title: Epigenetic regulation of the MGMT and hMSH6 DNA repair genes in cells resistant to methylating agents.
abstract: We investigated the relationship between DNA cytosine methylation and the expression of two genes associated with resistance to DNA methylation damage. Variants of RajiMex- cells acquired resistance to N-methyl-N-nitrosourea by either reactivating a previously silent O6-methylguanine-DNA methyltransferase (MGMT) gene or by repressing the hMSH6 mismatch repair gene. DNA sequencing and measurements of mRNA and enzyme levels revealed that MGMT activity was not correlated with methylation of the core MGMT promoter. Treatment with the demethylating agent 5-azadeoxycytidine reduced MGMT mRNA and enzyme levels, indicating that methylation of some nonpromoter sequences may be required for MGMT gene expression. In contrast, both hMSH6 mRNA and protein levels were increased by 5-azadeoxycytidine treatment of an N-methyl-N-nitrosourea-resistant variant that did not express detectable hMSH6, which implies that this gene was transcriptionally silenced by cytosine methylation. This could be substantiated by in vitro modification of the CpG sites in the hMSH6 promoter with restriction methylase M.SssI, which abolished the transcription of a reporter gene under its control in a transient transfection assay. Taken together, our data show that treatment with chemical methylating agents alters gene expression patterns through increased CpG methylation of genomic DNA, and thereby permits the emergence and selection of clones that are resistant to these agents due to increased repair or tolerance of O6-methylguanine.
authors.affiliation: Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Bearzatto
authors.forename: M
authors.initials: M
authors.lastname: Szadkowski
authors.forename: P
authors.initials: P
authors.lastname: Macpherson
authors.forename: J
authors.initials: J
authors.lastname: Jiricny
authors.forename: P
authors.initials: P
authors.lastname: Karran
mesh_terms: D000477:Alkylating Agents / Q000494:pharmacology; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D001374:Azacitidine / Q000031:analogs & derivatives / Q000494:pharmacology; D015153:Blotting, Western; D018899:CpG Islands / Q000235:genetics; D003596:Cytosine / Q000378:metabolism; D019175:DNA Methylation; D004260:DNA Repair / Q000187:drug effects / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D000077209:Decitabine; D004305:Dose-Response Relationship, Drug; D019008:Drug Resistance, Neoplasm; D020868:Gene Silencing; D006801:Humans; D008156:Luciferases / Q000378:metabolism; D008770:Methylnitrosourea / Q000494:pharmacology; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000235:genetics* / Q000378:metabolism; D010957:Plasmids; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D000477:Alkylating Agents; D000964:Antimetabolites, Antineoplastic; D004268:DNA-Binding Proteins; C094443:G-T mismatch-binding protein; D012333:RNA, Messenger; D008770:Methylnitrosourea; D000077209:Decitabine; D003596:Cytosine; D008156:Luciferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D001374:Azacitidine