A distinct region of the MGMT CpG island critical for transcriptional regulation is preferentially methylated in glioblastoma cells and xenografts.

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl DNA adducts such as those induced by alkylating agents. Loss of MGMT expression through transcriptional silencing by hypermethylation of its CpG island (CGI) is found in diverse human cancers including glioblastomas. Glioblastomas that have MGMT methylation respond to temozolomide, an alkylating agent, resulting in improved survival. Consequently, assessment of MGMT methylation has become a therapy response and prognostic indicator. However, it is not clear whether the region of the MGMT CGI commonly analysed is the critical region involved in transcriptional control. We measured methylation levels at each CpG site for the entire MGMT CGI using bisulfite modification and pyrosequencing, and compared them with MGMT mRNA expression in glioblastoma cell lines, xenografts and normal brain tissues (41 samples). Two critical regions were identified (DMR1 and DMR2). DMR2 encompasses the commonly analysed region and was always methylated when DMR1 was methylated. A luciferase reporter assay showed that substitutions of several specific CpG sites within DMR2 significantly attenuated the promoter activity of the MGMT CGI. Our results indicate that several CpG sites within DMR2 play a critical role in the transcriptional control of MGMT, making DMR2 the optimal target for methylation testing. However, given the highly variable patterns of MGMT methylation associated with transcriptional silencing observed in this region among the tumours in this study, methylation levels need to be measured at a number of individual CpGs within DMR2 to confidently predict transcriptional silencing and thus sensitivity to alkylating agents.

High-resolution array-based comparative genomic hybridization of medulloblastomas and supratentorial primitive neuroectodermal tumors.

Medulloblastomas and supratentorial primitive neuroectodermal tumors are aggressive childhood tumors. We report our findings using array comparative genomic hybridization (CGH) on a whole-genome BAC/PAC/cosmid array with a median clone separation of 0.97 Mb to study 34 medulloblastomas and 7 supratentorial primitive neuroectodermal tumors. Array CGH allowed identification and mapping of numerous novel, small regions of copy number change to genomic sequence in addition to the large regions already known from previous studies. Novel amplifications were identified, some encompassing oncogenes MYCL1, PDGFRA, KIT, and MYB not previously reported to show amplification in these tumors. In addition, one supratentorial primitive neuroectodermal tumor had lost both copies of the tumor-suppressor genes CDKN2A and CDKN2B. Ten medulloblastomas had findings suggestive of isochromosome 17q. In contrast to previous reports using conventional CGH, array CGH identified 3 distinct breakpoints in these cases: Ch 17: 17940393-19251679 (17p11.2, n = 6), Ch 17: 20111990-23308272 (17p11.2-17q11.2, n = 4), and Ch 17: 38425359-39091575 (17q21.31, n = 1). Significant differences were found in the patterns of copy number change between medulloblastomas and supratentorial primitive neuroectodermal tumors, providing further evidence that these tumors are genetically distinct despite their morphologic and behavioral similarities.

Pathology and molecular genetics of astrocytic gliomas.

Astrocytic gliomas are the most common primary brain tumours. Here we summarize the characteristic neuropathological features of the different types of astrocytic neoplasms according to the World Health Organization classification of tumours of the nervous system. In addition, we report on the present state of the art concerning the molecular genetics of these tumours. Over the past 20 years a number of recurrent chromosomal,genetic and epigenetic alterations have been found to be associated with the different histological types and malignancy grades of astrocytic tumours. However, we are still far from understanding the complex mechanisms that underly tumour initiation and progression in the individual case. Furthermore, the clinical significance of molecular parameters for the diagnostic and prognostic assessment of astrocytic gliomas is still limited. Therefore further investigation of the molecular mechanisms underlying oncogenesis and progression of these most common brain tumours is necessary to improve their diagnostic assessment and to devise novel, individually tailored treatment strategies.

Structural genomic abnormalities of chromosomes 9 and 18 in myxopapillary ependymomas.

Myxopapillary ependymomas (MPEs) are low-grade neuroepithelial tumors typically occurring in the conus-cauda equina-filum terminale region. Limited molecular and cytogenetic analysis of MPEs has not demonstrated consistent abnormalities. In an attempt to clarify the chromosomal status of these tumors and identify commonly aberrant regions in the genome we have combined 3 molecular/cyto/genetic methods to study 17 MPEs. Comparative genomic hybridization of 7/17 tumors identified concurrent gain on chromosomes 9 and 18 as the most frequent finding. The majority of the 17 tumors were also studied using microsatellite analysis with marker spanning the whole chromosomes 9 and 18 and interphase-FISH with centromeric probes for both chromosomes. Our combined results were consistent with concurrent gain in both chromosomes 9 and 18 in 11/17 cases, gain of either chromosome 9 or 18 and imbalance in the other chromosome in 3/17 tumors and allelic imbalances of chromosomes 9 or 18 in 3/17 and 1/17 tumors, respectively. Other abnormalities observed included gain of chromosomes 3, 4, 7, 8, 11, 13, 17q, 20, and X and loss of chromosomes 10 and 22. Our findings represent some steps towards understanding the molecular mechanisms involved in the development of MPE.

Prognostic and predictive markers in recurrent high grade glioma; results from the BR12 randomised trial.

We evaluated the prognostic and predictive value of a range of molecular changes in the setting of a randomised trial comparing standard PCV (procarbazine, CCNU (1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea) and vincristine) chemotherapy with the standard temozolomide (TMZ) 5-day (200 mg/m2/day) schedule and a 21-day (100 mg/m2/day) schedule in chemo-naïve, high-grade glioma (non-oligodendroglial tumours; WHO (World Health Organisation) grades III and IV) patients at first progression following radiotherapy.354 samples (79.2%) from the first operation of the 447 randomised patients provided enough tumour DNA for some or all parts of the study. Genome-wide array comparative genomic hybridisation (aCGH), mutation analysis of IDH1/2 and TP53 and methylation analyses of the MGMT CpG-island was done.84% of grade III tumours and 17% of grade IV had IDH1 or IDH2 mutations that conferred a better prognosis in both; MGMT methylation (defined as average value across 16 CpGs ≥ 10%) occurred in 75% of tumours and was also associated with improved survival. Both were of independent prognostic value after accounting for clinical factors and tumour grade. None of the molecular changes investigated gave clear evidence of a predictive benefit of TMZ over PCV or 21-day TMZ over 5-day TMZ although power was limited and a role for MGMT methylation could not be ruled out. Loss of 1p and 19q was seen in only 4 patients although hemizygous loss of 1p36 occurred in 20%.The findings support reports that IDH1/2 mutations and MGMT methylation can be used in addition to tumour grade and clinical factors to predict survival in patients with recurrent high grade gliomas when treated with any of the therapy regimes used.