Serotonin transporter genotype and neuroanatomy in autism spectrum disorders

Patterns of altered neural synchrony in the default mode network in autism spectrum disorder revealed with magnetoencephalography (MEG): Relationship to clinical symptomatology

Disrupted neural synchrony may be a primary electrophysiological abnormality in autism spectrum disorders (ASD), altering communication between discrete brain regions and contributing to abnormalities in patterns of connectivity within identified neural networks. Studies exploring brain dynamics to comprehensively characterize and link connectivity to large-scale cortical networks and clinical symptoms are lagging considerably. Patterns of neural coherence within the Default Mode Network (DMN) and Salience Network (SN) during resting state were investigated in 12 children with ASD (M_Age  = 9.2) and 13 age and gender-matched neurotypicals (NT) (M_Age  = 9.3) with magnetoencephalography. Coherence between 231 brain region pairs within four frequency bands (theta (4-7 Hz), alpha, (8-12 Hz), beta (13-30 Hz), and gamma (30-80 Hz)) was calculated. Relationships between neural coherence and social functioning were examined. ASD was characterized by lower synchronization across all frequencies, reaching clinical significance in the gamma band. Lower gamma synchrony between fronto-temporo-parietal regions was observed, partially consistent with diminished default mode network (DMN) connectivity. Lower gamma coherence in ASD was evident in cross-hemispheric connections between: angular with inferior/middle frontal; middle temporal with middle/inferior frontal; and within right-hemispheric connections between angular, middle temporal, and inferior/middle frontal cortices. Lower gamma coherence between left angular and left superior frontal, right inferior/middle frontal, and right precuneus and between right angular and inferior/middle frontal cortices was related to lower social/social-communication functioning. Results suggest a pattern of lower gamma band coherence in a subset of regions within the DMN in ASD (angular and middle temporal cortical areas) related to lower social/social-communicative functioning. Autism Res 2018, 11: 434-449. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.

LAY SUMMARY

Communication between different areas of the brain was observed in children with ASD and neurotypical children while awake, but not working on a task. Magnetoencephalography was used to measure tiny magnetic fields naturally generated via brain activity. The brains of children with ASD showed less communication between areas that are important for social information processing compared to the brains of neurotypical children. The amount of communication between these areas was associated with social and social communication difficulties.

Imaging genetics in neurodevelopmental psychopathology

Neurodevelopmental disorders are defined by highly heritable problems during development and brain growth. Attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorders (ASDs), and intellectual disability (ID) are frequent neurodevelopmental disorders, with common comorbidity among them. Imaging genetics studies on the role of disease-linked genetic variants on brain structure and function have been performed to unravel the etiology of these disorders. Here, we reviewed imaging genetics literature on these disorders attempting to understand the mechanisms of individual disorders and their clinical overlap. For ADHD and ASD, we selected replicated candidate genes implicated through common genetic variants. For ID, which is mainly caused by rare variants, we included genes for relatively frequent forms of ID occurring comorbid with ADHD or ASD. We reviewed case-control studies and studies of risk variants in healthy individuals. Imaging genetics studies for ADHD were retrieved for SLC6A3/DAT1, DRD2, DRD4, NOS1, and SLC6A4/5HTT. For ASD, studies on CNTNAP2, MET, OXTR, and SLC6A4/5HTT were found. For ID, we reviewed the genes FMR1, TSC1 and TSC2, NF1, and MECP2. Alterations in brain volume, activity, and connectivity were observed. Several findings were consistent across studies, implicating, for example, SLC6A4/5HTT in brain activation and functional connectivity related to emotion regulation. However, many studies had small sample sizes, and hypothesis-based, brain region-specific studies were common. Results from available studies confirm that imaging genetics can provide insight into the link between genes, disease-related behavior, and the brain. However, the field is still in its early stages, and conclusions about shared mechanisms cannot yet be drawn.

Reduced tract integrity of the model for social communication is a neural substrate of social communication deficits in autism spectrum disorder

BACKGROUND

Autism spectrum disorder (ASD) is a neurodevelopmental disorder with social communication deficits as one of the core symptoms. Recently, a five-level model for the social communication has been proposed in which white matter tracts corresponding to each level of the model are identified. Given that the model for social communication subserves social language functions, we hypothesized that the tract integrity of the model for social communication may be reduced in ASD, and the reduction may be related to social communication deficits.

METHODS

Sixty-two right-handed boys with ASD and 55 typically developing (TD) boys received clinical evaluations, intelligence tests, the Social Communication Questionnaire (SCQ), and MRI scans. Generalized fractional anisotropy (GFA) was measured by diffusion spectrum imaging to indicate the microstructural integrity of the tracts for each level of the social communication model. Group difference in the tract integrity and its relationship with the SCQ subscales of social communication and social interaction were investigated.

RESULTS

We found that the GFA values of the superior longitudinal fasciculus III (SLF III, level 1) and the frontal aslant tracts (FAT, level 2) were decreased in ASD compared to TD. Moreover, the GFA values of the SLF III and the FAT were associated with the social interaction subscale in ASD.

CONCLUSIONS

The tract integrity of the model for social communication is reduced in ASD, and the reduction is associated with impaired social interaction. Our results support that reduced tract integrity of the model for social communication might be a neural substrate of social communication deficits in ASD.

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.

Bone growth in juvenile rhesus monkeys is influenced by 5HTTLPR polymorphisms and interactions between 5HTTLPR polymorphisms and fluoxetine

Male rhesus monkeys received a therapeutic oral dose of the selective serotonin reuptake inhibitor (SSRI) fluoxetine daily from 1 to 3 years of age. Puberty is typically initiated between 2 and 3 years of age in male rhesus and reproductive maturity is reached at 4 years. The study group was genotyped for polymorphisms in the monoamine oxidase A (MAOA) and serotonin transporter (SERT) genes that affect serotonin neurotransmission. Growth was assessed with morphometrics at 4 month intervals and radiographs of long bones were taken at 12 month intervals to evaluate skeletal growth and maturation. No effects of fluoxetine, or MAOA or SERT genotype were found for growth during the first year of the study. Linear growth began to slow during the second year of the study and serotonin reuptake transporter (SERT) long polymorphic region (5HTTLPR) polymorphism effects with drug interactions emerged. Monkeys with two SERT 5HTTLPR L alleles (LL, putative greater transcription) had 25-39% less long bone growth, depending on the bone, than monkeys with one S and one L allele (SL). More advanced skeletal maturity was also seen in the LL group, suggesting earlier onset of puberty. An interaction between 5HTTLPR polymorphisms and fluoxetine was identified for femur and tibia growth; the 5HTTLPR effect was seen in controls (40% less growth for LL) but not in the fluoxetine treated group (10% less growth for LL). A role for serotonin in peripubertal skeletal growth and maturation has not previously been investigated but may be relevant to treatment of children with SSRIs.

Increased rate of amygdala growth in children aged 2 to 4 years with autism spectrum disorders: a longitudinal study

CONTEXT

Precocious amygdala enlargement is commonly observed in young children with autism. However, the age at which abnormal amygdala enlargement begins and the relative growth trajectories of the amygdala and total brain remain unclear.

OBJECTIVE

To determine whether the rate of amygdala growth is abnormal and disproportionate to total brain growth in very young children with autism spectrum disorders (ASDs).

DESIGN

Longitudinal structural magnetic resonance imaging study.

SETTING

Neuroimaging and diagnostic assessments were performed at an academic medical center. Participants were recruited from the community.

PARTICIPANTS

Baseline scans were acquired in 132 boys (85 with ASD and 47 control subjects with typical development [TD]; mean age, 37 months). Longitudinal magnetic resonance images were acquired in 70 participants (45 with ASD and 25 TD controls) 1 year later.

MAIN OUTCOME MEASURE

Amygdala volumes and total cerebral volumes (TCVs) were evaluated at both time points, and 1-year growth rates were calculated.

RESULTS

The amygdala was larger in children with ASD at both time points, but the magnitude of enlargement was greater at time 2. The TCV was also enlarged in the children with ASD by the same magnitude at both time points. When we controlled for TCV, amygdala enlargement remained significant at both time points. The rate of amygdala growth during this 1-year interval was faster in children with ASD than in TD controls. The rate of TCV growth did not differ between groups. Post hoc exploratory analyses revealed 3 patterns of amygdala and TCV growth rates in the ASD group.

CONCLUSIONS

Disproportionate amygdala enlargement is present by 37 months of age in ASD. The amygdala continues to grow at an increased rate, but substantial heterogeneity exists in amygdala and TCV growth patterns. Future studies aimed at clinical characterization of different growth patterns could have implications for choice and outcomes of treatment and behavioral therapy.

Imaging-genetics in autism spectrum disorder: advances, translational impact, and future directions

Autism Spectrum Disorder (ASD) refers to a group of heterogeneous neurodevelopmental disorders that are unified by impairments in reciprocal social communication and a pattern of inflexible behaviors. Recent genetic advances have resolved some of the complexity of the genetic architecture underlying ASD by identifying several genetic variants that contribute to the disorder. Different etiological pathways associated with ASD may converge through effects on common molecular mechanisms, such as synaptogenesis, neuronal motility, and axonal guidance. Recently, with more sophisticated techniques, neuroimaging, and neuropathological studies have provided some consistency of evidence that altered structure, activity, and connectivity within complex neural networks is present in ASD, compared to typically developing children. The imaging-genetics approach promises to help bridge the gap between genetic variation, resultant biological effects on the brain, and production of complex neuropsychiatric symptoms. Here, we review recent findings from the developing field of imaging-genetics applied to ASD. Studies to date have indicated that relevant risk genes are associated with alterations in circuits that mediate socio-emotional, visuo-spatial, and language processing. Longitudinal studies ideally focused on early development, in conjunction with investigation for gene-gene, and gene-environment interactions may move the promise of imaging-genetics in ASD closer to the clinical domain.

Autism in adults. New biologicial findings and their translational implications to the cost of clinical services

There is increasing evidence that children with autism spectrum disorder (ASD) have differences in brain growth trajectory. However, the neurobiological basis of ASD in adults is poorly understood. We report evidence that brain anatomy and aging in people with ASD is significantly different as compared to controls-so that in adulthood they no longer have a significantly larger overall brain volume, but they do have anatomical and functional abnormalities in frontal lobe, basal ganglia and the limbic system. Further we present preliminary evidence that females have significantly greater abnormalities in brain than males to express the same symptom severity of ASD (i.e. the female brain is "protective" against developing ASD). Also we present preliminary evidence that, in adults, clinical services for autism in the United Kingdom are experiencing very significantly increased demand; but that just over 50% of people seeking a diagnosis from one expert service do not have ASD. This consumes very significant health care resources, and so we need to identify new cost-effective methods to aid current diagnostic practice. We present initial evidence offering proof of concept that brain anatomy can be used to accurately distinguish adults with autism from healthy controls, and from some other neurodevelopmental disorders (ADHD). Hence further studies are required to determine if sMRI can become an aid to current diagnostic practice in young adults with ASD. Lastly we report evidence that differences in serotonin, glutamate and GABA may partially explain neuroanatomical and neurofunctional abnormalities in people with ASD, and that genetic influences on brain maturation vary across the lifespan (with 5-HT transporter polymorphisms having significant modulatory effects in children but not adults).

Density and function of central serotonin (5-HT) transporters, 5-HT1A and 5-HT2A receptors, and effects of their targeting on BTBR T+tf/J mouse social behavior

BTBR mice are potentially useful tools for autism research because their behavior parallels core social interaction impairments and restricted-repetitive behaviors. Altered regulation of central serotonin (5-HT) neurotransmission may underlie such behavioral deficits. To test this, we compared 5-HT transporter (SERT), 5-HT(1A) and 5-HT(2A) receptor densities among BTBR and C57 strains. Autoradiographic [(3) H] cyanoimipramine (1 nM) binding to SERT was 20-30% lower throughout the adult BTBR brain as compared to C57BL/10J mice. In hippocampal membrane homogenates, [(3) H] citalopram maximal binding (B(max) ) to SERT was 95 ± 13 fmol/mg protein in BTBR and 171 ± 20 fmol/mg protein in C57BL/6J mice, and the BTBR dissociation constant (K(D) ) was 2.0 ± 0.3 nM versus 1.1 ± 0.2 in C57BL/6J mice. Hippocampal 5-HT(1A) and 5-HT(2A) receptor binding was similar among strains. However, 8-OH-DPAT-stimulated [(35) S] GTPγS binding in the BTBR hippocampal CA(1) region was 28% higher, indicating elevated 5-HT(1A) capacity to activate G-proteins. In BTBR mice, the SERT blocker, fluoxetine (10 mg/kg) and the 5-HT(1A) receptor partial-agonist, buspirone (2 mg/kg) enhanced social interactions. The D(2) /5-HT(2) receptor antagonist, risperidone (0.1 mg/kg) reduced marble burying, but failed to improve sociability. Overall, altered SERT and/or 5-HT(1A) functionality in hippocampus could contribute to the relatively low sociability of BTBR mice.

Age-related temporal and parietal cortical thinning in autism spectrum disorders

Studies of head size and brain volume in autism spectrum disorders have suggested that early cortical overgrowth may be followed by prematurely arrested growth. However, the few investigations quantifying cortical thickness have yielded inconsistent results, probably due to variable ages and/or small sample sizes. We assessed differences in cortical thickness between high-functioning adolescent and young adult males with autism spectrum disorders (n = 41) and matched typically developing males (n = 40). We hypothesized thinner cortex, particularly in frontal, parietal and temporal regions, for individuals with autism spectrum disorders in comparison with typically developing controls. Furthermore, we expected to find an age × diagnosis interaction: with increasing age, more pronounced cortical thinning would be observed in autism spectrum disorders than typically developing participants. T(1)-weighted magnetization prepared rapid gradient echo 3 T magnetic resonance imaging scans were acquired from high-functioning males with autism spectrum disorders and from typically developing males matched group-wise on age (range 12-24 years), intelligence quotient (≥ 85) and handedness. Both gyral-level and vertex-based analyses revealed significantly thinner cortex in the autism spectrum disorders group that was located predominantly in left temporal and parietal regions (i.e. the superior temporal sulcus, inferior temporal, postcentral/superior parietal and supramarginal gyri). These findings remained largely unchanged after controlling for intelligence quotient and after accounting for psychotropic medication usage and comorbid psychopathology. Furthermore, a significant age × diagnosis interaction was found in the left fusiform/inferior temporal cortex: participants with autism spectrum disorders had thinner cortex in this region with increasing age to a greater degree than did typically developing participants. Follow-up within group comparisons revealed significant age-related thinning in the autism spectrum disorders group but not in the typically developing group. Both thinner temporal and parietal cortices during adolescence and young adulthood and discrepantly accelerated age-related cortical thinning in autism spectrum disorders suggest that a second period of abnormal cortical growth (i.e. greater thinning) may be characteristic of these disorders.

Do candidate genes discriminate patients with an autism spectrum disorder from those with attention deficit/hyperactivity disorder and is there an effect of lifetime substance use disorders?

OBJECTIVE

Autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD) are developmental disorders that overlap in a number of domains, sometimes complicating clinical distinction between both disorders. Although there is some evidence for a genetic overlap, there are no reports on genes that could differentiate between ASD and ADHD. Furthermore, it is not known whether this genetic overlap is influenced by co-morbid substance use disorders (SUD).

METHODS

A total of 110 adult patients with ASD (n=61) or ADHD (n=49) with or without a lifetime history of SUD participated in a study in which we genotyped polymorphisms in five known candidate genes for (one of) the disorders, i.e. the 5HTTLPR in SLC6A4/5-HTT, rs1800497 (TaqIA C>T) in DRD2, rs7794745 in CNTNAP2, rs1843809 in TPH2, and rs6565113 in CDH13. Genotyping was by Taqman-based analysis or by simple sequence length analysis, where appropriate.

RESULTS

ASD could be differentiated from ADHD with nominal statistical significance by the 5HTTLPR, and the polymorphisms in TPH2 and CNTNAP2. The results were independent of lifetime SUD status.

CONCLUSIONS

Serotonergic genes could prove to play an important role in differentiating between ASD and ADHD, but the results of this exploratory study need replication.

Communication, interventions, and scientific advances in autism: a commentary

Autism spectrum disorders (ASD) affect approximately 1 in 150 children across the U.S., and are characterized by abnormal social actions, language difficulties, repetitive or restrictive behaviors, and special interests. ASD include autism (autistic disorder), Asperger Syndrome, and Pervasive Developmental Disorder not otherwise specified (PDD-NOS or atypical autism). High-functioning individuals may communicate with moderate-to-high language skills, although difficulties in social skills may result in communication deficits. Low-functioning individuals may have severe deficiencies in language, resulting in poor communication between the individual and others. Behavioral intervention programs have been developed for ASD, and are frequently adjusted to accommodate specific individual needs. Many of these programs are school-based and aim to support the child in the development of their skills, for use outside the classroom with family and friends. Strides are being made in understanding the factors contributing to the development of ASD, particularly the genetic contributions that may underlie these disorders. Mutant mouse models provide powerful research tools to investigate the genetic factors associated with ASD and its co-morbid disorders. In support, the BTBR T+tf/J mouse strain incorporates ASD-like social and communication deficits and high levels of repetitive behaviors. This commentary briefly reviews the reciprocal relationship between observations made during evidence-based behavioral interventions of high- versus low-functioning children with ASD and the accumulating body of research in autism, including animal studies and basic research models. This reciprocity is one of the hallmarks of the scientific method, such that research may inform behavioral treatments, and observations made during treatment may inform subsequent research.

Conserved role for the serotonin transporter gene in rat and mouse neurobehavioral endophenotypes

The serotonin transporter knockout (SERT(-/-)) mouse, generated in 1998, was followed by the SERT(-/-) rat, developed in 2006. The availability of SERT(-/-) rodents creates the unique possibility to study the conservation of gene function across species. Here we summarize SERT(-/-) mouse and rat data, and discuss species (dis)similarities in neurobehavioral endophenotypes. Both SERT(-/-) rodent models show a disturbed serotonergic system, altered nociception, higher anxiety, decreased social behavior, as well as increased negative emotionality, behavioral inhibition and decision making. Used to model a wide range of psychiatric disorders, SERT(-/-) rodents may be particularly valuable in research on neurodevelopmental disorders such as depression, anxiety, and possibly autism. We conclude that SERT function is conserved across mice and rats and that their behavioral profile arises from common neurodevelopmental alterations. Because mice and rats have species-specific characteristics that confer differential research advantages, a comparison of the two models has heuristic value in understanding the mechanisms and behavioral outcome of SERT genetic variation in humans.