Genetic linkage studies

Genetic linkage was the first genome-wide method applied to ADHD. This method searches the genome for evidence that a segment of DNA is transmitted with a disorder within families. A review of the linkage literature found substantial disagreement about which chromosomal regions are linked to ADHD [67]. Although there is some overlap in “suggestive” findings, no finding met genome-wide significance [68]. To make sense of these results, Zhou et al. [69] applied Genome Scan Meta-Analysis. They found genome-wide significant linkage for a region on chromosome 16 between 64 Mb and 83 Mb. Because the linkage method only detects genetic variants that have large effects, the paucity of significant findings for other loci suggests that common DNA variants having a large effect on ADHD are unlikely to exist. Nearly all ADHD linkage studies have selected either sibling pairs or small families from outbred populations. Another approach is to assess for linkage in multigenerational population isolates. Arcos-Burgos et al. [70] used this strategy to study 16 multi-generational families from Colombia. In some of these families, they found evidence supporting linkage to chromosomes 4q13.2, 5q33.3, 8q11.23, 11q22, and 17p11. one region implicated LPHN3. For a review of supporting evidence, see ref. [71].

Candidate gene association studies

Early molecular genetic studies of ADHD sought to associate ADHD with genes that had some a priori plausibility as being involved in its etiology. Because the drugs that treat ADHD target dopaminergic or noradrenergic transmission, many studies examined “candidate genes” in these pathways. Results were frequently contradictory [26, 67]. In the meta-analyses of Gizer et al. [72], eight candidate DNA variants showed a statistically significant association with ADHD across multiple studies. These variants implicated six genes: the serotonin transporter gene (5HTT), the dopamine transporter gene (DAT1), the D4 dopamine receptor gene (DRD4), the D5 dopamine receptor gene (DRD5), the serotonin 1B receptor gene (HTR1B) and a gene coding for a synaptic vesicle regulating protein known as SNAP25. A meta-analysis covering all genetic association studies of adults with ADHD reported a significant association between adult ADHD and BAIAP2 (brain-specific angiogenesis inhibitor 1-associated protein 2). BAIAP2 is involved in neuronal proliferation, survival, and maturation and dendritic spine morphogenesis and may affect neuronal growth-cone guidance. These findings were significant even after Bonferroni correction [73]. For both the child and adult meta-analyses, the strength of each association, as measured by the odds ratio, is small, less than 1.5.

Many studies examined the dopamine transporter gene (SLC6A3), especially a 40-base pair variable number of tandem repeats regulatory polymorphism located in the 3′-untranslated region of the gene. This variant produces two common alleles with 9- and 10-repeats (9R and 10R). In humans, the 10R allele of this polymorphism has been associated with ADHD in youth [67] while the 9R allele is associated with ADHD in adults [74]. A meta-analysis showed that the 9R allele is associated with increased DAT activity in human adults as measured by positron emission tomography [75].

Genome-wide significant common variants

Genome-wide association studies (GWAS) scan the entire genome to detect common DNA variants having very small etiologic effects. By “common” we mean greater than 1% of the population. To do this, GWAS assay hundreds of thousands or even millions of single nucleotide polymorphisms (SNPs). Doing so has a statistical cost: to assert genome-wide statistical significance, an observed association must have a p value less than 0.00000005. This stringent p value needs very large samples.

The initial GWAS of ADHD [76,77,78,79,80,81,82,83,84,85,86] did not discover any DNA variants that achieved genome-wide significance, even when most of these samples were combined in meta-analysis having a sample size of 2064 trios (two parents and an ADHD child), 896 ADHD patients, and 2455 controls [87]. That study did find statistical significance for a group of candidate genes previously nominated by members of the International Multisite ADHD Genetics (IMAGE) project [88]. For a review of early GWAS studies, see Franke et al. [89]. Examination of the “molecular landscape” derived from the top findings from these initial GWAS studies along with other data concluded that genes regulating directed neurite outgrowth were strongly implicated in the etiology of ADHD [90]. Pathway and gene set analyses of GWAS data implicated pathways involved in the regulation of neurotransmitter release, neurite outgrowth and axon guidance as contributors to the etiology of ADHD [91,92,93].

A consortium of ADHD researchers completed a GWAS meta-analysis of 12 studies comprising 20,183 people with ADHD and 35,191 controls. For methodologic details about the studies contributing data to this meta-analysis, see Demontis et al. [94]. Twelve loci achieved genome-wide significance. None of the genome-wide significant SNPs showed significant heterogeneity between studies. Among the implicated genes, FOXP2 is especially notable because prior work had implicated it in adult ADHD (Ribases, 2012 #26445) and in speech and language disorders [95]. A FOXP2 knockout mouse study found that the gene regulates dopamine in ADHD-associated brain regions [96].

As described by Demontis et al. [94], other genes implicated by the genome-wide significant loci have relevant biological roles. DUSP6 regulates neurotransmitter homeostasis by affecting dopamine levels in the synapses. SEMA6D is expressed in the brain. It regulates neuronal wiring during embryonic development. ST3GAL3 harbors missense mutations associated with ID. LINC00461 is expressed in brain and includes variants associated with educational attainment. Another gene implicated at that locus is MEF2C, which has been associated with ID and several psychiatric disorders.

The consortium conducted several gene set analyses including three sets of genes regulated by FOXP2: (1) genes enriched in wild-type versus control FOXP2 knockout mouse brains; (2) genes showing differential expression in wild-type versus FOXP2 knockout mouse brains; and (3) genes enriched in basal ganglia or inferior frontal cortex from human fetal brain samples. None of these sets were associated with ADHD. Also, non-significant was a set of candidate genes for ADHD previously proposed by a panel of ADHD experts [88]. Among these, only SLC9A9 showed a weak association with ADHD. No Gene Ontology gene sets attained statistical significance but a set of genes showing high intolerance to loss of function did associate with ADHD.

Common variant ADHD as a polygenic disorder

The GWAS analyses also showed that much of ADHD’s heritability is due to the polygenic effects of many common variants each having very small effects. The SNP heritability was 0.22, which is about one-third of ADHD’s heritability computed from twin studies [97]. The polygenic architecture for ADHD was confirmed by estimating polygenic risk scores in one subset of the sample and showing that it predicted ADHD, in a dose-dependent manner, in a validation subset. As seen for other psychiatric disorders [98], the variance explained by these risk scores was low (5.5%).

Further evidence for the validity of the ADHD’s polygenic background comes from analyses showing that the relevant SNPs were enriched for annotations implicating conserved regions of the genome (which are known to have biological significance) and for regulatory elements specific to the central nervous system. The discovery of a polygenic susceptibility to ADHD does not show which DNA variants comprise the susceptibility. It does, however, support the idea that more genome-wide significant variants will be discovered in larger samples.

Martin et al. [99] showed that ADHD’s polygenic liability derived from a clinical sample predicted ASD traits in a population sample, which confirms twin study data [48, 51] and gene set analyses [100] showing genetic overlap between ADHD and ASDs. The polygenic liability score derived from Martin et al.’s ADHD case-control clinical sample also predicted both inattention and hyperactivity in the general population. This latter finding was replicated by Groen-Blokhuis et al. [101] who found that ADHD polygenic risk scores significantly predicted both parent and teacher ratings of attention in preschool- and school-aged children in the population. Likewise, Stergiakouli et al. [102] showed that the polygenic liability for ADHD traits in a population sample predicted ADHD clinical diagnoses in a case-control study. These results confirmed conclusions from twin studies that the liability for clinically defined ADHD is the extreme of a trait that varies continuously in the population [13].

Other polygenic score studies are confirming cross-disorder genetic associations previously predicted by family and twin studies. We have long known that ADHD co-occurs with conduct disorder. Both family and twin studies have implicated shared genes in this association [38, 103,104,105]. Consistent with this prior work, Hamshere et al. [106] reported a high polygenic risk for ADHD among children with comorbid conduct problems. In a large population study, Larsson et al. [107] reported that the relatives of ADHD individuals had an increased risk for schizophrenia and bipolar disorder. Consistent with that report, the polygenic risk score derived from a large GWAS of schizophrenia significantly discriminated ADHD cases from controls [108]. This discrimination was strongest for alleles that were risk alleles for both adult schizophrenia and adult bipolar disorder, which confirms prior family and twin data suggesting a genetic link between ADHD and bipolar disorder [109]. Moreover, a joint GWAS of ADHD and bipolar disorder reported a significant correlation between the polygenic scores of ADHD and bipolar disorder and also identified genome-wide significant loci for the two disorders [110]. Similarly, prior reports of familial co-transmission of ADHD and depression [54] have been extended by showing shared SNP heritability between the two disorders [98]. Using a novel drug challenge paradigm, Hart et al. [111] found that the polygenic scores for both schizophrenia and ADHD were associated with the euphoric response to amphetamine, which suggests that the genetic association between these disorders may be due to variants in the neural systems regulating the euphoric response to amphetamine.

Using GWAS results from many studies, it is possible to compute genetic correlations that indicate the degree to which the polygenic architectures of two disorders or traits overlap. When Demontis et al. [94] correlated ADHD’s polygenic risk with 220 disorders and traits, many highly significant correlations emerged. Figure 3 shows some of the most significant of these correlations (each passing the Bonferroni significance threshold). Some of these genetic correlations fit with prior expectations (e.g., with neuroticism, depression and the cross disorder GWAS). Others are consistent with the clinical epidemiology of ADHD (e.g., with obesity, IQ, smoking and school achievement). In some cases, these significant correlations offer new directions for understanding comorbidity. For example, some have interpreted the comorbidity between ADHD and obesity, which has been confirmed via meta-analysis [112], as being caused by the impulsivity associated with ADHD. The genetic correlation data suggest that shared genetic risk factors, and an underlying shared pathophysiology, account for this comorbidity.

Fig. 3 Genetic correlations of ADHD with other traits based on LD score regression Full size image

Some of the genetic correlations in Fig. 3 are entirely novel. These include ADHD’s genetic correlations with medical outcomes (lung cancer, coronary artery disease, parents’ age at death) and with demographics (number of children in the family, age first child born). There are, however, some consistent findings in the prior literature, which suggest that people with ADHD are more likely to have larger families [113] and more likely to die prematurely [114].