MATERIALS AND METHODS

All patients met the DSM–IV (American Psychiatric Association, 1994) criteria of schizophrenia. Consensus diagnosis was made independently by two authors of the present study (EFCC, RYLC) by face-to-face interview using the Structured Clinical Interview for DSM–IV. All patients were taking conventional antipsychotic medication together with, in some cases, anticholinergic medication (benzhexol in all subjects). Patients with a history of physical illness involving the central nervous system, substance and/or alcohol abuse were excluded.

Patients genomic DNA was collected by standard method. The primers were designed by Primer3 program basic on the human genomic sequence from the public databases (National Institute of Health, 2004). The sequences were: 5′-CATTCAAAGCTCCCCTTGAC-3′ and 5′GGGAGTAGGGAAGGAGATGC-3′. PCR was performed by using 100ng genomic DNA, 1x buffer, 1.5mM MgCl₂, 0.2mM dNTPs, 0.25μM of each primer and 1U Taq (ABgene). Running conditions consists 35 cycles of 94°C denaturing for 1 min, 56°C annealing for 1 min and 72°C extension for another 1 min. The PCR product (301bp) was then digested by the restriction enzyme MspI (New England BioLab). The A allele gives fragment of 301bp, while the G allele gives fragments of 166 and 135bp. The DNA fragments were detected by standard electrophoresis method. Perfect agreement (100%) on the genotyping processing was established between two laboratory technicians on the same sample.

Clinical symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS; Kay et al., 1987). The details of the complete procedure for each test are reported elsewhere (Chan et al., 2004; Halperin et al., 1991). Background cognition was assessed with the Wechsler Adult Intelligence Test–III Scale (WAIS–III; Wechsler, 1997a). Sustained attention was assessed by the Continuous Performance Test (Halperin et al., 1991). Verbal and visual memory was assessed by the Logical Memory Test and Visual Reproduction Test from the Wechsler Memory Scale–III (WMS–III; Wechsler, 1997b). Working memory was assessed by the Letter-Number Span Test (Gold et al., 1997) and 2-back test (Callicott et al., 1998). Executive function was assessed by verbal fluency and the Stroop Test.

The university and the corresponding hospital ethics committees approved the research plan and the recruitment procedure of the subjects with schizophrenia. Informed consent was obtained from all the subjects prior to the testing session and blood taking in accordance with the Declaration of Helsinki.

SPSS for window version 11.0 was used for statistical analysis. We computed a standardized score (z score) with the normative data from our laboratory. Multiple univariate analysis of covariance (ANCOVA) controlling for age, intellectual functioning, medication and duration of illness would be made whenever appropriate. Another set of non-parametric test of Kruskal-Wallis H Test would also be conducted to check with the consistency of the results. Statistical significance was considered to be attained at p < .05.