Mosaic analysis with double markers reveals tumor cell of origin in glioma

A long non-coding RNA signature in glioblastoma multiforme predicts survival

Long non-coding RNAs (lncRNAs) represent the leading edge of cancer research, and have been implicated in cancer biogenesis and prognosis. We aimed to identify lncRNA signatures that have prognostic values in glioblastoma multiforme (GBM). Using a lncRNA-mining approach, we performed lncRNA expression profiling in 213 GBM tumors from The Cancer Genome Atlas (TCGA), randomly divided into a training (n=107) and a testing set (n=106). We analyzed the associations between lncRNA signatures and clinical outcome in the training set, and validated the findings in the testing set. We also validated the identified lncRNA signature in another two independent GBM data sets from Gene Expression Omnibus (GEO), which contained specimens from 68 and 101 patients, respectively. We identified a set of six lncRNAs that were significantly associated with the overall survival in the training set (P≤0.01). Based on this six-lncRNA signature, the training-set patients could be classified into high-risk and low-risk subgroups with significantly different survival (HR=2.13, 95% CI=1.38-3.29; P=0.001). The prognostic value of this six-lncRNA signature was confirmed in the testing set and the two independent data sets. Further analysis revealed that the prognostic value of this signature was independent of age and O-6-methylguanine-DNA methyltransferase (MGMT) promoter methylation status. The identification of the prognostic lncRNAs indicates the potential roles of lncRNAs in GBM pathogenesis. This six-lncRNA signature may have clinical implications in the subclassification of GBM.

Expansion of oligodendrocyte progenitor cells following SIRT1 inactivation in the adult brain

Oligodendrocytes-the myelin-forming cells of the central nervous system-can be regenerated during adulthood. In adults, new oligodendrocytes originate from oligodendrocyte progenitor cells (OPCs), but also from neural stem cells (NSCs). Although several factors supporting oligodendrocyte production have been characterized, the mechanisms underlying the generation of adult oligodendrocytes are largely unknown. Here we show that genetic inactivation of SIRT1, a protein deacetylase implicated in energy metabolism, increases the production of new OPCs in the adult mouse brain, in part by acting in NSCs. New OPCs produced following SIRT1 inactivation differentiate normally, generating fully myelinating oligodendrocytes. Remarkably, SIRT1 inactivation ameliorates remyelination and delays paralysis in mouse models of demyelinating injuries. SIRT1 inactivation leads to the upregulation of genes involved in cell metabolism and growth factor signalling, in particular PDGF receptor α (PDGFRα). Oligodendrocyte expansion following SIRT1 inactivation is mediated at least in part by AKT and p38 MAPK-signalling molecules downstream of PDGFRα. The identification of drug-targetable enzymes that regulate oligodendrocyte regeneration in adults could facilitate the development of therapies for demyelinating injuries and diseases, such as multiple sclerosis.

The molecular and cell biology of pediatric low-grade gliomas

Pilocytic astrocytoma (PA) is the most common glial cell tumor arising in children. Sporadic cases are associated with KIAA1549:BRAF fusion rearrangements, while 15-20% of children develop PA in the context of the neurofibromatosis 1 (NF1) inherited tumor predisposition syndrome. The unique predilection of these tumors to form within the optic pathway and brainstem (NF1-PA) and cerebellum (sporadic PA) raises the possibility that gliomagenesis requires more than biallelic inactivation of the NF1 tumor suppressor gene or expression of the KIAA1549:BRAF transcript. Several etiologic explanations include differential susceptibilities of preneoplastic neuroglial cell types in different brain regions to these glioma-causing genetic changes, contributions from non-neoplastic cells and signals in the tumor microenvironment, and genomic modifiers that confer glioma risk. As clinically-faithful rodent models of sporadic PA are currently under development, Nf1 genetically-engineered mouse (GEM) models have served as tractable systems to study the role of the cell of origin, deregulated intracellular signaling, non-neoplastic cells in the tumor microenvironment and genomic modifiers in gliomagenesis. In this report, we highlight advances in Nf1-GEM modeling and review new experimental evidence that supports the emerging concept that Nf1- and KIAA1549:BRAF-induced gliomas arise from specific cell types in particular brain locations.

Oligodendrocyte progenitors balance growth with self-repulsion to achieve homeostasis in the adult brain

The adult CNS contains an abundant population of oligodendrocyte precursor cells (NG2⁺ cells) that generate oligodendrocytes and repair myelin, but how these ubiquitous progenitors maintain their density is unknown. Here we generated NG2-mEGFP mice and used in vivo two-photon imaging to study their dynamics in the adult brain. Time-lapse imaging revealed that NG2⁺ cells in the cortex are highly dynamic; they survey their local environment with motile filopodia, extend growth cones, and continuously migrate. They maintain unique territories through self-avoidance, and NG2⁺ cell loss through death, differentiation, or ablation triggered rapid migration and proliferation of adjacent cells to restore their density. NG2⁺ cells recruited to sites of focal CNS injury were similarly replaced by a proliferative burst surrounding the injury site. Thus, homeostatic control of NG2⁺ cell density through a balance of active growth and self-repulsion ensures that these progenitors are available to replace oligodendrocytes and participate in tissue repair.

Human glioma-initiating cells show a distinct immature phenotype resembling but not identical to NG2 glia

Glioma-initiating cells (GICs) represent a potential important therapeutic target because they are likely to account for the frequent recurrence of malignant gliomas; however, their identity remains unsolved. Here, we characterized the cellular lineage fingerprint of GICs through a combination of electrophysiology, lineage marker expression, and differentiation assays of 5 human patient-derived primary GIC lines. Most GICs coexpressed nestin, NG2 proteoglycan, platelet-derived growth factor receptor-α, and glial fibrillary acidic protein. Glioma-initiating cells could be partially differentiated into astrocytic but not oligodendroglial or neural lineages. We also demonstrate that GICs have a characteristic electrophysiologic profile distinct from that of well-characterized tumor bulk cells. Together, our results suggest that GICs represent a unique type of cells reminiscent of an immature phenotype that closely resembles but is not identical to NG2 glia with respect to marker expression and functional membrane properties.

Mosaic analysis with double markers reveals cell-type-specific paternal growth dominance

Genomic imprinting leads to preferred expression of either the maternal or paternal alleles of a subset of genes. Imprinting is essential for mammalian development, and its deregulation causes many diseases. However, the functional relevance of imprinting at the cellular level is poorly understood for most imprinted genes. We used mosaic analysis with double markers (MADM) in mice to create uniparental disomies (UPDs) and to visualize imprinting effects with single-cell resolution. Although chromosome 12 UPD did not produce detectable phenotypes, chromosome 7 UPD caused highly significant paternal growth dominance in the liver and lung, but not in the brain or heart. A single gene on chromosome 7, encoding the secreted insulin-like growth factor 2 (IGF2), accounts for most of the paternal dominance effect. Mosaic analyses implied additional imprinted loci on chromosome 7 acting cell autonomously to transmit the IGF2 signal. Our study reveals chromosome- and cell-type specificity of genomic imprinting effects.

The NF-κB RelB protein is an oncogenic driver of mesenchymal glioma

High-grade gliomas, such as glioblastomas (GBMs), are very aggressive, invasive brain tumors with low patient survival rates. The recent identification of distinct glioma tumor subtypes offers the potential for understanding disease pathogenesis, responses to treatment and identification of molecular targets for personalized cancer therapies. However, the key alterations that drive tumorigenesis within each subtype are still poorly understood. Although aberrant NF-κB activity has been implicated in glioma, the roles of specific members of this protein family in tumorigenesis and pathogenesis have not been elucidated. In this study, we show that the NF-κB protein RelB is expressed in a particularly aggressive mesenchymal subtype of glioma, and loss of RelB significantly attenuated glioma cell survival, motility and invasion. We find that RelB promotes the expression of mesenchymal genes including YKL-40, a marker of the MES glioma subtype. Additionally, RelB regulates expression of Olig2, a regulator of cancer stem cell proliferation and a candidate marker for the cell of origin in glioma. Furthermore, loss of RelB in glioma cells significantly diminished tumor growth in orthotopic mouse xenografts. The relevance of our studies for human disease was confirmed by analysis of a human GBM genome database, which revealed that high RelB expression strongly correlates with rapid tumor progression and poor patient survival rates. Thus, our findings demonstrate that RelB is an oncogenic driver of mesenchymal glioma tumor growth and invasion, highlighting the therapeutic potential of inhibiting the noncanonical NF-κB (RelB-mediated) pathway to treat these deadly tumors.

Inflammation and Gliomagenesis: Bi-Directional Communication at Early and Late Stages of Tumor Progression

Inflammation has been closely linked to various forms of cancer. Less is known about the role of inflammation in glioma, especially at the initiation stage. In this review, we first describe the unique features of the immune system in the brain. We then discuss the current understanding of the mechanisms by which glioma cells modulate the immune system, especially how bi-directional communications between immune cells and glioma cells create an immunosuppressed microenvironment that promotes tumor survival and growth. We also address the potential tumor-initiating roles of inflammation in glioma. Finally, we describe several immunotherapy approaches currently being developed to reverse these interactions and stimulate the immune system to eliminate glioma cells.

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.

NG2-cells are not the cell of origin for murine neurofibromatosis-1 (Nf1) optic glioma

Low-grade glial neoplasms (astrocytomas) represent one of the most common brain tumors in the pediatric population. These tumors frequently form in the optic pathway (optic pathway gliomas, OPGs), especially in children with the neurofibromatosis type 1 (NF1)-inherited tumor predisposition syndrome. To model these tumors in mice, we have previously developed several Nf1 genetically-engineered mouse strains that form optic gliomas. However, there are three distinct macroglial cell populations in the optic nerve (astrocytes, NG2+ (nerve/glial antigen 2) cells and oligodendrocytes). The presence of NG2+ cells in the optic nerve raises the intriguing possibility that these cells could be the tumor-initiating cells, as has been suggested for adult glioma. In this report, we used a combination of complementary in vitro and novel genetically-engineered mouse strains in vivo to determine whether NG2+ cells could give rise to Nf1 optic glioma. First, we show that Nf1 inactivation results in a cell-autonomous increase in glial fibrillary acidic protein+ (GFAP+), but not in NG2+, cell proliferation in vitro. Second, similar to the GFAP-Cre transgenic strain that drives Nf1 optic gliomagenesis, NG2-expressing cells also give rise to all three macroglial lineages in vivo. Third, in contrast to the GFAP-Cre strain, Nf1 gene inactivation in NG2+ cells is not sufficient for optic gliomagenesis in vivo. Collectively, these data demonstrate that NG2+ cells are not the cell of origin for mouse optic glioma, and support a model in which gliomagenesis requires Nf1 loss in specific neuroglial progenitors during embryogenesis.

ASYMMETRIC CELL DIVISION: IMPLICATIONS FOR GLIOMA DEVELOPMENT AND TREATMENT

Glioma is a heterogeneous disease process with differential histology and treatment response. It was previously thought that the histological features of glial tumors indicated their cell of origin. However, the discovery of continuous neuro-gliogenesis in the normal adult brain and the identification of brain tumor stem cells within glioma have led to the hypothesis that these brain tumors originate from multipotent neural stem or progenitor cells, which primarily divide asymmetrically during the postnatal period. Asymmetric cell division allows these cell types to concurrently self-renew whilst also producing cells for the differentiation pathway. It has recently been shown that increased symmetrical cell division, favoring the self-renewal pathway, leads to oligodendroglioma formation from oligodendrocyte progenitor cells. In contrast, there is some evidence that asymmetric cell division maintenance in tumor stem-like cells within astrocytoma may lead to acquisition of treatment resistance. Therefore cell division mode in normal brain stem and progenitor cells may play a role in setting tumorigenic potential and the type of tumor formed. Moreover, heterogeneous tumor cell populations and their respective cell division mode may confer differential sensitivity to therapy. This review aims to shed light on the controllers of cell division mode which may be therapeutically targeted to prevent glioma formation and improve treatment response.

Gene Regulation of Prominin-1 (CD133) in Normal and Cancerous Tissues

A pentaspan membrane glycoprotein prominin-1 (frequently called CD133 in human) is widely used as a surface marker to identify and isolate normal stem/progenitor cells from various organs, although it is also expressed in some types of differentiated cells. Since CD133 was identified as a universal marker to isolate cancer stem cells (CSCs) in tumors derived from multiple tissues, much attention has been directed toward the relationship between its gene regulation and identity of CSCs (i.e., cancer stemness). Prominin-1 (PROM1) gene possesses five alternative promoters yielding multiple first exons within the 5'-untranslated region (UTR) and also splicing variants affecting the open reading frame (ORF) sequence, implicating the complicated gene regulation in a context-dependent manner. This chapter aims to organize the accumulated findings on prominin-1 with a focus on its altered expression and regulation in normal and cancerous cells and to discuss potential regulatory networks underlying cancer stemness.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Pediatric glioma-associated KIAA1549:BRAF expression regulates neuroglial cell growth in a cell type-specific and mTOR-dependent manner

Tandem duplications involving the BRAF kinase gene have recently been identified as the most frequent genetic alteration in sporadic pediatric glioma, creating a novel fusion protein (f-BRAF) with increased BRAF activity. To define the role of f-BRAF in gliomagenesis, we demonstrate that f-BRAF regulates neural stem cell (NSC), but not astrocyte, proliferation and is sufficient to induce glioma-like lesions in mice. Moreover, f-BRAF-driven NSC proliferation results from tuberin/Rheb-mediated mammalian target of rapamycin (mTOR) hyperactivation, leading to S6-kinase-dependent degradation of p27. Collectively, these results establish mTOR pathway activation as a key growth regulatory mechanism common to both sporadic and familial low-grade gliomas in children.

Generation of Dhx9-deficient clones in T-cell development with a mitotic recombination technique

Mitotic recombination is an effective tool for generating mutant clones in somatic tissues. Because of difficulties associated with detecting and quantifying mutant clones in mice, this technique is limited to analysis of growth-related phenotypes induced by loss function of tumor suppressor genes. Here, we used the polymorphic CD45.1/CD45.2 alleles on chromosome 1 as pan-hematopoietic markers to track mosaic clones generated through mitotic recombination in developing T cells. We show that lineage-specific mitotic recombination can be induced and reliably detected as CD45.1 or CD45.2 homozygous clones from the CD45.1/CD45.2 heterozygous background. We have applied this system in the analysis of a lethal mutation in the Dhx9 gene. Mosaic analysis revealed a stage-specific role for Dhx9 during T-cell maturation. Thus, the experimental system described in this study offers a practical means for mosaic analysis of germline mutations in the hematopoietic system.

Dedifferentiation of neurons and astrocytes by oncogenes can induce gliomas in mice

Glioblastoma multiforme (GBM) is the most common and aggressive malignant primary brain tumor in humans. Here we show that gliomas can originate from differentiated cells in the central nervous system (CNS), including cortical neurons. Transduction by oncogenic lentiviral vectors of neural stem cells (NSCs), astrocytes, or even mature neurons in the brains of mice can give rise to malignant gliomas. All the tumors, irrespective of the site of lentiviral vector injection (the initiating population), shared common features of high expression of stem or progenitor markers and low expression of differentiation markers. Microarray analysis revealed that tumors of astrocytic and neuronal origin match the mesenchymal GBM subtype. We propose that most differentiated cells in the CNS upon defined genetic alterations undergo dedifferentiation to generate a NSC or progenitor state to initiate and maintain the tumor progression, as well as to give rise to the heterogeneous populations observed in malignant gliomas.

Transposon mutagenesis identifies genes that transform neural stem cells into glioma-initiating cells

Neural stem cells (NSCs) are considered to be the cell of origin of glioblastoma multiforme (GBM). However, the genetic alterations that transform NSCs into glioma-initiating cells remain elusive. Using a unique transposon mutagenesis strategy that mutagenizes NSCs in culture, followed by additional rounds of mutagenesis to generate tumors in vivo, we have identified genes and signaling pathways that can transform NSCs into glioma-initiating cells. Mobilization of Sleeping Beauty transposons in NSCs induced the immortalization of astroglial-like cells, which were then able to generate tumors with characteristics of the mesenchymal subtype of GBM on transplantation, consistent with a potential astroglial origin for mesenchymal GBM. Sequence analysis of transposon insertion sites from tumors and immortalized cells identified more than 200 frequently mutated genes, including human GBM-associated genes, such as Met and Nf1, and made it possible to discriminate between genes that function during astroglial immortalization vs. later stages of tumor development. We also functionally validated five GBM candidate genes using a previously undescribed high-throughput method. Finally, we show that even clonally related tumors derived from the same immortalized line have acquired distinct combinations of genetic alterations during tumor development, suggesting that tumor formation in this model system involves competition among genetically variant cells, which is similar to the Darwinian evolutionary processes now thought to generate many human cancers. This mutagenesis strategy is faster and simpler than conventional transposon screens and can potentially be applied to any tissue stem/progenitor cells that can be grown and differentiated in vitro.

The roots of cancer: stem cells and the basis for tumor heterogeneity

Recent studies of prostate cancer and other tumor types have revealed significant support, as well as unexpected complexities, for the application of concepts from normal stem cell biology to cancer. In particular, the cell of origin and cancer stem cell models have been proposed to explain the heterogeneity of tumors during the initiation, propagation, and evolution of cancer. Thus, a basis of intertumor heterogeneity has emerged from studies investigating whether stem cells and/or non-stem cells can serve as cells of origin for cancer and give rise to tumor subtypes that vary in disease outcome. Furthermore, analyses of putative cancer stem cells have revealed the genetically diverse nature of cancers and expanded our understanding of intratumor heterogeneity and clonal evolution. Overall, the principles that have emerged from these stem cell studies highlight the challenges to be surmounted to develop effective treatment strategies for cancer.

Developmental origins of brain tumors

Brain tumors are devastating owing to the high fatality rate and the devastating impact on life qualities of patients. Recent advancement of comparative transcriptome profiling tools and mouse genetic models has greatly deepened our understanding of the developmental origins of these tumors, which could lead to effective therapeutic strategies. We review recent progresses in three types of brain tumors: ependymoma, medulloblastoma, and malignant glioma. The conceptual framework established by these studies converged on three important aspects. First, subtypes in each tumor group originate from distinct cell types. Second, each cell-of-origin is uniquely susceptible to some but not other genetic mutations. Lastly, mutant stem cells may not transform until they differentiate into more restricted progenitor cell type. Overall, these findings indicate the existence of intricate interactions between gene mutations and developmental program for the formation of brain tumors.

The nuclear receptor TLX is required for gliomagenesis within the adult neurogenic niche

Neural stem cells (NSCs) continually generate functional neurons in the adult brain. Due to their ability to proliferate, deregulated NSCs or their progenitors have been proposed as the cells of origin for a number of primary central nervous system neoplasms, including infiltrating gliomas. The orphan nuclear receptor TLX is required for proliferation of adult NSCs, and its upregulation promotes brain tumor formation. However, it is unknown whether TLX is required for gliomagenesis. We examined the genetic interactions between TLX and several tumor suppressors, as well as the role of TLX-dependent NSCs during gliomagenesis, using mouse models. Here, we show that TLX is essential for the proliferation of adult NSCs with a single deletion of p21, p53, or Pten or combined deletion of Pten and p53. While brain tumors still form in Tlx mutant mice, these tumors are less infiltrative and rarely associate with the adult neurogenic niches, suggesting a non-stem-cell origin. Taken together, these results indicate a critical role for TLX in NSC-dependent gliomagenesis and implicate TLX as a therapeutic target to inhibit the development of NSC-derived brain tumors.

A restricted cell population propagates glioblastoma growth after chemotherapy

Glioblastoma multiforme is the most common primary malignant brain tumour, with a median survival of about one year. This poor prognosis is due to therapeutic resistance and tumour recurrence after surgical removal. Precisely how recurrence occurs is unknown. Using a genetically engineered mouse model of glioma, here we identify a subset of endogenous tumour cells that are the source of new tumour cells after the drug temozolomide (TMZ) is administered to transiently arrest tumour growth. A nestin-ΔTK-IRES-GFP (Nes-ΔTK-GFP) transgene that labels quiescent subventricular zone adult neural stem cells also labels a subset of endogenous glioma tumour cells. On arrest of tumour cell proliferation with TMZ, pulse-chase experiments demonstrate a tumour re-growth cell hierarchy originating with the Nes-ΔTK-GFP transgene subpopulation. Ablation of the GFP+ cells with chronic ganciclovir administration significantly arrested tumour growth, and combined TMZ and ganciclovir treatment impeded tumour development. Thus, a relatively quiescent subset of endogenous glioma cells, with properties similar to those proposed for cancer stem cells, is responsible for sustaining long-term tumour growth through the production of transient populations of highly proliferative cells.

Review: insights gained from modelling high-grade glioma in the mouse

High-grade gliomas (HGGs) are devastating primary brain tumours with poor outcomes. Advances towards effective treatments require improved understanding of pathogenesis and relevant model systems for preclinical testing. Mouse models for HGG provide physiologically relevant experimental systems for analysis of HGG pathogenesis. There are advantages and disadvantages to the different methodologies used to generate such models, including implantation, genetic engineering or somatic gene transfer approaches. This review highlights how mouse models have provided insights into the contribution of specific mutations to tumour initiation, progression and phenotype, the influence of tumour micro-environment, and the analysis of cell types that can give rise to glioma. HGGs are a heterogeneous group of tumours, and the complexity of diverse mutations within common signalling pathways as well as the developmental and cell-type context of transformation contributes to the overall diversity of glioma phenotype. Enhanced understanding of the mutations and cell types giving rise to HGG, along with the ability to design increasingly complex mouse models that more closely simulate the process of human gliomagenesis will continue to provide improved experimental systems for dissecting mechanisms of disease pathogenesis and for preclinical testing.

Cell lineage tracing techniques for the study of brain development and regeneration

Characterization of the means by which cells are generated and organized to make an organ as complex as the brain is a formidable task. Understanding how adult stem cells give rise to progeny that integrate into the existing structures during regeneration or in response to injury is equally challenging. Lineage tracing techniques are essential to studying cell behaviors such as proliferation, migration and differentiation, since they allow stem or precursor cells to be marked and their descendants followed and characterized over time. Here, we describe some of the key lineage tracing techniques available to date, highlighting advantages and drawbacks and focusing on their application in neural fate mapping. The more traditional methods are now joined by exciting new approaches to provide a vast array of tools at the disposal of neurobiologists.

MASTR: a technique for mosaic mutant analysis with spatial and temporal control of recombination using conditional floxed alleles in mice

Mosaic mutant analysis, the study of cellular defects in scattered mutant cells in a wild-type environment, is a powerful approach for identifying critical functions of genes and has been applied extensively to invertebrate model organisms. A highly versatile technique has been developed in mouse: MASTR (mosaic mutant analysis with spatial and temporal control of recombination), which utilizes the increasing number of floxed alleles and simultaneously combines conditional gene mutagenesis and cell marking for fate analysis. A targeted allele (R26(MASTR)) was engineered; the allele expresses a GFPcre fusion protein following FLP-mediated recombination, which serves the dual function of deleting floxed alleles and marking mutant cells with GFP. Within 24 hr of tamoxifen administration to R26(MASTR) mice carrying an inducible FlpoER transgene and a floxed allele, nearly all GFP-expressing cells have a mutant allele. The fate of single cells lacking FGF8 or SHH signaling in the developing hindbrain was analyzed using MASTR, and it was revealed that there is only a short time window when neural progenitors require FGFR1 for viability and that granule cell precursors differentiate rapidly when SMO is lost. MASTR is a powerful tool that provides cell-type-specific (spatial) and temporal marking of mosaic mutant cells and is broadly applicable to developmental, cancer, and adult stem cell studies.

Identification of novel pathways involved in the pathogenesis of human adamantinomatous craniopharyngioma

Activating mutations in the gene encoding β-catenin have been identified in the paediatric form of human craniopharyngioma (adamantinomatous craniopharyngioma, ACP), a histologically benign but aggressive pituitary tumour accounting for up to 10% of paediatric intracranial tumours. Recently, we generated an ACP mouse model and revealed that, as in human ACP, nucleocytoplasmic accumulation of β-catenin (β-cat(nc)) and over-activation of the Wnt/β-catenin pathway occurs only in a very small proportion of cells, which form clusters. Here, combining mouse genetics, fluorescence labelling and flow-sorting techniques, we have isolated these cells from tumorigenic mouse pituitaries and shown that the β-cat(nc) cells are enriched for colony-forming cells when cultured in stem cell-promoting media, and have longer telomeres, indicating shared properties with normal pituitary progenitors/stem cells (PSCs). Global gene profiling analysis has revealed that these β-cat(nc) cells express high levels of secreted mitogenic signals, such as members of the SHH, BMP and FGF family, in addition to several chemokines and their receptors, suggesting an important autocrine/paracrine role of these cells in the pathogenesis of ACP and a reciprocal communication with their environment. Finally, we highlight the clinical relevance of these findings by showing that these pathways are also up-regulated in the β-cat(nc) cell clusters identified in human ACP. As well as providing further support to the concept that pituitary stem cells may play an important role in the oncogenesis of human ACP, our data reveal novel disease biomarkers and potential pharmacological targets for the treatment of these devastating childhood tumours.

Innate neural stem cell heterogeneity determines the patterning of glioma formation in children

The concept that gliomas comprise a heterogeneous group of diseases distinguished by their developmental origin raises the intriguing possibility that neural stem cells (NSCs) from different germinal zones have differential capacities to respond to glioma-causing genetic changes. We demonstrate that lateral ventricle subventricular zone NSCs are molecularly and functionally distinct from those of the third ventricle. Consistent with a unique origin for pediatric low-grade glioma, third ventricle, but not lateral ventricle, NSCs hyperproliferate in response to mutations characteristic of childhood glioma. Finally, we demonstrate that pediatric optic gliomas in Nf1 genetically engineered mice arise from the third ventricle. Collectively, these observations establish the importance of innate brain region NSC heterogeneity in the patterning of gliomagenesis in children and adults.

Glioma revisited: from neurogenesis and cancer stem cells to the epigenetic regulation of the niche

Gliomas are the most incident brain tumor in adults. This malignancy has very low survival rates, even when combining radio- and chemotherapy. Among the gliomas, glioblastoma multiforme (GBM) is the most common and aggressive type, and patients frequently relapse or become refractory to conventional therapies. The fact that such an aggressive tumor can arise in such a carefully orchestrated organ, where cellular proliferation is barely needed to maintain its function, is a question that has intrigued scientists until very recently, when the discovery of the existence of proliferative cells in the brain overcame such challenges. Even so, the precise origin of gliomas still remains elusive. Thanks to new advents in molecular biology, researchers have been able to depict the first steps of glioma formation and to accumulate knowledge about how neural stem cells and its progenitors become gliomas. Indeed, GBM are composed of a very heterogeneous population of cells, which exhibit a plethora of tumorigenic properties, supporting the presence of cancer stem cells (CSCs) in these tumors. This paper provides a comprehensive analysis of how gliomas initiate and progress, taking into account the role of epigenetic modulation in the crosstalk of cancer cells with their environment.

Capturing the molecular and biological diversity of high-grade astrocytoma in genetically engineered mouse models

High-grade astrocytoma remains a significant challenge to the clinician and researcher alike. Intense study of the molecular pathogenesis of these tumors has allowed identification of frequent genetic alterations and critical core pathways in this disease. The use of novel mouse genetic tools to study the consequence of specific mutations in brain has led to the development of multiple representative genetically engineered mouse models that provided novel insights into gliomagenesis. As we learn more about the biology of high-grade astrocytoma from the study of these models, we anticipate that our improved understanding will eventually lead to greater success in clinical trials and improved outcome for patients.

Emerging insights into the molecular and cellular basis of glioblastoma

Glioblastoma is both the most common and lethal primary malignant brain tumor. Extensive multiplatform genomic characterization has provided a higher-resolution picture of the molecular alterations underlying this disease. These studies provide the emerging view that "glioblastoma" represents several histologically similar yet molecularly heterogeneous diseases, which influences taxonomic classification systems, prognosis, and therapeutic decisions.

Emerging insights into the molecular and cellular basis of glioblastoma

Glioblastoma is both the most common and lethal primary malignant brain tumor. Extensive multiplatform genomic characterization has provided a higher-resolution picture of the molecular alterations underlying this disease. These studies provide the emerging view that "glioblastoma" represents several histologically similar yet molecularly heterogeneous diseases, which influences taxonomic classification systems, prognosis, and therapeutic decisions.

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.

A review of the role of stem cells in the development and treatment of glioma

The neurosurgical management of patients with intrinsic glial cancers is one of the most rapidly evolving areas of practice. This has been fuelled by advances in surgical technique not only in cytoreduction but also in drug delivery. Further innovation will depend on a deeper understanding of the biology of the disease and an appreciation of the limitations of current knowledge. Here we review the controversial topic of cancer stem cells applied to glioma to provide neurosurgeons with a working overview. It is now recognised that the adult human brain contains regionally specified cell populations capable of self-renewal that may contribute to tumour growth and maintenance following accumulated mutational change. Tumour cells adapted to maintain growth demonstrate some stem-like characteristics and as such constitute a legitimate therapeutic target. Cellular reprogramming technologies raise the potential of developing stem cells as novel surgical tools to target disease and possibly ameliorate some of the consequences of treatment. Achieving these goals remains a significant challenge to neurosurgical oncologists, not least in challenging how we think about treating brain cancer. This review will briefly examine our understanding of adult stem cells within the brain, the evidence that they contribute to the development of brain tumours as tumour-initiating cells, and the potential implications for therapy. It will also look at the role stem cells may play in the future management of glioma.

MAPK pathway activation in pilocytic astrocytoma

Pilocytic astrocytoma (PA) is the most common tumor of the pediatric central nervous system (CNS). A body of research over recent years has demonstrated a key role for mitogen-activated protein kinase (MAPK) pathway signaling in the development and behavior of PAs. Several mechanisms lead to activation of this pathway in PA, mostly in a mutually exclusive manner, with constitutive BRAF kinase activation subsequent to gene fusion being the most frequent. The high specificity of this fusion to PA when compared with other CNS tumors has diagnostic utility. In addition, the frequency of alteration of this key pathway provides an opportunity for molecularly targeted therapy in this tumor. Here, we review the current knowledge on mechanisms of MAPK activation in PA and some of the downstream consequences of this activation, which are now starting to be elucidated both in vitro and in vivo, as well as clinical considerations and possible future directions.

Paediatric and adult malignant glioma: close relatives or distant cousins?

Gliomas in children differ from their adult counterparts by their distribution of histological grade, site of presentation and rate of malignant transformation. Although rare in the paediatric population, patients with high-grade gliomas have, for the most part, a comparably dismal clinical outcome to older patients with morphologically similar lesions. Molecular profiling data have begun to reveal the major genetic alterations underpinning these malignant tumours in children. Indeed, the accumulation of large datasets on adult high-grade glioma has revealed key biological differences between the adult and paediatric disease. Furthermore, subclassifications within the childhood age group can be made depending on age at diagnosis and tumour site. However, challenges remain on how to reconcile clinical data from adult patients to tailor novel treatment strategies specifically for paediatric patients.

Studying a complex tumor: potential and pitfalls

Glioblastoma multiforme is a histopathologically heterogeneous disease with few treatment options. Therapy based on genomic alterations is rapidly gaining popularity because of the high response rate and high specificity. DNA copy number and exon-sequencing studies of glioblastoma multiforme samples have revealed recurrent genomic alterations in genes such as TP53, EGFR, and IDH1, but to date, this has not resulted in novel glioblastoma multiforme therapies. Identification of expression subtypes has resulted in new insights such as the association between genomic abnormalities and expression signatures. This review describes the types of genomic studies that have been performed and that are underway, the most prominent results, and the implications of genomic research for the development of clinical treatment modalities.

Differential expression of 2',3'-cyclic-nucleotide 3'-phosphodiesterase and neural lineage markers correlate with glioblastoma xenograft infiltration and patient survival

PURPOSE

Glioblastoma multiforme (GBM) is a poorly treated human brain cancer with few established clinically useful molecular prognostic markers. We characterized glioblastoma stem-like cells (GSC) according to developmental neural lineage markers and correlated their expression with patient survival.

EXPERIMENTAL DESIGN

Immunoblot array of neural lineage markers classified five independently isolated human GSC lines into three classes exhibiting differential expression of oligodendrocyte progenitor cells (OPC), astrocyte progenitor cells (APC), and neural progenitor cells (NPC) markers. Immunodeficient mice were orthotopically implanted with each cell line to evaluate tumor infiltration and recipient survival. 2',3'-Cyclic-nucleotide 3'-phosphodiesterase (CNP) antigenic expression was used to evaluate a clinically annotated GBM tissue microarray with 115 specimens.

RESULTS

We report that molecular classification of patient-derived GSCs using neural lineage markers show association with differential xenograft invasiveness, and also show significant correlation to survival in both the mouse model and human patients. Orthotopic implantation into immunodeficient mice showed Ki-67 proliferative index independent xenograft infiltration: class I GSCs (OPC and NPC positive) established focal lesions, class II GSCs (NPC positive) formed minimally invasive lesions, and class III GSCs (APC positive) established highly infiltrative lesions. The OPC marker, CNP also exhibited high expression in focal xenografts versus low expression in invasive xenografts. Differential CNP expression correlated with mouse model survival, and CNP immunoassay of a large GBM tissue microarray also showed significant differential patient survival.

CONCLUSIONS

GSC classification with developmental neural lineage markers revealed CNP as a novel and potentially useful clinical prognosis marker, and suggests clinical importance for patient-specific GSC analysis.

On the origin of glioma

Glioma is the most frequent primary brain tumor of adults that has a presumably glial origin. Although our knowledge regarding molecular mechanisms and signaling pathways involved in gliomagenesis has increased immensely during the past decade, high-grade glioma remains a lethal disease with dismal prognosis. The failure of current therapies has to a large extent been ascribed the functional heterogeneity of glioma cells. One reason for this heterogeneity is most certainly the large number of variations in genetic alterations that can be found in high-grade gliomas. Another factor that may influence glioma heterogeneity could be the cell type from which the glioma is initiated. The cell of origin for glioma is still undefined, and additional knowledge about this issue may prove critical for a more complete understanding of glioma biology. Based on information from patients, developmental biology, and experimental glioma models, the most putative target cells include astrocytes, neural stem cells, and oligodendrocyte precursor cells, which are all discussed in more detail in this article. Animal modeling of glioma suggests that these three cell types have the capability to be the origin of glioma, and we have reason to believe that, depending on the initiating cell type, prognosis and response to therapy may be significantly different. Thus, it is essential to explore further the role of cellular origin in glioma.

The cellular origin for malignant glioma and prospects for clinical advancements

Glioma remains incurable despite great advancements in medicine. Targeting the cell of origin for gliomas could bring great hope for patients. However, as a collection of diverse diseases, each subtype of glioma could derive from a distinct cell of origin. To resolve such a complex problem, one must use multiple research approaches to gain deep insights. Here we review current evidence regarding the cell of origin from clinical observations, whole-genome molecular pathology and glioma animal models. We conclude that neural stem cells, glial progenitors (including oligodendrocyte progenitor cells) and astrocytes could all serve as cells of origin for gliomas, and that cells incurring initial mutations (cells of mutation) might not transform, while their progeny cells could instead transform and act as cells of origin. Further studies with multidisciplinary approaches are needed to link each subtype to a particular cell of origin, and to develop effective therapies that target the signaling network within these cells.

Regulation of glioblastoma multiforme stem-like cells by inhibitor of DNA binding proteins and oligodendroglial lineage-associated transcription factors

Tumor-initiating stem cells (also referred to as cancer stem cells, CSCs) are a subpopulation of cancer cells that play unique roles in tumor propagation, therapeutic resistance and tumor recurrence. It is increasingly important to understand how molecular signaling regulates the self-renewal and differentiation of CSCs. Basic helix-loop-helix (bHLH) transcription factors are critical for the differentiation of normal stem cells, yet their roles in neoplastic stem cells are not well understood. In glioblastoma neurosphere cultures that contain cancer stem cells (GBM-CSCs), the bHLH family member inhibitors of DNA binding protein 2 and 4 (Id2 and Id4) were found to be upregulated during the differentiation of GBM-CSCs in response to histone deacetylase inhibitors. In this study, we examined the functions of Id2 and Id4 in GBM neurosphere cells and identified Id proteins as efficient differentiation regulators of GBM-CSCs. Overexpression of Id2 and Id4 promoted the lineage-specific differentiation of GBM neurosphere cells as evidenced by the induction of neuronal/astroglial differentiation markers Tuj1 and GFAP and the inhibition of the oligodendroglial marker GalC. Id protein overexpression also reduced both stem cell marker expression and neurosphere formation potential, a biological marker of cancer cell "stemness." We further showed that Id2 and Id4 regulated GBM neurosphere differentiation through downregulating of another bHLH family member, the oligodendroglial lineage-associated transcription factors (Olig) 1 and 2. Our results provide evidence for distinct functions of Id proteins in neoplastic stem cells, which supports Id proteins and their downstream targets as potential candidates for differentiation therapy in CSCs.

Extensions of MADM (mosaic analysis with double markers) in mice

Mosaic Analysis with Double Markers (MADM) is a method for generating genetically mosaic mice, in which sibling mutant and wild-type cells are labeled with different fluorescent markers. It is a powerful tool that enables analysis of gene function at the single cell level in vivo. It requires transgenic cassettes to be located between the centromere and the mutation in the gene of interest on the same chromosome. Here we compare procedures for introduction of MADM cassettes into new loci in the mouse genome, and describe new approaches for expanding the utility of MADM. We show that: 1) Targeted homologous recombination outperforms random transgenesis in generation of reliably expressed MADM cassettes, 2) MADM cassettes in new genomic loci need to be validated for biallelic and ubiquitous expression, 3) Recombination between MADM cassettes on different chromosomes can be used to study reciprocal chromosomal deletions/duplications, and 4) MADM can be modified to permit transgene expression by combining it with a binary expression system. The advances described in this study expand current, and enable new and more versatile applications of MADM.

Lineage tracing

Lineage tracing is the identification of all progeny of a single cell. Although its origins date back to developmental biology of invertebrates in the 19(th) century, lineage tracing is now an essential tool for studying stem cell properties in adult mammalian tissues. Lineage tracing provides a powerful means of understanding tissue development, homeostasis, and disease, especially when it is combined with experimental manipulation of signals regulating cell-fate decisions. Recently, the combination of inducible recombinases, multicolor reporter constructs, and live-cell imaging has provided unprecedented insights into stem cell biology. Here we discuss the different experimental strategies currently available for lineage tracing, their associated caveats, and new opportunities to integrate lineage tracing with the monitoring of intracellular signaling pathways.

Kv1.1-dependent control of hippocampal neuron number as revealed by mosaic analysis with double markers

Megencephaly, or mceph, is a spontaneous frame-shift mutation of the mouse Kv1.1 gene. This mceph mutation results in a truncated Kv1.1 channel α-subunit without the channel pore domain or the voltage sensor. Interestingly, mceph/mceph mouse brains are enlarged and – unlike wild-type mouse brains – they keep growing throughout adulthood, especially in the hippocampus and ventral cortex. We used mosaic analysis with double markers (MADM) to identify the underlying mechanism. In mceph-MADM6 mice with only a small fraction of neurons homozygous for the mceph mutation, those homozygous mceph/mceph neurons in the hippocampus are more abundant than wild-type neurons produced by sister neural progenitors. In contrast, neither mceph/mceph astrocytes, nor neurons in the adjacent dorsal cortex (including the entorhinal and parietal cortex) exhibited overgrowth in the adult brain. The sizes of mceph/mceph hippocampal neurons were comparable to mceph/+ or wild-type neurons. Our mosaic analysis reveals that loss of Kv1.1 function causes an overproduction of hippocampal neurons, leading to an enlarged brain phenotype.

Neurofibromatosis 2011: a report of the Children's Tumor Foundation annual meeting

The 2011 annual meeting of the Children's Tumor Foundation, the annual gathering of the neurofibromatosis (NF) research and clinical communities, was attended by 330 participants who discussed integration of new signaling pathways into NF research, the appreciation for NF mutations in sporadic cancers, and an expanding pre-clinical and clinical agenda. NF1, NF2, and schwannomatosis collectively affect approximately 100,000 persons in US, and result from mutations in different genes. Benign tumors of NF1 (neurofibroma and optic pathway glioma) and NF2 (schwannoma, ependymoma, and meningioma) and schwannomatosis (schwannoma) can cause significant morbidity, and there are no proven drug treatments for any form of NF. Each disorder is associated with additional manifestations causing morbidity. The research presentations described in this review covered basic science, preclinical testing, and results from clinical trials, and demonstrate the remarkable strides being taken toward understanding of and progress toward treatments for these disorders based on the close interaction among scientists and clinicians.