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Relationship: 2811


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

Oxidative Stress leads to Increase, DNA strand breaks

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
Deposition of energy leading to occurrence of cataracts adjacent Moderate Low Vinita Chauhan (send email) Under development: Not open for comment. Do not cite
Deposition of Energy Leading to Learning and Memory Impairment adjacent Vinita Chauhan (send email) Under development: Not open for comment. Do not cite

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 Low NCBI
rat Rattus norvegicus Low NCBI
rabbit Oryctolagus cuniculus Low NCBI
bovine Bos taurus Low NCBI

Sex Applicability

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

Life Stage Applicability

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

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

Oxidative stress is an event that involves both a reduction in free radical scavengers and enzymes, and an increase in free radicals (Brennan et al., 2012). Oxidative stress needs to be maintained within an organism to avoid an excess of damage to biological structures, such as DNA. A redox homeostasis between the radicals and the scavengers is necessary. Between ROS and RNS, collectively known as RONS, ROS is particularly significant to oxidative damage and disease states. Increased ROS-mediated oxidative stress is defined by the state of a cell where ROS production has surpassed the ability of antioxidant/scavenging systems to stabilize the radicals (Kurutas 2016). Free radicals such as superoxide anion (O2•-) and hydroxyl radical (OH • -) are highly reactive molecules that have at least one unpaired electron in an outer ring, causing them to have a strong affinity for electrons in other molecules (Cabrera et al., 2011). A redox reaction occurs when the radical gains an electron from a nearby molecule (Kruk et al., 2016). The oxidation reaction can alter elements of the molecule’s structure. Depending on individual reactivity, RONS can travel varying distances following formation. Radicals such as singlet oxygen and hydroxyl radical are highly unstable and will react with molecules near their generation point, while radicals such as H2O2 are more stable and membrane permeable, meaning they can travel further to find electrons (Spector, 1990). Since DNA is mainly found in nucleus, ROS needs to reach the nucleus to induce breaks. Hydroxyl radicals, in addition to being highly reactive, are capable of causing DNA damage (Halliwell et al., 2021; Engwa et al., 2020). The regulation of these radicals is achieved by the antioxidant defense response (ADR), which includes enzymatic and non-enzymatic processes. The ADR is recruited to manage RONS levels, with antioxidants such as superoxide dismutase (SOD) functioning as the first line of defense (Engwa et al., 2020). These antioxidants act as scavengers to oxidants, reacting with them before reaching other structures within the cell such as DNA strands (Cabrera et al., 2011; Engwa et al., 2020). The backbone of DNA can fragment upon sustained exposure to ROS (Uwineza et al., 2019; Cannan et al., 2016). Due to low oxidation potentials, adenine and guanine are the DNA bases more prone to oxidation, with oxidation potentials (normal hydrogen electrode) at pH 7 of 1.3 eV and 1.42 eV compared to the 1.6 eV and 1.7 eV of cytosine and thymine (Fong, 2016; Halliwell et al., 2021; Poetsch, 2020). In fact, certain radicals even target guanine in a selective fashion, including carbonate anion radical (CO3•-) and singlet oxygen (1O2) (Halliwell et al., 2021). 

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

The strategy for collating the evidence to support the relationship is described in Kozbenko et al 2022. Briefly, a scoping review methodology was used to prioritize studies based on a population, exposure, outcome, endpoint statement.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Overall Weight of Evidence: Moderate 

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

The biological plausibility of the relationship between increased oxidative stress leading to increased DNA double strand breaks (DSBs) is highly supported by the literature. Evidence was collected from studies conducted using in vitro lens epithelial cell models and derived from humans, but also includes bovine and germ line cells (Spector, 1990; Stohs, 1995; Aitken et al., 2001; Spector, 1995). Since most of the evidence is derived from studies using a human cell model it limits the ability to compare between different taxonomies. (Cencer et al., 2018; Liu et al., 2013; Smith et al., 2015; Zhou et al., 2016; Ahmadi et al., 2022; Meng et al., 2021). 

 ROS that are generated specifically as a result of radiation are highly localized, increasing the likelihood of clustered regions of damage. Naturally generated ROS are more widespread and as a result less capable of generating clusters of damage. ROS will act on DNA bases to oxidize or delete them from the sequence, which create nicks on the strand (Cannan et al., 2016). This damage can occur to any DNA base but bases such as guanine and adenine are most vulnerable due to their low oxidation potentials (Fong, 2016). The mechanism through which the strand break occurs is a result of base excision repair (BER) happening at multiple sites that are too close together, resulting in the spontaneous conversion to DSBs prior to completion of repair. ROS damage to bases clustered together means that multiple sites of BER are happening very close together and while the strand may be able to support the damaged area for one repair, concurrent repairs make surrounding areas more fragile and the strand breaks at the nick sites are under added strain (Cannan et al., 2016). Endogenous damage to DNA as a result of radicals appears over time and mainly as isolated lesions, a pattern understood to be due to the diffusion of the radicals resulting in homogenous distribution patterns. This differs from the specific situations where radiation acts as the stressor to increase oxidative stress, as the radiation track will be highly localized and form radicals within that hit space. This leads to non-homologous lesions and clustered damage to the DNA (Ward et al., 1985). 

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


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
Modulating Factor (MF) MF Specification Effect(s) on the KER Reference(s)
Age  Reduced antioxidant capacities have been linked to aged lenses (in humans >30 years old). The development of a chemical barrier between the cortex and the nucleus is partially responsible, as it prevents GSH from protecting aged lens cells from ROS.  Prevention of RONS-mediated damage is primarily achieved by antioxidants, so a lowered capacity would likely lead to reduced damage mitigation abilities. 78% of lens over 30 had a low level of GSH in the center compared to 14% of lens under 30. Lens epithelial cells have an associated 3-fold increase in γ-H2AX (marker of DNA damage) when GSH-PX decreases by 2-fold.  Taylor & Davies, 1987; Cabrera & Chihuailaf, 2011; Quinlan & Hogg, 2018; Sweeney & Truscott, 1998; Meng & Fang, 2021 
Free radical scavengers  ROS-scavengers are essential components of the body’s natural defense against oxidative damage. Increased ROS production leads to increased incidence of electron donation by scavengers, thus reducing the overall level of free radical scavengers available to deal with ROS.  Isothiocyanates, such as sulforaphane (SFN), activate the release of more enzymatic scavengers. When SFN was added to in vitro LECs, LDH decreased to near unexposed cell levels and was associated with 3.3x less DNA strand breaks compared to the non-SFN cells following stressor exposure.  Taylor et al., 1987; Cabrera et al., 2011; Liu et al., 2013 
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

This KER is plausible in all life stages, sexes, and organisms with DNA. The majority of the evidence is from human in vitro studies that do not specify the sex. No in vivo evidence was found to support the relationship. 


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

Ahmadi, M. et al. (2021), “Early Responses to Low-Dose Ionizing Radiation in Cellular Lens Epithelial Models”, Radiation Research, Vol.197, Radiation Research Society, Indianapolis,  

Aitken, R.J. and C. Krausz. (2001), “Oxidative stress, DNA damage and the Y chromosome”, Reproduction, Vol.122/2001, Bioscientifica, Bristol,  

Annesley, S.J. and P.R. Fisher. (2019), “Mitochondria in Health and Disease”, Cells, Vol.8/7, MDPI, Basel,  

Brennan, L., R. McGreal and M. Kantorow. (2012), “Oxidative stress defense and repair systems of the ocular lens”, Frontiers in Bioscience – Elite, Vol.4/E(1), Frontiers in Bioscience, Singapore,  

Britton, S. et al. (2020), “ATM antagonizes NHEJ proteins assembly and DNA-ends synapsis at single-ended DNA double strand breaks”, Nucleic Acids Research, Vol.48/17, Oxford University Press, Oxford,  

Cabrera, M. and R. Chihuailaf. (2011), “Antioxidants and the integrity of ocular tissues”, Veterinary Medicine International, Vol.2011, Hindawi Limited, London,  

Cannan, W. and D. Pederson. (2016), “Mechanisms and consequences of double-strand DNA break formation in chromatin”, Journal of Cell Physiology, Vol.231/1, Wiley, Hoboken,  

Cencer, C. et al. (2018), “PARP-1/PAR Activity in Cultured Human Lens Epithelial Cells Exposed to Two Levels of UVB Light”, Photochemistry and Photobiology, Vol.94/1, Wiley-Blackwell, Hoboken,  

Climent, M. et al. (2020), “MicroRNA and ROS Crosstalk in Cardiac and Pulmonary Diseases”, International Journal of Molecular Science, Vol.21/12, MDPI, Basel,  

Dahm-Daphie, J., C. Sass, and W. Alberti. (2000), “Comparison of biological effects of DNA damage induced by ionizing radiation and hydrogen peroxide in CHO cells”, International Journal Radiation Biology, Vol.76/1, Informa, London,  

Engwa, G.A., F.N. Nweke and B.N. Nkeh-Chungag. (2020), “Free Radicals, Oxidative Stress-Related Diseases and Antioxidant Supplementation”, Alternative therapies in health and medicine, Vol.28/1, InnoVision Health Media, Eagan, pp.114-128.  

Feng, C. et al. (2016), “Lycopene protects human SH-SY5Y neuroblastoma cells against hydrogen peroxide-induced death via inhibition of oxidative stress and mitochondria-associated apoptotic pathways”, Molecular Medicine Reports, Vol.13/5, Spandidos Publications, Athens,  

Fong, C.W. (2016), “Platinum anti-cancer drugs: Free radical mechanism of Pt-DNA adduct formation and anti-neoplastic effect”, Free Radical Biology and Medicine, Vol.95/June 2016, Elsevier, Amsterdam,  

Halliwell, B. et al. (2021), “Hydroxyl radical is a significant player in oxidative DNA damage in vivo”, Chemical Society Reviews, Vol.50, Royal Society of Chemistry, London,  

Kay, J. et al. (2019), “Inflammation-induced DNA damage, mutations and cancer”, DNA Repair, Vol.83, Elsevier, Amsterdam,  

Jeggo, P.A., V. Geuting and M. Löbrich. (2011), “The role of homologous recombination in radiation-induced double-strand break repair”, Radiotherapy and Oncology, Vol.101/1, Elsevier, Amsterdam,  

Kozbenko, T. et al. (2022), “Deploying elements of scoping review methods for adverse outcome pathway development: a space travel case example”, International Journal of Radiation Biology, 1–12. 

Kurutas E. B. (2016), “The importance of antioxidants which play the role in cellular response against oxidative/nitrosative stress: current state”, Nutrition journal, Vol.15/1, Biomed Central, London,  

Kruk, J., K. Kubasik-Kladna and H. Aboul-Enein. (2016), “The Role Oxidative Stress in the Pathogenesis of Eye Diseases: Current Status and a Dual Role of Physical Activity”, Mini-Review in Medicinal Chemistry, Vol.16/3, Bentham Science Publishers, Sharjah,  

Li, Y. et al. (1998), “Response of lens epithelial cells to hydrogen peroxide stress and the protective effect of caloric restriction”, Experimental Cell Research, Vol.239/2, Elsevier, Amsterdam,  

Liu, H. et al. (2013), “Sulforaphane can protect lens cells against oxidative stress: Implications for cataract prevention”, Investigative Ophthalmology and Visual Science, Vol.54/8, Association for Research in Vision and Ophthalmology, Rockville,  

Meng, K. and C. Fang. (2021), “Knockdown of Tripartite motif-containing 22 (TRIM22) relieved the apoptosis of lens epithelial cells by suppressing the expression of TNF receptor-associated factor 6 (TRAF6)”, Bioengineered, Vol.12/1, Taylor & Francis, Oxfordshire,  

Nishida, M. et al. (2005), “Ga12/13- and Reactive Oxygen Species-dependent Activation of c-Jun NH2-terminal Kinase and p38 Mitogen-activated Protein Kinase by Angiotensin Receptor Stimulation in Rat Neonatal Cardiomyocytes”, Journal of Biological Chemistry, Vol.280/18, American Society for Biochemistry and Molecular Biology, Rockville,  

Poetsch, A.R. (2020), “The genomics of oxidative DNA damage, repair, and resulting mutagenesis”, Computational and Structural Biotechnology Journal, Vol.18, Elsevier, Amsterdam,  

Quinlan, R.A., and P.J. Hogg. (2018), “γ-Crystallin redox–detox in the lens”, Journal of Biological Chemistry, Vol.293/46, American Society for Biochemistry and Molecular Biology, Rockville, 

Scully, R. and A. Xie. (2013), “Double strand break repair functions of histone H2AX”, Mutation Research, Vol.750/1-2, Elsevier, Amsterdam,  

Smith, A. et al. (2015), “Ku80 counters oxidative stress-induced DNA damage and cataract formation in the human lens”, Investigative Ophthalmology and Visual Science, Vol.56/13, Association for Research in Vision and Ophthalmology, Rockville,  

Spector, A. et al. (1997), “Microperoxidases catalytically degrade reactive oxygen species and may be anti-cataract agents”, Experimental Eye Research, Vol.65/4, Academic Press Inc, Cambridge,  

Spector, A. et al. (1996), “Variation in cellular glutathione peroxidase activity in lens epithelial cells, transgenics and knockouts does not significantly change the response to H2O2 stress”, Experimental Eye Research, Vol.62/5, Academic Press Inc, Cambridge,  

Spector, A. (1995), “Oxidative stress‐induced cataract: mechanism of action”, The FASEB Journal, Vol.9/12, Federation of American Societies for Experimental Biology, Bethesda,  

Spector, A. (1990), “Oxidation and Aspects of Ocular Pathology”, CLAO Journal, Vol.16/1, Lippincott, Williams and Wilkins Ltd, Philadelphia, S8-S10.  

Stohs, S. (1995), “The role of free radicals in toxicity and disease”, Journal of Basic Clinical Physiology and Pharmacology, Vol.6/3-4, Walter de Gruyter GmbH, Berlin,  

Sweeney, M.H.J. and R.J.W. Truscott. (1998), “An Impediment to Glutathione Diffusion in Older Normal Human Lenses: a Possible Precondition for Nuclear Cataract”, Experimental Eye Research, Vol.67, Academic Press Inc, Cambridge, 

Taylor, A. and K. J. A. Davies (1987), “Protein oxidation and loss of protease activity may lead to cataract formation in the aged lens”, Free Radical Biology & Medicine, Vol. 3, Pergamon Journals Ltd, United States of America, pp. 371-377  

Uwineza, A. et al. (2019), “Cataractogenic load – A concept to study the contribution of ionizing radiation to accelerated aging in the eye lens”, Mutation Research - Reviews in Mutation Research, Vol.779, Elsevier, Amsterdam,  

Ward, J.F., W.F. Blakely & E.I. Joner. (1985), “Mammalian Cells Are Not Killed by DNA Single-Strand Breaks Caused by Hydroxyl Radicals from Hydrogen Peroxide”, Radiation Research, Vol.103/3, Radiation Research Society, Indianapolis, hptts://  

Wu, H. et al. (2021), “Lactate dehydrogenases amplify reactive oxygen species in cancer cells in response to oxidative stimuli”, Signal Transduction and Targeted Therapy, Vol.6/1, Nature Portfolio, Berlin,  

Yang, Y. et al. (1998), “The effect of catalase amplification on immortal lens epithelial cell lines”, Experimental Eye Research, Vol.67/6, Academic Press Inc, Cambridge,  

Yuan, J., R. Adamski and J. Chen. (2010), “Focus on histone variant H2AX: To be or not to be”, FEBS Letters, Vol.584/17, Wiley, Hoboken,  

Zhou, Y. et al. (2016), “Protective Effect of Rutin Against H2O2-Induced Oxidative Stress and Apoptosis in Human Lens Epithelial Cells”, Current Eye Research, Vol.41/7, Taylor & Francis, Oxfordshire,