O6-methylguanine-DNA methyltransferase activity in breast and brain tumors

Clinical relevance of MGMT in the treatment of cancer

Anumber of DNA-damaging chemotherapeutic agents attack the O(6) position on guanine, forming the most potent cytotoxic DNA adducts known. The DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT), encoded by the gene MGMT, repairs alkylation at this site and is responsible for protecting both tumor and normal cells from these agents. Cells and tissues vary greatly in AGT expression, not only between tissues but also between individuals. AGT activity correlates inversely with sensitivity to agents that form O(6)-alkylguanine DNA adducts, such as carmustine (BCNU), temozolomide, streptozotocin, and dacarbazine. The one exception is those tumors lacking mismatch repair, which renders them resistant to methylating agents. A recent study in patients with gliomas confirmed the correlation between low-level expression of the MGMT gene and response and survival after BCNU. An inhibitor to AGT, O(6)-benzylguanine (BG), depletes AGT in human tumors without associated toxicity and is now in phase II clinical trials. Finally, mutations within the active site region of the MGMT gene render the AGT protein resistant to BG inactivation. As a result, mutant MGMT gene transfer into hematopoietic stem cells has been shown to selectively protect the marrow from the combination of an alkylating agent and BG, while at the same time sensitizing tumor cells. MGMT remains a paradigm for development of new agents that modulate known mechanisms of drug resistance in cancer cells and raise the spectra of combinatorial therapies that encompass known drug resistance mechanisms.

An approach to the evaluation of the activity of the DNA repair enzyme O6-methylguanine-DNA-methyl-transferase in tumor tissue in vivo: syntheses of 6-benzyloxy-9-(2-[18F]fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-[18F]fluoroethyl)-7H-purin-2-yl-amine

The resistance of tumor cells to the cytostatic activity of methylating and chloroethylating anticancer drugs is determined by the level of expression of the DNA repair protein O6-methylguanine-DNA-methyl-transferase (MGMT). The synthesis of labelled 6-benzyloxy-9H-purin-2-ylamine derivatives should hence allow a quantification of the MGMT status of tumor and non-target tissue in vivo. 6-benzyloxy-9-(2-fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-fluoroethyl)-7H-purin-2-yl-amine were synthesized and evaluated in vitro, both showing an affinity of 1.8 microM. 6-benzyloxy-9-(2-[18F]fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-[18F]fluoroethyl)-7H-purin-2-yl-amine were synthesized by alkylation of 6-benzyloxy-9H-purin-2-ylamine with 1-[18F]fluoro-2-tosylethane in optimized yields of 41% and 20%, respectively. Biodistribution studies were performed in nude mice, carrying mex+ (MGMT expressing) and mex- tumors.

Repair of O(6)-methylguanine is not affected by thymine base pairing and the presence of MMR proteins

Methylation at the O(6)-position of guanine (O(6)-MeG) by alkylating agents is efficiently removed by O(6)-methylguanine-DNA methyltransferase (MGMT), preventing from cytotoxic, mutagenic, clastogenic and carcinogenic effects of O(6)-MeG-inducing agents. If O(6)-MeG is not removed from DNA prior to replication, thymine will be incorporated instead of cytosine opposite the O(6)-MeG lesion. This mismatch is recognized and processed by mismatch repair (MMR) proteins which are known to be involved in triggering the cytotoxic and genotoxic response of cells upon methylation. In this work we addressed three open questions. (1) Is MGMT able to repair O(6)-MeG mispaired with thymine (O(6)-MeG/T)? (2) Do MMR proteins interfere with the repair of O(6)-MeG/T by MGMT? (3) Does MGMT show a protective effect if it is expressed after replication of DNA containing O(6)-MeG? Using an in vitro assay we show that oligonucleotides containing O(6)-MeG/T mismatches are as efficient as oligonucleotides containing O(6)-MeG/C in competing for MGMT repair activity, indicating that O(6)-MeG mispaired with thymine is still subject to repair by MGMT. The addition of MMR proteins from nuclear extracts, or of recombinant MutSalpha, to the in vitro repair assay did not affect the repair of O(6)-MeG/T lesions by MGMT. This indicates that the presence of MutSalpha still allows access of MGMT to O(6)-MeG/T lesions. To elucidate the protective effect of MGMT in the first and second replication cycle after N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment, MGMT transfected CHO cells were synchronized and MGMT was inactivated by pulse-treatment with O(6)-benzylguanine (O(6)-BG). Thereafter, the recovered cells were treated with MNNG and subjected to clonogenic survival assays. Cells which expressed MGMT in the first and second cell cycle were more resistant than cells which expressed MGMT only in the second (post-treatment) cell cycle. Cells which did not express MGMT in both cell cycles were most sensitive. This indicates that repair of O(6)-MeG can occur both in the first and second cell cycle after alkylation protecting cells from the killing effect of the lesion.

Inactivation of O(6)-methylguanine-DNA methyltransferase by glucose-conjugated inhibitors

The DNA-repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a decisive determinant of resistance of tumor cells to methylating and chloroethylating anti-cancer drugs. Therefore, selective inhibition of MGMT in tumors is expected to cause tumor sensitization. Several inhibitors of MGMT have been developed which function in both tumors and normal tissue. To deplete MGMT preferentially in tumors, strategies to target the inhibitor to the tumor tissue need to be developed. Here, we report on the properties of glucose-conjugated MGMT inhibitors that might be useful for tumor targeting since tumor cells frequently over-express glucose transporter. O(6)-Benzylguanine (O6BG), 8-aza-O(6)-benzylguanine, O(6)-(4-bromothenyl)-guanine (O6BTG) and the corresponding spacer-linked beta-D-glucose conjugates were analyzed comparatively for MGMT-inhibitory activity. Substitution at the N9 position of the purine moiety resulted generally in a reduction in the efficiency with which the inhibitors blocked MGMT. However, the inhibitory activity of the O6BTG conjugates increased with increasing spacer length, and O6BTG conjugated with a C8 spacer with beta-D-glucose was nearly as effective as O6BTG on its own. MGMT was inhibited by the conjugates both in crude cell extracts and upon treatment of intact HeLa cells, indicating efficient uptake of the glucose conjugates into cells. Since the O6BTG-C8-D-glucose conjugate 8-[O(6)-(4-bromothenyl)-guan-9-yl]-octyl-beta-D-glucoside was highly efficient at MGMT inhibition in a non-toxic concentration range, the drug might be a useful tool for specific tumor sensitization.

Phenotypes of Drosophila homologs of human XPF and XPG to chemically-induced DNA modifications

DmXPF (mei9) and DmXPG (mus201) mutants are Drosophila homologs of the mammalian XPF and XPG genes, respectively. For Drosophila germ cells, causal correlations exist between the magnitude of a potentiating effect of a deficiency in these functions, measured as the M(NER-)/M(NER+) mutability ratio, and the type of DNA modification. M(NER-)/M(NER+) mutability ratios may vary with time interval between DNA adduct formation and repair, mutagen dose and depend also on the genetic endpoint measured. For forward mutations, there is no indication of any differential response of DmXPF compared to DmXPG. Subtle features appeared from a class-by-class comparison: (i) Methylating agents always produce higher M(NER-)/M(NER+) ratios than their ethylating analogs; (ii) M(NER-)/M(NER+) mutability ratios are significantly enhanced for cross-linking N-mustards, aziridine and di-epoxide compounds, but not for cross-linking nitrosoureas. The low hypermutability effects with bifunctional nitrogen mustards, aziridine and epoxide compounds are attributed to unrepaired mono-alkyl adducts; (iii) The efficient repair of mono-alkyl-adducts at ring nitrogens in wild-type germ cells is evident from the absence of a dose-response relationship for ethylene oxide, propylene imine and methyl methanesulfonate (MMS). These chemicals become powerful germline mutagens when the NER system is disrupted. Systematic studies of the type performed on germ cells are not available for somatic cells of Drosophila. The sparse data available show large differences in the response of germ cells and somatic cells. The bifunctional agent mechlorethamine (MEC) but not the monofunctional MMS or 2-chloroethylamine cause in NER(-) XXfemale symbol the highest potentiating effect on mitotic recombination. The causes of the discrepancy between the extraordinarily high activity of MEC in mus201 somatic cells and its low potentiating effect in germ cells is unknown at present.

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.

Risk assessment in first degree female relatives of breast cancer patients using the alkaline Comet assay

First degree female relatives (FDFRs) of breast cancer patients have been reported to have a 2- to 3-fold increase in breast cancer risk as compared with the general population. Assessment of genetic instability (DNA damage and repair efficiency) is an important parameter concerning mutagenesis and carcinogenesis. In an attempt to identify individuals at high risk of breast cancer in the FDFRs of breast cancer patients, two tests were used: the alkaline Comet assay on leucocytes and the micronucleus test (MNT) on buccal epithelial cells. In addition to FDFRs, two other categories of subjects were included: breast cancer patients and controls. The Comet assay was used to study basal DNA damage, DNA susceptibility to a mutagen (N-methyl N-nitro N-nitrosoguanidine) and DNA repair efficiency. In addition, the MNT served as an indicator of chromosome breakage/aneuploidy. A significant increase in DNA damage (basal and after treatment with a mutagen, as well as after allowing repair to take place) and micronucleus frequency was observed from controls to FDFRs and from FDFRs to breast cancer patients. There was considerable variability in the subjects with respect to both of these parameters. Outliers identified among the FDFRs based on 3 SD limits of DNA damage and micronucleus frequency were considered as high risk individuals.

Activity of O(6)-methylguanine-DNA methyltransferase in relation to p53 status and therapeutic response in ovarian cancer

The DNA-repair protein O(6)-methylguanine-DNA methyltransferase (alkyltransferase; MGMT) is a major determinant of resistance of cells to various alkylating cytostatic drugs. Its expression in tissues is highly variable, indicating complex regulatory mechanisms involved. Transfection-mediated expression of wild-type p53 has been shown to negatively regulate basal promoter activity of MGMT in vitro. To elucidate whether p53 is involved in regulation of MGMT in tumor tissue, we examined MGMT expression and the p53 status of 140 primary ovarian carcinomas and analyzed the data as to the correlation between MGMT and p53, as well as the survival response of the patients after chemotherapy. We show that MGMT expression is highly variable in ovarian carcinomas, ranging from zero level up to 2500 fmol/mg protein. MGMT activity was significantly lower in tumors with wild-type p53 (p53wt) than in tumors with mutant p53 (p53mt) (p = 0.045). As expected, the percentage of tumors with p53mt increased with increasing histologic grade of the tumors. Thus, p53mt was observed in 4, 45 and 64% of grades 1, 2 and 3 tumors, respectively (p = 0.001). Increase in p53mt was accompanied by an increase in MGMT activity, which was, on average, 460 +/- 66, 624 +/- 63 and 662 +/- 60 fmol/mg protein in grades 1, 2 and 3 tumors, respectively (p = 0.047). In addition, MGMT activity as well as p53mt were associated with the FIGO stage of the tumors. Mean MGMT activity was 472 +/- 48 fmol/mg for patients with FIGO stages I and II, as compared with 675 +/- 50 fmol/mg for patients with FIGO stages III and IV, (p = 0.0179). The percentage of p53mt was 27% and 54% in ovarian tumors with FIGO stages I/II and FIGO stages III/IV, respectively (p = 0.004). Thus, progression of ovarian tumors was clearly associated with increase of both MGMT activity and the percentage of p53mt. In tumors expressing low MGMT (<100 fmol/mg), p53mt was very rarely found. No significant association was observed between MGMT level in ovarian carcinomas and the survival of patients treated with cyclophosphamide and carboplatin. On the other hand, a clear correlation was found between histological type, grading, residual tumor mass and p53wt expression and duration of the patient's survival. The finding that p53wt expression was associated with low MGMT level in primary ovarian cancer supports the view that down-regulation of basal MGMT promoter activity by p53wt is also relevant in tumor cells in vivo. Int. J. Cancer (Pred. Oncol.) 84:388-395, 1999.

Relevance of DNA repair to carcinogenesis and cancer therapy

DNA-reactive carcinogens and anticancer drugs induce many structurally distinct cytotoxic and potentially mutagenic DNA lesions. The capability of normal and malignant cells to recognize and repair different DNA lesions is an important variable influencing the risk of mutation and cancer as well as therapy resistance. Using monoclonal antibody-based immunoanalytical assays, very low amounts of defined carcinogen-DNA adducts can be quantified in bulk genomic DNA, individual genes, and in the nuclear DNA of single cells. The kinetics of DNA repair can thus be measured in a lesion-, gene-, and cell type-specific manner, and the DNA repair profiles of malignant cells can be monitored in individual patients. Even structurally very similar DNa lesions may be repaired with extremely different efficiency. The miscoding DNA alkylation products O6-methylguanine (O6-MeGua) and O6-ethylguanine (O6-EtGua), for example, differ only by one CH2 group. These lesions are formed in DNA upon exposure to N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNU), both of which induce mammary adenocarcinomas in female rats at high yield. Unrepaired O6-alkylguanines cause transition mutations via mispairing during DNA replication. O6-MeGua is repaired at a similar slow rate in transcribed (H-ras, beta-actin) and inactive genes (IgE heavy chain; bulk DNA) of the target mammary epithelia (which express the repair protein O6-alkylguanine-DNA alkyltransferase at a very low level). O6-EtGua, however, via an alkyltransferase-independent mechanism, is excised approximately 20 times faster than O6-MeGua from the transcribed genes selectively. Correspondingly, G:C-->A:T transitions arising from unrepaired O6-MeGua at the second nucleotide of codon 12 (GGA) of the H-ras gene are frequently found in MeNU-induced mammary tumors, but are absent in their EtNU-induced counterparts.

O6-methylguanine-DNA methyltransferase activity and sensitivity to cyclophosphamide and cisplatin in human lung tumor xenografts

The DNA repair protein O6-methylguanine-DNA methyl-transferase (MGMT) is a main determinant of resistance of cells to the cytostatic effects of O6-alkylguanine-generating alkylating agents. The purpose of our study was to assay MGMT activity in cells of lung cancers and to correlate MGMT levels with chemotherapy response to cyclophosphamide (CTX) and cisplatin (DDP). MGMT levels were determined in 14 human lung tumor xenografts. There was a wide variation of MGMT expression in these tumors, ranging from 10 to 984 fmol/mg protein. There was also a wide range in the sensitivity of the xenografts to CTX and DDP, as measured by specific growth delay. When the MGMT levels of the different xenograft lines were compared with the corresponding responses to CTX and DDP, a close correlation was found between MGMT activity and CTX (lin reg., r = -0.83, p < 0.05). The higher the MGMT activity, the less pronounced was the growth-inhibiting effect of CTX. With DDP, no such correlation was found. Our results indicate that the in vivo response of tumors to CTX is related to the level of MGMT expression.

Bleomycin-induced DNA damage and its removal in lymphocytes of breast cancer patients studied by comet assay

The studies concerned the response to bleomycin treatment in peripheral blood lymphocytes (PBL) of breast cancer (BC) subjects. The level of BLM-induced DNA strand breaks was evaluated using alkaline comet assay followed by visual scoring. The sensitivity to genotoxic exposure as well as the time-course of damage removal were estimated and analysed in comparison to control (healthy) subjects. Despite high inter-individual variability, the differences between the BC and non-cancer groups still proved to be statistically significant. Lymphocytes of the BC subjects appeared to be more sensitive to BLM exposure as shown by higher level of DNA damage. The DNA repair capacity was weaker in PBL obtained from BC patients than that in lymphocytes of controls.

Inhibition of DNA repair as a means of increasing the antitumor activity of DNA reactive agents

Chemotherapeutic alkylnitrosoureas (BCNU, CCNU, streptozotocin) and alkyltriazenes (DTIC, temozolomide) produce a cytotoxic lesion at the O(6)-position of guanine. The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase removes damage from the O(6)-position in a single-step mechanism without co-factors. There is extensive evidence that this protein is one of the most important factors contributing to alkylnitrosourea and alkyltriazene treatment failure. There is an inverse correlation between the level of this protein and the sensitivity of cells to the cytotoxic effects of O(6)-alkylating agents. Attempts have been made to modulate AGT activity using anti-sense technology, methylating agents, O(6)-alkylguanines, and O(6)-benzylguanine analogs. O(6)-Benzylguanine and its analogs are clearly the most potent direct inactivators of the AGT protein. The mechanism involves O(6)-benzylguanine acting as a low-molecular weight substrate with transfer of the benzyl group to the cysteine residue within the active site of the repair protein. Pretreatment of cells with non-toxic doses of O(6)-benzylguanine results in an increase in the sensitivity to O(6)-alkylating agents. Animal studies revealed that the therapeutic index of BCNU increased when administered in combination with O(6)-benzylguanine. This drug is currently in phase I clinical trials. Evidence from animal studies indicates that myelosuppression may be the dose-limiting toxicity, thus, efforts are aimed at improving the therapeutic index by the stable expression of O(6)-benzylguanine-resistant AGT proteins into targeted normal tissue such as bone marrow. The successful modulation of alkyltransferases brings on an exciting new era for alkylnitrosoureas and alkyltriazenes.

Localization of O6-alkylguanine transferase in cancer susceptible cells of human female breast

The cell type specific distribution of O6-alkylguanine-DNA-alkyltransferase (AGT) protein was assessed using immunohistochemical localization in human female breast tissue sections and biochemical quantitation of fractionated cell extracts. The results demonstrated that the AGT protein is predominantly localized in luminal epithelial and myoepithelial cells of the intralobular mammary ducts. Western blot analysis revealed that the AGT level in epithelial cell rich organoid fraction was substantially higher than the whole tissue and fibro-collagenous stromal cell fraction of the normal breast. The quantitative activity measurements confirmed the occurrence of a statistically significant 2.7-fold (P = 0.05) and 4.0-fold (P = 0.04) enriched AGT activity in extracts prepared from the organoids compared to the whole tissue homogenate and fractionated stromal cells, respectively. The results suggest that the invariably high AGT level in malignant compared to the normal breast tissue could be due to AGT accumulation in luminal epithelial and myoepithelial cell mass, increasing as a consequence of the uncontrolled proliferation and differentiation of ductal cells in invasive carcinoma.

Protective effect of O6-methylguanine-DNA methyltransferase (MGMT) on the cytotoxic and recombinogenic activity of different antineoplastic drugs

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) removes alkyl groups from the O6 position of guanine in DNA and thus may protect cells against genotoxic effects of agents inducing this lesion. To analyze quantitatively the level of protection mediated by MGMT against antineoplastic drugs, we determined the cytotoxic and recombinogenic (sister-chromatid exchange inducing) effects of various chemotherapeutic agents in a pair of isogenic Chinese hamster cell lines deficient and proficient for MGMT, generated upon transfection with human MGMT cDNA. Furthermore, we compared the responses of the human cell lines HeLa MR (MGMT deficient) and HeLa S3 (MGMT proficient) to the various agents. It is shown that: (1) MGMT proficient cells are resistant in cell killing to the methylating drug streptozotocin and all the chloroethylating nitrosoureas tested. There was a marked agent specificity in protection. The level of resistance provoked by MGMT increased in the order BCNU < CCNU < ACNU < HeCNU < streptozotocin. (2) MGMT did not protect cells against killing induced by chlorambucil, cisplatin, melphalan, activated cyclophosphamide (malosfamide) and activated ifosfamide (4-hydroperoxy-ifosfamide). (3) MGMT caused protection against the recombinogenic effect of all nitrosoureas tested. The lowest level of protection was again observed for BCNU, followed by CCNU, ACNU < HeCNU < streptozotocin. (4) MGMT proficient cells did not exhibit resistance in SCE induction towards cyclophosphamide (activated by microsomes), 4-hydroperoxy-ifosfamide, mafosfamide, chlorambucil and melphalan. Some protection was afforded, however, against cisplatin (and transplatin). This effect was abolished by pretreatment of cells with O6-benzylguanine, which depletes MGMT, indicating that some lesion(s) induced by cisplatin giving rise to SCEs can be repaired by MGMT. Taken together, these results indicate that streptozotocin, HeCNU and ACNU are more selective than CCNU and BCNU in killing MGMT deficient cells, and that in the cases of cyclophosphamide, ifosfamide, chlorambucil, cisplatin and melphalan MGMT is not involved in mediating cytotoxic drug resistance.