Due to the multiplicity of neuron subtypes in the brain, solving the cellular basis of a complex neurologic disease such as childhood absence epilepsy could not be achieved until recently. Development of the bacteriophage PI-derived Cre/loxP recombination system enabled significant progress towards solving the cellular basis of memory and should also facilitate our goal of solving the cellular basis of childhood absence epilepsy. We will use this method to target human absence epilepsy gene mutations to specific neuron subtypes in the murine brain. We will focus on the childhood absence epilepsy-associated mutations recently discovered in the T-type calcium channel Cav3.2 gene. T-type calcium channels have been implicated in absence epilepsy. Furthermore, some disease mutations alter channel gating. Based on these findings, we hypothesize Cav3.2 mutations alter the firing properties of specific neuron subtypes in the brain to cause the characteristic 3Hz rhythmic discharge of spike-and-wave complexes, and the behavioral arrests afflicting children with absence epilepsy. To test this hypothesis, we will first, recreate the disease in mice using an epitope-tagged CACNA1H transgene (224 kb, genomic DNA) that encodes Cav3.2. Nucleotide mutations will be made to recreate the epilepsy-associated amino acid changes F161L and V831M, which alter Cav3.2 channel gating. Second, we will develop technologies for targeting the disease gene to specific neuron subtypes by adding a Cre recombinase delete-able transcriptional and translation silencing element to the transgene. We will assess gene silencing by breeding to mice with neuron subtype specific Cre recombinase transgenes. Because they contain cell-type specific promoters, the Cre transgenes express Cre recombinase protein in limited neuron subtypes. Only in these neurons will Cre delete the silencing element, cause transgene expression, and generate epitope tag staining. With this tool we plan to test whether abnormal burst firing in cortical pyramidal or reticular thalamic neurons cause absence epilepsy. If the characteristic signs of epilepsy are reproduced, the results will establish that mutations in Cav3.2 cause absence epilepsy. Once the silencing element system is created, future work to determine the specific neuron subtype whose dysfunction produces absence epilepsy will become possible. Identifying the neural substrate of absence epilepsy will facilitate work to identify other disease genes and potential drug targets.
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