PROJECT SUMMARY This proposal outlines a comprehensive series of experiments to assess the basis of overgrowth phenotypes associated with gain of function mutations in PIK3CA, using the mouse as a model system. Although well studied in cancer, recent data has revealed a role for PIK3CA mutation in several developmental disorders including MCAP, DMEG, CLOVES syndrome, epidermal nevi and seborrheic keratosis. Here will assess the developmental and signaling pathway disruptions caused by 3 different Pik3ca gain of function mutations, with particular emphasis on the developing brain. To date, all reported PIK3CA mutations are postzygotic or mosaic rather than germline mutations. Accordingly, we hypothesize that PIK3CA phenotypes correlate with both the severity of the mutation and level (and distribution) of mosaicism. To test this and other hypotheses, we will use standard conditional genetic approaches to express 3 patient-related Pik3ca gof mutations in embryonic CNS neuronal progenitors and their descendants. We will then generate ES cell chimeras, injecting ES cells constitutively expressing the same mutations into wild-type blastocysts to assess phenotype-mosaicism relationships both within the brain and throughout the entire body. We also propose to use in utero electroporation technology to more specifically target mosaic expression to the developing brain. Our studies will assess the developmental pathogenesis of brain pathology and characterize the associated epilepsy, a pressing clinically relevant phenotype. Finally, we will use pharmacological approaches to assess the underlying mechanisms driving acutely Pik3ca-dependent seizures in post-natal mice. These assays represent the first step toward developing molecularly rational epilepsy therapy in PIK3CA segmental brain overgrowth syndrome patients.