Progress on molecular biomarkers and classification of malignant gliomas

Abstract

Gliomas are the most common primary intracranial tumors in adults. Anaplastic gliomas (WHO grade III) and glioblastomas (WHO grade IV) represent the major groups of malignant gliomas in the brain. Several diagnostic, predictive, and prognostic biomarkers for malignant gliomas have been reported over the last few decades, and these markers have made great contributions to the accuracy of diagnosis, therapeutic decision making, and prognosis of patients. However, heterogeneity in patient outcomes may still be observed, which highlights the insufficiency of a classification system based purely on histopathology. Great efforts have been made to incorporate new information about the molecular landscape of gliomas into novel classifications that may potentially guide treatment. In this review, we summarize three distinctive biomarkers, three most commonly altered pathways, and three classifications based on microarray data in malignant gliomas.

Keywords

malignant glioma / molecular biomarker / IDH1 / MGMT / molecular classification

Introduction

Gliomas are the most common primary intracranial tumors in adults. In 2007, the World Health Organization (WHO) classified gliomas into astrocytomas, oligodendrogliomas, ependymomas, and others, based on morphology [1]. Apart from the most common pediatric glioma, pilocytic astrocytoma (WHO grade I), which is relatively well demarcated from the surrounding tissue and can thus be resected, most gliomas are diffused [2]. Diffuse gliomas are categorized into low-grade gliomas (LGGs, WHO grade II) and high-grade gliomas (HGGs, WHO grades III and IV), depending on their rate of growth. Anaplastic astrocytomas (WHO grade III) and glioblastomas (WHO grade IV) represent the major groups of malignant gliomas in the brain. The most malignant of the HGGs is glioblastoma multiforme (GBM). Primary GBMs (pGBMs) arise de novo in older patients and have a short duration of clinical symptoms [3], whereas secondary GBMs (sGBMs) develop from preceding grade II or III gliomas, which feature longer durations of symptoms and frequently affect patients younger than 40 years [4]. Allowing for individual variability, the survival of patients with glioblastoma has improved from an average of 10 months to 14 months after diagnosis over the past 5 years as a result of improvements in the standard of care [5]. On rare occasions, patients with GBM may survive for over 3 years. Several clinical and histopathological elements have been associated with a better prognosis for GBM, including younger age, good performance status, gross total resection, adjuvant treatments, giant-cell subtype, and oligodendroglial differentiation [6,7]. Heterogeneity in patient outcomes highlights the insufficiency of a classification system based purely on histopathology, which provides diagnostic information about the group as whole but limited information about individual patients. In the era of developing targeted therapies, a classification based on histopathology cannot provide sufficient insight to allow patient stratification. Great efforts have therefore been made to incorporate new information about the molecular landscape of gliomas into novel classifications that may potentially guide treatment. In addition, many biomarkers associated with patient outcomes and sensitivity to certain therapies (so-called prognostic and predictive factors) have been reported in recent decades by screening of high throughput microarray data followed by in vivo, in vitro, and clinical studies; such markers are likely to continue to be identified with further improvements in screening technology. Considering that a large number of reports on biomarkers and classifications of gliomas are available, we screened the most common and important ones and discuss them below.

Molecular biomarkers associated with malignant gliomas

Isocitrate dehydrogenase (IDH) 1/2 gene mutation

IDH catalyzes the oxidative carboxylation of isocitrate to α-ketoglutarate, resulting in the reduction of NADP to NADPH [8]. IDH 1 and 2 function in the cytoplasm and mitochondria, respectively, representing one of the most significant recent discoveries in the field of GBM. Recent findings demonstrate that IDH1 gene mutation is associated with younger patients and secondary GBMs and has better outcomes [9]. Subsequent studies have found that IDH1 mutation occurs in 60% to 90% of all WHO grade II and grade III diffused gliomas and sGBMs [10-14] but rarely in pGBMs and pilocytic astrocytomas [10]. IDH2 gene mutations are found in 5% of all gliomas [15]. IDH1/2 mutations have not been found in histologically tumor-like tissue, such as gliosis, adverse radiotherapy reactions, virus infection, infarction, or demyelination, thus improving the diagnostic accuracy of biopsies [16,17]. NADPH, which can prevent cells from oxidative stress, decreases after IDH mutation [10,18], while α-ketoglutarate, which can degrade the tumor-growth and angiogenesis promoter hypoxia-inducible factor (HIF)-1α, increases [19]. Heterozygotic IDH1/2 mutation enhances the activity of enzymes catalyzing the production of hydroxyglutarate, which may be associated with tumor formation [20,21]. Thus, IDH1/2 mutation is considered a diagnostic and prognostic marker and is one of the most exciting findings in glioma research.

Loss of heterozygosity (LOH) in chromosome 1p/19q

LOHs in the short arm of chromosome 1 (1p) and long arm of chromosome 19 (19q) are commonly found in oligodendrogliomas [22]. Although tumor suppressive genes have not been found on 1p and 19q, a recent exome-sequencing study of oligodendrogliomas revealed inactivating mutations of the CIC gene on 19q and the FUBP1 gene on 1p in a substantial fraction of oligodendrogliomas [23], providing important insights into the pathogenesis, diagnosis, prognosis, and treatment of these tumors. LOH in 1p/19q can be found in 90% of all oligodendrogliomas (WHO grade II), 60% of all anaplastic oligodendrogliomas (WHO grade III), 30% to 50% of all oligoastrocytomas, and less than 10% of all diffuse astrocytomas, including GBM [22]. There is a strong relationship between LOH of 1p/19q and oligodendroglial differentiation but this loss is not detected in all oligodendrogliomas; thus, the 1p/19q status cannot be inferred on the basis of histology [24]. Previous studies have confirmed the prognostic and predictive values of LOH in 1p/19q in relation to first-line chemotherapies [25-28].

To some extent, as a diagnostic, predictive, and prognostic marker, LOH in 1p/19q can shed more light on gliomas with oligo components.

Three most commonly altered pathways in malignant gliomas

Previous studies have identified the three most commonly altered pathways in GBMs as the receptor tyrosine kinase (RTK)/RAS/phosphoinositide 3-kinase (PI3K) (88%), p53 (87%), and retinoblastoma protein (RB) (77%) pathways [29].

Numerous growth factors are involved in the RTK/RAS/PI3K pathway, and abnormal activation of this signaling pathway may result from amplification or mutation of growth factor receptor genes, which may either be driven by activating downstream pathways or by loss or mutation of tumor suppressor genes.

Epidermal growth factor receptor (EGFR) is the most commonly altered factor in the aforementioned pathway [30,31]; it promotes cell proliferation by activating downstream mitogen-activated protein kinase and PI3K-Akt pathways [32]. EGFR gene amplification or mutation is found in 30% to 40% of all pGBMs [1]. About half of all EGFR-amplified GBMs express a truncated mutant variant, EGFRvIII, characterized by genomic deletion of exons 2 to 7, resulting in a constitutively active oncogenic form [33]. EGFR/EGFRvIII have been promoted as diagnostic markers, although their prognostic value remains unclear. Some studies have found that EGFR amplification has minimal effects on survival, while others have considered it as an indicator of poor prognosis in young and anaplastic gliomas [34,35].

PTEN is a tumor suppressor gene located on the long arm of chromosome 10, which can interact with the tumor-promoting PI3K-Akt pathway [36]. Mutations or chromosomal loss of PTEN is often found in malignant gliomas, while loss of heterogeneity (LOH) in 10q is common in pGBM, sGBM, and anaplastic astrocytoma but occurs less frequently in anaplastic oligodentrocytoma. PTEN mutations are found in 15% to 40% of all pGBMs but are absent in sGBMs and other gliomas [37]. To date, LOH in 10q and mutations of PTEN are considered indicators of poor outcomes in malignant gliomas, and loss of 10q has been associated with tumor progression in most studies [38].

The p53 and RB pathways are two other main tumor suppressor pathways. The p53 protein can activate p21 transcription, which can then block cell cycle progression in the G₁ phase by binding and inhibiting the function of the cyclin D family of proteins [39]. The RB protein can prevent cell entry into the S phase by inactivating the E2F family of transcription factors. p53 and RB can be direct targets of mutations; inactivation of cell cycle control pathways can also be achieved indirectly by mutation or overexpression of other signaling molecules in these pathways.

As the aforementioned pathways are commonly altered in gliomas, some of them are potential therapeutic targets. Some monoclonal antibodies against RTKs have been administered in clinics.

O-6-Methylguanine-DNA methyltransferase (MGMT)

MGMT is a DNA repair enzyme that can remove alkyl groups from the O6 position of guanine, a function that underlies the development of resistance to alkylating agent therapy in some patients [40]. Methylation of cytosines in the MGMT promoter CG-dinucleotide-rich sites can silence its expression relative to normal tissue, promoting reactions with alkylating agents [41,42]. MGMT promoter methylation can be found in all grades of gliomas and has been identified as an independent prognostic factor in anaplastic gliomas and GBMs, including aged GBM patients, in some [25,42-45], but not all studies [46]. Some researchers have found that the methylation status varies during disease progression and is a prognostic factor in primary but not secondary tumors [47]. MGMT promoter methylation has been reported to have predictive value in GBM patients treated with temozolomide [41,48] but not in those with anaplastic gliomas [25,49].

Molecular classification of malignant gliomas

Phillips’ classification

In 2006, Phillips et al. [50] identified a gene signature including 35 genes by analyzing microarrays of 76 WHO grade III and grade IV astrocytomas; this signature allowed classification of patients proneural (PN), proliferative (Prolif), and mesenchymal (Mes) subclasses. Higher expression levels of neuron-associated markers have been observed in PN, which is linked to significantly longer survival times (174.5 weeks). Prolif is characterized by the overexpression of proliferation-associated markers (proliferating cell nuclear antigen and topoisomerase II α), and Mes displays overexpression of angiogenesis markers, including vascular endothelial growth factor (VEGF), VEGF receptor (VEGFR) 1, VEGFR2, and the endothelial marker platelet/endothelial cell adhesion molecule 1. These two subclasses have shorter survival times (60.5 and 65.0 weeks, respectively). Upon recurrence, tumors frequently shift toward the Mes subclass. The chromosomal locations of genes distinguishing these tumor subclasses parallel DNA copy-number differences among the subclasses.

Researchers first identified probe sets with expression levels most strongly correlated with survival (Spearman r of log-transformed expression intensity values versus survival times>0.45 or<-0.45), followed by two-way agglomerative clustering of the resulting 108 probe sets and 76 samples. This analysis identified three discrete groups of sample sets that differed markedly in survival-related gene expressions. To determine markers for each of the three tumor subclasses, probe sets most strongly overexpressed by each tumor subgroup compared with the remaining subclasses were identified. Using the most robust markers for each of the three tumor subsets, a set of 35 genes, referred to as the gene signature, was derived; this signature could be used for either hierarchical clustering or k-means clustering to assign tumors to subclasses.

Researchers have also found a clear association between tumor grade and subtype; GBMs represent a mix of the three subtypes, while grade III diffuse gliomas almost invariably belong to the PN subtype, regardless of their inclusion of oligodendroglioma or astrocytoma components.

Classification of the Cancer Genome Atlas (TCGA)

TCGA analyzes over 500 primary, untreated glioblastoma specimens at the DNA (gene copy number, gene sequencing, and epigenetic methylation), mRNA (gene expression profile), and microRNA (small RNAs that can regulate expression) levels [51]. Cross-platform analyses are still ongoing, and new genetic alterations have been detected, providing evidence that glioblastomas can be subdivided into several subtypes.

The “classical” subtype, which is characterized by highly proliferative cells, shows uniform gains in chromosome 7 accompanied by losses in chromosome 10 (93%) and frequent focal losses in chromosome 9p21.3 (95%). These chromosomal events lead to amplification of the EGFR gene and losses in PTEN and CDKN2A gene loci, whereas alterations in TP53, neurofibromatosis-1 (NF1), platelet-derived growth factor receptor α (PDGFRA), and IDH1 genes were rare. Classical GBM is sensitive to classical radiation and chemotherapies, probably because the p53 DNA-damage response remains intact in this group of patients. Such tumors may also be responsive to inhibitors of Mdm2, the negative regulator of p53. Expression levels of the neural precursor and stem cell marker genes NES, NOTCH3, JAG1, and LFNG in the Notch signaling pathway, and SMO, GAS1, and GLI2 in the sonic hedgehog signaling pathways are elevated in classical GBM.

The second subtype is defined by an expression profile associated with mesenchyme and angiogenesis and overexpression of several genes, including the CHI3L1/YKL40 and MET genes, genes for the astrocytic markers CD44 and MERTK, and genes in the tumor necrosis factor super family and nuclear factor κB pathways. This mesenchymal group shows frequent inactivation of the NF1 (37%), TP53 (32%), and PTEN (32%) genes. Tumors of this subtype are sensitive to aggressive chemoradiotherapies and may be responsive to inhibitors of Ras, PI3K, and angiogenesis.

A third “proneural” subtype shows an expression profile reminiscent of gene activation in neuronal development with high expression levels of oligodendrocytic (PDGFRA, OLIG2, TCF3, and NKX2-2) and proneural (SOX, DCX, DLL3, ASCL1, and TCF4) development genes. Patients in this group are younger, and 30% of them demonstrate overexpression or amplification/mutation of the PDGFRA gene. Mutations in the IDH1 gene are signature genetic alterations. Frequent mutations in TP53 (54%) and PIK3CA/PIK3R1 (19%) genes and significant amplification of chromosome 7 and losses in chromosome 10 (50%) have been observed; however, this subtype presents less frequently than the classical subtype. Identification of IDH1/2 gene mutations in LGGs also suggests that sGBMs may belong to this subtype. This subtype may be most responsive to inhibitors of the HIF, PI3K, and PDGFRA pathways. Survival in the proneural subtype is slightly better than in the other glioblastoma subtypes, even though tumors of this type are the least responsive to aggressive classical therapies.

The fourth subtype, termed the “neural” subtype, is less well defined and has a gene expression profile most similar to that found in normal brain tissue, with activation of neuronal markers such as NEFL, GABRA1, SYT1, and SLC12A5. However, these tumors demonstrate a low degree of infiltration by normal cells, excluding the possibility of bias in the expression analyses. Nevertheless, their expression signature is suggestive of cells with a differentiated phenotype.

Comparison of the gene expression patterns of the four GBM subtypes with those of primary murine astrocytes, oligodendrocytes, neurons, and microglia suggests that the subtypes may reflect different cell origins, a hypothesis that merits further confirmation.

Glioma CpG island methylator phenotype (G-CIMP)

A CpG island methylator phenotype (CIMP) was first characterized in human colorectal cancer by Toyota and colleagues [52] as cancer-specific CpG island hypermethylation of a subset of genes in a subset of tumors.

In a recent study, Noushmehr et al. [53] determined DNA methylation profiles in a set of 272 TCGA GBM samples. They initially relied on the Illumina GoldenGate platform, using both the standard Cancer Panel I and a custom-designed array, and subsequently migrated to the more comprehensive Infinium platform upon availability. They then selected the most variant probes on each platform and performed consensus clustering to identify GBM subgroups. Three DNA methylation clusters were identified using either the GoldenGate or Infinium data with 97% concordance (61/63). Cluster 1 formed a particularly tight cluster on both platforms with a highly characteristic DNA methylation profile, reminiscent of the CIMP described in colorectal cancer. GBM samples showed similar concerted methylation changes at a subset of loci. Cluster 1 tumors were thus designated as having a G-CIMP.

G-CIMP-positive samples are associated with sGBM or rGBM and are tightly associated with IDH1 mutation. G-CIMP tumors also show a relative lack of copy-number variations commonly observed in GBM, including EGFR amplification, gains in chromosome 7, and losses in chromosome 10. Integration of the DNA methylation data with gene expression data shows that G-CIMP-positive tumors represent a subset of proneural tumors. G-CIMP-positive tumors show better outcomes within GBMs as well as within the proneural subset, consistent with prior reports on IDH1 mutant tumors. Interestingly, of the five discordant cases of G-CIMP-positive IDH1-wild-type tumors studied, two patients survived for over 5 years after diagnosis, which suggests that a G-CIMP-positive status may confer favorable outcomes independent of IDH1 mutation status. However, studies with more discordant cases are necessary to clarify the effects of G-CIMP status on survival compared with IDH1 mutation. The improved prognosis conferred by proneural tumors can largely be accounted for by the G-CIMP-positive subset. These findings indicate that G-CIMP could be used to further refine expression-defined groups into additional subtypes with clinical implications.

RBP1 and G0S2 are two genes that show the strongest evidence of epigenetic silencing in G-CIMP tumors. RBP1 has previously been reported to be epigenetically silenced in cancer cell lines and primary tumors, and the association of the encoded protein with retinoic acid receptors (RARs) has been well characterized. G0S2 gene expression is regulated by retinoic acid (RA) and encodes a protein that promotes apoptosis in primary cells, suggesting a tumor-suppressor role. The vitamin A metabolite RA is important for both embryonic and adult growth and plays diverse roles involving neuronal development and differentiation mediated by RARs.

Dissecting gene expressions and DNA methylation alterations in G-CIMP tumors among lower-grade gliomas will be helpful in developing a better understanding of the roles of mutant IDH1 and G-CIMP DNA methylation on tumor grade and patient survival.

Conclusions

Recent findings on the biomarkers and classification systems of gliomas have extraordinarily expanded our understanding of the disease.

Human brain tumors represent a heterogeneous group of tumors with respect to both tumor characteristics and patient outcomes. While classification of human brain tumors to date has relied largely on histological characteristics, tools for the molecular analysis of brain tumors are rapidly emerging in the field of molecular pathology. As discussed above, several molecular classification systems are available, and collective evidence suggests the existence of two major groups of gliomas. One group exhibits relative overexpression of genes with functional ontology related to cell motility, the extracellular matrix, and cell adhesion (mesenchymal), whereas the second group demonstrates relative overexpression of genes known to be associated with neural development (proneural). With respect to the two major subtypes, a clear association between tumor grade and subtype may be observed; GBMs represent a mix of mesenchymal and proneural subtypes, while grade II and grade III diffuse gliomas are almost invariably proneural. Consistent with these findings, proneural GBMs tend to show independent clinical and molecular evidence of “secondary” GBM, including younger patient age, higher rates of p53 and IDH1 mutation, and lower rates of EGFR amplification and losses in chromosome 10.

Transcriptional profiling of these tumors has identified molecular subtypes with complex gene signatures, resulting in the identification of prognostic markers independent of existing clinical and pathological criteria. These subtypes represent an instrumental classification system, and all other systems can be compared with it. Further subclassification and different refinement methods have been proposed, but the methodologies for developing these classification systems differ and there is currently no consensus regarding the optimal classification method. Next-generation sequencing will likely identify more robust biomarkers. Such an approach is poised to provide rapid advances in the field of molecular neuropathology. Further identification of prognostic and predictive markers will allow customization of therapy to match the genetic makeup of individual patients.

References

[1] Louis DN, Ohgaki H, Wiestler OD, Cavenee WK, Burger PC, Jouvet A, Scheithauer BW, Kleihues P. The 2007 WHO classification of tumours of the central nervous system. Acta Neuropathol2007; 114(2): 97-109

[2] Jones DT, Gronych J, Lichter P, Witt O, Pfister SM. MAPK pathway activation in pilocytic astrocytoma. Cell Mol Life Sci2012; 69(11): 1799-1811

[3] Biernat W, Huang H, Yokoo H, Kleihues P, Ohgaki H. Predominant expression of mutant EGFR (EGFRvIII) is rare in primary glioblastomas. Brain Pathol2004; 14(2): 131-136

[4] Ohgaki H, Kleihues P. Population-based studies on incidence, survival rates, and genetic alterations in astrocytic and oligodendroglial gliomas. J Neuropathol Exp Neurol2005; 64(6): 479-489

[5] Van Meir EG, Hadjipanayis CG, Norden AD, Shu HK, Wen PY, Olson JJ. Exciting new advances in neuro-oncology: the avenue to a cure for malignant glioma. CA Cancer J Clin2010; 60(3): 166-193

[6] Lawrence YR, Mishra MV, Werner-Wasik M, Andrews DW, Showalter TN, Glass J, Shen X, Symon Z, Dicker AP. Improving prognosis of glioblastoma in the 21st century: who has benefited most? Cancer2012; 118(17): 4228-4234

[7] Wang Y, Li S, Chen L, You G, Bao Z, Yan W, Shi Z, Chen Y, Yao K, Zhang W, Kang C, Jiang T. Glioblastoma with an oligodendroglioma component: distinct clinical behavior, genetic alterations, and outcome. Neuro-oncol2012; 14(4): 518-525

[8] Geisbrecht BV, Gould SJ. The human PICD gene encodes a cytoplasmic and peroxisomal NADP(+)-dependent isocitrate dehydrogenase. J Biol Chem1999; 274(43): 30527-30533

[9] Parsons DW, Jones S, Zhang X, Lin JC, Leary RJ, Angenendt P, Mankoo P, Carter H, Siu IM, Gallia GL, Olivi A, McLendon R, Rasheed BA, Keir S, Nikolskaya T, Nikolsky Y, Busam DA, Tekleab H, Diaz LA Jr, Hartigan J, Smith DR, Strausberg RL, Marie SK, Shinjo SM, Yan H, Riggins GJ, Bigner DD, Karchin R, Papadopoulos N, Parmigiani G, Vogelstein B, Velculescu VE, Kinzler KW. An integrated genomic analysis of human glioblastoma multiforme. Science2008; 321(5897): 1807-1812

[10] Yan H, Parsons DW, Jin G, McLendon R, Rasheed BA, Yuan W, Kos I, Batinic-Haberle I, Jones S, Riggins GJ, Friedman H, Friedman A, Reardon D, Herndon J, Kinzler KW, Velculescu VE, Vogelstein B, Bigner DD. IDH1 and IDH2 mutations in gliomas. N Engl J Med2009; 360(8): 765-773

[11] Watanabe T, Nobusawa S, Kleihues P, Ohgaki H. IDH1 mutations are early events in the development of astrocytomas and oligodendrogliomas. Am J Pathol2009; 174(4): 1149-1153

[12] Nobusawa S, Watanabe T, Kleihues P, Ohgaki H. IDH1 mutations as molecular signature and predictive factor of secondary glioblastomas. Clin Cancer Res2009; 15(19): 6002-6007

[13] van den Bent MJ, Dubbink HJ, Marie Y, Brandes AA, Taphoorn MJ, Wesseling P, Frenay M, Tijssen CC, Lacombe D, Idbaih A, van Marion R, Kros JM, Dinjens WN, Gorlia T, Sanson M. IDH1 and IDH2 mutations are prognostic but not predictive for outcome in anaplastic oligodendroglial tumors: a report of the European Organization for Research and Treatment of Cancer Brain Tumor Group. Clin Cancer Res2010; 16(5): 1597-1604

[14] Shibahara I, Sonoda Y, Kanamori M, Saito R, Yamashita Y, Kumabe T, Watanabe M, Suzuki H, Kato S, Ishioka C, Tominaga T. IDH1/2 gene status defines the prognosis and molecular profiles in patients with grade III gliomas. Int J Clin Oncol2012; 17(6): 551-561

[15] Hartmann C, Meyer J, Balss J, Capper D, Mueller W, Christians A, Felsberg J, Wolter M, Mawrin C, Wick W, Weller M, Herold-Mende C, Unterberg A, Jeuken JW, Wesseling P, Reifenberger G, von Deimling A. Type and frequency of IDH1 and IDH2 mutations are related to astrocytic and oligodendroglial differentiation and age: a study of 1,010 diffuse gliomas. Acta Neuropathol2009; 118(4): 469-474

[16] Capper D, Sahm F, Hartmann C, Meyermann R, von Deimling A, Schittenhelm J. Application of mutant IDH1 antibody to differentiate diffuse glioma from nonneoplastic central nervous system lesions and therapy-induced changes. Am J Surg Pathol2010; 34(8): 1199-1204

[17] Horbinski C, Kofler J, Kelly LM, Murdoch GH, Nikiforova MN. Diagnostic use of IDH1/2 mutation analysis in routine clinical testing of formalin-fixed, paraffin-embedded glioma tissues. J Neuropathol Exp Neurol2009; 68(12): 1319-1325

[18] Ichimura K, Pearson DM, Kocialkowski S, Bäcklund LM, Chan R, Jones DT, Collins VP. IDH1 mutations are present in the majority of common adult gliomas but rare in primary glioblastomas. Neuro-oncol2009; 11(4): 341-347

[19] Zhao S, Lin Y, Xu W, Jiang W, Zha Z, Wang P, Yu W, Li Z, Gong L, Peng Y, Ding J, Lei Q, Guan KL, Xiong Y. Glioma-derived mutations in IDH1 dominantly inhibit IDH1 catalytic activity and induce HIF-1alpha. Science2009; 324(5924): 261-265

[20] Dang L, White D W, Gross S, Bennett B D, Bittinger M A, Driggers E M, Fantin V R, Jang H G, Jin S, Keenan M C, Marks K M, Prins R M, Ward P S, Yen K E, Liau L M, Rabinowitz J D, Cantley L C, Thompson C B, Vander Heiden M G, Su S M. Cancer-associated IDH1 mutations produce 2-hydroxyglutarate. Nature2009;462(7274):739-744.

[21] Reitman ZJ, Jin G, Karoly ED, Spasojevic I, Yang J, Kinzler KW, He Y, Bigner DD, Vogelstein B, Yan H. Profiling the effects of isocitrate dehydrogenase 1 and 2 mutations on the cellular metabolome. Proc Natl Acad Sci USA2011; 108(8): 3270-3275

[22] Jeuken JW, von Deimling A, Wesseling P. Molecular pathogenesis of oligodendroglial tumors. J Neurooncol2004; 70(2): 161-181

[23] Bettegowda C, Agrawal N, Jiao Y, Sausen M, Wood LD, Hruban RH, Rodriguez FJ, Cahill DP, McLendon R, Riggins G, Velculescu VE, Oba-Shinjo SM, Marie SK, Vogelstein B, Bigner D, Yan H, Papadopoulos N, Kinzler KW. Mutations in CIC and FUBP1 contribute to human oligodendroglioma. Science2011; 333(6048): 1453-1455

[24] Scheie D, Cvancarova M, Mørk S, Skullerud K, Andresen PA, Benestad I, Helseth E, Meling T, Beiske K. Can morphology predict 1p/19q loss in oligodendroglial tumours? Histopathology2008; 53(5): 578-587

[25] Wick W, Hartmann C, Engel C, Stoffels M, Felsberg J, Stockhammer F, Sabel MC, Koeppen S, Ketter R, Meyermann R, Rapp M, Meisner C, Kortmann RD, Pietsch T, Wiestler OD, Ernemann U, Bamberg M, Reifenberger G, von Deimling A, Weller M. NOA-04 randomized phase III trial of sequential radiochemotherapy of anaplastic glioma with procarbazine, lomustine, and vincristine or temozolomide. J Clin Oncol2009; 27(35): 5874-5880

[26] Cairncross G, Berkey B, Shaw E, Jenkins R, Scheithauer B, Brachman D, Buckner J, Fink K, Souhami L, Laperierre N, Mehta M, Curran W. Phase III trial of chemotherapy plus radiotherapy compared with radiotherapy alone for pure and mixed anaplastic oligodendroglioma: Intergroup Radiation Therapy Oncology Group Trial 9402. J Clin Oncol2006; 24(18): 2707-2714

[27] van den Bent MJ, Carpentier AF, Brandes AA, Sanson M, Taphoorn MJ, Bernsen HJ, Frenay M, Tijssen CC, Grisold W, Sipos L, Haaxma-Reiche H, Kros JM, van Kouwenhoven MC, Vecht CJ, Allgeier A, Lacombe D, Gorlia T. Adjuvant procarbazine, lomustine, and vincristine improves progression-free survival but not overall survival in newly diagnosed anaplastic oligodendrogliomas and oligoastrocytomas: a randomized European Organisation for Research and Treatment of Cancer phase III trial. J Clin Oncol2006; 24(18): 2715-2722

[28] Kouwenhoven MCM, Kros JM, French PJ, Biemond-ter Stege EM, Graveland WJ, Taphoorn MJB, Brandes AA, van den Bent MJ. 1p/19q loss within oligodendroglioma is predictive for response to first line temozolomide but not to salvage treatment. Eur J Cancer2006; 42(15): 2499-2503

[29] Cancer Genome Atlas Research Network. Comprehensive genomic characterization defines human glioblastoma genes and core pathways. Nature2008; 455(7216): 1061-1068

[30] Wong AJ, Ruppert JM, Bigner SH, Grzeschik CH, Humphrey PA, Bigner DS, Vogelstein B. Structural alterations of the epidermal growth factor receptor gene in human gliomas. Proc Natl Acad Sci USA1992; 89(7): 2965-2969

[31] Hurtt MR, Moossy J, Donovan-Peluso M, Locker J. Amplification of epidermal growth factor receptor gene in gliomas: histopathology and prognosis. J Neuropathol Exp Neurol1992; 51(1): 84-90

[32] Lino MM, Merlo A. PI3Kinase signaling in glioblastoma. J Neurooncol2011; 103(3): 417-427

[33] Gan HK, Kaye AH, Luwor RB. The EGFRvIII variant in glioblastoma multiforme. J Clin Neurosci2009; 16(6): 748-754

[34] Hatanpaa KJ, Burma S, Zhao D, Habib AA. Epidermal growth factor receptor in glioma: signal transduction, neuropathology, imaging, and radioresistance. Neoplasia2010; 12(9): 675-684

[35] Aldape KD, Ballman K, Furth A, Buckner JC, Giannini C, Burger PC, Scheithauer BW, Jenkins RB, James CD. Immunohistochemical detection of EGFRvIII in high malignancy grade astrocytomas and evaluation of prognostic significance. J Neuropathol Exp Neurol2004; 63(7): 700-707

[36] Knobbe CB, Merlo A, Reifenberger G. Pten signaling in gliomas. Neuro Oncol2002; 4(3): 196-211

[37] Ohgaki H, Kleihues P. Genetic alterations and signaling pathways in the evolution of gliomas. Cancer Sci2009; 100(12): 2235-2241

[38] Koul D. PTEN signaling pathways in glioblastoma. Cancer Biol Ther2008; 7(9): 1321-1325

[39] Busch C, Geisler J, Knappskog S, Lillehaug JR, Lønning PE. Alterations in the p53 pathway and p16INK4a expression predict overall survival in metastatic melanoma patients treated with dacarbazine. J Invest Dermatol2010; 130(10): 2514-2516

[40] Silber JR, Bobola MS, Blank A, Chamberlain MC. O(6)-methylguanine-DNA methyltransferase in glioma therapy: promise and problems. Biochim Biophys Acta2012; 1826(1): 71-82

[41] Hegi ME, Diserens AC, Gorlia T, Hamou MF, de Tribolet N, Weller M, Kros JM, Hainfellner JA, Mason W, Mariani L, Bromberg JE, Hau P, Mirimanoff RO, Cairncross JG, Janzer RC, Stupp R. MGMT gene silencing and benefit from temozolomide in glioblastoma. N Engl J Med2005; 352(10): 997-1003

[42] Olson RA, Brastianos PK, Palma DA. Prognostic and predictive value of epigenetic silencing of MGMT in patients with high grade gliomas: a systematic review and meta-analysis. J Neurooncol2011; 105(2): 325-335

[43] Gorlia T, van den Bent MJ, Hegi ME, Mirimanoff RO, Weller M, Cairncross JG, Eisenhauer E, Belanger K, Brandes AA, Allgeier A, Lacombe D, Stupp R. Nomograms for predicting survival of patients with newly diagnosed glioblastoma: prognostic factor analysis of EORTC and NCIC trial 26981-22981/CE.3. Lancet Oncol2008; 9(1): 29-38

[44] Rivera AL, Pelloski CE, Gilbert MR, Colman H, De La Cruz C, Sulman EP, Bekele BN, Aldape KD. MGMT promoter methylation is predictive of response to radiotherapy and prognostic in the absence of adjuvant alkylating chemotherapy for glioblastoma. Neuro-oncol2010; 12(2): 116-121

[45] Minniti G, Salvati M, Arcella A, Buttarelli F, D’Elia A, Lanzetta G, Esposito V, Scarpino S, Maurizi Enrici R, Giangaspero F. Correlation between O6-methylguanine-DNA methyltransferase and survival in elderly patients with glioblastoma treated with radiotherapy plus concomitant and adjuvant temozolomide. J Neurooncol2011; 102(2): 311-316

[46] Combs SE, Rieken S, Wick W, Abdollahi A, von Deimling A, Debus J, Hartmann C. Prognostic significance of IDH-1 and MGMT in patients with glioblastoma: one step forward, and one step back? Radiat Oncol2011; 6(1): 115

[47] Brandes AA, Franceschi E, Tosoni A, Bartolini S, Bacci A, Agati R, Ghimenton C, Turazzi S, Talacchi A, Skrap M, Marucci G, Volpin L, Morandi L, Pizzolitto S, Gardiman M, Andreoli A, Calbucci F, Ermani M. O(6)-methylguanine DNA-methyltransferase methylation status can change between first surgery for newly diagnosed glioblastoma and second surgery for recurrence: clinical implications. Neuro-oncol2010; 12(3): 283-288

[48] Hegi ME, Diserens AC, Godard S, Dietrich PY, Regli L, Ostermann S, Otten P, Van Melle G, de Tribolet N, Stupp R. Clinical trial substantiates the predictive value of O-6-methylguanine-DNA methyltransferase promoter methylation in glioblastoma patients treated with temozolomide. Clin Cancer Res2004; 10(6): 1871-1874

[49] van den Bent MJ, Dubbink HJ, Sanson M, van der Lee-Haarloo CR, Hegi M, Jeuken JW, Ibdaih A, Brandes AA, Taphoorn MJ, Frenay M, Lacombe D, Gorlia T, Dinjens WN, Kros JM. MGMT promoter methylation is prognostic but not predictive for outcome to adjuvant PCV chemotherapy in anaplastic oligodendroglial tumors: a report from EORTC Brain Tumor Group Study 26951. J Clin Oncol2009; 27(35): 5881-5886

[50] Phillips HS, Kharbanda S, Chen R, Forrest WF, Soriano RH, Wu TD, Misra A, Nigro JM, Colman H, Soroceanu L, Williams PM, Modrusan Z, Feuerstein BG, Aldape K. Molecular subclasses of high-grade glioma predict prognosis, delineate a pattern of disease progression, and resemble stages in neurogenesis. Cancer Cell2006; 9(3): 157-173

[51] Verhaak RG, Hoadley KA, Purdom E, Wang V, Qi Y, Wilkerson MD, Miller CR, Ding L, Golub T, Mesirov JP, Alexe G, Lawrence M, O’Kelly M, Tamayo P, Weir BA, Gabriel S, Winckler W, Gupta S, Jakkula L, Feiler HS, Hodgson JG, James CD, Sarkaria JN, Brennan C, Kahn A, Spellman PT, Wilson RK, Speed TP, Gray JW, Meyerson M, Getz G, Perou CM, Hayes DN. Integrated genomic analysis identifies clinically relevant subtypes of glioblastoma characterized by abnormalities in PDGFRA, IDH1, EGFR, and NF1. Cancer Cell2010; 17(1): 98-110

[52] Toyota M, Ahuja N, Ohe-Toyota M, Herman JG, Baylin SB, Issa JP. CpG island methylator phenotype in colorectal cancer. Proc Natl Acad Sci USA1999; 96(15): 8681-8686

[53] Noushmehr H, Weisenberger DJ, Diefes K, Phillips HS, Pujara K, Berman BP, Pan F, Pelloski CE, Sulman EP, Bhat KP, Verhaak RG, Hoadley KA, Hayes DN, Perou CM, Schmidt HK, Ding L, Wilson RK, Van Den Berg D, Shen H, Bengtsson H, Neuvial P, Cope LM, Buckley J, Herman JG, Baylin SB, Laird PW, Aldape K. Identification of a CpG island methylator phenotype that defines a distinct subgroup of glioma. Cancer Cell2010; 17(5): 510-522

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg