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Relationship: 2816

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Modified Proteins leads to Cataracts

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Modified Proteins

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Cataracts

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

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Deposition of energy leading to occurrence of cataracts	adjacent	Moderate	Low	Vinita Chauhan (send email)	Open for citation & comment	Under Review

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) 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. More help

Term	Scientific Term	Evidence	Link
human	Homo sapiens	Moderate	NCBI
mouse	Mus musculus	High	NCBI
rat	Rattus norvegicus	Moderate	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Female	Moderate
Male	Moderate

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
All life stages	High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

The maintenance of the correct structure and location of lens proteins is crucial for the proper refraction of light in the eye. Any modifications to the proteins of the lens can result in a reduction in lens transparency and cataract formation through the mechanism of protein aggregation (Zhao et al., 2015). Cataracts are a progressive condition in which the lens of the eye develops opacities and becomes cloudy, resulting in blurred vision as well as glare and haloes around lights (National Eye Institute, 2022). For this AOP, a cataract is defined when over 5% of the lens is opacified. Under normal conditions, lens proteins work to support the eye through chaperones, gap junctional, and structural functions (Ghosh & Chauhan, 2019; NCRP, 2016). Light enters the eye and passes through the crystallin proteins of the lens, which are responsible for 90% of the proteins in a mature lens. These proteins are carefully arranged as to limit their interference with the light, and the lens cells remove their organelles once they are mature to reduces light-scattering (Moreau & King, 2012; Toyama & Hetzer, 2013). Proteins play other roles in the creation of a transparent medium. Beta- and γ-crystallins are structural proteins that ensure the proper inter-protein interactions occur for the maintenance of nuclear transparency, and alpha crystallin proteins chaperone other proteins, including beta- and γ-crystallins, around the lens (Ghosh & Chauhan, 2019; Toyama & Hetzer, 2013). Lens epithelial cells (LEC) rely on proteins, such as connexin43, to act as phenotypic markers to help organize the cells within the lens following proliferation, preventing the cells from improperly layering within the eye. LECs are packed with crystallin proteins. If the connexin43 proteins are altered, that would impair their ability to help organize the LECs properly, resulting in all the proteins found within those LECs to be disoriented compared to the proteins of neighbouring cells (Berthoud et al., 2014). This improper layering of the cells leads to modified transparency in the lens as a result of the disorganization of the many crystallin proteins within the LEC. Connexin proteins typically join chaperone proteins in a complex and repair misfolded proteins (NCRP, 2016). Proteins can be modified from exposure to stressors, and depending on the type of protein, the alteration will also differ. Following modification, proteins will be unable to correctly perform their roles within the lens, such as preventing aggregation via proper chaperone and structural actions. (Ghosh & Chauhan, 2019; Toyama & Hetzer, 2013; NCRP, 2016). Protein aggregation occurs, which is worsened by the inability of the proteins to form complexes to repair themselves, and this leads to reduced lens transparency and increased cataract incidence.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

The strategy for collating the evidence to support the relationship is described in Kozbenko et al 2022. Briefly, a scoping review methodology was used to prioritize studies based on a population, exposure, outcome, endpoint statement.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Overall Weight of Evidence: Moderate

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field 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. More help

There is strong biological plausibility to support the link between modified proteins and cataracts. A review focusing on modified proteins and cataracts is particularly relevant as it discusses different types of protein modifications, and the resulting effect on increased human lens opacity (Truscott, 2005). Several other studies discuss multiple types of protein alterations that can cause increased cataracts/lens opacity, often attributed to improper protein function (Hamada et al., 2014; NCRP, 2016; Ghosh & Chauhan, 2019). Currently, the majority of the empirical evidence to support this relationship is derived from studies conducted in adult male subjects, therefore there is limited opportunity to comment on sex or age effects on this relationship (Fujii et al., 1986; Menard et al., 1986). There is also limited information for taxonomic comparisons of rats, humans, and mice related to modified proteins leading to cataracts, with only one paper listed for each species. However, it is evident that all three species have evidence to support the causal connectivity of this relationship (Menard et al., 1986; Truscott, 2005; Fujii et al., 1986).

Once lens fiber cells are damaged, and the intracellular proteins are modified, these modifications are permanent, as the cells lack the organelles needed to undergo protein turnover (Toyama & Hetzer, 2013). These protein modifications can in turn cause protein aggregation, which are high molecular weight proteins, and these can modify the multi-layering of cells (Bron et al., 2000; Moreau & King, 2012; NCRP, 2016). Among the different types of amino acids, tryptophan, histidine, and cysteine are all at risk for modifications from oxidative processes (Balasubramanian, 2000). Oxidized proteins can have modified water-solubility (Hamada et al., 2014; Moreau & King, 2012). The protein content in the lens of the eye needs to be optimal to ensure that the lens transparency can appropriately contribute to the refractive medium of the lens, meaning that the water solubility of proteins is crucial and can dictate the development of lens opacities. Proteins can become water-insoluble when they undergo post-translational modifications, shifting the solubility fraction of the lens proteins (Hamada et al., 2014). They are unable to aggregate while in their natural water-soluble state and must first undergo modifications to decrease solubility and generate a build-up of proteins (Moreau & King, 2012). Alpha-crystalline proteins, when modified, are unable to chaperone other proteins to the correct locations, aggravating the protein aggregation in the eye (Blakely et al., 2010; Uwineza et al., 2019). The binding of these chaperone proteins to the molten globule region of other proteins leads to the formation of high molecular weight proteins, a common protein type seen in cataract patients (NCRP, 2016; Truscott, 2005; Moreau & King, 2012). High-molecular-weight crystalline aggregation causes light to scatter at a higher rate than normal, increasing lens opacity (Uwineza et al., 2019; Bron et al., 2000; Toyama & Hetzer, 2013). Furthermore, when connexin proteins, which form intercellular channels between LECs and lens fiber cells (Tjahjono et al., 2020), are unable to perform their function, the LECs will improperly layer. Modifications to these proteins has been linked to human cataract development (NCRP, 2016).

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

This relationship is poorly supported. However, there is some empirical evidence from studies using stressors such as γ- and X-rays that cause protein modifications resulting in lens opacification and cataract development. These studies are derived from in vivo mouse and rat models using whole lenses (Fujii et al., 1986; Menard et al., 1986).

Incidence Concordance

There is low evidence to support an incidence concordance relationship between modified proteins and cataract development. Following the exposure of the lenses in vivo to 15.3 Gy γ-rays, the level of soluble proteins dropped to 0.07x control levels and were associated with observed opacities being larger than those in control lenses (Menard et al., 1986). Ample evidence has shown that protein modifications, particularly phosphorylation, may be associated with cataracts. These studies used human and animal models with pre-existing cataracts, and showed the presence of phosphorylated crystallin, MDM2 and tyrosine proteins (Wang et al. 2020; Hui-Ju et al. 2013; Chandrasekher et al. 2004).

Time Concordance

There is low evidence in the literature to support time concordance between modified proteins and increased lens opacity/cataract development. High dose (>2 Gy) in vivo studies have shown that cataracts first appeared 6 ½ weeks post-modification. Modified D/L amino acid conformation ratio of lens proteins was observed in vivo in whole lenses as early as 11 days post 15 Gy X-irradiation, while lens opacities were shown to occur as early as eight weeks post-irradiation. (Fujii et al., 1986).

Essentiality

Modified proteins been found to increase cataracts above background levels. Therefore, although radiation is not essential for the development of cataracts, it is essential for promoting it above this normal level (Menard et al., 1986). There is low evidence in the literature in the form of knock-out and knock-in studies to support the essentiality of protein aggregation in the development of opacities. The return of the lens protein solubility ratio to near control levels resulted in the opacity level of the lens more closely resembling the control lens than the unshielded treatment lens, following in vivo 15.3 Gy γ-irradiation on whole lenses (Menard et al., 1986).

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

N/A

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Modulating Factor (MF)	MF Specification	Effect(s) on the KER	Reference(s)
Age	≥ 40 years old (human) has higher incidence of lens opacity	Proteins naturally change and degrade over time however they do not get removed from within the lens’ center. This leads to a higher level of modified protein accumulation within the lens in older individuals. Protein accumulation/aggregation is linked to light scattering and cataracts.	Hains & Truscott, 2010; NCRP, 2016
5-cholesten-3b,25-diol (VP1-001)	Administration of compound	VP1-001 reversed α-crystallin aggregation in vivo, resulting in decreased lens opacity.	Molnar et al., 2019; Wang et al., 2022

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

The following tables provide representative examples of the relationship, unless otherwise indicated, all data is statistically significant.

Incidence Concordance

Reference	Experiment Description	Result
Menard et al., 1986	In vivo, rats received head-only exposure to 15.3 Gy γ-rays, proteins detected with Lowry assay and size-exclusion liquid chromatography, lens opacity assessed by slit-lamp eye examinations.	In rats exposed in vivo to 15.3 Gy γ-rays, the water-soluble protein make-up in the lens decreased 13.6x (indicating increased levels of modified proteins) and dense cataracts were observed, while controls developed minimal opacification.

Time Concordance

Reference	Experiment Description	Result
Fujii et al., 1986	In vivo, mice received whole-body exposure to 15 Gy X-rays D/L ratio of proteins was determined with gas-liquid chromatography and cataracts determined by the observation of lens opacification.	In mice exposed in vivo to 15 Gy X-rays the ratio of D/L conformation lens proteins increased 1.5x 60 days post-irradiation. Lens opacity increased at the same point in time.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

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?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

N/A

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

This KER is plausible in all life stages, sexes, and organisms that have a clear lens for vision. The majority of the evidence is from in vivo studies (adult mice, and rats) and human cohorts. No in vitro evidence was found to support the relationship.

References

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

Balasubramanian, D. (2000), “Ultraviolet Radiation and Cataract”, Journal of Ocular Pharmacology and Therapeutics, Vol.16/3, Mary Ann Liebert Inc, Larchmont, https://doi.org/10.1089/JOP.2000.16.285.

Berthoud, V. et al. (2014), “Roles and regulation of lens epithelial cell connexins”, FEBS Letters, Vol. 588/8, Elsevier, England, https://doi.org/10.1016/j.febslet.2013.12.024

Blakely, E. et al. (2010), “Radiation Cataractogenesis: Epidemiology and Biology”, Radiation Research, Vol.173/5, Radiation Research Society, Indianapolis, https://doi.org/10.1667/RRXX19.1.

Bron, A. et al. (2000), “The Ageing Lens”, Ophthalmologica, Vol.214, Karger Publishers, Basel, https://doi.org/10.1159/000027475.

Chandrasekher, G., and& Sailaja, D. (2004),. “Alterations in lens protein tyrosine phosphorylation and phosphatidylinositol 3-kinase signaling during selenite cataract formation”,., Current eye research, Vol.28(2),. https://doi.org/10.1076/ceyr.28.2.135.26232

Fujii, N. et al. (1986), “D-Amino Acid in Irradiated and Aged Mouse”, Journal of Radiation Research, Vol.27, Oxford University Press, Oxford, https://doi.org/10.1269/JRR.27.183.

Ghosh, K. and P. Chauhan. (2019), “Crystallins and Their Complexes” in Macromolecular Protein Complexes II: Structure and Function, Subcellular Biochemistry, Vol.93, Springer Nature Switzerland AG, Handel, https://doi.org/10.1007/978-3-030-28151-9.

Hamada, N. et al. (2014), “Emerging issues in radiogenic cataracts and cardiovascular disease”, Journal of Radiation Research, Vol.55/5, Oxford University Press, Oxford, https://doi.org/10.1093/jrr/rru036.

Hains, B. and R. Truscott. (2010), “Age-dependent deamidation of lifelong proteins in the human lens”, Investigative Ophthalmology and Visual Science, Vol.51/6, Association for Research in Vision and Ophthalmology, Rockville, https://doi.org/10.1167/iovs.09-4308.

Kozbenko, T. et al. (2022), “Deploying elements of scoping review methods for adverse outcome pathway development: a space travel case example”, International Journal of Radiation Biology, 1–12. https://doi.org/10.1080/09553002.2022.2110306

Lin, H., et al. (2013), “An Increase in Phosphorylation and Truncation of Crystallin With the Progression of Cataracts”, Current Therapeutic Research, Vol. 74, https://doi.org/10.1016/j.curtheres.2012.10.003.

Menard, T. et al. (1986), “Radioprotection against cataract formation by WR-77913 in gamma-irradiated rats”, International Journal of Radiation Oncology Biology Physics, Vol.12, Elsevier, Amsterdam, https://doi.org/10.1016/0360-3016(86)90199-9.

Molnar, K. S. et al. (2019), "Mechanism of action of VP1-001 in cryAB(R120G)-associated and age-related cataracts”, Investigative Ophthalmology & Visual Science, Vol. 60/10, The Association for Research in Vision and Ophthalmology, United States, https://doi.org/ 10.1167/iovs.18-25647

Moreau, K. and J. King. (2012), “Protein misfolding and aggregation in cataract disease and prospects for prevention”, Trends in Molecular Medicine, Vol.18/5, Elsevier Ltd, London, https://doi.org/10.1016/j.molmed.2012.03.005.

National Eye Institute (2022), Cataracts, https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/cataracts (accessed November 29, 2022).

NCRP (2016), “Guidance on radiation dose limits for the lens of the eye”, NCRP Commentary, Vol.26, National Council on Radiation Protection and Measurements Publications, Bethesda.

Tjahjono, N. et al. (2020), “Connexin 50-R205G mutation perturbs lens epithelial cell proliferation and differentiation”, Investigative Ophthalmology & Visual Science, Vol. 61/3, The Association for Research in Vision and Ophthalmology, United States, https://doi.org/10.1167/iovs.61.3.25

Toyama, B. and M. Hetzer. (2013), “Protein homeostasis: Live long, won't prosper”, Nature Reviews Molecular Cell Biology, Vol.14/1, Nature Portfolio, Berlin, https://doi.org/10.1038/nrm3496.

Truscott, R. (2005), “Age-related nuclear cataract - Oxidation is the key”, Experimental Eye Research, Vol.80/5, Academic Press Inc, Cambridge, https://doi.org/10.1016/J.EXER.2004.12.007.

Uwineza, A. et al. (2019), “Cataractogenic load – A concept to study the contribution of ionizing radiation to accelerated aging in the eye lens”, Mutation Research-Reviews in Mutation Research, Vol.779, Elsevier, Amsterdam, https://doi.org/10.1016/J.MRREV.2019.02.004.

Wang, K. et al. (2022), “Oxysterol compounds in mouse mutant αA and αB-crystallin lenses ca improve the optical properties of the lens”, Investigative Ophthalmology & Visual Science, Vol. 63/5, The Association for Research in Vision and Ophthalmology, United States, https://doi.org/10.1167/iovs.63.5.15

Zhao, L. et al. (2015), “Lanosterol reverses protein aggregation in cataracts”, Nature, Vol.523, Nature Portfolio, Berlin, https://doi.org/10.1038/nature14650.