Editor's Highlight: Genetic Targets of Acute Toluene Inhalation in Drosophila melanogaster

Abstract

Interpretation and use of data from high-throughput assays for chemical toxicity require links between effects at molecular targets and adverse outcomes in whole animals. The well-characterized genome of Drosophila melanogaster provides a potential model system by which phenotypic responses to chemicals can be mapped to genes associated with those responses, which may in turn suggest adverse outcome pathways associated with those genes. To determine the utility of this approach, we used the Drosophila Genetics Reference Panel (DGRP), a collection of ∼200 homozygous lines of fruit flies whose genomes have been sequenced. We quantified toluene-induced suppression of motor activity in 123 lines of these flies during exposure to toluene, a volatile organic compound known to induce narcosis in mammals via its effects on neuronal ion channels. We then applied genome-wide association analyses on this effect of toluene using the DGRP web portal (http://dgrp2.gnets.ncsu.edu), which identified polymorphisms in candidate genes associated with the variation in response to toluene exposure. We tested ∼2 million variants and found 82 polymorphisms located in or near 66 candidate genes that were associated with phenotypic variation for sensitivity to toluene at P < 5 × 10-5, and human orthologs for 52 of these candidate Drosophila genes. None of these orthologs are known to be involved in canonical pathways for mammalian neuronal ion channels, including GABA, glutamate, dopamine, glycine, serotonin, and voltage sensitive calcium channels. Thus this analysis did not reveal a genetic signature consistent with processes previously shown to be involved in toluene-induced narcosis in mammals. The list of the human orthologs included Gene Ontology terms associated with signaling, nervous system development and embryonic morphogenesis; these orthologs may provide insight into potential new pathways that could mediate the narcotic effects of toluene.

Keywords: DGRP.; fruit fly; genome-wide association; motor activity; narcosis; volatile organic compound.

FIG. 1

Schematic description of data analysis process.

FIG. 2

Profiles of the time course of motor activity of 4 representative strains of flies from the DGRP. Activity counts per 10-min block are shown for flies in air (blue) and toluene (red) across the 120 ten-min blocks (20 h) of measurement. Toluene exposure began after a 60-min acclimation period and ended 4 h later. Activity reached a maximum after exposure in each strain and declined late in the observation period as the flies died. The N values above each panel represent the numbers of flies in the air and toluene groups respectively.

FIG. 3

Four effects of toluene distributed across the 123 lines of DGRP flies, ordered by the magnitude of the effect. The difference scores on the ordinate are displayed in rank order for each endpoint to show the proportions of flies that were more (values > 1) or less (values < 1) active in toluene than in air. Relative to activity in air, activity was suppressed by toluene in 104 of the 123 lines during exposure, and was increased after toluene exposure in 75 of the 123 lines. The rank-ordered sequences of the lines differed across endpoints.