Autism and nonsyndromic mental retardation associated with a de novo mutation in the NLGN4X gene promoter causing an increased expression level

Implications of Cell Adhesion Molecules in Autism Spectrum Disorder Pathogenesis

Autism spectrum disorder (ASD) is a neurodevelopmental illness that leads to repetitive behavior and debilitates social communication. Genetic changes such as susceptible genes and environmental factors promote ASD pathogenesis. Mutations in neuroligins (NLGNs) and neurexin (NRXNs) complex which encode cell adhesion molecules have a significant part in synapses formation, transcription, and excitatory-inhibitory balance. The ASD pathogenesis could partly, at the least, be related to synaptic dysfunction. Here, the NRXNs and NLGNs genes and signaling pathways involved in the synaptic malfunction that causes ASD have been reviewed. Besides, a new insight of NLGNs and NRXNs genes in ASD will be conferred.

The non-coding genome in Autism Spectrum Disorders

Autism Spectrum Disorders (ASD) are a group of neurodevelopmental disorders (NDDs) characterized by difficulties in social interaction and communication, repetitive behavior, and restricted interests. While ASD have been proven to have a strong genetic component, current research largely focuses on coding regions of the genome. However, non-coding DNA, which makes up for ∼99% of the human genome, has recently been recognized as an important contributor to the high heritability of ASD, and novel sequencing technologies have been a milestone in opening up new directions for the study of the gene regulatory networks embedded within the non-coding regions. Here, we summarize current progress on the contribution of non-coding alterations to the pathogenesis of ASD and provide an overview of existing methods allowing for the study of their functional relevance, discussing potential ways of unraveling ASD's "missing heritability".

X-linked ichthyosis: New insights into a multi-system disorder

Background

X-linked ichthyosis (XLI) is a rare genetic condition almostexclusively affecting males; it is characterised by abnormal desquamation and retentionhyperkeratosis, and presents with polygonal brown scales. Most cases resultfrom genetic deletions within Xp22.31 spanning the STS (steroid sulfatase)gene, with the remaining cases resulting from STS-specific mutations. For manyyears it has been recognised that individuals with XLI are at increased risk ofcryptorchidism and corneal opacities.

Methods

We discuss emerging evidence that such individuals are alsomore likely to be affected by a range of neurodevelopmental and psychiatrictraits, by cardiac arrhythmias, and by rare fibrotic and bleeding-relatedconditions. We consider candidate mechanisms that may confer elevatedlikelihood of these individual conditions, and propose a novel commonbiological risk pathway.

Results

Understanding the prevalence, nature and co-occurrence ofcomorbidities associated with XLI is critical for ensuring early identificationof symptoms and for providing the most effective genetic counselling andmultidisciplinary care for affected individuals.

Conclusion

Future work in males with XLI, and in new preclinical andcellular model systems, should further clarify underlying pathophysiologicalmechanisms amenable to therapeutic intervention.

Comparative mapping of selected structural determinants on the extracellular domains of cholinesterase-like cell-adhesion molecules

Cell adhesion generally involves formation of homophilic or heterophilic protein complexes between two cells to form transcellular junctions. Neural cell-adhesion members of the α/β-hydrolase fold superfamily of proteins use their extracellular or soluble cholinesterase-like domain to bind cognate partners across cell membranes, as illustrated by the neuroligins. These cell-adhesion molecules currently comprise the synaptic organizers neuroligins found in all animal phyla, along with three proteins found only in invertebrates: the guidance molecule neurotactin, the glia-specific gliotactin, and the basement membrane protein glutactin. Although these proteins share a cholinesterase-like fold, they lack one or more residues composing the catalytic triad responsible for the enzymatic activity of the cholinesterases. Conversely, they are found in various subcellular localisations and display specific disulfide bonding and N-glycosylation patterns, along with individual surface determinants possibly associated with recognition and binding of protein partners. Formation of non-covalent dimers typical of the cholinesterases is documented for mammalian neuroligins, yet whether invertebrate neuroligins and their neurotactin, gliotactin and glutactin relatives also form dimers in physiological conditions is unknown. Here we provide a brief overview of the localization, function, evolution, and conserved versus individual structural determinants of these cholinesterase-like cell-adhesion proteins. This article is part of the special issue entitled 'Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield'.

Autism-associated variants of neuroligin 4X impair synaptogenic activity by various molecular mechanisms

BACKGROUND

Several genetic alterations, including point mutations and copy number variations in NLGN genes, have been associated with psychiatric disorders, such as autism spectrum disorder (ASD) and X-linked mental retardation (XLMR). NLGN genes encode neuroligin (NL) proteins, which are adhesion molecules that are important for proper synaptic formation and maturation. Previously, we and others found that the expression level of murine NL1 is regulated by proteolytic processing in a synaptic activity-dependent manner.

METHODS

In this study, we analyzed the effects of missense variants associated with ASD and XLMR on the metabolism and function of NL4X, a protein which is encoded by the NLGN4X gene and is expressed only in humans, using cultured cells, primary neurons from rodents, and human induced pluripotent stem cell-derived neurons.

RESULTS

NL4X was found to undergo proteolytic processing in human neuronal cells. Almost all NL4X variants caused a substantial decrease in the levels of mature NL4X and its synaptogenic activity in a heterologous culture system. Intriguingly, the L593F variant of NL4X accelerated the proteolysis of mature NL4X proteins located on the cell surface. In contrast, other variants decreased the cell-surface trafficking of NL4X. Notably, protease inhibitors as well as chemical chaperones rescued the expression of mature NL4X.

LIMITATIONS

Our study did not reveal whether these dysfunctional phenotypes occurred in individuals carrying NLGN4X variant. Moreover, though these pathological mechanisms could be exploited as potential drug targets for ASD, it remains unclear whether these compounds would have beneficial effects on ASD model animals and patients.

CONCLUSIONS

These data suggest that reduced amounts of the functional NL4X protein on the cell surface is a common mechanism by which point mutants of the NL4X protein cause psychiatric disorders, although different molecular mechanisms are thought to be involved. Furthermore, these results highlight that the precision medicine approach based on genetic and cell biological analyses is important for the development of therapeutics for psychiatric disorders.

Transforming growth factor beta 1 (TGFβ1) polymorphisms and unexplained infertility: A genetic association study

The prevalence of infertility is increasing and worrisome. About 10 to 30% of infertility is classified as idiopathic or unexplained infertility (UI).TGF-β is multifunctional and immunoregulatry cytokine which regulates both implantation and adhesion of trophoblasts to the extracellular matrix during pregnancy. The aim of the current study was to investigate the association between two polymorphisms rs1800470 (C29T) and rs1800471 (G74C) of the TGF-β1 gene in Iranian patients with unexplained infertility. A total of 250 UI patients and 484 healthy individuals with no history of infertility were included in the study. The amplification and sequencing of target DNA fragments were done using PCR and automated sequencing methods, respectively. The effects of these polymorphisms on both TGF-β1 structure and function of mRNA and protein were analyzed using new in-silico tools. The frequency distribution of the alleles, genotypes, and haplotypes of both rs1800470 and rs1800471 polymorphisms had a statistically significant difference between subjects and controls. CC genotype of TGF-β1 rs1800470 (29C→T) increase the risk of UI in male UI patients. Moreover, C alleles of TGF-β1 rs1800471 was associated with increased risk of UI in female UI patients. Couples, subgroup analysis revealed a significant association between TGF-β1 polymorphisms (rs1800470, rs1800471) and the risk of UI in male, female, and all UI patients. The frequency of TG and CG haplotypes were statistically different in both UI and healthy subjects group (P < 0.05). RS1800471 polymorphisms changed the secondary structure of TGF-β1 mRNA and resulted in the removal of one mRNA arm and creation of two new arms. Taken together, the results of the current study suggest that TGF-β1 functional polymorphisms may play an important role in the susceptibility to UI in Iranian population. According to in silico analysis, polymorphisms in TGF-β1 can reduce mRNA half-life and, therefore, reduced TGF-β1 expression. .

Novel Microdeletion in the X Chromosome Leads to Kallmann Syndrome, Ichthyosis, Obesity, and Strabismus

Background

A large deletion in Xp22.3 can result in contiguous gene syndromes, including X-linked ichthyosis (XLI) and Kallmann syndrome (KS), presenting with short stature, chondrodysplasia punctata, intellectual disability, and strabismus. XLI and KS are caused by the deletion of STS and ANOS1, respectively.

Method

Two KS patients with XLI were screened to identify possible pathogenic mutations using whole exome sequencing. The clinical characteristics, molecular genetics, treatment outcomes, and genotype–phenotype association for each patient were analyzed.

Results

We identified a novel 3,923 kb deletion within the Xp22.31 region (chrX: 5810838–9733877) containing STS, ANOS1, GPR143, NLGN4X, VCX-A, PUDP, and PNPLA4 in patient 1, who presented with KS, XLI, obesity, hyperlipidemia, and strabismus. We identified a novel 5,807 kb deletion within the Xp22.31-p22.33 regions (chrX: 2700083–8507807) containing STS, ANOS1, and other 24 genes in patient 2, who presented with KS, XLI, obesity, and strabismus. No developmental delay, abnormal speech development, or autistic behavior were noticed in either patient.

Conclusion

We identified two novel microdeletions in the X chromosome leading to KS and XLI. These findings contribute to the understanding of the molecular mechanisms that drive contiguous gene syndromes. Our research confirmed that the Kallmann-Ichthyosis phenotype is caused by microdeletions at the chromosome level.

A framework for an evidence-based gene list relevant to autism spectrum disorder

Autism spectrum disorder (ASD) is often grouped with other brain-related phenotypes into a broader category of neurodevelopmental disorders (NDDs). In clinical practice, providers need to decide which genes to test in individuals with ASD phenotypes, which requires an understanding of the level of evidence for individual NDD genes that supports an association with ASD. Consensus is currently lacking about which NDD genes have sufficient evidence to support a relationship to ASD. Estimates of the number of genes relevant to ASD differ greatly among research groups and clinical sequencing panels, varying from a few to several hundred. This Roadmap discusses important considerations necessary to provide an evidence-based framework for the curation of NDD genes based on the level of information supporting a clinically relevant relationship between a given gene and ASD.

Y chromosome gene copy number and lack of autism phenotype in a male with an isodicentric Y chromosome and absent NLGN4Y expression

We describe a unique male with a dicentric Y chromosome whose phenotype was compared to that of males with 47,XYY (XYY). The male Y-chromosome aneuploidy XYY is associated with physical, behavioral/cognitive phenotypes, and autism spectrum disorders. We hypothesize that increased risk for these phenotypes is caused by increased copy number/overexpression of Y-encoded genes. Specifically, an extra copy of the neuroligin gene NLGN4Y might elevate the risk of autism in boys with XYY. We present a unique male with the karyotype 46,X,idic(Y)(q11.22), which includes duplication of the Y short arm and proximal long arm and deletion of the distal long arm, evaluated his physical, behavioral/cognitive, and neuroimaging/magnetoencephalography (MEG) phenotypes, and measured blood RNA expression of Y genes. The proband had tall stature and cognitive function within the typical range, without autism features. His blood RNA showed twofold increase in expression of Yp genes versus XY controls, and absent expression of deleted Yq genes, including NLGN4Y. The M100 latencies were similar to findings in typically developing males. In summary, the proband had overexpression of a subset of Yp genes, absent NLGN4Y expression, without ASD findings or XYY-MEG latency findings. These results are consistent with a role for NLGN4Y overexpression in the etiology of behavioral phenotypes associated with XYY. Further investigation of NLGN4Y as an ASD risk gene in XYY is warranted. The genotype and phenotype(s) of this subject may also provide insight into how Y chromosome genes contribute to normal male development and the male predominance in ASD.

Autism-associated neuroligin-4 mutation selectively impairs glycinergic synaptic transmission in mouse brainstem synapses

Loss-of-function mutations of the human postsynaptic cell-adhesion protein neuroligin-4 have been repeatedly associated with autism, but the precise synaptic function of neuroligin-4 that may account for its role in autism remains unclear. Here, we show in murine brainstem synapses that neuroligin-4 is selectively required for glycinergic synaptic transmission in mice.

In human patients, loss-of-function mutations of the postsynaptic cell-adhesion molecule neuroligin-4 were repeatedly identified as monogenetic causes of autism. In mice, neuroligin-4 deletions caused autism-related behavioral impairments and subtle changes in synaptic transmission, and neuroligin-4 was found, at least in part, at glycinergic synapses. However, low expression levels precluded a comprehensive analysis of neuroligin-4 localization, and overexpression of neuroligin-4 puzzlingly impaired excitatory but not inhibitory synaptic function. As a result, the function of neuroligin-4 remains unclear, as does its relation to other neuroligins. To clarify these issues, we systematically examined the function of neuroligin-4, focusing on excitatory and inhibitory inputs to defined projection neurons of the mouse brainstem as central model synapses. We show that loss of neuroligin-4 causes a profound impairment of glycinergic but not glutamatergic synaptic transmission and a decrease in glycinergic synapse numbers. Thus, neuroligin-4 is essential for the organization and/or maintenance of glycinergic synapses.

A strategic plan to identify key neurophysiological mechanisms and brain circuits in autism

Autism and Autism Spectrum Disorder (ASD) cover a large variety of clinical profiles which share two main dimensions: social and communication impairment and repetitive behaviors or restricted interests, which are present during childhood. There is now no doubt that genetic factors are a major component in the etiology of autism but precise physiopathological pathways are still being investigated. Furthermore, developmental trajectories combined with compensatory mechanisms will lead to various clinical and neurophysiological profiles which together constitute this Autism Spectrum Disorder. To better understand the pathophysiology of autism, comprehension of key neurophysiological mechanisms and brain circuits underlying the different bioclinical profiles is thus crucial. To achieve this goal we propose a strategy which investigates different levels of information processing from sensory perception to complex cognitive processing, taking into account the complexity of the stimulus and whether it is social or non-social in nature. In order to identify different developmental trajectories and to take into account compensatory mechanisms, we further propose that such protocols should be carried out in individuals from childhood to adulthood representing a wide variety of clinical forms.

Not all neuroligin 3 and 4X missense variants lead to significant functional inactivation

INTRODUCTION

Neuroligins are postsynaptic cell adhesion molecules that interact with neurexins to regulate the fine balance between excitation and inhibition of synapses. Recently, accumulating evidence, involving mutation analysis, cellular assays, and mouse models, has suggested that neuroligin (NLGN) mutations affect synapse maturation and function. Previously, four missense variations [p.G426S (NLGN3), p.G84R (NLGN4X), p.Q162K (NLGN4X), and p.A283T (NLGN4X)] in four different unrelated patients have been identified by PCR and direct sequencing.

METHODS

In this study, we analyzed the functional effect of these missense variations by in vitro experiment via the stable HEK293 cells expressing wild-type and mutant neuroligin.

RESULTS

We found that the four mutations did not significantly impair the expression of neuroligin 3 and neuroligin 4X, and also did not measurably inhibit the neurexin 1-neuroligin interaction. These variants might play a modest role in the pathogenesis of autism or might simply be unreported infrequent polymorphisms.

CONCLUSION

Our data suggest that these four previously described neuroligin mutations are not primary risk factors for autism.

Association Analysis of Noncoding Variants in Neuroligins 3 and 4X Genes with Autism Spectrum Disorder in an Italian Cohort

Since involved in synaptic transmission and located on X-chromosome, neuroligins 3 and 4X have been studied as good positional and functional candidate genes for autism spectrum disorder pathogenesis, although contradictory results have been reported. Here, we performed a case-control study to assess the association between noncoding genetic variants in NLGN3 and NLGN4X genes and autism, in an Italian cohort of 202 autistic children analyzed by high-resolution melting. The results were first compared with data from 379 European healthy controls (1000 Genomes Project) and then with those from 1061 Italian controls genotyped by Illumina single nucleotide polymorphism (SNP) array 1M-duo. Statistical evaluations were performed using Plink v1.07, with the Omnibus multiple loci approach. According to both the European and the Italian control groups, a 6-marker haplotype on NLGN4X (rs6638575(G), rs3810688(T), rs3810687(G), rs3810686(C), rs5916269(G), rs1882260(T)) was associated with autism (odd ratio = 3.58, p-value = 2.58 × 10⁻⁶ for the European controls; odds ratio = 2.42, p-value = 6.33 × 10⁻³ for the Italian controls). Furthermore, several haplotype blocks at 5-, 4-, 3-, and 2-, including the first 5, 4, 3, and 2 SNPs, respectively, showed a similar association with autism. We provide evidence that noncoding polymorphisms on NLGN4X may be associated to autism, suggesting the key role of NLGN4X in autism pathophysiology and in its male prevalence.

SHANK1 and autism spectrum disorders

Autism spectrum disorders (ASD) are highly heterogeneous pediatric developmental disorders with estimated heritability more than 70%. Although the genetic factors in ASD are mainly unknown, a large number of gene mutations have been found, especially in genes involved in neurogenesis. The Neurexin-Neuroligin-Shank (NRXN-NLGN-SHANK) pathway plays a key role in the formation, maturation and maintenance of synapses, consistent with the hypothesis of neurodevelopmental abnormality in ASD. Presynaptic NRXNs interact with postsynaptic NLGNs in excitatory glutamatergic synapses. SHANK proteins function as core components of the postsynaptic density (PSD) by interacting with multiple proteins. Recently, deletions and point mutations of the SHANK1 gene have been detected in ASD individuals, indicating the involvement of SHANK1 in ASD. This review focuses on the function of SHANK1 protein, Shank1 mouse models, and the molecular genetics of the SHANK1 gene in human ASD.

GABA/Glutamate synaptic pathways targeted by integrative genomic and electrophysiological explorations distinguish autism from intellectual disability

Phenotypic and genetic heterogeneity is predominant in autism spectrum disorders (ASD), for which the molecular and pathophysiological bases are still unclear. Significant comorbidity and genetic overlap between ASD and other neurodevelopmental disorders are also well established. However, little is understood regarding the frequent observation of a wide phenotypic spectrum associated with deleterious mutations affecting a single gene even within multiplex families. We performed a clinical, neurophysiological (in vivo electroencephalography-auditory-evoked related potentials) and genetic (whole-exome sequencing) follow-up analysis of two families with known deleterious NLGN4X gene mutations (either truncating or overexpressing) present in individuals with ASD and/or with intellectual disability (ID). Complete phenotypic evaluation of the pedigrees in the ASD individuals showed common specific autistic behavioural features and neurophysiological patterns (abnormal MisMatch Negativity in response to auditory change) that were absent in healthy parents as well as in family members with isolated ID. Whole-exome sequencing in ASD patients from each family identified a second rare inherited genetic variant, affecting either the GLRB or the ANK3 genes encoding NLGN4X interacting proteins expressed in inhibitory or in excitatory synapses, respectively. The GRLB and ANK3 mutations were absent in relatives with ID as well as in control databases. In summary, our findings provide evidence of a double-hit genetic model focused on excitatory/inhibitory synapses in ASD, that is not found in isolated ID, associated with an atypical in vivo neurophysiological pattern linked to predictive coding.

Common mechanisms of excitatory and inhibitory imbalance in schizophrenia and autism spectrum disorders

Autism Spectrum Disorders (ASD) and Schizophrenia (SCZ) are cognitive disorders with complex genetic architectures but overlapping behavioral phenotypes, which suggests common pathway perturbations. Multiple lines of evidence implicate imbalances in excitatory and inhibitory activity (E/I imbalance) as a shared pathophysiological mechanism. Thus, understanding the molecular underpinnings of E/I imbalance may provide essential insight into the etiology of these disorders and may uncover novel targets for future drug discovery. Here, we review key genetic, physiological, neuropathological, functional, and pathway studies that suggest alterations to excitatory/inhibitory circuits are keys to ASD and SCZ pathogenesis.

The complex genetics in autism spectrum disorders

Autism spectrum disorders (ASD) are a pervasive neurodevelopmental disease characterized by deficits in social interaction and nonverbal communication, as well as restricted interests and stereotypical behavior. Genetic changes/heritability is one of the major contributing factors, and hundreds to thousands of causative and susceptible genes, copy number variants (CNVs), linkage regions, and microRNAs have been associated with ASD which clearly indicates that ASD is a complex genetic disorder. Here, we will briefly summarize some of the high-confidence genetic changes in ASD and their possible roles in their pathogenesis.

Behavioral phenotypes in males with XYY and possible role of increased NLGN4Y expression in autism features

The male sex chromosome disorder, 47,XYY syndrome (XYY), is associated with increased risk for social-emotional difficulties, attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). We hypothesize that increased Y chromosome gene copy number in XYY leads to overexpression of Y-linked genes related to brain development and function, thereby increasing risk for these phenotypes. We measured expression in blood of two Y genes NLGN4Y and RPS4Y in 26 boys with XYY and 11 male controls and evaluated whether NLGN4Y expression correlates with anxiety, ADHD, depression and autistic behaviors (from questionnaires) in boys with XYY. The XYY cohort had increased risk of ASD behaviors on the social responsiveness scale (SRS) and increased attention deficits on the Conners' DSM-IV inattention and hyperactive scales. In contrast, there was no increase in reported symptoms of anxiety or depression by the XYY group. Peripheral expression of two Y genes in boys with XYY vs. typically developing controls was increased twofold in the XYY group. Results from the SRS total and autistic mannerisms scales, but not from the attention, anxiety or depression measures, correlated with peripheral expression of NLGN4Y in boys with XYY. Males with XYY have social phenotypes that include increased risk for autism-related behaviors and ADHD. Expression of NLGN4Y, a gene that may be involved in synaptic function, is increased in boys with XYY, and the level of expression correlates with overall social responsiveness and autism symptoms. Thus, further investigation of NLGN4Y as a plausible ASD risk gene in XYY is warranted.

Reciprocal signaling between translational control pathways and synaptic proteins in autism spectrum disorders

Autism spectrum disorder (ASD) is a heterogeneous group of heritable neurodevelopmental disorders. Symptoms of ASD, which include deficits in social interaction skills, impaired communication ability, and ritualistic-like repetitive behaviors, appear in early childhood and continue throughout life. Genetic studies have revealed at least two clusters of genes frequently associated with ASD and intellectual disability: those encoding proteins involved in translational control and those encoding proteins involved in synaptic function. We hypothesize that mutations occurring in these two clusters of genes interfere with interconnected downstream signaling pathways in neuronal cells to cause ASD symptomatology. In this review, we discuss the monogenic forms of ASD caused by mutations in genes encoding for proteins that regulate translation and synaptic function. Specifically, we describe the function of these proteins, the intracellular signaling pathways that they regulate, and the current mouse models used to characterize the synaptic and behavioral features associated with their mutation. Finally, we summarize recent studies that have established a connection between mRNA translation and synaptic function in models of ASD and propose that dysregulation of one has a detrimental impact on the other.

The social brain network and autism

Available research data in Autism suggests the role of a network of brain areas, often known as the ‘social brain’. Recent studies highlight the role of genetic mutations as underlying patho-mechanism in Autism. This mini review, discusses the basic concepts behind social brain networks, theory of mind and genetic factors associated with Autism. It critically evaluates and explores the relationship between the behavioral outcomes and genetic factors providing a conceptual framework for understanding of autism.

Etiologies underlying sex differences in Autism Spectrum Disorders

The male predominance of Autism Spectrum Disorders (ASD) is one of the best-known, and at the same time, one of the least understood characteristics of these disorders. In this paper we review genetic, epigenetic, hormonal, and environmental mechanisms underlying this male preponderance. Sex-specific effects of Y-linked genes (including SRY expression leading to testicular development), balanced and skewed X-inactivation, genes that escape X-inactivation, parent-of-origin allelic imprinting, and the hypothetical heterochromatin sink are reviewed. These mechanisms likely contribute to etiology, instead of being simply causative to ASD. Environments, both internal and external, also play important roles in ASD's etiology. Early exposure to androgenic hormones and early maternal immune activation comprise environmental factors affecting sex-specific susceptibility to ASD. The gene-environment interactions underlying ASD, suggested here, implicate early prenatal stress as being especially detrimental to boys with a vulnerable genotype.

Screening for intellectual disability using high-resolution CMA technology in a retrospective cohort from Central Brazil

Intellectual disability is a complex, variable, and heterogeneous disorder, representing a disabling condition diagnosed worldwide, and the etiologies are multiple and highly heterogeneous. Microscopic chromosomal abnormalities and well-characterized genetic conditions are the most common causes of intellectual disability. Chromosomal Microarray Analysis analyses have made it possible to identify putatively pathogenic copy number variation that could explain the molecular etiology of intellectual disability. The aim of the current study was to identify possible submicroscopic genomic alterations using a high-density chromosomal microarray in a retrospective cohort of patients with otherwise undiagnosable intellectual disabilities referred by doctors from the public health system in Central Brazil. The CytoScan HD technology was used to detect changes in the genome copy number variation of patients who had intellectual disability and a normal karyotype. The analysis detected 18 CNVs in 60% of patients. Pathogenic CNVs represented about 22%, so it was possible to propose the etiology of intellectual disability for these patients. Likely pathogenic and unknown clinical significance CNVs represented 28% and 50%, respectively. Inherited and de novo CNVs were equally distributed. We report the nature of CNVs in patients from Central Brazil, representing a population not yet screened by microarray technologies.

A non-syndromic intellectual disability associated with a de novo microdeletion at 7q and 18p, microduplication at Xp, and 18q partial trisomy detected using chromosomal microarray analysis approach

BACKGROUND

Chromosome abnormalities that segregate with a disease phenotype can facilitate the identification of disease loci and genes. The relationship between chromosome 18 anomalies with severe intellectual disability has attracted the attention of cytogeneticists worldwide. Duplications of the X chromosome can cause intellectual disability in females with variable phenotypic effects, due in part to variations in X-inactivation patterns. Additionally, deletions of the 7qter region are associated with a range of phenotypes.

RESULTS

We report the first case of de novo microdeletion at 7q and 18p, 18q partial trisomy, microduplication at Xp associated to intellectual disability in a Brazilian child, presenting a normal karyotype. Karyotyping showed any chromosome alteration. Chromosomal microarray analysis detected a de novo microdeletion at 18p11.32 and 18q partial trisomy, an inherited microdeletion at 7q31.1 and a de novo microduplication at Xp22.33p21.3.

CONCLUSIONS

Our report illustrates a case that presents complex genomic imbalances which may contribute to a severe clinical phenotypes. The rare and complex phenotypes have to be investigated to define the subsets and allow the phenotypes classification.

Sleep, plasticity and the pathophysiology of neurodevelopmental disorders: the potential roles of protein synthesis and other cellular processes

Sleep is important for neural plasticity, and plasticity underlies sleep-dependent memory consolidation. It is widely appreciated that protein synthesis plays an essential role in neural plasticity. Studies of sleep-dependent memory and sleep-dependent plasticity have begun to examine alterations in these functions in populations with neurological and psychiatric disorders. Such an approach acknowledges that disordered sleep may have functional consequences during wakefulness. Although neurodevelopmental disorders are not considered to be sleep disorders per se, recent data has revealed that sleep abnormalities are among the most prevalent and common symptoms and may contribute to the progression of these disorders. The main goal of this review is to highlight the role of disordered sleep in the pathology of neurodevelopmental disorders and to examine some potential mechanisms by which sleep-dependent plasticity may be altered. We will also briefly attempt to extend the same logic to the other end of the developmental spectrum and describe a potential role of disordered sleep in the pathology of neurodegenerative diseases. We conclude by discussing ongoing studies that might provide a more integrative approach to the study of sleep, plasticity, and neurodevelopmental disorders.

Neurobiology of autism gene products: towards pathogenesis and drug targets

RATIONALE

The genetic heterogeneity of autism spectrum disorders (ASDs) is enormous, and the neurobiology of proteins encoded by genes associated with ASD is very diverse. Revealing the mechanisms on which different neurobiological pathways in ASD pathogenesis converge may lead to the identification of drug targets.

OBJECTIVE

The main objective is firstly to outline the main molecular networks and neuronal mechanisms in which ASD gene products participate and secondly to answer the question how these converge. Finally, we aim to pinpoint drug targets within these mechanisms.

METHOD

Literature review of the neurobiological properties of ASD gene products with a special focus on the developmental consequences of genetic defects and the possibility to reverse these by genetic or pharmacological interventions.

RESULTS

The regulation of activity-dependent protein synthesis appears central in the pathogenesis of ASD. Through sequential consequences for axodendritic function, neuronal disabilities arise expressed as behavioral abnormalities and autistic symptoms in ASD patients. Several known ASD gene products have their effect on this central process by affecting protein synthesis intrinsically, e.g., through enhancing the mammalian target of rapamycin (mTOR) signal transduction pathway or through impairing synaptic function in general. These are interrelated processes and can be targeted by compounds from various directions: inhibition of protein synthesis through Lovastatin, mTOR inhibition using rapamycin, or mGluR-related modulation of synaptic activity.

CONCLUSIONS

ASD gene products may all feed into a central process of translational control that is important for adequate glutamatergic regulation of dendritic properties. This process can be modulated by available compounds but may also be targeted by yet unexplored routes.

The role of ionotropic glutamate receptors in childhood neurodevelopmental disorders: autism spectrum disorders and fragile x syndrome

Autism spectrum disorder (ASD) and Fragile X syndrome (FXS) are relatively common childhood neurodevelopmental disorders with increasing incidence in recent years. They are currently accepted as disorders of the synapse with alterations in different forms of synaptic communication and neuronal network connectivity. The major excitatory neurotransmitter system in brain, the glutamatergic system, is implicated in learning and memory, synaptic plasticity, neuronal development. While much attention is attributed to the role of metabotropic glutamate receptors in ASD and FXS, studies indicate that the ionotropic glutamate receptors (iGluRs) and their regulatory proteins are also altered in several brain regions. Role of iGluRs in the neurobiology of ASD and FXS is supported by a weight of evidence that ranges from human genetics to in vitro cultured neurons. In this review we will discuss clinical, molecular, cellular and functional changes in NMDA, AMPA and kainate receptors and the synaptic proteins that regulate them in the context of ASD and FXS. We will also discuss the significance for the development of translational biomarkers and treatments for the core symptoms of ASD and FXS.

Genetic association and gene expression profiles of TGFB1 and the contribution of TGFB1 to otosclerosis susceptibility

Otosclerosis (OTSC) is a common form of acquired hearing loss resulting from disturbed bone remodeling in the otic capsule of the middle ear. Transforming growth factor-beta1 (TGFB1) produced by osteoblasts is the most abundant growth factor in human bone. Previous studies have shown the contribution of single-nucleotide polymorphisms (SNPs) in TGFB1 toward the risk of developing OTSC in some ethnic populations. The present study was aimed at investigating the genetic association and expression profiles of TGFB1 in OTSC patients. Two SNPs (c.-800G > A and c.-509C > T) in the promoter region and three SNPs (c.29T > C, c.74G > C, and c.788C > T) in the coding region were genotyped in 170 cases and 170 controls. The genetic association analysis revealed the significant association between c.-509C > T (p = 0.0067; odds ratio [OR] = 1.562; 95% confidence interval [CI], 1.140-2.139) and OTSC. The increased minor allele "T" frequency in cases (0.42) compared to controls (0.31) indicates its possible role in the etiology of the disease. The minor allele frequencies for the SNPs c.-800G > A, c.29T > C, and c.74G >C were similar among the cases (0.04, 0.47, and 0.08, respectively) and controls (0.05, 0.42, 0.07, respectively). We found that c.788C > T was monomorphic in this population. Interestingly, a four-locus haplotype (G-T-T-G) from these SNPs was found to be significantly associated with OTSC (p = 0.0077). We identified a de novo heterozygous mutation c.-832G > A in the promoter region of TGFB1 in 1 patient. In a secondary analysis, we investigated the possibility of abnormal TGFB1 expression and irregular bone growth in OTSC by expression analysis of TGFB1 mRNA in disease tissue compared to control. We found relatively increased expression of TGFB1 mRNA in the stapes tissues of cases compared to controls (p = 0.0057). In conclusion, this study identified a risk variant c.-509C > T and a risk haplotype G-T-T-G in the TGFB1 gene that contribute toward the susceptibility to OTSC.

Autism genetics

Autism spectrum disorder (ASD) is a severe neuropsychiatric disease with strong genetic underpinnings. However, genetic contributions to autism are extremely heterogeneous, with many different loci underlying the disease to a different extent in different individuals. Moreover, the phenotypic expression (i.e., "penetrance") of these genetic components is also highly variable, ranging from fully penetrant point mutations to polygenic forms with multiple gene-gene and gene-environment interactions. Furthermore, many genes involved in ASD are also involved in intellectual disability, further underscoring their lack of specificity in phenotypic expression. We shall hereby review current knowledge on the genetic basis of ASD, spanning genetic/genomic syndromes associated with autism, monogenic forms due to copy number variants (CNVs) or rare point mutations, mitochondrial forms, and polygenic autisms. Finally, the recent contributions of genome-wide association and whole exome sequencing studies will be highlighted.

Lack of association between NLGN3, NLGN4, SHANK2 and SHANK3 gene variants and autism spectrum disorder in a Chinese population

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication, absence or delay in language development, and stereotyped or repetitive behaviors. Genetic studies show that neurexin-neuroligin (NRXN-NLGN) pathway genes contribute susceptibility to ASD, which include cell adhesion molecules NLGN3, NLGN4 and scaffolding proteins SHANK2 and SHANK3. Neuroligin proteins play an important role in synaptic function and trans-synaptic signaling by interacting with presynaptic neurexins. Shank proteins are scaffolding molecules of excitatory synapses, which function as central organizers of the postsynaptic density. Sequence level mutations and structural variations in these genes have been identified in ASD cases, while few studies were performed in Chinese population. In this study, we examined the copy numbers of four genes NLGN4, NLGN3, SHANK2, and SHANK3 in 285 ASD cases using multiplex fluorescence competitive polymerase chain reaction (PCR). We also screened the regulatory region including the promoter region and 5'/3' untranslated regions (UTR) and the entire coding region of NLGN4 in a cohort of 285 ASD patients and 384 controls by direct sequencing of genomic DNA using the Sanger method. DNA copy number calculation in four genes showed no deletion or duplication in our cases. No missense mutations in NLGN4 were identified in our cohort. Association analysis of 6 common SNPs in NLGN4 did not find significant difference between ASD cases and controls. These findings showed that these genes may not be major disease genes in Chinese ASD cases.

Absence of deficits in social behaviors and ultrasonic vocalizations in later generations of mice lacking neuroligin4

Mutations in NLGN4X have been identified in individuals with autism spectrum disorders and other neurodevelopmental disorders. A previous study reported that adult male mice lacking neuroligin4 (Nlgn4) displayed social approach deficits in the three-chambered test, altered aggressive behaviors and reduced ultrasonic vocalizations. To replicate and extend these findings, independent comprehensive analyses of autism-relevant behavioral phenotypes were conducted in later generations of the same line of Nlgn4 mutant mice at the National Institute of Mental Health in Bethesda, MD, USA and at the Institut Pasteur in Paris, France. Adult social approach was normal in all three genotypes of Nlgn4 mice tested at both sites. Reciprocal social interactions in juveniles were similarly normal across genotypes. No genotype differences were detected in ultrasonic vocalizations in pups separated from the nest or in adults during reciprocal social interactions. Anxiety-like behaviors, self-grooming, rotarod and open field exploration did not differ across genotypes, and measures of developmental milestones and general health were normal. Our findings indicate an absence of autism-relevant behavioral phenotypes in subsequent generations of Nlgn4 mice tested at two locations. Testing environment and methods differed from the original study in some aspects, although the presence of normal sociability was seen in all genotypes when methods taken from Jamain et al. (2008) were used. The divergent results obtained from this study indicate that phenotypes may not be replicable across breeding generations, and highlight the significant roles of environmental, generational and/or procedural factors on behavioral phenotypes.

Identification of rare X-linked neuroligin variants by massively parallel sequencing in males with autism spectrum disorder

BACKGROUND

Autism spectrum disorder (ASD) is highly heritable, but the genetic risk factors for it remain largely unknown. Although structural variants with large effect sizes may explain up to 15% ASD, genome-wide association studies have failed to uncover common single nucleotide variants with large effects on phenotype. The focus within ASD genetics is now shifting to the examination of rare sequence variants of modest effect, which is most often achieved via exome selection and sequencing. This strategy has indeed identified some rare candidate variants; however, the approach does not capture the full spectrum of genetic variation that might contribute to the phenotype.

METHODS

We surveyed two loci with known rare variants that contribute to ASD, the X-linked neuroligin genes by performing massively parallel Illumina sequencing of the coding and noncoding regions from these genes in males from families with multiplex autism. We annotated all variant sites and functionally tested a subset to identify other rare mutations contributing to ASD susceptibility.

RESULTS

We found seven rare variants at evolutionary conserved sites in our study population. Functional analyses of the three 3' UTR variants did not show statistically significant effects on the expression of NLGN3 and NLGN4X. In addition, we identified two NLGN3 intronic variants located within conserved transcription factor binding sites that could potentially affect gene regulation.

CONCLUSIONS

These data demonstrate the power of massively parallel, targeted sequencing studies of affected individuals for identifying rare, potentially disease-contributing variation. However, they also point out the challenges and limitations of current methods of direct functional testing of rare variants and the difficulties of identifying alleles with modest effects.

Developmental delays and reduced pup ultrasonic vocalizations but normal sociability in mice lacking the postsynaptic cell adhesion protein neuroligin2

Mutations in neurexin and neuroligin genes have been associated with neurodevelopmental disabilities including autism. Autism spectrum disorder is diagnosed by aberrant reciprocal social interactions, deficits in social communication, and repetitive, stereotyped patterns of behaviors, along with narrow restricted interests. Mouse models have been successfully used to study physiological and behavioral outcomes of mutations in the trans-synaptic neurexin-neuroligin complex. To further understand the behavioral consequences of Neuroligin2 (NLGN2) mutations, we assessed several behavioral phenotypes relevant to autism in neuroligin2 null (Nlgn2(-/-)), heterozygote (Nlgn2(+/-)), and wildtype (Nlgn2(+/+)) littermate control mice. Reduced breeding efficiency and high reactivity to handling was observed in Nlgn2(-/-) mice, resulting in low numbers of adult mice available for behavioral assessment. Consistent with previous findings, Nlgn2(-/-) mice displayed normal social behaviors, concomitant with reduced exploratory activity, impaired rotarod performance, and delays on several developmental milestones. No spontaneous stereotypies or repetitive behaviors were detected. Acoustic, tactile, and olfactory sensory information processing as well as sensorimotor gating were not affected. Nlgn2(-/-) pups isolated from mother and littermates emitted fewer ultrasonic vocalizations and spent less time calling than Nlgn2(+/+) littermate controls. The present findings add to the growing literature on the role of neurexins and neuroligins in physiology and behavior relevant to neurodevelopmental disorders.

Synaptic dysfunction in neurodevelopmental disorders associated with autism and intellectual disabilities

The discovery of the genetic causes of syndromic autism spectrum disorders and intellectual disabilities has greatly informed our understanding of the molecular pathways critical for normal synaptic function. The top-down approaches using human phenotypes and genetics helped identify causative genes and uncovered the broad spectrum of neuropsychiatric features that can result from various mutations in the same gene. Importantly, the human studies unveiled the exquisite sensitivity of cognitive function to precise levels of many diverse proteins. Bottom-up approaches applying molecular, biochemical, and neurophysiological studies to genetic models of these disorders revealed unsuspected pathogenic mechanisms and identified potential therapeutic targets. Moreover, studies in model organisms showed that symptoms of these devastating disorders can be reversed, which brings hope that affected individuals might benefit from interventions even after symptoms set in. Scientists predict that insights gained from studying these rare syndromic disorders will have an impact on the more common nonsyndromic autism and mild cognitive deficits.

Turner syndrome and sexual differentiation of the brain: implications for understanding male-biased neurodevelopmental disorders

Turner syndrome (TS) is one of the most common sex chromosome abnormalities. Affected individuals often show a unique pattern of cognitive strengths and weaknesses and are at increased risk for a number of other neurodevelopmental conditions, many of which are more common in typical males than typical females (e.g., autism and attention-deficit hyperactivity disorder). This phenotype may reflect gonadal steroid deficiency, haploinsufficiency of X chromosome genes, failure to express parentally imprinted genes, and the uncovering of X chromosome mutations. Understanding the contribution of these different mechanisms to outcome has the potential to improve clinical care for individuals with TS and to better our understanding of the differential vulnerability to and expression of neurodevelopmental disorders in males and females. In this paper, we review what is currently known about cognition and brain development in individuals with TS, discuss underlying mechanisms and their relevance to understanding male-biased neurodevelopmental conditions, and suggest directions for future research.

Autism, language and communication in children with sex chromosome trisomies

PURPOSE

Sex chromosome trisomies (SCTs) are found on amniocentesis in 2.3-3.7 per 1000 same-sex births, yet there is a limited database on which to base a prognosis. Autism has been described in postnatally diagnosed cases of Klinefelter syndrome (XXY karyotype), but the prevalence in non-referred samples, and in other trisomies, is unclear. The authors recruited the largest sample including all three SCTs to be reported to date, including children identified on prenatal screening, to clarify this issue.

DESIGN

Parents of children with a SCT were recruited either via prenatal screening or via a parental support group, to give a sample of 58 XXX, 19 XXY and 58 XYY cases. Parents were interviewed using the Vineland Adaptive Behavior Scales and completed questionnaires about the communicative development of children with SCTs and their siblings (42 brothers and 26 sisters).

RESULTS

Rates of language and communication problems were high in all three trisomies. Diagnoses of autism spectrum disorder (ASD) were found in 2/19 cases of XXY (11%) and 11/58 XYY (19%). After excluding those with an ASD diagnosis, communicative profiles indicative of mild autistic features were common, although there was wide individual variation.

CONCLUSIONS

Autistic features have not previously been remarked upon in studies of non-referred samples with SCTs, yet the rate is substantially above population levels in this sample, even when attention is restricted to early-identified cases. The authors hypothesise that X-linked and Y-linked neuroligins may play a significant role in the aetiology of communication impairments and ASD.

Functional dependence of neuroligin on a new non-PDZ intracellular domain

Neuroligins, a family of postsynaptic adhesion molecules, are important in synaptogenesis through a well-characterized trans-synaptic interaction with neurexin. In addition, neuroligins are thought to drive postsynaptic assembly through binding of their intracellular domain to PSD-95. However, there is little direct evidence to support the functional necessity of the neuroligin intracellular domain in postsynaptic development. We found that presence of endogenous neuroligin obscured the study of exogenous mutated neuroligin. We therefore used chained microRNAs in rat organotypic hippocampal slices to generate a reduced background of endogenous neuroligin. On this reduced background, we found that neuroligin function was critically dependent on the cytoplasmic tail. However, this function required neither the PDZ ligand nor any other previously described cytoplasmic binding domain, but rather required a previously unknown conserved region. Mutation of a single critical residue in this region inhibited neuroligin-mediated excitatory synaptic potentiation. Finally, we found a functional distinction between neuroligins 1 and 3.

Genetic basis of autism: is there a way forward?

PURPOSE OF REVIEW

This paper outlines some of the key findings from genetic research carried out in the last 12-18 months, which indicate that autism spectrum disorder (ASD) is a complex disorder involving interactions between genetic, epigenetic and environmental factors.

RECENT FINDINGS

The current literature highlights the presence of genetic and phenotypic heterogeneity in ASD with a number of underlying pathogenetic mechanisms. In this regard, there are at least three phenotypic presentations with distinct genetic underpinnings: autism plus phenotype characterized by syndromic ASD caused by rare, single-gene disorders; broad autism phenotype caused by genetic variations in single or multiple genes, each of these variations being common and distributed continually in the general population, but resulting in varying clinical phenotypes when it reaches a certain threshold through complex gene-gene and gene-environment interactions; and severe and specific phenotype caused by 'de-novo' mutations in the patient or transmitted through asymptomatic carriers of such mutation.

SUMMARY

Understanding the neurobiological processes by which genotypes become phenotypes, along with the advances in developmental neuroscience and neuronal networks at the cellular and molecular level, is paving the way for translational research involving targeted interventions of affected molecular pathways and early intervention programs that promote normal brain responses to stimuli and alter the developmental trajectory.

Cell adhesion molecules and their involvement in autism spectrum disorder

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders characterized by abnormalities in social interaction, language development and behavior. Recent genetic studies demonstrate that alterations in synaptic genes including those encoding cell adhesion molecules and their interaction partners play important roles in the pathogenesis of ASD. Systematic analyses of different cell adhesion molecule genes will help elucidate their normal functions and regulatory mechanisms in the establishment and maintenance of normal neural circuits and uncover genetic aberrations contributing to ASD.

Engineering DYRK1A overdosage yields Down syndrome-characteristic cortical splicing aberrations

Down syndrome (DS) associates with impaired brain functions, but the underlying mechanism(s) are yet unclear. The "gene dosage" hypothesis predicts that in DS, overexpression of a single gene can impair multiple brain functions through a signal amplification effect due to impaired regulatory mechanism(s). Here, we report findings attributing to impairments in the splicing process such a regulatory role. We have used DS fetal brain samples in search for initial evidence and employed engineered mice with MMU16 partial trisomy (Ts65Dn) or direct excess of the splicing-associated nuclear kinase Dyrk1A, overdosed in DS for further analyses. We present specific albeit modest changes in the DS brain's splicing machinery with subsequently amplified effects in target transcripts; and we demonstrate that engineered excess of Dyrk1A can largely recapitulate these changes. Specifically, in both the fetal DS brains and the Dyrk1A overdose models, we found ample modestly modified splicing-associated transcripts which apparently induced secondary enhancement in exon inclusion of key synaptic transcripts. Thus, DS-reduced levels of the dominant-negative TRKBT1 transcript, but not other TRKB mRNA transcripts, were accompanied by corresponding decreases in BDNF. In addition, the DS brains and Dyrk1A overdosage models showed selective changes in the transcripts composition of neuroligin mRNAs as well as reductions in the "synaptic" acetylcholinesterase variant AChE-S mRNA and corresponding increases in the stress-inducible AChE-R mRNA variant, yielding key synaptic proteins with unusual features. In cotransfected cells, Dyrk1A overdosage caused parallel changes in the splicing pattern of an AChE mini-gene, suggesting that Dyrk1A overdosage is both essential and sufficient to induce the observed change in the composition of AChE mRNA variants. Furthermore, the Dyrk1A overdosage animal models showed pronounced changes in the structure of neuronal nuclear speckles, where splicing events take place and in SR proteins phosphorylation known to be required for the splicing process. Together, our findings demonstrate DS-like brain splicing machinery malfunctioning in Dyrk1A overexpressing mice. Since individual splicing choices may alter cell fate determination, axon guidance, and synaptogenesis, these findings suggest the retrieval of balanced splicing as a goal for DS therapeutic manipulations early in DS development.

[Autism, genetics and synaptic function alterations]

Autism is a neurodevelopmental disorder characterized by a deficit of language and communication both associated with a restricted repertoire of activities and interests. The current prevalence of autistic disorder stricto sensu is estimated at 1/500 whereas autism spectrum disorders (ASD) increases up to 1/150 to 1/200. Mental deficiency (MD) and epilepsy are present in numerous autistic individuals. Consequently, autism is as a major public health issue. Autism was first considered as a non biological disease; however various rational approaches for analysing epidemiological data suggested the possibility of the influence of genetic factors. In 2003, this hypothesis was clearly illustrated by the characterization of genetic mutations transmitted through a mendelian manner. Subsequently, the glutamate synapse appeared as a preferential causal target in autism because the identified genes encoded proteins present in this structure. Strikingly, the findings that an identical genetic dysfunction of the synapse might also explain some MD suggested the possibility of a genetic comorbidity between these neurodevelopmental conditions. To date, various identified genes are considered indifferently as "autism" or "MD" genes. The characterization of mutations in the NLGN4X gene in patients with Asperger syndrome, autism without MD, or MD without autism, was the first example. It appears that a genetic continuum between ASD on one hand, and between autism and MD on the other hand, is present. Consequently, it is likely that genes already involved in MD will be found mutated in autistic patients and will represent future target for finding new factors in autism.

Environmental risk factors for autism: do they help cause de novo genetic mutations that contribute to the disorder?

Recent research has discovered that a number of genetic risk factors for autism are de novo mutations. Advanced parental age at the time of conception is associated with increased risk for both autism and de novo mutations. We investigated the hypothesis that other environmental factors associated with increased risk for autism might also be mutagenic and contribute to autism by causing de novo mutations. A survey of the research literature identified 9 environmental factors for which increased pre-conceptual exposure appears to be associated with increased risk for autism. Five of these factors--mercury, cadmium, nickel, trichloroethylene, and vinyl chloride--are established mutagens. Another four--including residence in regions that are urbanized, located at higher latitudes, or experience high levels of precipitation--are associated with decreased sun exposure and increased risk for vitamin D deficiency. Vitamin D plays important roles in repairing DNA damage and protecting against oxidative stress--a key cause of DNA damage. Factors associated with vitamin D deficiency will thus contribute to higher mutation rates and impaired repair of DNA. We note how de novo mutations may also help explain why the concordance rate for autism is so markedly higher in monozygotic than dizygotic twins. De novo mutations may also explain in part why the prevalence of autism is so remarkably high, given the evidence for a strong role of genetic factors and the low fertility of individuals with autism--and resultant selection pressure against autism susceptibility genes. These several lines of evidence provide support for the hypothesis, and warrant new research approaches--which we suggest--to address limitations in existing studies. The hypothesis has implications for understanding possible etiologic roles of de novo mutations in autism, and it suggests possible approaches to primary prevention of the disorder, such as addressing widespread vitamin D deficiency and exposure to known mutagens.