An Investigation of the Genetic and Neural Correlates of Executive Dysfunction in Female Fragile X Premutation Carriers

Fragile X syndrome is a neurodevelopmental disorder which represents one of the most common genetic risk factors for autism. Fragile X syndrome is the consequence of a large (>200) trinucleotide CGG repeat expansion, in the 5’ UTR region of the Fragile X mental retardation gene 1 (FMR1), located on the X chromosome. However, smaller FMR1 premutation (PM) expansions (55-199 CGG repeats) are more common (approximately 1 in 209 females and 1 in 430 males), and confer a risk of a number of medical, psychiatric and cognitive conditions, as well as Fragile X tremor-ataxia syndrome (FXTAS). Estimated to effect up to 40% of PM males, and up to 16% of PM females over 50 years of age, FXTAS is characterised by intention tremor, cerebellar gait ataxia, and cognitive dysfunction (specifically executive dysfunction and dementia), corresponding with changes in neuroanatomy (generalised atrophy as well as hyperintensities in the middle cerebellar peduncles, brainstem and corpus callosum). It is hypothesised that FXTAS is the consequence of an FMR1 mRNA toxic-gain of function, given that PM individuals tend to exhibit increased levels of FMR1 mRNA, compared to those with normal FMR1 alleles (<45 CGG repeats). However, there is increasing evidence of both neuroanatomical changes and cognitive dysfunction in PM-carriers without FXTAS. Specifically, cortical and subcortical atrophy, a reduction in white matter integrity, and compensatory patterns of neural activation, have been well documented in PM males without FXTAS, as well as executive dysfunction on tasks reliant on response inhibition and working memory processes. These changes indicate disruption to cortico-cerebellar processing. It does however; remain unclear whether PM females without FXTAS experience analogous deficits and whether genetic and neural changes (within the cortico-cerebellar network) influence PM-related dysfunction.
   Accordingly, the principal aim of this thesis was to investigate whether PM females without FXTAS, like their male counterparts, experience executive dysfunction on tasks reliant on response inhibition and working memory processes. To establish this, a range of neuropsychological and ocular motor saccadic paradigms were employed. Saccadic paradigms are a sensitive neuromotor tool for investigating cognitive function, with well-defined sensorimotor processes that produce precise and stereotyped output with known neural correlates. Inter-relationships between executive (dys)function, genetic (CGG, DNA methylation and FMR1 mRNA) and neural (structural and functional) markers were also explored, facilitating inferences concerning the biological source of cortico-cerebellar disruption.
   Herein, a series of six experimental chapters provide a comprehensive assessment of the biological correlates of executive dysfunction in PM females without FXTAS. Results revealed deficits of executive function were most prevalent in tasks requiring rapid resolution of responses (Chapter 2). This was most evident for saccade paradigms examining response inhibition and working memory processes, for which we reveal correlations between performance and genetic indices (Chapter 3 and 4). Correlations were also revealed between executive dysfunction (measures derived from an executive control saccade task), and FMR1 intron 1 methylation markers. These methylation markers can influence gene function through expression of long non-coding RNAs, and were also found to correspond with measures of cortical grey matter (Chapter 6). Meanwhile FMR1 mRNA levels were found to correlate with measures of white matter integrity (Chapter 7). Finally, clear dissociations were revealed between PM females without FXTAS and controls with respect to i) grey matter neural activation (Chapter 5); ii) grey matter structure (Chapter 6) and iii) white matter microstructure (Chapter 7) and executive dysfunction.
   Collectively, the biological findings from this work suggest two possible molecular mechanisms for cortico-cerebellar pathway disruption: i) changes in FMR1 intron 1 methylation which affect grey matter structure through long non-coding RNAs, and ii) increased FMR1 mRNA which alters white matter microstructure. This second pathway, in particular, may represent early FXTAS-related changes.