Limited genetic covariance between autistic traits and intelligence: findings from a longitudinal twin study

A Unifying Theory for Autism: The Pathogenetic Triad as a Theoretical Framework

This paper presents a unifying theory for autism by applying the framework of a pathogenetic triad to the scientific literature. It proposes a deconstruction of autism into three contributing features (an autistic personality dimension, cognitive compensation, and neuropathological risk factors), and delineates how they interact to cause a maladaptive behavioral phenotype that may require a clinical diagnosis. The autistic personality represents a common core condition, which induces a set of behavioral issues when pronounced. These issues are compensated for by cognitive mechanisms, allowing the individual to remain adaptive and functional. Risk factors, both exogenous and endogenous ones, show pathophysiological convergence through their negative effects on neurodevelopment. This secondarily affects cognitive compensation, which disinhibits a maladaptive behavioral phenotype. The triad is operationalized and methods for quantification are presented. With respect to the breadth of findings in the literature that it can incorporate, it is the most comprehensive model yet for autism. Its main implications are that (1) it presents the broader autism phenotype as a non-pathological core personality domain, which is shared across the population and uncoupled from associated features such as low cognitive ability and immune dysfunction, (2) it proposes that common genetic variants underly the personality domain, and that rare variants act as risk factors through negative effects on neurodevelopment, (3) it outlines a common pathophysiological mechanism, through inhibition of neurodevelopment and cognitive dysfunction, by which a wide range of endogenous and exogenous risk factors lead to autism, and (4) it suggests that contributing risk factors, and findings of immune and autonomic dysfunction are clinically ascertained rather than part of the core autism construct.

Ex-Gaussian, Frequency and Reward Analyses Reveal Specificity of Reaction Time Fluctuations to ADHD and Not Autism Traits

Both attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) have been linked to increased reaction time variability (RTV), a marker of attentional fluctuation. Here we test whether specificity to either trait emerges when we examine (1) detailed ex-Gaussian and frequency RTV subcomponents, (2) effects while controlling for the other trait and (3) improvement in the RTV measures following rewards or a faster event rate. 1110 children aged 7-10 years from a population-based sample completed a Go/No-Go task under three conditions (slow, fast and incentives). We measured RTV with standard deviation of RT (SDRT), ex-Gaussian distribution measures (Sigma and Tau), RT fluctuations in cycles of ~14-90 s in all conditions (Slow-4 and Slow-5), and RT fluctuations in cycles of 2-14 s in the fast condition (Slow-2 and Slow-3). Parent-rated ADHD and ASD traits were obtained. All refined RTV components were linked to ADHD traits only and not to ASD traits, while Sigma did not relate to either trait. Although both ADHD and ASD social-communication traits were associated with SDRT, the association with social-communication impairments disappeared when controlling for ADHD traits. A reward-induced improvement in RTV measures, indicating malleability, emerged in relation to ADHD traits but not ASD traits. Under closer inspection, specificity emerges of high RTV to ADHD traits. For the clinician, our findings indicate that attentional fluctuation in children with high ASD traits may be due to co-occurring ADHD traits and emphasise how the effectiveness of rewards does not generalise from ADHD to ASD traits.

Insufficient Evidence for "Autism-Specific" Genes

Despite evidence that deleterious variants in the same genes are implicated across multiple neurodevelopmental and neuropsychiatric disorders, there has been considerable interest in identifying genes that, when mutated, confer risk that is largely specific for autism spectrum disorder (ASD). Here, we review the findings and limitations of recent efforts to identify relatively “autism-specific” genes, efforts which focus on rare variants of large effect size that are thought to account for the observed phenotypes. We present a divergent interpretation of published evidence; discuss practical and theoretical issues related to studying the relationships between rare, large-effect deleterious variants and neurodevelopmental phenotypes; and describe potential future directions of this research. We argue that there is currently insufficient evidence to establish meaningful ASD specificity of any genes based on large-effect rare-variant data.

Assessing the evidence for shared genetic risks across psychiatric disorders and traits

Genetic influences play a significant role in risk for psychiatric disorders, prompting numerous endeavors to further understand their underlying genetic architecture. In this paper, we summarize and review evidence from traditional twin studies and more recent genome-wide molecular genetic analyses regarding two important issues that have proven particularly informative for psychiatric genetic research. First, emerging results are beginning to suggest that genetic risk factors for some (but not all) clinically diagnosed psychiatric disorders or extreme manifestations of psychiatric traits in the population share genetic risks with quantitative variation in milder traits of the same disorder throughout the general population. Second, there is now evidence for substantial sharing of genetic risks across different psychiatric disorders. This extends to the level of characteristic traits throughout the population, with which some clinical disorders also share genetic risks. In this review, we summarize and evaluate the evidence for these two issues, for a range of psychiatric disorders. We then critically appraise putative interpretations regarding the potential meaning of genetic correlation across psychiatric phenotypes. We highlight several new methods and studies which are already using these insights into the genetic architecture of psychiatric disorders to gain additional understanding regarding the underlying biology of these disorders. We conclude by outlining opportunities for future research in this area.

Child, Maternal and Demographic Factors Influencing Caregiver-Reported Autistic Trait Symptomatology in Toddlers

Current research on children's autistic traits in the general population relies predominantly on caregiver-report, yet the extent to which individual, caregiver or demographic characteristics are associated with informants' ratings has not been sufficiently explored. In this study, caregivers of 396 Singaporean two-year-olds from a birth cohort study completed the Quantitative Checklist for Autism in Toddlers. Children's gender, cognitive functioning and birth order, maternal age, and ethnic group membership were not significant predictors of caregiver-reported autistic traits. Poorer child language development and higher maternal depressive symptoms significantly predicted more social-communicative autistic traits, while lower maternal education predicted more behavioural autistic traits. Children's language and informants' educational level and depressive symptomatology may need to be considered in caregiver-reports of autistic traits.

Age-Dependent Pleiotropy Between General Cognitive Function and Major Psychiatric Disorders

BACKGROUND

General cognitive function predicts psychiatric illness across the life course. This study examines the role of pleiotropy in explaining the link between cognitive function and psychiatric disorder.

METHODS

We used two large genome-wide association study data sets on cognitive function-one from older age, n = 53,949, and one from childhood, n = 12,441. We also used genome-wide association study data on educational attainment, n = 95,427, to examine the validity of its use as a proxy phenotype for cognitive function. Using a new method, linkage disequilibrium regression, we derived genetic correlations, free from the confounding of clinical state between psychiatric illness and cognitive function.

RESULTS

We found a genetic correlation of .711 (p = 2.26e-12) across the life course for general cognitive function. We also showed a positive genetic correlation between autism spectrum disorder and cognitive function in childhood (rg = .360, p = .0009) and for educational attainment (rg = .322, p = 1.37e-5) but not in older age. In schizophrenia, we found a negative genetic correlation between older age cognitive function (rg = -.231, p = 3.81e-12) but not in childhood or for educational attainment. For Alzheimer's disease, we found negative genetic correlations with childhood cognitive function (rg = -.341, p = .001), educational attainment (rg = -.324, p = 1.15e-5), and with older age cognitive function (rg = -.324, p = 1.78e-5).

CONCLUSIONS

The pleiotropy exhibited between cognitive function and psychiatric disorders changed across the life course. These age-dependent associations might explain why negative selection has not removed variants causally associated with autism spectrum disorder or schizophrenia.

Common polygenic risk for autism spectrum disorder (ASD) is associated with cognitive ability in the general population

Cognitive impairment is common among individuals diagnosed with autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD). It has been suggested that some aspects of intelligence are preserved or even superior in people with ASD compared with controls, but consistent evidence is lacking. Few studies have examined the genetic overlap between cognitive ability and ASD/ADHD. The aim of this study was to examine the polygenic overlap between ASD/ADHD and cognitive ability in individuals from the general population. Polygenic risk for ADHD and ASD was calculated from genome-wide association studies of ASD and ADHD conducted by the Psychiatric Genetics Consortium. Risk scores were created in three independent cohorts: Generation Scotland Scottish Family Health Study (GS:SFHS) (n=9863), the Lothian Birth Cohorts 1936 and 1921 (n=1522), and the Brisbane Adolescent Twin Sample (BATS) (n=921). We report that polygenic risk for ASD is positively correlated with general cognitive ability (beta=0.07, P=6 × 10(-7), r(2)=0.003), logical memory and verbal intelligence in GS:SFHS. This was replicated in BATS as a positive association with full-scale intelligent quotient (IQ) (beta=0.07, P=0.03, r(2)=0.005). We did not find consistent evidence that polygenic risk for ADHD was associated with cognitive function; however, a negative correlation with IQ at age 11 years (beta=-0.08, Z=-3.3, P=0.001) was observed in the Lothian Birth Cohorts. These findings are in individuals from the general population, suggesting that the relationship between genetic risk for ASD and intelligence is partly independent of clinical state. These data suggest that common genetic variation relevant for ASD influences general cognitive ability.

Genetic variation in GABRB3 is associated with Asperger syndrome and multiple endophenotypes relevant to autism

BACKGROUND

Autism spectrum conditions (ASC) are associated with deficits in social interaction and communication, alongside repetitive, restricted, and stereotyped behavior. ASC is highly heritable. The gamma-aminobutyric acid (GABA)-ergic system has been associated consistently with atypicalities in autism, in both genetic association and expression studies. A key component of the GABA-ergic system is encoded by the GABRB3 gene, which has been previously implicated both in ASC and in individual differences in empathy.

METHODS

In this study, 45 genotyped single nucleotide polymorphisms (SNPs) within GABRB3 were tested for association with Asperger syndrome (AS), and related quantitative traits measured through the following tests: the Empathy Quotient (EQ), the Autism Spectrum Quotient (AQ), the Systemizing Quotient-Revised (SQ-R), the Embedded Figures Test (EFT), the Reading the Mind in the Eyes Test (RMET), and the Mental Rotation Test (MRT). Two-loci, three-loci, four-loci haplotype analyses, and one seven-loci haplotype analysis were also performed in the AS case-control sample.

RESULTS

Three SNPs (rs7180158, rs7165604, rs12593579) were significantly associated with AS, and two SNPs (rs9806546, rs11636966) were significantly associated with EQ. Two SNP-SNP pairs, rs12438141-rs1035751 and rs12438141-rs7179514, showed significant association with variation in the EFT scores. One SNP-SNP pair, rs7174437-rs1863455, was significantly associated with variation in the MRT scores. Additionally, a few haplotypes, including a 19 kb genomic region that formed a linkage disequilibrium (LD) block in our sample and contained several nominally significant SNPs, were found to be significantly associated with AS.

CONCLUSION

The current study confirms the role of GABRB3 as an important candidate gene in both ASC and normative variation in related endophenotypes.

Narrowly versus broadly defined autism spectrum disorders: differences in pre- and perinatal risk factors

This study examined the differential contribution of pre- and perinatal risks in narrowly versus broadly defined autism spectrum disorder (ASD) and across core symptom domains, IQ and co-morbid problems. Children with a DSM-IV diagnosis of autistic disorder (AD) (n = 121) or pervasive developmental disorder not otherwise specified (PDD-NOS) (n = 75) were compared to a typical control sample (n = 311). Diagnoses were based on extensive assessments between 12 and 49 months of age (M = 33.3, SD = 6.4) and re-evaluated at 43-98 months (M = 68.1, SD = 10.7) in 70% of the cases. Compared with controls, cases with ASD were more likely to be firstborn and show a suboptimal condition after birth. Case mothers reported more infections and more stress during pregnancy. Although the ASD subgroups showed mostly overlapping risks, cases with PDD-NOS differed from those with AD by higher exposure to smoking during pregnancy (SDP) and by a negative association of smoking with IQ, regardless of confounders. SDP appears to contribute more to broadly defined (PDD-NOS) than to narrowly defined ASD (AD). Findings suggest differences in etiological contributors between ASD phenotypes.

The relationship between sensory sensitivity and autistic traits in the general population

Individuals with Autism Spectrum Disorders (ASDs) tend to have sensory processing difficulties (Baranek et al. in J Child Psychol Psychiatry 47:591-601, 2006). These difficulties include over- and under-responsiveness to sensory stimuli, and problems modulating sensory input (Ben-Sasson et al. in J Autism Dev Disorders 39:1-11, 2009). As those with ASD exist at the extreme end of a continuum of autistic traits that is also evident in the general population, we investigated the link between ASD and sensory sensitivity in the general population by administering two questionnaires online to 212 adult participants. Results showed a highly significant positive correlation (r = .775, p < .001) between number of autistic traits and the frequency of sensory processing problems. These data suggest a strong link between sensory processing and autistic traits in the general population, which in turn potentially implicates sensory processing problems in social interaction difficulties.

Examining and interpreting the female protective effect against autistic behavior

Male preponderance in autistic behavioral impairment has been explained in terms of a hypothetical protective effect of female sex, yet little research has tested this hypothesis empirically. If females are protected, they should require greater etiologic load to manifest the same degree of impairment as males. The objective of this analysis was to examine whether greater familial etiologic load was associated with quantitative autistic impairments in females compared with males. Subjects included 3,842 dizygotic twin pairs from the Twins Early Development Study (TEDS) and 6,040 dizygotic twin pairs from the Child and Adolescent Twin Study of Sweden (CATSS). In both samples, we compared sibling autistic traits between female and male probands, who were identified as children scoring in the top 90th and 95th percentiles of the population autistic trait distributions. In both TEDS and CATSS, siblings of female probands above the 90th percentile had significantly more autistic impairments than the siblings of male probands above the 90th percentile. The siblings of female probands above the 90th percentile also had greater categorical recurrence risk in both TEDS and CATSS. Results were similar in probands above the 95th percentile. This finding, replicated across two nationally-representative samples, suggests that female sex protects girls from autistic impairments and that girls may require greater familial etiologic load to manifest the phenotype. It provides empirical support for the hypothesis of a female protective effect against autistic behavior and can be used to inform and interpret future gene finding efforts in autism spectrum disorders.

Evidence that autistic traits show the same etiology in the general population and at the quantitative extremes (5%, 2.5%, and 1%)

CONTEXT

Genetic factors play an important role in the etiology of both autism spectrum disorders and autistic traits. However, little is known about the etiologic consistency of autistic traits across levels of severity.

OBJECTIVE

To compare the etiology of typical variation in autistic traits with extreme scoring groups (including top 1%) that mimicked the prevalence of diagnosed autism spectrum disorders in the largest twin study of autistic traits to date.

DESIGN

Twin study using phenotypic analysis and genetic model-fitting in the total sample and extreme scoring groups (top 5%, 2.5%, and 1%).

SETTING

A nationally representative twin sample from the general population of England.

PARTICIPANTS

The families of 5968 pairs aged 12 years old in the Twins' Early Development Study. Main Outcome Measure  Autistic traits as assessed by the Childhood Autism Spectrum Test.

RESULTS

Moderate to high heritability was found for autistic traits in the general population (53% for females and 72% for males). High heritability was found in extreme-scoring groups. There were no differences in heritability among extreme groups or between the extreme groups and the general population. A continuous liability shift toward autistic trait affectedness was seen in the cotwins of individuals scoring in the top 1%, suggesting shared etiology between extreme scores and normal variation.

CONCLUSION

This evidence of similar etiology across normal variation and the extremes has implications for molecular genetic models of autism spectrum disorders and for conceptualizing autism spectrum disorders as the quantitative extreme of a neurodevelopmental continuum.

Autistic traits below the clinical threshold: re-examining the broader autism phenotype in the 21st century

Diagnosis, intervention and support for people with autism can be assisted by research into the aetiology of the condition. Twin and family studies indicate that autism spectrum conditions are highly heritable; genetic relatives of people with autism often show milder expression of traits characteristic for autism, referred to as the Broader Autism Phenotype (BAP). In the past decade, advances in the biological and behavioural sciences have facilitated a more thorough examination of the BAP from multiple levels of analysis. Here, the candidate phenotypic traits delineating the BAP are summarised, including key findings from neuroimaging studies examining the neural substrates of the BAP. We conclude by reviewing the value of further research into the BAP, with an emphasis on deriving heritable endophenotypes which will reliably index autism susceptibility and offer neurodevelopmental mechanisms that bridge the gap between genes and a clinical autism diagnosis.

Stability of autistic traits in the general population: further evidence for a continuum of impairment

OBJECTIVE

This study investigated the developmental course of autistic traits in a nationally representative sample of subjects 7 to 13 years of age.

METHOD

The parents of 6,539 children in the Avon Longitudinal Study of Parents and Children completed the Social and Communication Disorders Checklist at ages 7, 10, and 13. The phenotypic progression of autistic traits was assessed in the full sample and in high-scoring individuals (e.g., top 10%, 5%). Gender, IQ, and overall behavior difficulties were examined as potentially relevant influences on autistic trait trajectories.

RESULTS

Autistic traits were highly stable in the general population overall and in the high-scoring groups. In the full sample, there was no change in mean Social and Communication Disorders Checklist scores for female subjects ages 7 to 13 (p = .43). Scores for male subjects decreased slightly, but significantly, on the order of 0.1 standard deviations (p < .001). There was no mean change in parent-rated autistic traits within any of the high-scoring groups. IQ was not related to phenotypic progression; high parent-rated behavior problems predicted slight improvement in Social and Communication Disorders Checklist scores over the course of the study period in high-scoring individuals (p < .01).

CONCLUSIONS

These findings suggest that autistic traits are highly stable in the general population, even in individuals with the highest concentrations of autism-like behaviors. Phenotypic stability is consistent with expectations for individuals with autism spectrum disorders, providing further support for a phenomenologic continuum across the clinical threshold. Moreover, the gap between female and male risk for autistic symptomology is consistent over time.

DIA1R is an X-linked gene related to Deleted In Autism-1

Background

Autism spectrum disorders (ASDs) are frequently occurring disorders diagnosed by deficits in three core functional areas: social skills, communication, and behaviours and/or interests. Mental retardation frequently accompanies the most severe forms of ASDs, while overall ASDs are more commonly diagnosed in males. Most ASDs have a genetic origin and one gene recently implicated in the etiology of autism is the Deleted-In-Autism-1 (DIA1) gene.

Methodology/Principal Findings

Using a bioinformatics-based approach, we have identified a human gene closely related to DIA1, we term DIA1R (DIA1-Related). While DIA1 is autosomal (chromosome 3, position 3q24), DIA1R localizes to the X chromosome at position Xp11.3 and is known to escape X-inactivation. The gene products are of similar size, with DIA1 encoding 430, and DIA1R 433, residues. At the amino acid level, DIA1 and DIA1R are 62% similar overall (28% identical), and both encode signal peptides for targeting to the secretory pathway. Both genes are ubiquitously expressed, including in fetal and adult brain tissue.

Conclusions/Significance

Examination of published literature revealed point mutations in DIA1R are associated with X-linked mental retardation (XLMR) and DIA1R deletion is associated with syndromes with ASD-like traits and/or XLMR. Together, these results support a model where the DIA1 and DIA1R gene products regulate molecular traffic through the cellular secretory pathway or affect the function of secreted factors, and functional deficits cause disorders with ASD-like symptoms and/or mental retardation.