The molecular genetics of central nervous system tumors

Ribosomal Proteins RPS11 and RPS20, Two Stress-Response Markers of Glioblastoma Stem Cells, Are Novel Predictors of Poor Prognosis in Glioblastoma Patients

Glioblastoma stem cells (GSC) co-exhibiting a tumor-initiating capacity and a radio-chemoresistant phenotype, are a compelling cell model for explaining tumor recurrence. We have previously characterized patient-derived, treatment-resistant GSC clones (TRGC) that survived radiochemotherapy. Compared to glucose-dependent, treatment-sensitive GSC clones (TSGC), TRGC exhibited reduced glucose dependence that favor the fatty acid oxidation pathway as their energy source. Using comparative genome-wide transcriptome analysis, a series of defense signatures associated with TRGC survival were identified and verified by siRNA-based gene knockdown experiments that led to loss of cell integrity. In this study, we investigate the prognostic value of defense signatures in glioblastoma (GBM) patients using gene expression analysis with Probeset Analyzer (131 GBM) and The Cancer Genome Atlas (TCGA) data, and protein expression with a tissue microarray (50 GBM), yielding the first TRGC-derived prognostic biomarkers for GBM patients. Ribosomal protein S11 (RPS11), RPS20, individually and together, consistently predicted poor survival of newly diagnosed primary GBM tumors when overexpressed at the RNA or protein level [RPS11: Hazard Ratio (HR) = 11.5, p<0.001; RPS20: HR = 4.5, p = 0.03; RPS11+RPS20: HR = 17.99, p = 0.001]. The prognostic significance of RPS11 and RPS20 was further supported by whole tissue section RPS11 immunostaining (27 GBM; HR = 4.05, p = 0.01) and TCGA gene expression data (578 primary GBM; RPS11: HR = 1.19, p = 0.06; RPS20: HR = 1.25, p = 0.02; RPS11+RPS20: HR = 1.43, p = 0.01). Moreover, tumors that exhibited unmethylated O-6-methylguanine-DNA methyltransferase (MGMT) or wild-type isocitrate dehydrogenase 1 (IDH1) were associated with higher RPS11 expression levels [corr (IDH1, RPS11) = 0.64, p = 0.03); [corr (MGMT, RPS11) = 0.52, p = 0.04]. These data indicate that increased expression of RPS11 and RPS20 predicts shorter patient survival. The study also suggests that TRGC are clinically relevant cells that represent resistant tumorigenic clones from patient tumors and that their properties, at least in part, are reflected in poor-prognosis GBM. The screening of TRGC signatures may represent a novel alternative strategy for identifying new prognostic biomarkers.

Selection of DNA aptamers against glioblastoma cells with high affinity and specificity

Background

Glioblastoma is the most common and most lethal form of brain tumor in human. Unfortunately, there is still no effective therapy to this fatal disease and the median survival is generally less than one year from the time of diagnosis. Discovery of ligands that can bind specifically to this type of tumor cells will be of great significance to develop early molecular imaging, targeted delivery and guided surgery methods to battle this type of brain tumor.

Methodology/Principal Findings

We discovered two target-specific aptamers named GBM128 and GBM131 against cultured human glioblastoma cell line U118-MG after 30 rounds selection by a method called cell-based Systematic Evolution of Ligands by EXponential enrichment (cell-SELEX). These two aptamers have high affinity and specificity against target glioblastoma cells. They neither recognize normal astraglial cells, nor do they recognize other normal and cancer cell lines tested. Clinical tissues were also tested and the results showed that these two aptamers can bind to different clinical glioma tissues but not normal brain tissues. More importantly, binding affinity and selectivity of these two aptamers were retained in complicated biological environment.

Conclusion/Significance

The selected aptamers could be used to identify specific glioblastoma biomarkers. Methods of molecular imaging, targeted drug delivery, ligand guided surgery can be further developed based on these ligands for early detection, targeted therapy, and guided surgery of glioblastoma leading to effective treatment of glioblastoma.

Variation in DNA repair gene XRCC3 affects susceptibility to astrocytomas and glioblastomas

The gene XRCC3 (X-ray cross complementing group 3) has the task of repairing damage that occurs when there is recombination between homologous chromosomes. Repair of recombination between homologous chromosomes plays an important role in maintaining genome integrity, although it is known that double-strand breaks are the main inducers of chromosomal aberrations. Changes in the XRCC3 protein lead to an increase in errors in chromosome segregation due to defects in centrosomes, resulting in aneuploidy and other chromosomal aberrations, such as small increases in telomeres. We examined XRCC3 Thr241Met polymorphism using PCR-RFLP in 80 astrocytoma and glioblastoma samples. The individuals of the control group (N = 100) were selected from the general population of the São Paulo State. Odds ratio and 95%CI were calculated using a logistic regression model. Patients who had the allele Met of the XRCC3 Thr241Met polymorphism had a significantly increased risk of tumor development (odds ratio = 3.13; 95% confidence interval = 1.50-6.50). There were no significant differences in overall survival of patients. We suggest that XRCC3 Thr241Met polymorphism is involved in susceptibility for developing astrocytomas and glioblastomas.

Analysis of the polymorphisms XRCC1Arg194Trp and XRCC1Arg399Gln in gliomas

XRCC genes (X-ray cross-complementing group) were discovered mainly for their roles in protecting mammalian cells against damage caused by ionizing radiation. Studies determined that these genes are important in the genetic stability of DNA. Although the loss of some of these genes does not necessarily confer high levels of sensitivity to radiation, they have been found to represent important components of various pathways of DNA repair. To ensure the integrity of the genome, a complex system of DNA repair was developed. Base excision repair is the first defense mechanism of cells against DNA damage and a major event in preventing mutagenesis. Repair genes may play an important role in maintaining genomic stability through different pathways that are mediated by base excision. In the present study, we examined XRCC1Arg194Trp and XRCC1Arg399Gln polymorphism using PCR-RFLP in 80 astrocytoma and glioblastoma samples. Patients who had the allele Trp of the XRCC1Arg194Trp polymorphism had an increased risk of tumor development (OR = 8.80; confidence interval at 95% (95%CI) = 4.37-17.70; P < 0.001), as did the allele Gln of XRCC1Arg399Gln (OR = 1.01; 95%CI = 0.53-1.93; P = 0.971). Comparison of overall survival of patients did not show significant differences. We suggest that XRCC1Arg194Trp and XRCC1Arg399Gln polymorphisms are involved in susceptibility for developing astrocytomas and glioblastomas.

GSTP1 Ile105Val polymorphism in astrocytomas and glioblastomas

Glutathione S-transferases (GSTs) constitute a superfamily of ubiquitous multifunctional enzymes that are involved in the cellular detoxification of a large number of endogenous and exogenous chemical agents that have electrophilic functional groups. People who have deficiencies in this family of genes are at increased risk of developing some types of tumors. We examined GSTP1 Ile105Val polymorphism using PCR-RFLP in 80 astrocytoma and glioblastoma samples. Patients who had the Val allele of the GSTP1 Ile105Val polymorphism had an increased risk of tumor development (odds ratio = 8.60; 95% confidence interval = 4.74-17.87; P < 0.001). Overall survival of patients did not differ significantly. We suggest that GSTP1 Ile105Val polymorphisms are involved in susceptibility to developing astrocytomas and glioblastomas.

Differential expression of 12 histone deacetylase (HDAC) genes in astrocytomas and normal brain tissue: class II and IV are hypoexpressed in glioblastomas

BACKGROUND

Glioblastoma is the most lethal primary malignant brain tumor. Although considerable progress has been made in the treatment of this aggressive tumor, the clinical outcome for patients remains poor. Histone deacetylases (HDACs) are recognized as promising targets for cancer treatment. In the past several years, HDAC inhibitors (HDACis) have been used as radiosensitizers in glioblastoma treatment. However, no study has demonstrated the status of global HDAC expression in gliomas and its possible correlation to the use of HDACis. The purpose of this study was to evaluate and compare mRNA and protein levels of class I, II and IV of HDACs in low grade and high grade astrocytomas and normal brain tissue and to correlate the findings with the malignancy in astrocytomas.

METHODS

Forty-three microdissected patient tumor samples were evaluated. The histopathologic diagnoses were 20 low-grade gliomas (13 grade I and 7 grade II) and 23 high-grade gliomas (5 grade III and 18 glioblastomas). Eleven normal cerebral tissue samples were also analyzed (54 total samples analyzed). mRNA expression of class I, II, and IV HDACs was studied by quantitative real-time polymerase chain reaction and normalized to the housekeeping gene beta-glucuronidase. Protein levels were evaluated by western blotting.

RESULTS

We found that mRNA levels of class II and IV HDACs were downregulated in glioblastomas compared to low-grade astrocytomas and normal brain tissue (7 in 8 genes, p < 0.05). The protein levels of class II HDAC9 were also lower in high-grade astrocytomas than in low-grade astrocytomas and normal brain tissue. Additionally, we found that histone H3 (but not histone H4) was more acetylated in glioblastomas than normal brain tissue.

CONCLUSION

Our study establishes a negative correlation between HDAC gene expression and the glioma grade suggesting that class II and IV HDACs might play an important role in glioma malignancy. Evaluation of histone acetylation levels showed that histone H3 is more acetylated in glioblastomas than normal brain tissue confirming the downregulation of HDAC mRNA in glioblastomas.

Some speculation on the origin of glioblastoma

Glioblastoma, the most common primary brain tumor, is also the most deadly, with median survival of about one year, which is little improved over the last five decades. Its pathogenesis is a vexing problem. Despite extensive basic and clinical scientific research, little is known regarding the cause of this disease, the genetic factors which drive its course, or any strategies which may result in effective treatment. This persistent resistance to understanding suggests to the authors that some of the fundamental assumptions regarding the disease are likely to be flawed, and that a new paradigm must be sought to replace them. This manuscript is a review of some of what is known regarding this disease, and then presents a series of hypotheses which compromise an alternative view of glioblastoma.

Effects of cotransfection of antisense-EGFR and wild-type PTEN cDNA on human glioblastoma cells

The main molecular genetic changes identified in glioblastomas are overexpression/amplification of the epidermal growth factor receptor (EGFR) gene and mutation/ deletion of the tumor suppressor PTEN gene. These two genetic changes both play important roles in glial tumorigenesis and progression. In this study, we demonstrated that wild-type PTEN transfection inhibited the growth and transforming ability of U87MG cells by 69.3% and 73.5%, respectively. On the other hand, antisense-EGFR transfection inhibited the growth and transforming phenotype of these cells by 50.3% and 46.8%, respectively. However, cotransfection of U87MG cells with wild-type PTEN and antisense EGFR constructs could inhibit the cellular growth by 91.7%. The transforming phenotype of these cells was completely inhibited. In addition, these cotransfected cells showed a differentiated form and expressed much lower telomerase activity than cells transfected with wild-type PTEN or antisense-EGFR alone. In summary, these results suggest that cotransfection is a better approach to suppress glioma cell growth than wild-type PTEN transfer or antisense-EGFR transfection alone. This approach may prove useful as an adjunct therapy in the treatment of glioblastomas.

N-Myc downstream-regulated gene 2 (NDRG2) inhibits glioblastoma cell proliferation

The most severe form of brain glioma, glioblastoma (GBM), is highly malignant and usually resistant to chemotherapy. Therefore, discovery of new targets for gene therapy is important. Using subtraction cloning, we identified the human N-Myc downstream-regulated gene 2 (hNDRG2), located at chromosome 14q11.2, as a gene that is significantly suppressed in GBM tissues. Semiquantitative RT-PCR showed that the hNDRG2 gene transcript is expressed in normal brain tissue and low-grade gliomas but is present at low levels in 15 of 27 (56%) human GBM tissues and all of the 6 human glioblastoma cell lines examined. Furthermore, transfection of human glioblastoma U373 and U138 cells with a cDNA encoding hNDRG2 markedly reduced the cell proliferation. Our findings provide the first evidence to suggest that hNDRG2 may play a role in glioblastoma carcinogenesis.

Classification of glioblastoma multiforme in adults by molecular genetics

Recent advances in cytogenetic and molecular methodologies have elucidated certain principal characteristics of oncogenesis in glioblastoma multiforme. The earliest clues implicate gene sequence alterations, such as gene amplification and numerical gain or loss of function in specific chromosomes. Genetic classification and expression patterns have thus been constructed, conferring the likelihood of two types of glioblastoma, primary (de novo) as opposed to secondary (evolving from a pre-existing low-grade glioma). The former group of tumors exhibits more frequent occurrences of EGFR gene amplification, whereas the latter group relies strongly on TP53 gene inactivation. Many other tumor suppressor genes and oncogenes have been discovered. Most gene alterations induce cell cycle dysfunction on a complex molecular level. Further insight into tumor genesis by means of genomic assays may aid in predicting the clinical behavior of glioblastoma and in providing individualized potential targets for therapeutic agents.

Down-regulation of Bcl-2 and Bcl-xL expression with bispecific antisense treatment in glioblastoma cell lines induce cell death

The functions of the antiapoptotic proteins Bcl-2 and Bcl-xL were examined in glioblastoma cells. Expression of both Bcl-2 and Bcl-xL were found to be elevated in protein lysates from seven early passage cell lines derived from human glioblastoma tumors compared with non-neoplastic glial cells. Down-regulation of both bcl-2 and bcl-xL expression in glioblastoma cell lines U87 and NS008 with bcl-2/bcl-xL bispecific antisense oligonucleotide resulted in spontaneous cell death. The mechanism of cell death was partially caspase-dependent. Executioner caspase 6 and caspase 7, but not caspase 3, were involved in apoptosis induced by bcl-2/bcl-xL antisense treatment. Interestingly, western blots failed to demonstrate expression of caspase 3 in two of the seven glioblastoma cell lines examined. The data support the hypothesis that Bcl-2 and Bcl-xL are important in preventing cell death in glioblastoma cells. It also suggests that there are functional pathways capable of successful completion of caspase-dependent cell death in gliomas. These findings support a potential role of bcl-2/bcl-xL bispecifc antisense oligonucleotide therapy as a treatment strategy to enhance caspase-dependent cell death in patients with glioblastoma.

Metastasis to and from the central nervous system--the 'relatively protected site'

The brain has long been recognised as a site with a very low rate of metastases, despite the potential for cancers to be extremely locally aggressive. This feature contrasts with most of the rest of the body, where metastatic spread is much more common. The pathological behaviour of any tumour is governed by both its inherent composition and the composition of the matrix in which it is sited. Much work has been done in recent years to elucidate the factors within the central nervous system (CNS) that give the brain its unique properties. Tumour interactions with the blood-brain barrier, microglia, and various matrix proteins, cytokines, and growth factors have a central role. This review concentrates mainly on the process of tumour spread from the CNS and explores how the brain is a protected site. CNS metastases from extraneural sites are also briefly covered.

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.

Antisense epidermal growth factor receptor RNA transfection in human glioblastoma cells down-regulates telomerase activity and telomere length

Epidermal growth factor receptor is overexpressed and/or amplified in up to 50% of glioblastomas, suggesting an important role of this gene in glial tumorigenesis and progression. In the present study we demonstrated that epidermal growth factor receptor is involved in regulation of telomerase activity in glioblastoma. Antisense-epidermal growth factor receptor approach was used to inhibit epidermal growth factor receptor expression of glioblastoma U87MG cells. Telomerase activity in antisense-epidermal growth factor receptor cells decreased by up to 54 folds compared with control cells. Moreover, the telomere lengths of antisense-epidermal growth factor receptor cells were shortened. In addition, the tumorigenicity of antisense-epidermal growth factor receptor cells was significantly inhibited. Taken together, there were strong correlations between tumorigenicity and epidermal growth factor receptor expression levels, and between tumorigenicity and telomerase activity. These results provide evidence that epidermal growth factor receptor plays an important role in the regulation of telomerase activity of glioma cells. Our findings provide new insights into both the biological functions of epidermal growth factor receptor and the regulation of telomerase activity. The inhibition of telomerase activity triggered by antisense-epidermal growth factor receptor treatment may reflect yet another mechanism of antisense-epidermal growth factor receptor approach in tumour suppression.

Control of cell cycle exit and entry by protein kinase B-regulated forkhead transcription factors

AFX-like Forkhead transcription factors, which are controlled by phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, are involved in regulating cell cycle progression and cell death. Both cell cycle arrest and induction of apoptosis are mediated in part by transcriptional regulation of p27(kip1). Here we show that the Forkheads AFX (FOXO4) and FKHR-L1 (FOXO3a) also directly control transcription of the retinoblastoma-like p130 protein and cause upregulation of p130 protein expression. Detailed analysis of p130 regulation demonstrates that following Forkhead-induced cell cycle arrest, cells enter G(0) and become quiescent. This is shown by a change in phosphorylation of p130 to G(0)-specific forms and increased p130/E2F-4 complex formation. Most importantly, long-term Forkhead activation causes a sustained but reversible inhibition of proliferation without a marked increase in apoptosis. As for the activity of the Forkheads, we also show that protein levels of p130 are controlled by endogenous PI3K/PKB signaling upon cell cycle reentry. Surprisingly, not only nontransformed cells, but also cancer cells such as human colon carcinoma cells, are forced into quiescence by Forkhead activation. We therefore propose that Forkhead inactivation by PKB signaling in quiescent cells is a crucial step in cell cycle reentry and contributes to the processes of transformation and regeneration.

Genetic imbalances in pleomorphic xanthoastrocytoma detected by comparative genomic hybridization and literature review

Pleomorphic xanthoastrocytoma (PXA) is a rare, low-grade astrocytic tumor found in the central nervous system. Histologically, the tumor is characterized by markedly pleomorphic and lipidized cells. Although most of the patients have a favorable prognosis, a small number of cases undergoing recurrence or progression to anaplastic astrocytoma were reported. Very few genetic studies have been performed on PXA because of its rarity and the pathogenesis of this neoplasm is largely unknown. In order to provide an overview of genetic alterations in PXA, we performed comparative genomic hybridization to identify chromosomal imbalances (DNA gains and losses) in three cases of PXA. Genetic imbalance was detected on at least one chromosome for each case. One case, which revealed multiple genetic alterations, showed a poor prognosis. DNA gain on chromosome 7 and loss on 8p were demonstrated in two of three cases, suggesting that the candidate gene(s) located on these regions may play a role in the development of PXA. Further studies are needed to identify the residing candidate genes that are involved in the tumorigenesis of PXA. In addition, the histopathological features and previous genetic studies on PXA are reviewed.

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

AIMS

While it is well known that pilocytic astrocytomas are clinically distinct from diffuse astrocytomas, few comprehensive studies have focused on their genetic differences. The aim of this study was to examine pilocytic astrocytomas for genetic alterations that are commonly seen in diffuse astrocytomas.

METHODS AND RESULTS

By using molecular genetic and immunohistochemical techniques, we evaluated p16, p53, CDK4 and PTEN genes in 29 pilocytic astrocytomas. Mutation screening of p53 and PTEN was performed by single strand conformation polymorphism analysis followed by direct sequencing. Loss of heterozygosity (LOH) of p53, p16 and 10q23-25 loci was performed with microsatellite markers and genomic microsatellite instability (MSI) was also screened. Protein expression of p16, p53, CDK4 and PTEN was examined by immunohistochemistry. Five tumours were found to have single genetic alterations, which included a p53 mutation, a PTEN mutation, MSI at a single microsatellite marker of the p16 locus, and one single LOH at each p16 and 10q23 loci. Protein expressions of p16, CDK4 and PTEN were detected in 73%, 61% and 38% of tumours, respectively. Significantly and in sharp contrast to diffuse astrocytomas, no pilocytic astrocytoma in our series stained for p53 protein.

CONCLUSION

Pilocytic astrocytomas have neither MSI phenotype nor recurrent alterations of the p53 and p116 genes. However, altered expression of PTEN may be important in the genesis of pilocytic astrocytomas. We conclude that pilocytic astrocytomas are genetically distinct from diffuse astrocytomas. Lack of p53 mutation/immunostaining may serve as a diagnostic adjunct for differentiating pilocytic astrocytomas from diffuse astrocytomas in small neurosurgical biopsies.

Central neurocytomas are genetically distinct from oligodendrogliomas and neuroblastomas

AIMS

Central neurocytoma is a rare central nervous system tumour typically found in the lateral ventricles and at the septum pellucidum. Histologically, it resembles oligodendrogliomas and yet ultrastructurally, it shows neuronal differentiation. Its molecular oncogenesis is not known. The aim of this study was to examine whether major genetic events found in oligodendrogliomas and neuronal tumours, namely allelic deletions of chromosomes 1p and 19q and N-myc amplification, can be found in central neurocytomas. As there was one report describing gain of chromosome 7 in central neurocytomas, we also examined epidermal growth factor receptor (EGFR) amplification, as the EGFR gene is located at chromosome 7p.

METHODS AND RESULTS

Nine central neurocytomas and matched blood samples were examined for loss of heterozygosity (LOH) of 1p and 19q13.2-13.4 with 23 finely mapped microsatellite markers. N-myc amplification was studied by fluorescence in-situ hybridization using paraffin-embedded sections. EGFR amplification was tested for by differential PCR. Six of nine (67%) tumours showed LOH at one or more loci at 1p and 5/9 (56%) of cases showed LOH at 19q. However, common regions of deletion cannot be identified. The majority of informative markers are retained at 1p (84%) and 19q (86%). Only one tumour showed amplification of N-myc and none of the cases showed amplification of EGFR.

CONCLUSION

Central neurocytomas are genetically distinct from oligodendrogliomas, and chromosomes 1p and 19q probably do not play an important role in their pathogenesis. N-myc and EGFR amplification are rare.

Detection of chromosomal imbalances in central neurocytomas by using comparative genomic hybridization

OBJECT

Central neurocytomas are rare neuronal tumors commonly found in the intraventricular regions. Little is known about the tumorigenesis of these neoplasms. The aim of this study was to provide an overview of genetic imbalances in central neurocytomas.

METHODS

In this study, comparative genomic hybridization was used to identify DNA sequence copy number changes (losses and gains) in a series of 10 central neurocytomas. Tumor DNA and normal reference DNA were differentially labeled and allowed to cohybridize to normal metaphase chromosomes. After hybridization and fluorescent staining of the bound DNA, regions of gain or of loss of DNA sequences were detected as changes in the tumor/normal fluorescence intensity ratio along the target metaphase chromosomes. A gain of DNA sequence was detected in chromosomes 2p, 10q, and 18q. A protooncogene, Bcl2, which maps to 18q21, was evaluated by immunohistochemical analysis to determine its role in the formation of central neurocytomas.

CONCLUSIONS

In this study the authors identified recurrent genetic changes on chromosomes 2p, 10q, and 18q in central neurocytomas and highlighted chromosomal regions for additional mapping and cloning of candidate genes that are important in the development of central neurocytomas.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Establishment and characterization of a human cell line from paediatric cerebellar glioblastoma multiforme

Permanent glioma cell lines are invaluable tools in understanding the biology of glioblastomas. The present study reports the establishment of a clonal human cell line, GBM6840, derived from a biopsy of paediatric cerebellar glioblastoma multiforme. GBM6840 had a doubling time of 32 h and grew as a monolayer of large round cells that retained immunopositivity for glial fibrillary acidic protein and vimentin. Karyotypic analysis revealed a modal chromosome number of 68 and polysomies of chromosomes 3, 5 and 20, as well as the presence of 3-4 marker chromosomes. GBM6840 also showed anchorage-independent growth in soft agar and tumour formation in nude mice. The p16(CDKN2A) gene was transcriptionally silenced by hypermethylation, consistent with the lack of protein expression observed in the original tumour and cultured cells. Western blot analysis revealed normal protein expression of pRb and CDK4. It appears that p16 is the major component altered in the cell cycle pathway and may confer these cells unrestrained proliferation potential. Neither EGFR gene amplification nor over-expression of the protein was detected in the cultured cells. Over-expression of the p53 protein was observed in the majority of cells, despite undetectable mutation (exons 5-8) in the gene. One allele of the PTEN gene was found to be mutated during in vitro cultivation. Telomerase activity was demonstrated in the cultured cells but not in the original tumour, supporting the hypothesis that telomerase is required for the in vitro immortalization process.

Altered expression of the suppressors PML and p53 in glioblastoma cells with the antisense-EGF-receptor

Gene amplification and enhanced expression of the epidermal growth factor receptor (EGFR) represent the major molecular genetic alteration in glioblastomas and it may play an essential role in cell growth and in the carcinogenic process. On the other hand, the nuclear suppressor proteins PML and p53 are also known to play critical roles in cancer development and in suppressing cell growth. Here we report that, in glioblastoma cells with defective EGFR function, the expressions of both promyelocytic leukaemia (PML) and p53 were altered. Cells that were transfected with the antisense-cDNA of EGFR were found to have more cells in G1 and fewer cells in S phase. In addition, the transfected cells were found to be non-responsive to EGF-induced cell growth. Interestingly, the expression of the suppressors p53 and PML were found to be significantly increased by immunohistochemical assay in the antisense-EGFR cells. Moreover, the PML expression in many of the cells was converted from the nuclear dot pattern into fine-granulated staining pattern. In contrast, the expressions of other cell cycle regulated genes and proto-oncogene, including the cyclin-dependent kinase 4 (cdk4), retinoblastoma, p16INK4a and p21H-ras, were not altered. These data indicate that there are specific inductions of PML and p53 proteins which may account for the increase in G1 and growth arrest in antisense-EGFR treated cells. It also indicates that the EGF, p53 and PML transduction pathways were linked and they may constitute an integral part of an altered growth regulatory programme. The interactions and cross-talks of these critical molecules may be very important in regulating cell growth, differentiation and cellular response to treatment in glioblastomas.