Genetic linkage of attention-deficit/hyperactivity disorder on chromosome 16p13, in a region implicated in autism

Molecular Mechanisms Underlying NMDARs Dysfunction and Their Role in ADHD Pathogenesis

Attention deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, although the aetiology of ADHD is not yet understood. One proposed theory for developing ADHD is N-methyl-D-aspartate receptors (NMDARs) dysfunction. NMDARs are involved in regulating synaptic plasticity and memory function in the brain. Abnormal expression or polymorphism of some genes associated with ADHD results in NMDAR dysfunction. Correspondingly, NMDAR malfunction in animal models results in ADHD-like symptoms, such as impulsivity and hyperactivity. Currently, there are no drugs for ADHD that specifically target NMDARs. However, NMDAR-stabilizing drugs have shown promise in improving ADHD symptoms with fewer side effects than the currently most widely used psychostimulant in ADHD treatment, methylphenidate. In this review, we outline the molecular and genetic basis of NMDAR malfunction and how it affects the course of ADHD. We also present new therapeutic options related to treating ADHD by targeting NMDAR.

A Systematic Screening of ADHD-Susceptible Variants From 25 Chinese Parents-Offspring Trios

Attention-deficit/hyperactivity disorder (ADHD) is one of the most prevalent and heritable childhood behavioral disorders. Although a number of ADHD-susceptible regions had been identified, details about the variations of genes and their related patterns involved in ADHD are still lacking. In this study, we collected 25 Chinese parents-offspring trios, each of which consisted of a child diagnosed with ADHD and his/her unaffected parents, and analyzed the variations from whole-genome sequencing data. SNVs in reported ADHD-susceptible regions and on the genes whose functions were related to dopamine were screened, and we identified a set of variants with functional annotations which were specifically detected in ADHD children, including most SNVs in the gene coding region that might impair protein functions and a few SNVs in promoter or 3' untranslated region (3'-UTR) that might affect the regulation of relative gene expression in a transcriptional or posttranscriptional level. All the information may further contribute to the understanding, prediction, prevention, and treatment of ADHD in clinical.

[The Genetic Basis of ADHD - An Update]

The Genetic Basis of ADHD - An Update Abstract. Genetic risks play an important role in the etiology of attention-deficit/hyperactivity disorder (ADHD). This review presents the current state of knowledge concerning the genetic basis of the disorder. It discusses the results of twin- and family-based studies, linkage and association studies as well as recent findings resulting from Genome Wide Association Studies (GWAS). Furthermore, it elaborates on the relevance of polygenic risk scores, rare variants, and epigenetic alterations, especially in light of findings on genetic pleiotropy in the context of frequent psychiatric comorbidities in patients with ADHD.

The GRIN2B and GRIN2A Gene Variants Are Associated With Continuous Performance Test Variables in ADHD

Objective: To examine the association between variants of N-methyl-D-aspartate (NMDA) receptor subunit-encoding genes (GRIN2A and GRIN2B) and continuous performance test (CPT) variables in ADHD and healthy controls. Method: In all, 253 ADHD patients and 98 controls were recruited. The diagnosis, genotype, and diagnosis-genotype interaction effects for the CPT variables were examined. Results: Significant diagnosis effects were detected for all CPT variables. There were significant genotype and interaction effects on response time variability (RTV) by the GRIN2B variant. The C/C subgroup had higher RTV than the C/T + T/T subgroup in ADHD, but not in controls. There were significant genotype effects on omission errors by the GRIN2A variant. The G/G subgroup had more omission errors than the G/A + A/A subgroup in ADHD patients, but not in controls. Conclusion: These results suggest that the genetic variants of GRIN2B and GRIN2A confer an increased susceptibility to attentional impairment in ADHD patients.

The affective temperament traits and pregnancy-related depression in mothers may constitute risk factors for their children with attention deficit and hyperactivity disorder

We investigated whether the affective temperaments of mothers and maternal depression before and during the index pregnancy are related to the development of Attention-Deficit Hyperactivity Disorder (ADHD) in their children. One hundred and twenty children were screened for the diagnosis of ADHD. After exclusion and inclusion criteria were applied for children and their mothers, we compared the mothers of children with (n = 63) and without ADHD (n = 60) in terms of affective temperament traits, depression before and during the index pregnancy, and some environmental risk factors. The rate of boys were significantly higher among children with ADHD compared to healthy controls. The mothers of children with ADHD had significantly lower education levels, more cigarette consumption during pregnancy, and more depression rates before the pregnancy than those of healthy children. Male gender (p = .002), Hamilton Depression Rating Scale (HDRS) (p = .002), cyclothymic (p = .022), irritable (p = .035) and anxious temperament scores (p = .016) significantly predicted the association between the mothers and their children with ADHD. Our findings might suggest that male child gender, the severity of depression at index pregnancy, higher cyclothymic, irritable, and anxious temperament scores in mothers may constitute as important risk factors for the development of ADHD in their children.Impact statementWhat is already known on this subject? Although the effects of maternal depression on ADHD were extensively investigated, the relationship between affective temperament traits of mothers and ADHD in their children has not been sufficiently examined. To investigate the roles of maternal affective temperament traits and pregnancy-related depression on offspring ADHD would help us to understand the etiopathogenic bases of ADHD.What do the results of this study add? Cyclothymic, irritable and anxious temperaments of mothers were significantly associated with the offspring ADHD after adjusting for the gender, maternal smoking, prepartum and antenatal depression.What are the implications of these findings for clinical practice and/or further research? These findings might demonstrate that some maternal affective temperaments and depression during pregnancy are suggestive of an inherited predisposition to ADHD in offsprings. Longitudinal studies are required to demonstrate the relationship between maternal affective temperament features and the development of affective illness in children with ADHD.

Analysis of shared homozygosity regions in Saudi siblings with attention deficit hyperactivity disorder

AIM

Genetic and clinical complexities are common features of most psychiatric illnesses that pose a major obstacle in risk-gene identification. Attention deficit hyperactivity disorder (ADHD) is the most prevalent child-onset psychiatric illness, with high heritability. Over the past decade, numerous genetic studies utilizing various approaches, such as genome-wide association, candidate-gene association, and linkage analysis, have identified a multitude of candidate loci/genes. However, such studies have yielded diverse findings that are rarely reproduced, indicating that other genetic determinants have not been discovered yet. In this study, we carried out sib-pair analysis on seven multiplex families with ADHD from Saudi Arabia. We aimed to identify the candidate chromosomal regions and genes linked to the disease.

PATIENTS AND METHODS

A total of 41 individuals from multiplex families were analyzed for shared regions of homozygosity. Genes within these regions were prioritized according to their potential relevance to ADHD.

RESULTS

We identified multiple genomic regions spanning different chromosomes to be shared among affected members of each family; these included chromosomes 3, 5, 6, 7, 8, 9, 10, 13, 17, and 18. We also found specific regions on chromosomes 8 and 17 to be shared between affected individuals from more than one family. Among the genes present in the regions reported here were involved in neurotransmission (GRM3, SIGMAR1, CHAT, and SLC18A3) and members of the HLA gene family (HLA-A, HLA-DPA1, and MICC).

CONCLUSION

The candidate regions identified in this study highlight the genetic diversity of ADHD. Upon further investigation, these loci may reveal candidate genes that enclose variants associated with ADHD. Although most ADHD studies were conducted in other populations, our study provides insight from an understudied, ethnically interesting population.

[Monoamine metabolism in children with hyperkinetic disorder]

In this review article, the authors consider a role of interaction between noradrenergic and dopaminergic systems in children with hyperkinetic disorder without impairment of the operational system of intelligence. The uneven development of higher mental functions observed in these cases is determined by the specificity of the formation of functional systems and the inclusion of compensatory mechanisms as well.

Association of the GRIN2B rs2284411 polymorphism with methylphenidate response in attention-deficit/hyperactivity disorder

OBJECTIVE

We investigated the possible association between two NMDA subunit gene polymorphisms (GRIN2B rs2284411 and GRIN2A rs2229193) and treatment response to methylphenidate (MPH) in attention-deficit/hyperactivity disorder (ADHD).

METHODS

A total of 75 ADHD patients aged 6-17 years underwent 6 months of MPH administration. Treatment response was defined by changes in scores of the ADHD-IV Rating Scale (ADHD-RS), clinician-rated Clinical Global Impression-Improvement (CGI-I), and Continuous Performance Test (CPT). The association of the GRIN2B and GRIN2A polymorphisms with treatment response was analyzed using logistic regression analyses.

RESULTS

The GRIN2B rs2284411 C/C genotype showed significantly better treatment response as assessed by ADHD-RS inattention ( p=0.009) and CGI-I scores ( p=0.009), and there was a nominally significant association in regard to ADHD-RS hyperactivity-impulsivity ( p=0.028) and total ( p=0.023) scores, after adjusting for age, sex, IQ, baseline Clinical Global Impression-Severity (CGI-S) score, baseline ADHD-RS total score, and final MPH dose. The GRIN2B C/C genotype also showed greater improvement at the CPT response time variability ( p<0.001). The GRIN2A G/G genotype was associated with a greater improvement in commission errors of the CPT compared to the G/A genotype ( p=0.001).

CONCLUSIONS

The results suggest that the GRIN2B rs2284411 genotype may be an important predictor of MPH response in ADHD.

Risk factors for the existence of attention deficit hyperactivity disorder symptoms in children with autism spectrum disorders

Over the years, several authors have reported symptoms of attention deficit hyperactivity disorder (ADHD) in patients with autism spectrum disorders (ASD); however, studies on the risk factors of ADHD symptoms in children with ASD are lacking. The aim of this cross-sectional study was to identify the risk factors for the development of ADHD symptoms in children with ASD. The sample consisted of 67 children with ASD who were assessed with Conner's Parent Rating Scale-Revised (CPRS-R), and with a semi-structured detailed interview administered to parents, to collect a series of clinical data such as coexisting somatic and neuropsychiatric problems and familial and pre/peri/postpartum risk factors. We found that 55% of ASD children exceeded the cut-off of CPRS-R Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), total scale. The univariate analyses showed that children's age (P=0.048), motor delay (P=0.039), enuresis (P=0.014), allergies (P<0.01), comorbid oppositional defiant disorder (P=0.026) and intellectual disabilities comorbidities (P=0.034) were associated to the CPRS-R DSM-IV total score. Some familial predictors such as neuropsychiatric family history of intellectual disabilities (P=0.003) and psychosis (P=0.039) were related to the CPRS-R DSM-IV total score. In particular, a model including allergies (P=0.000) and family history of psychosis (P=0.03) explained 25% (corrected R²=0.25) of the variance of the DSM-IV ADHD score. In conclusion, we identified some risk factors associated with the development of ADHD symptoms in ASD children that need to be studied further.

Treadmill exercise ameliorates motor dysfunction through inhibition of Purkinje cell loss in cerebellum of valproic acid-induced autistic rats

Autism is a complex developmental disorder with impairments in social interaction, communication, repetitive behavior and motor skills. Exercise enhances cognitive function, ameliorates motor dysfunction, and provides protective profits against neurodegeneration. In the present study, we evaluated the effect of treadmill exercise on the motor coordination and Purkinje cell loss in relation with reactive astrocytes and microglial activation in the cerebellum using valproic acid (VPA)-induced autism rat model. On the 12th day of pregnancy, the pregnant rats in the VPA-exposed group received intraperitoneal injections of 600-mg/kg VPA. After birth, the rat pups were divided into four groups: the control group, the exercise group, the VPA-treated group, the VPA-treated and exercise group. The rat pups in the exercise groups were forced to run on a treadmill for 30 min once a day, 5 times a week for 4 weeks. In the present results, motor balance and coordination was disturbed by induction of autism, in contrast, treadmill exercise alleviated motor dysfunction in the autistic rats. Purkinje cell loss, reactive astrocytes, and microglial activation were occurred by induction of autism, in contrast, treadmill exercise enhanced survival rate of Purkinje neurons through inhibition of reactive astrocytes and microglia in the autistic rats. The present study showed that exercise may provide a potential therapeutic strategy for the alleviation of motor dysfunction in autistic patients.

Facial affect recognition in autism, ADHD and typical development

INTRODUCTION

Autism spectrum disorder (ASD) and Attention-Deficit Hyperactivity Disorder (ADHD) have been associated with facial affect recognition (FAR) alterations.

METHODS

This study examined accuracy and response times for general and specific FAR in whole face and eye-region stimuli. FAR was assessed in matched samples of children and adolescents with ASD (n = 35), ADHD (n = 32), and typical development (TD) (n = 32) aged 8.6-15.9 years (M = 11.6; SD = 2.0).

RESULTS

Compared to TD, the ASD group performed less accurate and showed longer response times for general and specific FAR, mostly driven by problems in neutral and happy face identification. The ADHD group responded faster than the ASD group for global FAR. No differences between ADHD and TD were found. Attentional distractibility had a significant effect on FAR performance in ASD and ADHD.

CONCLUSIONS

Findings confirm FAR alterations in ASD, but not ADHD, and endorse effects of attentional distractibility on FAR in ASD and ADHD. FAR and attention function training is clinically meaningful in ASD. Future studies should include control for visual attention and facial configuration skills, use naturalistic FAR material and also investigate implicit FAR.

Association of glutathione S-transferases M1, T1 and P1 gene polymorphisms with attention deficit and hyperactivity disorder in Korean children

Attention deficit and hyperactivity disorder (ADHD) is highly heritable disorder and common in school-age children characterized by inattention, hyperactivity and impulsivity. Although its heritability was estimated at 80-90% from family, adoption and twin studies, the molecular etiology of this disorder has not elucidated. Meanwhile, an impaired balance of oxidant-antioxidant status and increased oxidative stress is observed in ADHD, and it may imply a possible relationship between oxidative stress and etiology of ADHD. Glutathione S-transferase (GST) is antioxidant enzymes that play a key role in the cellular detoxification. In the present study, we examined the association between the genetic polymorphisms of GSTM1, GSTP1 and GSTT1, and ADHD in Korean children. Case-control study was conducted with 243 ADHD children and 327 controls. There were no significant associations between the polymorphisms and the incidence of ADHD (p>0.05). However, significant associations were observed in the stratified analyses. The frequency of GSTP1 Ile/Ile genotype is reached to the significant level in the hyperactivity subtype (88.2%) compared to controls (64.8%) (p=0.035) and the frequency of GSTT1-null genotype is significantly higher in the inattentive boys (p=0.005). Similarly, GSTT1-null genotype showed significant associations in combined subtype (p=0.016) and hyperactivity subtype (p=0.036) of the ADHD girls. Thus our result imply that the polymorphisms in the GST genes may affect ADHD, however, replication study for larger sample set and functional studies are crucial to confirm these findings.

Maternal stress and perinatal features in autism and attention deficit/hyperactivity disorder

BACKGROUND

We investigated the shared and non-shared perinatal risk factors for autism spectrum disorders (ASD) and attention deficit/hyperactivity disorder (ADHD) in a clinical sample. Additionally, we compared these groups regarding pre/postpartum maternal stress and the duration of breastfeeding.

METHODS

Children aged 3-18 years old with ASD (n = 100) were compared with age- and gender-matched children with ADHD (n = 100) and with age- and gender-matched healthy controls (n = 80).

RESULTS

Prematurity of the neonate and maternal stress/depressive mood in pregnancy were common risk factors shared by ASD and ADHD. Postpartum maternal depressive mood may be more specific to ASD, while shorter duration of breastfeeding may be related to ADHD.

CONCLUSIONS

ASD and ADHD may have some perinatal features in common. Identification of perinatal factors for ASD and ADHD carries clinical implications in terms of primary prevention.

Using Sibling Designs to Understand Neurodevelopmental Disorders: From Genes and Environments to Prevention Programming

Neurodevelopmental disorders represent a broad class of childhood neurological conditions that have a significant bearing on the wellbeing of children, families, and communities. In this review, we draw on evidence from two common and widely studied neurodevelopmental disorders-autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD)-to demonstrate the utility of genetically informed sibling designs in uncovering the nature and pathogenesis of these conditions. Specifically, we examine how twin, recurrence risk, and infant prospective tracking studies have contributed to our understanding of genetic and environmental liabilities towards neurodevelopmental morbidity through their impact on neurocognitive processes and structural/functional neuroanatomy. It is suggested that the siblings of children with ASD and ADHD are at risk not only of clinically elevated problems in these areas, but also of subthreshold symptoms and/or subtle impairments in various neurocognitive skills and other domains of psychosocial health. Finally, we close with a discussion on the practical relevance of sibling designs and how these might be used in the service of early screening, prevention, and intervention efforts that aim to alleviate the negative downstream consequences associated with disorders of neurodevelopment.

Motor Circuit Anatomy in Children with Autism Spectrum Disorder With or Without Attention Deficit Hyperactivity Disorder

This study examined the morphology of frontal-parietal regions relevant to motor functions in children with autism spectrum disorder (ASD) with or without attention deficit hyperactivity disorder (ADHD). We also explored its associations with autism severity and motor skills, and the impact of comorbid ADHD on these associations. Participants included 126 school-age children: 30 had ASD only, 33 had ASD with ADHD, and 63 were typically developing. High resolution 3T MPRAGE images were acquired to examine the cortical morphology (gray matter volume, GMV, surface area, SA, and cortical thickness, CT) in three regions of interest (ROI): precentral gyrus (M1), postcentral gyrus (S1), and inferior parietal cortex (IPC). Children with ASD showed abnormal increases in GMV and SA in all three ROIs: (a) increased GMV in S1 bilaterally and in right M1 was specific to children with ASD without ADHD; (b) all children with ASD (with or without ADHD) showed increases in the left IPC SA. Furthermore, on measures of motor function, impaired praxis was associated with increased GMV in right S1 in the ASD group with ADHD. Children with ASD with ADHD showed a positive relationship between bilateral S1 GMV and manual dexterity, whereas children with ASD without ADHD showed a negative relationship. Our findings suggest that (a) ASD is associated with abnormal morphology of cortical circuits crucial to motor control and learning; (b) anomalous overgrowth of these regions, particularly S1, may contribute to impaired motor skill development, and (c) functional and morphological differences are apparent between children with ASD with or without ADHD.

Association of LPHN3 rs6551665 A/G polymorphism with attention deficit and hyperactivity disorder in Korean children

Attention deficit hyperactivity disorder (ADHD) is a common and highly heritable disorder of school-age children. Its heritability was estimated at 80-90% but the genetic component underpinning this disorder remains to be disclosed. Recently, a highly consistent association between latrophilin3 (LPHN3) gene and ADHD was reported. In the present study, we examined the association between the LPHN3 rs6551665 A/G polymorphism and ADHD in Korea. The samples used in the study consisted of 150 ADHD children and 322 controls. The ADHD children were diagnosed according to DSM-IV. ADHD symptoms were evaluated with Dupaul Parent ADHD Rating Scales. LPHN3 rs6551665 SNP was determined by PCR-RFLP. Hardy-Weinberg equilibrium, genotype and allele frequency differences between the case and the control, and odds ratio were examined using the chi-square and exact tests. The LPHN3 gene locus was found to have no deviation from the Hardy-Weinberg expectation. We observed a significant association between the ADHD children and control group in genotype frequency (p=0.01) and allele frequency (p=0.02). The ADHD children appeared to have a surplus of GG genotype (OR 2.959, 95% CI 1.416-6.184, p=0.003) and G allele (OR 1.44, 95% CI 1.062-1.945, p=0.02). The association was more distinctive when analysis was confined to male samples (p=0.005), the OR of male controls and cases was 4.029 (95% CI 1.597-10.164, p=0.002) and the OR having G allele vs. A allele was 1.46 (95% CI 1.002-2.127, p=0.048). Thus our results imply that the LPHN3 rs6551665 GG genotype and G allele may provide a significant effect on the ADHD, although larger sample sizes and functional studies are necessary to further elucidate these findings.

Molecular genetic studies of ADHD and its candidate genes: a review

Attention-deficit/hyperactivity disorder (ADHD) is a common childhood-onset psychiatric disorder with high heritability. In recent years, numerous molecular genetic studies have been published to investigate susceptibility loci for ADHD. These results brought valuable candidates for further research, but they also presented great challenge for profound understanding of genetic data and general patterns of current molecular genetic studies of ADHD since they are scattered and heterogeneous. In this review, we presented a retrospective review of more than 300 molecular genetic studies for ADHD from two aspects: (1) the main achievements of various studies were summarized, including linkage studies, candidate-gene association studies, genome-wide association studies and genome-wide copy number variation studies, with a special focus on general patterns of study design and common sample features; (2) candidate genes for ADHD have been systematically evaluated in three ways for better utilization. The thorough summary of the achievements from various studies will provide an overview of the research status of molecular genetics studies for ADHD. Meanwhile, the analysis of general patterns and sample characteristics on the basis of these studies, as well as the integrative review of candidate ADHD genes, will propose new clues and directions for future experiment design.

RETRACTED: How does ASD symptomology correlate with ADHD presentations?

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor in Chief, after review by an independent panel of experts, due to evidence of a compromised peer review process and the failure of one author to disclose significant conflicts of interest. An independent peer-review process is a cornerstone of scientific integrity that allows for research to be scrutinized before publication to ensure that conclusions are anchored in sound methodology and objective interpretation of the results. Equally important is that the readership of research is fully informed about any potential competing interests that may have influenced the research process. This article is being retracted because it did not include a declaration of a conflict of interest of one author in relation to diagnostic tools which the paper endorses. The same author was also the Editor in Chief of the journal at the time of publication and evidence indicates that the paper was accepted without any independent peer review by external reviewers. The article is therefore retracted in accordance with the ethical guidelines of Elsevier: https://www.elsevier.com/about/policies/publishing-ethics and the Committee on Publication Ethics (COPE) https://publicationethics.org/files/retraction-guidelines.pdf.

The comorbidity of ADHD and autism spectrum disorder

ADHD and autism spectrum disorder are common psychiatric comorbidities to each another. In addition, there is behavioral, biological and neuropsychological overlap between the two disorders. There are also several important differences between autism spectrum disorder and ADHD. Treatment strategies for the comorbid condition will also be reviewed. Future areas of research and clinical need will be discussed.

Response time intra-subject variability: commonalities between children with autism spectrum disorders and children with ADHD

Despite the common co-occurrence of symptoms of attention deficit hyperactivity disorder (ADHD) in individuals with autism spectrum disorders (ASD), the underlying mechanisms are under-explored. A potential candidate for investigation is response time intra-subject variability (RT-ISV), a hypothesized marker of attentional lapses. Direct comparisons of RT-ISV in ASD versus ADHD are limited and contradictory. We aimed to examine whether distinct fluctuations in RT-ISV characterize children with ASD and with ADHD relative to typically developing children (TDC). We applied both a priori-based and data-driven strategies to RT performance of 46 children with ASD, 46 with ADHD, and 36 TDC (aged 7-11.9 years). Specifically, we contrasted groups relative to the amplitude of four preselected frequency bands as well as to 400 frequency bins from 0.006 to 0.345 Hz. In secondary analyses, we divided the ASD group into children with and without substantial ADHD symptoms (ASD(+) and ASD(-), respectively). Regardless of the strategy employed, RT-ISV fluctuations at frequencies between 0.20 and 0.345 Hz distinguished children with ADHD, but not children with ASD, from TDC. Children with ASD(+) and those with ADHD shared elevated amplitudes of RT-ISV fluctuations in frequencies between 0.18 and 0.345 Hz relative to TDC. In contrast, the ASD(-) subgroup did not differ from TDC in RT-ISV frequency fluctuations. RT-ISV fluctuations in frequencies 0.18-0.345 Hz (i.e., periods between 3 and 5 s) are associated with ADHD symptoms regardless of categorical diagnosis and may represent a biomarker. These results suggest that children with ADHD and those with ASD(+) share common underlying pathophysiological mechanisms of RT-ISV.

Brief report: prevalence of attention deficit/hyperactivity disorder among individuals with an autism spectrum disorder

Currently, both the DSM-IV-TR and ICD-10 preclude the diagnosis of attention deficit/hyperactivity disorder (ADHD) in cases that present with an autism spectrum disorder (ASD). This criterion will be removed in the upcoming DSM-V, but the relationship between ASD and ADHD, and in particular the prevalence of ADHD among the ASD population, remains controversial. Previous studies have reported clinically significant ADHD symptoms in one-third to three-quarters of ASD-affected individuals (probands). In our sample of 1,838 simplex children and adolescents with ASD, we found that less than 16% met clinically significant levels of ADHD symptoms, per parent report. When both parent and teacher reports were considered, the comorbidity rate was even lower, at 2%.

Attention deficit hyperactivity disorder

Over the last two decades, there have been numerous technical and methodological advances available to clinicians and researchers to better understand attention deficit hyperactivity disorder (ADHD) and its etiology. Despite the growing body of literature investigating the disorder's pathophysiology, ADHD remains a complex psychiatric disorder to characterize. This chapter will briefly review the literature on ADHD, with a focus on its history, the current genetic insights, neurophysiologic theories, and the use of neuroimaging to further understand the etiology. We address some of the major concerns that remain unclear about ADHD, including subtype instability, heterogeneity, and the underlying neural correlates that define the disorder. We highlight that the field of ADHD is rapidly evolving; the descriptions provided here will hopefully provide a sturdy foundation for which to build and improve our understanding of the disorder.

The relationship between autism spectrum disorders and attention-deficit/hyperactivity disorder: an overview

The autism spectrum has become a highly studied topic, perhaps the most researched of all developmental disorders. A host of related topics are being studied, with one of the most common being comorbidity of autism with other conditions such as epilepsy, sleep, and anxiety disorders. One of the most prevalent of these comorbid conditions is attention-deficit/hyperactivity disorder (AD/HD). A considerable amount of research has appeared on this topic with respect to symptom expression, prevalence of overlap, type of symptom overlap, and the effect of these two conditions co-occurrence on other symptoms and disorders. Given the substantial data base that has accrued, review and synthesis of these data are in order. This is the purpose of the present manuscript.

RETRACTED: A comparison of tantrum behavior profiles in children with ASD, ADHD and comorbid ASD and ADHD

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor in Chief, after review by an independent panel of experts, due to evidence of a compromised peer review process and the failure of one author to disclose significant conflicts of interest. An independent peer-review process is a cornerstone of scientific integrity that allows for research to be scrutinized before publication to ensure that conclusions are anchored in sound methodology and objective interpretation of the results. Equally important is that the readership of research is fully informed about any potential competing interests that may have influenced the research process. This article is being retracted because it did not include a declaration of a conflict of interest of one author in relation to diagnostic tools which the paper endorses. The same author was also the Editor in Chief of the journal at the time of publication and evidence indicates that the paper was accepted without any independent peer review by external reviewers. The article is therefore retracted in accordance with the ethical guidelines of Elsevier: https://www.elsevier.com/about/policies/publishing-ethics and the Committee on Publication Ethics (COPE) https://publicationethics.org/files/retraction-guidelines.pdf.

Polygenic transmission and complex neuro developmental network for attention deficit hyperactivity disorder: genome-wide association study of both common and rare variants

Attention-deficit hyperactivity disorder (ADHD) is a complex polygenic disorder. This study aimed to discover common and rare DNA variants associated with ADHD in a large homogeneous Han Chinese ADHD case-control sample. The sample comprised 1,040 cases and 963 controls. All cases met DSM-IV ADHD diagnostic criteria. We used the Affymetrix6.0 array to assay both single nucleotide polymorphisms (SNPs) and copy number variants (CNVs). Genome-wide association analyses were performed using PLINK. SNP-heritability and SNP-genetic correlations with ADHD in Caucasians were estimated with genome-wide complex trait analysis (GCTA). Pathway analyses were performed using the Interval enRICHment Test (INRICH), the Disease Association Protein-Protein Link Evaluator (DAPPLE), and the Genomic Regions Enrichment of Annotations Tool (GREAT). We did not find genome-wide significance for single SNPs but did find an increased burden of large, rare CNVs in the ADHD sample (P = 0.038). SNP-heritability was estimated to be 0.42 (standard error, 0.13, P = 0.0017) and the SNP-genetic correlation with European Ancestry ADHD samples was 0.39 (SE 0.15, P = 0.0072). The INRICH, DAPPLE, and GREAT analyses implicated several gene ontology cellular components, including neuron projections and synaptic components, which are consistent with a neurodevelopmental pathophysiology for ADHD. This study suggested the genetic architecture of ADHD comprises both common and rare variants. Some common causal variants are likely to be shared between Han Chinese and Caucasians. Complex neurodevelopmental networks may underlie ADHD's etiology.

Externalizing and tantrum behaviours in children with ASD and ADHD compared to children with ADHD

OBJECTIVE

Compare rates of externalizing in children with autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD) symptoms to children with ADHD.

METHOD

Parents/caregivers of 85 children with ASD and/or ADHD were surveyed about their children's behaviours using the Autism Spectrum Disorders-Comorbidity for Children and the Autism Spectrum Disorders-Behaviour Problem for Children.

RESULTS

Specific main effects analyses were then conducted. Children with ASD exhibited a higher number of externalizing (F(1, 83) = 83.34, p < 0.001) and tantrum behaviours (F(1,83) = 781.86, p < 0.001) than children without ASD.

CONCLUSIONS

ASD exacerbates the externalizing symptoms of ADHD during childhood. This study adds to the literature on the importance of assessing for a wide-range of possible behaviour problems in children presenting with ADHD symptomatology. The implications of these findings are discussed in the context of other research.

How Prevalent Are Autistic Traits Among Children With Attention-Deficit/Hyperactivity Disorder? A Qualitative Review of the Literature

BACKGROUND

Twin, family, and linkage studies have indicated that attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorders (ASDs) share a portion of their heritable etiology. This suggests that individuals with ADHD may manifest different forms of ASDs that may range from fully developed syndromic forms of the disorder to milder manifestations of ASD symptomatology, which will henceforth be referred to as autistic traits.

OBJECTIVE

The main purpose of this study was to conduct a literature search to examine the current body of knowledge regarding the prevalence of autistic traits (operationalized as the presence of autistic symptoms in the absence of a diagnosis of ASD) among children with ADHD and the associated morbidity of such traits.

METHOD

A systematic literature search in PubMed was conducted to discover all controlled studies published in the English language that systematically assessed the presence of autistic traits in children with ADHD who did not meet the criteria for ASDs. Three articles met our inclusion and exclusion criteria and were included in this qualitative review.

RESULTS

Only three scientific papers that met our a priori inclusion and exclusion criteria were identified. These articles described findings obtained from clinical samples and identified a prevalence of autistic traits among children with ADHD that ranged from 7% to 60%. As compared with children with ADHD without these traits, the presence of autistic traits in children with ADHD was associated with more severe dysfunction in a wide range of non-overlapping domains and in the social and communication domains in particular.

CONCLUSIONS

Although this study was limited by the small amount of available literature about this subject, these findings suggest that a sizeable subset of children with ADHD manifest autistic traits and that the presence of these traits is associated with increased dysfunction and social and communication deficits.

Association between the GRM7 rs3792452 polymorphism and attention deficit hyperacitiveity disorder in a Korean sample

Background

The purpose of this study was to investigate the association between the ionotropic and glutamate receptors, N-methyl D-asparate 2A (GRIN2A) and 2B (GRIN2B), and the metabotropic glutamate receptor mGluR7 (GRM7) gene polymorphisms and attention-deficit hyperactivity disorder (ADHD) in Korean population.

Methods

We conducted a case–control analysis of 202 ADHD subjects and 159 controls, performed a transmission disequilibrium test (TDT) on 149 trios, and compared scores from the continuous performance test (CPT), the Children’s Depression Inventory (CDI), and the State-Trait Anxiety Inventory for Children (STAIC) according to the genotype of the glutamate receptor genes.

Results

There were no significant differences in the genotype or allele frequencies of the GRIN2A rs8049651, GRIN2B rs2284411, or GRM7 rs37952452 polymorphisms between the ADHD and control groups. For 148 ADHD trios, the TDT analysis also showed no preferential transmission of the GRIN2A rs8049651 or GRIN2B rs2284411 polymorphisms. However, the TDT analysis of the GRM7 rs3792452 polymorphism showed biased transmission of the G allele (χ2 = 4.67, p = 0.031). In the ADHD probands, the subjects with GG genotype in the GRM7 rs37952452 polymorphism had higher mean T-scores for omission errors on the CPT than did those with the GA or AA genotype (t = 3.38, p = 0.001). In addition, the ADHD subjects who were homozygous for the G allele in the GRM7 rs37952452 polymorphism had higher STAIC-T (t = 5.52, p < 0.001) and STAIC-S (t = 2.74, p = 0.007) scores than did those with the GA or AA genotype.

Conclusions

These results provide preliminary evidence of an association between the GRM7 rs37952452 polymorphism and selective attention deficit and anxiety found within the Korean ADHD population.

ADHD and autism: differential diagnosis or overlapping traits? A selective review

According to DSM-IV TR and ICD-10, a diagnosis of autism or Asperger Syndrome precludes a diagnosis of attention-deficit/hyperactivity disorder (ADHD). However, despite the different conceptualization, population-based twin studies reported symptom overlap, and a recent epidemiologically based study reported a high rate of ADHD in autism and autism spectrum disorders (ASD). In the planned revision of the DSM-IV TR, dsm5 (www.dsm5.org), the diagnoses of autistic disorder and ADHD will not be mutually exclusive any longer. This provides the basis of more differentiated studies on overlap and distinction between both disorders. This review presents data on comorbidity rates and symptom overlap and discusses common and disorder-specific risk factors, including recent proteomic studies. Neuropsychological findings in the areas of attention, reward processing, and social cognition are then compared between both disorders, as these cognitive abilities show overlapping as well as specific impairment for one of both disorders. In addition, selective brain imaging findings are reported. Therapeutic options are summarized, and new approaches are discussed. The review concludes with a prospectus on open questions for research and clinical practice.

LPHN3 and attention-deficit/hyperactivity disorder: interaction with maternal stress during pregnancy

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous behavioral disorder, complex both in etiology and clinical expression. Both genetic and environmental factors have been implicated, and it has been suggested that gene-environment interactions may play a pivotal role in the disorder. Recently, a significant association was reported between ADHD and LPHN3 (which codes for latrophilin 3), and replicated in independent samples.

METHODS

We have examined the association between tag single nucleotide polymorphisms (SNPs) in LPHN3 within the region previously implicated in ADHD. Family based association tests (FBAT) were conducted (n = 380 families) with the categorical diagnosis of ADHD, behavioral and cognitive phenotypes related to ADHD, and response to treatment (given a fixed dose of methylphenidate, 0.5 mg/day). Stratified FBAT analyses, based on maternal smoking and stress during pregnancy, was conducted.

RESULTS

Whereas limited association was observed in the total sample, highly significant interaction between four LPHN3 tag SNPs (rs6551665, rs1947274, rs6858066, rs2345039) and maternal stress during pregnancy was noted. Analysis conducted in the sub-group of mothers exposed to minimal stress during pregnancy showed significant associations with ADHD, behavioral and cognitive dimensions related to ADHD, as well as treatment response. Although extensive association was observed with the candidate SNPs, the findings are partially inconsistent with previously published results with the opposite alleles over-transmitted in these studies.

CONCLUSIONS

These results provide evidence for the interaction between a genetic and environmental factor independently shown to be associated with ADHD. If confirmed in independent large studies, they may present a step forward in unraveling the complex etiology of ADHD.

Social-emotional development through a behavior genetics lens: infancy through preschool

The field of developmental behavior genetics has added significantly to the collective understanding of what factors influence human behavior and human development. Research in this area has helped to explain not only how genes and environment contribute to individual differences but also how the interplay between genes and environment influences behavior and human development. The current chapter provides a background of the theory and methodology behind behavior genetic research and the field of developmental behavior genetics. It also examines three specific developmental periods as they relate to behavior genetic research: infancy, toddlerhood, and early preschool. The behavior genetic literature is reviewed for key socioemotional developmental behaviors that fit under each of these time periods. Temperament, attachment, frustration, empathy, and aggression are behaviors that develop in early life that were examined here. Thus, the general purpose of this chapter is to provide an overview of how genes and environment, as well as the interplay between them, relate to early socioemotional behaviors.

Treatment for co-occurring attention deficit/hyperactivity disorder and autism spectrum disorder

Interest in the co-occurrence of attention deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) has grown in the last decade. Research on clinical populations supports the frequent co-occurrence of ADHD traits (e.g., hyperactivity) in individuals with ASD and ASD traits (e.g., social communication deficits) in individuals with ADHD. Similar trends in co-occurring traits have been observed in population-based samples, as well as family and genetic studies of affected individuals. Despite increased interest in co-occurring ADHD and ASD, relatively little research has been devoted to treatment considerations. The vast majority of intervention research has examined pharmacological treatment using traditional ADHD medications. Relatively few psychosocial interventions have directly addressed co-occurring symptoms. Treatment development will benefit from enhanced understanding of the phenomenon of co-occurring ADHD and ASD. Key topics for future research include examining developmental trajectories of co-occurring disorders, comorbid psychiatric conditions, deficits in social skills, and the nature of executive functioning impairment in individuals with co-occurring ADHD and ASD. In the current review, research in these areas is reviewed along with recommendation for future study. Given that clinicians are routinely observing and treating individuals with co-occurring symptoms, further research will yield needed information to inform intervention development and maximize benefits for affected individuals.

Validity of DSM-IV attention deficit/hyperactivity disorder symptom dimensions and subtypes

Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) criteria for attention deficit/hyperactivity disorder (ADHD) specify two dimensions of inattention and hyperactivity-impulsivity symptoms that are used to define three nominal subtypes: predominantly hyperactive-impulsive type (ADHD-H), predominantly inattentive type (ADHD-I), and combined type (ADHD-C). To aid decision making for DSM-5 and other future diagnostic systems, a comprehensive literature review and meta-analysis of 546 studies was completed to evaluate the validity of the DSM-IV model of ADHD. Results indicated that DSM-IV criteria identify individuals with significant and persistent impairment in social, academic, occupational, and adaptive functioning when intelligence, demographic factors, and concurrent psychopathology are controlled. Available data overwhelmingly support the concurrent, predictive, and discriminant validity of the distinction between inattention and hyperactivity-impulsivity symptoms, and indicate that nearly all differences among the nominal subtypes are consistent with the relative levels of inattention and hyperactivity-impulsivity symptoms that define the subtypes. In contrast, the DSM-IV subtype model is compromised by weak evidence for the validity of ADHD-H after first grade, minimal support for the distinction between ADHD-I and ADHD-C in studies of etiological influences, academic and cognitive functioning, and treatment response, and the marked longitudinal instability of all three subtypes. Overall, we conclude that the DSM-IV ADHD subtypes provide a convenient clinical shorthand to describe the functional and behavioral correlates of current levels of inattention and hyperactivity-impulsivity symptoms, but do not identify discrete subgroups with sufficient long-term stability to justify the classification of distinct forms of the disorder. Empirical support is stronger for an alternative model that would replace the subtypes with dimensional modifiers that reflect the number of inattention and hyperactivity-impulsivity symptoms at the time of assessment. (PsycINFO Database Record (c) 2012 APA, all rights reserved).

Examining autistic traits in children with ADHD: does the autism spectrum extend to ADHD?

We examined to what extent increased parent reports of autistic traits in some children with Attention Deficit Hyperactivity Disorder (ADHD) are the result of ADHD-related symptoms or qualitatively similar to the core characteristics of autism spectrum disorders (ASD). Results confirm the presence of a subgroup of children with ADHD and elevated ratings of core ASD traits (ADHD(+)) not accounted for by ADHD or behavioral symptoms. Further, analyses revealed greater oppositional behaviors, but not greater ADHD severity or anxiety, in the ADHD(+) subgroup compared to those with ADHD only. These results highlight the importance of specifically examining autistic traits in children with ADHD for better characterization in studies of the underlying physiopathology and treatment.

Detection and characterization of copy number variation in autism spectrum disorder

There now exist multiple lines of evidence pointing to a significant genetic component underlying the aetiology of autism spectrum disorders (ASDs). The advent of methodologies for scanning the human genome at high resolution, coupled with the recognition of copy number variation (CNV) as a prevalent source of genomic variation, has led to new strategies in the identification of clinically relevant loci. Balanced genomic changes, such as translocations and inversions, also contribute to ASD, but current studies have shown that screening with microarrays has up to fivefold increase in diagnostic yield. Recent work by our group and others has shown unbalanced genomic alterations that are likely pathogenic in upwards of 10% of cases, highlighting an important role for CNVs in the genetic aetiology of ASD. A trend in our empirical data has shifted focus for discovery of candidate loci towards individually rare but highly penetrant CNVs instead of looking for common variants of low penetrance. This strategy has proven largely successful in identifying ASD-susceptibility candidate loci, including gains and losses at 16p11.2, SHANK2, NRXN1, and PTCHD1. Another emerging and intriguing trend is the identification of the same genes implicated by rare CNVs across neurodevelopmental disorders, including schizophrenia, attention deficit hyperactivity disorder, and intellectual disability. These observations indicate that similar pathways may be involved in phenotypically distinct outcomes. Although interrogation of the genome at high resolution has led to these novel discoveries, it has also made cataloguing, characterization, and clinical interpretation of the increasing amount of CNV data difficult. Herein, we describe the history of genomic structural variation in ASD and how CNV discovery has been used to pinpoint novel ASD-susceptibility loci. We also discuss the overlap of CNVs across neurodevelopmental disorders and comment on the current challenges of understanding the relationship between CNVs and associated phenotypes in a clinical context.

Quantitative and molecular genetics of ADHD

ADHD is a common and highly heritable disorder. Family, twin, and adoption studies confirm a strong genetic influence in risk for ADHD and there has been a great deal of interest in identifying the genetic factors involved. Quantitative genetic studies find that genetic risk for ADHD is continuously distributed throughout the population, that there are both shared and unique genetic influences on inattention and hyperactivity-impulsivity, and that ADHD shares genetic risk factors with commonly co-occurring clinical syndromes and traits. ADHD is found at all ages and the underlying genetic architecture is similar across the lifespan. In terms of specific genetic findings, there is consistent evidence of monoamine neurotransmitter involvement with the best evidence coming from genetic markers in or near the dopamine D4 and D5 receptor genes. Recent genome-wide association studies have identified new association findings, including genes involved in cell division, cell adhesion, neuronal migration, and neuronal plasticity. However, as yet, none of these pass genome-wide levels of significance. Finally, recent data confirm an important role for rare copy number variants, including those that are found in schizophrenia and autism. Future work should use genetic association data to determine the nature of the cognitive, neuronal and cellular processes that mediate genetic risks on behaviour, and identify environmental factors that interact with genetic risks for ADHD.

ADHDgene: a genetic database for attention deficit hyperactivity disorder

With a worldwide prevalence of ~5%, attention deficit hyperactivity disorder (ADHD) has become one of the most common psychiatric disorders. The polygenetic nature of ADHD indicates that multiple genes jointly contribute to the development of this complex disease. Studies aiming to explore genetic susceptibility of ADHD have been increasing in recent years. There is a growing need to integrate the genetic data from various genetic studies to provide a comprehensive data set and uniform access for convenience of in-depth data mining. So far, there has been no such effort for ADHD. To address the genetic complexity of ADHD, we developed the ADHDgene database by integrating ADHD-related genetic factors by profound literature reading. Based on the data from the literature, extended functional analysis, including linkage disequilibrium analysis, pathway-based analysis and gene mapping were performed to provide new insights into genetic causes of ADHD. Moreover, powerful search tools and a graphical browser were developed to facilitate the navigation of the data and data connections. As the first genetic database for ADHD, ADHDgene aims to provide researchers with a central genetic resource and analysis platform for ADHD and is freely available at http://adhd.psych.ac.cn/.

Atypical behaviors and comorbid externalizing symptoms equally predict children with attention-deficit/hyperactivity disorder's social functioning

The goal of the current study was to determine within a clinical sample what differentiates children with ADHD who experience social functioning difficulties from those who appear to have healthy social functioning. Participants for this study included 62 children (mean age = 11.3 years) with a DSM-IV diagnosis of ADHD confirmed by a comprehensive clinical diagnostic assessment. Multiple indicators of children's social functioning were collected via parent report including: social skills, social adaptability, peer difficulties, and social quality of life. Parent reports of children's externalizing, internalizing, and atypical behaviors were also collected. Results indicated that both externalizing symptoms and atypical behaviors predicted children with ADHD's social functioning, even after controlling for ADHD symptoms severity. No association was found between internalizing symptoms and social functioning. The current study provides initial data suggesting that atypical behaviors found in children with ADHD are as powerful as comorbid externalizing symptoms in predicting social functioning difficulties. Due to the shared variance from relying solely on parent report, it will be critical for future research to replicate our findings using multi-informant data such as peer and teacher reports which provide unique information on children's social functioning.

ADHD and genetic syndromes

A high rate of Attention Deficit/Hyperactivity Disorder (ADHD)-like characteristics has been reported in a wide variety of disorders including syndromes with known genetic causes. In this article, we review the genetic and the neurobiological links between ADHD symptoms and some genetic syndromes such as: Fragile X Syndrome, Neurofibromatosis 1, DiGeorge Syndrome, Tuberous Sclerosis Complex, Turner Syndrome, Williams Syndrome and Klinefelter Syndrome. Although each syndrome may arise from different genetic abnormalities with multiple molecular functions, the effects of these abnormalities may give rise to common effects downstream in the biological pathways or neural circuits, resulting in the presentation of ADHD symptoms. Early diagnosis of ADHD allows for earlier treatment, and has the potential for a better outcome in children with genetic syndromes.

Development of a multi-dimensional scale for PDD and ADHD

A novel assessment scale, the multi-dimensional scale for pervasive developmental disorder (PDD) and attention-deficit/hyperactivity disorder (ADHD) (MSPA), is reported. Existing assessment scales are intended to establish each diagnosis. However, the diagnosis by itself does not always capture individual characteristics or indicate the level of support required, since inter-individual differences are substantial and co-morbidity is common. The MSPA consists of 14 domains and each domain is rated by a nine-point quantitative scale. The clinical and behavioral features are projected onto a radar-chart, which facilitates understanding of the disorders both by the patients themselves and by those in their surroundings. We assessed 179 patients and analyzed features by six diagnostic subgroups, which showed relationships between features and diagnoses. The inter-rater reliability was satisfactory.

The impact of study design and diagnostic approach in a large multi-centre ADHD study: Part 2: Dimensional measures of psychopathology and intelligence

BACKGROUND

The International Multi-centre ADHD Genetics (IMAGE) project with 11 participating centres from 7 European countries and Israel has collected a large behavioural and genetic database for present and future research. Behavioural data were collected from 1068 probands with ADHD and 1446 unselected siblings. The aim was to describe and analyse questionnaire data and IQ measures from all probands and siblings. In particular, to investigate the influence of age, gender, family status (proband vs. sibling), informant, and centres on sample homogeneity in psychopathological measures.

METHODS

Conners' Questionnaires, Strengths and Difficulties Questionnaires, and Wechsler Intelligence Scores were used to describe the phenotype of the sample. Data were analysed by use of robust statistical multi-way procedures.

RESULTS

Besides main effects of age, gender, informant, and centre, there were considerable interaction effects on questionnaire data. The larger differences between probands and siblings at home than at school may reflect contrast effects in the parents. Furthermore, there were marked gender by status effects on the ADHD symptom ratings with girls scoring one standard deviation higher than boys in the proband sample but lower than boys in the siblings sample. The multi-centre design is another important source of heterogeneity, particularly in the interaction with the family status. To a large extent the centres differed from each other with regard to differences between proband and sibling scores.

CONCLUSIONS

When ADHD probands are diagnosed by use of fixed symptom counts, the severity of the disorder in the proband sample may markedly differ between boys and girls and across age, particularly in samples with a large age range. A multi-centre design carries the risk of considerable phenotypic differences between centres and, consequently, of additional heterogeneity of the sample even if standardized diagnostic procedures are used. These possible sources of variance should be counteracted in genetic analyses either by using age and gender adjusted diagnostic procedures and regional normative data or by adjusting for design artefacts by use of covariate statistics, by eliminating outliers, or by other methods suitable for reducing heterogeneity.

Understanding the complex etiologies of developmental disorders: behavioral and molecular genetic approaches

OBJECTIVE

This article has 2 primary goals. First, a brief tutorial on behavioral and molecular genetic methods is provided for readers without extensive training in these areas. To illustrate the application of these approaches to developmental disorders, etiologically informative studies of reading disability (RD), math disability (MD), and attention-deficit hyperactivity disorder (ADHD) are then reviewed. Implications of the results for these specific disorders and for developmental disabilities as a whole are discussed, and novel directions for future research are highlighted.

METHOD

Previous family and twin studies of RD, MD, and ADHD are reviewed systematically, and the extensive molecular genetic literatures on each disorder are summarized. To illustrate 4 novel extensions of these etiologically informative approaches, new data are presented from the Colorado Learning Disabilities Research Center, an ongoing twin study of the etiology of RD, ADHD, MD, and related disorders.

CONCLUSIONS

RD, MD, and ADHD are familial and heritable, and co-occur more frequently than expected by chance. Molecular genetic studies suggest that all 3 disorders have complex etiologies, with multiple genetic and environmental risk factors each contributing to overall risk for each disorder. Neuropsychological analyses indicate that the 3 disorders are each associated with multiple neuropsychological weaknesses, and initial evidence suggests that comorbidity between the 3 disorders is due to common genetic risk factors that lead to slow processing speed.

Analysis of GWAS top hits in ADHD suggests association to two polymorphisms located in genes expressed in the cerebellum

Attention deficit/hyperactivity disorder (ADHD) is a common psychiatric disorder influenced by genetic factors. Several chromosomal regions with potential linkage and candidate genes associations have been reported, but findings are often inconsistent and non-replicated. The few genome-wide association studies (GWAS) carried out so far differ for study design and phenotypes analyzed, and did not detect any association significant at the genome-wide level. In the present study we examined the top SNPs reported in the GWAS by Neale et al. 2008 in an independent cohort. Although our sample size is smaller (415 trios vs. 909), the power was sufficient to confirm the role of candidate markers in ADHD if a true association exists. Two out of 36 top SNPs were significant at alpha = 0.05 in our sample, although none was still significant after correction for multiple tests. These two SNPs are both located in genes coding for as yet uncharacterized proteins expressed in the cerebellum, XKR4 in 8q12.1, and FAM190A in 4q22.1. Three other FAM190A SNPs have TDT P-values of <10(-5) in our sample, a level of significance only reached by a total of five SNPs in our genome-wide data. While these findings could be due to chance, we cannot exclude that these markers are indeed associated to disease risk. Remarkably, brain imaging studies have shown reduction of the posterior inferior cerebellar lobules volume of ADHD boys and girls compared to controls, persistent with age and not present in unaffected siblings, suggesting that the cerebellum may be directly related to pathophysiology of ADHD.

Identifying loci for the overlap between attention-deficit/hyperactivity disorder and autism spectrum disorder using a genome-wide QTL linkage approach

OBJECTIVE

The genetic basis for autism spectrum disorder (ASD) symptoms in children with attention-deficit/hyperactivity disorder (ADHD) was addressed using a genome-wide linkage approach.

METHOD

Participants of the International Multi-Center ADHD Genetics study comprising 1,143 probands with ADHD and 1,453 siblings were analyzed. The total and subscale scores of the Social Communication Questionnaire (SCQ) were used as quantitative traits for multipoint regression-based linkage analyses on 5,407 autosomal single-nucleotide polymorphisms applying MERLIN-regress software, both without and with inclusion of ADHD symptom scores as covariates.

RESULTS

The analyses without ADHD symptom scores as covariates resulted in three suggestive linkage signals, i.e., on chromosomes 15q24, 16p13, and 18p11. Inclusion of ADHD symptom scores as covariates resulted in additional suggestive loci on chromosomes 7q36 and 12q24, whereas the LOD score of the locus on chromosome 15q decreased below the threshold for suggestive linkage. The loci on 7q, 16p, and 18p were found for the SCQ restricted and repetitive subscale, that on 15q was found for the SCQ communication subscale, and that on 12q for the SCQ total score.

CONCLUSIONS

Our findings suggest that QTLs identified in this study are ASD specific, although the 15q QTL potentially has pleiotropic effects for ADHD and ASD. This study confirms that genetic factors influence ASD traits along a continuum of severity, as loci potentially underlying ASD symptoms in children with ADHD were identified even though subjects with autism had been excluded from the IMAGE sample, and supports the hypothesis that differential genetic factors underlie the three ASD dimensions.