Association between attention deficit hyperactivity disorder and the DXS7 locus

Advances in molecular genetic studies of attention deficit hyperactivity disorder in China

SUMMARY

Attention deficit hyperactivity disorder (ADHD) is a common psychiatric condition in children worldwide that typically includes a combination of symptoms of inattention and hyperactivity/impulsivity. Genetic factors are believed to be important in the development and course of ADHD so many candidate genes studies and genome-wide association studies (GWAS) have been conducted in search of the genetic mechanisms that cause or influence the condition. This review provides an overview of gene association and pharmacogenetic studies of ADHD from mainland China and elsewhere that use Han Chinese samples. To date, studies from China and elsewhere remain inconclusive so future studies need to consider alternative analytic techniques and test new biological hypotheses about the relationship of neurotransmission and neurodevelopment to the onset and course of this disabling condition.

Association study between the dopamine-related candidate gene polymorphisms and ADHD among Saudi Arabia population via PCR technique

Attention deficit hyperactivity disorder (ADHD) is one of the most common childhood behavioral disorders characterized by inattention, hyperactivity and impulsivity. In Saudi Arabia the prevalence of combined ADHD is 16.4 %. ADHD etiology is not clear and not completely understood. There are several evidences for involvement of dopaminergic, serotonergic and noradrenergic neurotransmitter systems in the pathogenesis of ADHD. Monoamine Oxidase A (MAOA) is involved in the degradation of all three of these neurotransmitters. Dopamine Transporter 1 (DAT1) plays an important role in controlling blood levels of dopamine. The aim of the present study is to investigate the association between ADHD and polymorphisms of MAOA 30 bp-promoter VNTR and DAT1 40 bp 3' UTRVNTR in Saudi population. PCR technique was employed to detect polymorphisms of MAOA and DAT1 genes in a sample of 120 ADHD subjects and 160 controls. Alleles and genotypes frequencies for both of MAOA and DAT1 polymorphisms were compared among ADHD subjects against controls. Association between ADHD and alleles as well as genotypes for each studied polymorphisms was tested by odds ratio (OR) test and the magnitude of this association was estimated by 95 % confidence interval (95 % CI). A significant association was found between two MAOA genotypes 3/4 and 3/2 with ADHD (P < 0.01, OR = 3, 4.9) as a risk effect. No significant association was found with MAOA alleles. Among DAT1 polymorphisms two alleles (7 and 11 repeats) (P < 0.01, OR = 2.5 and 3.3) as well as two genotypes (11/11 and 11/7) (P < 0.01, OR = 4, 3) showed significant association with ADHD as a risk effect. On the contrary, 9 and 10 repeats revealed significant association as a protective effect as well as 10/10 and 10/9 genotypes. These findings support the hypothesis that some of the MAOA and DAT1 polymorphisms have a causative role in the development of ADHD in the Saudi population. Another polymorphism did not give rise to support this hypothesis. This is the first report investigated the association between MAOA and DAT1 polymorphism at molecular level in Saudi Arabia population as well as Arab world. Therefore further studies are needed to generalize obtained results at Saudi Arabia.

Why are autism spectrum conditions more prevalent in males?

Autism Spectrum Conditions (ASC) are much more common in males, a bias that may offer clues to the etiology of this condition. Although the cause of this bias remains a mystery, we argue that it occurs because ASC is an extreme manifestation of the male brain. The extreme male brain (EMB) theory, first proposed in 1997, is an extension of the Empathizing-Systemizing (E-S) theory of typical sex differences that proposes that females on average have a stronger drive to empathize while males on average have a stronger drive to systemize. In this first major update since 2005, we describe some of the evidence relating to the EMB theory of ASC and consider how typical sex differences in brain structure may be relevant to ASC. One possible biological mechanism to account for the male bias is the effect of fetal testosterone (fT). We also consider alternative biological theories, the X and Y chromosome theories, and the reduced autosomal penetrance theory. None of these theories has yet been fully confirmed or refuted, though the weight of evidence in favor of the fT theory is growing from converging sources (longitudinal amniocentesis studies from pregnancy to age 10 years old, current hormone studies, and genetic association studies of SNPs in the sex steroid pathways). Ultimately, as these theories are not mutually exclusive and ASC is multi-factorial, they may help explain the male prevalence of ASC.

Investigation of the serotonin 2C receptor gene in attention deficit hyperactivity disorder in UK samples

Background

Attention deficit hyperactivity disorder (ADHD) is a common, childhood-onset neurodevelopmental disorder that is more frequent in males than females. Several genes on the X chromosome have been studied as candidate risk factors for ADHD including the 5-HT_2C receptor (HTR2C) gene. Association between polymorphisms in HTR2C and ADHD were reported in a recent study.

Findings

In this study we investigated the association between ADHD and two polymorphisms C-759T (rs3813929) and G-697C (rs518147) in the promoter region of the HTR2C gene using a sample of 180 UK ADHD probands and their parents. We have shown that the -697G allele was significantly over-transmitted to affected ADHD probands (P = 0.017). No association was detected between the C-759T polymorphism and ADHD. Haplotype analysis of the two markers revealed no significantly increased transmission of any haplotype to ADHD.

Conclusion

The findings provide evidence that the G-allele of the G-697C HTR2C polymorphism may be involved in the development of ADHD. The results replicate one of the findings published recently.

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.

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.

The monoamine oxidase B gene exhibits significant association to ADHD

Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric condition with strong genetic basis. Recent work in China indicated that ADHD may be linked to Xp1-2 in the Han Chinese population. The gene encoding monoamine oxidase B (MAOB), the main enzyme degrading dopamine in the human brain, is located in this region. The current study sequenced the exons and the 5' and 3' flanking regions of the MAOB gene and found four common variants including 2276C>T and 2327C>T in exon 15, rs1799836 in intron 13 and rs1040399 in 3'-UTR. We assessed the association of these variants with ADHD in 548 trios collected from 468 males and 80 females probands. TDT analysis showed that alleles of each polymorphism were preferentially transmitted to probands (rs1799836, P = 3.28E-15; rs1040399, P = 1.87E-6; 2276T>C or 2327T>C, P = 2.20E-6) and haplotype-based TDT analyses also found distorted transmission. In conclusion, this study provides the strongest evidence for the involvement of MAOB gene in the etiology of ADHD to date, at least in Han Chinese population.

Parent-of-origin effects in attention-deficit hyperactivity disorder

The goal of the present study was to investigate parent-of-origin effects in attention-deficit hyperactivity disorder (ADHD). Parent-of-origin effects in ADHD may be due to differences in the relative quantity of risk factors transmitted by each parent. Alternatively, parent-of-origin effects may be produced by qualitative differences in the risks transmitted, such as those carried on the sex chromosomes or regulated by genomic imprinting. 60 children with maternal-only history of ADHD and 131 children with paternal-only history of ADHD were compared on three domains for which prior evidence suggested parent-of-origin effects may exist: core symptoms, disruptive behaviours and depression. Dependent variables were derived from previously validated, age-appropriate and standardized parent and teacher interviews and questionnaires. Depression levels were rated using the Child Depression Inventory. Consistent with previous research and the predictions derived from threshold models of ADHD etiology, the maternal history group received higher ratings of behavioural disorder (ADHD, conduct disorder and oppositional symptoms) than the paternal history group. Parent-of-origin effects were also observed for depression, with the paternal history group rating themselves as significantly more depressed than children in the maternal history group, particularly girls. Heightened paternal transmission relative to maternal is suggestive of genomic imprinting, and the interaction with proband sex indicates the involvement of the sex chromosomes or sex-specific physiological or hormonal factors. Interpretations of these data in terms of environmental and genetic factors, including epigenetic and sex-linked hypotheses, are explored.

Family-based association study of serotonergic candidate genes and attention-deficit/hyperactivity disorder in a German sample

Alterations in the serotonergic pathway have been implicated in the pathogenesis of attention-deficit/hyperactivity disorder (ADHD). The aim of this study was to investigate seven genetic variants in three genes (serotonin transporter (5-HTT), serotonin receptor 1B (5-HTR1B) and serotonin receptor 2A (5-HTR2A)), which have previously been shown to be associated with ADHD. The polymorphisms under investigation were the 5-HTTLPR, the VNTR in intron 2 and the 3'UTR SNP in 5-HTT, the 5-HTR1B variations 861G>C and 102T>C, and the 5-HTR2A variations His452Tyr and 1438G>A. We genotyped these variants in a sample of 102 families with 229 children with ADHD according to DSM-IV criteria. Among the affected children, 69% fulfilled criteria for the combined type, 27% for the predominantly inattentive type, and 4% for the predominantly hyperactive-impulsive type. Associations were tested by the pedigree transmission disequilibrium test (PDT). All investigated polymorphisms in serotonergic candidate genes showed no association to ADHD in our sample. Earlier studies of these polymorphisms had also shown inconsistent results, with some studies reporting significant associations and others demonstrating no association. This discordance between studies may reflect variation in patient ascertainment criteria, genetic heterogeneity, too low statistical power for the expected effects or false positive results in the initial reports. We cannot rule out the possibility that other variations in the investigated genes contribute to the etiology of ADHD.

Association between polymorphisms in serotonin 2C receptor gene and attention-deficit/hyperactivity disorder in Han Chinese subjects

Attention deficit hyperactivity disorder (ADHD) is much more frequent in males than females, so several genes on the X chromosome (e.g., MAOA and MAOB) have been pursued as candidates for influencing risk for the disorder. HTR2C is also located on the X chromosome. In the current study, we examined the relationship between the C-759T and G-697C polymorphisms of HTR2C and ADHD in 488 Han Chinese families. Transmission Disequilibrium Test (TDT) analysis showed that the -759C allele, the -697G allele, and haplotype -759C/-697G were significantly over-transmitted to affected probands, while haplotypes -759C/-697C and -759T/-697C were under-transmitted. When families were divided into three subtypes according to the diagnosis of probands, the -697G allele and haplotype -759C/-697G were significantly over transmitted to ADHD-C probands, while haplotype -759T/-697C was under-transmitted to these individuals; however, no biased transmission of any allele or haplotype was observed for probands with ADHD-I, suggesting that different subtypes of ADHD have different genetic influences. Our findings highlight the need to explore the role of 5-HT2C receptor dysfunction in the pathogenesis of ADHD.

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.

Attention deficit disorder: not just for children

Abstract Attention deficit/hyperactivity disorder (ADHD) has long been described in children who demonstrate developmentally inappropriate symptoms of inattention, impulsivity and motor restlessness. In adults, symptoms are known to persist and the validity of adult ADHD as an entity is now recognized. There is an associated high proportion of other serious psychiatric comorbidities, especially substance abuse, mood and anxiety disorders. Advances have been made into the aetiology and management of ADHD. Many of these focus on the dopamine and noradrenaline pathways.

Association analysis of the monoamine oxidase A and B genes with attention deficit hyperactivity disorder (ADHD) in an Irish sample: preferential transmission of the MAO-A 941G allele to affected children

Pharmacological and genetic studies suggest the importance of the dopaminergic, serotonergic, and noradrenergic systems in the pathogenesis of attention deficit hyperactivity disorder (ADHD). Monoamine oxidases A and B (MAO-A and MAO-B) degrade biogenic amines such as dopamine and serotonin and thereby control the levels of these neurotransmitters in the central nervous system. We examined four polymorphisms in the MAO-A gene (30 bp promoter VNTR, CA microsatellite in intron 2, 941G/T SNP in exon 8, and A/G SNP in intron 12) as well as two markers in the MAO-B gene (CA microsatellite in intron 2 and T/C SNP in intron 13) for association with ADHD in an Irish sample of 179 nuclear families. TDT analysis of the examined MAO-A markers revealed a significant association of the more active MAO-A 941G allele with the disorder (chi2 = 5.1, P = 0.03, OR = 1.7). In addition, haplotype analysis revealed a significantly increased transmission of a haplotype consisting of the shorter allele of the promoter VNTR (allele 1), the 6-repeat allele of the CA microsatellite and the G-allele of the 941G/T SNP (famhap global statistic 34.54, P = 0.01) to ADHD cases. No significant distortion in the number of transmitted alleles was observed between the two examined MAO-B polymorphisms and ADHD. These findings suggest the importance of the 941G/T MAO-A polymorphism in the development of ADHD at least in the Irish population.

Attention deficit hyperactivity disorder: Current status and future perspectives

Attention deficit hyperactivity disorder (ADHD) is a common neurobehavioural syndrome of childhood onset. It can be diagnosed by a competent paediatric neuropsychiatrist, although a multidisciplinary team approach is preferable. As yet, there is no single treatment strategy proven to cure this syndrome and no long-lasting documented effects once the adopted line of treatment is withdrawn.

A combined therapeutic approach that includes pharmaceutical and nonpharmaceutical treatments is used in the management of ADHD in the developed countries – an approach that cannot be used in the developing countries, which do not have well structured health services. The need for further research is paramount, as a deeper understanding of the complex neural network is an important preliminary step.

This article argues that multiple paradoxes found within the concept of ADHD will open new horizons in the aetiopathogenesis, molecular basis and treatment approaches of this syndrome.

Genetics of adult attention-deficit/hyperactivity disorder

Results of behavioral genetic investigations using family twin and adoption studies converge with those of molecular genetic studies in showing that genes influence susceptibility to'attention-deficit/hyperactivity disorder (ADHD). These finding suggest that genetic mechanisms that predispose individuals to ADHD are complex. It seems likely that the disorder is caused by the combined actions of several genes. It is equally clear that aberrant genes create a vulnerability to the disorder that is not expressed in all environments. The literature about the genetics of adult ADHD is relatively small, but it suggests not only that the persistent form of ADHD is familial, but that it is more familial than the nonpersistent form. More work from twin and molecular genetic studies is needed to determine if the increased familiality of persistent ADHD reflects the actions of genes or of familial environmental causes.

Family-based and association studies of monoamine oxidase A and attention deficit hyperactivity disorder (ADHD): preferential transmission of the long promoter-region repeat and its association with impaired performance on a continuous performance test (TOVA)

Monoamine oxidase A (MAO A) is located on the X chromosome and metabolizes biogenic amines including dopamine, norepinephrine and serotonin. A functional promoter-region polymorphism of this gene has been described that has been studied in a number of mental illnesses but not in attention deficit hyperactivity disorder (ADHD). In the current study, we examined the MAO A promoter-region polymorphism initially in 133 triads and observed preferential transmission of the long alleles from 74 heterozygote mothers to ADHD probands (chi(2) = 4.37, P = 0.036, df = 1). We also examined the role of this polymorphism in a computerized continuous performance test, the TOVA. Significant differences were observed on errors of commission (chi(2) = 7.021, P = 0.008) and patients carrying the long MAO A allele made significantly more such errors. Errors of commission are a measure of impulsivity. However, following Ritalin (methylphenidate) administration the association between this polymorphism and errors of commission was markedly attenuated and no longer significant at the P < 0.05 level. We also analyzed the provisional association by the case-control design. A significant difference in allele frequency was observed between 110 male probands vs 202 male controls (Pearson chi(2) = 7.94, P = 0.047). Similarly results were obtained when 19 female probands were compared to female controls (genotype chi(2) = 21.28; P = 0.0032, 3 df and allele chi(2) = 30.88, P= 0.0007, 2 df). All three complementary approaches employed (family-based, case-control and quantitative trait design) suggest a role for the MAO A promoter-region polymorphism in conferring risk for ADHD in our patient population.

Association analysis of monoamine oxidase A and attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a highly heritable disorder. Although the causes of ADHD are unknown, dopaminergic, serotonergic and nor-adrenergic pathways have been strongly implicated. Monoamine Oxidase A (MAOA) is involved in the degradation of all three of these neurotransmitters and therefore has been suggested as a strong candidate gene for ADHD. Animal and human studies have implicated MAOA and 5-HT in impulsive and aggressive behavior. We therefore additionally postulated that MAOA might be associated with a subtype of ADHD where aggressive and impulsive features are especially prominent. We have tested this hypothesis by genotyping two polymorphisms (the 30-bp VNTR in the promoter and the Fnu4HI 941T-->G) in MAOA that are associated with altered MAOA function. Our sample consisted of 171 British Caucasian children 6-16 years of age fulfilling DSM-III R, DSM-IV or ICD-10 criteria for ADHD/Hyperkinetic Disorder. Using case control analysis and then the TDT, no association was found between these two MAOA polymorphisms and ADHD. Case control analysis of the VNTR showed an association with a subgroup of children with comorbid conduct problems (OR = 2.0, 95% CI = 1.09, 3.5), and TDT analysis indicated a statistical trend toward association. Our findings highlight the importance of phenotype definition and the need for the MAOA VNTR to be further examined.

Linkage studies between attention-deficit hyperactivity disorder and the monoamine oxidase genes

Attention-deficit hyperactivity disorder (ADHD) is a prevalent behavioral disorder in children and the etiology of this disorder is not clear. Molecular genetic and pharmacological studies suggest the involvement of dopaminergic and noradrenergic neurotransmitter systems in ADHD, e.g., several reports have found association between ADHD and the dopamine receptor gene DRD-4, the dopamine transporter gene DAT1, and the catecholamine clearance enzyme catechol-O-methyltransferase. Monoamine oxidase (MAO) A and B genes encode enzymes that participate in the metabolism of neurotransmitters of the dopaminergic and noradrenergic systems. MAO inhibitors have been shown to be effective in the treatment of ADHD. Our previous studies showed an association between ADHD and the DXS7 locus, which is located in close vicinity to the MAO genes on chromosome X. These findings suggest that there might be linkage between ADHD and MAO genes. To test this hypothesis, we used the transmission/disequilibrium test (TDT) to test for linkage between a VNTR polymorphism at the MAOA(CA)(n) or MAOB(GT)(n) locus and DSM-III-R-diagnosed ADHD in 82 nuclear families of the Chinese population. The TDT analysis revealed linkage between ADHD and the MAOA(CA)(n) locus (chi-square = 15.25, df = 7, P < 0.05), but not the MAOB(GT)(n) locus (chi-square = 11.18, df = 7, P > 0.05). The data showed that ADHD was in linkage with the MAOA gene and suggested that MAOA might be a susceptibility factor for ADHD.