Blood was taken for each individual in the FXS group to estimate FMRP percentage analysis was based on the percentage of peripheral lymphocytes containing FMRP, as assessed by immunostaining techniques(Kimball Genetics,( 22)).
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One female and three males showed evidence of mosaicism(with both premutation and full mutation) the remaining individuals were diagnosed with full mutation. FXS diagnosis was confirmed via Southern blot DNA analysis(Kimball Genetics, Denver, CO). Participants, aged 15-25 years, included 30 individuals with FXS and a comparison group of 25 individuals without FXS. We also sought to investigate the relationship between neural system habituation and individual differences in level of FMRP and autism symptoms. As such, differences in habituation could be attributed primarily to FXS and not to behavioral symptoms. We sought to describe effects that were associated with FXS regardless of sex, and irrespective of IQ, autism symptoms and adaptive functioning therefore, our comparison group was matched to the FXS group on these criteria. We included individuals with FXS from both sexes because previous fMRI studies have examined either only females( 14) or only males( 15) although both reported aberrant neural systems underlying gaze processing.
We included faces with direct and averted eye gaze to test a secondary hypothesis that habituation is abnormal for direct but not averted eye gaze. We used an fMRI paradigm with repeated presentations of faces to examine neural system habituation to eye gaze in FXS. Given the association between FMRP and synaptic plasticity, we hypothesized that individuals with FXS would display aberrant neural system habituation. Neural system habituation is routinely measured by quantifying changes in the magnitude of fMRI activation to repeated stimuli, as demonstrated in typically developing individuals within systems underlying emotion processing( 21). These deficits may be associated with aberrant neural system habituation and understanding the extent of any habituation impairment will be important for developing behavioral treatments. We know little about neural system habituation in FXS, however deficits in synaptic plasticity have been established( 4). Habituation to anxiety provoking stimuli is critical for successful desensitization and neural system plasticity is essential for habituation( 20). Such treatments have been shown to be effective in reducing symptoms of social anxiety disorder( 18) and in behavioral shaping, a technique showing promise in FXS( 19). If social anxiety underlies eye gaze avoidance in FXS, then reducing anxiety with behavioral interventions such as exposure therapy may prove therapeutic. Therefore, eye gaze avoidance may be linked to emotional responses to eye gaze, such as social anxiety symptoms known to be present behaviorally in FXS( 17). Functional magnetic resonance imaging (fMRI) studies of gaze processing( 14 15), indicate abnormal activation in brain regions supporting visual and social processing (fusiform and superior temporal gyri)( 14), as well as regions underlying emotion processing (insula and amygdala)( 15 16).
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Eye gaze aversion in individuals with FXS is also associated with changes in skin conductance( 8), cortisol reactivity( 9) and pupillary reactivity( 10), suggesting hyperarousal( 11).Įye gaze avoidance in FXS may also be linked to abnormalities in the neural circuitry underlying face/gaze processing as suggested by aberrant morphology in the fusiform gyrus, superior temporal gyrus and amygdala( 12 13). In the case of eye contact avoidance, individuals with FXS are sensitive to gaze initiation and find eye contact aversive whereas, in general, individuals with ASD are insensitive to social gaze( 7). Despite similarities, specific characteristics of some social deficits are unique to FXS. Reduced levels of FMRP are associated with cognitive impairment( 5) as well as social deficits that overlap with characteristics of ASD and social anxiety disorder( 6). FMRP is involved in regulation of synaptic plasticity and dendritic pruning, both critical in neurodevelopment( 3 4). FXS results from a trinucleotide CGG repeat expansion (locus Xq27.3) leading to hypermethylation of the fragile X mental retardation 1 gene (FMR1) promoter region and reduced levels of FMR1 protein (FMRP)( 2). Fragile X syndrome (FXS), the most common inherited cause of intellectual disability, is the leading monogenic cause of autism spectrum disorder (ASD)( 1).
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