Association between CLOCK 3111T/C and preferred circadian phase in Korean patients with bipolar disorder

Genes associated with circadian rhythms have been suggested to play an important role in the pathophysiology of bipolar disorder. A single nucleotide polymorphism (SNP) in the 3′-flanking region of CLOCK (3111T/C; rs1801260) has been reported to be associated with sleep disturbances and an increased recurrence rate in patients with bipolar disorder. We examined the association of CLOCK 3111T/C with bipolar disorder in 260 patients and 350 controls in a Korean population. CLOCK 3111T/C showed significant allelic and genotypic associations with bipolar disorder (P = 0.012, P = 0.033, respectively). Morningness/eveningness (M/E) was evaluated using the Composite Scale of Morningness (CSM) in 108 patients with bipolar disorder. In the subgroup analysis of the highest and lowest 25th percentile of M/E scores, significantly more C allele carriers were found among extreme evening types than among extreme morning types (P = 0.041). After correcting for age, C allele carriers had lower M/E scores than those carrying the T/T genotype, but the association was not statistically significant. We also analyzed the association between age at onset (AAO) and CLOCK 3111T/C in the bipolar disorder group, and no association was found. Despite the relatively small sample sizes, these results support a possible role of the CLOCK 3111T/C SNP in bipolar disorder. Further studies with larger samples and more polymorphisms are necessary.