Brain derived neurotrophic factor (BDNF) Val66Met polymorphism is not risk factor for bipolar disorder

Bipolar disorder (BPD) is a psychiatric disease, characterized by the cycles of mania and depression. Several genetic studies investigated BDNF gene Val66Met polymorphism as risk factor for BPD, but results were inconclusive. Therefore, present meta-analysis was performed to reevaluate the BDNF Val66Met polymorphism and BPD association. Four databases (Pubmed, Springer Link, Science Direct and Google Scholar) were searched for eligible studies up to March 31,2018. Pooled odds ratios (OR) with 95% confidence intervals (CI) were calculated to estimate the strength of the association. All statistical analyses were done by MetaAnalyst and Mix program. Forty studies with a total of 28,787 subjects (10,085 cases and 18,702 controls) were included in this meta-analysis. Overall, pooled analysis indicated that there was no significant association between BDNF Val66Met polymorphism and BPD risk under all five genetic models (ORA vs.G =0.99, 95%CI= 0.94-1.03, p=0.49; ORAG vs. GG= 0.1.02, 95%CI= 0.95-1.07, p= 0.57; ORAA vs. GG = 0.98, 95%CI=0.89-1.08, p=0.75; ORAA+AG vs. GG= 1.0, 95%CI= 0.94-1.06, p= 0.89;ORAA vs. AG+GG= 0.96, 95%CI= 0.89-1.05, p= 0.47). Similarly, no significant association was observed in ethnicity based subgroup analysis in both Asian and Caucasian population. However, significant association was found in subtype analysis between BDNF Val66Met and BPDII (ORAA+AG vs. GG= 1.21, 95%CI= 1.06-1.37, p= 0.003) but not with BPDI. These findings suggested that the BDNF Val66Met polymorphism confer no genetic susceptibility to BPD I but risk for BPDII.