TRANSCRIPTIONAL DYSREGULATION OF T-TYPE CALCIUM CHANNELS IN CHILDHOOD ABSENCE EPILEPSY

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PROJECT SUMMARY/ABSTRACT The goal of my research is to identify pathophysiological mechanisms of Childhood Absence Epilepsy (CAE) using transgenic mouse models. CAE is the most common pediatric epilepsy, and over one-third of cases are pharmaco-resistant. While clinical absence episodes are non-convulsive, the generalized 3-5 Hz spike-wave seizures (SWs) are accompanied by partial loss of consciousness and behavioral arrest, occur hundreds of times per day, and are linked to significant attention and cognitive deficits. This is not a benign disorder. Electrophysiological evidence in human and mouse models show SWs are generated within the thalamocortical circuit of the brain, where elevated T-type calcium currents have a well-characterized role in rebound burst firing activity that is thought to drive CAE seizure activity. Data from several monogenic mouse models of CAE, beginning with the P/Q-type calcium channel mutant tottering, support the hypothesis that abnormal synaptic input onto thalamic neurons elevates low threshold T-currents prior to SWs onset. However, the molecular mechanisms mediating the downstream remodeling of T-channel activity in thalamic neurons are not yet clearly defined. The experiments proposed in this application will dissect the roles of specific T-channel isoforms within the thalamocortical circuit that lead to hyperexcitability and spike-wave seizures. I will test three hypotheses that defective synaptic release due to P/Q-type calcium channel mutations elevate T-currents in postsynaptic thalamic cells by altering 1) transcription of T-type subunit genes, 2) their splice form ratios, and/or 3) transcription of auxiliary genes for T-type channels that encode functionally interacting proteins that regulate T-channel expression. To achieve these goals, I propose the following 2 research aims: 1) Determine the level and pattern of thalamic Cacna1g and Cacna1h channel isoform expression in Cacna1a mutant model tottering, and 2) Investigate the role of an established T-channel modifier gene, Stac1, in T-current elevation and SWs. The first aim will investigate the T-channel isoform expression patterns within identified thalamocortical circuit neurons and determine whether they have expression ratio abnormalities before and after seizure onset in tottering mice. The second aim will investigate whether Stac1 contributes to SWs activity, whether Stac1 deletion modifies thalamic T-channel expression in vivo, and whether deletion of Stac1 will modify or prevent seizures in tottering mice. In preliminary studies, I have obtained quantitative evidence using the sensitive, single cell resolution RNAscope in situ hybridization method that mRNA for Cacna1g, the predominant T-type channel alpha-subunit expressed in thalamic relay nuclei, is elevated in tottering thalamus in concordance with electrophysiological evidence. I have also identified that deletion of Stac1, a modifier of surface expression of T-type channel subtype expressed in thalamic neurons, generates a CAE phenotype. These studies may help understand the molecular mechanisms of T-type calcium channel remodeling and expand the genome of thalamocortical rhythm disorders.