BACKGROUND: A number of epilepsy syndromes are characterized by sleep-activated epileptiform discharges, however drivers of this process are not well understood. Previous research has found that thalamic injury in early life may increase the odds of sleep-activated spikes. Benign childhood epilepsy with centrotemporal spikes (BECTS) is among the most common pediatric-onset epilepsy syndromes, characterized by sleep-potentiated spike activity, a focal sensorimotor seizure semiology, and deficits in language, attention, and behavioral functioning. Though ictal and interictal electro-clinical activity resolves during mid-adolescence, adverse psychosocial outcomes may persist. Previous findings from monozygotic twin and neuroimaging studies suggest a multifactorial pattern of disease and raise suspicion for structural changes in thalamocortical connectivity focal to the seizure onset zone, though this has not been explored. OBJECTIVE: This research aims to (1) assess white matter differences in focal thalamocortical connectivity between BECTS cases and healthy controls using validated probabilistic tractography methods, (2) assess the association between spike burden and white matter connectivity focal to the seizure onset zone, and (3) evaluate longitudinal changes in thalamocortical connectivity across four cases. METHODS: 42 subjects ages 6-15 years were recruited between November 2015 and February 2018, including 23 BECTS cases and 19 healthy controls. Subjects underwent 3 Tesla structural and diffusion-weighted magnetic resonance imaging (2mm x 2mm x 2mm) with 64 gradient directions (b-value=2000) and 72 electrode sleep-deprived electroencephalographic (EEG) recordings. Seed and target regions of interest (ROIs) were created within each hemisphere using the Desikan-Killiany atlas, with the thalamus set as a seed ROI, and SOZ cortex and non-SOZ (NSOZ) cortex as target ROIs. Probabilistic tractography was executed using PROBTRACKX2 with 500 streamlines per seed voxel, 0.5 millimeter steps, and a curvature threshold of 0.2. All streamlines reaching the target ROI were summed and normalized by seed voxel count. Results for BECTS and healthy controls were plotted by age. The slope of thalamocortical connectivity versus age was computed for each group and compared between groups using nonparametric bootstrap analysis. Additionally, the association between SOZ connectivity and spike burden was assessed in a subgroup analysis using a linear regression model, controlling for age. RESULTS: A significant difference in the developmental trajectory of thalamocortical connectivity to the SOZ in BECTS cases compared to healthy controls was found (p=0.014), where the increase in connectivity with age observed in healthy controls was not present in BECTS children. These results did not extend to NSOZ thalamocortical connections (p=0.192). Longitudinal results support these observations, where all BECTS cases who underwent repeat imaging (N=4) showed a decrease in thalamocortical connectivity to the SOZ over the follow-up period. No relationship was found between thalamocortical connectivity and spike burden (p=0.840). CONCLUSIONS: These findings suggest that children with BECTS show subtle alterations in thalamocortical white matter development focal to the seizure onset zone. Thalamocortical connectivity to the SOZ does not appear to directly mediate non-REM sleep spike potentiation in BECTS. Limitations of this study include the potential for selection bias and limited power to detect sample differences. Additional research is needed to further characterize thalamocortical network changes and electrographic and neuropsychological correlates.

The subplate layer of the cerebral cortex is comprised of a heterogeneous population of cells and contains some of the earliest-generated neurons. Subplate plays a fundamental role in cortical development. In the embryonic brain, subplate cells contribute to the guidance and areal targeting of corticofugal and thalamic axons. At later stages, these cells are involved in the maturation and plasticity of the cortical circuitry and the establishment of functional modules. In my thesis, I aimed to further characterize the embryonic murine subplate by establishing a gene expression profile of this population at embryonic day 15.5 (E15.5) using laser capture microdissection combined with microarrays. I found over 250 transcripts with presumed higher expression in the subplate at E15.5. Using quantitative RT-PCR, in situ hybridization and immunohistochemistry, I have confirmed specific expression in the E15.5 subplate for 13 selected genes which have not been previously associated with this compartment. In the reeler mutant, the expression pattern of a majority of these genes was shifted in accordance with the altered position of subplate cells. These genes belong to several functional groups and likely contribute to the maturation and electrophysiological properties of subplate cells and to axonal growth and guidance. The roles of two selected genes - cadherin 10 (Cdh10) and Unc5 homologue c (Unc5c) - were explored in more detail. Preliminary results suggest an involvement of Cdh10 in subplate layer organization while Unc5c could mediate the waiting period of subplate corticothalamic axons in the internal capsule. Finally, I compared the expression of a selection of subplate-specific genes (subplate markers) between mouse and rat and found some surprising species differences. Confirmed subplate markers were used to monitor subplate injury in a rat model of preterm hypoxiaischemia and it appeared that deep cortical layers including subplate showed an increased vulnerability over upper layers. Further characterization of subplate-specific genes will allow us to broaden our understanding of molecular mechanisms underlying subplate properties and functions in normal and pathological development.