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Alkylation, DNA leads to Increase, Heritable mutations in offspring
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||Carole Yauk (send email)||Open for citation & comment||WPHA/WNT Endorsed|
Life Stage Applicability
Key Event Relationship Description
Pre-meiotic male germ cells are outside of the blood-testis barrier and thus are exposed if there is systemic distribution. Exposure of pre-meiotic male germ cells to DNA alkylating agents results in DNA alkyl adducts. Replication of DNA with alkyl adducts thus can cause mutations in these cells. Fertilization of an egg by sperm containing mutations causes an increase in the number of mutations that are transmitted to their offspring.
Evidence Collection Strategy
Evidence Supporting this KER
Alkylating agents are prototypical mutagens in laboratory animals. It is established that these agents, especially those chemicals that preferentially cause O-alkylation in DNA, induce heritable mutations. ENU (N-ethyl-N-nitrosourea) is a prototypical agent used to derive offspring with de novo mutations inherited from exposed males (e.g., http://ja.brc.riken.jp/lab/mutants/genedriven.htm). In fact, ENU mutagenicity is a standard bench tool for genetic screens used to identify new mutations associated with a phenotype of interest.
A variety of alkylating agents are positive in the mouse specific locus test demonstrating that they cause heritable mutations in offspring as a result of exposure of pre-meiotic male germ cells. These agents include ENU, methyl nitrosourea (MNU), procarbazine and melphalan. This has been thoroughly reviewed by Marchetti and Wyrobek (2005) and Witt and Bishop (1996). It should be noted that procarbazine and melphalan predominantly cause N-alkyl adducts and yield a weaker response in the specific locus test assay in male pre-meiotic germs (these agents yield higher responses in post-meiotic stages of spermatogenesis).
Uncertainties and Inconsistencies
As described above, not all alkylating agents cause heritable mutations as a result of mutations arising in spermatogonia. O-alkylation is critical, and the size of the alkyl group is important, with ENU exhibiting an order of magnitude greater response than MNU. Although there are no inconsistencies based on knowledge of the spectrum of adducts expected for specific alkylating agents, the database on which this KER is assessed is nearly exclusively centered on ENU. Moreover, a key data gap includes evidence of the effect of alkylating agents in the offspring of exposed humans.
Very little data are available on exposed humans despite the fact that humans may be exposed to high doses of alkylating agents during chemotherapy. Thus far the evidence has not supported that the cancer treatments pose heritable mutagenic hazards based on assessment of cancer (Madanat-Harjuoja et al., 2011), minisatellite mutations (Tawn et al., 2013) and congenital anomalies (Signorello et al., 2012) in offspring, or minisatellite mutation analysis in sperm ( Zheng et al., 2000; Armour et al., 199). However, cancer therapies are complex combinations of drugs that include agents that generally induce N-alkylation rather than O-alkylation. It has been suggested that the search for human germ cell mutagens has been flawed by lack of appropriate power, focus on the wrong agents, and using the wrong tools (DeMarini, 2012).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
That alkylation of DNA causes heritable mutations has been demonstrated specifically in flies, fish, and rodents. However, it is assumed that alkylating agents would act broadly on virtually any DNA sequence, in any organism, in any cell type. Thus, as long as the species has male germ cells, this KER would be relevant to that species.
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