Characterization of the Fundulus heteroclitus embryo transcriptional response and development of a gene expression-based fingerprint of exposure for the alternative flame retardant, TBPH (bis (2-ethylhexyl)-tetrabromophthalate)

Abstract

Although alternative Flame Retardant (FR) chemicals are expected to be safer than the legacy FRs they replace, their risks to human health and the environment are often poorly characterized. This study used a small volume, fish embryo system to reveal potential mechanisms of action and diagnostic exposure patterns for TBPH (bis (2-ethylhexyl)-tetrabromophthalate), a component of several widely-used commercial products. Two different concentration of TBPH were applied to sensitive early life stages of an ecologically important test species, Fundulus heteroclitus (Atlantic killifish), with a well-annotated genome. Exposed fish embryos were sampled for transcriptomics or chemical analysis of parent compound and primary metabolite or observed for development and survival through larval stage. Global transcript profiling using RNA-seq was conducted (n = 16 per treatment) to provide a non-targeted and statistically robust approach to characterize TBPH gene expression patterns. Transcriptomic analysis revealed a dose-response in the expression of genes associated with a surprisingly limited number of biological pathways, but included the aryl hydrocarbon receptor signal transduction pathway, which is known to respond to several toxicologically-important chemical classes. A transcriptional fingerprint using Random Forests was developed that was able to perfectly discriminate exposed vs. non-exposed individuals in test sets. These results suggest that TBPH has a relatively low potential for developmental toxicity (at least in fishes), despite concerns related to its structural similarities to endocrine disrupting chemicals and that the early life stage Fundulus system may provide a convenient test system for exposure characterization. More broadly, this study advances the usefulness of a biological testing and analysis system utilizing non-targeted transcriptomics profiling and early developmental endpoints that complements current screening methods to characterize chemicals of ecological and human health concern.

Keywords: AHR; Killifish; RNA-seq; TBPH; Transcriptomics.

Fig. 1.

Dose-dependent expression of the top 6 significant transcripts among three groups. The six bar-plots show the mean expression level of the top 6 significant transcripts, one for each transcript as indicated by transcript ID at top-left of each plot. In each bar-plot, x-axis shows experimental groups and y-axis is mean expression level in RPKM (Reads Per Kilobase of transcript per Million mapped reads). The height of each box indicates estimated mean expression level and the corresponding error bar shows the 95% confidence interval of the mean estimate (mean ± standard error (S.E)). The mean expression of these six transcripts were statistically significant in both control-TBPH_LOW and control-TBPH_HIGH comparisons at the 5%.FDR cutoff.

Fig. 2.

Gene expression level heat map of top 13 differentially expressed transcripts. in the heatmap plot, each column stands for an experimental sample, and each row stands for a transcript as annotated on the right size The left pane, consisting of the first 16 samples, is of the control group, the middle pane of the next 16 samples is of TBPH_LOW group, and the right pane of the remaining 16 samples is of TBPH_HIGH group. In the row label, ctl stands for the control group, while LH is for the combined TBPH_LOW and TBPH_HIGH group. The head map colors indicate relative expression level with green representing the lowest expression, and red for the highest expression level. The plot clearly shows dose-dependent response of these genes with increasing expression level from the control, TBPH_LOW, and TBPH_HIGH.