A unified genetic theory for sporadic and inherited autism

Fruit flies and intellectual disability

Mental retardation--known more commonly nowadays as intellectual disability--is a severe neurological condition affecting up to 3% of the general population. As a result of the analysis of familial cases and recent advances in clinical genetic testing, great strides have been made in our understanding of the genetic etiologies of mental retardation. Nonetheless, no treatment is currently clinically available to patients suffering from intellectual disability. Several animal models have been used in the study of memory and cognition. Established paradigms in Drosophila have recently captured cognitive defects in fly mutants for orthologs of genes involved in human intellectual disability. We review here three protocols designed to understand the molecular genetic basis of learning and memory in Drosophila and the genes identified so far with relation to mental retardation. In addition, we explore the mental retardation genes for which evidence of neuronal dysfunction other than memory has been established in Drosophila. Finally, we summarize the findings in Drosophila for mental retardation genes for which no neuronal information is yet available. All in all, this review illustrates the impressive overlap between genes identified in human mental retardation and genes involved in physiological learning and memory.

Copy-number variations associated with autism spectrum disorder

Autism spectrum disorder (ASD) is a clinically heterogeneous developmental disorder with a strong genetic component. Rare genetic disorders and various chromosomal abnormalities are thought to account for approximately 10% of people with ASD. The etiology of the remaining cases remains unknown. Recent advances in array-based technology have increased the resolution in detecting submicroscopic deletions and duplications, referred to as copy-number variations. ASD-associated copy-number variations, which are considered to be present in individuals with ASD but not in unaffected individuals, have been extensively investigated. These data will provide us with an opportunity not only to search for genes causing or contributing to ASDs but also to understand the genetics of ASD.

Autism: many genes, common pathways?

Autism is a heterogeneous neurodevelopmental syndrome with a complex genetic etiology. It is still not clear whether autism comprises a vast collection of different disorders akin to intellectual disability or a few disorders sharing common aberrant pathways. Unifying principles among cases of autism are likely to be at the level of brain circuitry in addition to molecular pathways.

Copy-number variations associated with neuropsychiatric conditions

Neuropsychiatric conditions such as autism and schizophrenia have long been attributed to genetic alterations, but identifying the genes responsible has proved challenging. Microarray experiments have now revealed abundant copy-number variation--a type of variation in which stretches of DNA are duplicated, deleted and sometimes rearranged--in the human population. Genes affected by copy-number variation are good candidates for research into disease susceptibility. The complexity of neuropsychiatric genetics, however, dictates that assessment of the biomedical relevance of copy-number variants and the genes that they affect needs to be considered in an integrated context.

Mutations in the gene encoding CADM1 are associated with autism spectrum disorder

The unified idea on the molecular pathogenesis of Autism Spectrum Disorder (ASD) is still unknown although mutations in genes encoding neuroligins and SHANK3 have been shown in a small part of the patients. RA175/SynCAM1/CADM1(CADM1), a member of immunoglobulin superfamily, is another synaptic cell adhesion molecule. To clarify the idea that impaired synaptogenesis underlies the pathogenesis of ASD, we examined the relationship between mutations in the CADM1 gene and ASD. We found two missense mutations, C739A(H246N) and A755C(Y251S), in the CADM1 gene of male Caucasian ASD patients and their family members. Both mutations were located in the third immunoglobulin domain, which is essential for trans-active interaction. The mutated CADM1 exhibited less amount of high molecular weight with the matured oligosaccharide, defective trafficking to the cell surface, and more susceptibility to the cleavage and or degradation. Our findings provide key support for the unified idea that impaired synaptogenesis underlies the pathogenesis of ASD.

Disruption of the neurexin 1 gene is associated with schizophrenia

Deletions within the neurexin 1 gene (NRXN1; 2p16.3) are associated with autism and have also been reported in two families with schizophrenia. We examined NRXN1, and the closely related NRXN2 and NRXN3 genes, for copy number variants (CNVs) in 2977 schizophrenia patients and 33 746 controls from seven European populations (Iceland, Finland, Norway, Germany, The Netherlands, Italy and UK) using microarray data. We found 66 deletions and 5 duplications in NRXN1, including a de novo deletion: 12 deletions and 2 duplications occurred in schizophrenia cases (0.47%) compared to 49 and 3 (0.15%) in controls. There was no common breakpoint and the CNVs varied from 18 to 420 kb. No CNVs were found in NRXN2 or NRXN3. We performed a Cochran-Mantel-Haenszel exact test to estimate association between all CNVs and schizophrenia (P = 0.13; OR = 1.73; 95% CI 0.81-3.50). Because the penetrance of NRXN1 CNVs may vary according to the level of functional impact on the gene, we next restricted the association analysis to CNVs that disrupt exons (0.24% of cases and 0.015% of controls). These were significantly associated with a high odds ratio (P = 0.0027; OR 8.97, 95% CI 1.8-51.9). We conclude that NRXN1 deletions affecting exons confer risk of schizophrenia.

Early behavioral intervention, brain plasticity, and the prevention of autism spectrum disorder

Advances in the fields of cognitive and affective developmental neuroscience, developmental psychopathology, neurobiology, genetics, and applied behavior analysis have contributed to a more optimistic outcome for individuals with autism spectrum disorder (ASD). These advances have led to new methods for early detection and more effective treatments. For the first time, prevention of ASD is plausible. Prevention will entail detecting infants at risk before the full syndrome is present and implementing treatments designed to alter the course of early behavioral and brain development. This article describes a developmental model of risk, risk processes, symptom emergence, and adaptation in ASD that offers a framework for understanding early brain plasticity in ASD and its role in prevention of the disorder.

Autism: definition, neurobiology, screening, diagnosis

Autism (ie, the autism spectrum disorders) is now recognized in 1 in 150 children. This article highlights the definition, neurobiology, screening, and diagnosis of autism. The genetics, immunology, imaging, and neurophysiology of autism are reviewed, with particular emphasis on areas that impact pediatricians. Early recognition of the social deficits that characterize autism is key to maximizing the potential of these children.

What's new in autism?

This review on autism spectrum disorder (ASD) focusses on recent insights in the clinical picture, such as continuity of the phenotype and the concept of broader phenotype, on epidemiology and on clinical issues relevant to physicians, including new methods for early screening and diagnosis, psychiatric and somatic co-morbidity, and the expansion of so-called complementary and alternative treatments. ASD is a disorder with mainly genetic causes and recent insights show that a variety of genetic mechanisms may be involved, i.e. single gene disorders, copy number variations and polygenic mechanisms. Technological advances in genetics have lead to a number of promising findings, which, together with other lines of fundamental research, suggest that ASD may be a disorder of connectivity in the brain, at least in a subgroup of patients. It is possible that part of the genetic load in autism actually reflects gene-environment interaction, but there is no evidence for purely environmental causes in a substantial number of cases. Clinical research suggests that ASD may be a multi-system disorder in at least a subgroup of subjects, affecting the gastro-intestinal (GI) tract, the immune system and perhaps other systems. Behavioural treatments remain the cornerstone of management, and are mainly aimed at stimulation of the domains of impaired development and reducing secondary behaviours. These treatments are constantly being refined, but the main progress in this area may be the increase of research on effectiveness.

A review of recent reports on autism: 1000 studies published in 2007

From 1000 studies published in 2007 on all aspects of autism, those that reached clear conclusions or included quantitative data were selected for this review. Possible etiologies include elemental metals, especially the inconsistent evidence regarding mercury from the vaccine preservative thimerosal, not used after 2001, and chromosomes and genes with the conclusion that autism has a complex genetic architecture. Also, various parental conditions are considered, as are many different abnormalities in the central nervous system, especially underconnectivity within the cortex. Furthermore, deficiencies in mirror neurons have been proposed, leading to the "theory of mind" explanation that autistic children tend to disregard others. In addition, various global deficiencies, like an increase in inhibitory synaptic transmission, are proposed. Characteristics of these children include selective (inward) attention; underresponsiveness; stereotyped repetitive motor behavior; increased head size, weight, and height; various cognitive and communicative disorders; and also epilepsy. Therapy has emphasized risperidone, but some atypical antipsychotic medications have been helpful, as have robotic aids, massage, hyperbaric oxygen, and music. Nearly every conceivable problem that a child could have can be observed in these unfortunate children.

Potential for early-life immune insult including developmental immunotoxicity in autism and autism spectrum disorders: focus on critical windows of immune vulnerability

Early-life immune insults (ELII) including xenobiotic-induced developmental immunotoxicity (DIT) are important factors in childhood and adult chronic diseases. However, prenatal and perinatal environmentally induced immune alterations have yet to be considered in depth in the context of autism and autism spectrum disorders (ASDs). Numerous factors produce early-life-induced immune dysfunction in offspring, including exposure to xenobiotics, maternal infections, and other prenatal-neonatal stressors. Early life sensitivity to ELII, including DIT, results from the heightened vulnerability of the developing immune system to disruption and the serious nature of the adverse outcomes arising after disruption of one-time immune maturational events. The resulting health risks extend beyond infectious diseases, cancer, allergy, and autoimmunity to include pathologies of the neurological, reproductive, and endocrine systems. Because these changes may include misregulation of resident inflammatory myelomonocytic cells in tissues such as the brain, they are a potential concern in cases of prenatal-neonatal brain pathologies and neurobehavioral deficits. Autism and ASDs are chronic developmental neurobehavioral disorders that are on the rise in the United States with prenatal and perinatal environmental factors suspected as contributors to this increase. Evidence for an association between environmentally associated childhood immune dysfunction and ASDs suggests that ELII and DIT may contribute to these conditions. However, it is not known if this linkage is directly associated with the brain pathologies or represents a separate (or secondary) outcome. This review considers the known features of ELII and DIT and how they may provide important clues to prenatal brain inflammation and the risk of autism and ASDs.

Gender-selective toxicity of thimerosal

A recent report shows a correlation of the historical use of thimerosal in therapeutic immunizations with the subsequent development of autism; however, this association remains controversial. Autism occurs approximately four times more frequently in males compared to females; thus, studies of thimerosal toxicity should take into consideration gender-selective effects. The present study was originally undertaken to determine the maximum tolerated dose (MTD) of thimersosal in male and female CD1 mice. However, during the limited MTD studies, it became apparent that thimerosal has a differential MTD that depends on whether the mouse is male or female. At doses of 38.4-76.8mg/kg using 10% DMSO as diluent, seven of seven male mice compared to zero of seven female mice tested succumbed to thimerosal. Although the thimerosal levels used were very high, as we were originally only trying to determine MTD, it was completely unexpected to observe a difference of the MTD between male and female mice. Thus, our studies, although not directly addressing the controversy surrounding thimerosal and autism, and still preliminary due to small numbers of mice examined, provide, nevertheless, the first report of gender-selective toxicity of thimerosal and indicate that any future studies of thimerosal toxicity should take into consideration gender-specific differences.

A novel 6.14 Mb duplication of chromosome 8p21 in a patient with autism and self mutilation

Autism spectrum disorders (ASDs) are a group of neurodevelopmental disorders with a strong genetic etiology. Cytogenetic abnormalities have been detected in 5-10% of the patients with autism. In this study, we present the clinical, cytogenetic and array-comparative genomic hybridization (array-CGH) evaluation of a 13-year-old male with severe developmental delay, facial dysmorphic features, autism and self mutilation. The patient was found to carry a de novo duplication of chromosome region 8p21 of minimally 6.14 and maximally 6.58 Mb as ascertained by bacterial artificial chromosome (BAC)-based array-CGH. Hitherto, only a few patients with autism with cytogenetically visible duplications involving the chromosome 8p21 region have been described, but the extent of these duplications has not been determined at the molecular level. This represents the smallest rearrangement of chromosomal region 8p21 as yet found in a patient with autism. For 11 of the 36 genes with known functions located within this duplication clear transcription in the brain was found. Of those the STMN4 and DPYSL2 genes are the most likely candidate genes to be involved in neuronal development, and, if altered in gene-dosage, in the autistic phenotype of our patient.

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.

Advances in autism genetics: on the threshold of a new neurobiology

Autism is a heterogeneous syndrome defined by impairments in three core domains: social interaction, language and range of interests. Recent work has led to the identification of several autism susceptibility genes and an increased appreciation of the contribution of de novo and inherited copy number variation. Promising strategies are also being applied to identify common genetic risk variants. Systems biology approaches, including array-based expression profiling, are poised to provide additional insights into this group of disorders, in which heterogeneity, both genetic and phenotypic, is emerging as a dominant theme.

The functional impact of structural variation in humans

Structural variation includes many different types of chromosomal rearrangement and encompasses millions of bases in every human genome. Over the past 3 years, the extent and complexity of structural variation has become better appreciated. Diverse approaches have been adopted to explore the functional impact of this class of variation. As disparate indications of the important biological consequences of genome dynamism are accumulating rapidly, we review the evidence that structural variation has an appreciable impact on cellular phenotypes, disease and human evolution.

Copy number variation in the autism genome

BACKGROUND

Autism spectrum disorders (ASDs) are among the most heritable of all neurodevelopmental disorders. Despite intense research there has been limited success in deciphering the etiology of ASDs.

OBJECTIVE

It has been shown that chromosomal rearrangements play an important role in ASDs. The recent development of techniques to screen the genome for genetic variation at ever-higher resolution has led to some crucial discoveries over the last year. This progress is described and discussed.

METHODS

This review provides an overview of genetic variation studies in ASD, with a focus on structural genetic variation.

RESULTS/CONCLUSION

Screening for copy number variation is an important approach in ASD research. With the introduction of next-generation sequencing, the pace of ASD genetics will increase in the near future.

Understanding genetic factors in idiopathic scoliosis, a complex disease of childhood

Idiopathic scoliosis (AIS) is the most common pediatric spinal deformity, affecting ~3% of children worldwide. AIS significantly impacts national health in the U. S. alone, creating disfigurement and disability for over 10% of patients and costing billions of dollars annually for treatment. Despite many investigations, the underlying etiology of IS is poorly understood. Twin studies and observations of familial aggregation reveal significant genetic contributions to IS. Several features of the disease including potentially strong genetic effects, the early onset of disease, and standardized diagnostic criteria make IS ideal for genomic approaches to finding risk factors. Here we comprehensively review the genetic contributions to IS and compare those findings to other well-described complex diseases such as Crohn's disease, type 1 diabetes, psoriasis, and rheumatoid arthritis. We also summarize candidate gene studies and evaluate them in the context of possible disease aetiology. Finally, we provide study designs that apply emerging genomic technologies to this disease. Existing genetic data provide testable hypotheses regarding IS etiology, and also provide proof of principle for applying high-density genome-wide methods to finding susceptibility genes and disease modifiers.

Association between microdeletion and microduplication at 16p11.2 and autism

BACKGROUND

Autism spectrum disorder is a heritable developmental disorder in which chromosomal abnormalities are thought to play a role.

METHODS

As a first component of a genomewide association study of families from the Autism Genetic Resource Exchange (AGRE), we used two novel algorithms to search for recurrent copy-number variations in genotype data from 751 multiplex families with autism. Specific recurrent de novo events were further evaluated in clinical-testing data from Children's Hospital Boston and in a large population study in Iceland.

RESULTS

Among the AGRE families, we observed five instances of a de novo deletion of 593 kb on chromosome 16p11.2. Using comparative genomic hybridization, we observed the identical deletion in 5 of 512 children referred to Children's Hospital Boston for developmental delay, mental retardation, or suspected autism spectrum disorder, as well as in 3 of 299 persons with autism in an Icelandic population; the deletion was also carried by 2 of 18,834 unscreened Icelandic control subjects. The reciprocal duplication of this region occurred in 7 affected persons in AGRE families and 4 of the 512 children from Children's Hospital Boston. The duplication also appeared to be a high-penetrance risk factor.

CONCLUSIONS

We have identified a novel, recurrent microdeletion and a reciprocal microduplication that carry substantial susceptibility to autism and appear to account for approximately 1% of cases. We did not identify other regions with similar aggregations of large de novo mutations.

Structural variation of chromosomes in autism spectrum disorder

Structural variation (copy number variation [CNV] including deletion and duplication, translocation, inversion) of chromosomes has been identified in some individuals with autism spectrum disorder (ASD), but the full etiologic role is unknown. We performed genome-wide assessment for structural abnormalities in 427 unrelated ASD cases via single-nucleotide polymorphism microarrays and karyotyping. With microarrays, we discovered 277 unbalanced CNVs in 44% of ASD families not present in 500 controls (and re-examined in another 1152 controls). Karyotyping detected additional balanced changes. Although most variants were inherited, we found a total of 27 cases with de novo alterations, and in three (11%) of these individuals, two or more new variants were observed. De novo CNVs were found in approximately 7% and approximately 2% of idiopathic families having one child, or two or more ASD siblings, respectively. We also detected 13 loci with recurrent/overlapping CNV in unrelated cases, and at these sites, deletions and duplications affecting the same gene(s) in different individuals and sometimes in asymptomatic carriers were also found. Notwithstanding complexities, our results further implicate the SHANK3-NLGN4-NRXN1 postsynaptic density genes and also identify novel loci at DPP6-DPP10-PCDH9 (synapse complex), ANKRD11, DPYD, PTCHD1, 15q24, among others, for a role in ASD susceptibility. Our most compelling result discovered CNV at 16p11.2 (p = 0.002) (with characteristics of a genomic disorder) at approximately 1% frequency. Some of the ASD regions were also common to mental retardation loci. Structural variants were found in sufficiently high frequency influencing ASD to suggest that cytogenetic and microarray analyses be considered in routine clinical workup.