MGMT: Key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents
Introduction
Mutagens in the environment [1], in tobacco smoke [2] and food [3], as well as endogenous metabolic products [4] generate reactive electrophilic species that alkylate DNA. Despite their carcinogenic potential, monofunctional alkylating agents are used, due to their cytotoxic properties, in chemotherapy of various cancers such as Hodgkin's disease, non-Hodgkin's lymphoma, malignant melanoma, neuroblastoma, soft tissue sarcomas, pancreatic (islet cell) cancer, carcinoid tumors, astrocytoma, glioblastoma and brain metastasis from solid tumors. Examples of methylating anticancer drugs are procarbazine (PCB, PCZ, N-methyl-hydrazine, Natulan®, Matulane®), dacarbazine (DIC, imidazole carboxamide, dimethyl-triazeno-imidazole-carboxamide, DTIC®-Dome), streptozotocin (STZ, NSC 85998, Zanosar®) and temozolomide (TMZ, SCHS2.365, NSC 362856, Temodal®, Temodar®). PCB and DTIC require metabolic activation to form their DNA-reactive metabolite [5], [6] while STZ alkylates DNA without metabolic activation [7]. TMZ undergoes spontaneous hydrolysis at physiological pH to generate the active metabolite, 5-(3-methyl triazen-1-yl)imidazole-4-carboxamide (MTIC) [8] (Fig. 1). The O6-chloroethylating agents, which include carmustine (BCNU, BiCNU®), lomustine (CCNU, CeeNU®), nimustine (ACNU) and fotemustine (Muphoran®) are used in the treatment of glioblastoma and to a lesser extent in treating malignant melanoma, gastrointestinal and pancreatic cancer, Hodgkin's and non-Hodgkin's lymphoma. Various combinations of methylating and chloroethylating agents together with other anticancer drugs have been used [9], [10], [11] and trials are still ongoing, e.g. in glioma therapy [12].
All of the above-mentioned mutagens and chemotherapeutics react with DNA via an SN1 mechanism to form 12 base adducts and phosphotriester (Fig. 2). The SN1 reaction follows a first-order kinetics that is dependent on the formation of an electrophilic carbonium ion, which covalently binds to nucleophilic sites on DNA. Alkyl DNA base adducts have different stabilities. Thus, N3-methyladenine (N3MeA) and N3-methylguanine (N3MeG) are readily hydrolyzed, while other adducts, e.g. N7-methylguanine (N7MeG) are stable for longer times (T1/2 in vitro: 40–80 h) [13]. O6MeG is more stable and persists in DNA in the absence of O6-methylguanine-DNA methyltransferase (MGMT) [14], [15], [16]. N7MeG and N3MeA are the most frequent methyl adducts comprising 80–85% and 8–18% of total alkyl adducts, respectively. However, O6MeG accounting for O.3 (for methyl methanesulfonate) up to 8% (for methylnitrosourea) of the total DNA methyl adducts is the most critical lesion since it is pre-mutagenic and pre-toxic. Another pre-mutagenic methylation lesion is O4-methylthymine (O4MeT), which is induced at a much lower level (<0.4%) [13]. In this review, agents inducing O6-alkylguanine in DNA are collectively termed O6-alkylating agents (O6AA).
O6MeG and O4MeT are repaired by MGMT (for extensive reviews on MGMT see [17], [18], [19], [20], [21]). If not repaired, O6MeG can give rise to cell death, chromosomal aberrations, mutations and cancer. How much O4MeT contributes to these end points is not precisely known. This review will focus on the important role this DNA repair protein plays in preventing these detrimental endpoints. We will also briefly discuss other alkylation damage defense and processing functions (ABH, BER, MMR) and, finally, the mechanisms behind these endpoints triggered by the MGMT repaired lesions O6MeG and O6-chloroethylguanine.
Section snippets
Single step damage reversal by MGMT
MGMT (also referred to as ATase, AGT, AGAT; E.C. 2.1.1.63) repairs O6-alkylation adducts in a one-step alkyl transfer reaction that transfers the alkyl group from the oxygen in the DNA to a cystein residue in the catalytic pocket of MGMT, thereby restoring DNA and inactivating MGMT. As one MGMT molecule can repair only one alkyl adduct, the cells capacity for removing DNA O6-alkylguanine adducts depends on the total number of MGMT molecules per cell and the rate at which the cell can
One lesion, six endpoints and one defense
Although O6MeG comprises only a small fraction of DNA methyl adducts, it has attracted considerable attention because of its pleiotropic biological effects. O6MeG is responsible for causing point mutations, sister chromatid exchanges (SCEs), chromosomal aberrations, tumor initiation, tumor progression and cell kill. These endpoints, and the role of MGMT in protecting against them, will be discussed below.
Acknowledgements
This work was supported by Deutsche Forschungsgemeinschaft, grants KA724/13-1 and 13-2 and SFB 432/B7. We are grateful to Sankar Mitra and Geoff Margison for helpful comments and proofreading of the manuscript. We acknowledge Julia Hammerling for apoptosis measurements.
References (217)
Genotoxicity of tobacco smoke and tobacco smoke condensate: a review
Mutat. Res.
(2004)- et al.
Genotoxicity of heat-processed foods
Mutat. Res.
(2005) - et al.
Genotoxicity of streptozotocin
Mutat. Res.
(2002) - et al.
Phase II study of temozolomide and thalidomide with radiation therapy for newly diagnosed glioblastoma multiforme
Int. J. Radiat. Oncol. Biol. Phys.
(2004) Distribution of methyl and ethyl adducts following alkylation with monofunctional alkylating agents
Mutat. Res.
(1990)- et al.
DNA repair and chromosomal stability in the alkylating agent-hypersensitive Chinese hamster cell line 27-1
Mutat. Res.
(1990) Repair of O(6)-alkylguanine by alkyltransferases
Mutat. Res.
(2000)Properties of mammalian O6-alkylguanine-DNA transferases
Mutat. Res.
(1990)- et al.
Nuclear translocation of mismatch repair proteins MSH2 and MSH6 as a response of cells to alkylating agents
J. Biol. Chem.
(2000) - et al.
Intra- and intercellular variations in the repair efficiency of O6-methylguanine, and their contribution to kinetic complexity
Mutat. Res.
(2004)