Genetic and epigenetic markers of gliomas

Malignant gliomas are aggressive and highly invasive tumors. Various genetic and epigenetic changes are common for these tumors. Mostly they concern the genes involved in cell-cycle regulation, apoptotic pathways, cell invasion, angiogenesis, and cell metabolism. The role of epigenetic mechanisms in glioma malignant transformation, despite recent progress, is uncertain and remains under intense study. This review describes the mechanisms of epigenetic regulation of gene expression, including posttranslational modifications of histones, DNA methylation in promoter regions, and microRNA regulation. The genetic and epigenetic factors driving the pathogenesis of gliomas in their possible mutual influence and the potential epigenetic targets that can be used for diagnostics and new therapeutic approaches are also discussed.     

The pro-migratory and pro-invasive role of the procoagulant tissue factor in malignant gliomas

During the infiltration process, glioma cells are known to migrate along preexisting anatomical structures such as blood vessels, axonal fiber tracts and the subependymal space, thereby widely invading surrounding CNS tissue. This phenomenon represents a major obstacle for the clinical treatment of these tumours. Several extracellular key factors and intracellular signaling pathways have been previously linked to the highly aggressive, invasive phenotype observed in malignant gliomas. The glioblastoma (GBM) which is the most malignant form of these tumors, is histologically characterized by areas of tumor necroses and pseudopalisading cells, the latter likely representing tumor cells actively migrating away from the hypoxic-ischemic core of the tumor. It is believed that intravascular thromboses play a major role in the emergence of hypoxia and intratumoral necroses in GBMs. One of the most highly upregulated prothrombotic factor in malignant gliomas is tissue factor (TF), a 47 kDa type I transmembrane protein belonging to the cytokine receptor superfamily. In a recent study, we provided evidence that TF/FVIIa signaling via the protease-activated receptor 2 (PAR-2) promotes cell growth, migration and invasion of glioma cells. In this point of view article we outline the key molecular players involved in migration and invasion of gliomas, highlight the potential role of TF for the pro-migratory and pro-invasive phenotype of these tumors and discuss the underlying mechanisms on the cellular level and in the tumor microenvironment.     

The pro-migratory and pro-invasive role of the procoagulant tissue factor in malignant gliomas

During the infiltration process, glioma cells are known to migrate along preexisting anatomical structures such as blood vessels, axonal fiber tracts and the subependymal space, thereby widely invading surrounding CNS tissue. This phenomenon represents a major obstacle for the clinical treatment of these tumors. Several extracellular key factors and intracellular signaling pathways have been previously linked to the highly aggressive, invasive phenotype observed in malignant gliomas. The glioblastoma (GBM), which is the most malignant form of these tumors, is histologically characterized by areas of tumor necroses and pseudopalisading cells, the latter likely representing tumor cells actively migrating away from the hypoxic- ischemic core of the tumor. It is believed that intravascular thromboses play a major role in the emergence of hypoxia and intratumoral necroses in GBMs. One of the most highly upregulated prothrombotic factor in malignant gliomas is tissue factor (TF), a 47 kDa type I transmembrane protein belonging to the cytokine receptor superfamily. In a recent study, we provided evidence that TF/FVIIa signaling via the protease-activated receptor 2 (PAR-2) promotes cell growth, migration and invasion of glioma cells. In this Commentary & View, we outline the key molecular players involved in migration and invasion of gliomas, highlight the potential role of TF for the pro-migratory and pro-invasive phenotype of these tumors and discuss the underlying mechanisms on the cellular level and in the tumor microenvironment.Key words: brain tumor, blood coagulation, hypoxia, MAP kinase, cancer stem cells, tumor invasion     

Brain Tumor Stem-Like Cells Identified by Neural Stem Cell Marker CD15

In recent years, a small number of cells that have stem cell properties were identified in human gliomas called brain tumor stem cells (BTSCs), which were thought to mainly contribute to the initiation and development of gliomas and could be identified by the surface marker CD133. However, recent studies indicated that the expression of CD133 might be regulated by environmental conditions such as hypoxia and that there might be CD133^- BTSCs. Genetic mouse models demonstrated that some gliomas originated from transformed neural stem cells (NSCs). Therefore, we investigated the expression of CD15, a surface marker for NSCs, in tumor spheres derived from astrocytoma and ependymoma. CD15⁺ cells isolated from these tumor spheres had properties of BTSCs including self-renewal, multidifferentiation, and the ability to recapitulate the phenocopy of primary tumors. CD15 exhibited stable expression in long-term cultured tumor spheres, which sustained BTSCs properties, whereas CD133 expression decreased significantly in late passages. Furthermore, CD15⁺CD133^- cells isolated from early or late passages of tumor spheres showed similar characteristics of BTSCs. Examination of glioma samples by immunohistochemistry showed that CD15 was expressed in a subset of human brain tumors. Therefore, CD15 can be used as a marker of stem-like cells derived from brain tumors that might contain CD133^- BTSCs.     

Human glioblastoma U87MG cells transduced with a dominant negative p53 (TP53) adenovirus construct undergo radiation-induced mitotic catastrophe

Human gliomas are among the most aggressive tumors, and they respond poorly to treatment. The efficacy of surgical, radiation and chemotherapy treatment of these tumors is limited by the development of resistance. Interventions aimed at altering the response of these tumors to radiation or chemotherapy treatments are needed to improve survival rate and prognosis. Glioblastomas are generally p53 (TP53) functional tumors; however, DNA repair pathways are activated in these tumors instead of the pathways to apoptosis. Thus resistance to treatment is seen in the ability of these tumors to overcome cell death. We present data that demonstrate that U87MG glioblastoma cells transduced with a dominant-negative p53 adenovirus construct become sensitized to radiation-induced mitotic catastrophe through abrogation of G(2)/M checkpoint control and overaccumulation of cyclin B1. These findings suggest that interventions abrogating the G(2)/M checkpoint sensitize these cells to radiation-induced mitotic catastrophe and may represent a novel mechanism to increase the efficacy of radiation in wild-type p53 gliomas that are resistant to apoptosis.     

Molecular biology of glioma tumorigenesis

Gliomas are the most common intracranial malignant tumors in humans, and high-grade gliomas in particular pose a unique challenge due to their propensity for proliferation and tissue invasion. Our understanding of glioma oncogenesis, proliferation, and invasion has been greatly advanced in the past 10 years as researchers have gained a better understanding of the molecular biology of these tumors. This article highlights glioma histopathology, as well as cytogenetic and molecular alterations associated with the pathogenesis of human gliomas. It is hoped that better understanding of the molecular pathogenesis of gliomas will improve tumor classification as well as lead to novel targets for therapy and prognostic markers.     

成人胶质瘤诊断的分子遗传学研究进展

胶质瘤是目前中枢神经系统中常见的恶性肿瘤,由于脑组织的特殊性,胶质瘤呈弥漫浸润性生长,恶性程度高,手术难以完整切除且易复发。2007年(第四版)WHO中枢神经系统肿瘤病理学和遗传学对胶质瘤进行了详细的组织学分类,但是循证医学发现依靠组织学的病理诊断标准并不能对胶质瘤的临床表现和预后评估作出精准的判断。近年来全世界都在开展胶质瘤相关的遗传学研究,许多遗传学分子改变被发现,如异柠檬酸脱氢酶(IDH)突变、染色体1p/19q缺失、TP53突变、ATRX突变和TERT启动子突变等,组织学诊断受到了挑战。因此更多的病理科和神经外科医生结合组织形态和遗传学改变对胶质瘤作出"综合性"诊断,使得病理诊断更接近胶质瘤的生物学本质,以便更精准的指导临床治疗。     

Glioma models.

Gliomas are primary central nervous system tumors that arise from astrocytes, oligodendrocytes or their precursors. Gliomas can be classified into several groups according to their histologic characteristics, the most malignant of the gliomas is glioblastoma multiforme. In contrast to the long-standing and well-defined histopathology, the underlying molecular and genetic bases for gliomas are only just emerging. Many genetic alterations have been identified in human gliomas, however, establishing unequivocal correlation between these genetic alterations and gliomagenesis requires accurate animal models for this disease. Here we are reviewing the existing animal models for gliomas with different strategies and our current knowledge on the important issues about this disease, such as activation of signal transduction pathways, disruption of cell cycle arrest pathways, cell-of-origin of gliomas, and therapeutic strategies.     

Glioma models

Gliomas are primary central nervous system tumors that arise from astrocytes, oligodendrocytes or their precursors. Gliomas can be classified into several groups according to their histologic characteristics, the most malignant of the gliomas is glioblastoma multiforme. In contrast to the long-standing and well-defined histopathology, the underlying molecular and genetic bases for gliomas are only just emerging. Many genetic alterations have been identified in human gliomas, however, establishing unequivocal correlation between these genetic alterations and gliomagenesis requires accurate animal models for this disease. Here we are reviewing the existing animal models for gliomas with different strategies and our current knowledge on the important issues about this disease, such as activation of signal transduction pathways, disruption of cell cycle arrest pathways, cell-of-origin of gliomas, and therapeutic strategies.     

Potential adenovirus-mediated gene therapy of glioma cancer

Malignant gliomas are typically characterized by rapid cell proliferation and a marked propensity to invade and damage surrounding tissues. They are the main brain tumors notoriously resistant to currently available therapies, since they fail to undergo apoptosis upon anticancer treatments. With recent advances in neuroscience and improved understanding of the molecular mechanisms of invasive migration, gene therapy provides a new strategy for treating glioma cancer. Brain tumor gene therapy using viral vectors and stem cells has shown promise in animal model and human patient studies. Here, we review recent studies on engineering adenoviral vectors that can be used as therapy for brain tumors. The new findings presented in this study are essential for the further exploration of this cancer and they represent an approach for developing a newer and more effective therapeutic approach in the clinical treatment of human glioma cancer.     

Glioma proteomics: Status and perspectives

High grade gliomas are the most common brain tumors in adults and their malignant nature makes them the fourth biggest cause of cancer death. Major efforts in neuro-oncology research are needed to reach similar progress in treatment efficacy as that achieved for other cancers in recent years. In addition to the urgent need to identify novel effective drug targets against malignant gliomas, the search for glioma biomarkers and grade specific protein signatures will provide a much needed contribution to diagnosis, prognosis, treatment decision and assessment of treatment response. Over the past years glioma proteomics has been attempted at different levels, including proteome analysis of patient biopsies and bodily fluids, of glioma cell lines and animal models. Here we provide an extensive review of the outcome of these studies in terms of protein identifications (protein numbers and regulated proteins), with an emphasis on the methods used and the limitations of the studies with regard to biomarker discovery. This is followed by a perspective on novel technologies and on the potential future contribution of proteomics in a broad sense to understanding glioma biology.     

Role of NOD- like Receptors in Glioma Angiogenesis: Insights into future therapeutic interventions

Gliomas are the most common solid tumors among central nervous system tumors. Most glioma patients succumb to their disease within two years of the initial diagnosis. The median survival of gliomas is only 14.6 months, even after aggressive therapy with surgery, radiation, and chemotherapy. Gliomas are heavily infiltrated with myeloid- derived cells and endothelial cells. Increasing evidence suggests that these myeloid- derived cells interact with tumor cells promoting their growth and migration. NLRs (nucleotide-binding oligomerization domain (NOD)-containing protein like receptors) are a class of pattern recognition receptors that are critical to sensing pathogen and danger associated molecular patterns. Mutations in some NLRs lead to autoinflammatory diseases in humans. Moreover, dysregulated NLR signaling is central to the pathogenesis of several cancers, autoimmune and neurodegenerative diseases. Our review explores the role of angiogenic factors that contribute to upstream or downstream signaling pathways leading to NLRs. Angiogenesis plays a significant role in the pathogenesis of variety of tumors including gliomas. Though NLRs have been detected in several cancers including gliomas and NLR signaling contributes to angiogenesis, the exact role and mechanism of involvement of NLRs in glioma angiogenesis remain largely unexplored. We discuss cellular, molecular and genetic studies of NLR signaling and convergence of NLR signaling pathways with angiogenesis signaling in gliomas. This may lead to re-appropriation of existing anti-angiogenic therapies or development of future strategies for targeted therapeutics in gliomas.     

Toward a molecular classification of the gliomas: histopathology, molecular genetics, and gene expression profiling

As many as 100,000 new cases of brain tumor are diagnosed each year in the United States. About half of these are primary gliomas and the remaining half are metastatic tumors and non-glial primary tumors. Currently, gliomas are classified based on phenotypic characteristics. Recent progress in the elucidation of genetic alterations found in gliomas have raised the exciting possibility of using genetic and molecular analyses to resolve some of the problematic issues currently associated with the histological approach to glioma classification. Recently, immunohistochemical studies using novel proliferation markers have significantly advanced the assessment of tumor growth potential and the grading criteria of some tumor subtypes. Preliminary studies using cDNA array technologies suggest that the profiling of gene expression patterns may provide a novel and meaningful approach to glioma classification and subclassification. Furthermore, cDNA array technologies may also be used to identify candidate genes involved in glioma tumor development, invasion, and progression. This review summarizes current glioma classification schemes that are based on histopathological characteristics and discusses the potential for using cDNA array technology in the molecular classification of gliomas.     

Malignant glioma: lessons from genomics, mouse models, and stem cells

Eighty percent of malignant tumors that develop in the central nervous system are malignant gliomas, which are essentially incurable. Here, we discuss how recent sequencing studies are identifying unexpected drivers of gliomagenesis, including mutations in isocitrate dehydrogenase 1 and the NF-κB pathway, and how genome-wide analyses are reshaping the classification schemes for tumors and enhancing prognostic value of molecular markers. We discuss the controversies surrounding glioma stem cells and explore how the integration of new molecular data allows for the generation of more informative animal models to advance our knowledge of glioma's origin, progression, and treatment.     

Amplification of KIT, PDGFRA, VEGFR2, and EGFR in gliomas

Receptor tyrosine kinase aberrations are implicated in the genesis of gliomas. We investigated expression and amplification of KIT, PDGFRA, VEGFR2, and EGFR in 87 gliomas consisting of astrocytomas, anaplastic astrocytomas, oligodendrogliomas, or oligoastrocytomas in tumor samples collected at the time of the diagnosis and in samples of the same tumors at tumor recurrence. Gene amplifications were investigated using either chromogenic in situ hybridization or fluorescence in situ hybridization, and protein expression using immunohistochemistry. In samples collected at glioma diagnosis, KIT and PDGFRA amplifications were more frequent in anaplastic astrocytomas than in astrocytomas, oligodendrogliomas, and oligoastrocytomas [28% versus 5% (P = 0.012) and 33% versus 2% (P = 0.0008), respectively]. VEGFR2 amplifications occurred in 6% to 17% of the gliomas at diagnosis, and EGFR amplifications in 0% to 12%. Amplified KIT was more frequently present in recurrent gliomas than in newly diagnosed gliomas (P = 0.0066). KIT amplification was associated with KIT protein expression and with presence of PDGFRA and EGFR amplifications both at the time of the first glioma diagnosis and at tumor recurrence, and with VEGFR2 amplification at tumor recurrence. Three (4%) primary gliomas and 10 (14%) recurrent gliomas that were evaluable for coamplification of KIT, PDGFRA, and VEGFR2 showed amplification of at least two of these genes; the amplicon contained amplified KIT in all 13 cases. In conclusion, besides glioblastoma, amplified KIT, PDGFRA, and VEGFR may also occur in lower-grade gliomas and in their recurrent tumors. It is currently not known whether specific tyrosine kinase inhibitors are effective in the treatment of such gliomas.     

Development of an arterial tree in C6 gliomas but not in A375 melanomas

The microcirculation of tumors is severely disturbed. Tumors are usually supplied by fragile capillaries and do not possess the natural hierarchy of blood vessels. The detection of specific markers for arterial and venous endothelial cells (ECs) now enables us to study the vascular tree in tumors. We have injected rat C6 glioma and human A375 melanoma cells into 3.5- to 4-day-old avian embryos. After 10-12 days of reincubation the tumor cells formed solid tumors vascularized by host ECs. In contrast to the melanomas, the gliomas induced an almost normal vascular tree with arterial and venous vessels. The arterial vessels express the arterial EC marker ephrin-B2, and possess a media of smooth muscle alpha-actin (alphaSMA)-positive cells. Venular vessels in the gliomas are ephrin-B2-negative/alphaSMA-positive. Although the gliomas may represent a rare case of vascular tree induction in tumors, the results underline the heterogeneity of tumor-induced angiogenesis. This has an impact on tumor blood flow and thereby also on the efficacy of chemotherapy and radiotherapy.     

Advances in glioma models using in vivo electroporation to highjack neurodevelopmental processes

Glioma is the most prevalent type of neurological malignancies. Despite decades of efforts in neurosurgery, chemotherapy and radiation therapy, glioma remains one of the most treatment-resistant brain tumors with unfavorable outcomes. Recent progresses in genomic and epigenetic profiling have revealed new concepts of genetic events involved in the etiology of gliomas in humans, meanwhile, revolutionary technologies in gene editing and delivery allows to code these genetic “events” in animals to genetically engineer glioma models. This approach models the initiation and progression of gliomas in a natural microenvironment with an intact immune system and facilitates probing therapeutic strategies. In this review, we focus on recent advances in in vivo electroporation-based glioma modeling and outline the established genetically engineered glioma models (GEGMs).     

Quantitation of angiogenesis and its correlation with vascular endothelial growth factor expression in astrocytic tumors

OBJECTIVE: To quantitate tumor angiogenesis by establishing intratumoral microvessel density (IMD), to study vascular endothelial growth factor (VEGF) expression in different grades of astrocytomas and to correlate VEGF expression with tumor angiogenesis. STUDY DESIGN: Forty cases of astrocytic neoplasms (10 of each grade) were assessed for tumor angiogenesis and VEGF expression. The panendothelial marker CD31 was used to highlight microvessels. Tumor angiogenesis was quantitated as IMD count per square millimeter in areas of high vascularity, or "hot spots," using an image analyzer. VEGF expression was studied in sections of the tumors. IMD counts per square millimeter and VEGF expression were correlated with histologic grade. The angiogenic potential of tumors as reflected by IMD counts per square millimeter was correlated with the intensity of VEGF expression. RESULTS: Vascular proliferation in high grade gliomas was significantly higher as compared to that in low grade gliomas. IMD count per square millimeter revealed a positive correlation with histologic grade in high grade gliomas. Pilocytic astrocytoma and low grade astrocytoma as a group had comparable IMD counts per square millimeter. VEGF expression paralleled IMD counts in rare high grade gliomas only. CONCLUSION: Malignant progression in astrocytoma is heralded and accompanied by increased angiogenesis. VEGF is an important angiogenic factor in high grade gliomas since its expression parallels the increased IMD counts in these tumors. In contrast, in low grade gliomas, angiogenic factors other than VEGF may contribute to vascular proliferation. The results emphasize the role of antiangiogenic therapy as an optimal tool in therapeutic strategies as they become available.