O6-methylguanine-DNA methyltransferase in tumors and cells of the oligodendrocyte lineage

The future of high-grade glioma: Where we are and where are we going

High-grade glioma (HGG) are optimally treated with maximum safe surgery, followed by radiotherapy (RT) and/or systemic chemotherapy (CT). Recently, the treatment of newly diagnosed anaplastic glioma (AG) has changed, particularly in patients with 1p19q codeleted tumors. Results of trials currenlty ongoing are likely to determine the best standard of care for patients with noncodeleted AG tumors. Trials in AG illustrate the importance of molecular characterization, which are germane to both prognosis and treatment. In contrast, efforts to improve the current standard of care of newly diagnosed glioblastoma (GB) with, for example, the addition of bevacizumab (BEV), have been largely disappointing and furthermore molecular characterization has not changed therapy except in elderly patients. Novel approaches, such as vaccine-based immunotherapy, for newly diagnosed GB are currently being pursued in multiple clinical trials. Recurrent disease, an event inevitable in nearly all patients with HGG, continues to be a challenge. Both recurrent GB and AG are managed in similar manner and when feasible re-resection is often suggested notwithstanding limited data to suggest benefit from repeat surgery. Occassional patients may be candidates for re-irradiation but again there is a paucity of data to commend this therapy and only a minority of selected patients are eligible for this approach. Consequently systemic therapy continues to be the most often utilized treatment in recurrent HGG. Choice of therapy, however, varies and revolves around re-challenge with temozolomide (TMZ), use of a nitrosourea (most often lomustine; CCNU) or BEV, the most frequently used angiogenic inhibitor. Nevertheless, no clear standard recommendation regarding the prefered agent or combination of agents is avaliable. Prognosis after progression of a HGG remains poor, with an unmet need to improve therapy.

DNA methylation biomarkers: cancer and beyond

Biomarkers are naturally-occurring characteristics by which a particular pathological process or disease can be identified or monitored. They can reflect past environmental exposures, predict disease onset or course, or determine a patient's response to therapy. Epigenetic changes are such characteristics, with most epigenetic biomarkers discovered to date based on the epigenetic mark of DNA methylation. Many tissue types are suitable for the discovery of DNA methylation biomarkers including cell-based samples such as blood and tumor material and cell-free DNA samples such as plasma. DNA methylation biomarkers with diagnostic, prognostic and predictive power are already in clinical trials or in a clinical setting for cancer. Outside cancer, strong evidence that complex disease originates in early life is opening up exciting new avenues for the detection of DNA methylation biomarkers for adverse early life environment and for estimation of future disease risk. However, there are a number of limitations to overcome before such biomarkers reach the clinic. Nevertheless, DNA methylation biomarkers have great potential to contribute to personalized medicine throughout life. We review the current state of play for DNA methylation biomarkers, discuss the barriers that must be crossed on the way to implementation in a clinical setting, and predict their future use for human disease.

Oligodendrogliomas Lacking O6-Methylguanine-DNA-Methyltransferase Expression

O6-Methylguanine-DNA-Methyltransferase (MGMT) is a DNA repair protein considered to be a chemosensitivity predictor. We evaluated the immunohistochemical MGMT expression in 28 consecutive oligodendroglial tumors (21 oligodendrogliomas, 5 mixed oligoastrocytomas, and 2 glioblastomas with prominent oligodendroglial features; 13 treated with CCNU) and compared it with that of 13 glioblastomas. Twenty-six (93%) oligodendroglial tumors were MGMT-negative, 2 (7%) were MGMT-positive. Twelve (92%) patients treated with CCNU had MGMT-negative lesions and their median survival was 73 months; 1 patient had an MGMT-positive oligodendroglioma and is alive at 28 months. Three (23%) glioblastomas were MGMT-negative and 10 (77%) MGMT-positive. The lower MGMT expression in oligodendroglial tumors compared to glioblastomas (P < 0.05), which have different chemosensitivity, suggests a possible role of MGMT in the determination of chemoresistance. Nevertheless, the heterogeneous outcome of our MGMT-negative oligodendroglial tumors treated with CCNU, indicates that MGMT expression alone is insufficient to predict the response to alkylating drugs, presumably because of the numerous mechanisms involved.

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

PURPOSE

O6-methylguanine-methyltransferase (MGMT) promoter methylation has been shown to predict survival of patients with glioblastomas if temozolomide is added to radiotherapy (RT). It is unknown if MGMT promoter methylation is also predictive to outcome to RT followed by adjuvant procarbazine, lomustine, and vincristine (PCV) chemotherapy in patients with anaplastic oligodendroglial tumors (AOT).

PATIENTS AND METHODS

In the European Organisation for the Research and Treatment of Cancer study 26951, 368 patients with AOT were randomly assigned to either RT alone or to RT followed by adjuvant PCV. From 165 patients of this study, formalin-fixed, paraffin-embedded tumor tissue was available for MGMT promoter methylation analysis. This was investigated with methylation specific multiplex ligation-dependent probe amplification.

RESULTS

In 152 cases, an MGMT result was obtained, in 121 (80%) cases MGMT promoter methylation was observed. Methylation strongly correlated with combined loss of chromosome 1p and 19q loss (P = .00043). In multivariate analysis, MGMT promoter methylation, 1p/19q codeletion, tumor necrosis, and extent of resection were independent prognostic factors. The prognostic significance of MGMT promoter methylation was equally strong in the RT arm and the RT/PCV arm for both progression-free survival and overall survival. In tumors diagnosed at central pathology review as glioblastoma, no prognostic effect of MGMT promoter methylation was observed.

CONCLUSION

In this study, on patients with AOT MGMT promoter methylation was of prognostic significance and did not have predictive significance for outcome to adjuvant PCV chemotherapy. The biologic effect of MGMT promoter methylation or pathogenetic features associated with MGMT promoter methylation may be different for AOT compared with glioblastoma.

Oligodendrogliomas: molecular biology and treatment

Oligodendroglial tumors continue to receive much attention because of their relative sensitivity to chemotherapy. The histological diagnosis of oligodendroglial tumors is subject to considerable interobserver variation. The revised 2007 World Health Organization classification of brain tumors no longer accepts the diagnosis "mixed anaplastic oligoastrocytoma" if necrosis is present; these tumors should be considered glioblastomas (perhaps with oligodendroglial features). The 1p/19q codeletion that is associated with sensitivity to chemotherapy is mediated by an unbalanced translocation of 19p to 1q. Randomized studies have shown that patients with 1p/19q codeleted tumors also have a better outcome with radiotherapy. Histologically more atypical tumors are less likely to have this 1p/19q codeletion; here, other alterations usually associated with astrocytic tumors are often found. Some patients with tumors with classic histological features but no 1p/19q codeletion still have a very favorable prognosis. Currently, the best approach for newly diagnosed anaplastic oligodendroglial tumors is unclear. Early adjuvant chemotherapy does not provide a better outcome than chemotherapy at the time of progression. The value of combined chemoirradiation with temozolomide has not been proven in these tumors, and could at least theoretically be associated with greater neurotoxicity. Tumors with 1p and 19q loss can also be managed with early chemotherapy, while deferring radiotherapy to the time of further progression. The presently available second-line chemotherapy results are modest, and better salvage treatments are necessary. The molecular explanation for the greater sensitivity of 1p/19q codeleted tumors is still unclear, and this could, in part, be explained by more frequent MGMT promoter gene methylation.

Frequent promoter hypermethylation and low expression of the MGMT gene in oligodendroglial tumors

Allelic losses on the chromosome arms 1p and 19q have been associated with favorable response to chemotherapy and good prognosis in anaplastic oligodendroglioma patients, but the molecular mechanisms responsible for this relationship are as yet unknown. The DNA repair enzyme O(6)-methylguanine DNA methyltransferase (MGMT) may cause resistance to DNA-alkylating drugs commonly used in the treatment of anaplastic oligodendrogliomas and other malignant gliomas. We report on the analysis of 52 oligodendroglial tumors for MGMT promoter methylation, as well as mRNA and protein expression. Using sequencing of sodium bisulfite-modified DNA, we determined the methylation status of 25 CpG sites within the MGMT promoter. In 46 of 52 tumors (88%), we detected MGMT promoter hypermethylation as defined by methylation of more than 50% of the sequenced CpG sites. Real-time reverse transcription-PCR showed reduced MGMT mRNA levels relative to non-neoplastic brain tissue in the majority of tumors with hypermethylation. Similarly, immunohistochemical analysis showed either no or only small fractions of MGMT positive tumor cells. MGMT promoter hypermethylation was significantly more frequent and the percentage of methylated CpG sites in the investigated MGMT promoter fragment was significantly higher in tumors with loss of heterozygosity on chromosome arms 1p and 19q as compared to tumors without allelic losses on these chromosomes arms. Taken together, our data suggest that MGMT hypermethylation and low or absent expression are frequent in oligodendroglial tumors and likely contribute to the chemosensitivity of these tumors.

Relationship between expression of O6-methylguanine-DNA methyltransferase, glutathione-S-transferase pi in glioblastoma and the survival of the patients treated with nimustine hydrochloride: an immunohistochemical analysis

Drug resistance is one of the important factors that determine tumor response to chemotherapy. Several candidates for resistance to various chemotherapeutic agents have been elucidated. O6-methylguanine-DNA methyltransferase (MGMT) removes methylation damage induced by nitrosourea from the O6 position of DNA guanines before cell injury. Glutathione-S-transferase (GST) pi is also involved in nitrosourea resistance. We examined the expression of MGMT and GST pi in 18 glioblastomas (GBM) using immunohistochemistry and compared the results with patients' survival after administration of 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU)-based chemotherapy. According to the Kaplan-Meier's method, although median progression free survival (PFS) of eight patients whose tumors retained high MGMT (3+ approximately 2+), and 10 patients whose tumors showed low MGMT expression (1+ approximately 0) were nine and 15 months, respectively (p = 0.09), median overall survival (OS) of the two groups were 12 and 22 months, respectively, which were significantly different (p = 0.01). GST pi expression in GBM was not a prognostic factor. It is suggested that GBM with strong staining of MGMT activity may show more resistance to ACNU-based chemotherapy compared to that with low MGMT. The simple immunohistochemical analysis of MGMT in GBM can be a useful method to determine whether ACNU or another treatment regimen should be recommended.

Response of short-term cultures derived from human malignant glioma to aziridinylbenzoquinone, etoposide and doxorubicin: an in vitro phase II trial

The relative resistance of malignant glioma to chemotherapy makes the identification of new cytotoxic drugs critically important. The use of short-term cultures derived from these tumors to screen drugs at doses that can be attained within human intracranial tumors provides a model system that should be capable of identifying effective drugs suitable for clinical evaluation. The sensitivity of a panel of short-term cultures derived from 22 malignant astrocytoma and four malignant oligodendroglioma was assessed to aziridinylbenzoquinone (AZQ), etoposide and doxorubicin (DOX) using a [(35)S] methione uptake assay. The ID(50) of each culture was compared to the levels of drug which could be achieved in the tumor using standard doses. There was marked heterogeneity between cultures in response to each drug. Whilst there was no evidence that cultures derived from grade III astrocytoma were more sensitive to any of the drugs than cultures derived from grade IV astrocytoma, cultures derived from oligodendroglioma tended to be more sensitive to the alkylating agent AZQ, but not to either of the other drugs. The sensitivity of these short-term cultures at concentrations that can be achieved in situ corresponded well with the clinical efficacy of AZQ and etoposide. Although DOX appeared to be toxic to human gliomas cells in vitro, its limited penetration into the intact brain would seem to preclude its use i.v., but it is likely to be effective if local drug delivery techniques could be employed. The study suggests that short-term cultures derived from malignant glioma should be used to screen investigational agents for potential clinical efficacy.

Anticancer drug resistance in primary human brain tumors

The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.

Clinical response of O6-methylguanine-DNA methyltransferase levels to 1,3-(2-chloroethyl)-1-nitrosourea chemotherapy in glioma patients

Adjuvant nitrosourea chemotherapy fails to prolong patient survival significantly as many tumors demonstrate resistance to these drugs. It has been documented in cell lines that O6-methylguanine-DNA methyltransferase (MGMT) plays an important role in chloroethylnitrosourea (CENU) drug resistance.

The authors evaluated MGMT expression in 22 glioma specimens by using an immunofluorescence assay and compared the results with clinical response of the patients to CENU-based chemotherapy.

The patients were treated with CENU after evidence of progressive disease following surgery and radiotherapy. Eight tumor samples had no detectable MGMT, whereas other samples had from 9989 to 982,401 molecules/nucleus. In one group (12 patients), the tumor decreased in size or was stable (effective group), whereas in the other group (10 patients), the tumor demonstrated continuous growth during chemotherapy (progressive group). The median time to progression (TTP) was 6.7 months with a median survival of 13 months. The Mer− patients (MGMT < 60,000 molecules/nucleus) appeared to have more chance of stable disease or response to CENU therapy than the Mer+ patients (MGMT > 60,000 molecules/nucleus) (chi-square = 4.791, p = 0.0286). In patients with glioblastomas multiforme (GBMs), the TTP of Mer+ patients was shorter than that of Mer− patients (t = 2.04, p = 0.049). As a corollary, the MGMT levels were significantly higher in GBM tumors from the progressive group than those from the effective group (t = -2.26, p = 0.029). The TTP and survival time in the effective GBM group were also longer than those in the progressive GBM group. However, there was no significant correlation between MGMT levels and either the survival time (r = 0.04, p = 0.8595) or TTP (r = 0.107, p = 0.6444).

Results from this study suggested that MGMT positivity is indicative of more aggressive disease that progresses more rapidly when exposed to CENU therapy. However, MGMT-negative tumors are not always sensitive to CENU agents, suggesting that other factors may also be important.