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

5-HTP inhibits eosinophilia via intracellular endothelial 5-HTRs; SNPs in 5-HTRs associate with asthmatic lung function

Background

Previous research showed that 5-hydroxytryptophan (5HTP), a metabolic precursor of serotonin, reduces allergic lung inflammation by inhibiting eosinophil migration across endothelial monolayers.

Objective

It is unknown if serotonin receptors are involved in mediating this 5HTP function or if serotonin receptor (HTR) single nucleotide polymorphisms (SNPs) associate with lung function in humans.

Methods

Serotonin receptor subtypes were assessed by qPCR, western blot, confocal microscopy, pharmacological inhibitors and siRNA knockdown. HTR SNPs were assessed in two cohorts.

Results

Pharmacological inhibition or siRNA knockdown of the serotonin receptors HTR1A or HTR1B in endothelial cells abrogated the inhibitory effects of 5HTP on eosinophil transendothelial migration. In contrast, eosinophil transendothelial migration was not inhibited by siRNA knockdown of HTR1A or HTR1B in eosinophils. Surprisingly, these HTRs were intracellular in endothelial cells and an extracellular supplementation with serotonin did not inhibit eosinophil transendothelial migration. This is consistent with the inability of serotonin to cross membranes, the lack of selective serotonin reuptake receptors on endothelial cells, and the studies showing minimal impact of selective serotonin reuptake inhibitors on asthma. To extend our HTR studies to humans with asthma, we examined the CHIRAH and GALA cohorts for HTR SNPs that affect HTR function or are associated with behavior disorders. A polygenic index of SNPs in HTRs was associated with lower lung function in asthmatics.

Conclusions

Serotonin receptors mediate 5HTP inhibition of transendothelial migration and HTR SNPs associate with lower lung function. These results may serve to aid in design of novel interventions for allergic inflammation.

Attention-Deficit/Hyperactivity Disorder Predominantly Inattentive Subtype/Presentation: Research Progress and Translational Studies

Research on the predominantly inattentive attention-deficit/hyperactivity disorder (ADHD-PI) subtype/presentation is important given its high prevalence, but paradoxically it is under-recognized and undertreated. The temporal stability of the inattention symptom could impact the high worldwide prevalence of ADHD-PI. Some evidence suggests differences in the nature of attentional deficit in ADHD-PI vs. that in other subtypes. Impairments in neuropsychological, neurocognitive, and social functioning are also evident in ADHD-PI, which could be specific to the subtype (e.g., processing speed, social perception, and skills), or differ from others in severity. Neuroimaging studies have also revealed ADHD-PI-specific neuropathological abnormalities and those that are shared with other subtypes. ADHD-PI is highly comorbid with learning and internalizing (e.g., anxiety and depression) disorders. There is no solid evidence for ADHD-PI-specific genetic etiologies and differential responses of subtypes to ADHD medications. Translational studies have used the Wistar Kyoto/NCrl substrain which requires further characterizations as an ADHD-PI model. Overall, ADHD-PI research has been conducted in the context of the Diagnostic and Statistical Manual, which arguably does not conform to the widely recognized "dimensional" view of ADHD. The Research Domain Criteria has been proposed to provide a novel framework for understanding the nature of neuropsychiatric illnesses and ultimately improve their diagnosis and treatment.

X-Chromosome Effects on Attention Networks: Insights from Imaging Resting-State Networks in Turner Syndrome

Attention deficit hyperactivity disorder (ADHD) is strongly affected by sex, but sex chromosomes' effect on brain attention networks and cognition are difficult to examine in humans. This is due to significant etiologic heterogeneity among diagnosed individuals. In contrast, individuals with Turner syndrome (TS), who have substantially increased risk for ADHD symptoms, share a common genetic risk factor related to the absence of the X-chromosome, thus serving as a more homogeneous genetic model. Resting-state functional MRI was employed to examine differences in attention networks between girls with TS (n = 40) and age- sex- and Tanner-matched controls (n = 33). We compared groups on resting-state functional connectivity measures from data-driven independent components analysis (ICA) and hypothesis-based seed analysis. Using ICA, reduced connectivity was observed in both frontoparietal and dorsal attention networks. Similarly, using seeds in the bilateral intraparietal sulcus (IPS), reduced connectivity was observed between IPS and frontal and cerebellar regions. Finally, we observed a brain-behavior correlation between IPS-cerebellar connectivity and cognitive attention measures. These findings indicate that X-monosomy contributes affects to attention networks and cognitive dysfunction that might increase risk for ADHD. Our findings not only have clinical relevance for girls with TS, but might also serve as a biological marker in future research examining the effects of the intervention that targets attention skills.

Brain imaging genetics in ADHD and beyond - Mapping pathways from gene to disorder at different levels of complexity

Attention-deficit/hyperactivity disorder (ADHD) is a common and often persistent neurodevelopmental disorder. Beyond gene-finding, neurobiological parameters, such as brain structure, connectivity, and function, have been used to link genetic variation to ADHD symptomatology. We performed a systematic review of brain imaging genetics studies involving 62 ADHD candidate genes in childhood and adult ADHD cohorts. Fifty-one eligible research articles described studies of 13 ADHD candidate genes. Almost exclusively, single genetic variants were studied, mostly focussing on dopamine-related genes. While promising results have been reported, imaging genetics studies are thus far hampered by methodological differences in study design and analysis methodology, as well as limited sample sizes. Beyond reviewing imaging genetics studies, we also discuss the need for complementary approaches at multiple levels of biological complexity and emphasize the importance of combining and integrating findings across levels for a better understanding of biological pathways from gene to disease. These may include multi-modal imaging genetics studies, bioinformatic analyses, and functional analyses of cell and animal models.

Elucidating X chromosome influences on Attention Deficit Hyperactivity Disorder and executive function

OBJECTIVE

To identify distinct behavioral and cognitive profiles associated with ADHD in Turner syndrome (TS), relative to idiopathic ADHD and neurotypical controls, in order to elucidate X-linked influences contributing to ADHD.

METHODS

We used a multilevel-model approach to compare 49 girls with TS to 37 neurotypical females, aged 5-12, on established measures of behavior (BASC-2) and neurocognitive function (NEPSY). We further compared girls with TS to BASC-2 and NEPSY age-matched reference data obtained from children with idiopathic ADHD.

RESULTS

Within the TS group, 51% scored at or above the "at-risk" range for ADHD-associated behaviors on the BASC-2 (TS/+ADHD). The BASC-2 behavioral profile in this TS/+ADHD-subgroup was comparable to a reference group of boys with ADHD with respect to attentional problems and hyperactivity. However, the TS/+ADHD-subgroup had significantly higher hyperactivity scores relative to a reference sample of girls with ADHD (p = 0.016). The behavioral profile in TS was associated with significantly lower attention and executive function scores on the NEPSY relative to neurotypical controls (p = 0.015); but was comparable to scores from a reference sample of children with idiopathic ADHD. Deficits in attention and executive function were not observed in girls with TS having low levels of ADHD-associated behavior (TS/-ADHD).

CONCLUSIONS

ADHD-associated behavioral and cognitive problems in TS are prevalent and comparable in severity to those found in children with idiopathic ADHD. The ADHD phenotype in TS also appears relatively independent of cognitive features typically associated with TS, like visuospatial weaknesses. These findings suggest that X-linked haploinsufficiency and downstream biological effects contribute to increased risk for ADHD.

Serotonin mediated immunoregulation and neural functions: Complicity in the aetiology of autism spectrum disorders

Serotonergic system has long been implicated in the aetiology of autism spectrum disorders (ASD), since platelet hyperserotonemia is consistently observed in a subset of autistic patients, who respond well to selective serotonin reuptake inhibitors. Apart from being a neurotransmitter, serotonin functions as a neurotrophic factor directing brain development and as an immunoregulator modulating immune responses. Serotonin transporter (SERT) regulates serotonin level in lymphoid tissues to ensure its proper functioning in innate and adaptive responses. Immunological molecules such as cytokines in turn regulate the transcription and activity of SERT. Dysregulation of serotonergic system could trigger signalling cascades that affect normal neural-immune interactions culminating in neurodevelopmental and neural connectivity defects precipitating behavioural abnormalities, or the disease phenotypes. Therefore, we suggest that a better understanding of the cross talk between serotonergic genes, immune systems and serotonergic neurotransmission will open wider avenues to develop pharmacological leads for addressing the core ASD behavioural deficits.

Sex differences in attention Deficit Hyperactivity Disorder: candidate genetic and endocrine mechanisms

Attention Deficit Hyperactivity Disorder (ADHD) is a developmental condition characterised by severe inattention, pathological impulsivity and hyperactivity; it is relatively common affecting up to 6% of children, and is associated with a risk of long-term adverse educational and social consequences. Males are considerably more likely to be diagnosed with ADHD than females; the course of the disorder and its associated co-morbidities also appear to be sensitive to sex. Here, I discuss fundamental biological (genetic and endocrine) mechanisms that have been shown to, or could theoretically, contribute towards these sexually dimorphic phenomena. Greater understanding of how and why the sexes differ with respect to ADHD vulnerability should allow us to identify and characterise novel protective and risk factors for the disorder, and should ultimately facilitate improved diagnosis, prognosis and treatment.

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.

BAIAP2 exhibits association to childhood ADHD especially predominantly inattentive subtype in Chinese Han subjects

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is a common chronic neurodevelopmental disorder with a high heritability. Much evidence of hemisphere asymmetry has been found for ADHD probands from behavioral level, electrophysiological level and brain morphology. One previous research has reported possible association between BAIAP2, which is asymmetrically expressed in the two cerebral hemispheres, with ADHD in European population. The present study aimed to investigate the association between BAIAP2 and ADHD in Chinese Han subjects.

METHODS

A total of 1,397 ADHD trios comprised of one ADHD proband and their parents were included for family-based association tests. Independent 569 ADHD cases and 957 normal controls were included for case-control studies. Diagnosis was performed according to the DSM-IV criteria. Nine single nucleotide polymorphisms (SNPs) of BAIAP2 were chosen and performed genotyping for both family-based and case-control association studies.

RESULTS

Transmission disequilibrium tests (TDTs) for family-based association studies showed significant association between the CA haplotype comprised by rs3934492 and rs9901648 with predominantly inattentive type (ADHD-I). For case-control study, chi-square tests provided evidence for the contribution of SNP rs4969239, rs3934492 and rs4969385 to ADHD and its two clinical subtypes, ADHD-I and ADHD-C. However, only the associations for ADHD and ADHD-I retained significant after corrections for multiplicity or logistic regression analyses adjusting the potential confounding effect of gender and age.

CONCLUSIONS

These above results indicated the possible involvement of BAIAP2 in the etiology of ADHD, especially ADHD-I.

Impact of serotonin (5-HT)2C receptors on executive control processes

Although the serotonin (5-hydroxytryptamine, 5-HT) neurotransmitter system has been implicated in modulating executive control processes such as attention, response inhibition, and behavioral flexibility, the contributions of particular serotonin receptors remain unclear. Here, using operant-based behavioral paradigms, we demonstrate that mice with genetically ablated 5-HT2C receptors (2CKO mice) display deficits in executive functions. 2CKO mice were impaired in the acquisition of a visuospatial attention task as assessed in the 5-choice serial reaction time task (5-CSRTT). In this task, 2CKO mice exhibited marked impairment of attentional processes, with normal response inhibition. We assessed dynamic changes in neurotransmitter levels within the nucleus accumbens (NAc) by in vivo microdialysis in task-performing animals. Extracellular dopamine concentrations were elevated in the NAc of 2CKO mice during task performance, indicating that 5-HT2C receptors impact dopamine homeostasis during a visuospatial attention task. These findings raise the possibility that disinhibition of mesolimbic dopamine pathways contributes to impaired attention and perturbed task performance in 2CKO mice. Additionally, in a spatial reversal learning task, 2CKO mice failed to improve their performance over a series of reversals, indicating that intact 5-HT2C receptor signaling is required to accurately respond to repeated changes in reward contingencies. In contrast to the 2CKO phenotype in the 5-CSRTT, wild-type mice treated with the 5-HT2C receptor antagonist SB242084 exhibited diminished response inhibition, suggesting differing effects of acute pharmacological blockade and constitutive loss of 5-HT2C receptor activity. Altogether, these findings provide insights into the serotonergic regulation of executive control processes and suggest that impaired 5-HT2C receptor signaling during development may predispose to executive function disorders.

Bioinformatic analyses to select phenotype affecting polymorphisms in HTR2C gene

OBJECTIVE

Single nucleotide polymorphisms (SNPs) in serotonin related genes influence mental disorders, responses to pharmacological and psychotherapeutic treatments. In planning association studies, researchers that want to investigate new SNPs have to select some among a large number of candidates. Our aim is to guide researchers in the selection of the most likely phenotype affecting polymorphisms. Here, we studied serotonin receptor 2C (HTR2C) SNPs because, till now, only relatively few of about 2000 are investigated.

METHODS

We used the most updated and assessed bioinformatic tools to predict which variations can give rise to biological effects among 2450 HTR2C SNPs.

RESULTS

We suggest 48 SNPs that are worth considering in future association studies in the field of psychiatry, psychology and pharmacogenomics. Moreover, our analyses point out the biological level probably affected, such as transcription, splicing, miRNA regulation and protein structure, thus allowing to suggest future molecular investigations.

CONCLUSIONS

Although few association studies are available in literature, their results are in agreement with our predictions, showing that our selection methods can help to guide future association studies.

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.

Candidate gene analysis in an on-going genome-wide association study of attention-deficit hyperactivity disorder: suggestive association signals in ADRA1A

OBJECTIVES

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable, common developmental disorder. Although a few confirmed associations have emerged from candidate gene studies, these have shown the same limitations that have become evident in the study of other complex diseases, often with inconsistent and nonreplicated results across different studies.

METHODS

In this report, 27 ADHD candidate genes were explored in greater depth using high-density tag single nucleotide polymorphism (SNP) genotyping. Association with 557 SNPs was tested using the transmission disequilibrium test in 270 nuclear pedigrees selected from an ongoing ADHD genetic study that includes all disease subtypes.

RESULTS

SNPs in seven genes including SLC1A3, SLC6A3, HTR4, ADRA1A, HTR2A, SNAP25, and COMT showed a nominal level of association with ADHD (P values <0.05), but none remained significant after a stringent correction for the total number of tests performed.

CONCLUSION

The strongest signal emerged from SNPs in the promoter region (rs3808585) and in an intron (rs17426222, rs4732682, rs573514) of ADRA1A, all located within the same haplotype block. Some of the SNPs in HTR2A and COMT have already been reported by others, whereas other SNPs will need confirmation in independent samples.

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.

Attention Deficit Hyperactivity Disorder comorbid oppositional defiant disorder and its predominately inattentive type: evidence for an association with COMT but not MAOA in a Chinese sample

Background

There are three childhood disruptive behavior disorders (DBDs), attention deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD). The most common comorbid disorder in ADHD is ODD. DSM-IV describes three ADHD subtypes: predominantly inattentive type (ADHD-IA), predominantly hyperactive-impulsive type (ADHD-HI), and combined type (ADHD-C). Prior work suggests that specific candidate genes are associated with specific subtypes of ADHD in China. Our previous association studies between ADHD and functional polymorphisms of COMT and MAOA, consistently showed the low transcriptional activity alleles were preferentially transmitted to ADHD-IA boys. Thus, the goal of the present study is to test the hypothesis that COMT Val158Met and MAOA-uVNTR jointly contribute to the ODD phenotype among Chinese ADHD boys.

Methods

171 Chinese boys between 6 and 17.5 years old (mean = 10.3, SD = 2.6) with complete COMT val158met and MAOA-uVNTR genotyping information were studied. We used logistic regression with genotypes as independent variables and the binary phenotype as the dependent variable. We used p < 0.05 as the level of nominal statistical significance. Bonferroni correction procedures were used to adjust for multiple comparisons.

Results

Our results highlight the potential etiologic role of COMT in the ADHD with comorbid ODD and its predominately inattentive type in male Chinese subjects. ADHD with comorbid ODD was associated with homozygosity of the high-activity Val allele, while the predominantly inattentive ADHD subtype was associated with the low-activity Met allele. We found no evidence of association between the MAOA-uVNTR variant and ADHD with comorbid ODD or the ADHD-IA subtype.

Conclusion

Our study of attention deficit hyperactivity disorder comorbid oppositional defiant disorder and its predominately inattentive type highlights the potential etiologic role of COMT for ADHD children in China. But we failed to observe an interaction between COMT and MAOA, which suggests that epistasis between COMT and MAOA genes does not influence the phenotype of ADHD-IA with comorbid ODD in a clinical sample of Chinese male subjects. To confirm our findings further studies with a larger number of subjects and healthy controls are needed.

Linkage analysis of attention deficit hyperactivity disorder

Results of behavioral genetic and molecular genetic studies have converged to suggest that both genes contribute to the development of ADHD. Although prior linkage studies have produced intriguing results, their results have been inconsistent, with no clear pattern of results emerging across studies. We genotyped 5,980 SNPs across the genome in 1,187 individuals from families with children diagnosed with ADHD. We then performed two nonparametric linkage analyses on ADHD families: (1) an affected sibling pair linkage analysis on 217 families with 601 siblings diagnosed with ADHD and (2) a variance components linkage analysis using the number of ADHD symptoms as the phenotype on 260 families with 1,100 phenotyped siblings. The affection status linkage analysis had a maximum LOD score of 1.85 on chromosome 8 at 54.2 cM. The maximum LOD score in the variance components linkage analysis was 0.8 on chromosome 8 at 93.4 cM. The absence of regions of significant or suggestive linkage in these data suggest that there are no genes of large effect contributing to the ADHD phenotype.

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.

Role of the serotonin system in ADHD: treatment implications

A limited number of studies have considered whether the activity of serotonin (5-hydroxytryptamine [5-HT]) contributes to the problems experienced by youngsters with attention-deficit/hyperactivity disorder (ADHD). The aim of this article is to review this work and propose interpretations. Peripheral measures of 5-HT and its metabolite do not point to a widespread association with the diagnosis. However, separate consideration of the major domains of dysfunction (motor activity, inattention and impulsivity) support a more differentiated assessment. The marked innervation of motor regions of the brain by 5-HT projections and the clear involvement of 5-HT systems in the control of locomotion in animals suggests a likely node for dysfunction in ADHD. The few relevant studies do not show evidence of this, but more attention should be accorded to the issue. The situation is different for attention-related processes; here, there are deficiencies in perceptual sensitivity and the appropriate designation of saliency to stimulation. These are attributable, in part, to altered 5-HT activity. Marked and opposite changes of 5-HT responsivity are associated with behavioral and cognitive impulsivity. There is also a growing series of studies demonstrating preferential transmission of various genetic markers for 5-HT receptors that are expressed in ADHD. Currently, the heterogeneity of methods in this young discipline restricts the possibilities of definition of these markers and the types of ADHD in which they are expressed.

ADHD genetics: 2007 update

Attention-deficit/hyperactivity disorder (ADHD) is highly heritable. Confirmed association has been reported for several candidate genes, including DAT1, DRD4, SNAP-25, DRD5, 5HTT, HTR1B, and DBH; however, these confer relatively small risk. Family-based linkage studies have identified a number of chromosomal regions containing potential ADHD predisposing loci, some overlapping in two or more studies, including 5p, 6q, 7p, 11q, 12q, and 17p. New large-scale studies that apply recent technological advances to perform high-density genotyping of the entire genome, in combination with information on the haplotype structure of the human genome, now allow testing of single-nucleotide polymorphism association with disease phenotype without any a priori hypothesis. They may contribute further to our understanding of the genetic factors involved in ADHD. The heterogeneous complex ADHD phenotype, as well as epigenetic factors may be contributing to the challenge of genetic studies. Samples that include limited age ranges may have better success at uncovering genes whose expression is limited to specific developmental stages.

No allele variation of the MAOA gene promoter in male Chinese subjects with attention deficit hyperactivity disorder

The aim of the current study was to examine any difference in the repeat sequence of the monoamine oxidase A (MAOA) gene promoter between males with attention deficit hyperactivity disorder (ADHD) and randomly selected subjects in a community. The role of the MAOA gene promoter in ADHD cases was also investigated. The total of 244 participants included 57 male ADHD subjects as the case group and 187 males, also in southern Taiwan, as the community controls. There was no significantly different distribution in the repeat sequence of the MAOA gene promoter (chi2 = 3.895, d.f. = 3, p = 0.273), and no significantly different distribution of 'long-form' and 'short-form' alleles between the ADHD group and the male community group was noted (chi2 = 2.484, d.f. = 1, p = 0.115). Some aspects of clinical response are mentioned in the discussion of this study which are worth exploring further in the future.