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Relationship: 1070

Title

The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

Inhibition, UROD leads to Accumulation, Highly carboxylated porphyrins

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

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 organisation to predicted outcomes at higher levels of organisation 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 WoE 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. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help
AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Aryl hydrocarbon receptor activation leading to uroporphyria adjacent Moderate Moderate Allie Always (send email) Open for citation & comment TFHA/WNT Endorsed

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help
Term Scientific Term Evidence Link
mouse Mus musculus High NCBI
rat Rattus norvegicus High NCBI
human Homo sapiens High NCBI
chicken Gallus gallus Moderate NCBI

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help
Sex Evidence
Unspecific Not Specified

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help
Term Evidence
Adult High

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

Through the normal heme biosynthesis pathway, uroporphyrinogen is converted to coproporphyrinogen by uroporphyrinogen decarboxylase (UROD)[1]. In the event that UROD activity is reduced (due to genetic disorders or chemical inhibition) uroporphyrinogen, and other porphyrinogen substrates of UROD, are preferentially oxidized to highly stable porphyrins by the phase one metabolizing enzyme CYP1A2 (in mammals;CYP1A5 in birds)[2][3][4] . Uroporphyrin and hepta- and hexa-carboxylic acid porphyrins (highly carboxylated porphyrins)[5] accumulate in the liver, kidneys, spleen, skin and blood leading to a heme disorder known as porphyria [6][7].

Evidence Supporting this KER

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help

The WOE for this KER is strong in mammals and Moderate in birds.

Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help
Hepatic uroporphyrinogen accumulation versus inhibition of uroporphyrinogen decarboxylase activity from individual mice treated with iron and HCB. Control: ○, Treated: ∆. (Source: Lambrecht, R.W. et al. (1988) Biochem. J. 253 (1), 131-138.)

It is well established that porphyrin accumulation, which is a result of uroporphyrin oxidation (UROX), and UROD inhibition go hand in hand[8]. Because CYP1A2/5 binds a broad range of substrates, significant UROX only occurs when there is an excess of uroporphyrinogen, which occurs when UROD is inhibited. Each of the four acetic acid substituents of porphyrinogen is decarboxylated in sequence with the consequent formation of hepta-, hexa-, and pentacarboxylic porphyrinogens as intermediates[9]. Oxidation of these intermediates results in their corresponding, highly stable porphyrins.

Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help

Uroporphyrin accumulation in avian models is less consistently accompanied by decreased UROD activity, and when it does occur, it is less marked than in mammals[13][14]. Although numerous studies show both a decrease in UROD activity and porphyrin accumulation in avian species, Lambrecht et al.[14] reported the accumulation of porphyrins in chicken embryo hepatocytes and Japanese quail liver without a decrease in UROD activity. They also note that the modest reduction in UROD activity (often less than 50%) is not enough to explain the extent of porphyrin accumulation observed and suggests there may be another mechanism at play. Alternatively, the difference between avian and mammals in regard to UROD inhibition may lie in the time-course of the response rather than its mechanism[19].

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help
Time-scale
This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help
Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help
Known Feedforward/Feedback loops influencing this KER
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

Induction of CYP1A2 increases its availability and consequently its ability to compete with UROD to oxidize uroporphyrinogen. At least one of these oxidation products is believed to be a competitive inhibitor of UROD. Therefore, UROD inhibition potentiates the oxidation of uroporphyrinogens by CYP1A2 to porphyrins leading to increased porphyrin accumulation and in turn UROD inhibition.

Domain of Applicability

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help

Chemical induces porphyrin accumulation has been demonstrated in, rats, mice and chicken[18][4][2]. Human porphyria cutanea tarda is also characterized biochemically by an increase in porphyrinogen oxidation leading to accumulation of porphyrins[15]. The correlation between reduced UROD activity and HCP accumulation in mammals is well defined[15][16][17] but is less consistent in avian models[14].

References

List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help
  1. Smith, A. G., Clothier, B., Carthew, P., Childs, N. L., Sinclair, P. R., Nebert, D. W., and Dalton, T. P. (2001) Protection of the Cyp1a2(-/-) null mouse against uroporphyria and hepatic injury following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol. Appl. Pharmacol. 173 (2), 89-98.
  2. 2.0 2.1 Jacobs, J. M., Sinclair, P. R., Bement, W. J., Lambrecht, R. W., Sinclair, J. F., and Goldstein, J. A. (1989). Oxidation of uroporphyrinogen by methylcholanthrene-induced cytochrome P-450. Essential role of cytochrome P-450d. Biochem. J 258 (1), 247-253.
  3. Lambrecht, R. W., Sinclair, P. R., Gorman, N., and Sinclair, J. F. (1992). Uroporphyrinogen oxidation catalyzed by reconstituted cytochrome P450IA2. Arch. Biochem. Biophys. 294 (2), 504-510.
  4. 4.0 4.1 Sinclair, P. R., Gorman, N., Walton, H. S., Sinclair, J. F., Lee, C. A., and Rifkind, A. B. (1997). Identification of CYP1A5 as the CYP1A enzyme mainly responsible for uroporphyrinogen oxidation induced by AH receptor ligands in chicken liver and kidney. Drug Metab. Dispos. 25 (7), 779-783.
  5. Marks, G. S., Powles, J., Lyon, M., McCluskey, S., Sutherland, E., and Zelt, D. (1987). Patterns of porphyrin accumulation in response to xenobiotics. Parallels between results in chick embryo and rodents. Ann. N. Y. Acad. Sci. 514, 113-127.
  6. Frank, J., and Poblete-Gutierrez, P. (2010) Porphyria cutanea tarda--when skin meets liver. Best. Pract. Res. Clin Gastroenterol. 24(5), 735-745.
  7. Doss, M., Schermuly, E., and Koss, G. (1976). Hexachlorobenzene porphyria in rats as a model for human chronic hepatic porphyrias. Ann. Clin Res. 8 Suppl 17, 171-181.
  8. Smith, A. G., and Elder, G. H. (2010) Complex gene-chemical interactions: hepatic uroporphyria as a paradigm. Chem. Res. Toxicol. 23 (4), 712-723.
  9. Elder, G. H., and Roberts, A. G. (1995). Uroporphyrinogen decarboxylase. J Bioenerg. Biomembr. 27 (2), 207-214.
  10. Phillips, J. D., Bergonia, H. A., Reilly, C. A., Franklin, M. R., and Kushner, J. P. (2007) A porphomethene inhibitor of uroporphyrinogen decarboxylase causes porphyria cutanea tarda. Proc. Natl. Acad. Sci. U. S. A 104 (12), 5079-5084.
  11. Sano, S., Kawanishi, S., and Seki, Y. (1985) Toxicity of polychlorinated biphenyl with special reference to porphyrin metabolism. Environ. Health Perspect. 59, 137-143.
  12. Sinclair, P. R., Gorman, N., Trask, H. W., Bement, W. J., Szakacs, J. G., Elder, G. H., Balestra, D., Sinclair, J. F., and Gerhard, G. S. (2003). Uroporphyria caused by ethanol in Hfe(-/-) mice as a model for porphyria cutanea tarda. Hepatology 37 (2), 351-358.
  13. James, C. A., and Marks, G. S. (1989). Inhibition of chick embryo hepatic uroporphyrinogen decarboxylase by components of xenobiotic-treated chick embryo hepatocytes in culture. Can. J Physiol Pharmacol. 67 (3), 246-249.
  14. 14.0 14.1 14.2 Lambrecht, R. W., Sinclair, P. R., Bement, W. J., Sinclair, J. F., Carpenter, H. M., Buhler, D. R., Urquhart, A. J., and Elder, G. H. (1988) Hepatic uroporphyrin accumulation and uroporphyrinogen decarboxylase activity in cultured chick-embryo hepatocytes and in Japanese quail (Coturnix coturnix japonica) and mice treated with polyhalogenated aromatic compounds. Biochem. J. 253 (1), 131-138.
  15. 15.0 15.1 15.2 Caballes F.R., Sendi, H., and Bonkovsky, H. L. (2012). Hepatitis C, porphyria cutanea tarda and liver iron: an update. Liver Int. 32 (6), 880-893.
  16. 16.0 16.1 Mylchreest, E., and Charbonneau, M. (1997) Studies on the mechanism of uroporphyrinogen decarboxylase inhibition in hexachlorobenzene-induced porphyria in the female rat. Toxicol. Appl. Pharmacol. 145 (1), 23-33.
  17. 17.0 17.1 Seki, Y., Kawanishi, S., and Sano, S. (1987). Mechanism of PCB-induced porphyria and yusho disease. Ann. N. Y. Acad. Sci. 514, 222-234.
  18. Nakano, K., Ishizuka, M., Sakamoto, K. Q., and Fujita, S. (2009). Absolute requirement for iron in the development of chemically induced uroporphyria in mice treated with 3-methylcholanthrene and 5-aminolevulinate. Biometals 22 (2), 345-351.
  19. Lambrecht, R. W., Jacobs, J. M., Sinclair, P. R., & Sinclair, J. F. (1990). Inhibition of uroporphyrinogen decarboxylase activity. The role of cytochrome P-450-mediated uroporphyrinogen oxidation. Biochemical Journal269(2), 437-441.