Objectives: Bipolar disorder (BPD) is an affective disorder characterized by episodes of depression and mania. Cognitive impairment in patients with BPD is common and recent data suggest that it may represent a trait-like feature of the illness, as it persists during periods of wellness and co-segregates in families of bipolar patients. This suggests an underlying genetic predisposition towards cognitive impairment in patients with BPD; however, there have been few studies investigating the effects of candidate genes on this symptom domain. The catechol-O-methyl transferase (COMT) gene is located on chromosome 22q11 and is involved in the metabolism of dopamine and norepinephrine. A large body of evidence suggests that COMT is associated with cognitive performance in patients with schizophrenia and in healthy volunteers but there have been no reports of its relationship to cognition in BPD.
Methods: We genotyped 52 Caucasian bipolar I probands and 102 Caucasian healthy controls across four single nucleotide polymorphisms (SNPs) within the COMT gene and administered a cognitive screening battery. We first assessed the relationship between COMT genotype and diagnosis and then tested for effects on cognition.
Results: We observed a modest but significant association between SNP rs165599 and BPD I, with the g allele being over-represented in cases versus controls (odds ratio, OR = 2.41; allelic p = 0.02; genotypic p = 0.04). Further, we found a relationship between the risk allele at this SNP and poorer performance on measures of verbal memory [California Verbal Learning Test (CVLT) Trials 1–5; p = 0.005, η2 = 0.07] particularly with regard to prefrontal aspects of learning (CVLT semantic cluster; p = 0.037, η2 = 0.04) in patients with BPD and in healthy controls. The common Val158Met polymorphism was not associated with diagnosis (p = 0.32) or cognition in our sample.
Conclusions: These data suggest that COMT genetic variation at SNP rs165599 is associated with BPD I and influences prefrontal aspects of verbal memory in bipolar patients and healthy controls.