Genetic Studies of Mental Illness: Are Children Being Left Behind?

Understanding the genetic architecture of psychiatric disorders is paramount to linking psychopathologies to their genetic underpinnings. In turn, this knowledge can inform strategies for identifying high-risk individuals, early intervention, and development of personalized treatment approaches.1,2 Over the past 2 decades, owing to lowering per capita costs and relative ease of analysis, a plethora of studies have used single nucleotide polymorphism genotyping and genome-wide association studies (GWASs) to unravel common and rare risk loci underlying psychiatric disorders and their endophenotypes.3 In contrast to the single allele focus of classical Mendelian inheritance, mental illnesses are often polygenic in nature with multiple common genetic variants, each contributing a small, but meaningful added risk. By interrogating the entire genome, GWASs have allowed the functional assessment of promising candidate genes in in vivo as well as in vitro models of psychiatric disease. Further, these findings have spawned the approach of calculating polygenic risk scores, a promising strategy for inferring genetic susceptibility to the development of psychopathology by taking into account the polygenic structure of psychiatric disorders.

Family Environment, Neurodevelopmental Risk, and the Environmental Influences on Child Health Outcomes (ECHO) Initiative: Looking Back and Moving Forward.

The family environment, with all its complexity and diverse components, plays a critical role in shaping neurodevelopmental outcomes in children. Herein we review several domains of the family environment (family socioeconomic status, family composition and home environment, parenting behaviors and interaction styles, parental mental health and functioning, and parental substance use) and discuss how these domains influence neurodevelopment, with particular emphasis on mental health outcomes. We also highlight a new initiative launched by the National Institutes of Health, the Environmental influences on Child Health Outcomes (ECHO) program. We discuss the role that ECHO will play in advancing our understanding of the impact of the family environment on children's risk for psychiatric outcomes. Lastly, we conclude with important unanswered questions and controversies in this area of research, highlighting how ECHO will contribute to resolving these gaps in our understanding, clarifying relationships between the family environment and children's mental health.

Resting-state connectivity within and across neural circuits in anorexia nervosa.

INTRODUCTION: Obsessional thoughts and ritualized eating behaviors are characteristic of Anorexia Nervosa (AN), leading to the common suggestion that the illness shares neurobiology with obsessive-compulsive disorder (OCD). Resting-state functional connectivity MRI (rs-fcMRI) is a measure of functional neural architecture. This longitudinal study examined functional connectivity in AN within the limbic cortico-striato-thalamo-cortical (CSTC) loop, as well as in the salience network, the default mode network, and the executive control network (components of the triple network model of psychopathology). METHODS: Resting-state functional connectivity MRI scans were collected in unmedicated female inpatients with AN (n = 25) and healthy controls (HC; n = 24). Individuals with AN were scanned before and after weight restoration and followed for one month after hospital discharge. HC were scanned twice over the same timeframe. RESULTS: Using a seed-based correlation approach, individuals with AN had increased connectivity within the limbic CSTC loop when underweight, only. There was no significant association between limbic CSTC connectivity and obsessive-compulsive symptoms or prognosis. Exploratory analyses of functional network connectivity within the triple network model showed reduced connectivity between the salience network and left executive control network among AN relative to HC. These abnormalities persisted following weight restoration. CONCLUSIONS: The CSTC findings suggest that the neural underpinnings of obsessive-compulsive symptoms may differ from those of OCD. The inter-network abnormalities warrant examination in relation to illness-specific behaviors, namely abnormal eating behavior. This longitudinal study highlights the complexity of the neural underpinnings of AN.

Evidence for Thalamocortical Circuit Abnormalities and Associated Cognitive Dysfunctions in Underweight Individuals with Anorexia Nervosa.

Anorexia nervosa (AN) is characterized by extremely low body weight resulting from pathological food restriction, and carries a mortality rate among the highest of any psychiatric illness. AN, particularly during the acute, underweight state of the illness, has been associated with abnormalities across a range of brain regions, including the frontal cortex and basal ganglia. Few studies of AN have investigated the thalamus, a key mediator of information flow through frontal-basal ganglia circuit loops. We examined both thalamic surface morphology using anatomical MRI and thalamo-frontal functional connectivity using resting-state functional MRI. Individuals with AN (n=28) showed localized inward deformations of the thalamus relative to healthy controls (HC, n=22), and abnormal functional connectivity between the thalamus and the dorsolateral and anterior prefrontal cortices. Alterations in thalamo-frontal connectivity were associated with deficits in performance on tasks probing cognitive control (Stroop task) and working memory (Letter-Number Sequencing (LNS) task). Our findings suggest that abnormalities in thalamo-frontal circuits may have a role in mediating aspects of cognitive dysfunction in underweight individuals with AN.

Neural Correlates of Aggression in Medication-Naive Children with ADHD: Multivariate Analysis of Morphometry and Tractography.

Aggression is widely observed in children with attention deficit/hyperactivity disorder (ADHD) and has been frequently linked to frustration or the unsatisfied anticipation of reward. Although animal studies and human functional neuroimaging implicate altered reward processing in aggressive behaviors, no previous studies have documented the relationship between fronto-accumbal circuitry-a critical cortical pathway to subcortical limbic regions-and aggression in medication-naive children with ADHD. To address this, we collected behavioral measures and parental reports of aggression and impulsivity, as well as structural and diffusion MRI, from 30 children with ADHD and 31 healthy controls (HC) (mean age, 10±2.1 SD). Using grey matter morphometry and probabilistic tractography combined with multivariate statistical modeling (partial least squares regression and support vector regression), we identified anomalies within the fronto-accumbal circuit in childhood ADHD, which were associated with increased aggression. More specifically, children with ADHD showed reduced right accumbal volumes and frontal-accumbal white matter connectivity compared with HC. The magnitude of the accumbal volume reductions within the ADHD group was significantly correlated with increased aggression, an effect mediated by the relationship between the accumbal volume and impulsivity. Furthermore, aggression, but not impulsivity, was significantly explained by multivariate measures of fronto-accumbal white matter connectivity and cortical thickness within the orbitofrontal cortex. Our multi-modal imaging, combined with multivariate statistical modeling, indicates that the fronto-accumbal circuit is an important substrate of aggression in children with ADHD. These findings suggest that strategies aimed at probing the fronto-accumbal circuit may be beneficial for the treatment of aggressive behaviors in childhood ADHD.

Evidence for limited D1 and D2 receptor coexpression and colocalization within the dorsal striatum of the neonatal mouse.

The striatum is the major input nucleus of the basal ganglia involved in reward processing, goal-directed behaviors, habit learning, and motor control. The striatum projects to the basal ganglia output nuclei via the "direct" and "indirect" pathways, which can be distinguished by their projection fields and their opposing effects on behavior. In adult animals, the functional opposition is modulated by the differential actions of D1 and D2 dopamine receptors (D1R, D2R), the expression of which is largely separated between these pathways. To determine whether a similar degree of separation exists earlier in development, we used dual-label immunohistochemistry to map dorsal-striatal D1R and D2R expression at the promoter level in postnatal day 0 (PD0) Drd1a-tdTomato/Drd2-GFP BAC transgenic mice, and at the receptor level by costaining for native D1R and D2R in wildtype (WT) PD0 animals. To assess for potential molecular interactions between D1R and D2R we also employed a recently developed proximity-ligation assay (PLA). Limited coexpression and colocalization of the D1R and D2R proteins was found in clusters of neurons endemic to the "patch" compartment as identified by costaining with tyrosine hydroxylase, but not outside these clusters. Moreover, in contrast to our recent findings where we failed to detect a D1R-D2R PLA signal in the adult striatum, in PD0 striatum we did identify a clear PLA signal for this pair of receptors. This colocalization at close proximity points to a possible role for D1R/D2R-mediated crosstalk in early striatal ontogeny.

Genetic variation in COMT activity impacts learning and dopamine release capacity in the striatum.

A common genetic polymorphism that results in increased activity of the dopamine regulating enzyme COMT (the COMT Val(158) allele) has been found to associate with poorer cognitive performance and increased susceptibility to develop psychiatric disorders. It is generally assumed that this increase in COMT activity influences cognitive function and psychiatric disease risk by increasing dopamine turnover in cortical synapses, though this cannot be directly measured in humans. Here we explore a novel transgenic mouse model of increased COMT activity, equivalent to the relative increase in activity observed with the human COMT Val(158) allele. By performing an extensive battery of behavioral tests, we found that COMT overexpressing mice (COMT-OE mice) exhibit cognitive deficits selectively in the domains that are affected by the COMT Val(158) allele, stimulus-response learning and working memory, functionally validating our model of increased COMT activity. Although we detected no changes in the level of markers for dopamine synthesis and dopamine transport, we found that COMT-OE mice display an increase in dopamine release capacity in the striatum. This result suggests that increased COMT activity may not only affect dopamine signaling by enhancing synaptic clearance in the cortex, but may also cause changes in presynaptic dopamine function in the striatum. These changes may underlie the behavioral deficits observed in the mice and might also play a role in the cognitive deficits and increased psychiatric disease risk associated with genetic variation in COMT activity in humans.

Selective overexpression of dopamine D3 receptors in the striatum disrupts motivation but not cognition.

BACKGROUND: Evidence indicating an increase in dopamine D2 receptor (D2R) density and occupancy in patients with schizophrenia comes from positron emission tomography studies using ligands that bind both D2Rs and dopamine D3 receptors (D3Rs), questioning the role of D3Rs in the pathophysiology of the disease. Dopamine D3 receptor positron emission tomography ligands have recently been developed and antagonists with preferential affinity for D3R versus D2R are undergoing clinical evaluation. To determine if an increase in D3Rs in the striatum could produce phenotypes relevant to schizophrenia, we generated a transgenic model of striatal D3R overexpression. METHODS: A bi-transgenic system was used to generate mice with increased D3Rs selectively in the striatum. Mice with overexpression of D3R were subjected to an extensive battery of behavioral tests, including several relevant to schizophrenia. Ligand binding and quantitative reverse transcription polymerase chain reaction methods were used to quantify the effect of D3R overexpression on dopamine D1 receptors (D1Rs) in the striatum. RESULTS: Mice with overexpression of D3R show no abnormalities in basic behavioral functions or cognitive tests but do display a deficit in incentive motivation. This was associated with a reduction in striatal D1R ligand binding, driven by a downregulation at the level of transcription. Both motivation and D1R expression were rescued by switching off the transgene in adulthood. CONCLUSIONS: Overexpression of D3Rs in the striatum of mice does not elicit cognitive deficits but disrupts motivation, suggesting that changes in D3Rs may be involved in the negative symptoms of schizophrenia. These data imply that it will be important to evaluate the effects of D3R antagonists on motivational symptoms, which are not improved by currently available antipsychotic medications.

Effects of a short-course MDMA binge on dopamine transporter binding and on levels of dopamine and its metabolites in adult male rats.

Although the recreational drug 3,4-methylenedioxymethamphetamine (MDMA) is often described as a selective serotonergic neurotoxin, some research has challenged this view. The objective of this study was to determine the influence of MDMA on subsequent levels of two different markers of dopaminergic function, the dopamine transporter (DAT) as well as dopamine and its major metabolites. In experiment I, adult male Sprague-Dawley rats were administered either a low or moderate dose MDMA binge (2.5 or 5.0mg/kg×4 with an inter-dose interval of 1h) or saline, and were killed 1 week later. The moderate dose dramatically reduced [(3)H]WIN 35,428 binding to striatal DAT by 73.7% (P≤0.001). In experiment II, animals were binged with a higher dose of MDMA (10mg/kg×4) to determine the drug's effects on concentrations of serotonin (5-HT), dopamine, and their respective major metabolites 5-hydroxyindoleacetic acid (5-HIAA), dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in the striatum and frontal cortex 1 week later. As expected, MDMA significantly reduced 5-HT and 5-HIAA (≥50%) in these structures, while only a marginal decrease in dopamine was noted in the striatum. In contrast, levels of DOPAC (34.3%, P<0.01) and HVA (33.5%, P<0.001) were reduced by MDMA treatment, suggesting a decrease in dopamine turnover. Overall, these findings indicate that while serotonergic markers are particularly vulnerable to MDMA-induced depletion, significant dopaminergic deficits may also occur under some conditions. Importantly, DAT expression may be more vulnerable to perturbation by MDMA than dopamine itself.

The Nature of 3, 4-Methylenedioxymethamphetamine (MDMA)-Induced Serotonergic Dysfunction: Evidence for and Against the Neurodegeneration Hypothesis.

High doses of the recreational drug 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") have been well-documented to reduce the expression of serotonergic markers in several forebrain regions of rats and nonhuman primates. Neuroimaging studies further suggest that at least one of these markers, the plasma membrane serotonin transporter (SERT), may also be reduced in heavy Ecstasy users. Such effects, particularly when observed in experimental animal models, have generally been interpreted as reflecting a loss of serotonergic fibers and terminals following MDMA exposure. This view has been challenged, however, based on the finding that MDMA usually does not elicit glial cell reactions known to occur in response to central nervous system (CNS) damage. The aim of this review is to address both sides of the MDMA-neurotoxicity controversy, including recent findings from our laboratory regarding the potential of MDMA to induce serotonergic damage in a rat binge model. Our data add to the growing literature implicating neuroregulatory mechanisms underlying MDMA-induced serotonergic dysfunction and questioning the need to invoke a degenerative response to explain such dysfunction.

Effects of 3,4-methylenedioxymethamphetamine (MDMA) on serotonin transporter and vesicular monoamine transporter 2 protein and gene expression in rats: implications for MDMA neurotoxicity.

3,4-Methylenedioxymethamphetamine (MDMA; 'Ecstasy') is a popular recreational drug used worldwide. This study aimed to determine the effects of this compound on the expression of nerve terminal serotonergic markers in rats. Experiment 1 investigated MDMA-induced changes in levels of the serotonin transporter (SERT) and the vesicular monoamine transporter 2 (VMAT-2) in the hippocampus, a region with sparse dopaminergic innervation, after lesioning noradrenergic input with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). Adult male Sprague-Dawley rats were administered 100 mg/kg DSP-4 or saline 1 week prior to either an MDMA (10 mg/kg x 4) or saline binge. Two weeks following the binge treatment, the DSP-4/MDMA group unexpectedly showed little change in hippocampal VMAT-2 protein expression compared with DSP-4/Saline controls, despite large reductions in SERT levels in all regions examined in the MDMA-treated animals. Furthermore, animals treated with binge MDMA (Experiment 2) showed a striking decrease in SERT gene expression (and a lesser effect on VMAT-2) measured by quantitative RT-PCR in pooled dorsal and median raphe tissue punches, when compared with saline-treated controls. These results demonstrate that MDMA causes substantial regulatory changes in the expression of serotonergic markers, thus questioning the need to invoke distal axotomy as an explanation of MDMA-related serotonergic deficits.

Repeated adolescent MDMA ("Ecstasy") exposure in rats increases behavioral and neuroendocrine responses to a 5-HT2A/2C agonist.

MDMA (3,4-methylenedioxymethamphetamine) is a popular recreational drug among adolescents. The present study aimed to determine the effects of repeated intermittent administration of 10 mg/kg MDMA during adolescence on behavioral (Experiment 1) and neuroendocrine (Experiment 2) responses of rats to the 5-HT(2A/2C) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and on [(3)H]ketanserin binding to 5-HT(2A) receptors. In the first experiment, MDMA pretreatment increased the frequency of head twitches and back muscle contractions, but not wet-dog shakes, to a high-dose DOI challenge. In the second experiment, both the prolactin and corticosterone responses to DOI were potentiated in MDMA-pretreated animals. No changes were found in 5-HT(2A) receptor binding in the hypothalamus or other forebrain areas that were examined. These results indicate that intermittent adolescent MDMA exposure enhances sensitivity of 5-HT(2A/2C) receptors in the CNS, possibly through changes in downstream signaling mechanisms.