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.     

A low symptomatic form of neurodegeneration in younger carriers of the FMR1 premutation, manifesting typical radiological changes

Fragile X-associated tremor/ataxia (FXTAS) is a late onset disorder caused by a premutation in the FMR1 gene, in which neurological symptoms are associated with white matter (wm) changes, especially within the middle cerebellar peduncles (MCP sign), seen on magnetic resonance images (MRIs). We report a discrepancy between obvious radiological presentations and minimal clinical involvement in two younger male premutation carriers. These carriers, aged 52 and 39 years, showed distinct MCP sign, but reported no neurological symptoms. If this discrepancy represents the initial stage of FXTAS, our findings suggest the possibility of early diagnosis from MRI scans.     

The fragile X‐associated tremor ataxia syndrome (FXTAS) in Indonesia

Fragile X‐associated disorders caused by the premutation of the FMR1 gene, includes the fragile X‐associated tremor/ataxia syndrome (FXTAS). FXTAS affects more than 40% of premutation males over the age of 50 and 75% over the age of 80. FMR1 molecular analysis was done using PCR and confirmed by Southern Blot. Three premutation males were diagnosed FXTAS using quantification based on the standard neurological examination. Cognitive impairment was assessed using Raven and WAIS‐R test. MRI was done to identify the middle cerebellar peduncle (MCP) sign, white matter disease and/or cerebral atrophy. Three cases of FXTAS are identified, of five individuals older than 50 years in one family tree two met criteria for definite FXTAS and the third with sub‐clinical symptoms, although cognitive and radiological criteria are met. These cases are the first identified FXTAS cases in rural Indonesia. In addition with lack of routine medical follow‐up, complications of FXTAS, such as hypertension may go unrecognized and untreated, which may further exacerbate the central nervous system (CNS) findings of FXTAS.     

Penetrance of FMR1 premutation associated pathologies in fragile X syndrome families

Within the past few years, there has been a significant change in identifying and characterizing the FMR1 premutation associated phenotypes. The premutation has been associated with elevated FMR1 mRNA levels and slight to moderate reductions in FMRP levels. Furthermore, it has been established that ∼20% of female premutation carriers present primary ovarian insufficiency (POI) and that fragile X-associated tremor/ataxia syndrome (FXTAS) occurs in one-third of all male premutation carriers older than 50 years. Besides POI and FXTAS, new disorders have recently been described among individuals (especially females) with the FMR1 premutation. Those pathologies include thyroid disease, hypertension, seizures, peripheral neuropathy, and fibromyalgia. However there are few reports related to FXTAS penetrance among female premutation carriers or regarding these disorders recently associated to the FMR1 premutation. Therefore, we have evaluated 398 fragile X syndrome (FXS) families in an attempt to provide an estimation of the premutation associated phenotypes penetrance. Our results show that signs of FXTAS are detected in 16.5% of female premutation carriers and in 45.5% of premutated males older than 50 years. Furthermore, among females with the FMR1 premutation, penetrance of POI, thyroid disease and chronic muscle pain is 18.6, 15.9 and 24.4%, respectively. The knowledge of this data might be useful for accurate genetic counselling as well as for a better characterization of the clinical phenotypes of FMR1 premutation carriers.     

Size bias of fragile X premutation alleles in late-onset movement disorders

Background

Fragile X‐associated tremor/ataxia syndrome (FXTAS), caused by premutation expansions (55–200 CGG repeats) of the FMR1 gene, shares clinical features with other movement disorders, particularly in the domains of gait ataxia, intention tremor and parkinsonism. However, the prevalence of FXTAS within other diagnostic categories is not well defined.

Methods

A meta‐analysis was conducted of all published (n = 14) genetic screens for expanded FMR1 alleles to assess the prevalence and CGG‐repeat size bias of FMR1 premutation alleles in those populations.

Results

In men with late‐onset cerebellar ataxia, the prevalence of premutation alleles (1.5%; 16/1049) was 13 times greater than expected based on its prevalence in the general population (2%; 16/818 for age of onset >50 years; odds ratio 12.4; 95% confidence interval 1.6 to 93.5). Meta‐analysis of CGG‐repeat data for screened patients with premutation alleles shows a shift to larger repeat size than in the general population (p<0.001). 86% (19/22) of premutation alleles were larger than 70 repeats in the patients screened, whereas only approximately 22% of premutation alleles are larger than 70 repeats in the general population.

Conclusions

Expanded FMR1 alleles contribute to cases of late‐onset sporadic cerebellar ataxia, suggesting that FMR1 genetic testing should be carried out in such cases. The biased distribution of FMR1 allele sizes has substantial implications for genetic counselling of carriers with smaller alleles who are at a low risk of developing FXTAS, and suggests that the estimated prevalence of FXTAS among men >50 years of age in the general population may be two to threefold lower than the initial figure of 1 in 3000.     

Premutation CGG-repeat expansion of the Fmr1 gene impairs mouse neocortical development

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late adult-onset neurodegenerative disorder caused by a premutation CGG-trinucleotide repeat expansion (55-200 CGG repeats) within the 5'-untranslated region of the FMR1 gene. Although FXTAS generally affects premutation carriers over 50 years of age, cognitive and psychological symptoms can appear in carriers during childhood, suggesting that the FMR1 premutation affects brain function early in life. Recent work with cultured hippocampal neurons from a premutation (Fmr1 CGG knock-in) mouse model revealed impaired development of early postnatal neurons, consistent with the developmental clinical involvement of premutation carriers. In the current work, we show that the presence of premutation CGG-repeat expansions in the mouse Fmr1 gene alters embryonic neocortical development. Specifically, embryonic premutation mice display migration defects in the neocortex and altered expression of neuronal lineage markers. The current data demonstrate that premutation alleles of the Fmr1 gene are associated with defects in developmental programs operating during prenatal stages of brain formation and provide further evidence that the FMR1 premutation has a neurodevelopmental component.     

Sleep apnea in fragile X premutation carriers with and without FXTAS

This report seeks to establish the prevalence of sleep apnea in patients with the fragile X mental retardation 1 (FMR1) premutation with and without fragile X-associated tremor/ataxia syndrome (FXTAS) and to determine any correlation between CGG repeat and FMR1 mRNA levels with sleep apnea prevalence. Demographic and medical data from 430 (229 males, 201 females) participants were used in this analysis. Participants included premutation carriers with (n = 118) and without FXTAS (n = 174) as well as controls without the premutation (n = 123). Logistic regression models were employed to estimate the odds ratio of sleep apnea relative to controls, adjusted for age and gender, and also to examine potential association with CGG size and FMR1 mRNA expression level. The observed proportion of sleep apnea in premutation carriers with and without FXTAS and controls are 31.4% (37/118), 8.6% (15/174), and 13.8% (17/123), respectively. The adjusted odds of sleep apnea for premutation carriers with FXTAS is about 3.4 times that compared to controls (odds ratio, OR = 3.4, 95% confidence interval (CI) 1.8-7.4; P = 0.001), and similarly relative to premutation carriers without FXTAS (OR = 2.9, 95% CI 1.2-6.9; P = 0.014). The risk of sleep apnea was not different between controls and premutation carriers without FXTAS. The presence of sleep apnea is not associated with CGG repeat numbers nor FMR1 mRNA expression level among premutation carriers. Our data supports a higher prevalence and risk of sleep apnea in patients with FXTAS. We recommend that all patients diagnosed with FXTAS be screened for sleep apnea given the negative and perhaps accelerative impact sleep apnea may have on their FXTAS progression.     

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.     

Fragile X‐associated tremor/ataxia syndrome (FXTAS) in grey zone carriers

The grey zone (GZ; 45–54 CGG repeats in the FMR1 gene) is considered a normal allele; however, several studies have found a high frequency of GZ in movement disordered populations. Here, we describe neurological features of fragile X‐associated tremor/ataxia syndrome (FXTAS) in two carriers of GZ alleles, although FXTAS has been defined as occurring only in premutation carriers (55–200 CGG repeats). Both patients had family members who had premutation and were diagnosed with FXTAS. The presence of relatively high GZ alleles with elevated fragile X mental retardation 1 mRNA (FMR1‐mRNA) combined with a family history of FXTAS that may represent a facilitating genetic background for FXTAS are the factors that led to the presence of FXTAS in these individuals with a GZ allele. Further research into clinical involvement of GZ alleles is recommended and the definition of FXTAS may require revision.     

Social anxiety and autism spectrum traits among adult FMR1 premutation carriers

Behavioral symptoms and traits have been proposed as early markers in neurodegenerative diseases. The aim of this study was to evaluate social anxiety and autism in FMR1 premutation carriers using the Social Phobia Inventory (SPIN) and the Autism‐Spectrum Quotient (AQ) questionnaires. Fifty‐nine premutation carriers were compared with 50 controls. The SPIN test showed statistically significant differences between female but not male carriers. The AQ questionnaire found statistically significant differences between premutation carriers and controls in the total AQ as well as in the social skills and attention switching subdomains. A gender effect was only observed for the social skills subdomain. Spearman's correlation analysis revealed a moderately positive correlation with the total AQ scores as well as the social skills and communication subdomains. Our results show that fragile X‐associated tremor/ataxia syndrome (FXTAS) patients have higher AQ scores. Moreover, this is the first study to find statistically significant differences between FXTAS and no‐FXTAS premutation carriers in the communication and the imagination subdomains, suggesting that FXTAS patients present a broader autistic phenotype than premutation carriers without FXTAS. Based on our results, a wide range of behavioral/psychiatric traits should be included within the broader phenotypic presentation of individuals with the FMR1 premutation.     

Reduced telomere length in older men with premutation alleles of the fragile X mental retardation 1 gene

Reduced telomere length has recently been reported in T lymphocytes of individuals with trisomy 21 Down syndrome (DS) and dementia. Shorter telomeres also have been documented in dyskeratosis congenita, cell senescence, Alzheimer disease, and neoplastic transformation. These observations suggest that similar shortening may occur in people with fragile X-associated tremor/ataxia syndrome (FXTAS), which frequently is accompanied by dementia. To test this hypothesis, telomere length has been quantified in T lymphocytes from older male carriers of premutation FMR1 alleles, with or without FXTAS, and FXTAS with dementia. Shorter telomeres (relative to age-matched controls) were observed in 5/5 individuals with FXTAS and dementia, in 2/2 individuals with FXTAS without dementia, and in 3/3 individuals with the fragile X premutation only (P values ranged from <0.001 to <0.05; Student's t-test), indicating that telomere shortening is associated with the premutation expansion of the FMR1 gene. The current study design allowed simultaneous comparisons among control, premutation, FXTAS, and FXTAS with dementia samples, and showed nearly equal degrees of shortening relative to controls among the three premutation sample groups. Thus, telomere shortening may serve as a biomarker for cellular dysregulation that may precede the development of the symptoms of FXTAS.     

Determinants of cerebellar and cerebral volume in the general elderly population

In a population-based study of 3962 community-dwelling nondemented elderly we investigated the relation of age, sex, cardiovascular risk factors, and the presence of infarcts with cerebellar volume, and its interrelationship with cerebral volumes. Cerebellar and cerebral gray and white matter were segmented using Freesurfer version 4.5 (http://surfer.nmr.mgh.harvard.edu/). We used linear regression analyses to model the relationship between age, sex, cardiovascular risk factors, brain infarcts, white matter lesions (WMLs) and cerebellar and cerebral volume. Smaller cerebellar volumes with increasing age were mainly driven by loss of white matter. Diabetes, higher serum glucose and lower cholesterol levels were related to smaller cerebellar volume. No association was found between hypertension, smoking, apolipoprotein E (ApoE) genotype, and cerebellar volume. Supratentorial lacunar infarcts and WMLs were related to smaller cerebellar volume. Infratentorial infarcts were related to smaller cerebellar white matter volume and total cerebral volume. This study suggests that determinants of cerebellar volume do not entirely overlap with those established for cerebral volume. Furthermore, presence of infarcts or WMLs in the cerebrum can affect cerebellar volume.     

Prevalence of CGG expansions of the FMR1 gene in a US population-based sample

The primary goal of this study was to calculate the prevalence of the premutation of the FMR1 gene and of the "gray zone" using a population-based sample of older adults in Wisconsin (n = 6,747 samples screened). Compared with past research, prevalence was relatively high (1 in 151 females and 1 in 468 males for the premutation and 1 in 35 females and 1 in 42 males for the gray zone as defined by 45-54 CGG repeats). A secondary study goal was to describe characteristics of individuals found to have the premutation (n = 30, 7 males and 23 females). We found that premutation carriers had a significantly higher rate of divorce than controls, as well as higher rates of symptoms that might be indicative of fragile X-associated tremor ataxia syndrome (FXTAS; numbness, dizziness/faintness) and fragile X primary ovarian insufficiency (FXPOI; age at last menstrual period). Although not statistically significant, premutation carriers were twice as likely to have a child with disability.     

CGG repeat length correlates with age of onset of motor signs of the fragile X-associated tremor/ataxia syndrome (FXTAS).

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurological disorder among carriers of premutation CGG-repeat expansions within the FMR1 gene. Principal features of FXTAS include progressive action tremor and gait ataxia, with associated features of parkinsonism, peripheral neuropathy, dysautonomia, and cognitive decline. Although both clinical and neuropathologic features of FXTAS are known to be highly associated with CGG repeat length, the relationship between repeat length and age-of-onset is not known. To address this issue, the ages of onset of action tremor and gait ataxia were documented by history for 93 male carriers. For this cohort, the mean ages of onset were 62.6 +/- 8.1 years (range, 39-78 years) for tremor, and 63.6 +/- 7.3 years (range, 47-78 years) for ataxia; the mean CGG repeat number was 88.5 +/- 14 (range, 60-133). Analysis of the relationship between clinical onset and molecular measures revealed significant correlations between CGG repeat number and onset of both tremor (P = 0.001) and ataxia (P = 0.002), as well as overall onset (P < 0.0001). Our findings indicate that the CGG repeat number is a potential predictor of the age of onset of core motor features of FXTAS.     

White matter changes in basis pontis in small expansion FMR1 allele carriers with parkinsonism

Examples of white matter hyperintensities (wmh) on magnetic resonance images in a basis pontis are presented in two male carriers, each of whom carry a small CGG expansion fragile X mental retardation (FMR1) allele. One carried a premutation (PM) allele of 85 CGG repeats and the other, a gray zone (GZ) allele of 47 repeats. Both were originally diagnosed with idiopathic Parkinson's disease (iPD). Similar changes are also shown in one PM carrier of 99 repeats affected with mild tremor and imbalance, who was ascertained through a fragile X syndrome family. These examples draw attention to the occurrence of wmh in a basis pontis in the carriers of small CGG expansions presenting with tremor and ataxia. Moreover, the presence of this change in GZ, as well as PM, allele carriers originally diagnosed with iPD supports our earlier suggestion that both these alleles may contribute to the neurodegenerative changes in this disorder which, in the examples presented, have been reflected by wmh, most prominent in the cerebellar peduncles and/or pontine area. © 2011 Wiley‐Liss, Inc.     

Effects of age on tissues and regions of the cerebrum and cerebellum.

Normal volunteers, aged 30 to 99 years, were studied with MRI. Age was related to estimated volumes of: gray matter, white matter, and CSF of the cerebrum and cerebellum; gray matter, white matter, white matter abnormality, and CSF within each cerebral lobe; and gray matter of eight subcortical structures. The results were: 1) Age-related losses in the hippocampus were significantly accelerated relative to gray matter losses elsewhere in the brain. 2) Among the cerebral lobes, the frontal lobes were disproportionately affected by cortical volume loss and increased white matter abnormality. 3) Loss of cerebral and cerebellar white matter occurred later than, but was ultimately greater than, loss of gray matter. It is estimated that between the ages of 30 and 90 volume loss averages 14% in the cerebral cortex, 35% in the hippocampus, and 26% in the cerebral white matter. Separate analyses were conducted in which genetic risk associated with the Apolipoprotein E epsilon4 allele was either overrepresented or underrepresented among elderly participants. Accelerated loss of hippocampal volume was observed with both analyses and thus does not appear to be due to the presence of at-risk subjects. MR signal alterations in the tissues of older individuals pose challenges to the validity of current methods of tissue segmentation, and should be considered in the interpretation of the results.     

Screening for FMR-1 premutations in 122 older Flemish males presenting with ataxia

Recently, Hagerman et al described the occurrence of a late-onset neurological disorder in five male carriers of the fragile-X (FMR-1) premutation. The major characteristics of this disorder, designated the Fragile-X Tremor Ataxia Syndrome (FXTAS), are progressive intention tremor, cerebellar ataxia and cognitive decline. Most cases of FXTAS published thus far were ascertained through families with a known fragile-X proband. Since cerebellar ataxia is one of the main cardinal features, we performed FMR-1 premutation screening in 122 male patients, older than 50 years, who were referred to us for testing of the spinocerebellar ataxia (SCA 1, 2, 3, 6, 7) genes and who were found to be negative. In this group of patients, we found five patients with an FMR-1 premutation. In four of them, a definite diagnosis of FXTAS could be made, based on the proposed diagnostic clinical and radiological criteria for FXTAS. In light of these figures, we recommend that FMR-1 analysis should be included in the molecular diagnostic work-up in the group of male ataxia patients older than 50 years.     

Expanded clinical phenotype of women with the FMR1 premutation

Fragile X-associated tremor/ataxia syndrome (FXTAS) is generally considered to be uncommon in older female carriers of premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene; however, neither prevalence, nor the nature of the clinical phenotype, has been well characterized in female carriers. In this study, we evaluated 146 female carriers (mean, 42.3 years; range, 20-75 years) with and without core features of FXTAS (tremor; gait ataxia), and 69 age-matched controls (mean, 45.8 years; range, 21-78 years). Compared with controls, carriers with definite or probable FXTAS had greater medical co-morbidity, with increased prevalence of thyroid disease (P = 0.0096), hypertension (P = 0.0020), seizures (P = 0.0077), peripheral neuropathy (P = 0.0040), and fibromyalgia (P = 0.0097), in addition to the typical symptoms of FXTAS-tremor (P < 0.0001) and ataxia (P < 0.0001). The non-FXTAS premutation group had more complaints of chronic muscle pain (P = 0.0097), persistent paraesthesias in extremities (P < 0.0001), and history of tremor (P < 0.0123) than controls. The spectrum of clinical involvement in female carriers with FXTAS is quite broad, encompassing a number of medical co-morbidities as well as the core movement disorder. The remarkable degree of thyroid dysfunction (17% in the non-FXTAS group and 50% in the FXTAS group) warrants consideration of thyroid function studies in all female premutation carriers, particularly those with core features of FXTAS.     

FXTAS in an unmethylated mosaic male with fragile X syndrome from Chile

Carriers of an FMR1 premutation allele (55–200 CGG repeats) often develop the neurodegenerative disorders, fragile X‐associated tremor/ataxia syndrome (FXTAS). Neurological signs of FXTAS, parkinsonism and rapid onset of cognitive decline have not been reported in individuals with an unmethylated full mutation (FM). Here, we report a Chilean family affected with FXS, inherited from a parent carrier of an FMR1 unmethylated full mosaic allele, who presented with a fast progressing FXTAS. This case suggests that the definition of FXTAS may need to be broadened to not only include those with a premutation but also those with an expanded allele in FM range with a lack of methylation leading to elevated FMR1‐mRNA expression levels and subsequent RNA toxicity.