Decomposing the autism phenotype into familial dimensions

Autism spectrum heterogeneity: fact or artifact?

The current diagnostic practices are linked to a 20-fold increase in the reported prevalence of ASD over the last 30 years. Fragmenting the autism phenotype into dimensional "autistic traits" results in the alleged recognition of autism-like symptoms in any psychiatric or neurodevelopemental condition and in individuals decreasingly distant from the typical population, and prematurely dismisses the relevance of a diagnostic threshold. Non-specific socio-communicative and repetitive DSM 5 criteria, combined with four quantitative specifiers as well as all their possible combinations, render limitless variety of presentations consistent with the categorical diagnosis of ASD. We propose several remedies to this problem: maintain a line of research on prototypical autism; limit the heterogeneity compatible with a categorical diagnosis to situations with a phenotypic overlap and a validated etiological link with prototypical autism; reintroduce the qualitative properties of autism presentations and of current dimensional specifiers, language, intelligence, comorbidity, and severity in the criteria used to diagnose autism in replacement of quantitative "social" and "repetitive" criteria; use these qualitative features combined with the clinical intuition of experts and machine-learning algorithms to differentiate coherent subgroups in today's autism spectrum; study these subgroups separately, and then compare them; and question the autistic nature of "autistic traits".

Language delay aggregates in toddler siblings of children with autism spectrum disorder

BACKGROUND

Language delay is extremely common in children with autism spectrum disorder (ASD), yet it is unclear whether measurable variation in early language is associated with genetic liability for ASD. Assessment of language development in unaffected siblings of children with ASD can inform whether decreased early language ability aggregates with inherited risk for ASD and serves as an ASD endophenotype.

METHODS

We implemented two approaches: (1) a meta-analysis of studies comparing language delay, a categorical indicator of language function, and language scores, a continuous metric, in unaffected toddlers at high and low familial risk for ASD, and (2) a parallel analysis of 350 unaffected 24-month-olds in the Infant Brain Imaging Study (IBIS), a prospective study of infants at high and low familial risk for ASD. An advantage of the former was its detection of group differences from pooled data across unique samples; an advantage of the latter was its sensitivity in quantifying early manifestations of language delay while accounting for covariates within a single large sample.

RESULTS

Meta-analysis showed that high-risk siblings without ASD (HR-noASD) were three to four times more likely to exhibit language delay versus low-risk siblings without ASD (LR-noASD) and had lower mean receptive and expressive language scores. Analyses of IBIS data corroborated that language delay, specifically receptive language delay, was more frequent in the HR-noASD (n = 235) versus LR-noASD group (n = 115). IBIS language scores were continuously and unimodally distributed, with a pathological shift towards decreased language function in HR-noASD siblings. The elevated inherited risk for ASD was associated with lower receptive and expressive language scores when controlling for sociodemographic factors. For receptive but not expressive language, the effect of risk group remained significant even when controlling for nonverbal cognition.

CONCLUSIONS

Greater frequency of language delay and a lower distribution of language scores in high-risk, unaffected toddler-aged siblings support decreased early language ability as an endophenotype for ASD, with a more pronounced effect for receptive versus expressive language. Further characterization of language development is warranted to refine genetic investigations of ASD and to elucidate factors influencing the progression of core autistic traits and related symptoms.

Eighteen-year-old man with autism, obsessive compulsive disorder and a SHANK2 variant presents with severe anorexia that responds to high-dose fluoxetine

The SHANK2 gene codes for a protein involved in organising the postsynaptic density and disruptions have been associated with autism spectrum disorders (ASDs). ASDs are frequently comorbid with intellectual disability and anxiety disorders and emerging evidence suggests potentially common aetiologies. Here, we report the case of an 18-year-old man with ASD who presented with severe anorexia due to fear of food contamination, food avoidance and stereotypies attributable to underlying obsessive compulsive disorder (OCD). The patient was found to be heterozygous for c.2518C>T (p.Pro840Ser), a likely damaging coding variant in the proline rich region of SHANK2 Interestingly, the patient's disordered eating behaviour began to improve only after high-dose fluoxetine was initiated to target OCD symptoms. Overall, this case highlights the utility of molecular genetic testing in clinical psychiatry and provides an example of how genetic information can inform clinicians in the treatment of complex neuropsychiatric syndromes.

The phenotypic manifestations of rare genic CNVs in autism spectrum disorder

Significant evidence exists for the association between copy number variants (CNVs) and Autism Spectrum Disorder (ASD); however, most of this work has focused solely on the diagnosis of ASD. There is limited understanding of the impact of CNVs on the 'sub-phenotypes' of ASD. The objective of this paper is to evaluate associations between CNVs in differentially brain expressed (DBE) genes or genes previously implicated in ASD/intellectual disability (ASD/ID) and specific sub-phenotypes of ASD. The sample consisted of 1590 cases of European ancestry from the Autism Genome Project (AGP) with a diagnosis of an ASD and at least one rare CNV impacting any gene and a core set of phenotypic measures, including symptom severity, language impairments, seizures, gait disturbances, intelligence quotient (IQ) and adaptive function, as well as paternal and maternal age. Classification analyses using a non-parametric recursive partitioning method (random forests) were employed to define sets of phenotypic characteristics that best classify the CNV-defined groups. There was substantial variation in the classification accuracy of the two sets of genes. The best variables for classification were verbal IQ for the ASD/ID genes, paternal age at birth for the DBE genes and adaptive function for de novo CNVs. CNVs in the ASD/ID list were primarily associated with communication and language domains, whereas CNVs in DBE genes were related to broader manifestations of adaptive function. To our knowledge, this is the first study to examine the associations between sub-phenotypes and CNVs genome-wide in ASD. This work highlights the importance of examining the diverse sub-phenotypic manifestations of CNVs in ASD, including the specific features, comorbid conditions and clinical correlates of ASD that comprise underlying characteristics of the disorder.

A molecular genetic study of autism and related phenotypes in extended pedigrees

Background

Efforts to uncover the risk genotypes associated with the familial nature of autism spectrum disorder (ASD) have had limited success. The study of extended pedigrees, incorporating additional ASD-related phenotypes into linkage analysis, offers an alternative approach to the search for inherited ASD susceptibility variants that complements traditional methods used to study the genetics of ASD.

Methods

We examined evidence for linkage in 19 extended pedigrees ascertained through ASD cases spread across at least two (and in most cases three) nuclear families. Both compound phenotypes (i.e., ASD and, in non-ASD individuals, the broad autism phenotype) and more narrowly defined components of these phenotypes, e.g., social and repetitive behavior, pragmatic language, and anxiety, were examined. The overarching goal was to maximize the aggregate information available on the maximum number of individuals and to disaggregate syndromic phenotypes in order to examine the genetic underpinnings of more narrowly defined aspects of ASD behavior.

Results

Results reveal substantial between-family locus heterogeneity and support the importance of previously reported ASD loci in inherited, familial, forms of ASD. Additional loci, not seen in the ASD analyses, show evidence for linkage to the broad autism phenotype (BAP). BAP peaks are well supported by multiple subphenotypes (including anxiety, pragmatic language, and social behavior) showing linkage to regions overlapping with the compound BAP phenotype. Whereas 'repetitive behavior’, showing the strongest evidence for linkage (Posterior Probability of Linkage = 62% at 6p25.2-24.3, and 69% at 19p13.3), appears to be linked to novel regions not detected with other compound or narrow phenotypes examined in this study.

Conclusions

These results provide support for the presence of key features underlying the complexity of the genetic architecture of ASD: substantial between-family locus heterogeneity, that the BAP appears to correspond to sets of subclinical features segregating with ASD within pedigrees, and that different features of the ASD phenotype segregate independently of one another. These findings support the additional study of larger, even more individually informative pedigrees, together with measurement of multiple, behavioral- and biomarker-based phenotypes, in both affected and non-affected individuals, to elucidate the complex genetics of familial ASD.

Infant siblings and the investigation of autism risk factors

Infant sibling studies have been at the vanguard of autism spectrum disorders (ASD) research over the past decade, providing important new knowledge about the earliest emerging signs of ASD and expanding our understanding of the developmental course of this complex disorder. Studies focused on siblings of children with ASD also have unrealized potential for contributing to ASD etiologic research. Moving targeted time of enrollment back from infancy toward conception creates tremendous opportunities for optimally studying risk factors and risk biomarkers during the pre-, peri- and neonatal periods. By doing so, a traditional sibling study, which already incorporates close developmental follow-up of at-risk infants through the third year of life, is essentially reconfigured as an enriched-risk pregnancy cohort study. This review considers the enriched-risk pregnancy cohort approach of studying infant siblings in the context of current thinking on ASD etiologic mechanisms. It then discusses the key features of this approach and provides a description of the design and implementation strategy of one major ASD enriched-risk pregnancy cohort study: the Early Autism Risk Longitudinal Investigation (EARLI).

Language performance in siblings of nonverbal children with autism

The study focuses on language and cognitive abilities of siblings of the linguistically most affected children with autism (i.e. siblings of nonverbal children - SIBS-ANV). Twenty-eight SIBS-ANV (17 boys), ages 4-9 years, took part in the study. All children attended regular schools, and none had received a diagnosis of autism. Controls were 27 typically developing children (SIBS-TD; 16 boys) matched to the SIBS-ANV on age, family background, socioeconomic status and type of school they attended. Significant IQ differences, as well as language differences as measured on the Clinical Evaluation of Language Fundamentals (CELF), emerged between SIBS-ANV and SIBS-TD. However, differences in the language scores mostly disappeared when PIQ and FSIQ were controlled for. Furthermore, grammatical analysis of spontaneous speech samples produced in the course of testing did not reveal any significant differences between the groups. These results add to recent work suggesting that language deficits may not be part of the Broad Autism Phenotype (BAP). It further suggests that the cognitive deficit characteristic of nonverbal people with autism may be familial.

Novel method for combined linkage and genome-wide association analysis finds evidence of distinct genetic architecture for two subtypes of autism

The Autism Genome Project has assembled two large datasets originally designed for linkage analysis and genome-wide association analysis, respectively: 1,069 multiplex families genotyped on the Affymetrix 10 K platform, and 1,129 autism trios genotyped on the Illumina 1 M platform. We set out to exploit this unique pair of resources by analyzing the combined data with a novel statistical method, based on the PPL statistical framework, simultaneously searching for linkage and association to loci involved in autism spectrum disorders (ASD). Our analysis also allowed for potential differences in genetic architecture for ASD in the presence or absence of lower IQ, an important clinical indicator of ASD subtypes. We found strong evidence of multiple linked loci; however, association evidence implicating specific genes was low even under the linkage peaks. Distinct loci were found in the lower IQ families, and these families showed stronger and more numerous linkage peaks, while the normal IQ group yielded the strongest association evidence. It appears that presence/absence of lower IQ (LIQ) demarcates more genetically homogeneous subgroups of ASD patients, with not just different sets of loci acting in the two groups, but possibly distinct genetic architecture between them, such that the LIQ group involves more major gene effects (amenable to linkage mapping), while the normal IQ group potentially involves more common alleles with lower penetrances. The possibility of distinct genetic architecture across subtypes of ASD has implications for further research and perhaps for research approaches to other complex disorders as well.

Mutations in the SHANK2 synaptic scaffolding gene in autism spectrum disorder and mental retardation

Using microarrays, we identified de novo copy number variations in the SHANK2 synaptic scaffolding gene in two unrelated individuals with autism-spectrum disorder (ASD) and mental retardation. DNA sequencing of SHANK2 in 396 individuals with ASD, 184 individuals with mental retardation and 659 unaffected individuals (controls) revealed additional variants that were specific to ASD and mental retardation cases, including a de novo nonsense mutation and seven rare inherited changes. Our findings further link common genes between ASD and intellectual disability.

Animal models of autism spectrum disorders: information for neurotoxicologists

Recent findings derived from large-scale datasets and biobanks link multiple genes to autism spectrum disorders. Consequently, novel rodent mutants with deletions, truncations and in some cases, overexpression of these candidate genes have been developed and studied both behaviorally and biologically. At the Annual Neurotoxicology Meeting in Rochester, NY in October of 2008, a symposium of clinicians and basic scientists gathered to present the behavioral features of autism, as well as strategies to model those behavioral features in mice and primates. The aim of the symposium was to provide researchers with up-to-date information on both the genetics of autism and how they are used in differing in vivo and in vitro animal models as well as to provide a background on the environmental exposures being tested on several animal models. In addition, researchers utilizing complementary approaches, presented on cell culture, in vitro or more basic models, which target neurobiological mechanisms, including Drosophila. Following the presentation, a panel convened to explore the opportunities and challenges of using model systems to investigate genetic and environment interactions in autism spectrum disorders. The following paper represents a summary of each presentation, as well as the discussion that followed at the end of the symposium.

PDD symptoms in ADHD, an independent familial trait?

The aims of this study were to investigate whether subtle PDD symptoms in the context of ADHD are transmitted in families independent of ADHD, and whether PDD symptom familiality is influenced by gender and age. The sample consisted of 256 sibling pairs with at least one child with ADHD and 147 healthy controls, aged 5-19 years. Children who fulfilled criteria for autistic disorder were excluded. The Children's Social Behavior Questionnaire (CSBQ) was used to assess PDD symptoms. Probands, siblings, and controls were compared using analyses of variance. Sibling correlations were calculated for CSBQ scores after controlling for IQ, ADHD, and comorbid anxiety. In addition, we calculated cross-sibling cross-trait correlations. Both children with ADHD and their siblings had higher PDD levels than healthy controls. The sibling correlation was 0.28 for the CSBQ total scale, with the CSBQ stereotyped behavior subscale showing the strongest sibling correlation (r = 0.35). Sibling correlations remained similar in strength after controlling for IQ and ADHD, and were not confounded by comorbid anxiety. Sibling correlations were higher in female than in male probands. The social subscale showed stronger sibling correlations in elder than in younger sibling pairs. Cross-sibling cross-trait correlations for PDD and ADHD were weak and not-significant. The results confirm that children with ADHD have high levels of PDD symptoms, and further suggest that the familiality of subtle PDD symptoms in the context of ADHD is largely independent from ADHD familiality.

Genome-wide linkage analyses of quantitative and categorical autism subphenotypes

BACKGROUND

The search for susceptibility genes in autism and autism spectrum disorders (ASD) has been hindered by the possible small effects of individual genes and by genetic (locus) heterogeneity. To overcome these obstacles, one method is to use autism-related subphenotypes instead of the categorical diagnosis of autism since they may be more directly related to the underlying susceptibility loci. Another strategy is to analyze subsets of families that meet certain clinical criteria to reduce genetic heterogeneity.

METHODS

In this study, using 976 multiplex families from the Autism Genome Project consortium, we performed genome-wide linkage analyses on two quantitative subphenotypes, the total scores of the reciprocal social interaction domain and the restricted, repetitive, and stereotyped patterns of behavior domain from the Autism Diagnostic Interview-Revised. We also selected subsets of ASD families based on four binary subphenotypes, delayed onset of first words, delayed onset of first phrases, verbal status, and IQ > or = 70.

RESULTS

When the ASD families with IQ > or = 70 were used, a logarithm of odds (LOD) score of 4.01 was obtained on chromosome 15q13.3-q14, which was previously linked to schizophrenia. We also obtained a LOD score of 3.40 on chromosome 11p15.4-p15.3 using the ASD families with delayed onset of first phrases. No significant evidence for linkage was obtained for the two quantitative traits.

CONCLUSIONS

This study demonstrates that selection of informative subphenotypes to define a homogeneous set of ASD families could be very important in detecting the susceptibility loci in autism.

What is new in autism?

PURPOSE OF REVIEW

Autism is now recognized in one out of 150 children. This review highlights the topics within the growing autism literature that are shaping current thinking on autism and advancing research and clinical understanding of autism spectrum disorders.

RECENT FINDINGS

The role of single-stranded microdeletions and epigenetic influences on brain development has dramatically altered our understanding of the etiology of the autisms. Recent research has focused on the role of synapse structure and function as central to the development of autism and suggests possible targets of interventions. Brain underconnectivity has been a focus in recent imaging studies and has become a central theme in conceptualizing autism. Despite increased awareness of autism there is no 'epidemic' and no one cause for autism. Data from the sibling studies are identifying early markers of autism and defining the broader autism phenotype.

SUMMARY

Larger datasets in genetics, a focus on the early signs of autism, and increased recognition of the importance of defining subgroups of children with autism are leading to a greater understanding of the etiologies of autism. A growing interest in defining the molecular biology of social cognition, which is at the core of autism, will lead to expansion of our presently limited choices of mechanistically based interventions.