Evidence for Cross Species Extrapolation of Mammalian-Based High-Throughput Screening Assay Results

Abstract

High-throughput screening (HTS) and computational technologies have emerged as important tools for chemical hazard identification. The US Environmental Protection Agency (EPA) launched the Toxicity ForeCaster (ToxCast) Program, which has screened thousands of chemicals in hundreds of mammalian-based HTS assays for biological activity. The data are being used to prioritize toxicity testing on those chemicals likely to lead to adverse effects. To use HTS assays in predicting hazard to both humans and wildlife, it is necessary to understand how broadly these data may be extrapolated across species. The US EPA Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS; https://seqapass.epa.gov/seqapass/ ) tool was used to assess conservation of the 484 protein targets represented in the suite of ToxCast assays and other HTS assays. To demonstrate the utility of the SeqAPASS data for guiding extrapolation, case studies were developed which focused on targets of interest to the US Endocrine Disruptor Screening Program and the Organisation for Economic Cooperation and Development. These case studies provide a line of evidence for conservation of endocrine targets across vertebrate species, with few exceptions, and demonstrate the utility of SeqAPASS for defining the taxonomic domain of applicability for HTS results and identifying organisms for suitable follow-up toxicity tests.

Figure 1.

Boxplots depicting SeqAPASS (v3.0) data illustrating the percent similarity across species compared to human (Homo sapiens) androgen receptor (AR), examining the primary amino acid sequences (a) and the ligand binding domain (b). In each plot, the o represents the human AR and • represents the species with the highest percent similarity within the specified taxonomic group. The top and bottom of each box represent the 75^th and 25^th percentiles, respectively. The top and bottom whiskers extend up to 1.5 times the interquartile range. The mean and median values for each taxonomic group are represented by horizontal thick and thin black lines on the box, respectively. The dashed line indicates the cut-off for predictions of intrinsic susceptibility. Species from taxonomic groups that cross the susceptibility cut-off or those found above the cut-off are predicted as likely to be susceptible to chemicals that act on the query species protein target. The protein target is therefore conserved in species predicted to be susceptible.

Figure 1.

Boxplots depicting SeqAPASS (v3.0) data illustrating the percent similarity across species compared to human (Homo sapiens) androgen receptor (AR), examining the primary amino acid sequences (a) and the ligand binding domain (b). In each plot, the o represents the human AR and • represents the species with the highest percent similarity within the specified taxonomic group. The top and bottom of each box represent the 75^th and 25^th percentiles, respectively. The top and bottom whiskers extend up to 1.5 times the interquartile range. The mean and median values for each taxonomic group are represented by horizontal thick and thin black lines on the box, respectively. The dashed line indicates the cut-off for predictions of intrinsic susceptibility. Species from taxonomic groups that cross the susceptibility cut-off or those found above the cut-off are predicted as likely to be susceptible to chemicals that act on the query species protein target. The protein target is therefore conserved in species predicted to be susceptible.