SLC6A4 markers modulate platelet 5-HT level and specific behaviors of autism: a study from an Indian population

Comprehensive systematic review and meta-analysis of the association between common genetic variants and autism spectrum disorder

BACKGROUND

Autism spectrum disorder (ASD) is neurodevelopmental disorder characterized by stereotyped behavior and deficits in communication and social interactions. To date, numerous studies have investigated the associations between genetic variants and ASD risk. However, the results of these published studies lack a clear consensus. In the present study, we performed a systematic review on the association between genetic variants and ASD risk. Meanwhile, we conducted a meta-analysis on available data to identify the association between the single nucleotide polymorphisms (SNPs) of candidate genes and ASD risk.

METHODS

We systematically searched public databases including English and Chinese from their inception to August 1, 2022. Two independent reviewers extracted data and assessed study quality. Odds ratio and 95 % confidence interval were used as effect indexes to evaluate the association between the SNPs of candidate genes and the risk of ASD. Heterogeneity was explored through subgroup, sensitivity, and meta-regression analyses. Publication bias was assessed by using Egger's and Begg's tests for funnel plot asymmetry. In addition, TSA analysis were performed to confirm the study findings.

RESULTS

We summarized 84 SNPs of 32 candidate genes from 81 articles included in the study. Subsequently, we analyzed 16 SNPs of eight genes by calculating pooled ORs, and identified eight significant SNPs of contactin associated protein 2 (CNTNAP2), methylentetrahydrofolate reductase (MTHFR), oxytocin receptor (OXTR), and vitamin D receptor (VDR). Results showed that seven SNPs, including the CNTNAP2 rs2710102 (homozygote, heterozygote, dominant and allelic models) and rs7794745 (heterozygote and dominant models), MTHFR C677T (homozygote, heterozygote, dominant, recessive and allelic models) and A1298C (dominant and allelic models), OXTR rs2254298 (homozygote and recessive models), VDR rs731236 (homozygote, dominant, recessive and allelic models) and rs2228570 (homozygote and recessive models), were showed to be correlated with an increased ASD risk. By contrast, the VDR rs7975232 was correlated with a decreased the risk of ASD under the homozygote and allelic models.

CONCLUSION

Our study summarized research evidence on the genetic variants of ASD and provides a broad and detailed overview of ASD risk genes. The C677T and A1298C polymorphisms of MTHFR, rs2710102 and rs7794745 polymorphisms of CNTNAP2, rs2254298 polymorphism of OXTR, and rs731236 and rs2228570 polymorphisms of VDR were genetic risk factors. The rs7975232 polymorphism of VDR was a genetic protective factor for ASD. Our study provides novel clues to clinicians and healthcare decision-makers to predict ASD susceptibility.

Genetic and neural mechanisms of sleep disorders in children with autism spectrum disorder: a review

Background

The incidence of sleep disorders in children with autism spectrum disorder (ASD) is very high. Sleep disorders can exacerbate the development of ASD and impose a heavy burden on families and society. The pathological mechanism of sleep disorders in autism is complex, but gene mutations and neural abnormalities may be involved.

Methods

In this review, we examined literature addressing the genetic and neural mechanisms of sleep disorders in children with ASD. The databases PubMed and Scopus were searched for eligible studies published between 2013 and 2023.

Results

Prolonged awakenings of children with ASD may be caused by the following processes. Mutations in the MECP2, VGAT and SLC6A1 genes can decrease GABA inhibition on neurons in the locus coeruleus, leading to hyperactivity of noradrenergic neurons and prolonged awakenings in children with ASD. Mutations in the HRH1, HRH2, and HRH3 genes heighten the expression of histamine receptors in the posterior hypothalamus, potentially intensifying histamine's ability to promote arousal. Mutations in the KCNQ3 and PCDH10 genes cause atypical modulation of amygdala impact on orexinergic neurons, potentially causing hyperexcitability of the hypothalamic orexin system. Mutations in the AHI1, ARHGEF10, UBE3A, and SLC6A3 genes affect dopamine synthesis, catabolism, and reuptake processes, which can elevate dopamine concentrations in the midbrain. Secondly, non-rapid eye movement sleep disorder is closely related to the lack of butyric acid, iron deficiency and dysfunction of the thalamic reticular nucleus induced by PTCHD1 gene alterations. Thirdly, mutations in the HTR2A, SLC6A4, MAOA, MAOB, TPH2, VMATs, SHANK3, and CADPS2 genes induce structural and functional abnormalities of the dorsal raphe nucleus (DRN) and amygdala, which may disturb REM sleep. In addition, the decrease in melatonin levels caused by ASMT, MTNR1A, and MTNR1B gene mutations, along with functional abnormalities of basal forebrain cholinergic neurons, may lead to abnormal sleep-wake rhythm transitions.

Conclusion

Our review revealed that the functional and structural abnormalities of sleep-wake related neural circuits induced by gene mutations are strongly correlated with sleep disorders in children with ASD. Exploring the neural mechanisms of sleep disorders and the underlying genetic pathology in children with ASD is significant for further studies of therapy.

Oxytocin and serotonin in the modulation of neural function: Neurobiological underpinnings of autism-related behavior

Autism spectrum disorders (ASD) is a group of generalized neurodevelopmental disorders. Its main clinical features are social communication disorder and repetitive stereotyped behavioral interest. The abnormal structure and function of brain network is the basis of social dysfunction and stereotyped performance in patients with autism spectrum disorder. The number of patients diagnosed with ASD has increased year by year, but there is a lack of effective intervention and treatment. Oxytocin has been revealed to effectively improve social cognitive function and significantly improve the social information processing ability, empathy ability and social communication ability of ASD patients. The change of serotonin level also been reported affecting the development of brain and causes ASD-like behavioral abnormalities, such as anxiety, depression like behavior, stereotyped behavior. Present review will focus on the research progress of serotonin and oxytocin in the pathogenesis, brain circuit changes and treatment of autism. Revealing the regulatory effect and neural mechanism of serotonin and oxytocin on patients with ASD is not only conducive to a deeper comprehension of the pathogenesis of ASD, but also has vital clinical significance.

Synthesis of genetic association studies on autism spectrum disorders using a genetic model-free approach

BACKGROUND

Autism spectrum disorder (ASD) is a clinically and genetically heterogeneous group of neurodevelopmental disorders. Despite the extensive efforts of scientists, the etiology of ASD is far from completely elucidated. In an effort to enlighten the genetic architecture of ASDs, a meta-analysis of all available genetic association studies (GAS) was conducted.

METHODS

We searched in the Human Genome Epidemiology Navigator (HuGE Navigator) and PubMed for available case-control GAS of ASDs. The threshold for meta-analysis was two studies per genetic variant. The association between genotype distribution and ASDs was examined using the generalized linear odds ratio (ORG). For variants with available allele frequencies, the examined model was the allele contrast.

RESULTS

Overall, 57 candidate genes and 128 polymorphisms were investigated in 159 articles. In total 28 genetic polymorphisms have been shown to be associated with ASDs, that are harbored in 19 genes. Statistically significant results were revealed for the variants of the following genes adenosine deaminase (ADA), bone marrow stromal cell antigen-1 (CD157/BST1), Dopamine receptor D1 (DRD1), engrailed homolog 2 (EN2), met proto-oncogene (MET), methylenetetrahydrofolate reductase (MTHFR), solute carrier family 6 member 4 (SLC6A4), Synaptosomal-associated protein, 25kDa (SNAP-25) and vitamin D receptor (VDR). In the allele contrast model of cases versus healthy controls, significant associations were observed for Adrenoceptor Alpha 1B (ADRA1B), acetyl serotonin O - methyltransferase (ASMT), complement component 4B (C4B), dopamine receptor D3 (DRD3), met proto-oncogene (MET), neuroligin 4, X-linked (NLGN4), neurexin 1 (NRXN1), oxytocin receptor (OXTR), Serine/Threonine-Protein Kinase PFTAIRE-1 (PFTK1), Reelin (RELN) and Ras-like without CAAX 2 (RIT2).

CONCLUSION

These significant findings provide further evidence for genetic factors' implication in ASDs offering new perspectives in means of prevention and prognosis.

A systematic review of common genetic variation and biological pathways in autism spectrum disorder

Background

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition characterized by persistent deficits in social communication and interaction. Common genetic variation appears to play a key role in the development of this condition. In this systematic review, we describe the relationship between genetic variations and autism. We created a gene dataset of the genes involved in the pathogenesis of autism and performed an over-representation analysis to evaluate the biological functions and molecular pathways that may explain the associations between these variants and the development of ASD.

Results

177 studies and a gene set composed of 139 were included in this qualitative systematic review. Enriched pathways in the over-representation analysis using the KEGG pathway database were mostly associated with neurotransmitter receptors and their subunits. Major over-represented biological processes were social behavior, vocalization behavior, learning and memory. The enriched cellular component of the proteins encoded by the genes identified in this systematic review were the postsynaptic membrane and the cell junction.

Conclusions

Among the biological processes that were examined, genes involved in synaptic integrity, neurotransmitter metabolism, and cell adhesion molecules were significantly involved in the development of autism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-021-00662-z.

Maternal serotonin transporter genotype and offsprings' clinical and cognitive measures of ADHD and ASD

Serotonin (5-HT) is an important factor for prenatal neurodevelopment whereby its neurotrophic actions can be regulated through maternal-fetal interactions. We explored if maternal 5-HTTLPR genotype is associated with clinical and cognitive measures of attention-deficit/hyperactivity disorder (ADHD) and comorbid autism spectrum disorder (ASD) in typically-developing and ADHD-diagnosed offspring, beyond classical inheritance and environmental- and comorbidity-mediators/confounders. Family-based variance decomposition analyses were performed incorporating 6-31 year-old offsprings' as well as parental genotypes of 462 ADHD and control families from the NeuroIMAGE cohort. Dependent measures were offsprings' ADHD symptom- and ASD trait-scores and cognitive measures including executive functioning (including response inhibition and cognitive flexibility), sustained attention, reward processing, motor control, and emotion recognition. Offsprings' stereotyped behavior was predicted by an interaction between maternal 5-HTTLPR genotype and offsprings' sex. Furthermore, offspring of mothers with low-expressing genotypes demonstrated larger reward-related reductions in reaction time. While specifically adult male offspring of these mothers reported a faster reversal learning with less errors, specifically young female offspring of these mothers were more accurate in identifying happy faces. Adult offspring from the mothers with low-expressing 5-HTTLPR genotypes were also slower in identifying happy faces. However, this association seemed to be mediated by offsprings' high anxiety levels. In sum, we found some support for a role of the maternal 5-HT system in modulating fetal brain development and behavior. Offsprings' cognitive measures might be more sensitive to small alterations within the maternal 5-HT system than their ADHD and ASD clinical phenotypes. Further studies are needed to specify the association between maternal genotype and risk for neurodevelopmental disorders.

Rare Opportunities for Insights Into Serotonergic Contributions to Brain and Bowel Disorders: Studies of the SERT Ala56 Mouse

Altered structure, expression, and regulation of the presynaptic serotonin (5-HT) transporter (SERT) have been associated with multiple neurobehavioral disorders, including mood disorders, obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). Opportunities to investigate mechanistic links supporting these associations were spurred with the identification of multiple, rare human SERT coding variants in a study that established a male-specific linkage of ASD to a linkage marker on chromosome 17 which encompassed the location of the SERT gene (SLC6A4). We have explored the most common of these variants, SERT Ala56, in vitro and in vivo. Results support a tonic elevation of 5-HT transport activity in transfected cells and human lymphoblasts by the variant in vitro that leads to an increased 5-HT clearance rate in vivo when studied in the SERT Ala56 mouse model, along with altered sensitivity to SERT regulatory signaling pathways. Importantly, hyperserotonemia, or an elevated whole blood 5-HT, level, was found in SERT Ala56 mice, reproducing a well-replicated trait observed in a significant fraction of ASD subjects. Additionally, we found multiple biochemical, physiological, and behavioral alterations in the SERT Ala56 mice that can be analogized to those observed in ASD and its medical comorbidities. The similarity of the functional impact of the SERT Ala56 variant to the consequences of p38α MAPK activation, ascribed to the induction of a biased conformation of the transporter toward an outward-facing conformation, has resulted in successful efforts to restore normal behavioral and bowel function via pharmacological and genetic p38α MAPK targeting. Moreover, the ability of the inflammatory cytokine IL-1β to enhance SERT activity via a p38α MAPK-dependent pathway suggests that the SERT Ala56 conformation mimics that of a chronic inflammatory state, supporting findings in ASD of elevated inflammatory cytokine levels. In this report, we review studies of the SERT Ala56 variant, discussing opportunities for continued insight into how chronically altered synaptic 5-HT homeostasis can drive reversible, functional perturbations in 5-HT sensitive pathways in the brain and periphery, and how targeting the SERT regulome, particularly through activating pathways such as those involving IL-1β/p38α MAPK, may be of benefit for neurobehavioral disorders, including ASD.

Expression analysis of selected genes involved in tryptophan metabolic pathways in Egyptian children with Autism Spectrum Disorder and learning disabilities

Autism Spectrum Disorder (ASD) and learning disabilities are neurodevelopmental disabilities characterized by dramatically increasing incidence rates, yet the exact etiology for these disabilities is not identified. Impairment in tryptophan metabolism has been suggested to participate in the pathogenesis of ASD, however, further validation of its involvement is required. Additionally, its role in learning disabilities is still uninvestigated. Our objective was to evaluate some aspects of tryptophan metabolism in ASD children (N = 45) compared to children with learning disabilities (N = 44) and healthy controls (N = 40) by measuring the expression levels of the MAOA, HAAO and AADAT genes using real-time RT-qPCR. We also aimed to correlate the expression patterns of these genes with parental ages at the time of childbirth, levels of serum iron, and vitamin D3 and zinc/copper ratio, as possible risk factors for ASD. Results demonstrated a significant decrease in the expression of the selected genes within ASD children (p < 0.001) relative to children with learning disabilities and healthy controls, which significantly associated with the levels of our targeted risk factors (p < 0.05) and negatively correlated to ASD scoring (p < 0.001). In conclusion, this study suggests that the expression of the MAOA, HAAO and AADAT genes may underpin the pathophysiology of ASD.

Autism spectrum disorder in India: a scoping review

Autism is a grand challenge in global mental health to be dealt with on a priority basis. Phenotypic knowledge, biological understanding, and evidence-based intervention studies are all from western countries. We know very little about autism in the low- and middle-income countries (LMIC). Lack of infrastructure and difficulties in operationalizing research has widened the knowledge gap. We performed a comprehensive scoping review of research in Autism Spectrum Disorder in India to have an overall impression, identify gaps, and formulate evidence-based recommendations for further study. We searched PubMed, SCOPUS, and Cochrane Library to identify relevant Indian studies. A hundred and fifty-nine publications met the inclusion criteria. Most of the research contribution in autism is from few tertiary care medical centres, technological institutes, and not-for-profit organizations. We identified various themes of research like clinical profile, interventions, biomarkers, psychological, social, epidemiological, and risk factors. Evidence-based intervention studies, translation and adaptation of standard diagnostic instruments, and qualitative research on the experience of autism appeared to be state of the art. However, epidemiological studies, biomarkers identification, risk assessment studies were of low quality. There is a need for nationwide studies with representative sampling on epidemiology, biomarkers, and risk factors for a complete evaluation of the actual burden and biology of autism in India. Also, there is a need to design implementation research to evaluate the effectiveness of evidence-based interventions in routine healthcare settings. We recommend that future research should fill these gaps in understanding autism and improving its outcome in India.

Genetic Knockout of the Serotonin Reuptake Transporter Results in the Reduction of Dendritic Spines in In vitro Rat Cortical Neuronal Culture

Dysregulation of the serotonergic system has been reported to have a significant role in several neurological disorders including depression, autism and substance abuse disorders. Changes in the expression of the serotonin transporter (SERT) through polymorphisms in the regulatory regions of the SERT gene have been associated, but not yet been conclusively linked to, neuropsychiatric disorders. In turn, dendritic spine structure and function are critical for neuronal function and the disruption of dendritic spine formation at glutamatergic synapses is a hallmark of several neuropsychiatric disorders. To understand the effect of SERT depletion on dendritic spine formation, neuronal cultures were established from the cortex of postnatal day 0-1 SERT knockout (KO) rats. Cortical neurons were subsequently allowed to mature to 21 days in vitro, and dendritic spine density was assessed using immunocytochemical co-labelling of drebrin and microtubule associated protein 2. Genetic knockout of the SERT had a gene-dose effect on dendritic spine densities of cortical neurons. The results of this paper implicate SERT function with the formation of dendritic spines at glutamatergic synapses, thereby offering insight into the aetiology of several neuropathologies.

Upregulation of miR-361-3p suppresses serotonin-induced proliferation in human pulmonary artery smooth muscle cells by targeting SERT

Background

Abnormal proliferation of pulmonary artery smooth muscle cells (PASMCs) is a key mechanism in pulmonary arterial hypertension (PAH). Serotonin (5-hydroxytryptamine, 5-HT) can induce abnormal proliferation of PASMCs. The role of miR-361-3p in serotonin-induced abnormal PASMCs proliferation remains unclear.

Methods

The miR-361-3p level was analyzed in plasma from PAH patients and normal controls and in human PASMCs (hPASMCs) using RT-PCR. The hPASMCs were transfected with an miR-361-3p mimic and then treated with serotonin. Untransfected hPASMCs were used as the control. Cell proliferation was evaluated using an MTS assay and 5-ethynyl-2′-deoxyuridine (EdU) staining. The cell cycle stages were evaluated using flow cytometry. The association between miR-361-3p and serotonin transporter (SERT) was determined using a luciferase reporter assay and anti-AGO2 RNA immunoprecipitation assay. The protein expression was evaluated via western blotting.

Results

The miR-361-3p level was lower in plasma from PAH patients than in plasma from the any of the normal control subjects. The mean pulmonary arterial pressure, pulmonary vascular resistance and pulmonary vascular resistance index were higher in PAH patients whose miR-361-3p level was lower than the median value for patients than in those whose miR-361-3p level was higher than the median. Serotonin treatment reduced miR-361-3p expression in the hPASMCs. MiR-361-3p overexpression suppressed cell proliferation, promoted apoptosis, induced G1 arrest, and decreased the phosphorylation level of ERK1/2 in serotonin-treated hPASMCs. SERT was identified as an miR-361-3p target. Its overexpression alleviated the effect of miR-361-3p overexpression on serotonin-induced hPASMC proliferation and upregulation of phosphorylated ERK1/2.

Conclusions

The miR-361-3p level is lower in the plasma of PAH patients. Upregulation of miR-361-3p suppresses serotonin-induced proliferation of hPASMCs by targeting SERT. Our results suggest that miR-361-3p is a potential therapeutic target in PAH.

Significant associations between 5-hydroxytryptaminetransporter-linked promoter region polymorphisms of the serotonin transporter (solute carrier family 6 member 4) gene and Thai patients with autism spectrum disorder

Autism spectrum disorder (ASD) is a form of pervasive developmental disorder manifested by impairment in social interactions and repetitive behaviors. Although genetic contribution is strongly suspected in autism, the specific genetic factors remain unidentified. Hyperserotoninemia has been reported in some autistic patients, and several studies have demonstrated an association between 5-hydroxytryptamine-transporter-linked promoter region (5-HTTLPR) polymorphisms and rs25531 single nucleotide polymorphism in the serotonin transporter gene (solute carrier family 6 member 4; SLC6A4) and ASD, indicating a possible involvement of the serotonin system in the etiology of ASD.To explore this situation further, a case-control association study of 5-HTTLPR and rs25531 polymorphisms on Thai ASD patients was conducted. A total of 188 ASD cases fulfilling the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) criteria (156 males and 32 females) and a total of 250 normal controls were recruited from the same ethnic backgrounds. 5-HTTLPR polymorphisms (Long, L; Short, S) and rs25531 (A/G) single nucleotide polymorphism were genotyped and compared between the patients and normal controls using chi-square statistics.The L/L genotype was more common in patients than in the controls (13.8% vs 5.2%, P = .006), and the LA haplotype was found in patients more than the controls (16.9% vs 12.2%, P = .048). When male patients were analyzed alone (156 individuals), the associations were also statistically significant with P = .017 for L/L genotype, and P = .019 for LA haplotype distribution.Our findings support previous reports suggesting an association between the 5-HTTLPR and rs25531 polymorphisms of SLC6A4 and patients with ASD.

The role of 5-HTTLPR in autism spectrum disorder: New evidence and a meta-analysis of this polymorphism in Latin American population with psychiatric disorders

The autism spectrum disorder (ASD) is a complex disorder encompassing a broad phenotypic and genotypic variability. The short (S)/long (L) 5-HTTLPR polymorphism has a functional role in the regulation of extracellular serotonin levels and both alleles have been associated to ASD. Most studies including European, American, and Asian populations have suggested an ethnical heterogeneity of this polymorphism; however, the short/long frequencies from Latin American population have been under-studied in recent meta-analysis. Here, we evaluated the 5-HTTLPR polymorphism in Colombian individuals with idiopathic ASD and reported a non-preferential S or L transmission and a non-association with ASD risk or symptom severity. Moreover, to recognize the allelic frequencies of an under-represented population we also recovered genetic studies from Latin American individuals and compared these frequencies with frequencies from other ethnicities. Results from meta-analysis suggest that short/long frequencies in Latin American are similar to those reported in Caucasian population but different to African and Asian regions.

The Neurochemistry of Autism

Autism spectrum disorder (ASD) refers to complex neurobehavioral and neurodevelopmental conditions characterized by impaired social interaction and communication, restricted and repetitive patterns of behavior or interests, and altered sensory processing. Environmental, immunological, genetic, and epigenetic factors are implicated in the pathophysiology of autism and provoke the occurrence of neuroanatomical and neurochemical events relatively early in the development of the central nervous system. Many neurochemical pathways are involved in determining ASD; however, how these complex networks interact and cause the onset of the core symptoms of autism remains unclear. Further studies on neurochemical alterations in autism are necessary to clarify the early neurodevelopmental variations behind the enormous heterogeneity of autism spectrum disorder, and therefore lead to new approaches for the treatment and prevention of autism. In this review, we aim to delineate the state-of-the-art main research findings about the neurochemical alterations in autism etiology, and focuses on gamma aminobutyric acid (GABA) and glutamate, serotonin, dopamine, N-acetyl aspartate, oxytocin and arginine-vasopressin, melatonin, vitamin D, orexin, endogenous opioids, and acetylcholine. We also aim to suggest a possible related therapeutic approach that could improve the quality of ASD interventions. Over one hundred references were collected through electronic database searching in Medline and EMBASE (Ovid), Scopus (Elsevier), ERIC (Proquest), PubMed, and the Web of Science (ISI).

Gender-Specific Effect of 5-HT and 5-HIAA on Threshold Level of Behavioral Symptoms and Sex-Bias in Prevalence of Autism Spectrum Disorder

Platelet hyperserotonemia in a subset of Autism Spectrum Disorder (ASD) probands, efficacy of selective serotonin reuptake inhibitors (SSRIs) in reducing behavioral deficits and gender-bias in normal serotonin (5-hydroxy tryptamine or 5-HT) synthesis suggest disruption in stringent regulation of serotonin metabolism in ASD. Therefore, we investigated the changes in 5-HT and 5-hydroxy indole acetic acid (5-HIAA) in ASD probands to assess its effect on the behavior of male and female probands. ASD cases (n = 215) were examined using childhood autism rating scale (CARS). Platelet 5-HT (104 cases and 26 controls) and platelet/plasma 5-HIAA (73 cases and 17 controls) were estimated using high performance liquid chromatography coupled with electrochemical detector (HPLC-ECD). In male probands, we observed increase in platelet 5-HT content in association with increase in the score for adaptive responses and increase in platelet 5-HIAA levels with concomitant decline in the score for intellectual response. Age did not influence the neurochemical parameters, but imitation, listening responses and nonverbal communication scores decreased with age. Conversely in female probands, plasma 5-HIAA level significantly attenuated with age, when platelet 5-HT content remained unchanged. Interestingly, platelet/plasma 5-HT and plasma 5-HIAA were higher in female controls. Female probands displayed severe autism-associated behaviors. Overall results indicate gender-bias in 5-HT and 5-HIAA regulation, which probably increases the threshold level of ASD phenotypes in the females, thereby affecting ASD prevalence in a sex-specific manner.

Extreme enhancement or depletion of serotonin transporter function and serotonin availability in autism spectrum disorder

A variety of human and animal studies support the hypothesis that serotonin (5-hydroxytryptamine or 5-HT) system dysfunction is a contributing factor to the development of autism in some patients. However, many questions remain about how developmental manipulation of various components that influence 5-HT signaling (5-HT synthesis, transport, metabolism) persistently impair social behaviors. This review will summarize key aspects of central 5-HT function important for normal brain development, and review evidence implicating perinatal disruptions in 5-HT signaling in the pathophysiology of autism spectrum disorder. We discuss the importance, and relative dearth, of studies that explore the possible correlation to autism in the interactions between important intrinsic and extrinsic factors that may disrupt 5-HT homeostasis during development. In particular, we focus on exposure to 5-HT transport altering mechanisms such as selective serotonin-reuptake inhibitors or genetic polymorphisms in primary or auxiliary transporters of 5-HT, and how they relate to neurological stores of serotonin and its precursors. A deeper understanding of the many mechanisms by which 5-HT signaling can be disrupted, alone and in concert, may contribute to an improved understanding of the etiologies and heterogeneous nature of this disorder. We postulate that extreme bidirectional perturbations of these factors during development likely compound or synergize to facilitate enduring neurochemical changes resulting in insufficient or excessive 5-HT signaling, that could underlie the persistent behavioral characteristics of autism spectrum disorder.

Association Between 5-HTTLPR Polymorphism and the Risk of Autism: A Meta-Analysis Based on Case-Control Studies

Background: Recently, many case-control studies have reported the association between 5-HTTLPR polymorphism and autism risk. However, the results are inconclusive and conflicting. To investigate the genetic association of 5-HTTLPR polymorphism and autism risk, we conducted a comprehensive meta-analysis based on previous case-control studies. Methods: Literature search was performed through PubMed, Embase, Web of Knowledge and CNKI databases until June 27, 2018. The strength of the association was assessed by relative risk (RR) and its corresponding 95% confidence interval (CI). Fixed or random effect model was selected based on the results of heterogeneity test. Further, subgroup analyses were conducted to explore the association of 5-HTTLPR polymorphism and autism risk in different population. Results: Eleven studies with 930 cases and 1234 controls were identified. Although there was a significant association between 5-HTTLPR polymorphism and autism risk under the dominant model after removing the studies causing heterogeneity, the significance did not exist after Bonferroni's correction. Subgroup analyses also showed similar results after Bonferroni's correction. In addition, there was no obvious publication bias in our meta-analysis. Conclusions: Our present meta-analysis does not support a direct effect of 5-HTTLPR polymorphism on autism risk according to present results. Further analyses of the effect of genetic networks and more well designed studies with larger sample size are required.

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.

Associations of endocrine stress-related gene polymorphisms with risk of autism spectrum disorders: Evidence from an integrated meta-analysis

Autism spectrum disorders (ASD) are related to serotonin transporter (5-HTT) and catechol-O-methyl transferase (COMT) as two most monoaminergic polymorphic variations. However, multiple studies assessing rs4680 and 5-HTTLPR variants in ASD have reported inconsistent results. Therefore, we conducted an integrated meta-analysis to combine case-control and transmission/disequilibrium test (TDT) studies to determine whether COMT and 5-HTT are associated with ASD. We searched multiple electronic databases (PubMed, EmBase and Web of Science) to identify studies assessing the rs4680 and 5-HTTLPR variants in ASD from Jan 1997 to Dec 2016. Then allelic data from case-control and TDT studies were analyzed by the Catmap package in the R software. A total of 5 studies were eligible for the meta-analysis of rs4680, including 3 case-control, 1 TDT and 1 TDT & case-control studies. Meanwhile, 22 studies of 5-HTTLPR were available, including 16 TDT, 4 case-control and 2 TDT & case-control studies. The current meta-analysis included 814 ASD cases, 741 controls and 311 families related to rs4680; 749 ASD cases, 1,118 controls and 1,861 families relevant to 5-HTTLPR were also evaluated. For rs4680, the pooled OR was 1.18 (95% CI = 0.87-1.59, P = 0.29, P_{heterogeneity}  < 0.00001). There was no significant association of rs4680 with risk of ASD between the two subgroups. For 5-HTTLPR, the pooled OR was 1.05 (95% CI = 0.92-1.20, P = 0.4652, P_{heterogeneity}  < 0.00001). Meanwhile, we found no significant risk in individual case-control or TDT studies. The above findings indicated that neither COMT rs4680 nor 5-HTT 5-HTTLPR polymorphism significantly affects ASD risk. Autism Res 2017, 10: 1722-1736. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.

LAY SUMMARY

Our results showed no evidence of significant association of either COMT rs4680 or 5-HTT 5-HTTLPR variants with ASD, showing that these two genes may not be major susceptible genetic factors in ASD occurrence, and may have a reciprocal action with each other in combination with environmental factors. These findings further provide evidence that a single gene variant may not dictate autism occurrence, but possibly contributes to a specific phenotype or subtype of ASD.

Genetic variants of MAOB affect serotonin level and specific behavioral attributes to increase autism spectrum disorder (ASD) susceptibility in males

Serotonergic system participates in various developmental processes and modulation of behaviour. Autism Spectrum Disorder (ASD) is characterized by a range of behavioral symptoms scaling from mild to severe. Abnormal 5-HT synthesis and signalling, platelet hyperserotonemia and amelioration of repetitive behaviours by SSRI are some of the key findings, which reinforced the hypothesis that serotonergic genes might act as ASD susceptible genes. Therefore, genes encoding monoamine oxidases A/B (MAOA/MAOB) received special attention as these genes are located on the X-chromosome and the gene products are responsible for 5-HT degradation. In the present study, we conducted population-based association analysis of eight markers of MAOB with ASD in a study cohort of 203 cases and 236 controls form India and examined its effect on platelet 5-HT content and behaviour. Gender-specific changes were observed for the contrasting LD between pair of markers among cases and controls. Case-control analysis demonstrated over-distribution of major C allele of rs2283728 and rs2283727 in male and female ASD cases respectively. Haplotypic distribution and interaction among markers showed more robust effect in male cases. Interestingly, male ASD cases displayed higher platelet 5-HT content in comparison to the respective controls. Quantitative trait analysis revealed significant correlation of genetic variants and haplotypes of MAOB markers, rs1799836 and rs6324 with increased platelet 5-HT level and CARS scores for specific behavioral symptoms respectively in males. This study suggests that MAOB increases ASD risk in males, possibly through its sex-specific regulatory effect on 5-HT metabolism and behavior.

Peer social interaction is facilitated in juvenile rhesus monkeys treated with fluoxetine

Fluoxetine improves social interactions in children with autism, social anxiety and social phobia. It is not known whether this effect is mediated directly or indirectly by correcting the underlying pathology. Genetics may also influence the drug effect. Polymorphisms of the MAOA (monoamine oxidase A) gene interact with fluoxetine to influence metabolic profiles in juvenile monkeys. Juvenile nonhuman primates provide an appropriate model for studying fluoxetine effects and drug*gene interactions in children. Male rhesus monkeys 1-3 years of age living in permanent social pairs were treated daily with a therapeutic dose of fluoxetine or vehicle (n = 16/group). Both members of each social pair were assigned to the same treatment group. They were observed for social interactions with their familiar cagemate over a 2-year dosing period. Subjects were genotyped for MAOA variable number of tandem repeats (VNTR) polymorphisms categorized for high or low transcription rates (hi-MAOA, low-MAOA). Fluoxetine-treated animals spent 30% more time in social interaction than vehicle controls. Fluoxetine significantly increased the duration of quiet interactions, the most common type of interaction, and also of immature sexual behavior typical of rhesus in this age group. Specific behaviors affected depended on MAOA genotype of the animal and its social partner. When given fluoxetine, hi-MOAO monkeys had more social invitation and initiation behaviors and low-MAOA subjects with low-MAOA partners had more grooming and an increased frequency of some facial and vocal expressive behaviors. Fluoxetine may facilitate social interaction in children independent of remediation of psychopathology. Common genetic variants may modify this effect.

Serotonin mediated immunoregulation and neural functions: Complicity in the aetiology of autism spectrum disorders

Serotonergic system has long been implicated in the aetiology of autism spectrum disorders (ASD), since platelet hyperserotonemia is consistently observed in a subset of autistic patients, who respond well to selective serotonin reuptake inhibitors. Apart from being a neurotransmitter, serotonin functions as a neurotrophic factor directing brain development and as an immunoregulator modulating immune responses. Serotonin transporter (SERT) regulates serotonin level in lymphoid tissues to ensure its proper functioning in innate and adaptive responses. Immunological molecules such as cytokines in turn regulate the transcription and activity of SERT. Dysregulation of serotonergic system could trigger signalling cascades that affect normal neural-immune interactions culminating in neurodevelopmental and neural connectivity defects precipitating behavioural abnormalities, or the disease phenotypes. Therefore, we suggest that a better understanding of the cross talk between serotonergic genes, immune systems and serotonergic neurotransmission will open wider avenues to develop pharmacological leads for addressing the core ASD behavioural deficits.