A polygenic risk score associated with measures of depressive symptoms among older adults

It has been suggested that depression is a polygenic trait, arising from the influences of multiple loci with small individual effects. The aim of this study is to generate a polygenic risk score (PRS) to examine the association between genetic variation and depressive symptoms. Our analytic sample included N = 10,091 participants aged 50 and older from the Health and Retirement Study (HRS). Depressive symptoms were measured by Center for Epidemiological Studies-Depression scale (CESD) scores assessed on up to nine occasions across 18 years. We conducted a genome-wide association analysis for a discovery set (n = 7,000) and used the top 11 single-nucleotide polymorphisms, all with p < 10(-5) to generate a weighted PRS for our replication sample (n = 3,091). Results showed that the PRS was significantly associated with mean CESD score in the replication sample (β = .08, p = .002). The R(2) change for the inclusion of the PRS was .003. Using a multinomial logistic regression model, we also examined the association between genetic risk and chronicity of high (4+) CESD scores. We found that a one-standard-deviation increase in PRS was associated with a 36 percent increase in the odds of having chronically high CESD scores relative to never having had high CESD scores. Our findings are consistent with depression being a polygenic trait and suggest that the cumulative influence of multiple variants increases an individual's susceptibility for chronically experiencing high levels of depressive symptoms.

Measurement invariance of internalizing and externalizing behavioral syndrome factors in a non-Western sample

This study examined the measurement structure of Child Behavior Checklist internalizing and externalizing syndrome scales in 1,146 eleven-year-old children from a birth cohort in Mauritius. We tested for measurement invariance at configural, metric, and scalar levels by gender and religioethnicity (Creole, Hindu, Muslim). A pared-down model representing five primary factors and two secondary factors met all three forms of invariance, supporting the validity of their use for group comparisons among Mauritian children. As rated by their parents, girls were higher than boys on Somatic Complaints and lower on Aggressive Behavior, Attention Problems, and Externalizing. Creoles were higher than Muslims and Hindus on all seven factors. Hindus were higher than Muslims on Somatic Complaints and lower on Aggressive Behavior. To our knowledge, this is the first study to demonstrate strict invariance of a Child Behavior Checklist-based internalizing and externalizing factor structure among subgroups within a society.

Reexamining the association between fertility and longevity: testing the disposable soma theory in a modern human sample of twins

The disposable soma theory proposes a trade-off between fertility and longevity but existing findings on this association have been mixed. This study used data from 15,622 twins born between 1901 and 1925 ascertained from the population-based Swedish Twin Registry to test the child-longevity association and whether it is accounted for by individual-level factors or by genetic and environmental factors shared by family members. Based on survival analysis, both women and men with children had significantly longer survival relative to the childless, with a slightly higher relative advantage in men. Adjustments for demographic factors and cotwin fertility did not mediate the parenting-survival association, indicating that this association is attributable to individual-level factors associated with fertility rather than family-level environmental or genetic factors shared by cotwins. These results, derived from a large, population-based sample, are inconsistent with the disposable soma theory as applied to modern human populations.

Evidence for multiple genetic factors underlying the DSM-IV criteria for alcohol dependence

To determine the number of genetic factors underlying the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria for alcohol dependence (AD), we conducted structural equation twin modeling for seven AD criteria, plus two summary screening questions, in 7133 personally interviewed male and female twins from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders, who reported lifetime alcohol consumption. The best-fit twin model required three genetic and two unique environmental common factors, and criterion-specific unique environmental factors. The first genetic factor was defined by high loadings for the probe question about quantity and frequency of alcohol consumption, and tolerance criterion. The second genetic factor loaded strongly on the probe question about self-recognition of alcohol-related problems and AD criteria for loss of control, desire to quit, preoccupation and activities given up. The third genetic factor had high loadings for withdrawal and continued use despite the problems criteria. Genetic factor scores derived from these three factors differentially predicted patterns of comorbidity, educational status and other historical/clinical features of AD. The DSM-IV syndrome of AD does not reflect a single dimension of genetic liability, rather, these criteria reflect three underlying dimensions that index risk for: (i) tolerance and heavy use; (ii) loss of control with alcohol associated social dysfunction and (iii) withdrawal and continued use despite problems. While tentative and in need of replication, these results, consistent with the rodent literature, were validated by examining predictions of the genetic factor scores and have implications for gene-finding efforts in AD.

Shared genetic contributions to early-onset drinking and drinking to cope motives

INTRODUCTION

Recent evidence from empirical studies indicates that individuals who begin drinking at an early age may be more likely to use alcohol to cope with negative mood states and stress; however, the mechanisms underlying this association are unclear. One possibility is that early drinking directly increases risk for drinking to cope (DTC). Alternatively, the association between early drinking and DTC may be indirect, attributable to overlapping genetic or environmental factors. No prior genetically informative study has investigated the sources of covariation underlying the early-onset drinking-DTC association.

METHOD

Early-onset drinking (before age 15) was assessed using structured clinical interviews in a sample of 7130 male and female participants aged 19-56 years from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD, Kendler & Prescott, 2006). DTC was assessed using the mood management scale of the alcohol use inventory (Horn & Wanberg, 1983). The sources of the covariation between early first drink and DTC were estimated using bivariate twin modeling.

RESULTS

Early drinking onset was reported by 28% of males and 16% of females and was associated with significantly higher DTC scores (phenotypic correlation: males = .19, females = .22). Results from bivariate twin models indicated that the association between early-onset drinking and DTC was completely attributable to shared genetic factors that contribute to both behaviors.

CONCLUSIONS

Greater DTC among early-onset drinkers may not reflect a direct causal process, as shared biological pathways may explain vulnerability to stress-related drinking seen among early-onset drinkers.

Interactive effects of childhood maltreatment and recent stressful life events on alcohol consumption in adulthood

OBJECTIVE

Childhood maltreatment is associated with early alcohol use initiation, alcohol-related problem behaviors, and alcohol use disorders in adulthood. Heavy drinking risk among individuals exposed to childhood maltreatment could be partly attributable to stress sensitization, whereby early adversity leads to psychobiological changes that heighten sensitivity to subsequent stressors and increase risk for stress-related drinking. We addressed this issue by examining whether the association between past-year stressful life events and past-year drinking density, a weighted quantity-frequency measure of alcohol consumption, was stronger among adults exposed to childhood maltreatment.

METHOD

Drinking density, stressful life events, and child maltreatment were assessed using structured clinical interviews in a sample of 4,038 male and female participants ages 20-58 years from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders. Stress sensitization was examined using hierarchical multiple regression analyses to test whether stressful events moderated the association between maltreatment and drinking density. Analyses were stratified by sex and whether the impact was different for independent stressful events or dependent stressful events as related to a participant's actions.

RESULTS

Independent stressful events were associated with heavier drinking density among women exposed to maltreatment. In contrast, drinking density was roughly the same across independent stressful life events exposure among women not exposed to maltreatment. There was little evidence for Maltreatment × Independent Stressor interactions in men or Maltreatment × Dependent Stressor interactions in either gender.

CONCLUSIONS

Early maltreatment may have direct effects on vulnerability to stress-related drinking among women, particularly in association with stressors that are out of one's control.

Meta-analyses of genome-wide linkage scans of anxiety-related phenotypes

Genetic factors underlying trait neuroticism, reflecting a tendency towards negative affective states, may overlap genetic susceptibility for anxiety disorders and help explain the extensive comorbidity amongst internalizing disorders. Genome-wide linkage (GWL) data from several studies of neuroticism and anxiety disorders have been published, providing an opportunity to test such hypotheses and identify genomic regions that harbor genes common to these phenotypes. In all, 11 independent GWL studies of either neuroticism (n=8) or anxiety disorders (n=3) were collected, which comprised of 5341 families with 15 529 individuals. The rank-based genome scan meta-analysis (GSMA) approach was used to analyze each trait separately and combined, and global correlations between results were examined. False discovery rate (FDR) analysis was performed to test for enrichment of significant effects. Using 10 cM intervals, bins nominally significant for both GSMA statistics, P(SR) and P(OR), were found on chromosomes 9, 11, 12, and 14 for neuroticism and on chromosomes 1, 5, 15, and 16 for anxiety disorders. Genome-wide, the results for the two phenotypes were significantly correlated, and a combined analysis identified additional nominally significant bins. Although none reached genome-wide significance, an excess of significant P(SR)P-values were observed, with 12 bins falling under a FDR threshold of 0.50. As demonstrated by our identification of multiple, consistent signals across the genome, meta-analytically combining existing GWL data is a valuable approach to narrowing down regions relevant for anxiety-related phenotypes. This may prove useful for prioritizing emerging genome-wide association data for anxiety disorders.

The effects of age at drinking onset and stressful life events on alcohol use in adulthood: a replication and extension using a population-based twin sample

BACKGROUND

A study by Dawson and colleagues (Alcohol Clin Exp Res 2007; 31:69) using data from National Epidemiologic Survey on Alcohol and Related Condition found earlier drinking onset age, and higher levels of past-year stressful life events (SLE) were associated with higher past-year alcohol consumption. The aims of our study were as follows: (i) to attempt to replicate this interaction; (ii) to extend it by examining sex and event dependence as potential moderators of the effect; and (iii) to estimate the roles of genetic and environmental factors in mediating the overlap of early drinking onset and SLE in their relations with alcohol consumption.

METHODS

Data were from 1,382 female and 2,218 male drinkers interviewed as part of the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders. Regression models were used to evaluate the main and interactive effects of early drinking onset and moderate or severe past-year SLE on past-year drinking density (PYDD), a weighted quantity-frequency measure of alcohol consumption. Analyses adjusted for demographic covariates and were stratified by sex and whether SLE were independent or dependent on the person's actions, as rated by interviewers. Structural twin models were used to estimate the degree to which early drinking onset, SLE, and their interaction accounted for additive genetic, common environmental and individual-specific variance in PYDD.

RESULTS

We replicated the prior finding of a main effect of higher alcohol consumption among individuals reporting earlier drinking onset. Age at drinking onset accounted for about 5% of the variation in PYDD, and this association was mostly attributable to overlapping genetic influences. Evidence for an interaction between onset age and SLE was generally weak, possibly because of lower power and other methodological differences from Dawson and colleagues' study. However, there was some evidence consistent with an interaction of higher PYDD among early drinking men who experienced independent SLE and early drinking women with dependent SLE.

CONCLUSIONS

We confirmed prior findings of an association between early age at drinking onset with higher past-year drinking among young- and middle-aged adults and found limited evidence supporting a replication for higher stress-related drinking among early-onset drinkers. The association is consistent with early onset and stress-related drinking being attributable to overlapping genetic liability. Among early drinkers, our results suggest sex differences in consumption with regard to event dependence.

Clarifying the relationship between religiosity and psychiatric illness: the impact of covariates and the specificity of buffering effects

Previous analyses in a large population-based sample of female twins indicated that three dimensions of religiosity – personal devotion, personal conservatism and institutional conservatism – were, in different ways, significantly related to current depressive symptoms and substance use and lifetime psychiatric and substance use disorders. Furthermore, personal devotion, but neither personal conservatism nor institutional conservatism, buffered the depressogenic effects of stressful life events (SLEs). We here explore further these results, using linear, logistic and Cox regression models. Eight personality and six demographic variables had distinct patterns of association with the three dimensions. Personal devotion was positively associated with years of education, age, and optimism and negatively correlated with neuroticism. Personal conservatism was negatively associated with education, income, age, mastery and positively correlated with neuroticism. Institutional conservatism was negatively correlated with self-esteem and parental education. Covarying for these 14 variables produced little change in their association with psychiatric and substance use outcomes. The impact of the dimensions of religiosity differed as a function of the SLE category. High levels of both personal devotion and institutional conservatism protected against the depressogenic effects of death and personal illness. High levels of personal conservatism were associated with increased sensitivity to relationship problems. These results suggest that the association between religiosity and low risk for symptoms of depression and substance use may be in part causal. The relationship between dimensions of religiosity and response to SLEs is complex but probably of importance in clarifying the nature of the coping process.

The Differential Heritability of Regular Tobacco Use Based on Method of Administration

Several large studies have demonstrated that the liability to smoke cigarettes is strongly genetically influenced. However, the role of genetic and environmental risk factors in the use of other common forms of tobacco use has yet to be studied. Data on the regular use of cigarettes, cigars, pipes, dip (moist snuff), and chewing tobacco from 2634 male twins were analyzed with ACE structural equation models. Twin similarity for regular cigarette and dip use was largely genetic in origin. However, twin resemblance for chewing tobacco was just about equally the result of genes and shared environment, and twin similarity for use of pipes and cigars was entirely the result of shared environmental factors. Thus, the genetic influences on the liability for regular tobacco use appear to vary based on tobacco type. The causes for the use of different forms of tobacco are complex and worthy of further study.

Different phenotypic and genotypic presentations in alcohol dependence: age at onset matters

OBJECTIVE

Several theoretical typology models have been proposed to classify alcoholism into more homogeneous subtypes using various criteria, for which age at onset of alcohol dependence is shared across many models. We investigated the evidence for the distinction between early- versus late-onset alcoholism by examining relevant phenotypic and genotypic variables.

METHOD

Data are from 1,248 individuals with alcohol dependence, who were interviewed to collect detailed clinical information. Early versus late onset of alcohol dependence was defined by the age at onset of 22 years. Odds ratio (OR) and Cohen's d were calculated as effect size for comparisons of clinical features between the two groups. We adjusted interviewed age and gender in logistic regression models. Case-control genetic analyses were conducted for the association between HTR1B, SLC6A4, DRD2, and OPRμ1 genes and subgroups of alcohol dependence using a sample of 530 controls screened for alcohol problems.

RESULTS

Early-onset alcoholism exhibited significantly (p < .01) different clinical characteristics from late-onset alcoholism, including higher severity in alcohol dependence symptoms (d = 0.22) and maximum drinking quantity within 24 hours (d = 0.40), more rapid progression from regular drinking to meet alcohol dependence diagnosis (d = 1.73), higher expectancies for alcohol (d = 0.22-0.47), more comorbidity with externalizing disorders (ORs = 2.8-2.9), and greater prevalence of family alcohol use problems (d = 0.26-0.43). In addition, markers in the HTR1B and OPRμ1 genes showed genetic associations with subgroups of alcohol dependence (ORs = 1.5-2.4).

CONCLUSIONS

Our findings support that subgroups of alcohol dependence defined by onset age have phenotypic and genetic differences. The early-onset subgroup had more severe features for almost every aspect we examined. Coupled with genetic association findings, age at onset of alcohol dependence may serve as a simple but important clinical marker with implications for future etiological research and intervention.

Heritability of Parkinson disease in Swedish twins: a longitudinal study

Previous twin studies report no heritability of Parkinson's disease (PD) based on cross sectional information. Here, we apply a longitudinal design and re-evaluate cross sectional data in the population-based Swedish Twin Registry (STR) using clinical as well as hospital discharge and cause of death diagnoses. In the longitudinal analyses (based on 46,436 individuals), we identified 542 twins with PD and 65 twins with Parkinsonism. Concordance rates for PD were 11% for monozygotic and 4% for same-sexed dizygotic twin pairs, with a heritability estimate of 34%. Concordance rates for PD or parkinsonism were 13% for monozygotic and 5% for same-sexed dizygotic twin pairs, with a heritability estimate of 40%. In the cross sectional analyses (based on 49,814 individuals), we identified 287 twins with PD and 79 twins with parkinsonism. Concordance rates for PD were 4% for monozygotic and same-sexed dizygotic twin pairs and 0 for opposite-sexed twin pairs. Concordance rates for PD or parkinsonism were somewhat higher but the heritability estimate was nonsignificant. Our longitudinal analyses demonstrate that PD and parkinsonism are modestly heritable.

A twin association study of nicotine dependence with markers in the CHRNA3 and CHRNA5 genes

Twin and family studies have provided overwhelming evidence for the genetic basis of individual differences in tobacco initiation (TI), regular smoking (RS) and nicotine dependence (ND). However, only a few genes have been reliably associated with ND. We used a finite mixture distribution model to examine the significance and effect size of the association of previously identified and replicated specific variants in the CHRNA5 and CHRNA3 receptor genes with ND, against the background of genetic and environmental risk factors for ND. We hypothesize that additional phenotypic information in relatives who have not been genotyped can be used to increase the power of detecting the genetic variant. The nicotine measures were assessed by personal interview in female, male and opposite sex twin pairs (N = 4,153) from the population-based Virginia Twin Registry. Three SNPs in the CHRNA5 and CHRNA3 receptor genes, previously shown to be significantly associated with ND in this sample, were replicated in the augmented analyses; they accounted for less than one percent of the genetic variance in liability to ND, which is estimated to be over 50% of the phenotypic variance. The significance of these effects was increased by adding twins with phenotype but without genotype data, but gains are limited and variable. The SNPs associated with ND did not show a significant association with either TI or RS and appear to be specific to the addictive stage of ND, characterized by current smoking and smoking a large amount of cigarettes per day. Furthermore, these SNPs did not appear to be associated with the remaining items comprising the FTND scale. This study confirmed a significant contribution of the CHRNA receptor on different forms of tobacco dependence. However, the genetic variant only accounted for little of the total genetic variance for liability to ND. Including phenotypic data on ungenotyped relatives can improve the statistical power to detect the effects of genetic variants when they contribute to individual differences in the phenotype.

The influence of gene-environment interactions on alcohol consumption and alcohol use disorders: a comprehensive review

Since 2005, a rapidly expanding literature has evaluated whether environmental factors such as socio-cultural context and adversity interact with genetic influences on drinking behaviors. This article critically reviews empirical research on alcohol-related genotype-environment interactions (GxE) and provides a contextual framework for understanding how genetic factors combine with (or are shaped by) environmental factors to influence the development of drinking behaviors and alcohol use disorders. Collectively, evidence from twin, adoption, and molecular genetic studies indicates that the degree of importance of genetic influences on risk for drinking outcomes can vary in different populations and under different environmental circumstances. However, methodological limitations and lack of consistent replications in this literature make it difficult to draw firm conclusions regarding the nature and effect size of alcohol-related GxE. On the basis of this review, we describe several methodological challenges as they relate to current research on GxE in drinking behaviors and provide recommendations to aid future research.

The influence of gene-environment interactions on alcohol consumption and alcohol use disorders: a comprehensive review

Since 2005, a rapidly expanding literature has evaluated whether environmental factors such as socio-cultural context and adversity interact with genetic influences on drinking behaviors. This article critically reviews empirical research on alcohol-related genotype-environment interactions (GxE) and provides a contextual framework for understanding how genetic factors combine with (or are shaped by) environmental factors to influence the development of drinking behaviors and alcohol use disorders. Collectively, evidence from twin, adoption, and molecular genetic studies indicates that the degree of importance of genetic influences on risk for drinking outcomes can vary in different populations and under different environmental circumstances. However, methodological limitations and lack of consistent replications in this literature make it difficult to draw firm conclusions regarding the nature and effect size of alcohol-related GxE. On the basis of this review, we describe several methodological challenges as they relate to current research on GxE in drinking behaviors and provide recommendations to aid future research.

Toward a comprehensive developmental model for alcohol use disorders in men

The multiple risk factors for alcohol use (AU) and alcohol use disorders (AUDs) are interrelated through poorly understood pathways, many of which begin in childhood. In this report, the authors seek to develop an empirical, broad-based developmental model for the etiology of AU and AUDs in men. We assessed 15 risk factors in four developmental tiers in 1,794 adult male twins from the Virginia population based twin registry. The best fitting model explained 39% of the variance in late adolescent AU, and 30% of the liability to lifetime symptoms of AUD. AU and AUDs can be best understood as arising from the action and interaction of two pathways reflecting externalizing genetic/temperamental and familial/social factors. Peer group deviance was important in each pathway. Internalizing symptoms played a more minor role. Familial/social factors were especially important influences on AU, while genetic/temperamental factors were more critical for AUDs. We conclude that AU and AUDs in men are complex traits influenced by genetic, family, temperamental, and social factors, acting and interacting over developmental time.

Accounting for the association between childhood maltreatment and alcohol-use disorders in males: a twin study

BACKGROUND

An association between childhood maltreatment and subsequent alcohol abuse and/or dependence (AAD) has been found in multiple studies of females. Less is known about the association between childhood maltreatment and AAD among males, and the mechanisms that underlie this association in either gender. One explanation is that childhood maltreatment increases risk for AAD. An alternative explanation is that the same genetic or environmental factors that increase a child's risk for being maltreated also contribute to risk for AAD in adulthood.

METHOD

Lifetime diagnosis of AAD was assessed using structured clinical interviews in a sample of 3527 male participants aged 19-56 years from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders. The sources of childhood maltreatment-AAD association were estimated using both a matched case-control analysis of twin pairs discordant for childhood maltreatment and bivariate twin modeling.

RESULTS

Approximately 9% of participants reported childhood maltreatment, defined as serious neglect, molestation, or physical abuse occurring before the age of 15 years. Those who experienced childhood maltreatment were 1.74 times as likely to meet AAD criteria compared with males who did not experience childhood maltreatment. The childhood maltreatment-AAD association largely reflected environmental factors in common to members of twin pairs. Additional exploratory analyses provided evidence that AAD risk associated with childhood maltreatment was significantly attenuated after adjusting for measured family-level risk factors.

CONCLUSIONS

Males who experienced childhood maltreatment had an increased risk for AAD. Our results suggest that the childhood maltreatment-AAD association is attributable to broader environmental adversity shared between twins.

Neurotransmitter and neuromodulator genes associated with a history of depressive symptoms in individuals with alcohol dependence

BACKGROUND

Depressive symptoms are common among individuals with alcohol use disorders and impact treatment outcome. Substantial overlap exists among the neurobiological systems proposed in the pathophysiology of depressive and alcohol use disorders; however, specific genetic effects contributing to risk for depressive comorbidity remain poorly understood.

METHODS

This study examines the association of depressive symptom scores for lifetime depression (the sum of DSM-IV major depression co-endorsed criteria for lifetime depression) with markers in 120 candidate genes in 554 alcohol-dependent individuals. The candidate genes code for molecules involved in dopamine, serotonin, glutamate, gamma-aminobutyric acid (GABA), and opioid neurotransmission, cell signaling, pharmacokinetics, stress biology, and behavioral control. Analyses were conducted at the single marker level with experimentwise permutation to control for multiple testing.

RESULTS

Results revealed nominal associations for markers in 20 genes. Following experimentwise permutation, markers in the corticotropin-releasing hormone-binding protein (CRHBP) the μ-opioid receptor (OPRM1) and the β1 subunit of GABA A (GABA(A)) receptors (GABRB1) met or exceeded the significance threshold. None of the markers associated with depressive symptom scores were significantly associated with alcohol dependence symptom scores.

CONCLUSION

These findings suggest potential risk genes for depressive symptoms in alcohol-dependent individuals.

Limited associations of dopamine system genes with alcohol dependence and related traits in the Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD)

BACKGROUND

Over 50 years of evidence from research has established that the central dopaminergic reward pathway is likely involved in alcohol dependence (AD). Additional evidence supports a role for dopamine (DA) in other disinhibitory psychopathology, which is often comorbid with AD. Family and twin studies demonstrate that a common genetic component accounts for most of the genetic variance in these traits. Thus, DA-related genes represent putative candidates for the genetic risk that underlies not only AD but also behavioral disinhibition. Many linkage and association studies have examined these relationships with inconsistent results, possibly because of low power, poor marker coverage, and/or an inappropriate correction for multiple testing.

METHODS

We conducted an association study on the products encoded by 10 DA-related genes (DRD1-D5, SLC18A2, SLC6A3, DDC, TH, COMT) using a large, ethnically homogeneous sample with severe AD (n = 545) and screened controls (n = 509). We collected genotypes from linkage disequilibrium (LD)-tagging single nucleotide polymorphisms (SNPs) and employed a gene-based method of correction. We tested for association with AD diagnosis in cases and controls and with a variety of alcohol-related traits (including age-at-onset, initial sensitivity, tolerance, maximum daily drinks, and a withdrawal factor score), disinhibitory symptoms, and a disinhibitory factor score in cases only. A total of 135 SNPs were genotyped using the Illumina GoldenGate and Taqman Assays-on-Demand protocols.

RESULTS

Of the 101 SNPs entered into standard analysis, 6 independent SNPs from 5 DA genes were associated with AD or a quantitative alcohol-related trait. Two SNPs across 2 genes were associated with a disinhibitory symptom count, while 1 SNP in DRD5 was positive for association with the general disinhibitory factor score.

CONCLUSIONS

Our study provides evidence of modest associations between a small number of DA-related genes and AD as well as a range of alcohol-related traits and measures of behavioral disinhibition. While we did conduct gene-based correction for multiple testing, we did not correct for multiple traits because the traits are correlated. However, false-positive findings remain possible, so our results must be interpreted with caution.

The relationship between genetic influences on alcohol dependence and on patterns of alcohol consumption

BACKGROUND

Genetic factors impact substantially both on alcohol consumption (AC) and on the risk for alcohol dependence (AD). However, we know little about the degree to which measures of AC index the genetic risk for AD.

METHODS

We assessed a lifetime history of AD by DSM-IV criteria and four measures of AC at the time of heaviest drinking (drink frequency, regular quantity, maximum quantity, and drunk frequency) in 5,073 adult twins from same-sex pairs from the Virginia Twin Registry. Structural models were fitted using Mx.

RESULTS

We found evidence for different genetic structure in the sexes. In women, genetic risk for AD and for the four measures of AC was entirely shared. In men, the AC measures captured 85% of the genetic risk for AD. In women, the genetic relationship with AD was strongest for drunk frequency and in men for both drunk frequency and regular quantity.

CONCLUSIONS

In a population-based sample of twins, four relatively simple measures of AC obtained for the time of lifetime heaviest drinking were able to capture all (in women) or a very large proportion (in men) of the genetic risk for the complex multi-dimensional construct of AD. If replicated, these results have practical implications for studies aiming to assess genetic risk for AD.

A systematic gene-based screen of chr4q22-q32 identifies association of a novel susceptibility gene, DKK2, with the quantitative trait of alcohol dependence symptom counts

Studies of alcohol dependence (AD) have consistently found evidence of linkage on chromosome 4q21-q32. A genome-wide linkage scan in the Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD) sample also provided its strongest evidence of linkage on chromosome 4q22-q32 using an index of AD severity based on the count of DSM-IV AD symptoms (ADSX; LOD = 4.59). We conducted a systematic, gene-centric association study using 518 LD-tagging single nucleotide polymorphisms (SNPs) in the 65 known and predicted genes within the 1-LOD interval surrounding the linkage peak. Case-only regression analysis with the quantitative variable of ADSX was performed in the 562 genetically independent cases; nominal support for association was demonstrated by 32 tagging SNPs in 14 genes. We did not observe study-wide significance, but gene-wise correction for multiple testing with the Nyholt procedure yielded empirical evidence of association with two genes, DKK2 (dickkopf homolog 2) (P = 0.007) and EGF (epidermal growth factor) (P = 0.025) in the IASPSAD sample. Three SNPs in DKK2 (rs427983; rs419558; rs399087) demonstrated empirical significance. Assessment of possible replication in 847 cases of European descent from a large independent sample, the Collaborative Study of the Genetics of Alcoholism, yielded replication for DKK2 but not EGF. We observed genotypic and phenotypic replication for DKK2 with the three SNPs yielding significant association with ADSX in the IASPSAD sample. Haplotype-specific expression measurements in post-mortem tissue samples suggested a functional role for DKK2. This evidence notwithstanding, replication is needed before confidence can be placed in these findings.

Empirically defined subtypes of alcohol dependence in an Irish family sample

Alcohol dependence (AD) is clinically and etiologically heterogeneous. The goal of this study was to explore AD subtypes among a sample of 1221 participants in the Irish Affected Sib Pair Study of Alcohol Dependence, all of whom met DSM-IV criteria for AD. Variables used to identify the subtypes included major depressive disorder, antisocial personality disorder, illicit drug dependence (cannabis, sedatives, stimulants, cocaine, opioids, and hallucinogens), nicotine dependence, the personality traits of neuroticism and novelty seeking, and early alcohol use. Using latent class analysis, a 3-class solution was identified as the most parsimonious description of the data. Individuals in a Mild class were least likely to have comorbid psychopathology, whereas a severe class had highest probabilities of all comorbid psychopathology. The third class was characterized by high probabilities of major depression and higher neuroticism scores, but lower likelihood of other comorbid disorders than seen in the severe class. Overall, sibling pair resemblance for class was stronger within than between classes, and was greatest for siblings within the severe class, suggesting a stronger familial etiology for this class. These findings are consistent with the affective regulation and behavioral disinhibition subtypes of alcoholism, and are in line with prior work suggesting familial influences on subtype etiology.

The Norwegian Institute of Public Health twin study of mental health: examining recruitment and attrition bias

All Norwegian twin pairs born 1967-1974 and still living in Norway in 1992 were invited to a health questionnaire study (Q1). 2,570 pairs (65%) participated. These cohorts and the twin cohorts born 1967-1979 were invited to a new questionnaire study (Q2) in 1998. This time 3,334 pairs (53%) participated. Almost all pairs having participated in the 1998 study were invited to an interview study of mental health (MHS), taking place 1999-2004. 1,391 complete pairs (44%) participated. The questionnaire studies included extensive data on somatic health with fewer items on mental health and demography. Health-related and demographic information available from the Medical Birth Registry on all invited twins was applied to predict participation to the first study. A few registry variables indicating poor health predicted nonparticipation in Q1. Health information and demography from Q1 were tested as predictors of participation in the follow-up study (Q2). Monozygosity, female sex, being unmarried, having no children, and high education predicted participation, whereas few indicators of poor mental and somatic health and unhealthy lifestyle moderately predicted nonparticipation in Q2. No health indicators reported in Q2 predicted further participation. Standard genetic twin analyses of indicators of various mental disorders from Q2, validated by diagnostic data from the MHS, did not indicate differences in genetic/environmental covariance structures between participants and nonparticipants in MHS. In general the results show a moderate selection towards good mental and somatic health. Attrition from Q2 to the MHS does not appear to affect twin analyses of mental health related variables.