Dopa decarboxylase gene polymorphisms and attention deficit hyperactivity disorder (ADHD): no evidence for association in the Irish population

Testing candidate genes for attention-deficit/hyperactivity disorder in fruit flies using a high throughput assay for complex behavior

Fruit flies are important model organisms for functional testing of candidate genes in multiple disciplines, including the study of human diseases. Here we use a high-throughput locomotor activity assay to test the response on activity behavior of gene disruption in Drosophila melanogaster. The aim was to investigate the impact of disruption of 14 candidate genes for human attention-deficit/hyperactivity disorder (ADHD) on fly behavior. By obtaining a range of correlated measures describing the space of variables for behavioral activity we show, that some mutants display similar phenotypic responses, and furthermore, that the genes disrupted in those mutants had common molecular functions; namely processes related to cGMP activity, cation channels and serotonin receptors. All but one of the candidate genes resulted in aberrant behavioral activity, suggesting involvement of these genes in behavioral activity in fruit flies. Results provide additional support for the investigated genes being risk candidate genes for ADHD in humans.

Expanding the toolbox of ADHD genetics. How can we make sense of parent of origin effects in ADHD and related behavioral phenotypes?

Genome-wide association (GWA) studies have shown that many different genetic variants cumulatively contribute to the risk of psychiatric disorders. It has also been demonstrated that various parent-of-origin effects (POE) may differentially influence the risk of these disorders. Together, these observations have provided important new possibilities to uncover the genetic underpinnings of such complex phenotypes. As POE so far have received little attention in neuropsychiatric disorders, there is still much progress to be made. Here, we mainly focus on the new and emerging role of POE in attention-deficit hyperactivity disorder (ADHD). We review the current evidence that POE play an imperative role in vulnerability to ADHD and related disorders. We also discuss how POE can be assessed using statistical genetics tools, expanding the resources of modern psychiatric genetics. We propose that better comprehension and inspection of POE may offer new insight into the molecular basis of ADHD and related phenotypes, as well as the potential for preventive and therapeutic interventions.

The DOPA decarboxylase (DDC) gene is associated with alerting attention

DOPA decarboxylase (DDC) is involved in the synthesis of dopamine, norepinephrine and serotonin. It has been suggested that genes involved in the dopamine, norepinephrine, and cholinergic systems play an essential role in the efficiency of human attention networks. Attention refers to the cognitive process of obtaining and maintaining the alert state, orienting to sensory events, and regulating the conflicts of thoughts and behavior. The present study tested seven single nucleotide polymorphisms (SNPs) within the DDC gene for association with attention, which was assessed by the Attention Network Test to detect three networks of attention, including alerting, orienting, and executive attention, in a healthy Han Chinese sample (N=451). Association analysis for individual SNPs indicated that four of the seven SNPs (rs3887825, rs7786398, rs10499695, and rs6969081) were significantly associated with alerting attention. Haplotype-based association analysis revealed that alerting was associated with the haplotype G-A-T for SNPs rs7786398-rs10499695-rs6969081. These associations remained significant after correcting for multiple testing by max(T) permutation. No association was found for orienting and executive attention. This study provides the first evidence for the involvement of the DDC gene in alerting attention. A better understanding of the genetic basis of distinct attention networks would allow us to develop more effective diagnosis, treatment, and prevention of deficient or underdeveloped alerting attention as well as its related prevalent neuropsychiatric disorders.

Parent-of-origin effects of FAS and PDLIM1 in attention-deficit/hyperactivity disorder

BACKGROUND

Previous studies have suggested that there may be a parent-of-origin effect for attention-deficit/hyperactivity disorder(ADHD) candidate genes. The objective of the present study was to investigate parent-of-origin effects using a genome-wide association analysis of the International Multicentre ADHD Genetics (IMAGE) study sample.

METHODS

Family-based association analysis for ADHD using 846 ADHD probands and their parents was performed using the PLINK program, and parent-of-origin effects were studied using a Z score for the difference in paternal versus maternal odds ratios.

RESULTS

We identified 44 single nucleotide polymorphisms (SNPs) showing parent-of-origin effects at a significance level of p < 0.001. The most significant SNP, rs7614907, is at position 3q13.33 in the CDGAP gene (p = 0.000064 for parent-of-origin effect). Furthermore, 2 genes (FAS and PDLIM1) showed moderate parent-of-origin effects (p = 0.00086 for rs9658691 and p = 0.00077 for rs11188249) and strong maternal transmission (p = 0.000059 for rs9658691 and p = 0.0000068 for rs11188249). In addition, ZNF775 showed a moderate parent-of-origin effect (p = 0.00036 for rs7790549) and strong paternal transmission (p = 0.000041 for rs7790549).

LIMITATIONS

We only had 1 sample available for analysis.

CONCLUSION

These results suggest several genes or regions with moderate parent-of-origin effects, and these findings will serve as a resource for replication in other populations to elucidate the potential role of these genetic variants in ADHD.

Epigenetics: genetics versus life experiences

Epigenetics is the field of research that examines alterations in gene expression caused by mechanisms other than changes in DNA sequence. ADHD is highly heritable; however, epigenetics are considered relevant in potentially explaining the variance not accounted for by genetic influence. In this chapter, some of the well-known processes of epigenetics, such as chromosome organization, DNA methylation, and effects of transcriptional factors are reviewed along with studies examining the role of these processes in the pathophysiology of ADHD. Potential epigenetic factors conferring risk for ADHD at various developmental stages, such as alcohol, tobacco, toxins, medications, and psychosocial stressor are discussed. Animal studies investigating ADHD medications and changes in CNS Gene/Protein Expression are also explored since they provide insight into the neuronal pathways involved in ADHD pathophysiology. The current limited data suggest that identification of the epigenetic processes involved in ADHD is extremely important and may lead to potential interventions that may be applied to modify the expression of deleterious, as well as protective, genes.

Molecular genetics of attention deficit hyperactivity disorder

Although twin studies demonstrate that ADHD is a highly heritable condition, molecular genetic studies suggest that the genetic architecture of ADHD is complex. The handful of genome-wide linkage and association scans that have been conducted thus far show divergent findings and are, therefore, not conclusive. Similarly, many of the candidate genes reviewed here (ie, DBH, MAOA, SLC6A2, TPH-2, SLC6A4, CHRNA4, GRIN2A) are theoretically compelling from neurobiological systems perspective but available data are sparse and inconsistent. However, candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder, with meta-analyses supportive of a role of the genes coding for DRD4, DRD5, SLC6A3, SNAP-25, and HTR1B in the etiology of ADHD.

A high-density single-nucleotide polymorphism screen of 23 candidate genes in attention deficit hyperactivity disorder: suggesting multiple susceptibility genes among Chinese Han population

Attention deficit hyperactivity disorder (ADHD) is a common childhood-onset behavioral disorder with a definite genetic component. The search for genes predisposing to ADHD has focused on genes involved in the regulation of monoamine systems. In this study, we emphasized genes that underlie various aspects of dopamine, norepinephrine and serotonin neurotransmissions and performed a comprehensive association analysis by screening with 245 single-nucleotide polymorphisms (SNPs) of 23 candidate genes in a sample of Chinese Han descent. A total of 182 DSM-IV ADHD children and 184 healthy controls were genotyped and analyzed with an average density of one SNP every 6.1 kb. Both single-SNP and multi-marker haplotype analyses were implemented to exploit association signal for ADHD and its diagnostic subtypes. Empirical P-values were derived on the basis of 5000 permutations to evaluate gene-wide statistical significance. MAOA yielded highly suggestive evidence of association (empirical P<0.01, OR=1.94) with ADHD. For inattentive ADHD, MAOA, DDC and SYP showed suggestive evidence of association (empirical P<0.05). ADRA2C achieved suggestive significance (empirical P<0.05) for ADHD combined type. Additionally, for six genes (SNAP25, NET1, DBH, CHRNA4, DRD3 and SYT1) we detected one or more SNPs with nominal P-values</=0.05. This study has identified several genes as promising susceptibility loci for ADHD. Replication efforts and further investigations remain necessary to provide definite proof of association.

Exploration of 19 serotoninergic candidate genes in adults and children with attention-deficit/hyperactivity disorder identifies association for 5HT2A, DDC and MAOB

Attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric disorder in which different genetic and environmental susceptibility factors are involved. Several lines of evidence support the view that at least 30% of ADHD patients diagnosed in childhood continue to suffer the disorder during adulthood and that genetic risk factors may play an essential role in the persistence of the disorder throughout lifespan. Genetic, biochemical and pharmacological studies support the idea that the serotonin system participates in the etiology of ADHD. Based on these data, we aimed to analyze single nucleotide polymorphisms across 19 genes involved in the serotoninergic neurotransmission in a clinical sample of 451 ADHD patients (188 adults and 263 children) and 400 controls using a population-based association study. Several significant associations were found after correcting for multiple testing: (1) the DDC gene was strongly associated with both adulthood (P=0.00053; odds ratio (OR)=2.17) and childhood ADHD (P=0.0017; OR=1.90); (2) the MAOB gene was found specifically associated in the adult ADHD sample (P=0.0029; OR=1.90) and (3) the 5HT2A gene showed evidence of association only with the combined ADHD subtype both in adults (P=0.0036; OR=1.63) and children (P=0.0084; OR=1.49). Our data support the contribution of the serotoninergic system in the genetic predisposition to ADHD, identifying common childhood and adulthood ADHD susceptibility factors, associations that are specific to ADHD subtypes and one variant potentially involved in the continuity of the disorder throughout lifespan.

Parent of origin effects in attention/deficit hyperactivity disorder (ADHD): analysis of data from the international multicenter ADHD genetics (IMAGE) program

There are conflicting reports suggesting that the parental origin of transmitted risk alleles may play a role in the etiology of attention deficit/hyperactivity disorder (ADHD). A recent report by Hawi and colleagues observed a generalized paternal over-transmission of alleles associated with ADHD. This was not replicated in more recent studies. Using data from a large multicenter study we examined the overall and gene-specific parent of origin effect in 554 independent SNPs across 47 genes. Transmission disequilibrium and explicit parent of origin test were performed using PLINK. Overall parent of origin effect was tested by Chi-square. There was no overall parent of origin effect in the IMAGE sample (chi(1)(2) = 1.82, P = 0.117). Five markers in three genes, DDC, TPH2, and SLC6A2 showed nominal association (P < 0.01) with ADHD combined subtype when restricted to maternal or paternal transmission only. Following the initial report by Hawi and co-workers three studies, including this one, found no evidence to support an overall parent of origin effect for markers associated with ADHD. We cannot however, exclude gene-specific parent of origin effect in the etiology ADHD.

Dopamine system genes and ADHD: a review of the evidence

The search for genes influencing the development of attention-deficit/hyperactivity disorder (ADHD) has identified a number of associated genes within, or influencing, the dopamine neurotransmitter system. The focus on this system as the site of genetic susceptibility was prompted by information from animal models, particularly transgenics, as well as the mechanism of action of the psychostimulants, the primary pharmacological treatment for ADHD. Thus far, genes in the dopamine system reported as associated with ADHD, by at least one study, include the dopamine transporter, the dopamine receptors D1, D4 and D5, as well as genes encoding proteins that control the synthesis, degradation and release of dopamine. For some of these genes, replication across studies provides evidence supporting the relationship; however, for others, the data is far from conclusive and further work is needed. The quick progress in the genetic findings was initially surprising given the complexity of the phenotype and the relatively small sample sizes used in the initial studies. However, the high heritability of ADHD, as indicated by twin studies, may have contributed to the success. The genes studied so far are estimated to contribute only weakly or moderately to the risk for the development of ADHD. This may be because these genes, in fact, make only a small contribution. However, few studies have comprehensively examined the genetic information across the gene. This will lead to underestimates of risk if the polymorphism(s) tested is/are not the functional change(s) actually contributing to the genetic susceptibility and if linkage disequilibrium between tested marker(s) and causal variant(s) is weak, or if there is substantial allelic heterogeneity. While the studies thus far are very promising, virtually nothing is known on precisely how genetic variation in these genes actually contributes to risk; thus, functional studies are now required.

Genetics of attention deficit hyperactivity disorder

Results of behavioral genetic and molecular genetic studies have converged to suggest that both genetic and nongenetic factors contribute to the development of attention deficit hyperactivity disorder (ADHD). Family, twin, and adoption studies provide compelling evidence that genes play a strong role in mediating susceptibility to ADHD. In contrast to a handful of genome-wide scans conducted thus far, many candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder. Yet, even these associations are small and consistent with the idea that the genetic vulnerability to ADHD is mediated by many genes of small effects. These small effects emphasize the need for future candidate gene studies to implement strategies that will provide enough statistical power to detect such small effects.

Genomic imprinting of Dopa decarboxylase in heart and reciprocal allelic expression with neighboring Grb10

By combining a tissue-specific microarray screen with mouse uniparental duplications, we have identified a novel imprinted gene, Dopa decarboxylase (Ddc), on chromosome 11. Ddc_exon1a is a 2-kb transcript variant that initiates from an alternative first exon in intron 1 of the canonical Ddc transcript and is paternally expressed in trabecular cardiomyocytes of the embryonic and neonatal heart. Ddc displays tight conserved linkage with the maternally expressed and methylated Grb10 gene, suggesting that these reciprocally imprinted genes may be coordinately regulated. In Dnmt3L mutant embryos that lack maternal germ line methylation imprints, we show that Ddc is overexpressed and Grb10 is silenced. Their imprinting is therefore dependent on maternal germ line methylation, but the mechanism at Ddc does not appear to involve differential methylation of the Ddc_exon1a promoter region and may instead be provided by the oocyte mark at Grb10. Our analysis of Ddc redefines the imprinted Grb10 domain on mouse proximal chromosome 11 and identifies Ddc_exon1a as the first example of a heart-specific imprinted gene.

Genetische Befunde bei der Aufmerksamkeitsdefizit- und Hyperaktivitätsstörung (ADHS)

Zusammenfassung: Die Aufmerksamkeitsdefizit- und Hyperaktivitätsstörung (ADHS) ist mit einer Prävalenz von 3-7% eine häufige kinder- und jugendpsychiatrische Störung. Auf der Basis formalgenetischer Studien ergibt sich eine Heritabilitätsschätzung von 60-80% für ADHS mit einem ca. 5-fach erhöhten Risiko für erstgradige Verwandte von Betroffenen. Bislang vier Genomscans lieferten potentiell relevante chromosomale Regionen, insbesondere den einheitlichen Kopplungsbefund auf 5p13. Aus einer Vielzahl von Assoziationsstudien zu Kandidatengenen deuten aktuelle Metaanalysen auf die Relevanz der Gene der dopaminergen Rezeptoren DRD4 und DRD5 sowie des serotonergen Rezeptors HTR1B und des Synaptosomal Assoziierten Proteins (SNAP-25). In Tiermodellen liegen vorwiegend Paradigmen für Hyperaktivität vor; diese sind in knockout- und Quantitative Trait Loci (QTL) Designs mit viel versprechenden Ergebnissen zum dopaminergen System untersucht worden. Es ist davon auszugehen, dass erst das Zusammenwirken verschiedener Gen-Varianten mit jeweils moderatem bis hin zu kleinem Effekt den Phänotyp ADHS bedingen (Oligo-/ Polygenie) und bei verschiedenen Betroffenen unterschiedliche Kombinationen von prädisponierenden Gen-Polymorphismen zu ADHS führen können. Entsprechend sind für molekulargenetische Studien große Fallzahlen notwendig und die bisherigen Befunde als vorläufig zu interpretieren. Zukunftsweisend für die molekulargenetische Aufklärung von ADHS sind SNP-basierte Genomscans, mit denen 10 000-1 000 000 einzelne Polymorphismen (SNPs) gleichzeitig untersucht werden können. Tiermodelle liefern Hinweise auf die Funktion relevanter Kandidatengene und tragen zur Erweiterung der bislang teilweise widersprüchlichen Kenntnisse zur Neurobiologie des ADHS bei.

The analysis of 51 genes in DSM-IV combined type attention deficit hyperactivity disorder: association signals in DRD4, DAT1 and 16 other genes

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder, starting in early childhood and persisting into adulthood in the majority of cases. Family and twin studies have demonstrated the importance of genetic factors and candidate gene association studies have identified several loci that exert small but significant effects on ADHD. To provide further clarification of reported associations and identify novel associated genes, we examined 1,038 single-nucleotide polymorphisms (SNPs) spanning 51 candidate genes involved in the regulation of neurotransmitter pathways, particularly dopamine, norepinephrine and serotonin pathways, in addition to circadian rhythm genes. Analysis used within family tests of association in a sample of 776 DSM-IV ADHD combined type cases ascertained for the International Multi-centre ADHD Gene project. We found nominal significance with one or more SNPs in 18 genes, including the two most replicated findings in the literature: DRD4 and DAT1. Gene-wide tests, adjusted for the number of SNPs analysed in each gene, identified associations with TPH2, ARRB2, SYP, DAT1, ADRB2, HES1, MAOA and PNMT. Further studies will be needed to confirm or refute the observed associations and their generalisability to other samples.

Intronic variants in the dopa decarboxylase ( DDC ) gene are associated with smoking behavior in European-Americans and African-Americans

We report here a study considering association of alleles and haplotypes at the DOPA decarboxylase (DDC) locus with the DSM-IV diagnosis of nicotine dependence (ND) or a quantitative measure for ND using the Fagerstrom Test for Nicotine Dependence (FTND). We genotyped 18 single nucleotide polymorphisms (SNPs) spanning a region of approximately 210 kb that includes DDC and the genes immediately flanking DDC in 1,590 individuals from 621 families of African-American (AA) or European-American (EA) ancestry. Evidence of association (family-based tests) was observed with several SNPs for both traits (0.0002<or=P<or=0.04). The most significant result was obtained for the relationship of FTND score to SNP rs12718541 (AA families: P=0.002; EA families: P=0.03; all families: P=0.0002) which is in the same intron as the splice site for a neuronal isoform of human DDC lacking exons 10-15. Haplotype analysis did not reveal any SNP combination with stronger evidence for association than rs12718541 alone. Although sequence analysis suggests that rs12718541 may be an intronic splicing enhancer, further studies are needed to determine whether a direct link exists between an alternatively spliced form of DDC and predisposition to ND. These findings confirm a previous report of association of DDC with ND, localize the causative variants to the 3' end of the coding region and extend the association to multiple population groups.

The genetics of attention deficit hyperactivity disorder

Over the past 15 years, considerable progress has been made in understanding the etiology of childhood Attention Deficit Hyperactivity Disorder (ADHD), largely due to the publication of numerous twin studies which are consistent in suggesting substantial genetic influences (i.e., heritabilities ranging from 60% to 90%), non-shared environmental influences that are small-to-moderate in magnitude (i.e., ranging from 10% to 40%), and little-to-no shared environmental influences. Following from these quantitative genetic findings, numerous molecular genetic studies of association and linkage between ADHD and a variety of candidate genes have been conducted during the past 10 years. The majority of the candidate genes studied underlie various facets of the dopamine, norepinephrine, and serotonin neurotransmitter systems, although the etiological role of candidate genes outside of neurotransmitter systems (e.g., involved in various aspects of brain and nervous system development) have also been examined. In this paper, we review recent findings from candidate gene studies of childhood ADHD and highlight those candidate genes for which associations are most replicable and which thus appear most promising. We conclude with a consideration of some of the emerging themes that will be important in future studies of the genetics of ADHD.

Méta-analyse des gènes candidats dans le trouble déficit attentionnel avec hyperactivité (TDAH)

Attention-deficit hyperactivity disorder (ADHD) is a common behavioral disorder observed during childhood, detected in 3% to 5% of school-age children. The disorder is characterised by marked inattention, hyperactivity, and impulsiveness. In most cases, symptoms can be treated by catecholamine-releasing drugs, such as methylphenidate. Children with ADHD are at higher risk for substance abuse and oppositional, conduct and mood disorders. Familial and adoption studies shed light on the genetic vulnerability of ADHD. Twin studies estimated the broad heritability to range between 40% and 90%. The mode of transmission is yet unknown, but is likely polygenic. Molecular genetic studies in ADHD should contribute to a greater understanding of the pathophysiology of the disorder (genetics of the vulnerability), and could help to select a more rational type of treatment (pharmacogenetic). Family-based association studies already performed are reviewed in this manuscript. Association studies, using haplotype relative risk (HRR) or transmission disequilibrium test (TDT) have focused on candidate genes which code for proteins potentially involved in the etiopathogenesis of the disorder. Genes involved in dopamine, serotonin, and noradrenalin systems have thus been assessed for their role in core features of ADHD, such as motor overactivity, inattention, and impulsiveness. According to a meta-analysis, the DAT1 gene, an obvious candidate gene in ADHD vulnerability, does not appear to be involved (OR = 1.13, p = 0.21). On the other hand, DRD4 (OR = 1.26, p = 0.01) and DRD5 (OR = 1.4, p = 0.01) are significantly associated to ADHD according to the present meta-analysis, confirming previous ones. Recent studies showed a trend for an association between one allele of the 5-HTT (considering case-control studies) and DBH (OR = 1.27, p = 0.06) genes and ADHD, but these positive findings have to be replicated. ADHD is a complex disorder with potentially many different risk factors. Genetic and phenotypic heterogeneity could explain why some association studies are positive, whereas others are negative. For instance, different developmental pathways are likely to lead to similar clinical outcomes. More clear-cut phenotypes, such as ADHD with conduct disorder, or ADHD with bipolar disorder, could be more homogenous, the genes involved being therefore more easy to detect. These phenotypes are beginning to be specifically studied in molecular genetics. In addition, the development of pharmacogenetics could help to identify predictors of clinical response for a specific type of treatment, which would be clearly helpful in clinical practice.

Haplotype analysis indicates an association between the DOPA decarboxylase (DDC) gene and nicotine dependence

DOPA decarboxylase (DDC; also known as L-amino acid decarboxylase; AADC) is involved in the synthesis of dopamine, norepinephrine and serotonin. Because the mesolimbic dopaminergic system is implicated in the reinforcing effects of many drugs, including nicotine, the DDC gene is considered a plausible candidate for involvement in the development of vulnerability to nicotine dependence (ND). Further, this gene is located within the 7p11 region that showed a 'suggestive linkage' to ND in our previous genome-wide scan in the Framingham Heart Study population. In the present study, we tested eight single nucleotide polymorphisms (SNPs) within DDC for association with ND, which was assessed by smoking quantity (SQ), the heaviness of smoking index (HSI) and the Fagerstrom test for ND (FTND) score, in a total of 2037 smokers and non-smokers from 602 nuclear families of African- or European-American (AA or EA, respectively) ancestry. Association analysis for individual SNPs using the PBAT-GEE program indicated that SNP rs921451 was significantly associated with two of the three adjusted ND measures in the EA sample (P=0.01-0.04). Haplotype-based association analysis revealed a protective T-G-T-G haplotype for rs921451-rs3735273-rs1451371-rs2060762 in the AA sample, which was significantly associated with all three adjusted ND measures after correction for multiple testing (min Z=-2.78, P=0.006 for HSI). In contrast, we found a high-risk T-G-T-G haplotype for a different SNP combination in the EA sample, rs921451-rs3735273-rs1451371-rs3757472, which showed a significant association after Bonferroni correction with the SQ and FTND score (max Z=2.73, P=0.005 for FTND). In summary, our findings provide the first evidence for the involvement of DDC in the susceptibility to ND and, further, reveal the racial specificity of its impact.

Attention-deficit hyperactivity disorder (ADHD)

Approaches to the diagnosis and treatment of attention-deficit hyperactivity disorder (ADHD) are undergoing a major change as a result of information from studies on the genetics of ADHD and the use of new neuroimaging technologies. Moreover, pharmacogenomics, although still in its infancy, will provide a basis for much more sophisticated treatment strategies for ADHD, particularly once more information is available about the genetics of ADHD. Even at this point in time, there is some pertinent information available that, although not ready for application in clinical settings, nonetheless provides a broader perspective for the clinician. In terms of etiology, ADHD is a neuropsychiatric disorder. There is a genetic basis in about 80% of the cases, involving a number of different genes, and in about 20% of the cases, ADHD is the result of an acquired insult to the brain. Some individuals likely have both genetic and acquired forms. Although medication works well in many cases of ADHD, optimal treatment of ADHD requires integrated medical and behavioral treatment. The family plays a crucial role in the management of children with ADHD. Because there is often a very high degree of comorbidity between ADHD and learning disabilities, teachers also have a great deal to contribute in the day-to-day management of these children. Early recognition and treatment prevent the development of more serious psychopathology in adolescence and adulthood.

Molecular genetics of attention-deficit hyperactivity disorder (ADHD): an update

Attention-deficit hyperactivity disorder (ADHD) is a heritable and behavioral condition of childhood, affecting 5-10% of school-age children worldwide. Affected patients exhibit various behavioral problems such as carelessness, restlessness, disobedience and failure to stay quiet in class. The etiology of ADHD is not known. However, family, twin and adoption studies have provided strong evidence for a genetic etiology of the disorder. A genome-wide scan has identified six chromosomal loci with LOD scores suggestive of linkage. Animal studies suggest the involvement of the brain dopamine pathway and its alteration in ADHD but there is no direct evidence to support this hypothesis. In addition, there are at least 20 candidate genes of small effect that have been studied but none of them appear to be the major gene causing ADHD. Medical intervention along with psychosocial therapy proved to be beneficial for controlling ADHD, although some undesirable side effects have been encountered during medical treatment. In the future, identification of environmental factors, study of additive gene effects and the interaction of genes and environmental factors may provide better insight into the pathophysiology of ADHD. This may lead to an effective new treatment strategy.

Pedigree disequilibrium test (PDT) replicates association and linkage between DRD4 and ADHD in multigenerational and extended pedigrees from a genetic isolate

Association/linkage between dopamine D4 receptor (DRD4) polymorphisms and attention-deficit/hyperactivity disorder (ADHD) has been suggested by case-control- and nuclear-family-based studies. Here, we present a candidate gene analysis for DRD4 using 14 extended and multigenerational families segregating ADHD derived from the 'Paisa' community of Antioquia, Colombia, a genetic isolate. Two DRD4 polymorphisms (a 120 bp tandem duplication at the promoter and a 48 bp-VNTR at exon 3), reported associated to ADHD, were genotyped. Parametric and non-parametric linkage analyses, and a family-based association test (FBAT), the pedigree disequilibrium test (PDT), were applied to search for evidence of association/linkage. Two-point LOD scores were significantly negative, with values ranging from -3.21 (P=0.011158) to -7.66 (P=0.000091 at theta=0). Non-parametrical analysis resulted in nonsignificant evidence for linkage. The PDT showed a moderate trend toward significance of association/linkage between the 7-repeat (7R) allele at the 48 bp VNTR and ADHD (P=0.0578). Furthermore, the haplotype analysis shows a significant association/linkage of the 7R-240 bp haplotype (P=0.0467) with ADHD. Results suggest that either a moderate DRD4 genetic effect, or linkage disequilibrium of DRD4 with an ADHD disease locus in the vicinity or the linkage to a phenotypic component of the ADHD spectrum could be underlying this association/linkage. These results provide further evidence for the association of ADHD to genetic variation in or near to DRD4 and replicate the previously reported association between ADHD and the 7R allele.

A whole-genome scan in 164 Dutch sib pairs with attention-deficit/hyperactivity disorder: suggestive evidence for linkage on chromosomes 7p and 15q

A genome scan was performed on 164 Dutch affected sib pairs (ASPs) with attention-deficit/hyperactivity disorder (ADHD). All subjects were white and of Dutch descent and were phenotyped according to criteria set out in the Diagnostic and Statistical Manual Of Mental Disorders, 4th edition. Initially, a narrow phenotype was defined, in which all the sib pairs met the full ADHD criteria (117 ASPs). In a broad phenotype, additional sib pairs were included, in which one child had an autistic-spectrum disorder but also met the full ADHD criteria (164 ASPs). A set of 402 polymorphic microsatellite markers with an average intermarker distance of 10 cM was genotyped and analyzed using the Mapmaker/sibs program. Regions with multipoint maximum likelihood scores (MLSs) >1.5 in both phenotypes were fine mapped with additional markers. This genome scan indicated several regions of interest, two of which showed suggestive evidence for linkage. The most promising chromosome region was located at 15q, with an MLS of 3.54 under the broad phenotype definition. This region was previously implicated in reading disability and autism. In addition, MLSs of 3.04 and 2.05 were found for chromosome regions 7p and 9q in the narrow phenotype. Except for a region on chromosome 5, no overlap was found with regions mentioned in the only other independent genome scan in ADHD reported to date.

Possible parent-of-origin effect of Dopa decarboxylase in susceptibility to bipolar affective disorder

Dopa decarboxylase (DDC) catalyses the synthesis of both dopamine and serotonin as well as trace amines suggested to possess neuromodulating capabilities. We have previously reported evidence suggesting an association between DDC and bipolar affective disorder (BPAD) [Børglum et al., 1999]. To further investigate the possible role of DDC in BPAD, we analyzed a 1- and a 4-bp deletion variant-both of putative functional significance-in two new samples: a case-control sample with 140 cases and 204 controls, and 100 case-parents trios. We also tested for association in subjects with familial disease in both the new and the previously investigated samples. The previously reported association was not replicated in either of the new samples. However, a preponderance of the 1-bp deletion was increased by analysis of the familial cases separately for all case-control samples investigated, indicating a possible association with familial disease (combined analysis, P = 0.02). In the trio sample, a preferential paternal transmission of the 4-bp deletion was observed (P = 0.006). DDC is located next to the imprinted gene GRB10, which is expressed specifically from the paternal allele in fetal brains. Increased transmission of paternal DDC alleles has also been suggested in attention deficit hyperactivity disorder. We suggest that DDC might confer susceptibility to BPAD predominantly when paternally transmitted.

Genes de suscetibilidade no transtorno de déficit de atenção e hiperatividade

O transtorno de déficit de atenção e hiperatividade (TDAH) é um dos transtornos mais comuns da infância e adolescência, afetando entre 3% a 6% das crianças em idade escolar. Essa patologia caracteriza-se por sintomas de desatenção, hiperatividade e impulsividade, apresentando ainda uma alta heterogeneidade clínica. Embora as causas precisas do TDAH não estejam esclarecidas, a influência de fatores genéticos é fortemente sugerida pelos estudos epidemiológicos, cujas evidências impulsionaram um grande número de investigações com genes candidatos. Atualmente, apesar da ênfase dada a este tópico, nenhum gene pode ser considerado necessário ou suficiente ao desenvolvimento do TDAH, e a busca de genes que influenciam este processo ainda é o foco de muitas pesquisas. O objetivo desse artigo é, portanto, sumarizar e discutir os principais resultados das pesquisas com genes candidatos no TDAH.