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Increasing evidence suggests that autism is associated with abnormal white-matter (WM) anatomy and impaired brain ‘connectivity’. While myelin plays a critical role in synchronized brain communication, its aetiological role in autistic symptoms has only been indirectly addressed by WM volumetric, relaxometry and diffusion tensor imaging studies. A potentially more specific measure of myelin content, termed myelin water fraction (MWF), could provide improved sensitivity to myelin alteration in autism.
We performed a cross-sectional imaging study that compared 14 individuals with autism and 14 age- and IQ-matched controls. T1 relaxation times (T1), T2 relaxation times (T2) and MWF values were compared between autistic subjects, diagnosed using the Autism Diagnostic Interview – Revised (ADI-R), with current symptoms assessed using the Autism Diagnostic Observation Schedule (ADOS) and typical healthy controls. Correlations between T1, T2 and MWF values with clinical measures [ADI-R, ADOS, and the Autism Quotient (AQ)] were also assessed.
Individuals with autism showed widespread WM T1 and MWF differences compared to typical controls. Within autistic individuals, worse current social interaction skill as measured by the ADOS was related to reduced MWF although not T1. No significant differences or correlations with symptoms were observed with respect to T2.
Autistic individuals have significantly lower global MWF and higher T1, suggesting widespread alteration in tissue microstructure and biochemistry. Areas of difference, including thalamic projections, cerebellum and cingulum, have previously been implicated in the disorder; however, this is the first study to specifically indicate myelin alteration in these regions.
Velo-cardio-facial syndrome (VCFS) is associated with deletions at chromosome 22q11, abnormalities in brain anatomy and function, and schizophrenia-like psychosis. Thus it is assumed that one or more genes within the deleted region are crucial to brain development. However, relatively little is known about how genetic variation at 22q11 affects brain structure and function. One gene on 22q11 is catechol-O-methyltransferase (COMT): an enzyme that degrades dopamine and contains a functional polymorphism (Val158Met) affecting enzyme activity. Here, we investigated the effect of COMT Val158Met polymorphism on brain anatomy and cognition in adults with VCFS.
The COMT Val158Met polymorphism was genotyped for 26 adults with VCFS on whom DNA was available. We explored its effects on regional brain volumes using hand tracing approaches; on regional grey- and white-matter density using computerized voxel-based analyses; and measures of attention, IQ, memory, executive and visuospatial function using a comprehensive neuropsychological test battery.
After corrections for multiple comparisons Val-hemizygous subjects, compared with Met-hemizygotes, had a significantly larger volume of frontal lobes. Also, Val-hemizygotes had significantly increased grey matter density in cerebellum, brainstem, and parahippocampal gyrus, and decreased white matter density in the cerebellum. No significant effects of COMT genotype on neurocognitive performance were found.
COMT genotype effects on brain anatomy in VCFS are not limited to frontal regions but also involve other structures previously implicated in VCFS. This suggests variation in COMT activity is implicated in brain development in VCFS.
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