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Aop: 8

AOP Title

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Short name:

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Nuclear receptor induced TH Catabolism and Developmental Hearing Loss

Authors

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Katie Paul Friedman, National Center for Computational Toxicology, US EPA, RTP, NC USA <paul-frieidman@epa.gov>

Mary E. Gilbert, National Health and Environmental Effects Research Laboratory, US EPA, RTP, NC USA <gilbert.mary@epa.gov>

Kevin M. Crofton, National Center for Computational Toxicology, US EPA, RTP, NC USA <crofton.kevin@epa.gov>

Point of Contact

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Katie Paul Friedman

Contributors

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• Katie Paul Friedman
• Kevin Crofton
• Mary Gilbert

Status

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This AOP was last modified on September 19, 2017 09:54

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Revision dates for related pages

Page	Revision Date/Time
Activation, Pregnane-X receptor, NR1l2	September 16, 2017 10:14
Loss, Cochlear function	September 16, 2017 10:14
Thyroxine (T4) in neuronal tissue, Decreased	September 17, 2017 17:11
Thyroxine (T4) in serum, Decreased	September 17, 2017 18:37
Induction, Upregulation of glucuronyltransferase activity	September 16, 2017 10:14
Increase, Biliary excretion TH glucuronide	September 16, 2017 10:14
Hippocampal anatomy, Altered	September 16, 2017 10:14
Hippocampal Physiology, Altered	September 17, 2017 17:17
Hippocampal gene expression, Altered	September 17, 2017 17:14
Decrease, Tissue thyroid hormone concentration leads to Decrease, TR-regulated cochlear proteins	December 03, 2016 16:37
Decrease, TR-regulated cochlear proteins leads to Damage, Structure of the cochlea	December 03, 2016 16:37
Damage, Structure of the cochlea leads to Loss, Cochlear function	December 03, 2016 16:37
Activation, Pregnane-X receptor, NR1l2 leads to Up Regulation, Hepatic transporter gene expression	December 03, 2016 16:37
Up Regulation, Hepatic transporter gene expression leads to Increase, Hepatic transport of parent T4	December 03, 2016 16:37
Decrease, Serum thyroxine (T4) leads to Decrease, Tissue thyroid hormone concentration	December 03, 2016 16:37
Activation, Pregnane-X receptor, NR1l2 leads to Induction, Upregulation of glucuronyltransferase activity	December 03, 2016 16:37
Activation, Constitutive androstane receptor, NR1l3 leads to Induction, Upregulation of glucuronyltransferase activity	December 03, 2016 16:37
Induction, Upregulation of glucuronyltransferase activity leads to Decrease, Serum thyroxine (T4)	December 03, 2016 16:37
Increase, Biliary excretion TH glucuronide leads to Decrease, Serum thyroxine (T4)	December 03, 2016 16:37
Induction, Upregulation of glucuronyltransferase activity leads to Increase, Biliary excretion TH glucuronide	December 03, 2016 16:38

Abstract

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Data from rodent studies demonstrate that thyroid hormone disruption during cochlear development culminates in ototoxicity. Developmental exposure of rats to polychlorinated biphenyls (PCBs) results in a low-frequency hearing loss in adult offspring (Goldey et al., 1995a; Herr et al., 1996; 2001; Crofton and Rice, 1999; Laskey et al., 2002). A body of work now supports the hypothesis that this ototoxicity results from PCB-induced hypothyroxinemia during a critical period of auditory development. Evidence for this hypothesis includes: a correlation between the severity of functional auditory impairment and the degree of thyroid hormone depletion (Goldey et al., 1995a; 1995b; Goldey and Crofton, 1998; Crofton, 2004), a cross-fostering study demonstrating that the critical exposure period is postnatal (Crofton et al., 2000a), and amelioration of the hearing loss following postnatal thyroxine replacement (Goldey and Crofton, 1998). Below an adverse outcome pathway is described for chemicals that activate xenobiotic nuclear receptors, including AhR, CAR, and PXR, leading to thyroid hormone disruption during cochlear development and resulting in permanent auditory loss.

This AOP is a revision and update of the original started on the Chemical Mode of Action wiki sponsored by WHO/IPCS. This MOA was described and published by Crofton and Zoeller 2005).

Background (optional)

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This AOP is an update of the WHO/IPCS MOA developed in 2005 by Crofton and Zoeller (Crit Rev Toxicol 2005).

Crofton KM, Zoeller RT. Mode of action: neurotoxicity induced by thyroid hormone disruption during development--hearing loss resulting from exposure to PHAHs. Crit Rev Toxicol. 2005 Oct-Nov;35(8-9):757-69. PMID: 6417043

Summary of the AOP

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Stressors

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Describes stressors known to trigger the MIE and provides evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. The evidence supporting the stressor will typically consist of a brief description and citation of literature showing that particular stressors can trigger the MIE.

Instructions

To add a stressor associated with an AOP, under “Summary of the AOP” click ‘Add Stressor’ will bring user to the “New Aop Stressor” page. In the Name field, user can search for stressor by name. Choosing a stressor from the resulting drop down populates the field. Selection of an Evidence level from the drop down menu and add any supporting evidence in the text box. Click ‘Add stressor’ to add the stressor to the AOP page.

Molecular Initiating Event

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Key Events

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Adverse Outcome

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Relationships Between Two Key Events (Including MIEs and AOs)

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Network View

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Life Stage Applicability

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Taxonomic Applicability

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Indicate the relevant domain of applicability in terms of taxa.

Instructions

To add a taxonomic term to an AOP page, under “Taxonomic Applicability” select ‘add taxonomic term.’ User will be directed to a page entitled “Adding Taxonomic Term to AOP.” The user can search for and select an existing term from the drop down list of existing terms to populate the “Term” field. If a relevant term does not exist, click ‘Request New Taxon Term’ to request a term from AOP-Wiki administrators. Click ‘Add taxonomic term’ to add this term to the AOP page. Evidence can be left blank and added later.

To edit a taxonomic term on an AOP page, under “Taxonomic Applicability” click ‘Edit.’  User will be directed to a page entitled “Editing AOP Taxonomic Term” where they can edit the Evidence field using the drop down menu. Clicking ‘Update taxonomic term’ will update the Evidence field and redirect the user back to the AOP page.

Sex Applicability

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Graphical Representation

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Click to download graphical representation template

Overall Assessment of the AOP

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This section addresses the relevant domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and weight of evidence for the overall hypothesised AOP (i.e., including the MIE, KEs and AO) as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). It draws upon the evidence assembled for each KER as one of several components which contribute to relative confidence in supporting information for the entire hypothesised pathway. An important component in assessing confidence in supporting information as a basis to consider regulatory application of AOPs beyond that described in Section 6 is the essentiality of each of the key events as a component of the entire pathway. This is normally investigated in specifically-designed stop/reversibility studies or knockout models (i.e., those where a key event can be blocked or prevented). Assessment of the overall AOP also contributes to the identification of KEs for which confidence in the quantitative relationship with the AO is greatest (i.e., to facilitate determining the most sensitive predictor of the AO).

Instructions

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Domain of Applicability

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The relevant domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Domain of applicability is informed by the “Description” and “Taxonomic Relevance” section of each KE description and the “Description of the KER” section of each KER description. The relevant domain of applicability of the AOP as a whole will most often be defined based on the most narrowly restricted of its KEs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the domain of applicability of the AOP as a whole would generally be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE descriptions, the rationale for defining the relevant domain of applicability of the overall AOP should be briefly summarised on the AOP page.

Instructions

To edit the “Domain of Applicability” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Domain of Applicability” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page.  The new text should appear under the “Domain of Applicability” section on the AOP page.

Essentiality of the Key Events

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The essentiality of various of the KEs is influential in considering confidence in an overall hypothesised AOP for potential regulatory application being secondary only to biological plausibility of KERs (Meek et al., 2014; 2014a). The defining question for determining essentiality (included in Annex 1) relates to whether or not downstream KEs and/or the AO is prevented if an upstream event is experimentally blocked. It is assessed, generally, then, on the basis of direct experimental evidence of the absence/reduction of downstream KEs when an upstream KE is blocked or diminished (e.g., in null animal models or reversibility studies). Weight of evidence for essentiality of KEs would be considered high if there is direct evidence from specifically designed experimental studies illustrating essentiality for at least one of the important key events [e.g., stop/reversibility studies, antagonism, knock out models, etc.) moderate if there is indirect 25 evidence that experimentally induced change of an expected modulating factor attenuates or augments a key event (e.g., augmentation of proliferative response (KEupstream) leading to increase in tumour formation (KEdownstream or AO)) and weak if there is no or contradictory experimental evidence of the essentiality of any of the KEs (Annex 1).

Instructions

To edit the “Essentiality of the Key Events” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Essentiality of the Key Events” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page.  The new text should appear under the “Essentiality of the Key Events” section on the AOP page.

Weight of Evidence Summary

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Summary Table
Provide an overall summary of the weight of evidence based on the evaluations of the individual linkages from the Key Event Relationship pages.

Concordance of dose-response relationships

Multiple studies provide limited (2-3 doses) dose-response data for many of the key events. These studies demonstrate similar magnitudes of effect on circulating hormones for doses of PCBs that are within an order of magnitude (3-25 mg/kg/day for Aroclor 1254) (e.g.,Morse et al., 1996; Goldey et al., 1998). Very limited data are available correlating any of the key events. One exception is the relationship between circulating serum T4 concentrations during development and the magnitude of hearing loss (Crofton, 2004). There is a very good correlation between total serum T4 concentrations on postnatal day (PND) 14 and hearing loss assessed in adult offspring of PCB exposed dams (Figure 2). All of these events occur within a 2-3 fold dose range.

 

Temporal concordance among the key events and the adverse outcome Strength, consistency, and specificity of association of adverse effect and initiating event Biological plausibility, coherence, and consistency of the experimental evidence Alternative mechanism(s) or MIE(s) that logically present themselves and the extent to which they may detract from the AOP Uncertainties, inconsistencies, and data gaps

 

 

 

 

 

 

 

Quantitative Considerations

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The extent of quantitative understanding of the various KERs in the overall hypothesised AOP is also critical in consideration of potential regulatory application. For some applications (e.g. doseresponse analysis in in depth risk assessment), quantitative characterisation of downstream KERs may be essential while for others, quantitative understanding of upstream KERs may be important (e.g., QSAR modelling for category formation for testing). Because evidence that contributes to quantitative understanding of the KER is generally not mutually exclusive with the empirical support for the KER, evidence that contributes to quantitative understanding should generally be considered as part of the evaluation of the weight of evidence supporting the KER (see Annex 1, footnote b). General guidance on the degree of quantitative understanding that would be characterised as weak, moderate, or strong is provided in Annex 2.

Instructions

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Considerations for Potential Applications of the AOP (optional)

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At their discretion, the developer may include in this section discussion of the 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. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale. Detailing such considerations can aid the process of transforming narrative descriptions of AOPs into practical tools. In this context, it is necessarily beneficial to involve members of the regulatory risk assessment community on the development and assessment team. The Network view which is generated based on assessment of weight of evidence/degree of confidence in the hypothesized AOP taking into account the elements described in Section 7 provides a useful summary of relevant information as a basis to consider appropriate application in a regulatory context. Consideration of application needs then, to take into consideration the following rank ordered qualitative elements: Confidence in biological plausibility for each of the KERs Confidence in essentiality of the KEs Empirical support for each of the KERs and overall AOP The extent of weight of evidence/confidence in both these qualitative elements and that of the quantitative understanding for each of the KERs (e.g., is the MIE known, is quantitative understanding restricted to early or late key events) is also critical in determining appropriate application. For example, if the confidence and quantitative understanding of each KER in a hypothesised AOP are low and or low/moderate and the evidence for essentiality of KEs weak (Section 7), it might be considered as appropriate only for applications with less potential for impact (e.g., prioritisation, category formation for testing) versus those that have immediate implications potentially for risk management (e.g., in depth assessment). If confidence in quantitative understanding of late key events is high, this might be sufficient for an in depth assessment. The analysis supporting the Network view is also essential in identifying critical data gaps based on envisaged regulatory application.

Instructions

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References

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List the bibliographic references to original papers, books or other documents used to support the AOP.

Instructions

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