title: Spectrum of mutations induced by methyl and ethyl methanesulfonate at the hprt locus of normal and tag expressing Chinese hamster fibroblasts.
abstract: This work describes the isolation and characterization of methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS) induced 6-thioguanine-resistant mutants in normal and Escherichia coli tag gene expressing Chinese hamster fibroblast, RJKO, cells. It was previously shown that increased removal of 3-alkylated adenine, effected by 3-methyladenine DNA glycosylase I (Tag), reduces the frequencies of hprt mutations induced by alkylating agents which produce mostly N-alkylation (MMS and EMS) to half the normal rate. In order to identify which type of mutation is suppressed by increased 3-alkyladenine repair we have determined the DNA base sequence changes of the hprt cDNA in 61 independent MMS- and EMS-induced mutant clones. For both cell types and irrespective of the agent used, the majority of mutations were GC to AT transitions originating in the non-transcribed strand. Only 6/55 base substitutions occurred at AT base pairs: five AT to GC transitions and one AT to CG transversion. Six mutations were found to be deletions. These results indicate that 3-alkylated adenines in DNA are not directly premutagenic. The fact that the mutation frequency is reduced by increased 3-alkyladenine removal might be explained by postulating the existence in mammalian cells of an SOS-like response turned on by cytotoxic lesions like 3-alkyladenine, or, alternatively, that increased removal of 3-alkyladenine increases the number of single-strand breaks in DNA, which stalls DNA replication and allows a prolonged time for DNA repair by the alkyltransferase.
authors.affiliation: Biotechnology Centre of Oslo, Norway.
authors.forename: A
authors.initials: A
authors.lastname: Klungland
authors.forename: K
authors.initials: K
authors.lastname: Laake
authors.forename: E
authors.initials: E
authors.lastname: Hoff
authors.forename: E
authors.initials: E
authors.lastname: Seeberg
mesh_terms: D000478:Alkylation; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D045647:DNA Glycosylases; D017931:DNA Primers / Q000737:chemistry; D004260:DNA Repair; D005020:Ethyl Methanesulfonate / Q000633:toxicity*; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics*; D008741:Methyl Methanesulfonate / Q000633:toxicity*; D008969:Molecular Sequence Data; D016296:Mutagenesis / Q000187:drug effects; D009699:N-Glycosyl Hydrolases / Q000378:metabolism*; D013866:Thioguanine / Q000494:pharmacology; D014162:Transfection
chemical_list: D017931:DNA Primers; D005020:Ethyl Methanesulfonate; D008741:Methyl Methanesulfonate; D007041:Hypoxanthine Phosphoribosyltransferase; C021565:3-methyladenine-DNA glycosylase; D045647:DNA Glycosylases; D009699:N-Glycosyl Hydrolases; D013866:Thioguanine