs) have an expansion of 55-200 CGG repeats in the FMR1 gene. Male fXPCs are at risk for developing a neurodegenerative motor disorder (FXTAS) often accompanied by inhibitory control impairments, even in fXPCs without motor symptoms. Inhibitory control impairments might precede, and thus indicate elevated risk for motor impairment associated with FXTAS. We tested whether inhibitory impairments are observable in fXPCs by assessing oculomotor performance.

METHOD

Participants were males aged 18-48 years asymptomatic for FXTAS. FXPCs (n = 21) and healthy age-matched controls (n = 22) performed four oculomotor tasks. In a Fixation task, participants fixated on a central cross and maintained gaze position when a peripheral stimulus appeared. In a Pursuit task, participants maintained gaze on a square moving at constant velocity. In a Prosaccade task, participants fixated on a central cross, then looked at a peripheral stimulus. An Antisaccade task was identical to the Prosaccade task, except participants looked in the direction opposite the stimulus. Inhibitory cost was the difference in saccade latency between the Antisaccade and Prosaccade tasks.

RESULTS

Relative to controls, fXPCs had longer saccade latency in the Antisaccade task. In fXPCs, inhibitory cost was positively associated with vermis area in lobules VI-VII.

CONCLUSION

Antisaccades require inhibitory control to inhibit reflexive eye movements. We found that eye movements are sensitive to impaired inhibitory control in fXPCs asymptomatic for FXTAS. Thus, eye movements may be useful in assessing FXTAS risk or disease progression.

Towards an Understanding of Neuropsychiatric Manifestations in Fragile X Premutation Carriers

Fragile X-associated disorders (FXD) are a group of disorders caused by expansion of non-coding CGG repeat elements in the fragile X (FMR1) gene. One of these disorders, fragile X syndrome (FXS), is the most common heritable cause of intellectual disability, and is caused by large CGG repeat expansions (>200) resulting in silencing of the FMR1 gene. An increasingly recognized number of neuropsychiatric FXD have recently been identified that are caused by 'premutation' range expansions (55-200). These disorders are characterized by a spectrum of neuropsychiatric manifestations ranging from an increased risk of neurodevelopmental, mood and anxiety disorders to neurodegenerative phenotypes such as the fragile X-associated tremor ataxia syndrome (FXTAS). Here, we review advances in the clinical understanding of neuropsychiatric disorders in premutation carriers across the lifespan and offer guidance for the detection of such disorders by practicing psychiatrists and neurologists.

Investigation of memory, executive functions, and anatomic correlates in asymptomatic FMR1 premutation carriers

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset movement disorder associated with FMR1 premutation alleles. Asymptomatic premutation (aPM) carriers have preserved cognitive functions, but they present subtle executive deficits. Current efforts are focusing on the identification of specific cognitive markers that can detect aPM carriers at higher risk of developing FXTAS. This study aims at evaluating verbal memory and executive functions as early markers of disease progression while exploring associated brain structure changes using diffusion tensor imaging. We assessed 30 aPM men and 38 intrafamilial controls. The groups perform similarly in the executive domain except for decreased performance in motor planning in aPM carriers. In the memory domain, aPM carriers present a significant decrease in verbal encoding and retrieval. Retrieval is associated with microstructural changes of the white matter (WM) of the left hippocampal fimbria. Encoding is associated with changes in the WM under the right dorsolateral prefrontal cortex, a region implicated in relational memory encoding. These associations were found in the aPM group only and did not show age-related decline. This may be interpreted as a neurodevelopmental effect of the premutation, and longitudinal studies are required to better understand these mechanisms.

High apolipoprotein E4 allele frequency in FXTAS patients

PURPOSE

Fragile X-associated tremor/ataxia syndrome is a late-onset neurodegenerative disorder that occurs in FMR1 premutation carriers. It is well known that the apolipoprotein E ε4 allele is a risk factor for neurodegenerative disease. The main goal of this work was to evaluate the apolipoprotein E genotypes and allelic distribution among patients with fragile X-associated tremor/ataxia syndrome.

METHODS

A total of 44 unrelated FMR1 premutation carriers (22 presenting with fragile X-associated tremor/ataxia syndrome and 22 without fragile X-associated tremor/ataxia syndrome) were genotyped.

RESULTS

All the apolipoprotein E ε4/4 genotype carriers detected (100%), and six of the seven apolipoprotein E ε4/3 genotype carriers (85.7%) are patients presenting with fragile X-associated tremor/ataxia syndrome symptoms, whereas only 40% of the apolipoprotein E ε3/3 genotype carriers belong to the fragile X-associated tremor/ataxia syndrome group. The results showed that the presence of the apolipoprotein E ε4 allele increases the risk of developing fragile X-associated tremor/ataxia syndrome (odds ratio = 12.041; P = 0.034).

CONCLUSION

On the basis of these results, we conclude that the presence of at least one apolipoprotein E ε4 allele might act as a genetic factor predisposing individuals to develop fragile X-associated tremor/ataxia syndrome.

The neurology and corresponding genetics of fragile X disorders: insights into the genetics of neurodegeneration

There have been significant advances in understanding how the fragile X gene (FMR1) can lead to distinct neurological syndromes. Clinical features of two disorders – fragile X syndrome and fragile X-associated tremor ataxia syndrome (FXTAS) – are highlighted in this article. These two disorders – one a neurodevelopmental disorder, the other a neurodegenerative disorder – are caused by a single expanded CGG repeat sequence within the FMR1 gene. Minor differences in repeat length result in the markedly different phenotypes. Understanding the action of FMR1 in FXTAS and fragile X syndrome has yielded significant insights into the genetics of neurodegeneration. Moreover, the genetic model in FXTAS is similar to several other neurologic genetic disorders, suggesting there are common pathways shared by many phenotypically diverse progressive neurodegenerative disorders. Finally, it is possible that targeted therapies for disorders such as FXTAS may also be effective in other neurodegenerative disorders that share similar mechanisms of pathogenesis.

Ages of Onset of Mood and Anxiety Disorders in Fragile X Premutation Carriers

OBJECTIVE

FMR1 premutation carriers of both genders have a high lifetime prevalence of anxiety and depressive disorders, however little is known regarding the onset ages of these conditions. This study compared onset ages of mood and anxiety disorders in premutation carriers with typical onset ages of the same disorders in the general population.

METHODS

Eighty-one premutation carriers (42% men; average age 62, SD 10) with and without FXTAS completed the Structured Clinical Interview for DSM-IV-TR. Onset ages of mood and anxiety disorders were compared to the corresponding typical population onset ages using the signed rank test.

RESULTS

Overall median onset ages of MDD (46 years old, p < 0.0001), panic disorder (40 years old, p = 0.0067), and specific phobia (11.5 years old, p = 0.0003) were significantly higher in premutation carriers compared to the general population. Median MDD onset ages in male carriers (52 years old) and those with FXTAS (49.5 years old) were significantly higher relative to the general population (median 32, both p < 0.0001). Tremor and ataxia emerged significantly later than MDD and the anxiety disorders studied.

CONCLUSION

Depressive and anxiety disorders in premutation carriers have a later onset compared to the general population, but precede the onset of motor symptoms. This may be due to progressive mRNA toxicity in the limbic system, white matter changes leading to neuronal dysconnectivity, and interaction with environmental factors. Psychosocial factors may be protective. Further research is needed to understand the full spectrum of psychiatric phenotypes in FMR1 premutation carriers.

COGNITIVE DYSFUNCTION IN FMR1 PREMUTATION CARRIERS

Premutation carriers of the fragile X mental retardation gene (especially men) older than 50 may develop a neurodegenerative disease, the fragile X-associated tremor/ataxia syndrome (FXTAS). Carriers may present with varied cognitive impairments. Attention, working memory, declarative and procedural learning, information processing speed, and recall are among the cognitive domains affected. Executive dysfunction is a prominent deficit, which has been demonstrated mostly in men with FXTAS. In more advanced stages of FXTAS, both men and women may develop a mixed cortical-subcortical dementia, manifested by psychomotor slowing and deficits in attention, retrieval, recall, visuospatial skills, occasional apraxia, as well as overt personality changes. Studies have shown dementia rates as high as 37-42% in older men with FXTAS, although more research is needed to understand the prevalence and risk factors of dementia in women with FXTAS. Neuropsychiatric symptoms are common and reflect the dysfunction of underlying frontal-subcortical neural circuits, along with components of the cerebellar cognitive affective syndrome. These include labile or depressed mood, anxiety, disinhibition, impulsivity, and (rarely) psychotic symptoms. In this paper we review the information available to date regarding the prevalence and clinical picture of FXTAS dementia. Differential diagnosis may be difficult, given overlapping motor and non-motor signs with several other neurodegenerative diseases. Anecdotal response to cholinesterase inhibitors and memantine has been reported, while symptomatic treatments can address the neuropsychiatric manifestations of FXTAS dementia.

Male carriers of the FMR1 premutation show altered hippocampal-prefrontal function during memory encoding

Previous functional MRI (fMRI) studies have shown that fragile X mental retardation 1 (FMR1) fragile X premutation allele carriers (FXPCs) exhibit decreased hippocampal activation during a recall task and lower inferior frontal activation during a working memory task compared to matched controls. The molecular characteristics of FXPCs includes 55–200 CGG trinucleotide expansions, increased FMR1 mRNA levels, and decreased FMRP levels especially at higher repeat sizes. In the current study, we utilized MRI to examine differences in hippocampal volume and function during an encoding task in young male FXPCs. While no decreases in either hippocampal volume or hippocampal activity were observed during the encoding task in FXPCs, FMRP level (measured in blood) correlated with decreases in parahippocampal activation. In addition, activity in the right dorsolateral prefrontal cortex during correctly encoded trials correlated negatively with mRNA levels. These results, as well as the established biological effects associated with elevated mRNA levels and decreased FMRP levels on dendritic maturation and axonal growth, prompted us to explore functional connectivity between the hippocampus, prefrontal cortex, and parahippocampal gyrus using a psychophysiological interaction analysis. In FXPCs, the right hippocampus evinced significantly lower connectivity with right ventrolateral prefrontal cortex (VLPFC) and right parahippocampal gyrus. Furthermore, the weaker connectivity between the right hippocampus and VLPFC was associated with reduced FMRP in the FXPC group. These results suggest that while FXPCs show relatively typical brain response during encoding, faulty connectivity between frontal and hippocampal regions may have subsequent effects on recall and working memory.

Abnormal dendrite and spine morphology in primary visual cortex in the CGG knock-in mouse model of the fragile X premutation

The fragile X mental retardation 1 gene (Fmr1) is polymorphic for CGG trinucleotide repeat number in the 5'-untranslated region, with repeat lengths <45 associated with typical development and repeat lengths >200 resulting in hypermethylation and transcriptional silencing of the gene and mental retardation in the fragile X Syndrome (FXS). Individuals with CGG repeat expansions between 55 and 200 are carriers of the fragile X premutation (PM). PM carriers show a phenotype that can include anxiety, depression, social phobia, and memory deficits. They are also at risk for developing fragile X-associated tremor/ataxia syndrome (FXTAS), a late onset neurodegenerative disorder characterized by tremor, ataxia, cognitive impairment, and neuropathologic features including intranuclear inclusions in neurons and astrocytes, loss of Purkinje cells, and white matter disease. However, very little is known about dendritic morphology in PM or in FXTAS. Therefore, we carried out a Golgi study of dendritic complexity and dendritic spine morphology in layer II/III pyramidal neurons in primary visual cortex in a knock-in (KI) mouse model of the PM. These CGG KI mice carry an expanded CGG trinucleotide repeat on Fmr1, and model many features of the PM and FXTAS. Compared to wild-type (WT) mice, CGG KI mice showed fewer dendritic branches proximal to the soma, reduced total dendritic length, and a higher frequency of longer dendritic spines. The distribution of morphologic spine types (e.g., stubby, mushroom, filopodial) did not differ between WT and KI mice. These findings demonstrate that synaptic circuitry is abnormal in visual cortex of mice used to model the PM, and suggest that such changes may underlie neurologic features found in individuals carrying the PM as well as in individuals with FXTAS.

Comprehensive neurocognitive endophenotyping strategies for mouse models of genetic disorders

There is a need for refinement of the current behavioral phenotyping methods for mouse models of genetic disorders. The current approach is to perform a behavioral screen using standardized tasks to define a broad phenotype of the model. This phenotype is then compared to what is known concerning the disorder being modeled. The weakness inherent in this approach is twofold: First, the tasks that make up these standard behavioral screens do not model specific behaviors associated with a given genetic mutation but rather phenotypes affected in various genetic disorders; secondly, these behavioral tasks are insufficiently sensitive to identify subtle phenotypes. An alternate phenotyping strategy is to determine the core behavioral phenotypes of the genetic disorder being studied and develop behavioral tasks to evaluate specific hypotheses concerning the behavioral consequences of the genetic mutation. This approach emphasizes direct comparisons between the mouse and human that facilitate the development of neurobehavioral biomarkers or quantitative outcome measures for studies of genetic disorders across species.

Fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an under-recognized disorder that is a significant cause of late-adult-onset ataxia. The etiology is expansion of a trinucleotide repeat to the premutation range (55-200 CGG repeats) in the fragile X mental retardation 1 (FMR1) gene. Expansion to >200 CGGs causes fragile X syndrome, the most common heritable cause of cognitive impairment and autism. Core features of FXTAS include progressive action tremor and gait ataxia; with frequent, more variable features of cognitive decline, especially executive dysfunction, parkinsonism, neuropathy, and autonomic dysfunction. MR imaging shows generalized atrophy and frequently abnormal signal in the middle cerebellar peduncles. Autopsy reveals intranuclear inclusions in neurons and astrocytes and dystrophic white matter. FXTAS is likely due to an RNA toxic gain-of-function of the expanded-repeat mRNA. The disorder typically affects male premutation carriers over age 50, and, less often, females. Females also are at increased risk for primary ovarian insufficiency, chronic muscle pain, and thyroid disease. Treatment targets specific symptoms, but progression of disability is relentless. Although the contribution of FXTAS to the morbidity and mortality of the aging population requires further study, the disorder is likely the most common single-gene form of tremor and ataxia in the older adult population.

Neuropsychological findings from older premutation carrier males and their noncarrier siblings from families with fragile X syndrome

OBJECTIVE

Carriers of the FMR1 premutation allele are at a significantly increased risk for a late-onset neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). The primary features of FXTAS are late-onset intention tremor and gait ataxia. Previous reports have shown global deficits in neuropsychological measures among males with FXTAS, particularly those related to executive functioning. The purpose of this study was to investigate the neuropsychological profile among older males with the premutation who are at risk for FXTAS.

METHOD

Premutation carriers, 66 with motor symptoms and 23 without, and 18 noncarrier siblings were recruited from pedigrees diagnosed with fragile X syndrome, all over age 50. Subjects were examined with a neurological test battery to identify symptoms of FXTAS and a neuropsychological test battery to investigate cognitive and behavioral profiles. Linear regression and ANCOVA were used to determine the effect of the premutation on outcome measures adjusting for age and education.

RESULTS

We identified a significant decrease in scores of general intelligence and a marginally significant decrease in scores of logical memory among premutation carrier males with motor symptoms compared to the noncarrier male siblings. We did not identify deficits in executive functioning in our sample of premutation carrier males with motor symptoms.

CONCLUSIONS

Similar to other reports, we found that the FMR1 premutation is associated with deficits in general intelligence and memory among older males with symptoms of FXTAS. However, our results differed in that we found no evidence of premutation-associated executive dysfunction. We provide possible explanations for this difference.

Widespread non-central nervous system organ pathology in fragile X premutation carriers with fragile X-associated tremor/ataxia syndrome and CGG knock-in mice

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an adult-onset neurodegenerative disorder generally presenting with intention tremor and gait ataxia, but with a growing list of co-morbid medical conditions including hypothyroidism, hypertension, peripheral neuropathy, and cognitive decline. The pathological hallmark of FXTAS is the presence of intranuclear inclusions in both neurons and astroglia. However, it is unknown to what extent such inclusions are present outside the central nervous system (CNS). To address this issue, we surveyed non-CNS organs in ten human cases with FXTAS and in a CGG repeat knock-in (CGG KI) mouse model known to possess neuronal and astroglial inclusions. We find inclusions in multiple tissues from FXTAS cases and CGG KI mice, including pancreas, thyroid, adrenal gland, gastrointestinal, pituitary gland, pineal gland, heart, and mitral valve, as well as throughout the associated autonomic ganglia. Inclusions were observed in the testes, epididymis, and kidney of FXTAS cases, but were not observed in mice. These observations demonstrate extensive involvement of the peripheral nervous system and systemic organs. The finding of intranuclear inclusions in non-CNS somatic organ systems, throughout the PNS, and in the enteric nervous system of both FXTAS cases as well as CGG KI mice suggests that these tissues may serve as potential sites to evaluate early intervention strategies or be used as diagnostic factors.

Mouse models of fragile X-associated tremor ataxia

OBJECTIVE

To describe the development of mouse models of fragile X-associated tremor/ataxia (FXTAS) and the behavioral, histological and molecular characteristics of these mice.

METHOD

This paper compares the pathophysiology and neuropsychological features of FXTAS in humans to the major mouse models of FXTAS. Specifically, the development of a transgenic mouse line carrying an expanded CGG trinucleotide repeat in the 5'-untranslated region (5'-UTR) of the Fmr1 gene is described along with a description of the characteristic intranuclear ubiquitin-positive inclusions and the behavioral sequella observed in these mice.

RESULTS

CGG KI mice model many of the important features of FXTAS, although some aspects are not well modeled in mice. Aspects of FXTAS that are modeled well include elevated levels of Fmr1 mRNA, reduced levels of Fmrp, the presence of intranuclear inclusions that develop with age and show similar distributions within neurons, and neuropsychological and cognitive deficits, including poor motor function, impaired memory and evidence of increased anxiety. Features of FXTAS that are not well modeled in these mice include intentional tremors that are observed in some FXTAS patients but have not been reported in CGG KI mice. In addition, although intranuclear inclusions in astrocytes are very prominent in FXTAS, there are relatively few observed in CGG KI mice. A number of additional features of FXTAS have not been systematically examined in mouse models yet, including white matter disease, hyperintensities in T2-weighted magnetic resonance imaging, and brain atrophy, although these are currently under investigation in our laboratories.

CONCLUSIONS

The available mouse model has provided valuable insights into the molecular biology and pathophysiology of FXTAS and will be particularly useful for developing and testing new therapeutic treatments in the future.

Fragile X-associated tremor/ataxia syndrome: clinical phenotype, diagnosis, and treatment

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder caused by a CGG repeat expansion in the premutation range (55-200) in the fragile X mental retardation 1 gene. Onset is typically in the early seventh decade, and men are principally affected. The major signs are cerebellar gait ataxia, intention tremor, frontal executive dysfunction, and global brain atrophy. Other frequent findings are parkinsonism (mild), peripheral neuropathy, psychiatric symptoms (depression, anxiety, and agitation), and autonomic dysfunction. The clinical presentation is heterogeneous, with individuals presenting with varied dominating signs, such as tremor, dementia, or neuropathy. Magnetic resonance imaging shows atrophy and patchy white matter lesions in the cerebral hemispheres and middle cerebellar peduncles. The latter has been designated the middle cerebellar peduncle sign, which occurs in about 60% of affected men, and is relatively specific for FXTAS. Affected females generally have less severe disease, less cognitive decline, and some symptoms different from that of men, for example, muscle pain. Management of FXTAS is complex and includes assessment of the patient's neurological and medical deficits, treatment of symptoms, and provision of relevant referrals, especially genetic counseling. Treatment is empirical, based on anecdotal experience and on knowledge of what works for symptoms of other disorders that also exist in FXTAS. Presently, the disorder is underrecognized because the first published report was only in 2001 and because the presentation is variable and mainly consists of a combination of signs common in the elderly. However, accurate diagnosis is critical for the patient and for the family because they need education regarding their genetic and health risks.

Rare intranuclear inclusions in the brains of 3 older adult males with fragile x syndrome: implications for the spectrum of fragile x-associated disorders

The FMR1 gene is polymorphic for the length of CGG trinucleotide repeat expansions in the 5' untranslated region. Premutation (55-200 CGG repeats) and full-mutation (>200 CGG repeats) alleles give rise to their respective disorders by different pathogenic mechanisms: RNA gain-of-function toxicity leads to fragile X-associated tremor/ataxia syndrome in the premutation range, and transcriptional silencing and absence of fragile X mental retardation protein (FMRP) lead to fragile X syndrome in the full-mutation range. However, for the latter, incomplete silencing and/or size-mosaicism might result in some contribution to the disease process from residual messenger RNA production. To address this possibility, we examined the brains of 3 cases of fragile X syndrome for the presence of intranuclear inclusions in the hippocampal dentate gyrus. We identified low levels (0.1%-1.3%) of intranuclear inclusions in all 3 cases. Quantitative reverse transcription-polymerase chain reaction for FMR1 messenger RNA and immunofluorescence for FMRP revealed low but detectable levels of both RNA and protein in the 3 cases, consistent with the presence of small numbers of inclusions. The intranuclear inclusions were only present in FMRP-immunoreactive cells. The small numbers of inclusions and very low levels of both FMR1 RNA and protein suggest that the clinical course in these 3 subjects would not have been influenced by contributions from RNA toxicity.

A patient with fragile x-associated tremor/ataxia syndrome presenting with executive cognitive deficits and cerebral white matter lesions

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that primarily affects males who are carriers of a premutation of a CGG expansion in the FMR1 gene. In Asian populations, FXTAS has rarely been reported. Here, we report the case of a Japanese FXTAS patient who showed predominant executive cognitive deficits as the main feature of his disease. In contrast, the patient exhibited only very mild symptoms of intention tremor and ataxia, which did not interfere with daily activities. A gene analysis revealed that the patient carried a premutation of a CGG expansion (111 CGG repeats) in the FMR1 gene. The mRNA expression level of FMR1 in the patient was 1.5-fold higher than in controls. On brain MRI scans, fluid-attenuated inversion recovery images showed high-intensity lesions in the middle cerebellar peduncles and the cerebral white matter, with a frontal predominance. The present case extends previous notions regarding the cognitive impairment in FXTAS patients. Recognizing FXTAS patients with predominant cognitive impairment from various ethnic backgrounds would contribute to our understanding of the phenotypic variation of this disease.

Altered zinc transport disrupts mitochondrial protein processing/import in fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder that affects individuals who are carriers of small CGG premutation expansions in the fragile X mental retardation 1 (FMR1) gene. Mitochondrial dysfunction was observed as an incipient pathological process occurring in individuals who do not display overt features of FXTAS (1). Fibroblasts from premutation carriers had lower oxidative phosphorylation capacity (35% of controls) and Complex IV activity (45%), and higher precursor-to-mature ratios (P:M) of nDNA-encoded mitochondrial proteins (3.1-fold). However, fibroblasts from carriers with FXTAS symptoms presented higher FMR1 mRNA expression (3-fold) and lower Complex V (38%) and aconitase activities (43%). Higher P:M of ATPase β-subunit (ATPB) and frataxin were also observed in cortex from patients that died with FXTAS symptoms. Biochemical findings observed in FXTAS cells (lower mature frataxin, lower Complex IV and aconitase activities) along with common phenotypic traits shared by Friedreich's ataxia and FXTAS carriers (e.g. gait ataxia, loss of coordination) are consistent with a defective iron homeostasis in both diseases. Higher P:M, and lower ZnT6 and mature frataxin protein expression suggested defective zinc and iron metabolism arising from altered ZnT protein expression, which in turn impairs the activity of mitochondrial Zn-dependent proteases, critical for the import and processing of cytosolic precursors, such as frataxin. In support of this hypothesis, Zn-treated fibroblasts showed a significant recovery of ATPB P:M, ATPase activity and doubling time, whereas Zn and desferrioxamine extended these recoveries and rescued Complex IV activity.

A voxel-based morphometry study of grey matter loss in fragile X-associated tremor/ataxia syndrome

Fragile X-associated tremor/ataxia syndrome is a neurodegenerative disorder that primarily affects older male premutation carriers of the fragile X mental retardation gene. Although its core symptoms are mainly characterized by motor problems such as intention tremor and gait ataxia, cognitive decline and psychiatric problems are also commonly observed. Past radiological and histological approaches have focused on prominent neurodegenerative changes in specific brain structures including the cerebellum and limbic areas. However, quantitative investigations of the regional structural abnormalities have not been performed over the whole brain. In this study, we adopted the voxel-based morphometry method together with regions of interest analysis for the cerebellum to examine the pattern of regional grey matter change in the male premutation carriers with and without fragile X-associated tremor/ataxia syndrome. In a comparison with healthy controls, we found striking grey matter loss of the patients with fragile X-associated tremor/ataxia syndrome in multiple regions over the cortical and subcortical structures. In the cerebellum, the anterior lobe and the superior posterior lobe were profoundly reduced in both vermis and hemispheres. In the cerebral cortex, clusters of highly significant grey matter reduction were found in the extended areas in the medial surface of the brain, including the dorsomedial prefrontal cortex, anterior cingulate cortex and precuneus. The other prominent grey matter loss was found in the lateral prefrontal cortex, orbitofrontal cortex, amygdala and insula. Although the voxel-wise comparison between the asymptomatic premutation group and healthy controls did not reach significant difference, a regions of interest analysis revealed significant grey matter reduction in anterior subregions of the cerebellar vermis and hemisphere in the asymptomatic premutation group. Correlation analyses using behavioural scales of the premutation groups showed significant associations between grey matter loss in the left amygdala and increased levels of obsessive-compulsiveness and depression, and between decreased grey matter in the left inferior frontal cortex and anterior cingulate cortex and poor working memory performance. Furthermore, regression analyses revealed a significant negative effect of CGG repeat size on grey matter density in the dorsomedial frontal regions. A significant negative correlation with the clinical scale for the severity of fragile X-associated tremor/ataxia syndrome was found in a part of the vermis. These observations reveal the anatomical patterns of the neurodegenerative process that underlie the motor, cognitive and psychiatric problems of fragile X-associated tremor/ataxia syndrome, together with incipient structural abnormalities that may occur before the clinical onset of this disease.