Bipolar disorder is a chronic mental illness with high prevalence of co-occurring alcohol use disorder. Linkage studies have revealed several candidate genes in the dopaminergic and serotonergic pathways which may be associated with both bipolar and alcohol use disorders. We investigated the relationship between polymorphisms in candidate genes and alcohol use disorder comorbidity in bipolar patients. We performed a retrospective study of a genomic database consisting of 278 bipolar disorder patients. Diagnosis of bipolar disorder was according to the Structured Clinical Interview for DSM-IV Axis I disorders (SCID-I). RFLP analysis of single nucleotide polymorphisms were performed in dopamine (DRD1, DRD2 and DRD3) and serotonin receptor and transporter genes (5HTTLPR, 5HT1B, 5HT2A, 5HT2C). There were 179 (64%) females in the database. Seventy-one (25.5%) of the bipolar patients were diagnosed as comorbid alcohol use disorder. Chi-square analysis indicated that in female bipolar patients, there was a significant difference in genotype frequency between the bipolar patients with comorbid alcohol use disorder and non-comorbid bipolar patients for the Ser23Cys (rs6318) polymorphism of the 5HT2C gene. Overall, the results indicate a possible association between 5HT2C and alcohol use disorder comorbidity.
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