Molecular Pathways of Glioblastoma and Glioblastoma Stem Cells

IntroductionGlioblastoma (GBM, WHO grade IV) is a type of highly malignant brain tumor that infiltrates the brain extensively and remains virtually incurable despite being treated with gross total resection and post-operative adjuvant radiation and chemotherapy.The vast majority of patients with GBM will always develop tumor recurrence.The tumor's location, its unique feature of high motility, and its protection by the blood brain barrier make certain therapies that are effective for some other cancers ineffective against brain tumors.Overall, the 5-year survival rate is less than 10%, with a final mortality rate of close to 100 percent.The molecular mechanisms that underlie persistent tumorigenesis and treatment resistance are still poorly understood.A genome-wide expression profile analysis revealed that besides those genes associated with cell proliferation, inflammation, angiogenesis, and extracellular matrix (ECM) remodeling, a series of genes linked with neuroepithelial stem cells, mesenchymal stem cells, skeletal/cartilage development, morphogenesis, and organogenesis, were determined to be overexpressed when compared with normal brain tissue, implicating that a tissue regeneration/repair-like program is constantly activated in GBM tumors.A subset of GBM develops from lower-grade gliomas and can thus be clinically classified as ''secondary,'' whereas some GBM occur with no prior evidence of a lower-grade tumor and can be clinically classified as ''primary.''Substantial genetic differences between these groups of GBM have been identified.Moreover, a molecular classification study indicated that both treatment-refractory and untreated primary GBM tumors are clustered in a group segregated from treated and untreated secondary GBM tumors, and supports the view that GBM subtypes may have derived from a distinct cell-oforigin, which is resistant to conventional therapy, therefore allowing for re-seeding tumor with molecular properties similar to untreated tumors.Thus, post-treatment tumor recurrence may mimic the scenario of post-injury tissue repair.Many adult tissues undergo renewal after injury, and hence require a new supply of cells originating from specialized tissue stem cells with the capability to undergo self-renewal and differentiation to repair damaged tissue.Recently, glioblastoma stem cells (GSC) or glioblastoma stem-like cells (GSLC), a minor subpopulation within tumor mass, were isolated and characterized as tumor-initiating cells and were hypothesized to be responsible for post-treatment recurrence because of their enhanced radio-/chemo-resistant phenotype and ability to reconstitute the original tumor tissue when grafted into mice.In contrast to the hyperproliferative, www.intechopen.comManagement of CNS Tumors 28 inflammatory, and hyperangiogeneic properties seen in GBM tumors, molecular analysis by gene expression profiling revealed that GSC possess neuroectodermal properties and express molecular signatures of radial glial cells (RGC) and neural crest cells (NCC), as well as portray a migratory, quiescent, and slow-growing phenotype that characterizes tumor suppressor properties.Based on the tumor stem cell model and theory, conventional cell cycle-targeted radio-chemotherapy, which aims to kill fast-growing tumor cells, would then be unable to eliminate post-operative remaining tumorigenic cells that possess quiescent stem cell properties.Thus, in order to prevent tumor recurrence, a strategy targeting essential gene pathways of GSC must be identified and incorporated into the standard treatment regimen.Identifying intrinsic and extrinsic cues, by which GSC maintain tumorigenic capacity and antiapoptotic feature to sustain tumorigenesis may highlight novel therapeutic strategies to greatly diminish the recurrence rate of GBM and provide potentially curative strategies for treating brain cancers.In this chapter, we review molecular properties of GBM tumors and GSC.We also summarize molecular signaling pathways that have been relatively well-studied in GSC and are essential for maintaining GSC stemness, tumorigenic capacity, and radio-chemoresistant phenotype. Molecular properties of glioblastoma2.1 Genetic and clinical pathways to glioblastomas GBM remains refractory to conventional therapy.The histopathologic features that distinguish it from lower-grade astrocytic tumors are the presence of cellular atypia, mitotic figures, necrotic foci with peripheral cellular pseudopalisading, and microvascular hyperplasia (1).Two subgroups of GBM have been established based on clinical experience and have been affiliated with distinct genetic mechanisms of tumorigenesis.Secondary GBM, also known as progressive GBM, develop slowly through progression from low-grade glial tumors (WHO grade II) or anaplastic glial tumors (WHO grade III) and frequently display p53 mutation (chromosome 17) (~65%) and amplification or overexpression of platelet-derived growth factor receptor (PDGFR), but not epidermal growth factor receptor (EGFR) (2-3).Additionally, progression to secondary GBM often accompanies an allelic loss at chromosome 19q, 17p, and 10q, and a loss of expression of deleted-in-colorectalcarcinoma gene (DCC) (~50%) but rarely include PTEN mutations (5%) (4-6).The p53 mutation is usually found in the low-grade lesions, indicating p53 alteration is an early event in astrocytoma progression (7).PDGFR amplification or overexpression is also present at the early stages suggesting that it may have a role in the progression of these tumors.In contrast, loss of heterozygosity for the retinoblastoma-1 (RB1) gene was found in high-grade astrocytomas (25%) but not in low-grade astrocytomas, indicating disruption of the RB pathway is likely a significant event in the malignant transformation to GBM (8).On the other hand, primary GBM, also known as de novo GBM, seem to develop rapidly and manifest high-grade lesion from the outset and are genetically characterized by EGFR amplification/overexpression (chromosome 7) (~60%), a simultaneous loss of chromosome 10, but rarely a concurrent p53 mutation.The most common EGFR gene mutation in primary GBMs is EGFRvIII, a variant lacking exons 2-7 (corresponding to cDNA nucleotides 275-1075 encoding amino acids 6-273), which results in a truncated cell surface receptor with ligand-independent constitutive tyrosine kinase activity (9)(10)(11).This mutation presumably occurs through alternative splicing or gene rearrangements (12-13) and leads to the loss of binding activation by its normal ligand, EGF and TGF-a (14-15).Mouse double www.intechopen.comMolecular Pathways of Glioblastoma and Glioblastoma Stem Cells 29 minute 2 (MDM2) amplification that neutralizes p53 activity (16), is observed in more than 50% of primary GBM, but rarely in secondary GBM.Additionally, CDKN2A (p16INK4a) deletion, PTEN mutation, Rb protein alterations and loss of all or a portion of chromosome 10 are frequently seen in primary GBM (17-18).p16INK4a deletion is infrequent in secondary GBMs and its deletion and p53 mutation appear to be two mutually exclusive events in GBMs (19).Primary GBMs account for the vast majority of cases (60%) and typically occur in the elderly (>50 years old), whereas secondary GBMs, are less common (40%) and typically develop in younger patients (<45 y) (4).Primary and secondary GBMs are indistinguishable to the neurosurgeon as well as neuropathologist, and the clinical management of these two GBM subtypes is identical.To date, temozolomide (TMZ) administered daily with radiation therapy (RT) for six weeks, followed by adjuvant TMZ for six months, has become standard therapy for patients with newly diagnosed GBM. Genetic characteristics of GBM link to prognosisThe overall prognosis for patients with GBM is extremely poor.However, a small proportion of patients show prolonged survival.A study indicates that different gliomaassociated genomic aberrations may serve as prognostic markers in combination with histopathological findings (17).The use of comparative genomic hybridization (CGH)-based analysis of 20 primary GBMs suggests that loss of chromosome 10 and gain/amplification in chromosome 7 are most frequently observed in primary GBMs and are associated with microvascularization and poor prognosis (17).In contrast, the combination of chromosome 1p and 17p13-p14 and 19q deletions are associated with a longer survival time (5,17,(20)(21).The analysis of loss of heterozygosity (LOH) on chromosomes 19q, 1p, and 13q, using polymorphic microsatellite markers, however, has indicated that LOH on chromosome 19q was frequently found in secondary GBMs (50%) but rarely detected in primary GBMs (20), suggesting that tumor suppressor gene(s) located on chromosome 19q are frequently involved in the progression from a low-grade astrocytoma to secondary glioblastoma, but do not play a major role in the evolution of primary glioblastomas.Clinical trials indicated that patients whose tumor had a methylated promoter for the gene encoding O-6methylguanine-DNA methyltransferase (MGMT), were more likely to benefit from the addition of TMZ to .A recent study further showed that pattern of, and time to, recurrence after TMZ concomitant with and adjuvant to radiotherapy are strictly correlated with MGMT methylation status (21).Recently, genomewide mutational analysis of GBM revealed somatic mutations of cytosolic isocitrate dehydrogenase 1 gene (IDH1), which catalyzes the oxidative decarboxylation of isocitrate to -ketoglutarate, most frequently in WHO grade II and III astrocytomas and secondary GBM but rarely in primary GBM (22-23), and patient tumors with IDH1 or related mitochondrial IDH2 mutations had a improved clinical prognosis than those with wild-type IDH genes (25,27).It is suggested that IDH mutation is a highly prognosis predictor and selective molecular signature of secondary GBM (28-29). Molecular classification of glioblastoma subtypesIdentification of chromosomal abnormalities and cancer-associated genes in solid tumors is becoming easier as genome-wide analysis technologies improve and as the genome sequence is being completed.These technologies allow for genome-wide data acquisition in study of cancer genetics and biology, particularly in analysis of complex expression www.intechopen.com