ITGB3 shows genetic and expression interaction with SLC6A4

The role of integrin beta in schizophrenia: a preliminary exploration

Integrins are transmembrane heterodimeric (αβ) receptors that transduce mechanical signals between the extracellular milieu and the cell in a bidirectional manner. Extensive research has shown that the integrin beta (β) family is widely expressed in the brain and that they control various aspects of brain development and function. Schizophrenia is a relatively common neurological disorder of unknown etiology and has been found to be closely related to neurodevelopment and neurochemicals in neuropathological studies of schizophrenia. Here, we review literature from recent years that shows that schizophrenia involves multiple signaling pathways related to neuronal migration, axon guidance, cell adhesion, and actin cytoskeleton dynamics, and that dysregulation of these processes affects the normal function of neurons and synapses. In fact, alterations in integrin β structure, expression and signaling for neural circuits, cortex, and synapses are likely to be associated with schizophrenia. We explored several aspects of the possible association between integrin β and schizophrenia in an attempt to demonstrate the role of integrin β in schizophrenia, which may help to provide new insights into the study of the pathogenesis and treatment of schizophrenia.

A perspective on molecular signalling dysfunction, its clinical relevance and therapeutics in autism spectrum disorder

Intellectual disability (ID) and autism spectrum disorder (ASD) are neurodevelopmental disorders that have become a primary clinical and social concern, with a prevalence of 2-3% in the population. Neuronal function and behaviour undergo significant malleability during the critical period of development that is found to be impaired in ID/ASD. Human genome sequencing studies have revealed many genetic variations associated with ASD/ID that are further verified by many approaches, including many mouse and other models. These models have facilitated the identification of fundamental mechanisms underlying the pathogenesis of ASD/ID, and several studies have proposed converging molecular pathways in ASD/ID. However, linking the mechanisms of the pathogenic genes and their molecular characteristics that lead to ID/ASD has progressed slowly, hampering the development of potential therapeutic strategies. This review discusses the possibility of recognising the common molecular causes for most ASD/ID based on studies from the available models that may enable a better therapeutic strategy to treat ID/ASD. We also reviewed the potential biomarkers to detect ASD/ID at early stages that may aid in diagnosis and initiating medical treatment, the concerns with drug failure in clinical trials, and developing therapeutic strategies that can be applied beyond a particular mutation associated with ASD/ID.

Evidence that ITGB3 promoter variants increase serotonin blood levels by regulating platelet serotonin transporter trafficking

Elevated serotonin (5-HT) blood levels, the first biomarker identified in autism research, has been consistently found in 20-30% of patients with Autism Spectrum Disorder (ASD). Hyperserotonemia is mainly due to greater 5-HT uptake into platelets, mediated by the 5-HT transporter (SERT) located at the platelet plasma membrane. The protein complex involved in platelet SERT trafficking and externalization includes integrin β3, the beta subunit of the platelet membrane adhesive GP IIb/IIIa. Integrin β3 is encoded by the ITGB3 gene, previously identified as a quantitative trait locus (QTL) for 5-HT blood levels in ASD at single nucleotide polymorphism (SNP) rs2317385. The present study aims to identify the functional ITGB3 gene variants contributing to hyperserotonemia. ITGB3 gene sequencing in 20 individuals selected on the basis of rs2317385 genotypes defined four haplotypes encompassing six SNPs located in the ITGB3 gene promoter region, all in linkage disequilibrium with rs2317385. Luciferase assays in two hematopoietic cell lines, K-562 and HEL 92.1.7, demonstrate that ITGB3 gene promoter activity is enhanced by the presence of the C allele at rs55827077 specifically during differentiation into megakaryocytes (P < 0.01), with modulatory effects by flanking SNPs. This same allele is strongly associated with (a) higher 5-HT blood levels in 176 autistic individuals (P < 0.001), (b) greater platelet integrin β3 protein expression (P < 0.05) and (c) enhanced SERT trafficking from the cytosol toward the platelet plasma membrane (P = 4.05 × 10-11). Our results support rs55827077 as the functional ITGB3 gene promoter variant contributing to elevated 5-HT blood levels in ASD and define a mechanistic chain of events linking ITGB3 to hyperserotonemia.

p38α MAPK signaling drives pharmacologically reversible brain and gastrointestinal phenotypes in the SERT Ala56 mouse

Autism spectrum disorder (ASD) is a common neurobehavioral disorder with limited treatment options. Activation of p38 MAPK signaling networks has been identified in ASD, and p38 MAPK signaling elevates serotonin (5-HT) transporter (SERT) activity, effects mimicked by multiple, hyperfunctional SERT coding variants identified in ASD subjects. Mice expressing the most common of these variants (SERT Ala56) exhibit hyperserotonemia, a biomarker observed in ASD subjects, as well as p38 MAPK-dependent SERT hyperphosphorylation, elevated hippocampal 5-HT clearance, hypersensitivity of CNS 5-HT_1A and 5-HT_2A/2C receptors, and behavioral and gastrointestinal perturbations reminiscent of ASD. As the α-isoform of p38 MAPK drives SERT activation, we tested the hypothesis that CNS-penetrant, α-isoform-specific p38 MAPK inhibitors might normalize SERT Ala56 phenotypes. Strikingly, 1-week treatment of adult SERT Ala56 mice with MW150, a selective p38α MAPK inhibitor, normalized hippocampal 5-HT clearance, CNS 5-HT_1A and 5-HT_2A/2C receptor sensitivities, social interactions, and colonic motility. Conditional elimination of p38α MAPK in 5-HT neurons of SERT Ala56 mice restored 5-HT_1A and 5-HT_2A/2C receptor sensitivities as well as social interactions, mirroring effects of MW150. Our findings support ongoing p38α MAPK activity as an important determinant of the physiological and behavioral perturbations of SERT Ala56 mice and, more broadly, supports consideration of p38α MAPK inhibition as a potential treatment for core and comorbid phenotypes present in ASD subjects.

Analysis of neuroanatomical differences in mice with genetically modified serotonin transporters assessed by structural magnetic resonance imaging

Background

The serotonin (5-HT) system has long been implicated in autism spectrum disorder (ASD) as indicated by elevated whole blood and platelet 5-HT, altered platelet and brain receptor and transporter binding, and genetic linkage and association findings. Based upon work in genetically modified mice, 5-HT is known to influence several aspects of brain development, but systematic neuroimaging studies have not previously been reported. In particular, the 5-HT transporter (serotonin transporter, SERT; 5-HTT) gene, Slc6a4, has been extensively studied.

Methods

Using a 7-T MRI and deformation-based morphometry, we assessed neuroanatomical differences in an Slc6a4 knockout mouse on a C57BL/6 genetic background, along with an Slc6a4 Ala56 knockin mouse on two different genetic backgrounds (129S and C57BL/6).

Results

Individually (same sex, same background, same genotype), the only differences found were in the female Slc6a4 knockout mouse; all the others had no significant differences. However, an analysis of variance across the whole study sample revealed a significant effect of Slc6a4 on the amygdala, thalamus, dorsal raphe nucleus, and lateral and frontal cortices.

Conclusions

This work shows that an increase or decrease in SERT function has a significant effect on the neuroanatomy in 5-HT relevant regions, particularly the raphe nuclei. Notably, the Slc6a4 Ala56 knockin alone appears to have an insignificant, but suggestive, effect compared to the KO, which is consistent with Slc6a4 function. Despite the small number of 5-HT neurons and their localization to the brainstem, it is clear that 5-HT plays an important role in neuroanatomical organization.

Electronic supplementary material

The online version of this article (10.1186/s13229-018-0210-z) contains supplementary material, which is available to authorized users.

The Gain-of-Function Integrin β3 Pro33 Variant Alters the Serotonin System in the Mouse Brain

Engagement of integrins by the extracellular matrix initiates signaling cascades that drive a variety of cellular functions, including neuronal migration and axonal pathfinding in the brain. Multiple lines of evidence link the ITGB3 gene encoding the integrin β3 subunit with the serotonin (5-HT) system, likely via its modulation of the 5-HT transporter (SERT). The ITGB3 coding polymorphism Leu33Pro (rs5918, Pl^A2) produces hyperactive αvβ3 receptors that influence whole-blood 5-HT levels and may influence the risk for autism spectrum disorder (ASD). Using a phenome-wide scan of psychiatric diagnoses, we found significant, male-specific associations between the Pro33 allele and attention-deficit hyperactivity disorder and ASDs. Here, we used knock-in (KI) mice expressing an Itgb3 variant that phenocopies the human Pro33 variant to elucidate the consequences of constitutively enhanced αvβ3 signaling to the 5-HT system in the brain. KI mice displayed deficits in multiple behaviors, including anxiety, repetitive, and social behaviors. Anatomical studies revealed a significant decrease in 5-HT synapses in the midbrain, accompanied by decreases in SERT activity and reduced localization of SERTs to integrin adhesion complexes in synapses of KI mice. Inhibition of focal adhesion kinase (FAK) rescued SERT function in synapses of KI mice, demonstrating that constitutive active FAK signaling downstream of the Pro32Pro33 integrin αvβ3 suppresses SERT activity. Our studies identify a complex regulation of 5-HT homeostasis and behaviors by integrin αvβ3, revealing an important role for integrins in modulating risk for neuropsychiatric disorders.SIGNIFICANCE STATEMENT The integrin β3 Leu33Pro coding polymorphism has been associated with autism spectrum disorders (ASDs) within a subgroup of patients with elevated blood 5-HT levels, linking integrin β3, 5-HT, and ASD risk. We capitalized on these interactions to demonstrate that the Pro33 coding variation in the murine integrin β3 recapitulates the sex-dependent neurochemical and behavioral attributes of ASD. Using state-of-the-art techniques, we show that presynaptic 5-HT function is altered in these mice, and that the localization of 5-HT transporters to specific compartments within the synapse, disrupted by the integrin β3 Pro33 mutation, is critical for appropriate reuptake of 5-HT. Our studies provide fundamental insight into the genetic network regulating 5-HT neurotransmission in the CNS that is also associated with ASD risk.

Reverse Pathway Genetic Approach Identifies Epistasis in Autism Spectrum Disorders

Although gene-gene interaction, or epistasis, plays a large role in complex traits in model organisms, genome-wide by genome-wide searches for two-way interaction have limited power in human studies. We thus used knowledge of a biological pathway in order to identify a contribution of epistasis to autism spectrum disorders (ASDs) in humans, a reverse-pathway genetic approach. Based on previous observation of increased ASD symptoms in Mendelian disorders of the Ras/MAPK pathway (RASopathies), we showed that common SNPs in RASopathy genes show enrichment for association signal in GWAS (P = 0.02). We then screened genome-wide for interactors with RASopathy gene SNPs and showed strong enrichment in ASD-affected individuals (P < 2.2 x 10-16), with a number of pairwise interactions meeting genome-wide criteria for significance. Finally, we utilized quantitative measures of ASD symptoms in RASopathy-affected individuals to perform modifier mapping via GWAS. One top region overlapped between these independent approaches, and we showed dysregulation of a gene in this region, GPR141, in a RASopathy neural cell line. We thus used orthogonal approaches to provide strong evidence for a contribution of epistasis to ASDs, confirm a role for the Ras/MAPK pathway in idiopathic ASDs, and to identify a convergent candidate gene that may interact with the Ras/MAPK pathway.

Integrins in synapse regulation

Integrins are a large family of extracellular matrix (ECM) receptors. In the developing and adult brain, many integrins are present at high levels at synapses. The tetrapartite structure of synapses - which comprises presynaptic and postsynaptic neurons, the ECM and glial processes - places synaptic integrins in an excellent position to sense dynamic changes in the synaptic environment and use this information to coordinate further changes in synapse structure and function that will shape neural circuit properties. Recent developments in our understanding of the cellular and physiological roles of integrins, which range from control of neural process outgrowth and synapse formation to regulation of synaptic plasticity and memory, enable us to attempt a synthesis of synaptic integrin function.

Distortion of the normal function of synaptic cell adhesion molecules by genetic variants as a risk for autism spectrum disorders

Synaptic cell adhesion molecules (SCAMs) are a functional category of cell adhesion molecules that connect pre- and postsynapses by the protein-protein interaction via their extracellular cell adhesion domains. Countless numbers of common genetic variants and rare mutations in SCAMs have been identified in the patients with autism spectrum disorders (ASDs). Among these, NRXN and NLGN family proteins cooperatively function at synaptic terminals both of which genes are strongly implicated as risk genes for ASDs. Knock-in mice carrying a single rare point mutation of NLGN3 (NLGN3 R451C) discovered in the patients with ASDs display a deficit in social interaction and an enhancement of spatial learning and memory ability reminiscent of the clinical phenotype of ASDs. NLGN4 knockout (KO) and NRXN2α KO mice also show a deficit in sociability as well as some specific neuropsychiatric behaviors. In this review, we selected NRXNs/NLGNs, CNTNAP2/CNTNAP4, CNTN4, ITGB3, and KIRREL3 as strong ASD risk genes based on SFARI score and summarize the protein structures, functions at synapses, representative discoveries in human genetic studies, and phenotypes of the mutant model mice in light of the pathophysiology of ASDs.

The serotonin system in autism spectrum disorder: From biomarker to animal models

Elevated whole blood serotonin, or hyperserotonemia, was the first biomarker identified in autism spectrum disorder (ASD) and is present in more than 25% of affected children. The serotonin system is a logical candidate for involvement in ASD due to its pleiotropic role across multiple brain systems both dynamically and across development. Tantalizing clues connect this peripheral biomarker with changes in brain and behavior in ASD, but the contribution of the serotonin system to ASD pathophysiology remains incompletely understood. Studies of whole blood serotonin levels in ASD and in a large founder population indicate greater heritability than for the disorder itself and suggest an association with recurrence risk. Emerging data from both neuroimaging and postmortem samples also indicate changes in the brain serotonin system in ASD. Genetic linkage and association studies of both whole blood serotonin levels and of ASD risk point to the chromosomal region containing the serotonin transporter (SERT) gene in males but not in females. In ASD families with evidence of linkage to this region, multiple rare SERT amino acid variants lead to a convergent increase in serotonin uptake in cell models. A knock-in mouse model of one of these variants, SERT Gly56Ala, recapitulates the hyperserotonemia biomarker and shows increased brain serotonin clearance, increased serotonin receptor sensitivity, and altered social, communication, and repetitive behaviors. Data from other rodent models also suggest an important role for the serotonin system in social behavior, in cognitive flexibility, and in sensory development. Recent work indicates that reciprocal interactions between serotonin and other systems, such as oxytocin, may be particularly important for social behavior. Collectively, these data point to the serotonin system as a prime candidate for treatment development in a subgroup of children defined by a robust, heritable biomarker.

Integrin β3 Haploinsufficiency Modulates Serotonin Transport and Antidepressant-Sensitive Behavior in Mice

Converging lines of evidence have identified genetic interactions between the serotonin transporter (SERT) gene and ITGB3, which encodes the β3 subunit that forms the αIIbβ3 and αvβ3 integrin receptor complexes. Here we examine the consequences of haploinsufficiency in the mouse integrin β3 subunit gene (Itgb3) on SERT function and selective 5-hydroxytryptamine (5-HT) reuptake inhibitor (SSRI) effectiveness in vivo. Biochemical fractionation studies and immunofluorescent staining of murine brain slices reveal that αvβ3 receptors and SERTs are enriched in presynaptic membranes from several brain regions and that αvβ3 colocalizes with a subpopulation of SERT-containing synapses in raphe nuclei. Notably, we establish that loss of a single allele of Itgb3 in murine neurons is sufficient to decrease 5-HT uptake by SERT in midbrain synaptosomes. Pharmacological assays to elucidate the αvβ3-mediated mechanism of reduced SERT function indicate that decreased integrin β3 subunit expression scales down the population size of active SERT molecules and, as a consequence, lowers the effective dose of SSRIs. These data are consistent with the existence of a subpopulation of SERTs that are tightly modulated by integrin αvβ3 and significantly contribute to global SERT function at 5-HT synapses in the midbrain. Importantly, our screen of a normal human population for single nucleotide polymorphisms in human ITGB3 identified a variant associated with reductions in integrin β3 expression levels that parallel our mouse findings. Thus, polymorphisms in human ITGB3 may contribute to the differential responsiveness of select patients to SSRIs.

CHL1, ITGB3 and SLC6A4 gene expression and antidepressant drug response: results from the Munich Antidepressant Response Signature (MARS) study

Aim: The identification of antidepressant drugs (ADs) response biomarkers in depression is of high clinical importance. We explored CHL1 and ITGB3 expression as tentative response biomarkers. Materials & methods: In vitro sensitivity to ADs, as well as gene expression and genetic variants of the candidate genes CHL1, ITGB3 and SLC6A4 were measured in lymphoblastoid cell lines (LCLs) of 58 depressed patients. Results: An association between the clinical remission of depression and the basal expression of CHL1 and ITGB3 was discovered. Individuals whose LCLs expressed higher levels of CHL1 or ITGB3 showed a significantly better remission upon AD treatment. In addition individuals with the CHL1 rs1516338 TT genotype showed a significantly better remission after 5 weeks AD treatment than those carrying a CC genotype. No association between the in vitro sensitivity of LCLs toward AD and the clinical remission could be detected. Conclusion: CHL1 expression in patient-derived LCLs correlated with the clinical outcome. Thus, it could be a valid biomarker to predict the success of an antidepressant therapy.

Original submitted 8 December 2014; Revision submitted 2 March 2015

Association of the FGA and SLC6A4 genes with autistic spectrum disorder in a Korean population

BACKGROUND

Autism spectrum disorder (ASD) is a neurobiological disorder characterized by distinctive impairments in cognitive function, language, and behavior. Linkage and population studies suggest a genetic association between solute carrier family 6 member 4 (SLC6A4) variants and ASD.

METHOD

Logistic regression was used to identify associations between single-nucleotide polymorphisms (SNPs) and ASD with 3 alternative models (additive, dominant, and recessive). Linear regression analysis was performed to determine the influence of SNPs on Childhood Autism Rating Scale (CARS) scores as a quantitative phenotype.

RESULTS

In the present study, we examined the associations of SNPs in the SLC6A4 gene and the fibrinogen alpha chain (FGA) gene. Logistic regression analysis showed a significant association between the risk of ASD and rs2070025 and rs2070011 in the FGA gene. The gene-gene interaction between SLC6A4 and FGA was not significantly associated with ASD susceptibility. However, polymorphisms in both SLC6A4 and the FGA gene significantly affected the symptoms of ASD.

CONCLUSION

Our findings indicate that FGA and SLC6A4 gene interactions may contribute to the phenotypes of ASD rather than the incidence of ASD.

The role of β3 integrin gene variants in Autism Spectrum Disorders--diagnosis and symptomatology

Autism Spectrum Disorders (ASDs) represent a group of very complex early-onset neurodevelopmental diseases. In this study, we analyzed 5 SNPs (rs2317385, rs5918, rs15908, rs12603582, rs3809865) at the β3 integrin locus (ITGB3), which has been suggested as a possible susceptibility gene, both as single markers and as part of haplotypes in 209 ASD children and their biological parents. We tested for association with the following: a) DSM-IV ASD diagnosis; b) clinical symptoms common in ASD patients (repetitive behaviors, echolalia, seizures and epilepsy, mood instability, aggression, psychomotor agitation, sleep disorders); and c) dimensional scores obtained with the Autism Screening Questionnaire and the Childhood Autism Rating Scale. These hypotheses were investigated using family-based tests, logistic regression models and analysis of covariance. The family-based tests showed an association with the H5 haplotype (composed by GTCGA alleles, the order of SNPs as above), which was transmitted less often than expected by chance (P=0.006; Pcorr=0.036). The analyses of the clinical symptoms showed a trend for an association with rs12603582 (P=0.008; Pcorr=0.064) and positive results for the haplotype composed of rs15908 and rs12603582 (Pglcorr=0.048; Pindcorr=0.015), both in symptoms of echolalia. Other nominal associations with different variants were found and involved epilepsy/seizures, aggression symptoms and higher ASQ scores. Although our positive results are not definitive, they suggest small effect associations of the ITGB3 gene with both ASD diagnosis and symptoms of echolalia. Other studies are nonetheless needed to fully understand the involvement of this locus on the etiology of ASDs and its different clinical aspects.

Blood serotonin levels in autism spectrum disorder: a systematic review and meta-analysis

Elevated blood serotonin (5-HT) levels were the first biomarker identified in autism research. Many studies have contrasted blood 5-HT levels in autistic patients and controls, but different measurement protocols, technologies, and biomaterials have been used through the years. We performed a systematic review and meta-analysis to provide an overall estimate of effect size and between-study heterogeneity, while verifying whether and to what extent different methodological approaches influence the strength of this association. Our literature search strategy identified 551 papers, from which 22 studies providing patient and control blood 5-HT values were selected for meta-analysis. Significantly higher 5-HT levels in autistic patients compared to controls were recorded both in whole blood (WB) [O.R.=4.6; (3.1-5.2); P=1.0×10(-12]), and in platelet-rich plasma (PRP) [O.R.=2.6 (1.8-3.9); P=2.7×10(-7)]. Predictably, studies measuring 5-HT levels in platelet-poor plasma (PPP) yielded no significant group difference [O.R.=0.54 (0.2-2-0); P=0.36]. Altogether, elevated 5-HT blood levels were recorded in 28.3% in WB and 22.5% in PRP samples of autistic individuals, as reported in 15 and 4 studies, respectively. Studies employing HPLC vs fluorometric assays yield similar cumulative effect sizes, but the former display much lower variability. In summary, despite some limitations mainly due to small study sample sizes, our results significantly reinforce the reliability of elevated 5-HT blood levels as a biomarker in ASD, providing practical indications potentially useful for its inclusion in multi-marker diagnostic panels for clinical use.

Neuroimaging endophenotypes in animal models of autism spectrum disorders: lost or found in translation?

RATIONALE

Autism spectrum disorder(s) (ASDs) is a neurodevelopmental disorder characterized by stereotyped behaviours and impairments in communication and social interactions. This heterogeneity has been a major obstacle in uncovering the aetiology and biomarkers of ASDs. Rodent models with genetic modifications or environmental insults have been created to study particular endophenotypes and bridge the gap between genetics and behavioural phenotypes. Translational neuroimaging modalities with their ability to screen the brain noninvasively and yield structural, biochemical and functional information provide a unique platform for discovery and evaluation of such endophenotypes in preclinical and clinical research.

OBJECTIVES

We reviewed literature on translational neuroimaging in rodent models of ASDs. The most prominent models will be described and the respective neuroimaging endophenotypes will be discussed with reference to human data. A perspective on future directions of translational neuroimaging in animal models of ASDs will be given.

RESULTS AND CONCLUSIONS

To date, we experience a proliferation of rodent models which recapitulate specific liabilities identified in ASDs patients. Translational neuroimaging in these models is emerging but is skewed towards magnetic resonance imaging (MRI) modalities. Volumetric and structural assessments of the brain are dominating and a host of endophenotypes have been reported that allude to findings in ASDs patients but with only few to converge among the models. Caveats of current studies are the diverging biological conditions related to genetic background and age of the animals. It is anticipated that longitudinal and functional assessments will gain much importance and will help elucidating mechanistic relationship between behavioural and structural endophenotypes.

Genetic effects on cerebellar structure across mouse models of autism using a magnetic resonance imaging atlas

Magnetic resonance imaging (MRI) of autism populations is confounded by the inherent heterogeneity in the individuals' genetics and environment, two factors difficult to control for. Imaging genetic animal models that recapitulate a mutation associated with autism quantify the impact of genetics on brain morphology and mitigate the confounding factors in human studies. Here, we used MRI to image three genetic mouse models with single mutations implicated in autism: Neuroligin-3 R451C knock-in, Methyl-CpG binding protein-2 (MECP2) 308-truncation and integrin β3 homozygous knockout. This study identified the morphological differences specific to the cerebellum, a structure repeatedly linked to autism in human neuroimaging and postmortem studies. To accomplish a comparative analysis, a segmented cerebellum template was created and used to segment each study image. This template delineated 39 different cerebellar structures. For Neuroligin-3 R451C male mutants, the gray (effect size (ES) = 1.94, FDR q = 0.03) and white (ES = 1.84, q = 0.037) matter of crus II lobule and the gray matter of the paraflocculus (ES = 1.45, q = 0.045) were larger in volume. The MECP2 mutant mice had cerebellar volume changes that increased in scope depending on the genotype: hemizygous males to homozygous females. The integrin β3 mutant mouse had a drastically smaller cerebellum than controls with 28 out of 39 cerebellar structures smaller. These imaging results are discussed in relation to repetitive behaviors, sociability, and learning in the context of autism. This work further illuminates the cerebellum's role in autism.

Rare coding variants of the adenosine A3 receptor are increased in autism: on the trail of the serotonin transporter regulome

BACKGROUND

Rare genetic variation is an important class of autism spectrum disorder (ASD) risk factors and can implicate biological networks for investigation. Altered serotonin (5-HT) signaling has been implicated in ASD, and we and others have discovered multiple, rare, ASD-associated variants in the 5-HT transporter (SERT) gene leading to elevated 5-HT re-uptake and perturbed regulation. We hypothesized that loci encoding SERT regulators harbor variants that impact SERT function and/or regulation and therefore could contribute to ASD risk. The adenosine A3 receptor (A3AR) regulates SERT via protein kinase G (PKG) and other signaling pathways leading to enhanced SERT surface expression and catalytic activity.

METHODS

To test our hypothesis, we asked whether rare variants in the A3AR gene (ADORA3) were increased in ASD cases vs. controls. Discovery sequencing in a case-control sample and subsequent analysis of comparison exome sequence data were conducted. We evaluated the functional impact of two variants from the discovery sample on A3AR signaling and SERT activity.

RESULTS

Sequencing discovery showed an increase of rare coding variants in cases vs. controls (P=0.013). While comparison exome sequence data did not show a significant enrichment (P=0.071), combined analysis strengthened evidence for association (P=0.0025). Two variants discovered in ASD cases (Leu90Val and Val171Ile) lie in or near the ligand-binding pocket, and Leu90Val was enriched individually in cases (P=0.040). In vitro analysis of cells expressing Val90-A3AR revealed elevated basal cGMP levels compared with the wildtype receptor. Additionally, a specific A3AR agonist increased cGMP levels across the full time course studied in Val90-A3AR cells, compared to wildtype receptor. In Val90-A3AR/SERT co-transfections, agonist stimulation elevated SERT activity over the wildtype receptor with delayed 5-HT uptake activity recovery. In contrast, Ile171-A3AR was unable to support agonist stimulation of SERT. Although both Val90 and Ile171 were present in greater numbers in these ASD cases, segregation analysis in families showed incomplete penetrance, consistent with other rare ASD risk alleles.

CONCLUSIONS

Our results validate the hypothesis that the SERT regulatory network harbors rare, functional variants that impact SERT activity and regulation in ASD, and encourages further investigation of this network for other variation that may impact ASD risk.

Genetic association and gene-gene interaction analyses suggest likely involvement of ITGB3 and TPH2 with autism spectrum disorder (ASD) in the Indian population

BACKGROUND

Serotoninergic dysfunction leads to neurodevelopmental abnormalities and behavioral impairments. Platelet hyperserotoninemia is reported as the best identified endophenotype for autism spectrum disorders. Therefore, in the present study we investigate the association of TPH2, the rate limiting enzyme in 5-HT biosynthesis and ITGB3, a serotonin quantitative trait locus with ASD in the Indian population.

METHODS

Population and family-based genetic association and gene-gene interaction analyses were performed to evaluate the role of ITGB3 and TPH2 markers in ASD etiology.

RESULTS

Association tests using ITGB3 markers revealed significant paternal overtransmission of T allele of rs5918 to male probands. Interestingly for TPH2, we observed significant overrepresentation of A-A (rs11179000-rs4290270), G-A (rs4570625-rs4290270), G-G-A (rs4570625-rs11179001-rs4290270) and A-G-A (rs11179000-rs11179001-rs4290270) haplotypes in the controls and maternal preferential transmission of A-A (rs11179001-rs7305115), T-A-A (rs4570625-rs11179001-rs7305115) and T-A-A (rs11179000-rs11179001-rs7305115) and nontransmission of G-G-A (rs4570625-rs11179001-rs7305115) haplotypes to the affected offspring. Moreover, interaction of ITGB3 marker, rs15908 with TPH2 markers was found to be significant and influenced by the sex of the probands. Predicted individual risk, which varied from very mild to moderate, supports combined effect of these markers in ASD.

CONCLUSION

Overall results of the present study indicate likely involvement of ITGB3 and TPH2 in the pathophysiology of ASD in the Indian population.

Neuroanatomical Assessment of the Integrin β3 Mouse Model Related to Autism and the Serotonin System Using High Resolution MRI

The integrinβ3 (ITGβ3) gene has been associated with both autism and the serotonin system. The purpose of this study was to examine the volumetric differences in the brain of an ITGβ3 homozygous knockout mouse model compared with a corresponding wild-type mouse using high resolution magnetic resonance imaging and detailed statistical analyses. The most striking difference found was an 11% reduction in total brain volume. Moreover, 32 different regions were found to have significantly different relative volumes (percentage total brain volume) in the ITGβ3 mouse. A number of interesting differences relevant to autism were discovered including a smaller corpus callosum volume and bilateral decreases in the hippocampus, striatum, and cerebellum. Relative volume increases were also found in the frontal and parieto-temporal lobes as well as in the amygdala. Particularly intriguing were the changes in the lateral wings of the dorsal raphe nuclei since that nucleus is so integral to the development of many different brain regions and the serotonin system in general.

Autism-associated gene expression in peripheral leucocytes commonly observed between subjects with autism and healthy women having autistic children

Autism spectrum disorder (ASD) is a severe neuropsychiatric disorder which has complex pathobiology with profound influences of genetic factors in its development. Although the numerous autism susceptible genes were identified, the etiology of autism is not fully explained. Using DNA microarray, we examined gene expression profiling in peripheral blood from 21 individuals in each of the four groups; young adults with ASD, age- and gender-matched healthy subjects (ASD control), healthy mothers having children with ASD (asdMO), and asdMO control. There was no blood relationship between ASD and asdMO. Comparing the ASD group with control, 19 genes were found to be significantly changed. These genes were mainly involved in cell morphology, cellular assembly and organization, and nerve system development and function. In addition, the asdMO group possessed a unique gene expression signature shown as significant alterations of protein synthesis despite of their nonautistic diagnostic status. Moreover, an ASD-associated gene expression signature was commonly observed in both individuals with ASD and asdMO. This unique gene expression profiling detected in peripheral leukocytes from affected subjects with ASD and unaffected mothers having ASD children suggest that a genetic predisposition to ASD may be detectable even in peripheral cells. Altered expression of several autism candidate genes such as FMR-1 and MECP2, could be detected in leukocytes. Taken together, these findings suggest that the ASD-associated genes identified in leukocytes are informative to explore the genetic, epigenetic, and environmental background of ASD and might become potential tools to assess the crucial factors related to the clinical onset of the disorder.

Mold allergen sensitization in adult asthma according to integrin β3 polymorphisms and Toll-like receptor 2/+596 genotype

BACKGROUND

Integrin β3 (ITGB3) and Toll-like receptor 2 (TLR2) are candidate genes for asthma and sensitization to mold allergens. Integrin β3 forms a complex with TLR2, and this biological interaction is required for the response of monocytes to TLR2 agonists such as fungal glucan.

OBJECTIVE

To study whether genetic interaction between single nucleotide polymorphisms (SNPs) in genes encoding the TLR2-ITGB3 complex enhances susceptibility to mold sensitization.

METHODS

Association analysis was conducted in 1243 adults (524 with asthma) who participated in the follow-up of the Epidemiological Study on the Genetics and Environment of Asthma. Allergic sensitization to mold allergens was determined by skin prick testing. Association of mold sensitization with 14 ITGB3 SNPs was tested under an additive genetic model. Interaction between ITGB3 SNPs and TLR2/+596, which was previously shown to be associated with asthma, was studied.

RESULTS

A positive skin prick test to mold was found in 115 subjects with asthma (22.0%) and in 61 subjects without asthma (8.5%). The ITGB3 rs2056131 A allele was associated with mold sensitization in subjects with asthma with an odds ratio (95% CI) of 0.60 (0.43-0.83; P = .001). Ten other ITGB3 SNPs were significantly associated with mold sensitization in TLR2/+596TT subjects with asthma (P = .03-.002), whereas much weaker associations were found in carriers of the TLR2/+596 C allele (P = .60-.04). Interaction between TLR2/+596 and these ITGB3 SNPs was statistically significant (P interaction = .05-.001).

CONCLUSION

TLR2/+596 genotype may influence the association between ITGB3 SNPs and mold sensitization in adults with asthma.

Absence of preference for social novelty and increased grooming in integrin β3 knockout mice: initial studies and future directions

Elevated whole blood serotonin 5-HT, or hyperserotonemia, is a common biomarker in autism spectrum disorder (ASD). The integrin β3 receptor subunit gene (ITGB3) is a quantitative trait locus for whole blood 5-HT levels. Recent work shows that integrin β3 interacts with the serotonin transporter (SERT) in both platelets and in the midbrain. Furthermore, multiple studies have now reported gene-gene interaction between the integrin β3 and SERT genes in association with ASD. Given the lack of previous data on the impact of integrin β3 on brain or behavioral phenotypes, we sought to compare mice with decreased or absent expression of the integrin β3 receptor subunit (Itgb3 +/- and -/-) with wildtype littermate controls in behavioral tasks relevant to ASD. These mice did not show deficits in activity level in the open field or anxiety-like behavior on the elevated plus maze, two potential confounds in the evaluation of mouse social behavior. In the three-chamber social test, mice lacking integrin β3 were shown to have normal sociability but did not show a preference for social novelty. Importantly, the absence of integrin β3 did not impair olfaction or the ability to recall familiar social odors. Additionally, mice lacking integrin β3 showed increased grooming behavior in novel environments. These preliminary studies reveal altered social and repetitive behavior in these mice, which suggests that the integrin β3 subunit may be involved in brain systems relevant to ASD. Further work is needed to fully characterize these behavioral changes and the underlying brain mechanisms.

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.

Family-based association study of ITGB3 in autism spectrum disorder and its endophenotypes

The integrin-β 3 gene (ITGB3), located on human chromosome 17q21.3, was previously identified as a quantitative trait locus (QTL) for 5-HT blood levels and has been implicated as a candidate gene for autism spectrum disorder (ASD). We performed a family-based association study in 281 simplex and 12 multiplex Caucasian families. ITGB3 haplotypes are significantly associated with autism (HBAT, global P=0.038). Haplotype H3 is largely over-transmitted to the affected offspring and doubles the risk of an ASD diagnosis (HBAT P=0.005; odds ratio (OR)=2.000), at the expense of haplotype H1, which is under-transmitted (HBAT P=0.018; OR=0.725). These two common haplotypes differ only at rs12603582 located in intron 11, which reaches a P-value of 0.072 in single-marker FBAT analyses. Interestingly, rs12603582 is strongly associated with pre-term delivery in our ASD patients (P=0.008). On the other hand, it is SNP rs2317385, located at the 5' end of the gene, that significantly affects 5-HT blood levels (Mann-Whitney U-test, P=0.001; multiple regression analysis, P=0.010). No gene-gene interaction between ITGB3 and SLC6A4 has been detected. In conclusion, we identify a significant association between a common ITGB3 haplotype and ASD. Distinct markers, located toward the 5' and 3' ends of the gene, seemingly modulate 5-HT blood levels and autism liability, respectively. Our results also raise interest into ITGB3 influences on feto-maternal immune interactions in autism.

The serotonergic system: its role in pathogenesis and early developmental treatment of autism

Autism is a severe childhood disorder already presenting in the first 3 years of life and, therefore, strongly correlated with neurodevelopmental alterations in prenatal, as well as postnatal period. Neurotransmitters hold a pivotal role in development by providing the stimulation needed for synapses and neuronal networks to be formed during the critical period of neuroplasticity. Aberrations of the serotonergic system modify key processes in the developing brain and are strongly implicated in the pathophysiology of developmental disorders. Evidence for the role of serotonin in autism emerges from neuropathological, imaging and genetic studies. Due to its developmental arrest, autism requires early intervention that would, among others, target the disrupted serotonergic system and utilize brain plasticity to elicit clinically important brain changes in children.

Association and gene-gene interaction of SLC6A4 and ITGB3 in autism

Autism is a heritable neurodevelopmental disorder with substantial genetic heterogeneity. Studies point to possible links between autism and two serotonin related genes: SLC6A4 and ITGB3 with a sex-specific genetic effect and interaction between the genes. Despite positive findings, inconsistent results have complicated interpretation. This study seeks to validate and clarify previous findings in an independent dataset taking into account sex, family-history (FH) and gene-gene effects. Family-based association analysis was performed within each gene. Gene-gene interactions were tested using extended multifactor dimensionality reduction (EMDR) and MDR-phenomics (MDR-P) using sex of affecteds and FH as covariates. No significant associations with individual SNPs were found in the datasets stratified by sex, but associations did emerge when we stratified by family history. While not significant in the overall dataset, nominally significant association was identified at RS2066713 (P = 0.006) within SLC6A4 in family-history negative (FH-) families, at RS2066713 (P = 0.038) in family-history positive (FH+) families but with the opposite risk allele as in the FH- families. For ITGB3, nominally significant association was identified at RS3809865 overall (P = 0.040) and within FH+ families (P = 0.031). However, none of the associations survived the multiple testing correction. MDR-P confirmed gene-gene effects using sex of affecteds (P = 0.023) and family history (P = 0.014, survived the multiple testing corrections) as covariates. Our results indicate the extensive heterogeneity within these two genes among families. The potential interaction between SLC6A4 and ITGB3 may be clarified using family history as an indicator of genetic architecture, illustrating the importance of covariates as markers of heterogeneity in genetic analyses.

Modeling rare gene variation to gain insight into the oldest biomarker in autism: construction of the serotonin transporter Gly56Ala knock-in mouse

Alterations in peripheral and central indices of serotonin (5-hydroxytryptamine, 5-HT) production, storage and signaling have long been associated with autism. The 5-HT transporter gene (HTT, SERT, SLC6A4) has received considerable attention as a potential risk locus for autism-spectrum disorders, as well as disorders with overlapping symptoms, including obsessive-compulsive disorder (OCD). Here, we review our efforts to characterize rare, nonsynonymous polymorphisms in SERT derived from multiplex pedigrees carrying diagnoses of autism and OCD and present the initial stages of our effort to model one of these variants, Gly56Ala, in vivo. We generated a targeting vector to produce the Gly56Ala substitution in the Slc6a4 locus by homologous recombination. Following removal of a neomycin resistance selection cassette, animals exhibiting germline transmission of the Ala56 variant were bred to establish a breeding colony on a 129S6 background, suitable for initial evaluation of biochemical, physiological and behavioral alterations relative to SERT Gly56 (wild-type) animals. SERT Ala56 mice were achieved and exhibit a normal pattern of transmission. The initial growth and gross morphology of these animals is comparable to wildtype littermate controls. The SERT Ala56 variant can be propagated in 129S6 mice without apparent disruption of fertility and growth. We discuss both the opportunities and challenges that await the physiological/behavioral analysis of Gly56Ala transgenic mice, with particular reference to modeling autism-associated traits.

Multiplex ligation-dependent probe amplification for genetic screening in autism spectrum disorders: efficient identification of known microduplications and identification of a novel microduplication in ASMT

Background

It has previously been shown that specific microdeletions and microduplications, many of which also associated with cognitive impairment (CI), can present with autism spectrum disorders (ASDs). Multiplex ligation-dependent probe amplification (MLPA) represents an efficient method to screen for such recurrent microdeletions and microduplications.

Methods

In the current study, a total of 279 unrelated subjects ascertained for ASDs were screened for genomic disorders associated with CI using MLPA. Fluorescence in situ hybridization (FISH), quantitative polymerase chain reaction (Q-PCR) and/or direct DNA sequencing were used to validate potential microdeletions and microduplications. Methylation-sensitive MLPA was used to characterize individuals with duplications in the Prader-Willi/Angelman (PWA) region.

Results

MLPA showed two subjects with typical ASD-associated interstitial duplications of the 15q11-q13 PWA region of maternal origin. Two additional subjects showed smaller, de novo duplications of the PWA region that had not been previously characterized. Genes in these two novel duplications include GABRB3 and ATP10A in one case, and MKRN3, MAGEL2 and NDN in the other. In addition, two subjects showed duplications of the 22q11/DiGeorge syndrome region. One individual was found to carry a 12 kb deletion in one copy of the ASPA gene on 17p13, which when mutated in both alleles leads to Canavan disease. Two subjects showed partial duplication of the TM4SF2 gene on Xp11.4, previously implicated in X-linked non-specific mental retardation, but in our subsequent analyses such variants were also found in controls. A partial duplication in the ASMT gene, located in the pseudoautosomal region 1 (PAR1) of the sex chromosomes and previously suggested to be involved in ASD susceptibility, was observed in 6–7% of the cases but in only 2% of controls (P = 0.003).

Conclusion

MLPA proves to be an efficient method to screen for chromosomal abnormalities. We identified duplications in 15q11-q13 and in 22q11, including new de novo small duplications, as likely contributing to ASD in the current sample by increasing liability and/or exacerbating symptoms. Our data indicate that duplications in TM4SF2 are not associated with the phenotype given their presence in controls. The results in PAR1/PAR2 are the first large-scale studies of gene dosage in these regions, and the findings at the ASMT locus indicate that further studies of the duplication of the ASMT gene are needed in order to gain insight into its potential involvement in ASD. Our studies also identify some limitations of MLPA, where single base changes in probe binding sequences alter results. In summary, our studies indicate that MLPA, with a focus on accepted medical genetic conditions, may be an inexpensive method for detection of microdeletions and microduplications in ASD patients for purposes of genetic counselling if MLPA-identified deletions are validated by additional methods.

The neurobiology of autism

Improving clinical tests are allowing us to more precisely classify autism spectrum disorders and diagnose them at earlier ages. This raises the possibility of earlier and potentially more effective therapeutic interventions. To fully capitalize on this opportunity, however, will require better understanding of the neurobiological changes underlying this devastating group of developmental disorders. It is becoming clear that the normal trajectory of neurodevelopment is altered in autism, with aberrations in brain growth, neuronal patterning and cortical connectivity. Changes to the structure and function of synapses and dendrites have also been strongly implicated in the pathology of autism by morphological, genetic and animal modeling studies. Finally, environmental factors are likely to interact with the underlying genetic profile, and foster the clinical heterogeneity seen in autism spectrum disorders. In this review we attempt to link the molecular pathways altered in autism to the neurodevelopmental and clinical changes that characterize the disease. We focus on signaling molecules such as neurotrophin, Reelin, PTEN and hepatocyte growth factor, neurotransmitters such as serotonin and glutamate, and synaptic proteins such as neurexin, SHANK and neuroligin. We also discuss evidence implicating oxidative stress, neuroglial activation and neuroimmunity in autism.

Multifactor dimensionality reduction-phenomics: a novel method to capture genetic heterogeneity with use of phenotypic variables

Complex human diseases do not have a clear inheritance pattern, and it is expected that risk involves multiple genes with modest effects acting independently or interacting. Major challenges for the identification of genetic effects are genetic heterogeneity and difficulty in analyzing high-order interactions. To address these challenges, we present MDR-Phenomics, a novel approach based on the multifactor dimensionality reduction (MDR) method, to detect genetic effects in pedigree data by integration of phenotypic covariates (PCs) that may reflect genetic heterogeneity. The P value of the test is calculated using a permutation test adjusted for multiple tests. To validate MDR-Phenomics, we compared it with two MDR-based methods: (1) traditional MDR pedigree disequilibrium test (PDT) without consideration of PCs (MDR-PDT) and (2) stratified phenotype (SP) analysis based on PCs, with use of MDR-PDT with a Bonferroni adjustment (SP-MDR). Using computer simulations, we examined the statistical power and type I error of the different approaches under several genetic models and sampling scenarios. We conclude that MDR-Phenomics is more powerful than MDR-PDT and SP-MDR when there is genetic heterogeneity, and the statistical power is affected by sample size and the number of PC levels. We further compared MDR-Phenomics with conditional logistic regression (CLR) for testing interactions across single or multiple loci with consideration of PC. The results show that CLR with PC has only slightly smaller power than does MDR-Phenomics for single-locus analysis but has considerably smaller power for multiple loci. Finally, by applying MDR-Phenomics to autism, a complex disease in which multiple genes are believed to confer risk, we attempted to identify multiple gene effects in two candidate genes of interest--the serotonin transporter gene (SLC6A4) and the integrin beta 3 gene (ITGB3) on chromosome 17. Analyzing four markers in SLC6A4 and four markers in ITGB3 in 117 white family triads with autism and using sex of the proband as a PC, we found significant interaction between two markers--rs1042173 in SLC6A4 and rs3809865 in ITGB3.

Integrin beta 3 genotype influences asthma and allergy phenotypes in the first 6 years of life

BACKGROUND

The integrin beta3 gene (ITGB3) encodes a subunit of the platelet and monocyte-specific fibrinogen receptor and the widely expressed vitronectin receptor, which have diverse roles in cell migration, adhesion, and signaling. Previous work from our laboratory reported associations between single nucleotide polymorphisms (SNPs) in ITGB3 and asthma and allergic sensitization in 4 populations.

OBJECTIVE

To examine whether SNPs in ITGB3 are associated with the development of asthma and allergic phenotypes in early life.

METHODS

We typed 13 SNPs in 206 children participating in a birth cohort study and tested for associations with asthma and allergy phenotypes in the first 6 years of life.

RESULTS

Our study revealed significant associations between SNPs in ITGB3 and asthma, wheezing, and IgE levels, suggesting an early role for this gene in the development of asthma and allergy. In particular, SNPs at the 3' end of the gene were significantly associated with IgE levels beginning at 1 year of age, whereas a SNP in intron 1 showed significant interaction effects with viral respiratory illness in infancy on asthma susceptibility.

CONCLUSION

Our results suggest that genetic variation in ITGB3 contributes to asthma susceptibility and allergic sensitization, and that the effects of this gene begin early in life. Similar to our earlier study, different SNPs in the gene are associated with asthma and IgE.

CLINICAL IMPLICATIONS

ITGB3 may play an important role in the development of asthma and allergy and may represent a potential therapeutic target for the treatment of these disorders.

Evidence for epistasis between SLC6A4 and ITGB3 in autism etiology and in the determination of platelet serotonin levels

Autism is a neurodevelopmental disorder of unclear etiology. The consistent finding of platelet hyperserotonemia in a proportion of patients and its heritability within affected families suggest that genes involved in the serotonin system play a role in this disorder. The role in autism etiology of seven candidate genes in the serotonin metabolic and neurotransmission pathways and mapping to autism linkage regions (SLC6A4, HTR1A, HTR1D, HTR2A, HTR5A, TPH1 and ITGB3) was analyzed in a sample of 186 nuclear families. The impact of interactions among these genes in autism was assessed using the multifactor-dimensionality reduction (MDR) method in 186 patients and 181 controls. We further evaluated whether the effect of specific gene variants or gene interactions associated with autism etiology might be mediated by their influence on serotonin levels, using the quantitative transmission disequilibrium test (QTDT) and the restricted partition method (RPM), in a sample of 109 autistic children. We report a significant main effect of the HTR5A gene in autism (P = 0.0088), and a significant three-locus model comprising a synergistic interaction between the ITGB3 and SLC6A4 genes with an additive effect of HTR5A (P < 0.0010). In addition to the previously reported contribution of SLC6A4, we found significant associations of ITGB3 haplotypes with serotonin level distribution (P = 0.0163). The most significant models contributing to serotonin distribution were found for interactions between TPH1 rs4537731 and SLC6A4 haplotypes (P = 0.002) and between HTR1D rs6300 and SLC6A4 haplotypes (P = 0.013). In addition to the significant independent effects, evidence for interaction between SLC6A4 and ITGB3 markers was also found. The overall results implicate SLC6A4 and ITGB3 gene interactions in autism etiology and in serotonin level determination, providing evidence for a common underlying genetic mechanism and a molecular explanation for the association of platelet hyperserotonemia with autism.