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Activating point mutations within the TERT promoter (C228T or C250T) account for the most frequent alteration in aggressive brain tumors. Presence of these alterations results in the generation of binding sites for E-twenty-six (ETS) transcription factors accompanied by enhanced TERT expression. Accordingly, TERT promoter mutations foster cellular immortalization and subsequently tumor aggressiveness. Due to the limitation of treatment options in aggressive brain tumors, including glioblastoma and medulloblastoma, new therapeutic targets need to be discovered. As we previously described a strong interaction of oncogenic MEK/ETS signaling and TERT promoter mutations, we hypothesize that inhibition of these factors halters cell immortalization in TERT-driven brain tumors. Our study included three TERT promoter wild-type (TERTwt), six mutated (TERTmut) glioblastoma and three TERTmut medulloblastoma cell models and tested the effect of MEK inhibitors (U0126 and trametinib) and the ETS inhibitor YK-4-279 on cell viability and clone formation. Cellular senescence upon treatment was evaluated by beta-galactosidase assays. Impact on TERT mRNA expression and TERT promoter activity were analyzed by quantitative real-time PCR and luciferase reporter assays, respectively. Furthermore, the effects on MAPK- and PI3K pathway activation were evaluated by Western blot. Amongst the investigated inhibitors, tumor cells harboring C228T mutation were distinctly more sensitive against trametinib as compared to TERTwt and C250T TERTmut cells. Similar effects were observed on clonogenicity upon long-term exposure to this inhibitor. Regarding MAPK signaling activation, trametinib treatment completely blocked ERK phosphorylation in every cell model, while activation of ETS1 was more effectively reduced in C228T TERTmutcells. Accordingly, exposure to trametinib reduced TERT expression and promoter activity accompanied by induction of cellular senescence in cells with C228T mutation. Impact of trametinib is currently investigated in preclinical experiments using TERTmut brain tumor models. Summarizing, MEK inhibition represents a novel strategy to overcome cell immortalization especially in C228T TERTmut brain tumors.

CSIG-30. TARGETING TELOMERASE VIA MEK INHIBITION IN AGGRESSIVE TERT PROMOTER MUTATED BRAIN TUMORS