TREM2 inhibition triggers antitumor cell activity of myeloid cells in glioblastoma.

Triggering receptor expressed on myeloid cells 2 (TREM2) plays important roles in brain microglial function in neurodegenerative diseases, but the role of TREM2 in the GBM TME has not been examined. Here, we found that TREM2 is highly expressed in myeloid subsets, including macrophages and microglia in human and mouse GBM tumors and that high TREM2 expression correlates with poor prognosis in patients with GBM. TREM2 loss of function in human macrophages and mouse myeloid cells increased interferon-γ-induced immunoactivation, proinflammatory polarization, and tumoricidal capacity. In orthotopic mouse GBM models, mice with chronic and acute Trem2 loss of function exhibited decreased tumor growth and increased survival. Trem2 inhibition reprogrammed myeloid phenotypes and increased programmed cell death protein 1 (PD-1)+CD8+ T cells in the TME. Last, Trem2 deficiency enhanced the effectiveness of anti-PD-1 treatment, which may represent a therapeutic strategy for patients with GBM.

Gonadal sex patterns p21-induced cellular senescence in mouse and human glioblastoma.

Males exhibit higher incidence and worse prognosis for the majority of cancers, including glioblastoma (GBM). Disparate survival may be related to sex-biased responses to treatment, including radiation. Using a mouse model of GBM, we show that female cells are more sensitive to radiation, and that senescence represents a major component of the radiation therapeutic response in both sexes. Correlation analyses revealed that the CDK inhibitor p21 and irradiation induced senescence were differentially regulated between male and female cells. Indeed, female cellular senescence was more sensitive to changes in p21 levels, a finding that was observed in wildtype and transformed murine astrocytes, as well as patient-derived GBM cell lines. Using a novel Four Core Genotypes model of GBM, we further show that sex differences in p21-induced senescence are patterned during early development by gonadal sex. These data provide a rationale for the further study of sex differences in radiation response and how senescence might be enhanced for radiation sensitization. The determination that p21 and gonadal sex are required for sex differences in radiation response will serve as a foundation for these future mechanistic studies.

CDC20 regulates sensitivity to chemotherapy and radiation in glioblastoma stem cells.

Glioblastoma stem cells (GSCs) are an important subpopulation in glioblastoma, implicated in tumor growth, tumor recurrence, and radiation resistance. Understanding the cellular mechanisms for chemo- and radiation resistance could lead to the development of new therapeutic strategies. Here, we demonstrate that CDC20 promotes resistance to chemotherapy and radiation therapy. CDC20 knockdown does not increase TMZ- and radiation-induced DNA damage, or alter DNA damage repair, but rather promotes cell death through accumulation of the pro-apoptotic protein, Bim. Our results identify a CDC20 signaling pathway that regulates chemo- and radiosensitivity in GSCs, with the potential for CDC20-targeted therapeutic strategies in the treatment of glioblastoma.

Competitive binding of E3 ligases TRIM26 and WWP2 controls SOX2 in glioblastoma.

The pluripotency transcription factor SOX2 is essential for the maintenance of glioblastoma stem cells (GSC), which are thought to underlie tumor growth, treatment resistance, and recurrence. To understand how SOX2 is regulated in GSCs, we utilized a proteomic approach and identified the E3 ubiquitin ligase TRIM26 as a direct SOX2-interacting protein. Unexpectedly, we found TRIM26 depletion decreased SOX2 protein levels and increased SOX2 polyubiquitination in patient-derived GSCs, suggesting TRIM26 promotes SOX2 protein stability. Accordingly, TRIM26 knockdown disrupted the SOX2 gene network and inhibited both self-renewal capacity as well as in vivo tumorigenicity in multiple GSC lines. Mechanistically, we found TRIM26, via its C-terminal PRYSPRY domain, but independent of its RING domain, stabilizes SOX2 protein by directly inhibiting the interaction of SOX2 with WWP2, which we identify as a bona fide SOX2 E3 ligase in GSCs. Our work identifies E3 ligase competition as a critical mechanism of SOX2 regulation, with functional consequences for GSC identity and maintenance.

Defining phenotypic and functional heterogeneity of glioblastoma stem cells by mass cytometry.

Most patients with glioblastoma (GBM) die within 2 years. A major therapeutic goal is to target GBM stem cells (GSCs), a subpopulation of cells that contribute to treatment resistance and recurrence. Since their discovery in 2003, GSCs have been isolated using single-surface markers, such as CD15, CD44, CD133, and α6 integrin. It remains unknown how these single-surface marker-defined GSC populations compare with each other in terms of signaling and function and whether expression of different combinations of these markers is associated with different functional capacity. Using mass cytometry and fresh operating room specimens, we found 15 distinct GSC subpopulations in patients, and they differed in their MEK/ERK, WNT, and AKT pathway activation status. Once in culture, some subpopulations were lost and previously undetectable ones materialized. GSCs that highly expressed all 4 surface markers had the greatest self-renewal capacity, WNT inhibitor sensitivity, and in vivo tumorigenicity. This work highlights the potential signaling and phenotypic diversity of GSCs. Larger patient sample sizes and antibody panels are required to confirm these findings.

Diffusion Histology Imaging Combining Diffusion Basis Spectrum Imaging (DBSI) and Machine Learning Improves Detection and Classification of Glioblastoma Pathology.

PURPOSE: Glioblastoma (GBM) is one of the deadliest cancers with no cure. While conventional MRI has been widely adopted to examine GBM clinically, accurate neuroimaging assessment of tumor histopathology for improved diagnosis, surgical planning, and treatment evaluation remains an unmet need in the clinical management of GBMs. EXPERIMENTAL DESIGN: We employ a novel diffusion histology imaging (DHI) approach, combining diffusion basis spectrum imaging (DBSI) and machine learning, to detect, differentiate, and quantify areas of high cellularity, tumor necrosis, and tumor infiltration in GBM. RESULTS: Gadolinium-enhanced T1-weighted or hyperintense fluid-attenuated inversion recovery failed to reflect the morphologic complexity underlying tumor in patients with GBM. Contrary to the conventional wisdom that apparent diffusion coefficient (ADC) negatively correlates with increased tumor cellularity, we demonstrate disagreement between ADC and histologically confirmed tumor cellularity in GBM specimens, whereas DBSI-derived restricted isotropic diffusion fraction positively correlated with tumor cellularity in the same specimens. By incorporating DBSI metrics as classifiers for a supervised machine learning algorithm, we accurately predicted high tumor cellularity, tumor necrosis, and tumor infiltration with 87.5%, 89.0%, and 93.4% accuracy, respectively. CONCLUSIONS: Our results suggest that DHI could serve as a favorable alternative to current neuroimaging techniques in guiding biopsy or surgery as well as monitoring therapeutic response in the treatment of GBM.

GATA2 Regulates Constitutive PD-L1 and PD-L2 Expression in Brain Tumors.

Encouraging clinical results using immune checkpoint therapies to target the PD-1 axis in a variety of cancer types have paved the way for new immune therapy trials in brain tumor patients. However, the molecular mechanisms that regulate expression of the PD-1 pathway ligands, PD-L1 and PD-L2, remain poorly understood. To address this, we explored the cell-intrinsic mechanisms of constitutive PD-L1 and PD-L2 expression in brain tumors. PD-L1 and PD-L2 expression was assessed by flow cytometry and qRT-PCR in brain tumor cell lines and patient tumor-derived brain tumor-initiating cells (BTICs). Immunologic effects of PD-L2 overexpression were evaluated by IFN-γ ELISPOT. CD274 and PDCD1LG2 cis-regulatory regions were cloned from genomic DNA and assessed in full or by mutating and/or deleting regulatory elements by luciferase assays. Correlations between clinical responses and PD-L1 and PD-L2 expression status were evaluated in TCGA datasets in LGG and GBM patients. We found that a subset of brain tumor cell lines and BTICs expressed high constitutive levels of PD-L1 and PD-L2 and that PD-L2 overexpression inhibited neoantigen specific T cell IFN-γ production. Characterization of novel cis-regulatory regions in CD274 and PDCD1LG2 lead us to identify that GATA2 is sufficient to drive PD-L1 and PD-L2 expression and is necessary for PD-L2 expression. Importantly, in TCGA datasets, PD-L2 correlated with worse clinical outcomes in glioma patients.. By perturbing GATA2 biology, targeted therapies may be useful to decrease inhibitory effects of PD-L2 in the microenvironment.

Establishing Primary Human Glioblastoma Adherent Cultures from Operative Specimens.

This chapter describes a method for isolation, maintenance, and propagation of primary glioblastoma (GBM) cells in adherent monolayer cultures from patient tumor specimens. This method enables the establishment of GBM cultures with stem or progenitor-like cell characteristics, including self-renewal capacity, differentiation along restricted neural lineages, and tumor-initiating potential when orthotopically injected into immunocompromised mice. This experimentally tractable model system is therefore suitable for a wide variety of analyses in vitro as well as in vivo. Key examples of biological analyses that can be performed using these cells are also described.

Biological and therapeutic implications of multisector sequencing in newly diagnosed glioblastoma.

Background: Diagnostic workflows for glioblastoma (GBM) patients increasingly include DNA sequencing-based analysis of a single tumor site following biopsy or resection. We hypothesized that sequencing of multiple sectors within a given tumor would provide a more comprehensive representation of the molecular landscape and potentially inform therapeutic strategies. Methods: Ten newly diagnosed, isocitrate dehydrogenase 1 (IDH1) wildtype GBM tumor samples were obtained from 2 (n = 9) or 4 (n = 1) spatially distinct tumor regions. Tumor and matched blood DNA samples underwent whole-exome sequencing. Results: Across all 10 tumors, 51% of mutations were clonal and 3% were subclonal and shared in different sectors, whereas 46% of mutations were subclonal and private. Two of the 10 tumors exhibited a regional hypermutator state despite being treatment naïve, and remarkably, the high mutational load was predominantly limited to one sector in each tumor. Among the canonical cancer-associated genes, only telomerase reverse transcriptase (TERT) promoter mutations were observed in the founding clone in all tumors. Reconstruction of the clonal architecture in different sectors revealed regionally divergent evolution, and integration of data from 2 sectors increased the resolution of inferred clonal architecture in a given tumor. Predicted therapeutic mutations differed in presence and frequency between tumor regions. Similarly, different sectors exhibited significant divergence in the predicted neoantigen landscape. Conclusions: The substantial spatial heterogeneity observed in different GBM tumor sectors, especially in spatially restricted hypermutator cases, raises important caveats to our current dependence on single-sector molecular information to guide either targeted or immune-based treatments.

An NAD+-dependent transcriptional program governs self-renewal and radiation resistance in glioblastoma.

Accumulating evidence suggests cancer cells exhibit a dependency on metabolic pathways regulated by nicotinamide adenine dinucleotide (NAD+). Nevertheless, how the regulation of this metabolic cofactor interfaces with signal transduction networks remains poorly understood in glioblastoma. Here, we report nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting step in NAD+ synthesis, is highly expressed in glioblastoma tumors and patient-derived glioblastoma stem-like cells (GSCs). High NAMPT expression in tumors correlates with decreased patient survival. Pharmacological and genetic inhibition of NAMPT decreased NAD+ levels and GSC self-renewal capacity, and NAMPT knockdown inhibited the in vivo tumorigenicity of GSCs. Regulatory network analysis of RNA sequencing data using GSCs treated with NAMPT inhibitor identified transcription factor E2F2 as the center of a transcriptional hub in the NAD+-dependent network. Accordingly, we demonstrate E2F2 is required for GSC self-renewal. Downstream, E2F2 drives the transcription of members of the inhibitor of differentiation (ID) helix-loop-helix gene family. Finally, we find NAMPT mediates GSC radiation resistance. The identification of a NAMPT-E2F2-ID axis establishes a link between NAD+ metabolism and a self-renewal transcriptional program in glioblastoma, with therapeutic implications for this formidable cancer.

High incidence of TERT mutation in brain tumor cell lines.

TERT promoter gene mutations are highly recurrent in malignant glioma. However, little information exists regarding their presence in experimental brain tumor models. To better characterize systems in which TERT mutation studies could be appropriately modeled experimentally, the TERT promoter was examined by conventional sequencing in primary brain tumor initiating cells (BTIC), two matched recurrent BTIC lines, a panel of established malignant glioma cell lines, and two meningioma cell lines. Telomerase gene expression was examined by quantitative PCR. We found that all glioblastoma BTIC lines harbored a TERT mutation, which was retained in two patient-matched recurrent BTIC. The TERT C228T or C250T mutation was found in 33/35 (94 %) of established malignant glioma cell lines and both meningioma cell lines examined. Brain tumor cell lines expressed variably high telomerase levels. Thus, a high percentage of glioma cell lines, as well as two meningioma cell lines, harbors TERT mutations. These data characterize tractable, accessible models with which to further explore telomerase biology in these tumor types.

A CDC20-APC/SOX2 Signaling Axis Regulates Human Glioblastoma Stem-like Cells.

Glioblastoma harbors a dynamic subpopulation of glioblastoma stem-like cells (GSCs) that can propagate tumors in vivo and is resistant to standard chemoradiation. Identification of the cell-intrinsic mechanisms governing this clinically important cell state may lead to the discovery of therapeutic strategies for this challenging malignancy. Here, we demonstrate that the mitotic E3 ubiquitin ligase CDC20-anaphase-promoting complex (CDC20-APC) drives invasiveness and self-renewal in patient tumor-derived GSCs. Moreover, CDC20 knockdown inhibited and CDC20 overexpression increased the ability of human GSCs to generate brain tumors in an orthotopic xenograft model in vivo. CDC20-APC control of GSC invasion and self-renewal operates through pluripotency-related transcription factor SOX2. Our results identify a CDC20-APC/SOX2 signaling axis that controls key biological properties of GSCs, with implications for CDC20-APC-targeted strategies in the treatment of glioblastoma.

PDE7B is a novel, prognostically significant mediator of glioblastoma growth whose expression is regulated by endothelial cells.

Cell-cell interactions between tumor cells and constituents of their microenvironment are critical determinants of tumor tissue biology and therapeutic responses. Interactions between glioblastoma (GBM) cells and endothelial cells (ECs) establish a purported cancer stem cell niche. We hypothesized that genes regulated by these interactions would be important, particularly as therapeutic targets. Using a computational approach, we deconvoluted expression data from a mixed physical co-culture of GBM cells and ECs and identified a previously undescribed upregulation of the cAMP specific phosphodiesterase PDE7B in GBM cells in response to direct contact with ECs. We further found that elevated PDE7B expression occurs in most GBM cases and has a negative effect on survival. PDE7B overexpression resulted in the expansion of a stem-like cell subpopulation in vitro and increased tumor growth and aggressiveness in an in vivo intracranial GBM model. Collectively these studies illustrate a novel approach for studying cell-cell interactions and identifying new therapeutic targets like PDE7B in GBM.