Pilocytic astrocytomas do not show most of the genetic changes commonly seen in diffuse astrocytomas

Type 1 Insulin-Like Growth Factor Receptor Nuclear Localization in High-Grade Glioma Cells Enhances Motility, Metabolism, and In Vivo Tumorigenesis

Gliomas are the most frequent solid tumors in children. Among these, high-grade gliomas are less common in children than in adults, though they are similar in their aggressive clinical behavior. In adults, glioblastoma is the most lethal tumor of the central nervous system. Insulin-like growth factor 1 receptor (IGF1R) plays an important role in cancer biology, and its nuclear localization has been described as an adverse prognostic factor in different tumors. Previously, we have demonstrated that, in pediatric gliomas, IGF1R nuclear localization is significantly associated with high-grade tumors, worst clinical outcome, and increased risk of death. Herein we explore the role of IGF1R intracellular localization by comparing two glioblastoma cell lines that differ only in their IGF1R capacity to translocate to the nucleus. In vitro, IGF1R nuclear localization enhances glioblastoma cell motility and metabolism without affecting their proliferation. In vivo, IGF1R has the capacity to translocate to the nucleus and allows not only a higher proliferation rate and the earlier development of tumors but also renders the cells sensitive to OSI906 therapy. With this work, we provide evidence supporting the implications of the presence of IGF1R in the nucleus of glioma cells and a potential therapeutic opportunity for patients harboring gliomas with IGF1R nuclear localization.

hTERT Transduction Extends the Lifespan of Primary Pediatric Low-Grade Glioma Cells While Preserving the Biological Response to NGF

The neurotrophin nerve growth factor (NGF) modulates the growth of human gliomas and is able to induce cell differentiation through the engagement of tropomyosin receptor kinase A (TrkA) receptor, although the role played in controlling glioma survival has proved controversial. Unfortunately, the slow growth rate of low-grade gliomas (LGG) has made it difficult to investigate NGF effects on these tumors in preclinical models. In fact, patient-derived low-grade human astrocytoma cells duplicate only a limited number of times in culture before undergoing senescence. Nevertheless, replicative senescence can be counteracted by overexpression of hTERT, the catalytic subunit of telomerase, which potentially increases the proliferative potential of human cells without inducing cancer-associated changes. We have extended, by hTERT transduction, the proliferative in vitro potential of a human LGG cell line derived from a pediatric pilocytic astrocytoma (PA) surgical sample. Remarkably, the hTERT-transduced LGG cells showed a behavior similar to that of the parental line in terms of biological responses to NGF treatment, including molecular events associated with induction of NGF-related differentiation. Therefore, transduction of LGG cells with hTERT can provide a valid approach to increase the in vitro life-span of patient-derived astrocytoma primary cultures, characterized by a finite proliferative potential.

Type 1 IGF Receptor Localization in Paediatric Gliomas: Significant Association with WHO Grading and Clinical Outcome

Nuclear localization of insulin-like growth factor receptor type 1 (IGF-1R) has been described as adverse prognostic factor in some cancers. We studied the expression and localization of IGF-1R in paediatric patients with gliomas, as well as its association with World Health Organization (WHO) grading and survival. We conducted a single cohort, prospective study of paediatric patients with gliomas. Samples were taken at the time of the initial surgery; IGF-1R expression and localization were characterized by immunohistochemistry (IHC), subcellular fractionation and western blotting. Tumours (47/53) showed positive staining for IGF-1R by IHC. IGF-1R nuclear labelling was observed in 10/47 cases. IGF-1R staining was mostly non-nuclear in low-grade tumours, while IGF-1R nuclear labelling was predominant in high-grade gliomas (p = 0.0001). Survival was significantly longer in patients with gliomas having non-nuclear IGF-1R localization than in patients with nuclear IGF-1R tumours (p = 0.016). In gliomas, IGF-1R nuclear localization was significantly associated with both high-grade tumours and increased risk of death. Based on a prospective design, we provide evidence of a potential usefulness of intracellular localization of IGF-1R as prognostic factor in paediatric patients with gliomas.

Comparative transcriptomics reveals similarities and differences between astrocytoma grades

Background

Astrocytomas are the most common primary brain tumors distinguished into four histological grades. Molecular analyses of individual astrocytoma grades have revealed detailed insights into genetic, transcriptomic and epigenetic alterations. This provides an excellent basis to identify similarities and differences between astrocytoma grades.

Methods

We utilized public omics data of all four astrocytoma grades focusing on pilocytic astrocytomas (PA I), diffuse astrocytomas (AS II), anaplastic astrocytomas (AS III) and glioblastomas (GBM IV) to identify similarities and differences using well-established bioinformatics and systems biology approaches. We further validated the expression and localization of Ang2 involved in angiogenesis using immunohistochemistry.

Results

Our analyses show similarities and differences between astrocytoma grades at the level of individual genes, signaling pathways and regulatory networks. We identified many differentially expressed genes that were either exclusively observed in a specific astrocytoma grade or commonly affected in specific subsets of astrocytoma grades in comparison to normal brain. Further, the number of differentially expressed genes generally increased with the astrocytoma grade with one major exception. The cytokine receptor pathway showed nearly the same number of differentially expressed genes in PA I and GBM IV and was further characterized by a significant overlap of commonly altered genes and an exclusive enrichment of overexpressed cancer genes in GBM IV. Additional analyses revealed a strong exclusive overexpression of CX3CL1 (fractalkine) and its receptor CX3CR1 in PA I possibly contributing to the absence of invasive growth. We further found that PA I was significantly associated with the mesenchymal subtype typically observed for very aggressive GBM IV. Expression of endothelial and mesenchymal markers (ANGPT2, CHI3L1) indicated a stronger contribution of the micro-environment to the manifestation of the mesenchymal subtype than the tumor biology itself. We further inferred a transcriptional regulatory network associated with specific expression differences distinguishing PA I from AS II, AS III and GBM IV. Major central transcriptional regulators were involved in brain development, cell cycle control, proliferation, apoptosis, chromatin remodeling or DNA methylation. Many of these regulators showed directly underlying DNA methylation changes in PA I or gene copy number mutations in AS II, AS III and GBM IV.

Conclusions

This computational study characterizes similarities and differences between all four astrocytoma grades confirming known and revealing novel insights into astrocytoma biology. Our findings represent a valuable resource for future computational and experimental studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1939-9) contains supplementary material, which is available to authorized users.

Current Understanding of BRAF Alterations in Diagnosis, Prognosis, and Therapeutic Targeting in Pediatric Low-Grade Gliomas

The mitogen-activated protein kinase (MAPK) pathway is known to play a key role in the initiation and maintenance of many tumors as well as normal development. This often occurs through mutation of the genes encoding RAS and RAF proteins which are involved in signal transduction in this pathway. BRAF is one of three RAF kinases which act as downstream effectors of growth factor signaling leading to cell cycle progression, proliferation, and survival. Initially reported as a point mutation (V600E) in the majority of metastatic melanomas, other alterations in the BRAF gene have now been reported in a variety of human cancers including papillary thyroid cancer, colon carcinomas, hairy cell leukemia, and more recently in gliomas. The identification of oncogenic mutations in the BRAF gene have led to a revolution in the treatment of metastatic melanoma using targeted molecular therapies that affect the MAPK pathway either directly through BRAF inhibition or downstream through inhibition of MEK. This review describes the molecular biology of BRAF in the context of pediatric low-grade gliomas, the role of BRAF as a diagnostic marker, the prognostic implications of BRAF, and evidence for therapeutic targeting of BRAF.

Transcriptional analysis of aggressiveness and heterogeneity across grades of astrocytomas

Astrocytoma is the most common glioma, accounting for half of all primary brain and spinal cord tumors. Late detection and the aggressive nature of high-grade astrocytomas contribute to high mortality rates. Though many studies identify candidate biomarkers using high-throughput transcriptomic profiling to stratify grades and subtypes, few have resulted in clinically actionable results. This shortcoming can be attributed, in part, to pronounced lab effects that reduce signature robustness and varied individual gene expression among patients with the same tumor. We addressed these issues by uniformly preprocessing publicly available transcriptomic data, comprising 306 tumor samples from three astrocytoma grades (Grade 2, 3, and 4) and 30 non-tumor samples (normal brain as control tissues). Utilizing Differential Rank Conservation (DIRAC), a network-based classification approach, we examined the global and individual patterns of network regulation across tumor grades. Additionally, we applied gene-based approaches to identify genes whose expression changed consistently with increasing tumor grade and evaluated their robustness across multiple studies using statistical sampling. Applying DIRAC, we observed a global trend of greater network dysregulation with increasing tumor aggressiveness. Individual networks displaying greater differences in regulation between adjacent grades play well-known roles in calcium/PKC, EGF, and transcription signaling. Interestingly, many of the 90 individual genes found to monotonically increase or decrease with astrocytoma grade are implicated in cancer-affected processes such as calcium signaling, mitochondrial metabolism, and apoptosis. The fact that specific genes monotonically increase or decrease with increasing astrocytoma grade may reflect shared oncogenic mechanisms among phenotypically similar tumors. This work presents statistically significant results that enable better characterization of different human astrocytoma grades and hopefully can contribute towards improvements in diagnosis and therapy choices. Our results also identify a number of testable hypotheses relating to astrocytoma etiology that may prove helpful in developing much-needed biomarkers for earlier disease detection.

The molecular biology of WHO grade I astrocytomas

World Health Organization (WHO) grade I astrocytomas include pilocytic astrocytoma (PA) and subependymal giant cell astrocytoma (SEGA). As technologies in pharmacologic neo-adjuvant therapy continue to progress and as molecular characteristics are progressively recognized as potential markers of both clinically significant tumor subtypes and response to therapy, interest in the biology of these tumors has surged. An updated review of the current knowledge of the molecular biology of these tumors is needed. We conducted a Medline search to identify published literature discussing the molecular biology of grade I astrocytomas. We then summarized this literature and discuss it in a logical framework through which the complex biology of these tumors can be clearly understood. A comprehensive review of the molecular biology of WHO grade I astrocytomas is presented. The past several years have seen rapid progress in the level of understanding of PA in particular, but the molecular literature regarding both PA and SEGA remains nebulous, ambiguous, and occasionally contradictory. In this review we provide a comprehensive discussion of the current understanding of the chromosomal, genomic, and epigenomic features of both PA and SEGA and provide a logical framework in which these data can be more readily understood.

Pathological and molecular advances in pediatric low-grade astrocytoma

Pediatric low-grade astrocytomas are the most common brain tumors in children. They can have similar microscopic and clinical features, making accurate diagnosis difficult. For patients whose tumors are in locations that do not permit full resection, or those with an intrinsically aggressive biology, more effective therapies are required. Until recently, little was known about the molecular changes that drive the initiation and growth of pilocytic and other low-grade astrocytomas beyond the association of a minority of cases, primarily in the optic nerve, with neurofibromatosis type 1. Over the past several years, a wide range of studies have implicated the BRAF oncogene and other members of this signaling cascade in the pathobiology of pediatric low-grade astrocytoma. In this review, we attempt to summarize this rapidly developing field and discuss the potential for translating our growing molecular knowledge into improved diagnostic and prognostic biomarkers and new targeted therapies.

Differences in molecular genetics between pediatric and adult malignant astrocytomas: age matters

The microscope - the classical tool for the investigation of cells and tissues - remains the basis for the classification of tumors throughout the body. Nowhere has this been more true than in the grading of astrocytomas. In spite of the fact that our parents warned us not to judge a book by its cover, we have continued to assume that adult and pediatric malignant gliomas that look the same, will have the same mutations, and thus respond to the same therapy. Rapid advances in molecular biology have permitted us the opportunity to go inside the cell and characterize the genetic events that underlie the true molecular heterogeneity of adult and pediatric brain tumors. In this paper, we will discuss some of the important clinical differences between pediatric and adult gliomas, with a focus on the molecular analysis of these different age groups.

Long-term epilepsy-associated tumors

The term long-term epilepsy associated tumor (LEAT) encompasses lesions identified in patients investigated for long histories (often 2 years or more) of drug-resistant epilepsy. They are generally slowly growing, low grade, cortically based tumors, more often arising in younger age groups and in many cases exhibit neuronal in addition to glial differentiation. Gangliogliomas and dysembryoplastic neuroepithelial tumors predominate in this group. LEATs are further united by cyto-architectural changes that may be present in the adjacent cortex which have some similarities to developmental focal cortical dysplasias (FCD); these are now grouped as FCD type IIIb in the updated International League Against Epilepsy (ILAE) classification. In the majority of cases, surgical treatments are beneficial from both perspectives of managing the seizures and the tumor. However, in a minority, seizures may recur, tumors may show regrowth or recurrence, and rarely undergo anaplastic progression. Predicting and identifying tumors likely to behave less favorably are key objectives of the neuropathologist. With immunohistochemistry and modern molecular pathology, it is becoming increasingly possible to refine diagnostic groups. Despite this, some LEATs remain difficult to classify, particularly tumors with "non-specific" or diffuse growth patterns. Modification of LEAT classification is inevitable with the goal of unifying terminological criteria applied between centers for accurate clinico-pathological-molecular correlative data to emerge. Finally, establishing the epileptogenic components of LEAT, either within the lesion or perilesional cortex, will elucidate the cellular mechanisms of epileptogenesis, which in turn will guide optimal surgical management of these lesions.

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.

Impact of molecular biology studies on the understanding of brain tumors in childhood

Pediatric brain tumors are the second most common form of childhood malignancy. Brain tumors are a very heterogenous group of tumors and the pathogenesis of many of these tumors is yet to be clearly elucidated. Current diagnostic tools include histopathology and immunohistochemistry, but classification based on these means has significant limitations. As our understanding of the molecular biology of individual tumors continues to increase it has led to the identification of reliable and increasingly available molecular biomarkers. Molecular techniques are likely to complement current standard means of investigation and help not only overcome diagnostic challenges but may also result in better disease classification and risk stratification, leading to more personalized therapeutic approaches.

Interplay among BRAF, p16, p53, and MIB1 in pediatric low-grade gliomas

BRAF rearrangements and BRAF V600E point mutations are recurring events in pediatric low-grade gliomas. However, their clinical significance, including possible interactions between these markers and other glioma biomarkers, is unclear. In this study a retrospective cohort of 198 pediatric low-grade gliomas (including 40 treated with adjuvant therapy) was analyzed for BRAF rearrangements, BRAF V600E, p16/CDKN2A deletion, p53 expression, and MIB1 proliferation index. In tumors with BRAF rearrangement, homozygous p16 deletion correlated with shorter progression-free survival (P = .04). A high MIB1 proliferation index trended toward worse response to adjuvant radiotherapy compared to BRAF-rearranged, p16-intact tumors (P = .08). On multivariate analysis, the 2 most consistently powerful independent adverse prognostic markers were midline location (P = .0001) and p16 deletion (P = .03). Tumors with BRAF V600E had a strong trend toward an increased risk for progression (hazard ratio = 2.48, P = .07), whereas those with BRAF rearrangement had a milder trend toward reduced risk (hazard ratio = .54, P = .15). These data suggest that p16 deletion adversely impacts the outcomes of BRAF-driven gliomas, that high proliferation index may be a better marker of progression risk than BRAF, that BRAF rearrangement and BRAF V600E might not necessarily produce comparable outcomes, and that none of these markers is stronger than tumor location in determining prognosis in pediatric low-grade gliomas.

Genetic aberrations leading to MAPK pathway activation mediate oncogene-induced senescence in sporadic pilocytic astrocytomas

PURPOSE

Oncogenic BRAF/Ras or NF1 loss can potentially trigger oncogene-induced senescence (OIS) through activation of the mitogen-activated protein kinase (MAPK) pathway. Somatic genetic abnormalities affecting this pathway occur in the majority of pilocytic astrocytomas (PA), the most prevalent brain neoplasm in children. We investigated whether OIS is induced in PA.

EXPERIMENTAL DESIGN

We tested expression of established senescence markers in three independent cohorts of sporadic PA. We also assessed for OIS in vitro, using forced expression of wild-type and V600E-mutant BRAF in two astrocytic cell lines: human telomerase reverse transcriptase (hTERT)-immortalized astrocytes and fetal astrocytes.

RESULTS

Our results indicate that PAs are senescent as evidenced by marked senescence-associated acidic β-galactosidase activity, low KI-67 index, and induction of p16(INK4a) but not p53 in the majority of 52 PA samples (46 of 52; 88.5%). Overexpression of a number of senescence-associated genes [CDKN2A (p16), CDKN1A (p21), CEBPB, GADD45A, and IGFBP7] was shown at the mRNA level in two independent PA tumor series. In vitro, sustained activation of wild-type or mutant BRAF induced OIS in both astrocytic cell lines. Loss of p16(INK4a) in immortalized astrocytes abrogated OIS, indicative of the role of this pathway in mediating this phenomenon in astrocytes. OIS is a mechanism of tumor suppression that restricts the progression of benign tumors. We show that it is triggered in PAs through p16(INK4a) pathway induction following aberrant MAPK activation.

CONCLUSIONS

OIS may account for the slow growth pattern in PA, the lack of progression to higher-grade astrocytomas, and the high overall survival of affected patients.

OLIG2 is differentially expressed in pediatric astrocytic and in ependymal neoplasms

The bHLH transcription factor, OLIG2, is universally expressed in adult human gliomas and, as a major factor in the development of oligodendrocytes, is expressed at the highest levels in low-grade oligodendroglial tumors. In addition, it is functionally required for the formation of high-grade astrocytomas in a genetically relevant murine model. The pediatric gliomas have genomic profiles that are different from the corresponding adult tumors and accordingly, the expression of OLIG2 in non-oligodendroglial pediatric gliomas is not well documented within specific tumor types. In the current study, the pattern of OLIG2 expression in a spectrum of 90 non-oligodendroglial pediatric gliomas varied from very low levels in the ependymomas (cellular and tanycytic) to high levels in pilocytic astrocytoma, and in the diffuse-type astrocytic tumors (WHO grades II–IV). With dual-labeling, glioblastoma had the highest percentage of OLIG2 expressing cells that were also Ki-67 positive (mean = 16.3%) whereas pilocytic astrocytoma WHO grade I and astrocytoma WHO grade II had the lowest (0.9 and 1%, respectively); most of the Ki-67 positive cells in the diffuse-type astrocytomas (WHO grade II–III) were also OLIG2 positive (92–94%). In contrast to the various types of pediatric astrocytic tumors, all ependymomas WHO grade II, regardless of site of origin, showed at most minimal OLIG2 expression, suggesting that OLIG2 function in pediatric gliomas is cell lineage dependent.

Association of molecular alterations, including BRAF, with biology and outcome in pilocytic astrocytomas

Pilocytic astrocytoma (PA) is the most common glioma in the pediatric population. PAs can exhibit variable behavior that does not always correlate with location. Although oncogenic rearrangements of the BRAF gene have recently been described in PAs, it is not clear whether such alterations have an impact on outcome. An institutional cohort of 147 PAs (118 with outcome data) from both cerebellar and non-cerebellar locations (spine, diencephalon, midbrain, brainstem, and cortex) was utilized in this study. Parameters included quantification of characteristic morphologic variables as well as genes and molecular loci previously shown to be of relevance in high-grade gliomas, including 1p, 9p, 10q, 17p, 19q, and BRAF. Neither 1p, 9p, and 10q nor 19q showed significant association with outcome in PAs, although p16 deletion was more common in PAs of the midbrain, brainstem, and spinal cord. Loss of heterozygosity on 17p13 correlated with increased risk of recurrence in cerebellar tumors. BRAF gene rearrangements were more common in cerebellar tumors than non-cerebellar tumors and associated with classic biphasic histology in the cerebellum. However, clinical outcome was independent of BRAF status. The molecular biology of PAs differs according to location, yet BRAF rearrangements do not appear to produce PAs with different behavior. Nevertheless, such tumors may have altered sensitivity to pathway-specific adjuvant therapy. Additionally, deletion on 17p13 may be an adverse prognostic biomarker in cerebellar tumors.

Brain tumors across the age spectrum: biology, therapy, and late effects

The clinical difference between brain tumors in adults and children is striking. Compared with adults, pediatric tumor types (mostly glial and neuronal) are more sensitive to adjuvant irradiation and chemotherapy. Pediatric tumors more often require craniospinal irradiation based on their propensity to disseminate within the neuraxis. The spectrum of side effects is broader in the child based on age and extent of treatment: radiation therapy brings increased risk of severe long-term sequelae affecting neurologic, endocrine, and cognitive function. In this review of glioma, ependymoma, and medulloblastoma, we highlight the differences between adults and children, including the higher incidence of spinal cord ependymoma and supratentorial high-grade glioma in the adult and a higher incidence of medulloblastoma in the child. With the exception of completely resected low-grade glioma, radiation therapy remains a standard of care for most patients. In some settings, the radiation oncologist should suggest further surgery or additional adjuvant therapy in an effort to optimize local tumor control. An effort is underway to better characterize adult and pediatric brain tumors biologically with an emphasis on improving our understanding of tumor genesis, malignant transformation, and some of the similarities and differences between tumor types and their response to conventional therapy.

Profiling critical cancer gene mutations in clinical tumor samples

BACKGROUND

Detection of critical cancer gene mutations in clinical tumor specimens may predict patient outcomes and inform treatment options; however, high-throughput mutation profiling remains underdeveloped as a diagnostic approach. We report the implementation of a genotyping and validation algorithm that enables robust tumor mutation profiling in the clinical setting.

METHODOLOGY

We developed and implemented an optimized mutation profiling platform ("OncoMap") to interrogate approximately 400 mutations in 33 known oncogenes and tumor suppressors, many of which are known to predict response or resistance to targeted therapies. The performance of OncoMap was analyzed using DNA derived from both frozen and FFPE clinical material in a diverse set of cancer types. A subsequent in-depth analysis was conducted on histologically and clinically annotated pediatric gliomas. The sensitivity and specificity of OncoMap were 93.8% and 100% in fresh frozen tissue; and 89.3% and 99.4% in FFPE-derived DNA. We detected known mutations at the expected frequencies in common cancers, as well as novel mutations in adult and pediatric cancers that are likely to predict heightened response or resistance to existing or developmental cancer therapies. OncoMap profiles also support a new molecular stratification of pediatric low-grade gliomas based on BRAF mutations that may have immediate clinical impact.

CONCLUSIONS

Our results demonstrate the clinical feasibility of high-throughput mutation profiling to query a large panel of "actionable" cancer gene mutations. In the future, this type of approach may be incorporated into both cancer epidemiologic studies and clinical decision making to specify the use of many targeted anticancer agents.

Impact of morphology, MIB-1, p53 and MGMT on outcome in pilocytic astrocytomas

Pilocytic astrocytoma (PA) is the most common glioma in the pediatric population. PAs can exhibit variable behavior that does not always correlate with location, yet at present there is no way to predict which tumors will be more aggressive. To address this problem, an institutional cohort of 147 PAs (118 with outcome data) from both cerebellar and noncerebellar locations (spine, diencephalon, midbrain, brainstem and cortex) was utilized. Parameters included quantification of characteristic morphologic variables as well as genes previously shown to be of relevance in high-grade gliomas, including MIB-1, p53 and MGMT. In this cohort, the classic biphasic appearance was most common in cerebellar tumors, whereas noncerebellar tumors were predominantly microcystic. Associations with outcome suggest that the presence of degenerative atypia may be a favorable factor in PAs. Oligodendroglial morphology and the absence of leptomeningeal invasion are adverse histologic factors, but only in cerebellar tumors. Conversely, MIB-1 proliferation index and p53 and MGMT expression do not correlate with outcome. Morphologic biomarkers thus do exist for PAs, but the utility of each biomarker varies according to location. These results suggest that PAs differ fundamentally according to location; therefore, biological behavior may not simply depend on extent of resection.

Update on diagnostic practice: tumors of the nervous system

CONTEXT

Changes in the practice of diagnosing brain tumors are formally reflected in the evolution of the World Health Organization classification. Beyond this classification, the practice of diagnostic pathology is also changing with the availability of new tests and the introduction of new treatment options.

OBJECTIVE

Glioblastomas, oligodendrogliomas, glioneuronal tumors, and primitive pediatric tumors are discussed in an exemplary way to illustrate these changes.

DATA SOURCES

Review of relevant publications through Medline database searches.

CONCLUSIONS

The example of glioblastomas shows how new predictive markers may help identify subgroups of tumors that respond to certain therapy regimens. The development of new treatment strategies also leads to different questions in the assessment of brain tumors, as seen in the example of pseudoprogression or the changes in tumor growth pattern in patients taking bevacizumab. Oligodendrogliomas illustrate how the identification of 1p/19q loss as a cytogenetic aberration aids our understanding of these tumors and changes diagnostic practice but also introduces new challenges in classification. Glioneuronal tumors are an evolving group of lesions. Besides a growing list of usually low-grade entities with well-defined morphologic features, these also include more poorly defined cases in which a component of infiltrating glioma is often associated with focal neuronal elements. The latter is biologically interesting but of uncertain clinical significance. Oligodendrogliomas and glioneuronal tumors both illustrate the importance of effective communication between the pathologist and the treating oncologist in the discussion of these patients. Finally, the discussion of primitive pediatric tumors stresses the clinical importance of the distinction between different entities, like atypical teratoid rhabdoid tumor, "central" (supratentorial) primitive neuroectodermal tumor, "peripheral" primitive neuroectodermal tumor, and medulloblastoma. In medulloblastomas, the recognition of different variants is emerging as a prognostic factor that may in the future also predict therapy responsiveness.

Duplication of 7q34 is specific to juvenile pilocytic astrocytomas and a hallmark of cerebellar and optic pathway tumours

Background:

Juvenile pilocytic astrocytomas (JPA), a subgroup of low-grade astrocytomas (LGA), are common, heterogeneous and poorly understood subset of brain tumours in children. Chromosomal 7q34 duplication leading to fusion genes formed between KIAA1549 and BRAF and subsequent constitutive activation of BRAF was recently identified in a proportion of LGA, and may be involved in their pathogenesis. Our aim was to investigate additional chromosomal unbalances in LGA and whether incidence of 7q34 duplication is associated with tumour type or location.

Methods and results:

Using Illumina-Human-Hap300-Duo and 610-Quad high-resolution-SNP-based arrays and quantitative PCR on genes of interest, we investigated 84 paediatric LGA. We demonstrate that 7q34 duplication is specific to sporadic JPA (35 of 53 – 66%) and does not occur in other LGA subtypes (0 of 27) or NF1-associated-JPA (0 of 4). We also establish that it is site specific as it occurs in the majority of cerebellar JPA (24 of 30 – 80%) followed by brainstem, hypothalamic/optic pathway JPA (10 of 16 – 62.5%) and is rare in hemispheric JPA (1 of 7 – 14%). The MAP-kinase pathway, assessed through ERK phosphorylation, was active in all tumours regardless of 7q34 duplication. Gain of function studies performed on hTERT-immortalised astrocytes show that overexpression of wild-type BRAF does not increase cell proliferation or baseline MAPK signalling even if it sensitises cells to EGFR stimulation.

Conclusions and interpretation:

Our results suggest that variants of JPA might arise from a unique site-restricted progenitor cell where 7q34 duplication, a hallmark of this tumour-type in association to MAPK-kinase pathway activation, potentially plays a site-specific role in their pathogenesis. Importantly, gain of function abnormalities in components of MAP-Kinase signalling are potentially present in all JPA making this tumour amenable to therapeutic targeting of this pathway.

Pediatric brain tumors: current treatment strategies and future therapeutic approaches

Pediatric CNS tumors are the most common solid tumors of childhood and the second most common cancer after hematological malignancies accounting for approximate 20 to 25% of all primary pediatric tumors. With over 3,000 new cases per year in the United States, childhood CNS tumors are the leading cause of death related to cancer in this population. The prognosis for these patients has improved over the last few decades, but current therapies continue to carry a high risk of significant side effects, especially for the very young. Currently a combination of surgery, radiation, and chemotherapy is often used in children greater than 3 years of age. This article will outline current and future therapeutic strategies for the most common pediatric CNS tumors, including primitive neuroectodermal tumors such as medulloblastoma, as well as astrocytomas and ependymomas.

Histopathologic predictors of pilocytic astrocytoma event-free survival

Pilocytic astrocytoma (PA) is the most common pediatric brain tumor. Most arise in the cerebellum, but they also can develop in the brainstem and optic nerve, where gross total resection (GTR) is not possible. In the absence of GTR, significant variability in both clinical behavior and histology exists. To identify potential markers associated with poor clinical outcome, we retrospectively assessed pathological features in 107 patients with PAs. We identified four pathological features (necrosis, oligodendroglioma-like features, vascular hyalinization, and calcification) that showed a significant correlation with decreased event-free survival (EFS). Similar to previous reports, we also found that PAs involving the optic pathway were associated with worse EFS compared with those arising in other locations. In contrast, mitotic index, p53 immunoreactivity and hyperactivation of several mitogenic signaling pathways (MAPK, CREB, mTOR) did not demonstrate a statistically significant relationship with EFS. Lastly, we did find a statistical trend between EFS and the number of CD68+ cells, suggesting that non-neoplastic elements of the tumor microenvironment may influence subsequent growth and clinical recurrence. Collectively, the identification of specific histopathologic features associated with clinical outcome may improve our ability to determine which PAs are more likely to exhibit clinical progression and require more vigilant observation.

Clinicopathological and immunohistochemical features of three pilomyxoid astrocytomas: comparative study with 11 pilocytic astrocytomas

Pilomyxoid astrocytoma, first described by Tihan et al., was recently included as an established variant of pilocytic astrocytoma in the World Health Organization classification of CNS tumors. Histologically, it much resembles pilocytic astrocytoma, but monomorphic myxoid tumor of pilocytic cells with prominent angiocentric growth pattern without Rosenthal fibers or eosinophilic granular bodies is characteristic of pilomyxoid astrocytoma. Pilomyxoid astrocytoma is thought to be more aggressive with more frequent local recurrence as well as cerebrospinal spread. The authors recently encountered a case of pilomyxoid astrocytoma, therefore the purpose of the present study was undertake a retrospective review of pilocytic astrocytomas previously diagnosed during the past 10 years. Consequently, two of them were found to have histological features suggestive of pilomyxoid astrocytoma and both involved multiple recurrence, suggesting aggressive behavior in comparison to pilocytic astrocytoma. Therefore, knowledge of this entity is essential to surgical pathologists and clinicians for patient management.

Treatment of pediatric brain tumors

Over the past decades considerable advances have been made in neurosurgery, radiotherapy and chemotherapy resulting in improved survival and cure rates for children with brain tumors. Here we review four of the most common subtypes of pediatric brain tumors, low-grade and high-grade astrocytomas, medulloblastomas and ependymomas, highlighting their molecular features regarding their tumor biology, and promising potential therapeutic targets that may hold promise for finding new "molecular targeted" drugs. Importantly, appropriate clinical trial design will play a critical role in the evaluation of new and novel treatment approaches for pediatric brain tumors.

BRAF gene duplication constitutes a mechanism of MAPK pathway activation in low-grade astrocytomas

The molecular pathogenesis of pediatric astrocytomas is still poorly understood. To further understand the genetic abnormalities associated with these tumors, we performed a genome-wide analysis of DNA copy number aberrations in pediatric low-grade astrocytomas by using array-based comparative genomic hybridization. Duplication of the BRAF protooncogene was the most frequent genomic aberration, and tumors with BRAF duplication showed significantly increased mRNA levels of BRAF and a downstream target, CCND1, as compared with tumors without duplication. Furthermore, denaturing HPLC showed that activating BRAF mutations were detected in some of the tumors without BRAF duplication. Similarly, a marked proportion of low-grade astrocytomas from adult patients also had BRAF duplication. Both the stable silencing of BRAF through shRNA lentiviral transduction and pharmacological inhibition of MEK1/2, the immediate downstream phosphorylation target of BRAF, blocked the proliferation and arrested the growth of cultured tumor cells derived from low-grade gliomas. Our findings implicate aberrant activation of the MAPK pathway due to gene duplication or mutation of BRAF as a molecular mechanism of pathogenesis in low-grade astrocytomas and suggest inhibition of the MAPK pathway as a potential treatment.

Angiogenesis and gliomas: current issues and development of surrogate markers

Despite significant improvements, current therapies have yet to cure infiltrative gliomas. Glioma progression is strongly dependent on the development of a new vascular network that occurs primarily by angiogenesis. Hypoxia and genetic anomalies within a glioma trigger the angiogenic switch, thus upregulating angiogenic factors and downregulating antiangiogenic factors. The main factors indicative of angiogenesis are now well known, and more recently, differences based on grade and subtype have been reported. New data also indicate a potential role for postnatal vasculogenesis with bone marrow endothelial progenitors in addition to angiogenesis in tumor vascular development. All of these factors may have therapeutic implications. Antiangiogenic therapies are presently being developed; more than 80 trials are ongoing. Initial results indicate that epidermal growth factor receptor inhibitors, anti-metalloproteases, and thalidomide do not demonstrate strong anti-tumor activity. Thus, antiangiogenic agents combined with conventional therapies and second-generation antiangiogenic drugs for targeting multiple molecular pathways are presently being tested. Clinical experience also demonstrates the failure of conventional imaging to monitor these new approaches accurately. New advances in the design of surrogate markers for angiogenesis have been reported for both magnetic resonance and molecular imaging techniques. This article summarizes the mechanisms of the angiogenic switch based on tumor grade and subtype, reviews completed and ongoing clinical trials, and details the present and the future of surrogate markers for angiogenesis in gliomas.

Molecular analysis of pediatric brain tumors

The identification of molecular genetic abnormalities in specific types of pediatric brain tumors is beginning to play a role in the stratification of patients into treatment groups. The finding of an INI1 alteration in an atypical teratoid/rhabdoid tumor or malignant neoplasm with overlapping histologic features will be required for entry onto disease-specific protocols within the Children's Oncology Group. Refinement in the classification of medulloblastoma and malignant glioma patients will likely depend on the genetic and signaling pathways that characterize these tumors. Advances in this area will depend on the ability to identify new disease genes, validate prognostic markers, and develop biologically based therapeutic strategies to tailor treatment.

Current concepts in the molecular genetics of pediatric brain tumors: implications for emerging therapies

BACKGROUND

The revolution in molecular biology that has taken place over the past 2 decades has provided researchers with new and powerful tools for detailed study of the molecular mechanisms giving rise to the spectrum of pediatric brain tumors. Application of these tools has greatly advanced our understanding of the molecular pathogenesis of these lesions.

REVIEW

After familiarizing readers with some promising new techniques in the field of oncogenomics, this review will present the current state of knowledge as it pertains to the molecular biology of pediatric brain neoplasms. Along the way, we hope to highlight specific instances where the detailed mechanistic knowledge acquired thus far may be exploited for therapeutic advantage.

Tumour vascularity is of prognostic significance in adult, but not paediatric astrocytomas

Astrocytomas are the commonest type of brain tumours in adults and children. Although the most reliable prognostic indicators have been shown consistently to be patient age and tumour histological grade, biological progression in these tumours is inevitable and the overall prognosis has remained poor. Due to the evidence that vascular changes are important histological features of astrocytomas, the aim of this study was to investigate prognostic significance of tumour vascularity in paediatric and adult astrocytomas. Study population consisted of 70 patients (45 adult and 25 children) with histologically proven diagnosis of astrocytoma with no history of previous therapy. Histological quantification of tumour vascularity was performed using three different methods: microvessel density, vascular grading and Chalkley counting. Histological classification and grading were also assessed using the World Health Organization system. In contrast to the results in paediatric astrocytomas, tumour vascularity in adult tumours correlated significantly with postoperative survival by univariate analysis (P < 0.05). Microvessel density appeared to be an independent indicator of prognosis by multivariate analysis (P = 0.001). Likewise, patients with microvessel density of 70 or greater had significantly shorter survival than the remaining group (P < 0.001). Patient age and tumour histological grade were also correlated with survival. We conclude that histological quantification of tumour vascularity is a significant prognosticator in adult astrocytomas, but not in children. Our data do not support the validity of applications of antiangiogenic agents in paediatric astrocytic tumours, particularly pilocytic astrocytomas.

Fluorescence in situ hybridization (FISH) in diagnostic and investigative neuropathology

Over the last decade, fluorescence in situ hybridization (FISH) has emerged as a powerful clinical and research tool for the assessment of target DNA dosages within interphase nuclei. Detectable alterations include aneusomies, deletions, gene amplifications, and translocations, with primary advantages to the pathologist including its basis in morphology, its applicability to archival, formalin-fixed paraffin-embedded (FFPE) material, and its similarities to immunohistochemistry. Recent technical advances such as improved hybridization protocols, markedly expanded probe availability resulting from the human genome sequencing initiative, and the advent of high-throughput assays such as gene chip and tissue microarrays have greatly enhanced the applicability of FISH. In our lab, we currently utilize only a limited battery of DNA probes for routine diagnostic purposes, with determination of chromosome 1p and 19q dosage in oligodendroglial neoplasms representing the most common application. However, research applications are numerous and will likely translate into a growing list of clinically useful markers in the near future. In this review, we highlight the advantages and disadvantages of FISH and familiarize the reader with current applications in diagnostic and investigative neuropathology.

Pilocytic astrocytoma presenting as primary diffuse leptomeningeal gliomatosis: report of a unique case and review of the literature

We describe a 25-year-old male patient with primary diffuse leptomeningeal gliomatosis (PDLG) presenting with gait ataxia, positive Lhermitte's sign, double vision, and right abducens nerve palsy. Spinal magnetic resonance imaging showed extended intradural, extramedullary, contrast-enhancing masses with compression of the myelon. Spinal leptomeningeal biopsy revealed a pilocytic astrocytoma WHO grade I. Despite chemotherapy with vincristin and carboplatin, the patient died 2 months after admission. A thorough autopsy showed no evidence for primary neoplasms in brain, spine and optic nerve. Sequence analysis of tumor protein 53 gene (TP53) revealed a missense mutation in exon 5, and expression of phosphatase and tensin homolog (mutated in multiple advanced cancers 1) (PTEN) protein was not detected, which may have contributed to astrocytoma development. To our knowledge, this is the first definitive case of pilocytic astrocytoma presenting as PDLG.

Molecular diagnostics in central nervous system tumors

Central nervous system (CNS) neoplasms can be diagnostically challenging, due to remarkably wide ranges in histologic appearance, biologic behavior, and therapeutic approach. Nevertheless, accurate diagnosis is the critical first step in providing optimal patient care. As with other oncology-based specialties, there is a rapidly expanding interest and enthusiasm for identifying and utilizing new biomarkers to enhance the day-to-day practice of surgical neuropathology. In this regard, the field is primed by recent advances in basic research, elucidating the molecular mechanisms of tumorigenesis and progression in the most common adult and pediatric brain tumors. Thus far, few have made the transition into routine clinical practice, the most notable example being 1p and 19q testing in oligodendroglial tumors. However, the field is rapidly evolving and many other biomarkers are likely to emerge as useful ancillary diagnostic, prognostic, or therapeutic aids. The goal of this article is to highlight the most common genetic alterations currently implicated in CNS tumors, focusing most on those that are either already in common use in ancillary molecular diagnostics testing or are likely to become so in the near future.

Estrogen receptor beta (ERbeta) is expressed in brain astrocytic tumors and declines with dedifferentiation of the neoplasm

PURPOSE

Estrogen receptor beta (ERbeta) is the second identified receptor mediating the effects of estrogen on target tissues. The role of ERbeta in cancer pathobiology is largely unknown, because specific antibodies have not been available until recently. Initial studies have shown that ERbeta expression declines in breast, ovarian, prostatic, and colon carcinomas. Tamoxifen, a synthetic anti-estrogen compound that is a mixed agonist/antagonist of estrogen receptor alpha (ERalpha) and a pure antagonist of ERbeta, has moderate beneficial effects in human astrocytic neoplasms. However, most published studies agree that glial tumors do not express ERalpha. The purpose of this study was to explore the expression of ERbeta in astrocytic neoplasms.

METHODS

ERbeta expression was monitored immunohistochemically in 56 cases of astrocytomas of all grades (grade I-IV) and in adjacent non-neoplastic brain tissue.

RESULTS

Moderate or strong nuclear immunopositivity was obtained in non-neoplastic astrocytes and in low-grade astrocytomas, whereas the majority of high-grade tumors were immunonegative or displayed weak immunoreactivity. The progressive decline in ERbeta expression paralleled the increase in tumor grade.

CONCLUSIONS

In as much as ERbeta is possibly the only ER expressed in astrocytes, its decreased expression may play an important role in astrocytic tumor initiation and in the potential response of glial neoplasms to tamoxifen.

Molecular pathogenesis of childhood brain tumors

In the last decade, the molecular biology revolution has advanced considerably. These advances have enhanced our understanding of the genetic underpinnings of human brain tumors in general, and pediatric brain tumors in particular. We now know that many pediatric brain tumors arise from disturbances in developmentally regulated signaling pathways. The medulloblastoma, a tumor in which the developmental Hedgehog and WNT pathways have gone awry, is a prime example of this. New techniques in genetic engineering have allowed for the creation of sophisticated mouse models of brain tumors that recapitulate the human disease. Many laboratories are now using cDNA microarrays to study the expression level of thousands of genes that may be aberrantly expressed in brain tumors when compared to normal control cells. In the next decade, the use of several new molecular techniques to establish brain tumor diagnoses will likely become standard tools in the diagnostics and treatment stratification of children with central nervous system tumors.

Génétique des gliomes, vers une classification moléculaire

Gliomas result from specific genetic alterations--such as activation of specific oncogenes and/or inactivation of specific tumor suppressor genes. These alterations affect specific pathways involved in either signal transduction or cell cycle control, leading to phenotypic changes such as uncontrolled proliferation, inhibition of apoptosis, genetic instability, invasive properties. Tumoral progression includes multiple molecular pathways of clinical relevance: early alterations (p53 mutations for astrocytomas, 1p and 19q loss for oligodendrogliomas) and late alterations (EGF-R amplification, PTEN and P16/CDKN2A inactivation). Genetic profile is not only of diagnostic--but also prognostic relevance, as shown by 1p associated to 19q loss in oligodendrogliomas which is predictive of better prognosis and higher response rate.

High-molecular-weight tropomyosins localize to the contractile rings of dividing CNS cells but are absent from malignant pediatric and adult CNS tumors

Tropomyosin has been implicated in the control of actin filament dynamics during cell migration, morphogenesis, and cytokinesis. In order to gain insight into the role of tropomyosins in cell division, we examined their expression in developing and neoplastic brain tissue. We found that the high-molecular-weight tropomyosins are downregulated at birth, which correlates with glial cell differentiation and withdrawal of most cells from the cell cycle. Expression of these isoforms was restricted to proliferative areas in the embryonic brain and was absent from the adult, where the majority of cells are quiescent. However, they were induced under conditions where glial cells became proliferative in response to injury. During cytokinesis, these tropomyosin isoforms were associated with the contractile ring. We also investigated tropomyosin expression in neoplastic CNS tissues. Low-grade astrocytic tumors expressed high-molecular-weight tropomyosins, while highly malignant CNS tumors of diverse origin did not (P </= 0.001). Furthermore, high-molecular-weight tropomyosins were absent from the contractile ring in highly malignant astrocytoma cells. Our findings suggest a role for high-molecular-weight tropomyosins in astrocyte cytokinesis, although highly malignant CNS tumors are still able to undergo cell division in their absence. Additionally, the correlation between high-molecular-weight tropomyosin expression and tumor grade suggests that tropomyosins are potentially useful as indicators of CNS tumor grade.

Genetic alterations commonly found in diffusely infiltrating cerebral gliomas are rare or absent in pleomorphic xanthoastrocytomas

Pleomorphic xanthoastrocytoma (PXA) is a rare, usually well-circumscribed and superficially located neoplasm that preferentially arises in the cerebral cortex of children and young adults. The molecular aberrations that are associated with these tumors have not been studied systematically so far. We here report on a molecular genetic analysis of 62 PXAs (46 PXAs of World Health Organization [WHO] grade II and 16 PXAs with anaplastic features) for alterations of 5 candidate genes known to be frequently aberrant in diffusely infiltrating astrocytic gliomas, i.e. TP53, CDKN2A (p16(INK4a)), CDK4, MDM2, and EGFR. Only 3 PXAs (5%) carried a TP53 mutation. None of the 62 PXAs had lost both copies of the CDKN2A gene. The CDK4, MDM2, or EGFR genes were not amplified in any of the tumors. Fourteen PXAs were additionally analyzed for loss of heterozygosity (LOH) at microsatellite markers located on the chromosomes/chromosomal arms 1, gp, 9p, 10, 17, 19q, and 22q. Two PXAs (14%) had LOH at all informative markers on 9p, while 1 PXA demonstrated an interstitial area of allelic imbalance between D22S533 and D22S417 at 22q11.2-q13.3. Further analysis of 10 PXAs for inactivation of the CDKN2A. p14(ARF), and CDKN2B (p15(INK4b)) genes on 9p21 did not reveal any homozygous deletion, mutation, promoter hypermethylation, or complete loss of mRNA expression. Taken together, our results indicate that the chromosomal and genetic aberrations in PXAs are different from those typically associated with the diffusely infiltrating astrocytic and oligodendroglial gliomas. These genetic differences likely contribute to the more favorable behavior of PXAs and may be helpful for the molecular differential diagnosis of cerebral gliomas.

Influence of p53 mutations on prognosis of patients with glioblastoma

BACKGROUND

The influence of p53 mutations on the biology of astrocytic tumors is controversial. p53 is thought to be inactivated in the early stage of gliomagenesis; however, what role its inactivation plays in the malignancy of gliomas remains unknown. To understand the significance of p53 inactivation, the authors identified the locus of p53 gene mutation in glioma samples at different stages of progression and studied the correlation between the mutation and clinical behavior.

METHODS

Samples from newly diagnosed gliomas, including pure and mixed astrocytomas, were analyzed for p53 mutations using a yeast functional assay. To determine the locus of the gene mutations, DNA sequencing was performed.

RESULTS

The incidence of p53 mutations was higher in anaplastic astrocytomas (AA, 48%) than glioblastomas (GBM, 31%). There was no significant difference in the average ages of GBM patients with and without p53 mutations (54.9 years +/- 2.3 and 53.2 years +/- 4.6, respectively). In GBM patients, the mutation did not affect progression free survival or overall survival. Astrocytomas and GBM differed in the distribution of p53 mutation loci.

CONCLUSIONS

The p53 gene mutation does not markedly affect the survival of GBM patients. The difference in the location of p53 mutations between AA and GBM suggests that in gliomas, the p53 mutation may contribute not only to tumorigenesis (as an early event) but also to progression to malignancy (as a late event).

Mouse models of neurofibromatosis 1 and 2

The neurofibromatoses represent two of the most common inherited tumor predisposition syndromes affecting the nervous system. Individuals with neurofibromatosis 1 (NF1) are prone to the development of astrocytomas and peripheral nerve sheath tumors whereas those affected with neurofibromatosis 2 (NF2) develop schwannomas and meningiomas. The development of traditional homozygous knockout mice has provided insights into the roles of the NF1 and NF2 genes during development and in differentiation, but has been less instructive regarding the contribution of NF1 and NF2 dysfunction to the pathogenesis of specific benign and malignant tumors. Recent progress employing novel mouse targeting strategies has begun to illuminate the roles of the NF1 and NF2 gene products in the molecular pathogenesis of NF-associated tumors.

Astrocyte-specific inactivation of the neurofibromatosis 1 gene (NF1) is insufficient for astrocytoma formation

Individuals with the neurofibromatosis 1 (NF1) inherited tumor syndrome develop low-grade gliomas (astrocytomas) at an increased frequency, suggesting that the NF1 gene is a critical growth regulator for astrocytes. In an effort to determine the contribution of the NF1 gene product, neurofibromin, to astrocyte growth regulation and NF1-associated astrocytoma formation, we generated astrocyte-specific Nf1 conditional knockout mice (Nf1(GFAP)CKO) by using Cre/LoxP technology. Transgenic mice were developed in which Cre recombinase was specifically expressed in astrocytes by embryonic day 14.5. Successive intercrossing with mice bearing a conditional Nf1 allele (Nf1flox) resulted in GFAP-Cre Nf1flox/flox (Nf1(GFAP)CKO) animals. No astrocytoma formation or neurological impairment was observed in Nf1(GFAP)CKO mice after 20 months, but increased numbers of proliferating astrocytes were observed in several brain regions. To determine the consequence of Nf1 inactivation at different developmental times, the growth properties of embryonic day 12.5 and postnatal day 2 Nf1 null astrocytes were analyzed. Nf1 null astrocytes exhibited increased proliferation but lacked tumorigenic properties in vitro and did not form tumors when injected into immunocompromised mouse brains in vivo. Collectively, our results suggest that loss of neurofibromin is not sufficient for astrocytoma formation in mice and that other genetic or environmental factors might influence NF1-associated glioma tumorigenesis.