ADHD is one of the most prevalent childhood-onset psychiatric disorders, affecting 5.3% of children and adolescents worldwide (Polanczyk et al., 2007). The rising rates of ADHD diagnosis and the accompanying rise of stimulant drug use have led to a public debate over the validity of the diagnosis. This problem is now being addressed by a multidisciplinary approach involving natural scientists, clinicians, social scientists, and ethicists (Singh, 2008). It is important to mention that the prevalence rate of ADHD is highly affected by methodological variables (i.e., diagnostic criteria, definition of impairment, source of information) and moderately affected by the geographic location (lower prevalence rate is reported in Africa and the Middle-East compared to North America or Europe, but no significant differences were found in prevalence rates between North America, Europe, South America, Asia, or Oceania) (Polanczyk et al., 2007). The prevalence of ADHD decreases with age, but this disorder still affects approximately 3% of adults, with male predominance in every age range (Faraone, 2004). The characteristic inattention and impulsivity/hyperactivity problems can be present together or separately in the diagnostic subtypes of the DSM-IV (combined, inattentive, and impulsive/hyperactive subtypes, American Psychiatric Association, 1994), symptoms are also listed in the review by Castellanos and Tannock (2002). In the ICD-10 Classification of Mental and Behavioural Disorders, the diagnosis of hyperkinetic disorder is divided to subtypes based on the presence or absence of conduct disorder (CD), listing the disturbance of activity and attention and hyperkinetic conduct disorder separately (World Health Organization, 1993). There are many other frequent comorbid conditions beside CD in ADHD children, such as oppositional defiant disorder (ODD), learning disorders, mood disorders, and anxiety disorders (Biederman, 2005). In adults with ADHD antisocial personality disorder (APD) and substance abuse have also high prevalence besides mood and anxiety disorders (Biederman, 2005). However complex and heterogeneous this disorder might seem, ADHD is still among the best validated diagnoses in child psychiatry. The 76% heritability estimate (Faraone et al., 2005) prompted researchers to reveal the underlying specific genetic factors of this disorder.
The dopamine hypothesis of ADHD is based on data from pharmacological and neuroimaging studies, and from animal models (Madras et al., 2005). The most frequently used drug in ADHD pharmacotherapy is methylphenidate, which blocks DAT and NET, and increases the level of extracellular dopamine. In an elegant SPECT study, Krause et al. (2000) showed that four weeks of methylphenidate treatment reduced the striatal DAT density in adult ADHD patients to the level of the control subjects. DAT1 knockout mice have been shown to exhibit hyperactivity, possibly through a hyperdopaminergic state (Gainetdinov et al., 1999). A more recent animal study showed that a targeted overexpression of DAT in the nucleus accumbens did not result in a change of motor activity but increased impulsivity and risk-taking behaviors in delayed reward experiments (Adriani et al., 2009). As a result, the most widely studied gene in ADHD is DAT1. A meta-analysis of the DAT1 3′ UTR VNTR from 2005 revealed a small but significant effect for the 10-repeat allele (OR of family-based studies = 1.13, 95% CI 1.03 – 1.24) (Faraone et al., 2005). The latest meta-analysis of case-control and family-based studies – taking into account the genetic heterogeneity as well – showed similar effect (OR = 1.12, 95% CI 1.00 – 1.27) (Gizer et al., 2009). Two additional DAT1 polymorphisms have been reported to have modest effects in ADHD: The 3- (or 6-) repeat allele of the intron 8 VNTR and the G-allele of rs27072 were indicated as ADHD risk alleles (Gizer et al., 2009). In addition, the 10-3(6) haplotype created from the two VNTRs was shown to be overtransmitted to combined type ADHD children in a family-based study of the International Multicenter ADHD Gene (IMAGE) project (Asherson et al., 2007). In terms of genes coding for synthesizing and catabolizing enzymes (TH, MAOA, COMT) or for the NET, no significant effect has been reported in the pooled analyses (Faraone et al., 2005; Gizer et al., 2009).
For the dopamine receptor genes, a meta-analysis revealed significant associations between the 7-repeat allele of the DRD4 exon 3 VNTR (OR = 1.33, 95% CI 1.15 – 1.54), the DRD4 −521 (rs1800955) T-allele (OR = 1.21, 95% CI 1.04 – 1.41), and the DRD5 148-bp allele (OR = 1.23, 95% CI 1.06 – 1.43) (Gizer et al., 2009). Taking into account the studies in Asian populations, the DRD5 148-bp allele and the DRD4 7-repeat allele were confirmed as risk factors; in addition, the DRD4 5-repeat allele also conferred increased risk in this meta-analysis (Li et al., 2006). Using haplotype analyses of the DRD4 polymorphisms resulted in mixed outcomes, with only the rs747302 C-allele being common in the indicated risk haplotypes (Barr et al., 2001; Mill et al., 2003; Lowe et al., 2004; Kereszturi et al., 2007). Gene × gene interaction findings showing the involvement of the DRD4 7-repeat allele and DAT1 10/10 genotype in ADHD have been reported in South-American populations with small sample sizes (Roman et al., 2001; Carrasco et al., 2006). Larger scale ADHD studies concentrated on gene × environment analyses. The accumulated studies assessing the DRD4 and DAT1 VNTRs in ADHD have not replicated convincingly the original interaction of the DRD4 7-repeat allele with season of birth (Thapar et al., 2007), and the interactions of the DAT1 10/10 genotype with maternal smoking or alcohol consumption during pregnancy (Wermter et al., 2010).
Increasing number of quantitative trait analyses have been reported using inattention or hyperactivity severity scales. In population-based cohorts, the DRD4 7-repeat allele was related to higher ADHD scores when the low-scoring and high-scoring groups were compared (Curran et al., 2001); however, in the Dunedin-study sample (a large, unselected birth cohort), this finding was not supported (Mill et al., 2002b). Later studies of ADHD children linked the DRD4 5′ UTR to inattention symptoms (Lasky-Su et al., 2008), whereas the DAT1 10-repeat allele was shown to be related to hyperactivity (Mill et al., 2005b). These results are in accordance with the predominant prefrontal localization of DRD4 (attention) and the key role of DAT in the basal ganglia (locomotion). In terms of gene × environment interaction findings, the association of increased hyperactive-impulsive symptoms with DAT1 10-repeat allele and with exposure to prenatal smoking has not been supported by the most recent studies (Thapar et al., 2007). However, in the English and Romanian Adoptees longitudinal study, the DAT1 high risk haplotype (10/10 and 3/3 genotypes together) and severe institutional deprivation were associated with increased ADHD scores, and a gene × environment interaction was also detected. Among those who experienced extended periods (more than 6 months) of psychosocial and nutritional deprivation in Romanian orphanages, children with the DAT1 high risk haplotype had significantly higher ADHD scores in adolescence compared to those with low genetic or environmental risk (Stevens et al., 2009). Also, COMT genotype was shown to interact with low socioeconomic status in a general population-based study of pre-adolescents: children with the Val/Val genotype showed higher ADHD scores compared to other genotype groups in the low socioeconomic status families (Nobile et al., 2010).
Another useful tool in genetic research is to investigate disorder subtypes. Persistent ADHD has been recently proposed as a specific subgroup because adult ADHD seems to have a higher rate of familiality (Faraone, 2004). Investigating the genetic risk factors over the course of ADHD showed that by age 25, a larger number of the DRD4 7-repeat allele carriers (7+) had persistent ADHD compared to the 7-repeat absent group (7−) (76% vs. 66%, Biederman et al., 2009). The authors did not observe any genetic effect for the DAT1 3′ UTR VNTR. However, a large-scale collaborative European study showed that the DAT1 9/9 genotype of the 3′ UTR VNTR and the 9-3(6) haplotype of the two VNTRs were associated with persistent ADHD (Franke et al., 2010). This finding might provide explanation for increased DAT density observed in adult ADHD (Krause et al., 2000) because both the 9- and the 3-repeat alleles have been associated with higher levels of DAT expression (see the functional polymorphism section for more information).
The DRD4 and DAT1 genetic findings can be easily built into neurobiological models, and polymorphisms of these genes have been widely investigated in relation to ADHD endophenotypes. Endophenotypes are intermediate phenotypes between diagnostic classifications and the causative biological factors, representing quantitative and heritable traits that are found in unaffected relatives of the affected individuals (Almasy and Blangero, 2001). As proposed by Castellanos and Tannock (2002), these phenotypes should be related to specific brain processes. These authors suggested locomotor hyperactivity, delay aversion (preference for a smaller but sooner-received reward over a larger but later-received reward), and executive function (e.g., working memory and response inhibition) deficits for ADHD endophenotypes. The characteristic frequent lapses of attention could be measured by intra-individual reaction time variability in tasks that require sustained attention (for example, continuous performance tests, see “Attentional performance” section). The familiality of the implicated phenotypes can be assessed by comparing affected and unaffected siblings. For example, analyzing reaction-time variability and accuracy parameters during a Go/NoGo test, a collaborative European study reported that unaffected siblings showed intermediate scores between the ADHD children and the controls (Uebel et al., 2010). Interestingly, in the incentive condition, the reaction-time was faster for ADHD children and their unaffected siblings, but not for controls, whereas the accuracy was improved in all of the groups, suggesting a familial motivational dysfunction in ADHD. In terms of the reward processes associated with ADHD, another study reported that ADHD children and their siblings chose smaller, sooner-received rewards over larger, later-received rewards, confirming the familiality of delay aversion (Marco et al., 2009).
In terms of dopaminergic genetic effects on specific neurobiological endophenotypes, the DRD4 7-repeat allele was associated with poor performance on intelligence measures, interference control, and working memory tasks (Loo et al., 2008), supporting the involvement of DRD4 in executive functions in the PFC. Another interesting result was presented after the analysis of two independent birth cohorts: dopaminergic genetic effect on IQ could be detected only among those diagnosed with ADHD. Carriers of the DRD4 7-repeat allele or the DAT1 10/10 genotype had lower IQs compared to those without either risk genotype, whereas those carrying both DRD4 and DAT1 risk factors had the lowest IQ (Mill et al., 2006). Concerning the DAT1 3′ UTR VNTR findings in relation to neuropsychological measures, such as sustained attention and executive functions, mostly negative findings have been reported (reviewed by Rommelse et al., 2008). These researchers also conducted a large-scale neuropsychological study among ADHD patients and their siblings in a wide age range (5–19 years) and assessed many SNPs within the DAT1 gene in addition to the two VNTRs; however, they did not find any significant associations between DAT1 genotypes or haplotypes and neuropsychological performance.
Another type of endophenotypes comes from imaging studies that measure brain region volumes with magnetic resonance imaging (MRI) or that measure brain activity with functional MRI (fMRI). Several structural changes have been described in ADHD children compared to controls, such as smaller total brain volume, a reduction in the size of the cortical lobes and the caudate nucleus (Valera et al., 2007). A large-scale MRI study reported a decreased cortical thickness in the right PFC and in the posterior parietal cortex in ADHD children compared to controls. In addition, this study reported a DRD4 genetic effect in both groups: the ADHD 7+ group had the thinnest cortex, followed by the ADHD 7− group, then by the healthy 7+ group, and finally by the healthy 7− group (Shaw et al., 2007). It is important to note that this regional thinning was most apparent in childhood and largely resolved by late adolescence. Comparative fMRI studies of ADHD patients and controls have shown a reduction in the activity level of the basal ganglia thalamo-cortical circuits (involving the cortical areas, amygdala, hippocampus, and basal ganglia) in the resting state and during neurocognitive tasks, such as the Go/NoGo, stop-signal, or Stroop tests. Whereas, in reward-related settings, a reduction in the level of activity was observed in the nucleus accumbens (reviewed by Durston et al., 2009). A dopaminergic gene effect was observed in ADHD boys and their unaffected male siblings but was not observed in control boys: The DAT1 9-repeat allele carriers showed greater levels of activity in the striatum during cognitive control trials among probands at risk for ADHD (Durston et al., 2008).
TS is a childhood-onset neuropsychiatric disorder that is characterized by multiple motor tics, i.e., involuntary, rapid, non-rhythmic skeletal movements and vocalizations. The prevalence of TS varies in different age-groups and is presently estimated as 1% of school-age children, whereas the prevalence of tic disorders (chronic motor or vocal tics) vary between 6 to 12% among children (Singer, 2005). Similar to ADHD, TS is more frequent in males (3:1 ratio). Family studies have shown that TS is highly heritable, and small-scale twin studies indicate that TS has 80–90% heritability (O’Rourke et al., 2009). TS is rarely present without comorbid conditions, and most often, it is accompanied by OCD and/or ADHD (Singer, 2005). This pattern of comorbidity indicates that genetic risk factors closely connected with TS may be responsible for a spectrum of disorders, including OCD on one side of the spectrum and ADHD on the other. At the two endpoints of the spectrum, OCD and ADHD can exist separately with unique etiologies (O’Rourke et al., 2009). The most widely studied candidate genes in TS belong to the dopamine system; however, serotonergic genes have also been analyzed because of the frequent comorbidity with OCD.
The dopamine hypothesis of TS is based on pharmacological and neuroimaging evidence. Classic antipsychotic (neuroleptic) drugs, such as haloperidol, can effectively suppress tics through DRD2 antagonism (Singer, 2005). Most SPECT studies have shown increased DAT densities in the striatum of TS patients compared to controls (Albin and Mink, 2006), and post-mortem analyses have revealed elevated DAT and DRD2 levels in the frontal brain regions (Yoon et al., 2007a). In the early 1990s, all of the investigated dopaminergic genes were excluded from TS pathology because of the assumed autosomal dominant inheritance in the linkage studies. Comings et al. (1996) suggested that polygenic inheritance was involved in TS, and subsequent genetic analyses were based on complex inheritance (e.g., by studying allele transmission in families).
The DRD2 TaqI A1-allele has been implicated in TS by a series of case-control association studies (summarized by Comings et al., 1996). To date, only one case-control study from Taiwan supported this finding (Lee et al., 2005), and family-based studies did not observe over-transmission of the A1-allele (Nothen et al., 1994; Diaz-Anzaldua et al., 2004). For the DRD4 VNTR, case-control studies (Cruz et al., 1997; Comings et al., 1999; Yoon et al., 2007b) and family-based studies (Grice et al., 1996; Hebebrand et al., 1997; Diaz-Anzaldua et al., 2004; Tarnok et al., 2007) resulted in contradictory findings. There are only a few published studies that investigated the involvement of the dopamine synthesizing and catabolizing enzyme genes with mostly negative findings for TH and COMT, and positive findings for MAOA (reviewed by O’Rourke et al., 2009). For the DAT1 3′ UTR VNTR, the categorical analyses showed a significant association (Comings et al., 1996) and a tendency towards an association with the 10-repeat allele (Diaz-Anzaldua et al., 2004). Later studies did not observe significant associations with TS diagnosis (Tarnok et al., 2007; Yoon et al., 2007b). However, after applying a dimensional approach, the 9-repeat allele was associated with a greater tic severity (Tarnok et al., 2007). Based on the results of the largest SPECT studies where the 9-repeat allele was linked to a higher DAT density, we may speculate that this gene variant is a risk factor for TS and/or for tic severity. Further studies should apply quantitative trait and endophenotype analyses for TS to yield more consistent results, in a manner similar to that done in ADHD studies during the last decade.