Oncogenes and tumour suppressor genes in human central nervous system tumours

Human central nervous system (CNS) tumours may arise from both developing and adult brain. Primitive neuroectodermal tumours (PNETs) are the most malignant tumours arising from developing brain, and constitute approximately one third of brain tumours in children. Other common paediatric CNS tumours are ependymomas and pilocytic astrocytomas. In adult brain, astrocytic tumours including astrocytoma, anaplastic astrocytoma and glioblastoma are the most common brain tumours accounting for over 50% of primary intracranial malignancies. Glioblastomas are the most malignant brain tumours in adults. There is extensive evidence for the involvement of both oncogenes and tumour suppressor genes in the oncogenesis and progression of primary CNS tumours. In order to understand the nature of molecular abnormalities during oncogenesis and progression, we have investigated abnormalities of some tumour suppressor genes and oncogenes in such tumours.

Loss of one wild type TP53 allele combined with mutation of the retained allele has been shown in adult astrocytic gliomas. Involvement of the TP53 gene in the development of paediatric tumours had not been investigated. We examined 33 cases of paediatric CNS tumours for loss of alleles on chromosome 17 and for mutations of the TP53 gene. We found that 5 PNET cases had loss of chromosome 17p together with isochromosome l7q. Direct sequencing of the TP53 gene transcript from exon 1-11 showed no mutations. The high incidence of 17p loss with (iso)17q together with the absence of TP53 gene mutations suggests that there may be one or more gene(s) on 17p and 17q involved in these tumours Amplification of the MDM2 gene has been reported in human sarcomas and in anaplastic astrocytomas and glioblastomas. We examined 30 glioblastoma cell lines for MDM2 amplification and carried out a mutation analysis of TP53. Two cell lines showed amplification and over-expression of MDM2 but had no TP53 mutations. Co-amplification of CDK4 was found in one of these cell lines. At least one cell line had a TP53 mutation but no amplification of MDM2. These findings support the hypothesis that over-expression of MDM2, secondary to gene amplification, is an alternative mechanism to mutation of TP53.

The EGFR gene (7p12) has been found both amplified and rearranged in human glioblastomas. The most common rearrangements lead to the loss of exons 2-7 from the transcript. This results in the loss of the major part of the extracellular ligand-binding domain. In order to understand the significance of this mutated receptor in glioma progression, we cloned this truncated EGFR transcript. Studies in CHO transfectants demonstrated that the mutated receptor was constitutively activated. Studies of chromosome 7 in 121 glioblastomas by centromere-FISH analysis combined with microsatellite analysis (28 markers) along the chromosome demonstrated polysomy as well as complex rearrangements of chromosome 7 in glioblastomas. Our data also indicated that the amplicon of the region around the EGFR gene is complicated and EGFR is not always amplified. An extended series of 246 human astrocytic gliomas has been used to study the 7p12 region in detail. Our results showed that amplicons frequently include several unknown genes - sometimes without EGFR. The levels of mRNA expression are consistently raised when some of these genes are amplified. These unknown genes may form independent targets for the amplification process.