Natural variation in the sequestosome-related gene, sqst-5, underlies zinc homeostasis in Caenorhabditis elegans

Abstract

Zinc is an essential trace element that acts as a co-factor for many enzymes and transcription factors required for cellular growth and development. Altering intracellular zinc levels can produce dramatic effects ranging from cell proliferation to cell death. To avoid such fates, cells have evolved mechanisms to handle both an excess and a deficiency of zinc. Zinc homeostasis is largely maintained via zinc transporters, permeable channels, and other zinc-binding proteins. Variation in these proteins might affect their ability to interact with zinc, leading to either increased sensitivity or resistance to natural zinc fluctuations in the environment. We can leverage the power of the roundworm nematode Caenorhabditis elegans as a tractable metazoan model for quantitative genetics to identify genes that could underlie variation in responses to zinc. We found that the laboratory-adapted strain (N2) is resistant and a natural isolate from Hawaii (CB4856) is sensitive to micromolar amounts of exogenous zinc supplementation. Using a panel of recombinant inbred lines, we identified two large-effect quantitative trait loci (QTL) on the left arm of chromosome III and the center of chromosome V that are associated with zinc responses. We validated and refined both QTL using near-isogenic lines (NILs) and identified a naturally occurring deletion in sqst-5, a sequestosome-related gene, that is associated with resistance to high exogenous zinc. We found that this deletion is relatively common across strains within the species and that variation in sqst-5 is associated with zinc resistance. Our results offer a possible mechanism for how organisms can respond to naturally high levels of zinc in the environment and how zinc homeostasis varies among individuals. Author summaryZinc, although an essential metal, can be toxic if organisms are exposed to concentrations that are too high or too low. To prevent toxicity, organisms have evolved mechanisms to regulate zinc uptake from the environment. Here, we leveraged genetic variation between two strains of the roundworm Caenorhabditis elegans with different responses to high exogenous zinc to identify genes that might be involved in maintaining proper zinc levels. We identified four loci that contributed to differential zinc responses. One of these loci was the sequestosome-related gene sqst-5. We discovered that targeted deletions of sqst-5 caused an increase in resistance to zinc. Although SQST-5 contains a conserved zinc-binding protein domain, it has yet to be directly implicated in the C. elegans zinc response pathway. We identified two common forms of genetic variation in sqst-5 among 328 distinct strains, suggesting that variation in sqst-5 must have emerged multiple times, perhaps in response to an environment of high zinc. Overall, our study suggests a natural context for the evolution of zinc response mechanisms.