Properties of mammalian O6-alkylguanine-DNA transferases

Landscape of Germline Genetic Variants in AGT, MGMT, and TP53 in Mexican Adult Patients with Astrocytoma

Astrocytoma is the most common type of primary brain tumor. The risk factors for astrocytoma are poorly understood; however, germline genetic variants account for 25% of the risk of developing gliomas. In this study, we assessed the risk of astrocytoma associated with variants in AGT, known by its role in angiogenesis, TP53, a well-known tumor suppressor and the DNA repair gene MGMT in a Mexican population. A case-control study was performed in 49 adult Mexican patients with grade II-IV astrocytoma. Sequencing of exons and untranslated regions of AGT, MGMT, and TP53 from was carried in an Ion Torrent platform. Individuals with Mexican Ancestry from the 1000 Genomes Project were used as controls. Variants found in our cohort were then assessed in a The Cancer Genome Atlas astrocytoma pan-ethnic validation cohort. Variants rs1926723 located in AGT (OR 2.74, 1.40-5.36 95% CI), rs7896488 in MGMT (OR 3.43, 1.17-10.10 95% CI), and rs4968187 in TP53 (OR 2.48, 1.26-4.88 95% CI) were significantly associated with the risk of astrocytoma after multiple-testing correction. This is the first study where the AGT rs1926723 variant, TP53 rs4968187, and MGMT rs7896488 were found to be associated with the risk of developing an astrocytoma.

Mechanisms of Genome Maintenance in Plants: Playing It Safe With Breaks and Bumps

Maintenance of genomic integrity is critical for the perpetuation of all forms of life including humans. Living organisms are constantly exposed to stress from internal metabolic processes and external environmental sources causing damage to the DNA, thereby promoting genomic instability. To counter the deleterious effects of genomic instability, organisms have evolved general and specific DNA damage repair (DDR) pathways that act either independently or mutually to repair the DNA damage. The mechanisms by which various DNA repair pathways are activated have been fairly investigated in model organisms including bacteria, fungi, and mammals; however, very little is known regarding how plants sense and repair DNA damage. Plants being sessile are innately exposed to a wide range of DNA-damaging agents both from biotic and abiotic sources such as ultraviolet rays or metabolic by-products. To escape their harmful effects, plants also harbor highly conserved DDR pathways that share several components with the DDR machinery of other organisms. Maintenance of genomic integrity is key for plant survival due to lack of reserve germline as the derivation of the new plant occurs from the meristem. Untowardly, the accumulation of mutations in the meristem will result in a wide range of genetic abnormalities in new plants affecting plant growth development and crop yield. In this review, we will discuss various DNA repair pathways in plants and describe how the deficiency of each repair pathway affects plant growth and development.

MGMT Epigenetics: The Influence of Gene Body Methylation and Other Insights Derived from Integrated Methylomic, Transcriptomic, and Chromatin Analyses in Various Cancer Types

BACKGROUND

MGMT (O6-methylguanine-DNA methyltransferase) is primarily responsible for limiting the activity of some widely used chemotherapeutic agents, including temozolomide (TMZ) and carmustine (BCNU). The gene encoding this protein is epigenetically regulated, and assessment of methylation at its promoter region is used to predict glioma patients' response to TMZ.

METHODS

In this report, we employed a bioinformatic approach to elucidate MGMT's epigenetic regulation. Integrated for the analysis were genome-wide methylation and transcription datasets for > 8,600 human tissue (representing 31 distinct cancer types) and 500 human cancer cell line samples. Also crucial to the interpretation of results were publicly available data from the ENCODE Project: tracks for histone modifications (via ChIP-seq) and DNase I hypersensitivity (via DNaseseq), as well as methylation and transcription data for representative cell lines (HeLa-S3, HMEC, K562).

RESULTS AND DISCUSSION

We were able to validate (perhaps more comprehensively) the contrasting influences of CpG methylation at promoter region and at gene body on MGMT transcription. While the MGMT promoter is populated by CpG sites whose methylation levels displayed high negative correlation (R) with MGMT mRNA counts, the gene body harbors CpG sites exhibiting high positive R values. The promoter CpG sites with very high negative R's across cancer types include cg12981137, cg12434587, and cg00618725. Among the notable gene body CpG sites (high positive R's across cancer types) are cg00198994 (Intron 1), cg04473030 (Intron 2), and cg07367735 (Intron 4). For certain cancer types, such as melanoma, gene body methylation appears to be a better predictor of MGMT transcription (compared to promoter methylation). In general, the CpG methylation v. MGMT expression R values are higher in cell lines relative to tissues. Also, these correlations are noticeably more prominent in certain cancer types such as colorectal, adrenocortical, esophageal, skin, and head and neck cancers, as well as glioblastoma. As expected, hypomethylation at the promoter region is associated with more open chromatin, and enrichment of histone marks H3K4m1, H3K4m2, H3K4m3, and H3K9ac.

CONCLUSION

Overall, our analysis illustrated the contrasting influence of promoter and gene body methylation on MGMT expression. These observations may help improve diagnostic assays for MGMT.

In silico profiling and structural insights of zinc metal ion on O6-methylguanine methyl transferase and its interactions using molecular dynamics approach

O6-methylguanine DNA methyl transferase (MGMT) is a metalloenzyme participating in the repair of alkylated DNA. In this research, we performed a comparative study for evaluating the impact of zinc metal ion on the behavior and interactions of MGMT in the both enzymatic forms of apo MGMT and holo MGMT. DNA and proliferating cell nuclear antigen (PCNA), as partners of MGMT, were utilized to evaluate molecular interactions by virtual microscopy of molecular dynamics simulation. The stability and conformational alterations of each forms (apo and holo) MGMT-PCNA, and (apo and holo) MGMT-DNA complexes were calculated by MM/PBSA method. A total of seven systems including apo MGMT, holo MGMT, free PCNA, apo MGMT-PCNA, holo MGMT-PCNA, apo MGMT-DNA, and holo MGMT-DNA complexes were simulated. In this study, we found that holo MGMT was more stable and had better folding and functional properties than that of apo MGMT. Simulation analysis of (apo and holo) MGMT-PCNA complexes displayed that the sequences of the amino acids involved in the interactions were different in the two forms of MGMT. The important amino acids of holo MGMT involved in its interaction with PCNA included E92, K101, A119, G122, N123, P124, and K125, whereas the important amino acids of apo MGMT included R128, R135, S152, N157, Y158, and L162. Virtual microscopy of molecular dynamics simulation showed that the R128 and its surrounding residues were important amino acids involved in the interaction of holo MGMT with DNA that was exactly consistent with X-ray crystallography structure. In the apo form of the protein, the N157 and its surrounding residues were important amino acids involved in the interaction with DNA. The binding free energies of - 387.976, - 396.226, - 622.227, and - 617.333 kcal/mol were obtained for holo MGMT-PCNA, apo MGMT-PCNA, holo MGMT-DNA, and apo MGMT-DNA complexes, respectively. The principle result of this research was that the area of molecular interactions differed between the two states of MGMT. Therefore, in investigations of metalloproteins, the metal ion must be preserved in their structures. Finally, it is recommended to use the holo form of metalloproteins in in vitro and in silico researches.

Repair and translesion synthesis of O 6-alkylguanine DNA lesions in human cells

O ⁶-alkyl-2'-deoxyguanosine (O ⁶-alkyl-dG) lesions are among the most mutagenic and prevalent alkylated DNA lesions that are associated with cancer initiation and progression. In this study, using a shuttle vector-based strand-specific PCR-competitive replication and adduct bypass assay in conjunction with tandem MS for product identification, we systematically assessed the repair and replicative bypass of a series of O ⁶-alkyl-dG lesions, with the alkyl group being a Me, Et, nPr, iPr, nBu, iBu, or sBu, in several human cell lines. We found that the extent of replication-blocking effects of these lesions is influenced by the size of the alkyl groups situated on the O ⁶ position of the guanine base. We also noted involvement of distinct DNA repair pathways and translesion synthesis polymerases (Pols) in ameliorating the replication blockage effects elicited by the straight- and branched-chain O ⁶-alkyl-dG lesions. We observed that O ⁶-methylguanine DNA methyltransferase is effective in removing the smaller alkyl groups from the O ⁶ position of guanine, whereas repair of the branched-chain lesions relied on nucleotide excision repair. Moreover, these lesions were highly mutagenic during cellular replication and exclusively directed G→A mutations; Pol η and Pol ζ participated in error-prone bypass of the straight-chain lesions, whereas Pol κ preferentially incorporated the correct dCMP opposite the branched-chain lesions. Together, these results uncover key cellular proteins involved in repair and translesion synthesis of O ⁶-alkyl-dG lesions and provide a better understanding of the roles of these types of lesions in the etiology of human cancer.

Temozolomide and Pituitary Tumors: Current Understanding, Unresolved Issues, and Future Directions

Temozolomide, an alkylating agent, initially used in the treatment of gliomas was expanded to include pituitary tumors in 2006. After 12 years of use, temozolomide has shown a notable advancement in pituitary tumor treatment with a remarkable improvement rate in the 5-year overall survival and 5-year progression-free survival in both aggressive pituitary adenomas and pituitary carcinomas. In this paper, we review the mechanism of action of temozolomide as alkylating agent, its interaction with deoxyribonucleic acid repair systems, therapeutic effects in pituitary tumors, unresolved issues, and future directions relating to new possibilities of targeted therapy.

The proline rich domain of p53 is dispensable for MGMT-dependent DNA repair and cell survival following alkylation damage

In addition to promoting cell death and senescence, p53 also has important cellular survival functions. A mutant p53, lacking a proline-rich domain (p53^ΔP), that is deficient in controlling both cell death and cell cycle arrest, was employed to determine the biological means by which p53 mediates survival upon DNA damage. While p53^ΔP and p53^-/- cells were equally resistant to many DNA damaging agents, p53^ΔP cells showed an exquisite resistance to high doses of the alkylating agent Diazald (N-Methyl-N-(p-tolylsulfonyl)nitrosamide), as compared to cells completely deficient for p53 function. We determined that p53^ΔP was capable of transcribing the repair gene, MGMT (O6-methylguanine-DNA methyltransferase) after irradiation or alkylation damage, resulting in DNA repair and cell survival. Consistent with these observations, p53^ΔP mice show enhanced survival after IR relative to p53^-/- mice. Suppression or deletion of MGMT expression in p53^ΔP cells inhibited DNA repair and survival after alkylation damage, whereas MGMT overexpression in p53-deficient cells facilitated DNA repair and conferred survival advantage. This study shows that when cell death and cell cycle arrest pathways are inhibited, p53 can still mediate MGMT-dependent repair, to promote cell survival upon DNA damage.

Repair of O6-methylguanine adducts in human telomeric G-quadruplex DNA by O6-alkylguanine-DNA alkyltransferase

O⁶-alkylguanine-DNA alkyltransferase (AGT) is a single-cycle DNA repair enzyme that removes pro-mutagenic O⁶-alkylguanine adducts from DNA. Its functions with short single-stranded and duplex substrates have been characterized, but its ability to act on other DNA structures remains poorly understood. Here, we examine the functions of this enzyme on O⁶-methylguanine (6mG) adducts in the four-stranded structure of the human telomeric G-quadruplex. On a folded 22-nt G-quadruplex substrate, binding saturated at 2 AGT:DNA, significantly less than the ∼5 AGT:DNA found with linear single-stranded DNAs of similar length, and less than the value found with the telomere sequence under conditions that inhibit quadruplex formation (4 AGT:DNA). Despite these differences, AGT repaired 6mG adducts located within folded G-quadruplexes, at rates that were comparable to those found for a duplex DNA substrate under analogous conditions. Repair was kinetically biphasic with the amplitudes of rapid and slow phases dependent on the position of the adduct within the G-quadruplex: in general, adducts located in the top or bottom tetrads of a quadruplex stack exhibited more rapid-phase repair than did adducts located in the inner tetrad. This distinction may reflect differences in the conformational dynamics of 6mG residues in G-quadruplex DNAs.

6-Carboxyfluorescein and structurally similar molecules inhibit DNA binding and repair by O⁶-alkylguanine DNA alkyltransferase

Human O⁶-alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O⁶-alkylguanine and O⁴-alkylthymine adducts in single-stranded and duplex DNAs. These activities protect normal cells and tumor cells against drugs that alkylate DNA; drugs that inactivate AGT are under test as chemotherapeutic enhancers. In studies using 6-carboxyfluorescein (FAM)-labeled DNAs, AGT reduced the fluorescence intensity by ∼40% at binding saturation, whether the FAM was located at the 5' or the 3' end of the DNA. AGT protected residual fluorescence from quenching, indicating a solute-inaccessible binding site for FAM. Sedimentation equilibrium analyses showed that saturating AGT-stoichiometries were higher with FAM-labeled DNAs than with unlabeled DNAs, suggesting that the FAM provides a protein binding site that is not present in unlabeled DNAs. Additional fluorescence and sedimentation measurements showed that AGT forms a 1:1 complex with free FAM. Active site benzylation experiments and docking calculations support models in which the primary binding site is located in or near the active site of the enzyme. Electrophoretic analyses show that FAM inhibits DNA binding (IC₅₀∼76μM) and repair of DNA containing an O⁶-methylguanine residue (IC₅₀∼63μM). Similar results were obtained with other polycyclic aromatic compounds. These observations demonstrate the existence of a new class of non-covalent AGT-inhibitors. After optimization for binding-affinity, members of this class might be useful in cancer chemotherapy.

Mutations that probe the cooperative assembly of O⁶-alkylguanine-DNA alkyltransferase complexes

O(6)-Alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O(6)-alkylguanine and O(4)-alkylthymine adducts present in DNA that has been exposed to alkylating agents. AGT binds DNA cooperatively, and models of cooperative complexes predict that residues 1-7 of one protein molecule and residues 163-169 of a neighboring protein are closely juxtaposed. To test these models, we used directed mutagenesis to substitute triplets of alanine for triplets of native residues across these two sequences. Six of eight designed mutants expressed AGT at detectable levels. All mutant AGTs that were expressed were folded compactly, bound DNA with stoichiometries equivalent to that of the wild-type protein, and were able to protect Escherichia coli to varying degrees from the potent alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). All mutations attenuated DNA binding cooperativity, but unexpectedly, they also reduced the affinity of AGT for DNA. This suggests that the protein-protein and protein-DNA interactions of AGT are strongly coupled. When normalized for differences in AGT expression, cells expressing mutants KDC(3-5)-AAA, DCE(4-6)-AAA, and KEW(165-167)-AAA were significantly more susceptible to MNNG than wild-type cells. This is the first evidence, to the best of our knowledge, of a role for residues at the protein-protein interface and, by implication, cooperative protein-protein interactions in the cell-protective mechanisms of AGT.

Tumor O(6)-methylguanine-DNA methyltransferase inactivation by oral lomeguatrib

PURPOSE

A major mechanism of resistance to chlorethylnitrosureas and methylating agents involves the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT). We sought to determine the dose of oral 6-(4-bromo-2-thienyl) methoxy purin-2-amine (lomeguatrib), a pseudosubstrate inactivator of MGMT, required to render active protein undetectable 12 hours after dosing in prostate, primary central nervous system (CNS), and colorectal cancer patients.

EXPERIMENTAL DESIGN

Lomeguatrib was administered orally as a single dose (20-160 mg) approximately 12 hours before tumor resection. Dose escalation was projected to continue until grade 2 toxicity or until complete inactivation of tumor MGMT was encountered. Total MGMT protein levels were quantified by ELISA, and active protein levels were quantified by biochemical assay. MGMT promoter methylation was determined in glioblastoma DNA by methylation-specific PCR.

RESULTS

Thirty-seven patients were dosed with lomeguatrib, and 32 informative tumor samples were obtained. Mean total MGMT level varied between tumor types: 554 +/- 404 fmol/mg protein (+/-SD) for prostate cancer, 87.4 +/- 40.3 fmol/mg protein for CNS tumors, and 244 +/- 181 fmol/mg protein for colorectal cancer. MGMT promoter hypermethylation did not correlate with total protein expression. Consistent total MGMT inactivation required 120 mg of lomeguatrib in prostate and colorectal cancers. Complete consistent inactivation in CNS tumors was observed only at the highest dose of lomeguatrib (160 mg).

CONCLUSIONS

Total MGMT inactivation can be achieved in prostate, primary CNS, and colorectal cancers with a single administration of 120 or 160 mg lomeguatrib. The dose needed did not correlate with mean total MGMT protein concentrations. One hundred twenty to 160 mg/d of lomeguatrib should be administered to achieve total MGMT inactivation in future studies.

Prognostic and predictive value of p53 in low MGMT expressing glioblastoma treated with surgery, radiation and adjuvant temozolomide chemotherapy

OBJECTIVE

To assess the prognostic and predictive significance of p53 protein expression in low O6-methylguanine-DNA methyltransferase (MGMT) expressing glioblastoma multiform (GBM) treated with combined therapy.

METHODS

The authors reviewed the clinical outcomes of 46 low MGMT expressing GBM patients who had undergone surgery, conventional local radiotherapy and temozolomide chemotherapy. Correlation between p53 expression level and clinical outcomes were analysed with univariate and multivariate Cox model.

RESULTS

Patients with low p53 expression had a significantly improved progression free survival (PFS) (p=0.015) and overall survival (OS) (p=0.047) compared to those with high expression. On both univariate and multivariate analyses, low p53 expression persisted as a significant independent favorable prognostic factor for PFS (p=0.017). Pre-operative Karnofsky performance status score (p=0.029), tumor resection extent (p=0.045) and p53 expression level (p=0.038) were significant independent prognostic factors for OS.

CONCLUSION

In these low MGMT expressing GBM patients with combined treatment, low p53 expression was a significant independent favorable prognostic factor for both PFS and OS. In addition to MGMT, p53 may be another stratification variable in the future therapeutic trials.

IFN-beta sensitizes neuroblastoma to the antitumor activity of temozolomide by modulating O6-methylguanine DNA methyltransferase expression

Although temozolomide has shown clinical activity against neuroblastoma, this activity is likely limited by the DNA repair enzyme O6-methylguanine DNA methyltransferase (MGMT). We hypothesized that IFN-beta could sensitize neuroblastoma cells to the cytotoxic effects of temozolomide through its ability to down-regulate MGMT expression. In vitro proliferation of three neuroblastoma cell lines treated with IFN-beta and temozolomide alone or in combination was examined. Antitumor activity was assessed in both localized and disseminated neuroblastoma xenografts using single-agent and combination therapy, with continuous delivery of IFN-beta being established by a liver-targeted adeno-associated virus-mediated approach. Two neuroblastoma cell lines (NB-1691 and SK-N-AS) were found to have high baseline levels of MGMT expression, whereas a third cell line (CHLA-255) had low levels. Temozolomide had little effect on in vitro proliferation of the neuroblastoma cell lines with high MGMT expression, but pretreatment with IFN-beta significantly decreased MGMT expression and cell counts (NB-1691: 36 +/- 3% of control, P = 0.0008; SK-N-AS: 54 +/- 7% control, P = 0.003). In vivo, NB-1691 tumors in CB17-SCID mice treated with the combination of IFN-beta and temozolomide had lower MGMT expression and a significantly reduced tumor burden, both localized [percent initial tumor volume: 2,516 +/- 680% (control) versus 1,272 +/- 330% (temozolomide), P = 0.01; 1,348 +/- 220%, P = 0.03 (IFN-beta); 352 +/- 110%, P = 0.0001 (combo)] and disseminated [bioluminescent signal: control (1.32e10 +/- 6.5e9) versus IFN-beta (2.78e8 +/- 3.09e8), P = 0.025, versus temozolomide (2.06e9 +/- 1.55e9), P = 0.1, versus combination (2.13e7 +/- 7.67e6), P = 0.009]. IFN-beta appears to sensitize neuroblastoma cells to the cytotoxic effects of temozolomide through attenuation of MGMT expression. Thus, IFN-beta and temozolomide may be a useful combination for treating children with this difficult disease.

BCNU-sequestration by metallothioneins may contribute to resistance in a medulloblastoma cell line

PURPOSE

Resistance of neoplastic cells to the alkylating drug BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea] has been correlated with expression of O (6)-methylguanine-DNA methyltransferase, which repairs the O (6)-chloroethylguanine produced by the drug. Other possible mechanisms of resistance include raised levels of glutathione or increased repair of the DNA interstrand cross-links formed by BCNU. Transcriptional profiling revealed the upregulation of several metallothionein (MT) genes in a BCNU-resistant medulloblastoma cell line [D341 MED (OBR)] relative to its parental line. Previous studies have shown that MTs, through their reactive thiol groups can quench nitrogen mustard-derived alkylating drugs. In this report, we evaluate whether MTs can also quench BCNU.

METHODS

To demonstrate the binding of BCNU to MT, we used an assay that measured the release of the MT-bound divalent cations (Zn(2+), Cd(2+)) upon their displacement by the drug. We also measured the decomposition rates of BCNU at those reaction conditions.

RESULTS

The rate of release of the cations was higher in pH 7.4 than at pH 7.0, which is likely a result of more rapid decomposition of BCNU (thus faster release of MT-binding intermediate) at pH 7.4 than at pH 7.0.

CONCLUSION

We demonstrate that resistance to BCNU may be a result of elevated levels of MTs which act by sequestering the drug's decomposition product(s).

Inhibition of the 5' to 3' exonuclease activity of hEXO1 by 8-oxoguanine

The mismatch repair pathway is responsible for maintaining genomic stability by correcting base-base mismatches and insertion/deletion loops that arise mainly via replication errors. Additionally, the mismatch repair pathway performs a central role in the cellular response to both alkylation and reactive oxygen species induced DNA damage. An important step in mismatch processing is the recruitment of hEXO1, a 5' to 3' exonuclease, by hMSH2-hMSH6 to remove the nascent DNA strand. However, very little is currently known about the capacity of hEXO1 to exonucleolytically process damaged DNA bases. Therefore, we examined whether hEXO1 can degrade double-stranded DNA substrates containing alkylated or oxidized nucleotides. Our results demonstrated that hEXO1 is capable of degrading duplex DNA containing an O6-methylguanine (O6-meG) adduct paired with either a C or a T. Additionally, the hMSH2-hMSH6 complex stimulated hEXO1 exonuclease activity on the O6-meG/T and O6-meG/C DNA substrates. In contrast, hEXO1 exonuclease activity was significantly blocked by the presence of an 8-oxoguanine adduct in both single and double stranded DNA substrates. Further, hMSH2-hMSH6 was not able to alleviate the nucleolytic block caused by the 8-oxoguanine adduct in heteroduplex DNA.

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.

Kinetics of O(6)-methyl-2'-deoxyguanosine repair by O(6)-alkylguanine DNA alkyltransferase within K-ras gene-derived DNA sequences

O(6)-Methyl-2'-deoxyguanosine (O(6)-Me-dG) is a potent mutagenic DNA adduct that can be induced by a variety of methylating agents, including tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). O(6)-Me-dG is directly repaired by the specialized DNA repair protein, O(6)-alkylguanine DNA alkyltransferase (AGT), which transfers the O(6)-alkyl group from the modified guanine to a cysteine thiol within the active site of the protein. Previous investigations suggested that AGT repair of O(6)-alkylguanines may be sequence-dependent as a result of flanking nucleobase effects on DNA conformation and energetics. In the present work, a novel high-performance/pressure liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI+-MS/MS)-based approach was developed to analyze the kinetics of AGT-mediated repair of O(6)-Me-dG adducts placed at different sites within the double-stranded DNA sequence representing codons 8-17 of the K-ras protooncogene, 5'-G1TA G2TT G3G4A G5CT G6G7T G8G9C G10TA G11G12C AAG13 AG14T-3', where G5, G6, G7, G8, G9, G10, or G11 was replaced with O(6)-Me-dG. The second guanine of K-ras codon 12 (G7 in our numbering system) is a major mutational hotspot for G --> A transitions observed in lung tumors of smokers and in neoplasms induced in laboratory animals by exposure to methylating agents. O(6)-Me-dG-containing duplexes were incubated with human recombinant AGT protein, and the reactions were quenched at specific times. Following acid hydrolysis to release purines, isotope dilution HPLC-ESI-MS/MS was used to determine the amounts of O(6)-Me-G remaining in DNA. The relative extent of demethylation for O(6)-Me-dG adducts located at G5, G6, G7, G8, G9, G10, or G11 following a 10 s incubation with AGT showed little variation as a function of sequence position. Furthermore, the second-order rate constants for the repair of O(6)-Me-dG adducts located at the first and second positions of the K-ras codon 12 (5'-G6G7T-3') were similar (1.4 x 10(7) M(-1) s(-1) vs 7.4 x 10(6) M(-1) s(-1), respectively), suggesting that O(6)-Me-dG repair by AGT is not the determining factor for K-ras codon 12 mutagenesis following exposure to methylating agents. The new HPLC-ESI-MS/MS assay developed in this work is a valuable tool which will be used to further explore the role of local sequence environment and endogenous DNA modifications in shaping mutational spectra of NNK and other methylating agents.

Lomeguatrib, a potent inhibitor of O6-alkylguanine-DNA-alkyltransferase: phase I safety, pharmacodynamic, and pharmacokinetic trial and evaluation in combination with temozolomide in patients with advanced solid tumors

PURPOSE

A major mechanism of resistance to temozolomide involves the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (ATase). The main aims of this phase I trial were to determine an ATase-depleting dose (ADD) of lomeguatrib, a potent pseudosubstrate inhibitor, and to define a suitable dose of temozolomide to be used in combination with lomeguatrib in patients with advanced cancer.

EXPERIMENTAL DESIGN

Lomeguatrib was administered at dose levels of 10 to 40 mg/m2 days 1 to 5, as a single agent, and also in combination with temozolomide. Once the ADD of lomeguatrib was identified, the dose of temozolomide in combination was increased, in successive patient cohorts, from 50 to 175 mg/m2 on days 1 to 5 of a 28-day cycle to define the maximal tolerated dose and dose-limiting toxicity of the combination.

RESULTS

Thirty-eight patients with advanced solid tumors were enrolled. More than 95% ATase depletion within 4 hours of the first dose was achieved in peripheral blood mononuclear cells at lomeguatrib doses of > or =10 mg/m2/d i.v. or > or =20 mg/m2/d orally, and tumor biopsies showed > or =92% ATase depletion. At the ADD of lomeguatrib i.v., the maximal tolerated dose of temozolomide in combination was 150 mg/m2 days 1 to 5. The dose limiting toxicity of the combination of lomeguatrib and temozolomide was myelosuppression. The toxicity of lomeguatrib alone was minimal. In 23 patients with measurable disease, one complete response was seen and 12 patients had stable disease for at least 3 months.

CONCLUSION

This first administration of lomeguatrib to man successfully established an oral ADD of lomeguatrib and identified a combination regimen with temozolomide suitable for future clinical evaluation.

O6-methylguanine DNA methyltransferase and p53 status predict temozolomide sensitivity in human malignant glioma cells

Temozolomide (TMZ) is a methylating agent which prolongs survival when administered during and after radiotherapy in the first-line treatment of glioblastoma and which also has significant activity in recurrent disease. O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme attributed a role in cancer cell resistance to O6-alkylating agent-based chemotherapy. Using a panel of 12 human glioma cell lines, we here defined the sensitivity to TMZ in acute cytotoxicity and clonogenic survival assays in relation to MGMT, mismatch repair and p53 status and its modulation by dexamethasone, irradiation and BCL-X(L). We found that the levels of MGMT expression were a major predictor of TMZ sensitivity in human glioma cells. MGMT activity and clonogenic survival after TMZ exposure are highly correlated (p < 0.0001, r2 = 0.92). In contrast, clonogenic survival after TMZ exposure does not correlate with the expression levels of the mismatch repair proteins mutS homologue 2, mutS homologue 6 or post-meiotic segregation increased 2. The MGMT inhibitor O6-benzylguanine sensitizes MGMT-positive glioma cells to TMZ whereas MGMT gene transfer into MGMT-negative cells confers protection. The antiapoptotic BCL-X(L) protein attenuates TMZ cytotoxicity in MGMT-negative LNT-229 but not in MGMT-positive LN-18 cells. Neither ionizing radiation (4 Gy) nor clinically relevant concentrations of dexamethasone modulate MGMT activity or TMZ sensitivity. Abrogation of p53 wild-type function strongly attenuates TMZ cytotoxicity. Conversely, p53 mimetic agents designed to stabilize the wild-type conformation of p53 sensitize glioma cells for TMZ cytotoxicity. Collectively, these results suggest that the determination of MGMT expression and p53 status will help to identify glioma patients who will or will not respond to TMZ.

Brain tumor cell lines resistant to O6-benzylguanine/1,3-bis(2-chloroethyl)-1-nitrosourea chemotherapy have O6-alkylguanine-DNA alkyltransferase mutations

The chemotherapeutic activity of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU or carmustine) may be improved by the addition of O6-benzylguanine (O6-BG). The reaction of O6-BG with O6-alkylguanine-DNA alkyltransferase (AGT) prevents the repair of O6-chloroethyl lesions caused by BCNU. In clinics, the combination of O6-BG and BCNU is now being tested for the treatment of brain tumors. However, the effectiveness of this drug regimen may be limited by drug resistance acquired during treatment. To understand the possible mechanisms of resistance of brain tumor cells to the O6-BG/BCNU combination, we generated medulloblastoma cell lines (D283 MED, D341 MED, and Daoy) resistant to the combination of O6-BG and BCNU [O6-BG/BCNU resistant (OBR)]. DNA sequencing showed that all of the parent cell lines express wild-type AGTs, whereas every OBR cell line exhibited mutations that potentially affected the binding of O6-BG to the protein as evidenced previously by in vitro mutagenesis and structural studies of AGT. The D283 MED (OBR), Daoy (OBR), and D341 MED (OBR) cell lines expressed G156C, Y114F, and K165T AGT mutations, respectively. We reported previously that rhabdomyosarcoma TE-671 (OBR) also expresses a G156C mutation. These data suggest that the clonal selection of AGT mutants during treatment with O6-BG plus an alkylator may produce resistance to this intervention in clinical settings.

DNA damage and repair: consequences on dose-responses

Damage to DNA is considered to be the main initiating event by which genotoxins cause hereditary effects and cancer. Single or double strand breaks, bases modifications or deletions, intra- or interstrand DNA-DNA or DNA-protein cross-links constitute the major lesions formed in different proportions according to agents and to DNA sequence context. They can result in cell death or in mutational events which in turn may initiate malignant transformation. Normal cells are able to repair these lesions with fidelity or by introducing errors. Base excision (BER) and nucleotide excision (NER) repair are error-free processes acting on the simpler forms of DNA damage. A specialized form of BER involves the removal of mismatched DNA bases occurring as errors of DNA replication or from miscoding properties of damaged bases. Severe damage will be repaired according to several types of recombinational processes: homologous, illegitimate and site-specific recombination pathways. The loss of repair capacity as seen in a number of human genetic diseases and mutant cell lines leads to hypersensitivity to environmental agents. Repair-defective cells show qualitative (mutation spectrum) and quantitative alterations in dose-effect relationships. For such repair-deficient systems, direct measurements at low doses are possible and the extrapolation from large to low doses fits well with the linear or the linear-quadratic no-threshold models. Extensive debate still takes place as to the shape of the dose-response relationships in the region at which genetic effects are not directly detectable in repair-proficient normal cells. Comparison of repair mutants and wild-type organisms pragmatically suggests that, for many genotoxins and tissues, very low doses may have no effect at all in normal cells.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

The in vitro influence of sulfated bis-lactobionic acid amides on O6-alkylguanine-DNA alkyltransferase, DNase I, nucleic acid synthesis and chromatin structure

The influence of four sulfated bis-lactobionic acid amides (BLAA) of molecular weights between 2388 and 2514 on O6-alkylguanine-DNA alkyltransferase (AT), DNase I and nucleic acid synthesis as well as on nucleoid sedimentation and the viscosity of alkaline lysates of chicken embryo cells was studied in vitro. The activities of AT and DNase I were inhibited by BLAA in a dose-dependent manner. Depending on the polyanion used, concentrations depleting AT activity by 50% ranged between 3.5 and 7.0 microM, whereas BLAA concentrations of almost 250-320 microM were needed to halve DNase I activity. At concentrations above 8 microM, BLAA decreased scheduled DNA synthesis in a dose-dependent fashion whereas RNA synthesis remained unchanged even at the highest BLAA concentrations used (2 mM). In chicken embryo brain cells BLAA exerted a biphasic effect on the nucleoid sedimentation and the viscosity of alkaline cell lysates reflecting a decrease in chromatin compactness at lower BLAA concentrations (10-100 microM) and an increase in chromatin compactness at higher polyanion concentrations (> or = 200 microM). The remarkably high sensitivity of the nuclear enzyme AT deserves further investigation in regard to the fate of the polyanions within cells and tissues.

Mammalian cells share a common pathway for the relief of DNA replication arrest by O6-alkyl guanine, incorporated 6-thioguanine and UV photoproducts

We previously reported the cloning of a mammalian gene that restores UV resistance to a postreplication recovery defective and mex- Indian muntjac mutant cell line, SVM, by improving daughter-strand DNA replication on a UV-damaged template. The improved replication was, however, found to be error-prone, as judged by a hypermutable phenotype (Bouffler et al. (1990) Somatic Cell Mol. Genet., 16, 507-516). We now report that this gene also increases the resistance of SVM to the cytotoxic effects of methyl- and ethyl-nitrosourea, though not to dimethyl sulphate, by a similar postreplication recovery process. The gene does not increase the activity of O6-alkylguanine-DNA-alkyltransferase in the cell. We conclude that at least one mechanism of postreplication recovery in mammalian cells allows UV photoproducts and O6-alkylguanine lesions to be tolerated by the replication complex. The fact that the gene also confers resistance to 6-thioguanine suggests that, once incorporated, this base analogue can disrupt normal DNA replication and that a single mechanism can allow replication to proceed beyond 3 diverse DNA lesions.

A mutational hotspot in the aprt gene of Chinese hamster cells

Early work with adenine phosphoribosyltransferase-deficient mutants of CHO cells suggested that a site existed in the third exon of this gene which was preferentially susceptible to mutation by ethyl methanesulphonate. To determine whether this was real we analysed a large collection of induced mutants, and generated a high-density mutational spectrum for this exon. In addition, 4 sites outside exon 3 were analysed by blot. 37 mutations were found in 19 available sites, six of which were at nucleotide 1365, 1 of 2 sites in the putative hotspot (P less than 0.02). One other site, 1308, also was mutated in 6 cell lines and may also be preferentially mutable.

Inhibition of O6-alkylguanine-DNA alkyltransferase and DNase I activities in vitro by some alkylating substances and antineoplastic agents

The specificities of the DNA repair enzyme O6-alkylguanine-DNA alkyltransferase from brain and liver cells of the chick embryo and of DNase I were demonstrated in vitro by their response to substrate DNA pretreated with monofunctional alkylating agents of different O6-guanine alkylating ability and some antineoplastic agents. Treatment of DNA with ethidium bromide, Hoechst 33258, doxorubicin, Fe2+/bleomycin, and suramin resulted in a dose-dependent diminution of alkyltransferase activity (DE50 approximately 5 micrograms/ml, 15 micrograms/ml, 5 micrograms/ml, 5 micrograms/ml, 100 micrograms/ml, respectively). Apart from bleomycin, comparable results were obtained with DNase I. Thermal denaturation of the substrate DNA reduced both alkyltransferase and DNase I activity. No effect was seen with X-irradiation. Cisplatin decreased only DNase I activity. Some topoisomerase II and/or gyrase inhibitors remained without significant effects on the alkyltransferase reaction whereas DNA catabolism by DNase I was diminished in a dose-dependent manner (DE50 between 6.5 and 19 micrograms/ml).