In the meta-analysis of 3,957 cases (2,191 with a narrow phenotype) and 3,428 controls, genome-wide significant evidence for association to MDD was not observed for 2,391,203 genotyped or imputed HapMap II SNPs, suggesting that if any common SNPs are associated with MDD, their individual genotypic relative risks (GRRs) are likely to be small. Such associations could be detected in future, larger GWAS meta-analyses, a strategy that has succeeded for dozens of other common diseases.43, 44 In samples of one or a few thousand cases, many such loci will produce unimpressive results, but the regions with the strongest evidence for association are statistically most likely to be true associations. We discuss here the three genes in which P-values of approximately P < 10−6 were observed in the primary meta-analysis: ATP6V1B2, SP4 and GRM7.
