Project Summary Despite intense efforts, the long-term cure rates of childhood and adult solid tumors such as rhabdoid tumors are not satisfactory. Resistance to intensive chemotherapy is nearly universal, and targets for molecular therapies are largely undefined. For example, inactivating mutations of SMARCB1 cause rhabdoid tumors, but their molecular pathophysiology and genetics remain poorly understood. We have now found that the majority of childhood and distinct adult solid tumors, including rhabdoid tumors, express PGBD5, a DNA transposase- derived human gene. We have now found that human rhabdoid and numerous solid tumors are characterized by previously unknown somatic deletions and inversions involving PGBD5-specific signal sequences (PSS) at their breakpoints. Remarkably, PGBD5's DNA transposase enzymatic activity is both necessary and sufficient for cell transformation, and induces recurrent and site-specific genomic rearrangements, leading to cooperative inactivation of tumor suppressor genes. The central hypothesis of this proposal is that defining the genetic and epigenetic mechanisms PGBD5-induced tumorigenesis will identify fundamental causes of PGBD5-induced solid tumors and refractory rhabdoid tumors in particular. The applicant will test this hypothesis by investigating the molecular mechanisms of PGBD5-induced cell transformation and rhabdoid tumorigenesis using primary human and genetically-engineered mouse tumors. Aim 1 will elucidate genetic and epigenetic mechanisms of PGBD5-induced cell transformation, with the goal of identifying PGBD5 targets that are necessary for malignant transformation, and molecular mechanisms that control their induction. Aim 2 will pursue the preliminary evidence that PGBD5-induced genomic rearrangements cooperate with SMARCB1 mutations in rhabdoid tumors and determine the function of PGBD5 using developmentally faithful mouse rhabdoid tumor models in vivo. Successful completion of this project is expected to yield essential molecular mechanisms of PGBD5-induced tumorigenesis, thus providing key insights into a fundamental biological and clinical problem, which should have broad and lasting significance for understanding and treating solid tumors.