Investigating arsenic susceptibility from a genetic perspective in Drosophila reveals a key role for glutathione synthetase

Abstract

Chronic exposure to arsenic-contaminated drinking water can lead to a variety of serious pathological outcomes. However, differential responsiveness within human populations suggests that interindividual genetic variation plays an important role. We are using Drosophila to study toxic metal response pathways because of unrivalled access to varied genetic approaches and significant demonstrable overlap with many aspects of mammalian physiology and disease phenotypes. Genetic analysis (via chromosomal segregation and microsatellite marker-based recombination) of various wild-type strains exhibiting relative susceptibility or tolerance to the lethal toxic effects of arsenite identified a limited X-chromosomal region (16D-F) able to confer a differential response phenotype. Using an FRT-based recombination approach, we created lines harboring small, overlapping deficiencies within this region and found that relative arsenite sensitivity arose when the dose of the glutathione synthetase (GS) gene (located at 16F1) was reduced by half. Knockdown of GS expression by RNA interference (RNAi) in cultured S2 cells led to enhanced arsenite sensitivity, while GS RNAi applied to intact organisms dramatically reduced the concentration of food-borne arsenite compatible with successful growth and development. Our analyses, initially guided by observations on naturally occurring variants, provide genetic proof that an optimally functioning two-step glutathione (GSH) biosynthetic pathway is required in vivo for a robust defense against arsenite; the enzymatic implications of this are discussed in the context of GSH supply and demand under arsenite-induced stress. Given an identical pathway for human GSH biosynthesis, we suggest that polymorphisms in GSH biosynthetic genes may be an important contributor to differential arsenic sensitivity and exposure risk in human populations.

FIG. 1.

Viability of Drosophila melanogaster strains in sodium arsenite. Seeded embryos were scored for percent adult eclosion on arsenite-containing food and normalized to values obtained when seeded on arsenite-free food. Two strains that showed relative resistance (R) and sensitivity (S) to arsenite are marked by arrows and were selected for further analysis (though others could also have been investigated).

FIG. 2.

Relative adult eclosion percentages for male and female progeny resulting from reciprocal crosses of the Oregon R 1970 and PVM strains. Embryos were seeded on arsenite-free or arsenite-containing food and the sex of eclosing adults scored as a percentage of the total flies hatching at that concentration. Strains are designated as resistant (R) or sensitive (S) based on the data in Figure 1; the crossing scheme shown below represents the expected genotype of progeny from the reciprocal crosses if the arsenite tolerance/sensitivity gene(s) were X-linked.

FIG. 3.

Recombination mapping between Oregon R 1970 and PVM using strain-specific microsatellite markers on the X chromosome to locate an arsenite tolerance/sensitivity locus. (A) Crossing scheme shows X-chromosomal constitution of F1 heterozygous females backcrossed to PVM males—recombination on the female X leads to male progeny that contain either the resistant (R) or the sensitive (S) arsenite-response allele. (B) Microsatellite mapping in the 16DF region of the X chromosome shows mobility difference depending on parental source (R or S), allowing genotypic frequency to be scored in F2 males either selected for survival on 1mM arsenite or not selected (i.e., raised on normal food). (C) “R”-derived allele frequencies were scored for a variety of markers located along the X chromosome. As expected, when not selected for arsenite resistance, markers segregated in an approximately 1:1 ratio. When the R allele approaches 100% representation in resistant males, very close linkage to the arsenite-responsive allele(s) is anticipated.

FIG. 4.

X-chromosomal overlapping deficiencies created using the FLP-FRT recombination system to aid in identification of an arsenite response locus. Recovered deficiencies are shown both above and below a physical map of the 16C–17A chromosomal region, while the annotated genetic organization of the region from FlyBase is shown directly below it. *Haploinsufficient; ^Homozygous viable.

FIG. 5.

Arsenite sensitivity of X-chromosomal deficiency lines. (A) Arsenite dose-response assay on parental w¹¹¹⁸/Binsinscy strain to identify an experimental concentration threshold for testing toxic effects on Df lines. We chose to use 0.25mM arsenite for the sensitivity assays. Each bar represents the relative viability of emerging w¹¹¹⁸/Binsinscy adults exposed to the specified concentration of arsenite-supplemented food when compared to emerging w¹¹¹⁸/Binsinscy adults exposed to nonsupplemented food. *p < 0.01, **p < 0.005. (B) Viability ratio of various Df lines compared to that of the isogenic parental strain (w¹¹¹⁸/Binsinscy) tested on 0.25mM arsenite-supplemented food. *p < 0.05, **p < 0.01. (C) Deficiency #11 encompasses a region containing five annotated genes: CG6835 and CG32495 encode the enzyme GS, CG6788 and CG32496 are genes encoding cell adhesion molecules, and CG7772 encodes a protein with carbonate dehydratase activity.

FIG. 6.

RNAi-induced knockdown of GS expression in S2 cells. (A) dsRNA was targeted to the 5′ region of CG6835 and CG32495 and cell viability measured under differing concentrations of arsenite-supplemented growth medium. *p < 1.0 × 10⁻⁶ for 35 and 45μM As WT versus 35 and 45μM As GS RNAi. (B) RT-PCR analysis of GS transcript levels after targeting the 5′ region. Results have been normalized to actin. *p < 0.01. (C) dsRNA was targeted to the 3′ region of CG6835 and CG32495 and viability measured as described. *p < 1.0 × 10⁻⁷ for 35 and 45μM As WT versus 35 and 45μM As GS RNAi. (D) Real-time RT-PCR analysis of GS transcript levels after targeting the 3′ region. Results have been normalized to actin. *p < 0.01.

FIG. 7.

Effects of RNAi-induced knockdown of GS in flies. (A) Embryos containing the Gal4-inducible GS^{RNAi[5′/3′]} transgene were tested for their ability to eclose as adults in the presence or absence of ubiquitously expressed Gal4 on differing concentrations of arsenite-supplemented food. Survival is expressed relative to the non-Gal4–expressing GS^{RNAi[5′/3′]} line on control (0mM arsenite) food. # p = 0.05, * p < 0.05, ** p < 0.01. (B) Real-time RT-PCR analysis of GS transcription in GS^{RNAi[5′/3′]} flies in the presence or absence of a Gal4 transgene. *p < 0.01.