The subjects included 257 unrelated healthy volunteers (65 males and 192 females; age, 37.3 ± 11.9 years (mean ± SD)) recruited from the staff of several mental and general hospitals around Tokyo. They had no history of major psychiatric illness. The research protocol was approved by the ethics committee of the University of Tokyo. Written informed consent was obtained from all the subjects.
After blood collection, the subjects filled out the Revised NEO Personality Inventory (NEO PI-R), a self-report inventory based on the five-factor model of personality: Neuroticism, Extraversion, Openness to Experience, Agreeableness, and Conscientiousness [25]. To further investigate the relation with anxiety, the State-Trait Anxiety Inventory (STAI), an instrument for measuring anxiety differentiating between the temporary condition of State Anxiety and the more general and long-standing quality of Trait Anxiety [26], was also completed by the subjects. Genomic DNA was extracted from leukocytes by using a standard method. The -241A/G, -141C Ins/Del, and Ser311Cys polymorphisms in the DRD2 gene, the Ser9Gly polymorphism of the DRD3 gene, and the Val81Met and PstI site polymorphisms in the TH gene were genotyped. Genotyping of the -241A/G and -141C Ins/Del polymorphisms of the DRD2 gene was performed as described by Arinami et al. [11]; Ser311Cys of the DRD2 gene by Higuchi et al. [27]; the DRD3 gene by Ebstein et al. [18]; the Val81Met of the TH gene by Ishiguro et al. [23]; and the PstI site of the TH gene by Furlong et al. [24].
The associations between gene polymorphisms and the scores for NEO PI-R or STAI were statistically analyzed by one-way analysis of covariance (ANCOVA) adjusting sex and age. Epistasis was assessed using two-way ANCOVA adjusting sex and age between the polymorphisms of independent two genes. Independence between the genes was assessed by means of chi-square test in contingency tables with marginals defined by genotype counts. Bonferroni correction was conducted for multiple testing. The statistical package SPSS for Windows [28] was used for all analyses.
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