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Relationship: 2841

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

Increase, DNA strand breaks leads to Increase, Neural Remodeling

Upstream event

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

Increase, DNA strand breaks

Downstream event

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

Increase, Neural Remodeling

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 Learning and Memory Impairment	adjacent	Moderate	Low	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
rat	Rattus norvegicus	Low	NCBI

Sex Applicability

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

Sex	Evidence
Male	Moderate
Female	Low

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
Juvenile	Low
Adult	Moderate

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 single strand breaks (SSBs) and double strand breaks (DSBs) can lead to cell cycle arrest and apoptosis (Madabhushi, Pan and Tsai, 2014; Michaelidesova et al., 2019). In proliferative cells like neural stem/progenitor cells this will reduce neurogenesis within the brain (Alt and Schwer, 2018; Lee and McKinnon, 2007; Michaelidesova et al., 2019). Although the role of DSBs is less well-characterized in mature neurons (Lee and McKinnon, 2007; Thadathil fsylet al., 2019), some evidence suggests that unrepaired DNA strand breaks could also have deleterious effects in these neurons (Wang et al., 2017). Furthermore, there is evidence that DNA strand breaks can induce changes to neural plasticity and synaptic activity through changes in gene expression (Konopka and Atkin, 2022; Thadathil et al., 2019). This can occur via changes in N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor activity or changes in the expression of early response genes (ERGs) that encode transcription factors controlling processes like neurite outgrowth, synapse development and maturation and the balance between excitatory and inhibitory synapses (Konopka and Atkin, 2022).

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 between DNA strand breaks leading to neural remodeling is supported by literature.

Neural remodeling due to DNA strand breaks can occur through apoptosis (Abner and McKinnon, 2004; Desai et al., 2022; Madabhushi, Pan and Tsai, 2014; Michaelidesova et al., 2019; Wang et al., 2017; Zhu et al., 2019). Newly post-mitotic neurons with DSBs may undergo checkpoint mediated apoptosis as a mechanism to prevent their incorporation into the nervous system as mature neurons (Alt and Schwer, 2018; Lee and McKinnon, 2007). In response to DSBs, developing neural progenitor cells and a trace number of neural stem cells will undergo cell cycle arrest at critical stages. In the mammalian genome, DNA strand breaks can regulate checkpoint activation through the activation of phosphoinositide 3-kinase (PI3K)-related family of serine/threonine kinases (PIKK), ataxia telangiectasia mutated (ATM) and ATM/RAD3-related (ATR), that can phosphorylate many downstream proteins (Wang et al., 2017). Specifically, DSBs can activate ATM which phosphorylates p53, which can then act on apoptosis factors, p53-upregulated modulator of apoptosis (PUMA), CD95 (Fas/APO1) and apoptotic peptidase activating factor 1 (Apaf1) (Zhu et al., 2019). Activation of this pathway in proliferating cells like neuronal precursors can reduce neurogenesis (Wang et al., 2017; Michaelidesova et al., 2019).

DNA strand breaks can also lead to changes in synaptic activity, neural plasticity, proliferation, and differentiation. Neurons communicate electrically and chemically through synaptic contacts. Neural plasticity refers to the ability of the nervous system to modify its structure, function or connections in response to stimuli. DNA damage can modulate the activity and expression of glutamate receptors, including NMDA/AMPA, which are involved in synaptic activity, plasticity and neuronal activation in the central nervous system. The changes in receptor activity and expression modulate neuronal gene expression and lead to changes in plasticity (Konopka and Atkin, 2022). Additionally, changes in ERG expression following DNA damage can lead to certain neural remodeling changes, such as neurite outgrowth, synapse development and maturation (Konopka and Atkin, 2022). As well, inhibition of the cell cycle by DNA strand breaks can impair neurogenesis through decreased differentiation and proliferation of neural stem cells (NSCs) (Michaelidesova et al., 2019).

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 for this KER comes from in vivo mouse models as well as in vitro mouse- and human-derived cell models. Stressors used included X-ray radiation (Barazzuol, Ju and Jeggo, 2017; Barazzuol et al., 2015; Huang et al., 2021; Zhang et al., 2017), 60Co gamma ray radiation (Zanni et al., 2015), 137Cs gamma rays (Acharya et al., 2010), and 6 MV photon radiation (Schmal et al., 2019). Neural remodeling can be determined through various endpoints, including neuronal apoptosis, morphology, proliferation, differentiation and altered synaptic activity. Markers of DNA strand breaks in this KER include p53 binding protein 1 (53BP1), phosphorylation of H2AX (γ-H2AX), and phosphorylation of ATM (p-ATM).

Dose Concordance

Several studies demonstrate dose concordance for the relationship between DNA strand breaks leading to neural remodeling. Adult mice irradiated with X-rays showed increased 53BP1 foci in the lateral ventricle at doses as low as 0.1 Gy, while neuronal apoptosis and impaired neurogenesis were observed as low as 1 Gy (Barazzuol, Ju and Jeggo, 2017). Juvenile and adult male mice irradiated with 0.5 to 2 Gy of 6 MV photons showed increased DSBs in neurons at the same doses as impaired neurogenesis (Schmal et al., 2019). X-ray irradiation of mice aged 2 to 4 months with 50 mGy, 100 mGy and 200 mGy showed increased DSBs in the cerebellum, as well as apoptosis in the subventricular zone (SVZ) (Barazzuol et al., 2015).

Time Concordance

Many studies demonstrate that DNA strand breaks occur prior to neural remodeling. Studies frequently measure increased DNA DSBs through γ-H2AX foci or 53BP1 foci within 1 h post-irradiation and increased neuronal apoptosis 4 to 6 h post-irradiation (Acharya et al., 2010; Barazzuol, Ju and Jeggo, 2017; Barazzuol et al., 2015; Zhang et al., 2017). Studies have also measured neural remodeling at later timepoints than this. For example, Zhang et al. (2017) showed increased apoptosis at 2 days after 12 Gy of X-ray irradiation in HT22 hippocampal neurons, and Acharya et al. (2010) demonstrated decreased human NSC (hNSC) differentiation at 2 days and decreased cell numbers at 3 days following 5 Gy of 137Cs gamma rays.

Incidence Concordance

Few studies show greater levels of DNA strand breaks than neural remodeling. In hNSCs irradiated with 5 Gy of 137Cs gamma rays, γ-H2AX foci increased 30-fold, while apoptosis increased 2- to 3-fold and hNSC differentiation decreased 0.5-fold (Acharya et al., 2010). A study using multiple doses of X-rays (50, 100 and 200 mGy) demonstrated greater increases to DNA strand breaks than to apoptosis. At 200 mGy, 53BP1 foci increased 30-fold and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)+ cells increased 10-fold (Barazzuol et al., 2015).

Essentiality

Multiple studies show that inhibition of DNA strand breaks can reduce neural remodeling. Treatment of HT22 hippocampal neuronal cells with minocycline inhibited the expression of γ-H2AX and the p-ATM/ATM ratio as well as reduced apoptosis following X-ray exposure (Zhang et al., 2017). Similarly, mesenchymal stem cell conditioned media (MSC-CM) reduced the expression of γ-H2AX and reduced apoptosis, reversing the changes induced by X-ray radiation (Huang et al., 2021). Lithium chloride was also shown to reduce γ-H2AX levels and increase proliferation in neural stem cells irradiated with 60Co gamma rays (Zanni et al., 2015).

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

None identified.

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	Details	Effects on the KER	References
Drug	MSC-CM	Treatment reduced the expression of γ-H2AX and reduced apoptosis	Huang et al., 2021
	Lithium chloride	Reduced the level of γ-H2AX and increased proliferation of neural stem cells.	Zanni et al., 2015
	Minocycline (an antibiotic shown to reduce radiation-induced memory loss)	Treatment inhibited the increase in γ-H2AX and p-ATM and reduced apoptosis.	Zhang et al., 2017
Genetics	DNA ligase IV-null mutation	Mice with this mutation show greatly increased levels of apoptosis compared to wild-type mice due to reduced DNA repair following irradiation.	Barazzuol et al., 2015
Age	Hippocampal neurogenesis is more pronounced in younger mice.	Proliferative potential of neuronal precursors in the hippocampus, determined by Ki-67 immunostaining, was significantly reduced in juvenile mice but not significantly affected in adult mice after irradiation.	Schmal et al., 2019

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 table below provides some representative examples of quantitative linkages between the two key events. All data is statistically significant unless otherwise stated.

Dose Concordance

Reference	Experiment Description	Result
Schmal et al., 2019	In vivo. Juvenile and adult male mice were whole-body irradiated with various doses (5, 10, 15 or 20 fractions of 0.1 Gy) of 6 MV photons. DNA DSBs were determined by 53BP1 immunofluorescence in mature neurons. Neural remodeling was assessed by the level of DCX+ neuroprogenitor cells and transcription factor SRY (sex-determining-region-Y) box 2 (SOX2)+ stem/progenitor cells in the subgranular zone (SGZ) of the hippocampal dentate gyrus.	At 72h post-irradiation of juvenile mice, 53BP1 foci increased 1.5-fold at 0.5 Gy and 2.7-fold at 2 Gy, while DCX+ cells decreased 0.9-fold at 0.5 Gy and 0.7-fold at 2 Gy. At 72h post-irradiation of adult mice, 53BP1 foci increased 1.2-fold (non-significant) at 0.5 Gy and 2-fold at 2 Gy, while DCX+ cells decreased 0.9-fold at 0.5 Gy and 0.8-fold at 2 Gy. SOX2+ cells did not change at 72 h post-irradiation, but decreased 0.6-fold in juvenile mice and 0.8-fold in adult mice at 2 Gy after 1 month.
Barazzuol et al., 2015	In vivo. C57BL/6 mice aged 2-4 months were irradiated with 50 mGy, 100 mGy and 200 mGy of X-rays. Apoptosis in the SVZ was determined with a TUNEL assay. DSBs in the cerebellum were quantified with 53BP1 immunofluorescence.	53BP1 foci increased linearly from 0.05 foci/cell at 0 Gy to 1.3 foci per cell at 200 mGy. The number of TUNEL+ cells increased linearly from 5 cells/section at 0 Gy to about 50 cells/section at 200 mGy.
Barazzuol, Ju and Jeggo, 2017	In vivo. C57BL/6 mice were irradiated with various doses of X-rays (0.5 Gy/min). Immunofluorescence was used to detect TUNEL+ cells (apoptotic), Ki67+ cells (proliferating) and DCX+ cells (neuron progenitors). 53BP1 was also detected by immunofluorescence.	53BP1 foci increased over 10-fold at 0.1 Gy and about 80-fold at 2 Gy in the lateral ventricle. At 1, 2, and 3 Gy, Ki67+ cells in the lateral ventricle decreased 0.2-fold, and TUNEL+ cells were increased in the lateral ventricle. At 2 Gy, DCX+ cells decreased to less than 0.1-fold.

Time Concordance

Reference	Experiment Description	Result
Acharya et al., 2010	In vitro. hNSCs were irradiated with 5 Gy of 137Cs gamma rays (2.2 Gy/min. γ-H2AX foci for DSBs were quantified with immunofluorescence. Apoptosis of hNSCs was measured using fluorescence-activated cell sorting (FACS) for poly (ADP-ribose) polymerase (PARP) cleavage (early marker) and annexin V binding (late marker). Differentiation of hNSCs was measured by β-III-Tubulin staining and cell numbers were measured by SYBR green fluorescence.	γ-H2AX foci were increased from 5% to 95% as early as 0.3 h post-irradiation. PARP+ cells were increased 3-fold at 6 h post-irradiation, while annexin V+ cells were increased 2-fold 48 h post-irradiation. hNSC differentiation was decreased 0.5-fold 2 days post-irradiation. hNSC cell numbers were decreased 0.3-fold 3 days post-irradiation.
Barazzuol et al., 2015	In vivo. C57BL/6 mice aged 2-4 months were irradiated with 50 mGy, 100 mGy and 200 mGy of X-rays. Apoptosis in the SVZ was determined with a TUNEL assay. DSBs in the cerebellum were quantified with 53BP1 immunofluorescence.	The earliest increase in 53BP1 foci was observed at 0.25 h post-irradiation. The earliest increase in TUNEL+ cells was observed at 6 h post-irradiation.
Zhang et al., 2017	In vitro. Cells from the HT22 mouse hippocampal neuronal cell line were irradiated with 12 Gy of X-rays (4 Gy/min). γ-H2AX and p-ATM protein expression were determined with western blot. Apoptosis was determined with flow cytometry using annexin V and propidium iodide staining.	At 30 minutes post-irradiation, γ-H2AX increased 3.2-fold and the ratio of p-ATM/ATM increased 4.4-fold. Apoptosis increased over 10-fold at 48 h post-irradiation.
Barazzuol, Ju and Jeggo, 2017	In vivo. C57BL/6 mice were irradiated with various doses of X-rays (0.5 Gy/min). Immunofluorescence was used to detect TUNEL+ cells (apoptotic), Ki67+ cells (proliferating) and DCX+ cells (neuron progenitors). 53BP1 was also detected by immunofluorescence.	A peak in 53BP1 foci occurred at 0.5 h post-irradiation. Changes to TUNEL+ cells, Ki67+ cells and DCX+ cells were observed at 6 h post-irradiation.

Incidence Concordance

Reference	Experimental Description	Results
Acharya et al., 2010	In vitro. hNSCs were irradiated with 5 Gy of 137Cs gamma rays (2.2 Gy/min. γ-H2AX foci for DSBs were quantified with immunofluorescence. Apoptosis of hNSCs was measured using FACS for PARP cleavage (early marker) and annexin V binding (late marker). Differentiation of hNSCs was measured by β-III-Tubulin staining and cell numbers were measured by SYBR green fluorescence.	γ-H2AX foci were increased about 20-fold. PARP+ cells were increased 3-fold and annexin V+ cells were increased 2-fold. hNSC differentiation was decreased by 0.5-fold. hNSC cell numbers were decreased 0.3-fold.
Barazzuol et al., 2015	In vivo. C57BL/6 mice aged 2-4 months were irradiated with 50 mGy, 100 mGy and 200 mGy of X-rays. Apoptosis in the SVZ was determined with a TUNEL assay. DSBs in the cerebellum were quantified with 53BP1 immunofluorescence.	53BP1 foci increased almost 30-fold between 0 Gy and 200 mGy. The number of TUNEL+ cells increased about 10-fold between 0 Gy and 200 mGy.

Response-response Relationship

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

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

Some studies suggest that neuron activity can generate DNA DSBs. Specifically, it has been shown that γ-H2AX foci can be formed by the activation of NMDA a https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/epstein-barr-virus.html nd AMPA glutamate receptors (reviewed by Konopka and Atkin, 2022). Activity induced DSBs in mature neurons subsequently influence gene expression and neuronal activity (Alt and Schwer, 2018).

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

Evidence for this relationship is derived from studies that use human-derived cells and mouse models, with most of the evidence in mice. There is in vivo evidence in male animals. Most evidence is from adult models.

References

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

Abner, C. W. and P. J. McKinnon. (2004), "The DNA double-strand break response in the nervous system", DNA Repair, Vol. 3/8–9, Elsevier, Amsterdam, https://doi.org/10.1016/j.dnarep.2004.03.009.

Acharya, M. M. et al. (2010), "Consequences of ionizing radiation-induced damage in human neural stem cells", Free Radical Biology and Medicine, Vol. 49/12, Elsevier, Amsterdam, https://doi.org/10.1016/j.freeradbiomed.2010.08.021.

Alt, F. W. and B. Schwer. (2018), "DNA double-strand breaks as drivers of neural genomic change, function, and disease", DNA Repair, Vol. 71, Elsevier, Amsterdam, https://doi.org/10.1016/j.dnarep.2018.08.019.

Barazzuol, L., L. Ju, and P. A. Jeggo. (2017), “A coordinated DNA damage response promotes adult quiescent neural stem cell activation”, PLoS biology, 15(5), PLOS, San Francisco, https://doi.org/10.1371/journal.pbio.2001264

Barazzuol, L. et al. (2015), "Endogenous and X-ray-induced DNA double strand breaks sensitively activate apoptosis in adult neural stem cells", Journal of Cell Science, Vol. 128/19, The Company of Biologists, Cambridge, https://doi.org/10.1242/jcs.171223.

Desai, R. I. et al. (2022), "Impact of spaceflight stressors on behavior and cognition: A molecular, neurochemical, and neurobiological perspective", Neuroscience & Biobehavioral Reviews, Vol. 138, Elsevier, Amsterdam, https://doi.org/10.1016/j.neubiorev.2022.104676.

Huang, Y. et al. (2021), "Mesenchymal Stem Cell-Conditioned Medium Protects Hippocampal Neurons From Radiation Damage by Suppressing Oxidative Stress and Apoptosis", Dose-Response, Vol. 19/1, SAGE Publications https://doi.org/10.1177/1559325820984944.

Konopka, A. and J. D. Atkin. (2022), "The Role of DNA Damage in Neural Plasticity in Physiology and Neurodegeneration", Frontiers in Cellular Neuroscience, Vol. 16, Frontiers, https://doi.org/10.3389/fncel.2022.836885.

Lee, Y. and P. J. McKinnon. (2007), "Responding to DNA double strand breaks in the nervous system", Neuroscience, Vol. 145/4, Elsevier, Amsterdam, https://doi.org/10.1016/j.neuroscience.2006.07.026.

Madabhushi, R., L. Pan and L.-H. Tsai. (2014), "DNA Damage and Its Links to Neurodegeneration", Neuron, Vol. 83/2, Elsevier, Amsterdam, https://doi.org/10.1016/j.neuron.2014.06.034.

Michaelidesova, A. et al. (2019), "Effects of Radiation Therapy on Neural Stem Cells", Genes, Vol. 10/9, MDPI, Basel, https://doi.org/10.3390/genes10090640.

Schmal, Z. et al. (2019), "DNA damage accumulation during fractionated low-dose radiation compromises hippocampal neurogenesis", Radiotherapy and Oncology, Vol. 137, Elsevier, Amsterdam, https://doi.org/10.1016/j.radonc.2019.04.021.

Thadathil, N. et al. (2019), "DNA double-strand breaks: a potential therapeutic target for neurodegenerative diseases", Chromosome Research, Vol. 27/4, Springer Nature, https://doi.org/10.1007/s10577-019-09617-x.

Wang, H. et al. (2017), "Chronic oxidative damage together with genome repair deficiency in the neurons is a double whammy for neurodegeneration: Is damage response signaling a potential therapeutic target?", Mechanisms of Ageing and Development, Vol. 161, Elsevier, Amsterdam, https://doi.org/10.1016/j.mad.2016.09.005.

Zanni, G. et al. (2015), "Lithium increases proliferation of hippocampal neural stem/progenitor cells and rescues irradiation-induced cell cycle arrest in vitro", Oncotarget, Vol. 6/35, https://doi.org/10.18632/oncotarget.5191.

Zhang, L. et al. (2017), "The inhibitory effect of minocycline on radiation-induced neuronal apoptosis via AMPKα1 signaling-mediated autophagy", Scientific Reports, Vol. 7/1, Springer Nature, https://doi.org/10.1038/s41598-017-16693-8.

Zhu, L.-S. et al. (2019), "Emerging Perspectives on DNA Double-strand Breaks in Neurodegenerative Diseases", Current Neuropharmacology, Vol. 17/12, Bentham Science Publishers, https://doi.org/10.2174/1570159X17666190726115623.