Aop: 131


Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [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

Aryl hydrocarbon receptor activation leading to uroporphyria

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
AHR activation-uroporphyria

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help


List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

Authours: Amani Farhat1, Gillian Manning, Jason OBrien2, and Sean W. Kennedy3

Environment and Climate Change Canada

Contact Information:




Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Allie Always   (email point of contact)


List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Amani Farhat
  • Jason M. O'Brien
  • Allie Always


The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Open for citation & comment WPHA/WNT Endorsed 1.7 Included in OECD Work Plan
This AOP was last modified on May 08, 2022 11:33
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Activation, AhR March 22, 2018 14:00
Oxidation, Uroporphyrinogen September 16, 2017 10:16
Inhibition, UROD May 30, 2018 12:49
Accumulation, Highly carboxylated porphyrins May 07, 2018 07:56
Uroporphyria May 07, 2018 07:53
Induction, CYP1A2/CYP1A5 May 30, 2018 14:24
Activation, AhR leads to Induction, CYP1A2/CYP1A5 May 07, 2018 08:09
Induction, CYP1A2/CYP1A5 leads to Oxidation, Uroporphyrinogen May 31, 2018 15:43
Oxidation, Uroporphyrinogen leads to Inhibition, UROD May 30, 2018 10:58
Inhibition, UROD leads to Accumulation, Highly carboxylated porphyrins May 30, 2018 12:40
Accumulation, Highly carboxylated porphyrins leads to Uroporphyria May 07, 2018 08:03
Dibenzo-p-dioxin November 29, 2016 18:42
Polychlorinated biphenyl November 29, 2016 18:42
Hexachlorobenzene November 29, 2016 18:42
Polycyclic aromatic hydrocarbons (PAHs) February 09, 2017 15:43


In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

Hepatic uroporphyria is a disorder where the disturbance of heme biosynthesis results in accumulation and excretion of uroporphyrin, heptacarboxyl- and hexacarboxyl porphyrin: collectively referred to as highly carboxylated porphyrins (HCPs)[1][2][3]. The disorder is due to a homozygous mutation in uroporphyrinogen decarboxylase (UROD), an enzyme involved in the heme biosynthesis pathway [4], or may be chemically induced, which involves the inhibition of UROD. This adverse outcome pathway (AOP) describes the linkages leading to chemically induced porphyria through the activation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor.  AHR activation leads to the induction of cytochrome P450 1A2, a phase I metabolizing enzyme, which in turn results in excessive oxidation of uroporphyrinogen.  This oxidation produces a UROD inhibitor, preventing the conversion of uroporphyrinogen to coprouroporphyrinogen and increasing the synthesis of the UROD inhibitor in a positive feedback loop. The accumulation of uroporphyrinogen leads to its preferential oxidation and accumulation of HCP in various organs (Uroporphyria).  This AOP was developed in accordance with OECD guidelines and demonstrates a high degree of confidence as a qualitative AOP. The quantitative understanding of this AOP however is not yet complete, preventing the accurate prediction of uroporphyria from lower level key events.

Background (optional)

This optional subsection 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. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Heme is a cyclic tetrapyrrole cofactor containing Fe2+ porphyrin-containing ferroprotein that forms various hemoproteins such as hemoglobin, cytochromes and catalases [62]. Its biosynthesis mostly occurs in the liver and involves 8 separate steps. Porphyria is a disorder in which the disturbance of any of the steps of heme biosynthesis results in accumulation and excretion of porphyrins[2]. A variety of porphyrias exist depending on which enzyme in the pathway is deficient. This AOP describes a situation in which the 5th step of heme biosynthesis, uroporphyrinogen decarboxylase (UROD), which converts uroporphyrinogen to coproporphyrinogen, is inhibited.

Hepatic uroporphyria is viewed somewhat differently by clinicians and toxicologists. For the former it is mostly a sporadic disease (porphyria cutanea tarda; PCT) occurring sometimes in patients exposed to a variety of insults such as alcohol, estrogens, hepatitis viruses, HIV and on dialysis. Importantly, very early on it was found that lowering body iron stores by bleeding or now chelators causes remission [61]. In some northern European and US patients, carrying the hemochromatosis mutation is a risk factor but in other patients other iron susceptibility genes may contribute. Carrying a UROD mutation (lowering activity) is also a risk factor but still dependent on other susceptibility factors to see porphyria. To reproduce these findings experimentally has proved challenging but now possible. For toxicologists hepatic uroporphyria has mostly been seen as a toxic, but unique and curious endpoint of polychlorinated ligands of the AHR.  Experimentally, TCDD in mice is the most potent agent consistent with AHR mode of action but is more difficult in rats and other organisms. Hexachlorobenzene (HCB) has been greatly studied for its porphyria-inducing abilities and a large incident of porphyria in some young people in Turkey 60 years ago was ascribed to susceptible individuals who had consumed HCB. It is controversial whether HCB is a weak AHR ligand. Evidence of porphyria in people exposed accidentally or occupationally to accepted AHR ligands such as TCDD and PCBs is thin. Importantly, iron status can profoundly modify experimental uroporphyria induced by these chemicals especially in mice. In fact iron overload alone of mice will eventually produce a strong hepatic uroporphyria which is markedly genetically determined and toxicity can be ameliorated by chelators resembling PCT. Thus hepatic porphyria could alternatively be viewed as an iron AOP. At an overall level hepatic uroporphyria in animals and patients is the outcome of complex genetic traits and external stimuli in which in some traditional toxicological circumstances binding of a chemical to the AHR may have a major contribution[67] but in others may not.

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 18 Activation, AhR Activation, AhR
2 KE 850 Induction, CYP1A2/CYP1A5 Induction, CYP1A2/CYP1A5
3 KE 844 Oxidation, Uroporphyrinogen Oxidation, Uroporphyrinogen
4 KE 845 Inhibition, UROD Inhibition, UROD
5 KE 846 Accumulation, Highly carboxylated porphyrins Accumulation, Highly carboxylated porphyrins
6 AO 369 Uroporphyria Uroporphyria

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the 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. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. 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 Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove 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 ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and 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


The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. 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. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help
Life stage Evidence
Adult High
Juvenile High

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. 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
herring gull Larus argentatus Moderate NCBI
Japanese quail Coturnix japonica Low NCBI
Common Starling Common Starling Moderate NCBI

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Sex Evidence
Unspecific High

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

Overall, this AOP can most accurately be applied to mammalian species past the embryonic and infant stage of development.  It is also representative of a solid toxicity pathway in avian species, however the contribution of the defining key event (UROD inhibition) is not as well understood; it is not as dramatically and consistently inhibited as it is with mammals.  There is minimal evidence supporting the applicability of this AOP in fish, and none in alternate species.  Details and supporting evidences are summarized below.

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

Life Stage Applicability, Taxonomic Applicability, Sex Applicability Elaborate on the domains of applicability listed in the summary section above. Specifically, provide the literature supporting, or excluding, certain domains.

Life Stage Applicability: Uroporphyria occurs following chemical exposure in juvenile or adult individuals. Fetal exposure to dioxin-like compounds causes developmental abnormalities and embryolethality rather than HCP accumulation[15][16][17][18][19]. Turkish children under the age of two that were exposed to HCB through breastmilk passed away from a condition called "pink sore”[20].

Taxonomic Applicability: Although the AHR is highly conserved in evolution[21], chemical-induced uroporphyria has only been detected in birds[1][2][3] and mammals[22] , including an accidental outbreak in humans due to hexachlorobenzen-contaminated grain in the 1950s[20]. Fish are less susceptible to chemical-induced uroporphyria, but elevated levels of HCP have been documented in highly contaminated environments[23].

Sex Applicability: Although this AOP applies broadly to both males and females, sexual dimorphism for uroporphyria has been observed in rats exposed to hexachlorobenzene (HCB). Hepatic uroporphyrin III was markedly increased in female rats exposed to HCB whereas exposed males showed levels of hepatic porphyrins similar to controls[24].

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. 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.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help

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.

Every Key event in this AOP is absolutely essential for downstream events to occur. A summary of evidence for essentiality of each key event is given below.

Molecular Initiating Event: AHR activation (Essentiality=strong)

  • Mice with a high-affinity Ahr allele (C57BL/6J ) are much more sensitive to uroporphyria than mice with low-affinity Ahr allele (DBA/2)[25][26][27][28][29];
  • The Ah locus influences the susceptibility of C57BL/6J mice to HCB-induced porphyria[30];
  • Ahr knockout mice (C57BL/6) are resistant to development of porphyria, even in the presence of iron loading[25];
  • Primary hepatocytes of avian species indicate that species that are highly sensitive to AHR activation are more sensitive to uroporphyrin accumulation than species with lower sensitivity to AHR activation[31].

Key Event 1: CYP1A2/Cyp1A5 induction (Essentiality=strong)

  • CYP1A2 knockout in mice prevents chemical-induced uroporphyria[32][33][34];
  • CYP1A2 knockout prevents porphyria in genetically predisposed mice (Hfe-/-, Urod-/+) that normally develop porphyria in absence of external stimuli[35];
  • CYP1A2 levels are correlated with the extent of urophorphyrin accumulation in mice[36];
  • 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and non-ortho substituted PCBs that are potent inducers of CYP1A4/5 cause accumulation of only HCPs in chicken embryonic hepatocytes cultures, whereas PCBs that do not induce CYP1A4/5 cause a porphyrin pattern that is not consistent with inhibition of UROD[37];
  • Common tern (Sterna hirundo) embryonic hepatocyte cultures, which are ~50 to > 1600 times less sensitive than chicken embryonic hepatocyte cultures to CYP1A5 induction by TCDD and PCBs, do not accumulate HCPs upon chemical exposure[31].

It should be noted that a recent study by Davies et al.[25] found that both C57BL/6J mice (susceptible to chemical-induced porphyria) and DBA/2 mice (resistant to porphyria due to polymorphism in AHR gene) showed increased expression of CYP1A2 when exposed to TCDD, even though the DBA/2 strain did not develop porphyria. Furthermore AHR-/- mice showed a mild uroporphyric response in the presence of iron loading and 5-aminolevulinic acid (a heme precursor). These findings suggest that the induction of CYP1A2 is not crucial for chemical-induced porphyria, but a basal level of expression is absolutely essential.

Key Event 2: Uroporphyrinogen oxidation (UROX) (Essentiality=strong)

  • Uroporphyria is characterized biochemically by increased formation of HCPs derived by oxidation of the porphyrinogen substrates of uroporphyrinogen decarboxylase (UROD); secondary to decreased activity of this enzyme in the liver[22];
  • Uroporphomethane, derived from oxidizing a single carbon bridge in uroporphyrinogen, has been identified as the UROD inhibitor that leads to chemically- and genetically-induced uroporphyria in mice[38];
  • UROX activity is positively correlated with uroporphyrin levels in mice[36].

Key Event 3: Uroporphyrinogen decarboxylase (UROD) inhibition (Essentiality=strong)

  • Mutations in the UROD gene that reduce or eliminate UROD activity lead to porphyria in mammals; a decrease in hepatic UROD activity of at least 70% is necessary to observe symptoms from overproduction of porphyrins[22];
  • A marked progressive decrease in UROD enzyme activity is a common feature in animal models of chemical-induced porphyria[22][34][39][40][41];
  • Liver cytosol UROD activity in female rats exposed to HCB was decreased more than 70% and correlated with elevated hepatic uroporphyrin levels, whereas male rats, which did not develop porphyria, showed UROD activity similar to controls[24];
  • UROD activity is inversely proportional to uroporphyrin levels in mice[36];
  • In chicken hepatocytes, the strongest inducers of porphyrin accumulation were also the strongest inhibitors of UROD activity[41];
  • Reduced UROD enzyme activity, not protein levels, is characteristic of uroporphyria in humans and rats[24][42][43].

Key Event 4: Highly carboxylated porphyrin (HCP) accumulation (Essentiality=strong)

  • Under normal heme biosynthesis, porphyrins are only present in trace amounts in the liver; however, in the absence of UROD activity, the oxidation of Uroporphorynogen to uroporphyrins dominates, leading to an accumulation of HCPs;
  • Porphyrins are strongly fluorescent compounds resulting in a characteristic red fluorescence of hepatic tissue under UV light that is proportional to the level of porphyrins[44][45]. Increased urinary excretion of porphyrins is also indicative of their accumulation and can lead to dark red/brown urine[22]. HCPs also accumulate in the skin causing solar hypersensitivity and increased skin fragility[46];
  • HCP accumulation was observed in avian embryo hepatocyte cultures following exposure potent AHR agonists (dioxin-like compounds)[37][47][48][49] and in the livers of Japanese quails and chickens exposed to PCBs[50][51][52];
  • HCP accumulation was evident in mice treated with polyhalogenated aromatic compounds[36] or TCDD[25].

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help

Summary Table

Dose concordance

Table 1 demonstrates that upstream KEs (monooxygenase activity/quantity) are significantly affected at lower doses than downstream KEs (porphyrin levels). After a 6 month recovery period, CYP450 and hepatic porphyrin levels were dramatically reduced, however, they did not return to normal. Furthermore, urinary porphyrin excretion remained maximally elevated[53]

Uroporphyria Table 1 TCDD recovery.png

Temporal Concordance

Table 2 demonstrates that upstream KEs (CYP1A2 expression and UROD inhibition) are significantly affected at earlier time-points than downstream KEs (porphyrin levels). These studies also show that upstream KEs are more sensitive to change than downstream KEs; ddY mice showed a 44% reduction in UROD activity but did not develop uroporphyria[25][54].

Key Events Relationships

Table 3 shows a sampling of the literature that demonstrates changes in KEs at multiple levels of organization leading to uroporphyria. The use of animal models resistant to porphyria (low AHR affinity or AHR/CYP1A2 knockout) illustrates the essentiality of these KEs in for downstream effects.

Uroporphyria Table 3 KER Summary.png


CYPs other than CYP1A2 are able of catalyzing uroporphyrinogen oxidation, raising doubts on the essentiality of CYP1A2 for this pathway. For instance, Phillips et al.[35] were able to generate mild uroporphyria in a Cyp1A2-/- mouse model that is genetically predisposed (Hfe-/-, Urod-/+) to develop porphyria.

The essentiality of CYP1A2 induction in human porphyria cutanea tarda is unclear. UROX activity in human liver microsomes was not correlated with CYP1A2 content[66]. Furthermore, there is contradictory evidence regarding the association between CYP1A2 polymorphism and susceptibility to porphyria cutanea tarda [63-64]. It may be possible that in patients with a genetic variation in UROD causing an inherent reduction in activity, the activity of CYP1A2 is less important.

UROD inhibition is not always observed and/or is less pronounced in avian models of porphyria, mainly in quail [47]. It is suggested that a mechanism other than UROD inhibition explain the extent of porphyrin accumulation in birds. Therefore, the applicability of this AOP to avian remains uncertain.

The characterization of the UROD inhibitor isolated by Phillips [38] has been criticized by Danton[66]. Therefore, UROD inhibitor has yet to be identified.

AhR binding stressors under certain conditions do not lead to adverse effect in particular mammalian strains[25].

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

Summary Table 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.

The overall quantitative understanding of this AOP is moderate for mammals and poor for alternate species. Quantitative models have been developed that predict the AHR transactivation potential of various compounds [55][56][57], but the extent of AHR activation necessary to produce porphyria is not known. It has been established that a reduction in UROD activity of at least 70% is required to lead to overt uroporphyrin in mammals[58][24][54]. Additionally, numerous in vitro systems have been developed to study porphyrin accumulation and UROD inhibition simultaneously; therefore, this KER provides the most feasible target for a predictive, quantitative model. However, care must be taken when reading across to other species; UROD inhibition is not always observed in avian models of porphyria, and when it is, it is less pronounced[59][60][47].

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.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 or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help

This AOP was developed with the intended purpose of chemical screening as well as ecological risk assessment.  There are numerous in vitro assays for each key event up to the level of UROD activity.  There is sufficient evidence that a 70% inhibition of UROD activity significantly increases the risk of developing uroporphyria in mammals, making it a promising target assay in the battery of chemical screening tools.   Furthermore, there has recently been significant advances in the understanding of differences in avian sensitivity to AHR agonists, and a similar effort is underway for fish.  Sequencing the AHR ligand binding domain of any bird species (and potentially fish species) allows for its classification as low, medium or high sensitivity, which aids in the chemical risk assessment of DLCs and other AHR agonists.  There is also potential use for this AOP in risk management, as minimum allowable environmental levels can be customized to the sensitivity of the native species in the area under consideration.


List the bibliographic references to original papers, books or other documents used to support the AOP. More help
  1. 1.0 1.1 Fox, G. A., Norstrom, R. J., Wigfield, D. C., and Kennedy, S. W. (1988) Porphyria in herring gulls: A biochemical response to chemical contamination of great lakes food chains. ‘’Environmental Toxicology and Chemistry’’ ‘’’7’’’ (10), 831-839
  2. 2.0 2.1 Kennedy, S. W., and Fox, G. A. (1990) Highly carboxylated porphyrins as a biomarker of polyhalogenated aromatic hydrocarbon exposure in wildlife: Confirmation of their presence in Great Lakes herring gull chicks in the early 1970s and important methodological details. Chemosphere 21 (3), 407-415.
  3. 3.0 3.1 Kennedy, S. W., Fox, G. A., Trudeau, S. F., Bastien, L. J., and Jones, S. P. (1998) Highly carboxylated porphyrin concentration: A biochemical marker of PCB exposure in herring gulls. Marine Environmental Research 46 (1-5), 65-69.
  4. Thunell, S. (2000) Porphyrins, porphyrin metabolism and porphyrias. I. Update. Scand. J. Clin. Lab Invest 60 (7), 509-540.
  5. Baba, T., Mimura, J., Nakamura, N., Harada, N., Yamamoto, M., Morohashi, K., and Fujii-Kuriyama, Y. (2005) Intrinsic function of the aryl hydrocarbon (dioxin) receptor as a key factor in female reproduction. Mol. Cell Biol. 25 (22), 10040-10051.
  6. Denison, M. S., Soshilov, A. A., He, G., DeGroot, D. E., and Zhao, B. (2011) Exactly the same but different: promiscuity and diversity in the molecular mechanisms of action of the aryl hydrocarbon (dioxin) receptor. Toxicol. Sci. 124 (1), 1-22.
  7. Fernandez-Salguero, P. M., Pineau, T., Hilbert, D. M., McPhail, T., Lee, S. S., Kimura, S., Nebert, D. W., Rudikoff, S., Ward, J. M., and Gonzalez, F. J. (1995) Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor. Science 268 (5211), 722-726.
  8. Ichihara, S., Yamada, Y., Ichihara, G., Nakajima, T., Li, P., Kondo, T., Gonzalez, F. J., and Murohara, T. (2007) A role for the aryl hydrocarbon receptor in regulation of ischemia-induced angiogenesis. Arterioscler. Thromb. Vasc. Biol. 27 (6), 1297-1304.
  9. Lahvis, G. P., Lindell, S. L., Thomas, R. S., McCuskey, R. S., Murphy, C., Glover, E., Bentz, M., Southard, J., and Bradfield, C. A. (2000) Portosystemic shunting and persistent fetal vascular structures in aryl hydrocarbon receptor-deficient mice. Proc. Natl. Acad. Sci U. S. A 97 (19), 10442-10447.
  10. Mimura, J., Yamashita, K., Nakamura, K., Morita, M., Takagi, T. N., Nakao, K., Ema, M., Sogawa, K., Yasuda, M., Katsuki, M., and Fujii-Kuriyama, Y. (1997) Loss of teratogenic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking the Ah (dioxin) receptor. Genes Cells 2 (10), 645-654.
  11. Omiecinski, C. J., Vanden Heuvel, J. P., Perdew, G. H., and Peters, J. M. (2011) Xenobiotic metabolism, disposition, and regulation by receptors: from biochemical phenomenon to predictors of major toxicities. Toxicol. Sci. 120 Suppl 1, S49-S75.
  12. Schmidt, J. V., Su, G. H., Reddy, J. K., Simon, M. C., and Bradfield, C. A. (1996) Characterization of a murine Ahr null allele: involvement of the Ah receptor in hepatic growth and development. Proc. Natl. Acad. Sci U. S. A 93 (13), 6731-6736.
  13. Thackaberry, E. A., Gabaldon, D. M., Walker, M. K., and Smith, S. M. (2002) Aryl hydrocarbon receptor null mice develop cardiac hypertrophy and increased hypoxia-inducible factor-1alpha in the absence of cardiac hypoxia. Cardiovasc. Toxicol. 2 (4), 263-274.
  14. Zhang, N., Agbor, L. N., Scott, J. A., Zalobowski, T., Elased, K. M., Trujillo, A., Duke, M. S., Wolf, V., Walsh, M. T., Born, J. L., Felton, L. A., Wang, J., Wang, W., Kanagy, N. L., and Walker, M. K. (2010) An activated renin-angiotensin system maintains normal blood pressure in aryl hydrocarbon receptor heterozygous mice but not in null mice. Biochem. Pharmacol. 80 (2), 197-204.
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  16. Carro, T., Taneyhill, L. A., and Ottinger, M. A. (2013) The effects of an environmentally relevant 58 congener polychlorinated biphenyl (PCB) mixture on cardiac development in the chick embryo. Environ. Toxicol. Chem. 23(6), 1325-31
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  19. Wells, P. G., Lee, C. J., McCallum, G. P., Perstin, J., and Harper, P. A. (2010) Receptor- and reactive intermediate-mediated mechanisms of teratogenesis. Handb. Exp. Pharmacol. (196), 131-162.
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  25. 25.0 25.1 25.2 25.3 25.4 Davies, R., Clothier, B., Robinson, S. W., Edwards, R. E., Greaves, P., Luo, J., Gant, T. W., Chernova, T., and Smith, A. G. (2008) Essential role of the AH receptor in the dysfunction of heme metabolism induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin. Chem. Res. Toxicol. 21 (2), 330-340.
  26. Jones, K. G., and Sweeney, G. D. (1977) Association between induction of aryl hydrocarbon hydroxylase and depression of uroporphyrinogen decarboxylase activity. Res. Commun. Chem. Pathol. Pharmacol. 17 (4), 631-637.
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  29. Smith, A. G., Clothier, B., Robinson, S., Scullion, M. J., Carthew, P., Edwards, R., Luo, J., Lim, C. K., and Toledano, M. (1998) Interaction between iron metabolism and 2,3,7,8-tetrachlorodibenzo-p-dioxin in mice with variants of the Ahr gene: a hepatic oxidative mechanism. Mol. Pharmacol. 53 (1), 52-61.
  30. Hahn, M. E., Gasiewicz, T. A., Linko, P., and Goldstein, J. A. (1988) The role of the Ah locus in hexachlorobenzene-induced porphyria. Studies in congenic C57BL/6J mice. Biochem. J. 254 (1), 245-254.
  31. 31.0 31.1 Lorenzen, A., Shutt, J. L., and Kennedy, S. W. (1997) Sensitivity of common tern (Sterna hirundo) embryo hepatocyte cultures to CYP1A induction and porphyrin accumulation by halogenated aromatic hydrocarbons and common tern egg extracts. Archives of Environmental Contamination and Toxicology 32 (2), 126-134.
  32. Greaves, P., Clothier, B., Davies, R., Higginson, F. M., Edwards, R. E., Dalton, T. P., Nebert, D. W., and Smith, A. G. (2005) Uroporphyria and hepatic carcinogenesis induced by polychlorinated biphenyls-iron interaction: absence in the Cyp1a2(-/-) knockout mouse. Biochem. Biophys. Res. Commun. 331 (1), 147-152.
  33. Sinclair, P. R., Gorman, N., Dalton, T., Walton, H. S., Bement, W. J., Sinclair, J. F., Smith, A. G., and Nebert, D. W. (1998) Uroporphyria produced in mice by iron and 5-aminolaevulinic acid does not occur in Cyp1a2(-/-) null mutant mice. Biochem. J. 330 ( Pt 1'), 149-153.
  34. 34.0 34.1 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.
  35. Phillips, J. D., Kushner, J. P., Bergonia, H. A., and Franklin, M. R. (2011) Uroporphyria in the Cyp1a2-/- mouse. Blood Cells Mol. Dis. 47 (4), 249-254.
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