Expression Quantitative Trait Locus rs6356 Is Associated with Susceptibility to Heroin Addiction by Potentially Influencing TH Gene Expression in the Hippocampus and Nucleus Accumbens.

Opioid addiction is a complicated and highly heritable brain disease. Dysfunction in dopaminergic signaling is involved in the pathogenesis of addictive disorders. Encoding a dopamine synthetase, the tyrosine hydroxylase (TH) gene has long been an interesting candidate in genetic association studies for opioid addiction. However, the mechanisms underlying associations of risk gene variants and opioid addiction remain unknown. In the present study, we first analyzed the association between TH gene variants and susceptibility and traits of heroin addiction in 801 patients with heroin addiction and 930 healthy controls. Methylation levels in the promoter region of the TH gene were detected and compared between the heroin addiction and healthy control groups. To reveal the potential mechanism of the association of TH gene variants and heroin addiction, correlations between the risk TH single nucleotide polymorphism (SNPs) for heroin addiction and the methylation and expression levels of the TH gene were examined. Our results demonstrated that SNP rs6356 was associated with susceptibility to heroin addiction. CpG TH_15 was hypermethylated in the heroin addiction group compared with the healthy control group. Notably, SNP rs6356 was correlated in an allele-specific manner with expression of the TH gene in the hippocampus and nucleus accumbens but not with methylation levels of CpG TH_15. Our findings suggest that the eQTL rs6356 was associated with susceptibility to heroin addiction by potentially affecting the expression of the TH gene in brain regions in the mesocorticolimbic dopamine system, including the hippocampus and nucleus accumbens.