SNPs to proxy for serum calcium were obtained from a GWAS meta-analysis of 39,400 individuals of European descent from 17 population-based cohorts [
34]. Genetic instruments were constructed by obtaining SNPs shown to robustly (
p < 1 × 10
−7) and independently to associate (
r2 < 0.01) with serum calcium levels that were replicated (one-sided
p < 0.05) in an independent meta-analysis of up to 21,679 individuals of European descent. In total, seven SNPs located in or near
CASR (rs1801725),
DGKD (rs1550532),
GCKR (rs780094),
GATA3 (rs10491003),
CARS (rs7481584),
DGKH (rs7336933), and
CYP24A1 (rs1570669) were independently replicated. Summary data on rs1801725 were not available in the PRACTICAL OncoArray analysis so we used a proxy SNP located in
CASR (rs17251221) in high linkage disequilibrium with rs1801725 (
r2 = 0.85), using the 1000 Genomes Project CEU database as a reference [
39]. As an initial test for horizontal pleiotropy (a single locus influencing multiple phenotypes through independent biological pathways; a violation of the “exclusion restriction criterion”), we examined associations of calcium SNPs with thousands of other traits in a large catalogue of summary genetic association statistics from previously published GWAS (MR-Base;
http://www.mrbase.org) [
40]. After applying a Bonferroni correction to account for multiple “look ups” of phenotypic traits with all 7 SNPs examined (
p < 0.05/x, where x represents the number of phenotypic trait “look ups” performed; 859 to 1,060 look-ups performed with corresponding corrected
p value thresholds: 5.8 × 10
−5 to 4.7 × 10
−5 across seven SNPs), we identified two SNPs (rs780094, rs1550532) that associated with multiple traits in MR-Base. rs780094 was robustly associated (
p < 4.8 × 10
−5) with various measures of lipids, insulin, and anthropometric traits and rs1550532 was robustly associated (
p < 4.8 × 10
−5) with inflammatory bowel disease; these traits have all been hypothesized to influence prostate cancer risk [
41‐
44]. Additionally, rs1550532 was strongly associated with levels of multiple “unknown metabolites” from untargeted GWAS of metabolomic studies [
45]. Given that these two SNPs could influence prostate cancer risk through biological pathways independent of calcium (i.e. horizontal pleiotropy), we removed them from our genetic instrument. Consequently, our genetic instrument for calcium used five SNPs that we assessed as being exclusively associated with serum calcium (rs17251221, rs10491003, rs7481584, rs7336933, rs1570669).