OECD Series on Adverse Outcome Pathways

An Adverse Outcome Pathway (AOP) describes a logical sequence of causally linked events at different levels of biological organisation, which follows exposure to a chemical and leads to an adverse health effect in humans or wildlife. AOPs are the central element of a toxicological knowledge framework, promoted by member countries through OECD, built to support chemical risk assessment based on mechanistic reasoning. These AOPs are available in the AOP Wiki, an interactive and virtual encyclopaedia for AOP development. Following their development and review, the endorsed AOPs are published the OECD Series on Adverse Outcome Pathways. As scientific knowledge progresses, the publication of an AOP in this series does not preclude the regular update or new contributions to a given AOP in the AOP Wiki. While the AOP Wiki is a dynamic tool, only impactful changes to the AOP will be reflected in subsequent updates of the published AOP. The number 1 in the OECD Series on Adverse Outcome Pathways is the Users’ Handbook, which is a supplement to the Guidance Document for developing and assessing AOPs. This handbook contains an updated template for AOP development and provides focused and practical instructions for both AOP developers and reviewers.


Adverse Outcome Pathway on Aryl hydrocarbon receptor activation leading to uroporphyria

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). The disorder is due to a homozygous mutation in uroporphyrinogen decarboxylase (UROD), an enzyme involved in the heme biosynthesis pathway, or may be chemically induced, which involves the inhibition of UROD. This 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 metabolising 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).


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