Mission: not impossible? Candidate gene studies in child psychiatric disorders

Etiologic subtypes of attention-deficit/hyperactivity disorder: brain imaging, molecular genetic and environmental factors and the dopamine hypothesis

Multiple theories of Attention-Deficit/Hyper-activity Disorder (ADHD) have been proposed, but one that has stood the test of time is the dopamine deficit theory. We review the narrow literature from recent brain imaging and molecular genetic studies that has improved our understanding of the role of dopamine in manifestation of symptoms of ADHD, performance deficits on neuropsychological tasks, and response to stimulant medication that constitutes the most common treatment of this disorder. First, we consider evidence of the presence of dopamine deficits based on the recent literature that (1) confirms abnormalities in dopamine-modulated frontal-striatal circuits, reflected by size (smaller-than-average components) and function (hypoactivation); (2) clarifies the agonist effects of stimulant medication on dopaminergic mechanisms at the synaptic and circuit level of analysis; and (3) challenges the most-widely accepted ADHD-related neural abnormality in the dopamine system (higher-than-normal dopamine transporter [DAT] density). Second, we discuss possible genetic etiologies of dopamine deficits based on recent molecular genetic literature, including (1) multiple replications that confirm the association of ADHD with candidate genes related to the dopamine receptor D4 (DRD4) and the DAT; (2) replication of differences in performance of neuropsychological tasks as a function of the DRD4 genotype; and (3) multiple genome-wide linkage scans that demonstrate the limitations of this method when applied to complex disorders but implicate additional genes that may contribute to the genetic basis of ADHD. Third, we review possible environmental etiologies of dopamine deficits based on recent studies of (1) toxic substances that may affect the dopamine system in early development and contribute substantially to the etiology of ADHD; (2) fetal adaptations in dopamine systems in response to stress that may alter early development with lasting effects, as proposed by the developmental origins of health and disease hypothesis; and (3) gene-environment interactions that may moderate selective damage or adaptation of dopamine neurons. Based on these reviews, we identify critical issues about etiologic subtypes of ADHD that may involve dopamine, discuss methods that could be used to address these issues, and review old and new theories that may direct research in this area in the future.

No evidence of association of two 5HT transporter gene polymorphisms and attention deficit hyperactivity disorder

OBJECTIVES

Three studies to date have found evidence (or a trend for evidence) of linkage and association between the long allele of the 44 base pair repeat insertion/deletion 5-HTT functional polymorphism (5-HTTLPR) and attention deficit hyperactivity disorder (ADHD). In an attempt to replicate these findings, we examined this polymorphism and a variable number tandem repeat in the second intron of 5-HTT for association with ADHD.

METHODS

One hundred and fifty children who met diagnostic criteria for ADHD and their parents (where available) were genotyped for these polymorphisms. Analysis was undertaken using the transmission disequilibrium test and haplotype analysis, as well as case-control comparisons using a control group of 121 individuals.

RESULTS

No association between either the 5-HTTLPR or the variable number tandem repeat (VNTR) in ADHD was found (extended transmission disequilibrium test; P=0.37 and P=0.62, respectively). Haplotype analysis was also non-significant. Further analysis revealed no evidence of association in the subgroups of those without conduct disorder and in medication non-responders.

CONCLUSIONS

Failure to replicate findings from previous studies may be due to a lack of statistical power. However, given recent findings by Kent et al. (2002) of association with another polymorphism in the 5HTT gene, we hypothesise that previous positive findings may have arisen by the LPR and VNTR being in linkage disequilibrium with the true susceptibility polymorphism.

Preferential transmission of interleukin-1 receptor antagonist alleles in attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder, where family data support substantial heritability.(1) To date, association studies focussed mainly on genes regulating dopaminergic neurotransmission.(2)Interleukin-1 (IL-1) activity in the brain has been implicated with differentiation of dopaminergic neurons(3,4) and modulation of central monoaminergic reactivity.(5) We investigated the role of interleukin-1 receptor antagonist (IL-1Ra) gene variable number tandem repeat (VNTR) polymorphism,(6) in a sample of 86 children with DSM-IV ADHD and their parents. Transmission disequilibrium analysis showed increased transmission of the IL-1Ra 4-repeat allele (chi(2) = 4.07, P = 0.04) and decreased transmission of the 2-repeat allele (chi(2) = 4.59, P = 0.03) to affected children. The 4-repeat allele was associated with a significantly increased risk for ADHD (chi(2) = 4.46, df 1, P = 0.035, RR = 1.292, 95% CI 1.01-1.66). The IL-1Ra 2-repeat allele was associated with a significantly decreased risk for ADHD (chi(2) = 4.65, df 1, P = 0.03, RR = 0.763, 95% CI 0.59-0.98). If replicated, this finding may point to a role for brain cytokine activity in the etiopathogenesis of ADHD.

Susceptibility genes for a trait measure of attention deficit hyperactivity disorder: a pilot study in a non-clinical sample of twins

Attention deficit hyperactivity disorder (ADHD) is a highly heritable disorder, and molecular genetic studies are underway, with most researchers focusing on identifying susceptibility genes in clinical samples with ADHD. An alternative approach is to search for quantitative trait loci underlying the trait measure of ADHD in non-clinical samples. Positive findings of association of the dopamine transporter DAT1 480 bp allele (allele 10) and the DRD4 7 repeat allele with clinical ADHD have been previously reported. In this pilot study, we examined these polymorphisms in a selected population-based sample of twins (50 high scoring pairs, 42 low scoring pairs). There was a trend for an increase in the frequency of the dopamine receptor DRD4 7 repeat allele in the high-scoring concordant monozygotic twins (odds ratio=1.4). Although this result was not statistically significant, the frequency of the 7 repeat allele was similar to that reported for our clinic sample of ADHD patients drawn from the same geographical area. There was a non-significant trend for an increased frequency of the DAT1 allele 10 (odds ratio=1.3). These results suggest that a molecular genetic study based on a questionnaire-derived measure of ADHD in a non-clinical sample is feasible and the results appear to be comparable with those from studies of clinical cases. However, sample size and power are key issues to consider when using this approach.

Approaches to gene mapping in complex disorders and their application in child psychiatry and psychology

BACKGROUND

Twin studies demonstrate the importance of genes and environment in the aetiology of childhood psychiatric and neurodevelopmental disorders. Advances in molecular genetics enable the identification of genes involved in complex disorders and enable the study of molecular mechanisms and gene--environment interactions.

AIMS

To review the role of molecular genetics studies in childhood behavioural and developmental traits.

METHOD

Molecular approaches to complex disorders are reviewed, with examples from autism, reading disability and attention-deficit hyperactivity disorder (ADHD).

RESULTS

The most robust finding in ADHD is the association of a variable number tandem repeat polymorphism in exon 3 of the DRD4 gene. Other replicated associations with ADHD are outlined in the text. In autism, there is a replicated linkage finding on chromosome 7. Linkage studies in reading disability have confirmed a locus on chromosome 6 and strongly suggest one on chromosome 15.

CONCLUSIONS

In the next 5--0 years susceptibility genes for these disorders will be established. Describing their relationship to biological and behavioural function will be a far greater challenge.

Examining for association between candidate gene polymorphisms in the dopamine pathway and attention-deficit hyperactivity disorder: a family-based study

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable childhood-onset psychiatric condition characterized by developmentally inappropriate inattention, hyperactivity, and impulsiveness. The pathophysiology of ADHD is currently unknown. However, the therapeutic effects of stimulant medication together with findings from animal and neuroimaging studies as well as from several molecular genetic studies of the dopamine receptor D4 gene and dopamine transporter gene have implicated involvement of the dopaminergic system. To test the dopaminergic hypothesis further, we have looked for association between ADHD and alleles of seven dopamine-related candidate genes using a family-based association approach in a sample of 150 children diagnosed with ADHD. We tested polymorphisms in genes encoding three dopamine receptors (DRD3, DRD4, and DRD5) and four dopamine-relevant enzymes: tyrosine hydroxylase [tyrosine hydroxylase (TH)], dopamine beta hydroxylase (DbetaH), catechol-O-methyltransferase (COMT), and monoamine oxidase A (MAOA). We were unable to detect a significant association with any of the polymorphisms genotyped, although there was a trend for preferential transmission of the DRD5 148 bp marker allele and the MAOA 122 bp marker allele. We conclude that none of the alleles we have tested makes a major contribution to ADHD, although much larger samples are required to exclude small effects.

Association study of a dopamine transporter polymorphism and attention deficit hyperactivity disorder in UK and Turkish samples

Molecular genetic studies in attention deficit hyperactivity disorder (ADHD) have focussed on candidate genes within the dopamine system, which is thought to be the main site of action of stimulant drugs, the primary pharmacological treatment of the disorder. Of particular interest are findings with the dopamine transporter gene (DAT1), since stimulant drugs interact directly with the transporter protein. To date, there have been eight published association studies of ADHD with a 480 base-pair allele of a variable number tandem repeat (VNTR) polymorphism in the 3'-untranslated region of the gene, five that support an association and three against. We have analysed the same VNTR marker in a dataset of UK Caucasian children and an independent dataset of Turkish Caucasian children with DSM-IV ADHD, using the transmission disequilibrium test (TDT). Results from the UK (chi(2) = 8.97, P = 0.001, OR = 1.95), but not the Turkish sample (chi(2) = 0.93, P = 0.34) support association and linkage between genetic variation at the DAT1 locus and ADHD. When considered alongside evidence from other published reports, there is only modest evidence for the association, consistent with a very small main effect for the 480-bp allele (chi(2) = 3.45, P = 0.06, OR = 1.15), however we find significant evidence of heterogeneity between the combined dataset (chi(2) = 22.64, df = 8, P = 0.004).

Attention deficit hyperactivity disorder (ADHD) and the dopamine D4 receptor gene: evidence of association but no linkage in a UK sample

Recent studies report association and linkage between attention deficit hyperactivity disorder (ADHD) and the 7-repeat allele of a 48 base-pair repeat in the dopamine D4 receptor gene (DRD4). We examined the frequency of this allele in a sample of probands with DSM-IV ADHD using a case-control design, as well as the transmission disequilibrium test (TDT) and haplotype-based haplotype relative risk (HHRR) in the subset of probands with DNA available from both parents. One hundred and thirty-two ADHD probands were compared with 189 controls (chi(2) = 6.17, 1 df, P = 0.01, OR = 1.73, 95% CI = 1.11--2.71). A total of 85 complete trios were available for within-family tests of association and linkage. Fifty-two heterozygous parents carrying one copy of the 7-repeat were informative for the TDT (29 transmitted vs 23 non-transmitted, chi(2) = 0.69). Analysis of the entire sample of 132 probands using TRANSMIT provided no additional evidence for excess transmission of the 7-repeat allele (58 transmitted vs 54 non-transmitted). HHRR gave similar results. We conclude that the case-control findings are likely to be falsely positive, resulting from genetic stratification. However we can not rule out alternative explanations of low statistical power and gene-environment correlation.

Genes and attention deficit hyperactivity disorder

The initial molecular genetic studies of attention deficit hyperactivity disorder (ADHD) evaluated two candidate genes (DAT and DRD4) suggested by dopamine theories of this common disorder and its treatment with stimulant medication. The initial reports of weak associations with ADHD have been replicated by many (but not all) investigators, as is expected for genes with small effects. This literature is reviewed, along with emerging literature generated by active research groups investigating additional genes that might contribute to the genetic basis of this complex disorder.