title: Influence of DNA repair on nonlinear dose-responses for mutation.
abstract: Recent evidence has challenged the default assumption that all DNA-reactive alkylating agents exhibit a linear dose-response. Emerging evidence suggests that the model alkylating agents methyl- and ethylmethanesulfonate and methylnitrosourea (MNU) and ethylnitrosourea observe a nonlinear dose-response with a no observed genotoxic effect level (NOGEL). Follow-up mechanistic studies are essential to understand the mechanism of cellular tolerance and biological relevance of such NOGELs. MNU is one of the most mutagenic simple alkylators. Therefore, understanding the mechanism of mutation induction, following low-dose MNU treatment, sets precedence for weaker mutagenic alkylating agents. Here, we tested MNU at 10-fold lower concentrations than a previous study and report a NOGEL of 0.0075 µg/ml (72.8nM) in human lymphoblastoid cells, quantified through the hypoxanthine (guanine) phosphoribosyltransferase assay (OECD 476). Mechanistic studies reveal that the NOGEL is dependent upon repair of O(6)-methylguanine (O(6)MeG) by the suicide enzyme O(6)MeG-DNA methyltransferase (MGMT). Inactivation of MGMT sensitizes cells to MNU-induced mutagenesis and shifts the NOGEL to the left on the dose axis.
authors.affiliation: Institute of Life Science, College of Medicine, Swansea University, SA2 8PP, UK.
authors.forename: Adam D
authors.initials: AD
authors.lastname: Thomas
authors.forename: Gareth J S
authors.initials: GJ
authors.lastname: Jenkins
authors.forename: Bernd
authors.initials: B
authors.lastname: Kaina
authors.forename: Owen G
authors.initials: OG
authors.lastname: Bodger
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Tomaszowski
authors.forename: Paul D
authors.initials: PD
authors.lastname: Lewis
authors.forename: Shareen H
authors.initials: SH
authors.lastname: Doak
authors.forename: George E
authors.initials: GE
authors.lastname: Johnson
mesh_terms: D001483:Base Sequence; D002460:Cell Line; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors; D017931:DNA Primers; D004260:DNA Repair; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007041:Hypoxanthine Phosphoribosyltransferase / Q000235:genetics; D009153:Mutagens / Q000633:toxicity*; D009154:Mutation; D016133:Polymerase Chain Reaction; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors
chemical_list: D017931:DNA Primers; D009153:Mutagens; D025521:Tumor Suppressor Proteins; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D007041:Hypoxanthine Phosphoribosyltransferase; D045643:DNA Repair Enzymes