Normal Neural Stem Cells Research Articles

Interferon-beta inhibits human glioma stem cell growth by modulating immune response and cell cycle related signaling pathways

Malignant Glioma is characterized by strong self-renewal potential and immature differentiation potential. The main reason is that malignant glioma holds key cluster cells, glioma stem cells (GSCs). GSCs contribute to tumorigenesis, tumor progression, recurrence, and treatment resistance. Interferon-beta (IFN-β) is well known for its anti-proliferative efficacy in diverse cancers. IFN-β also displayed potent antitumor effects in malignant glioma. IFN-β affect both GSCs and Neural stem cells (NSCs) in the treatment of gliomas. However, the functional comparison, similar or different effects of IFN-β on GSCs and NSCs are rarely reported. Here, we studied the similarities and differences of the responses to IFN-β between human GSCs and normal NSCs. We found that IFN-β preferentially inhibited GSCs over NSCs. The cell body and nucleus size of GSCs increased after IFN-β treatment, and the genomic analysis revealed the enrichment of the upregulated immune response, cell adhesion genes and down regulated cell cycle, ribosome pathways. Several typical cyclin genes, including cyclin A2 (CCNA2), cyclin B1 (CCNB1), cyclin B2 (CCNB2), and cyclin D1 (CCND1), were significantly downregulated in GSCs after IFN-β stimulation. We also found that continuous IFN-β stimulation after passage further enhanced the inhibitory effect. Our study revealed how genetic diversity resulted in differential effects in response to IFN-β treatment. These results may contribute to improve the applications of IFN-β in anti-cancer immunotherapy. In addition, these results may also help to design more effective pharmacological strategies to target cancer stem cells while protecting normal neural stem cells.

Crumbs complex-directed apical membrane dynamics in epithelial cell ingression.

Epithelial cells often leave their tissue context and ingress to form new cell types or acquire migratory ability to move to distant sites during development and tumor progression. Cells lose their apical membrane and epithelial adherens junctions during ingression. However, how factors that organize apical-basal polarity contribute to ingression is unknown. Here, we show that the dynamic regulation of the apical Crumbs polarity complex is crucial for normal neural stem cell ingression. Crumbs endocytosis and recycling allow ingression to occur in a normal timeframe. During early ingression, Crumbs and its complex partner the RhoGEF Cysts support myosin and apical constriction to ensure robust ingression dynamics. During late ingression, the E3-ubiquitin ligase Neuralized facilitates the disassembly of the Crumbs complex and the rapid endocytic removal of the apical cell domain. Our findings reveal a mechanism integrating cell fate, apical polarity, endocytosis, vesicle trafficking, and actomyosin contractility to promote cell ingression, a fundamental morphogenetic process observed in animal development and cancer.

Subventricular zone adult mouse neural stem cells require insulin receptor forself-renewal.

SummaryThe insulin receptor (INSR) is an evolutionarily conserved signaling protein that regulates development and cellular metabolism. INSR signaling promotes neurogenesis in Drosophila; however, a specific role for the INSR in maintaining adult neural stem cells (NSCs) in mammals has not been investigated. We show that conditionally deleting the Insr gene in adult mouse NSCs reduces subventricular zone NSCs by ∼70% accompanied by a corresponding increase in progenitors. Insr deletion also produced hyposmia caused by aberrant olfactory bulb neurogenesis. Interestingly, hippocampal neurogenesis and hippocampal-dependent behaviors were unperturbed. Highly aggressive proneural and mesenchymal glioblastomas had high INSR/insulin-like growth factor (IGF) pathway gene expression, and isolated glioma stem cells had an aberrantly high ratio of INSR:IGF type 1 receptor. Moreover, INSR knockdown inhibited GBM tumorsphere growth. Altogether, these data demonstrate that the INSR is essential for a subset of normal NSCs, as well as for brain tumor stem cell self-renewal.

ADAR1-mediated RNA editing links ganglioside catabolism to glioblastoma stem cell maintenance.

Glioblastoma (GBM) is the most common and lethal primary malignant brain tumor, containing GBM stem cells (GSCs) that contribute to therapeutic resistance and relapse. Exposing potential GSC vulnerabilities may provide therapeutic strategies against GBM. Here, we interrogated the role of adenosine-to-inosine (A-to-I) RNA editing mediated by adenosine deaminase acting on RNA 1 (ADAR1) in GSCs and found that both ADAR1 and global RNA editomes were elevated in GSCs compared with normal neural stem cells. ADAR1 inactivation or blocking of the upstream JAK/STAT pathway through TYK2 inhibition impaired GSC self-renewal and stemness. Downstream of ADAR1, RNA editing of the 3′-UTR of GM2A, a key ganglioside catabolism activator, proved to be critical, as interference with ganglioside catabolism and disruption of ADAR1 showed a similar functional impact on GSCs. These findings reveal that RNA editing links ganglioside catabolism to GSC self-renewal and stemness, exposing a potential vulnerability of GBM for therapeutic intervention.

Single Cell RNA Analysis of Glioma Neural Stem Cells Shows Disturbed Amino Acids Metabolism and Axonal Growth Cone Dynamics in Glioblastoma Multiforme

Glioblastoma multiforme(GBM) is a group of aggressive tumors of the central nervous system. Despite advancements in the treatment of GBM, patients diagnosed with these tumors typically have a poor prognosis and poor quality of life as the disease develops. The single-cell RNA high-throughput sequencing processed data for Glioma cancer stem cells were taken from GEO and analyzed to find out the underlying expression differences at the gene level between glioma neural stem cells(GSCs) and Normal neural stem cells(NSCs). In the current study, we have performed an RNA-sequencing analysis between GSCs and NSCs to better understand the origin of GBM. We have performed bioinformatics analysis on the transcriptional profile of 134 samples which consisted of 75 GSCs and 59 NSCs obtained from the NCBI bio project(PRJNA546254). First, an exploratory analysis was performed which showed significant variation patterns between GSCs and NSCs. Subsequently, Deseq2 differential gene expression analysis identified 1436 differentially expressed genes be-tween GSCs and NSCs[(padj. value <0.05, log2 fold change (>=+/-1.5)]. This study reveals genes like MAOA, MAOB, GATM, GLDC, AMT, and SHMT1 as the key features contributing to the disturbed processes of Glycine, threonine, and serine amino acid metabolism, axonal cone growth curve, and cell migration in Glioma. Conclusively, our study also depicts gene expression changes in amyloid beta-binding protein in between GSCs and NSCs which plays an important role in tumor microenvironment formation. Besides, the results presented here reveal new insight into the progression of GBM and the identification of novel genes involved in gliomagenesis.

Repression of differentiation genes by Hes transcription factors fuels neural tumour growth in Drosophila.

Neural stem cells (NSC) in divide asymmetrically to generate one cell that retains stem cell identity and another that is routed to differentiation. Prolonged mitotic activity of the NSCs gives rise to the plethora of neurons and glial cells that wire the brain and nerve cord. Genetic insults, such as excess of Notch signaling, perturb the normal NSC proliferation programs and trigger the formation of NSC hyperplasias, which can subsequently progress to malignancies. Hes proteins are crucial mediators of Notch signaling, and in the NSC context they act by repressing a cohort of early pro-differentiation transcription factors. Downregulation of these pro-differentiation factors makes NSC progeny cells susceptible to adopting an aberrant stem cell program. We have recently shown that Hes overexpression in Drosophila leads to NSC hyperplasias that progress to malignant tumours after allografting to adult hosts. We have combined genetic analysis, tissue allografting and transcriptomic approaches to address the role of Hes genes in NSC malignant transformation. We show that the E (spl) genes are important mediators in the progression of Notch hyperplasias to malignancy, since allografts lacking the E (spl) genes grow much more slowly. We further present RNA profiling of Hes-induced tumours at two different stages after allografting. We find that the same cohort of differentiation-promoting transcription factors that are repressed in the primary hyperplasias continue to be downregulated after transplantation. This is accompanied by an upregulation of stress-response genes and metabolic reprogramming. The combination of dedifferentiation and cell physiology changes most likely drive tumour growth.

STEM-10. UPSTREAM REGULATORS OF THE GLIOMA STEM CELL PHENOTYPE

Abstract Glioblastoma (GBM) is the most common and deadly primary brain tumor in adults. Recurrence of the disease is attributed in part to the presence of Glioma Stem Cells (GSC), which are resistant to chemo- and radiotherapy and can initiate tumor formation. Molecularly, GSCs resemble the mesenchymal subtype that is associated with worse prognosis. GSCs share many characteristics with Neural Stem Cells (NSCs) including proliferative potential, migratory capacity, telomerase activity, diverse progeny and similar gene signature, however differ fundamentally from NSCs in their tumor forming ability. RNA-Seq analysis of GSCs and NSCs illustrates significant enrichment in GSCs of transcription factors (TFs) known to be dysregulated in cancer, chief among them being SIX1, a developmental TF with documented roles in progression of multiple cancers. Overexpression of SIX1 in A172 GBM cells enhances proliferation, promotes resistance to radiotherapy and alters expression of a core set of 4 developmental TFs (POU3F2, SALL2, OLIG2 and SOX2) capable to reprogram differentiated GBM cells into GSCs (Suva et al., 2014). Analysis of SIX1 in surgical samples from glioma patients illustrates that high expression of SIX1 correlates with tumor grade, finding corroborated in the TCGA and CGGA data sets. Surprisingly, primary NSCs, after extended time in culture, increase their proliferation rate, acquire a mesenchymal transcriptional signature akin to GSCs, including high expression of SIX1, and form deadly tumors when implanted into the brains of mice. Comparing the epigenetic landscape of transformed and normal NSCs we identify a significant enrichment of accessible chromatin at promoter and enhancer loci in transformed NSCs, including at regulatory regions of SIX1 and of genes that define mesenchymal GBM. These data suggest that SIX1 may represent an upstream regulator of the GSC phenotype and may drive malignant transformation of NSCs. Genetic and epigenetic loss of function analyses are ongoing to test this hypothesis.

P04.19 A combined use of a G-quadruplex oligonucleotide (GQ) and neuro-inducing small molecules inhibit glioblastoma cell proliferation

Abstract BACKGROUND According to one of the theories, gliomas can occur as a result of dysregulation of stem cell division in the subventricular region of the brain. The CD133 membrane marker is a characteristic of both normal and tumor neural stem cells therefore it can be used to isolate a stem cell population from tumor tissue. Tumor cells actively proliferate which suggests that their possible differentiation may be achieved by inhibiting of their division as these two processes are mutually exclusive. For this purpose, G-quadruplex oligonucleotides together with neural-inducers such as a brain-derived neurotrophic factor (BDNF) may be used. MATERIAL AND METHODS Five cell cultures obtained from human glioblastoma tissues were analyzed for expression of CD133 using RT-qPCR. From cell culture with the highest level of CD133 using immunomagnetic separation CD133+ and CD133- cultures were received. CD133fr/peGFP-c1 recombinant DNA consisted of a CD133 second extracellular loop fragment and a peGFP-c1 vector was constructed to determine the localization of prominin-1, that is known as CD133 when found on cell membrane, using confocal microscopy. On chosen cell cultures an oligonucleotide bi-(AID-1-T) and its combination with BDNF were tested. The mechanism of GQ’s action is cytostatic and its non-toxicity properties were proved by flow cytometry. For evaluating the proliferative activity of cells MTT assay was performed on 10th and 20th days after exposure to the factors. RESULTS Cell culture G01 was chosen for further research as it had the highest level of the CD133. Colocalization of CD133 and GFP demonstrated a membrane localization of CD133 in cells with high expression level of this marker. MTT assay on 10th day after exposure to bi-(AID-1-T) as well as its combination with BDNF on cell culture G01 CD133- showed total inhibition of cell proliferation. The same combinations tested on G01 CD133+ cell culture demonstrated no difference in proliferative activity. After 20 days after exposure to bi-(AID-1-T) and combination of bi-(AID-1-T) with BDNF the significant decrease of G01 CD133+ cells’ proliferation was observed. When tested on whole glioblastoma cell culture G01 these combinations also showed significant inhibition of cell proliferation. CONCLUSION We showed that glioblastoma cells upon transfection with recombinant DNA, that contains a fragment of CD133, mainly have a membrane localization of this marker. It was observed that CD133+ cells are more stable to external influence that can be a proof of the fact that CD133 is charactered for glioblastoma stem cells. We tested the effect of an GQ bi-(AID-1-T) and its combination with BDNF and showed that BDNF is necessary for blocking proliferation of glioblastoma cells. Altogether, the results may be used for further research as it reveals a potential treatment for patients with glioblastoma. Grant №075-15-2020-809 (13.1902.21.0030).

Anticarin β Inhibits Human Glioma Progression by Suppressing Cancer Stemness via STAT3.

Glioma is the most common form of malignant brain cancer. It is very difficult to cure malignant glioma because of the presence of glioma stem cells, which are a barrier to cure, have high tumorigenesis, associated with drug resistance, and responsible for relapse by regulating stemness genes. In this study, our results demonstrated that anticarin β, a natural compound from Antiaris toxicaria, can effectively and selectively suppress proliferation and cause apoptosis in glioma cells, which has an IC50 that is 100 times lower than that in mouse normal neural stem cells. Importantly, cell sphere formation assay and real time-quantitative analysis reveal that anticarin β inhibits cancer stemness by modulating related stemness gene expression. Additionally, anticarin β induces DNA damage to regulate the oncogene expression of signal transducer and activator of transcription 3 (STAT3), Akt, mitogen-activated protein kinases (MAPKs), and eventually leading to apoptosis. Furthermore, anticarin β effectively inhibits glioma growth and prolongs the lifts pan of tumor-bearing mice without systemic toxicity in the orthotopic xenograft mice model. These results suggest that anticarin β is a promising candidate inhibitor for malignant glioma.

A Fast and Efficient Approach to Obtaining High-Purity Glioma Stem Cell Culture.

Glioma is the most common and malignant primary brain tumor. Patients with malignant glioma usually have a poor prognosis due to drug resistance and disease relapse. Cancer stem cells contribute to glioma initiation, progression, resistance, and relapse. Hence, quick identification and efficient understanding of glioma stem cells (GSCs) are of profound importance for therapeutic strategies and outcomes. Ideally, therapeutic approaches will only kill cancer stem cells without harming normal neural stem cells (NSCs) that can inhibit GSCs and are often beneficial. It is key to identify the differences between cancer stem cells and normal NSCs. However, reports detailing an efficient and uniform protocol are scarce, as are comparisons between normal neural and cancer stem cells. Here, we compared different protocols and developed a fast and efficient approach to obtaining high-purity glioma stem cell by tracking observation and optimizing culture conditions. We examined the proliferative and differentiative properties confirming the identities of the GSCs with relevant markers such as Ki67, SRY-box containing gene 2, an intermediate filament protein member nestin, glial fibrillary acidic protein, and s100 calcium-binding protein (s100-beta). Finally, we identified distinct expression differences between GSCs and normal NSCs including cyclin-dependent kinase 4 and tumor protein p53. This study comprehensively describes the features of GSCs, their properties, and regulatory genes with expression differences between them and normal stem cells. Effective approaches to quickly obtaining high-quality GSCs from patients should have the potential to not only help understand the diseases and the resistances but also enable target drug screening and personalized medicine for brain tumor treatment.

Abstract 2947: Establishment and characterization of a series of murine cancer homograft models derived from induced cancer stem cells (iCSCs)

Abstract Background: Both leukemia and solid tumors show considerable heterogeneity at histological and functional levels, as well as in their responses to therapies. For many years, genetic lesions during clonal evolution were considered to be responsible for such heterogeneity. However, recent studies suggest a new paradigm - that tumors show hierarchy in which cancer stem cells (CSCs) have the ability to self-renew and give rise to differentiated tumor cells, and are responsible for the overall organization of a tumor. CSCs have important roles in tumor development, relapse and metastasis due to their intrinsic self-renewal characteristics and tumorigenic properties. Identification and characterization of these crucial cells will allow earlier detection of malignancies and better prediction of tumor behavior, and ultimately lead to preventive and therapeutic clinical strategies against CSCs. Murine tumor models derived from CSCs provide a valuable tool for research of tumor cells derived from distinct cells of origin. Methods: We have generated a series of murine cancer homograft models by retroviral transduction of proto-oncogenes Myc (N-Myc or c-Myc) into hematopoietic stem cells, progenitor B cells or bone marrow stromal cells isolated from wild-type or Ink4a/Arf knockout (Ink4a/Arf KO) mice, and overexpression of H-RasV12 in normal neural stem cells isolated from Ink4a/Arf KO mice. Models were developed by transplanting tumor cells derived from distinct iCSCs into recipient animals. Results: Five murine cancer homograft models were successfully developed, including one precursor B acute lymphoblastic leukemia/lymphoma (pre-B ALL/LBL) model established through overexpression of N-Myc in hematopoietic stem cells isolated from wild-type C57BL/6 mice; two pre-B ALL/LBL models established through overexpression of c-Myc in progenitor B cells isolated from Ink4a/Arf KO C57BL/6 mice; one osteosarcoma established through overexpression of c-Myc in bone marrow stromal cells isolated from Ink4a/Arf KO C57BL/6 mice; and one glioblastoma model established through overexpression of H-RasV12 in normal neural stem cells isolated from Ink4a/Arf KO C57BL/6 mice. Characterization of in vivo growth, histopathology, immunohistochemistry, immune profiling and responses to immune, cytotoxic and targeted therapies were performed. The results indicated that tumors derived from distinct iCSCs shared phenotypic and functional characteristics with the corresponding human tumors. Conclusions: We generated and characterized a series of murine cancer homograft models derived from distinct iCSCs for elucidating detailed mechanisms of tumorigenesis and for preclinical evaluation of novel therapeutic strategies for cancer eradication. Citation Format: Jessie J. Wang, Likun Zhang, Annie X. An, Henry Q.X. Li, Davy Xuesong Ouyang. Establishment and characterization of a series of murine cancer homograft models derived from induced cancer stem cells (iCSCs) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2947.

RARE-22. THERAPEUTIC TARGETING OF PURINE METABOLISM IN DIPG

Diffuse intrinsic pontine glioma is a universally lethal disease primarily impacting pediatric patients. There are currently no targeted therapies increasing overall for patients with these tumors; therefore, our lab set out to elucidate metabolic dependencies in DIPG patient-derived cell lines with the ultimate goal of identifying novel therapeutic targets. Through untargeted metabolomics and gene expression analyses, we have identified the purine metabolism gene ATIC to be important for DIPG tumor cell survival and proliferation. Anti-folate drugs such as methotrexate target de novo purine biosynthesis and are used to treat other pediatric cancers; however, we have identified a small molecule inhibitor of ATIC that may offer clinical benefits over other inhibitors of this pathway. In vitro cell viability experiments have demonstrated DIPG cell lines are much more sensitive to the ATIC inhibitor relative to normal neural stem cells and glial cell lines. Furthermore, we have started in vivo studies on pre-clinical mouse models of DIPG with promising results. Treatment with the ATIC inhibitor has significantly increased overall survival relative to control and vehicle treated mice. The dosage we started at was well tolerated in these mice so we are following up on this in vivo work through dose-escalation studies as well as combination treatment strategies. Mechanistically, the ATIC inhibitor works differently than anti-folate compounds such as methotrexate; therefore we are also elucidating why cancer cells are much more sensitive to this compound.

Aberrations of Genomic Imprinting in Glioblastoma Formation.

In human glioblastoma (GBM), the presence of a small population of cells with stem cell characteristics, the glioma stem cells (GSCs), has been described. These cells have GBM potential and are responsible for the origin of the tumors. However, whether GSCs originate from normal neural stem cells (NSCs) as a consequence of genetic and epigenetic changes and/or dedifferentiation from somatic cells remains to be investigated. Genomic imprinting is an epigenetic marking process that causes genes to be expressed depending on their parental origin. The dysregulation of the imprinting pattern or the loss of genomic imprinting (LOI) have been described in different tumors including GBM, being one of the earliest and most common events that occurs in human cancers. Here we have gathered the current knowledge of the role of imprinted genes in normal NSCs function and how the imprinting process is altered in human GBM. We also review the changes at particular imprinted loci that might be involved in the development of the tumor. Understanding the mechanistic similarities in the regulation of genomic imprinting between normal NSCs and GBM cells will be helpful to identify molecular players that might be involved in the development of human GBM.

The RNA m6A Reader YTHDF2 Maintains Oncogene Expression and Is a Targetable Dependency in Glioblastoma Stem Cells

Glioblastoma is a universally lethal cancer driven by glioblastoma stem cells (GSC). Here, we interrogated N 6-methyladenosine (m6A) mRNA modifications in GSCs by methyl RNA immunoprecipitation followed by sequencing and transcriptome analysis, finding transcripts marked by m6A often upregulated compared with normal neural stem cells (NSC). Interrogating m6A regulators, GSCs displayed preferential expression, as well as in vitro and in vivo dependency, of the m6A reader YTHDF2, in contrast to NSCs. Although YTHDF2 has been reported to destabilize mRNAs, YTHDF2 stabilized MYC and VEGFA transcripts in GSCs in an m6A-dependent manner. We identified IGFBP3 as a downstream effector of the YTHDF2-MYC axis in GSCs. The IGF1/IGF1R inhibitor linsitinib preferentially targeted YTHDF2-expressing cells, inhibiting GSC viability without affecting NSCs and impairing in vivo glioblastoma growth. Thus, YTHDF2 links RNA epitranscriptomic modifications and GSC growth, laying the foundation for the YTHDF2-MYC-IGFBP3 axis as a specific and novel therapeutic target in glioblastoma. SIGNIFICANCE: Epitranscriptomics promotes cellular heterogeneity in cancer. RNA m6A landscapes of cancer and NSCs identified cell type-specific dependencies and therapeutic vulnerabilities. The m6A reader YTHDF2 stabilized MYC mRNA specifically in cancer stem cells. Given the challenge of targeting MYC, YTHDF2 presents a therapeutic target to perturb MYC signaling in glioblastoma.This article is highlighted in the In This Issue feature, p. 211.

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MBRS-31. COMBINING IRRADIATION AND ANTI-CD47 TO ENHANCE THE TREATMENT OF GROUP 3 MEDULLOBLASTOMA

Medulloblastoma (MB) is the most common malignant primary pediatric brain tumor. The Group 3 molecular subgroup of Medulloblastoma (Group 3 MB) is the deadliest with only 30% long term survival. Irradiation for Group 3 Medulloblastoma is required for long term survival of children. Methods to enhance the effect of irradiation against Group 3 MB are an active area of investigation. Immunotherapy using the anti-CD47 treatment has shown promise in treating Group 3 MB. We recently demonstrated that irradiation significantly enhanced anti-CD47-mediated phagocytosis of high-grade glioma cells in vitro. Furthermore, mice engrafted with human high-grade glioma that received anti-CD47 combined with irradiation showed a significant increase in the survival rate and a significant decrease in tumor growth than those that received a single treatment. We have now extended these studies to demonstrate the enhancement of anti-CD47-dependent phagocytosis of human Group 3 MB with irradiation. We also analyzed normal human neural stem cells exposed to the same treatments to assess for the potential toxicity that uniquely exists with this treatment combination.

IMMU-03. EFFECT OF COMBINING IRRADIATION AND ANTI-CD47 TREATMENT ON THE PHAGOCYTOSIS OF NORMAL BRAIN CELLS

Abstract Immunotherapy using CD47 blockade has shown promise in treating adult and pediatric malignant primary central nervous system tumors. We recently demonstrated that irradiation significantly enhanced anti-CD47-dependent phagocytosis of high-grade glioma cells in vitro. Furthermore, mice engrafted with high-grade human glioma that received anti-CD47 combined with irradiation showed a significant increase in the survival rate and a significant decrease in tumor growth than those that received a single treatment. However, the use of irradiation may result in potential toxicity to normal CNS cells that are not susceptible to macrophage phagocytosis in anti-CD47 monotherapy. We have now extended these studies to test the effect of combining anti-CD47 and irradiation on macrophage-mediated phagocytosis of central nervous system cells. We analyzed phagocytosis of normal human neural stem cells exposed to different irradiation doses in combination with anti-CD47 treatment to assess for the potential toxicity that uniquely exists with this treatment combination.

CBIO-17. IDENTIFYING A NETWORK OF ESSENTIAL & TUMORIGENIC GENES IN GLIOBLASTOMA USING WHOLE-GENOME CRISPR Cas9 SCREENING

Abstract GBM is the most common primary malignant brain tumor in adults, with a 100% recurrence rate and a median survival of 21 months. As such, advances in therapy are desperately needed. Genomic approaches have shown that GBM has high intra-tumoral heterogeneity. However, a comprehensive understanding of determinants of growth is required to identify new therapeutic targets. CRISPR-Cas9 screening technology has enabled whole-genome screens that allow objective identification of genes governing specific phenotypes. Here, we performed a genome-wide CRISPR knockout screen in H4 human glioma cells to identify genes that drive proliferation. Our screen identified ~150 novel essential growth genes. From this list, we identified 5 genes that were previously unstudied, show significant elevations in expression at the RNA and protein levels (p< 0.05), and show significant survival benefit in patient datasets (p< 0.05) – PSMB3, CHCHD4, THOC1, SPDYE5, HSPA1. Our validation experiments showed that knocking out these genes resulted in cell death in multiple GBM patient-derived xenograft (PDX) lines. In addition, animals with knockout cells implanted demonstrated extended survival (p< 0.01). Furthermore, overexpression of these genes in a normal neural stem cell line resulted in transformation to a cancer phenotype, as evidenced by sphere formation in a soft agar assay (p< 0.01). Further investigation of one of these genes, PSMB3, which is a subunit of the proteasome, showed that ubiquitinated proteins were significantly increased after PSMB3 knockdown, suggesting that disruption of the proteasome system was the likely cause of cell death. We performed a ubiquitin immunoprecipitation (IP) to identify which genes were being uniquely ubiquitinated in GBM. Results showed pathways involving retinoblastoma genes, DNA repair genes, and DNA replication. Together, this data suggests that both our CRISPR screens and our ubiquitin IP have yielded promising and novel therapeutic targets for GBM, a disease desperately in need of new strategies.

EXTH-40. THERAPEUTICALLY TARGETING DE NOVO PURINE BIOSYNTHESIS IN DIFFUSE INTRINSIC PONTINE GLIOMA

Abstract Diffuse intrinsic pontine glioma (DIPG) is an incurable brainstem malignancy in children with median survival less than 1 year and 5-year overall survival only 2 percent. Little progress has been made in treating this deadly disease due to its inoperable location and treatments aimed at targets defined in adult gliomas. Despite recent advances in genetic characterization of DIPGs there are still no targeted therapies that significantly improve overall survival. We recently generated a metabolic profile for DIPG elucidating an upregulation in purine metabolism, specifically in de novo purine biosynthesis (DNPB). Normally nucleotide salvage maintains cellular purine levels by recycling degraded bases, however DNPB is needed when purine levels are depleted. Purine metabolism provides the basic components of nucleotides needed for tumor proliferation and thus considered a high-priority target in cancer treatment. DNPB is a sequential ten step enzymatic process resulting in the production of inosine monophosphate. The last step in DNPB is carried out by the bifunctional enzyme ATIC which is upregulated in DIPG cell lines, and in patient tumors. Our preliminary data demonstrates DIPG cell lines are sensitive to pharmacological inhibition and genetic ablation of multiple enzymes in the DNPB pathway. Strikingly, cell viability could be rescued by purine supplementation when inhibiting this pathway except when ATIC is inhibited indicating the mechanism of cell death for ATIC inhibition is independent of purine nucleotide levels. Furthermore, there is a therapeutic window for targeting ATIC in DIPG cell lines relative to normal neural stem cells and normal human astrocytes. Metabolic flux experiments have demonstrated DNPB is upregulated in DIPG cell lines and the reason these cells are more sensitive to ATIC inhibition is likely related to the rapid accumulation of a cytotoxic metabolite upstream of ATIC. In vivo studies are currently underway in pre-clinical mouse models for DIPG.

Niche-derived soluble DLK1 promotes glioma growth

Tumor cell behaviors associated with aggressive tumor growth such as proliferation, therapeutic resistance, and stem cell characteristics are regulated in part by soluble factors derived from the tumor microenvironment. Tumor-associated astrocytes represent a major component of the glioma tumor microenvironment, and astrocytes have an active role in maintenance of normal neural stem cells in the stem cell niche, in part via secretion of soluble delta-like noncanonical Notch ligand 1 (DLK1). We found that astrocytes, when exposed to stresses of the tumor microenvironment such as hypoxia or ionizing radiation, increased secretion of soluble DLK1. Tumor-associated astrocytes in a glioma mouse model expressed DLK1 in perinecrotic and perivascular tumor areas. Glioma cells exposed to recombinant DLK1 displayed increased proliferation, enhanced self-renewal and colony formation abilities, and increased levels of stem cell marker genes. Mechanistically, DLK1-mediated effects on glioma cells involved increased and prolonged stabilization of hypoxia-inducible factor 2alpha, and inhibition of hypoxia-inducible factor 2alpha activity abolished effects of DLK1 in hypoxia. Forced expression of soluble DLK1 resulted in more aggressive tumor growth and shortened survival in a genetically engineered mouse model of glioma. Together, our data support DLK1 as a soluble mediator of glioma aggressiveness derived from the tumor microenvironment.