Contribution of 5-HT2A receptor gene -1438A>G polymorphism to outcome of attention-deficit/hyperactivity disorder in adolescents

Neuroinflammation as a risk factor for attention deficit hyperactivity disorder

Attention Deficit Hyperactivity Disorder (ADHD) is a persistent, and impairing pediatric-onset neurodevelopmental condition. Its high prevalence, and recurrent controversy over its widespread identification and treatment, drive strong interest in its etiology and mechanisms. Emerging evidence for a role for neuroinflammation in ADHD pathophysiology is of great interest. This evidence includes 1) the above-chance comorbidity of ADHD with inflammatory and autoimmune disorders, 2) initial studies indicating an association with ADHD and increased serum cytokines, 3) preliminary evidence from genetic studies demonstrating associations between polymorphisms in genes associated with inflammatory pathways and ADHD, 4) emerging evidence that early life exposure to environmental factors may increase risk for ADHD via an inflammatory mechanism, and 5) mechanistic evidence from animal models of maternal immune activation documenting behavioral and neural outcomes consistent with ADHD. Prenatal exposure to inflammation is associated with changes in offspring brain development including reductions in cortical gray matter volume and the volume of certain cortical areas -parallel to observations associated with ADHD. Alterations in neurotransmitter systems, including the dopaminergic, serotonergic and glutamatergic systems, are observed in ADHD populations. Animal models provide strong evidence that development and function of these neurotransmitters systems are sensitive to exposure to in utero inflammation. In summary, accumulating evidence from human studies and animal models, while still incomplete, support a potential role for neuroinflammation in the pathophysiology of ADHD. Confirmation of this association and the underlying mechanisms have become valuable targets for research. If confirmed, such a picture may be important in opening new intervention routes.

Lack of genetic association of 5-HTR2A 102 T/C and -1438A/G polymorphisms with Tourette syndrome in a family-based association study in a Chinese Han population

INTRODUCTION

Our purpose is to investigate whether polymorphisms of 102 T/C and -1438A/G in 5HTR2A are associated with Tourette syndrome (TS) in Chinese Han population.

METHODS

A total of 178 TS trios were recruited in this study. After the allelic and genotypic distributions of two polymorphisms were genotyped using polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP), we compared their genetic distributions with what is expected with Hardy-Weinberg to explore whether there might be an association of these polymorphisms with TS by haplotype relative risk (HRR) and transmission disequilibrium test (TDT) statistics.

RESULTS

Our results showed that no significant associations were found between the HTR2A 102 T/C and -1438A/G polymorphisms and TS (for HTR2A 102 T/C: TDT = 2.041, df = 1, P = 0.175; HRR = 1.468, χ(2)  = 1.905, P = 0.168, 95% confidence interval: 0.850-2.535; for HTR2A: -1438A/G, TDT = 0.093, df = 1, P = 0.819; HRR = 0.965, χ(2)  = 0.018, P = 0.894, 95% confidence interval: 0.574-1.624).

DISCUSSION

Our study suggested that the HTR2A 102T/C and -1438A/G polymorphisms may not be associated with susceptibility to TS, and thus do not play a major role in the development of TS in the Chinese Han population. However, these results need to be confirmed in a larger sample collected from different populations.

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 between SYP with attention-deficit/hyperactivity disorder in Chinese Han subjects: differences among subtypes and genders

Dysfunction of neurotransmitters has been suggested to be involved in the etiology of attention-deficit/hyperactivity disorder (ADHD). Hence, genes encoding proteins involved in the vesicular release process of those neurotransmitters are attractive candidates in ADHD genetics. One of these genes is SYP, which encodes synaptophysin, a protein known to participate in regulating neurotransmitter release and synaptic plasticity. Several studies have reported an association between SYP and ADHD, but more work is needed to refine the association. In the present study, we attempt to investigate their association in Chinese Han subjects by family-based and case-control studies. Transmission disequilibrium tests (TDTs) in 1112 trios found significant association between SYP and the predominantly inattentive subtype (ADHD-I), especially for males with ADHD-I, both from single nucleotide polymorphism (SNP) and haplotypic analyses. Chi-square tests in 1682 ADHD probands and 957 comparison subjects indicated possible association of SYP with female ADHD and female ADHD-I. However, the associated alleles and haplotypes between males and females were reversed. In conclusion, our results suggested that SYP may be primarily associated with ADHD-I and its genetic mechanism may be gender-specific. Thus, it is necessary to take subtype and gender into account in ADHD genetic studies.

The relationship between the presence of ADHD and certain candidate gene polymorphisms in a Turkish sample

Due to the high heritability of attention-deficit hyperactivity disorder (ADHD), parents of children with ADHD appear to represent a good sample group for investigating the genetics of the disorder. The aim of this study was to investigate the association between ADHD and six polymorphisms in five candidate genes [5-HT2A (rs6311), NET1 (rs2242447), COMT (rs4818), NTF3 (rs6332), SNAP-25 (rs3746544) and (rs1051312)]. We included 228 parents of children diagnosed with ADHD and 109 healthy parents as the control group. The polymorphisms were genotyped using polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) assays and analyzed using the chi-square test and the multinomial logit model. SNAP-25 (rs3746544) polymorphism was associated with loading for ADHD, while 5-HT2A (rs6311) and NET1 (rs2242447) polymorphisms were associated with ADHD. On the other hand, there was no significant association between the SNAP-25 (rs1051312), NTF3 (rs6332), or COMT (rs4818) gene polymorphisms and ADHD. In addition, we found that even if variation in the SNAP-25 gene alone does not affect the phenotype, it may nevertheless lead to the emergence of a clinical ADHD picture in the presence of other genetic factors. Our findings suggest that a combination of NET1 (rs2242447) and SNAP-25 (rs3746544) is a risk factor for ADHD. Problems associated with the noradrenergic and serotonergic systems and SNAP-25 may play a role, both alone and in interaction with one another, in the pathophysiological mechanisms of ADHD.

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.

Epigenetics in Developmental Disorder: ADHD and Endophenotypes

Heterogeneity in attention-deficit/hyperactivity disorder (ADHD), with complex interactive operations of genetic and environmental factors, is expressed in a variety of disorder manifestations: severity, co-morbidities of symptoms, and the effects of genes on phenotypes. Neurodevelopmental influences of genomic imprinting have set the stage for the structural-physiological variations that modulate the cognitive, affective, and pathophysiological domains of ADHD. The relative contributions of genetic and environmental factors provide rapidly proliferating insights into the developmental trajectory of the condition, both structurally and functionally. Parent-of-origin effects seem to support the notion that genetic risks for disease process debut often interact with the social environment, i.e., the parental environment in infants and young children. The notion of endophenotypes, markers of an underlying liability to the disorder, may facilitate detection of genetic risks relative to a complex clinical disorder. Simple genetic association has proven insufficient to explain the spectrum of ADHD. At a primary level of analysis, the consideration of epigenetic regulation of brain signalling mechanisms, dopamine, serotonin, and noradrenaline is examined. Neurotrophic factors that participate in the neurogenesis, survival, and functional maintenance of brain systems, are involved in neuroplasticity alterations underlying brain disorders, and are implicated in the genetic predisposition to ADHD, but not obviously, nor in a simple or straightforward fashion. In the context of intervention, genetic linkage studies of ADHD pharmacological intervention have demonstrated that associations have fitted the "drug response phenotype," rather than the disorder diagnosis. Despite conflicting evidence for the existence, or not, of genetic associations between disorder diagnosis and genes regulating the structure and function of neurotransmitters and brain-derived neurotrophic factor (BDNF), associations between symptoms-profiles endophenotypes and single nucleotide polymorphisms appear reassuring.

Achieving remission as a routine goal of pharmacotherapy in attention-deficit hyperactivity disorder

Remission should be the goal of attention-deficit hyperactivity disorder (ADHD) treatment. However, there is no universally accepted definition of remission in ADHD, although clinical studies use a number of criteria. This article examines current research into the concept of remission in ADHD by reviewing the literature for definition and achievement of remission in children and adults with ADHD. Results demonstrate that the concept of remission in ADHD has been proposed by several study groups, using thresholds of validated rating scales to indicate syndromic, symptomatic and functional remission. Several studies have demonstrated the achievement of remission in ADHD children utilizing methylphenidate delivered by an osmotic, controlled-release formulation (OROS®) and atomoxetine. However, none has defined a time period over which these criteria must be met for an individual with ADHD to be considered 'in remission'. Standardized remission criteria in ADHD will provide a tool for assessing the effectiveness of treatments for ADHD, and raise treatment standards.

Molecular genetics of attention-deficit/hyperactivity disorder: an overview

As heritability is high in attention-deficit/hyperactivity disorder (ADHD), genetic factors must play a significant role in the development and course of this disorder. In recent years a large number of studies on different candidate genes for ADHD have been published, most have focused on genes involved in the dopaminergic neurotransmission system, such as DRD4, DRD5, DAT1/SLC6A3, DBH, DDC. Genes associated with the noradrenergic (such as NET1/SLC6A2, ADRA2A, ADRA2C) and serotonergic systems (such as 5-HTT/SLC6A4, HTR1B, HTR2A, TPH2) have also received considerable interest. Additional candidate genes related to neurotransmission and neuronal plasticity that have been studied less intensively include SNAP25, CHRNA4, NMDA, BDNF, NGF, NTF3, NTF4/5, GDNF. This review article provides an overview of these candidate gene studies, and summarizes findings from recently published genome-wide association studies (GWAS). GWAS is a relatively new tool that enables the identification of new ADHD genes in a hypothesis-free manner. Although these latter studies could be improved and need to be replicated they are starting to implicate processes like neuronal migration and cell adhesion and cell division as potentially important in the aetiology of ADHD and have suggested several new directions for future ADHD genetics 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.

The genetics of attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a complex neurodevelopmental disorder that almost certainly represents the common outcome of multiple causal pathways and it is now generally accepted that genetic factors make a significant contribution to these pathways. Behavioral studies suggest a heritability of approximately 0.76. While molecular genetic approaches have identified a range of potential candidate genes, it is now clear that the genetics of ADHD are characterized by a number of genes each of which makes a small but significant contribution to the overall risk. Several genome-wide linkage studies have been conducted and, although there are considerable differences in findings between studies, several regions have been supported across several studies (bin 16.4, 5p13, 11q22-25, 17p11). The contribution of several candidate genes has been supported by meta-analyses (DRD4, DRD5, DAT1, HTR1B and SNAP25). Genome-wide association scans are starting to appear but have not yet had sufficient power to produce conclusive results. Gene-environment interactions, which are as yet relatively understudied, are likely to be of importance in fully understanding the role of genes in ADHD and will be discussed.

Gene expression profiles in the prefrontal cortex of SHR rats by cDNA microarrays

We have undertaken cDNA microarrays to identify differentially expressed genes in the prefrontal cortex (PFC) of spontaneously hypertensive-rat (SHR), a rodent model of attention deficit hyperactivity disorder (ADHD) versus control Wistar-Kyoto (WKY) rats. The analysis of the gene expression profiles indicated that 57 genes were up-regulated and 97 genes were down-regulated in the PFC of SHR. These predominately expressed genes included genes involved in neural development, immunity, transcription factor, monoamine neurotransmitter, metabolism, signal transduction, apoptosis and so on. Although more detailed analyses are necessary, it is anticipated that further study of genes identified will provide insights into their specific roles 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.

The serotonin 2A receptor gene in alcohol dependence and tobacco smoking

Alcohol and nicotine abuse and dependence are multifactorial traits that frequently co-occur, where 80-90% of alcohol-dependent individuals who seek treatment smoke. Nicotine is the main compound of tobacco and one of its effects is to increase the release of serotonin. Variations in the serotonergic system may influence some aspects of smoking. The serotonin receptor gene HTR2A has been a candidate gene with some evidence for association with alcohol and nicotine dependencies. The polymorphism HTR2A A-1438G is a functional SNP, and the presence of the A allele increases the transcriptional activity of the gene. The aim of the present study was to test for possible associations between the A-1438G polymorphism in the serotonin receptor gene (HTR2A) with tobacco smoking combined or not with alcohol dependence. The polymorphic site was genotyped in three groups of European-derived Brazilians: individuals with co-occurrence of alcohol dependence and tobacco smoking (n=113), non-alcoholic smokers (n=120) and non-smoking controls (n=115). A higher frequency of the A allele was observed in the two groups of smokers than in the non-smoking controls (chi(2)=6.53, p=0.04). Combining these groups in comparison with the control group, the difference is more significant (chi(2)=6.45, p=0.01). These results support previous evidence for association between HTR2A polymorphisms and substance use disorders.

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.

Retinoblastoma and mental retardation microdeletion syndrome: clinical characterization and molecular dissection using array CGH

We describe three patients with retinoblastoma, dysmorphic features and developmental delay. Patients 1 and 2 have high and broad forehead, deeply grooved philtrum, thick anteverted lobes and thick helix. Patient 1 also has dolicocephaly, sacral pit/dimple and toe crowding; patient 2 shows intrauterine growth retardation and short fifth toe. Both patients have partial agenesis of corpus callosum. Patient 3 has growth retardation, microcephaly, thick lower lip and micrognathia. Using array-comparative genomic hybridization (CGH), we identified a 13q14 de novo deletion in patients 1 and 2, while patient 3 had a 7q11.21 maternally inherited deletion, probably not related to the disease. Our results confirm that a distinct facial phenotype is related to a 13q14 deletion. Patients with retinoblastoma and malformations without a peculiar facial phenotype may have a different deletion syndrome or a casual association of mental retardation and retinoblastoma. Using array-CGH, we defined a critical region for mental retardation and dysmorphic features. We compared this deletion with a smaller one in a patient with retinoblastoma (case 4) and identified two distinct critical regions, containing 30 genes. Four genes appear to be good functional candidates for the neurological phenotype: NUFIP1 (nuclear fragile X mental retardation protein 1), HTR2A (serotonin receptor 2A), PCDH8 (prothocaderin 8) and PCDH17 (prothocaderin 17).

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.