Repair of O(6)-alkylguanine by alkyltransferases

Epigenetic regulation of O6-methylguanine-DNA methyltransferase gene expression by histone acetylation and methyl-CpG binding proteins

Transcriptional silencing of the DNA repair gene, O6-methylguanine-DNA methyltransferase (MGMT) in a proportion of transformed cell lines is associated with methylated CpG hotspots in the MGMT 5′ flank. The goal of the study was to evaluate the mechanism by which CpG methylation of theMGMT promoter region influenced silencing of the gene. Analysis of histone acetylation status in two regions of the promoter using chromatin immunoprecipitation assay showed that a higher level of histone acetylation was associated with expression in three MGMT-expressing cell lines (HeLa CCL2, HT29, and Raji) compared with three MGMT-silenced cell lines (HeLa S3, BE, and TK6). To determine how the modulation of CpG methylation and histone acetylation influenced MGMT expression, we exposed the cells to 5-aza-2′deoxycytidine (5-Aza-dC), inhibitor of DNA methylation, which strongly up-regulated MGMT expression in three MGMT-silenced cell lines whereas trichostatin A, inhibitor of histone deacetylase, weakly induced MGMT. However, combined treatment with 5-Aza-dC and trichostatin A significantly up-regulated MGMT RNA expression to a greater extent than in cells treated with either agent alone suggesting that histone deacetylation plays a role in MGMT silencing but that CpG methylation has a dominant effect. Consistent with enhanced MGMT expression, 5-Aza-dC increased the association of acetylated histone H3 and H4 bound to the MGMT promoter. Chromatin immunoprecipitation analysis of methyl-CpG binding domain containing proteins detected a greater amount of MeCP2, MBD1, and CAF-1 bound to the MGMT promoter in MGMT-silenced cells. Our findings implicate specific MBD proteins in methylation-mediated transcriptional silencing of MGMT.

Clinical trial substantiates the predictive value of O-6-methylguanine-DNA methyltransferase promoter methylation in glioblastoma patients treated with temozolomide

PURPOSE

In the setting of a prospective clinical trial, we determined the predictive value of the methylation status of the O-6-methylguanine-DNA methyltransferase (MGMT) promoter for outcome in glioblastoma patients treated with the alkylating agent temozolomide. Expression of this excision repair enzyme has been associated with resistance to alkylating chemotherapy.

EXPERIMENTAL DESIGN

The methylation status of MGMT in the tumor biopsies was evaluated in 38 patients undergoing resection for newly diagnosed glioblastoma and enrolled in a Phase II trial testing concomitant and adjuvant temozolomide and radiation. The epigenetic silencing of the MGMT gene was determined using methylation-specific PCR.

RESULTS

Inactivation of the MGMT gene by promoter methylation was associated with longer survival (P = 0.0051; Log-rank test). At 18 months, survival was 62% (16 of 26) for patients testing positive for a methylated MGMT promoter but reached only 8% (1 of 12) in absence of methylation (P = 0.002; Fisher's exact test). In the presence of other clinically relevant factors, methylation of the MGMT promoter remains the only significant predictor (P = 0.017; Cox regression).

CONCLUSIONS

This prospective clinical trial identifies MGMT-methylation status as an independent predictor for glioblastoma patients treated with a methylating agent. The association of the epigenetic inactivation of the DNA repair gene MGMT with better outcome in this homogenous cohort may have important implications for the design of future trials and supports efforts to deplete MGMT by O-6-benzylguanine, a noncytotoxic substrate of this enzyme.

The Werner syndrome protein confers resistance to the DNA lesions N3-methyladenine and O6-methylguanine: implications for WRN function

The Werner syndrome (WS) protein (WRN), a DNA helicase/exonuclease, is required for genomic stability and avoidance of cancer. Current evidence suggests that WRN is involved in the resolution of stalled and/or collapsed replication forks. This function is indicated, in part, by replication defects in WS cells and by hypersensitivity to agents causing major structural aberrations in DNA that block replication. We show here that antisense suppression of WRN in two human glioma cell lines reproduces hallmarks of the drug cytotoxicity profile of WS cells, namely, hypersensitivity to 4-nitroquinoline 1-oxide, camptothecin and hydroxyurea. We also show that antisense-treated cells are hypersensitive to methyl-lexitropsin, a site-specific alkylating agent that produces mainly N3-methyladenine, a cytotoxic and replication-blocking lesion. Antisense-treated cells are hypersensitive to O(6)-methylguanine adducts as well, but only when repair by O(6)-methylguanine-DNA methyltransferase is lacking. Our results illustrate the drug sensitivity caused by deficiency of WRN in a uniform genetic background. They extend the WRN DNA damage sensitivity spectrum to methyl base adducts that can result in blocked replication, and suggest that WRN may be required for resumption of processive replication when incomplete repair of DNA damage leaves blocking lesions at forks. The evidence that highly disparate lesions fall within the purview of WRN, and that abrogating DNA repair can reveal dependence on WRN, suggests that WRN may protect the genome from the lethal, mutagenic and carcinogenic effects of widely diverse DNA damage arising from endogenous processes and environmental agents.

Immunohistochemical study for O6-methylguanine-DNA methyltransferase in the non-neoplastic and neoplastic components of gliomas

Although the expression O6-methylguanine-DNA methyltransferase (MGMT) is an important hallmark for decision of nitrosourea chemotherapy for glioma patients, no immunohistochemical method for analysis of MGMT has been standardized yet. Gliomas usually contain non-neoplastic cells even deep in the tumor. It is not known which of these components expresses MGMT. To clarify this point, we investigated MGMT expression in the non-neoplastic cells in autopsy and surgical specimens by immunohistochemistry. High grade gliomas were also studied to find a cut-off point for treatment decision. MGMT immunohistochemistry in the normal brain or brain with non-neoplastic disease revealed nuclear staining in some endothelial cells, inflammatory cells, ependymal cells, astrocytes and oligodendroglias. Some cells were double stained with CD68 (macrophages or microglias). The neurons were consistently MGMT-negative. High grade gliomas always contained an MGMT-positive non-neoplastic component. Although, the endothelial cells were easily distinguished from the neoplastic cells, other cells were often mistaken for tumor cells. The population of MGMT-positive non-neoplastic cells was usually less than 10%. We set a cut off-point at 10% between the positive and negative groups because the statistical difference in the overall survival was most distinct at this value. In 51 high grade glioma patients, who received both radiotherapy and chemotherapy with nimustine (ACNU), the median overall survival of the MGMT-negative group (23 months) was significantly longer than that of the MGMT-positive group (14 months) (P < 0.009). Multivariate analysis revealed that the negative MGMT expression was a significant prognostic variable next to the degree of surgical removal for the overall survival. In the MGMT-positive group, addition of platinum-based chemotherapy did not improve the survival.

Phase II study of temozolomide and cisplatin as primary treatment prior to radiotherapy in newly diagnosed glioblastoma multiforme patients with measurable disease. A study of the Spanish Medical Neuro-Oncology Group (GENOM)

This phase II study evaluates the activity of temozolomide and cisplatin administered before radiation therapy in newly diagnosed glioblastoma multiforme patients, in terms of response, time to progression and survival.

PATIENTS AND METHODS

Forty patients with measurable disease after surgery, a Karnofsky status > 60, and Barthel Index > 10 were included. They were treated with three cycles of temozolomide 200 mg/m2/day for 5 days and cisplatin 100 mg/m2 on day 1. Conventional focal radiation therapy to 60 Gy was administered after response evaluation.

RESULTS

Three patients were not evaluable for central reviewed response but all 40 were evaluable for toxicity, time to progression and survival. Objective responses by Macdonald criteria on an intent to treat basis were 45% including complete response in three patients (7.5%), and partial response in 15 patients (37.5%). Responses were seen in biopsy-only patients (33.4%) as well as in partial surgery patients (52%). Median survival for all patients was 12.5 months. Biopsy-only patients had a median survival of 12.8 months. Grade 3 to 4 neutropenia was the most important toxicity, and occurred in 37.5% of patients. A delay in 18.2% and a dose reduction in 9.6% of cycles were necessary due to myelosuppression on day 28. Two patients had neutropenic fever resulting in one treatment-related death. Eighty-two percent of patients received radiotherapy.

CONCLUSION

This regimen has significant activity, as it induces objective responses even in biopsy-only patients, appearing to improve their median survival. A better combination schedule is needed to improve the toxicity profile.

A genotoxic screen: rapid analysis of cellular dose-response to a wide range of agents that either damage DNA or alter genome maintenance pathways

SNP analysis has come to the forefront of genomics since the mouse and human genomes have been sequenced. High throughput functional screens are necessary to evaluate these sequence databases. Described here is a genotoxic screen: a rapid method that determines the cellular dose-response to a wide range of agents that either damage DNA or alter basic cellular pathways important for maintaining genomic integrity. Importantly, a single person utilizing standard tissue culture equipment may perform these assays composed of 20 agents that attack genomic integrity or maintenance at many different levels. Thus, a small lab may perform this screen to determine the integrity of a wide range of DNA repair, chromatin metabolism, and response pathways without the limitations of investigator bias. A genotoxic screen will be useful when analyzing cells with either known genetic alterations (generated directly by the investigator or derived from individuals with known mutations) or unknown genetic alterations (cells with spontaneous mutations such as cancer-derived cells). Screening many genotoxins at one time will aid in determining the biological importance of these altered genes. Here we show the dose-response curves of mouse embryonic stem (ES) cells and HeLa cells exposed to 20 genotoxic agents. ES cells were chosen since they are amenable to genetic alteration by the investigator. HeLa cells were chosen since they were derived from cancer and are commonly used. Comparing the dose-response curves of these two cell lines show their relative sensitivity to these agents and helps define their genotoxic profile. As a part of phenomics, a large genotoxic profile database for cancer-derived cells, when integrated with other databases such as expression profiles and comparative genomic hybridization, may aid in maximizing the effectiveness of developing anti-cancer protocols.

Phase 1 study of 28-day, low-dose temozolomide and BCNU in the treatment of malignant gliomas after radiation therapy

We conducted a study to determine the dose-limiting toxicity of an extended dosing schedule of temozolomide (TMZ) when used with a fixed dose of BCNU, or 1,3-bis(2-chloroethyl)-1-nitrosourea (carmustine), taking advantage of TMZ's ability to deplete O6-alkylguanine-DNA-alkyltransferase and the synergistic activity of these two agents. Patients with malignant gliomas who had undergone radiation therapy were eligible. Patients were treated with TMZ for 28 days, followed by a 28-day rest (1 cycle). The TMZ was started at 50 mg/m2 and increased in 10-mg/m2 increments; a fixed dose of BCNU (150 mg/m2) was given within 72 h of starting TMZ. A standard phase 1 dose-escalation scheme was used with 3 patients per cohort. Fourteen glioblastoma patients and 10 anaplastic astrocytoma patients were treated. The dose-limiting toxicity was myelosuppression at 90 mg/m2 of TMZ. The total number of cycles given was 73 (median number was 2). Six patients (25%) required a dose reduction in BCNU, and six were removed from study for hematologic toxicity after cycle 1; three patients overlapped. The median time to progression and overall survival were, respectively, 82 and 132 weeks for anaplastic astrocytomas and 14 and 69 weeks for glioblastomas. We conclude that the combination of BCNU and the extended dosing schedule of TMZ is feasible and that the maximal tolerated dose of a 28-day course of TMZ is 80 mg/m2 when combined with a fixed dose of BCNU at 150 mg/m2. This is the recommended dose for phase 2, but myelosuppression after cycle 1 suggests that long-term treatment may be difficult.

Mutational-reporter transgenes rescued from mice lacking either Mgmt, or both Mgmt and Msh6 suggest that O6-alkylguanine-induced miscoding does not contribute to the spontaneous mutational spectrum

O6-methylguanine methyltransferase, Mgmt, constitutes the first line of defense against O6-alkylguanine, which can result in G : C to A : T transitions upon DNA replication. Mgmt has been found in organisms as diverse as archaebacteria and mammals. This evolutionary conservation suggests that all organisms may be exposed to either endogenous or environmental alkylating agents. We thus hypothesized that tissues of Mgmt-/- mice would exhibit elevated mutant frequencies. Employing the Big Blue trade mark transgenic system, we evaluated lacI mutants rescued from liver and small intestinal DNA of young Mgmt-/- mice. Interestingly, while there was a small difference between Mgmt-/- mice and controls with respect to lacI mutant frequency, no differences attributable to Mgmt deficiency were apparent in the mutational spectra. Although mutations stemming from O6-guanine alkylations would be predicted to be cumulative, we found no evidence of an Mgmt-dependent alteration in mutation spectrum in DNA samples from 12 month-old mice. To optimize our ability to detect mutations resulting from O6-alkylguanine-induced G : T mismatches, mice with combined deficiencies of Mgmt and the DNA mismatch repair molecule, Msh6, were analysed. In spite of this strategy, we observed no significant differences between Mgmt-/- Msh6-/- and Msh6-/- mouse lacI mutations, except for a trend towards a greater percentage (of total transitions) of G : C to A : T changes in Mgmt-/-Msh6-/- livers. Therefore, despite the striking evolutionary conservation of Mgmt, deficiency of this gene did not significantly impact the spontaneous lacI mutational spectrum in vivo.

Active-site alkylation destabilizes human O6-alkylguanine DNA alkyltransferase

O(6)-alkylguanine-DNA alkyltransferase (AGT) repairs pro-mutagenic O(6)-alkylguanine and O(4)-alkylthymine lesions in DNA. The alkylated form of the protein is not reactivated; instead, it is rapidly ubiquitinated and degraded. Here, we show that alkylation destabilizes the native fold of the protein by 0.5-1.2 kcal/mole and the DNA-binding function by 0.8-1.4 kcal/mole. On this basis, we propose that destabilization of the native conformational ensemble acts as a signal for ubiquitination.

Labeling of fusion proteins with synthetic fluorophores in live cells

A general approach for the sequential labeling of fusion proteins of O(6)-alkylguanine-DNA alkyltransferase (AGT) with different fluorophores in mammalian cells is presented. AGT fusion proteins with different localizations in the cell can be labeled specifically with different fluorophores, and the fluorescence labeling can be used for applications such as multicolor analysis of dynamic processes and fluorescence resonance energy transfer measurements. The facile access to a variety of different AGT substrates as well as the specificity of the labeling reaction should make the approach an important tool to study protein function in live cells.

DNA repair in hybrid fish of the genus Xiphophorus

The genus Xiphophorus is an important vertebrate model for investigating the etiology and genetics of both spontaneous and induced cancers. Xiphophorus are comprised of 23 species most of which can be crossed to produce fertile interspecies hybrid progeny. The Xiphophorus gene map is well developed and allows genetic associations to be studied among cohorts of progeny derived from backcrossing interspecies hybrid animals to one of the parental strains. In interspecies cross-progeny from select Xiphophorus backcrosses, ionizing radiation, ultraviolet light (UVB), and exposure to methylnitrosourea (MNU) have all been shown to induce tumors. Induced tumor types represented in various models include melanoma, fibrosarcoma, schwannoma, retinoblastoma, etc. The well-established backcross hybrid genetics make Xiphophorus fish an excellent system to study the contribution of DNA repair capability to induced tumorigenesis. DNA repair pathways represent multigenic traits that must be tightly regulated to insure genome fidelity. Herein we review initial DNA repair studies that assess repair capacities among different Xiphophorus species and interspecies hybrids. Assessment of both base excision repair (BER) and nucleotide excision repair (NER) have yielded consistent results indicating reduced DNA repair function in hybrid fish tissues. These data provide molecular support for potential reduced fitness in hybrid fish under conditions of environmental stress and may present a plausible explanation for absence of interspecies hybridization in sympatric environments. In addition, they support the role of direct DNA damage and its repair in the initiation of tumors in Xiphophorus hybrids.

Exon 5 polymorphisms in the O6-alkylguanine DNA alkyltransferase gene and lung cancer risk in non-smokers exposed to second-hand smoke

PURPOSE

The objective of the study was to examine the association of three exon 5 variants in the O(6)-alkylguanine DNA alkyltransferase (AGT) gene involved in the repair of the mutagenic DNA lesion O(6)-alkylguanine formed by nitrosamines, with lung cancer risk in never-smokers.

EXPERIMENTAL DESIGN

Exon 5 of the AGT gene was sequenced in genomic DNA from 136 cases and 133 hospital- or population-based controls for whom questionnaire information on second-hand smoke and diet was available to determine the frequencies of the Gly(160)Arg, Ile(143)Val, and Lys(178)Arg variant alleles.

RESULTS

No codon (160)Arg variant alleles were found in the study population. The codon (143)Val and (178)Arg variant alleles, present at allele frequencies of 0.07, showed 100% linkage. The odds ratio (OR) of lung cancer for these variant carriers was 2.05 [95% confidence interval (CI) 1.03-4.07]. The risk varied between the different lung cancer pathologies with an increased risk for adenocarcinoma (OR 2.67, 95% CI 1.21-5.87) or small cell carcinoma (OR 4.83, 95% CI 0.91-25.7) but not for squamous cell carcinoma (OR 1.07, 95% CI 0.27-4.18). Compared with individuals carrying the mutant alleles unexposed to second-hand smoke, the OR for exposed variant carriers was 1.95 (95% CI 0.53-1.15); a similar interaction, although not significative, was observed for low consumption of cruciferous vegetables and for green vegetables and tomatoes.

CONCLUSIONS

These results point toward a role of AGT polymorphisms in lung cancer susceptibility among never-smokers, in particular among subjects exposed to environmental carcinogens.

The role of surrogate markers in the clinical development of antiretroviral therapy: a model for early evaluation of targeted cancer drugs

Both CD4 cell counts and measurements of plasma HIV-1 RNA (i.e. viral load) have become established surrogate markers for predicting treatment and disease outcome in HIV infection, and are instrumental for the evaluation of new antiretroviral drugs in clinical trials. Recently, HIV drug-resistance testing has also become available and has been shown to have prognostic value in providing guidance with antiretroviral therapy. The identification of robust surrogate markers is also an essential requirement for the clinical development of targeted anticancer agents, which unlike their cytotoxic counterparts, are often devoid of the toxicities that have been traditionally used to monitor the efficacy of chemotherapy. In particular, biological or molecular markers that are predictive of a drug effect need to be integrated into early efficacy trials of targeted therapies in order to confirm that the drug is in fact "hitting" the intended target. The full clinical significance of many of the altered cell types or polymorphisms, which are selected by cytostatic agents, remains to be elucidated. However, molecular genotyping of these targets, akin to drug resistance testing for HIV infection, may constitute an important strategy to assist with the selection and monitoring of targeted chemotherapy in cancer patients. Thus, lessons from HIV/AIDS on the value of surrogate makers may assist with the development and optimization of targeted cancer therapy.

Molecular heterogeneity of diffuse large B-cell lymphoma: implications for disease management and prognosis

Diffuse large B-cell lymphoma (DLBCL) accounts for approximately 40% of all B-cell non-Hodgkin lymphomas of the Western world. According to the "WHO classification of tumours of the haematopoietic and lymphoid tissues", the term DLBCL is likely to include more than one disease entity, as suggested by the marked variability of the clinical presentation and response to treatment of this disease. Such heterogeneity may reflect the occurrence of distinct molecular subtypes of DLBCL as well as differences in the host's immune function. In immunocompetent hosts, approximately 50% DLBCL carry one of two primary molecular lesions defining two distinct genotypic subgroups, characterized by activation of either the BCL-6 or the BCL-2 proto-oncogene. Conversely, the remaining DLBCL of immunocompetent hosts display one of several molecular lesions, each associated with a small subset of cases and including activation of the proto-oncogenes REL, MUC-1, BCL-8 and c-MYC. The molecular pathogenesis of immunodeficiency-associated DLBCL differs substantially from that of DLBCL in immunocompetent hosts. In fact, EBV infection is present in a large fraction of immunodeficiency-associated DLBCL, whereas it is consistently negative in DLBCL of immunocompetent hosts, probably reflecting the critical role of disruption of the immune system in this disease. Finally, the application of DNA microarray technology to DLBCL has led to the distinction of two disease variants: a germinal center like DLBCL and an activated peripheral B-cell like DLBCL. Overall the molecular features of DLBCL may identify prognostic categories of the disease and may represent a powerful tool for therapeutic stratification.

Interaction of human and bacterial AlkB proteins with DNA as probed through chemical cross-linking studies

The Escherichia coli AlkB protein was recently found to repair cytotoxic DNA lesions 1-methyladenine and 3-methylcytosine by using a novel iron-catalyzed oxidative demethylation mechanism. Three human homologs, ABH1, ABH2 and ABH3, have been identified, and two of them, ABH2 and ABH3, were shown to have similar repair activities to E.coli AlkB. However, ABH1 did not show any repair activity. It was suggested that ABH3 prefers single-stranded DNA and RNA substrates, whereas AlkB and ABH2 can repair damage in both single- and double-stranded DNA. We employed a chemical cross-linking approach to probe the structure and substrate preferences of AlkB and its three human homologs. The putative active site iron ligands in these proteins were mutated to cysteine residues. These mutant proteins were used to cross-link to different DNA probes bearing thiol-tethered bases. Disulfide-linked protein-DNA complexes can be trapped and analyzed by SDS-PAGE. Our results show that ABH2 and ABH3 have structural and functional similarities to E.coli AlkB. ABH3 shows preference for the single-stranded DNA probe. ABH1 failed to cross-link to the probes tested. This protein, unlike other AlkB proteins, does not seem to interact with DNA in its E.coli expressed form.

Role of DNA mismatch repair in apoptotic responses to therapeutic agents

Deficiencies in DNA mismatch repair (MMR) have been found in both hereditary cancer (i.e., hereditary nonpolyposis colorectal cancer) and sporadic cancers of various tissues. In addition to its primary roles in the correction of DNA replication errors and suppression of recombination, research in the last 10 years has shown that MMR is involved in many other processes, such as interaction with other DNA repair pathways, cell cycle checkpoint regulation, and apoptosis. Indeed, a cell's MMR status can influence its response to a wide variety of chemotherapeutic agents, such as temozolomide (and many other methylating agents), 6-thioguanine, cisplatin, ionizing radiation, etoposide, and 5-fluorouracil. For this reason, identification of a tumor's MMR deficiency (as indicated by the presence of microsatellite instability) is being utilized more and more as a prognostic indicator in the clinic. Here, we describe the basic mechanisms of MMR and apoptosis and investigate the literature examining the influence of MMR status on the apoptotic response following treatment with various therapeutic agents. Furthermore, using isogenic MMR-deficient (HCT116) and MMR-proficient (HCT116 3-6) cells, we demonstrate that there is no enhanced apoptosis in MMR-proficient cells following treatment with 5-fluoro-2'-deoxyuridine. In fact, apoptosis accounts for only a small portion of the induced cell death response.

O6-methylguanine-DNA-methyltransferase expression and gene polymorphisms in relation to chemotherapeutic response in metastatic melanoma

In a retrospective study, O⁶-methylguanine-DNA-methyltransferase (MGMT) expression was analysed by immunohistochemistry using monoclonal human anti-MGMT antibody in melanoma metastases in patients receiving dacarbazine (DTIC) as single-drug therapy or as part of combination chemotherapy with DTIC–vindesine or DTIC–vindesine–cisplatin. The correlation of MGMT expression levels with clinical response to chemotherapy was investigated in 79 patients with metastatic melanoma. There was an inverse relationship between MGMT expression and clinical response to DTIC-based chemotherapy (P=0.05). Polymorphisms in the coding region of the MGMT gene were also investigated in tumours from 52 melanoma patients by PCR/SSCP and nucleotide sequence analyses. Single-nucleotide polymorphisms (SNPs) in exon 3 (L53L and L84F) and in exon 5 (I143V/K178R) were identified. There were no differences in the frequencies of these polymorphisms between these melanoma patients and patients with familial melanoma or healthy Swedish individuals. Functional analysis of variants MGMT-I143V and -I143V/K178R was performed by in vitro mutagenesis in Escherichia coli. There was no evidence that these variants decreased the MGMT DNA repair activity compared to the wild-type protein. All melanoma patients with the MGMT 53/84 polymorphism except one had tumours with high MGMT expression. There was no significant correlation between any of the MGMT polymorphisms and clinical response to chemotherapy, although an indication of a lower response rate in patients with SNPs in exon 5 was obtained. Thus, MGMT expression appears to be more related to response to chemotherapy than MGMT polymorphisms in patients with metastatic melanoma.

Cytotoxicity, DNA damage, and apoptosis induced by new fotemustine analogs on human melanoma cells in relation to O6-methylguanine DNA-methyltransferase expression

Fotemustine is a third generation chloroethylnitrosourea that has demonstrated significant antitumoral effects in malignant melanoma. However, its use is somewhat limited by its toxic side effects and chemoresistance caused by direct repair of O6-alkyl groups by the enzyme O6-methylguanine DNA-methyltransferase (MGMT). The aim of this work was to determine to what extent the expression of MGMT influences cytotoxicity, DNA damage, and apoptosis induced by new nitrososulfamide analogs of fotemustine (compounds 4 and 8), which have previously demonstrated interesting antiproliferative properties. We carried out complementary strategies that consisted of MGMT cDNA transfection in CAL77 Mer- melanoma cells and of MGMT inhibition with O6-benzylguanine (BG) in A375 Mer+ melanoma cells. MGMT-transfected cells were 7 to 9 times less sensitive to fotemustine than parent cells, whereas no difference between the transfected and parent cells was observed for nitrososulfamide analogs. The cytotoxicity of these analogs vis à vis a MGMT-proficient A375 melanoma cell line was approximately 3 times greater than that of fotemustine. Coincubation of these cells with O6-benzylguanine significantly increased the cytotoxicity of fotemustine and compound 8, whereas BG had little effect on the cytotoxicity of compound 4. Furthermore, DNA fragmentation determined by a comet assay was greater with nitrososulfamide analogs than with fotemustine. O6-benzylguanine increased DNA fragmentation for fotemustine and compound 8, but not for compound 4, which induced comets with a typical apoptotic appearance. The ability of this compound to induce apoptosis in the absence of BG was confirmed by a specific enzyme-linked immunosorbent assay apoptotic assay using a single-stranded DNA monoclonal antibody.

Enhanced DNA excision repair in CCRF-CEM cells resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea, quantitated using the single cell gel electrophoresis (Comet) assay

Enhanced DNA repair activity is important for the development of cellular resistance to alkylating agents. Here, we quantitated the kinetics of DNA excision repairs initiated by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in human leukemia CCRF-CEM cells. CEM cells that had been established resistant to BCNU (CEM-R) were evaluated in comparison with parental CEM cells (CEM-S). The excision repair kinetics were quantitated as the amount of DNA single strand breaks, which were generated from the incision/excision of the damaged DNA and were diminished by the rejoining of renewed DNA, using the single cell gel electrophoresis (Comet) assay. CEM-R cells were 10-fold more resistant to BCNU than CEM-S cells, and also showed cross-resistance to melphalan and cisplatin. In response to the treatment with BCNU, both CEM-S and CEM-R cells initiated an incision/excision reaction at the end of the incubation period, and completed the rejoining process within 4 hr. While CEM-S cells could not repair the damage induced by the high concentration of BCNU, CEM-R cells completed the repair process regardless of BCNU concentrations, suggesting enhanced excision repairs in CEM-R cells. The excision repair activity of CEM-R cells was increased with regard to the incision reaction and to the rate of the repair. Similar results were obtained using ultraviolet C, suggesting enhanced nucleotide excision repair in CEM-R cells. Thus, the enhanced DNA excision repairs were successfully quantitated in the resistant leukemic cell line using the Comet assay. The evaluation of the repair activity may predict the sensitivity of cancer cells to chemotherapy and provide a clue to overcome the resistance.

Genetic susceptibility to thymic lymphomas and K-ras gene mutation in mice after exposure to X-rays and N-ethyl-N-nitrosourea

PURPOSE

Ras activation is one of the major mechanisms for the development of murine thymic lymphomas by radiation and chemical carcinogens. To gain insight into the relationship between genetic susceptibility and ras gene mutation, the frequency and spectrum of ras gene mutation was examined in thymic lymphomas from susceptible and resistant mice.

MATERIALS AND METHODS

K- and N-ras mutations in thymic lymphomas that arose in X-ray-irradiated and N-ethyl-N-nitrosourea (ENU)-treated mice of susceptible C57BL/6, rather resistant C3H and their hybrid B6C3F1 were analysed by polymerase chain reaction-single-strand conformation polymorphism and subsequent DNA sequencing.

RESULTS

C57BL/6 exhibited a higher incidence of thymic lymphomas after exposure to X-rays and ENU than C3H, with B6C3F1 being intermediate. K-ras gene mutations occurred frequently in the pathogenesis of ENU-induced thymic lymphomas in susceptible C57BL/6 as opposed to resistant C3H. The ras mutations were more frequent in ENU-induced thymic lymphomas than X-ray-induced thymic lymphomas, and with the latter, there was no clear evidence for strain differences, suggesting that the genetic susceptibility to X-rays was independent of ras activation. The mutations of K-ras in thymic lymphomas from C57BL/6 were predominantly GGT to GAT in codon 12, whereas this mutation type was never found in those from C3H. No strain difference was observed in the nucleotide sequence or expression levels of O(6)-alkylguanine alkyltransferase, indicating that this enzyme did not account for the genetic susceptibility to ras activation.

CONCLUSIONS

The results indicate that there is a clear strain and carcinogen dependency of K-ras mutation and that the frequency of ras mutation might determine the genetic susceptibility to ENU-induced lymphomagenesis, whereas pathways independent of ras activation might determine the susceptibility to X-ray-induced lymphomagenesis.

Synthesis and preliminary biological evaluation of radiolabeled O6-benzylguanine derivatives, new potential PET imaging agents for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in breast cancer

Novel radiolabeled O(6)-benzylguanine (O(6)-BG) derivatives, 2-amino-6-O-[(11)C]-[(methoxymethyl)benzyloxy]-9-methyl purines ([(11)C]p-O(6)-AMMP, 1a; [(11)C]m-O(6)-AMMP, 1b; [(11)C]o-O(6)-AMMP, 1c), 2-amino-6-O-benzyloxy-9-[(11)C]-[(methoxycarbonyl)methyl]purine ([(11)C]ABMMP, 2), and 2-amino-6-O-benzyloxy-9-[(11)C]-[(4'-methoxycarbonyl)benzyl]purine ([(11)C]ABMBP, 3), have been synthesized for evaluation as new potential positron emission tomography (PET) imaging agents for the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) in breast cancer. The appropriate precursors for radiolabeling were obtained in two to three steps from starting material 2-amino-6-chloropurine with moderate to excellent chemical yields. Tracers were prepared by O-[(11)C]methylation of hydroxymethyl or acid precursors using [(11)C]methyl triflate. Pure target compounds were isolated by solid-phase extraction (SPE) purification procedure in 45-65% radiochemical yields (decay corrected to end of bombardment), and a synthesis time of 20-25 min. The activity of unlabeled standard samples of 1-3 was evaluated via an in vitro AGT oligonucleotide assay. Preliminary findings from biological assay indicate the synthesized analogs have similar strong inhibitory effectiveness on AGT in comparison with the parent compound O(6)-BG. The results warrant further evaluation of these radiotracers as new potential PET imaging agents for the DNA repair protein AGT in breast cancer in vivo.

Who's on first in the cellular response to DNA damage?

Cellular DNA-repair pathways involve proteins that have roles in other DNA-metabolic processes, as well as those that are dedicated to damage removal. Several proteins, which have diverse functions and are not known to have roles in DNA repair, also associate with damaged DNA. These newly discovered interactions could either facilitate or hinder the recognition of DNA damage, and so they could have important effects on DNA repair and genetic integrity. The outcome for the cell, and ultimately for the organism, might depend on which proteins arrive first at sites of DNA damage.

Directed evolution of O6-alkylguanine-DNA alkyltransferase for efficient labeling of fusion proteins with small molecules in vivo

We report here the generation of mutants of the human O(6)-alkylguanine-DNA alkyltransferase (hAGT) for the efficient in vivo labeling of fusion proteins with synthetic reporter molecules. Libraries of hAGT were displayed on phage, and mutants capable of efficiently reacting with the inhibitor O(6)-benzylguanine were selected based on their ability to irreversibly transfer the benzyl group to a reactive cysteine residue. Using synthetic O(6)-benzylguanine derivatives, the selected mutant proteins allow for a highly efficient covalent labeling of hAGT fusion proteins in vivo and in vitro with small molecules and therefore should become important tools for studying protein function in living cells. In addition to various applications in proteomics, the selected mutants also yield insight into the interaction of the DNA repair protein hAGT with its inhibitor O(6)-benzylguanine.

The selectivity and inhibition of AlkB

AlkB is one of four proteins involved in the adaptive response to DNA alkylation damage in Escherichia coli and is highly conserved from bacteria to humans. Recent analyses have verified the prediction that AlkB is a member of the Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase family of enzymes. AlkB mediates repair of methylated DNA by direct demethylation of 1-methyladenine and 3-methylcytosine lesions. Other members of the Fe(II) and 2OG-dependent oxygenase family, including those involved in the hypoxic response, are targets for therapeutic intervention. Assays measuring 2OG turnover were used to investigate the selectivity of AlkB. 1-Methyladenosine, 1-methyl-2'-deoxyadenosine, 3-methylcytidine, and 3-methyl-2'-deoxycytidine all stimulated 2OG turnover by AlkB but were not demethylated indicating an uncoupling of 2OG and prime substrate oxidation and that oligomeric DNA is required for hydroxylation and subsequent demethylation. In contrast the equivalent unmethylated nucleosides did not stimulate 2OG turnover indicating that the presence of a methyl group in the substrate is important in initiating oxidation of 2OG. Stimulation of 2OG turnover by 1-methyladenosine was highly dependent on the presence of a reducing agent, ascorbate or dithiothreitol. Following the observation that AlkB is inhibited by high concentrations of 2OG, analogues of 2OG, including 2-mercaptoglutarate, were found to specifically inhibit AlkB. The flavonoid quercetin inhibits both AlkB and the 2OG oxygenase factor-inhibiting hypoxia-inducible factor (FIH) in vitro. FIH inhibition by quercetin occurs in the presence of excess iron indicating a specific interaction, while the inhibition of AlkB by quercetin is, predominantly, due to nonspecific iron chelation.

DNA-binding mechanism of O6-alkylguanine-DNA alkyltransferase. Effects of protein and DNA alkylation on complex stability

The mutagenic and cytotoxic effects of many endogenous and exogenous alkylating agents are mitigated by the actions of O(6)-alkylguanine-DNA alkyltransferase (AGT). In humans this protein protects the integrity of the genome, but it also contributes to the resistance of tumors to DNA-alkylating chemotherapeutic agents. Here we report properties of the interaction between AGT and short DNA oligonucleotides. We show that although AGT sediments as a monomer in the absence of DNA, it binds cooperatively to both single-stranded and double-stranded deoxyribonucleotides. This strong cooperative interaction is only slightly perturbed by active site mutation of AGT or by alkylation of either AGT or DNA. The stoichiometry of complex formation with 16-mer oligonucleotides, assessed by analytical ultracentrifugation and electrophoretic mobility shift assays, is 4:1 on single-stranded and duplex DNA and is unchanged by several active site mutations or by protein or DNA alkylation. These results have significant implications for the mechanisms by which AGT locates and interacts with repairable alkyl lesions to effect DNA repair.

Delayed repletion of O6-methylguanine-DNA methyltransferase resulting in failure to protect the human glioblastoma cell line SF767 from temozolomide-induced cytotoxicity

OBJECT

Temozolomide (TMZ)-induced O6-methylguanine (MG) DNA lesions, if not removed by MG-DNA methyltransferase (MGMT), mispair with thymine, trigger rounds of futile mismatch repair (MMR), and in glioma cells lead to prolonged G2-M arrest and ultimately cell death. Depletion of MGMT by O6-benzylguanine (BG) sensitizes tumor cells to TMZ, and this combination is currently used in clinical trials. The use of the TMZ+BG combination in gliomas, however, is complicated by the prolonged TMZ-induced G2-M arrest, which may delay activation of poorly defined cell death pathways and allow for MGMT repletion and reversal of toxicity.

METHODS

To address these issues, the actions of TMZ were monitored in DNA MMR-proficient SF767 glioma cells depleted of MGMT by BG, and in cells in which BG was removed at various times after TMZ exposure. In MGMT-depleted cells, TMZ exposure led to DNA single-strand breaks and phosphorylation of cdc2, followed by G2-M arrest, induction of p53/p21, and DNA double-strand breaks. Although DNA single-strand breaks, phosphorylation of cdc2, and G2-M arrest could be reversed by repletion of MGMT up to 5 days after TMZ exposure, TMZ-induced cytotoxicity could only be prevented if MGMT was replenished within 24 hours of the onset of G2-M arrest, and before the creation of DNA double-strand breaks.

CONCLUSIONS

These results indicate that although SF767 glioma cells undergo a prolonged G2-M arrest in response to TMZ, their ability to escape TMZ-induced cytotoxicity by MGMT repletion is limited to an approximately 24-hour period after the onset of G2-M arrest.

Contribution of the Atm protein to maintaining cellular homeostasis evidenced by continuous activation of the AP-1 pathway in Atm-deficient brains

Maintenance of genome stability is essential for keeping cellular homeostasis. The DNA damage response is a central component in maintaining genome integrity. Among of the most cytotoxic DNA lesions are double strand breaks (DSBs) caused by ionizing radiation or radiomimetic chemicals. ATM is missing or inactivated in patients with ataxia-telangiectasia. Ataxia-telangiectasia patients display a pleiotropic phenotype and suffer primarily from progressive ataxia caused by degeneration of cerebellar Purkinje and granule neurons. Additional features are immunodeficiency, genomic instability, radiation sensitivity, and cancer predisposition. Disruption of the mouse Atm locus creates a murine model of ataxia-telangiectasia that exhibits most of the clinical features of the human disease but very mild neuronal abnormality. The ATM protein is a multifunctional protein kinase, which serves as a master regulator of cellular responses to DSBs. There is growing evidence that ATM may be involved in addition to the DSB response in other processes that maintain processes in cellular homeostasis. For example, mounting evidence points to increased oxidative stress in the absence of ATM. Here we report that the AP-1 pathway is constantly active in the brains of Atm-deficient mice not treated with DNA damaging agents. A canonical activation (increased phosphorylation of mitogen-activated protein kinase kinase-4, c-Jun N-terminal kinase, and c-Jun) of the AP-1 pathway was found in Atm-deficient cerebra, whereas induction of the AP-1 pathway in Atm-deficient cerebella is likely to mediate elevated expression of c-Fos and c-Jun. Although Atm(+/+) mice are capable of responding to ionizing radiation by activating stress responses such as the AP-1 pathway, Atm-deficient mice display higher basal AP-1 activity but gradually lose their ability to activate AP-1 DNA-binding activity in response to ionizing radiation. Our results further demonstrate that inactivation of the ATM gene results in a state of constant stress.

Synthesis and preliminary biological evaluation of 6-O-[11C]-[(methoxymethyl)benzyl]guanines, new potential PET breast cancer imaging agents for the DNA repair protein AGT

Novel radiolabeled O(6)-benzylguanine derivatives, 6-O-[(11)C]-[(methoxymethyl)benzyl]guanines ([(11)C]p-O(6)-MMBG, 1a; [(11)C]m-O(6)-MMBG, 1b; ([(11)C]o-O(6)-MMBG, 1c), have been synthesized for evaluation as new potential positron emission tomography (PET) breast cancer imaging agents for DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (AGT).

A general method for the covalent labeling of fusion proteins with small molecules in vivo

Characterizing the movement, interactions, and chemical microenvironment of a protein inside the living cell is crucial to a detailed understanding of its function. Most strategies aimed at realizing this objective are based on genetically fusing the protein of interest to a reporter protein that monitors changes in the environment of the coupled protein. Examples include fusions with fluorescent proteins, the yeast two-hybrid system, and split ubiquitin. However, these techniques have various limitations, and considerable effort is being devoted to specific labeling of proteins in vivo with small synthetic molecules capable of probing and modulating their function. These approaches are currently based on the noncovalent binding of a small molecule to a protein, the formation of stable complexes between biarsenical compounds and peptides containing cysteines, or the use of biotin acceptor domains. Here we describe a general method for the covalent labeling of fusion proteins in vivo that complements existing methods for noncovalent labeling of proteins and that may open up new ways of studying proteins in living cells.

Effect of nucleotide excision repair on ENU-induced mutation in female germ cells of Drosophila melanogaster

The role of nucleotide excision repair (NER) in the repair of alkylation damage in the germ cells of higher eukaryotes has been studied mainly by treating postmeiotic male germ cells. Little is known about repair in actively repairing female germ cells. In this study, we treated NER-deficient (ner(-)) mus201(D1) Drosophila females with N-ethyl-N-nitrosourea (ENU) and determined both the mutant frequencies in the multiple locus recessive lethal (RL) test and in the single locus vermilion gene and determined the ENU mutation spectrum in the vermilion gene. The results show that ENU is mutagenic in all cell stages and that the induced frequencies increase with cell maturation, from oogonia to mature oocytes. In addition, the induced spectrum consists mainly of A:T-->T:A transversions (43.8%), A:T-->G:C transitions (21.9%), and A:T-->C:G transversions (15.6%). G:C-->A:T (3.1%) transitions, other transversions (9.4%), frameshifts (3.1%), and deletions (3.1%) were also found. Comparison of these results with those previously obtained for repair-proficient (ner(+)) female germ cells reveal: 1) Differences in the RL and vermilion mutation frequencies for ner(+) and ner(-) germ cells, indicating that NER is involved in the repair of ENU-induced damage to these cells. 2) At least 15.6% of mutations in ner(-) cells may be the consequence of N-ethylation damage and mutations of this type were not detected in ner(+) cells. 3) Although differences were found in transition frequencies between ENU-treated ner(+) and ner(-) germ cells (52.2% vs. 25%), suggesting that a functional NER is involved in processing O-ethylated damage, the role of NER in repairing O-ethylated adducts is uncertain.

Inhibition of telomerase increases resistance of melanoma cells to temozolomide, but not to temozolomide combined with poly (adp-ribose) polymerase inhibitor

In the present study, we have investigated the influence of telomerase inhibition in chemosensitivity of melanoma cells to temozolomide (TMZ), a methylating agent with promising antitumor activity against metastatic melanoma. In fact, telomerase, a ribonucleoprotein enzyme expressed in the majority of tumors, is presently considered an attractive target for anticancer therapy, with the double aim of reducing tumor growth and increasing chemosensitivity of cancer cells. Susceptibility to TMZ and to other antitumor agents used for treatment of metastatic melanoma was initially assessed in melanoma lines with different basal levels of telomerase activity. Thereafter, chemosensitivity was investigated after inhibition of telomerase by means of stable transfection of a catalytically inactive, dominant-negative mutant of hTERT (DN-hTERT). This study shows for the first time that: a) susceptibility to TMZ of melanoma lines derived from the same patient did not depend on basal telomerase activity; b) inhibition of telomerase by DN-hTERT resulted in reduced growth rate and increased resistance to TMZ and to the chloroethylating agent carmustine, increased sensitivity to cisplatin, and no change in response to tamoxifen or to a selective N3-adenine methylating agent; c) inhibition of poly(ADP-ribose) polymerase (PARP), an enzyme involved in the repair of N-methylpurines, restored sensitivity of DN-hTERT clones to TMZ. These results indicate that a careful selection of the antitumor agent has to be made when antitelomerase therapy is combined with chemotherapy. Moreover, the data presented here suggest that TMZ + PARP inhibitor combination is active against telomerase-suppressed and slowly growing tumors.

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

DNA alkylation and repair in the large bowel: animal and human studies

O6-methylguanine (O6-MeG), a procarcinogenic DNA adduct that arises from exposure to methylating agents, has been detected in human colorectal DNA at levels comparable to those that cause adverse effects in model systems. O6-MeG levels vary within the colon, being higher in the cancer-prone regions of the large bowel. In rats and mice, O6-MeG persistence in colon DNA is associated with the induction of colon tumors after treatment with methylating agents. These tumors frequently contain K-ras GC-->AT transition mutations, which is consistent with the mutagenic properties of O6-MeG: such mutations are also commonly found in human colorectal cancers. O6-Alkylguanine adducts are removed by the DNA repair protein, O6-alkylguanine DNA-alkyltransferase (MGMT). MGMT overexpression in transgenic mice reduces the formation of K-ras GC-->AT mutations and tumors induced by methylating agents. Interindividual variations in human colon MGMT activity are large and large bowel tumors can occur in regions of low activity. Low MGMT activity in normal mucosa has been associated with the occurrence of K-ras GC-->AT mutations, whereas reduced MGMT expression and an increased frequency of K-ras GC-->AT mutations in colorectal cancers have been linked to MGMT promoter methylation. MGMT activity is also lower in adenomas than in adjacent normal tissue but only in those adenomas with this specific mutation. These results are entirely consistent with the hypothesis that GC-->AT mutations in the K-ras oncogene result from the formation and persistence of O6-alkylguanine lesions in colorectal DNA. Human exposure to endogenous or exogenous alkylating agents may thus be an environmental determinant of colorectal cancer risk.

Paradoxical enhancement of the toxicity of 1,2-dibromoethane by O6-alkylguanine-DNA alkyltransferase

The presence of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) paradoxically increases the mutagenicity and cytotoxicity of 1,2-dibromoethane (DBE) in Escherichia coli. This enhancement of genotoxicity did not occur when the inactive C145A mutant of human AGT (hAGT) was used. Also, hAGT did not enhance the genotoxicity of S-(2-haloethyl)glutathiones that mimic the reactive product of the reaction of DBE with glutathione, which is catalyzed by glutathione S-transferase. These experiments support a mechanism by which hAGT activates DBE. Studies in vitro showed a direct reaction between purified recombinant hAGT and DBE resulting in a loss of AGT repair activity and a formation of an hAGT-DBE conjugate at Cys(145). A 2-hydroxyethyl adduct was found by mass spectrometry to be present in the Gly(136)-Arg(147) peptide from tryptic digests of AGT reacted with DBE. Incubation of AGT with DBE and oligodeoxyribonucleotides led to the formation of covalent AGT-oligonucleotide complexes. These results indicate that DBE reacts at the active site of AGT to generate an S-(2-bromoethyl) intermediate, which forms a highly reactive half-mustard at Cys(145). In the presence of DNA, the DNA-binding function of AGT facilitates formation of DNA adducts. In the absence of DNA, the intermediate undergoes hydrolytic decomposition to form AGT-Cys(145)-SCH(2)CH(2)OH.

AlkB-mediated oxidative demethylation reverses DNA damage in Escherichia coli

The bacterial AlkB protein is known to be involved in cellular recovery from alkylation damage; however, the function of this protein remains unknown. AlkB homologues have been identified in several organisms, including humans, and a recent sequence alignment study has suggested that these proteins may belong to a superfamily of 2-oxoglutarate-dependent and iron-dependent oxygenases (2OG-Fe(ii)-oxygenases). Here we show that AlkB from Escherichia coli is indeed a 2-oxoglutarate-dependent and iron-dependent DNA repair enzyme that releases replication blocks in alkylated DNA by a mechanism involving oxidative demethylation of 1-methyladenine residues. This mechanism represents a new pathway for DNA repair and the third type of DNA damage reversal mechanism so far discovered.

The DNA repair protein, O(6)-methylguanine-DNA methyltransferase is a proteolytic target for the E6 human papillomavirus oncoprotein

We have previously shown that O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that protects tissues against toxic and carcinogenic effects of alkylating agents, is degraded through ubiquitination-dependent proteolysis. Here, we investigated the role of the human papillomavirus (HPV) E6 protein in MGMT degradation. In three pairs of isogenic human tumor cell lines in which a member of each pair expressed the E6 protein through stable transfection (HCT116/HCT116-E6, MCF7/MCF7-E6, and RKO/RKO-E6), we found a consistent 40-55% reduction in the MGMT protein level and its activity in all E6-expressing cells compared with the parent cells (P=<0.05). E6 expression did not, however, alter the levels of MGMT mRNA. Addition of the recombinant MGMT (rMGMT) protein to extracts of HCT116/E6 cells resulted in the binding of E6 to MGMT. Further, the purified E6 protein promoted the degradation of rMGMT in rabbit reticulocyte lysates. Immunoprecipitation assays showed the presence of a ternary protein complex between MGMT, E6, and the cellular ubiquitin-ligase E6-associated protein (E6-AP). Transient transfection of the p53-null H1299 lung tumor cells with an E6 construct also down-regulated the MGMT. The MGMT protein also showed structural features that are compatible for interaction with the E6, and E6-AP components. Collectively, these data suggest that the oncogenic E6 proteins enhance the ubiquitin-dependent proteolysis of MGMT.

Phase I study of temozolomide in relapsed/refractory acute leukemia

PURPOSE

To determine the dose-limiting toxicity and maximum-tolerated dose of temozolomide in patients with acute leukemia.

PATIENTS AND METHODS

Twenty patients (16 with acute myelogenous leukemia, two with acute lymphoblastic leukemia, and two with chronic myelogenous leukemia in blastic phase) received 43 cycles of temozolomide. Patients began treatment at two different dose levels: 200 mg/m(2)/d for 7 days or 200 mg/m(2)/d for 9 days.

RESULTS

Prolonged aplasia was the dose-limiting toxicity, and the maximum-tolerated dose was 7 days of temozolomide. Overall treatment was well tolerated: hospitalization was required in only nine of 43 courses, and there were no treatment-related deaths. Two patients obtained a complete response, and two others met criteria for complete response except for platelet recovery. Overall, nine of 20 patients had a significant decrease in bone marrow blasts after temozolomide treatment.

CONCLUSION

Temozolomide was well tolerated and had significant antileukemic activity when administered as a single agent. Further studies of temozolomide in hematologic malignancies are indicated.

Cigarette smoking and lung cancer: chemical mechanisms and approaches to prevention

Much is now known about the carcinogens in cigarette smoke, their conversion to forms that react with DNA, and the miscoding properties of the resulting DNA adducts that cause the many genetic changes known to exist in human lung cancer. The chronic exposure of pulmonary DNA to a multitude of metabolically activated carcinogens is consistent with our current understanding of cancer as a disease resulting from many changes in key genes regulating growth. This review illustrates how this solid foundation of knowledge can be used to find new ways to prevent lung cancer. Three prevention-related topics are discussed: human uptake of tobacco carcinogens as a way of assessing risk and investigating mechanisms; individual differences in the metabolic activation and detoxification of carcinogens, which may relate to cancer susceptibility; and chemoprevention of lung cancer in smokers and ex-smokers. These new approaches are necessary as adjuncts to education and cessation efforts, which despite some success have not eliminated tobacco smoking.

Analysis of O(6)-methylguanine-DNA methyltransferase in melanoma tumours in patients treated with dacarbazine-based chemotherapy

In a retrospective study we analysed the levels of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) in melanoma metastases in patients receiving dacarbazine (DTIC) either as a single drug or as part of combination chemotherapy regimens, and related the expression levels to the clinical response to treatment. Biopsies of subcutaneous and lymph node metastases obtained before chemotherapy in 65 patients with disseminated malignant melanoma were examined for MGMT protein levels by immunohistochemistry using a monoclonal anti-human MGMT antibody. All patients received chemotherapy with DTIC, given either as a single drug or in combination with vindesine and in some cases cisplatin. DTIC as single agent was given to 44 patients, while 21 received combination chemotherapy. Objective responses to chemotherapy were seen in 12 patients, while 53 patients failed to respond to treatment. The expression of MGMT was determined according to the proportion of antibody-stained tumour cells, using a cut-off level of 50%. In 12 of the patients more than one metastasis was analysed, and in seven of these cases the MGMT expression differed between tumours in the same individual. Among the responders a larger proportion (six out of 12, 50%) had tumours containing less than 50% MGMT-positive tumour cells than among the non-responders (12 out of 53, 23%). These data are consistent with the hypothesis that MGMT contributes to resistance to DTIC-based treatment, although the difference between responders and non-responders with respect to the proportion of MGMT-positive tumour cells was not statistically significant (P = 0.077).

Pharmacokinetic study of cystemustine, administered on a weekly schedule in cancer patients

BACKGROUND

Cystemustine is a chloroethylnitrosourea mostly active in humans against glioma and melanoma. The present report describes the results of a new phase I trial with cystemustine administered on a weekly schedule. The pharmacokinetic and pharmacodynamic properties of cystemustine were investigated.

PATIENTS AND METHODS

Forty-three patients entered this study. Cystemustine was administered at dose levels ranging from 30 to 60 mg/m2. The drug was given on days 1, 8, 15 and 22, followed by a 4-week rest period.

RESULTS

Thrombocytopenia was the dose-limiting toxicity and appeared to be reversible, but probably cumulative. This toxicity appeared dose-related, both in frequency and severity. The maximum tolerated dose was 60 mg/m2. Nonhematological toxicity was generally mild. Three partial responses were observed at dose levels of 50 and 60 mg/m2. Pharmacokinetics analysis showed mono- or biphasic cystemustine blood disposition with a mean a half-life of 4 min and mean terminal half-life of 49 min.

CONCLUSIONS

There was a clear linear relationship between the area under the blood drug concentration-time curve (AUC) and the dose of cystemustine (P < 0.001). There was also a significant relationship between the AUC and the toxic effects of cystemustine on platelets, granulocytes and leukocytes (P < 0.001). A reasonable starting dose for phase II studies is 40 mg/m2, with dose escalation based on blood cell counts.

O6-alkylguanine-DNA alkyltransferase: role in carcinogenesis and chemotherapy

The DNA in human cells is continuously undergoing damage as consequences of both endogenous processes and exposure to exogenous agents. The resulting structural changes can be repaired by a number of systems that function to preserve genome integrity. Most pathways are multicomponent, involving incision in the damaged DNA strand and resynthesis using the undamaged strand as a template. In contrast, O(6)-alkylguanine-DNA alkyltransferase is able to act as a single protein that reverses specific types of alkylation damage simply by removing the offending alkyl group, which becomes covalently attached to the protein and inactivates it. The types of damage that ATase repairs are potentially toxic, mutagenic, recombinogenic and clastogenic. They are generated by certain classes of carcinogenic and chemotherapeutic alkylating agents. There is consequently a great deal of interest in this repair system in relation to both carcinogenesis and cancer chemotherapy.

The polymorphic human glutathione transferase T1-1, the most efficient glutathione transferase in the denitrosation and inactivation of the anticancer drug 1,3-bis(2-chloroethyl)-1-nitrosourea

A member of the Theta class of human glutathione transferases (GST T1-1) was found to display the greatest catalytic activity towards the cytostatic drug 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) of the GSTs studied. In this investigation (the most extensive to date), enzymes from four classes of the soluble human GSTs were heterologously expressed, purified, and kinetically characterized. From the 12 enzymes examined, only GST M2-2, GST M3-3 and GST T1-1 had significant activities with BCNU. This establishes that the activity is not a characteristic of a particular class of GSTs. Although GST M3-3 was previously reported to have the greatest activity with BCNU, the current investigation demonstrates that GST M2-2 is equally active and that GST T1-1 has an approximately 20-fold higher specific activity than either of the Mu class enzymes. A more rigorous kinetic analysis of GST T1-1 gave the following parameters with BCNU: a k(cat) of 0.035 +/-0.003s(-1) and a K(M) of 1.0 +/- 0.1mM. The finding that GST T1-1 has the highest activity towards BCNU is significant since GST T1-1 is expressed in the brain, a common target for BCNU treatment. Furthermore, the existence of a GST T1-1 null allele in up to 60% in some populations, may influence both the sensitivity of tumors to chemotherapy and the severity of adverse side-effects in patients treated with this agent.

Overexpression of Ogt reduces MNU and ENU induced transition, but not transversion, mutations in E. coli

Studies of alkylation-induced mutations in Escherichia coli FX-11 revealed that both N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) produced tRNA suppressor mutations (G:C to A:T) but only ENU produced a significant number of backmutations (A:T to G:C, A:T to T:A and A:T to C:G). Further, the ENU-induced transversions were absent in a UmuC-defective strain. This suggested that transition mutations could result from alkylation of guanine or thymine at the O(6)- and O(4)-positions, respectively, but that transversions might result from alkylation of thymine at the O(2)-position. To test this idea, the gene encoding O(6)-alkylguanine-DNA methyltransferase (ogt) was recombined into a plasmid to overexpress the cellular levels of this enzyme. Ogt protein can de-alkylate O(6)-alkylguanine and O(4)-alkylthymine, but not O(2)-alkylthymine. Cells harboring the plasmid (or a control plasmid lacking the ogt gene) were exposed to different concentrations of MNU or ENU and the resulting mutations were analyzed. With either MNU or ENU, the frequency of GlnV(o) suppressors was reduced about 70-fold in the Ogt-overexpressing cells, suggesting that Ogt eliminated O(6)-alkylguanine. Similarly, GlnU(o) suppressor frequencies were substantially reduced. In contrast, the reduction in frequency for the backmutations was slight, only about 2.5-fold with MNU and less than two-fold for ENU. However, DNA sequence analysis of the backmutations showed that only A:T to G:C transitions were affected by overexpression of Ogt, suggesting repair of O(4)-alkylthymine. The frequency of transversions, in comparison, was essentially unaltered. These results implicate O(2)-alkylthymine as a likely candidate for transversion mutagenesis induced by ENU.