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AOP: 301


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

Inhibition of Cystathionine Beta synthase leading to impaired the early development of anterior-posterior axis

Short name
A name that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
Homocysteine, anterior-posterior axis

Graphical Representation

A graphical representation of the AOP.This graphic should list all KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs. More help
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

Chang Seon Ryu1, Kichul Choe1, Baeckkyoung Sung1, Seungyun Baik1, Sang Kyum Kim2 and Young Jun Kim1

1Environmental Safety Group, Korea Institute of Science and Technology (KIST) Europe, Campus E 7.1, Saarbruecken, Germany

2College of Pharmacy, Chungnam National University, daehakro 99, daejeon, Republic of Korea

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
Young Jun Kim   (email point of contact)


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  • Young Jun Kim


This field is used to identify coaches who supported the development of the AOP.Each coach selected must be a registered author. More help


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 16:03

Revision dates for related pages

Page Revision Date/Time
Inhibition of cystathionine beta-synthase June 14, 2019 09:39
Increased Homocysteine level June 14, 2019 09:45
Increased, Plasma HCY level July 03, 2019 05:31
Induced, dysfunction of microcirculation July 03, 2019 05:34
Impaired,anterior-posterior axis development July 03, 2019 05:35
CBS inhibition leads to Homocysteine increases June 14, 2019 09:53
Homocysteine increases leads to Plasma HCY level July 03, 2019 05:37
Plasma HCY level leads to dysfunction of microcirculation July 03, 2019 05:37
dysfunction of microcirculation leads to anterior-posterior axis development July 03, 2019 05:37
Aminooxyacetic acid June 14, 2019 09:57
S-(4-Carboxybutyl)-D,L-homocysteine; 5-(3-Amino-3-carboxypropyl)sulfanyl-pentanoic acid June 14, 2019 09:57


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

This AOP describes an adverse outcome that results from the inhibition of homocysteine (Hcy) catabolism.  Hcy is a non-proteinogenic intermediary amino acid formed by the conversion of methionine to cysteine.  Hcy is metabolized via two major pathways that remethylated via two remethylation pathway methionine synthase (MS) and betaine homocysteine methyltransferase (BHMT) and transsulfuration pathway enzyme, cystathionine beta-synthase (CBS). Impairment of re-methylation inhibition and/or transsulfuration lead to increment systemic concentration as known as hyperhomocysteinemia.  CBS is responsible for 50% of Hcy clearance (Noga et al. 2003), together CBS is responsible for the generation of hydrogen sulfide (H2S) from cysteine (Carter and Morton, 2016).    Hyperhomocysteinemia leads to increment in Hcy to the neuronal system.  CBS gene was expressed in liver and kidney, skeletal, cardiac and nervous systems (Robert et al. 2003;  Namekata et al. 2004). Genetic deficiency of CBS in fish is critical for axis development (Prabhudesai et al. 2018), but there is not enough evidence in early neuronal development impairment in fish by chemical inhibition. Aminooxyacetic acid is a widely used CBS inhibitor that can reduce Hcy clearance and H2S formation in the brain, has not only inhibition of CBS but also cystathione gamma-lyase. So far no selective pharmacological SBS inhibitor is currently available (Asimakopoulou et al. 2013).


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

Several observations indicate the involvement of hyperhomocysteinemia in neurodegeneration. Hcy levels may play a role in neuronal death via stimulation of glutamate receptors. Homocysteine is an agonist for metabotropic glutamate receptors as well as for NMDA (N-methyl-D-aspartate, as a partial antagonist) and AMPA (amino-3-hydroxy-5-methyl-4-isoxazolepropionate)/Kainate ionotropic glutamate receptors (Lazarewicz et al. 2003; Poddar and Paul 2009). Hcy further mediated subsequent Ca2+ efflux to biphasic activation of p38  mitogen-activated protein kinase  (MAPK) (poddar and Paul 2013). Second possible mechanism of Hcy toxicity is free radical species production, and downregulation of antioxidant enzymes such as superoxide dismutase and peroxidase (Moat et al. 2000; Liu et al. 2013)  Hcy has also been reported to modulate the expression of pro-inflammatory molecules, C-reactive protein in vascular smooth muscle cells (Pang et al. 2014). Moreover, Hcy is able to inhibit neurogenesis in the hippocampus and subventricular zone of the murine adult brain (Wang et al. 2012; Rabaneda et al. 2008).    Hyperhomocysteinemia accelerates the dopaminergic cell death, probably due to the fact that hyperhomocysteinemia could cause a severe reduction in dopamine turnover in the striatum (De Lau et al. 2005).

Acknowledgements: This research was supported by the National Research Council of Science & Technology(NST) grant by the Korea government (MSIP) (No. CAP-17-01-KIST Europe)



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

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 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
KE 1661 Increased Homocysteine level Homocysteine increases
KE 1665 Increased, Plasma HCY level Plasma HCY level
KE 1666 Induced, dysfunction of microcirculation dysfunction of microcirculation
AO 1667 Impaired,anterior-posterior axis development anterior-posterior axis development

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

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (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

Prototypical Stressors

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help
Life stage Evidence
Birth to < 1 month High

Taxonomic Applicability

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Sex Applicability

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Sex Evidence
Mixed Moderate

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

This AOP is under development supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CAP-17-01-KIST Europe).

To do

Expected duration

Building the AOP frame

Development of KEs

3 month

Production of experimental data

18 month

Overall assessment of the AOP

Biological domain of applicability

3 month

Essentiality of all KEs

3 month

Evidence supporting all KERs

5 month

Quantitative WoE considerations

5 month

Quantitative understanding for each KER

6 month

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

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

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

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors.  More help

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

This AOP is designed to detect changes of Hcy concentration in early developmental condition including fish larvae by chemical inhibition of CBS.  Accumulation of neuronal damage in fish is the should be measured together in in vivo experiment for clarifing the adverse outcome. In addition, screening of selective potential chemical inhibitor of CBS will be performed in in vitro cell-based assay in different species.


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

Abbott MH, Folstein SE, Abbey H, Pyeritz RE. (1987) Psychiatric manifestations of homocystinuria due to cystathionine beta-synthase deficiency: prevalence, natural history, and relationship to neurologic impairment and vitamin B6-responsiveness. Am J Med Genet. 1987 Apr; 26(4):959-69.

SJ Lee, SH Park, JF Chung, WR Choi, and HK Huh(2017) Homocysteine-induced peripheral microcirculation dysfunction in zebrafish and its attenuation by L-arginine. Oncotarget. 2017 Aug 29; 8(35): 58264–58271.

Noga AA, Stead LM, Zhao Y, Brosnan ME, Brosnan JT, Vance DE (2003) Plasma homocysteine is regulated by phospholipid methylation. J. Biol. Chem. 278, 5952–5955.

Carter RN, Morton NM.  (2016) Cysteine and hydrogen sulphide in the regulation of metabolism: insights from genetics and pharmacology  J Pathol. 238(2):321-32.

Lazarewicz JW1, Ziembowicz A, Matyja E, Stafiej A, Zieminska E (2003) Homocysteine-evoked 45Ca release in the rabbit hippocampus is mediated by both NMDA and group I metabotropic glutamate receptors: in vivo microdialysis study.  Neurochem Res. 28(2):259-69.

Poddar R, Paul S. (2009) Homocysteine-NMDA receptor-mediated activation of extracellular signal-regulated kinase leads to neuronal cell death. J Neurochem. 110(3):1095-106

Pang X, Liu J, Zhao J, Mao J, Zhang X, Feng L (2014) Homocysteine induces the expression of C-reactive protein via NMDAr-ROS-MAPK-NF-KB signal pathway in rat vascular smooth muscle cells. Atherosclerosis. 236:73–81

De Lau LM, Koudstaal PJ, van Meurs JB, Uitterlinden AG, Hofman A, Breteler MM (2005) Methylenterahydrofolate reductase C677T genotype and PD. Annu. Neurol. 57:927–930.

Liu HH, Shih TS, Huang HR, Huang SC, Lee LH, Huang YC. (2013) Plasma homocysteine is associated with increased oxidative stress and antioxidant enzyme activity in welders. ScientificWorldJournal. 370487.

Moat SJ, Bonham JR, Cragg RA, Powers HJ.(2000)  Elevated plasma homocysteine elicits an increase in antioxidant enzyme activity.  Free Radic Res. 2000 Feb;32(2):171-9.

Wang J, Bai X, Chen Y, Zhao Y, Liu X. (2012) Homocysteine induces apoptosis of rat hippocampal neurons by inhibiting 14-3-3ε expression and activating calcineurin. PLoS One. 7(11):e48247.

Rabaneda LG1, Carrasco M, López-Toledano MA, Murillo-Carretero M, Ruiz FA, Estrada C, Castro C. (2008) Homocysteine inhibits proliferation of neuronal precursors in the mouse adult brain by impairing the basic fibroblast growth factor signaling cascade and reducing extracellular regulated kinase 1/2-dependent cyclin E expression. FASEB J. 22(11):3823-35.

Prabhudesai et al. (2018) Cystathionine β-Synthase Is Necessary for Axis Development in Vivo. Front Cell Dev Biol. 2018 Feb 16;6:14

Asimakopoulou A, Panopoulos P, Chasapis CT, Coletta C, Zhou Z, Cirino G, Giannis A, Szabo C, Spyroulias GA, Papapetropoulos A. (2013) Selectivity of commonly used pharmacological inhibitors for cystathionine β synthase (CBS) and cystathionine γ lyase (CSE).  Br J Pharmacol. 169(4):922-32.

J Histochem Cytochem. 2003 Mar;51(3):363-71. Expression of the cystathionine beta synthase (CBS) gene during mouse development and immunolocalization in adult brain. Robert K1, Vialard F, Thiery E, Toyama K, Sinet PM, Janel N, London J.

J Biol Chem. 2004 Dec 17;279(51):52961-9. Epub 2004 Oct 4. Abnormal lipid metabolism in cystathionine beta-synthase-deficient mice, an animal model for hyperhomocysteinemia. Namekata K1, Enokido Y, Ishii I, Nagai Y, Harada T, Kimura H.