Regulation of cell cycle and differentiation by ASCL1 in Glioblastoma
Glioblastoma multiforme (GBM) is the most aggressive primary brain tumour in adults, as well as the most common. The current standard therapy is maximal safe resection, followed by radiotherapy in combination with the alkylating agent, temozolomide. Despite this multi-model treatment approach, median survival is just 14.6 months and new therapies are urgently needed. Glioblastoma stem cells (GSCs) are a highly tumourigenic subpopulation of GBM cells believed to promote therapeutic resistance along with angiogenesis and metastasis. Cancer stem cells share crucial characteristics with normal stem cells such as their ability to self- renew, maintain proliferation and differentiate multi-potently. One strategy to target GSCs is to force them to differentiate into post-mitotic cells, as this would cause them to lose their long-term repopulation potential and would therefore limit tumour growth. In this study, I investigated if the transcription factor ASCL1 could drive neuronal differentiation in GSCs. ASCL1 is a key regulator of neurogenesis in the developing CNS and is sufficient to reprogramme fibroblasts, astrocytes and induced pluripotent cells into neurons. However, ASCL1 also promotes proliferation, and in GSCs, it has been shown to drive tumourigenesis by upregulating Wnt signaling. Given its role in two opposing functions, ASCL1 is tightly regulated by multi-site phosphorylation on serine-proline resides. It is phosphorylated when driving proliferation in cycling cells and is un(der)phosphorylated when activating the transcription of its downstream targets involved in differentiation. I found that endogenous ASCL1 was phosphorylated in GSC lines and hypothesized that dephosphorylating ASCL1 may drive differentiation. I tested this by overexpressing a phosphomutant form of ASCL1, and found that it drove cell cycle exit through the downregulation of Cyclin D2, CDK4 and CDK6. However, ASCL1 did not drive overt differentiation which suggests GSCs may not respond to differentiation cues.