Short-term transcriptomic points-of-departure are consistent with chronic points-of departure for three organophosphate pesticides across mouse and fathead minnow.
New approach methods (NAMs) can reduce the need for chronic, animal-based toxicity and carcinogenicity bioassays. Here, we apply benchmark dose (BMD) response modeling (BMDExpress 2.3) to transcriptomic data after short-term exposures to three organophosphate pesticides (OPPs) to estimate chronic adverse effect levels in mouse. Specifically, mouse-liver were collected for RNA sequencing after 7-days of exposure, respectively, to several dose-levels of well-known OPPs: fenthion, methidathion and parathion. Transcriptional points-of-departure (TPODs) were compared to chronic, apical points-of-departures (APOD) for each OPP to inform more efficient means of estimating chemical potency and risk. The mouse-liver TPODs for fenthion, methidathion, and parathion were 0.009, 0.093, and 0.046 mg/Kg-day, respectively. These short-term TPODs reflected the relative potencies of the most sensitive, apical effects from the chronic studies with fenthion identified as the most potent OPP. Moreover, the TPODs were 2.3 to 17-fold more sensitive than the chronic APODs for mouse, suggesting utility in using short-term gene response for estimating long-term chemical risk. Further investigation into OPPs' modes of action identified significant impacts on acetylcholinesterase mRNA abundance (FDR p-value <0.05, |FC|>2) as well as enrichment of canonical pathways (IPA, z-score>|2|) associated with organism death and neurological and immune dysfunctions, which have been known to be affected by OPPs. Despite bioassay-related differences, our results indicate the conservation of key events potentially important to OPPs biological activity and potency across species. While additional research is needed, these results build confidence in using short-term, molecular-based assays for the characterization of toxicity and risk, thereby reducing reliance on chronic, rodent-based studies. Overall design: B6C3F1 male mouse livers (mix of left lateral, caudate, and right medial lobes) were collected for RNA sequencing after 7-days of exposure to several dose-levels of well-known organophosphate pesticides: fenthion at 2.5, 5, 10, 20 ppm; methidathion at 10, 25, 50, 100 ppm; and parathion at at 20, 40, 80 ppm (n=7/dose/OPP). OPPs were incorporated into AIN-93G rodent diet (or not for controls) and provided ad libitum with consumption monitored.