Attention-deficit hyperactivity disorder (ADHD) is a prevalent behavioral disorder in children and the etiology of this disorder is not clear. Molecular genetic and pharmacological studies suggest the involvement of dopaminergic and noradrenergic neurotransmitter systems in ADHD, e.g., several reports have found association between ADHD and the dopamine receptor gene DRD-4, the dopamine transporter gene DAT1, and the catecholamine clearance enzyme catechol-O-methyltransferase. Monoamine oxidase (MAO) A and B genes encode enzymes that participate in the metabolism of neurotransmitters of the dopaminergic and noradrenergic systems. MAO inhibitors have been shown to be effective in the treatment of ADHD. Our previous studies showed an association between ADHD and the DXS7 locus, which is located in close vicinity to the MAO genes on chromosome X. These findings suggest that there might be linkage between ADHD and MAO genes. To test this hypothesis, we used the transmission/disequilibrium test (TDT) to test for linkage between a VNTR polymorphism at the MAOA(CA)(n) or MAOB(GT)(n) locus and DSM-III-R-diagnosed ADHD in 82 nuclear families of the Chinese population. The TDT analysis revealed linkage between ADHD and the MAOA(CA)(n) locus (chi-square = 15.25, df = 7, P < 0.05), but not the MAOB(GT)(n) locus (chi-square = 11.18, df = 7, P > 0.05). The data showed that ADHD was in linkage with the MAOA gene and suggested that MAOA might be a susceptibility factor for ADHD.

Previous studies have suggested that the serotonergic (5-HT) system might be involved in the development of Attention-deficit hyperactivity disorder (ADHD). ADHD is frequently characterized by aggressive and impulsive behavior, a major symptom associated with reduction in serotonergic function. The tryptophan hydroxylase (TPH) gene is a reasonable candidate for ADHD because it encodes the rate-limiting enzyme in the process of 5-HT biosynthesis. In this study, we examined the relationship between the A218C polymorphism in TPH gene and ADHD. Sixty-nine ADHD patients and their biological parents were investigated. The A218C polymorphism in intron 7 of TPH gene was detected by PCR-RFLP method. No allele or genotype concerned with this A218C polymorphism was found to be associated with ADHD when analyzed with the haplotype relative risk method. Therefore, our data indicate that the TPH gene A218C polymorphism may not be a susceptibility factor of ADHD in the Chinese Han population.

Attention deficit hyperactivity disorder (ADHD) is a prevalent disorder in children. The etiology of this disease is not clear. Genetics studies have suggested the involvement of the dopamine DRD-4 receptor gene and dopamine transporter gene (DAT1). Clinical studies have shown that monoamine oxidase-B (MAO-B) inhibitors are effective in the treatment of ADHD. These findings suggest that monoamine oxidase (MAO) genes might be involved in the origin of ADHD. In the present work, the DXS7 locus of chromosome X, which is closely linked to MAO genes, was selected as a marker to study the possible association between ADHD and MAO genes in the Chinese population. Haplotype-based haplotype relative risk (HHRR) and the transmission disequilibrium test (TDT) methods were employed to analyze the association and the linkage disequilibrium, respectively. Significant association (X(2) = 15.86; 1 df; P < 0.001) and linkage (X(2) = 14.88; 1 df; P < 0.001) were detected between the 157-bp allele of the DXS7 locus and the DSM-III-R-diagnosed ADHD (N = 72) in trios composed of father, mother, and affected offspring. The data suggested that ADHD was associated and in linkage with DXS7 locus.