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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 O6-methylguanine (O6MeG) by the suicide enzyme O6MeG-DNA methyltransferase (MGMT). Inactivation of MGMT sensitizes cells to MNU-induced mutagenesis and shifts the NOGEL to the left on the dose axis. [ABSTRACT FROM AUTHOR]

A recent shift by the scientific and regulatory community, towards accepting the existence of non-linear dose responses for certain DNA reactive genotoxic agents, has unveiled a myriad of questions regarding their biological basis. The mechanisms responsible for `genotoxic tolerance' at low doses are wide ranging but poorly understood, yet this information is essential when analysing non-linear dose responses for hazard and risk assessment. For DNA reactive genotoxins, non-linear dose responses can arise from many diŠerent biological mechanisms, including DNA repair. Recent work from our group explored the contributory role of DNA repair to nonlinear genotoxic dose responses, in human cells exposed alkylating agents. Here we discuss the involvement of the repair enzymes methylpurine DNA-glycosylase and methyl-guanine methyl-transferase in modulating the non-linear dose responses observed in human cells exposed to ethyl methanesulfonate (EMS) and N-methyl-N-nitrosourea, respectively. We also discuss the exposure of binary mixtures, and how combinations of the dissimilar acting agents Benomyl and EMS at their no observed genotoxic eŠect levels, induce a significant increase in micronuclei. [ABSTRACT FROM AUTHOR]