Abstracts

Stargazer mice are mutated in the [italic]Cacng2[/italic] gene and have frequent spontaneous absence seizures. In addition to [italic]Cacng2[/italic], there are seven closely-related genes encoding CACNG (gamma) proteins. We have focused our studies on the CACNG4 protein as it is closely related to CACNG2 and is expressed in the brain. By introducing a lacZ targeted mutation into the [italic]Cacng4[/italic] gene, we were able to both disrupt the gene and follow chimeric protein expression in the mouse brain. We introduced the LacZ gene into the carboxy terminus of [italic]Cacng4[/italic] and generated homozygous mice with the targeted deletion of the [italic]Cacng4[/italic] gene. This mutation was combined with the [italic]Cacng2[/italic] mutations in the stargazer, waggler and stargazer3J allelic series. EEGs were recorded from implanted electrodes to measure spontaneous absence seizure activity. Brain sections were stained with X-Gal to reveal the regions with LacZ (beta-galactosidase) activity. The homozygous [italic]Cacng4[/italic] targeted mutant appeared normal and had no absence seizure activity. However, by constructing double mutants carrying both this mutation and a mutation in the [italic]Cacng2[/italic] gene, we were able to detect increased seizure activity compared to the single mutants. The LacZ staining revealed that the chimeric gamma4 [ndash]LacZ protein is expressed, especially in the caudate putamen, the habenulae, the inferior colliculus, the CA3 region of the hippocampus and the Purkinje cell layer of the cerebellum. Staining was also observed within the thalamus, but there was no staining within any of the cortical regions. The [italic]Cacng4[/italic] targeted mutant revealed no obvious phenotype, including no incidence of seizures. However when this mutation was introduced onto the [italic]Cacng2[/italic] mutant background, the double homozygotes had increased absence seizure activity indicating that the CACNG4 protein does have a role in seizure suppression but this could only be revealed in a compromised [italic]Cacng2[/italic] background. [italic]Cacng4[/italic] is expressed in the thalamus, and this region, along with the cortex, has been implicated in the etiology of absence seizures. (Supported by NS32801 (V.A.L.).)