Common variant c.-22 + 155C > T of BDNF as a genetic risk factor of opium addiction

Opioid use disorder is largely genetic in nature. The common genetic variants present in population might be modulating the risk by affecting expression level of genes in the brain. Here, we study common variants in promoter region of brain-derived neurotrophic factor (BDNF) and Dopamine receptor D2 (DRD2) in a highly prevalent opium addiction geographical region. We identify highly expressed isoforms using CAGE datasets and its associated promoter. Common variants were extracted from promoters and genotyped in addicts compared to controls. The associated variants obtained through hitchhiking events were removed and eQTL analysis was performed. We identified two mutations (rs7944119:G > T and rs13306221:C > T) in the promoter of BDNF to be significantly associated with the addiction. In the dominant inheritance model, both rs7944119 and rs13306221 increases the risk of addiction. Of these two, rs7944119 was in linkage disequilibrium with rs13306221 and showed association due to hitchhiking. The rs13306221-T was associated with a lower expression level of the short BDNF isoform in the Cerebellar cortex. This finding suggests that rs13306221 (c.-22 + 155C > T) could increase risk of addiction by decreasing the expression level of the short isoform of BDNF, therefore, changes in the expression of the BDNF might not be the effect, but rather a cause of opium addiction disorder. Or, subjects with less expression of BDNF are more prone to addiction and addiction further decreases expression of BDNF short isoform in the brain. Although, it should be explored further in more detail.