PROJECT SUMMARY/ABSTRACT Myoclonic Atonic Epilepsy (MAE), a rare developmental and epileptic encephalopathy (DEE) characterized by frequent severe seizures, developmental delay, and intellectual disability, has been linked to loss of function variants in the SLC6A1 gene. The SLC6A1 gene encodes the γ-aminobutyric acid (GABA) transporter 1 (GAT- 1), the most abundant GABA transporter in the brain which is responsible for the reuptake of GABA at the synapse. In humans and rodents, SLC6A1 DEE variants result in reduced GABA uptake and lead to physiological, behavioral, cognitive, and developmental abnormalities including neural network hyperexcitability and epilepsy. However, the mechanisms by which GAT-1 haploinsufficiency (HI) results in epilepsy remain unclear. An in vitro human model of SLC6A1 HI is therefore critical to elucidate disease mechanisms and test potential therapeutics. The objective of this project is to use our newly developed 2-D and 3-D human induced pluripotent stem cell (iPSC)-derived in vitro models of SLC6A1 HI to explore alterations in GABAergic signaling, early cortical development and neuronal excitability. We hypothesize that SLC6A1 HI alters brain development and GABAergic signaling leading to neuronal network hyperexcitability in MAE patients. This hypothesis is based in part on our unexpected finding of severe interneuron migration defects in SLC6A1 heterozygous and homozygous CRISPR knockout and patient ganglionic eminence-like organoids. Novel methods, including a new 3-D medial ganglionic eminence-like organoid model which I have developed and transcription factor-induced GABAergic neurons (iGNs) derived from CRISPR gene edited iPSCs will be used to test this hypothesis. I will assess neuronal morphology, gene and protein expression, neural progenitor proliferation/differentiation/migration, interneuron maturation and GABAergic synaptogenesis to evaluate the developmental consequences of SLC6A1 HI. Functional assays, including whole-cell patch-clamp recordings, calcium imaging, multielectrode array recordings, and local field potential recordings, will be conducted to examine electrophysiological activity at single-cell and network resolutions, while GAT-1 transport activity will be measured using a tritiated GABA uptake assay. Based on our preliminary data and previous work, a 50% or greater reduction in GAT1 expression and function is anticipated in patient and het/homozygous CRISPR knockout lines, as well as delayed migration of SLC6A1 HI and knockout cells compared to controls. Deficits in GABAergic neuron maturation, synaptogenesis, and network formation are also expected. The results of this study will yield significant insight into SLC6A1-related DEE mechanisms and will provide the broader scientific community with a platform to study a wide range of genetic epilepsies and to test future therapeutic targets. The proposed experiments, mentoring team and training plan will provide me with the necessary growth a...