title: Sublinear response in lacZ mutant frequency of Muta™ Mouse spermatogonial stem cells after low dose subchronic exposure to N-ethyl-N-nitrosourea.
abstract: The transgenic rodent mutation assay was used to compare the dose-response relationship of lacZ mutant frequency (MF) in spermatogonial stem cells exposed acutely or subchronically to N-ethyl-N-nitrosourea (ENU). Muta(™) Mouse males were exposed orally to 0, 25, 50, or 100 mg/kg ENU for acute exposures and 0, 1, 2, or 5 mg/(kg day) for 28-day subchronic exposures. LacZ MF was measured in sperm collected 70 days post-exposure to target spermatogonial stem cells. Dose-response data were fit to linear, quadratic, exponential, or power models. Acute exposure resulted in a dose-dependent increase in MF that was significant (P < 0.05) at all doses tested and was best described by a quadratic dose-response model that was linear in the low dose range. In contrast, similar total doses fragmented over a 28-day subchronic exposure only resulted in a significant increase in lacZ MF at the highest dose tested. Therefore, the subchronic no observable genotoxic effect level (NOGEL) was 2 mg/(kg day) (or 56 mg/kg total dose). The subchronic dose-response was best described by the exponential and power models, which were sublinear in the low dose range. Benchmark dose lower confidence limits (BMDLs) for acute and subchronic exposure were 3.0 and 1.0 mg/(kg day) (or 27.4 mg/kg total dose), respectively. These findings are supportive of a saturable DNA repair mechanism as the mutagenic mode of action for ENU in spermatogonia and imply that sufficiently low exposures would not cause appreciable genotoxic effects over background. This may have important implications for the quantitative risk assessment of germ cell mutagens.
authors.affiliation: Environmental Health Science and Research Bureau, Health Canada, Tunney's Pasture, 0803A, Ottawa, ON, K1A 0K9, Canada.
authors.forename: Jason M
authors.initials: JM
authors.lastname: O'Brien
authors.forename: Mike
authors.initials: M
authors.lastname: Walker
authors.forename: Ahalya
authors.initials: A
authors.lastname: Sivathayalan
authors.forename: George R
authors.initials: GR
authors.lastname: Douglas
authors.forename: Carole L
authors.initials: CL
authors.lastname: Yauk
authors.forename: Francesco
authors.initials: F
authors.lastname: Marchetti
mesh_terms: D053687:Adult Stem Cells / Q000187:drug effects*; D000818:Animals; D004249:DNA Damage / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005038:Ethylnitrosourea / Q000633:toxicity*; D007763:Lac Operon; D008297:Male; D008822:Mice, Transgenic; D009153:Mutagens / Q000633:toxicity; D009154:Mutation; D059645:Mutation Rate; D059550:Toxicity Tests, Subchronic / Q000379:methods*
chemical_list: D009153:Mutagens; D005038:Ethylnitrosourea
keywords: N-ethyl-N-nitrosourea; dose-response modeling; germ cell mutation; spermatogonia; threshold; transgenic rodent mutation assay