Oncogenic Role of Merlin/NF2 in Glioblastoma

Glioblastoma is the most common and aggressive primary brain tumor in adults, with a poor prognosis because of its resistance to radiotherapy and chemotherapy. Merlin/NF2 (neurofibromatosis type 2) is a tumor suppressor found to be mutated in most nervous system tumors; however, it is not mutated in glioblastomas. Merlin associates with several transmembrane receptors and intracellular proteins serving as an anchoring molecule. Additionally, it acts as a key component of cell motility. By selecting subpopulations of U251 glioblastoma cells, we observed that high expression of phosphorylated Merlin at serine 518 (S518-Merlin), Notch1 and epidermal growth factor receptor (EGFR) correlated with increased cell proliferation and tumorigenesis. These cells were defective in cell-contact inhibition with changes in Merlin phosphorylation directly affecting Notch1, EGFR expression as well as downstream targets Hes1 and Ccnd. Of note, we identified a function for S518-Merlin which is distinct from what has been reported when the expression of Merlin is diminished in relation to EGFR and Notch expression, providing first-time evidence that demonstrates that the phosphorylation of Merlin at S518 in glioblastoma promotes oncogenic properties that are not only the result of inactivation of the tumor suppressor role of Merlin, but also, an independent process implicating a Merlin-driven regulation of Notch1 and EGFR.