Project Summary Uveal melanoma (UM) is the most common intraocular tumor in adults. Nearly half of UM patients develop metastatic disease and survive one year or less. No effective therapy exists. Current interventions impair or destroy vision, yet do not improve morbidity or mortality caused by metastatic disease. Clinical trials of cytotoxic chemotherapeutics and immune checkpoint inhibitors have shown little efficacy. Mutant constitutively active forms of Gαq/11 that drive oncogenesis in ~90% of UM patients thus far have been undruggable. Inhibitors of signaling molecules downstream of these oncoproteins have failed to demonstrate significant clinical benefit. Effective therapy may require breakthroughs that enable direct targeting of constitutively active Gαq/11 or necessary but as yet unidentified downstream signaling cascades. This project fills these gaps by showing for the first time that constitutively active Gαq/11 can be trapped pharmacologically in the inactive GDP-bound state, thereby attenuating downstream signaling. The inhibitor causes Gαq/11-driven UM cells to arrest growth, die, or re-differentiate into melanocytic cells, whereas it has no effect on BRAF-driven UM cells. Inhibitor-treated UM cells has revealed a novel oncogenesis mechanism in which signaling by constitutively active Gαq/11 antagonizes epigenetic silencing mediated by polycomb repressive complex 2 (PRC2) to drive de-differentiation. Based on these breakthroughs, the following Aims will be pursued: 1) Identify novel druggable targets that mediate signaling between constitutively active Gαq/11 and PRC2 in UM cells; 2) Determine whether the Gαq/11 inhibitor provides vision-sparing therapeutic benefit in mouse models of primary and metastatic UM; and 3) Set the stage for clinical trials by determining which clinical subclasses of human primary UM tumors respond ex vivo to the Gαq/11 inhibitor. In summary, this project provides unprecedented opportunity to determine whether direct pharmacological inhibition of mutant constitutively active Gαq/11 could provide the first effective and potentially vision-sparing therapeutic option for treating UM.