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AOP: 651

Title

A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the AO. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Suppression, Estrogen receptor (ER) activity leads to Impaired, Spermatogenesis via epigenetic modifications

Short name

A name that succinctly summarises the information from the title. This name should not exceed 90 characters. More help

Suppression ER activity leads to Impaired, Spermatogenesis

The current version of the Developer's Handbook will be automatically populated into the Handbook Version field when a new AOP page is created.Authors have the option to switch to a newer (but not older) Handbook version any time thereafter. More help

Handbook Version v2.8

Graphical Representation

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Click to download graphical representation template Explore AOP in a Third Party Tool

Authors

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Of the content populated in the AOP-Wiki: John R. Frisch and Travis Karschnik, General Dynamics Information Technology; Daniel L. Villeneuve, US Environmental Protection Agency, Risk Assessment Support Division; Scott Lynn, US Environmental Protection Agency, Office of Chemical Safety and Pollution Prevention.

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section. More help

John Frisch (email point of contact)

Contributors

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John Frisch

Coaches

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OECD Information Table

Provides users with information concerning how actively the AOP page is being developed and whether it is part of the OECD Workplan and has been reviewed and/or endorsed. OECD Project: Assigned upon acceptance onto OECD workplan. This project ID is managed and updated (if needed) by the OECD. OECD Status: For AOPs included on the OECD workplan, ‘OECD status’ tracks the level of review/endorsement of the AOP . This designation is managed and updated by the OECD. Journal-format Article: The OECD is developing co-operation with Scientific Journals for the review and publication of AOPs, via the signature of a Memorandum of Understanding. When the scientific review of an AOP is conducted by these Journals, the journal review panel will review the content of the Wiki. In addition, the Journal may ask the AOP authors to develop a separate manuscript (i.e. Journal Format Article) using a format determined by the Journal for Journal publication. In that case, the journal review panel will be required to review both the Wiki content and the Journal Format Article. The Journal will publish the AOP reviewed through the Journal Format Article. OECD iLibrary published version: OECD iLibrary is the online library of the OECD. The version of the AOP that is published there has been endorsed by the OECD. The purpose of publication on iLibrary is to provide a stable version over time, i.e. the version which has been reviewed and revised based on the outcome of the review. AOPs are viewed as living documents and may continue to evolve on the AOP-Wiki after their OECD endorsement and publication. More help

OECD Project #	OECD Status	Reviewer's Reports	Journal-format Article	OECD iLibrary Published Version

This AOP was last modified on July 28, 2026 11:22

Revision dates for related pages

Page	Revision Date/Time
Suppression, Estrogen receptor (ER) activity	July 28, 2026 10:42
Epigenetic modification process	July 28, 2026 10:49
Impaired, Spermatogenesis	July 28, 2026 10:54
Suppression, Estrogen receptor (ER) activity leads to Epigenetic modification process	July 28, 2026 11:13
Epigenetic modification process leads to Impaired, Spermatogenesis	July 28, 2026 11:18

Abstract

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Estrogen receptors (ER) are nuclear transcription factors involved in regulation of many physiological processes in vertebrates through signalling. There are two main classic types of nuclear estrogen receptors, estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) receptors and a more recently discovered membrane receptor, G protein-coupled estrogen receptor 1 (GPER1; Chen et al. (2022)). Binding by estrogen activates estrogen receptor activity. Suppression of estrogen receptor activity occurs when a stressor hinders estrogen binding to estrogen receptors or when estrogen levels are diminished, resulting in lowered estrogen receptor activity.

Epigenetic modifications are alterations that affect gene expression without altering the underlying gene sequence. Among some of the more common processes that lead to epigenetic modifications are DNA methylation and alteration of histone proteins. DNA methylation involves addition of methyl groups to DNA which causes increased or decreased gene expression. Histone proteins are small proteins that assist gene expression, DNA repair, and DNA replication; histones can be modified by the addition of methyl, acetyl, or phosphate groups, which accelerates or slows DNA expression, repair, and replication.

Spermatogenesis is the process in which germ cells develop into mature sperm in testes in male animals. Impaired spermatogenesis can result in lower mature sperm counts, problems in proper sperm function such as sperm motility, or other sperm abnormalities. For review of normal epigenetic modifications in spermatogenesis, see Gunes and Kulac (2013), for review of how disruption of epigenetic modifications disrupts spermatogenesis, see Cui et al. (2025).

This AOP contributes to the scientific understanding of the mechanistic foundations for linking suppression of estrogen receptor activity to the adverse outcome impaired spermatogenesis, with sperm motility, morphology, and abnormalities representing apical endpoints in guideline tests associated with the Endocrine Disruptor Screening Program (US EPA 1998; OECD 2001).

AOP Development Strategy

Context

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This AOP was part of an Environmental Protection Agency effort to develop AOPs that establish scientifically supported causal linkages between alternative endpoints measured using new approach methodologies (NAMs) and guideline apical endpoints measured in Tier 1 and Tier 2 test guidelines (U.S. EPA, 2025) employed by the Endocrine Disruptor Screening Program (EDSP). A series of key events that represent significant, measurable, milestones connecting molecular initiation to apical endpoints indicative of adversity were identified based on scientific review articles and empirical studies. Additionally, scientific evidence supporting the causal relationships between each pair of key events was assembled and evaluated. The present effort focused primarily on empirical studies with laboratory mammals.

Strategy

Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components. Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). More help

The scope of the aforementioned EPA project was to develop AOP(s) relevant to apical endpoints employed in the test guidelines, based on mechanisms consistent with empirical studies. The literature used to support this AOP and its constituent pages began with the test guidelines and followed to primary, secondary, and/or tertiary works concerning the relevant underlying biology. KE and KER page creation and re-use was determined using Handbook principles where page re-use was preferred.

Summary of the AOP

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Events:

Molecular Initiating Events (MIE)

An MIE is a specialised KE that represents the beginning (point of interaction between a prototypical stressor and the biological system) of an AOP. More help

Key Events (KE)

A measurable event within a specific biological level of organisation. More help

Adverse Outcomes (AO)

An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help

Type	Event ID	Title	Short name

MIE

1046

Suppression, Estrogen receptor (ER) activity

2152

Epigenetic modification process

1758

Impaired, Spermatogenesis

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP.Each table entry acts as a link to the individual KER description page. More help

Title	Adjacency	Evidence	Quantitative Understanding

Suppression, Estrogen receptor (ER) activity leads to Epigenetic modification process	adjacent	Moderate
Epigenetic modification process leads to Impaired, Spermatogenesis	adjacent	Moderate

Network View

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (WoE) summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Prototypical Stressors

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Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help

Life stage	Evidence
Adult, reproductively mature	Moderate

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available. More help

Term	Scientific Term	Evidence	Link
mammals	mammals	High	NCBI
Vertebrates	Vertebrates	Moderate	NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help

Sex	Evidence
Male	High

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help

1. Support for Biological Plausibility of Key Event Relationships: Is there a mechanistic relationship between KEup and KEdown consistent with established biological knowledge?
Key Event Relationship (KER)	Level of Support Strong = Extensive understanding of the KER based on extensive previous documentation and broad acceptance. Moderate = Support of essentiality from empirical studies, with some uncertainty in how tightly upstream events are linked to downstream events.
Relationship 3833: Suppression, Estrogen receptor (ER) activity leads to epigenetic modifications	Moderate support. The relationship between suppression of estrogen receptor activity and epigenetic modifications is broadly accepted and supported by animal data, particularly laboratory mammal data. Given the complexity of molecular-level interactions, it is difficult to link suppression of ER signalling to specific epigenetic modifications.
Relationship 3834: Epigenetic modifications leads to impaired, Spermatogenesis	Moderate support. The relationship between epigenetic modifications and impaired spermatogenesis is broadly accepted and supported by animal data, particularly laboratory mammal data. Although specific epigenetic modifications have been linked to specific spermatogenesis impairments, it is difficult to establish causality.
Overall	Moderate support. Extensive understanding of the relationships between events from empirical studies from animals, particularly laboratory mammals.

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

Life Stage: Adult, reproductively mature for process producing fully functional sperm.

Sex: Applies to males as related to sperm.

Taxonomic: Largely studied in laboratory mammals, plausible for all vertebrates as estrogen receptors have been evolutionarily conserved.

Essentiality of the Key Events

The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently, evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence. The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs. More help

2. Essentiality of Key Events: Are downstream KEs and/or the AO prevented if an upstream KE is blocked?
Key Event (KE)	Level of Support Strong = Direct evidence from specifically designed experimental studies illustrating essentiality and direct relationship between key events.
MIE 1046 Suppression, Estrogen receptor (ER) activity	Strong support. Suppression of estrogen receptor activity leads to epigenetic modifications. Evidence is available from exposure to endocrine-disrupting compounds and toxicants. Best evidence for essentiality for estrogen receptor activity is from stressor exposure studies decreasing estrogen receptor activity leading to altered histone messenger RNA, protein and methylation.
KE 2152 Epigenetic modifications	Strong support. Epigenetic modifications lead to impaired spermatogenesis. Evidence is available from exposure to endocrine-disrupting compounds and toxicants. Best evidence for essentiality of epigenetic modifications is from studies showing a specific epigenetic modification leading to a specific impairment of spermatogenesis (reviewed by Cui et al. 2025).
AO 1758 Impaired, Spermatogenesis	This is the final event of the AOP.
Overall	Strong support. Direct evidence from empirical studies from animals, particularly laboratory mammals for all key events.

Evidence Assessment

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

3. Empirical Support for Key Event Relationship: Does empirical evidence support that a change in KEup leads to an appropriate change in KEdown?
Key Event Relationship (KER)	Level of Support Strong = Experimental evidence from exposure to toxicant shows consistent change in both events across taxa and study conditions.
Relationship 3833: Suppression, Estrogen receptor (ER) activity leads to epigenetic modifications	Strong support. Suppression of estrogen receptor activity leads to epigenetic modifications. Evidence is available from endocrine disruptor and toxicant studies. Suppression of estrogen receptor activity occurred earlier in the time-course of exposure than epigenetic modifications, and the concentrations of stressors that suppressed estrogen receptor activity were equal to or lower than the concentrations that led to epigenetic modifications. Therefore, the data support a causal relationship.
Relationship 3834: Epigenetic modifications leads to impaired, Spermatogenesis	Strong support. Epigenetic modifications leads to impaired spermatogenesis. Evidence is available from endocrine disruptor and toxicant studies. Epigenetic modifications occurred earlier in the time-course of exposure than impaired spermatogenesis, and the concentrations of stressors that caused epigenetic modifications were equal to or lower than the concentrations that led to impaired spermatogenesis. Therefore, the data support a causal relationship.
Overall	Strong support. Evidence from empirical studies shows consistent relationships in upstream and downstream events, with upstream events occurring earlier in the time-course of exposure and at equal or lower concentrations than downstream events, supporting causal relationships.

Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help

Modulating Factor (MF)	Influence or Outcome	KER(s) involved

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors. More help

Considerations for Potential Applications of the AOP (optional)

Addressess potential applications of an AOP to support regulatory decision-making.This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. More help

References

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

Chen P, Li B, Ou-Yang L. 2022. Role of estrogen receptors in health and disease. Frontiers in Endrocrinology 13:839005.

Cui Y, Deng J, Zhang Y, Du L, Jiang F, Li C, Chen W, Zhang H, He Z. 2025. Epigenetic regulation by DNA methylation, histone modifications and chromatin remodeling complexes in controlling spermatogenesis and their dysfunction with male infertility. Cellular and Molecular Life Sciences 82(1): 343.

Gunes S, Kulac T. 2013. The role of epigenetics in spermatogenesis. Turkish Journal of Urology 39(3): 181-187.

Organisation for Economic Co-operation and Development. 2001. Test No. 416: Two-Generation Reproduction Toxicity, OECD Guidelines for the Testing of Chemicals, Section 4. https://www.oecd.org/content/dam/oecd/en/publications/reports/2001/01/test-no-416-two-generation-reproduction-toxicity_g1gh2941/9789264070868-en.pdf (retrieved 15 July 2026)

U.S. Environmental Protection Agency. 1998. Health Effects Test Guidelines OPPTS 870.3800 Reproduction and Fertility Effects. https://ntp.niehs.nih.gov/sites/default/files/iccvam/suppdocs/feddocs/epa/epa_870_3800.pdf (retrieved 24 December 2025)

U.S. Environmental Protection Agency. 2025. EDSP Test Guidelines and Guidance Document. https://www.epa.gov/test-guidelines-pesticides-and-toxic-substances/edsp-test-guidelines-and-guidance-document (retrieved 25 July 2025).