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AOP: 566

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

Decreased, GnRH pulsatility/release leading to estradiol availability, increased via impaired ovulation

Short name
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Decreased GnRH release leading to increased E2
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 v2.7

Graphical Representation

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

Authors

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Anna Lanzoni

Martina Panzarea

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
Travis Karschnik   (email point of contact)

Contributors

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  • Travis Karschnik
  • Ana-Andreea Cioca
  • anna lanzoni
  • Martina Panzarea

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 January 14, 2025 17:54

Revision dates for related pages

Page Revision Date/Time
Decreased, GnRH pulsatility/release January 02, 2025 17:04
Decreased, LH Surge January 13, 2025 16:37
Impaired ovulation January 03, 2025 15:38
Increased plasma estradiol to progesterone ratio (estrogen dominance/unopposed estrogen) January 03, 2025 16:02
Estradiol availability, increased November 21, 2024 17:40
Persistent vaginal cornification January 13, 2025 15:51
Decreased, GnRH pulsatility/release leads to Decreased, LH Surge January 03, 2025 14:46
Decreased, LH Surge leads to Impaired ovulation January 10, 2025 14:30
Impaired ovulation leads to Plasma estradiol/progesterone ratio, increase January 14, 2025 17:29
Plasma estradiol/progesterone ratio, increase leads to Increased E2 availability January 14, 2025 17:29
Plasma estradiol/progesterone ratio, increase leads to PVC January 14, 2025 17:30

Abstract

A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

Due to the multiplicity of possible MIEs (Kisspeptin decrease, gamma-aminobutyric acid-ergic (GABAergic) modulation, neuropeptides and vasopressin role, etc.), it was decided to develop this AOP starting from the common relevant KE, the reduced availability of GnRH at pituitary level. However, discussion on plausible MIEs is included under Annex B.4.

Ovarian hormones regulate normal human endometrial cell proliferation, regeneration and function and therefore they are implicated in endometrial carcinogenesis directly or via influencing other hormones and metabolic pathways. The role of unopposed estrogen in the pathogenesis of EC has received considerable attention, together with other hormones, such as androgens and GnRH.

One of the key homeostatic hormonal loops in this system is provided by the ovarian hormones, E2 and P4, that modulate the activity of the neuronal network controlling the release of GnRH. The hypothalamic GnRH neurons release GnRH in an episodic manner into the pituitary portal circulation to generate distinct pulses of luteinising hormone (LH) and follicle-stimulating hormone (FSH) throughout the ovarian cycle. Thus, the brain and pituitary produce an on-going pulsatile pattern of gonadotropin secretion that slows on oestrous to allow appropriate follicular development and a surge pattern of secretion at mid-cycle to initiate ovulation.

Numerous studies have reported that the oestrous-stage decline in LH pulse frequency results from the post-ovulatory secretion of P4 (Soules et al., 1984; Smith et al., 1989; Goodman 2015) and the administration of P4 was found to dramatically slow GnRH pulse generator activity in the mouse (McQuillan et al., 2019). Thus, it seems very likely that P4 is the key gonadal hormone exerting a negative feedback influence upon the pulse generator during the cycle and does so to bring about the post-ovulatory slowing of pulsatility.

As follicles grow, estrogen synthesis increases in the female ovary. This in turn promotes GnRH pulses in the hypothalamus. GnRH binds to its receptor expressed by pituitary gonadotropic cells and induces the release of 2 gonadotropins, LH and FSH. In turn, LH and FSH stimulate gametogenesis and steroidogenesis in the gonads (Duffy et al., 2018). An LH surge is needed and responsible for the downstream pathways that induce ovulation; this includes resumption of meiosis in the oocyte and cellular changes that allow rupture of the follicle to release the egg for fertilisation. It increases intrafollicular proteolytic enzymes, weakening the wall of the ovary and allowing for passage of the mature follicle (Robker et al., 2018).

The suppression of GnRH availability, due to the impairment of regulatory systems or destruction of the peptide, results in a failure of response to pre-ovulatory level of estrogen to produce LH surges. Without the LH surge, the downstream pathways are not able to function and as a result ovulation does not occur. If the LH surge is delayed, then ovulation may be delayed as well and fails to occur within the correct time window. This can have a negative impact on the reproductive health of females and perturb the oestrous cycle.

In most cases, if ovulation is blocked or delayed, the ratio of estradiol/progesterone (E2/P4) remains high due to lack of P4 increase that is initiated after ovulation. As a result, ovarian and circulating steroid hormone levels remain in the ‘pre-ovulatory’ state, i.e. high E2, and low P4. In addition, with ovulation disruption, formation of corpus lutea is delayed or inhibited. This overall disrupts the cycle and can lead to persistent oestrous.

Persistent oestrous is characterised by the lack of corpus lutea formation, and observation of cysts and antral follicles. Morphologically, it is demonstrated by persistent vaginal cornification (PVC). It is considered persistent if at least two cycles were perturbed with the appearance of PVC (Finch, 2014; Stewart et al., 2022). A prolonged increased circulating E2/P4 ratio leads to an increase of E2 bioavailability in a variety of estrogenic-responsive organs, including the uterus due to insufficient counterbalance by P4. However, compensatory mechanisms (e.g. intracrine networks) may differ across different tissues. The degree to which E2/P4 ratio should increase to overwhelm these compensatory responses has not been established.

AOP Development Strategy

Context

Used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development.The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. More help

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

Please refer to the EFSA Scientific Opinion for an overview of the Context and Strategy of the AOP development.

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 530 Decreased, GnRH pulsatility/release Decreased, GnRH pulsatility/release
KE 531 Decreased, LH Surge Decreased, LH Surge
KE 1695 Impaired ovulation Impaired ovulation
KE 2303 Increased plasma estradiol to progesterone ratio (estrogen dominance/unopposed estrogen) Plasma estradiol/progesterone ratio, increase
KE 2251 Estradiol availability, increased Increased E2 availability
KE 2306 Persistent vaginal cornification PVC

Relationships Between Two Key Events (Including MIEs and AOs)

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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

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

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

Taxonomic Applicability

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Term Scientific Term Evidence Link
mammals mammals NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help
Sex Evidence
Male
Female

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

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

Sex Applicability: Males and Females

Taxonomic Applicablity: Restricted to mammals.

Life Stages Applicability: Adulthood.

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

Evidence Assessment

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

KER title

Biological Plausibility

Empirical Support[KT1] 

Essentiality

Brief Explanation (summary)

KER 1:

KE1 => KE2 reduced GnRH availability pulsatory release leads to decrease/delayed LH surge

H

H

H

Biological Plausibility.

There is an extensive understanding based on extensive previous documentation and broad acceptance that LH surge depends by GnRH release and availability, it is considered a consolidated scientific concept with a well-established mechanistic basis. As consequence, the KER is considered a high biological plausible KER.  

Empirical Support. There are multiple studies showing dependent change in both events following exposure to a range of specific stressors i.e., atrazine, tributyltin (TBT), endopeptidases, GABA modulators and light stimulation. Also, the available evidence support a concordance in terms of temporal, dose- response and incidence with no or few critical data gaps or conflicting data (see section “empirical evidence” of this KER). The empirical support was therefore considered high.

Essentiality. There is enough direct evidence supporting the essentiality of the current KE, blocking the GnRH receptor with a competitive antagonist led to a full blockage of the LH surge in ewes (Karsch et al., 1997) and rats (Wu, 1997 and Lasdun, 1089). Moreover, different studies demonstrates that the selective deletion of estrogen receptor alpha from kisspeptin neurons (neurons that innervates the GnRH neurons) as well lesions of the AVPv nucleus, resulted in an abolishment of the LH surge leading to persistent estrous (Dubois, 2015). As a matter of facts, estrogens are acting on the kisspeptin neurons, the GnRh neurons have not the estrogen receptor.

Indirect evidence (i.e., Goodman 2015) is also available.

The essentiality is therefore weighted as high.

KER 2:

KE2 => KE3:  Reduced LH surge leads to delayed ovulation

H

H

H

Biological Plausibility. The biological plausibility of this KERs is linked to the physiological role of LH in mammals which is a consolidated scientific concept (dogma). The biological plausibility is therefore high.

Empirical support. There are multiple studies showing dependent change in both events following exposure to a range of specific stressors i.e., atrazine, TCDD, PFOS, and GnRH antagonists. Also, the available evidence (see section “empirical evidence” of this KER) supports a concordance in terms of temporal, dose- response and incidence with no or few critical data gaps or conflicting data.

The empirical support was therefore considered high.

Essentiality.  KO animal models were used to demonstrate that both LH and the LHCGR is necessary to trigger ovulation. This is also corroborated by the evidence that LH or LHCGR mutations found in women from different families presents lack of ovulation and issue on menstrual cycle.

Therefore, there is enough direct evidence supporting the essentiality of this KEs. The essentiality is considered high.

KER3 –

KE3 => KE4: Delayed ovulation leads to estrogen dominance

H

M

M

Biological Plausibility. The biological plausibility of this KERs is linked to the physiology of reproductive system.  Being a consolidated scientific concept, the biological plausibility is high.

Empirical support. There are few studies showing dependent change in both events. Moreover, the analytical methods used to measure the levels of hormones in plasma, make difficult a comparison of the E2/P4 (Estradiol/Progesteron) ratio among different studies. The empirical support was therefore considered moderate.

Essentiality. 

From the biological perspective it is well established that a lack of ovulation perturbs the estrous cycle due to the absence of the expected increase in progesterone secretion with the formation of corpus lutea. This alters the balance between estrogens and progesterone expected after ovulation.

However, it is noted that the empirical evidence found in support of the current AOP development, clearly reported that in the knockout strains of LHβ or its receptor LHCGR, where the ovulation was found to be destroyed, both estradiol and progesterone are decreased in the serum or ovary. This evidence is not supporting the essentiality of this KEs and its impact on the later KE in this AOP i.e., estrogen dominance.

The essentiality is therefore weighted as moderate.

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

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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

Dubois SL, Acosta-Martínez M, DeJoseph MR, Wolfe A, Radovick S, Boehm U, Urban JH and Levine JE, 2015. Positive, but not negative feedback actions of estradiol in adult female mice require estrogen receptor α in kisspeptin neurons. Endocrinology, 156:1111-1120. doi: 10.1210/en.2014-1851

Duffy DM, Ko C, Jo M, Brannstrom M and Curry TE, 2019. Ovulation: Parallels With Inflammatory Processes. Endocr Rev, 40:369-416. doi: 10.1210/er.2018-00075

Finch CE, 2014. The menopause and aging, a comparative perspective. J Steroid Biochem Mol Biol, 142:132-141. doi: 10.1016/j.jsbmb.2013.03.010

Karsch FJ, Bowen JM, Caraty A, Evans NP and Moenter SM, 1997. Gonadotropin-releasing hormone requirements for ovulation. Biol Reprod, 56:303-309. doi: 10.1095/biolreprod56.2.303

Lasdun A, Reznik S, Molineaux CJ and Orlowski M, 1989. Inhibition of endopeptidase 24.15 slows the in vivo degradation of luteinizing hormone-releasing hormone. J Pharmacol Exp Ther, 251:439-447

McQuillan HJ, Han SY, Cheong I and Herbison AE, 2019. GnRH Pulse Generator Activity Across the Estrous Cycle of Female Mice. Endocrinology, 160:1480-1491. doi: 10.1210/en.2019-00193

Robker RL, Hennebold JD and Russell DL, 2018. Coordination of Ovulation and Oocyte Maturation: A Good Egg at the Right Time. Endocrinology, 159:3209-3218. doi: 10.1210/en.2018-00485

Stewart CA, Stewart MD, Wang Y, Mullen RD, Kircher BK, Liang R, Liu Y and Behringer RR, 2022. Chronic Estrus Disrupts Uterine Gland Development and Homeostasis. Endocrinology, 163. doi: 10.1210/endocr/bqac011

Wu TJ, Pierotti AR, Jakubowski M, Sheward WJ, Glucksman MJ, Smith AI, King JC, Fink G and Roberts JL, 1997. Endopeptidase EC 3.4.24.15 presence in the rat median eminence and hypophysial portal blood and its modulation of the luteinizing hormone surge. J Neuroendocrinol, 9:813-822. doi: 10.1046/j.1365-2826.1997.00637.x