Interestingly, 9 SNPs at 7p12.2 (8 SNPs in DDC gene and 1 SNP in GRB10 gene) reached genome-wide significance level (P<5×10−7) in the SAGE sample (Table 4). Two of them, including a nonsynonymous SNP, had replicable cis-acting regulatory effects on the DDC transcript expression, which increased the possibility that DDC might play a functional role in risk for disease. DOPA decarboxylase (DDC; also known as L-amino acid decarboxylase; AADC) is involved in the synthesis of dopamine, norepinephrine and serotonin. DDC encodes a protein that is responsible for catalyzing the decarboxylation of L-3,4-dihydroxyphenylalanine (DOPA) to dopamine, L-5-hydroxytryptophan to serotonin and L-tryptophan to tryptamine (Hopfer et al., 2001). Chromosome 7 has been previously reported to have linkage with MaxDrinks (Saccone et al., 2005) by using the COGA sample. DDC is a potential gene that has association with nicotine dependence and smoking behavior (Yu et al., 2006). Agrawal et al. (2011) reported that multiple markers in DDC were associated with alcohol consumption and suggested the potential role of the serotonin synthesis pathway in alcohol consumption in a sample of 827 young women. DDC
