Glioblastoma (GBM), a highly aggressive and malignant form of primary adult brain cancer, poses significant therapeutic challenges. Despite our improved understanding of the cellular and molecular mechanisms underlying tumorigenesis and the evolution of GBM, targeted molecular therapies have failed to improve patient survival outcomes. The failure of standard treatments and targeted therapies is mainly attributed to the acquisition of phenotypic plasticity of tumor cells and GBM stem-like cells. Epigenetic modifications and their mediators have emerged as crucial regulators of phenotypic plasticity, influencing tumor heterogeneity, therapy resistance and disease progression. Here, we summarize and provide insights into epigenetic regulation of GBM plasticity and specifically, focus on the roles played by DNA- and histone modifiers and non-coding RNAs in driving phenotypic plasticity and resistance. We also delve into their dynamics in response to standard therapies and the challenges for targeting them to overcome phenotypic plasticity and resistance in GBM.

Glioblastoma (GBM) is a fatal adult brain tumor with an extremely poor prognosis. GBM poses significant challenges for targeted therapies due to its intra- and inter-tumoral heterogeneity, a highly immunosuppressive microenvironment, diffuse infiltration into normal brain parenchyma, protection by the blood-brain barrier and acquisition of therapeutic resistance. Recent studies have implicated epigenetic modifiers as key players driving tumorigenesis, resistance, and progression of GBM. While the vast majority of GBM research on epigenetic modifiers thus far has focused predominantly on elucidating the functional roles and targeting of DNA methyltransferases and histone deacetylases, emerging evidence indicates that histone acetyltransferases (HATs) also play a key role in mediating plasticity and therapeutic resistance in GBM. Here, we will provide an overview of HATs, their dual roles and functions in cancer as both tumor suppressors and oncogenes and focus specifically on their implications in GBM resistance. We also discuss the technical challenges in developing selective HAT inhibitors and highlight their promise as potential anti-cancer therapeutics for treating intractable cancers such as GBM.

A key feature distinguishing high-grade glioma (HG) from low-grade glioma (LG) is the extensive neovascularization and endothelial hyperproliferation. Prior work has shown that tumor-associated vasculature from HG is molecularly and functionally distinct from normal brain vasculature and expresses higher levels of protumorigenic factors that promote glioma growth and progression. However, it remains unclear whether vessels from LG also express protumorigenic factors, and to what extent they functionally contribute to glioma growth. Here, we profile the transcriptomes of glioma-associated vascular cells (GVC) from IDH-mutant (mIDH) LG and IDH-wild-type (wIDH) HG and show that they exhibit significant molecular and functional differences. LG-GVC show enrichment of extracellular matrix–related gene sets and sensitivity to antiangiogenic drugs, whereas HG-GVC display an increase in immune response–related gene sets and antiangiogenic resistance. Strikingly, conditioned media from LG-GVC inhibits the growth of wIDH glioblastoma cells, whereas HG-GVC promotes growth. In vivo cotransplantation of LG-GVC with tumor cells reduces growth, whereas HG-GVC enhances tumor growth in orthotopic xenografts. We identify ASPORIN (ASPN), a small leucine-rich repeat proteoglycan, highly enriched in LG-GVC as a growth suppressor of wIDH glioblastoma cells in vitro and in vivo. Together, these findings indicate that GVC from LG and HG are molecularly and functionally distinct and differentially regulate tumor growth. Implications: This study demonstrated that vascular cells from IDH-mutant LG and IDH-wild-type HG exhibit distinct molecular signatures and have differential effects on tumor growth via regulation of ASPN-TGFβ1-GPM6A signaling.

Analysis of tumor gene expression is an important approach for the classification and identification of therapeutic vulnerabilities. However, targeting glioblastoma (GBM) based on molecular subtyping has not yet translated into successful therapies. Here, we present an integrative approach based on molecular pathways to expose new potentially actionable targets. We used gene set enrichment analysis to conduct an unsupervised clustering analysis to condense the gene expression data from bulk patient samples and patient-derived gliomasphere lines into new gene signatures. We identified key targets that are predicted to be differentially activated between tumors and were functionally validated in a library of gliomasphere cultures. Resultant cluster-specific gene signatures associated not only with hallmarks of cell cycle and stemness gene expression, but also with cell type–specific markers and different cellular states of GBM. Several upstream regulators, such as PIK3R1 and EBF1 were differentially enriched in cells bearing stem cell like signatures and bear further investigation. We identified the transcription factor E2F1 as a key regulator of tumor cell proliferation and self-renewal in only a subset of gliomasphere cultures predicted to be E2F1 signaling dependent. Our in vivo work also validated the functional significance of E2F1 in tumor formation capacity in the predicted samples. E2F1 inhibition also differentially sensitized E2F1-dependent gliomasphere cultures to radiation treatment. Our findings indicate that this novel approach exploring cancer pathways highlights key therapeutic vulnerabilities for targeting GBM.Significance: Molecular classification of GBM has not yet resulted in the development of effective therapies. We have developed an integrative approach to identify molecular targets differentially utilized by individual tumors. This approach could lead to patient- and tumor-specific therapeutics.

One of the hallmarks of glioblastoma (GBM) is extensive neovascularization. In addition to supplying blood and nutrients, vascular endothelial (VE) cells provide trophic support to GBM cells via paracrine signaling, the precise mechanisms of which are being unraveled. Here, using patient-derived GBM and VE cells as well as orthotopic GBM mouse models, we report that Endocan ( ESM1 ), an endothelial-secreted proteoglycan, confers enhanced proliferative, migratory, and angiogenic properties to GBM cells and regulates their spatial identity. Mechanistically, Endocan exerts at least part of its functions via direct binding and activation of the PDGFRA receptor. Subsequent downstream signaling enhances chromatin accessibility of the Myc promoter and upregulates Myc expression inducing highly stable phenotypic changes in GBM cells. Furthermore, Endocan confers a radioprotection phenotype in GBM cells, both in vitro and in vivo . Inhibition of Endocan-PDGFRA signaling with ponatinib increases survival in the Esm1 wild-type but not in the Esm1 knock-out mouse GBM model. Our findings identify Endocan and its downstream signaling axis as a potential target to subdue the recurrence of GBM and further highlight the importance of vascular to tumor cell signaling for GBM biology. Significance statement: Identification of the Endocan/PDGFRA/Myc axis demonstrates an important role of VE cells in GBM malignancy. The contribution of Endocan to the development of GBM cell populations with different phenotypes reveal an additional pathway underlying the origin of GBM intratumoral heterogeneity. Targeting Endocan-mediated crosstalk may enhance the efficacy of GBM treatment.