No abstract

TMZ is well tolerated and provides a substantial rate of response in LGOT. Chromosome 1p loss is correlated with radiographic response and could be a helpful marker for guiding therapeutic decision making in LGOT.

This exclusive association suggests a new mechanism of tumorigenesis. Perhaps the IDH1/IDH2 mutation is a prerequisite for the occurrence of the t(1;19) translocation, or it is required for the 1p19q codeleted cells to acquire a tumor phenotype.

The O(6)-methylguanine-DNA methyltransferase gene (MGMT) is methylated in several cancers, including gliomas. However, the functional role of cysteine-phosphate-guanine (CpG) island (CGI) methylation in MGMT silencing is still controversial. The aim of this study was to investigate whether MGMT CGI methylation correlates inversely with RNA expression of MGMT in glioblastomas and to determine the CpG region whose methylation best reflects the level of expression. The methylation level of CpG sites that are potentially related to expression was investigated in 54 glioblastomas by pyrosequencing, a highly quantitative method, and analyzed with respect to their MGMT mRNA expression status. Three groups of patients were identified according to the methylation pattern of all 52 analyzed CpG sites. Overall, an 85% rate of concordance was observed between methylation and expression (p < 0.0001). When analyzing each CpG separately, six CpG sites were highly correlated with expression (p < 0.0001), and two CpG regions could be used as surrogate markers for RNA expression in 81.5% of the patients. This study indicates that there is good statistical agreement between MGMT methylation and expression, and that some CpG regions better reflect MGMT expression than do others. However, if transcriptional repression is the key mechanism in explaining the higher chemosensitivity of MGMT-methylated tumors, a substantial rate of discordance should lead clinicians to be cautious when deciding on a therapeutic strategy based on MGMT methylation status alone.

BACKGROUND.In contrast to oligodendrogliomas, molecular predictors of prognosis have not been consistently found in glioblastomas. However, genetic studies show that glioblastomas consist of several genetic subtypes and raise the possibility that molecular alterations could be predictive of survival. METHODS.A search for loss of heterozygosity (LOH) on chromosome 1p, 9p, 10q, 19q, EGFR (epidermal growth factor receptor), CDK4, and MDM2 (mouse double minute) amplifications, CDKN2A (INK4A/ARF) homozygous deletions, p53 expression, was performed in a series of 220 primary glioblastomas. The molecular alterations were then correlated with each other to identify distinct molecular pathways and with clinical parameters and the course of the disease to identify prognostic markers. RESULTS.Nonrandom associations were found between EGFR amplification and LOH10q, LOH9p, and INK4A/ARF deletion, LOH1p and LOH19q, and MDM2 and CDK4 amplification, whereas mutual exclusions were found between p53 expression and EGFR amplification, LOH 9p/INK4A/ARF homozygous deletion, and MDM2 and CDK4 amplification. Age (P ϭ 4.10 -5 ) and performance status (P ϭ .003) were the main predictors of outcome. In contrast, molecular markers were of limited impact: MDM2 amplification correlated with poor outcome on both univariate and multivariate analysis (P ϭ .01) and EGFR amplification with good prognosis on multivariate analysis (P ϭ .02). CONCLUSION.Despite their limited prognostic impact, the genetic markers investigated here outline distinct molecular pathways involved in glioblastoma tumorigenesis and warrant broader molecular screening.

The methylation status of the O6-methylguanine-methyltransferase promoter (MGMTP) was evaluated in 68 low-grade gliomas treated by neoadjuvant temozolomide. Methylated MGMTP was detected in 63 of 68 (92.6 %) patients and was a favorable predictor of progression-free survival as compared with unmethylated MGMTP tumors (p < 0.0001). Assessment of MGMTP status could help identifying low-grade gliomas patients more likely to respond to chemotherapy or to benefit from MGMT depletion strategies.

Extensive genomic and gene expression studies have been performed in gliomas, but the epigenetic alterations that characterize different subtypes of gliomas remain largely unknown. Here, we analyzed the methylation patterns of 807 genes (1536 CpGs) in a series of 33 low-grade gliomas (LGGs), 36 glioblastomas (GBMs), 8 paired initial and recurrent gliomas, and 9 controls. This analysis was performed with Illumina's Golden Gate Bead methylation arrays and was correlated with clinical, histological, genomic, gene expression, and genotyping data, including IDH1 mutations. Unsupervised hierarchical clustering resulted in 2 groups of gliomas: a group corresponding to de novo GBMs and a group consisting of LGGs, recurrent anaplastic gliomas, and secondary GBMs. When compared with de novo GBMs and controls, this latter group was characterized by a very high frequency of IDH1 mutations and by a hypermethylated profile similar to the recently described glioma CpG island methylator phenotype. MGMT methylation was more frequent in this group. Among the LGG cluster, 1p19q codeleted LGG displayed a distinct methylation profile. A study of paired initial and recurrent gliomas demonstrated that methylation profiles were remarkably stable across glioma evolution, even during anaplastic transformation, suggesting that epigenetic alterations occur early during gliomagenesis. Using the Cancer Genome Atlas data set, we demonstrated that GBM samples that had an LGG-like hypermethylated profile had a high rate of IDH1 mutations and a better outcome. Finally, we identified several hypermethylated and downregulated genes that may be associated with LGG and GBM oncogenesis, LGG oncogenesis, 1p19q codeleted LGG oncogenesis, and GBM oncogenesis.

RASSF1A is a major tumor suppressor gene located at 3p21.3. We investigated the role of aberrant promoter region hypermethylation of RASSF1A in a large series of adult gliomas. RASSF1A was frequently methylated in both primary tumors (36/63; 57%) and tumor cell lines (7/7; 100%). Hypermethylation of RASSF1A in glioma cell lines correlated with loss of expression and treatment with a demethylating agentreactivated RASSF1A gene expression. Furthermore, reexpression of RASSF1A suppressed the growth of glioma cell line H4 in vitro. Next, we investigated whether other members of the RASSF gene family were also inactivated by methylation. NORE1B and RASSF3 were not methylated in gliomas, while NORE1A and RASSF5/ AD037 demonstrated methylation in glioma cell lines but not in primary tumors. We then investigated the methylation status of three other candidate 3p21.3 tumor suppressor genes. CACNA2D2 and SEMA3B were not frequently methylated, but the BLU gene located just centromeric to RASSF1 was frequently methylated in glioma cell lines (7/7) and in 80% (35/44) of glioma tumors. In these tumor cell lines, BLU expression was restored after treatment with a demethylating agent. Loss of BLU gene expression in glioma tumors correlated with BLU methylation. There was no association between RASSF1A and BLU methylation. RASSF1A methylation increased with tumor grade, while BLU methylation was seen at similar frequencies in all grades. Our data implicate RASSF1A and BLU promoter methylation in the pathogenesis of adult gliomas, while other RASSF family members and CACNA2D2 and SEMA3B appear to have only minor roles. In addition, RASSF1A and BLU methylation appear to be independent and specific events and not due to region-wide changes in DNA methylation.