The hierarchical factor model of ADHD: invariant across age and national groupings?

OBJECTIVE: To examine the factor structure of attention-deficit/hyperactivity disorder (ADHD) in a clinical sample of 1,373 children and adolescents with ADHD and their 1,772 unselected siblings recruited from different countries across a large age range. Hierarchical and correlated factor analytic models were compared separately in the ADHD and sibling samples, across three different instruments and across parent and teacher informants. Specific consideration was given to factorial invariance analyses across different ages and different countries in the ADHD sample. METHOD: A sample of children and adolescents between 5 and 17 years of age with ADHD and their unselected siblings was assessed. Participants were recruited from seven European countries and Israel. ADHD symptom data came from a clinical interview with parents Parental Account of Childhood Symptoms and questionnaires from parents and teachers (Conners Parent and Teacher). RESULTS: A hierarchical general factor model with two specific factors best represented the structure of ADHD in both the ADHD and unselected sibling groups, and across informants and instruments. The model was robust and invariant with regard to age differences in the ADHD sample. The model was not strongly invariant across different national groups in the ADHD sample, likely reflecting severity differences across the different centers and not any substantial difference in the clinical presentation of ADHD. CONCLUSIONS: The results replicate previous studies of a model with a unitary ADHD component and separable specific traits of inattention and hyperactivity/impulsivity. The unique contribution of this study was finding support for this model across a large developmental and multinational/multicultural sample and its invariance across ages.

Autism symptoms in Attention-Deficit/Hyperactivity Disorder: a familial trait which correlates with conduct, oppositional defiant, language and motor disorders.

It is hypothesised that autism symptoms are present in Attention-Deficit/Hyperactivity Disorder (ADHD), are familial and index subtypes of ADHD. Autism symptoms were compared in 821 ADHD probands, 1050 siblings and 149 controls. Shared familiality of autism symptoms and ADHD was calculated using DeFries-Fulker analysis. Autism symptoms were higher in probands than siblings or controls, and higher in male siblings than male controls. Autism symptoms were familial, partly shared with familiality of ADHD in males. Latent class analysis using SCQ-score yielded five classes; Class 1(31%) had few autism symptoms and low comorbidity; Classes 2-4 were intermediate; Class 5(7%) had high autism symptoms and comorbidity. Thus autism symptoms in ADHD represent a familial trait associated with increased neurodevelopmental and oppositional/conduct disorders.

Association between tryptophan hydroxylase 2, performance on a continuance performance test and response to methylphenidate in ADHD participants.

The main objective of this study was to examine neuropsychological mechanisms mediating the association between tryptophan hydroxylase 2 (TPH2) and attention deficit hyperactivity disorder (ADHD). A continuous performance test (T.O.V.A.) was administered to 344 participants diagnosed with DSM IV ADHD who were also genotyped for eight TPH2 intronic SNPs. Association between TPH2 (single SNPs and haplotypes), ADHD, and performance on the T.O.V.A. were tested using robust family-based association tests as implemented in two statistical genetic programs: UNPHASED and PBAT. Association was only observed between an eight locus haplotype and ADHD DSM IV combined type III (global P = 0.036). Robust association was observed between TPH2 single SNPs (and haplotypes) and performance on the T.O.V.A., especially Errors of Omission (eight locus haplotypes, global P = 0.038). Significant associations were also observed between TPH2 and improvement (before-after scores) in T.O.V.A. Total Response Variability scores following acute methylphenidate challenge (eight locus haplotypes, global P = 0.009). Using the MFBAT program, significant multivariate association was observed between single SNPs and haplotypes [eight locus haplotypes and all four T.O.V.A. variables (PBAT-GEE P = 0.013)]. The two most common TPH2 eight locus haplotypes were in a Yin Yang configuration and the Yang haplotype was the risk haplotype for both DSM IV ADHD and deficits in neuropsychological performance. The current investigation shows that risk for ADHD conferred by TPH2 variants is partially mediated by serotonergic mechanisms impacting some facets of executive function. Importantly, improvement in T.O.V.A. performance, especially on Response Time Variability, following methylphenidate was also associated with TPH2.

Intelligence in DSM-IV combined type attention-deficit/hyperactivity disorder is not predicted by either dopamine receptor/transporter genes or other previously identified risk alleles for attention-deficit/hyperactivity disorder.

A major goal of genetic studies of attention deficit hyperactivity disorder (ADHD) is to identify individual characteristics that might help segregate the disorder's inherent heterogeneity. [Mill et al. (2006); Arch Ger Psychiatry 63:462-469] recently reported a potentially important association between two dopamine-related risk polymorphisms (DRD4 variable number tandem repeat (VNTR) in exon 3 and DAT1 VNTR in the 3' UTR) and lowered IQ in ADHD. The objective of the current study was to replicate the [Mill et al. (2006); Arch Ger Psychiatry 63:462-469] findings in a clinical sample and to extend the analysis to a large range of alternative SNP markers of putative ADHD risk alleles identified in a recent study [Brookes et al. (2006); Mol Genet 11:934-953]. Participants were 1081 children and adolescents with a research-confirmed combined type ADHD diagnosis and 1300 unaffected siblings who took part in the International Multi-centre ADHD Genetics (IMAGE) project. They were recruited from multiple settings from across Europe: Belgium, Britain, Germany, Ireland, Israel, Netherlands, Spain and Switzerland. The results were that ADHD was associated with reduced IQ. However, there was no association between the two dopamine-related risk polymorphisms and IQ in either the probands or their siblings. Furthermore, other selected genetic markers previously demonstrated to be associated with ADHD in this sample were not associated with IQ. This large scale study with a clinically ascertained and regorously diagnosed sample failed to replicate the association between genetic polymorphisms in the dopamine system and IQ in ADHD. We also observed no association of other SNPs with IQ in ADHD.

Genetic heterogeneity in ADHD: DAT1 gene only affects probands without CD.

Previous studies have found heterogeneous association between DAT1-3'-UTR-VNTR and attention deficit hyperactivity disorder (ADHD). Various proportions of conduct disorder (CD) comorbidity in their ADHD samples may partially explain the observational discrepancies. Evidence for this comes from family and twin studies which found ADHD probands with CD (ADHD + CD) are genetically different from those without CD (ADHD - CD). Genotypes of 20 DAT1 markers were analyzed in 576 trios, consisting of 141 ADHD + CD and 435 ADHD - CD. In addition to the classical TDT test, a specific genetic heterogeneity test was performed to identify variants that have different transmission patterns in the two phenotypic subgroups. After multiple-test correction, rs40184 and rs2652511 were significant in TDT tests. Further heterogeneity test found the two SNPs had a significant transmission pattern difference between ADHD + CD and ADHD - CD children, indicating that DAT1 has a significantly greater genetic influence on ADHD without CD. Although the result needs further replications, it does highlight the importance of selecting genetically homogeneous samples for molecular genetic analyses of ADHD.

DSM-IV combined type ADHD shows familial association with sibling trait scores: a sampling strategy for QTL linkage.

Attention deficit hyperactivity disorder (ADHD) is a discrete clinical syndrome characterized by the triad of inattention, hyperactivity, and impulsivity in the context of marked impairments. Molecular genetic studies have been successful in identifying genetic variants associated with ADHD, particularly with DSM-IV inattentive and combined subtypes. Quantitative trait locus (QTL) approaches to linkage and association mapping have yet to be widely used in ADHD research, although twin studies investigating individual differences suggest that genetic liability for ADHD is continuously distributed throughout the population, underscoring the applicability of quantitative dimensional approaches. To investigate the appropriateness of QTL approaches, we tested the familial association between 894 probands with a research diagnosis of DSM-IV ADHD combined type and continuous trait measures among 1,135 of their siblings unselected for phenotype. The sibling recurrence rate for ADHD combined subtype was 12.7%, yielding a sibling recurrence risk ratio (lambda(sib)) of 9.0. Estimated sibling correlations around 0.2-0.3 are similar to those estimated from the analysis of fraternal twins in population twin samples. We further show that there are no threshold effects on the sibling risk for ADHD among the ADHD probands; and that both affected and unaffected siblings contributed to the association with ADHD trait scores. In conclusion, these data confirm the main requirement for QTL mapping of ADHD by demonstrating that narrowly defined DSM-IV combined type probands show familial association with dimensional ADHD symptom scores amongst their siblings.

Partial replication of a DRD4 association in ADHD individuals using a statistically derived quantitative trait for ADHD in a family-based association test.

BACKGROUND: Previous research found an association between single nucleotide polymorphisms (SNPs) in the promoter region of DRD4 and statistically derived phenotypes generated from attention-deficit/hyperactivity disorder (ADHD) symptoms. We sought to replicate this finding by using the same methodology in an independent sample of ADHD individuals. METHODS: Four SNPs were genotyped in and around DRD4 in 2631 individuals in 642 families. We developed a quantitative phenotype at each SNP by weighting nine inattentive and nine hyperactive-impulsive symptoms. The weights were selected to maximize the heritability at each SNP. Once a quantitative phenotype was generated at each SNP, the screening procedure implemented in PBAT was used to select and test the five SNPs/genetic model combinations with the greatest power to detect an association for DRD4. RESULTS: One of the four SNPs was associated with the quantitative phenotypes generated from the ADHD symptoms (corrected p-values = .02). A rank ordering of the correlation between each of the ADHD symptoms and the quantitative phenotype suggested that hyperactive-impulsive symptoms were more strongly correlated with the phenotype; however, including inattentive symptoms was necessary to achieve a significant result. CONCLUSIONS: This study partially replicated a previous finding by identifying an association between rs7124601 and a quantitative trait generated from ADHD symptoms. The rs7124601 is in linkage disequilibrium (LD) with the SNPs identified previously. In contrast to the previous study, this finding suggests that both hyperactive-impulsive and inattentive symptoms are important in the association.

Confirmation that a specific haplotype of the dopamine transporter gene is associated with combined-type ADHD.

OBJECTIVE: The primary purpose of this study was to confirm the association of a specific haplotype of the dopamine transporter gene and attention deficit hyperactivity disorder (ADHD), which could be one source of the heterogeneity seen across published studies. METHOD: The authors previously reported the association of ADHD with a subgroup of chromosomes containing specific alleles of two variable-number tandem repeat polymorphisms within the 3' untranslated region and intron 8 of the dopamine transporter gene. They now report on this association in a sample of ADHD combined-type probands. RESULTS: The original observations were confirmed, with an overall odds ratio of 1.4 across samples. CONCLUSIONS: These data challenge results of meta-analyses suggesting that dopamine transporter variation does not have an effect on the risk for ADHD, and they indicate that further investigation of functional variation in the gene is required.

An autonomic nervous system biofeedback modality for the treatment of attention deficit hyperactivity disorder--an open pilot study.

OBJECTIVE: To study the effect of a new non-invasive technique of non-cognitive biofeedback called Autonomic Nervous System Biofeedback Modality on the behavioral and attention parameters of a sample of children with attention deficit hyperactivity disorder. METHOD: 19 subjects attending regular schools, who met DSM-IV criteria for attention deficit hyperactivity disorder, received four sessions of Autonomic Nervous System Biofeedback Modality treatment. The heart rate variability was measured before and after the treatment, as were measures of efficacy including Conners' Teacher Questionnaires (28 items), the Child Behavior Check List for parents and teachers and Continuous Performance Test. RESULTS: Positive treatment effect was observed in all the subjects. The parent's Child Behavior Check List and Conners' Teacher Questionnaires of the whole group showed statistically significant differences. The teacher's Child Behavior Check List showed positive change not reaching statistical significance. A positive correlation between heart rate variability changes and improvement of symptoms of attention deficit hyperactivity disorder was found. CONCLUSION: These are preliminary findings of apparent efficacy of Autonomic Nervous System Biofeedback Modality treatment for attention deficit hyperactivity disorder in children. Future controlled trials are warranted.

Heart rate variability during a continuous performance test in children with problems of attention.

BACKGROUND: Children with impulsive behavior and poor self regulation have been shown to have low parasympathetic tone. High vagal tone is associated with attention to novel stimuli. OBJECTIVE: To study if Heart Rate Variability, an index of vagal tone, is a mediator of attention. METHOD: 77 children who performed a Continuous Performance test (TOVA test) had their EKG recorded for Heart Rate Variability Measurements. Subjects were assigned to groups according to their performance on the TOVA test and a general linear model for repeated measures applied. Pearson Correlations were applied for TOVA scores and HRV Values at four epochs. RESULTS: No individual correlations were found between Attention Scores and HRV. However, there was a significant group difference showing that good performers had a higher "vagal" tone than poor performers. CONCLUSION: The parasympathetic system as measured through HRV is not a mediator of attention. HRV may be an indicator of better health and ability to self regulate.

Population differences in the International Multi-Centre ADHD Gene Project.

The International Multi-Centre ADHD Gene sample consists of 674 families from eight countries (Belgium, England, Germany, Holland, Ireland, Israel, Spain, and Switzerland) ascertained from clinics for combined-type attention definity hyperactivity disorder in an offspring. 863 SNPs were successfully genotyped across 47 autosomal genes implicated in psychiatric disorders yielding a single nucleotide polymorphism (SNP) density of approximately one SNP per 2.5 kb. A global test of heterogeneity showed 269 SNPs nominally significant (expected 43). Inclusion of the Israeli population accounted for approximately 70% of these nominally significant tests. Hardy-Weinberg equilibrium tests suggest that combining all these populations would induce stratification, but that the Northern European populations (Belgium, England, Germany, Holland, and Ireland) could be appropriate. Tag SNPs were generated using pair-wise and aggressive tagging from Carlson et al. [2004] and de Bakker et al. [2005], respectively, in each population and applied to the other populations. Cross-population performance across Northern Europe was consistent with within population comparisons. Smaller sample size for each population tended to yield more problems for the generation of aggressive tags and the application of pair-wise tags. Any case-control sample employing an Israeli sample with Northern Europeans must consider stratification. A Northern European tag set, however, appears to be appropriate for capturing the variation across populations.

Linkage to chromosome 1p36 for attention-deficit/hyperactivity disorder traits in school and home settings.

BACKGROUND: Limited success has been achieved through previous attention-deficit/hyperactivity disorder (ADHD) linkage scans, which were all designed to map genes underlying the dichotomous phenotype. The International Multi-centre ADHD Genetics (IMAGE) project performed a whole genome linkage scan specifically designed to map ADHD quantitative trait loci (QTL). METHODS: A set of 1094 single selected Caucasian ADHD nuclear families was genotyped on a highly accurate and informative single nucleotide polymorphism (SNP) panel. Two quantitative traits measuring the children's symptoms in home and school settings were collected and standardized according to a population sample of 8000 children to reflect the developmental nature and gender prevalence difference of ADHD. Univariate linkage test was performed on both traits and their mean score. RESULTS: A significant common linkage locus was found at chromosome 1p36 with a locus-specific heritability of 5.1% and a genomewide empirical p < .04. Setting-specific suggestive linkage signals were also found: logarithm of odds (LOD) = 2.2 at 9p23 for home trait and LOD = 2.6 at 11q21 for school trait. CONCLUSIONS: These results indicate that given large samples with proper phenotypic measures, searching for ADHD genes with a QTL strategy is an important alternative to using the clinical diagnosis. The fact that our linkage region 1p36 overlaps with the dyslexia QTL DYX8 further suggests it is potentially a pleiotropic locus for ADHD and dyslexia.

Association of the dopamine D5 receptor with attention deficit hyperactivity disorder (ADHD) and scores on a continuous performance test (TOVA).

Towards further clarifying the role of dopamine D5 receptor (DRD5) microsatellite polymorphism in the etiology of ADHD, we used a robust family based strategy to test for association between DRD5 and this disorder. Additionally, a neuropsychological mechanism by which this allele may confer risk was explored by examining the relationship between DRD5 genotype and scores on a continuous performance test. DNA was obtained from 164 probands and their parents. Additionally, the majority of these probands were administered a computerized continuous performance test, the Test Of Variables of Attention (TOVA). We first confirmed preferential transmission (TDT chi(2) = 7.02, P = 0.008) of the 148 base pair allele in 155 informative transmissions (94 transmitted and 61 non-transmitted 148 bp allele). Additionally, we used a family-based association test (FBAT) and observed significant multivariate association using FBAT between TOVA scores before methylphenidate administration and the DRD5 microsatellite polymorphism across all four TOVA variables: multi-allelic, multivariate test chi(2) = 16.32, P = 0.037 when the 148 bp allele was compared to all others (collapsed genotype) that was also significant (chi(2) = 59.20, P = 0.025) when all 14 alleles (full genotype) were analyzed. Following methylphenidate, no significant association was observed (chi(2) = 12.08, P = 0.147 for 148 bp versus all others) and, similarly, for all 14 alleles (chi(2) = 47.18, P = 0.343). In summary, the main finding of this report is that the DRD5 repeat polymorphism confers a small but significant risk for ADHD consistent with previous reports. Provisional results in this single investigation suggest that the DRD5 microsatellite also affects performance scores on the TOVA.