Identification of Toxcast Intended Gene Targets Relevant to Neurotoxicity/Developmental Neurotoxicity and Putative Mechanisms of Neurotoxicity.
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Background and Purpose: Our goal was to evaluate ToxCastTM unique asay gene targets for biological relevancy to neurotoxicity/developmental neurotoxicity, and to compare the results to putative mechanisms of action of previously identified neurotoxicants.
Methods: A panel of EPA neurotoxicologists examined the function of the non-redundant assay gene targets included in ToxCast assays (Invitro db version 3.5, 2022). A relevance category scheme was created and was applied to the ToxCast intended gene targets. The categories indicated a gene target’s role in the nervous system as follows: 1 = Directly relevant to neural function; 2 = Relevant to neurodevelopmental processes; 3 = General cellular process found in all cells; 3A = General cellular process that haves a function in neurodevelopment; 4 = Not directly related to neural function.
A list of 548 identified neurotoxicants was curated from: Schaumburg and Spencer’s Experimental and Clinical Neurotoxicology (2000), and Casarett and Doull’s Toxicology: The Basic Science of Poisons (2008). Molecular initiating event (MIE) or key events (KE) for the chemicals were identified from a manual literature curation of databases (PubChem, PubMed, CompTox Chemicals Dashboard, and Google Scholar).
Each chemical’s MIEs or KEs were then cross-referenced with the intended assay gene targets, to identify congruence or indicate gene targets not covered by the ToxCast assay suite.
Results: Eighty unique gene assay targets (16.6%) had direct relevance to neural function (Category 1). Sixteen gene targets (3.3%) were relevant to neurodevelopmental processes (Category 2). The largest subset of assay gene targets (303; 63.0%) represented general cellular processes important to all cells. (Category 3). However, 97 gene assay targets (20.1% of all gene assay targets; 32% of category 3 gene assay targets) were critical for proper neurodevelopment (Category 3A). In total (category 1 +2 +3A), the neural-relevant gene assay targets represent about 40% of all ToxCast assay gene targets. Finally, 82 intended gene assay targets (17.0 %) were characterized as not being relevant for neural function (Category 4).
A literature review for 508 chemicals identified MIEs for 375 compounds and KEs for 173 compounds. Some MIEs were not related to an assay gene target. Mechanisms of neurotoxicity such as protein adducts are not covered by existing high-throughput assays. The largest class of KEs for the chemicals involved oxidative stress. Other KEs targeted DNA or RNA pathways and inhibition of protein synthesis. The highest agreement between neurotoxicant MIEs and intended gene targets was in the direct neural relevance category (375 of 548 neurotoxicants). Assay gene targets related to neural development were indicated for 3 of 548 compounds. General cellular process assay gene targets were indicated for 235 of 548 chemicals. Only 4 gene assay targets classified as not directly relevant to neurotoxicity were involved in neurotoxicant MIEs (0.18%). Numerous MIEs/KEs associated with the neurotoxicant list are not included in the current ToxCast assay’s targets.
Conclusions: We demonstrated that the ToxCast assay has a large percentage (~40%) of assay gene targets that are relevant for the neurotoxic chemicals we examined. However, coverage is limited for some potential MIEs/KEs due to the cellular and biological diversity of the nervous system. Increased coverage for additional receptor subtypes, ion channels and mechanisms of oxidative stress are possible areas for future development. This abstract does not reflect US EPA policy.