P02.02.b loss of elavl2 promotes maintenance and tumorigenic competence in glioblastoma

Abstract BACKGROUND IDH1 wild type glioblastoma (GBM) is the most frequent and aggressive brain tumor in adults. It has been proven that it is driven by glioma stem cells (GSCs) that govern tumor growth and determine treatment resistance. GSCs transit between active and quiescent states, whose balance is likely dependent on both cell-autonomous and microenvironmental signals. MATERIAL AND METHODS transgenic cell lines were produced by retroviral infection following standard protocols, as well as zebrafish growths and orthotopic mouse brain transplants. Transgenic Drosophila lines were crossed with conventional procedures. Phenotypic evaluations, iClip protocol were conducted with standard methods as well. RESULTS in our study, we identified the gene ELAVL2, encoding the HuB protein, as commonly lost in GBM. We demonstrated that the ELAVL2 deletion occurs collaterally with the CDKN2A/B deletion, very close in the 9p21 region, but it is also present as an independent event. Rescue of HuB expression on a panel of ELAVL2-null GSCs affects their invasive ability and shifts their cell state toward differentiation, with loss of self-renewal and multipotency. A reduction in the proportion of quiescent cells determines an increased cytotoxicity of the drug temozolomide (TMZ).We validated these results in vivo, in two different animal models, showing that the tumorigenic potential of xenotransplanted ELAVL2-rescued GSCs is greatly reduced with respect to the ELAVL2-null GSCs in zebrafish and in the mice brain. We also analyzed the effects of ELAVL2 silencing, through RNAi, in a GBM genetic model in D. melanogaster, observing an increased brain volume when Rbp9, the ELAVL2 ortholog in Drosophila, is silenced. Profiling of HuB target mRNAs in GSCs highlighted genes associated with PTEN and stem cells related pathways. PTEN is upregulated in ELAVL2-rescued GSCs and mediates part of the effects of HuB, as PTEN knock-down in the same cells interferes with cell growth and TMZ-sensitivity. In our Rbp9-silenced Drosophila genetic model PTEN overexpression rescues the Rbp9 phenotype, reducing brain volume. CONCLUSION our results, both in vitro and in vivo, indicate that ELAVL2 acts as a tumor suppressor in GBM and its selective loss increases the fitness of GSCs, thus resulting in increased GBM aggressiveness. ELAVL2 promotes activation and neuronal commitment of quiescent GSCs and PTEN is one of the mediators of ELAVL2 in tumor development.