Contributions of DNA repair and damage response pathways to the non-linear genotoxic responses of alkylating agents

Abstract

From a risk assessment perspective, DNA-reactive agents are conventionally assumed to have genotoxic risks at all exposure levels, thus applying a linear extrapolation for low-dose responses. New approaches discussed here, including more diverse and sensitive methods for assessing DNA damage and DNA repair, strongly support the existence of measurable regions where genotoxic responses with increasing doses are insignificant relative to control. Model monofunctional alkylating agents have in vitro and in vivo datasets amenable to determination of points of departure (PoDs) for genotoxic effects. A session at the 2013 Society of Toxicology meeting provided an opportunity to survey the progress in understanding the biological basis of empirically-observed PoDs for DNA alkylating agents. Together with the literature published since, this review discusses cellular pathways activated by endogenous and exogenous alkylation DNA damage. Cells have evolved conserved processes that monitor and counteract a spontaneous steady-state level of DNA damage. The ubiquitous network of DNA repair pathways serves as the first line of defense for clearing of the DNA damage and preventing mutation. Other biological pathways discussed here that are activated by genotoxic stress include post-translational activation of cell cycle networks and transcriptional networks for apoptosis/cell death. The interactions of various DNA repair and DNA damage response pathways provide biological bases for the observed PoD behaviors seen with genotoxic compounds. Thus, after formation of DNA adducts, the activation of cellular pathways can lead to the avoidance a mutagenic outcome. The understanding of the cellular mechanisms acting within the low-dose region will serve to better characterize risks from exposures to DNA-reactive agents at environmentally-relevant concentrations.

BPDE-induced genotoxicity: relationship between DNA adducts, mutagenicity in the in vitro PIG-A assay, and the transcriptional response to DNA damage in TK6 cells

“…This is of particular interest in case of low, for human exposure relevant levels. As stated in the introduction, there have been numerous contributions stating that for alkylating agents there may be a dose range where DNA adducts are measurable but do not result in increased mutation frequencies; for recent review see (Klapacz et al 2016 ). Nevertheless, this raises the question whether this is a general phenomenon or restricted to certain DNA lesions.…”

“…Especially directly genotoxic, i.e., DNA-reactive agents or their DNA-reactive metabolites, are generally assumed to represent risk factors at any concentration, following a linear dose–response also in the low concentration range, implying that even one or a few DNA lesions may result in mutations, and thus, may increase tumor risk. This assumption has repeatedly been challenged during the last years, due to observations that, for example, in case of alkylating compounds such as EMS DNA lesions are linear also in the low dose range, while increases in mutation frequencies follow a non-linear dose–response relationship (Doak et al 2007 ; Gocke and Müller 2009 ; Jenkins et al 2010 ; Pottenger et al 2009 ; for recent review see Klapacz et al 2016 ). This raised the question whether this may apply for all genotoxic substances, due to, for example, complete repair, as well as other DNA damage response systems in cases of low levels of DNA damage, which are overwhelmed in case of higher levels of DNA adducts (Greim and Albertini 2014 ; Klapacz et al 2016 ).…”

“…This assumption has repeatedly been challenged during the last years, due to observations that, for example, in case of alkylating compounds such as EMS DNA lesions are linear also in the low dose range, while increases in mutation frequencies follow a non-linear dose–response relationship (Doak et al 2007 ; Gocke and Müller 2009 ; Jenkins et al 2010 ; Pottenger et al 2009 ; for recent review see Klapacz et al 2016 ). This raised the question whether this may apply for all genotoxic substances, due to, for example, complete repair, as well as other DNA damage response systems in cases of low levels of DNA damage, which are overwhelmed in case of higher levels of DNA adducts (Greim and Albertini 2014 ; Klapacz et al 2016 ). Clearly, with respect to a potential impairment of genomic stability, a distinction has to be made between the occurrence of DNA adducts, which may be repaired, and there conversion into mutations, i.e., irreversible alteration of the genetic information.…”

Next-generation DNA damage sequencing

“…133 Exogenous alkylating agents also contribute to the occurrence of O 6 -MedG in the genome, including natural and anthropogenic components of air, water, and food, as well as several chemotherapeutic agents. 134 Fig . 6 Genome-wide mapping method for BPDE-dG NER repair events.…”

The safety evaluation of food flavoring substances: the role of genotoxicity studies

“…Theoretically, genetic damage poses a safety concern only if (a) interaction with genetic material is likely to occur in vivo; (b) the genetic interaction, which is a stochastic event, occurs at a relevant genetic locus in a coding or otherwise functional DNA sequence (rather than as a silent DNA modification); (c) repair is insufficient (DNA repair capacity is exceeded); and (d) the phenotype of the genetic damage has biological consequences (i.e. leads to cancer, germ cell damage, or other cell/tissue disruption) (Vogelstein et al 2013;Klapacz et al 2016;Liu et al 2016;Basu 2018).…”