PROJECT SUMMARY Epilepsy is a major cause of morbidity, mortality, disability, and expense, and affects over 470,000 children in the U.S. While many medications to control seizures have been developed, about 30% of patients do not respond to medications, and to date, there are no medications that can prevent or halt the progression of epilepsy. Recently, many genetic causes of epilepsy have been identified, providing insights into pathways involved in epileptogenesis. Mutations causing hyperactivity of the mTOR pathway (so-called “mTORpathies”) have emerged as an important cause of cerebral malformations and epilepsy, including tuberous sclerosis complex, focal cortical dysplasia, and polyhydramnios, megalencephaly, and symptomatic epilepsy (PMSE) syndrome, which is associated with severe epilepsy and intellectual disability and is caused by a loss-of- function in the STRADA gene. Inhibition of the mTOR pathway with rapamycin or its analogs can help in mTORopathies, but many individuals still have seizures that do not respond. Prior studies generated stem cells from individuals with PMSE and differentiated them into excitatory neurons in 2-D culture and dorsally-fated human cerebral organoids, 3-D neural structures that resemble the developing cortex and predominantly contain excitatory neurons. The PMSE organoids demonstrated mTOR hyperactivity, increased size, abnormal morphology, delayed neuronal differentiation, and an increase in outer radial glia, a progenitor cell that is responsible for the expanded size of the human brain. These findings explain megalencephaly in PMSE, but the intermediate mechanisms by which STRADA loss-of-function results in these observed phenotypes is not known. Furthermore, the effect of STRADA loss-of-function on inhibitory interneurons that originate from ventral forebrain structures is not known. Abnormal inhibitory interneurons are a major cause of epilepsy and autism, both prominent features of mTORopathies. This proposal will test the central hypothesis that loss of STRADA causes cortical malformation by mTOR complex 1 hyperactivity, and that inhibitory interneuron development is also impaired, resulting in epilepsy. This innovative proposal will use dorsally-fated organoids to identify which signaling pathway alterations downstream of STRADA result in altered cortical development, and will determine whether compensatory feedback signals hinder the effectiveness of rapamycin treatment (Aim 1). We will also generate fusions between ventrally-fated and dorsally-fated organoids to study interneuron development, interneuron migration, cell-type specific transcriptional alterations (Aim 2), as well as neuronal network hyperexcitability that can result in epilepsy (Aim 3). The proposed study will have significant impact because understanding the signaling pathways and cell types responsible for pathogenesis in PMSE will provide a platform to develop mechanistically driven therapies that can halt or reverse epileptog...