Megalocephaly (or MEG) is a developmental brain disorder characterized by generalized brain overgrowth. It occurs in a growing number of developmental and metabolic disorders and is associated with severe childhood neurological complications including epilepsy, intellectual disability, autism, and hydrocephalus and Chiari malformations. MEG shows significant overlap with more severe but segmental brain disorders including hemimegalencephaly (HMEG) and focal cortical dysplasia (FCD) that collectively constitute the most common cause of intractable epilepsy in children. Activating mutations in key genes within the critical PI3K- AKT-mTOR signaling network (including PIK3CA, PIK3R2, AKT3 and CCND2) have been recently identified in MEG, HMEG and FCD, suggesting that these phenotypes constitute a single broad spectrum of developmental brain disorders with shared molecular etiologies and neuropathological features. The goal of this proposal is to interrogate the molecular basis of these disorders in affected human-derived cells and tissues to define the mutational spectrum, levels of mosaicism, tissue distribution and pathway dysregulation in these disorders. First, I propose to perform single cell sequencing of affected neurons from HMEG and FCD human brain samples obtained from epilepsy surgery to test for known and candidate genes. Second, I will globally assay PI3K-AKT pathway proteins in affected human brain tissues with mutations in key PI3K-AKT pathway genes using high throughput proteomics to identify pathway dysregulation and localize specific sub-pathways and downstream targets. Third, the PI3K AKT genes discovered so far explain ~75% of children with the two most common MEG syndromes, ~25% with HMEG-FCD and none with rare MEG syndromes. I propose studies to identify additional MEG-HMEG-FCD causative genes using whole exome and whole genome sequencing on multiple tissues from affected individuals. My career goal is to become a physician-scientist devoted to integrating cutting-edge genomic and proteomic techniques on human-derived brain tissues, including single cell sequencing, with accurate and quantitative phenotyping to further our understanding of the molecular and biologic basis of human developmental brain disorders. I propose a five-year research program that will incorporate didactic and research training under the mentorship of Dr. William B. Dobyns at the Center for Integrative Brain Research (CIBR). My advisory committee and network of collaborators, combined with the resources at the Seattle Children's Research Institute and University of Washington, will provide the environment necessary for my successful completion of this proposal and transition to an independently funded physician-scientist.