Sublinear response in lacZ mutant frequency of Muta™ Mouse spermatogonial stem cells after low dose subchronic exposure to N-ethyl-N-nitrosourea
- DOI
- Language of the publication
- English
- Date
- 2015-01-17
- Type
- Article
- Author(s)
- O'Brien, Jason M.
- Walker, Mike
- Sivathayalan, Ahalya
- Douglas, George R.
- Yauk, Carole L.
- Marchetti, Francesco
- Publisher
- Wiley
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.
Plain language summary
Health Canada is responsible for assessing the health risks posed by exposure to chemicals. One important component of this risk assessment is to characterize the relationship between the levels of exposure and any associated toxic effects (dose-response). The dose-response relationship is used by risk assessors to determine acceptable exposure levels. Currently, there is contention in the regulatory community about the characteristics of the dose-response for chemicals that cause DNA damage, especially at low doses and in germ cells (sperm and egg). In this study, a transgenic rodent mutation assay was used to characterize the dose-response relationship between exposure to the model DNA-damaging chemical N-ethyl-N-nitrosourea (ENU) and DNA mutations in the germ cells of exposed male mice. The dose-response after a single exposure to high doses (acute exposure) was also compared to the dose-response in mice exposed to similar total doses fractionated over a 28-day period (sub-chronic exposure). The results showed a linear dose-response in germ cell mutation for mice exposed acutely to ENU, where all dose groups had significantly higher mutation compared to control animals, and the amount of mutation was proportional to the dose received. In contrast, the dose-response after sub-chronic exposure was non-linear, such that only the highest dose group had significantly more mutation than control animals. The degree of mutation in all other dose groups was indistinguishable from controls. This outcome suggests that germ cells have a coping mechanism to prevent mutation from low levels of ENU exposure. This is contrary to the current paradigm, which assumes that there are no levels of exposure that do not result in mutation. The results from this study have important implications for the quantitative risk assessment of DNA-damaging chemicals. Additionally, this work demonstrates the utility of the transgenic rodent mutation assay for quantitative dose-response analysis in germ cells.
Subject
- Health,
- Health and safety