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Increase, DNA strand breaks leads to Increase, Chromosomal aberrations
Key Event Relationship Overview
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||Carole Yauk (send email)||Open for comment. Do not cite||EAGMST Under Review|
|Direct deposition of ionizing energy leading to lung cancer||non-adjacent||High||Low||Vinita Chauhan (send email)||Under development: Not open for comment. Do not cite||EAGMST Under Review|
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
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 Supporting this KER
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 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).
Uncertainties and Inconsistencies
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
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
DNA strand breaks and subsequent chromosomal aberrations can occur in any eukaryotic and prokaryotic cell.
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.
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.