2.4.1  Catechol-O-methyltransferase (COMT)
COMT catalyzes a major step in the degradation of dopamine, norepinephrine, and epinephrine; about 60% of the DA degradation in the PFC is performed by COMT.[68] The most popular marker for the COMT gene is the Val/Met functional SNP (rs4680). In spite of the negative findings from a Hong Kong meta-analysis[69] and a recent systematic review, [70] this marker is currently the most actively researched SNP listed on the ADHD gene database[71] (http://adhd.psych.ac.cn/). In China, Qian and colleagues[72],[73] did notfind an association between COMT and ADHD but did find some sexspecific associations: compared to controls, the Met allele was preferentially transmitted to boys with ADHD and the Val allele was preferentially transmitted to girls with ADHD; they also found that male ADHD comorbid with ODD was associated with homozygosity of the high-activity Val allele, while the ADHD-I was associated with the low-activity Met allele. Other studies in Chinese samples also failed to identify a significant association between ADHD and COMT.[74],[75],[76],[77] Zhang and colleagues[78] reported that the rs6267 of COMTwas not associated with the susceptibility to ADHD but it was associated to some of the clinical characteristics of ADHD.

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