To the extent possible under law, AOP-Wiki has waived all copyright and related or neighboring rights to KE:97

Event: 97

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Alkylation, DNA

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Alkylation, DNA

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Cell term
eukaryotic cell

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
DNA alkylation deoxyribonucleic acid increased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Alkylation of DNA leading to heritable mutations MolecularInitiatingEvent Carole Yauk (send email) Open for citation & comment TFHA/WNT Endorsed
DNA alkylation -> cancer 1 MolecularInitiatingEvent Carole Yauk (send email) Open for adoption
DNA alkylation -> cancer 2 MolecularInitiatingEvent Carole Yauk (send email) Not under active development
Alkylation of DNA leading to reduced sperm count MolecularInitiatingEvent Carole Yauk (send email) Under development: Not open for comment. Do not cite


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
mouse Mus musculus High NCBI
Syrian golden hamster Mesocricetus auratus High NCBI
rat Rattus norvegicus High NCBI
Homo sapiens Homo sapiens High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help

Sex Applicability

No help message More help
Term Evidence
Mixed High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

The event involves DNA alkylation to form a variety of different DNA adducts (i.e., alkylated nucleotides). Alkylation occurs at various sites in DNA and can include alkylation of adenine- Nl, - N3, - N7, guanine- N3, - O6, - N7, thymine-O2, - N3, - O4, cytosine- O2, -N3, and the phosphate (diester) group (reviewed in detail in Beranek 1990). In addition, alkylation can involve modification with different sizes of alkylation groups (e.g., methyl, ethyl, propyl). It should be noted that many of these adducts are not stable or are readily repaired (discussed in more detail below). A small proportion of adducts are stable and remain bound to DNA for long periods of time.

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

There is no OECD guideline for measurement of alkylated DNA, although technologies for their detection are established. Reviews of modern methods to measure DNA adducts include Himmelstein et al,. 2009 and Philips et al., 2000.

High performance liquid chromatography (HPLC) methods can be used to measure whether an agent is capable of alkylating DNA in somatic cells. Alkyl adducts in somatic cells can be measured using immunological methods (described in Nehls et al. 1984), as well as HPLC (methods in de Groot et al. 1994) or a combination of 32P post-labeling, HPLC and immunologic detection (Kang et al. 1992). We note that mass spectrometry provides structural specificity and confirmation of the structure of DNA adducts.

DNA alkylation can also be measured using a modified comet assay. This method involves the digestion of alkylated DNA bases with 3–methyladenine DNA glycosylase (Collins et al., 2001; Hasplova et al., 2012) followed by the standard comet assay to detect where alkyl adducts occur. The advantage of this method is that the alkaline version of the comet assay, as a core method, has an in vivo OECD guideline.

Finally, structure-activity relationships (SARs) have been developed to predict the possibility that a chemical will alkylate DNA (e.g., Vogel and Ashby, 1994; Benigni, 2005; Dai et al., 1989; Lewis and Griffith, 1987).

Measurement of alkylation in male germ cells:

In rodent testes, studies have detected adducts in situ by immuhistocytological staining. For example, fixed testes are incubated with O6-EtGua -specific mouse monoclonal antibody and subsequently with a labeled anti-mouse IgG F antibody. Nuclear DNA is counterstained with DAPI 4,6-diamidino- 2-phenylindole. Fluorescence signals from immunostained O6-EtGua residues in DNA are visualized by fluorescence microscopy and quantitated using an image analysis system. Methods are described in (Seiler et al. 1997). An immunoslot blot assay for detection of O6-EtGua has been described previously in (Mientjes et al. 1996).

Alternatively, rodents have also been exposed to radio-labeled alkylating agents. Examples from the literature include [2-3H] ENU, [1-3H]di-ethyl sulfate, or [1-3H]ethyl-methane sulfonate. Following treatment with the labeled chemical, testis and other tissues of interest are removed. Germ cells are released from tubuli by pushing out the contents with forceps. Using this procedure all germ-cell stages are liberated from the tubuli, with the possible exception of part of the population of stem-cell spermatogonia that remain attached to the walls of the tubuli. DNA is then extracted from germ cells, empty testis tubuli and other tissues of interest. DNA adduct formation is determined after neutral and acid hydrolysis of DNA followed by separation of the various ethylation products using HPLC (described in van Zeeland et al. 1990).

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

Alkylated DNA has been measured in somatic cells in a variety of species, from prokaryotic organisms, to rodents in vivo, to human cells in culture. Theoretically, DNA alkylation can occur in any cell type in any organism.

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.

Alkylating agents are prototypical DNA-reactive compounds and have been extensively studied for decades (reviewed in Beranek 1990). The chemicals can be direct-acting electrophiles, or can be converted from non-reactive substances to reactive metabolites via metabolism. A prototypical alkylating agent is N-ethyl-N-nitrosourea (chemical formula C3H7N3O2) (ENU). ENU is rapidly absorbed following oral exposure and intraperitoneal injections and distributed widely across the tissues. ENU is unstable and readily reacts with somatic and germ cell DNA in mice, rats, flies and hamsters, to alkylate DNA. Very generally, mono-functional (referring to the transfer of a single alkyl group) alkylating agents include: 1. Alkyl sulfates: e.g., diethyl (DES) and dimethyl sulfate (DMS); 2. Alkyl alkanesulfonates: e.g., methyl (MMS) and ethyl methanesulfonate (EMS); 3. Nitrosamides: e.g., methyl (MNU) and ethyl nitrosourea (ENU), methyl- (MNNG) and ethyl-N'-nitro-N-nitrosoguanidine (ENNG), and the indirect-acting (i.e., requiring metabolic activation) dimethyl (DMN) and diethyl nitrosamines (DEN).

ENU is the most widely studied and understood alkylating agent and as such has been instrumental in contributing to the knowledgebase in this field. Immunohistochemistry studies clearly indicate the presence of alkylated DNA following exposure to ENU in both somatic cells and spermatogonia (Kamino et al. 1995; Seiler et al. 1997; van Zeeland et al. 1990).


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help

Benigni, R. (2005), "Structure-activity relationship studies of chemical mutagens and carcinogens: mechanistic investigations and prediction and approaches", Chem. Rev., 105: 1767-1800.

Beranek, D.T. (1990), "Distribution of methyl and ethyl adducts following alkylation with monofunctional alkylating agents", Mutation Res., 231: 11-30.

Collins, A.R., M. Dusinská and A. Horská (2001), "A Detection of alkylation damage in human lymphocyte DNA with the comet assay". Acta Biochim Pol., 48: 611-4.

Dai, Q.H. and R.G. Zhong (1989), "Quantitative pattern recognition for structure-carcinogenic activity relationship of N-nitroso compounds based upon Di-region theory", Sci China B., 32:776-790.

de Groot, A.J., J.G. Jansen, C.F. van Valkenburg and A.A. van Zeeland (1994), "Molecular dosimetry of 7-alkyl- and O6-alkylguanine in DNA by electrochemical detection", Mutat Res., 307: 61-6.

Hašplová, K., A. Hudecová, Z. Magdolénová, M. Bjøras, E. Gálová, E. Miadoková and M. Dušinská (2012), "DNA alkylation lesions and their repair in human cells: modification of the comet assay with 3-methyladenine DNA glycosylase (AlkD)", Toxicol Lett., 208: 76-81.

Himmelstein, M.W., P.J. Boogaard, J. Cadet, P.B. Farmer, J.J. Kim, E.A. Martin, R. Persaud and D.E. Shuker (2009), "Creating context for the use of DNA adduct data in cancer risk assessment: II. Overview of methods of identification and quantitation of DNA damage", Crit. Rev. Toxicol., 39: 679-94.

Kamino, K., F. Seiler, M. Emura, J. Thomale, M.F. Rajewsky and U. Mohr (1995), "Formation of O6-ethylguanine in spermatogonial DNA of adult Syrian golden hamster by intraperitoneal injection of diethylnitrosamine", Exp. Toxicol. Pathol., 47: 443-445.

Kang, H.I., C. Konishi, G. Eberle, M.F. Rajewsky, T. Kuroki and N.H. Huh (1992), "Highly sensitive, specific detection of O6-methylguanine, O4-methylthymine, and O4-ethylthymine by the combination of high-performance liquid chromatography prefractionation, 32P postlabeling, and immunoprecipitation", Cancer Res., 52: 5307-5312.

Lewis, D.F. and V.S. Griffiths (1987), "Molecular electrostatic potential energies and methylation of DNA bases: a molecular orbital-generated quantitative structure-activity relationship", Xenobiotica, 17: 769-776.

Mientjes, E.J., K. Hochleitner, A. Luiten-Schuite, J.H. van Delft, J. Thomale, F. Berends, M.F. Rajewsky, P.H. Lohman and R.A. Baan (1996), "Formation and persistence of O6-ethylguanine in genomic and transgene DNA in liver and brain of lambda(lacZ) transgenic mice treated with N-ethyl-N-nitrosourea", Carcinogenesis, 17: 2449-2454.

Nehls, P., M.F. Rajewsky, E. Spiess, D. Werner (1984), "Highly sensitive sites for guanine-O6 ethylation in rat brain DNA exposed to N-ethyl-N-nitrosourea in vivo", EMBO J., 3:327-332.

Phillips, D.H., P.B. Farmer, F.A. Beland, R.G. Nath, M.C. Poirier, M.V. Reddy and K.W. Turteltaub (2000), "Methods of DNA adduct determination and their application to testing compounds for genotoxicity", Environ. Mol. Mutagen., 35: 222-233.

Scherer, E., A.A. Jenner and L. den Engelse (1987), "Immunocytochemical studies on the formation and repair of O6-alkylguanine in rat tissues", IARC Sci. Publ., 84: 55-58.

Sega, G.A., C.R. Rohrer, H.R. Harvey and A.E. Jetton (1986), "Chemical dosimetry of ethyl nitrosourea in the mouse testis", Mutat. Res., 159: 65-74.

Seiler, F., K. Kamino, M. Emura, U. Mohr and J. Thomale (1997), "Formation and persistence of the miscoding DNA alkylation product O6-ethylguanine in male germ cells of the hamster", Mutat. Res., 385: 205-211.

van Zeeland, A.A., A. de Groot and A. Neuhäuser-Klaus (1990), "DNA adduct formation in mouse testis by ethylating agents: a comparison with germ-cell mutagenesis", Mutat. Res. 231: 55-62.

Vogel, E.W., Ashby, J. (1994), "Structure-activity relationships: experimental approaches." In: Methods to asses DNA Damage and repair: Interspecies comparisons. Edited by R.T. Tardiff, P.H.M. Lohman and G.N. Wogan, SCOPE, Wiley and Sons LTD.