Examination of genetic variation in GABRA2 with conduct disorder and alcohol abuse and dependence in a longitudinal study

Early-life stress influences acute and sensitized responses of adult mice to cocaine by interacting with GABAA α2 receptor expression

Early-life stress (ELS) is known to exert long-term effects on brain function, with resulting deleterious consequences for several aspects of mental health, including the development of addiction to drugs of abuse. One potential mechanism in humans is suggested by findings that ELS interacts with polymorphisms of the GABRA2 gene, encoding α2 subunits of GABAA receptors, to increase the risk for both post-traumatic stress disorder and vulnerability to cocaine addiction. We used a mouse model, in which the amount of material for nest building was reduced during early postnatal life, to study interactions between ELS and expression of α2-containing GABAA receptors in influencing cocaine-related behaviour. Breeding of parents heterozygous for a deletion of α2 resulted in litters containing homozygous knockout (α2), heterozygous knockout (α2) and wild-type (α2) offspring. Following the ELS procedure, the mice were allowed to develop to adulthood before being tested for the acute effect of cocaine on locomotor stimulation, behavioural sensitization to repeated cocaine and to cocaine-conditioned activity. Exposure to ELS resulted in increased acute locomotor stimulant effects of cocaine across all genotypes, with the most marked effects in α2 mice (which also showed increased activity following vehicle). Repeated cocaine administration to nonstressed mice resulted in sensitization in α2 and α2 mice, but, in keeping with previous findings, not in α2 mice. Previous exposure to ELS reduced sensitization in α2 mice, albeit not significantly, and abolished sensitization in α2 mice. Conditioned activity was elevated following ELS in all animals, independently of genotype. Thus, while the enhanced acute effects of cocaine following ELS being most marked in α2 mice suggests a function of α2-containing GABAA receptors in protecting against stress, the interaction between ELS and genotype in influencing sensitization may be more in keeping with ELS reducing expression of α2-containing GABAA receptors. The ability of ELS to increase cocaine-conditioned locomotor activity appears to be independent of α2-containing GABAA receptors.

Genetic influences on conduct disorder

Conduct disorder (CD) is a moderately heritable psychiatric disorder of childhood and adolescence characterized by aggression toward people and animals, destruction of property, deceitfulness or theft, and serious violation of rules. Genome-wide scans using linkage and association methods have identified a number of suggestive genomic regions that are pending replication. A small number of candidate genes (e.g., GABRA2, MAOA, SLC6A4, AVPR1A) are associated with CD related phenotypes across independent studies; however, failures to replicate also exist. Studies of gene-environment interplay show that CD genetic predispositions also contribute to selection into higher-risk environments, and that environmental factors can alter the importance of CD genetic factors and differentially methylate CD candidate genes. The field's understanding of CD etiology will benefit from larger, adequately powered studies in gene identification efforts; the incorporation of polygenic approaches in gene-environment interplay studies; attention to the mechanisms of risk from genes to brain to behavior; and the use of genetically informative data to test quasi-causal hypotheses about purported risk factors.

Post-GWAS in Psychiatric Genetics: A Developmental Perspective on the "Other" Next Steps

As psychiatric genetics enters an era where gene identification is finally yielding robust, replicable genetic associations and polygenic risk scores, it is important to consider next steps and delineate how that knowledge will be applied to ultimately ameliorate suffering associated with substance use and psychiatric disorders. Much of the post-genome-wide association study discussion has focused on the potential of genetic information to elucidate the underlying biology and use this information for the development of more effective pharmaceutical treatments. In this review we focus on additional areas of research that should follow gene identification. By taking genetic findings into longitudinal, developmental studies, we can map the pathways by which genetic risk manifests across development, elucidating the early behavioral manifestations of risk, and studying how various environments and interventions moderate that risk across developmental stages. The delineation of risk across development will advance our understanding of mechanism, sex differences and risk and resilience processes in different racial/ethnic groups. Here, we review how the extant twin study literature can be used to guide these efforts. Together, these new lines of research will enable us to develop more informed, tailored prevention and intervention efforts.

Impulsivity Dimensions and Risky Sex Behaviors in an At-Risk Young Adult Sample

Impulsivity is a personality-based risk factor that has been well studied in relation to risky sexual behavior. Recent conceptualizations of impulsivity have proposed multidimensional facets comprised of premeditation, perseverance, sensation seeking, negative urgency, and positive urgency (UPPS-P model). Prior studies have found that these facets are associated with risky sexual behavior in adolescent and college student samples, but no prior studies have evaluated them in clinical samples. The current study examined how impulsivity-related traits related to two different risky sexual behaviors in a clinical sample of at-risk young adults who had both conduct disorder and substance use disorder symptoms as adolescents (n = 529). Lack of premeditation was also tested as a moderator of the relationship between facets of impulsivity and both risky sex outcomes. Results demonstrated that sensation seeking, negative urgency, and positive urgency were correlated with risky sex behaviors. Additionally, multiple regression analyses indicated that sensation seeking was uniquely associated with the number of sexual partners in the past 5 years, whereas positive urgency was uniquely associated with unprotected sex while under the influence. Finally, a significant interaction between lack of premeditation and negative urgency suggests that at-risk young adults with both high negative urgency and lack of premeditation were the likeliest to have the most sexual partners in the past 5 years. This study adds to the current understanding of the relationship between reward- and affect-driven facets of impulsivity and risky sexual behaviors and may lend utility to the development of interventions for at-risk populations.

GABAA Receptor Subtype Mechanisms and the Abuse-Related Effects of Ethanol: Genetic and Pharmacological Evidence

Ethanol's reinforcing and subjective effects, as well as its ability to induce relapse, are powerful factors contributing to its widespread use and abuse. A significant mediator of these behavioral effects is the GABA_A receptor system. GABA_A receptors are the target for γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the CNS. Structurally, they are pentameric, transmembrane chloride ion channels comprised of subunits from at least eight different families of distinct proteins. The contribution of different GABA_A subunits to ethanol's diverse abuse-related effects is not clear and remains an area of research focus. This chapter details the clinical and preclinical findings supporting roles for different α, β, γ, and δ subunit-containing GABA_A receptors in ethanol's reinforcing, subjective/discriminative stimulus, and relapse-inducing effects. The reinforcing properties of ethanol have been studied the most systematically, and convergent preclinical evidence suggests a key role for the α5 subunit in those effects. Regarding ethanol's subjective/discriminative stimulus effects, clinical and genetic findings support a primary role for the α2 subunit, whereas preclinical evidence implicates the α5 subunit. At present, too few studies investigating ethanol relapse exist to make any solid conclusions regarding the role of specific GABA_A subunits in this abuse-related effect.

A GABRA2 polymorphism improves a model for prediction of drinking initiation

BACKGROUND

Survival analysis was used to explore the addition of a single nucleotide polymorphism (SNP) and covariates (sex, interview age, and ancestry) on a previously published model's ability to predict onset of drinking. A SNP variant of rs279871, in the chromosome 4 gene encoding gamma-aminobutyric acid receptor (GABRA2), was selected due to its associations with alcoholism in young adults and with behaviors that increased risk for early drinking.

METHODS

A subsample of 674 adolescents (ages 14-17) participating in the Collaborative Study on the Genetics of Alcoholism (COGA) was examined using a previously derived Cox proportional hazards model containing: 1) number of non-drinking related conduct disorder (CD) symptoms, 2) membership in a high-risk alcohol-dependent (AD) family, 3) most best friends drank (MBFD), 4) Achenbach Youth Self Report (YSR) externalizing score, and 5) YSR social problems score. The above covariates along with the SNP variant of GABRA2, rs279871, were added to this model. Five new prototype models were examined. The most parsimonious model was chosen based on likelihood ratio tests and model fit statistics.

RESULTS

The final model contained four of the five original predictors (YSR social problems score was no longer significant and hence dropped from subsequent models), the three covariates, and a recessive GABRA2 rs279871 TT genotype (two copies of the high-risk allele containing thymine). The model indicated that adolescents with the high-risk TT genotype were more likely to begin drinking than those without this genotype.

CONCLUSIONS

The joint effect of the gene (rs279871 TT genotype) and environment (MBFD) on adolescent alcohol initiation is additive, but not interactive, after controlling for behavior problems (CD and YSR externalizing score). This suggests that the impact of the high-risk TT genotype on the onset of drinking is affected by controlling for peer drinking and does not include genotype-by-environment interactions.

Unique and interactive effects of impulsivity facets on reckless driving and driving under the influence in a high-risk young adult sample

Risky driving behaviors are disproportionately high among young adults and impulsivity is a robust risk factor. Recent conceptualizations have proposed multidimensional facets of impulsivity comprised of negative urgency, premeditation, perseverance, sensation seeking, and positive urgency (UPPS-P model). Prior studies have found these facets are associated with risky driving behaviors in college student samples, but no prior studies have examined these facets in clinical samples. This study examined the unique and interactive effects of UPPS-P impulsivity facets on past-year risky driving behaviors in a sample of high-risk young adults (ages 18-30 years) with a history of substance use and antisocial behavior and their siblings (n=1,100). Multilevel Poisson regressions indicated that sensation seeking and negative urgency were uniquely and positively associated with both frequency of past-year reckless driving and driving under the influence. Moreover, lack of premeditation was uniquely and positively associated with reckless driving, whereas lack of perseverance was uniquely and positively associated with driving under the influence. Furthermore, lack of premeditation moderated and strengthened the positive association between sensation seeking and driving under the influence. These study findings suggest that assessing multiple facets of trait impulsivity could facilitate targeted prevention efforts among young adults with a history of externalizing psychopathology.

Risky driving and sexual behaviors as developmental outcomes of co-occurring substance use and antisocial behavior

PURPOSE

To examine the associations between substance use and antisocial behavior trajectories and seven risky behaviors over time.

METHOD

Data were collected from a high-risk sample of adolescents followed into young adulthood. Five trajectory classes, identified based on dual development of substance use and antisocial behavior symptoms, were used to predict three risky driving and four risky sexual behaviors.

RESULTS

In this high-risk sample (n=530), participants reported notably high overall rates of reckless driving (55.5%) and unprotected sex under the influence (44.8%) in the past year. Risky behaviors that are typically of low base rates in population-based studies were also elevated, with 8.8% reporting past-year driving under the influence (DUI) charge, 17.6% reporting lifetime sexually transmitted infection (STI), and 10.4% reporting lifetime injection drug use. The Dual Chronic class had the highest levels of all seven risky behaviors, and were 3-4 times more likely to report risky driving, lifetime STI, and injection drug use than the Relatively Resolved class. Rates of past-year reckless driving and DUI were elevated among classes with persistent antisocial behavior, whereas rates of DUI, DUI charge, and unprotected sex under the influence were elevated among classes with persistent substance use.

CONCLUSIONS

Young adults with persistent co-occurring substance use and antisocial behavior engage in multiple very costly risky behaviors. Differential associations between risky behaviors and trajectory classes highlight the need for targeted interventions.

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 inactivation of midkine modulates behavioural responses to ethanol possibly by enhancing GABA(A) receptor sensitivity to GABA(A) acting drugs

Midkine (MK) is a cytokine with important functions in dopaminergic neurons that is found upregulated in the prefrontal cortex of alcoholics. We have studied the behavioural effects of ethanol in MK genetically deficient (MK-/-) and wild type (MK+/+) mice. A low dose of ethanol (1.0g/kg), unable to cause conditioned place preference (CPP) in MK+/+ mice, induced a significant CPP in MK-/- mice, suggesting that MK prevents the rewarding effects of low doses of ethanol. However, this difference between genotypes is lost when a higher, rewarding, dose of ethanol (2.0g/kg) is used. Accordingly, the anxiolytic effects of 1.0mg/kg diazepam, other GABA(A) acting drug, were significantly enhanced in MK-/- mice compared to MK+/+ mice; however, 2.0mg/kg diazepam caused increased anxiolytic effects in MK+/+ mice. In addition, MK-/- mice showed a significant delayed recovery from ethanol (2.0g/kg)-induced ataxia whereas the sedative effects induced by ethanol (3.6g/kg), tested in a loss of righting reflex paradigm, were found to be similar in MK-/- and MK+/+ mice. The data indicate that MK differentially regulates the behavioural responses to ethanol. The results suggest that differences in the sensitivity of GABA(A) receptors to GABA(A) acting drugs caused by genetic inactivation of MK could underlie the different behavioural responses to ethanol in MK-/- mice. Overall, these results suggest that MK may be a novel genetic factor of importance in alcohol use disorders, and that potentiation of MK signalling pathway may be a promising therapeutic strategy in the treatment of these disorders.