This R03 project will generate two lines of knock-in mice with point mutations in the endogenous Kcnc3 gene, which encodes the Kv3.3 voltage-gated K+ channel. In humans, the analogous KCNC3 mutations cause distinct forms of spinocerebellar ataxia type 13 (SCA13). SCA13 is a rare autosomal dominant disease characterized by substantial atrophy of the cerebellum and locomotor deficits. Depending on the causative mutation, SCA13 presents as an early-onset neurodevelopmental disease or as an adult-onset neurodegenerative disease. The developmental form of SCA13 is evident in infancy or early childhood with intellectual disability, motor delay, persistent motor deficits, and severe cerebellar atrophy. The degenerative form of SCA13 emerges in adulthood with progressive ataxia accompanied by progressive loss of cerebellar volume. The product of the disease gene, Kv3.3, is highly expressed in fast-spiking cerebellar neurons, where it is an essential regulator of excitability. Disease-causing mutations alter Kv3.3 function, suggesting that changes in action potential firing trigger pathogenesis in SCA13. As a monogenic disorder, SCA13 provides an excellent opportunity to investigate the role of excitability in neuronal health and survival, and o identify mechanisms that translate altered excitability into pathogenic changes during development and aging. Animal models are essential to determine the consequences of SCA13 mutations in vivo and to investigate how different mutations in the same gene give rise to distinct clinical phenotypes. To identify mechanisms that contribute to pathogenesis in SCA13, a mammalian model system in which brain development, structure, and function are closely related to humans is crucial. Kcnc3 knock-in mice will provide essential tools for investigating the etiology of SCA13 and for translating the results of basic research into new therapeutic approaches for neurodevelopmental and neurodegenerative diseases. The knock-in mice generated in this project will be the first genetically accurate models of SCA13 in a mammalian model system. They will be made freely available as a resource for the research community. During the two year term of the R03 grant, we propose to accomplish the following Specific Aims: 1) to generate targeting vectors encoding the SCA13 adult- onset R421H or infant-onset F449L mutations in the mouse Kcnc3 gene; 2) to generate two lines of knock-in mice; and 3) to begin to characterize the phenotypes of the knock-in animals.