DIPG-19. FOXR2 is an oncogenic driver across pediatric and adult cancers

Abstract BACKGROUND: Understanding how aberrant transcription factors (TFs) hijack normal development to induce oncogenesis is a critical question in oncology. Forkhead box (FOX) proteins are a superfamily of transcriptional regulators characterized by a forkhead DNA-binding domain. Within this family, Forkhead Box R2 (FOXR2) has been identified as a candidate structural variant (SV) driver in a subset of pediatric cancers including CNS embryonal tumors and peripheral neuroblastoma. While FOXR2 has been shown to stabilize MYC isoforms, the mechanistic details through which it enhances tumor formation, other non-SV mechanisms of activating aberrant expression, and the true extent of its role as an oncogene across all cancers have not been systematically evaluated. METHODS: We applied an integrative approach using transcriptomics, epigenetics, in vitro cancer models, and in vivo mouse models to systematically evaluate the mechanisms by which FOXR2 is activated across human cancers. RESULTS: We performed a pan-cancer analysis of FOXR2 activation across over 10,000 adult and pediatric cancer samples, and surprisingly found FOXR2 to be aberrantly upregulated in 70% of all cancer types (including diffuse midline gliomas), and 8% of all individual tumors. FOXR2 expression occurred predominantly in the absence of rearrangement/fusions, single nucleotide variants, or copy number aberrations at the DNA level. Transcriptomic and epigenomic analyses show the vast majority of tumors (78%) aberrantly express FOXR2 through a previously undescribed epigenetic mechanism via hypomethylation of a novel promoter. Using both in vitro and in vivo models, we demonstrate that FOXR2 expression is both sufficient and necessary for transformation across multiple lineages, including DMGs. CONCLUSION: Taken together, this study demonstrates that FOXR2 is a novel and potent oncogene across pediatric and adult cancers, and highlights a new epigenetic mechanism by which its expression is activated.