PROJECT SUMMARY The pathogenesis of neurodevelopmental disorders is challenging to address for many reasons. The human brain is relatively inaccessible for direct evaluation, human autopsy and imaging studies provide limited opportunities to study molecular or cellular mechanisms, animal models sometimes do not replicate important disease features, and immortalized neuron-like cell lines cannot be used to model normal development. Induced pluripotent stem cells (iPSCs) provide a powerful new experimental tool to address these limitations. They can be created from patients with known developmental disorders caused by defined mutations, they provide a renewable resource, and they can be differentiated into specific cell lineages to provide species-specific and lineage-specific experimental models to study developmental mechanisms at the cellular and molecular levels. In the current proposal we address several fundamental questions that are broadly relevant to iPSC modeling for neurodevelopmental disorders. We focus on Lesch-Nyhan disease (LND) as a prototype disorder, because it has numerous features that make it an unusually tractable Mendelian disorder for iPSC modeling. LND and its milder variants are caused by mutations in the HPRT1 gene, resulting in deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). The neurobehavioral abnormalities in this disorder have been linked with developmental dysfunction of brain dopamine neurons. Aim 1 focuses on establishing a well-characterized bank of iPSCs across the spectrum of disease severity, to address issues related to how specific mutations causing different disease severity are reflected in iPSC models. Aim 2 focuses on establishing a well-characterized bank of isogenic iPSCs in which specific mutations have been introduced via gene editing methods, to address issues related to the genetic background of individuals from which iPSC models are derived. In Aim 3 we differentiate these lines into dopamine neurons, to address how iPSC models can reveal the timing and mechanisms of pathogenesis related to LND. Overall, this project will address fundamental questions that are broadly relevant for iPSC modeling of neurodevelopmental disorders, as well as more specific insights into the pathogenesis of the neurobehavioral abnormalities of LND.