Defective repair of alkylated DNA by human tumour and SV40-transformed human cell strains

MGMT inhibition regulates radioresponse in GBM, GSC, and melanoma

Radiotherapy is the standard treatment for glioblastoma (GBM), but the overall survival rate for radiotherapy treated GBM patients is poor. The use of adjuvant and concomitant temozolomide (TMZ) improves the outcome; however, the effectiveness of this treatment varies according to MGMT levels. Herein, we evaluated whether MGMT expression affected the radioresponse of human GBM, GBM stem-like cells (GSCs), and melanoma. Our results indicated a correlation between MGMT promoter methylation status and MGMT expression. MGMT-producing cell lines ACPK1, GBMJ1, A375, and MM415 displayed enhanced radiosensitivity when MGMT was silenced using siRNA or when inhibited by lomeguatrib, whereas the OSU61, NSC11, WM852, and WM266-4 cell lines, which do not normally produce MGMT, displayed reduced radiosensitivity when MGMT was overexpressed. Mechanistically lomeguatrib prolonged radiation-induced γH2AX retention in MGMT-producing cells without specific cell cycle changes, suggesting that lomeguatrib-induced radiosensitization in these cells is due to radiation-induced DNA double-stranded break (DSB) repair inhibition. The DNA-DSB repair inhibition resulted in cell death via mitotic catastrophe in MGMT-producing cells. Overall, our results demonstrate that MGMT expression regulates radioresponse in GBM, GSC, and melanoma, implying a role for MGMT as a target for radiosensitization.

Variant allelic frequencies of driver mutations can identify gliomas with potentially false-negative MGMT promoter methylation results

MGMT promoter methylation testing is required for prognosis and predicting temozolomide response in gliomas. Accurate results depend on sufficient tumor cellularity, but histologic estimates of cellularity are subjective. We sought to determine whether driver mutation variant allelic frequency (VAF) could serve as a more objective metric for cellularity and identify possible false-negative MGMT samples. Among 691 adult-type diffuse gliomas, MGMT promoter methylation was assessed by pyrosequencing (N = 445) or DNA methylation array (N = 246); VAFs of TERT and IDH driver mutations were assessed by next generation sequencing. MGMT results were analyzed in relation to VAF. By pyrosequencing, 56% of all gliomas with driver mutation VAF ≥ 0.325 had MGMT promoter methylation, versus only 37% with VAF < 0.325 (p < 0.0001). The mean MGMT promoter pyrosequencing score was 19.3% for samples with VAF VAF ≥ 0.325, versus 12.7% for samples with VAF < 0.325 (p < 0.0001). Optimal VAF cutoffs differed among glioma subtypes (IDH wildtype glioblastoma: 0.12-0.18, IDH mutant astrocytoma: ~0.33, IDH mutant and 1p/19q co-deleted oligodendroglioma: 0.3-0.4). Methylation array was more sensitive for MGMT promoter methylation at lower VAFs than pyrosequencing. Microscopic examination tended to overestimate tumor cellularity when VAF was low. Re-testing low-VAF cases with methylation array and droplet digital PCR (ddPCR) confirmed that a subset of them had originally been false-negative. We conclude that driver mutation VAF is a useful quality assurance metric when evaluating MGMT promoter methylation tests, as it can help identify possible false-negative cases.

Genomic and Epigenomic Features of Glioblastoma Multiforme and its Biomarkers

Glioblastoma multiforme is a serious and life-threatening tumor of central nervous system, characterized by aggressive behavior, poor prognosis, and low survival rate. Despite of the availability of aggressive antitumor therapeutic regimen for glioblastoma (radiotherapy followed by chemotherapeutic dose), recovery rate, and patients' survival ratio is attributed to the lack of selectivity of therapeutic drugs and less advancement in cancer therapeutics over last decade. Moreover, tools employed in conventional diagnosis of glioblastoma are more invasive and painful, making the process excruciating for the patients. These challenges urge for the need of novel biomarkers for diagnosis, prognosis, and prediction purpose with less invasiveness and more patient compliance. This article will explore the genetic biomarkers isocitrate dehydrogenase mutation, MGMT mutations, and EGFR that can be deployed as an analytical tool in diagnosis of disease and prognosis of a therapeutic course. The review also highlights the importance of employing novel microRNAs as prognostic biomarkers. Recent clinical advancements to treat GBM and to prevent relapse of the disease are also discussed in this article in the hope of finding a robust and effective method to treat GBM.

Crosstalk between repair pathways elicits double-strand breaks in alkylated DNA and implications for the action of temozolomide

Temozolomide (TMZ), a DNA methylating agent, is the primary chemotherapeutic drug used in glioblastoma treatment. TMZ induces mostly N-alkylation adducts (N7-methylguanine and N3-methyladenine) and some O6-methylguanine (O6mG) adducts. Current models propose that during DNA replication, thymine is incorporated across from O6mG, promoting a futile cycle of mismatch repair (MMR) that leads to DNA double-strand breaks (DSBs). To revisit the mechanism of O6mG processing, we reacted plasmid DNA with N-methyl-N-nitrosourea (MNU), a temozolomide mimic, and incubated it in Xenopus egg-derived extracts. We have shown that in this system, MMR proteins are enriched on MNU-treated DNA and we observed robust, MMR-dependent, repair synthesis. Our evidence also suggests that MMR, initiated at O6mG:C sites, is strongly stimulated in cis by repair processing of other lesions, such as N-alkylation adducts. Importantly, MNU-treated plasmids display DSBs in extracts, the frequency of which increases linearly with the square of alkylation dose. We suggest that DSBs result from two independent repair processes, one involving MMR at O6mG:C sites and the other involving base excision repair acting at a nearby N-alkylation adduct. We propose a new, replication-independent mechanism of action of TMZ, which operates in addition to the well-studied cell cycle-dependent mode of action.

The Role of Mismatch Repair in Glioblastoma Multiforme Treatment Response and Resistance

Mismatch repair (MMR) is a highly conserved DNA repair pathway that is critical for the maintenance of genomic integrity. This pathway targets base substitution and insertion-deletion mismatches, which primarily arise from replication errors that escape DNA polymerase proof-reading function. Here, the authors review key concepts in the molecular mechanisms of MMR in response to alkylation damage, approaches to detect MMR status in the clinic, and the clinical relevance of this pathway in glioblastoma multiforme treatment response and resistance.

siRNA nanoparticle suppresses drug-resistant gene and prolongs survival in an orthotopic glioblastoma xenograft mouse model

Temozolomide (TMZ) is the standard of care chemotherapy drug for treating glioblastomas (GBMs), the most aggressive cancer that affects people of all ages. However, its therapeutic efficacy is limited by the drug resistance mediated by a DNA repair protein, O⁶-methylguanine-DNA methyltransferase (MGMT), which eliminates the TMZ-induced DNA lesions. Here we report the development of an iron oxide nanoparticle (NP) system for targeted delivery of siRNAs to suppress the TMZ-resistance gene (MGMT). We show that our NP is able to overcome biological barriers, bind specifically to tumor cells, and reduce MGMT expression in tumors of mice bearing orthotopic GBM serially-passaged patient-derived xenografts. The treatment with sequential administration of this NP and TMZ resulted in increased apoptosis of GBM stem-like cells, reduced tumor growth, and significantly-prolonged survival as compared to mice treated with TMZ alone. This study introduces an approach that holds great promise to improve the outcomes of GBM patients.

MLH1-mediated recruitment of FAN1 to chromatin for the induction of apoptosis triggered by O6 -methylguanine

O⁶ -Methylguanines (O⁶ -meG), which are produced in DNA by the action of alkylating agents, are mutagenic and cytotoxic, and induce apoptosis in a mismatch repair (MMR) protein-dependent manner. To understand the molecular mechanism of O⁶ -meG-induced apoptosis, we performed functional analyses of FANCD2 and FANCI-associated nuclease 1 (FAN1), which was identified as an interacting partner of MLH1. Immunoprecipitation analyses showed that FAN1 interacted with both MLH1 and MSH2 after treatment with N-methyl-N-nitrosourea (MNU), indicating the formation of a FAN1-MMR complex. In comparison with control cells, FAN1-knockdown cells were more resistant to MNU, and the appearances of a sub-G₁ population and caspase-9 activation were suppressed. FAN1 formed nuclear foci in an MLH1-dependent manner after MNU treatment, and some were colocalized with both MLH1 foci and single-stranded DNA (ssDNA) created at damaged sites. Under the same condition, FANCD2 also formed nuclear foci, although it was dispensable for the formation of FAN1 foci and ssDNA. MNU-induced formation of ssDNA was dramatically suppressed in FAN1-knockdown cells. We therefore propose that FAN1 is loaded on chromatin through the interaction with MLH1 and produces ssDNA by its exonuclease activity, which contributes to the activation of the DNA damage response followed by the induction of apoptosis triggered by O⁶ -meG.

O6-Methylguanine-DNA Methyltransferase (MGMT): Challenges and New Opportunities in Glioma Chemotherapy

Chemoresistance has been a significant problem affecting the efficacy of drugs targeting tumors for decades. MGMT, known as O⁶-methylguanine-DNA methyltransferase, is a DNA repair enzyme that plays an important role in chemoresistance to alkylating agents. Hence, MGMT is considered a promising target for tumor treatment. Several methods are employed to detect MGMT, each with its own advantages and disadvantages. Some of the detection methods are; immunohistochemistry, methylation-specific PCR (MSP), pyrophosphate sequencing, MGMT activity test, and real-time quantitative PCR. Methylation of MGMT promoter is a key predictor of whether alkylating agents can effectively control glioma cells. The prognostic value of MGMT in glioma is currently being explored. The expression of MGMT gene mainly depends on epigenetic modification–methylation of CpG island of MGMT promoter. CpG island covers a length of 762 bp, with 98 CpG sites located at the 5' end of the gene, ranging from 480 to 1,480 nucleotides. The methylation sites and frequencies of CpG islands vary in MGMT-deficient tumor cell lines, xenografts of glioblastoma and in situ glioblastoma. Methylation in some regions of promoter CpG islands is particularly associated with gene expression. The change in the methylation status of the MGMT promoter after chemotherapy, radiotherapy or both is not completely understood, and results from previous studies have been controversial. Several studies have revealed that chemotherapy may enhance MGMT expression in gliomas. This could be through gene induction or selection of high MGMT-expressing cells during chemotherapy. Selective survival of glioma cells with high MGMT expression during alkylating agent therapy may change MGMT status in case of recurrence. Several strategies have been pursued to improve the anti-tumor effects of temozolomide. These include the synthesis of analogs of O⁶-meG such as O⁶-benzylguanine (O⁶-BG) and O⁶-(4-bromothenyl) guanine (O⁶-BTG), RNAi, and viral proteins. This review describes the regulation of MGMT expression and its role in chemotherapy, especially in glioma. Targeting MGMT seems to be a promising approach to overcome chemoresistance. Further studies exploring new agents targeting MGMT with better curative effect and less toxicity are advocated. We anticipate that these developments will improve the current poor prognosis of glioma patients.

MutT homologue 1 (MTH1) catalyzes the hydrolysis of mutagenic O6-methyl-dGTP

Nucleotides in the free pool are more susceptible to nonenzymatic methylation than those protected in the DNA double helix. Methylated nucleotides like O6-methyl-dGTP can be mutagenic and toxic if incorporated into DNA. Removal of methylated nucleotides from the nucleotide pool may therefore be important to maintain genome integrity. We show that MutT homologue 1 (MTH1) efficiently catalyzes the hydrolysis of O6-methyl-dGTP with a catalytic efficiency similar to that for 8-oxo-dGTP. O6-methyl-dGTP activity is exclusive to MTH1 among human NUDIX proteins and conserved through evolution but not found in bacterial MutT. We present a high resolution crystal structure of human and zebrafish MTH1 in complex with O6-methyl-dGMP. By microinjecting fertilized zebrafish eggs with O6-methyl-dGTP and inhibiting MTH1 we demonstrate that survival is dependent on active MTH1 in vivo. O6-methyl-dG levels are higher in DNA extracted from zebrafish embryos microinjected with O6-methyl-dGTP and inhibition of O6-methylguanine-DNA methyl transferase (MGMT) increases the toxicity of O6-methyl-dGTP demonstrating that O6-methyl-dGTP is incorporated into DNA. MTH1 deficiency sensitizes human cells to the alkylating agent Temozolomide, a sensitization that is more pronounced upon MGMT inhibition. These results expand the cellular MTH1 function and suggests MTH1 also is important for removal of methylated nucleotides from the nucleotide pool.

DNA repair in personalized brain cancer therapy with temozolomide and nitrosoureas

Alkylating agents have been used since the 60ties in brain cancer chemotherapy. Their target is the DNA and, although the DNA of normal and cancer cells is damaged unselectively, they exert tumor-specific killing effects because of downregulation of some DNA repair activities in cancer cells. Agents exhibiting methylating properties (temozolomide, procarbazine, dacarbazine, streptozotocine) induce at least 12 different DNA lesions. These are repaired by damage reversal mechanisms involving the alkyltransferase MGMT and the alkB homologous protein ALKBH2, and through base excision repair (BER). There is a strong correlation between the MGMT expression level and therapeutic response in high-grade malignant glioma, supporting the notion that O⁶-methylguanine and, for nitrosoureas, O⁶-chloroethylguanine are the most relevant toxic damages at therapeutically relevant doses. Since MGMT has a significant impact on the outcome of anti-cancer therapy, it is a predictive marker of the effectiveness of methylating anticancer drugs, and clinical trials are underway aimed at assessing the influence of MGMT inhibition on the therapeutic success. Other DNA repair factors involved in methylating drug resistance are mismatch repair, DNA double-strand break (DSB) repair by homologous recombination (HR) and DSB signaling. Base excision repair and ALKBH2 might also contribute to alkylating drug resistance and their downregulation may have an impact on drug sensitivity notably in cells expressing a high amount of MGMT and at high doses of temozolomide, but the importance in a therapeutic setting remains to be shown. MGMT is frequently downregulated in cancer cells (up to 40% in glioblastomas), which is due to CpG promoter methylation. Astrocytoma (grade III) are frequently mutated in isocitrate dehydrogenase (IDH1). These tumors show a surprisingly good therapeutic response. IDH1 mutation has an impact on ALKBH2 activity thus influencing DNA repair. A master switch between survival and death is p53, which often retains transactivation activity (wildtype) in malignant glioma. The role of p53 in regulating survival via DNA repair and the routes of death are discussed and conclusions as to cancer therapeutic options were drawn.

Treatment with 5-azacitidine delay growth of glioblastoma xenografts: a potential new treatment approach for glioblastomas

PURPOSE

Glioblastoma multiforme (GBM) is the most lethal primary brain tumor in adults. The epigenetically active ribonucleoside analog 5-azacitidine is a new therapy option that changes tumor cell chromatin, which is frequently modified by methylation and deacetylation in malignant gliomas.

METHODS

In vitro, we analyzed cell viability, cell apoptosis, and migration of human GBM cells. In vivo, we established subcutaneous and intracerebral GBM mouse models originating from U87MG, U373MG, and primary GBM cells as well as one patient-derived xenograft. Xenografts were treated with 5-azacitidine as well as valproic acid, bevacizumab, temozolomide, and phosphate buffered saline. The tumor sizes and Ki67 proliferation indices were determined. Glioma angiogenesis was examined immunohistochemically by expression analysis of endothelial cells (CD31) and pericytes (PDGFRβ).

RESULTS

In vitro, 5-azacitidine treatment significantly reduced human glioblastoma cell viability, increased cellular apoptosis, and reduced cellular migration. In vivo, 5-azacitidine significantly reduced growth in two intracerebral GBM models. Notably, this was also shown for a xenograft established from a patient surgery sample; whereas, epigenetically acting valproic acid did not show any growth reduction. Highly vascularized tumors responded to treatment, whereas low-vascularized xenografts showed no response. Furthermore, intracerebral glioblastomas treated with 5-azacitidine showed a clearly visible reduction of tumor angiogenesis and lower numbers of endothelial cells and tumor vessel pericytes.

CONCLUSIONS

Our data show significant growth inhibition as well as antiangiogenic effects in intracerebral as well as patient-derived GBM xenografts. This encourages to investigate in detail the multifactorial effects of 5-azacitidine on glioblastomas.

Alkylation and Carbamylation Effects of Lomustine and Its Major Metabolites and MGMT Expression in Canine Cells

DNA Alkylation is thought to be the reason for the efficacy of lomustine while carbamylation has been implicated as the cause for the side effects seen with lomustine treatment such as hepatotoxicity. In the alkylation study we show that lomustine and its metabolites form similar levels of the DNA adducts N⁷ hydroxyethylguanine and O⁶ hydroxyethyldeoxyguanosine. In terms of carbamylation, lomustine showed greater extent of carbamylation in the canine hepatocytes and lymphoma cell lines. The DNA repair enzyme O⁶ methylguanine DNA methyltransferase (MGMT) causes resistance of tumor cells to bifunctional nitrosourea, like lomustine. There is no data available regarding MGMT expression/activity in canine cells or tissues. Our study shows that there is low MGMT activity in the canine lymphoid cell line 17-71 while the GL-1 cells did not show any detectable enzyme activity or mRNA expression. The MGMT enzyme activity measured in canine hepatocytes is about 250-350 fmol/mg protein as compared to about 90 fmol/mg protein in 17-71 cells. We also show that MGMT mRNA expression in 17-71 cells and canine hepatocytes positively correlates with its enzyme activity in these cells.

DNA repair mechanisms in cancer development and therapy

DNA damage has been long recognized as causal factor for cancer development. When erroneous DNA repair leads to mutations or chromosomal aberrations affecting oncogenes and tumor suppressor genes, cells undergo malignant transformation resulting in cancerous growth. Genetic defects can predispose to cancer: mutations in distinct DNA repair systems elevate the susceptibility to various cancer types. However, DNA damage not only comprises a root cause for cancer development but also continues to provide an important avenue for chemo- and radiotherapy. Since the beginning of cancer therapy, genotoxic agents that trigger DNA damage checkpoints have been applied to halt the growth and trigger the apoptotic demise of cancer cells. We provide an overview about the involvement of DNA repair systems in cancer prevention and the classes of genotoxins that are commonly used for the treatment of cancer. A better understanding of the roles and interactions of the highly complex DNA repair machineries will lead to important improvements in cancer therapy.

p53 upregulated modulator of apoptosis sensitizes drug-resistant U251 glioblastoma stem cells to temozolomide through enhanced apoptosis

Malignant glioma is a highly aggressive brain tumor with a poor prognosis. Chemotherapy has been observed to prolong overall survival rate and temozolomide (TMZ), a promising chemotherapeutic agent for treating glioblastoma (GBM), possesses the most effective clinical activity at present, although drug resistance limits its clinical outcome. Growing evidence supports the concept that initial and recurrent GBM may derive from glioblastoma stem cells, which may be responsible for drug resistance. However, the molecular mechanisms underlying this resistance remain to be elucidated. In the present study, a TMZ‑resistant GBM cell line, U251R, was developed and subsequently divided into two subpopulations according to the CD133 immunophenotype. No significant difference was identified in the expression of O6‑methylguanine‑DNA‑methyltransferase (MGMT) between CD133+ U251R cells and CD133‑ U251R cells, whereas the CD133+ cell population was more resistant to TMZ‑induced growth inhibition and cell death. TMZ achieves its cytotoxic effect by inducing DNA lesions and p53 upregulated modulator of apoptosis (PUMA) is an essential mediator of DNA damage‑induced apoptosis independently of p53 status. Therefore, whether PUMA effectively enhances growth suppression and induces apoptosis when combined with TMZ was investigated. Consequently, it was found that adenoviruses expressing wild‑type‑PUMA not only lead to the apoptosis of CD133+ U251R cells alone, but also significantly increase their sensitivity toward TMZ by elevating the Bcl‑2‑associated X protein/B‑cell lymphoma‑2 ratio without alterations in MGMT expression. Therefore, PUMA may be a suitable target for intervention to improve the therapeutic efficacy of TMZ.

Chemoresistance and chemotherapy targeting stem-like cells in malignant glioma

Glioblastoma remains a tumor with a dismal prognosis because of failure of current treatment. Glioblastoma cells with stem cell (GSC) properties survive chemotherapy and give rise to tumor recurrences that invariably result in the death of the patients. Here we summarize the current knowledge on chemoresistance of malignant glioma with a strong focus on GSC. Chemoresistant GSC are the most likely cause of tumor recurrence, but it remains controversial if GSC and under which conditions GSC are more chemoresistant than non-GSC within the tumor. Regardless of this uncertainty, the chemoresistance varies and it is mainly mediated by intrinsic factors. O6-methyl-guanidine methyltransferase (MGMT) remains the most potent mediator of chemoresistance, but disturbed mismatch repair system and multidrug resistance proteins contribute substantially. However, the intrinsic resistance by MGMT expression is regulated by extrinsic factors like hypoxia increasing MGMT expression and thereby resistance to alkylating chemotherapy. The search of new biomarkers helping to predict the tumor response to chemotherapy is ongoing and will complement the already known markers like MGMT.

MGMT gene silencing by promoter hypermethylation in gastric cancer in a high incidence area

PURPOSE

Inactivation of tumor suppressor and DNA repair genes by promoter hypermethylation does commonly occur in human cancers. O(6)-methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that removes methyl groups as well as larger adducts at the O(6) position of guanine. In the absence of MGMT activity, O(6)-methylguanine mispairs with thymine during DNA replication, resulting in G:C to A:T transitions. Promoter hypermethylation of the MGMT gene has been observed in various cancers, including gastric cancer. Here, we aimed at assessing the promoter hypermethylation, mutation and expression status of the MGMT gene in patients from a geographic region with a high incidence of gastric cancer (Kashmir, North India) and to investigate their association with various clinicopathological characteristics.

METHODS

In this study 82 gastric cancer samples and adjacent normal tissues were included. Mutations in the MGMT gene were detected by single stranded conformational polymorphism (SSCP) analysis and direct sequencing. Methylation-specific polymerase chain reaction (MS-PCR) and Western blot analyses were performed to detect promoter hypermethylation and concomitant (loss of) expression of the MGMT gene.

RESULTS

Promoter hypermethylation of the MGMT gene was found in 52.44% (43 of 82) of the tumor samples and loss of MGMT protein expression was detected in 45.12% (37 of 82) of the tumor samples. Hypermethylation and loss of expression were significantly associated with higher tumor grades (moderately/poorly differentiated) (P < 0.05) and higher tumor stages (III/IV) (P < 0.05). In addition, MGMT hypermethylation and loss of expression were found to be significantly associated with high salt tea consumption (P < 0.05).

CONCLUSIONS

Our results indicate that MGMT promoter hypermethylation and concomitant loss of MGMT protein expression may play an important role in the development of gastric cancer in the Kashmiri population. High salt tea consumption may be a risk factor.

Evaluation of methylation of MGMT (O⁶-methylguanine-DNA methyltransferase) gene promoter in sporadic colorectal cancer

The DNA repair gene O⁶-methylguanine-DNA methyltransferase (MGMT) is frequently methylated in colorectal cancer (CRC). The aim of this study was to demonstrate that MGMT methylation may be one of the candidate mediators of field cancerization in the colon mucosa. Therefore, quantitative methylation-specific polymerase chain reaction was performed on tumor itself and additional samples of 5 and 10 cm away from the tumor in 40 CRC patients. Moreover, colon mucosa was examined from 30 cases with no evidence of cancer as a control. MGMT promoter methylation was present in 27.5% of colorectal tumor specimens. Tumors that showed MGMT promoter methylation had substantial MGMT promoter methylation in their normal adjacent mucosa. The methylation was also observed in 36.36% (4/11) of normal samples with MGMT promoter methylation in the adjacent tumors, in 20.79% (6/29) of samples without MGMT methylation in the adjacent tumors, and in 6.66% (2/30) of control samples (p<0.006 and p<0.001 respectively). Finally, the mean of MGMT methylation levels was significantly higher in the cancerous group than in the control group (6.25±1.702 vs. 0.086±0.036, p<0.001). Some CRCs arise from a field defect defined by epigenetic inactivation of MGMT. Detection of such abnormality may ultimately be useful in risk assessment for CRCs.

Immunohistochemical analysis of O(6)-methylguanine-DNA methyltransferase in human melanoma in comparison with skin squamous cell carcinoma

Alkylating agents, often used for chemotherapy in patients with melanoma, can produce O(6)-alkylguanine (O(6)AG) which is related to tumor cell killing after treatment with alkylating agents. O(6)AG is effectively eliminated by O(6)-methylguanine-DNA methyltransferase (O(6)MGMT) and its level is correlative to the resistance to alkylating agents. However, little is known about the relationship of O(6)MGMT to the characteristics of melanoma. This study investigated the expression of O(6)MGMT in 12 melanomas and compared it with that in 11 skin squamous cell cancers (SCCs) immunohistochemically to evaluate the O(6)MGMT activity in melanoma and its clinical significance. All of the SCC samples had high O(6)MGMT expression, while the expression of O(6)MGMT in melanoma was diverse and 4 out of 12 samples had no or extremely low O(6)MGMT activity. Out of 6 lesions obtained from metastasis, 4 had a high O(6)MGMT activity. Two out of 3 cases with a low O(6)MGMT activity in each primary lesion did not show any evidence of metastasis or local recurrence. The evaluation of O(6)MGMT activity in melanoma may, therefore, be useful to determine the characteristics of tumor in each melanoma case. In addition, the present study implies the possibility of selective cancer chemotherapy for melanoma in the near future.

DNA damage-induced cell death: from specific DNA lesions to the DNA damage response and apoptosis

DNA damaging agents are potent inducers of cell death triggered by apoptosis. Since these agents induce a plethora of different DNA lesions, it is firstly important to identify the specific lesions responsible for initiating apoptosis before the apoptotic executing pathways can be elucidated. Here, we describe specific DNA lesions that have been identified as apoptosis triggers, their repair and the signaling provoked by them. We discuss methylating agents such as temozolomide, ionizing radiation and cisplatin, all of them are important in cancer therapy. We show that the potentially lethal events for the cell are O(6)-methylguanine adducts that are converted by mismatch repair into DNA double-strand breaks (DSBs), non-repaired N-methylpurines and abasic sites as well as bulky adducts that block DNA replication leading to DSBs that are also directly induced following ionizing radiation. Transcriptional inhibition may also contribute to apoptosis. Cells are equipped with sensors that detect DNA damage and relay the signal via kinases to executors, who on their turn evoke a process that inhibits cell cycle progression and provokes DNA repair or, if this fails, activate the receptor and/or mitochondrial apoptotic cascade. The main DNA damage recognition factors MRN and the PI3 kinases ATM, ATR and DNA-PK, which phosphorylate a multitude of proteins and thus induce the DNA damage response (DDR), will be discussed as well as the downstream players p53, NF-κB, Akt and survivin. We review data and models describing the signaling from DNA damage to the apoptosis executing machinery and discuss the complex interplay between cell survival and death.

The global DNA methylation surrogate LINE-1 methylation is correlated with MGMT promoter methylation and is a better prognostic factor for glioma

Gliomas are the most frequently occurring primary brain tumor in the central nervous system of adults. Glioblastoma multiformes (GBMs, WHO grade 4) have a dismal prognosis despite the use of the alkylating agent, temozolomide (TMZ), and even low grade gliomas (LGGs, WHO grade 2) eventually transform to malignant secondary GBMs. Although GBM patients benefit from promoter hypermethylation of the O⁶-methylguanine-DNA methyltransferase (MGMT) that is the main determinant of resistance to TMZ, recent studies suggested that MGMT promoter methylation is of prognostic as well as predictive significance for the efficacy of TMZ. Glioma-CpG island methylator phenotype (G-CIMP) in the global genome was shown to be a significant predictor of improved survival in patients with GBM. Collectively, we hypothesized that MGMT promoter methylation might reflect global DNA methylation. Additionally in LGGs, the significance of MGMT promoter methylation is still undetermined. In the current study, we aimed to determine the correlation between clinical, genetic, and epigenetic profiles including LINE-1 and different cancer-related genes and the clinical outcome in newly diagnosed 57 LGG and 54 GBM patients. Here, we demonstrated that (1) IDH1/2 mutation is closely correlated with MGMT promoter methylation and 1p/19q codeletion in LGGs, (2) LINE-1 methylation levels in primary and secondary GBMs are lower than those in LGGs and normal brain tissues, (3) LINE-1 methylation is proportional to MGMT promoter methylation in gliomas, and (4) higher LINE-1 methylation is a favorable prognostic factor in primary GBMs, even compared to MGMT promoter methylation. As a global DNA methylation marker, LINE-1 may be a promising marker in gliomas.

Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.

DNA damage induced by alkylating agents and repair pathways

The cytotoxic effects of alkylating agents are strongly attenuated by cellular DNA repair processes, necessitating a clear understanding of the repair mechanisms. Simple methylating agents form adducts at N- and O-atoms. N-methylations are removed by base excision repair, AlkB homologues, or nucleotide excision repair (NER). O⁶-methylguanine (MeG), which can eventually become cytotoxic and mutagenic, is repaired by O⁶-methylguanine-DNA methyltransferase, and O⁶MeG:T mispairs are recognized by the mismatch repair system (MMR). MMR cannot repair the O⁶MeG/T mispairs, which eventually lead to double-strand breaks. Bifunctional alkylating agents form interstrand cross-links (ICLs) which are more complex and highly cytotoxic. ICLs are repaired by complex of NER factors (e.g., endnuclease xeroderma pigmentosum complementation group F-excision repair cross-complementing rodent repair deficiency complementation group 1), Fanconi anemia repair, and homologous recombination. A detailed understanding of how cells cope with DNA damage caused by alkylating agents is therefore potentially useful in clinical medicine.

Epigenetic aberrations and therapeutic implications in gliomas

Almost all cancer cells have multiple epigenetic abnormalities, which combine with genetic changes to affect many cellular processes, including cell proliferation and invasion, by silencing tumor-suppressor genes. In this review, we focus on the epigenetic mechanisms of DNA hypomethylation and CpG island hypermethylation in gliomas. Aberrant hypermethylation in promoter CpG islands has been recognized as a key mechanism involved in the silencing of cancer-associated genes and occurs at genes with diverse functions related to tumorigenesis and tumor progression. Such promoter hypermethylation can modulate the sensitivity of glioblastomas to drugs and radiotherapy. As an example, the methylation of the O6-methylguanine DNA methyltransferase (MGMT) promoter is a specific predictive biomarker of tumor responsiveness to chemotherapy with alkylating agents. Further, we reviewed reports on pyrosequencing - a simple technique for the accurate and quantitative analysis of DNA methylation. We believe that the quantification of MGMT methylation by pyrosequencing might enable the selection of patients who are most likely to benefit from chemotherapy. Finally, we also evaluated the potential of de novo NY-ESO-1, the most immunogenic cancer/testis antigen (CTA) discovered thus far, as an immunotherapy target. The use of potent epigenetics-based therapy for cancer cells might restore the abnormally regulated epigenomes to a more normal state through epigenetic reprogramming. Thus, epigenetic therapy may be a promising and potent treatment for human neoplasia.

DNA ligase IV as a new molecular target for temozolomide

Temozolomide (TMZ) is a methylating agent used in chemotherapy against glioblastoma. This work was designed to clarify details in repair pathways acting to remove DNA double-strand breaks (DSBs) induced by TMZ. Cultured mouse embryonic fibroblasts were used which were deficient in DSB repair genes such as homologous recombination repair-related genes X-ray repair cross-complementing group 2 (XRCC2)and radiation sensitive mutant54 (Rad54), non-homologous end joining repair-related gene DNAligase IV (Lig4). Cell sensitivity to drug treatments was assessed using colony forming assays. The most effective molecular target which was correlated with TMZ cell sensitivity was Lig4. In addition, it was found that small interference RNAs (siRNA) for Lig4 efficiently enhanced cell lethality induced by TMZ in human glioblastoma A172 cells. These findings suggest that down regulation of Lig4 might provide a useful tool for cell sensitization during TMZ chemotherapy.

High incidence of MGMT promoter methylation in primary glioblastomas without correlation with TP53 gene mutations

O(6)-methylguanine DNA methyltransferase (MGMT) reduces cytotoxicity of therapeutic or environmental alkylating agents. MGMT promoter methylation has been associated with TP53 G: C to A:T transition mutations in various types of cancers, and with poor prognosis in patients who did not receive chemotherapy. Mutations of TP53 are more frequent in secondary than in primary glioblastoma, thus the expected MGMT promoter methylation was low in primary glioblastoma. Glioblastoma patients with MGMT promoter methylation showed better response to chemotherapy based on alkylating agents and longer survival than patients without MGMT methylation. We examined 32 primary glioblastomas, treated with radiotherapy and surgery, for TP53 mutation by direct sequencing and MGMT promoter methylation by methylation-specific PCR. MGMT promoter methylation and TP53 mutations were detected in 72% and 31% of primary glioblastoma, respectively. Although not statistically significant, the frequency of TP53 G:C to A:T mutations were higher in cases with (26%) than without (11%) MGMT promoter methylation (p=0.376). MGMT promoter methylation had no impact on patient survival. Our data indicate that MGMT promoter methylation occurs frequently in primary glioblastoma, but does not lead to G:C to A:T TP53 mutations, has no independent prognostic value and is not a predictive marker unless glioblastoma patients are treated with chemotherapy.

Transcriptional repression of O6-methylguanine DNA methyltransferase gene rendering cells hypersensitive to N,N'-bis(2-chloroethyl)-N-nitrosurea in camptothecin-resistant cells

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl-adducts from the O(6)-guanine in DNA and is a crucial defense against O(6)-alkylating agent-induced cytotoxicity. We demonstrated here that two camptothecin (CPT)-resistant cell lines (CPT30 and KB100) were more sensitive to N,N'-bis(2-chloroethyl)-N-nitrosurea (BCNU) than their parental cells. Enhanced sensitivity to BCNU in these two CPT-resistant cells involved transcriptional repression of the MGMT gene. The mechanism of MGMT gene down-regulation in CPT-resistant cells was not through gene abnormality, mRNA stability, and CpG island hypermethylation. However, the high level of methyl-CpG-binding protein 2 (MeCP2) and dimethylation of H3K9 in the promoter region were found in CPT30 and KB100 cells. Furthermore, increased MeCP2 binding on MGMT promoter was also found to be correlated with MGMT gene-silencing in short-term CPT treatment; thus, enhanced BCNU sensitivity was found in CPT-treated cells. Taken together, we suggest that CPT is able to suppress the transcription of the MGMT gene through recruiting of MeCP2 and H3K9 dimethylation, thus causing a synergistic interaction with BCNU. These findings provide a possible explanation regarding why the combination of CPT and BCNU results in a better objective response than single-use alone. In addition, this study supports a new indication for treating patients who are receiving refractory CPT derivatives with BCNU.

MGMT: a personal perspective

This review describes the history of studies on alkylation damage of mammalian genomes and its carcinogenic consequences that led to the discovery of a unique DNA repair protein, named MGMT. MGMT repairs O(6)-alkylguanine, a critical mutagenic lesion induced by alkylating agents. The follow-up studies in mammalian cells following the discovery of the ubiquitous repair protein in E. coli are summarized.

MGMT hypermethylation: a prognostic foe, a predictive friend

Alkylation of DNA at the O(6)-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. O(6)-alkylguanine-DNA alkyltransferase (AGT), also known as O(6)-methylguanine-DNA methyltransferase (MGMT), is the DNA repair protein responsible for removing alkylation adducts from the O(6)-position of guanine in DNA. The promoter CpG island hypermethylation-associated gene silencing of MGMT is associated with a wide spectrum of human tumors. This epigenetic inactivation of MGMT has two main consequences in human cancer. First, it uncovers a new mutator pathway that causes the accumulation of G-to-A transition mutations that can affect genes required for genomic stability. Second, there is a strong and significant positive correlation between MGMT promoter hypermethylation and increased tumor sensitivity to alkylating drugs. These findings underline the importance of MGMT promoter hypermethylation in basic and translational cancer research.

MGMT: key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in which MGMT has been modulated, revealed that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine are major mutagenic, carcinogenic, recombinogenic, clastogenic and killing lesions. O(6)MeG-induced clastogenicity and cell death require MutS alpha-dependent mismatch repair (MMR), whereas O(6)-chloroethylguanine-induced killing occurs independently of MMR. Extensive DNA replication is required for O(6)MeG to provoke cytotoxicity. In MGMT depleted cells, O(6)MeG induces apoptosis almost exclusively, barely any necrosis, which is presumably due to the remarkable ability of secondarily formed DNA double-strand breaks (DSBs) to trigger apoptosis via ATM/ATR, Chk1, Chk2, p53 and p73. Depending on the cellular background, O(6)MeG activates both the death receptor and the mitochondrial apoptotic pathway. The inter-individual expression of MGMT in human lymphocytes is highly variable. Given the key role of MGMT in cellular defense, determination of MGMT activity could be useful for assessing a patient's drug sensitivity. MGMT is expressed at highly variable amounts in human tumors. In gliomas, a correlation was found between MGMT activity, MGMT promoter methylation and response to O(6)AA. Although the human MGMT gene is inducible by glucocorticoids and genotoxins such as radiation and alkylating agents, the role of this induction in the protection against carcinogens and the development of chemotherapeutic alkylating drug resistance are still unclear. Modulation of MGMT expression in tumors and normal tissue is currently being investigated as a possible strategy for improving cancer therapy.

MGMT inhibitors--The Trinity College-Paterson Institute experience, a chemist's perception

The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (MGMT) can confer resistance to the cancer chemotherapeutic effects of the class of DNA damaging drugs generally referred to as the O(6)-alkylating agents. Inactivation of MGMT is thus a practical approach to improving the efficacy of such agents. An account is given of the collaboration between groups at Trinity College, Dublin and the Paterson Institute, Manchester which led to the development of the MGMT inactivating drug, Patrin (PaTrin-2, Lomeguatrib). The development of a simpler method of synthesis of O(6)-arylmethylguanines opened up the way to make a series of O(6)-heteroalkylmethyl analogues of the archetypal MGMT pseudosubstrate, O(6)-methylguanine. Of these, the furfuryl and thenyl compounds were the most active against recombinant Human MGMT in an in vitro assay. The 4-bromothenyl derivative was chosen for clinical trial as the most active compound. The MGMT active site tolerates O(6)-substituted guanines where the side chain can be quite large, but does not tolerate those with an aromatic or heteroaromatic ring with an 'ortho' substituent.

Tamoxifen accelerates proteasomal degradation ofO6-methylguanine DNA methyltransferase in human cancer cells

Tamoxifen, a synthetic triphenyl-ethylene compound, is a member of a class of anticancer drugs known as selective estrogen receptor modulators. It may block tumor growth by mimicking estrogen and binding to the estrogen receptors, preventing cancerous growth. Clinical studies have demonstrated that a combination chemo/hormonal therapy regimen with tamoxifen and O(6)-alkylating drugs increased the tumor response rate in cancer patients. The mechanism of action of this combined regimen remains undefined. In this study, we demonstrated that treatment of human colorectal HT-29 carcinoma cells with tamoxifen decreased the repair activity and expression level of O(6)-methylguanine DNA methyltransferase (MGMT) protein in a concentration- and time-dependent manner. This inhibition was also shown in other malignant human cells, regardless of their estrogen receptor status. Furthermore, MGMT inactivation by tamoxifen was associated with a significantly increased susceptibility of cells to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). No alteration in MGMT mRNA levels was observed in tamoxifen-treated cells. The half-life of MGMT protein was markedly decreased in the presence of tamoxifen. Tamoxifen-induced MGMT degradation could be blocked by MG-132, a proteasome inhibitor. An increased level of ubiquitinated MGMT protein was found after tamoxifen treatment. We conclude that tamoxifen decreased the MGMT protein level by accelerating protein degradation through the ubiquitin-dependent proteasomal pathway. These findings provide a strong rationale for combined chemo/hormonal therapy with tamoxifen and BCNU in the treatment of human cancers.

Promoter hypermethylation of DNA repair gene MGMT in laryngeal squamous cell carcinoma

The relationship between hypermethylation of CpG islands in the promoter regions of O6-methylguanine DNA methyltransferase (MGMT) genes and laryngeal squamous cell carcinoma was explored. Methylation-specific PCR and semi-quantitative RT-PCR were used to study the promoter methylation and mRNA expression of the MGMT gene in laryngeal carcinoma tissues, tissues adjacent to the tumor and normal laryngeal tissues. Hypermethylation of MGMT gene was detected in 16 samples of 46 (34.8%) laryngeal squamous cell carcinoma samples. However, the MGMT hypermethylation was not detected in all tissues adjacent to the tumors and normal tissues. No significant difference in MGMT gene hypermethylation was found in samples with different histological grades (chi2 = 3.130, P = 0.077) or in samples from patients with different TNM status (chi2 = 3.957, P = 0.138). No expression of MGMT mRNA was detected in all hypermethylated laryngeal carcinoma tissues. The expression of MGMT mRNA was detected in all unmethylated laryngeal carcinoma tissues, tissues adjacent to the tumors and normal tissues. It suggests that MGMT gene promoter hypermethylation is associated with MGMT gene transcription loss in laryngeal carcinoma tissues and possibly plays an important role in carcinogenesis of laryngeal tissues.

DNA hypermethylation status of multiple genes in soft tissue sarcomas

The aberrant methylation of promoter CpG islands is known to be a major inactivation mechanism of tumor-related genes. To determine the clinicopathological significance of gene promoter methylation in soft tissue sarcomas, we examined the promoter methylation status of 10 tumor-related genes in 65 soft tissue sarcomas and 19 adjacent non-neoplastic tissues by methylation-specific PCR. The methylation frequencies of tumor-related genes tested in soft tissue sarcomas were 17 (26%) for RASSF1A, 11 (17%) for DAP kinase, 10 (15%) for MGMT, nine (14%) for GSTP1, eight (12%) for PTEN, six (9%) for p16 and hMLH1, five (8%) for hMSH2, two (3%) for p14, and one (2%) for RB. Promoter methylation of these genes was not recognized in non-neoplastic tissues. All those cases of soft tissue sarcoma that had MGMT methylation, with the exception of one case of malignant peripheral nerve sheath tumor, showed large tumor size (> or = 10 cm) or recurrence. Moreover, eight of 10 cases with MGMT methylation revealed high American Joint Committee on Cancer stage. Seven of 10 cases (70%) with MGMT methylation showed a loss of MGMT expression by immunohistochemistry. In addition, MGMT methylation status had a statistically significant correlation with a loss of MGMT expression (P=0.014). In conclusion, although methylation of tumor-related genes was a relatively rare event in soft tissue sarcomas, methylation was tumor-specific. Of 10 tumor-related genes, cases with MGMT methylation had a tendency to be aggressive behavior. Moreover, MGMT methylation was closely associated with a loss of MGMT expression. Although our findings need to be extending to a large series, promoter methylation of tumor-related genes is likely to have an association with the pathogenesis of soft tissue sarcomas. Furthermore, MGMT methylation may be associated with tumor aggressiveness and the inactivation of MGMT gene.

Role of O6-methylguanine–DNA methyltransferase and effect of O6-benzylguanine on the anti-tumor activity of cis-diaminedichloroplatinum(II) in oral cancer cell lines

The DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) modulates the effectiveness of alkylating agents. However, the relationship between MGMT and the sensitivities to other agents has not been explored. In the present study, the association between MGMT expression and the cellular sensitivity to the platinum agent, CDDP was examined in four human oral cancer cell lines. CDDP depleted MGMT protein and mRNA levels in all four cell lines. Two cell lines with low MGMT expression were sensitive to an alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine and CDDP, whereas two other cell lines with high MGMT expression were resistant to both agents. Furthermore, the addition of the MGMT inhibitor, O6-benzylguanine (O6-BG), invariably enhanced CDDP sensitivity. CDDP depleted MGMT expression, and CDDP sensitivity was enhanced by O6-BG. These results provide valuable information about the relationship between MGMT expression and CDDP sensitivity in oral cancer chemotherapy.

Correlation of clinical features and methylation status of MGMT gene promoter in glioblastomas

In an effort to extend the potential relationship between the methylation status of MGMT promoter and response to CENU therapy, we examined the methylation status of MGMT promoter in 44 patients with glioblastomas. Tumor specimens were obtained during surgery before adjuvant treatment, frozen and stored at -80 degrees C until for DNA extraction process. DNA methylation patterns in the CpG island of the MGMT gene were determined in every tumor by methylation specific PCR (MSP). These results were then related to overall survival and response to alkylating agents using statistical analysis. Methylation of the MGMT promoter was detected in 68% of tumors, and 96.7% of methylated tumors exhibited also an unmethylated status. There was no relationship between the methylation status of the MGMT promoter and overall survival and response to alkylating agents. Our observations do not lead us to consider promoter methylation of MGMT gene as a prognostic factor of responsiveness to alkylating agents in glioblastomas.

Correlation between Promoter Hypermethylation of the O6-Methylguanine-Deoxyribonucleic Acid Methyltransferase Gene and Prognosis in Patients with High-grade Astrocytic Tumors Treated with Surgery, Radiotherapy, and 1-(4-Amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea-based Chemotherapy

OBJECTIVE

O 6-Methylguanine-deoxyribonucleic acid methyltransferase (MGMT) is a deoxyribonucleic acid repair protein associated with the chemoresistance of chloroethylnitrosoureas. We investigated whether MGMT promoter hypermethylation is associated with prognosis in patients with high-grade astrocytic tumors treated uniformly with surgery, radiotherapy, and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU)-based chemotherapy.

METHODS

Using the methylation-specific polymerase chain reaction, we assayed promoter hypermethylation of the MGMT gene in tumor deoxyribonucleic acid from 116 adult patients with supratentorial high-grade astrocytic tumors (42 anaplastic astrocytomas [AAs] and 74 glioblastomas multiforme [GBMs]). The Cox proportional hazards model was used in forward stepwise regression to assess the relative role of prognostic factors (i.e., age at surgery, sex, Karnofsky Performance Scale score, extent of surgical resection, methylation status of the MGMT promoter, and association between MGMT promoter methylation and survival).

RESULTS

MGMT promoter hypermethylation was confirmed in 19 (45.2%) of 42 AA patients and 33 (44.6%) of 74 GBM patients. It was significantly associated with both longer overall and progression-free survival time in AA but not GBM patients.

CONCLUSION

Our results demonstrate that MGMT promoter hypermethylation is associated with longer survival time in patients with AA who were treated with surgery, radiotherapy, and ACNU-based chemotherapy but not in patients with GBM.

Generating mutations but providing chemosensitivity: the role of O6-methylguanine DNA methyltransferase in human cancer

O(6)-methylguanine DNA methyltransferase (MGMT) is a key enzyme in the DNA repair network. MGMT removes mutagenic and cytotoxic adducts from O(6)-guanine in DNA, the preferred point of attack of many carcinogens (i.e. methylnitrosourea) and alkylating chemotherapeutic agents (i.e. BCNU, temozolamide, etc.). Hypermethylation of the CpG island located in the promoter region of MGMT is primarily responsible for the loss of MGMT function in many tumor types. The methylation-mediated silencing of MGMT has two consequences for cancer. First, tumors with MGMT methylation have a new mutator phenotype characterized by the generation of transition point mutations in genes involved in cancer etiology, such as the tumor suppressor p53 and the oncogene K-ras. Second, MGMT hypermethylation demonstrates the possibility of pharmacoepigenomics: methylated tumors are more sensitive to the killing effects of alkylating drugs used in chemotherapy. These recent results underscore the importance of MGMT in basic and translational cancer research.

Mechanisms of human DNA repair: an update

The human genome, comprising three billion base pairs coding for 30000-40000 genes, is constantly attacked by endogenous reactive metabolites, therapeutic drugs and a plethora of environmental mutagens that impact its integrity. Thus it is obvious that the stability of the genome must be under continuous surveillance. This is accomplished by DNA repair mechanisms, which have evolved to remove or to tolerate pre-cytotoxic, pre-mutagenic and pre-clastogenic DNA lesions in an error-free, or in some cases, error-prone way. Defects in DNA repair give rise to hypersensitivity to DNA-damaging agents, accumulation of mutations in the genome and finally to the development of cancer and various metabolic disorders. The importance of DNA repair is illustrated by DNA repair deficiency and genomic instability syndromes, which are characterised by increased cancer incidence and multiple metabolic alterations. Up to 130 genes have been identified in humans that are associated with DNA repair. This review is aimed at updating our current knowledge of the various repair pathways by providing an overview of DNA-repair genes and the corresponding proteins, participating either directly in DNA repair, or in checkpoint control and signaling of DNA damage.

Roles of MGMT and MLH1 proteins in alkylation-induced apoptosis and mutagenesis

To examine involvement of mismatch repair system in alkylation-induced apoptosis and mutagenesis, cell lines defective in the Mgmt gene encoding a DNA repair enzyme, O(6)-methylguanine-DNA methyltransferase, and/or the Mlh1 gene encoding a protein involved in mismatch repair were established from gene-targeted mice. Mgmt(-/-) cells are hypersensitive to the killing effect of N-methyl-N-nitrosourea (MNU) and this effect of MNU was overcome by introducing an additional mutation in the Mlh1 gene. Mgmt(-/-)Mlh1(-/-) cells are more resistant to MNU than are wild-type cells. When the human Mgmt cDNA sequence with a strong promoter was introduced, the wild-type cells acquired the same high level of resistance to MNU as that of Mgmt(-/-)Mlh1(-/-) cells. Although no apparent increase in MNU-induced mutant frequency was observed in such methyltransferase-overproducing wild-type cells, mutant frequency of Mgmt(-/-)Mlh1(-/-) cells became 10-fold higher after being treated with MNU. Mgmt(-/-)Mlh1(+/-) cells carrying approximately half the normal level of MLH1 protein showed a normal level of spontaneous mutant frequency, yet were still highly responsive to the mutagenic effect of the alkylating carcinogen. This haploinsufficient character of Mlh1 mutation was also observed in cell survival assays; Mgmt(-/-)Mlh1(+/-) cells were as resistant to MNU as were Mgmt(-/-)Mlh1(-/-) cells. While caspase-3 was induced in Mgmt(-/-)Mlh1(+/+) cells after treatment with MNU, no induction occurred in Mgmt(-/-)Mlh1(+/-) cells or in Mgmt(-/-)Mlh1(-/-) cells. The cellular content of MLH1 protein seems to be critical for determining if damaged cells enter into either a death or mutation-inducing pathway. The haploinsufficient phenotype of Mlh1-heterozygous cells may be explained by competition in heterodimer formation between MLH1 homologues.

CpG methylation-dependent repression of the human O6-methylguanine-DNA methyltransferase gene linked to chromatin structure alteration

The mechanism of inactivation of the O6-methylguanine-DNA methyltransferase (MGMT), responsible for repair of mutagenic and cytotoxic O6-alkylguanine, in Mex- tumor cells, is not completely understood. We have examined the role of CpG methylation in the human MGMT promoter in a luciferase (luc) reporter plasmid and associated alteration in chromatin structure. Methylation of 16% CpG sequences in promoter and flanking sequences in the plasmid with HpaII methylase reduced luciferase activity by 10-12-fold, while methylation of all CpG sites, including those in the luc coding sequence, as well as the promoter sequence blocked expression completely. Repression of luc expression due to partial but not complete CpG methylation could be reversed by histone deacetylase inhibitor trichostatin A (TSA). However, 5-azacytidine, which reverses CpG methylation, but not TSA, could reactivate silent MGMT gene in Mex- HeLa MR cells. Furthermore, chromatin immunoprecipitation (ChIP) assay showed reduced level of acetylation of H4 histone bound to the methylated promoter compared with the non-methylated promoter. These results suggest that complete repression of the MGMT gene in Mex- cells requires methylation of CpG sequences in both promoter and neighboring regions of the gene, resulting in inactive, condensed chromatin state of the gene.