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Relationship: 2813

Title

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

Energy Deposition leads to Increase, Oxidative DNA damage

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Energy Deposition

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Increase, Oxidative DNA damage

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	non-adjacent	Moderate	Moderate	Vinita Chauhan (send email)	Open for citation & comment

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
mouse	Mus musculus	Moderate	NCBI
rabbit	Oryctolagus cuniculus	Moderate	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Unspecific	Moderate

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

Energy can be deposited onto biomolecules stochastically from various forms of radiation. As radiation passes through an organism, it loses energy; potentially causing direct and indirect molecular-level damage in the process. The extent of damage occurs at various levels depending on ionization and non-ionization events (excitation of molecules). Reaction with water molecules can produce reactive oxygen species (ROS). Additionally, enzymes involved in reactive oxygen and nitrogen species (RONS) production can be directly upregulated (de Jager, Cockrell & Plessis, 2017). When one ROS interacts with the DNA, it produces DNA-protein cross-links, inter and intra-strand links, and tandem base lesions. When at least two ROS associate with DNA it produces oxidatively generated clustered DNA lesions (OCDLs), more complex damage. This can include single and double strand breaks, abasic sites, and oxidized bases (Cadet et al., 2012), which can lead to chromosomal aberrations, cytotoxicity, and oncogenic transformations (Stohs, 1995) as well as structural changes to the DNA, blocking polymerases (Zhang et al., 2010). Cells contain DNA repair mechanisms that help lessen the damage, but they are not perfect and can lead to insufficient repair , resulting in sustained damage (Eaton, 1995; Ainsbury et al., 2016; Markkanen, 2017).

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

As energy is deposited in an organism, it produces ROS (Pendergrass et al., 2010; Cheng, 2019). As their formation is highly regulated, any changes can be undesirable, inducing a state of oxidative stress where cellular defense mechanisms, such as antioxidants, are overwhelmed by ROS levels (Brennan & Kantorow, 2009). A low level of DNA damage constantly exists in healthy cells, with cells acquiring an estimated 70 000 lesions per day, mostly due to ROS produced during normal metabolism and base hydrolysis (Amente et al., 2019). This number increases under oxidative stress (Lee et al., 2004). If cells replicate, any damage to their DNA that is not correctly repaired is passed on to their descendants (Wolf et al., 2008). Furthermore, these mechanisms and outcomes may vary dependent on the stressor. Different stressors may interact and produce a greater than additive effect (Di Girolamo, 2010). For example, singlet oxygen plays an important role in activating mitogen-activated protein kinases (MAPKs), which act as signal transducers to initiate DNA damage.

Throughout this process, DNA repair pathways are also activated. These include the nucleotide excision repair (NER) pathway (Mesa, 2013), and the base excision repair (BER) pathway (Cheng et al., 2019). They can repair certain amounts of damage but may become overwhelmed when faced with large numbers of DNA lesions (Lee et al., 2004). Different lesions are also repaired at different rates or with different amounts of fidelity, which can affect the amount of residual damage. For example, DNA single strand breaks are usually repaired quickly (Collins, 2014), while double strand breaks are more complex and are therefore less likely to be repaired correctly (Schoenfeld et al., 2012; Markkanen 2017). The efficiency and effectiveness of the repair pathways will influence the amount of residual oxidative DNA damage.

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

The empirical evidence supporting this KER mostly measures different indicators of oxidative DNA damage, namely 8-OH-DG, 8-OH G, cyclobutane pyrimidine dimers, and multiple chromophores such as NADH. Research was conducted primarily in human cells and mice, mostly using UV or X-rays as a stressor. It is widely accepted that the deposition of energy, at all doses, results in immediate ionization events, followed by downstream events to DNA molecules.

Dose Concordance

There is a limited amount of data supporting dose concordance. One study found that human LECs exposed to 0 – 5 Gy X-rays displayed a gradual increase in 8-OH-DG concentration, reaching 2.25x control at the maximum dose (Bahia et al., 2018).

Time Concordance

There are limited studies to demonstrate a consistent trend, this is due to the differences in the oxidative DNA damage marker being measured across studies. However, one study by Pendergrass et al. (2010) found that in mice exposed to 11 Gy X-rays at 2 Gy/min, the amount of 8-OH G positive DNA fragments increased from a total number of 5 to 60 from the control at 9.5 and 11 months (female, adult C57BL/6 mouse)

Essentiality

Radiation exposure has been found to increase levels of oxidative DNA damage above background levels. Bahia et al. (2018) found that human LECs exposed to 5 Gy X-rays had a level of 8-hydroxyguanosine, an oxidative DNA damage indicator, that was twice as high as in sham-irradiated cells. Similarly, Pendergrass et al. (2010) found that mice exposed to 11 Gy X-rays had 3.6 times as many 8-OH G positive DNA fragments as unirradiated mice. In both cases, radiation exposure resulted in significant increase in markers of oxidative DNA damage above baseline levels. This indirectly highlights that radiation promotes an environment of oxidative stress that can then lead to downstream modification to the DNA molecules and, in the absence of radiation, there is no further initiation of oxidative stress induced DNA damage. This is also supported by studies where a dose-response relationship is assessed; it is noted that at lower levels of deposited energy there is less oxidative DNA damage (Bahia et al., 2018). Since deposited energy initiates events immediately, the removal of deposited energy, a physical stressor, also supports the essentiality of the key event. Studies that do not deposit energy are observed to have no downstream effects.

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

There are several uncertainties for this KER.

Some of the data indicates that oxidative DNA damage increases as the time since exposure (Pendergrass et al., 2010; Mesa and Bassnett, 2013). However, other data found a very slight decrease (Mesa and Bassnett, 2013).
Certain studies found that doses less than 0.5 Gy decrease ROS levels in a non-significant manner. This is thought to be due to radio-tolerance, where low doses induce defense mechanisms, such as glutathione or superoxide dismutase. As the dose is low, these defenses can overcome the effects of radiation, but as doses increase, they become overwhelmed, leading to increases in ROS levels (Bahia et al., 2018). These changes subsequently cause a similar pattern in DNA oxidative damage that dips between 0 and 0.5 Gy, where it begins to slowly increase (Bahia et al., 2018; Cheng et al., 2019).

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)
Antioxidants	Increased concentration, examples of antioxidants studied include glutathione and superoxide dismutase	Antioxidants scavenge ROS, resulting in a decrease in oxidative DNA damage.	Pendergrass et al., 2010; Bahia et al., 2018
UV absorbing contact lenses	Examples include senofilcon A	Helps to protect the eye against high doses of UVA, therefore decreasing oxidative DNA damage.	Giblin et al., 2012
Xeroderma pigmentosum	Presence of the genetic condition	Increases sensitivity to UV-induced oxidative DNA damage by affecting the nucleotide excision repair system.	Di Girolamo, 2010
lncRNA H19	Knockdown of lncRNA H19	Increases sensitivity to UVB-induced oxidative DNA damage by affecting the nucleotide excision repair system.	Cheng et al., 2019
Low radiation doses	Radiotolerance	Cells may display radio-tolerance by activating ROS scavenger defense mechanisms at low doses, resulting in a decrease in ROS levels and therefore a decrease in oxidative DNA damage, compared to the control. However, at higher doses these defenses are overwhelmed, and ROS levels rise.	Bahia et al., 2018
Replication rate	Increased replication	Cells that are actively replicating have increased rates of photolesion repair, and therefore, lower rates of oxidative DNA damage, as opposed to quiescent cells.	Mesa & Bassnett, 2013

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

The following tables provide representative examples of the relationship, unless otherwise indicated, all data is statistically significant.

Dose Concordance

Reference	Experiment Description	Result
Bahia et al., 2018	In vitro. Human lens epithelial cells were exposed to X rays at either 1.62 cGy/min or 38.2 cGy/min. 20 min HPLC-CoulArray analysis was used to determine 8-hydroxy deoxyguanosine (8-OH-DG) concentration.	Human LECs exposed to 0 – 5 Gy X-rays displayed a gradual increase in 8-OH-DG concentration, reaching 2.25x control at the maximum dose.
Cheng et al., 2019	In vitro. Human SRA01/04 lens epithelial cells exposed to UVB (312 nm peak, 280-320 nm range), the 8-OHdG ELISA assay was used to measure the amount of 8-OHdG present.	In cells exposed to UVB, the addition of siRNA2, a type of H19-siRNA, caused a 1.2x increase in 8-OHdG relative to control. Similarly, the addition of H19 into a pcDNA vector caused a 1.25x decrease in 8-OHdG relative to control.

Incidence Concordance

No data found.

Time Concordance

Reference	Experiment Description	Result
Pendergrass et al., 2010	In vivo. Female, adult, C57BL/6 mouse lenses received whole-body irradiation with 11 Gy X-rays at 2 Gy/min. Immunofluorescence was then used to determine the number of 8-OH-dG positive cortical nuclear fragments beneath the central zone.	In mice lenses immediately exposed in vivo to 11 Gy X-rays the level of 8-OH G positive DNA fragments increased to 3.5x control 9.5 months post-irradiation.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

As the time since irradiation increases, damage levels slowly increase during the first few months, but begin to rise more quickly as time passes (Pendergrass et al., 2010; Mesa and Bassnett, 2013).

Time-scale

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

N/A

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 in vivo female mice and rabbits, and female human and mice in vitro models.

References

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

Ainsbury, E. A. et al. (2021), “Radiation-induced lens opacities: Epidemiological, clinical and experimental evidence, methodological issues, research gaps and strategy”, Environment international, Vol. 146, Elsevier Ltd, Netherlands, https://doi.org/10.1016/j.envint.2020.106213

Amente, S. et al. (2019), “Genome-wide mapping of 8-oxo-7,8-dihydro-2’-deoxyguanosine reveals accumulation of oxidatively-generated damage at DNA replication origins within transcribed long genes of mammalian cells”, Nucleic Acids Research 2019, Vol. 47/1, Oxford University Press, England, https://doi.org/10.1093/nar/gky1152

Bahia, S. et al. (2018), “Oxidative and nitrative stress-related changes in human lens epithelial cells following exposure to X-rays”, International journal of radiation biology, Vol. 94/4, England, https://doi.org/10.1080/09553002.2018.1439194

Brennan, L. A. and M. Kantorow (2009), “Mitochondrial function and redox control in the aging eye: Role of MsrA and other repair systems in cataract and macular degernerations”, Experimental Eye Research, Vol. 88/2, Elsevier Ltd, England, https://doi.org/10.1016/j.exer.2008.05.018

Cadet, J. et al. (2012), “Oxidatively generated complex DNA damage: tandem and clustered lesions”, Cancer letters, Vol. 327/1, Elsevier Ireland Ltd, Ireland, https://doi.org/10.1016/j.canlet.2012.04.005

Cheng, T. et al. (2019), “lncRNA H19 contributes to oxidative damage repair in the early age-related cataract by regulating miR-29a/TDG axis”, Journal of cellular and molecular medicine, Vol. 23/9, Wiley Subscription Services, Inc. England, https://doi.org/10.1111/jcmm.14489

Collins, A. R. (2014), “Measuring oxidative damage to DNA and its repair with the comet assay”, Biochimica et biophysica acta. General subjects, Vol. 1840/2, Elsevier B.V., https://doi.org/10.1016/j.bbagen.2013.04.022

de Jager, T.L., Cockrell, A.E., Du Plessis, S.S. (2017), “Ultraviolet Light Induced Generation of Reactive Oxygen Species”, in Ultraviolet Light in Human Health, Diseases and Environment. Advances in Experimental Medicine and Biology, Springer, Cham, https://doi.org/10.1007/978-3-319-56017-5_2

Di Girolamo, N. (2010), “Signalling pathways activated by ultraviolet radiation: role in ocular and cutaneous health”, Current pharmaceutical Design, Vol. 16/12, Benthem Science Publishers Ltd, https://doi.org/10.2174/1381-6128/10

Eaton, J. W. (1995), “UV-mediated cataractogenesis: a radical perspective”, Documenta ophthalmologica, Vol. 88/3-4, Springer, Dordrecht, https://doi.org/10.1007/BF01203677

Giblin, F. J. et al. (2012), “A class I UV-blocking (senofilcon A) soft contact lens prevents UVA-induced yellow fluorescence and NADH loss in the rabbit lens nucleus in vivo”, Experimental eye research, Vol. 102, Elsevier Ltd, England, https://doi.org/10.1016/j.exer.2012.06.007

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. https://doi.org/10.1080/09553002.2022.2110306

Lee, J. et al. (2004), “Reactive oxygen species, aging, and antioxidative nutraceuticals”, Comprehensive reviews in food science and food safety, Vol. 3/1, Blackwell Publishing Ltd, Oxford, https://doi.org/10.1111/j.1541-4337.2004.tb00058.x

Markkanen, E. (2017), “Not breathing is not an option: How to deal with oxidative DNA damage”, DNA repair, Vol. 59, Elsevier B.V., Netherlands, https://doi.org/10.1016/j.dnarep.2017.09.007

Mesa, R. and S. Bassnett (2013), “UV-B induced DNA damage and repair in the mouse lens”, Investigative ophthalmology & visual science, Vol. 54/10, the Association for Research in Vision and Ophthalmology, United States, https://doi.org/10.1167/iovs.13-12644

Pendergrass, W. et al. (2010), “X-ray induced cataract is preceded by LEC loss, and coincident with accumulation of cortical DNA, and ROS; similarities with age-related cataracts”, Molecular vision, Vol. 16, United States, pp. 1496-1513

Schoenfeld, M. P. et al. (2012), “A hypothesis on biological protection from space radiation through the use of new therapeutic gases as medical counter measures”, Medical gas research, Vol. 2/1, BioMed Central Ltd, India, https://doi.org/10.1186/2045-9912-2-8

Stohs, S. J. (1995), “The role of free radicals in toxicity and disease”, Journal of Basic and Clinical Physiology and Pharmacology, Vol. 6/3-4, https://doi.org/10.1515/JBCPP.1995.6.3-4.205

Wolf, N. et al. (2008), “Radiation cataracts: mechanisms involved in their long delayed occurrence but then rapid progression”, Molecular vision, Molecular Vision, United States, pp. 274-285

Zhang, Y. et al. (2010), “Oxygen-induced changes in mitochondrial DNA and DNA repair enzymes in aging rat lens”, Mechanisms of ageing and development, Vol. 131/11, Elsevier Ireland Ltd, Clare, https://doi.org/10.1016/j.mad.2010.09.003