O6-methylguanine-DNA methyltransferase activity is associated with response to alkylating agent therapy and with MGMT promoter methylation in glioblastoma and anaplastic glioma

Background

CpG methylation in the O⁶-methylguanine-DNA methyltransferase (MGMT) promoter is associated with better outcome following alkylating agent chemotherapy in glioblastoma (GBM) and anaplastic glioma (AG). To what extent improved response reflects low or absent MGMT activity in glioma tissue has not been unequivocally assessed. This information is central to developing anti-resistance therapies.

Methods

We examined the relationship of MGMT activity in 91 GBMs and 84 AGs with progression-free survival (PFS) following alkylator therapy and with promoter methylation status determined by methylation-specific PCR (MSP).

Results

Cox regression analysis revealed that GBMs with high activity had a significantly greater risk for progression in dichotomous (P ≤ 0.001) and continuous (P ≤ 0.003) models, an association observed for different alkylator regimens, including concurrent chemo-radiation with temozolomide. Analysis of MGMT promoter methylation status in 47 of the GBMs revealed that methylated tumors had significantly lower activity (P ≤ 0.005) and longer PFS (P ≤ 0.036) compared to unmethylated tumors, despite overlapping activities. PFS was also significantly greater in methylated vs. unmethylated GBMs with comparable activity (P ≤ 0.005), and among unmethylated tumors with less than median activity (P ≤ 0.026), suggesting that mechanisms in addition to MGMT promote alkylator resistance. Similar associations of MGMT activity with PFS and promoter methylation status were observed for AGs.

Conclusions

Our results provide strong support for the hypotheses that MGMT activity promotes alkylator resistance and reflects promoter methylation status in malignant gliomas.

General significance

MGMT activity is an attractive target for anti-resistance therapy regardless of methylation status.

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Largest study to date of association of MGMT activity with treatment response.

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MGMT activity is inversely associated with alkylator response in malignant gliomas.

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Mean activity is significantly lower in MGMT promoter-methylated tumors.

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Better response in methylated tumors is unlikely due to lower MGMT activity alone.

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Supports the use of MGMT inhibitors to improve responsiveness to alkylator therapy.

Repair of 3-methyladenine and abasic sites by base excision repair mediates glioblastoma resistance to temozolomide

Alkylating agents have long played a central role in the adjuvant therapy of glioblastoma (GBM). More recently, inclusion of temozolomide (TMZ), an orally administered methylating agent with low systemic toxicity, during and after radiotherapy has markedly improved survival. Extensive in vitro and in vivo evidence has shown that TMZ-induced O(6)-methylguanine (O(6)-meG) mediates GBM cell killing. Moreover, low or absent expression of O(6)-methylguanine-DNA methyltransferase (MGMT), the sole human repair protein that removes O(6)-meG from DNA, is frequently associated with longer survival in GBMs treated with TMZ, promoting interest in developing inhibitors of MGMT to counter resistance. However, the clinical efficacy of TMZ is unlikely to be due solely to O(6)-meG, as the agent produces approximately a dozen additional DNA adducts, including cytotoxic N3-methyladenine (3-meA) and abasic sites. Repair of 3-meA and abasic sites, both of which are produced in greater abundance than O(6)-meG, is mediated by the base excision repair (BER) pathway, and occurs independently of removal of O(6)-meG. These observations indicate that BER activities are also potential targets for strategies to potentiate TMZ cytotoxicity. Here we review the evidence that 3-meA and abasic sites mediate killing of GBM cells. We also present in vitro and in vivo evidence that alkyladenine-DNA glycosylase, the sole repair activity that excises 3-meA from DNA, and Ape1, the major human abasic site endonuclease, mediate TMZ resistance in GBMs and represent potential anti-resistance targets.

Minimally cytotoxic doses of temozolomide produce radiosensitization in human glioblastoma cells regardless of MGMT expression

Concurrent treatment with the methylating agent temozolomide during radiotherapy has yielded the first significant improvement in the survival of adult glioblastomas (GBM) in the last three decades. However, improved survival is observed in a minority of patients, most frequently those whose tumors display CpG methylation of the O(6)-methylguanine (O(6)-meG)-DNA methyltransferase (MGMT) promoter, and adult GBMs remain invariably fatal. Some, although not all, preclinical studies have shown that temozolomide can increase radiosensitivity in GBM cells that lack MGMT, the sole activity in human cells that removes O(6)-meG from DNA. Here, we systematically examined the temozolomide dose dependence of radiation killing in established GBM cell lines that differ in ability to remove O(6)-meG or tolerate its lethality. Our results show that minimally cytotoxic doses of temozolomide can produce dose-dependent radiosensitization in MGMT-deficient cells, MGMT-proficient cells, and MGMT-deficient cells that lack mismatch repair, a process that renders cells tolerant of the lethality of O(6)-meG. In cells that either possess or lack MGMT activity, radiosensitization requires exposure to temozolomide before but not after radiation and is accompanied by formation of double-strand breaks within 45 minutes of radiation. Moreover, suppressing alkyladenine-DNA glycosylase, the only activity in human cells that excises 3-methyladenine from DNA, reduces the temozolomide dose dependence of radiosensitization, indicating that radiosensitization is mediated by 3-methyladenine as well as by O(6)-meG. These results provide novel information on which to base further mechanistic study of radiosensitization by temozolomide in human GBM cells and to develop strategies to improve the outcome of concurrent temozolomide radiotherapy.

Human glioma cell sensitivity to the sequence-specific alkylating agent methyl-lexitropsin

PURPOSE

Defining the cytotoxicity of individual adducts in DNA is necessary for mechanistic understanding of human brain tumor resistance to therapeutic alkylating agents and for design of DNA repair-related antiresistance strategies. Our purpose is to characterize the sensitivity of human glioma cells to methyl-lexitropsin (Me-lex), a sequence-specific alkylator that produces 3-methyladenine (3-meA) as the predominant (>90%) DNA lesion.

EXPERIMENTAL DESIGN

We quantitated the Me-lex cytotoxicity of 10 human glioma cell lines that differ in O(6)-methylguanine (O(6)-meG)-DNA methyltransferase (MGMT) and mismatch repair activity. We used antisense suppression of alkyladenine DNA glycosylase (AAG) and Ape1 to assess the contribution of 3-meA and abasic sites to lethality and measured abasic sites.

RESULTS

(a) The LD(10) for Me-lex varied widely among the cell lines. (b) MGMT-proficient lines were more resistant than MGMT-deficient lines, an unexpected finding because Me-lex produces very little O(6)-meG. (c) Suppression of AAG increased Me-lex killing and reduced abasic site content. (d) Suppression of Ape1 increased Me-lex killing and increased abasic site content. (e) Ablation of MGMT had no effect on Me-lex cytotoxicity.

CONCLUSIONS

(a) Me-lex is cytotoxic in human glioma cells and AAG promotes resistance, indicating that 3-meA is a lethal lesion in these cells. (b) Abasic sites resulting from 3-meA repair are cytotoxic and Ape1 promotes resistance to these derivative lesions. (c) A factor(s) associated with MGMT expression, other than repair of O(6)-meG, contributes to Me-lex resistance. (d) Me-lex may have clinical utility in the adjuvant therapy of gliomas. (e) AAG and Ape1 inhibitors may be useful in targeting alkylating agent resistance.

Apurinic endonuclease activity in adult gliomas and time to tumor progression after alkylating agent-based chemotherapy and after radiotherapy

PURPOSE

Apurinic/apyrimidinic endonuclease (Ap endo) is a key DNA repair enzyme that cleaves DNA at cytotoxic abasic sites caused by alkylating agents and radiation. We have observed that human glioma cells deficient in Ap endo activity are hypersensitive to clinically used alkylators (Silber et al., Clin Cancer Res 2002;8:3008.). Here we examine the association of glioma Ap endo activity with clinical response after alkylating agent-based chemotherapy or after radiotherapy.

EXPERIMENTAL DESIGN

Cox proportional hazards regression models were used to analyze the relationship of Ap endo activity with time to tumor progression (TTP).

RESULTS

In a univariate model with Ap endo activity entered as a continuous variable, the hazard ratio (HR) for progression after alkylator therapy in 30 grade III gliomas increased by a factor of 1.061 for every 0.01 increase in activity (P = 0.013). Adjusting for age, gender, extent of resection, and prior treatment strengthened slightly the association (HR = 1.094; P = 0.003). Similarly, the HR for progression after radiotherapy in 44 grade II and III tumors increased by a factor of 1.069 (P = 0.008). Adjusting for the aforementioned variables had little effect on the association. In contrast, we observed no association between activity and TTP in grade IV gliomas after either alkylator therapy in 34 tumors or radiotherapy in 26 tumors.

CONCLUSIONS

Our data suggest that Ap endo activity mediates resistance to alkylating agents and radiation and may be a useful predictor of progression after adjuvant therapy in a subset of gliomas.

The apurinic/apyrimidinic endonuclease activity of Ape1/Ref-1 contributes to human glioma cell resistance to alkylating agents and is elevated by oxidative stress

Alkylating agents are standard components of adjuvant chemotherapy for gliomas. We provide evidence here that Ape1/Ref-1, the major mammalian apurinic/apyrimidinic endonuclease (Ap endo), contributes to alkylating agent resistance in human glioma cells by incising DNA at abasic sites. We show that antisense oligonucleotides directed against Ape1/Ref-1 in SNB19, a human glioma cell line lacking O(6)-methylguanine-DNA-methyltransferase, mediate both reduction in Ape1/Ref-1 protein and Ap endo activity and concurrent reduction in resistance to methyl methanesulfonate and the clinical alkylators temozolomide and 1,3-(2-chloroethyl)-1-nitrosourea. An accompanying increase in the level of abasic sites indicates that the DNA repair activity of Ape1/Ref-1 contributes to resistance. Conversely, we also show that exposure of SNB19 cells to HOCl, a generator of reactive oxygen species (ROS), results in elevated Ape1/Ref-1 protein and Ap endo activity, enhanced alkylator resistance, and reduced levels of abasic sites. Given current evidence that heightened oxidative stress prevails within brain tumors, the finding that ROS increase resistance to clinical alkylators in glioma cells may have significance for the response of gliomas to alkylating agent-based chemotherapy. Our results may also be relevant to the design of therapeutic regimens using concurrent ionizing radiation (a generator of ROS) and alkylating agent-based chemotherapy.

O6-methylguanine-DNA methyltransferase-deficient phenotype in human gliomas: frequency and time to tumor progression after alkylating agent-based chemotherapy

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) contributes to the resistance of human brain tumor cell lines and xenografts to methylating and chloroethylating agents. We assayed MGMT in 174 newly diagnosed or recurrent gliomas to (a) quantitate changes in MGMT activity associated with alkylating agent-based chemotherapy; and (b) assess the contribution of MGMT to clinical outcome. Glioma MGMT activity ranged 300-fold, averaging 3,800+/-7,200 molecules/cell. Twenty-four percent of tumors lacked detectable activity [Methyl repair-deficient (Mer-) phenotype, defined here as <151 molecules/cell or <0.25 fmol/10(6) cells]. Tumors treated with surgery alone and tumors recurring after surgery and radiotherapy did not differ significantly in frequency of the Mer- phenotype (29% versus 24%). However, the frequency of the Mer- phenotype among tumors recurring after surgery, radiation, and alkylating agent-based chemotherapy was 7-fold lower than in tumors treated with surgery alone (4.3% versus 29%; P < or = 0.02) and 6-fold lower than in tumors recurring after surgery and radiation (4.3% versus 24%; P < or = 0.05). In contrast to gliomas, there was no relationship of alkylating agent-based therapy with the frequency of the Mer- phenotype in paired histologically normal brain. These data suggest that alkylating agents, either alone or synergistically with radiotherapy, selectively kill Mer- glioma cells in situ. Importantly, Mer- and Mer+ tumors did not differ in time to tumor progression following treatment with alkylating agents, indicating that although Mer- glioma cells may be differentially killed by alkylators, factors other than Mer phenotype were the principal determinants of time to clinical progression. Nonetheless, our results support the possibility that complete ablation of glioma MGMT with substrate analogue inhibitors could improve the efficacy of alkylating agent-based chemotherapy.

O6-methylguanine-DNA methyltransferase activity in adult gliomas: relation to patient and tumor characteristics

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) confers resistance to therapeutic methylating and chloroethylating agents in human brain tumor-derived cell lines. In this work, we assayed MGMT activity in 152 adult gliomas to establish correlates with patient and tumor characteristics. We also assayed MGMT in histologically normal brain adjacent to 87 tumors to characterize changes in activity accompanying neurocarcinogenesis. MGMT activity was detectable in 76% (115 of 152) of tumors, ranging approximately 300-fold from 0.30 to 89 fmol/10(6) cells (180-57,000 molecules/cell). Mean activity was 6.6 +/- 13 fmol/10(6) cells and varied 4-fold among diagnostic groups. The mean for oligodendrogliomas was 2-fold lower (P < 0.03), and for mixed oligodendroglioma-astrocytomas, the mean was 4-fold lower (P < 0.006) than for astroglial tumors. Twenty-five % of gliomas had no detectable MGMT activity (Mer- phenotype; < 0.25 fmol/10(6) cells or 150 molecules/cell). Glioma MGMT was inversely correlated with age (P < 0.01), consistent with the observed age dependence in the progenitor tissue of brain tumors (J. R. Silber et al., Proc. Natl. Acad. Sci. USA, 93: 6941-6946, 1996). Neither MGMT activity nor proportion of Mer- tumors differed by sex. Glioma MGMT was correlated with degree of aneuploidy (P < 0.006) but not with fraction of S-phase cells. Mean activity in tumors was 5-fold higher than in adjacent histologically normal brain (5.0 +/- 7.6 versus 1.1 +/- 1.9 fmol/10(6) cells; P < 0.001). Notably, elevation of tumor activity was observed in 62% of tissue pairs, ranging from 2-fold to > 105-fold. Moreover, 64% of Mer- normal tissue was accompanied by Mer+ tumor. These observations indicate that expression of MGMT activity is frequently activated and/or increased during human neurocarcinogenesis, and that the enhancement is not related to proliferation per se. Significantly, enhanced MGMT activity may heighten the resistance of brain tumors to therapeutic alkylating agents.

Lack of the DNA repair protein O6-methylguanine-DNA methyltransferase in histologically normal brain adjacent to primary human brain tumors

Exposure to exogenous alkylating agents, particularly N-nitroso compounds, has been associated with increased incidence of primary human brain tumors, while intrinsic risk factors are currently unknown. The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) is a major defense against the carcinogenicity of N-nitroso compounds and other alkylators. We report here that in 55% (64/117) of cases, histologically normal brain tissue adjacent to primary human brain tumors lacked detectable MGMT activity [methyl excision repair-defective (Mer-) status]. The incidence of Mer- status in normal brain tissue from brain tumor patients was age-dependent, increasing from 21% in children 0.25-19 years of age to 75% in adults over 50. In contrast, Mer- status was found in 12% (5/43) of normal brain specimens from patients operated for conditions other than primary brain tumors and was not age-dependent. The 4.6-fold elevation in incidence of Mer- status in brain tumor patients is highly significant (chi2 = 24; p < or = 0.001). MGMT activity was independent of age in the lymphocytes of brain tumor patients and was present in lymphocytes from six of nine tumor patients whose normal brain specimen was Mer-. DNA polymerase beta, apurinic/apyrimidinic endonuclease, and lactate dehydrogenase activities were present in all specimens tested, including Mer- specimens from brain tumor patients. Our data are consistent with a model of carcinogenesis in human brain in which epigenetically regulated lack of MGMT is a predisposing factor and alkylation-related mutagenesis is a driving force.