Toxicogenetic studies in Drosophila: Using fruit flies to study arsenic toxicity

Arsenic contamination of drinking water supplies around the world is considered the worst environmental disaster of recent times. Chronic consumption of arsenic can lead to an array of serious pathological outcomes, in some of which methylation of the metal may be a crucial component in determining toxicity. Differential responsiveness within human populations suggests inter-individual genetic variation also plays an important role. We have used Drosophila melanogaster as a model to study arsenic response pathways because of unrivalled access to varied genetic approaches and significant overlap with many aspects of mammalian physiology and disease phenotypes. Genetic analysis of various strains exhibiting relative susceptibility or resistance to arsenite toxicity resulted in the identification of a chromosomal region able to confer a differential response phenotype. We created fly lines harboring small, overlapping deficiencies in this region and found that relative arsenite sensitivity arose when the glutathione synthetase (GS) gene dose was reduced by half. Knock-down of GS expression by RNA interference both in S2 cells and in vivo led to highly enhanced arsenite sensitivity. These analyses provide genetic proof that an optimally functioning glutathione (GSH) biosynthetic pathway is required for a robust defense against arsenite. Moreover, they unexpectedly highlight a step previously considered to be without regulatory significance; the implications of this are discussed in the context of GSH supply and demand under arsenite-induced stress. Recent work has shown that Drosophila does not possess an arsenic methylation pathway comparable to the human. Since methylated arsenicals (MAs) may be key players in the carcinogenic activity of arsenic, we have "humanized" Drosophila through the introduction of the human arsenic(III) methyltransferase (hAS3MT) gene expressed under easily manipulated regulatory control. Transgenic flies can be induced to express an antigenically cross-reactive form with arsenic methyltransferase activity of the hAS3MT enzyme and its expression does not affect the development or viability when exposed or unexposed to arsenic. Preliminary results using an in vivo genotoxicity assay have shown that production of MAs induces tumorigenesis in Drosophila. This model is ready for use in exploring mechanisms of arsenic genotoxicity and/or carcinogenicity in many informative genetic backgrounds, as well as the effects of different polymorphic variants of AS3MT found in human populations.