Evidence for an interaction between age at first drink and genetic influences on DSM-IV alcohol dependence symptoms

BACKGROUND

Research suggests that individuals who start drinking at an early age are more likely to subsequently develop alcohol dependence. Twin studies have demonstrated that the liability to age at first drink and to alcohol dependence are influenced by common genetic and environmental factors, however, age at first drink may also environmentally mediate increased risk for alcohol dependence. In this study, we examine whether age at first drink moderates genetic and environmental influences, via gene x environment interactions, on DSM-IV alcohol dependence symptoms.

METHODS

Using data on 6,257 adult monozygotic and dizygotic male and female twins from Australia, we examined the extent to which age at first drink (i) increased mean alcohol dependence symptoms and (ii) whether the magnitude of additive genetic, shared, and nonshared environmental influences on alcohol dependence symptoms varied as a function of decreasing age. Twin models were fitted in Mx.

RESULTS

Risk for alcohol dependence symptoms increased with decreasing age at first drink. Heritable influences on alcohol dependence symptoms were considerably larger in those who reported an age at first drink prior to 13 years of age. In those with later onset of alcohol use, variance in alcohol dependence was largely attributable to nonshared environmental variance (and measurement error). This evidence for unmeasured gene x measured environment interaction persisted even when controlling for the genetic influences that overlapped between age at first drink and alcohol dependence symptoms.

CONCLUSIONS

Early age at first drink may facilitate the expression of genes associated with vulnerability to alcohol dependence symptoms. This is important to consider, not only from a public health standpoint, but also in future genomic studies of alcohol dependence.

Common variants in the trichohyalin gene are associated with straight hair in Europeans

Hair morphology is highly differentiated between populations and among people of European ancestry. Whereas hair morphology in East Asian populations has been studied extensively, relatively little is known about the genetics of this trait in Europeans. We performed a genome-wide association scan for hair morphology (straight, wavy, curly) in three Australian samples of European descent. All three samples showed evidence of association implicating the Trichohyalin gene (TCHH), which is expressed in the developing inner root sheath of the hair follicle, and explaining approximately 6% of variance (p=1.5x10(-31)). These variants are at their highest frequency in Northern Europeans, paralleling the distribution of the straight-hair EDAR variant in Asian populations.

Alcohol consumption indices of genetic risk for alcohol dependence

BACKGROUND

Previous research has reported a significant genetic correlation between heaviness of alcohol consumption and alcohol dependence (AD), but this association might be driven by the influence of AD on consumption rather than the reverse. We test the genetic overlap between AD symptoms and a heaviness of consumption measure among individuals who do not have AD. A high genetic correlation between these measures would suggest that a continuous measure of consumption may have a useful role in the discovery of genes contributing to dependence risk.

METHODS

Factor analysis of five alcohol use measures was used to create a measure of heaviness of alcohol consumption. Quantitative genetic analyses of interview data from the 1989 Australian Twin Panel (n = 6257 individuals; M = 29.9 years) assessed the genetic overlap between heaviness of consumption, DSM-IV AD symptoms, DSM-IV AD symptom clustering, and DSM-IV alcohol abuse.

RESULTS

Genetic influences accounted for 30%-51% of the variance in the alcohol measures and genetic correlations were .90 or higher for all measures, with the correlation between consumption and dependence symptoms among nondependent individuals estimated at .97 (95% confidence interval: .80-1.00).

CONCLUSIONS

Heaviness of consumption and AD symptoms have a high degree of genetic overlap even among nondependent individuals in the general population, implying that genetic influences on dependence risk in the general population are acting to a considerable degree through heaviness of use and that quantitative measures of consumption will likely have a useful role in the identification of genes contributing to AD.

Common variants in TMPRSS6 are associated with iron status and erythrocyte volume

We report a genome-wide association study to iron status. We identify an association of SNPs in TPMRSS6 to serum iron (rs855791, combined P = 1.5×10⁻²⁰), transferrin saturation (combined P = 2.2×10⁻²³), and erythrocyte mean cell volume (MCV, combined P = 1.1×10⁻¹⁰). We also find suggestive evidence of association with blood haemoglobin levels (combined P = 5.3×10⁻⁷). These findings demonstrate the involvement of TMPRSS6 in control of iron homeostasis and in normal erythropoiesis.

Genetic linkage findings for DSM-IV nicotine withdrawal in two populations

Nicotine withdrawal (NW) is both an important contributor to difficulty quitting cigarettes and because of mood-related withdrawal symptoms a problem of particular relevance to psychiatry. Twin-studies suggest that genetic factors influence NW (heritability = 45%). Only one previous linkage study has published findings on NW [Swan et al. (2006); Am J Med Genet Part B 141B:354-360; LOD = 2.7; Chr. 6 at 159 cM]. As part of an international consortium, genome-wide scans (using over 360 autosomal microsatellite markers) and telephone diagnostic interviews were conducted on 289 Australian (AUS) and 161 Finnish (FIN, combined (COMB) N = 450 families) families ascertained from twin registries through index-cases with a lifetime history of cigarette smoking. The statistical approach used an affected-sib-pair design (at least two adult full siblings reported a history of DSM-IV NW) and conducted the linkage analyses using MERLIN. Linkage signals with LOD scores >1.5 were found on two chromosomes: 6 (FIN: LOD = 1.93 at 75 cM) and 11 at two different locations (FIN: LOD = 3.55 at 17 cM, and AUS: LOD = 1.68 with a COMB: LOD = 2.30 at 123 cM). The multipoint LOD score of 3.55 on chromosome 11p15 in FIN met genomewide significance (P = 0.013 with 1,000 simulations). At least four strong candidate genes lie within or near this peak on chromosome 11: DRD4, TPH, TH, and CHRNA10. Other studies have reported that chromosome 11 may harbor genes associated with various aspects of smoking behavior. This study adds to that literature by highlighting evidence for NW.

Parental alcoholism and offspring behavior problems: findings in Australian children of twins

We examine the impact of rearing by an alcoholic parent on risk for child behavior problems using data on 2492 offspring drawn from two ongoing studies of children of female and male same- and opposite-sex twin pairs. Results of regression models predicting child behavior problems from parent and co-twin lifetime history of alcohol use disorder (AUD) provide support for genetic but not environmental transmission of externalizing and a measure of total problem behaviors. Results for internalizing behavior were inconclusive with respect to transmission of risk.

An examination of the overlap between genetic and environmental risk factors for intentional weight loss and overeating

OBJECTIVE

To further our understanding of how intentional weight loss (IWL) and overeating are related, we examined the shared genetic and environmental variance between lifetime IWL and overeating.

METHOD

Interview data were available for 1,976 female twins (both members of 439 and 264 pairs of monozygotic and dizygotic twins, respectively), mean age = 40.61, SD = 4.72. We used lifetime diagnostic data for eating disorders obtained from a semistructured psychiatric telephone interview, examined in a bivariate twin analysis. Both lifetime behaviors were measured on a 3-point scale, where absence of IWL or overeating formed one anchor on the scale and lifetime anorexia nervosa (AN) and bulimia nervosa (BN) formed the opposite anchors, respectively.

RESULTS

In line with previous findings, a higher body mass index was significantly associated with the lifetime presence of IWL and/or overeating (odds ratio = 1.13, 95% confidence interval (CI): 1.08-1.19). The best fitting twin model contained additive genetic and nonshared environmental influence influencing both IWL and overeating, with correlations between these influences of 0.61 (95% CI: 0.35-0.92) and 0.24 (95% CI: 0.07-0.42), respectively.

DISCUSSION

About 37% of genetic risk factors were considered to overlap between IWL and overeating, and with only 6% of overlap between environmental risk factors. Thus, considerable independence of risk factors was indicated.

Accurate, Large-Scale Genotyping of 5HTTLPR and Flanking Single Nucleotide Polymorphisms in an Association Study of Depression, Anxiety, and Personality Measures

BACKGROUND

The length polymorphism repeat in the promoter region of the serotonin transporter gene (5HTTLPR) is one of the most studied polymorphisms for association with a range of psychiatric and personality phenotypes. However, the original 5HTTLPR assay is prone to bias toward short allele calling.

METHODS

We designed new assays for the 5HTTLPR suitable for large-scale genotyping projects and we genotyped 13 single nucleotide polymorphisms (SNPs) in a 38-kilobase region around the 5HTTLPR, including SNP rs25531, a polymorphism of the 5HTTLPR long allele. Association analysis was conducted for major depression and/or anxiety disorder in unrelated cases (n = 1161) and control subjects (n = 1051) identified through psychiatric interviews administered to a large population sample of Australian twin families. Participants had been scored for personality traits several years earlier (n > or = 2643 unrelated individuals).

RESULTS

We identified a two-SNP haplotype proxy for 5HTTLPR; the CA haplotype of SNPs rs4251417 and rs2020934 is coupled with the short allele of 5HTTLPR (r(2) = .72). We found evidence for association (p = .0062, after accounting for multiple testing) for SLC6A4 SNPs rs6354 and rs2020936 (positioned in a different linkage disequilibrium [LD] block about 15.5 kb from 5HTTLPR) with anxiety and/or depression and neuroticism, with the strongest association for recurrent depression with onset in young adulthood (odds ratio = 1.55, 95% confidence interval = 1.16-2.06).

CONCLUSIONS

The associated SNPs are in the same LD block as the variable number of tandem repeats serotonin transporter intron 2 marker, for which association has previously been reported.

Timing of first alcohol use and alcohol dependence: evidence of common genetic influences

AIMS

To estimate the magnitude of genetic and environmental influences on timing of first alcohol use and alcohol dependence (AD) and to quantify the overlap in these influences across the two alcohol-related outcomes.

PARTICIPANTS

The sample consisted of 5382 twins (2691 complete pairs), aged 24-36 years, from the Australian Twin Registry.

MEASUREMENTS

History of alcohol use and DSM-IV alcohol dependence were assessed by structured telephone interview.

FINDINGS

In both sexes, the relationship between age at first alcohol use and risk for AD followed a linear trend, such that the highest rates of AD were observed in individuals who began drinking at an earlier than average age (14 years or younger). Heritability estimates for timing of first alcohol use and AD were 36% and 53%, respectively. Shared environmental factors accounted for 15% of variance in initiation. There was no evidence of shared environmental influences on AD. The genetic correlation between timing of first alcohol use and AD was 0.59.

CONCLUSIONS

Findings highlight the substantial role of genetics in the development of AD and the early manifestation of that genetic risk in the timing of alcohol use initiation which, unlike AD, is also influenced to a modest degree by shared environmental factors. The considerable overlap in heritable influences-and the virtual absence of overlap in individual-specific environmental influences-on initiation of alcohol use and AD indicates that the association between age at first drink and AD is attributable in large part to common genetic sources of variance.

Relationship between body mass index, alcohol use, and alcohol misuse in a young adult female twin sample

OBJECTIVE

The aim of this study was to examine the relationships between body mass index (BMI) and alcohol use and misuse in young adulthood in a sample of black and white female twins.

METHOD

Cox proportional hazards and logistic regression models were used to examine the relationships between BMI category and first alcohol use, current weekly alcohol use, and current weekly heavy episodic drinking in 3,514 (14.06% black) young adult female twins. Analyses were conducted separately in black and white women.

RESULTS

After adjusting for relevant covariates, in white women obesity was protective against alcohol use (hazard ratio = 0.83; 95% confidence interval [CI]: 0.73-0.93) and against weekly drinking (odds ratio [OR] = 0.36; 95% CI: 0.24-0.53) and weekly heavy episodic drinking (OR = 0.51; 95% CI: 0.31-0.82) among ever drinkers compared with women of ideal weight. Overweight women living with their parents were less likely to be weekly drinkers (OR = 0.31; 95% CI: 0.16-0.61), whereas overweight women who were not living with their parents were less likely to be weekly heavy episodic drinkers (OR = 0.51; 95% CI: 0.31-0.82). Among black women, obesity was not associated with any of the drinking outcomes; however, black women who were overweight and who reported that the majority of their friends were not weekly drinkers had greater odds of reporting weekly drinking than those of ideal weight (OR = 2.91; 95% CI: 1.33-6.39).

CONCLUSIONS

Obese white women were less likely to ever use alcohol, be weekly drinkers, or be weekly heavy episodic drinkers than their ideal-weight peers. Body weight may affect drinking behavior in young women, and this effect may differ by race. Future research is needed to identify mediators and moderators of this relationship as well as to explore racial differences in the effect of body weight on drinking behavior.

The role of harsh discipline in explaining sex differences in conduct disorder: a study of opposite-sex twin pairs

In the current study, two hypotheses about the role of harsh discipline (HD) in explaining the sex difference in the prevalence of conduct disorder (CD) were evaluated: that boys exhibit more CD than girls because (1) they are exposed to more HD and/or (2) there is a greater association between HD and CD in boys. These hypotheses were evaluated in a sample of male and female adult twins from different families (N = 3,502) as well as a sample of adult twin brothers and sisters (N = 655) in order to examine the extent to which sex differences remained after controlling for between-family differences. Retrospective reports of HD experienced between ages 6-13 and DSM-IV CD symptoms experienced before age 18 were obtained via structured psychiatric telephone interviews. Boys reported higher mean levels of HD and CD than girls, both between and within families, and the results of model-fitting analyses suggested that differences in the use of harsh disciplinary practices for sons versus daughters may partially explain the sex difference in the prevalence of CD. Between families, the relation between HD and CD was greater for girls than boys, but within families, there was no evidence of a sex difference in the relation between HD and CD. Inconsistent between-family and within-family results suggest that factors that differ between families are confounded with sex differences in the relation between HD and CD. A more stringent test of sex differences involves eliminating these between-family differences by studying boys and girls within the same family.

Common genetic influences on the timing of first use for alcohol, cigarettes, and cannabis in young African-American women

The risks associated with early age at initiation for alcohol, cigarette, and cannabis use are well documented, yet the timing of first use has rarely been studied in genetically informative frameworks, leaving the relative contributions of genetic and environmental factors to age at initiation largely unknown. The current study assessed overlap in heritable and environmental influences on the timing of initiation across these three substances in African-American women, using a sample of 462 female twins (100 monozygotic and 131 dizygotic pairs) from the Missouri Adolescent Female Twin Study. Mean age at the time of interview was 25.1 years. Ages at first use of alcohol, cigarettes, and cannabis were gathered in diagnostic interviews administered over the telephone. Standard genetic analyses were conducted with substance use initiation variables categorized as never, late, and early onset. Variance in the timing of first use was attributable in large part to genetic sources: 44% for alcohol, 62% for cigarettes, and 77% for cannabis. Genetic correlations across substances ranged from 0.25 to 0.70. Shared environmental influences were modest for alcohol (10%) and absent for cigarettes and cannabis. Findings contrast with reports from earlier studies based on primarily Caucasian samples, which have suggested a substantial role for shared environment on substance use initiation when measured as lifetime use. By characterizing onset as timing of first use, we may be tapping a separate construct. Differences in findings may also reflect a distinct etiological pathway for substance use initiation in African-American women that could not be detected in previous studies.

Alcohol criteria endorsement and psychiatric and drug use disorders among DUI offenders: greater severity among women and multiple offenders

PURPOSE

Data from the Collaborative Study on the Genetics of Alcoholism (COGA), a high-risk family study of alcohol dependence, were used to examine differences in alcohol diagnostic criteria endorsement and psychiatric and drug use disorders by gender and by number of DUI offenses.

RESULTS

Individuals with two or more DUIs exhibited greater severity of alcohol dependence than those with none or one DUI. This severity was characterized in three ways: (1) higher endorsement of alcohol diagnostic criterion items, with evidence of greater severity among women, (2) higher prevalence of co-occurring lifetime psychiatric disorders, and (3) higher rates of drug use and of dependence on cocaine, stimulants, and, for women only, marijuana and opiates.

CONCLUSIONS

By examining gradations of disorder within a combination of two high-risk indicators, DUI and family vulnerability, this study provides useful information for clinical research about individuals with chronic and severe alcohol problems. In addition, the observed gender differences in this high-risk sample will contribute to the literature on alcohol dependence among women at the more severe end of the dependence spectrum.

Genetic and environmental contributions to BMI in adolescent and young adult women

The objective of this study was to determine the genetic and environmental contributions to variation in BMI over time in European-American (EA) and African-American (AA) adolescent and young adult women. Self-reported BMI (kg/m(2)) data from 2,816 EA (1,306 twin pairs, 56.5% monozygotic (MZ)) and 404 AA (178 twin pairs, 42.7% MZ) women at baseline (T1; median age 15 years) and 3,225 EA (1,511 twin pairs, 55.3% MZ) and 539 AA (252 pairs, 43.3% MZ) women at follow-up (T2; median age 22 years) from a Midwestern US, population-based twin registry were used to construct biometrical genetic models. For EA women, the majority of the variance in BMI was attributable to additive genetic effects at both time points (82% for each), with the remaining variance attributable to nonshared environment. Genetic and nonshared environment correlations between adolescent and young adult BMI were 0.87 and 0.23, respectively. Among AA women, nonadditive genetic effects comprised 68% of the variance at T1 and 73% at T2, and were highly correlated (r(D) = 0.94). The proportions of variance attributable to nonshared environment at T1 (29%) and T2 (25%) were more modestly correlated (r(E) = 0.31). The remaining variance in AA women could be attributed to additive genetic effects. Additive vs. nonadditive genetic effects contribute differentially to BMI in AA vs. EA adolescent and young adult women. Additional research is needed to better characterize the environmental and genetic factors related to BMI in persons of different races to aid understanding of the complex determinants of body weight in individuals.

Simultaneous cannabis and tobacco use and cannabis-related outcomes in young women

Compared to those who reported a lifetime co-occurrence of cannabis and tobacco use, individuals who report simultaneous use of cannabis and tobacco are more likely to also report higher rates of substance-related problems and psychopathology. In a sample of young women, we examine (a) co-occurring use, or whether regular cigarette smoking is associated with increased cannabis involvement and (b) simultaneous use, a special form of co-occurring use where cannabis and cigarettes are typically used on the same occasion to test whether those who use cannabis and tobacco simultaneously are also more likely to report greater cannabis involvement and (c) the extent to which latent genetic and environmental factors contribute to simultaneous use in those with a history of co-occurring cannabis use and regular cigarette smoking. Women (N=3427) who report regular cigarette smoking are 4.5-9.5 times more likely to report co-occurring cannabis use and other stages of cannabis involvement, including DSM-IV cannabis abuse and dependence. In those women who report co-occurring regular cigarette smoking and lifetime cannabis use (N=1073), simultaneous use of cannabis and tobacco was associated with increased likelihood of negative cannabis-related outcomes. Simultaneous users were 1.6 times more likely to meet criteria for DSM-IV cannabis abuse, even after controlling for early covariates and for prior stages of cannabis involvement. Simultaneous use was not heritable, and twin similarity was attributable to shared environmental factors (31%). While our study does not determine causality between simultaneous tobacco-cannabis use and cannabis involvement, results indicate that simultaneous use is potentially a marker for more severe psychosocial consequences associated with cannabis use.

Can we identify genes for alcohol consumption in samples ascertained for heterogeneous purposes?

BACKGROUND

Previous studies have identified evidence of genetic influence on alcohol use in samples selected to be informative for alcoholism research. However, there are a growing number of genome-wide association studies (GWAS) using samples unselected for alcohol consumption (i.e., selected on other traits and forms of psychopathology), which nevertheless assess consumption as a risk factor. Is it reasonable to expect that genes contributing to variation in alcohol consumption can be identified in such samples?

METHODS

An exploratory approach was taken to determine whether linkage analyses for heaviness of alcohol consumption, using a sample collected for heterogeneous purposes, could replicate previous findings. Quantity and frequency measures of consumption were collected in telephone interviews from community samples. These measures, and genotyping, were available for 5,441 individuals (5,067 quasi-independent sibling pairs). For 1,533 of these individuals, data were collected on 2 occasions, about 8.2 years apart, providing 2 datasets that maximize data collected at either a younger or an older age. Analyses were conducted to address the question of whether age and heavier levels of alcohol consumption effects outcome. Linkage results were compared in the younger and older full samples, and with samples in which approximately 10, 20, and 40 of drinkers from the lower end of the distribution of alcohol consumption were dropped.

RESULTS

Linkage peaks varied for the age differentiated samples and for percentage of light drinkers retained. Larger peaks (LOD scores >2.0) were typically found in regions previously identified in linkage studies and/or containing proposed candidate genes for alcoholism including AGT, CARTPT, OPRD1, PIK3R1, and PDYN.

CONCLUSIONS

The results suggest that GWAS assessing alcohol consumption as a covariate for other conditions will have some success in identifying genes contributing to consumption-related variation. However, sample characteristics, such as participant age, and trait distribution, may have substantial effects on the strength of the genetic signal. These results can inform forthcoming GWAS where the same restrictions apply.

Genome-wide association study of smoking initiation and current smoking

For the identification of genes associated with smoking initiation and current smoking, genome-wide association analyses were carried out in 3497 subjects. Significant genes that replicated in three independent samples (n = 405, 5810, and 1648) were visualized into a biologically meaningful network showing cellular location and direct interaction of their proteins. Several interesting groups of proteins stood out, including glutamate receptors (e.g., GRIN2B, GRIN2A, GRIK2, GRM8), proteins involved in tyrosine kinase receptor signaling (e.g., NTRK2, GRB14), transporters (e.g., SLC1A2, SLC9A9) and cell-adhesion molecules (e.g., CDH23). We conclude that a network-based genome-wide association approach can identify genes influencing smoking behavior.

Developing a quantitative measure of alcohol consumption for genomic studies on prospective cohorts

OBJECTIVE

The purpose of this study was to develop a quantitative measure of alcohol consumption for gene-mapping studies.

METHOD

Using a sample of 3,787 young-adult twin women and an independent sample of 489 men and women from a college drinking study, we developed an alcohol-consumption factor score indexed by (1) maximum typical consumption (log-transformed quantity frequency [LQNTFRQ]), (2) maximum drinks in a 24-hours period (LMAXALC), (3) frequency of drinking five or more drinks per day (FIVE), and (4) frequency of drinking to intoxication (INTOX). We tested (1) for factorial and psychometric equivalence across samples and genders; (2) for construct validity and its equivalence, across samples and genders, using measures of tobacco and cannabis use and family history of alcoholism; and (3) to determine the heritability of the alcohol-consumption factor score using a genetic psychometric model.

RESULTS

A single-factor model fit well with factor loadings ranging from .60 to .90. With rare exception, we found support for measurement invariance across the two samples and across genders. Measures of nicotine and cannabis use as well as family history of alcoholism were associated, to a similar extent across samples and genders, with the underlying alcohol-consumption factor. Psychometric twin modeling revealed that each of the alcohol-consumption measures (h2=34%-47%) and the underlying factor score (h2=50%) were heritable, with the remainder of the variance attributable to individual-specific environmental factors. This moderately heritable alcohol-consumption factor also accounted for a majority of the genetic variance in LQNTFRQ, LMAXALC, FIVE, and INTOX.

CONCLUSIONS

Quantitative measures of alcohol consumption with the favorable attributes of measurement invariance, construct validity, and moderate heritability can greatly enhance future gene-mapping efforts, supplementing information afforded by conventional diagnostic measures of alcohol abuse/dependence.

Accumulation of trauma over time and risk for depression in a twin sample

BACKGROUND

Few genetically informative studies have examined the effects of different types of trauma on risk for depression over time. The aim of the present study was to examine the relative contributions over time of assaultive trauma, non-assaultive trauma, and familial effects to risk for depression.

METHOD

Histories of depression and trauma were obtained during structured diagnostic interviews with 5266 (mean age 29.9 years, s.d.=2.4) members of a volunteer Australian twin panel from the general population. Age at first onset of a DSM-IV major depressive episode was the dependent variable. Associations of depression with traumatic events were examined while accounting for the temporal sequence of trauma and depression and familial effects.

RESULTS

Assaultive traumatic events that occurred during childhood had the strongest association with immediate and long-term risk for depression, and outweighed familial effects on childhood-onset depression for most twins. Although men and women endorsed equal rates of assaultive trauma, women reported a greater accumulation of assaultive events at earlier ages than men, whereas men reported a greater accumulation of non-assaultive events at all ages.

CONCLUSIONS

Early exposure to assaultive trauma can influence risk for depression into adulthood. Concordance for early trauma is a significant contributor to the familiality of early-onset depression.

ADH single nucleotide polymorphism associations with alcohol metabolism in vivo

We have previously found that variation in alcohol metabolism in Europeans is linked to the chromosome 4q region containing the ADH gene family. We have now typed 103 single nucleotide polymorphisms (SNPs) across this region to test for allelic associations with variation in blood and breath alcohol concentrations after an alcohol challenge. In vivo alcohol metabolism was modelled with three parameters that identified the absorption and rise of alcohol concentration following ingestion, and the rate of elimination. Alleles of ADH7 SNPs were associated with the early stages of alcohol metabolism, with additional effects in the ADH1A, ADH1B and ADH4 regions. Rate of elimination was associated with SNPs in the intragenic region between ADH7 and ADH1C, and across ADH1C and ADH1B. SNPs affecting alcohol metabolism did not correspond to those reported to affect alcohol dependence or alcohol-related disease. The combined SNP associations with early- and late-stage metabolism only account for approximately 20% of the total genetic variance linked to the ADH region, and most of the variance for in vivo alcohol metabolism linked to this region is yet to be explained.

Flexible Mx specification of various extended twin kinship designs

The extended twin kinship design allows the simultaneous testing of additive and nonadditive genetic, shared and individual-specific environmental factors, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission (Eaves et al., 1999). It also handles the contribution of these sources of variance to the (co)variation of multiple phenotypes. Keller et al. (2008) extended this comprehensive model for family resemblance to allow or a flexible specification of assortment and vertical transmission. As such, it provides a general framework which can easily be reduced to fit subsets of data such as twin-parent data, children-of-twins data, etc. A flexible Mx specification of this model that allows handling of these various designs is presented in detail and applied to data from the Virginia 30,000. Data on height, body mass index, smoking status, church attendance, and political affiliation were obtained from twins and their families. Results indicate that biases in the estimation of variance components depend both on the types of relative available for analysis, and on the underlying genetic and environmental architecture of the phenotype of interest.

Genetic and environmental influences on the co-morbidity between depression, panic disorder, agoraphobia, and social phobia: a twin study

BACKGROUND

Major depression (MD) and anxiety disorders such as panic disorder (PD), agoraphobia (AG), and social phobia (SP) are heritable and highly co-morbid. However, the relative importance of genetic and environmental etiology of the covariation between these disorders, particularly the relationship between PD and AG, is less clear.

METHODS

This study measured MD, PD, and AG in a population sample of 5,440 twin pairs and 1,245 single twins, about 45% of whom were also scored for SP. Prevalences, within individual co-morbidity and twin odds ratios for co-morbidity, are reported. A behavioral genetic analysis of the four disorders using the classical twin design was conducted.

RESULTS

Odds ratios for MD, PD, AG, and SP in twins of individuals diagnosed with one of the four disorders were increased. Heritability estimates under a threshold-liability model for MD, PD, AG, and SP respectively were .33 (CI: 0.30-0.42), .38 (CI: 0.24-0.55), .48 (CI: 0.37-0.65), and .39 (CI: 0.16-0.65), with no evidence for any variance explained by the common environment shared by twins. We find that a common genetic factor explains a moderate proportion of variance in these four disorders. The genetic correlation between PD and AG was .83.

CONCLUSION

MD, PD, AG, and SP strongly co-aggregate within families and common genetic factors explain a moderate proportion of variance in these four disorders. The high genetic correlation between PD and AG and the increased odds ratio for PD and AG in siblings of those with AG without PD suggests a common genetic etiology for PD and AG.

Decomposing group differences of latent means of ordered categorical variables within a genetic factor model

A genetic factor model is introduced for decomposition of group differences of the means of phenotypic behavior as well as individual differences when the research variables under consideration are ordered categorical. The model employs the general Genetic Factor Model proposed by Neale and Cardon (Methodology for genetic studies of twins and families, 1992) and, more specifically, the extension proposed by Dolan et al. (Behav Genet 22: 319-335, 1992) which enables decomposition of group differences of the means associated with genetic and environmental factors. Using a latent response variable (LRV) formulation (Muthén and Asparouhov, Latent variable analysis with categorical outcomes: multiple-group and growth modeling in Mplus. Mplus web notes: No. 4, Version 5, 2002), proportional differences of response categories between groups are modeled within the genetic factor model in terms of the distributional differences of latent response variables assumed to underlie the observed ordered categorical variables. Use of the proposed model is illustrated using a measure of conservatism in the data collected from the Australian Twin Registry.

Autosomal linkage analysis for cannabis use behaviors in Australian adults

Cannabis is the most commonly used illicit drug in developed and in developing nations. Twin studies have highlighted the role of genetic influences on early stages of cannabis use, such as a lifetime history of use, early-onset use and frequent use, however, we are not aware of any genomic studies that have examined these phenotypes. Using data on 2314 families consisting of 5600 adult Australian offspring and their parents, all of whom were scanned using 1399 unique autosomal markers, we conducted autosomal linkage analyses for lifetime history of cannabis initiation, early-onset cannabis use and frequency of use, using a variance components approach in the linkage package MERLIN. Suggestive evidence for linkage was found on chromosome 18 (LOD 2.14 for frequency of use, LOD 1.97 for initiation, at 90-97 cM) and also on chromosome 19 (LOD 1.92 for early-onset at 17 cM). These LOD scores did not meet genome-wide significance. Further replication of these linkage regions in other samples will be required, although these initial results suggest further targeted efforts on chromosome 18 may yield interesting candidate genes for early stages of cannabis-related behaviors.