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Alkylation, DNA leads to Increase, Mutations
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Alkylation of DNA in male pre-meiotic germ cells leading to heritable mutations||non-adjacent||High||Moderate||Evgeniia Kazymova (send email)||Open for citation & comment||WPHA/WNT Endorsed|
|Alkylation of DNA leading to cancer 1||non-adjacent||High||Moderate||Arthur Author (send email)||Open for adoption|
Life Stage Applicability
Key Event Relationship Description
Alkylated DNA may be ‘misread’ during DNA replication, leading to insertion of incorrect nucleotides. Upon replication, these changes become fixed as mutations. Subsequent replication propagates these mutations to daughter cells. Mutations in stem cells are of the greatest concern, as these will persist throughout the organism’s lifetime. Thus, increased mutations will be found in the cells of organisms that possess alkylated DNA.
Evidence Collection Strategy
Evidence Supporting this KER
Alkylating agents can cause a variety of adducts and DNA damage (e.g., alkali labile sites, DNA strand breaks, etc.) that are potentially mutagenic and clastogenic. This KER focuses on the probability that an alkyl DNA adduct will lead to a mutation.
Not all adducts are equally mutagenic. Very generally, chemicals that preferentially cause O-alkylation in DNA induce DNA sequence changes, whereas chemicals that cause N-alkylation of DNA are more efficient inducers of structural chromosomal aberrations (reviewed in Beranek 1990). Indeed, a review of the biological significance of N7 alkyl-guanine adducts concluded that these adducts simply be used to confirm exposure to target tissue (Boysen et al., 2009), because the vast majority of studies shows that these adducts do not cause mispairing. A variety of work has demonstrated that N7-alkylguanine adducts can be bypassed essentially error free (e.g., Philippin et al., 2014; Shrivastav et al., 2010). Moreover, alkylation can involve modification with different sizes of alkylation groups (e.g., methyl, ethyl, propyl). Although response to these is qualitatively similar with respect to the key events, in general, larger alkylating groups tend to be more mutagenic (Beranek, 1990). It is widely known that chemicals that preferentially cause O-alkylation in DNA induce mutations. ENU (N-ethyl-N-nitrosourea) is a prototypical O-alkylating agent and the most studied male germ cell mutagen.
Alkylating agents are prototypical somatic and male germ cell mutagens.
Uncertainties and Inconsistencies
As described above, not all alkyl adducts are mutagenic. The proportion of oxygen-alkylation and the type of mutation (with ethylation > methylation) will govern mutagenicity, but there are few empirical data to support this. There are no inconsistencies or uncertainties for ENU or iPMS; other alkylating agents (EMS, MMS) have yielded some discrepancies in the transgenic rodent mutation assay. However, the experimental protocols applied were sub-standard (the OECD TG for this analysis was revised and published in 2013). Overall, more work is needed on alkylating agents other than ENU to fill important data gaps.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Alkylating agents are well-established to cause mutation in virtually any cell type in any organism.
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