Genome-Wide Association Study to Identify Genes Related to Renal Mercury Concentrations in Mice

Abstract

Background: Following human mercury (Hg) exposure, the metal accumulates in considerable concentrations in kidney, liver, and brain. Although the toxicokinetics of Hg have been studied extensively, factors responsible for interindividual variation in humans are largely unknown. Differences in accumulation of renal Hg between inbred mouse strains suggest a genetic interstrain variation regulating retention or/and excretion of Hg. A.SW, DBA/2 and BALB/C mouse strains accumulate higher amounts of Hg than B10.S.

Objectives: We aimed to find candidate genes associated with regulation of renal Hg concentrations.

Methods: A.SW, B10.S and their F1 and F2 offspring were exposed for 6 weeks to 2.0 mg Hg/L drinking water. Genotyping with microsatellites was conducted on 84 F2 mice for genome-wide scanning with ion pair reverse-phase high-performance liquid chromatography (IP RP HPLC). Quantitative trait loci (QTL) were established. Denaturing HPLC was used to detect single nucleotide polymorphisms for haplotyping and fine mapping in 184 and 32 F2 mice, respectively. Candidate genes (Pprc1, Btrc and Nfkb2) verified by fine mapping and QTL were further investigated by real-time polymerase chain reaction. Genes enhanced by Pprc1 (Nrf1 and Nrf2) were included for gene expression analysis.

Results: Renal Hg concentrations differed significantly between A.SW and B10.S mice and between males and females within each strain. QTL analysis showed a peak logarithm of odds ratio score 5.78 on chromosome 19 (p = 0.002). Haplotype and fine mapping associated the Hg accumulation with Pprc1, which encodes PGC-1-related coactivator (PRC), a coactivator for proteins involved in detoxification. Pprc1 and two genes coactivated by Pprc1 (Nrf1 and Nrf2) had significantly lower gene expression in the A.SW strain than in the B10.S strain.

Conclusions: This study supports Pprc1 as a key regulator for renal Hg excretion.

Citation: Alkaissi H, Ekstrand J, Jawad A, Nielsen JB, Havarinasab S, Soderkvist P, Hultman P. 2016. Genome-wide association study to identify genes related to renal mercury concentrations in mice. Environ Health Perspect 124:920-926; http://dx.doi.org/10.1289/ehp.1409284.

Figure 1

Kidney mercury concentrations. Mercury (Hg) deposition in kidneys of male and female A.SW and B10.S mice exposed to 2 mg Hg/L drinking water for 6 weeks. Data obtained from previous study (Ekstrand et al. 2010). Figure is presented as mean ± SD, **p = 0.0041, ***p < 0.0001 (Welch’s test).

Figure 2

Quantitative trait loci on autosomes and effect plot. (A) Genome-wide scan (n = 44 male, 40 female F2 mice) on autosomes was performed to identify quantitative trait loci (QTL) associated with Hg accumulation in kidney. Logarithm of odds (LOD) scores (y-axis) indicate a high association with microsatellite D19Mit53 on chromosome 19; LOD score = 5.78, ***p = 0.0002. (B) Mean ± SD renal Hg concentration (ng/g wet weight) according to D19Mit53 genotype. AA, homozygous for the A.SW allele; BB, homozygous for the B10.S allele; AB, heterozygote; ****p < 0.0001 (Mann–Whitney test).

Figure 3

Fine mapping and QTL. (A) Markers used for fine mapping in haplotype position between 37.98 and 38.97 cM on chromosome 19 were homozygous for the A.SW allele on D19Mit53; 184 F2 mice were analyzed. (B) QTL associated with Hg accumulation in kidney based on fine mapping results on chromosome 19, on 32 F2 offspring homozygous for A.SW on D19Mit53. Btrc, Pprc1 and Nfkb2 all had a LOD score of 1.94.

Figure 4

SNP genotype in Pprc1. Genotype data of F2 offspring on two SNPs, rs30400427 (A.SW, guanine; B10.S, adenine) and rs30815571 (A.SW, adenine; B10.S, guanine) in Pprc1. The x-axis shows homozygous genotypes for A.SW (AA) and B10.S (BB) and a heterozygous genotype (AB). Graph is presented as median ± interquartile range, *p = 0.0299; **p = 0.0018 (Mann–Whitney test).

Figure 5

Hg excretion from kidney. Hypothetical conceptual model consistent with our findings. Mercury enters proximal tubular cells via OAT-1, OAT-3 transporter proteins (Bridges and Zalups 2005) already bound to sulfhydryl protein GSH or binds to the protein inside tubular cells. MRP1-3 export GSH-Hg complexes out of the cell into the tubular lumen and out with the urine (Bridges et al. 2008b; Toyama et al. 2007). Pprc1 encodes the protein PGC-1-related coactivator, which acts as a coactivator for Nrf1 and Nrf2 (via CREB)transcription factors. Nrf1 regulates production of GSH levels (Chen et al. 2003; Kwong et al. 1999; Yang et al. 2005), and Nrf2 regulates production of MRP-1 and MRP-2 (Toyama et al. 2007).