A meta-analytic evaluation of the endophenotype hypothesis: effects of measurement paradigm in the psychiatric genetics of impulsivity

Dual-systems models of the genetic architecture of impulsive personality traits: neurogenetic evidence of distinct but related factors

BACKGROUND

Dual-systems models, positing an interaction between two distinct and competing systems (i.e. top-down self-control, and bottom-up reward- or emotion-based drive), provide a parsimonious framework for investigating the interplay between cortical and subcortical brain regions relevant to impulsive personality traits (IPTs) and their associations with psychopathology. Despite recent developments in multivariate analysis of genome-wide association studies (GWAS), molecular genetic investigations of these models have not been conducted.

METHODS

Using IPT GWAS, we conducted confirmatory genomic structural equation models (GenomicSEM) to empirically evaluate dual-systems models of the genetic architecture of IPTs. Genetic correlations between dual-systems factors and relevant cortical and subcortical neuroimaging phenotypes (regional/structural volume, cortical surface area, cortical thickness) were estimated and compared.

RESULTS

GenomicSEM dual-systems models underscored important sources of shared and unique genetic variance between top-down and bottom-up constructs. Specifically, a dual-systems genomic model consisting of sensation seeking and lack of self-control factors demonstrated distinct but related sources of genetic influences (rg = 0.60). Genetic correlation analyses provided evidence of differential associations between dual-systems factors and cortical neuroimaging phenotypes (e.g. lack of self-control negatively associated with cortical thickness, sensation seeking positively associated with cortical surface area). No significant associations were observed with subcortical phenotypes.

CONCLUSIONS

Dual-systems models of the genetic architecture of IPTs tested were consistent with study hypotheses, but associations with relevant neuroimaging phenotypes were mixed (e.g. no associations with subcortical volumes). Findings demonstrate the utility of dual-systems models for studying IPT genetic influences, but also highlight potential limitations as a framework for interpreting IPTs as endophenotypes for psychopathology.

Dual-systems models of the genetic architecture of impulsive personality traits: Neurogenetic evidence of distinct but related factors

Background

Dual-systems models provide a parsimonious framework for understanding the interplay between cortical and subcortical brain regions relevant to impulsive personality traits (IPTs) and their associations with psychiatric disorders. Despite recent developments in multivariate analysis of genome-wide association studies (GWAS), molecular genetic investigations of these models have not been conducted.

Methods

Using extant IPT GWAS, we conducted confirmatory genomic structural equation models (GenomicSEM) to empirically evaluate dual-systems models of the genetic architecture of IPTs. Genetic correlations between results of multivariate GWAS of dual-systems factors and GWAS of relevant cortical and subcortical neuroimaging phenotypes (regional/structural volume, cortical surface area, cortical thickness) were calculated and compared.

Results

Evaluation of GenomicSEM model fit indices for dual-systems models suggested that these models highlight important sources of shared and unique genetic variance between top-down and bottom-up constructs. Specifically, a dual-systems genomic model consisting of sensation seeking and lack of self-control factors demonstrated distinct but related sources of genetic influences ( r _g =.60). Genetic correlation analyses provided evidence of differential associations between dual-systems factors and cortical neuroimaging phenotypes (e.g., lack of self-control negatively associated with cortical thickness, sensation seeking positively associated with cortical surface area). However, no significant associations were observed for subcortical phenotypes inconsistent with hypothesized functional localization of dual-systems constructs.

Conclusions

Dual-systems models of the genetic architecture of IPTs tested here demonstrate evidence of shared and unique genetic influences and associations with relevant neuroimaging phenotypes. These findings emphasize potential advantages in utilizing dual-systems models to study genetic influences for IPTs and transdiagnostic associations with psychiatric disorders.

[Ordalie, sensation-seeking and impulsivity. Critical analysis of definitions]

BACKGROUND

In the fields of psychology and psychiatry, the use of the terms impulsivity, sensation-seeking and ordalie to refer to risk-taking behaviors can sometimes be confusing.

OBJECTIVE

The objective of this study was to establish a clinical definition of the concepts of ordalie, sensation-seeking and impulsivity, in order to analyze the similarities and differences between these concepts.

METHODS

We prioritized literature review articles with or without meta-analysis from the Medline database and supplemented with the Google-Scholar database. The articles were included in this review if their objectives were in line with ours. The research was conducted in November 2018.

RESULTS

Twenty-seven articles were selected. There are similarities in the clinical definitions of these concepts with measurable heterogeneous constructions, and an exacerbation in adolescence for engagement in harmful behaviors, but there are also nuances that highlight their differences.

CONCLUSION

We were able to describe areas of divergence and convergence between these three concepts but not to establish a quantitative diagram of the areas of divergence and convergence. It would seem that the coexistence of sensation-seeking and impulsivity in the same individual could explain that individual's involvement in ordalique behaviors. Further studies approaching this hypothesis would seem useful in terms of preventing risk-taking behaviors such as addictive behaviors.

Highs and lows: Genetic susceptibility to daily events

Why people differ in their susceptibility to external events is essential to our understanding of personality, human development, and mental disorders. Genes explain a substantial portion of these differences. Specifically, genes influencing the serotonin system are hypothesized to be differential susceptibility factors, determining a person’s reactivity to both positive and negative environments. We tested whether genetic variation in the serotonin transporter (5-HTTLPR) is a differential susceptibility factor for daily events. Participants (N = 326, 77% female, mean age = 25, range = 17–36) completed smartphone questionnaires four times a day over four to five days, measuring stressors, uplifts, positive and negative affect. Affect was predicted from environment valence in the previous hour on a within-person level using three-level autoregressive linear mixed models. The 5-HTTLPR fulfilled all criteria of a differential susceptibility factor: Positive affect in carriers of the short allele (S) was less reactive to both uplifts and stressors, compared to homozygous carriers of the long allele (L/L). This pattern might reflect relative affective inflexibility in S-allele carriers. Our study provides insight into the serotonin system’s general role in susceptibility and highlights the need to assess the whole spectrum of naturalistic experiences.

Male Ejaculatory Endophenotypes: Revealing Internal Inconsistencies of the Concept in Heterosexual Copulating Rats

Distinct manifestations of sexual behavior are conceived as separate phenotypes. Each sexual phenotype is assumed to be associated with a characteristic brain. These notions have justified the phenotyping of heterosexual copulator males based upon their ejaculation's latencies (EL) or frequencies (i.e., cumulative ejaculation number; EN). For instance, men and male rats showing premature, normal or retarded ejaculation are assumed to be distinctive endophenotypes. This concept, nonetheless, contradicts past and recent evidence that supports that sexual behavior is highly variable within each sex, and that the brain sexual functional morphology represents an intricate sexual phenotypic mosaic. Hence, for ejaculatory male endophenotypes to be considered as a valid biological concept, it must show internal consistency at various levels of organization (including genetic architectures), after being challenged by intrinsic and/or extrinsic factors. We then judged the internal consistency of the presumed ejaculatory endophenotypes by assessing whether copulatory behavior and the expression of copulation relevant genes and brain limbic structures are specific to each of the presumed EL- or EN-ejaculatory endophenotypes. To do this, copulating male rats were first phenotyped in groups consistently displaying short, average or long ejaculation latencies or very high, high, average, low or very low EN, based in their copulatory performance. Then, the internal consistency of the presumed EL- or EN-endophenotypes was tested by introducing as covariates of phenotyping other copulatory parameters (e.g., number of intromissions) in addition to EL or EN, or by analyzing the expression levels of genes encoding for estrogen receptor alpha, progesterone receptor, androgen receptor, aromatase, DNA methyl-transferase 3a and DNA methyl-transferase 1 in the amygdala, medial preoptic area, ventromedial hypothalamus and olfactory bulb. We found that even though there were group-level differences in all the variables that were studied, these differences did not add-up to create the presumed EL- or EN-ejaculatory endophenotypes. In fact, the extensive overlapping of copulatory parameters and expression levels of copulation relevant genes in limbic structures across EL- or EN-phenotyped copulating male rats, is not consistent with the hypothesis that distinct ejaculatory endophenotypes exist and that they are associated with specific brain characteristics.

Impulsive Personality Traits Mediate the Relationship Between Adult Attention-Deficit/Hyperactivity Symptoms and Alcohol Dependence Severity

BACKGROUND

While the role of attention-deficit/hyperactivity disorder (ADHD) as a risk factor for developing alcohol use disorder (AUD) has been established, the underlying pathways connecting the two are still not fully understood. Overlapping constructs such as impulsivity may explain the increased risk for developing AUD in individuals with ADHD.

METHODS

In this study, we assessed whether adult ADHD symptoms increase the odds of having a diagnosis of AUD. Furthermore, we tested whether facets of impulsivity explained the relationship between ADHD symptoms and alcohol dependence (AD) severity.

RESULTS

In a logistic regression of 749 participants (464 = AD, 285 = controls), overall adult ADHD symptoms, and more specifically, symptoms of hyperactivity/restlessness and problems with self-concept, increased the odds of having a diagnosis of AD. Within the AD sample, we found that impulsivity mediated the relationship between adult ADHD symptoms and AD severity. In particular, negative and positive urgency meditated the relationship of overall adult ADHD symptoms, and symptoms of hyperactivity/restlessness and problems with self-concept with AD severity.

CONCLUSIONS

These results highlight the importance of looking at cohorts of ADHD symptoms and facets of impulsivity to assess the risk of developing AUD. They also suggest potential avenues for intervention strategies in individuals with preexisting adult ADHD symptoms who are seeking treatment for AUD.

A Test-Replicate Approach to Candidate Gene Research on Addiction and Externalizing Disorders: A Collaboration Across Five Longitudinal Studies

This study presents results from a collaboration across five longitudinal studies seeking to test and replicate models of gene-environment interplay in the development of substance use and externalizing disorders (SUDs, EXT). We describe an overview of our conceptual models, plan for gene-environment interplay analyses, and present main effects results evaluating six candidate genes potentially relevant to SUDs and EXT (MAOA, 5-HTTLPR, COMT, DRD2, DAT1, and DRD4). All samples included rich longitudinal and phenotypic measurements from childhood/adolescence (ages 5-13) through early adulthood (ages 25-33); sample sizes ranged from 3487 in the test sample, to ~600-1000 in the replication samples. Phenotypes included lifetime symptom counts of SUDs (nicotine, alcohol and cannabis), adult antisocial behavior, and an aggregate externalizing disorder composite. Covariates included the first 10 ancestral principal components computed using all autosomal markers in subjects across the data sets, and age at the most recent assessment. Sex, ancestry, and exposure effects were thoroughly evaluated. After correcting for multiple testing, only one significant main effect was found in the test sample, but it was not replicated. Implications for subsequent gene-environment interplay analyses are discussed.

Clashing Diagnostic Approaches: DSM-ICD Versus RDoC

Since at least the middle of the past century, one overarching model of psychiatric classification has reigned supreme, namely, that of the Diagnostic and Statistical Manual of Mental Disorders and the International Statistical Classification of Diseases and Related Health Problems (herein referred to as DSM-ICD). This DSM-ICD approach embraces an Aristotelian view of mental disorders as largely discrete entities that are characterized by distinctive signs, symptoms, and natural histories. Over the past several years, however, a competing vision, namely, the Research Domain Criteria (RDoC) initiative launched by the National Institute of Mental Health, has emerged in response to accumulating anomalies within the DSM-ICD system. In contrast to DSM-ICD, RDoC embraces a Galilean view of psychopathology as the product of dysfunctions in neural circuitry. RDoC appears to be a valuable endeavor that holds out the long-term promise of an alternative system of mental illness classification. We delineate three sets of pressing challenges--conceptual, methodological, and logistical/pragmatic--that must be addressed for RDoC to realize its scientific potential. We conclude with a call for further research, including investigation of a rapprochement between Aristotelian and Galilean approaches to psychiatric classification.

Mechanisms in the relation between GABRA2 and adolescent externalizing problems

Conduct problems, alcohol problems and hyperactive-inattentive symptoms co-occur at a high rate and are heritable in adolescence. The γ-aminobutyric acid A receptor, α2 gene (GABRA2) is associated with a broad spectrum of externalizing problems and disinhibitory-related traits. The current study tested whether two important forms of disinhibition in adolescence, impulsivity and sensation seeking, mediated the effects of GABRA2 on hyperactive-inattentive symptoms, conduct problems, and alcohol problems. Participants were assessed at two waves (11-17 and 12-18 years old; N = 292). Analyses used the GABRA2 SNP, rs279858, which tags the two complementary (yin-yang) GABRA2 haplotypes. Multiple informants reported on adolescents' impulsivity and sensation seeking and adolescents self-reported their hyperactive-inattentive symptoms, conduct problems and lifetime alcohol problems. Impulsivity mediated the effect of GABRA2 on alcohol problems, hyperactive-inattentive symptoms, and conduct problems, whereas sensation seeking mediated the effect of GABRA2 on alcohol problems (AA/AG genotypes conferred risk). GABRA2 directly predicted adolescent alcohol problems, but the GG genotype conferred risk. Results suggest that there may be multiple pathways of risk from GABRA2 to adolescent externalizing problems, and suggest important avenues for future research.

Genetic influences on delay discounting in smokers: examination of a priori candidates and exploration of dopamine-related haplotypes

RATIONALE

Delay discounting is a behavioral economic index of impulsivity that reflects a person's relative preference for small immediate rewards versus larger delayed rewards. Elevated delay discounting is robustly linked to addictive disorders and has been increasingly investigated as a viable endophenotype for genetic influences on addiction.

OBJECTIVE

The aim of this study is to examine associations between delay discounting and two a priori loci, rs4680 in COMT and rs1800497 in ANKK1, and three exploratory haplotypes proximal to rs1800497 in a sample of daily smokers.

METHODS

Participants were 713 (60.2 % male) daily smokers of European ancestry who completed a delay discounting assessment and provided a DNA sample.

RESULTS

Significant associations were detected between greater discounting of medium magnitude rewards (~$55) and the G allele of rs4680, as well as the T allele of rs1800497. Exploratory haplotype analyses identified two haplotypes (rs1160467/rs1800497; rs6277/rs1079597) significantly associated with delay discounting rates. However, the rs1160467/rs1800497 haplotype associations appeared to be entirely attributable to variation in rs1800497, suggesting that the association of rs1800497 with discounting is best understood at the individual SNP level. Similarly, the rs6277/rs1079597 haplotype findings suggested that the association was specific to rs1079597.

CONCLUSIONS

This study provides further evidence that rs4680 and rs1800497 genotypes are significantly associated with delay discounting preferences and does so among smokers for the first time. The study also provides evidence of specificity for the rs1800497 association and identifies a novel locus, rs1079597, as a genetic contributor to higher delay discounting rates.

Genome-wide scans of genetic variants for psychophysiological endophenotypes: a methodological overview

This article provides an introductory overview of the investigative strategy employed to evaluate the genetic basis of 17 endophenotypes examined as part of a 20-year data collection effort from the Minnesota Center for Twin and Family Research. Included are characterization of the study samples, descriptive statistics for key properties of the psychophysiological measures, and rationale behind the steps taken in the molecular genetic study design. The statistical approach included (a) biometric analysis of twin and family data, (b) heritability analysis using 527,829 single nucleotide polymorphisms (SNPs), (c) genome-wide association analysis of these SNPs and 17,601 autosomal genes, (d) follow-up analyses of candidate SNPs and genes hypothesized to have an association with each endophenotype, (e) rare variant analysis of nonsynonymous SNPs in the exome, and (f) whole genome sequencing association analysis using 27 million genetic variants. These methods were used in the accompanying empirical articles comprising this special issue, Genome-Wide Scans of Genetic Variants for Psychophysiological Endophenotypes.