ABSTRACT Many epilepsy syndromes associated with severe, early-onset seizures result from de novo variants in genes involved in early brain development. Recent studies have also identified somatic variants in focal epilepsy associated with cortical malformations, including hemimegalencephaly and the more common focal cortical dysplasia (FCD) type 2. These post-zygotically acquired variants arise during neurogenesis and are therefore present in only a fraction of cells. Expanding on these early discoveries implicating somatic variants in epilepsy, we recently identified brain-specific somatic mutations in SLC35A2 in individuals with refractory neocortical epilepsy. Germline variants in SLC35A2 were previously implicated in a rare X-linked developmental and epileptic encephalopathy. Our data suggest that somatic variants in SLC35A2 may also be responsible for approximately 17% of intractable non-lesional focal epilepsy cases. The number of cells harboring a pathogenic SLC35A2 variant allele appears to correlate with disease severity, and several of the cases have FCD type 1a (FCD1a) pathology. Pathogenic variants in SLC35A2, both somatic and germline, prevent the Golgi-localized transporter from moving UDP-Galactose (UDP-Gal) into the Golgi apparatus for use in the formation of essential galactosylated glycans. There is theoretical, experimental, and observational data suggesting that Gal supplementation may be able to restore glycosylation to the cell to provide therapeutic benefit. In this study, we will define the functional consequences of SLC35A2 variants in epilepsy. Given that not all cells carry the variant allele in the individuals with a somatic SLC35A2 variant, in Aim 1 we seek to use resected human brain tissue specimens to identify the specific cell types in the brain harboring the variant alleles. This will allow us to determine which cell types contribute to SLC35A2 epilepsy and whether cell-type-specific burden dictates the pathological observations. In Aim 2 we will evaluate the effects of the variants on cell-type-specific glycosylation in human induced pluripotent stem cell (hiPSC)-derived neural progenitor cells and mature glutamatergic neurons, a cell type that we have preliminary data to support involvement in the epileptogenic processes. Since nearly all patients with an SLC35A2 variant have seizures, and a significant fraction has FCD1a, in Aim 3 we will also characterize the effects of the variants on individual and neural network activity, neural migration, and neurodevelopment in both human (e.g., hiPSC-derived neurons, 3-D organoids) and mouse models (e.g., mouse neural progenitors and in utero electroporation). In Aims 2 and 3, we will assess the effectiveness of Gal to restore glycosylation and reverse effects on neuronal development or activity. Collectively, these studies will translate our exciting initial discovery implicating a novel pathway underlying a significant fraction of individuals suffering from intractab...