ABSTRACT During the previous funding period, we generated a conditional mouse model of the recurrent sodium channel mutation SCN8A-p.R1872W that is responsible for the rare, devastating disorder Developmental Epileptic Encephalopathy (DEE). We demonstrated that expression of the mutant channel in excitatory neurons of the forebrain reproduces the seizures and sudden death of the human disorder. We also developed an effective treatment for Scn8a-DEE by intracerebroventricular administration of an antisense oligonucleotide (ASO) that reduced both mutant and wildtype transcript level throughout the brain, resulting in delayed seizure onset and extended lifespan. We will build on this important preclinical result by developing improved, longeracting treatments that will minimize the side effects of excess Scn8a reduction. First, AAV viral delivery of an Scn8a- shRNA will limit effects to specific brain regions only. Second, the multiple nucleotide substitutions in our engineered Scn8a-N1768D mutant make it feasible to specifically inactivate the mutant allele by CRISPR/Cas targeting. The goal of these experiments is to minimize the potential side effects of Scn8a deficiency and to avoid the repeated treatments required for ASO therapy. Finally, expression of Scn8aR1872W in cerebellar Purkinje cells results in elevated neuronal excitability. We will characterize mice with specific expression of the mutant allele in Purkinje cells to gain insight into the role of the cerebellum in the movement and cognitive co- morbidities that accompany SCN8A-DEE. The work described here will extend basic understanding of SCN8A pathophysiology and provide pre-clinical evidence regarding future translational potential of regional or mutant- allele-specific down-regulation of pathogenic variants of SCN8A.