Abstract
Glioblastoma (GBM), the most common and lethal adult primary brain tumour, exhibits cellular heterogeneity and a subpopulation of tumour cells exhibits the neural precursor phenotype and drives tumourigenesis. These findings suggest that tumours may represent aberrant organogenesis with growth caused by unlimited proliferation and failure of differentiation of malignant precursor cells. As GBM-neural stem (GNS) cells have precursor cell properties, promotion of differentiation represents a potential strategy for treatment. A key regulator of neuronal differentiation is ASCL1 during normal development and adult neurogenesis. Microarray analysis of ASCL1 expression in primary GNS cultures (n = 33) revealed two subgroups of GNS cultures as having either high or low expression. Evidence suggests that GNS cells with high expression of ASCL1 exhibit neuronal lineage commitment whereas GNS cells with low expression of ASCL1 do not undergo lineage commitment. This was confirmed using immunocytochemistry, quantitative real-time PCR and Western blot analysis. Furthermore, a dependent relationship was observed between ASCL1 subgroups and GNS subgroups responsive to inhibitors of Notch signaling. Treatment of the responder subgroup with a gamma-secretase inhibitor resulted in an increase in neuronal lineage markers and concomitant decrease in stem cell frequencies, as measured by immunocytochemistry and in vitro limiting dilution assays, respectively. Dominant-negative studies of ASCL1 suggest that ASCL1 is necessary for these effects in vitro and have been validated in vivo. This study aims to determine whether neuronal differentiation by blocking Notch signaling is mediated by ASCL1