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Differentiating pathway-specific from nonspecific effects in high-throughput toxicity data: A foundation for prioritizing adverse outcome pathway development
Citation:
Fay, K., Dan Villeneuve, J. Swintek, S. Edwards, M. Nelms, B. Blackwell, AND G. Ankley. Differentiating pathway-specific from nonspecific effects in high-throughput toxicity data: A foundation for prioritizing adverse outcome pathway development. TOXICOLOGICAL SCIENCES. Society of Toxicology, RESTON, VA, 163(2):500-515, (2018).
Impact/Purpose:
One of the EPA’s priorities in toxicity testing is to advance the field away from whole animal exposure studies to high-throughput, primarily in vitro, chemical screening. To this purpose, the US EPA’s ToxCast program was established (Dix et al. 2007, Judson et al. 2010, Kavlock et al. 2012). This program has screened over 9000 chemicals for hundreds of biological effects, generating a tremendous amount of data. For the data to be useful for hazard assessment, the in vitro effects must be related to outcomes at the organism or population levels. Adverse Outcome Pathways (AOPs) describe a sequential chain of causally-linked events at different levels of biological organization, ultimately resulting in adverse health or an ecotoxicological effect. As such, AOPs offer a mechanistic means to connect biological effects measured in vitro to outcomes relevant for chemical risk assessment. The AOP discovery and development task (CSS 12.01) recognizes the need to develop AOPs for ToxCast assay targets. This work utilizes the statistical evaluation of ToxCast data conducted by Judson et al. (2016), to prioritize those assays providing pathway-specific effects data for AOP development. Judson et al. statistically defined concentration thresholds for ToxCast chemicals above which they elicit general cytotoxic or effects. This meta-analysis identifies the ToxCast assays frequently responding to chemicals at concentrations well below their ?‘cytotoxic burst’ concentrations, ostensibly providing more pathway-specific effects information. This analysis offers a means to prioritize ToxCast biological targets for AOP development, which builds on FY16 products and supports the FY18 product: application of AOPs to complex mixture assessments.
Description:
The U.S. Environmental Protection Agency’s ToxCast program has screened thousands of chemicals for biological activity, primarily using high-throughput in vitro bioassays. Adverse outcome pathways (AOPs) offer a means to link pathway-specific biological activities with potential apical effects relevant to risk assessors. Thus, efforts are underway to develop AOPs relevant to pathway-specific perturbations detected in ToxCast assays. Previous work identified a ?‘cytotoxic burst’ (CTB) phenomenon wherein large numbers of the ToxCast assays begin to respond at or near test chemical concentrations that elicit cytotoxicity, and a statistical approach to defining the bounds of the CTB was developed. To focus AOP development on the molecular targets corresponding to ToxCast assays indicating pathway-specific effects, we conducted a meta-analysis to identify which assays most frequently respond at concentrations below the CTB. A preliminary list of potentially important, target-specific assays was determined by ranking assays by the fraction of chemical hits below the CTB compared to the number of chemicals tested. Additional priority assays were identified using a diagnostic-odds-ratio approach which gives greater ranking to assays with high specificity but low responsivity. Combined, the two prioritization methods identified several novel targets (e.g., peripheral benzodiazepine and progesterone receptors) to prioritize for AOP development, and affirmed the importance of a number of existing AOPs aligned with ToxCast targets (e.g., thyroperoxidase, estrogen receptor, aromatase). The prioritization approaches did not appear to be influenced by inter-assay differences in chemical bioavailability and the outcomes were robust based on a variety of different parameters used to define the CTB.
