Transcriptional networks controlled by SOX2 in glioblastoma stem cells

Abstract

Glioblastoma is the most common and aggressive primary malignant brain tumor in adults. It constitutes 45,2% of all malignant central nervous system (CNS) tumors and 80% of all primary malignant CNS tumors. The current standard of care for glioblastoma includes maximal surgical resection, radiation and chemotherapy. Despite steady advances in new treatments to improve survival rates, the overall survival of patients with glioblastoma has not improved over the last decades, remaining at 12 to 18 months from diagnosis. At the cellular level glioblastoma is composed by heterogeneous cell populations, among which the glioma stem-like cells (GSCs) exhibits self-renewal potential and the ability to reconstitute the original tumor upon orthotopic implantation. GSCs are the culprit of glioma chemo- and radio-resistance ultimately leading to relapse. The elucidation of the transcriptome and the molecular pathways involved in the generation and maintenance of GSCs is critical to understand the molecular underpinnings of glioblastoma malignancy and could allow the identification of relevant and novel therapeutic targets. SOX2, a critical transcription regulator of embryonic and neural stem cell function, is deregulated in GSCs and is highly expressed in glioblastoma. However the precise molecular pathways regulated by SOX2 in GSCs remain poorly understood. We hypothesized that SOX2 plays a prominent role in driving the growth, treatment resistance and recurrence of glioblastoma cells, through the orchestration of different transcriptional pathways. In this work we performed a genome-wide analysis of SOX2-regulated coding and non-coding transcripts in GSCs. We identified 2048 differentially expressed coding transcripts regulated by SOX2. These genes are involved in different biological process, such as cell adhesion and cell-cell signaling, and in canonical pathways related with intracellular signaling cascades and amino-acid metabolism pathways associated with GSC propagation. SOX2 regulates 261 non-coding transcripts differentially expressed in GSCs, including miRNAs and lncRNAs amongst other. We identified 2 interesting miRNAs regulated by SOX2. miR-301a-3p is over-expressed in glioblastoma tissues, positively correlates with SOX2 expression and participates in the invasive properties of GSCs, acting as an onco-miR. miR-425-5p is significantly overexpressed in glioblastoma tissue and also correlates with SOX2 expression. Moreover, SOX2 activates its expression by directly binding to its promoter. Inhibition of miR-425-5p expression results in inhibition of neurosphere formation, cell proliferation and finally apoptotic cell death. Inhibition of mir-425-5p in vivo leads to a significant increase in overall median survival time of mice bearing orthotopic glioma xenografts. In summary this work integrates for the first time the coding and non-coding transcriptome controlled by SOX2 in GSCs, defining miR-301 and miR-425 as novel onco-miRs in GSCs and gaining new insights about the molecular circuitries governing glioblastoma biology.