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Sally A. Mayasich, National Health and Environmental Effects Research Laboratory, US EPA, Duluth, MN, USA <email@example.com>
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>
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OECD Project 1.29: A catalog of putative AOPs that will enhance the utility of US EPA Toxcast high throughput screening data for hazard identification
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This putative AOP describes the potential for an adverse outcome resulting from the inhibition of Type III iodothyronine deiodinase (DIO3) during amphibian metamorphosis. Initial development of this AOP is based on literature in which amphibian deiodinases are genetically disrupted and prediction from tissue expression patterns. Chemical inhibition of DIO3, the molecular-initiating event (MIE), results in decreased transformation of thyroxine (T4) to the inactive form, 3,3’,5’-triiodothyronine (reverse T3, or rT3) and also decreased transformation of T3 to inactive form T2 in peripheral tissues. Thyroid hormones (THs), including appropriate levels of the inactive rT3 form, are essential for normal sequential development of amphibian tissues and organs, and activities of the three deiodinases found in amphibians, as in mammals, function in a highly regulated balance. Therefore, chemicals that interfere with the DIO3 catalyzing reaction of T4 inner-ring deiodination (IRD) to rT3 have the potential to cause overabundance of T4 as well as the active T3 form, potentially resulting in altered metamorphic development. Adverse consequences of rT3 insufficiency may vary based on timing of exposure and produce different effects at different developmental stages. In the African clawed frog, Xenopus laevis, DIO3 seems to be predominant during the early pre-metamorphosis development phase, protecting tissues from the actions of TH. Inhibition of DIO3 could alter T4/T3/rT3 feedback balance causing events that normally occur during pro-metamorphosis and post-metamorphic climax to occur too early and result in alterations in limb development, intestinal remodeling, gill resorption and/or tail resorption.
Summary of the AOP
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Molecular Initiating Event
|Molecular Initiating Event||Support for Essentiality|
|Deiodinase 3, Inhibition||Weak|
|Event||Support for Essentiality|
|Triiodothyronine (T3) in tissues, Increased||Moderate|
|Amphibian metamorphosis, Altered|
Relationships Among Key Events and the Adverse Outcome
|Event||Description||Triggers||Weight of Evidence||Quantitative Understanding|
|Deiodinase 3, Inhibition||Directly Leads to||Triiodothyronine (T3) in tissues, Increased||Weak||Weak|
|Triiodothyronine (T3) in tissues, Increased||Directly Leads to||Amphibian metamorphosis, Altered||Moderate||Weak|
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Life Stage Applicability
|African clawed frog||Xenopus laevis||Strong||NCBI|
Overall Assessment of the AOP
Domain of Applicability
Life Stage Applicability,
Elaborate on the domains of applicability listed in the summary section above. Specifically, provide the literature supporting, or excluding, certain domains.
Essentiality of the Key Events
Molecular Initiating Event Summary,
Key Event Summary
Provide an overall assessment of the essentiality for the key events in the AOP. Support calls for individual key events can be included in the molecular initiating event, key event, and adverse outcome tables above.
Weight of Evidence Summary
Provide an overall summary of the weight of evidence based on the evaluations of the individual linkages from the Key Event Relationship pages.
Provide an overall discussion of the quantitative information available for this AOP. Support calls for the individual relationships can be included in the Key Event Relationship table above.
Considerations for Potential Applications of the AOP (optional)
Becker, K.B., Stephens, K.C., Davey, J.C., Schneider, M.J., Galton, V.A. (1997). “The Type 2 and Type 3 iodothyronine deiodinases play important roles in coordinating development in Rana catesbeiana tadpoles.” Endocrinology 138(7): 2989-2997.
Galton VA, de Waard E, Parlow AF, St Germain DL, Hernndez, A. (2014) “Life without deiodinases.” Endocrinology. 155(10): 4081–4087.
Galton, V.A., Schneider, M.J., Clark, A.S., St. Germain, D.L. (2009). “Life without thyroxine to 3,5,3’-triiodothyronine conversion: studies in mice devoid of the 5’-deiodinases.” Endocrinology 150(6): 2957–2963.
Hernandez, A., Martinez ME, Fiering S, Galton VA, St Germain D (2006). Type 3 deiodinase is critical for the maturation and function of the thyroid axis. J Clin Invest 116:476–484.
Morvan-Dubois, G., Demeneix, B.A., Sachs, L.M. (2008). “Xenopus laevis as a model for studying thyroid hormone signaling: From development to metamorphosis.” Mol Cell Endocrinol. 293: 71-79.
Morvan-Dubois, G., Sebillot, A., Kuiper, G.G.J.M., Verhoelst, C.H.J., Darras, V.M., Visser, T.J., Demeneix, B.A. (2006). “Deiodinase activity is present in Xenopus laevis during early embryogenesis.” Endocrinolgy 147(10): 4941-4949.
Huang, H., Marsh-Armstrong, N., Brown, D.D. (1999). Metamorphosis is inhibited in transgenic Xenopus laevis tadpoles that overexpress type III deiodinase. Proc. Nat. Acad. Sci. USA 96: 962-967.