TB-06 MOLECULAR MECHANISM OF BRAIN TUMOUR FORMATION DRIVEN BY SUPRATENTORIAL EPENDYMOMA-SPECIFIC YAP1 FUSION GENES

YAP1 fusion-positive supratentorial ependymomas predominantly occur in infants, but the molecular mechanisms of oncogenesis are unknown. Here we show YAP1-MAMLD1 fusions but not YAP1 wildtype are sufficient to drive malignant transformation of neural progenitors in the developing cerebral cortex in mice, and the resulting tumours share histo-molecular characteristics of human ependymomas. Nuclear localization of YAP1-MAMLD1 protein is associated with its oncogenicity and is mediated by the nuclear localization signal of MAMLD1 in a YAP1-Ser127 phosphorylation-independent manner. Chromatin immunoprecipitation-sequencing analyses of human YAP1-MAMLD1-positive ependymoma reveal enrichment of NFI and TEAD transcription factor binding site motifs in YAP1-bound regulatory elements, hypothesizing the important role of these transcription factors in YAP1-MAMLD1-driven tumourigenesis. Indeed, co-immunoprecipitation assays revealed physical interactions of TEADs and NFIA/B with the YAP1 and MAMLD1 domains of the fusion protein, respectively. Mutation of the TEAD binding site in the YAP1 fusion or repression of NFI targets prevents tumour induction in mice. Together, these results demonstrate that the YAP1-MAMLD1 fusion functions as an oncogenic driver of ependymoma through recruitment of TEADs and NFIs, indicating a rationale for preclinical studies to block the interaction between YAP1 fusions and NFI and TEAD transcription factors.