Stressful life events and the brain-derived neurotrophic factor gene in bipolar disorder

BACKGROUND

Gene-environment interactions may contribute to the high heritability of bipolar affective disorder. The aim of the present study was to examine the interplay between the BDNF Val(66)Met polymorphism and stressful life events (SLEs) in bipolar disorder.

METHOD

A total of 1085 participants were recruited, including 487 bipolar I cases and 598 psychiatrically healthy controls. All participants completed the List of Threatening Life Events Questionnaire; bipolar subjects reported the events that occurred 6 months leading up to their worst manic episode and 6 months prior to their worst depressive episode, controls recorded events experienced 6 months before interview. The sample was genotyped for the BDNF Val(66)Met polymorphism (rs6265).

RESULTS

Both Met carrier BDNF genotype and SLEs were significantly associated with the worst depressive episode of bipolar disorder. For the worst depressive episodes the effects of SLEs were also significantly moderated by BDNF genotype (gene x environment interaction).

LIMITATIONS

The use of a self report questionnaire to measure stressful life events may increase recall inaccuracies, therefore caution should be taken when interpreting these results.

DISCUSSION

The findings of this study highlight the importance of the interplay between genes and the environment in bipolar disorder.

Gene variants associated with schizophrenia in a Norwegian genome-wide study are replicated in a large European cohort

We have performed a genome-wide association study (GWAS) of schizophrenia in a Norwegian discovery sample of 201 cases and 305 controls (TOP study) with a focused replication analysis in a larger European sample of 2663 cases and 13,780 control subjects (SGENE-plus study). Firstly, the discovery sample was genotyped with Affymetrix Genome-Wide Human SNP Array 6.0 and 572,888 markers were tested for schizophrenia association. No SNPs in the discovery sample attained genome-wide significance (P<8.7 x 10(-8)). Secondly, based on the GWAS data, we selected 1000 markers with the lowest P values in the discovery TOP sample, and tested these (or HapMap-based surrogates) for association in the replication sample. Sixteen loci were associated with schizophrenia (nominal P value<0.05 and concurring OR) in the replication sample. As a next step, we performed a combined analysis of the findings from these two studies, and the strongest evidence for association with schizophrenia was provided for markers rs7045881 on 9p21, rs433598 on 16p12 and rs10761482 on 10q21. The markers are located in PLAA, ACSM1 and ANK3, respectively. PLAA has not previously been described as a susceptibility gene, but 9p21 is implied as a schizophrenia linkage region. ACSM1 has been identified as a susceptibility gene in a previous schizophrenia GWAS study. The association of ANK3 with schizophrenia is intriguing in light of recent associations of ANK3 with bipolar disorder, thereby supporting the hypothesis of an overlap in genetic susceptibility between these psychopathological entities.

Genome-wide association study of migraine implicates a common susceptibility variant on 8q22.1

Migraine is a common episodic neurological disorder, typically presenting with recurrent attacks of severe headache and autonomic dysfunction. Apart from rare monogenic subtypes, no genetic or molecular markers for migraine have been convincingly established. We identified the minor allele of rs1835740 on chromosome 8q22.1 to be associated with migraine (p=5.12 × 10⁻⁹, OR 1.23 [1.150-1.324]) in a genome-wide association study of 2,748 migraineurs from three European headache clinics and 10,747 population-matched controls. The association was replicated in 3,202 cases and 40,062 controls for an overall meta-analysis p-value of 1.60 × 10⁻¹¹ (OR 1.18 [1.127 – 1.244]). rs1835740 is located between the astrocyte elevated gene 1 (MTDH/AEG-1) and plasma glutamate carboxypeptidase (PGCP). In an expression quantitative trait study in lymphoblastoid cell lines transcript levels of the MTDH/AEG-1 were found to have a significant correlation to rs1835740. Our data establish rs1835740 as the first genetic risk factor for migraine.

Genome-wide association study of major recurrent depression in the U.K. population

OBJECTIVE

Studies of major depression in twins and families have shown moderate to high heritability, but extensive molecular studies have failed to identify susceptibility genes convincingly. To detect genetic variants contributing to major depression, the authors performed a genome-wide association study using 1,636 cases of depression ascertained in the U.K. and 1,594 comparison subjects screened negative for psychiatric disorders.

METHOD

Cases were collected from 1) a case-control study of recurrent depression (the Depression Case Control [DeCC] study; N=1346), 2) an affected sibling pair linkage study of recurrent depression (probands from the Depression Network [DeNT] study; N=332), and 3) a pharmacogenetic study (the Genome-Based Therapeutic Drugs for Depression [GENDEP] study; N=88). Depression cases and comparison subjects were genotyped at Centre National de Génotypage on the Illumina Human610-Quad BeadChip. After applying stringent quality control criteria for missing genotypes, departure from Hardy-Weinberg equilibrium, and low minor allele frequency, the authors tested for association to depression using logistic regression, correcting for population ancestry.

RESULTS

Single nucleotide polymorphisms (SNPs) in BICC1 achieved suggestive evidence for association, which strengthened after imputation of ungenotyped markers, and in analysis of female depression cases. A meta-analysis of U.K. data with previously published results from studies in Munich and Lausanne showed some evidence for association near neuroligin 1 (NLGN1) on chromosome 3, but did not support findings at BICC1.

CONCLUSIONS

This study identifies several signals for association worthy of further investigation but, as in previous genome-wide studies, suggests that individual gene contributions to depression are likely to have only minor effects, and very large pooled analyses will be required to identify them.

A genome-wide association study of neuroticism in a population-based sample

Neuroticism is a moderately heritable personality trait considered to be a risk factor for developing major depression, anxiety disorders and dementia. We performed a genome-wide association study in 2,235 participants drawn from a population-based study of neuroticism, making this the largest association study for neuroticism to date. Neuroticism was measured by the Eysenck Personality Questionnaire. After Quality Control, we analysed 430,000 autosomal SNPs together with an additional 1.2 million SNPs imputed with high quality from the Hap Map CEU samples. We found a very small effect of population stratification, corrected using one principal component, and some cryptic kinship that required no correction. NKAIN2 showed suggestive evidence of association with neuroticism as a main effect (p<10⁻⁶) and GPC6 showed suggestive evidence for interaction with age (p≈10⁻⁷). We found support for one previously-reported association (PDE4D), but failed to replicate other recent reports. These results suggest common SNP variation does not strongly influence neuroticism. Our study was powered to detect almost all SNPs explaining at least 2% of heritability, and so our results effectively exclude the existence of loci having a major effect on neuroticism.

Genome-wide association study of recurrent major depressive disorder in two European case-control cohorts

Major depressive disorder (MDD) is a highly prevalent disorder with substantial heritability. Heritability has been shown to be substantial and higher in the variant of MDD characterized by recurrent episodes of depression. Genetic studies have thus far failed to identify clear and consistent evidence of genetic risk factors for MDD. We conducted a genome-wide association study (GWAS) in two independent datasets. The first GWAS was performed on 1022 recurrent MDD patients and 1000 controls genotyped on the Illumina 550 platform. The second was conducted on 492 recurrent MDD patients and 1052 controls selected from a population-based collection, genotyped on the Affymetrix 5.0 platform. Neither GWAS identified any SNP that achieved GWAS significance. We obtained imputed genotypes at the Illumina loci for the individuals genotyped on the Affymetrix platform, and performed a meta-analysis of the two GWASs for this common set of approximately half a million SNPs. The meta-analysis did not yield genome-wide significant results either. The results from our study suggest that SNPs with substantial odds ratio are unlikely to exist for MDD, at least in our datasets and among the relatively common SNPs genotyped or tagged by the half-million-loci arrays. Meta-analysis of larger datasets is warranted to identify SNPs with smaller effects or with rarer allele frequencies that contribute to the risk of MDD.

Meta-analysis and imputation refines the association of 15q25 with smoking quantity

Smoking is a leading global cause of disease and mortality. We established the Oxford-GlaxoSmithKline study (Ox-GSK) to perform a genome-wide meta-analysis of SNP association with smoking-related behavioral traits. Our final data set included 41,150 individuals drawn from 20 disease, population and control cohorts. Our analysis confirmed an effect on smoking quantity at a locus on 15q25 (P = 9.45 x 10(-19)) that includes CHRNA5, CHRNA3 and CHRNB4, three genes encoding neuronal nicotinic acetylcholine receptor subunits. We used data from the 1000 Genomes project to investigate the region using imputation, which allowed for analysis of virtually all common SNPs in the region and offered a fivefold increase in marker density over HapMap2 (ref. 2) as an imputation reference panel. Our fine-mapping approach identified a SNP showing the highest significance, rs55853698, located within the promoter region of CHRNA5. Conditional analysis also identified a secondary locus (rs6495308) in CHRNA3.

Genome-wide pharmacogenetics of antidepressant response in the GENDEP project

OBJECTIVE

The purpose of this study was to identify genetic variants underlying the considerable individual differences in response to antidepressant treatment. The authors performed a genome-wide association analysis of improvement of depression severity with two antidepressant drugs.

METHOD

High-quality Illumina Human610-quad chip genotyping data were available for 706 unrelated participants of European ancestry treated for major depression with escitalopram (N=394) or nortriptyline (N=312) over a 12-week period in the Genome-Based Therapeutic Drugs for Depression (GENDEP) project, a partially randomized open-label pharmacogenetic trial.

RESULTS

Single nucleotide polymorphisms in two intergenic regions containing copy number variants on chromosomes 1 and 10 were associated with the outcome of treatment with escitalopram or nortriptyline at suggestive levels of significance and with a high posterior likelihood of true association. Drug-specific analyses revealed a genome-wide significant association between marker rs2500535 in the uronyl 2-sulphotransferase gene and response to nortriptyline. Response to escitalopram was best predicted by a marker in the interleukin-11 (IL11) gene. A set of 72 a priori-selected candidate genes did not show pharmacogenetic associations above a chance level, but an association with response to escitalopram was detected in the interleukin-6 gene, which is a close homologue of IL11.

CONCLUSIONS

While limited statistical power means that a number of true associations may have been missed, these results suggest that efficacy of antidepressants may be predicted by genetic markers other than traditional candidates. Genome-wide studies, if properly replicated, may thus be important steps in the elucidation of the genetic basis of pharmacological response.

Population-based linkage analysis of schizophrenia and bipolar case-control cohorts identifies a potential susceptibility locus on 19q13

Population-based linkage analysis is a new method for analysing genomewide single nucleotide polymorphism (SNP) genotype data in case-control samples, which does not assume a common disease, common variant model. The genome is scanned for extended segments that show increased identity-by-descent sharing within case-case pairs, relative to case-control or control-control pairs. The method is robust to allelic heterogeneity and is suited to mapping genes which contain multiple, rare susceptibility variants of relatively high penetrance. We analysed genomewide SNP datasets for two schizophrenia case-control cohorts, collected in Aberdeen (461 cases, 459 controls) and Munich (429 cases, 428 controls). Population-based linkage testing must be performed within homogeneous samples and it was therefore necessary to analyse the cohorts separately. Each cohort was first subjected to several procedures to improve genetic homogeneity, including identity-by-state outlier detection and multidimensional scaling analysis. When testing only cases who reported a positive family history of major psychiatric disease, consistent with a model of strongly penetrant susceptibility alleles, we saw a distinct peak on chromosome 19q in both cohorts that appeared in meta-analysis (P=0.000016) to surpass the traditional level for genomewide significance for complex trait linkage. The linkage signal was also present in a third case-control sample for familial bipolar disorder, such that meta-analysing all three datasets together yielded a linkage P=0.0000026. A model of rare but highly penetrant disease alleles may be more applicable to some instances of major psychiatric diseases than the common disease common variant model, and we therefore suggest that other genome scan datasets are analysed with this new, complementary method.

Pathway-based approaches to imaging genetics association studies: Wnt signaling, GSK3beta substrates and major depression

Several lines of evidence implicate glycogen synthase kinase 3 beta (GSK3beta) in mood disorders. We recently reported associations between GSK3beta polymorphisms and brain structural changes in patients with recurrent major depressive disorder (MDD). Here we provide supporting observations by showing that polymorphisms in additional genes encoding proteins directly related to GSK3beta biological functions are associated with similar regional grey matter (GM) volume changes in MDD patients. We tested specifically for associations with genetic variation in canonical Wnt signaling pathway genes and in genes that encode substrate proteins of GSK3beta. We applied a general linear model with non-stationary cluster-based inference to examine associations between polymorphisms and regional voxel-based morphometry GM volume differences in recurrent MDD patients (n=134) and in age-, gender-, and ethnicity-matched healthy controls (n=144) to test for genotype-by-MDD interactions. We observed associations for polymorphisms in 8/13 canonical Wnt pathway genes and 5/10 GSK3beta substrate genes, predominantly in the temporolateral and medial prefrontal cortices. Similar associations were not found for 100 unrelated polymorphisms tested. This work suggests that identifying SNPs related to genes that encode functionally-interacting proteins that modulate common anatomical regions offers a useful approach to increasing confidence in outcomes from imaging genetics association studies. This is of particular interest when replication datasets are not available. Our observations lend support to the hypothesis that polymorphisms in GSK3beta play a role in MDD susceptibility or expression, in part, by acting via the canonical Wnt signaling pathway and related substrates.

Plasma protein biomarkers for depression and schizophrenia by multi analyte profiling of case-control collections

Despite significant research efforts aimed at understanding the neurobiological underpinnings of psychiatric disorders, the diagnosis and the evaluation of treatment of these disorders are still based solely on relatively subjective assessment of symptoms. Therefore, biological markers which could improve the current classification of psychiatry disorders, and in perspective stratify patients on a biological basis into more homogeneous clinically distinct subgroups, are highly needed. In order to identify novel candidate biological markers for major depression and schizophrenia, we have applied a focused proteomic approach using plasma samples from a large case-control collection. Patients were diagnosed according to DSM criteria using structured interviews and a number of additional clinical variables and demographic information were assessed. Plasma samples from 245 depressed patients, 229 schizophrenic patients and 254 controls were submitted to multi analyte profiling allowing the evaluation of up to 79 proteins, including a series of cytokines, chemokines and neurotrophins previously suggested to be involved in the pathophysiology of depression and schizophrenia. Univariate data analysis showed more significant p-values than would be expected by chance and highlighted several proteins belonging to pathways or mechanisms previously suspected to be involved in the pathophysiology of major depression or schizophrenia, such as insulin and MMP-9 for depression, and BDNF, EGF and a number of chemokines for schizophrenia. Multivariate analysis was carried out to improve the differentiation of cases from controls and identify the most informative panel of markers. The results illustrate the potential of plasma biomarker profiling for psychiatric disorders, when conducted in large collections. The study highlighted a set of analytes as candidate biomarker signatures for depression and schizophrenia, warranting further investigation in independent collections.

Meta-analysis of genome-wide association data identifies a risk locus for major mood disorders on 3p21.1

The major mood disorders, which include bipolar disorder (BD) and major depressive disorder (MDD), are substantially heritable, but few risk loci have been identified. We performed a meta-analysis of 5 major mood disorder case-control samples, including over 13,600 unique individuals genotyped with approximately 500,000 to 1 million single nucleotide polymorphism (SNP) markers on high-density arrays. Allele-wise association results were meta-analyzed with a method that weights results by sample size. We found genome-wide significant evidence that SNPs in a region of chromosome 3p21.1were associated with major mood disorders. The SNP rs2251219 returned the smallest meta-analysis p-value, 3.63 × 10⁻⁸, with a pooled odds ratio of 0.87. Supportive results were observed in 2 out of 3 independent samples tested in a replication study. These results implicate one or more genes in this region in the etiology of major mood disorders and suggest that BD and MDD share genetic risk factors.

A large replication study and meta-analysis in European samples provides further support for association of AHI1 markers with schizophrenia

The Abelson helper integration site 1 (AHI1) gene locus on chromosome 6q23 is among a group of candidate loci for schizophrenia susceptibility that were initially identified by linkage followed by linkage disequilibrium mapping, and subsequent replication of the association in an independent sample. Here, we present results of a replication study of AHI1 locus markers, previously implicated in schizophrenia, in a large European sample (in total 3907 affected and 7429 controls). Furthermore, we perform a meta-analysis of the implicated markers in 4496 affected and 18,920 controls. Both the replication study of new samples and the meta-analysis show evidence for significant overrepresentation of all tested alleles in patients compared with controls (meta-analysis; P = 8.2 x 10(-5)-1.7 x 10(-3), common OR = 1.09-1.11). The region contains two genes, AHI1 and C6orf217, and both genes-as well as the neighbouring phosphodiesterase 7B (PDE7B)-may be considered candidates for involvement in the genetic aetiology of schizophrenia.

Influence of NOS1 on verbal intelligence and working memory in both patients with schizophrenia and healthy control subjects

CONTEXT

Human and animal studies have implicated the gene NOS1 in both cognition and schizophrenia susceptibility.

OBJECTIVE

To investigate whether a potential schizophrenia risk single-nucleotide polymorphism (rs6490121) identified in a recent genome-wide association study negatively influences cognition in patients with schizophrenia and healthy control subjects.

DESIGN

A comparison of both cases and controls grouped according to NOS1 genotype (GG vs AG vs AA) on selected measures of cognition in 2 independent samples. We tested for association between NOS1 rs6490121 and cognitive functions known to be impaired in schizophrenia (IQ, episodic memory, working memory, and attentional control) in an Irish sample. We then sought to replicate the significant results in a German sample.

SETTING

Unrelated patients from general adult psychiatric inpatient and outpatient services and unrelated healthy volunteers from the general population were ascertained.

PARTICIPANTS

Patients with DSM-IV-diagnosed schizophrenia and healthy control subjects from independent samples of Irish (cases, n = 349; controls, n = 230) and German (cases, n = 232; controls, n = 1344) nationality.

RESULTS

A main effect of NOS1 genotype on verbal IQ and working memory was observed in the Irish sample where the homozygous carriers of the schizophrenia risk G allele performed poorly compared with the other genotype groups. These findings were replicated in the German sample, again with the GG genotype carriers performing below other genotype groups. Post hoc analysis of additional IQ measures (full-scale and performance IQ) in the German sample revealed that NOS1 GG carriers underperformed on these measures also.

CONCLUSIONS

NOS1 is associated with clinically significant variation in cognition. Whether this is a mechanism by which schizophrenia risk is increased (eg, via an influence on cognitive reserve) is yet to be confirmed.

Genome-wide association study identifies 19p13.3 (UNC13A) and 9p21.2 as susceptibility loci for sporadic amyotrophic lateral sclerosis

We conducted a genome-wide association study among 2,323 individuals with sporadic amyotrophic lateral sclerosis (ALS) and 9,013 control subjects and evaluated all SNPs with P < 1.0 x 10(-4) in a second, independent cohort of 2,532 affected individuals and 5,940 controls. Analysis of the genome-wide data revealed genome-wide significance for one SNP, rs12608932, with P = 1.30 x 10(-9). This SNP showed robust replication in the second cohort (P = 1.86 x 10(-6)), and a combined analysis over the two stages yielded P = 2.53 x 10(-14). The rs12608932 SNP is located at 19p13.3 and maps to a haplotype block within the boundaries of UNC13A, which regulates the release of neurotransmitters such as glutamate at neuromuscular synapses. Follow-up of additional SNPs showed genome-wide significance for two further SNPs (rs2814707, with P = 7.45 x 10(-9), and rs3849942, with P = 1.01 x 10(-8)) in the combined analysis of both stages. These SNPs are located at chromosome 9p21.2, in a linkage region for familial ALS with frontotemporal dementia found previously in several large pedigrees.

Association of the dystrobrevin binding protein 1 gene (DTNBP1) in a bipolar case-control study (BACCS)

Recent studies suggest a degree of overlap in genetic susceptibility across the traditional categories of schizophrenia and bipolar disorder. There is some evidence for an association of the dystrobrevin binding protein 1 gene (DTNBP1) with schizophrenia, and, thus, this gene has also become a focus of further investigation in bipolar disorder (BD). The aim of our study is to explore the association of DTNBP1 with BD and with a sub phenotype, presence/absence of psychotic symptoms, in a sample of 515 patients with BD (ICD10/DSMIV) and 1,316 ethnically matched control subjects recruited from the UK. Seven DTNBP1 SNPs: rs2743852 (SNP C), rs760761 (P1320), rs1011313 (P1325), rs3213207 (P1635), rs2619539 (P1655), rs16876571 and rs17470454 were investigated using the SNPlex genotyping system and 1 SNP (rs2619522) genotypes were imputed. Association analyses were conducted in a sample of 452 cases and 956 controls. We found significant differences in genotypic and allelic frequencies of rs3213207 and rs760761 of DTNBP1 between bipolar patients and controls. We also showed a global haplotypic association and an association of a particular haplotype with BD. Our results are consistent with previous studies in term of a general association between DTNBP1 and bipolar disorder and provide additional evidence that a portion of the genotypic overlap between schizophrenia and bipolar affective disorder is attributable to this gene.

Common variants conferring risk of schizophrenia

Schizophrenia is a complex disorder, caused by both genetic and environmental factors and their interactions. Research on pathogenesis has traditionally focused on neurotransmitter systems in the brain, particularly those involving dopamine. Schizophrenia has been considered a separate disease for over a century, but in the absence of clear biological markers, diagnosis has historically been based on signs and symptoms. A fundamental message emerging from genome-wide association studies of copy number variations (CNVs) associated with the disease is that its genetic basis does not necessarily conform to classical nosological disease boundaries. Certain CNVs confer not only high relative risk of schizophrenia but also of other psychiatric disorders. The structural variations associated with schizophrenia can involve several genes and the phenotypic syndromes, or the 'genomic disorders', have not yet been characterized. Single nucleotide polymorphism (SNP)-based genome-wide association studies with the potential to implicate individual genes in complex diseases may reveal underlying biological pathways. Here we combined SNP data from several large genome-wide scans and followed up the most significant association signals. We found significant association with several markers spanning the major histocompatibility complex (MHC) region on chromosome 6p21.3-22.1, a marker located upstream of the neurogranin gene (NRGN) on 11q24.2 and a marker in intron four of transcription factor 4 (TCF4) on 18q21.2. Our findings implicating the MHC region are consistent with an immune component to schizophrenia risk, whereas the association with NRGN and TCF4 points to perturbation of pathways involved in brain development, memory and cognition.

Association of GSK3beta polymorphisms with brain structural changes in major depressive disorder

CONTEXT

Indirect evidence suggests that the glycogen synthase kinase-3beta (GSK3beta) gene might be implicated in major depressive disorder (MDD).

BACKGROUND

We evaluated 15 GSK3beta single-nucleotide polymorphisms (SNPs) to test for associations with regional gray matter (GM) volume differences in patients with recurrent MDD. We then used the defined regions of interest based on significant associations to test for MDD x genotype interactions by including a matched control group without any psychiatric disorder, including MDD.

DESIGN

General linear model with nonstationary cluster-based inference.

SETTING

Munich, Germany.

PARTICIPANTS

Patients with recurrent MDD (n = 134) and age-, sex-, and ethnicity-matched healthy controls (n = 143).

MAIN OUTCOME MEASURES

Associations between GSK3beta polymorphisms and regional GM volume differences.

RESULTS

Variation in GM volume was associated with GSK3beta polymorphisms; the most significant associations were found for rs6438552, a putative functional intronic SNP that showed 3 significant GM clusters in the right and left superior temporal gyri and the right hippocampus (P < .001, P = .02, and P = .02, respectively, corrected for multiple comparisons across the whole brain). Similar results were obtained with rs12630592, an SNP in high linkage disequilibrium. A significant SNP x MDD status interaction was observed for the effect on GM volumes in the right hippocampus and superior temporal gyri (P < .001 and P = .01, corrected, respectively).

CONCLUSIONS

The GSK3beta gene may have a role in determining regional GM volume differences of the right hippocampus and bilateral superior temporal gyri. The association between genotype and brain structure was specific to the patients with MDD, suggesting that GSK3beta genotypes might interact with MDD status. We speculate that this is a consequence of regional neocortical, glial, or neuronal growth or survival. In considering core cognitive features of MDD, the association of GSK3beta polymorphisms with structural variation in the temporal lobe and hippocampus is of particular interest in the context of other evidence for structural and functional abnormalities in the hippocampi of patients with MDD.

The PsyCoLaus study: methodology and characteristics of the sample of a population-based survey on psychiatric disorders and their association with genetic and cardiovascular risk factors

BACKGROUND

The Psychiatric arm of the population-based CoLaus study (PsyCoLaus) is designed to: 1) establish the prevalence of threshold and subthreshold psychiatric syndromes in the 35 to 66 year-old population of the city of Lausanne (Switzerland); 2) test the validity of postulated definitions for subthreshold mood and anxiety syndromes; 3) determine the associations between psychiatric disorders, personality traits and cardiovascular diseases (CVD), 4) identify genetic variants that can modify the risk for psychiatric disorders and determine whether genetic risk factors are shared between psychiatric disorders and CVD. This paper presents the method as well as sociodemographic and somatic characteristics of the sample.

METHODS

All 35 to 66 year-old persons previously selected for the population-based CoLaus survey on risk factors for CVD were asked to participate in a substudy assessing psychiatric conditions. This investigation included the Diagnostic Interview for Genetic Studies to elicit diagnostic criteria for threshold disorders according to DSM-IV and algorithmically defined subthreshold syndromes. Complementary information was collected on potential risk and protective factors for psychiatric disorders, migraine and on the morbidity of first-degree relatives, whereas the collection of DNA and plasma samples was already part of the original CoLaus survey.

RESULTS

A total of 3,691 individuals completed the psychiatric evaluation (67% participation). The gender distribution of the sample did not differ significantly from that of the general population in the same age range. Although the youngest 5-year band of the cohort was underrepresented and the oldest 5-year band overrepresented, participants of PsyCoLaus and individuals who refused to participate revealed comparable scores on the General Health Questionnaire, a self-rating instrument completed at the somatic exam.

CONCLUSION

Despite limitations resulting from the relatively low participation in the context of a comprehensive and time-consuming investigation, the PsyCoLaus study should significantly contribute to the current understanding of psychiatric disorders and comorbid somatic conditions by: 1) establishing the clinical relevance of specific psychiatric syndromes below the DSM-IV threshold; 2) determining comorbidity between risk factors for CVD and psychiatric disorders; 3) assessing genetic variants associated with common psychiatric disorders and 4) identifying DNA markers shared between CVD and psychiatric disorders.

A genome-wide investigation of SNPs and CNVs in schizophrenia

We report a genome-wide assessment of single nucleotide polymorphisms (SNPs) and copy number variants (CNVs) in schizophrenia. We investigated SNPs using 871 patients and 863 controls, following up the top hits in four independent cohorts comprising 1,460 patients and 12,995 controls, all of European origin. We found no genome-wide significant associations, nor could we provide support for any previously reported candidate gene or genome-wide associations. We went on to examine CNVs using a subset of 1,013 cases and 1,084 controls of European ancestry, and a further set of 60 cases and 64 controls of African ancestry. We found that eight cases and zero controls carried deletions greater than 2 Mb, of which two, at 8p22 and 16p13.11-p12.4, are newly reported here. A further evaluation of 1,378 controls identified no deletions greater than 2 Mb, suggesting a high prior probability of disease involvement when such deletions are observed in cases. We also provide further evidence for some smaller, previously reported, schizophrenia-associated CNVs, such as those in NRXN1 and APBA2. We could not provide strong support for the hypothesis that schizophrenia patients have a significantly greater "load" of large (>100 kb), rare CNVs, nor could we find common CNVs that associate with schizophrenia. Finally, we did not provide support for the suggestion that schizophrenia-associated CNVs may preferentially disrupt genes in neurodevelopmental pathways. Collectively, these analyses provide the first integrated study of SNPs and CNVs in schizophrenia and support the emerging view that rare deleterious variants may be more important in schizophrenia predisposition than common polymorphisms. While our analyses do not suggest that implicated CNVs impinge on particular key pathways, we do support the contribution of specific genomic regions in schizophrenia, presumably due to recurrent mutation. On balance, these data suggest that very few schizophrenia patients share identical genomic causation, potentially complicating efforts to personalize treatment regimens.

Discovering genetic polymorphism associated with gene expression levels across the whole genome

Genetic differences have been shown to contribute to gene expression variability. A complete evaluation of the associations between a whole genome scan with 550k Single Nucleotide Polymorphisms (SNPs) and 54k detectable expression levels (probesets) was performed on 176 human peripheral blood samples. The results are presented along with visualizations that reveal cis and trans gene expression regulatory effects. The algorithmic approach followed utilized a distributed computational system. The analysis was performed using a linear regression adjusting for all relevant covariates. Permutation testing on a random subset of the top results provided an indication of the significance levels adjusted for multiple testing and the non independence of SNPs due to linkage disequilibrium. The database of the produced results can be used as a resource to assess the functional impact of genetic polymorphisms to gene expression regulation. This resource is applicable across all disease areas.

Serotonin transporter gene and adverse life events in adult ADHD

Childhood attention deficit hyperactivity disorder (ADHD) symptomatology persists in a substantial proportion of cases into adult life. ADHD is highly heritable but the etiology of ADHD is complex and heterogeneous, involving both genetic and non-genetic factors. In the present article we analyzed the influence of both genetics and adverse life events on severity of ADHD symptoms in 110 adult ADHD patients. Subjects were genotyped for the norepinephrine transporter (NET), the catechol-O-methyltransferase (COMT), the serotonin transporter promoter polymorphism (SERTPR) and the more rare A/G variant within SERTPR. Three main outcomes were obtained: (1) adverse events showed a small but positive correlation with current ADHD severity; (2) NET, COMT and the A/G variant within SERTPR were not associated with ADHD severity; (3) taking into account stressors, the long (L) SERTPR variant showed a mild effect on ADHD, being associated with an increased severity, particularly as regard affective dysregulations; on the other hand, in subjects exposed to early stressors, it showed a protective effect, as compared to the short (S) variant. In conclusion, our data support the role of environmental factors in adult ADHD symptomatology. SERTPR may be involved in some features of the illness and act as a moderator of environmental influences in ADHD.

Large recurrent microdeletions associated with schizophrenia

Reduced fecundity, associated with severe mental disorders, places negative selection pressure on risk alleles and may explain, in part, why common variants have not been found that confer risk of disorders such as autism, schizophrenia and mental retardation. Thus, rare variants may account for a larger fraction of the overall genetic risk than previously assumed. In contrast to rare single nucleotide mutations, rare copy number variations (CNVs) can be detected using genome-wide single nucleotide polymorphism arrays. This has led to the identification of CNVs associated with mental retardation and autism. In a genome-wide search for CNVs associating with schizophrenia, we used a population-based sample to identify de novo CNVs by analysing 9,878 transmissions from parents to offspring. The 66 de novo CNVs identified were tested for association in a sample of 1,433 schizophrenia cases and 33,250 controls. Three deletions at 1q21.1, 15q11.2 and 15q13.3 showing nominal association with schizophrenia in the first sample (phase I) were followed up in a second sample of 3,285 cases and 7,951 controls (phase II). All three deletions significantly associate with schizophrenia and related psychoses in the combined sample. The identification of these rare, recurrent risk variants, having occurred independently in multiple founders and being subject to negative selection, is important in itself. CNV analysis may also point the way to the identification of additional and more prevalent risk variants in genes and pathways involved in schizophrenia.

Association study of brain-derived neurotrophic factor (BDNF) and LIN-7 homolog (LIN-7) genes with adult attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric disorder with a large genetic component that has been shown to persist into adulthood in 30-60% of childhood ADHD cases. Adult ADHD confers an increased risk of ADHD in relatives when compared to childhood ADHD, possibly due to a greater genetic liability than the childhood form. Brain-derived neurotrophic factor (BDNF) is a neurotrophin expressed in the brain throughout life and is involved in survival, differentiation, and synaptic plasticity of several neuronal systems including dopaminergic pathways. Mammalian LIN-7 homolog is selectively expressed in specific neuronal populations and is involved in the postsynaptic density of neuronal synapses. LIN-7 is also a positional candidate, as it lies immediately downstream of BDNF. We tested for association between five BDNF polymorphisms, two LIN-7 polymorphisms and adult ADHD. The sample consisted of 80 trios comprised of an adult ADHD proband and their biological parents and an independent sample of 121 adult ADHD cases and a corresponding number of sex, age, and ethnically matched controls (total 201 probands). Allelic and haplotype association was found between both BDNF and adult ADHD, and LIN-7 and adult ADHD. HapMap indicates BDNF and LIN-7 occur in different haplotype blocks, though some linkage disequilibrium exists between the SNPs in these adjacent genes. Further investigations into the pathologic mechanisms of BDNF and LIN-7 in adult ADHD are required.

Failure to replicate effect of Kibra on human memory in two large cohorts of European origin

It was recently suggested that the Kibra polymorphism rs17070145 has a strong effect on multiple episodic memory tasks in humans. We attempted to replicate this using two cohorts of European genetic origin (n = 319 and n = 365). We found no association with either the original SNP or a set of tagging SNPs in the Kibra gene with multiple verbal memory tasks, including one that was an exact replication (Auditory Verbal Learning Task, AVLT). These results suggest that Kibra does not have a strong and general effect on human memory.

Family history of depression is associated with younger age of onset in patients with recurrent depression

BACKGROUND

Genetic epidemiology data suggest that younger age of onset is associated with family history (FH) of depression. The present study tested whether the presence of FH for depression or anxiety in first-degree relatives determines younger age of onset for depression.

METHOD

A sample of 1022 cases with recurrent major depressive disorder (MDD) was recruited at the Max Planck Institute and at two affiliated hospitals. Patients were assessed using the Schedules for Clinical Assessment in Neuropsychiatry and questionnaires including demographics, medical history, questions on the use of alcohol and tobacco, personality traits and life events. Survival analysis and the Cox proportional hazard model were used to determine whether FH of depression signals earlier age of onset of depression.

RESULTS

Patients who reported positive FH had a significantly earlier age of onset than patients who did not report FH of depression (log-rank=48, df=1, p<0.0001). The magnitude of association of FH varies by age of onset, with the largest estimate for MDD onset before age 20 years (hazard ratio=2.2, p=0.0009), whereas FH is not associated with MDD for onset after age 50 years (hazard ratio=0.89, p=0.5). The presence of feelings of guilt, anxiety symptoms and functional impairment due to depressive symptoms appear to characterize individuals with positive FH of depression.

CONCLUSIONS

FH of depression contributes to the onset of depression at a younger age and may affect the clinical features of the illness.

Alpha-5/alpha-3 nicotinic receptor subunit alleles increase risk for heavy smoking

Twin studies indicate that additive genetic effects explain most of the variance in nicotine dependence (ND), a construct emphasizing habitual heavy smoking despite adverse consequences, tolerance and withdrawal. To detect ND alleles, we assessed cigarettes per day (CPD) regularly smoked, in two European populations via whole genome association techniques. In these approximately 7500 persons, a common haplotype in the CHRNA3-CHRNA5 nicotinic receptor subunit gene cluster was associated with CPD (nominal P=6.9 x 10(-5)). In a third set of European populations (n= approximately 7500) which had been genotyped for approximately 6000 SNPs in approximately 2000 genes, an allele in the same haplotype was associated with CPD (nominal P=2.6 x 10(-6)). These results (in three independent populations of European origin, totaling approximately 15 000 individuals) suggest that a common haplotype in the CHRNA5/CHRNA3 gene cluster on chromosome 15 contains alleles, which predispose to ND.

Investigation of the dopamine D5 receptor gene (DRD5) in adult attention deficit hyperactivity disorder

Several lines of evidence from neuroimaging, pharmacology and genetics support the involvement of the dopaminergic system in the etiology of Attention Deficit Hyperactivity Disorder (ADHD). Previous candidate gene studies have investigated the association between a dinucleotide (CA)(n) repeat polymorphism, located 18.5 kb from the start codon of the DRD5 gene, and ADHD. Association between the 148 bp allele and ADHD has been reported in some studies, however replication of the finding has not been consistent. We tested for an association between the (CA)(n) repeat and adult ADHD in a sample comprised of 119 families with adult ADHD probands and 88 unrelated adult ADHD cases with a corresponding number of controls matched for age, ethnicity and sex. In the family sample we found a non-significant trend for association between the 148 bp allele and ADHD (Z=1.91, p=0.055). An excess of non-transmissions was detected for the 150 and 152bp alleles (Z=-2.26, p=0.023; Z=-2.20, p=0.028). Quantitative analysis performed using the Brown Attention Deficit Disorder Scale (BADDS) showed association between the 150 bp allele and lower total score (p=0.011), and lower effort (p=0.008), activation (p=0.008) and attention (p=0.01) cluster scores. We did not replicate association findings in the case-control group, likely due to the lack of statistical power of this sample. Our findings add to the literature suggesting DRD5 (CA)(n) repeat has a modest effect in modulating susceptibility to adult ADHD but further studies are required.

LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia

Left-right asymmetrical brain function underlies much of human cognition, behavior and emotion. Abnormalities of cerebral asymmetry are associated with schizophrenia and other neuropsychiatric disorders. The molecular, developmental and evolutionary origins of human brain asymmetry are unknown. We found significant association of a haplotype upstream of the gene LRRTM1 (Leucine-rich repeat transmembrane neuronal 1) with a quantitative measure of human handedness in a set of dyslexic siblings, when the haplotype was inherited paternally (P=0.00002). While we were unable to find this effect in an epidemiological set of twin-based sibships, we did find that the same haplotype is overtransmitted paternally to individuals with schizophrenia/schizoaffective disorder in a study of 1002 affected families (P=0.0014). We then found direct confirmatory evidence that LRRTM1 is an imprinted gene in humans that shows a variable pattern of maternal downregulation. We also showed that LRRTM1 is expressed during the development of specific forebrain structures, and thus could influence neuronal differentiation and connectivity. This is the first potential genetic influence on human handedness to be identified, and the first putative genetic effect on variability in human brain asymmetry. LRRTM1 is a candidate gene for involvement in several common neurodevelopmental disorders, and may have played a role in human cognitive and behavioral evolution.

Factor structure and external validity of the PANSS revisited

Considerable controversy exists concerning the positive and negative syndrome scale (PANSS), one of the most widely used instruments in schizophrenia research. In this article we revisited the factor structure and external validity of the PANSS in a sample of 500 participants with DSM IV diagnoses of schizophrenia. We found that a model with six latent factors provided a relatively good fit, considered adequate by two rules of thumb. Five factors corresponded closely to those typically derived in other studies: Negative, Positive, Excited/Activation, Anxious-Depressed/Dysphoric, and Disorganized/Autistic preoccupation. The sixth factor seemed to have face validity and was labeled Withdrawn. With the exception of Anxious-Depressed/Dysphoric, Cronbach's Alpha ranged from 0.70 to 0.85 suggesting an acceptable internal consistency. External validity was studied through correlations with socio-demographic variables, DSM IV (subtype) diagnoses, clinical characteristics, and drug use. The many significant correlations suggested that the six PANSS scales measure meaningful aspects of schizophrenia. Furthermore, the pattern of correlations varied, providing evidence that the scales assessed partly different aspects of the disease. Our analyses also suggested that some of the controversy about the PANSS can possibly be attributed to methodological factors where the substantial cross-loadings of some PANSS items may play an important role.

The dopamine-4 receptor gene associated with binge eating and weight gain in women with seasonal affective disorder: an evolutionary perspective

BACKGROUND

We recently described a preliminary association between the hypofunctional seven-repeat allele of the dopamine-4 receptor gene (DRD4) and increased maximal lifetime body mass index in women with seasonal affective disorder (SAD). In this study, we examined whether binge eating behavior mediated this putative association.

METHODS

The study sample consisted of 131 women with winter SAD who reported increased intake of high-carbohydrate/high-fat foods during depressive episodes. We compared rates of binge eating behavior in the two genotypic groups defined by the presence or absence of the seven-repeat allele of DRD4.

RESULTS

Consistent with our working hypothesis, the proportion of binge eaters was significantly greater in probands with the seven-repeat allele (18 of 46, 39.1%) than in probands without this allele (14 of 85, 16.5%) [chi(2)(1)= 8.32, p = .004; odds ratio = 3.25, 95% confidence interval 1.43, 7.41].

CONCLUSIONS

Pending replication in other samples, these results point to a genetic factor that could help in the early identification and treatment of women at higher risk for seasonal weight gain associated with binge eating behavior. At a theoretic level, the current results suggest a novel link between evolutionary models of seasonal weight gain on the one hand and the DRD4 gene on the other.

Linkage disequilibrium analysis of the dopamine beta-hydroxylase gene in persistent attention deficit hyperactivity disorder

Numerous family, twin and adoption studies have reported a strong genetic component for attention deficit hyperactivity disorder (ADHD). In addition, an extensive amount of literature has implicated abnormalities of the dopaminergic system. In view of this evidence, genes that influence dopaminergic transmission have become prime candidates for molecular genetic investigations of ADHD. There are currently three studies (Daly et al., 1999; Roman et al., 2002; Wigg et al., 2002) that have found an association between the dopamine beta-hydroxylase gene (DBH) TaqI 2 allele and childhood ADHD. As such, we tested for association of the DBH TaqI 2 allele in two independent samples of patients with the persistent variant of ADHD. These consisted of 97 nuclear families, and 112 adult cases with controls carefully matched according to gender, age and ethnicity. Transmission Disequilibrium Test analysis revealed weak over-transmission of the 2 allele (35 transmissions versus 27 non-transmissions; chi2 = 1.03, 1 degree of freedom, P=0.31). The case-control sample did not support previous findings since the 2 allele was more frequent in our control sample (137 versus 116; chi2 = 3.63, 1 degree of freedom, P=0.057). Taken together, these results do not provide support for a role of the DBH TaqI marker in our persistent ADHD samples.

Adrenergic alpha 2C receptor genomic organization: association study in adult ADHD

Attention deficit hyperactivity disorder (ADHD) is a chronic psychiatric condition in children and follow up studies have indicated that one to two thirds of patients continue to suffer from ADHD during late adolescence and adulthood. Pharmacological interventions, such as the new selective noradrenergic reuptake inhibitor, atomoxetine, suggest the adrenergic system as candidate in ADHD. We examined for the presence of linkage disequilibrium between the alpha 2C adrenergic receptor (ADRA2C) microsatellite polymorphism upstream of the ADRA2C gene and adult ADHD in a sample of nuclear families. We also present new information on the genomic organization of the ADRA2C locus. The microsatellite polymorphism was typed in 128 small families and analyzed using the Transmission Disequilibrium Test (TDT). There was no preferential transmission of any allele. These findings exclude a strong effect of this ADRA2C marker on risk for ADHD in our sample of adult proband families.

Pharmacogenetics of antipsychotic-induced weight gain

Weight gain appears to be a serious side effect encountered during treatment with many antipsychotic drugs. Although the propensities of inducing weight gain vary considerably between antipsychotics, weight gain is mostly observed in atypical antipsychotics, increasingly prescribed for a variety of psychiatric disorders. Beside the psychological consequences weight gain may influence patients' compliance and secondary medical comorbidities related to being overweight may arise, including diabetes, hypertonia, respiratory problems, and some types of cancer. Obesity research generally suggests that a complex system of neurotransmitters, neuropeptides, hormones and immune related factors interact in neural circuits involving at least the hypothalamus, the solitary tract and cortical structures to regulate energy homeostasis and body weight. Antipsychotics that have weight gain inducing properties may disrupt associated pathways at any of these levels, although it remains unclear what the mechanisms of action might be. Given the potential deleterious effects of weight gain, individual predictors of weight gain would be extremely helpful at the beginning of pharmacological treatment with atypical antipsychotics, allowing obesity to be avoided or for counteractive steps such as dietary restrictions to be taken in predisposed individuals. So far, only a few predictors to detect individuals at high risk have been reported and these have limited power. It is likely that genetic factors play a major role in determining individual response to antipsychotics as well as their side effect profile. In this article, we have reviewed literature related to antipsychotic-induced weight gain and have discussed the major issues, before updating the reader on current obesity research findings. Finally, we emphasize previous studies relating to the pharmacogenetics of antipsychotic-induced weight gain.

Polymorphisms in glutamate decarboxylase genes: analysis in schizophrenia

The decrease of glutamic acid decarboxylase (GAD) has been reported as an important neurochemical alteration of the inhibitory GABAergic interneurons in schizophrenia. To our knowledge no studies have investigated the genetic variants influencing GAD expression. To search for markers contributing to the genetic susceptibility of schizophrenia, we typed two polymorphisms by polymerase chain reaction-restriction fragment length polymorphism in both GAD1 and GAD2 genes in 112 triad families and 46 case-controls. We used the Transmission Disequilibrium Test to perform the qualitative family-based analyses and found negative results (GAD1, chi2 = 0.273, 1 degree of freedom, P = 0.60; GAD2, chi2 = 0, 1 degree of freedom, P = 1). In addition there were no associations with GAD1 and GAD2 and quantitative measures of suicide behaviour in this sample. Although our results are negative, this was the first study to investigate GAD genes in schizophrenia, and further studies of these genes, particularly with schizophrenia subtypes, may prove valuable.

Childhood inattention and dysphoria and adult obesity associated with the dopamine D4 receptor gene in overeating women with seasonal affective disorder

There is significant evidence that altered dopamine activity plays a role in seasonal affective disorder (SAD). The current study examined three separate genetic hypotheses for SAD related to the 7-repeat allele (7R) of the dopamine-4 receptor gene (DRD4), a variant associated with decreased affinity for dopamine. We examined the possible contribution of 7R to the overall expression of SAD, attention deficit disorder (ADD) comorbidity, and body weight regulation. As part of an ongoing genetic study of increased eating behavior and mood in female subjects, 108 women with winter SAD and carbohydrate craving/weight gain were administered the Wender-Utah Rating Scale to measure childhood ADD symptomatology, and a questionnaire to assess maximal lifetime body mass index (BMI). To test for an association between 7R and the categorical diagnosis of SAD, the transmission disequilibrium test (TDT) was used in a subsample of probands providing familial DNA. Standard parametric tests were used to compare childhood ADD symptoms and maximal lifetime BMI across the two genotypic groups defined by the presence or absence of 7R. The TDT found no initial evidence for an association between 7R and the categorical diagnosis of SAD. However, 7R carriers reported significantly greater inattention and dysphoria in childhood (p=0.01 and 0.001, respectively) and a higher maximal lifetime BMI (p=0.007) than did probands without this allele. Furthermore, excluding probands with extreme obesity (maximal BMI >40), a strong correlation was found linking childhood inattentive symptoms and maximal lifetime BMI (r=0.35, p=0.001). In overeating women with SAD, the 7R allele of DRD4 may be associated with a unique developmental trajectory characterized by attentional deficits and dysphoria in childhood and mild to moderate obesity in adulthood. This developmental course may reflect different manifestations of the same underlying vulnerability related to central dopamine dysfunction. Given the possibility of population stratification when studying genotype/phenotype relationships, future use of genomic controls and replication of our findings in other overeating and/or ADD populations are needed to confirm these initial results.

No evidence of linkage or association between the norepinephrine transporter (NET) gene MnlI polymorphism and adult ADHD

Attention deficit hyperactivity disorder (ADHD) is a prevalent psychiatric condition in children and follow-up studies have indicated that one to two thirds of patients continue to suffer from ADHD during late adolescence and adulthood. The mechanism of action of the new selective noradrenergic reuptake inhibitor, atomoxetine, efficacious in the treatment of ADHD symptoms, suggests that the norepinephrine transporter (NET) may be involved in ADHD. The aim of this study was to investigate for the presence of linkage disequilibrium between the MnlI RFLP in the NET gene and adult ADHD in a sample of nuclear families. The MnlI polymorphism was typed in 128 trios and analyzed using the transmission disequilibrium test (TDT). There was no preferential transmission of either allele (chi(2) = 0.209, df = 1, P = 0.647).

Discovery of a null mutation in a human trace amine receptor gene

G-protein-coupled receptors (GPCRs) are important mediators of signal transduction, and mutations in GPCR-encoding genes can lead to disease states. Here we describe a null mutation in an orphan GPCR-encoding gene that is predicted to inactivate completely the encoded receptor. The TA(3) receptor is a putative member of the recently described mammalian trace amine receptor family, and it is expressed in the pituitary gland and skeletal muscle. We tested for the presence of the mutant form of TA(3) (named TA(3)-TR) in a normal population, as well as in two disease groups (ADHD and bipolar affective disorder). We found TA(3)-TR to be commonly expressed in all groups, with approximately 20% allele frequency. We did not find any statistically significant correlation between either disease and the presence of TA(3)-TR.

A drosophila model for attention deficit hyperactivity disorder (ADHD): No evidence of association with PRKG1 gene

Attention deficit hyperactivity disorder (ADHD) is a prevalent psychiatric condition in children and follow up studies have indicated that 22-33% of patients continue to suffer from ADHD during late adolescence and adulthood. The action of psychostimulant drugs may be determined by additional mechanisms beyond the dopamine transporter and receptors. We are exploring new methodology for discovering these mechanisms. For example, in Drosophila, such an additional determinant of psychostimulant action could be protein kinase G (PKG) that affects food-search behavior. Here we initiated studies with the human homologue of PKG, the PRKG1 gene. The aim of this study was to investigate for the presence of linkage disequilibrium between the protein kinase G gene (PRKG1) and adult ADHD in a sample of nuclear families. Genotyping data for the C2276T polymorphism were analyzed using the Transmission Disequilibrium Test (TDT). Sixty three nuclear families were informative for the TDT on C2276T polymorphism, which showed no preferential transmission of either allele (chi-square = 0.778, df = 1, p = 0.316). These findings exclude a direct involvement of this genetic marker of the Protein kinase G gene in the pathogenesis of ADHD.