PROJECT SUMMARY Tuberous Sclerosis Complex (TSC) is a neurodevelopmental disorder caused by mutations in the TSC1 or TSC2 genes. The protein products of TSC1 and 2 form a complex that is a key negative regulator of mTOR signaling. TSC is associated with an array of neurological and psychiatric problems that can include epilepsy, autism spectrum disorder, and intellectual disability. The neurological manifestations of TSC are amongst the most debilitating to patients, yet our knowledge of the neuropathophysiology of TSC is limited. Animal models of TSC have been valuable to address questions of basic biology, revealing alterations in neuronal development, morphology, and synaptic communication. The next major challenge is to translate these findings to humans. This will require defining the consequences of TSC mutations in a human genetic and developmental context. To achieve this we will establish a novel human neuronal model for TSC based on Cas9-mediated gene editing of human embryonic stem cells (hESCs). To this end we have generated an isogenic panel of hESCs with heterozygous, homozygous, and conditional loss of function mutations in the TSC1 or TSC2 genes. These cells will be differentiated into neural progenitors, neurons, and cerebral organoids to model the early stages of human cortical development when TSC-related phenotypes first arise. We will determine how mutations in TSC1 or 2 affect the development and function of human neurons using biochemical, genome-wide profiling, imaging, and electrophysiological approaches. Our findings will answer several key questions related to genotype-phenotype relationships in TSC and the developmental origin of the cortical malformations that are a hallmark of this disorder. In addition, the cell lines we generate will be a valuable resource for the research community to investigate disease mechanisms and test potential therapeutics for TSC and other “mTOR-opathies” directly in primary human cells.