Methylation detection by MassARRAY
Two assays covering DRD4 and COMT promoters were performed, as these promoters have been the most widely implicated in the context of epigenetics. Methylation assays were designed using Epidesigner software (http://www.epidesigner.com/). A total of 35 CpG units in the promoter region of DRD4were evaluated (Supplementary Figure 1). For the promoter region of COMT, 39 CpG units were investigated (Supplementary Figure 2). DNA samples were amplified by polymerase chain reaction technique using the designed primers (Supplementary Figures 1 and 2), and DNA methylation was detected by BGI Genomics Co., Ltd. The methylation levels were quantified using MassARRAY and methylation ratios were calculated using EpiTyper software. The level of methylation is the average of the degrees of methylation of all CpG units in one gene. The degrees of methylation were expressed by the methylation percentage [mC/(mC+C)] for each CpG unit (mC is methylated cytosine and C is unmethylated cytosine).