Pathogenic variants in the gene SCN3A, which encodes the voltage-gated sodium (Na+) channel subunit Nav1 .3, cause SCN3A-related neurodevelopmental disorder (SCN3A-NDD), a recently identified condition defined by treatment-resistant epilepsy, severe to profound intellectual disability (ID), and, surprisingly, malformation of cortical development (MCD; abnormal structural development of the cerebral cortex). There is no known role for Na+ channels in structural brain development nor any basis for brain malformation caused by Na+ channel variants, and it remains unclear how genetic variants in SCN3A lead to MCD, epilepsy, and/or ID. This proposal seeks to define the mechanistic underpinnings of this poorly understood disorder in order to define the normal physiological role of Nav1 .3 and to inform therapeutic discovery or preventative measures for SCN3A-NDD, a devastating disease which currently has no treatment or cure. The prominence of MCD among our uniquely large patient cohort combined with known high embryonic expression of SCN3A in the brain motivates my central hypothesis that pathogenic variants in SCN3A cause epilepsy and MCD in SCN3A-NDD by producing pathological Na+ currents in developing cortical neurons leading to perturbed excitability and altered neuronal migration. Proposed experiments will functionally assess specific patient variants in SCN3A in our cohort and establish the relationship between individual genetic variant, channel dysfunction, neuronal excitability, structural development, and clinical presentation. Aim 1 will ascertain the link between genetic variation in SCN3A and Na+ channel dysfunction by determining the biophysical properties of wild-type versus variant Nav1 .3 in a transiently transfected (HEK-293T) cell system. Aim 2 will assess how altered channel activity impacts neuronal electrical excitability, anatomy, and maturation via generation of human excitatory cortical neurons derived from control or SCN3A variant-expressing induced pluripotent stem cell (iPSC) lines. A novel Nav1 .3-selective compound PF-6651385 will be employed to determine potential to pharmacologically correct dysfunction at the level of the channel (Aim 1) and/or neuron (Aim 2). This proposal using advanced model systems to interrogate pathogenic mechanisms of SCN3A-NDD will advance my training towards a career in translational neuroscience while providing novel insight into a potentially fundamental role for a Na+ channel in brain development and elucidating pathomechanisms underlying SCN3A-NDD to progress towards targeted therapies for patients suffering from this disorder.