1
|
Paustenbach DJ, Brown SE, Heywood JJ, Donnell MT, Eaton DL. Risk characterization of N-nitrosodimethylamine in pharmaceuticals. Food Chem Toxicol 2024; 186:114498. [PMID: 38341171 DOI: 10.1016/j.fct.2024.114498] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2023] [Revised: 01/23/2024] [Accepted: 01/31/2024] [Indexed: 02/12/2024]
Abstract
Since 2018, N-nitrosodimethylamine (NDMA) has been a reported contaminant in numerous pharmaceutical products. To guide the pharmaceutical industry, FDA identified an acceptable intake (AI) of 96 ng/day NDMA. The approach assumed a linear extrapolation from the Carcinogenic Potency Database (CPDB) harmonic-mean TD50 identified in chronic studies in rats. Although NDMA has been thought to act as a mutagenic carcinogen in experimental animals, it has not been classified as a known human carcinogen by any regulatory agency. Humans are exposed to high daily exogenous and endogenous doses of NDMA. Due to the likelihood of a threshold dose for NDMA-related tumors in animals, we believe that there is ample scientific basis to utilize the threshold-based benchmark dose or point-of-departure (POD) approach when estimating a Permissible Daily Exposure limit (PDE) for NDMA. We estimated that 29,000 ng/kg/day was an appropriate POD for calculating a PDE. Assuming an average bodyweight of 50 kg, we expect that human exposures to NDMA at doses below 5800 ng/day in pharmaceuticals would not result in an increased risk of liver cancer, and that there is little, if any, risk for any other type of cancer, when accounting for the mode-of-action in humans.
Collapse
Affiliation(s)
- D J Paustenbach
- Paustenbach and Associates, 970 West Broadway, Suite E, Jackson, WY, USA
| | - S E Brown
- Paustenbach and Associates, 207 Canyon Blvd, Boulder, CO, USA.
| | - J J Heywood
- Paustenbach and Associates, 207 Canyon Blvd, Boulder, CO, USA
| | - M T Donnell
- Valeo Sciences LLC, 333 Corporate Drive, Suite 130, Ladera Ranch, CA, USA
| | - D L Eaton
- Professor Emeritus, Department of Environmental & Occupational Health Sciences, University of Washington, Seattle, WA, USA
| |
Collapse
|
2
|
Leelatian N, Hong CS, Bindra RS. The Role of Mismatch Repair in Glioblastoma Multiforme Treatment Response and Resistance. Neurosurg Clin N Am 2021; 32:171-180. [PMID: 33781500 DOI: 10.1016/j.nec.2020.12.009] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Mismatch repair (MMR) is a highly conserved DNA repair pathway that is critical for the maintenance of genomic integrity. This pathway targets base substitution and insertion-deletion mismatches, which primarily arise from replication errors that escape DNA polymerase proof-reading function. Here, the authors review key concepts in the molecular mechanisms of MMR in response to alkylation damage, approaches to detect MMR status in the clinic, and the clinical relevance of this pathway in glioblastoma multiforme treatment response and resistance.
Collapse
Affiliation(s)
- Nalin Leelatian
- Department of Pathology, Yale School of Medicine, 310 Cedar Street LH 108, New Haven, CT 06510, USA
| | - Christopher S Hong
- Department of Neurosurgery, Yale School of Medicine, 333 Cedar Street Tompkins 4, New Haven, CT 06510, USA
| | - Ranjit S Bindra
- Department of Therapeutic Radiology, Yale School of Medicine, 333 Cedar Street Hunter 2, New Haven, CT 06510, USA.
| |
Collapse
|
3
|
Sobol Z, Spellman RA, Thiffeault C, Dobo KL, Schuler M. Impact of cell cycle delay on micronucleus frequency in TK6 cells. ENVIRONMENTAL AND MOLECULAR MUTAGENESIS 2014; 55:64-69. [PMID: 23982927 DOI: 10.1002/em.21810] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2013] [Revised: 07/16/2013] [Accepted: 07/17/2013] [Indexed: 06/02/2023]
Abstract
Previous studies with TK6 cells have shown that extending the recovery period after pulse treatment allows for greater micronucleus expression for some compounds. This study explores the role of cell cycle delay in micronucleus expression after pulse treatment with three model genotoxins [mitomycin C, etoposide (ETOP), vinblastine]. Cells were treated for 4 hr and allowed to recover for 36 hr with samples removed at various time points during the recovery period and analyzed for cell cycle distribution, apoptosis and micronucleus frequency. Our results show that mitomycin C causes cell cycle delay for 20 hr after pulse treatment and cell cycle perturbation is no longer evident after 36 hr of recovery. The micronucleus frequency of cells sampled at 36 hr is doubled when compared with cells sampled at 20 hr after mitomycin C removal. When cells were treated with indirect acting genotoxins (ETOP, vinblastine), cell cycle perturbation was not observed at the 20 hr time point. Micronucleus frequency after treatment with either ETOP or vinblastine did not differ between the 20 hr and the 36 hr time point. All three compounds induced similar levels of apoptosis ranging from 4.5 to 5.6% with maximum induction occurring at the 36-hr time point. We conclude that TK6 cells exhibit extended cell cycle arrest after exposure to MMC and can go on to express micronuclei, after overcoming cell cycle arrest.
Collapse
Affiliation(s)
- Zhanna Sobol
- Pfizer Worldwide Research and Development, Genetic Toxicology Center of Expertise, Eastern Point Road, Groton, Connecticut
| | | | | | | | | |
Collapse
|
4
|
Immunohistochemical analysis of O(6)-methylguanine-DNA methyltransferase in human melanoma in comparison with skin squamous cell carcinoma. Med Mol Morphol 2013; 47:8-13. [PMID: 23460078 DOI: 10.1007/s00795-013-0030-3] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Accepted: 12/06/2012] [Indexed: 10/27/2022]
Abstract
Alkylating agents, often used for chemotherapy in patients with melanoma, can produce O(6)-alkylguanine (O(6)AG) which is related to tumor cell killing after treatment with alkylating agents. O(6)AG is effectively eliminated by O(6)-methylguanine-DNA methyltransferase (O(6)MGMT) and its level is correlative to the resistance to alkylating agents. However, little is known about the relationship of O(6)MGMT to the characteristics of melanoma. This study investigated the expression of O(6)MGMT in 12 melanomas and compared it with that in 11 skin squamous cell cancers (SCCs) immunohistochemically to evaluate the O(6)MGMT activity in melanoma and its clinical significance. All of the SCC samples had high O(6)MGMT expression, while the expression of O(6)MGMT in melanoma was diverse and 4 out of 12 samples had no or extremely low O(6)MGMT activity. Out of 6 lesions obtained from metastasis, 4 had a high O(6)MGMT activity. Two out of 3 cases with a low O(6)MGMT activity in each primary lesion did not show any evidence of metastasis or local recurrence. The evaluation of O(6)MGMT activity in melanoma may, therefore, be useful to determine the characteristics of tumor in each melanoma case. In addition, the present study implies the possibility of selective cancer chemotherapy for melanoma in the near future.
Collapse
|
5
|
Ishiguro K, Shyam K, Penketh PG, Baumann RP, Sartorelli AC, Rutherford TJ, Ratner ES. Expression of O6-Methylguanine-DNA Methyltransferase Examined by Alkyl-Transfer Assays, Methylation-Specific PCR and Western Blots in Tumors and Matched Normal Tissue. ACTA ACUST UNITED AC 2013; 4:919-931. [PMID: 23946891 DOI: 10.4236/jct.2013.44103] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The tumor selectivity of alkylating agents that produce guanine O6-chloroethyl (laromustine and carmustine) and O6-methyl (temozolomide) lesions, depends upon O6-methylguanine-DNA methyltransferase (MGMT) activity being lower in tumor than in host tissue. Despite the established role of MGMT as a tumor resistance factor, consensus on how to assess MGMT expression in clinical samples is unsettled. The aim of this study is to examine the relationship between the values derived from distinctive MGMT measurements in 13, 12, 6 and 2 pairs of human tumors and matched normal adjacent tissue from the colon, kidney, lung and liver, respectively, and in human cell lines. The MGMT measurements included (a) alkyl-transfer assays using [benzene-3H]O6-benzylguanine as a substrate to assess functional MGMT activity, (b) methylation-specific PCR (MSP) to probe MGMT gene promoter CpG methylations as a measure of gene silencing, and (c) western immunoblots to analyze the MGMT protein. In human cell lines, a strict negative correlation existed between MGMT activity and the extent of promoter methylation. In tissue specimens, by contrast, the correlation between these two variables was low. Moreover, alkyl-transfer assays identified 3 pairs of tumors and normal tissue with tumor-selective reduction in MGMT activity in the absence of promoter methylation. Cell line MGMT migrated as a single band in western analyses, whereas tissue MGMT was heterogeneous around its molecular size and at much higher molecular masses, indicative of multi-layered post-translational modifications. Malignancy is occasionally associated with a mobility shift in MGMT. Contrary to the prevalent expectation that MGMT expression is governed at the level of gene silencing, these data suggest that other mechanisms that can lead to tumor-selective reduction in MGMT activity exist in human tissue.
Collapse
Affiliation(s)
- Kimiko Ishiguro
- Division of Gynecologic Oncology, Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, USA
| | | | | | | | | | | | | |
Collapse
|
6
|
Strauss BS. Repair, mutation, and other topics. DNA Repair (Amst) 2012. [DOI: 10.1016/j.dnarep.2011.04.008] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2022]
|
7
|
Pegg AE. Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools. Chem Res Toxicol 2011; 24:618-39. [PMID: 21466232 DOI: 10.1021/tx200031q] [Citation(s) in RCA: 155] [Impact Index Per Article: 11.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.
Collapse
Affiliation(s)
- Anthony E Pegg
- Department of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, Pennsylvania State University College of Medicine , Pennsylvania 17033, United States.
| |
Collapse
|
8
|
Abstract
The genome of all organisms is constantly attacked by a variety of environmental and endogenous mutagens that cause cell death, apoptosis, senescence, genetic diseases and cancer. To mitigate these deleterious endpoints of genotoxic reactions, living organisms have evolved one or more mechanisms for repairing every type of naturally occurring DNA lesion. For example, double-strand breaks are rapidly religated by non-homologous end-joining. Homologous recombination is used for the high-fidelity repair of interstrand cross-links, double-strand breaks and other DNA injuries that disrupt the replication fork. Some genotoxic lesions inflicted by alkylating agents can be repaired by direct reversal of DNA damage. The base excision repair pathway takes advantage of multiple DNA glycosylases to remove modified or incorrect bases. Finally, the nucleotide excision repair machinery provides a versatile strategy to monitor DNA quality and eliminate all forms of helix-distorting DNA lesions, including a wide diversity of carcinogen adducts. The efficiency of DNA repair responses is enhanced by their coupling to transcription and coordination with the cell cycle circuit.
Collapse
|
9
|
Abstract
This review describes the history of studies on alkylation damage of mammalian genomes and its carcinogenic consequences that led to the discovery of a unique DNA repair protein, named MGMT. MGMT repairs O(6)-alkylguanine, a critical mutagenic lesion induced by alkylating agents. The follow-up studies in mammalian cells following the discovery of the ubiquitous repair protein in E. coli are summarized.
Collapse
Affiliation(s)
- Sankar Mitra
- Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, TX 77555, USA.
| |
Collapse
|
10
|
Sekiguchi M. Molecular devices for high fidelity of DNA replication and gene expression. PROCEEDINGS OF THE JAPAN ACADEMY. SERIES B, PHYSICAL AND BIOLOGICAL SCIENCES 2006; 82:278-296. [PMID: 25792791 PMCID: PMC4338819 DOI: 10.2183/pjab.82.278] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/21/2006] [Accepted: 10/12/2006] [Indexed: 06/04/2023]
Abstract
Certain types of DNA lesions, produced through cellular metabolic processes and also by external environmental stresses, are responsible for the induction of mutations as well as of cancer. Most of these lesions can be eliminated by DNA repair enzymes, and cells carrying the remaining DNA lesions are subjected to apoptosis. The persistence of damaged bases in RNA can cause errors in gene expression, and the cells appear to possess a mechanism which can prevent damaged RNA molecules from entering the translation process. We have investigated these processes for high fidelity of DNA replication and gene expression, by using both biochemical and genetic means. We herein describe (1) the molecular mechanisms for accurate DNA synthesis, (2) mammalian proteins for sanitizing the DNA precursor pool, (3) error avoidance mechanisms for gene expression under oxidative stress, and (4) the roles of DNA repair and apoptosis in the prevention of cancer.
Collapse
Affiliation(s)
- Mutsuo Sekiguchi
- Fukuoka Dental College, 2–15–1 Tamura, Sawara-ku, Fukuoka 814–0193, Japan (e-mail: )
| |
Collapse
|
11
|
Roos WP, Batista LFZ, Naumann SC, Wick W, Weller M, Menck CFM, Kaina B. Apoptosis in malignant glioma cells triggered by the temozolomide-induced DNA lesion O6-methylguanine. Oncogene 2006; 26:186-97. [PMID: 16819506 DOI: 10.1038/sj.onc.1209785] [Citation(s) in RCA: 379] [Impact Index Per Article: 21.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
Methylating drugs such as temozolomide (TMZ) are widely used in the treatment of brain tumours (malignant gliomas). The mechanism of TMZ-induced glioma cell death is unknown. Here, we show that malignant glioma cells undergo apoptosis following treatment with the methylating agents N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and TMZ. Cell death determined by colony formation and apoptosis following methylation is greatly stimulated by p53. Transfection experiments with O(6)-methylguanine-DNA methyltransferase (MGMT) and depletion of MGMT by O(6)-benzylguanine showed that, in gliomas, the apoptotic signal originates from O(6)-methylguanine (O(6)MeG) and that repair of O(6)MeG by MGMT prevents apoptosis. We further demonstrate that O(6)MeG-triggered apoptosis requires Fas/CD95/Apo-1 receptor activation in p53 non-mutated glioma cells, whereas in p53 mutated gliomas the same DNA lesion triggers the mitochondrial apoptotic pathway. This occurs less effectively via Bcl-2 degradation and caspase-9, -2, -7 and -3 activation. O(6)MeG-triggered apoptosis in gliomas is a late response (occurring >120 h after treatment) that requires extensive cell proliferation. Stimulation of cell cycle progression by the Pasteurella multocida toxin promoted apoptosis whereas serum starvation attenuated it. O(6)MeG-induced apoptosis in glioma cells was preceded by the formation of DNA double-strand breaks (DSBs), as measured by gammaH2AX formation. Glioma cells mutated in DNA-PK(cs), which is involved in non-homologous end-joining, were more sensitive to TMZ-induced apoptosis, supporting the involvement of DSBs as a downstream apoptosis triggering lesion. Overall, the data demonstrate that cell death induced by TMZ in gliomas is due to apoptosis and that determinants of sensitivity of gliomas to TMZ are MGMT, p53, proliferation rate and DSB repair.
Collapse
Affiliation(s)
- W P Roos
- Department of Toxicology, University of Mainz, Mainz, Germany
| | | | | | | | | | | | | |
Collapse
|
12
|
Ishiguro K, Shyam K, Penketh PG, Sartorelli AC. Role of O6-alkylguanine-DNA alkyltransferase in the cytotoxic activity of cloretazine. Mol Cancer Ther 2006; 4:1755-63. [PMID: 16275997 DOI: 10.1158/1535-7163.mct-05-0169] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Cloretazine (VNP40101M; 101M; 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine) is a sulfonylhydrazine prodrug that generates both chloroethylating and carbamoylating species on activation. To explore the molecular mechanisms underlying the broad anticancer activity observed in preclinical studies, cloretazine and chloroethylating-only [i.e., 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine] and carbamoylating-only (i.e., 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine) analogues were evaluated in five murine hematopoietic cell lines. These cell lines were separable into two groups by virtue of their sensitivity to 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine; the sensitive group included L1210, P388, and F-MEL leukemias (IC50s, 6-8 micromol/L) and the resistant group consisted of Ba/F3 bone marrow and WEHI-3B leukemia cells (IC50s, 50-70 micromol/L). Resistant cells expressed O6-alkylguanine-DNA alkyltransferase (AGT), whereas sensitive cells did not. A correlation existed between AGT expression and the functional status of p53; AGT- cells possessed defective p53, whereas AGT+ cells contained wild-type p53. Based on recent findings on regulation of AGT gene expression by others, we suspect that silencing of the AGT gene by promoter hypermethylation frequently occurs during tumor progression involving p53 inactivation. O6-Chloroethylguanine is the initial DNA lesion that progresses to lethal interstrand DNA cross-links. Cloretazine exhibited a much higher preference toward the O6-chloroethylation of guanine, as measured by the difference in IC50s to wild-type and AGT-transfected L1210 cells, than 1,3-bis(2-chloroethyl)-1-nitrosourea, which targets the same site in DNA. Preferential toxicity of cloretazine against AGT- tumor cells coupled with decreased toxicity to AGT+ cells in host tissues constitute the therapeutic basis for cloretazine.
Collapse
Affiliation(s)
- Kimiko Ishiguro
- Department of Pharmacology and Developmental Therapeutics Program, Cancer Center, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06520, USA
| | | | | | | |
Collapse
|
13
|
Maki Y, Murakami J, Asaumi JI, Tsujigiwa H, Nagatsuka H, Kokeguchi S, Fukui K, Kawai N, Yanagi Y, Kuroda M, Tanaka N, Matsubara N, Kishi K. Role of O6-methylguanine–DNA methyltransferase and effect of O6-benzylguanine on the anti-tumor activity of cis-diaminedichloroplatinum(II) in oral cancer cell lines. Oral Oncol 2005; 41:984-93. [PMID: 16043385 DOI: 10.1016/j.oraloncology.2005.05.011] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2005] [Accepted: 05/27/2005] [Indexed: 11/17/2022]
Abstract
The DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) modulates the effectiveness of alkylating agents. However, the relationship between MGMT and the sensitivities to other agents has not been explored. In the present study, the association between MGMT expression and the cellular sensitivity to the platinum agent, CDDP was examined in four human oral cancer cell lines. CDDP depleted MGMT protein and mRNA levels in all four cell lines. Two cell lines with low MGMT expression were sensitive to an alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine and CDDP, whereas two other cell lines with high MGMT expression were resistant to both agents. Furthermore, the addition of the MGMT inhibitor, O6-benzylguanine (O6-BG), invariably enhanced CDDP sensitivity. CDDP depleted MGMT expression, and CDDP sensitivity was enhanced by O6-BG. These results provide valuable information about the relationship between MGMT expression and CDDP sensitivity in oral cancer chemotherapy.
Collapse
Affiliation(s)
- Yu Maki
- Department of Oral and Maxillofacial Radiology, Field of Tumor Biology, Okayama University, Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, 2-5-1, Shikata-cho, Okayama 700-8525, Japan
| | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
14
|
Asakawa JI, Kuick R, Kodaira M, Nakamura N, Katayama H, Pierce D, Funamoto S, Preston D, Satoh C, Neel JV, Hanash S. A Genome Scanning Approach to Assess the Genetic Effects of Radiation in Mice and Humans. Radiat Res 2004; 161:380-90. [PMID: 15038760 DOI: 10.1667/rr3146] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
We used Restriction Landmark Genome Scanning (RLGS) to assess, on a genome-wide basis, the mutation induction rate in mouse germ cells after radiation exposure. Analyses of 1,115 autosomal NotI DNA fragments per mouse for reduced spot intensity, indicative of loss of one copy, in 506 progeny derived from X-irradiated spermatogonia (190, 237 and 79 mice in 0-, 3-, and 5-Gy groups, respectively), permitted us to identify 16 mutations affecting 23 fragments in 20 mice. The 16 mutations were composed of eight small changes (1-9 bp) at microsatellite sequences, five large deletions (more than 25 kb), and three insertions of SINE B2 or LINE1 transposable elements. The maximum induction rate of deletion mutations was estimated as (0.17 +/- 0.09) x 10(-5)/locus Gy(-1). The estimate is considerably lower than 1 x 10(-5)/locus Gy(-1), the mean induction rate of deletion mutations at Russell's 7 loci, which assumed that deletion mutations comprise 50% of all mutations. We interpret the results as indicating that the mean induction rate of mutations in the whole genome may be substantially lower than that at the 7 loci. We also demonstrate the applicability of RLGS for detection of human mutations, which allows direct comparisons between the two species.
Collapse
Affiliation(s)
- Jun-ichi Asakawa
- Department of Genetics, Radiation Effects Research Foundation, 5-2 Hijiyama-Park, Minami-ku, Hiroshima 732-0815, Japan.
| | | | | | | | | | | | | | | | | | | | | |
Collapse
|
15
|
Takagi Y, Takahashi M, Sanada M, Ito R, Yamaizumi M, Sekiguchi M. Roles of MGMT and MLH1 proteins in alkylation-induced apoptosis and mutagenesis. DNA Repair (Amst) 2003; 2:1135-46. [PMID: 13679151 DOI: 10.1016/s1568-7864(03)00134-4] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
To examine involvement of mismatch repair system in alkylation-induced apoptosis and mutagenesis, cell lines defective in the Mgmt gene encoding a DNA repair enzyme, O(6)-methylguanine-DNA methyltransferase, and/or the Mlh1 gene encoding a protein involved in mismatch repair were established from gene-targeted mice. Mgmt(-/-) cells are hypersensitive to the killing effect of N-methyl-N-nitrosourea (MNU) and this effect of MNU was overcome by introducing an additional mutation in the Mlh1 gene. Mgmt(-/-)Mlh1(-/-) cells are more resistant to MNU than are wild-type cells. When the human Mgmt cDNA sequence with a strong promoter was introduced, the wild-type cells acquired the same high level of resistance to MNU as that of Mgmt(-/-)Mlh1(-/-) cells. Although no apparent increase in MNU-induced mutant frequency was observed in such methyltransferase-overproducing wild-type cells, mutant frequency of Mgmt(-/-)Mlh1(-/-) cells became 10-fold higher after being treated with MNU. Mgmt(-/-)Mlh1(+/-) cells carrying approximately half the normal level of MLH1 protein showed a normal level of spontaneous mutant frequency, yet were still highly responsive to the mutagenic effect of the alkylating carcinogen. This haploinsufficient character of Mlh1 mutation was also observed in cell survival assays; Mgmt(-/-)Mlh1(+/-) cells were as resistant to MNU as were Mgmt(-/-)Mlh1(-/-) cells. While caspase-3 was induced in Mgmt(-/-)Mlh1(+/+) cells after treatment with MNU, no induction occurred in Mgmt(-/-)Mlh1(+/-) cells or in Mgmt(-/-)Mlh1(-/-) cells. The cellular content of MLH1 protein seems to be critical for determining if damaged cells enter into either a death or mutation-inducing pathway. The haploinsufficient phenotype of Mlh1-heterozygous cells may be explained by competition in heterodimer formation between MLH1 homologues.
Collapse
Affiliation(s)
- Yasumitsu Takagi
- Frontier Research Center, Fukuoka Dental College, Fukuoka 814-0193, Japan.
| | | | | | | | | | | |
Collapse
|
16
|
Abstract
DNA methylation is essential for embryonic development and important for transcriptional repression, as observed in several biological phenomena. These include genomic imprinting, X-inactivation and carcinogenesis. The basic mechanism by which DNA methylation silences transcription is generally understood, but there is still much to be learned about how DNA methyltransferase is targeted to a specific region of the gene. Silencing by DNA methylation occurs at an early stage of carcinogenesis, when the DNA repair genes, MGMT and hMLH1, are frequently inactivated, resulting in mutations in key cancer-related genes in cells. Mice defective in Mgmt and/or Mlh1 gave clear evidence of the significant roles of these proteins in carcinogenesis. Recently, it has been demonstrated that DNA methylation is linked to histone methylation in fungi and plants, although it remains unknown whether this mechanism occurs in mammalian systems.
Collapse
Affiliation(s)
- Tsunehiro Mukai
- Division of Molecular Biology and Genetics, Department of Biomolecular Sciences, Saga Medical School, 5-1-1 Nabeshima, Saga 849-8501, Japan.
| | | |
Collapse
|
17
|
Kohya N, Miyazaki K, Matsukura S, Yakushiji H, Kitajima Y, Kitahara K, Fukuhara M, Nakabeppu Y, Sekiguchi M. Deficient expression of O(6)-methylguanine-DNA methyltransferase combined with mismatch-repair proteins hMLH1 and hMSH2 is related to poor prognosis in human biliary tract carcinoma. Ann Surg Oncol 2002; 9:371-9. [PMID: 11986189 DOI: 10.1007/bf02573872] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
BACKGROUND O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that transfers methyl groups from O(6)-methylguanine to itself. Alkylation of DNA at the O(6) position of guanine is an important step in the induction of mutations in the organism by alkylating agents. The O(6)-methyl G:T mismatch is recognized by the mismatch-repair (MMR) pathway. The biliary duct is highly exposed to alkylating agents because of its anatomical location. METHODS We examined 39 surgically resected gallbladder carcinomas and 35 extrahepatic bile duct carcinomas and evaluated the expression of MGMT and MMR protein (hMLH1 and hMSH2) by immunohistochemical staining. RESULTS MGMT-negative staining was detected in 59.0% of gallbladder carcinoma specimens and 60.0% of extrahepatic bile duct carcinoma specimens. In gallbladder carcinoma, hMLH1- and hMSH2-negative staining was observed in 51.3% and 59.0%, respectively, whereas in extrahepatic bile duct carcinoma, the respective values were 57.1% and 65.7%. MGMT-negative staining correlated with hepatic invasion in gallbladder carcinoma and with poor prognosis in both types of tumor. Furthermore, a combined MGMT and MMR status was shown to be a more significant prognostic biomarker in both tumor types. CONCLUSIONS Combined MGMT and MMR is a possible prognostic marker that probably reflects an accumulation of genetic mutations.
Collapse
Affiliation(s)
- Naohiko Kohya
- Department of Surgery, Saga Medical School, Saga, Japan
| | | | | | | | | | | | | | | | | |
Collapse
|
18
|
Branda RF, O'Neill JP, Brooks EM, Trombley LM, Nicklas JA. The effect of folate deficiency on the cytotoxic and mutagenic responses to ethyl methanesulfonate in human lymphoblastoid cell lines that differ in p53 status. Mutat Res 2001; 473:51-71. [PMID: 11166026 DOI: 10.1016/s0027-5107(00)00138-x] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
Folic acid deficiency acts synergistically with alkylating agents to increase genetic damage at the HPRT locus in Chinese hamster ovary cells in vitro and in rat splenocytes in vivo. The present studies extend these observations to human cells and, in addition, investigate the role of p53 activity on mutation induction. The human lymphoblastoid cell lines TK6 and WTK1 are derived from the same parental cell line (WI-L2), but WTK1 expresses mutant p53. Treatment of folate-replete or deficient WTK1 and TK6 cells with increasing concentrations (0-50microg/ml) of ethyl methanesulfonate (EMS) resulted in significantly different HPRT mutation dose-response relationships (P<0.01), indicating that folate deficiency increased the EMS-induced mutant frequency in both cell lines, but with a greater effect in TK6 cells. Molecular analyses of 152 mutations showed that the predominant mutation (65%) in both cell types grown in the presence or absence of folic acid was a G>A transition on the non-transcribed strand. These transitions were mainly at non-CpG sites, particularly when these bases were flanked 3' by a purine or on both sides by G:C base pairs. A smaller number of G>A transitions occurred on the transcribed strand (C>T=14%), resulting in 79% total G:C>A:T transitions. There were more genomic deletions in folate-deficient (15%) as compared to replete cells (4%) of both cell types. Mutations that altered RNA splicing were common in both cell types and under both folate conditions, representing 33% of the total mutations. These studies indicate that cells expressing p53 activity exhibit a higher rate of mutation induction but are more sensitive to the toxic effects of alkylating agents than those lacking p53 activity. Folate deficiency tends to reduce toxicity but increase mutation induction after EMS treatment. The p53 gene product did not have a major influence on the molecular spectrum after treatment with EMS, while folate deficiency increased the frequency of deletions in both cell types.
Collapse
Affiliation(s)
- R F Branda
- Department of Medicine and The Vermont Cancer Center, University of Vermont, Burlington, VT 05405, USA.
| | | | | | | | | |
Collapse
|
19
|
Haracska L, Prakash S, Prakash L. Replication past O(6)-methylguanine by yeast and human DNA polymerase eta. Mol Cell Biol 2000; 20:8001-7. [PMID: 11027270 PMCID: PMC86410 DOI: 10.1128/mcb.20.21.8001-8007.2000] [Citation(s) in RCA: 111] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
O(6)-Methylguanine (m6G) is formed by the action of alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on DNA. m6G is a highly mutagenic and carcinogenic lesion, and it presents a block to synthesis by DNA polymerases. Here, we provide genetic and biochemical evidence for the involvement of yeast and human DNA polymerase eta (Poleta) in the replicative bypass of m6G lesions in DNA. The formation of MNNG-induced mutations is almost abolished in the rad30Delta pol32Delta double mutant of yeast, which lacks the RAD30 gene that encodes Poleta and the Pol32 subunit of DNA polymerase delta (Poldelta). Although Poldelta can function in the mutagenic bypass of m6G lesions, our biochemical studies indicate that Poleta is much more efficient in replicating through m6G than Poldelta. Both Poleta and Poldelta insert a C or a T residue opposite from m6G; Poleta, however, is more accurate, as it inserts a C about twice as frequently as Poldelta. Alkylating agents are used in the treatment of malignant tumors, including lymphomas, brain tumors, melanomas, and gastrointestinal carcinomas, and the clinical effectiveness of these agents derives at least in part from their ability to form m6G in DNA. Inactivation of Poleta could afford a useful strategy for enhancing the effectiveness of these agents in cancer chemotherapy.
Collapse
Affiliation(s)
- L Haracska
- University of Texas Medical Branch, Sealy Center for Molecular Science, Galveston, Texas 77555-1061, USA
| | | | | |
Collapse
|
20
|
Tomita-Mitchell A, Kat AG, Marcelino LA, Li-Sucholeiki XC, Goodluck-Griffith J, Thilly WG. Mismatch repair deficient human cells: spontaneous and MNNG-induced mutational spectra in the HPRT gene. Mutat Res 2000; 450:125-38. [PMID: 10838138 DOI: 10.1016/s0027-5107(00)00020-8] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
We have determined both the spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutational spectra in the HPRT gene of human cells (MT1) defective in the mismatch repair gene hMSH6 (GTBP). Eight of nine exons and nine of sixteen intronic flanking sequences were scanned, encompassing >900 bp of the HPRT gene. Mutant hotspots were detected and separated by differences in their melting temperatures using constant denaturant capillary electrophoresis (CDCE) or denaturing gradient gel electrophoresis (DGGE).A key finding of this work is that a high proportion of all HPRT inactivating mutations is represented by a small number of hotspots distributed over the exons and mRNA splice sites. Thirteen spontaneous hotspots and sixteen MNNG-induced hotspots accounted for 55% and 48% of all 6TG(R) point mutations, respectively. MNNG-induced hotspots were predominantly G:C-->A:T transitions. The spontaneous spectrum of cells deficient in hMSH6 contained transversions (A:T-->T:A, G:C-->T:A, A:T-->C:G), transitions (A:T-->G:C), a plus-one insertion, and a minus-one deletion. Curiously, G:C-->A:T transitions, which dominate human germinal and somatic point mutations were absent from the spontaneous hMSH6 spectra.
Collapse
Affiliation(s)
- A Tomita-Mitchell
- Division of Bioengineering and Environmental Health, Center for Environmental Health Sciences, 21 Ames Street, Room 16-743, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | | | | | | | | | | |
Collapse
|
21
|
Bignami M, O'Driscoll M, Aquilina G, Karran P. Unmasking a killer: DNA O(6)-methylguanine and the cytotoxicity of methylating agents. Mutat Res 2000; 462:71-82. [PMID: 10767619 DOI: 10.1016/s1383-5742(00)00016-8] [Citation(s) in RCA: 153] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
Abstract
Methylating agents are potent carcinogens that are mutagenic and cytotoxic towards bacteria and mammalian cells. Their effects can be ascribed to an ability to modify DNA covalently. Pioneering studies of the chemical reactivity of methylating agents towards DNA components and their effectiveness as animal carcinogens identified O(6)-methylguanine (O(6)meG) as a potentially important DNA lesion. Subsequent analysis of the effects of methylating carcinogens in bacteria and cultured mammalian cells - including the discovery of the inducible adaptive response to alkylating agents in Escherichia coli - have defined the contributions of O(6)meG and other methylated DNA bases to the biological effects of these chemicals. More recently, the role of O(6)meG in killing mammalian cells has been revealed by the lethal interaction between persistent DNA O(6)meG and the mismatch repair pathway. Here, we briefly review the results which led to the identification of the biological consequences of persistent DNA O(6)meG. We consider the possible consequences for a human cell of chronic exposure to low levels of a methylating agent. Such exposure may increase the probability that the cell's mismatch repair pathway becomes inactive. Loss of mismatch repair predisposes the cell to mutation induction, not only through uncorrected replication errors but also by methylating agents and other mutagens.
Collapse
Affiliation(s)
- M Bignami
- Istituto Superiore di Sanitá, Viale Regina Elena, 00161, Rome, Italy
| | | | | | | |
Collapse
|
22
|
Kawate H, Sakumi K, Tsuzuki T, Nakatsuru Y, Ishikawa T, Takahashi S, Takano H, Noda T, Sekiguchi M. Separation of killing and tumorigenic effects of an alkylating agent in mice defective in two of the DNA repair genes. Proc Natl Acad Sci U S A 1998; 95:5116-20. [PMID: 9560238 PMCID: PMC20223 DOI: 10.1073/pnas.95.9.5116] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. The enzyme O6-methylguanine-DNA methyltransferase repairs O6-methylguanine as well as a minor methylated base, O4-methylthymine, in DNA. Mouse lines deficient in the methyltransferase (MGMT) gene are hypersensitive to both the killing and to the tumorigenic effects of alkylating agents. We now show that these dual effects of an alkylating agent can be dissociated by introduction of an additional defect in mismatch repair. Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU. In contrast to MGMT-/- MLH1(+/+) mice with decrease in size of the thymus and hypocellular bone marrow after MNU administration, no conspicuous change was found in MGMT-/- MLH1(-/-) mice treated in the same manner. Thus, killing and tumorigenic effects of an alkylating agent can be dissociated by preventing mismatch repair pathways.
Collapse
Affiliation(s)
- H Kawate
- Department of Biochemistry, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, Japan
| | | | | | | | | | | | | | | | | |
Collapse
|
23
|
Kaina B, Ziouta A, Ochs K, Coquerelle T. Chromosomal instability, reproductive cell death and apoptosis induced by O6-methylguanine in Mex-, Mex+ and methylation-tolerant mismatch repair compromised cells: facts and models. Mutat Res 1997; 381:227-41. [PMID: 9434879 DOI: 10.1016/s0027-5107(97)00187-5] [Citation(s) in RCA: 133] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
O6-Methylguanine (O6-MeG) is induced in DNA by methylating environmental carcinogens and various cytostatic drugs. It is repaired by O6-methylguanine-DNA methyltransferase (MGMT). If not repaired prior to replication, the lesion generates gene mutations and leads to cell death, sister chromatid exchanges (SCEs), chromosomal aberrations and malignant transformation. To address the question of how O6-MeG is transformed into genotoxic effects, isogenic Chinese hamster cell lines either not expressing MGMT (phenotypically Mex-), expressing MGMT (Mex+) or exhibiting the tolerance phenotype (Mex-, methylation resistant) were compared as to their clastogenic response. Mex- cells were more sensitive than Mex+ cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced chromosomal breakage, with marked differences in sensitivity depending on recovery time. At early recovery time, when cells out of the first post-treatment mitosis were scored, aberration frequency was about 40% reduced in Mex+ as compared to Mex- cells. At later stages of recovery when cells out of the second post-treatment mitosis were analyzed, the frequency of aberrations increased strongly in Mex- cells whereas it dropped to nearly control level in Mex+ cells. From this we conclude that, in the first post-treatment replication cycle of Mex- cells, only a minor part of aberrations (< 40%) was due to O6-MeG whereas, in the second post-treatment replication cycle, the major part of aberrations (> 90%) was caused by the lesion. Thus, O6-MeG is a potent clastogenic DNA damage that needs two DNA replication cycles in order to be transformed with high efficiency into aberrations. The same holds true for sister chromatid exchanges (SCEs). MNNG is highly potent in inducing SCEs in Mex- cells in the second replication cycle after alkylation. Under these conditions, SCE induction is nearly completely prevented by the expression of MGMT. This is opposed to SCE induction in the first post-treatment replication cycle, where higher doses of MNNG were required to induce SCEs and no protective effect of MGMT was observed. This indicates that SCEs induced in the first replication cycle after alkylation are due to other lesions than O6-MeG. In methylation tolerant cells, which are characterized by impaired G-T mismatch binding and MSH2 expression, aberration frequency induced by MNNG was weakly reduced in the first and strongly reduced in the second post-treatment mitoses, as compared to CHO wild-type cells. The results indicate that mismatch repair of O6-MeG-T mispairs is decisively involved in O6-MeG born chromosomal instability and recombination. We also show that Mex+ and methylation tolerant cells are more resistant than Mex- cells with regard to induction of apoptosis, indicating O6-MeG to be also an apoptosis-inducing lesion. The data are discussed as to the mechanism of cytotoxicity, aberration and SCE formation in cells treated with a methylating agent.
Collapse
Affiliation(s)
- B Kaina
- Division of Applied Toxicology, University of Mainz, Germany
| | | | | | | |
Collapse
|
24
|
Sekiguchi M, Sakumi K. Roles of DNA repair methyltransferase in mutagenesis and carcinogenesis. THE JAPANESE JOURNAL OF HUMAN GENETICS 1997; 42:389-99. [PMID: 12503185 DOI: 10.1007/bf02766939] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to induction of mutation as well as cancer. An enzyme, O6-methylguanine-DNA methyltransferase, is present in various organisms, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from O6-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancer, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying extra copies of the foreign methyltransferase gene showed a decreased susceptibility to alkylating carcinogens, with regard to tumor formation. By means of gene targeting, mouse lines defective in both alleles of the methyltransferase gene were established. Administration of methylnitrosourea to these gene-targeted mice led to early death while normal mice treated in the same manner showed no untoward effects. Numerous tumors were formed in the gene-defective mice exposed to a low dose of methylnitrosourea, while none or only few tumors were induced in the methyltransferase-proficient mice. It seems apparent that the DNA repair methyltransferase plays an important role in lowering a risk of occurrence of cancer in organisms.
Collapse
Affiliation(s)
- M Sekiguchi
- Fukuoka Dental College, Fukuoka 814-01, Japan
| | | |
Collapse
|
25
|
Gustafson DL, Trotter BK, Snead D, Waldren CA. Expression of human O6-methyl guanine methyl transferase (MGMT) in post replication repair (PRR) deficient CHO-UV-1 cells: compensation for hypersensitivity to methylating and ethylating agents but not to mitomycin C. SOMATIC CELL AND MOLECULAR GENETICS 1997; 23:9-17. [PMID: 9217997 DOI: 10.1007/bf02679951] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The cDNA for human MGMT was transfected into and expressed in CHO cells and the post-replication repair deficient mutant CHO-UV-1 cell, both of which are devoid of endogenous MGMT activity. Expression of MGMT activity was demonstrated by measurement of activity and by immunoblot analysis. The mutant phenotype of UV-1 is characterized by extreme hypersensitivity to killing by methylating and ethylating agents as well as the antitumor antibiotic mitomycin C (MMC). MGMT expression conferred equivalent, supra-normal levels of resistance to killing by MNNG (N-methyl-N'-nitro-nitrosoguanidine) or EMS (ethyl methanesulfonate) on CHO and UV-1, but had no effect on the lethality of MMC. So, even though a mutated gene other than MGMT is known to underlie the pleiotropic phenotype of UV-1, expression of MGMT compensates for part of this phenotype. This result indicates that attempts to concordance map and clone the gene(s) responsible for the UV-1 phenotype can be complicated when using MNNG selection due to compensation by the MGMT gene. These results also indicate that the post-replication repair deficient phenotype characterized in CHO-UV-1 cells, will be masked in cells normally expressing MGMT due to MGMT-mediated resistance to methylating and ethylating agents.
Collapse
Affiliation(s)
- D L Gustafson
- Department of Radiological Health Sciences, Colorado State University, Fort Collins 80521, USA
| | | | | | | |
Collapse
|
26
|
Hansson J, Martenhed G, Egyházi S, Tani E, Platz A. Analysis of O6-methylguanine-DNA methyltransferase mRNA in fine needle biopsies from human melanoma metastases by reverse transcription and polymerase chain reaction. Eur J Cancer 1996; 32A:2319-26. [PMID: 9038616 DOI: 10.1016/s0959-8049(96)00304-8] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
O6-methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein, which removes alkyl groups from the O6 atom of guanine residues. Tumour cells which lack MGMT are sensitive to cytostatic drugs such as dacarbazine (DTIC), whose active species bind to this site. To explore whether analyses of MGMT expression can be used as a predictive test for clinical sensitivity to DTIC in melanomas, we developed a method to assay MGMT mRNA levels in cells obtained by fine needle aspiration biopsies of metastases. cDNA was synthesised from mRNA prepared from biopsy material. Polymerase chain reaction was performed using primers complementary to MGMT cDNA and to beta-actin, which served as an internal control. Analyses of 44 biopsies from 35 patients showed a considerable variation in MGMT mRNA, with 15 samples (34%) lacking detectable mRNA. In 6 out of 8 patients in whom more than one tumour was analysed, separate metastases had different levels of MGMT mRNA. There was no correlation between MGTM activity studied by a biochemical assay and MGMT mRNA levels when these were compared in 10 surgical biopsies.
Collapse
Affiliation(s)
- J Hansson
- Department of Oncology, Karolinska Hospital, Stockholm, Sweden
| | | | | | | | | |
Collapse
|
27
|
Iwakuma T, Shiraishi A, Fukuhara M, Kawate H, Sekiguchi M. Organization and expression of the mouse gene for DNA repair methyltransferase. DNA Cell Biol 1996; 15:863-72. [PMID: 8892758 DOI: 10.1089/dna.1996.15.863] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
06-Methylguanine-DNA methyltransferase (MGMT) is present in various organisms, from bacteria to human cells, and plays an important role in preventing mutations caused by alkylating substances. To understand better the regulatory mechanism involved in the expression of the gene and to construct a mouse model to investigate roles of the enzyme in carcinogenesis, the genomic sequence for mouse methyltransferase was isolated and characterized. The gene consists of 5 exons and spans over 180 kb, whereas mRNA for the enzyme was less than 1 kb. The promoter region for the gene is GC-rich, contains many Sp1 recognition sequences and lacks typical TATA and CCAAT boxes. Primer extension and S1 mapping revealed the existence of multiple transcription initiation sites, among which a major site was defined as +1. The putative promoter region was placed upstream of the chloramphenicol acetyltransferase (CAT) reporter gene and the construct was introduced into mouse NIH-3T3 cells. Deletion analyses revealed that a sequence from -262 to + 56 carries the basic promoter activity. In addition, an adjacent region, spanning from +56 to +95, carries an E2F-like element that greatly stimulates the frequency of transcription. Alteration of TTTTGGGGC to TTAACGGGC considerably reduced the activity.
Collapse
Affiliation(s)
- T Iwakuma
- Department of Biochemistry, Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan
| | | | | | | | | |
Collapse
|
28
|
Affiliation(s)
- M Hall
- Cancer Research Campaign Mammalian Cell DNA Repair Group, Department of Zoology, Cambridge, U.K
| | | |
Collapse
|
29
|
Sekiguchi M, Nakabeppu Y, Sakumi K, Tuzuki T. DNA-repair methyltransferase as a molecular device for preventing mutation and cancer. J Cancer Res Clin Oncol 1996; 122:199-206. [PMID: 8601571 DOI: 10.1007/bf01209646] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Alkylation of DNA at the 0(6) position of guanine is regarded as one o f the most critical events leading to induction of mutations and cancers in organisms. Once 0(6)-methylguanine is formed, it can pair with thymine during DNA replication, the result being a conversion of the guanine.cytosine to an adenine.thymine pair in DNA, and such mutations are often found in tumors induced by alkylating agents. To counteract such effects, organisms possess a mechanism to repair 0(6)-methylguanine in DNA. An enzyme, 0(6)-methylguanine-DNA methyltransferase, is present in various organism, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from 0(6)-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancers, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying the foreign methyltransferase gene with functional promoters had higher levels of methyltransferase activity and showed a decreased susceptibility to N-nitroso compounds in regard to liver carcinogenesis. Mouse lines deficient in the methyltransferase gene, which were established by gene targeting, exhibited an extraordinarily high sensitivity to an alkylating carcinogen.
Collapse
Affiliation(s)
- M Sekiguchi
- Medical Institute of Bioregulation, Kyushu University, Fukuoka, Japan
| | | | | | | |
Collapse
|
30
|
Landini P, Volkert MR. Transcriptional activation of the Escherichia coli adaptive response gene aidB is mediated by binding of methylated Ada protein. Evidence for a new consensus sequence for Ada-binding sites. J Biol Chem 1995; 270:8285-9. [PMID: 7713936 DOI: 10.1074/jbc.270.14.8285] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The Escherichia coli aidB gene is part of the adaptive response to DNA methylation damage. Genes belonging to the adaptive response are positively regulated by the ada gene; the Ada protein acts as a transcriptional activator when methylated in one of its cysteine residues at position 69. Through DNaseI protection assays, we show that methylated Ada (meAda) is able to bind a DNA sequence between 40 and 60 base pairs upstream of the aidB transcriptional startpoint. Binding of meAda is necessary to activate transcription of the adaptive response genes; accordingly, in vitro transcription of aidB is dependent on the presence of meAda. Unmethylated Ada protein shows no protection against DNaseI digestion in the aidB promoter region nor does it promote aidB in vitro transcription. The aidB Ada-binding site shows only weak homology to the proposed consensus sequences for Ada-binding sites in E. coli (AAANNAA and AAAGCGCA) but shares a higher degree of similarity with the Ada-binding regions from other bacterial species, such as Salmonella typhimurium and Bacillus subtilis. Based on the comparison of five different Ada-dependent promoter regions, we suggest that a possible recognition sequence for meAda might be AATnnnnnnG-CAA. Higher concentrations of Ada are required for the binding of aidB than for the ada promoter, suggesting lower affinity of the protein for the aidB Ada-binding site. Common features in the Ada-binding regions of ada and aidB are a high A/T content, the presence of an inverted repeat structure, and their position relative to the transcriptional start site. We propose that these elements, in addition to the proposed recognition sequence, are important for binding of the Ada protein.
Collapse
Affiliation(s)
- P Landini
- Department of Molecular Genetics and Microbiology, University of Massachusetts Medical School, Worcester 01655, USA
| | | |
Collapse
|
31
|
Egyházi S, Hansson J, Ringborg U. O6-methylguanine-DNA methyltransferase activities in biopsies of human melanoma tumours. Br J Cancer 1995; 71:37-9. [PMID: 7819045 PMCID: PMC2033437 DOI: 10.1038/bjc.1995.8] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Tumour samples obtained from one primary melanoma and several lymph node and skin metastases were analysed for O6-methylguanine-DNA methyltransferase (MGMT) activity. While lymph node and skin metastases had similar average MGMT activity, the variance was significantly higher in lymph node metastases. Variability in MGMT activity was frequently observed in different metastases in the same individual and to a lesser extent within metastases.
Collapse
Affiliation(s)
- S Egyházi
- Department of Experimental Oncology, Radiumhemmet, Karolinska Hospital, Stockholm, Sweden
| | | | | |
Collapse
|
32
|
Huang JC, Hsu DS, Kazantsev A, Sancar A. Substrate spectrum of human excinuclease: repair of abasic sites, methylated bases, mismatches, and bulky adducts. Proc Natl Acad Sci U S A 1994; 91:12213-7. [PMID: 7991608 PMCID: PMC45407 DOI: 10.1073/pnas.91.25.12213] [Citation(s) in RCA: 172] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Nucleotide-excision repair is the repair system for removing bulky lesions from DNA. Humans deficient in this repair pathway suffer from xeroderma pigmentosum (XP), a disease characterized by photodermatoses, including skin cancers. At the cellular level, XP patients fail to remove cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts induced by UV light, as well as other bulky DNA lesions caused by various genotoxic agents. XP cells are not particularly sensitive to ionizing radiation or to alkylating agents that cause mostly nonbulky DNA lesions. Therefore, it has generally been assumed that the human nucleotide-excision repair enzyme (excinuclease) is specific for bulky adducts. To determine the substrate range of human excinuclease we used the highly sensitive excision assay and tested bulky adducts, synthetic apurinic/apyrimidinic sites, N6-methyladenine, O6-methylguanine, and mismatches as potential substrates. We found that all of these "lesions" were removed by human excinuclease, although with vastly different efficiencies.
Collapse
Affiliation(s)
- J C Huang
- Department of Biochemistry and Biophysics, University of North Carolina School of Medicine, Chapel Hill 27599
| | | | | | | |
Collapse
|
33
|
Abstract
DNA mismatch repair is an important pathway of mutation avoidance. It also contributes to the cytotoxic effects of some kinds of DNA damage, and cells defective in mismatch repair are resistant, or tolerant, to the presence of some normally cytotoxic base analogues in their DNA. The absence of a particular mismatch binding function from some mammalian cells confers resistance to the base analogues O6-methylguanine and 6-thioguanine in DNA. Cells also acquire a spontaneous mutator phenotype as a consequence of this defect. Impaired mismatch binding can cause an instability in DNA microsatellite regions that comprise repeated dinucleotides. Microsatellite DNA instability is common in familial and sporadic colon carcinomas as well as in a number of other tumours. Several independent lines of investigation have identified defects in mismatch repair proteins that are causally related to these cancers.
Collapse
Affiliation(s)
- P Karran
- Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, UK
| | | |
Collapse
|
34
|
Thomale J, Hochleitner K, Rajewsky M. Differential formation and repair of the mutagenic DNA alkylation product O6-ethylguanine in transcribed and nontranscribed genes of the rat. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)42081-3] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
|
35
|
Ceccotti S, Dogliotti E, Gannon J, Karran P, Bignami M. O6-methylguanine in DNA inhibits replication in vitro by human cell extracts. Biochemistry 1993; 32:13664-72. [PMID: 8257700 DOI: 10.1021/bi00212a035] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
To study the effects of methylation damage on DNA replication in vitro, the plasmid pSVori containing the SV40 origin of replication was reacted with N-methyl-N-nitrosourea and used as a substrate for SV40 T antigen dependent replication by HeLa cell extracts. The plasmid was methylated with a range of N-methyl-N-nitrosourea concentrations that introduced an average of 0.3-2.5 O6-methylguanine and equal amounts of 3-methyladenine lesions per DNA molecule. When methylated plasmid was incubated with extract of Mex-HeLaMR cells under conditions favoring DNA replication, an impairment of replication was observed as the accumulation of incompletely replicated form II plasmid molecules. These extracts simultaneously performed a T antigen independent, DpnI-sensitive DNA repair synthesis that increased with increasing DNA damage. Subtraction of this repair DNA synthesis revealed that methylation inhibited overall replication. At low levels of methylation (< or = 1 O6-methylguanine and < or = 1 3-methyladenine lesion per plasmid), inhibition was transient, while more extensive damage resulted in apparently irreversible inhibition of replication. Removal of O6-methylguanine by pretreatment of the methylated plasmid with purified human O6-methylguanine-DNA methyltransferase restored replication to almost normal levels. When the methylated plasmid was replicated by extracts of Mex+ HeLaS3 cells proficient in the repair of O6-methylguanine, a lower level of inhibition and less repair DNA synthesis was observed. The inhibition of DNA synthesis and the stimulation of repair DNA synthesis are thus both largely due to the presence of O6-methylguanine in DNA.(ABSTRACT TRUNCATED AT 250 WORDS)
Collapse
Affiliation(s)
- S Ceccotti
- Laboratory of Comparative Toxicology and Ecotoxicology, Istituto Superiore di Sanità, Rome, Italy
| | | | | | | | | |
Collapse
|
36
|
Molecular and cellular characterization of Mex-/methylation-resistant phenotype. Gene and cDNA cloning, serum dependence, and tumor suppression of transfectant strains. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(19)36899-1] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
|
37
|
Hotta T, Saito Y, Mikami T, Kurisu K, Kiya K, Uozumi T, Isowa G, Ishizaki K, Ikenaga M. Interrelationship between O6-alkylguanine-DNA alkyltransferase activity and susceptibility to chloroethylnitrosoureas in several glioma cell lines. J Neurooncol 1993; 17:1-8. [PMID: 8120566 DOI: 10.1007/bf01054267] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
In order to study the dynamic relationship in glioma cells between O6-alkylguanine-DNA alkyltransferase (AGT) activity and resistance to the cytotoxic effect of chloroethylnitrosoureas (CENUs), we investigated the changes in sensitivity to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) after modulation of AGT activity. In ACNU-resistant rat glioma cell lines (9LR1, 9LR3, and 9LR12) and a human glioma cell (HNG-1), O6-methylguanine enhanced cytotoxicity to ACNU following a depletion of AGT activity. But no enhancement of cytotoxicity was seen in an ACNU-sensitive rat glioma cell line (9L). In the 9L and 9LR12 cells, equivalently sublethal doses of ACNU similarly depleted AGT activity but the regeneration rates of this repair protein were different. In the case of a 7-day pretreatment with human recombinant interferon-beta (HuIFN-beta), although it could modulate AGT activity in HNG-1 cells, no definite influence on cellular sensitivity to CENUs was observed. However, a 50-day pretreatment with HuIFN-beta conferred resistance to CENUs on them despite its effect to reduce AGT activity. Thus, diversity was seen in the relation between AGT activity and resistance to CENUs when AGT activity was modulated by HuIFN-beta. The results of this study suggest that AGT activity is one of factors affecting cellular sensitivity to CENUs but that alternative mechanisms of tolerance may be induced depending upon some environmental effects.
Collapse
Affiliation(s)
- T Hotta
- Department of Neurosurgery, Hiroshima University School of Medicine, Japan
| | | | | | | | | | | | | | | | | |
Collapse
|
38
|
|
39
|
Mitra S, Kaina B. Regulation of repair of alkylation damage in mammalian genomes. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1993; 44:109-42. [PMID: 8434121 DOI: 10.1016/s0079-6603(08)60218-4] [Citation(s) in RCA: 113] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Affiliation(s)
- S Mitra
- Biology Division, Oak Ridge National Laboratory, Tennessee 37831
| | | |
Collapse
|
40
|
Nakatsu Y, Hattori K, Hayakawa H, Shimizu K, Sekiguchi M. Organization and expression of the human gene for O6-methylguanine-DNA methyltransferase. Mutat Res 1993; 293:119-32. [PMID: 7678140 DOI: 10.1016/0921-8777(93)90063-m] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
O6-Methylguanine-DNA methyltransferase plays an important role in cellular defence against mutagens and carcinogens with alkylating activity. Certain tumor-derived cell lines, termed Mer-, are defective in the enzyme activity and have an increased sensitivity to alkylating agents. We cloned the genomic sequence coding for the human O6-methylguanine-DNA methyltransferase and elucidated the structure. The gene consisted of 5 exons and spanned more than 170 kb, while mRNA for the enzyme was 950 nucleotides long. No or only little mRNA for the enzyme was formed in Mer- cells, though there was no gross difference in the coding and promoter regions of the gene between Mer+ and Mer- cells. The putative promoter region, derived from Mer+ cells, was placed upstream of the chloramphenicol acetyltransferase reporter gene and the constructs were introduced into Mer+ and Mer- cells. In Mer- cells, a lowered level of transient expression of the gene was observed as compared with Mer+ cells, but this difference alone does not account for the in vivo difference of expression of the gene in the two types of cells; there might be difference in cis-acting elements. The DNA sequence in the 5' upstream region of the gene was extremely GC-rich and there were no consensus sequences, such as the TATA and CAAT boxes. There were lower levels of methylation in the putative promoter of various Mer- cells, as compared with findings in Mer+ cells. Methylation in this region may be involved in regulating expression of the gene.
Collapse
Affiliation(s)
- Y Nakatsu
- Department of Biochemistry, Faculty of Medicine, Kyushu University, Fukuoka, Japan
| | | | | | | | | |
Collapse
|
41
|
Sarkar A, Dolan ME, Gonzalez GG, Marton LJ, Pegg AE, Deen DF. The effects of O6-benzylguanine and hypoxia on the cytotoxicity of 1,3-bis(2-chloroethyl)-1-nitrosourea in nitrosourea-resistant SF-763 cells. Cancer Chemother Pharmacol 1993; 32:477-81. [PMID: 8258197 DOI: 10.1007/bf00685893] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
O6-Alkylguanine-DNA alkyltransferase (AGT) activity is associated with resistance of brain tumor cell lines to the cytotoxic effects of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). SF-763 cells exhibit high AGT activity and are resistant to BCNU. In this study, we compared the effects of the AGT inhibitor O6-benzylguanine (BG) on the cytotoxicity of BCNU in oxic and hypoxic SF-763 cells; we also measured AGT activity, ornithine decarboxylase (ODC) activity, and polyamine levels to determine if there was any correlation with cell survival as determined by colony-forming efficiency assay. Exponentially growing monolayer cells were pretreated with 10 microM BG for 2 h under oxic or hypoxic (95% nitrogen/5% CO2) conditions and then exposed to graded concentrations of BCNU for 1 h. BG significantly lowered AGT activity but had no cytotoxic effect in oxic or hypoxic cells; hypoxia alone was not cytotoxic. The cytotoxicity of BCNU was 4 times higher in BG-treated hypoxic cells than in oxic cells treated with BCNU alone; the BCNU doses required for a 1-log cell kill were 75 and 300 microM, respectively. ODC activity was lowered by hypoxia alone but was not significantly affected by BG in either hypoxic or oxic cells. Polyamine levels were not significantly affected by hypoxia or BG. These results indicate that pretreatment with BG dramatically lowers AGT activity and increases the cytotoxicity of BCNU in both oxic and hypoxic SF-763 cells. The mechanism of this enhanced cytotoxicity is apparently unrelated to ODC activity or polyamine levels.
Collapse
Affiliation(s)
- A Sarkar
- Department of Neurological Surgery, School of Medicine, University of California, San Francisco 94143
| | | | | | | | | | | |
Collapse
|
42
|
Kaina B, Fritz G, Coquerelle T. Contribution of O6-alkylguanine and N-alkylpurines to the formation of sister chromatid exchanges, chromosomal aberrations, and gene mutations: new insights gained from studies of genetically engineered mammalian cell lines. ENVIRONMENTAL AND MOLECULAR MUTAGENESIS 1993; 22:283-292. [PMID: 8223512 DOI: 10.1002/em.2850220418] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
O6-methyl- and O6-ethylguanine are the major premutagenic and precarcinogenic lesions induced in DNA by monofunctional alkylating agents, albeit formed in minor amounts. The involvement of these lesions in SCE and aberration formation is less clear. We have analyzed the contribution of O6-alkylguanine to SCE and aberration formation, as well as its toxic and point mutation inducing effect in transgenic Chinese hamster ovary (CHO) cell lines that express variable amounts of human O6-methylguanine-DNA methyltransferase (MGMT). Cells that overexpress MGMT (or the bacterial Ada protein) gained resistance to the formation of alkylation-induced SCEs and aberrations, as compared to MGMT deficient cells. A correlation was apparent between the level of protection for SCEs and cell killing, indicating that both phenomena are interrelated. The protective effects were dependent on the level of MGMT expression, the agent used for alkylation, and cell cycle progression. Our data suggest that at least 2 kinds of lesions are responsible for SCE and aberration formation, namely, O6-alkylguanine and one or various N-alkylation products. The probability that O6-methylguanine is converted into cytogenetic effects has been estimated to be about 1:30 for SCEs, and 1:147,000 and 1:22,000 for chromosomal aberrations in the first and second post-treatment mitosis, respectively. The induction of SCEs and likely also of aberrations by O6-methylguanine requires two replication cycles and is supposed to involve the formation of secondary DNA lesions. Increased repair of 3-methyladenine and 7-methylguanine in CHO cells that overexpress the N-methylpurine-DNA glycosylase (MPG) after transfection with the human MPG-cDNA did not give rise to protection against methylation-induced SCEs and aberrations, probably because of incomplete excision repair. MPG overexpressing cells reacted even more sensitively to methylating agents, suggesting apurinic sites formed as a result of MPG action to be SCE and aberration-inducing lesions.
Collapse
Affiliation(s)
- B Kaina
- Department of Genetics, Nuclear Research Center Karlsruhe, Germany
| | | | | |
Collapse
|
43
|
Wang Y, Kato T, Ayaki H, Ishizaki K, Tano K, Mitra S, Ikenaga M. Correlation between DNA methylation and expression of O6-methylguanine-DNA methyltransferase gene in cultured human tumor cells. Mutat Res 1992; 273:221-30. [PMID: 1372105 DOI: 10.1016/0921-8777(92)90083-f] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Approximately 20% of human tumor cell strains are deficient in a DNA repair protein, O6-methylguanine-DNA methyltransferase (MGMT), and are called Mer- strains. In an attempt to determine the molecular basis for the extinction of MGMT expression in Mer- human cells, the distribution of DNA methylation sites in and around the exon sequences of the repair gene was compared in 6 Mer+ (repair-proficient) and 12 Mer- cell lines. Southern blot analysis of the genomic DNA digested with isoschizomeric restriction endonucleases MspI and HpaII to detect 5-methylcytosine in CCGG sequences indicated that the DNA of all the Mer+ cells but of none of the Mer- cells is heavily methylated in the exon-containing regions. The methylation pattern contradicts the general belief that inactive genes are hypermethylated compared to hypomethylation of transcriptionally active genes. It appears that the regulation of the MGMT gene in human cells is much more complex than simply dictated by its methylation level.
Collapse
Affiliation(s)
- Y Wang
- Radiation Biology Center, Kyoto University, Japan
| | | | | | | | | | | | | |
Collapse
|
44
|
Shiota S, von Wronski MA, Tano K, Bigner DD, Brent TP, Mitra S. Characterization of cDNA encoding mouse DNA repair protein O6-methylguanine-DNA methyltransferase and high-level expression of the wild-type and mutant proteins in Escherichia coli. Biochemistry 1992; 31:1897-903. [PMID: 1371399 DOI: 10.1021/bi00122a001] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
A mouse cDNA clone encoding O6-methylguanine-DNA methyltransferase (MGMT), responsible for repair of mutagenic O6-alkylguanine in DNA, was cloned from a lambda gt11 library. On the basis of an open reading frame in cDNA, the mouse protein contains 211 amino acids with a molecular mass of 22 kDa. The size and the predicted N-terminal sequence of the mouse protein were confirmed experimentally. The deduced amino acid sequence of the mouse MGMT is 70% homologous to that of the human MGMT. Cysteine-149 was shown to be the only alkyl acceptor residue in the mouse protein, in confirmation of the prediction based on conserved sequences of different MGMTs. Mouse MGMT protein is recognized by some monoclonal antibodies specific for human MGMT. Site-directed mutagenesis was utilized to reclone the mouse cDNA in a T7 promoter-based vector for overexpression of the native repair protein in Escherichia coli. The mouse protein has a tetrapeptide sequence, Pro-Glu-Gly-Val at positions 56-59, absent in the human protein. Neither deletion of this tetrapeptide nor substitution of valine-169 with alanine affected the activity of the mutant proteins.
Collapse
Affiliation(s)
- S Shiota
- University of Tennessee, Oak Ridge Graduate School of Biomedical Sciences 37831
| | | | | | | | | | | |
Collapse
|
45
|
Fritz G, Kaina B. Genomic differences between O6-methylguanine-DNA methyltransferase proficient (Mex+) and deficient (Mex−) cell lines: Possible role of genetic and epigenetic changes in conversion of Mex+ into Mex−. Biochem Biophys Res Commun 1992; 183:1184-90. [PMID: 1348932 DOI: 10.1016/s0006-291x(05)80315-8] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
Cell lines that possess O6-methylguanine-DNA methyltransferase (MGMT) repair activity (Mex+ phenotype) or are deficient for MGMT (Mex-) are compared at genomic level. It is shown that 1) 208F rat cells do not contain the MGMT gene, as detected by Southern blot hybridization. 2) Mex- HeLa MR and CHO-9 cells express very low amounts of MGMT mRNA, as detected by PCR. The size of the MGMT specific PCR product was slightly smaller than that generated from Mex+ HeLa S3 cells. 3) HeLa MR, compared to various human Mex+ cell lines, shows a restriction fragment length polymorphism indicating mutational alteration of MGMT gene sequences. 4) Mex- cells (HeLa MR) and cells that express very low MGMT activity (GM637) exhibit hypomethylation of the MGMT gene as revealed by MspI/HpaII restriction digests. 5) Exposure of Mex- cells to 5-azacytidine and selection with N-hydroxyethyl-N-chloroethylnitrosourea (HeCNU) did not yield Mex+ revertants. With V79 cells treated with 5-azacytidine clones resistant to HeCNU were isolated. These cells were MGMT deficient and not cross-resistant to N-methyl-N'-nitro-N-nitrosoguanidine indicating the existance of a defence mechanism other than MGMT against chloroethylating agents. The data suggest down-regulation of MGMT transcription accompanied by decreasing CpG methylation, but in some cell lines also mutational alterations to be involved in extinction of the Mex+ phenotype.
Collapse
Affiliation(s)
- G Fritz
- Kernforschungszentrum Karlsruhe, Institut für Genetik und Toxikologie, Karlsruhe, Germany
| | | |
Collapse
|
46
|
Bianchi R, Citti L, Beghetti R, Romani L, D'Incalci M, Puccetti P, Fioretti MC. O6-methylguanine-DNA methyltransferase activity and induction of novel immunogenicity in murine tumor cells treated with methylating agents. Cancer Chemother Pharmacol 1992; 29:277-82. [PMID: 1537073 DOI: 10.1007/bf00685945] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
To investigate the mechanism of the generation of immunogenic tumor variants by mutagenic drugs, murine leukemia cells exhibiting different sensitivity to killing by the alkylator 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and different ability to repair O6-methyl-guanine in their DNA were treated in vitro with a series of methylating agents, including triazene derivatives, temozolomide, and streptozotocin. At the population level, we found that BCNU-resistant cells (L1210/BCNU) that appeared to be cross-resistant to killing by a dimethyltriazene and expressed high levels of O6-methylguanine-DNA methyltransferase activity (mer+ phenotype) failed to generate highly immunogenic variant sublines on repeated exposure to the methylating agents. In contrast, all cells (L1210) that were susceptible to DNA alkylation damage and deficient in O6-methylguanine repair (mer-) developed immunogenic variant sublines. A noticeable exception was represented by streptozotocin treatment, which was equally effective in mer+ and mer- cells. At the clonal level, a single exposure to streptozotocin or a triazene derivative resulted in a high incidence (33% and 50%, respectively) of immunogenic cell generation in mer- cells only. In mer+ cells, streptozotocin treatment led to a 33% incidence of immunogenic clones only when the cells were concurrently exposed to O6-methylguanine as a free base. The activity of O6-methylguanine-DNA methyltransferase in mer+ cells was greatly reduced by treatment with O6-methylguanine or streptozotocin, and the combination of the two drugs led to enzyme levels similar to those observed in mer- cells. Taken together, these data suggest that the mechanism of O6-alkylation may be operative in the induction of novel tumor-cell antigenicity by methylating agents.
Collapse
Affiliation(s)
- R Bianchi
- Dip. Medicina Sperimentale-Sez. Farmacologia, Perugia, Italy
| | | | | | | | | | | | | |
Collapse
|
47
|
Souliotis VL, Zongza V, Nikolopoulou V, Dimitriadis GJ. Measurement of O6-methylguanine-type adducts in DNA and O6-alkylguanine-DNA-alkyltransferase repair activity in normal and neoplastic human tissues. COMPARATIVE BIOCHEMISTRY AND PHYSIOLOGY. B, COMPARATIVE BIOCHEMISTRY 1992; 101:269-75. [PMID: 1499274 DOI: 10.1016/0305-0491(92)90189-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
1. Novel assays based on the use of the suicide repair enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) to repair O6-alkylguanine-type adducts in DNA have been used for the analysis of extracts of human biopsy specimens of gastric mucosa, urinary bladder mucosa, colon and circulating lymphocytes. 2. Examination of these extracts revealed no detectable amounts of the precarcinogenic adduct O6-meG. 3. AGT measurements were the same among the normal and the autologous samples of all patients examined, which limits its prognostic value as a tumour marker. 4. AGT measurements from the cancer samples were much higher compared with the measurements of the other two groups which proves that AGT is just a marker of tumour burden. 5. Finally, AGT measurements from lymphocytes show that AGT from normal individuals have about the same value as that from patients suffering from cancer in urinary bladder mucosa and colon, but is much lower than that of patients with cancer in gastric mucosa.
Collapse
Affiliation(s)
- V L Souliotis
- National Hellenic Research Foundation, Athens, Greece
| | | | | | | |
Collapse
|
48
|
Harris LC, Potter PM, Tano K, Shiota S, Mitra S, Brent TP. Characterization of the promoter region of the human O6-methylguanine-DNA methyltransferase gene. Nucleic Acids Res 1991; 19:6163-7. [PMID: 1956775 PMCID: PMC329112 DOI: 10.1093/nar/19.22.6163] [Citation(s) in RCA: 111] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
O6-methylguanine-DNA methyltransferase (MGMT) is a ubiquitous protein responsible for repair of O6-alkylguanine, a mutagenic, carcinogenic and toxic lesion. To characterize the elements responsible for the regulation of the MGMT gene, a 2.6 kb Sstl fragment isolated from a genomic clone, was shown to contain 5' flanking sequences of the gene. The promoter activity of this fragment as well as various subfragments were tested in NIH 3T3 mouse fibroblasts by transient expression of the bacterial chloramphenicol acetyltransferase (CAT) gene linked to these fragments. Maximal promoter activity was observed in a 1.2 kb 3' terminal fragment, which contains the first untranslated exon. The transcription initiation site was identified in this fragment by primer extension and S1 mapping. Sequence analysis of this fragment showed the absence of TATA and CAAT boxes but an abundance of extremely GC-rich sequences, including ten GC hexanucleotide motifs 5'CCGCCC. Reduced CAT expression with the minimal promoter sequence suggests the presence of multiple regulatory elements.
Collapse
Affiliation(s)
- L C Harris
- Department of Biochemical and Clinical Pharmacology, St Jude Children's Research Hospital, Memphis, TN 38101
| | | | | | | | | | | |
Collapse
|
49
|
Sakumi K, Shiraishi A, Hayakawa H, Sekiguchi M. Cloning and expresion of cDNA for rat O6-methylguanine-DNA methyltransferase. Nucleic Acids Res 1991; 19:5597-601. [PMID: 1945835 PMCID: PMC328962 DOI: 10.1093/nar/19.20.5597] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
cDNA for O6-methylguanine-DNA methyltransferase was isolated by screening rat liver cDNA libraries, using as a probe the human cDNA sequence for methyltransferase. The rat cDNA encodes a protein with 209 amino acid residues. The predicted amino acid sequence of the rat methyltransferase exhibits considerable homology with those of the human, yeast and bacterial enzymes, especially around putative methyl acceptor sites. When the cDNA was placed under control of the lac promoter and expressed in methyltransferase-deficient Escherichia coli (ada-, ogt-) cells, a characteristic methyltransferase protein was produced. The rat DNA methyltransferase thus expressed could complement the biological defects of the E. coli cell caused by lack of its own DNA methyltransferases; e.g. increased sensitivity to alkylating agents in terms of both cell death and mutation induction.
Collapse
Affiliation(s)
- K Sakumi
- Department of Biochemistry, Kyushu University, Fukuoka, Japan
| | | | | | | |
Collapse
|
50
|
Inducibility of the DNA repair gene encoding O6-methylguanine-DNA methyltransferase in mammalian cells by DNA-damaging treatments. Mol Cell Biol 1991. [PMID: 1875945 DOI: 10.1128/mcb.11.9.4660] [Citation(s) in RCA: 100] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The inducibility of the mammalian O6-methylguanine-DNA methyltransferase (MGMT) gene encoding the MGMT protein (EC 2.1.1.63) responsible for removal of the procarcinogenic and promutagenic lesion O6-alkylguanine from DNA was examined by an analysis of transcription of the MGMT gene following exposure of repair-competent (Mex+) and repair-deficient (Mex-) cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). While human and rodent Mex- cells (CHO-9, V79, HeLa MR) showed no detectable MGMT mRNA despite the presence of the gene in their genome, the amount of it in several Mex+ lines (NIH 3T3, HeLa S3, HepG2) paralleled their MGMT activity. However, none of these cell lines showed an increase in the MGMT mRNA level after treatment with various concentrations of MNNG. In contrast, MNNG-treated rat hepatoma cells, H4IIE and FTO-2B, both Mex+, had three- to fivefold more MGMT mRNA than the corresponding untreated controls as measured 12 to 72 h after alkylation. N-Methyl-N-nitrosourea, methyl methanesulfonate, N-hydroxyethyl-N-chloroethylnitrosourea, UV light, and X rays caused a similar accumulation of MGMT mRNA in rat hepatoma cells. Studies with inhibitors of RNA and protein synthesis indicate that the induced increase in the amount of MGMT mRNA was due to enhanced transcription of the gene. Furthermore, they revealed the turnover of the MGMT mRNA to be relatively low (half-life, greater than 7 h). Mutagen-induced increase of transcription of MGMT mRNA in H4IIE cells was accompanied by elevation of MGMT repair activity and resulted in reduction of mutation frequency after a challenge dose of MNNG. Although induction of MGMT mRNA transcription has been observed in two rodent hepatoma cell lines so far, this appears to be the first demonstration of inducibility of a mammalian gene encoding a clearly define DNA repair function. The transcription activation of the MGMT gene protects cells from the mutagenic effects of methylating agents.
Collapse
|