Victor Hesselbrock

Genome-wide search for genes affecting the risk for alcohol dependence.

Alcohol dependence is a leading cause of morbidity and premature death. Several lines of evidence suggest a substantial genetic component to the risk for alcoholism: sibs of alcoholic probands have a 3-8 fold increased risk of also developing alcoholism, and twin heritability estimates of 50-60% are reported by contemporary studies of twins. We report on the results of a six-center collaborative study to identify susceptibility loci for alcohol dependence. A genome-wide screen examined 291 markers in 987 individuals from 105 families. Two-point and multipoint nonparametric linkage analyses were performed to detect susceptibility loci for alcohol dependence. Multipoint methods provided the strongest suggestions of linkage with susceptibility loci for alcohol dependence on chromosomes 1 and 7, and more modest evidence for a locus on chromosome 2. In addition, there was suggestive evidence for a protective locus on chromosome 4 near the alcohol dehydrogenase genes, for which protective effects have been reported in Asian populations.

Variants in Nicotinic Receptors and Risk for Nicotine Dependence

OBJECTIVE A recent study provisionally identified numerous genetic variants as risk factors for the transition from smoking to the development of nicotine dependence, including an amino acid change in the alpha5 nicotinic cholinergic receptor (CHRNA5). The purpose of this study was to replicate these findings in an independent data set and more thoroughly investigate the role of genetic variation in the cluster of physically linked nicotinic receptors, CHRNA5-CHRNA3-CHRNB4, and the risk of smoking. METHOD Individuals from 219 European American families (N=2,284) were genotyped across this gene cluster to test the genetic association with smoking. The frequency of the amino acid variant (rs16969968) was studied in 995 individuals from diverse ethnic populations. In vitro studies were performed to directly test whether the amino acid variant in the CHRNA5 influences receptor function. RESULTS A genetic variant marking an amino acid change showed association with the smoking phenotype (p=0.007). This variant is within a highly conserved region across nonhuman species, but its frequency varied across human populations (0% in African populations to 37% in European populations). Furthermore, functional studies demonstrated that the risk allele decreased response to a nicotine agonist. A second independent finding was seen at rs578776 (p=0.003), and the functional significance of this association remains unknown. CONCLUSIONS This study confirms that at least two independent variants in this nicotinic receptor gene cluster contribute to the development of habitual smoking in some populations, and it underscores the importance of multiple genetic variants contributing to the development of common diseases in various populations.

A genome-wide association study of alcohol dependence

Excessive alcohol consumption is one of the leading causes of preventable death in the United States. Approximately 14% of those who use alcohol meet criteria during their lifetime for alcohol dependence, which is characterized by tolerance, withdrawal, inability to stop drinking, and continued drinking despite serious psychological or physiological problems. We explored genetic influences on alcohol dependence among 1,897 European-American and African-American subjects with alcohol dependence compared with 1,932 unrelated, alcohol-exposed, nondependent controls. Constitutional DNA of each subject was genotyped using the Illumina 1M beadchip. Fifteen SNPs yielded P < 10−5, but in two independent replication series, no SNP passed a replication threshold of P < 0.05. Candidate gene GABRA2, which encodes the GABA receptor α2 subunit, was evaluated independently. Five SNPs at GABRA2 yielded nominal (uncorrected) P < 0.05, with odds ratios between 1.11 and 1.16. Further dissection of the alcoholism phenotype, to disentangle the influence of comorbid substance-use disorders, will be a next step in identifying genetic variants associated with alcohol dependence.

Linkage disequilibrium between the beta frequency of the human EEG and a GABAA receptor gene locus.

Human brain oscillations represent important features of information processing and are highly heritable. A common feature of beta oscillations (13–28 Hz) is the critical involvement of networks of inhibitory interneurons as pacemakers, gated by γ-aminobutyric acid type A (GABAA) action. Advances in molecular and statistical genetics permit examination of quantitative traits such as the beta frequency of the human electroencephalogram in conjunction with DNA markers. We report a significant linkage and linkage disequilibrium between beta frequency and a set of GABAA receptor genes. Uncovering the genes influencing brain oscillations provides a better understanding of the neural function involved in information processing.

Allelic and haplotypic association of GABRA2 with alcohol dependence.

Alcohol dependence is a highly prevalent disorder that is associated with serious morbidity and mortality. Because the GABAA neurotransmitter receptor is an important mediator for several behavioral effects of alcohol, genes encoding GABA‐related proteins are functional candidates to influence risk of alcohol dependence. Two genome‐wide scans showed linkage of alcohol dependence to a region on chromosome 4p, which contains a cluster of genes encoding GABAA receptor subunits. A recent effort to fine map that region showed a haplotypic association of alcohol dependence to the gene encoding the GABAA receptor α‐2 subunit (GABRA2). We examined 10 single nucleotide polymorphisms (SNPs) spanning the coding region of this gene in samples of European American subjects with alcohol dependence (n = 446), and controls (n = 334) screened to exclude substance use disorders. There was evidence of association to alcohol dependence for seven adjacent markers spanning 98,000 bp in the middle and 3′‐portion of the GABRA2 gene (range of P‐values = 0.008–0.03). When the subset of the alcohol‐dependent subjects excluding those with a diagnosis of cocaine or opioid dependence or major depressive episode (n = 198) was examined, the strength of the association was increased across these 7 SNPs (range of P‐values = 0.002–0.007). Two common haplotypes in this region accounted for 90.8% of chromosomes. The more common haplotype was present in 55.6% of control group chromosomes versus 48.2% of alcohol‐dependent subjects (P = 0.007) and 45.8% of subjects with alcohol dependence but no co‐morbid drug dependence or depression (P = 0.003). These findings replicate and extend recently reported findings, which together underscore the potential contribution of polymorphic variation at the GABRA2 locus to the risk for alcohol dependence.

Genome-wide association study of alcohol dependence implicates a region on chromosome 11

BACKGROUND Alcohol dependence is a complex disease, and although linkage and candidate gene studies have identified several genes associated with the risk for alcoholism, these explain only a portion of the risk. METHODS We carried out a genome-wide association study (GWAS) on a case-control sample drawn from the families in the Collaborative Study on the Genetics of Alcoholism. The cases all met diagnostic criteria for alcohol dependence according to the Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition; controls all consumed alcohol but were not dependent on alcohol or illicit drugs. To prioritize among the strongest candidates, we genotyped most of the top 199 single nucleotide polymorphisms (SNPs) (p < or = 2.1 x 10(-4)) in a sample of alcohol-dependent families and performed pedigree-based association analysis. We also examined whether the genes harboring the top SNPs were expressed in human brain or were differentially expressed in the presence of ethanol in lymphoblastoid cells. RESULTS Although no single SNP met genome-wide criteria for significance, there were several clusters of SNPs that provided mutual support. Combining evidence from the case-control study, the follow-up in families, and gene expression provided strongest support for the association of a cluster of genes on chromosome 11 (SLC22A18, PHLDA2, NAP1L4, SNORA54, CARS, and OSBPL5) with alcohol dependence. Several SNPs nominated as candidates in earlier GWAS studies replicated in ours, including CPE, DNASE2B, SLC10A2, ARL6IP5, ID4, GATA4, SYNE1, and ADCY3. CONCLUSIONS We have identified several promising associations that warrant further examination in independent samples.

Genetic variation in the CHRNA5 gene affects mRNA levels and is associated with risk for alcohol dependence.

Alcohol dependence frequently co-occurs with cigarette smoking, another common addictive behavior. Evidence from genetic studies demonstrates that alcohol dependence and smoking cluster in families and have shared genetic vulnerability. Recently a candidate gene study in nicotine dependent cases and nondependent smoking controls reported strong associations between a missense mutation (rs16969968) in exon 5 of the CHRNA5 gene and a variant in the 3′-UTR of the CHRNA3 gene and nicotine dependence. In this study we performed a comprehensive association analysis of the CHRNA5–CHRNA3–CHRNB4 gene cluster in the Collaborative Study on the Genetics of Alcoholism (COGA) families to investigate the role of genetic variants in risk for alcohol dependence. Using the family-based association test, we observed that a different group of polymorphisms, spanning CHRNA5-CHRNA3, demonstrate association with alcohol dependence defined by Diagnostic and Statistical Manual of Mental Disorders, 4th edn (DSM-IV) criteria. Using logistic regression we replicated this finding in an independent case-control series from the family study of cocaine dependence. These variants show low linkage disequilibrium with the SNPs previously reported to be associated with nicotine dependence and therefore represent an independent observation. Functional studies in human brain reveal that the variants associated with alcohol dependence are also associated with altered steady-state levels of CHRNA5 mRNA.

Quantitative trait loci analysis of human event-related brain potentials: P3 voltage

The P3 event-related brain potential (ERP) is a positive-going voltage change of scalp-recorded electroencephalographic activity that occurs between 300-500 ms after stimulus onset. It is elicited when a stimulus is perceived, memory operations are engaged, and attentional resources are allocated toward its processing. Because this ERP component reflects fundamental cognitive processing, it has found wide utility as an assessment of human mental function in basic and clinical studies. In particular, P3 attributes are heritable and have demonstrated considerable promise as a means to identify individuals at genetic risk for alcoholism. We have conducted a quantitative linkage analysis on a large sample from families with a high density of affected individuals. The analyses suggest that several regions of the human genome contain genetic loci related to the generation of the P3 component of the ERP, which are possible candidate loci underlying the functional organization of human neuroelectric activity.

P300 decrements in teenagers with conduct problems: implications for substance abuse risk and brain development

BACKGROUND The purpose of this study was to evaluate the effects of conduct disorder problems, family history, gender, and age on P300 electroencephalographic potentials in teenagers. METHODS The 257 subjects, aged 15 to 20 years, were assigned to one of twelve groups defined by the crossing of three between-subjects factors: 1) gender; 2) ranking below vs above the median number of conduct disorder problems for their gender; and 3) no family history of alcohol or drug dependence vs familial alcohol dependence vs familial heroin or cocaine dependence. RESULTS P300 amplitude was smaller among subjects reporting a greater number of conduct problems prior to age 15 vs those reporting fewer problems of this type. No family history effects were detected. Another set of analyses examined the effects of age on conduct problem-related decrements in P300. Smaller P300 amplitudes within the posterior scalp region were associated with a greater number of conduct problems among subjects younger than 16.5 years. Among subjects greater than this median age, the effects of these behaviors were only apparent over the frontal scalp. CONCLUSIONS It is concluded that P300 decrements previously attributed to familial alcohol/substance dependence might be the result of a coincident increase in the prevalence of conduct disorder problems. The analysis of age interactions suggests that P300 amplitude decrements observed at posterior scalp sites among subjects with more conduct problems disappear at approximately 16 to 17 years of age. After that age, decrements in frontal brain function may begin to emerge in the subset of conduct problem subjects who are at risk for developing adult antisocial personality disorder.

Frontal P300 decrements, alcohol dependence, and antisocial personality disorder.

BACKGROUND The purpose of this study was to examine the independent and interactive effects of alcohol dependence, antisocial personality disorder (ASPD), and age on brain function. METHODS P300 event-related potentials (ERPs) were recorded from 393 alcohol-dependent and 170 non-alcohol-dependent adults while they performed a visual oddball task. The two subject groups were further subdivided based upon age and the presence/absence of ASPD. RESULTS Alcohol dependence was associated with a significant P300 amplitude decrement at anterior electrode sites only. Antisocial personality disorder was also associated with reduced P300 amplitudes at anterior electrode sites; however, the effects were only significant among subjects 30 years of age or younger. To validate this association between ASPD and P300 amplitude a correlational analysis was performed; the correlation between anterior P300 amplitude and the total number of childhood conduct disorder and adult ASPD symptoms was significant. CONCLUSIONS The P300 amplitude decrement found at anterior electrode sites among subjects with ASPD is consistent with the results of numerous ERP, neuroimaging, or neuropsychologic studies of anterior brain function. Our study is unique in suggesting that the effects of ASPD on anterior brain function are best detected during early adulthood. The study also suggests that the detrimental neurophysiologic effects of alcohol dependence predominantly involve the anterior brain.