This Key Event Relationship is licensed under the Creative Commons BY-SA license. This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.

Relationship: 1939


A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Increase, DNA strand breaks leads to Increase, Chromosomal aberrations

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Oxidative DNA damage leading to chromosomal aberrations and mutations non-adjacent High Low Brendan Ferreri-Hanberry (send email) Open for comment. Do not cite WPHA/WNT Endorsed
Deposition of energy leading to lung cancer non-adjacent High Low Brendan Ferreri-Hanberry (send email) Open for citation & comment WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER.  More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
rat Rattus norvegicus High NCBI
mouse Mus musculus High NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help
Sex Evidence
Unspecific High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER.  More help
Term Evidence
All life stages High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

DNA strand breaks (single and double) can arise from endogenous processes (e.g., topoisomerase reaction, excision repair, and VDJ recombination) and exogenous insults (e.g., replications stressors, ionizing radiation, and reactive oxygen species). Single strand breaks (SSBs) are generally repaired rapidly without error. However, multiple SSBs in close proximity to each other and interference of replication by unrepaired SSBs can lead to double strand breaks (DSB). DSB are more difficult to repair and are more toxic than SSB (Kuzminov, 2001). DSBs may lead to chromosomal breakages that may permanently alter the structure of chromosomes (i.e., chromosomal aberrations) and cause loss of DNA segments.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings.  Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help
Biological Plausibility
Addresses the biological rationale for a connection between KEupstream and KEdownstream.  This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured.   More help

DNA strand breaks are a necessity for chromosomal aberrations to occur. However, not all strand breaks lead to clastogenic events as most of them is repaired rapidly by a variety of different repair mechanisms. DNA DSBs are the critical damage because they lead to chromosome breakage. It is well-understood that unrepaired DSBs can lead to chromosomal aberrations. Studies have demonstrated DSBs leading to irreversible structural damage; for example, treatment of cultured cells with replication stress-inducing agents such as hydroxyurea induced micronuclei that are positive for gamma-H2AX, a marker of DSBs (Xu et al., 2010). The link between DSBs and the importance of DSB repair processes, such as non-homologous end joining (NHEJ) and homologous recombination (HR), in preventing chromosomal aberrations/genomic instability is extensively discussed in literature and many reviews are available (van Gent et al., 2001; Ferguson and Alt, 2001; Hoeijmakers, 2001; Iliakis et al., 2004; Povirk, 2006; Weinstock et al., 2006; Natarajan and Palitti, 2008; Lieber et al., 2010; Mehta and Haber, 2014; Ceccaldi et al., 2016; Chang et al., 2017; Sishc and Davis, 2017; Brunet and Jasin, 2018).

In addition, attempted repair of DSBs can lead to chromosomal aberrations such as translocations; NHEJ is a recognized source of oncogenic translocations in human cancers (Ferguson and Alt, 2001; Weinstock et al., 2006; Byrne et al., 2014; Brunet and Jasin, 2018), and a contributor to the carcinogenic process (Hoeijmakers, 2001; Sishc and Davis, 2017)Other types of chromosomal aberrations can serve as indicators of genomic instability that can contribute to a variety of adverse health effects including neurodegeneration (Madabhushi et al., 2014).

Uncertainties and Inconsistencies
Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

As described above, statistically significant increases in MN occur, in some cases, at lower concentrations than strand breaks measured with the comet assay (Platel et al., 2001; Watters et al., 2009; Kawaguchi et al., 2010). The two assays measure different endpoints at different time points; the MN test may appear to be more sensitive than the comet assay but it is difficult to directly compare these two assays.

Mughal et al. (2010) study compared different parameters of comet assay (tail moment, length, and % tail DNA) to MN frequency. Depending on the parameter, the observation of increase in strand breaks varied. For example, % tail DNA would show a visible increase in strand breaks at one concentration; however, no change would be observed in the tail moment calculated using the same data. Use of different parameters in presenting comet assay data may add subjectivity to the results that are reported in certain papers.

Rossner Jr. et al. exposed human embryonic lung fibroblasts (HEL12469) to 1, 10, and 25 µM of benzo[a]pyrene (B[a]P) for 24 hours and measured DSB (γH2AX immunodetection by Western blotting) and translocations (by fluorescence in situ hybridization of chromosomes 1, 2, 4, 5, 7, 17) (Rossner Jr. et al., 2014).

  • Increases in γH2AX were observed only at 25 µM B[a]P (~2.5 fold increase) after the 24h exposure.
  • Translocations were quantified and expressed as the genomic frequency of translocations per 100 cells (FG/100)
    • All concentrations of B[a]P induced an elevated frequency of translocations compared to the DMSO control (DMSO: ~0.19/100; 1 µM: ~0.53/100 cells; 10 µM: ~0.33/100; 25 µM: ~0.39/100)

In this study, the increase in translocations was detected at concentrations that did not induce an increase in γH2AX signal. This observation of the discordant relationship between γH2AX and translocations may be due to the differences in assay sensitivity. In addition, immunodetection by Western blotting cannot precisely measure small changes in protein content.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references.  More help
Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help
Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help
Known Feedforward/Feedback loops influencing this KER
Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

DNA strand breaks and subsequent chromosomal aberrations can occur in any eukaryotic and prokaryotic cell.


List of the literature that was cited for this KER description. More help

Brunet, E. & M. Jasin (2018), "Induction of chromosomal translocations with CRISPR-Cas9 and other nucleases: Understanding the repair mechanisms that give rise to translocations.", Adv. Exp. Med. Biol. 1044:15-25.

Byrne, M. et al. (2014), "Mechanisms of oncogenic chromosomal translocations.", Ann. N.Y. Acad. Sci., 1310:89-97.

Ceccaldi, R., B. Rondinelli & A.D. D'Andrea (2016), "Repair Pathway Choices and Consequences at the Double-Strand Break.", Trends Cell Biol. 26(1):52-64.

Chang, H. et al. (2017), "Non-homologous DNA end joining and alternative pathways to double‑strand break repair.", Nature Rev. Mol. Cell. Biol., 18:495-506.

Chernikova, S.B., R.L. Wells & M. Elkind (1999), "Wortmannin Sensitizes Mammalian Cells to Radiation by Inhibiting the DNA-Dependent Protein Kinase-Mediated Rejoining of Double-Strand Breaks.", Radiat. Res., 151:159-166.

Collins, A.R. et al. (2008), "The comet assay: topical issues.", Mutagenesis, 23:143-151.

Dertinger, S.D. et al. (2019), "Predictions of genotoxic potential, mode of action, molecular targets, and potency via a tiered multiflow® assay data analysis strategy.", Environ. Mol. Mutagen., 60(6):513-533

Ensminger, M. et al. (2014), "DNA breaks and chromosomal aberrations arise when replication meets base excision repair.", J. Cell Biol.,  206:29.

Ferguson, D.O. & F.W. Alt (2001), "DNA double strand break repair and chromosomal translocation: Lessons from animal models.", Oncogene 20(40):5572–5579.

Hoeijmakers, J.H. (2001), "Genome maintenance mechanisms for preventing cancer.", Nature, 411:366-374.

Iliakis, G. et al. (2019), "Defined Biological Models of High-LET Radiation Lesions.", Radiat. Protect Dosimet., 183:60-68.

Iliakis, G. et al. (2004), "Mechanisms of DNA double strand break repair and chromosome aberration formation.", Cytogenet. Genome Res. 104:14-20.

Kawaguchi, S. et al. (2010), "Is the comet assay a sensitive procedure for detecting genotoxicity?.", J. Nucleic Acids, 2010:541050.

Kuzminov, A. (2001), "Single-strand interruptions in replicating chromosomes cause double-strand breaks.", Proc. Natl. Acad. Sci. USA 95:8241-8246.

Lieber, M. et al. (2010), "Nonhomologous DNA End Joining (NHEJ) and Chromosomal Translocations in Humans.", Subcell Biochem., 50:279-296.

Madabhushi, R., Pan, L., Tsai, L. (2014) "DNA damage and its links to neurodegeneration.", Neuron, 83(2):266-282. doi: 10.1016/j.neuron.2014.06.034.

Mehta, A. & J. Haber (2014), "Sources of DNA Double-Strand Breaks and Models of Recombinational DNA Repair.", Cold Spring Harb. Perspect Biol., 6:a016428.

Mughal, A. et al. (2010), "Micronucleus and comet assay in the peripheral blood of juvenile rat: Establishment of assay feasibility, time of sampling and the induction of DNA damage.", Mutat. Res. Gen. Tox. En., 700:86-94.

Natarajan, A.T & F. Palitti (2008), "DNA repair and chromosomal alterations.", Mutat. Res., 657:3-7.

Platel, A. et al. (2011), "Study of oxidative DNA damage in TK6 human lymphoblastoid cells by use of the thymidine kinase gene-mutation assay and the in vitro modified comet assay: Determination of No-Observed-Genotoxic-Effect-Levels.", Mutat. Res., 726:151-159.

Platel, A. et al. (2009), "Study of oxidative DNA damage in TK6 human lymphoblastoid cells by use of the in vitro micronucleus test: Determination of No-Observed-Effect Levels.", Mutat. Res., 678:30-37.

Povirk, L. (2006), "Biochemical mechanisms of chromosomal translocations resulting from DNA double-strand breaks.", DNA Repair 5:1199-1212.

Rogakou, E.P. et al. (1999), "Megabase chromatin domains involved in DNA double-strand breaks in vivo.", J. Cell Biol., 146:905-916.

Rossner, Jr. P et al. (2014), "Nonhomologous DNA end joining and chromosome aberrations in human embryonic lung fibroblasts treated with environmental pollutants.", Mutat. Res., 763-764:28-38.

Rothfuss, A. et al. (1999), "Evaluation of mutagenic effects of hyperbaric oxygen (HBO) in vitro.", Environ. Mol. Mutagen., 34:291-296.

Sishc, B.J. & A.J. Davis (2017), "The Role of the Core Non-Homologous End Joining Factors in Carcinogenesis and Cancer.", Cancers (Basel), 9(7): pii E82.

Sudprasert, W., P. Navasumrit & M. Ruchirawat (2006), "Effects of low-dose gamma radiation on DNA damage, chromosomal aberration and expression of repair genes in human blood cells.", Int. J. Hyg. Environ.-Health, 206:503-511.

Trenz, K., J. Landgraf & G. Speit (2003), "Mutagen sensitivity of human lymphoblastoid cells with a BRCA1 mutation.", Breast Cancer Res. Treat., 78:69-79.

Trenz, K., P. Schutz & G. Speit (2005), "Radiosensitivity of lymphoblastoid cell lines with a heterozygous BRCA1 mutation is not detected by the comet assay and pulsed field gel electrophoresis.", Mutagenesis, 20:131-137.

Turner, H.C. et al. (2015), "Effect of Dose Rate on Residual c-H2AX Levels and Frequency of Micronuclei in X-Irradiated Mouse Lymphocytes.", Radiat. Res., 183:315-324.

van Gent, D., J.H. Hoeijmakers & R. Kanaar (2001), "Chromosomal Stability and the DNA Double-Stranded Break Connection.", Nature 2:196-206.

Watters, G.P. et al. (2009), "H2AX phosphorylation as a genotoxicity endpoint.", Mutat. Res., 670:50-58.

Weinstock, D. et al. (2006), "Modeling oncogenic translocations: Distinct roles for double-strand break repair pathways in translocation formation in mammalian cells.", DNA Repair 5:1065-1074.

Xu, B. et al. (2010), "Replication Stress Induces Micronuclei Comprising of Aggregated DNA Double-Strand Breaks.", PLoS One, 6:e18618.