Jonathan T. Haselman, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <firstname.lastname@example.org>
Sigmund J. Degitz, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <email@example.com>
Michael W. Hornung, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <firstname.lastname@example.org>
Point of Contact
- Jonathan Haselman
|Author status||OECD status||OECD project||SAAOP status|
|Under development: Not open for comment. Do not cite||Under Development||1.29||Included in OECD Work Plan|
This AOP was last modified on December 03, 2016 16:37
|Inhibition, Na+/I- symporter (NIS)||September 16, 2017 10:15|
|Decrease of Thyroidal iodide||September 16, 2017 10:17|
|Thyroid hormone synthesis, Decreased||September 17, 2017 18:27|
|Thyroxine (T4) in serum, Decreased||September 17, 2017 18:37|
|Decreased, Thyroxine (T4) in tissues||November 29, 2016 19:42|
|Altered, Amphibian metamorphosis||December 03, 2016 16:37|
|Decreased, Triiodothyronine (T3) in tissues||November 29, 2016 19:43|
|Perchlorate||November 29, 2016 18:42|
This AOP describes how intracellular iodine deficits in thyroid follicular cells via chemical inhibition of sodium-iodide symporter (NIS) decrease thyroid hormone (TH) synthesis and cause delayed amphibian metamorphosis, or in extreme cases, arrests development. Amphibian metamorphosis is mediated by TH and successful completion of metamorphosis is generally required for organism survival. NIS is a critical transport protein that mediates iodine uptake into thyroid follicular cells making it available for thyroperoxidase (see TPO AOP) to catalyze its covalent bonding to tyrosine residues of thyroglobulin. TPO subsequently couples the iodinated tyrosines to form thyroxine (T4). Conversion of T4 to the active hormone, triiodothyronine (T3), is catalyzed by type I or II deiodinase enzymes (see DIO1 and DIO2 pAOPs) located within the peripheral organs and tissues, which then binds to thyroid receptor (TR). Activated TR then stimulates gene expression that drives the anatomical and physiological changes encompassed by the metamorphic process including limb emergence and development, lung development, gill and tail resorption, gut remodeling, metabolic profile changes in the liver, skin keratinization, etc. The model NIS inhibitor, perchlorate, has been tested in amphibian model species Xenopus laevis using in vivo study designs aiming to characterize temporal profiles of glandular hormone levels in addition to serum hormone levels and associated thyroid gland histopathology. Although there are only a few studies in amphibians that directly address NIS inhibition, these studies provide a strong weight of evidence supporting the specificity and essentiality of NIS inhibition leading to well-supported essential key events downstream.
This optional section should be 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. Instructions To add background information, click Edit in the upper right hand menu on the AOP page. Under the “Background (optional)” field, a text editable form provides ability to edit the Background. Clicking ‘Update AOP’ will update these fields.
Summary of the AOP
Molecular Initiating Event
|Inhibition, Na+/I- symporter (NIS)||Inhibition, Na+/I- symporter (NIS)|
|Decrease of Thyroidal iodide||Thyroidal Iodide, Decreased|
|Thyroid hormone synthesis, Decreased||TH synthesis, Decreased|
|Thyroxine (T4) in serum, Decreased||T4 in serum, Decreased|
|Decreased, Thyroxine (T4) in tissues||Decreased, Thyroxine (T4) in tissues|
|Decreased, Triiodothyronine (T3) in tissues||Decreased, Triiodothyronine (T3) in tissues|
|Altered, Amphibian metamorphosis||Altered, Amphibian metamorphosis|
Relationships Between Two Key Events (Including MIEs and AOs)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 organization to predicted outcomes at higher levels of organization 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 weight of evidence 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. Description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for assessment of the AOP (section 7). The modified Bradford Hill considerations of biological plausibility and empirical support can be evaluated with regard to the predictive relationships/associations between pairs of KEs as a basis for considering weight of evidence of KERs (Section 7). The plausibility of the relationship between two KEs with respect to current understanding of normal (i.e., unperturbed biology) can be evaluated. Concordance of empirical evidence (i.e., dose-response, temporal and incidence concordance) can also be assessed and is usually based on consideration of these relationships following exposure to specific stressors that are believed to initiate the pathway. For example, temporal concordance can be evaluated by considering whether each “upstream” KE precedes the next “downstream” KE in the series. For empirical evidence derived for a specific stressor, dose-response and incidence concordance can also be evaluated to determine whether the pattern of results supports the hypothesized KER – i.e., does KEupstream occur at equivalent or lower doses and/or with less frequency than KEdownstream. Consistencies or inconsistencies in supporting data across different biological contexts and/or multiple studies can also help define confidence in the KER. Therefore, the suggested subsections of the KER description included in the current template are intended to aid the user in collecting relevant information that will support evaluation of the level of confidence in each KER, which in turn contributes to the assessment of the weight of evidence of the AOP, overall (section 7). By convention, KERs may take one of two forms. They may refer specifically to direct linkage between a pair of KEs that are adjacent in an AOP. Alternatively, a KER may refer to indirect linkages between a pair of KEs for which the relationship is thought to run through another KE or a gap in current understanding (i.e., non-adjacent KEs in an AOP; represented as dashed arrows in Figure 3). It is not necessary to describe a KER for every possible binary pair of KEs that could be indirectly linked. However, the option to provide KER descriptions for indirect KERs is particularly useful within the AOP-Wiki, because empirical evidence supporting the linkages among KEs in an AOP (see below) may often skip steps. For example, some KE measurements may be fairly difficult to make, such that they are rarely made in routine studies. While there may be sufficient data to establish the KE as part of the AOP, much of the available weight of evidence may ignore or “leap over” that particular KE. Including indirect KER descriptions allows the weight of evidence for these indirect relationships to be readily described and linked to other AOPs. Additionally, it can aid the process of developing and expanding putative AOPs where initial 19 linkages may span significant knowledge-gaps which are later filled in with additional KEs as more information becomes available and/or targeted research is completed. Instructions To add a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Add relationship.’ User will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. Select a Directness from the drop down menu. The fields “Evidence” and “Quantitative understanding” can be entered at the time of creation of the relationship, or can be added later. Upon selection of these options from their respective drop-down menus, click ‘Update Aop relationship.’ The new relationship should be listed on the AOP page. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Directness, Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update Aop relationship’ to update these fields and return to the AOP page.
Life Stage Applicability
|African clawed frog||Xenopus laevis||Strong||NCBI|
Graphical RepresentationClick to download graphical representation template
Overall Assessment of the AOP
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 To edit the “Overall Assessment of the AOP” 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 “Overall Assessment of the AOP” 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 “Overall Assessment of the AOP” section on the AOP page.
Domain of ApplicabilityThe 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 EventsThe 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 SummaryThis involves evaluation of the Overall AOP based on Relative Level of Confidence in the KERs, Essentiality of the KEs and Degree of Quantitative Understanding based on Annexes 1 and 2. Annex 1 (“Guidance for assessing relative level of confidence in the Overall AOP”) guides consideration of the weight of evidence or degree of confidence in the predictive relationship between pairs of KEs based on KER descriptions and support for essentiality of KEs. It is designed to facilitate assignment of categories of high, moderate or low against specific considerations for each a series of defined element based on current experience in assessing MOAs/AOPs. In addition to increasing consistency through delineation of defining questions for the elements and the nature of evidence associated with assignment to each of the categories, importantly, the objective of completion of Annex 1 is to transparently delineate the rationales for the assignment based on the specified considerations. While it is not necessary to repeat lengthy text which appears in earlier parts of the document, the entries for the rationales should explicitly express the reasoning for assignment to the categories, based on the considerations for high, moderate or low weight of evidence included in the columns for each of the relevant elements. 24 While the elements can be addressed separately for each of the KERs, the essentiality of the KEs within the AOP is considered collectively since their interdependence is often illustrated through prevention or augmentation of an earlier or later key event. Where it is not possible to experimentally assess the essentiality of the KEs within the AOP (i.e., there is no experimental model to prevent or augment the key events in the pathway), this should be noted. Identified limitations of the database to address the biological plausibility of the KERs, the essentiality of the KEs and empirical support for the KERs are influential in assigning the categories for degree of confidence (i.e., high, moderate or low). Consideration of the confidence in the overall AOP is based, then, on the extent of available experimental data on the essentiality of KEs and the collective consideration of the qualitative weight of evidence for each of the KERs, in the context of their interdependence leading to adverse effect in the overall AOP. Assessment of the overall AOP is summarized in the Network View, which represents the degree of confidence in the weight of evidence both for the rank ordered elements of essentiality of the key events and biological plausibility and empirical support for the interrelationships between KEs. The AOP-Wiki provides such a network graphic based on the information provided in the MIE, KE, AO, and KER tables. The Key Event Essentiality calls are used to determine the size of each key event node with larger sizes representing higher confidence for essentiality. The Weight of Evidence summary in the KER table is used to determine the width of the lines connecting the key events with thicker lines representing higher confidence. Instructions To edit the “Weight of Evidence Summary” 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 “Weight of Evidence Summary” 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 “Weight of Evidence Summary” section on the AOP page.
Quantitative ConsiderationsThe 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 To edit the “Quantitative Considerations” 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 “Quantitative Considerations” 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 “Quantitative Considerations” section on the AOP page.
Considerations for Potential Applications of the AOP (optional)
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 To edit the “Considerations for Potential Applications of the AOP” 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 “Considerations for Potential Applications of the AOP” 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 “Considerations for Potential Applications of the AOP” section on the AOP page.
Hornung, M.W., Degitz, S.J., Korte, L.M., Olson, J.M., Kosian, P.A., Linnum, A.L. and Tietge, J.E., 2010. Inhibition of thyroid hormone release from cultured amphibian thyroid glands by methimazole, 6-propylthiouracil, and perchlorate. Toxicological Sciences, 118(1), pp.42-51.
Tietge, J.E., Holcombe, G.W., Flynn, K.M., Kosian, P.A., Korte, J.J., Anderson, L.E., Wolf, D.C. and Degitz, S.J., 2005. Metamorphic inhibition of Xenopus laevis by sodium perchlorate: effects on development and thyroid histology. Environmental Toxicology and Chemistry, 24(4), pp.926-933.
Tietge, J.E., Butterworth, B.C., Haselman, J.T., Holcombe, G.W., Hornung, M.W., Korte, J.J., Kosian, P.A., Wolfe, M. and Degitz, S.J., 2010. Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis. Aquatic Toxicology, 98(1), pp.44-50.