Microsatellite deletion mapping on chromosome 10q and mutation analysis of MMAC1, FAS, and MXI1 in human glioblastoma multiforme

Systematic review of the receptor tyrosine kinase superfamily in neuroblastoma pathophysiology

BACKGROUND

Neuroblastoma is a devastating disease accounting for 15% of all childhood cancer deaths. Yet, our understanding of key molecular drivers such as receptor tyrosine kinases (RTKs) in this pathology remains poorly clarified. Here, we provide a systematic analysis of the RTK superfamily in the context of neuroblastoma pathogenesis.

METHODS

Statistical correlations for all RTK family members' expression to neuroblastoma patient survival across 10 independent patient cohorts were annotated, synthesized, and ranked using the R2: Genomics Analysis and Visualization Platform. Gene expression of selected members across different cancer cell lines was further analyzed in the Cancer Cell Line Encyclopedia, part of the Cancer Dependency Map portal (depmap portal ( http://depmap.org )). Finally, we provide a detailed literature review for highly ranked candidates.

RESULTS

Our analysis defined two subsets of RTKs showing robust associations with either better or worse survival, constituting potential novel players in neuroblastoma pathophysiology, diagnosis, and therapy. We review the available literature regarding the oncogenic functions of these RTKs, their roles in neuroblastoma pathophysiology, and potential utility as therapeutic targets.

CONCLUSIONS

Our systematic analysis and review of the RTK superfamily in neuroblastoma pathogenesis provides a new resource to guide the research community towards focused efforts investigating signaling pathways that contribute to neuroblastoma tumor establishment, growth, and/or aggressiveness and targeting these druggable molecules in novel therapeutic strategies.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

ADD3 Deletion in Glioblastoma Predicts Disease Status and Survival

Loss of heterozygosity (LOH) on chromosome 10 frequently occurs in gliomas. Whereas genetic loci with allelic deletion often implicate tumor suppressor genes, a putative tumor suppressor Adducin3 (ADD3) mapped to chromosome 10q25.2 was found to be preferentially downregulated in high-grade gliomas compared with low-grade lesions. In this study, we unveil how the assessment of ADD3 deletion provides clinical significance in glioblastoma (GBM). By deletion mapping, we assessed the frequency of LOH in forty-three glioma specimens using five microsatellite markers spanning chromosome 10q23-10qter. Data were validated in The Cancer Genome Atlas (TCGA) cohort with 203 GBM patients. We found that allelic loss in both D10S173 (ADD3/MXI1 locus) and D10S1137 (MGMT locus) were positively associated with tumor grading and proliferative index (MIB-1). However, LOH events at only the ADD3/MXI1 locus provided prognostic significance with a marked reduction in patient survival and appeared to have diagnostic potential in differentiating high-grade gliomas from low-grade ones. Furthermore, we showed progressive loss of ADD3 in six out of seven patient-paired gliomas with malignant progression, as well as in recurrent GBMs. These findings suggest the significance of ADD3/MXI1 locus as a promising marker that can be used to refine the LOH10q assessment. Data further suggest the role of ADD3 as a novel tumor suppressor, whereby the loss of ADD3 is indicative of a progressive disease that may at least partially account for rapid disease progression in GBM. This study revealed for the first time the downregulation of ADD3 on the genetic level resulting from copy number deletion.

The Role of 2-Oxoglutarate Dependent Dioxygenases in Gliomas and Glioblastomas: A Review of Epigenetic Reprogramming and Hypoxic Response

Gliomas are a heterogeneous group of cancers that predominantly arise from glial cells in the brain, but may also arise from neural stem cells, encompassing low-grade glioma and high-grade glioblastoma. Whereas better diagnosis and new treatments have improved patient survival for many cancers, glioblastomas remain challenging with a highly unfavorable prognosis. This review discusses a super-family of enzymes, the 2-oxoglutarate dependent dioxygenase enzymes (2-OGDD) that control numerous processes including epigenetic modifications and oxygen sensing, and considers their many roles in the pathology of gliomas. We specifically describe in more detail the DNA and histone demethylases, and the hypoxia-inducible factor hydroxylases in the context of glioma, and discuss the substrate and cofactor requirements of the 2-OGDD enzymes. Better understanding of how these enzymes contribute to gliomas could lead to the development of new treatment strategies.

Fas Versatile Signaling and Beyond: Pivotal Role of Tyrosine Phosphorylation in Context-Dependent Signaling and Diseases

The Fas/FasL system is known, first and foremost, as a potent apoptosis activator. While its proapoptotic features have been studied extensively, evidence that the Fas/FasL system can elicit non-death signals has also accumulated. These non-death signals can promote survival, proliferation, migration, and invasion of cells. The key molecular mechanism that determines the shift from cell death to non-death signals had remained unclear until the recent identification of the tyrosine phosphorylation in the death domain of Fas as the reversible signaling switch. In this review, we present the connection between the recent findings regarding the control of Fas multi-signals and the context-dependent signaling choices. This information can help explain variable roles of Fas signaling pathway in different pathologies.

Structural mutation analysis of PTEN and its genotype-phenotype correlations in endometriosis and cancer

The phosphatase and tensin homolog deleted on chromosome ten (PTEN) gene encodes a tumor suppressor phosphatase that has recently been found to be frequently mutated in patients with endometriosis, endometrial cancer and ovarian cancer. Here, we present the first computational analysis of 13 somatic missense PTEN mutations associated with these phenotypes. We found that a majority of the mutations are associated in conserved positions within the active site and are clustered within the signature motif, which contain residues that play a crucial role in loop conformation and are essential for catalysis. In silico analyses were utilized to identify the putative effects of these mutations. In addition, coarse-grained models of both wild-type (WT) PTEN and mutants were constructed using elastic network models to explore the interplay of the structural and global dynamic effects that the mutations have on the relationship between genotype and phenotype. The effects of the mutations reveal that the local structure and interactions affect polarity, protein structure stability, electrostatic surface potential, and global dynamics of the protein. Our results offer new insight into the role in which PTEN missense mutations contribute to the molecular mechanism and genotypic-phenotypic correlation of endometriosis, endometrial cancer, and ovarian cancer. Proteins 2016; 84:1625-1643. © 2016 Wiley Periodicals, Inc.

The role of factor inhibiting HIF (FIH-1) in inhibiting HIF-1 transcriptional activity in glioblastoma multiforme

Glioblastoma multiforme (GBM) accounts for about 38% of primary brain tumors in the United States. GBM is characterized by extensive angiogenesis induced by vascular growth factors and cytokines. The transcription of these growth factors and cytokines is regulated by the Hypoxia-Inducible-Factor-1(HIF-1), which is a key regulator mediating the cellular response to hypoxia. It is known that Factor Inhibiting HIF-1, or FIH-1, is also involved in the cellular response to hypoxia and has the capability to physically interact with HIF-1 and block its transcriptional activity under normoxic conditions. Delineation of the regulatory role of FIH-1 will help us to better understand the molecular mechanism responsible for tumor growth and progression and may lead to the design of new therapies targeting cellular pathways in response to hypoxia. Previous studies have shown that the chromosomal region of 10q24 containing the FIH-1 gene is often deleted in GBM, suggesting a role for the FIH-1 in GBM tumorigenesis and progression. In the current study, we found that FIH-1 is able to inhibit HIF-mediated transcription of GLUT1 and VEGF-A, even under hypoxic conditions in human glioblastoma cells. FIH-1 has been found to be more potent in inhibiting HIF function than PTEN. This observation points to the possibility that deletion of 10q23-24 and loss or decreased expression of FIH-1 gene may lead to a constitutive activation of HIF-1 activity, an alteration of HIF-1 targets such as GLUT-1 and VEGF-A, and may contribute to the survival of cancer cells in hypoxia and the development of hypervascularization observed in GBM. Therefore FIH-1 can be potential therapeutic target for the treatment of GBM patients with poor prognosis.

MicroRNA-155 promotes glioma cell proliferation via the regulation of MXI1

Gliomas are the most common and aggressive primary tumors in the central nervous system. Recently, Max interactor-1 (MXI1), an antagonist of c-Myc that is involved in brain tumor progression, has been reported to be deregulated in a variety of tumors including glioma. However, the mechanism of MXI1 deregulation in gliomas remains unclear. In this study, we show that the relative expression level of MXI1 is markedly down-regulated in glioma cell lines. Using integrated bioinformatic analysis and experimental confirmation, we identified several miRNAs by screening a panel of predicted miRNAs that may regulate the MXI1 3′UTR. The strongest inhibitory miRNA, miR-155, can attenuate the activity of a luciferase reporter gene that is fused with the MXI1 3′UTR and decrease the expression levels of MXI1 mRNA and protein in U87 glioma cells. The potential role of miR-155 in promoting glioma cell proliferation by targeting MXI1 was confirmed in various glioma cell lines by rescue experiments using MTT assays, EdU incorporation assay, and cell counting experiments. In addition, we determined that the level of MXI1 mRNA was inversely correlated with the expression of miR-155 in 18 sets of glioblastoma multiforme specimens. These findings reveal for the first time that the targeting of MXI1 by miR-155 may result in a reduction in MXI1 expression and promote glioma cell proliferation; this result suggests a novel function of miR-155 in targeting MXI1 in glioma-genesis.

Emerging biomarkers in glioblastoma

Glioblastoma, the most common primary brain tumor, has few available therapies providing significant improvement in survival. Molecular signatures associated with tumor aggressiveness as well as with disease progression and their relation to differences in signaling pathways implicated in gliomagenesis have recently been described. A number of biomarkers which have potential in diagnosis, prognosis and prediction of response to therapy have been identified and along with imaging modalities could contribute to the clinical management of GBM. Molecular biomarkers including O(6)-methlyguanine-DNA-methyltransferase (MGMT) promoter and deoxyribonucleic acid (DNA) methylation, loss of heterozygosity (LOH) of chromosomes 1p and 19q, loss of heterozygosity 10q, isocitrate dehydrogenase (IDH) mutations, epidermal growth factor receptor (EGFR), epidermal growth factor, latrophilin, and 7 transmembrane domain-containing protein 1 on chromosome 1 (ELTD1), vascular endothelial growth factor (VEGF), tumor suppressor protein p53, phosphatase and tensin homolog (PTEN), p16INK4a gene, cytochrome c oxidase (CcO), phospholipid metabolites, telomerase messenger expression (hTERT messenger ribonucleic acid [mRNA]), microRNAs (miRNAs), cancer stem cell markers and imaging modalities as potential biomarkers are discussed. Inclusion of emerging biomarkers in prospective clinical trials is warranted in an effort for more effective personalized therapy in the future.

Emerging biomarkers in glioblastoma

Glioblastoma, the most common primary brain tumor, has few available therapies providing significant improvement in survival. Molecular signatures associated with tumor aggressiveness as well as with disease progression and their relation to differences in signaling pathways implicated in gliomagenesis have recently been described. A number of biomarkers which have potential in diagnosis, prognosis and prediction of response to therapy have been identified and along with imaging modalities could contribute to the clinical management of GBM. Molecular biomarkers including O(6)-methlyguanine-DNA-methyltransferase (MGMT) promoter and deoxyribonucleic acid (DNA) methylation, loss of heterozygosity (LOH) of chromosomes 1p and 19q, loss of heterozygosity 10q, isocitrate dehydrogenase (IDH) mutations, epidermal growth factor receptor (EGFR), epidermal growth factor, latrophilin, and 7 transmembrane domain-containing protein 1 on chromosome 1 (ELTD1), vascular endothelial growth factor (VEGF), tumor suppressor protein p53, phosphatase and tensin homolog (PTEN), p16INK4a gene, cytochrome c oxidase (CcO), phospholipid metabolites, telomerase messenger expression (hTERT messenger ribonucleic acid [mRNA]), microRNAs (miRNAs), cancer stem cell markers and imaging modalities as potential biomarkers are discussed. Inclusion of emerging biomarkers in prospective clinical trials is warranted in an effort for more effective personalized therapy in the future.

Applications of emerging molecular technologies in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults. Median survival from the time of diagnosis is less than a year, with less than 5% of patients surviving 5 years. These tumors are thought to arise through two different pathways. Primary GBMs represent de novo tumors, while secondary GBMs represent the malignant progression of lower-grade astrocytomas. Moreover, despite improvements in deciphering the complex biology of these tumors, the overall prognosis has not changed in the past three decades. The hope for improving the outlook for these glial-based malignancies is centered on the successful clinical application of current high-throughput technologies. For example, the complete sequencing of the human genome has brought both genomics and proteomics to the forefront of cancer research as a powerful approach to systematically identify large volumes of data that can be utilized to study the molecular and cellular basis of oncology. The organization of these data into a comprehensive view of tumor growth and progression translates into a unique opportunity to diagnose and treat cancer patients. In this review, we summarize current genomic and proteomic alterations associated with GBM and how these modalities may ultimately impact treatment and survival.

Genetic pathways to primary and secondary glioblastoma

Glioblastoma is the most frequent and most malignant human brain tumor. The prognosis remains very poor, with most patients dying within 1 year after diagnosis. Primary and secondary glioblastoma constitute distinct disease subtypes, affecting patients of different age and developing through different genetic pathways. The majority of cases (>90%) are primary glioblastomas that develop rapidly de novo, without clinical or histological evidence of a less malignant precursor lesion. They affect mainly the elderly and are genetically characterized by loss of heterozygosity 10q (70% of cases), EGFR amplification (36%), p16(INK4a) deletion (31%), and PTEN mutations (25%). Secondary glioblastomas develop through progression from low-grade diffuse astrocytoma or anaplastic astrocytoma and manifest in younger patients. In the pathway to secondary glioblastoma, TP53 mutations are the most frequent and earliest detectable genetic alteration, already present in 60% of precursor low-grade astrocytomas. The mutation pattern is characterized by frequent G:C-->A:T mutations at CpG sites. During progression to glioblastoma, additional mutations accumulate, including loss of heterozygosity 10q25-qter ( approximately 70%), which is the most frequent genetic alteration in both primary and secondary glioblastomas. Primary and secondary glioblastomas also differ significantly in their pattern of promoter methylation and in expression profiles at RNA and protein levels. This has significant implications, particularly for the development of novel, targeted therapies, as discussed in this review.

Genetic Pathways to Glioblastoma

We conducted a population-based study on glioblastomas in the Canton of Zurich, Switzerland (population, 1.16 million) to determine the frequency of major genetic alterations and their effect on patient survival. Between 1980 and 1994, 715 glioblastomas were diagnosed. The incidence rate per 100,000 population/year, adjusted to the World Standard Population, was 3.32 in males and 2.24 in females. Observed survival rates were 42.4% at 6 months, 17.7% at 1 year, and 3.3% at 2 years. For all of the age groups, younger patients survived significantly longer, ranging from a median of 8.8 months (<50 years) to 1.6 months (>80 years). Loss of heterozygosity (LOH) 10q was the most frequent genetic alteration (69%), followed by EGFR amplification (34%), TP53 mutations (31%), p16(INK4a) deletion (31%), and PTEN mutations (24%). LOH 10q occurred in association with any of the other genetic alterations and was predictive of shorter survival. Primary (de novo) glioblastomas prevailed (95%), whereas secondary glioblastomas that progressed from low-grade or anaplastic gliomas were rare (5%). Secondary glioblastomas were characterized by frequent LOH 10q (63%) and TP53 mutations (65%). Of the TP53 mutations in secondary glioblastomas, 57% were in hotspot codons 248 and 273, whereas in primary glioblastomas, mutations were more equally distributed. G:C-->A:T mutations at CpG sites were more frequent in secondary than primary glioblastomas (56% versus 30%; P = 0.0208). This suggests that the acquisition of TP53 mutations in these glioblastoma subtypes occurs through different mechanisms.

Mutations of PIK3CA in anaplastic oligodendrogliomas, high-grade astrocytomas, and medulloblastomas

The phosphatidylinositol 3'-kinase pathway is activated in multiple advanced cancers, including glioblastomas, through inactivation of the PTEN tumor suppressor gene. Recently, mutations in PIK3CA, a member of the family of phosphatidylinositol 3'-kinase catalytic subunits, were identified in a significant fraction (25-30%) of colorectal cancers, gastric cancers, and glioblastomas and in a smaller fraction of breast and lung cancers. These mutations were found to cluster into two major "hot spots" located in the helical and catalytic domains. To determine whether PIK3CA is genetically altered in brain tumors, we performed a large-scale mutational analysis of the helical and catalytic domains. A total of 13 mutations of PIK3CA within these specific domains were identified in anaplastic oligodendrogliomas, anaplastic astrocytomas, glioblastoma multiforme, and medulloblastomas, whereas no mutations were identified in ependymomas or low-grade astrocytomas. These observations implicate PIK3CA as an oncogene in a wider spectrum of adult and pediatric brain tumors and suggest that PIK3CA may be a useful diagnostic marker or a therapeutic target in these cancers.

Mxi1-0, an alternatively transcribed Mxi1 isoform, is overexpressed in glioblastomas

The c-Myc transcription factor regulates expression of genes related to cell growth, division, and apoptosis. Mxi1, a member of the Mad family, represses transcription of c-Myc-regulated genes by mediating chromatin condensation via histone deacetylase and the Sin3 corepressor. Mxi1 is a c-Myc antagonist and suppresses cell proliferation in vitro. Here, we describe the identification of Mxi1-0, a novel Mxi1 isoform that is alternatively transcribed from an upstream exon. Mxi1-0 and Mxi1 have different amino-terminal sequences, but share identical Max- and DNA-binding domains. Both isoforms are able to bind Max, to recognize E-box binding sites, and to interact with Sin3. Despite these similarities and in contrast to Mxi1, Mxi1-0 is predominantly localized to the cytoplasm and fails to repress c-Myc-dependent transcription. Although Mxi1-0 and Mxi1 are coexpressed in both human and mouse cells, the relative levels of Mxi1-0 are higher in primary glioblastoma tumors than in normal brain tissue. This variation in the levels of Mxi1-0 and Mxi1 suggests that Mxi1-0 may modulate the Myc-inhibitory activity of Mxi1. The identification of Mxi1-0 as an alternatively transcribed Mxi1 isoform has significant implications for the interpretation of previous Mxi1 studies, particularly those related to the phenotype of the mxi1 knockout mouse.

Prognostic value of loss of heterozygosity around three candidate tumor suppressor genes on chromosome 10q in astrocytomas

We thoroughly examined loss of heterozygosity (LOH) around three candidate tumor suppressor genes on chromosome 10q to determine whether LOH of each tumor suppressor gene is associated with the previously defined clinical prognostic indices. We also examined whether LOH can help predict prognostic variables in astrocytomas. We selected samples from 40 astrocytomas (grades 2-4), performed Ki-67 immunostaining, and counted positive cells. Using DNA from aliquots of tumor blocks and leukocytes, we investigated LOH around the PTEN, NEURL, and DMBTI genes (10q23.3-26.1) with the silver staining procedure. We then statistically evaluated the relationship among histological features, regional LOH on chromosome 10q, and survival. The mean survival period for patients with LOH around PTEN was 7.2 months after surgery, while that for patients without LOH around PTEN was 21.4 months. Thus, LOH around PTEN was closely associated with a reduced overall survival (p = 0.0020) but LOH at NEURL or DMBTI was not (p > 0.05). The combined features of an increase in histological grading and Ki-67-positive cells and the presence of LOH around PTEN significantly correlated with poor prognosis. These factors may be useful predictors of survival, and LOH analysis of tumor suppressor genes on chromosome 10q can contribute greatly to the treatment of patients with astrocytoma.

Caveolin 1-mediated regulation of receptor tyrosine kinase-associated phosphatidylinositol 3-kinase activity by ceramide

Previous studies have indicated that proapoptotic stresses downregulate the phosphatidylinositol 3-kinase [PI(3)K]/Akt survival pathway via the activation of acid-sphingomyelinase (A-SMase) and ceramide production. Ceramide induces apoptosis and inhibits PI(3)K activity without altering expression, association, or phosphorylation of receptors, adapter proteins, or PI(3)K subunits. PI(3)K inhibition by ceramide is associated with recruitment of caveolin 1 to PI(3)K-associated receptor complexes within lipid raft microdomains. Overexpression of caveolin 1 alone is sufficient to alter PI(3)K activity and sensitizes fibroblasts to ceramide-induced cell death. Most importantly, antisense expression of caveolin 1 dramatically reduces ceramide-induced PI(3)K deregulation and results in a loss-of-function stress response similar to that in A-SMase-deficient cells. Stress-induced recruitment of caveolin 1 to receptor complexes was found to be dependent on A-SMase since cell lines deficient in A-SMase did not exhibit caveolin 1 association with PI(3)K receptor complexes. Thus, a genetic link between A-SMase activation and caveolin 1-induced inhibition of PI(3)K activity exists. These results led us to propose that stress-induced changes in raft microdomains lead to altered receptor tyrosine kinase signal transduction through the modulation of caveolin 1 by ceramide.

Loss of chromosome 10 is an independent prognostic factor in high-grade gliomas

Loss of heterozygosity (LOH) for chromosome 10 is the most frequent genetic abnormality observed in high-grade gliomas. We have used fluorescent microsatellite markers to examine a series of 83 patients, 34 with anaplastic astrocytoma (grade 3) and 49 with glioblastoma multiforme (grade 4), for LOH of chromosome 10. Genotype analysis revealed LOH for all informative chromosome 10 markers in 12 (35%) of patients with grade 3 and 29 (59%) grade 4 tumours respectively, while partial LOH was found in a further eight (24%) grade 3 and ten (20%) grade 4 tumours. Partial LOH, was confined to the long arm (10q) in six and the short arm (10p) in three cases, while alleles from both arms were lost in four cases. Five tumours (one grade 3 and four grade 4) showed heterogeneity with respect to loss at different loci. There was a correlation between any chromosome 10 loss and poorer performance status at presentation (chi2 P = 0.005) and with increasing age at diagnosis (Mann-Whitney U-test P = 0.034) but not with tumour grade (chi2 p= 0.051). A Cox multivariate model for survival duration identified age (proportional hazards (PH), P= 0.004), grade (PH, P= 0.012) and any loss of chromosome 10 (PH, P= 0.009) as the only independent prognostic variables. Specifically, LOH for chromosome 10 was able to identify a subgroup of patients with grade 3 tumours who had a significantly shorter survival time. We conclude that LOH for chromosome 10 is an independent, adverse prognostic variable in high-grade glioma.

Acquisition of the glioblastoma phenotype during astrocytoma progression is associated with loss of heterozygosity on 10q25-qter

Loss of heterozygosity on chromosome 10 (LOH#10) is the most frequent genetic alteration in glioblastomas and occurs in more than 80% of cases. We recently reported that PTEN (MMAC1) on 10q23.3 is mutated in approximately 30% of primary (de novo) glioblastomas but rarely in secondary glioblastomas that progressed from low-grade or anaplastic astrocytomas. Because secondary glioblastomas also show LOH#10, tumor suppressor genes other than PTEN are likely to be involved. We analyzed LOH on chromosomes 10 and 19, using polymorphic microsatellite markers in microdissected foci showing histologically an abrupt transition from low-grade or anaplastic astrocytoma to glioblastoma, suggestive of the emergence of a new tumor clone. When compared to the respective low-grade or anaplastic astrocytoma of the same biopsy, deletions were detected in 7 of 8 glioblastoma foci on 10q25-qter distal to D10S597, covering the DMBT1 and FGFR2 loci. Six of 8 foci showed LOH at one or two flanking markers of PTEN but did not contain PTEN mutations. LOH on 10p and 19q was found in only one case each. These data indicate that acquisition of a highly anaplastic glioblastoma phenotype with marked proliferative activity and lack of glial fibrillary acidic protein expression is associated with loss of a putative tumor suppressor gene on 10q25-qter.