Claudine Kraan

Neurobehavioural evidence for the involvement of the FMR1 gene in female carriers of fragile X syndrome

For years, premutation-carriers of fragile X-syndrome (FXS) were assumed free from any deleterious phenotype. In this review, we discuss the current literature on neurocognitive, emotional and neuromotor profiles emerging in females with the fragile-X premutation, and discuss phenotypic profiles in male premutation-carriers to gain insights into possible underlying mechanisms associated with FMR1 gene expression. We contend that this emerging phenotypic profile in females with the fragile-X premutation needs further investigation using experimentally-driven tasks sensitive to neural networks especially vulnerable to FMR1 gene expression. Further investigation of developmental aspects of the female carrier profile is needed to determine the extent to which emotional, cognitive and neurobehavioural challenges indicate at-risk profiles for later degenerative decline, or rather a stable developmental phenotype. These future research avenues will provide critical new information which will enable identification of women at greatest risk for subtle age-dependent neurobehavioural changes well before the onset of more serious clinical consequences alongside the identification of biomarkers which may be useful in establishing the efficacy of future therapeutic interventions.

The cognitive neuropsychological phenotype of carriers of the FMR1 premutation

The fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disorder affecting a subset of carriers of the FMR1 (fragile X mental retardation 1) premutation. Penetrance and expression appear to be significantly higher in males than females. Although the most obvious aspect of the phenotype is the movement disorder that gives FXTAS its name, the disorder is also accompanied by progressive cognitive impairment. In this review, we address the cognitive neuropsychological and neurophysiological phenotype for males and females with FXTAS, and for male and female unaffected carriers. Despite differences in penetrance and expression, the cognitive features of the disorder appear similar for both genders, with impairment of executive functioning, working memory, and information processing the most prominent. Deficits in these functional systems may be largely responsible for impairment on other measures, including tests of general intelligence and declarative learning. FXTAS is to a large extent a white matter disease, and the cognitive phenotypes observed are consistent with what some have described as white matter dementia, in contrast to the impaired cortical functioning more characteristic of Alzheimer’s disease and related disorders. Although some degree of impaired executive functioning appears to be ubiquitous among persons with FXTAS, the data suggest that only a subset of unaffected carriers of the premutation - both female and male - demonstrate such deficits, which typically are mild. The best-studied phenotype is that of males with FXTAS. The manifestations of cognitive impairment among asymptomatic male carriers, and among women with and without FXTAS, are less well understood, but have come under increased scrutiny.

Impaired response inhibition is associated with self-reported symptoms of depression, anxiety, and ADHD in female FMR1 premutation carriers

Fragile X Mental Retardation 1 (FMR1) premutation carriers (PM‐carriers) have a defective trinucleotide expansion on the FMR1 gene that is associated with continuum of neuropsychological and mental disorders. Currently, little is known about the distinct subcomponents of executive function potentially impaired in female PM‐carriers, and there have been no investigations into associations between executive function and incidences of mental disorders. A total of 35 female PM‐carriers confirmed by Asuragen triple primed PCR DNA testing and 35 age‐ and intelligence‐matched controls completed tests of executive function (i.e., response inhibition and working memory) and self‐reported on social anxiety, depression, and ADHD predominantly inattentive (ADHD‐PI) symptoms. Compared to controls, PM‐carriers were significantly elevated on self‐reported social anxiety and ADHD‐PI symptoms. Irrespective of mental symptoms, female PM‐carries performed significantly worse than controls on a response inhibition test, and further investigations revealed significant correlations between executive function performance and self‐reported symptoms of anxiety, depression and ADHD‐PI. Critically, among PM‐carriers with good executive function performance, no women exceeded threshold markers for probable caseness of mental disorder. However, rates of probable caseness were elevated in those with average performance (response inhibition: social anxiety: 41.7%; depression: 20%; ADHD: 44.4%; working memory: social anxiety: 27.3%; depression: 9.1%; ADHD: 18.2%) and highly elevated for those with poor executive function performance (response inhibition: social anxiety: 58.3%; depression: 80%; ADHD: 55.6%; working memory: social anxiety: 100%; depression: 50%; ADHD: 83.3%). These data suggest that subtle executive dysfunction may be a useful neuropsychological indicator for a range of mental disorders previously reported in female PM‐carriers.

Novel methylation markers of the dysexecutive-psychiatric phenotype in FMR1 premutation women

Objective: To examine the epigenetic basis of psychiatric symptoms and dysexecutive impairments in FMR1 premutation (PM: 55 to 199 CGG repeats) women. Methods: A total of 35 FMR1 PM women aged between 22 and 55 years and 35 age- and IQ-matched women controls (CGG <45) participated in this study. All participants completed a range of executive function tests and self-reported symptoms of psychiatric disorders. The molecular measures included DNA methylation of the FMR1 CpG island in blood, presented as FMR1 activation ratio (AR), and 9 CpG sites located at the FMR1 exon1/intron 1 boundary, CGG size, and FMR1 mRNA levels. Results: We show that FMR1 intron 1 methylation levels could be used to dichotomize PM women into greater and lower risk categories (p = 0.006 to 0.037; odds ratio = 14–24.8), with only FMR1 intron 1 methylation, and to a lesser extent AR, being significantly correlated with the likelihood of probable dysexecutive or psychiatric symptoms (p < 0.05). Furthermore, the significant relationships between methylation and social anxiety were found to be mediated by executive function performance, but only in PM women. FMR1 exon 1 methylation, CGG size, and FMR1 mRNA could not predict probable dysexecutive/psychiatric disorders in PM women. Conclusions: This is the first study supporting presence of specific epigenetic etiology associated with increased risk of developing comorbid dysexecutive and social anxiety symptoms in PM women. These findings could have implications for early intervention and risk estimate recommendations aimed at improving the outcomes for PM women and their families.

The cognitive abilities associated with verbal fluency task performance differ across fluency variants and age groups in healthy young and old adults

Despite their widespread use in research and clinical practice, the cognitive abilities purportedly assessed by different verbal fluency task variants remain unclear and may vary across different healthy and clinical populations. The overarching aim of this study was to identify which cognitive abilities contribute to phonemic, semantic, excluded letter, and alternating verbal fluency tasks and whether these contributions differ across younger and older healthy adults. Method: Ninety-six younger (18–36 years) and 83 older (65–87 years) healthy participants completed measures of estimated verbal intelligence, semantic retrieval, processing speed, working memory, and inhibitory control, in addition to phonemic, semantic, excluded letter, and alternating fluency tasks. Eight hierarchical multiple regressions were conducted across the four fluency variants and two age groups to identify which cognitive variables uniquely contributed to these fluency tasks. Results: In the younger group, verbal intelligence and processing speed contributed to phonemic fluency, semantic retrieval to semantic fluency, processing speed and working memory to excluded letter fluency, and semantic retrieval to alternating fluency. In contrast, in the older group, verbal intelligence contributed to phonemic fluency, no cognitive variables contributed to semantic fluency, inhibition to excluded letter fluency, and verbal intelligence to alternating fluency. Conclusions: Our findings highlight that both lower and higher order cognitive skills contribute to verbal fluency tasks; however, these contributions vary considerably across fluency variants and age groups. The heterogeneity of cognitive determinants of verbal fluency, across variants and age, may explain why older people performed less proficiently on semantic and excluded letter fluency tasks while no age effects were found for phonemic and alternating fluency. Interpretation of verbal fluency performances need to be tailored according to which verbal fluency variant and age group are used.

Cognitive-motor interference during postural control indicates at-risk cerebellar profiles in females with the FMR1 premutation

Recent investigations report a higher risk of motor symptoms in females with the FMR1 premutation (PM-carriers) than has hitherto been appreciated. Here we examined basic sensorimotor and postural control under different sensory and attentional dual-task demands. Physiological performance and postural sway measures from the Physiological Profile Assessment (Lord et al., 2003 [39]) were conducted in 28 female PM-carriers (mean age: 41.32±8.03) and 31 female controls with normal FMR1 alleles (mean age: 41.61±8.3). Multiple regression analyses were conducted to examine the moderating role of CGG-repeat length on the relation between age and postural sway under dual-task interference. In female PM-carriers, our results showed significantly poorer proprioceptive awareness, slower reaction time, and greater postural displacement when performing a concurrent verbal fluency task. Significantly, these findings showed age- and genetically-modulated changes in dual-task postural displacement in the medio-lateral direction in female PM-carriers. These findings highlight the sensitivity of postural control paradigms in identifying early cerebellar postural changes that may act as surrogate markers of future decline in female PM-carriers.

Exploring inhibitory deficits in female premutation carriers of fragile X syndrome: Through eye movements

There is evidence which demonstrates that a subset of males with a premutation CGG repeat expansion (between 55 and 200 repeats) of the fragile X mental retardation 1 gene exhibit subtle deficits of executive function that progressively deteriorate with increasing age and CGG repeat length. However, it remains unclear whether similar deficits, which may indicate the onset of more severe degeneration, are evident in female PM-carriers. In the present study we explore whether female PM-carriers exhibit deficits of executive function which parallel those of male PM-carriers. Fourteen female fragile X premutation carriers without fragile X-associated tremor/ataxia syndrome and fourteen age, sex, and IQ matched controls underwent ocular motor and neuropsychological tests of select executive processes, specifically of response inhibition and working memory. Group comparisons revealed poorer inhibitory control for female premutation carriers on ocular motor tasks, in addition to demonstrating some difficulties in behaviour self-regulation, when compared to controls. A negative correlation between CGG repeat length and antisaccade error rates for premutation carriers was also found. Our preliminary findings indicate that impaired inhibitory control may represent a phenotype characteristic which may be a sensitive risk biomarker within this female fragile X premutation population.

The Abilities Associated with Verbal Fluency Performance in a Young, Healthy Population Are Multifactorial and Differ Across Fluency Variants

Numerous variants of verbal fluency tasks exist within clinical and research domains that purport to measure “executive function.” However, to date, there has been a paucity of research examining what specific abilities are measured by these tasks. In this study, the relationships between a select group of cognitive constructs and phonemic, semantic, alternating, and excluded-letter verbal fluency tests were examined in 93 young healthy individuals (aged 18 to 35 years old). Forward-selection multiple regression analyses were performed for each fluency task. Phonemic fluency was associated with verbal intellectual function and processing speed; semantic fluency was associated with working memory and semantic word retrieval; excluded-letter fluency was associated with processing speed; and alternating fluency was associated with semantic word retrieval. These results highlight verbal intellectual function, processing speed, and semantic word-retrieval contributions to verbal fluency performances. The main conclusion from this study is that the abilities associated with verbal fluency performance in a young healthy population are multifactorial and differ across fluency variants. These findings progress our theoretical understanding of what is measured by different verbal fluency tasks and will assist interpretation of performance.

Symbolic sequence learning is associated with cognitive–affective profiles in female FMR1 premutation carriers

This study examines implicit sequence learning impairments that may indicate at‐risk cerebellar profiles proposed to underlie some aspects of subtle cognitive and affective dysfunctions found among female fragile X mental retardation 1 (FMR1) premutation (PM)‐carriers. A total of 34 female PM‐carriers and 33 age‐ and intelligence‐matched controls completed an implicit symbolically primed serial reaction time task (SRTT) previously shown to be sensitive to cerebellar involvement. Implicit learning scores indicated a preservation of learning in both groups; however, PM‐carriers demonstrated poorer learning through significantly elevated response latencies overall and at each specific block within the symbolic SRTT. Group comparisons also revealed a core deficit in response inhibition, alongside elevated inattentive symptoms in female PM‐carriers. Finally, strong and significant associations were observed between poor symbolic SRTT performance and executive, visuospatial and affective deficits in the PM‐carrier group. These associations remained strong even after controlling motor speed, and were not observed in age‐ and intelligence quotient‐matched participants. The findings implicate cerebellar non‐motor networks subserving the implicit sequencing of responses in cognitive–affective phenotypes previously observed in female PM‐carriers. We contend that symbolic SRTT performance may offer clinical utility in future pharmaceutical interventions in female PM‐carriers.

Brain structure and intragenic DNA methylation are correlated and predict executive dysfunction in fragile X premutation females

DNA methylation of the Fragile X mental retardation 1 (FMR1) exon 1/intron 1 boundary has been associated with executive dysfunction in female carriers of a FMR1 premutation (PM: 55–199 CGG repeats), whereas neuroanatomical changes have been associated with executive dysfunction in PM males. To our knowledge, this study for the first time examined the inter-relationships between executive function, neuroanatomical structure and molecular measures (DNA methylation and FMR1 mRNA levels in blood) in PM and control (<44 CGG repeats) females. In the PM group, FMR1 intron 1 methylation was positively associated with executive function and cortical thickness in middle and superior frontal gyri, and left inferior parietal gyrus. By contrast, in the control group, FMR1 intron 1 methylation was negatively associated with cortical thickness of the left middle frontal gyrus and superior frontal gyri. No significant associations were revealed for either group between FMR1 mRNA and neuroanatomical structure or executive function. In the PM group, the lack of any significant association between FMR1 mRNA levels and phenotypic measures found in this study suggests that either FMR1 expression is not well conserved between tissues, or that FMR1 intron 1 methylation is linked to neuroanatomical and cognitive phenotype in PM females via a different mechanism.