EPN-30: YAP1-MAMLD1 FUSIONS ALONE ARE SUFFICIENT TO FORM SUPRATENTORIAL EPENDYMOMA-LIKE TUMORS IN MICE

Oncogenic fusions containing RELA or YAP1 have recently been identified as genetic hallmarks of distinct molecular subgroups of supratentorial ependymomas (ST-EPN), designated ST-EPN-RELA and ST-EPN-YAP1, respectively. ST-EPN-YAP1 tumors, exclusively found in pediatric patients, are molecularly and clinically different from ST-EPN-RELA tumors, suggesting that they have to be treated differently. YAP1 acts as a transcriptional regulator in the HIPPO tumor suppressor pathway. The lack of adequate models for ST-EPN-YAP1 has so far hindered efforts to develop effective targeted therapies for these tumors. In an attempt to model this subgroup, the most frequent fusion type, YAP1-MAMLD1, was constructed and cloned upstream of IRES-Luciferase into the pT2K transposable vector. The resulting vector was injected together with the Tol2 transposase into the lateral ventricle of E13.5 wildtype mouse embryos followed by transfection using an electroporation-based in vivo gene transfer approach. After birth, YAP1-MAMLD1-expressing tumors, monitored using luciferase-based in vivo bioluminescence imaging, developed rapidly with 100% penetrance. The animals had to be sacrificed due to severe neurological symptoms on average at P20. Resulting tumors resembled molecular characteristics of their human counterparts. The YAP1-MAMLD1 protein was predominantly compartmentalized to the nuclei of tumor cells even when phosphorylated at serine residue 127, which normally retains YAP1 in the cytoplasm, implying constitutive activation of the YAP1 fusion protein. Conserved tumorigenic potential of transformed cells was confirmed by subsequent orthotopic transplantation of these tumor cells into immunocompromised recipient mice. We thus provide a novel model for ST-EPN-YAP1, which is currently entering first preclinical drug testings.