Andreas Warnke

Meta-analysis of genome-wide association studies of attention-deficit/hyperactivity disorder

OBJECTIVE Although twin and family studies have shown attention-deficit/hyperactivity disorder (ADHD) to be highly heritable, genetic variants influencing the trait at a genome-wide significant level have yet to be identified. As prior genome-wide association studies (GWAS) have not yielded significant results, we conducted a meta-analysis of existing studies to boost statistical power. METHOD We used data from four projects: a) the Childrens Hospital of Philadelphia (CHOP); b) phase I of the International Multicenter ADHD Genetics project (IMAGE); c) phase II of IMAGE (IMAGE II); and d) the Pfizer-funded study from the University of California, Los Angeles, Washington University, and Massachusetts General Hospital (PUWMa). The final sample size consisted of 2,064 trios, 896 cases, and 2,455 controls. For each study, we imputed HapMap single nucleotide polymorphisms, computed association test statistics and transformed them to z-scores, and then combined weighted z-scores in a meta-analysis. RESULTS No genome-wide significant associations were found, although an analysis of candidate genes suggests that they may be involved in the disorder. CONCLUSIONS Given that ADHD is a highly heritable disorder, our negative results suggest that the effects of common ADHD risk variants must, individually, be very small or that other types of variants, e.g., rare ones, account for much of the disorders heritability.

Molecular genetics of adult ADHD: converging evidence from genome-wide association and extended pedigree linkage studies

A genome-wide association (GWA) study with pooled DNA in adult attention-deficit/hyperactivity disorder (ADHD) employing ~500K SNP markers identifies novel risk genes and reveals remarkable overlap with findings from recent GWA scans in substance use disorders. Comparison with results from our previously reported high-resolution linkage scan in extended pedigrees confirms several chromosomal loci, including 16q23.1-24.3 which also reached genome-wide significance in a recent meta-analysis of seven linkage studies (Zhou et al. in Am J Med Genet Part B, 2008). The findings provide additional support for a common effect of genes coding for cell adhesion molecules (e.g., CDH13, ASTN2) and regulators of synaptic plasticity (e.g., CTNNA2, KALRN) despite the complex multifactorial etiologies of adult ADHD and addiction vulnerability.

Strong genetic evidence of DCDC2 as a susceptibility gene for dyslexia.

We searched for linkage disequilibrium (LD) in 137 triads with dyslexia, using markers that span the most-replicated dyslexia susceptibility region on 6p21-p22, and found association between the disease and markers within the VMP/DCDC2/KAAG1 locus. Detailed refinement of the LD region, involving sequencing and genotyping of additional markers, showed significant association within DCDC2 in single-marker and haplotype analyses. The association appeared to be strongest in severely affected patients. In a second step, the study was extended to include an independent sample of 239 triads with dyslexia, in which the association--in particular, with the severe phenotype of dyslexia--was confirmed. Our expression data showed that DCDC2, which contains a doublecortin homology domain that is possibly involved in cortical neuron migration, is expressed in the fetal and adult CNS, which--together with the hypothesized protein function--is in accordance with findings in dyslexic patients with abnormal neuronal migration and maturation.

Genome-wide copy number variation study associates metabotropic glutamate receptor gene networks with attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a common, heritable neuropsychiatric disorder of unknown etiology. We performed a whole-genome copy number variation (CNV) study on 1,013 cases with ADHD and 4,105 healthy children of European ancestry using 550,000 SNPs. We evaluated statistically significant findings in multiple independent cohorts, with a total of 2,493 cases with ADHD and 9,222 controls of European ancestry, using matched platforms. CNVs affecting metabotropic glutamate receptor genes were enriched across all cohorts (P = 2.1 × 10−9). We saw GRM5 (encoding glutamate receptor, metabotropic 5) deletions in ten cases and one control (P = 1.36 × 10−6). We saw GRM7 deletions in six cases, and we saw GRM8 deletions in eight cases and no controls. GRM1 was duplicated in eight cases. We experimentally validated the observed variants using quantitative RT-PCR. A gene network analysis showed that genes interacting with the genes in the GRM family are enriched for CNVs in ∼10% of the cases (P = 4.38 × 10−10) after correction for occurrence in the controls. We identified rare recurrent CNVs affecting glutamatergic neurotransmission genes that were overrepresented in multiple ADHD cohorts.

Genome-wide analysis of copy number variants in attention deficit hyperactivity disorder: the role of rare variants and duplications at 15q13.3

Objective: Attention deficit hyperactivity disorder (ADHD) is a common, highly heritable psychiatric disorder. Because of its multifactorial etiology, however, identifying the genes involved has been difficult. The authors followed up on recent findings suggesting that rare copy number variants (CNVs) may be important for ADHD etiology. Method: The authors performed a genome-wide analysis of large, rare CNVs (<1% population frequency) in children with ADHD (N=896) and comparison subjects (N=2,455) from the IMAGE II Consortium. Results: The authors observed 1,562 individually rare CNVs >100 kb in size, which segregated into 912 independent loci. Overall, the rate of rare CNVs >100 kb was 1.15 times higher in ADHD case subjects relative to comparison subjects, with duplications spanning known genes showing a 1.2-fold enrichment. In accordance with a previous study, rare CNVs >500 kb showed the greatest enrichment (1.28-fold). CNVs identified in ADHD case subjects were significantly enriched for loci implicated in autism and in schizophrenia. Duplications spanning the CHRNA7 gene at chromosome 15q13.3 were associated with ADHD in single-locus analysis. This finding was consistently replicated in an additional 2,242 ADHD case subjects and 8,552 comparison subjects from four independent cohorts from the United Kingdom, the United States, and Canada. Presence of the duplication at 15q13.3 appeared to be associated with comorbid conduct disorder. Conclusions: These findings support the enrichment of large, rare CNVs in ADHD and implicate duplications at 15q13.3 as a novel risk factor for ADHD. With a frequency of 0.6% in the populations investigated and a relatively large effect size (odds ratio=2.22, 95% confidence interval=1.5–3.6), this locus could be an important contributor to ADHD etiology.

Leptin levels in patients with anorexia nervosa are reduced in the acute stage and elevated upon short-term weight restoration

Circulating leptin concentrations are known to be low in acute anorexia nervosa (AN), which is characterized by low weight, amenorrhea and specific psychopathological features. In this study plasma leptin concentrations were determined during inpatient treatment of 23 adolescent females with AN using a sensitive radioimmunoassay (RIA) and set into relationship to leptin levels of females matched for age, body mass index (BMI; kg m−2) and/or percent body fat. At referral patients had leptin concentrations well below the female controls. Weight gains led to steep increases of leptin levels which peaked at values well in excess of those observed in controls matched for BMI. In patients who reached the final treatment stage and who were followed-up after discharge, levels subsequently fluctuated and finally dropped into or below the control range. The low leptin levels at referral are likely to be involved in the pathogenesis of amenorrhea and the reduced metabolic state of acutely ill patients. Peak leptin levels reached after weight gain are possibly the cause of increased energy expenditure during this stage of the disorder.

Genetics of early-onset obsessive-compulsive disorder

Obsessive–compulsive disorder (OCD) is characterized by recurrent, intrusive and disturbing thoughts as well as by repetitive stereotypic behaviors. Epidemiological data are similar in children and adults, i.e., between 1 and 3% of the general population suffer from OCD. Children with OCD are often seriously impaired in their development. OCD, especially of early onset, has been shown to be familial. Several candidate genes of predominantly neurotransmitter systems have been analyzed and a total of three genome-wide linkage scans have been performed until now. Analyses of candidate genes in linkage regions have not provided evidence for their involvement in OCD, with the exception of the glutamate transporter gene SLC1A1 on 9p24. Genome-wide association analyses are in progress and the results will promote further independent replication studies. The consideration of subtypes regarding age of onset, symptom dimensions and/or comorbid disorders is needed.

A common variant of the latrophilin 3 gene, LPHN3, confers susceptibility to ADHD and predicts effectiveness of stimulant medication

Attention-Deficit/Hyperactivity Disorder (ADHD) has a very high heritability (0.8), suggesting that about 80% of phenotypic variance is due to genetic factors. We used the integration of statistical and functional approaches to discover a novel gene that contributes to ADHD. For our statistical approach, we started with a linkage study based on large multigenerational families in a population isolate, followed by fine mapping of targeted regions using a family-based design. Family- and population-based association studies in five samples from disparate regions of the world were used for replication. Brain imaging studies were performed to evaluate gene function. The linkage study discovered a genome region harbored in the Latrophilin 3 gene (LPHN3). In the world-wide samples (total n=6360, with 2627 ADHD cases and 2531 controls) statistical association of LPHN3 and ADHD was confirmed. Functional studies revealed that LPHN3 variants are expressed in key brain regions related to attention and activity, affect metabolism in neural circuits implicated in ADHD, and are associated with response to stimulant medication. Linkage and replicated association of ADHD with a novel non-candidate gene (LPHN3) provide new insights into the genetics, neurobiology, and treatment of ADHD.

Transmission disequilibrium of polymorphic variants in the tryptophan hydroxylase-2 gene in attention-deficit/ hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most common behavioral disorder in childhood with substantial heritability. Pharmacological and molecular genetic studies as well as characterization of animal models have implicated serotonergic dysfunction in the pathophysiology of ADHD. Here, we investigated the effect of polymorphic variants in the gene of the tryptophan hydroxylase-2 (TPH2), the rate-limiting enzyme of serotonin (5-HT) synthesis in the brain, in children and adolescents with ADHD. We analyzed three single nucleotide polymorphisms (SNPs) in and downstream of the transcriptional control region of the TPH2 gene in 103 families with 225 affected children. Allelic association in families with more than one affected child was assessed using the pedigree disequilibrium test. Preferential transmissions were detected for the two SNPs in TPH2s regulatory region (rs4570625, P=0.049; rs11178997, P=0.034), but not for the third SNP in intron 2 (rs4565946, P=0.3517). Haplotype analysis revealed a strong trend of association between the regulatory region SNPs (rs4570625, rs11178997) and ADHD (P=0.064). Our results link potentially functional TPH2 variations to the pathophysiology of ADHD, and further support the relevance of 5-HT in disorders related to altered motor activity and cognitive processes.

Altered response control and anterior cingulate function in attention-deficit/hyperactivity disorder boys

OBJECTIVE To investigate mechanisms and structures underlying prefrontal response control and inhibition in boys suffering from attention-deficit/hyperactivity disorder (ADHD). METHOD Sixteen boys with ADHD and 19 healthy controls were investigated electrophysiologically during performance of a visual Go-Nogo task (Continuous Performance Test, CPT). An electrophysiological source localization method was employed to further analyze the data. RESULTS The ADHD boys showed a significantly diminished central Nogo-P3, due to a lack of Nogo-related frontalization of the positive brain electrical field in this group. This two-dimensional effect was associated with a significantly reduced activation of the anterior cingulate cortex (ACC) in the ADHD boys in the Nogo condition of the CPT. Both groups did not significantly differ regarding the amplitude of the Nogo-N2. CONCLUSIONS The results indicate deficits in prefrontal response control in unmedicated ADHD boys that do not seem to be specifically inhibitory in nature. A supposed dysfunction of the ACC in ADHD was confirmed.