The majority of previous research has relied on candidate genes, particularly those genes involved in neurotransmitter systems known to be involved in the etiology of addiction or depression. In contrast, genome-wide scans do not rely on prior hypotheses, and therefore represent a useful method by which to identify novel variants influencing the phenotype of interest. In addition, they can provide further support for previously implicated genetic loci. Recently, Sullivan and colleagues (Sullivan et al., 2009) reported results from a genome-wide association study (GWAS) on major depression. Although no SNP met criteria for genome-wide significance (p<5 × 10−8), four of their most significant markers were in the gene coding for the presynaptic protein piccolo (PCLO). Muglia et al. (2008) also conducted a GWAS analysis for major depression using two separate samples, and reported no markers meeting genome-wide significance criteria. However, secondary analyses by this research group suggested that genes previously implicated in mood disorders were significantly (p<0.0001) associated with depression when the two samples were combined.
