PROJECT SUMMARY Despite considerable advances in identifying genetic and epigenetic abnormalities in medulloblastoma, many patients succumb to the disease. In addition, patients who respond to traditional therapy suffer from cognitive and intellectual deficits. Therefore, there is a dire need to identify novel therapies for treating medulloblastoma. Rho family GTPases are targets in multiple cancers including medulloblastoma. Rho, Rac1 and Cdc42 are essential mediators of cell-cell adhesion and cellular motility signaling, which are dysregulated in medulloblastoma. These GTPases are involved in the regulation of the cytoskeleton, cell migration, cellular proliferation, and developmental signaling. The small GTPase Rac1 has recently been reported as a possible therapeutic target in medulloblastoma due to its regulation of Hedgehog signaling via the GLI1 and GLI2 transcription factors. We have recently reported that GLI1/GLI2 are in a novel complex that contains the epigenetic regulators UHRF1 and DNMT1. Therefore, Rac1 may play a previously unappreciated role in epigenetic regulation of medulloblastoma by controlling the GLI1/GLI2/UHRF1/DNMT1 complex. To test this, we will utilize a novel brain penetrant Rac1 inhibitor we developed termed GYS32661. We will test the hypothesis that GYS32661 will reduce GLI1/GLI2 nuclear localization, attenuate Shh signaling in medulloblastoma in vitro (Aim 1) and reduce medulloblastoma growth in vivo (Aim 2). Collectively, our studies will test whether Rac1 inhibition is a novel therapeutic strategy for treating medulloblastoma.