1. ¹ADHD Outpatient Clinic, Child and Adolescent Psychiatric Division, Hospital de Clínicas de Porto Alegre, Federal University of Rio Grande do Sul, Brazil
2. ²Department of Genetics, Federal University of Rio Grande do Sul, Brazil

Correspondence: Dr Luis Augusto Rhode, Serviço de Psiquiatria da Infância e Adolescência, Hospital de Clínicas de Porto Alegre, Rua Ramiro Barcelos, 2350, Porto Alegre, Rio Grande do Sul 90035-003, Brazil. Tel/fax: +55 51 33168294; E-mail: lrohde@terra.com.br

Attention-deficit/hyperactivity disorder (ADHD) is a very common and heterogeneous psychiatric disorder of childhood with marked inattentive, hyperactive and impulsive symptoms. In school-age children, the prevalence of ADHD is between 3 and 6%, and its symptoms persist in more than 60% of patients in adolescence and adulthood.^1,2

Family, twin and adoption studies strongly support a role for genetic components in the etiology of the disorder. These studies showed both an increased risk of ADHD in the relatives of probands and a moderate to high heritability. The mode of transmission is unclear, but it is likely to be due to many genes, each of them with a small effect.³ Based on the dopaminergic theories of ADHD, dopamine genes have been the initial candidates for molecular studies. The dopamine D4 receptor gene (DRD4) and the dopamine transporter gene (DAT1) were the most studied. In both genes, the main polymorphisms investigated in association studies with ADHD are the variable number of tandem repeats (VNTR). In the DRD4 gene, this VNTR is a 48 bp sequence in the third exon that can be repeated 2–11 times. This 48 bp polymorphism shows considerable ethnic variability, but 2-, 4- and 7-repeat alleles are the most common variants across different populations. Several studies that focused on DRD4 gene found an association of the 7-repeat allele with ADHD.⁴ The VNTR at DAT1 gene is a 40 bp sequence in the 3' untranslated region. In all, 10 different alleles can be found, according to the presence of 3–13 copies of the 40 bp unit. The 10-repeat allele, the most prevalent variant worldwide, has been implicated as the risk allele for ADHD.⁴ Two recent meta-analyses support a small effect of both genes in the disorder (Waldman et al, unpublished).⁵

Pharmacotherapy is a fundamental component in the treatment of ADHD. Many studies have clearly documented the efficacy of stimulants (eg, methylphenidate (MPH)) in reducing the symptoms of ADHD, as well as in improving the functioning of several other domains.⁶

Although the impressive literature documents both a strong participation of genetics in the etiology of the disorder and a high rate of response to stimulants, surprisingly few studies on pharmacogenetics of ADHD were conducted. In a pioneer study, Winsberg and Comings⁷ found that homozygosity of the 10-repeat allele at DAT1 gene was associated with a poor response to MPH in 30 African-American children with ADHD. However, potential confounding variables between the two groups (subjects with and without the homozygosity for the 10-repeat allele) were not extensively assessed in that study.

Recently, our group was able to replicate this previous finding in a sample of Brazilian ADHD boys.⁸ In a blind naturalistic study, 50 male ADHD youths were treated with MPH. While 75% (15/20) of the youths without 10/10 genotype demonstrated an improvement in the core symptoms of the disorder higher than 50% with MPH, only 47% (14/30) of the subjects with 10/10 genotype achieved the same level of improvement with medication (one-tailed P=0.04). In addition, the group without this genotype had a significantly higher increase in global functioning than the other group (one-tailed P<0.01). It is important to note that no significant differences were found between groups in age, ethnicity, educational level, IQ, baseline ADHD symptomatology and global functioning, type of ADHD and main comorbidities, median length of time between scales administration, and in the initial and final doses of MPH.⁸

These two studies add to the compelling literature indicating the role of DAT1 gene variability in the response to stimulants. Seeman and Mandras⁹ proposed a mechanism of action for stimulants to explain how these drugs act blocking the dopamine transporter. Since the expected effect of MPH when blocking the dopamine transporter is to increase levels of dopamine at the synaptic cleft, it is possible that an 'overactive' transporter is coded when 10 copies of the 40 bp sequence are present.⁴ In this regard, Fuke et al¹⁰ documented that luciferase expression was significantly higher when the 3'-UTR of the DAT1 gene contained the 10-repeat allele than when it contained the 7- or 9-repeat alleles. These findings suggest that this VNTR might modulate the expression of the dopamine transporter gene.

Other evidence for this hypothesis comes from a pilot study from our group, which integrates pharmacogenetics and neuroimaging (n=8 ADHD boys with at least moderate response to MPH). In this study, we were able to detect a significantly higher regional cerebral blood flow (rCBF) in medial frontal and left basal ganglia areas in children with homozygosity for the 10-repeat allele at DAT1 gene than in children without the 10/10 genotype (for both areas: P=0.08). In addition, a trend for a higher rCBF in the right and left frontal areas were detected in children with the 10/10 genotype than in children without this condition (for both areas: P=0.08) (Rohde et al, in press)¹¹. These findings seem to suggest that ADHD children with homozygosity for the 10-repeat allele at DAT1 (possibly encoding an overactive dopamine transporter) needed a higher cerebral flow (probably reflecting a higher dopaminergic activity) in brain regions associated with working memory and inhibitory behavior (frontal and basal ganglia areas) to achieve at least moderate response to MPH than children without the 10/10 genotype. Thus, it is possible to speculate that less extracellular dopamine would be available by MPH blockade of dopamine transporter when an overactive transporter is coded. In this case, response to MPH would depend on a higher dopaminergic release. These findings are in agreement with a recent investigation suggesting that individual differences in response to MPH are due in part to individual differences in dopamine release.¹²

Although very promising, these ADHD pharmacogenetic studies should be understood with some caution. Recently, two different groups report contrasting findings when assessing the effect of DAT1 gene on the response to MPH in ADHD children.^13,14 Kirley et al¹³ found a positive association between DAT1 10-repeat allele and response to MPH and Stein et al¹⁴ found a worse response to MPH associated with homozygosity of the 9-repeat allele at DAT1 gene in a sample of 43 ADHD children. Although the reasons for this discrepancy remain unclear, some possibilities might be addressed: (a) differences in the methodology among studies, such as the type of patients included (drug-naive patients vs subjects with previous use of MPH; patients with different types of comorbidities) and strategies to assess response to pharmacotherapy (retrospective vs prospective evaluation; use of different scales); (b) nonreplication of positive findings is expected in association studies when samples are not too large;¹⁵ (c) the observed effects attributed to the 10-repeat allele at DAT1 gene could represent associations with other regions of this gene or with undiscovered markers, in linkage disequilibrium with this polymorphism; and (d) the effect of different 40 bp alleles should not be excluded, as already documented for the 48 bp VNTR at the DRD4 gene.¹⁶ Recently, two independent groups described different types of the 40 bp sequence at the DAT1 gene (Swanson, personal communication). Therefore, the worse response to MPH associated with the DAT1 gene observed in some studies could be because of a particular 10-repeat allele.

In addition to DAT1, other genes have been related to response to MPH. Hamarman¹⁷ found in a sample of 45 ADHD children that subjects with the 7-repeat allele at the DRD4 gene achieved less normalization of symptoms and required 1.5 times more MPH to achieve improvement than those without the 7-repeat allele (respectively, P=0.02 and P<0.001). Seeger et al¹⁸ reported that patients with hyperkinetic disorder (an European designation for a subgroup of ADHD children) who presented both the 7-repeat allele at the DRD4 locus and homozygosity for the long allele (L, insertion of a 44 bp sequence) at the serotonin transporter gene (5-HTT), that is, individuals with the DRD4*7/5-HTT LL genotype, showed a reduced improvement in general functioning during MPH treatment. These two studies open avenues for the evaluation of the effect of other genes, not only related to the dopaminergic system, as well as the interaction among genes on the response to the pharmacological treatment of the disorder.

Pharmacogenetic studies of ADHD are in their infancy. Multisite collaborative efforts to obtain larger samples, as the one currently in development through the ADHD molecular genetics network, should be implemented to allow the evaluation of the effect of different genes (individually or in combination) on the response of stimulants and other pharmacological agents used to treat this disorder.