Genotyping
DNA was extracted from ethylenediaminetetraacetic acid (EDTA)-treated peripheral blood (3ml) collected from participants through venipuncture and stored at − 80 °C. There have been many studies of polymorphisms in dopaminergic reward pathway related gene and disease associations, and some SNPs may affect the susceptibility to or severity of various substance use disorders 29. Information on the DRD4 and COMT genes was downloaded from the HapMap database and the SNPs with a minimum allele frequency (MAF) ≥ 1%. Biologically SNPs, which affect gene function, are generally located in gene promoters, exons, and 5’ untranslated regions (5’ UTRs). Based on the above screening criteria, a total of 8 SNP units in the DRD4 and COMT genes were selected in this study. DRD4 rs1800955, rs747302, rs3758653 and COMT rs4680, rs4818, rs6267, rs4633, rs6269 were genotyped by the TaqMan method using the ABI PRISM 3730XL Sequence Detection System (ABI, Foster City, CA, USA) carried by BGI Genomics Co., Ltd (Beijing, China). Two-tailed Pearson’s chi-square (χ2) tests were used to compare the distribution of genotypes with those predicted under Hardy–Weinberg equilibrium.