Complementary square wave voltammetry and tandem mass spectrometry analysis to identify and detect compensatory genomic changes in nematodes due to nickel (II) exposure

Nickel is a toxic heavy metal that may cause negative health outcomes including cancer upon exposure [1]. Square wave voltammetry was used to assay DNA directly extracted from nickel-exposed nematodes that had originated from either high or low Ni-containing environments in order to assess the role of evolutionary genetics in the Ni toxicity process. Extracted DNA was immobilized on pyrolytic graphite (PG) electrodes following layer by layer (LbL) protocols and then electrochemically oxidized in the presence of Ru(bpy)³⁺ to generate electrocatalytic oxidative currents at ∼+1.05 V vs. SCE. Both C. elegans and P. pacificus nematodes were utilized, each with a volcanic or cosmopolitan soil source strain. DNA oxidative peak currents (I_p) increased for all nematode strains upon exposure to 50 μg/L Ni²⁺, but those originating from volcanic soils exhibited significantly (40–50%) lower I_p upon Ni²⁺ exposure compared to similarly exposed nematodes from cosmopolitan soils. Further SWV analysis was performed on DNA from a series of C. elegans N2xCB4856 recombinant inbred advanced intercross line populations (RIALs). A continuum of I_p magnitude was seen as a function of Ni²⁺ exposure among the RIAL strains indicating Ni-tolerance is complex and affected by multiple loci. The majority of the DNA from Ni-exposed individual RIAL strain cultures produced an increase in oxidative current in comparison to DNA from analogous Ni-unexposed cultures of the same RIAL strains. Liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis on acid hydrolyzed nematode DNA provided validation of the electrochemical findings. Guanine and oxidatively damaged guanine content were monitored via appropriate m/z mass transitions. Guanine content in all Ni-exposed RIAL DNA was lower, while oxidatively damaged guanine was elevated compared to unexposed nematodes in all but one analyzed RIAL. Combined, the complementary electrochemical and MS/MS data provide evidence that evolutionary genetics leads to genetic protection from environmental toxicants, suggesting a possibility that multiple genes are involved in the protection of the organism from Ni exposure.