Experiment
Differences in accumulation of renal mercury (Hg) between inbred strains of mice suggest a genetic interstrain variation regulating retention or/and excretion of Hg. A.SW, DBA/2 and BALB/C mouse strains accumulated higher amounts of Hg than B10.S. The goal of the current study was to find candidate genes associated with regulation of renal Hg concentrations.
F1 mice were derived by crossing female A.SW-H2s (A.SW) and male B10.S-H2s-T18b/SnJ (B10.S) mice. (A.SW-H2s x B10.S-H2-T18b/SnJ)F2 hybrids (154=males, 180=females) were generated by crossing the F1 mice. All mice (A.SW, B10.S, F1 and F2) were given 2.7 mg HgCl2/L in drinking water at age 8-10 weeks for 6 weeks before sacrifice (age 14-16 weeks). Radioactivity of the left kidney, obtained after sacrifice, was measured and used to quantify renal Hg accumulation. DNA was isolated from the tail, spleen or kidneys.
A genome wide scan was performed on 28 F2 mice selected at random from each group for a total of 84 mice (44 male and 40 female). Haplotype analysis was performed on 334 F2 mice to narrow the QTL region. QTL were identified based on LOD score profiles derived from genome wide single QTL scan by Haley-Knott regression using R/qtl software. Regression was based on data from the 84 F2 offspring for 96 microsatellite markers covering 19 autosomes with an average spacing of 20 cM. The genome wide significance threshold was based on 10,000 permutations.
A significant QTL was identified, Renmq1 (renal mercury accumulation QTL 1), mapped to Chromosome 19, peaking at marker D19Mit53 (38.46 cM) with a LOD score of 5.78, p<0.0002. Renal Hg accumulation was significantly higher in F2 mice that were homozygous for the A.SW allele of D19Mit53 than in heterozygotes for the B10.S allele, suggesting autosomal recessive inheritance.
Additional genotyping with 20 microsatellite markers spaced between 20.18 and 56.28 cM identified a DNA block between D19Mit67 (37.98) and D19Mit9 (38.97 cM) in which 32 F2 homozygous mice were further analyzed with fine mapping. Fine mapping narrowed the region to 19:45630547-19:46384795 Mb with a LOD score of 1.94. SNP analysis revealed 3 genes segregated between background strains: Btrc, Pprc1 and Nfkb2.
F2 mice that were homozygous for the A.SW allele (AA) at two SNPs (rs30400427 and rs30815571) in Pprc1 showed significantly higher Hg accumulation than heterozygous (AB) and homozygous (BB) mice. Findings suggest that Pprc1 is a plausible candidate gene for QTL Renmq1.
Suggestive QTL for renal Hg accumulation were also identified on Chr 8 (12.59 cM), Chr 13 (27.48 cM) and Chr 17 (55.48 cM) respectively, all of which had LOD scores of > or equal to 2.0. [Fig 2.A.]
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