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AOP: 460


A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the 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

Antagonism of Smoothened receptor leading to orofacial clefting

Short name
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Anatagonsim SMO leads to OFC

Graphical Representation

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Click to download graphical representation template Explore AOP in a Third Party Tool


The names and affiliations of the individual(s)/organisation(s) that created/developed the AOP. More help

Jacob I. Reynolds1 , Brian P. Johnson1,2 

1Department of Biomedical Engineering, Institute for Quantitative Health Science and Engineering, Michigan State University, East Lansing, MI

2Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section.   More help
Arthur Author   (email point of contact)


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  • Jacob Reynolds
  • Arthur Author


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Provides 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. OECD Status - Tracks the level of review/endorsement the AOP has been subjected to. OECD Project Number - Project number is designated and updated by the OECD. SAAOP Status - Status managed and updated by SAAOP curators. More help
Handbook Version OECD status OECD project
This AOP was last modified on April 29, 2023 13:02

Revision dates for related pages

Page Revision Date/Time
Antagonism, Smoothened receptor July 28, 2022 12:17
Decrease, GLI1/2 translocation to nucleus October 28, 2022 15:22
Decrease, GLI1/2 target gene expression March 22, 2023 10:38
Decrease, Cell proliferation December 07, 2020 06:55
Decrease, palatal shelf outgrowth August 05, 2022 11:20
Ororofacial clefting August 08, 2022 15:15
Decrease, Sonic Hedgehog second messenger production March 22, 2023 12:09
Decrease, Smoothend relocation and activation October 27, 2022 09:14
Apoptosis December 20, 2022 08:33
Antagonism Smoothened leads to OFC April 14, 2023 10:48
Antagonism Smoothened leads to Decrease, SMO relocation January 23, 2023 15:43
Decrease, SMO relocation leads to Decrease, GLI1/2 translocation January 23, 2023 15:46
Decrease, GLI1/2 translocation leads to Decrease, GLI1/2 target gene expression January 27, 2023 15:34
Decrease, GLI1/2 target gene expression leads to Decrease, SHH second messenger production February 06, 2023 11:18
Decrease, SHH second messenger production leads to Decrease, Cell proliferation February 10, 2023 09:43
Decrease, Cell proliferation leads to Decrease, outgrowth March 10, 2023 09:51
Decrease, outgrowth leads to OFC March 13, 2023 13:45
Apoptosis leads to Decrease, outgrowth April 11, 2023 10:25
Decrease, GLI1/2 target gene expression leads to Apoptosis April 06, 2023 12:47
Vismodegib July 14, 2022 13:04
Cyclopamine August 03, 2022 09:21
SANT-1 October 04, 2022 13:17
SANT-2 October 04, 2022 13:17
SANT-3 October 04, 2022 13:17
SANT-4 October 04, 2022 13:18
Piperonyl butoxide March 23, 2023 10:19


A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

The Sonic Hedgehog (SHH) is a major signaling pathway of intercellular signaling during embryonic development. Disruption of SHH during critical periods of development can lead to orofacial clefts (OFCs). In canonical SHH signaling, the SHH ligand binds to the Patched1 (PTCH1) receptor and relieves its’ suppression of Smoothened (SMO) receptor. Antagonism of SMO results in disruption the downstream SHH signaling cascade. Disruption to the signaling cascade causes a decrease in the translocation of the GLI1/2 transcription factors to the nucleus resulting in a decrease in expression of the GLI1/2 target genes. This decrease in gene expression which causes a reduction in production of SHH secondary messengers, namely Fgf10 and members of the BMP family. This reduction in secondary messengers leads to a decrease in cellular proliferation in the palatal shelves. This reduction in cellular proliferation lead to a decrease in palatal shelf outgrowth which ultimately results in a cleft. This AOP is intended to serve as a tool for risk assessment for drug and chemical exposures during embryonic development when disruption to SHH through antagonism of SMO occurs.

AOP Development Strategy


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. More help

Orofacial clefts (OFCs), encompassing cleft lip with or without palate (CL/P), and cleft palate only (CPO) represent the second most common birth defect in humans with a prevalence of 1-2/1,000 births (Lidral, Moreno et al. 2008). The etiology of OFCs is complex with approximately 50% of CPO and 70% of CL/P considered non-syndromic (2011). SHH signaling is required for normal facial development and plays a critical role in the growth of the facial processes that form the upper palate and lip (Bush and Jiang 2012, Kurosaka 2015). The epithelial derived SHH drives orofacial development through an induced gradient in the underlying mesenchyme  (Lan and Jiang 2009, Kurosaka 2015). This gradient of SHH induces cellular proliferation and outgrowth of the mesenchyme (Lan and Jiang 2009). The SHH pathway is sensitive to chemical disruption and can be disrupted at multiple places along the signaling cascade during critical windows for exposure and has been shown to cause OFCs (Lipinski and Bushman 2010, Heyne, Melberg et al. 2015). The targets of this disruption include ligand modification, ligand secretion, downstream sensing, and signal transduction (Jeong and McMahon 2002, Lauth, Bergström et al. 2007, Petrova, Rios-Esteves et al. 2013). Chemical modulators of the SHH pathway have been identified including the natural alkaloid cyclopamine, both natural and synthetic pharmaceuticals, and a chemical commonly found in pesticides (Lipinski, Dengler et al. 2007, Lipinski, Song et al. 2010, Wang, Lu et al. 2012, Everson, Sun et al. 2019, Rivera-González, Beames et al. 2021).


Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components.  Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). More help

This AOP was developed as part of a larger network of AOPs linking disruption of SHH signaling with OFCs (EAGMST workplan project 1.101.). Orofacial clefts (OFCs) are one of the most common human birth defects and occur in approximately 1 in 700 live births (Mossey, Little et al. 2009, Dixon, Marazita et al. 2011). Early orofacial development involves epithelial ectoderm derived SHH ligand driving tissue outgrowth through an induced gradient of SHH dependent transcription in the underlying mesenchyme, which is thought to drive mesenchymal proliferation (Lan and Jiang 2009, Kurosaka 2015). The SHH pathway is sensitive to chemical disruption at multiple molecular targets along the signaling cascade, with exposure during critical windows in development leading to OFCs (Lipinski and Bushman 2010, Heyne, Melberg et al. 2015). The molecular targets of this disruption include SHH ligand modification with cholesterol and palmitoylate, ligand secretion, mesenchymal reception, and signal transduction (Jeong and McMahon 2002, Lauth, Bergström et al. 2007, Petrova, Rios-Esteves et al. 2013). This AOP focuses on the disruption to SHH signaling resulting in antagonism of the SMO receptor. To select the key events for the AOP, we used existing knowledge of the pathway along with reviews of the SHH pathway to assemble a path that was physiologically plausible. Care was taken to select events that would be of direct regulatory relevance (i.e. a method to quantify exists). To identify sources and data for each KER, Pubmed was used. Initially results were screened for relevance off title/abstract and any of suspected relevance were reviewed in full to determine their applicability for the KER. Each KER includes a table of relevant search information (date, search terms, citations, etc). It is the hope of the authors that this AOP is used as a tool for risk assessment for drug and chemical exposures during embryonic development when disruption to SHH through antagonism of SMO occurs.

Summary of the AOP

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Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a prototypical stressor and the biological system) of an AOP. More help
Key Events (KE)
A measurable event within a specific biological level of organisation. More help
Adverse Outcomes (AO)
An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help
Type Event ID Title Short name
MIE 2027 Antagonism, Smoothened receptor Antagonism Smoothened
KE 2044 Decrease, Smoothend relocation and activation Decrease, SMO relocation
KE 2028 Decrease, GLI1/2 translocation to nucleus Decrease, GLI1/2 translocation
KE 2040 Decrease, GLI1/2 target gene expression Decrease, GLI1/2 target gene expression
KE 1262 Apoptosis Apoptosis
KE 2043 Decrease, Sonic Hedgehog second messenger production Decrease, SHH second messenger production
KE 1821 Decrease, Cell proliferation Decrease, Cell proliferation
KE 2041 Decrease, palatal shelf outgrowth Decrease, outgrowth
AO 2042 Ororofacial clefting OFC

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes 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. More help

Network View

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Prototypical Stressors

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Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help
Life stage Evidence
Embryo High

Taxonomic Applicability

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Term Scientific Term Evidence Link
mouse Mus musculus NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help
Sex Evidence
Unspecific High

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

Essentiality of the Key Events

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. More help

Evidence Assessment

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Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help
Modulating Factor (MF) Influence or Outcome KER(s) involved

Quantitative Understanding

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Considerations for Potential Applications of the AOP (optional)

Addressess 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. More help


List of the literature that was cited for this AOP. More help