Gamma-aminobutyric acid system genes--no evidence for a role in alcohol use and abuse in a community-based sample

Alcohol in Psoriasis-From Bench to Bedside

Alcohol affects the symptoms, compliance and comorbidities as well as the safety and efficacy of treatments in psoriatic patients. In this review, we aim to summarize and link clinical observations with a molecular background, such as signaling pathways at the cellular level and genetic variations, and to provide an overview of how this knowledge could influence our treatment selection and patient management.

Recent advances in genetic studies of alcohol use disorders

PURPOSE OF REVIEW

Alcohol use disorder (AUD) is a complex genetic disorder with very high heritability. This polygenic disorder not only results in increased morbidity and mortality, it is also a substantial social and economic burden on families and the nation. For past three decades, several genetic studies were conducted to identify genes and pathways associated with AUD. This review aims to summarize past efforts and recent advances in genetic association studies of AUD and related traits.

RECENT FINDINGS

Initial genetic association studies achieved a limted success and suffered from low power due to small sample sizes. AUD is a polygenic trait and data from several thousands individuals was required to identify the genetic factors of small effect sizes. The scenario changed recently with technological advances and significant reduction in cost of the genome wide association analyses (GWAS). This enabled researchers to generate genomic data on mega biobanks and cohorts with access to extensive clinical and non-clinical phenotypes. Public access to data from biobanks and collaborative efforts of researchers lead to identification of several novel loci associated with AUDs and related traits. Efforts are now underway to identify the causal variants under the GWAS loci to identify target genes and biological mechanisms underpining AUDs. Many GWAS variants occur in promoter or enhancer regions of the genes and are involved in regulation of gene expression of causal genes. This, large amounts of "omics" data from projects such as "ENCODE", RoadMap and GTEx is also helping researchers to integrate "multi-omics" data to interpret functional significance of GWAS variants.

SUMMARY

With current review, we aim to present the recent advances in genetic and molecular studies of AUDs. Recent successes in genetic studies of AUDs will definetely motivate researchers and lead to better therapeutic interventions for this complex disorder.

Effects of Common and Rare Chromosome 4 GABAergic Gene Variation on Alcohol Use and Antisocial Behavior

OBJECTIVE

Epidemiological estimates suggest that nearly half of individuals diagnosed with alcohol use disorder will be diagnosed with another mental health disorder, with strong associations involving other externalizing disorders. Molecular genetic studies investigating the relation between alcohol use disorder and externalizing behaviors (e.g., antisocial behavior) have focused on a cluster of chromosome 4 γ-aminobutyric acid (GABA) receptor genes (GABRG1-A2-A4-B1) but have generated varying results.

METHOD

The current study examined associations between common and rare variation in this region with alcohol use disorder and antisocial behavior using genetic sequencing data. Specifically, the University of California at San Francisco Family Alcoholism Sample (n = 1,610; 62% female) was used to conduct common and rare variant association tests in the GABRG1-A2-A4-B1 cluster with DSM-5 alcohol use disorder symptom counts, antisocial behavior, and a product term representing their interaction.

RESULTS

Gene-based analyses of rare variation resulted in a significant association between rare GABRA2 variation and the interaction term. Single-variant analysis yielded only nominally significant associations. The strongest association for alcohol use disorder (rs3756007) was located in GABRA2, the strongest association for antisocial behavior (rs11941860) was located in GABRG1, and the interaction term yielded top associations in GABRA2 (rs2119183) and the intergenic region between GABRA2 and GABRG1 (rs536599). Common and rare variant associations for the interaction remained similar when covarying for the effects of the other type of variation, suggesting that the significant rare variant signal is independent of common variant contributions.

CONCLUSIONS

The present study suggests that both rare and common variant associations in GABRA2 confer risk for alcohol use disorder and antisocial behaviors, indicating a potential liability toward externalizing behavior more broadly.

GABAA receptor polymorphisms in alcohol use disorder in the GWAS era

Alcohol use disorder (AUD) is a chronic, relapsing, neuro-psychiatric illness of high prevalence and with a serious public health impact worldwide. It is complex and polygenic, with a heritability of about 50%, and influenced by environmental causal heterogeneity. Risk factors associated with its etiology have a genetic component. GABA (γ-aminobutyric acid) is a major inhibitory neurotransmitter in mammalian brain. GABA_A receptors are believed to mediate some of the physiological and behavioral actions of alcohol. In this critical review, relevant genetic terms and type and methodology of the genetic studies are briefly explained. Postulated candidate genes that encode subunits of GABA_A receptors, with all the reported SNPs, are presented. Genetic studies and meta-analyses examining polymorphisms of the GABA_A receptor and their association with AUD predisposition are presented. The data are critically examined with reference to recent GWAS studies that failed to show relations between GABA_A receptors and AUD. Restrictions and perspectives of the different findings are discussed.

Intergenerational continuity in parents' and adolescents' externalizing problems: The role of life events and their interaction with GABRA2

We examine whether parental externalizing behavior has an indirect effect on adolescent externalizing behavior via elevations in life events, and whether this indirect effect is further qualified by an interaction between life events and adolescents' GABRA2 genotype (rs279871). We use data from 2 samples: the Child Development Project (CDP; n = 324) and FinnTwin12 (n = 802). In CDP, repeated measures of life events, mother-reported adolescent externalizing, and teacher-reported adolescent externalizing were used. In FinnTwin12, life events and externalizing were assessed at age 14. Parental externalizing was indexed by measures of antisocial behavior and alcohol problems or alcohol dependence symptoms in both samples. In CDP, parental externalizing was associated with more life events, and the association between life events and subsequent adolescent externalizing varied as a function of GABRA2 genotype (p ≤ .05). The association between life events and subsequent adolescent externalizing was stronger for adolescents with 0 copies of the G minor allele compared to those with 1 or 2 copies of the minor allele. Parallel moderation trends were observed in FinnTwin12 (p ≤ .11). The discussion focuses on how the strength of intergenerational pathways for externalizing psychopathology may differ as a function of adolescent-level individual differences.

Heritability and molecular-genetic basis of resting EEG activity: a genome-wide association study

Several EEG parameters are potential endophenotypes for different psychiatric disorders. The present study consists of a comprehensive behavioral- and molecular-genetic analysis of such parameters in a large community sample (N = 4,026) of adolescent twins and their parents, genotyped for 527,829 single nucleotide polymorphisms (SNPs). Biometric heritability estimates ranged from .49 to .85, with a median of .78. The additive effect of all SNPs (SNP heritability) varied across electrodes. Although individual SNPs were not significantly associated with EEG parameters, several genes were associated with delta power. We also obtained an association between the GABRA2 gene and beta power (p < .014), consistent with findings reported by others, although this did not survive Bonferroni correction. If EEG parameters conform to a largely polygenic model of inheritance, larger sample sizes will be required to detect individual variants reliably.

Genome-wide association data suggest ABCB1 and immune-related gene sets may be involved in adult antisocial behavior

Adult antisocial behavior (AAB) is moderately heritable, relatively common and has adverse consequences for individuals and society. We examined the molecular genetic basis of AAB in 1379 participants from a case-control study in which the cases met criteria for alcohol dependence. We also examined whether genes of interest were expressed in human brain. AAB was measured using a count of the number of Antisocial Personality Disorder criteria endorsed under criterion A from the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV). Participants were genotyped on the Illumina Human 1M BeadChip. In total, all single-nucleotide polymorphisms (SNPs) accounted for 25% of the variance in AAB, although this estimate was not significant (P=0.09). Enrichment tests indicated that more significantly associated genes were over-represented in seven gene sets, and most were immune related. Our most highly associated SNP (rs4728702, P=5.77 × 10(-7)) was located in the protein-coding adenosine triphosphate-binding cassette, sub-family B (MDR/TAP), member 1 (ABCB1). In a gene-based test, ABCB1 was genome-wide significant (q=0.03). Expression analyses indicated that ABCB1 was robustly expressed in the brain. ABCB1 has been implicated in substance use, and in post hoc tests we found that variation in ABCB1 was associated with DSM-IV alcohol and cocaine dependence criterion counts. These results suggest that ABCB1 may confer risk across externalizing behaviors, and are consistent with previous suggestions that immune pathways are associated with externalizing behaviors. The results should be tempered by the fact that we did not replicate the associations for ABCB1 or the gene sets in a less-affected independent sample.

Endophenotypes for Alcohol Use Disorder: An Update on the Field

The endophenotype concept was first proposed as a strategy to use (purportedly) genetically simpler phenotypes in gene identification studies for psychiatric disorders, and is distinct from the closely related concept of intermediate phenotypes. In the area of alcohol use disorder (AUD) research, two candidate endophenotypes have produced replicable genetic associations: level of response to alcohol and neurophysiology markers (e.g., event-related oscillations and event-related potentials). Additional candidate endophenotypes from the cognitive, sensory, and neuroimaging literatures show promise, although more evidence is needed to fully evaluate their potential utility. Translational approaches to AUD endophenotypes have helped characterize the underlying neurobiology and genetics of AUD endophenotypes and identified relevant pharmacological interventions. Future research that capitalizes on the polygenic nature of endophenotypes and emphasizes endophenotypes that may change across development will enhance the usefulness of this concept to understand the genetically-influenced pathways toward AUD.

Knowns and unknowns for psychophysiological endophenotypes: integration and response to commentaries

We review and summarize seven molecular genetic studies of 17 psychophysiological endophenotypes that comprise this special issue of Psychophysiology, address criticisms raised in accompanying Perspective and Commentary pieces, and offer suggestions for future research. Endophenotypes are polygenic, and possibly influenced by rare genetic variants. Because they are not simpler genetically than clinical phenotypes, they are unlikely to assist gene discovery for psychiatric disorder. Once genetic variants for clinical phenotypes are identified, associated endophenotypes are likely to provide valuable insights into the psychological and neural mechanisms important to disorder pathology. This special issue provides a foundation for informed future steps in endophenotype genetics, including the formation of large sample consortia capable of fleshing out the many genetic variants contributing to individual differences in psychophysiological measures.