Another biological pathway to ADHD was discovered through study of coloboma mice, which have a hemizygous deletion of chromosome 2q (Wilson, 2000). This deletion region includes the gene encoding SNAP-25, a neuron-specific protein implicated in exocytotic neurotransmitter release. Coloboma mice show spontaneous hyperactivity, delays in achieving complex neonatal motor abilities and learning deficiencies. These problems are not seen if the mice are given a functioning SNAP25 gene through a transgenic procedure. Treatment with mixed amphetamine salts (Adderall) but not methylphenidate (Concerta, Metadate, Ritalin) reverses the hyperactivity, which is consistent with the mechanism of action of these medications.
Methylphenidate treats ADHD by blocking the dopamine transporter. Mixed amphetamine salts block the dopamine transporter but also facilitates the non-vesicular release of dopamine through reverse transport, which would be expected to reverse the deficits in exocytotic neurotransmitter release caused by the coloboma mutation.
To test genes associated with these biological hypotheses, candidate gene studies have used case-control or family-based designs. Case-control designs compare allele frequencies between patients with ADHD and non-ADHD controls. Alleles that confer risk for ADHD should be more common among patients with ADHD. The family-based design compares the alleles that parents transmit to children with ADHD to alleles parents do not transmit. If an allele increases the risk for ADHD, it should be more common among the transmitted alleles than the nontransmitted alleles. From both study designs, it is possible to derive an odds ratio (OR) that assesses the magnitude of the association between ADHD and the putative risk alleles. An OR of 1.0 indicates no association; >1.0 indicates the allele increases risk for ADHD; and <1.0 indicates the allele decreases the risk for ADHD.
Faraone et al. (in press) reviewed the ADHD candidate gene literature and examined pooled ORs for candidate genes that had been examined in at least three case-control or family-based association studies. Table 2 shows seven genes that provide statistically significant evidence of association with ADHD. Other genes have been studied, showing either negative (catechol O-methyltransferase, norepinephrine transporter) or unclear (D2 and D3 dopamine receptors; tyrosine hydroxylase; noradrenergic receptors ADRA2A, 2C and 1C; monoamine oxidase A; serotonin 2A receptor; the A4 and A7 acetylcholine receptor subunits, and the 2A glutamate) results.
The ORs for the positive associations range from 1.13 to 1.45 (Table 2). These small ORs are consistent with the idea that many genes of small effect mediate the genetic vulnerability to ADHD. Moreover, they suggest one explanation for the frequent failure to replicate initial reports of association: Many individual studies are underpowered to find significant associations if the effects are modest (Lohmueller et al., 2003). These small and sometimes inconsistent effects emphasize the need for future molecular genetic studies to implement strategies that will provide enough statistical power to detect small effects.
To address this project, we and colleagues from multiple sites in Europe conceived the International Multi-site ADHD Genetics (IMAGE) project. The main aim of IMAGE is to generate a clinical and genetic resource of 1,400 sibling pairs and their biological parents. The sibling pair design will enable the use of linkage analysis to identify chromosomal regions containing genes of moderate-to-large effect and association strategies to identify genes of small effect. The IMAGE group uses a novel approach by including ADHD probands and all available siblings in one dataset.
In order to increase confidence in the diagnosis and decrease potential genetic and etiological heterogeneity, probands are recruited from ADHD treatment centers and selected for DSM-IV combined subtype. All siblings of probands are included in the study, whether or not they have ADHD, and continuous rating scale measures of symptoms will be used to map genes.
The use of quantitative measures to map genes for common disorders is known as quantitative trait locus (QTL) analysis and reflects the view that genetic influences on ADHD are continuously distributed throughout the population (Asherson and IMAGE Consortium, 2004). This means that genes that increase risk for ADHD are also expected to influence individual differences in ADHD symptoms throughout the entire population. Similar approaches have been used for other common traits such as blood pressure (Harrap et al., 2002), cholesterol level (Lin, 2003) and dyslexia (Cardon et al., 1994).
