Genetic and Cultural Transmission of Smoking Initiation: An Extended Twin Kinship Model

Annual Research Review: Towards a deeper understanding of nature and nurture: combining family-based quasi-experimental methods with genomic data

Distinguishing between the effects of nature and nurture constitutes a major research goal for those interested in understanding human development. It is known, for example, that many parent traits predict mental health outcomes in children, but the causal processes underlying such associations are often unclear. Family-based quasi-experimental designs such as sibling comparison, adoption and extended family studies have been used for decades to distinguish the genetic transmission of risk from the environmental effects family members potentially have on one another. Recently, these designs have been combined with genomic data, and this combination is fuelling a range of exciting methodological advances. In this review we explore these advances - highlighting the ways in which they have been applied to date and considering what they are likely to teach us in the coming years about the aetiology and intergenerational transmission of psychopathology.

Association between childhood trauma, parental bonding and antisocial personality disorder in adulthood: A machine learning approach

Childhood trauma (CT) and parental bonding (PB) have been correlated with later antisocial personality disorder (ASPD). Aiming to better understand this complex interaction we analyzed the data from a cross-sectional study that evaluated 346 male inpatient cocaine users, using both traditional statistical analysis and machine learning (ML) approaches. Childhood Trauma Questionnaire (CTQ), Parental Bonding Instrument (PBI), and Mini International Neuropsychiatric Interview (MINI) were applied. We found a markedly higher prevalence of mental illness in the ASPD group. The ML method and the traditional analysis showed that emotional and physical abuse were the factors with the strongest relationship with ASPD. Also, there were discrepancies between the findings of both methods regarding physical neglect and paternal care. Although this study does not allow definitive answers in this matter, we do propose that these two methods can aid in better comprehending how multiple variables interact with each other in the development of psychological disorders.

Notes on Three Decades of Methodology Workshops

Since 1987, a group of behavior geneticists have been teaching an annual methodology workshop on how to use state-of-the-art methods to analyze genetically informative data. In the early years, the focus was on analyzing twin and family data, using information of their known genetic relatedness to infer the role of genetic and environmental factors on phenotypic variation. With the rapid evolution of genotyping and sequencing technology and availability of measured genetic data, new methods to detect genetic variants associated with human traits were developed and became the focus of workshop teaching in alternate years. Over the years, many of the methodological advances in the field of statistical genetics have been direct outgrowths of the workshop, as evidence by the software and methodological publications authored by workshop faculty. We provide data and demographics of workshop attendees and evaluate the impact of the methodology workshops on scientific output in the field by evaluating the number of papers applying specific statistical genetic methodologies authored by individuals who have attended workshops.

Incorporating Polygenic Risk Scores in the ACE Twin Model to Estimate A-C Covariance

The assumption in the twin model that genotypic and environmental variables are uncorrelated is primarily made to ensure parameter identification, not because researchers necessarily think that these variables are uncorrelated. Although the biasing effects of such correlations are well understood, a method to estimate these parameters in the twin model would be useful. Here we explore the possibility of relaxing this assumption by adding polygenic scores to the (univariate) twin model. We demonstrate that this extension renders the additive genetic (A)—common environmental (C) covariance (σ_AC) identified. We study the statistical power to reject σ_AC = 0 in the ACE model and present the results of simulations.

Association Study of Opioid Receptor Delta-Type 1 (OPRD1) Gene Variants with Nicotine Dependence in an Iranian Population

Twins studies indicate that many individual factors are associated with genetic polymorphisms in tobacco use, dependence vulnerability, and the ability to quit smoking. Opioid receptor delta-type 1 (OPRD1) is one of the most important genes in the opioid pathway. Therefore, the current study aimed to investigate the association of variants located in the intron 1 of the OPRD1 gene, including rs2236857, rs2236855, and rs760589, with susceptibility to nicotine dependence among northern Iranians. DNA of 426 individuals, including 224 smokers and 202 healthy people, were extracted with the salting-out standard technique, qualified with Agarose gel, then quantified with Nanodrop, and finally genotyped by Amplification Refractory Mutation System (ARMS) PCR. All statistical analyses were performed by SNPAlyze version 8.1 and SPSS version 20. Results revealed no significant association of all three studied variants with the susceptibility to nicotine dependence in any models of inheritance. However, there were five haplotypes with an overall frequency higher than 0.05; no significant impact of any of them on nicotine dependence was observed. Altogether, rs2236857, rs2236855, and rs760589 were not associated with nicotine dependence among northern Iranians.

Estimation of Parental Effects Using Polygenic Scores

Offspring resemble their parents for both genetic and environmental reasons. Understanding the relative magnitude of these alternatives has long been a core interest in behavioral genetics research, but traditional designs, which compare phenotypic covariances to make inferences about unmeasured genetic and environmental factors, have struggled to disentangle them. Recently, Kong et al. (2018) showed that by correlating offspring phenotypic values with the measured polygenic score of parents' nontransmitted alleles, one can estimate the effect of "genetic nurture"-a type of passive gene-environment covariation that arises when heritable parental traits directly influence offspring traits. Here, we instantiate this basic idea in a set of causal models that provide novel insights into the estimation of parental influences on offspring. Most importantly, we show how jointly modeling the parental polygenic scores and the offspring phenotypes can provide an unbiased estimate of the variation attributable to the environmental influence of parents on offspring, even when the polygenic score accounts for a small fraction of trait heritability. This model can be further extended to (a) account for the influence of different types of assortative mating, (b) estimate the total variation due to additive genetic effects and their covariance with the familial environment (i.e., the full genetic nurture effect), and (c) model situations where a parental trait influences a different offspring trait. By utilizing structural equation modeling techniques developed for extended twin family designs, our approach provides a general framework for modeling polygenic scores in family studies and allows for various model extensions that can be used to answer old questions about familial influences in new ways.

The Power to Detect Cultural Transmission in the Nuclear Twin Family Design With and Without Polygenic Risk Scores and in the Transmitted-Nontransmitted (Alleles) Design

We compare the power of two different approaches to detect passive genotype-environment (GE) covariance originating from cultural and genetic transmission operating simultaneously. In the traditional nuclear twin family (NTF) design, cultural transmission is estimated from the phenotypic covariance matrices of the mono- and dizygotic twins and their parents. Here, phenotyping is required in all family members. A more recent method is the transmitted-nontransmitted (T-NT) allele design, which exploits measured genetic variants in parents and offspring to test for effects of nontransmitted alleles from parents. This design requires two-generation genome-wide data and a powerful genome-wide association study (GWAS) for the phenotype in addition to phenotyping in offspring. We compared the power of both designs. Using exact data simulation, we demonstrate three points: how the power of the T-NT design depends on the predictive power of polygenic risk scores (PRSs); that when the NTF design can be applied, its power to detect cultural transmission and GE covariance is high relative to T-NT; and that, given effect sizes from contemporary GWAS, adding PRSs to the NTF design does not yield an appreciable increase in the power to detect cultural transmission. However, it may be difficult to collect phenotypes of parents and the possible importance of gene × age interaction, and secular generational effects can cause complications for many important phenotypes. The T-NT design avoids these complications.

Causes of Variation in Food Preference in the Netherlands

Our current society is characterized by an increased availability of industrially processed foods with high salt, fat and sugar content. How is it that some people prefer these unhealthy foods while others prefer more healthy foods? It is suggested that both genetic and environmental factors play a role. The aim of this study was to (1) identify food preference clusters in the largest twin-family study into food preference to date and (2) determine the relative contribution of genetic and environmental factors to individual differences in food preference in the Netherlands. Principal component analysis was performed to identify the preference clusters by using data on food liking/disliking from 16,541 adult multiples and their family members. To estimate the heritability of food preference, the data of 7833 twins were used in structural equation models. We identified seven food preference clusters (Meat, Fish, Fruits, Vegetables, Savory snacks, Sweet snacks and Spices) and one cluster with Drinks. Broad-sense heritability (additive [A] + dominant [D] genetic factors) for these clusters varied between .36 and .60. Dominant genetic effects were found for the clusters Fruit, Fish (males only) and Spices. Quantitative sex differences were found for Meat, Fish and Savory snacks and Drinks. To conclude, our study convincingly showed that genetic factors play a significant role in food preference. A next important step is to identify these genes because genetic vulnerability for food preference is expected to be linked to actual food consumption and different diet-related disorders.

Nicholas (Nick) G. Martin and the Extended Twin Model

The extended twin model is a unique design in the genetic epidemiology toolbox that allows to simultaneously estimate multiple causes of variation such as genetic and cultural transmission, genotype-environment covariance and assortative mating, among others. Nick Martin has played a key role in the conception of the model, the collection of substantially large data sets to test the model, the application of the model to a range of phenotypes, the publication of the results including cross-cultural comparisons, the evaluation of bias and power of the design and the further elaborations of the model, such as the children-of-twins design.

Cross-Cultural Comparison of Genetic and Cultural Transmission of Smoking Initiation Using an Extended Twin Kinship Model

BACKGROUND

Considerable evidence from twin and adoption studies indicates that genetic and shared environmental factors play a role in the initiation of smoking behavior. Although twin and adoption designs are powerful to detect genetic and environmental influences, they do not provide information on the processes of assortative mating and parent-offspring transmission and their contribution to the variability explained by genetic and/or environmental factors.

METHODS

We examined the role of genetic and environmental factors in individual differences for smoking initiation (SI) using an extended kinship design. This design allows the simultaneous testing of additive and non-additive 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, while also estimating the regression of the prevalence of SI on age. A dichotomous lifetime 'ever' smoking measure was obtained from twins and relatives in the 'Virginia 30,000' sample and the 'Australian 25,000'.

RESULTS

Results demonstrate that both genetic and environmental factors play a significant role in the liability to SI. Major influences on individual differences appeared to be additive genetic and unique environmental effects, with smaller contributions from assortative mating, shared sibling environment, twin environment, cultural transmission, and resulting genotype-environment covariance. Age regression of the prevalence of SI was significant. The finding of negative cultural transmission without dominance led us to investigate more closely two possible mechanisms for the lower parent-offspring correlations compared to the sibling and DZ twin correlations in subsets of the data: (1) age × gene interaction, and (2) social homogamy. Neither of the mechanism provided a significantly better explanation of the data.

CONCLUSIONS

This study showed significant heritability, partly due to assortment, and significant effects of primarily non-parental shared environment on liability to SI.

Cultural evolutionary theory: How culture evolves and why it matters

Human cultural traits-behaviors, ideas, and technologies that can be learned from other individuals-can exhibit complex patterns of transmission and evolution, and researchers have developed theoretical models, both verbal and mathematical, to facilitate our understanding of these patterns. Many of the first quantitative models of cultural evolution were modified from existing concepts in theoretical population genetics because cultural evolution has many parallels with, as well as clear differences from, genetic evolution. Furthermore, cultural and genetic evolution can interact with one another and influence both transmission and selection. This interaction requires theoretical treatments of gene-culture coevolution and dual inheritance, in addition to purely cultural evolution. In addition, cultural evolutionary theory is a natural component of studies in demography, human ecology, and many other disciplines. Here, we review the core concepts in cultural evolutionary theory as they pertain to the extension of biology through culture, focusing on cultural evolutionary applications in population genetics, ecology, and demography. For each of these disciplines, we review the theoretical literature and highlight relevant empirical studies. We also discuss the societal implications of the study of cultural evolution and of the interactions of humans with one another and with their environment.

Sex differences in genetic and environmental contributions to alcohol consumption from early adolescence to young adulthood

AIMS

To estimate genetic and environmental contributions to alcohol consumption from early adolescence to young adulthood, and test whether gender moderates these effects.

DESIGN

Longitudinal twin cohort design.

SETTING

Population-based sample from Norway.

PARTICIPANTS

A total of 2862 male and female twins, aged 14-22 years, were assessed at one (n = 881), two (n = 898) or three (n = 1083) occasions. The percentage of females was between 56 and 63 in the different age groups (in the different waves).

MEASUREMENTS

Alcohol consumption was measured by two questionnaire items about frequency of alcohol use and frequency of being drunk.

FINDINGS

Additive genetic effects showed low to moderate contributions [proportion estimate, 95% confidence interval (CI) = range from 0.03 (0.00-0.14) to 0.49 (0.37-0.59) in males and from 0.09 (0.00-0.57) to 0.41 (0.24-0.58) in females] from adolescence to young adulthood, while environmental influences shared by twin pairs and contributing to twin similarity were moderate to highly influential during this developmental period [proportion estimate, 95% CI = range from 0.04 (0.00-0.13) to 0.45 (0.26-0.60) in males for shared environment in common with females, from 0.25 (0.09-0.42) to 0.54 (0.06-0.78) for shared environment specific to males and from 0.36 (0.20-0.52) to 0.51 (0.37-0.71) in females]. There was evidence of qualitative sex differences with shared environmental influences being largely sex-specific from middle adolescence onwards.

CONCLUSIONS

Alcohol consumption from early adolescence to young adulthood appears to be influenced to a small to moderate degree by genetic factors and to a moderate to high degree by shared environmental factors (e.g. rearing influences, shared friends). The shared environmental factors influencing alcohol consumption appear to be largely gender-specific.

The genetic relationship between cannabis and tobacco cigarette use in European- and African-American female twins and siblings

BACKGROUND

Use of cigarettes and cannabis frequently co-occurs. We examine the role of genetic and environmental influences on variation in and covariation between tobacco cigarette and cannabis use across European-American (EA) and African-American (AA) women.

METHODS

Data on lifetime cannabis and cigarette use were drawn from interviews of 956 AA and 3557 EA young adult female twins and non-twin same sex female full siblings. Twin modeling was used to decompose variance in and covariance between cigarette and cannabis use into additive genetic, shared, special twin and non-shared environmental sources.

RESULTS

Cigarette use was more common in EAs (75.3%, 95% C.I. 73.8-76.7%) than AAs (64.2%, 95% C.I. 61.2-67.2%) while cannabis use was marginally more commonly reported by AAs (55.5%, 95% C.I. 52.5-58.8%) than EAs (52.4%, 95% C.I. 50.7-54.0%). Additive genetic factors were responsible for 43-66% of the variance in cigarette and cannabis use. Broad shared environmental factors (shared+special twin) played a more significant role in EA (23-29%) than AA (2-15%) women. In AA women, the influence of non-shared environment was more pronounced (42-45% vs. 11-19% in EA women). There was strong evidence for the same familial influences underlying use of both substances (rA=0.82-0.89; rC+T=0.70-0.75). Non-shared environmental factors were also correlated but less so (rE=0.48-0.66). No racial/ethnic differences were apparent in these sources of covariation.

CONCLUSION

Heritability of cigarette and cannabis use is comparable across racial/ethnic groups. Differences in the contribution of shared and non-shared environmental influences indicate that different factors may shape substance use in EA and AA women.

Genetic and environmental contributions to initiation of cigarette smoking in young African-American and European-American women

BACKGROUND

Distinctions in the relative contributions of genetic and environmental factors to initiation of cigarette smoking may explain, in part, the differences between African Americans and European Americans in the prevalence of smoking. The current investigation is the first to compare heritable and environmental influences on smoking initiation between African-American and European-American women.

METHODS

Data were drawn from Missouri Adolescent Female Twin Study participants and female Missouri Family Study participants (n=4498; 21% African-American, the remainder European-American). Mean ages at first and last assessments were 17.0 (SD=3.5) and 24.0 (SD=3.2), respectively. Twin-sibling modeling was conducted to estimate the proportion of variance in smoking initiation (i.e., ever trying a cigarette) attributable to additive genetic, shared environmental, special twin environmental, and unique environmental factors.

RESULTS

Additive genetic influences accounted for approximately half of the variance in smoking initiation in both African-American and European-American women. In the African-American subsample, the remaining variance was attributable primarily to unique environmental factors (46%; 95% CI: 28-71%). In the European-American subsample, only 12% (95% CI: 8-16%) of the variance was attributable to unique environmental factors, with the remainder accounted for by shared environmental (13%; 95% CI: 0-41%) and special twin environmental (24%; 95% CI: 0-52%) factors.

CONCLUSIONS

The estimated heritability of smoking initiation is substantial and nearly identical for African-American and European-American women, but the type of environmental factors that contribute to risk differ by race/ethnicity. Whereas the primary environmental influences on European-American women's smoking initiation are at the family level, those that impact African-American women's smoking initiation are primarily individual-specific.

Spousal resemblance for smoking: Underlying mechanisms and effects of cohort and age

BACKGROUND

In this study we ask why spouses resemble each other in smoking behaviour and assess if such resemblance depends on period of data collection or age. Spousal similarity may reflect different, not mutually exclusive, processes. These include phenotypic assortment (choice of spouse is based on phenotype) or social homogamy at the time spouses first meet, and marital interaction during the relationship.

METHODS

Ever and current smoking were assessed between 1991 and 2013 in surveys of the Netherlands Twin Register for 14,230 twins and 1,949 of their spouses (mean age 31.4 [SD=14.0]), and 11,536 parents of twins (53.4 [SD=8.6]). Phenotypic assortment and social homogamy were examined cross-sectionally by calculating the probability of agreement between twins and their spouses, twins and their co-twin's spouse and spouses of both twins as a function of zygosity. Marital interaction was tested by investigating the association between relationship duration and spousal resemblance.

RESULTS

Between 1991 and 2013 smoking declined in all age groups for both genders. Spousal resemblance for ever and current smoking was higher when data were more recent. For ever smoking, a higher age of men was associated with lower spousal resemblance. Phenotypic assortment was supported for both smoking measures, but social homogamy could not be excluded. No effect of marital interaction was found.

CONCLUSIONS

Differences in smoking prevalence across time and age influence spousal similarity. Individuals more often choose a spouse with similar smoking behaviour (phenotypic assortment) causing higher genotypic similarity between them. Given the heritability of smoking this increases genetic risk of smoking in offspring.

Genetic and environmental influences on adolescents' smoking involvement: a multi-informant twin study

Studying monozygotic and dizygotic adolescent twin pairs of both sexes reared together, the present study examined the extent to which the variance in smoking involvement is attributable to genetic and environmental effects, and to what extent there are sex differences in the etiology. Questionnaire data on how often the adolescent had ever smoked tobacco was collected from a population-based twin sample consisting of seven national birth cohorts (ages 12-18), their mothers, and their fathers (N = 1,394 families). The data was analyzed with multivariate genetic modeling, using a multi-informant design. The etiological structure of smoking involvement was best represented in an ACE common pathway model, with smoking defined as a latent factor loading onto all three informants' reports. Estimates could be set equal across sexes. Results showed that adolescent lifetime smoking involvement was moderately heritable (37 %). The largest influence was from the shared environment (56 %), while environmental effects unique to each twin had minimal influence (7 %).

The DNA methylation signature of smoking: an archetype for the identification of biomarkers for behavioral illness

Smoking is perhaps the foremost public health challenge in the United States and in the world. In a series of rapidly emerging studies, we and others have demonstrated that cigarette smoking is associated with changes in the DNA methylation signature of peripheral blood cells. The changes associated with this type of substance use are both dose and time dependent. These changes in DNA methylation are also accompanied by changes in gene transcription and protein expression whose patterns are furthermore indicative of increased vulnerability to other forms of complex illness. In the past, our efforts to translate this knowledge into actionable information has been stymied by a lack of methods through which to systematically to assess these changes. The rapid advance of DNA methylation assessment technologies changes that dynamic and presents the possibility that methylation-based clinical tools to aid the ascertainment of smoking status or effectiveness of treatment can be developed. In this chapter, we will review the latest advances in this field and discuss how these advances allow us insight as to methods through which to prevent smoking and shed insight into optimizing strategies through which to identify biomarkers for other behavioral illnesses which share similar contributions from environmental and gene- environmental interaction effects.

Estimation and partition of heritability in human populations using whole-genome analysis methods

Understanding genetic variation of complex traits in human populations has moved from the quantification of the resemblance between close relatives to the dissection of genetic variation into the contributions of individual genomic loci. However, major questions remain unanswered: How much phenotypic variation is genetic; how much of the genetic variation is additive and can be explained by fitting all genetic variants simultaneously in one model, and what is the joint distribution of effect size and allele frequency at causal variants? We review and compare three whole-genome analysis methods that use mixed linear models (MLMs) to estimate genetic variation. In all methods, genetic variation is estimated from the relationship between close or distant relatives on the basis of pedigree information and/or single nucleotide polymorphisms (SNPs). We discuss theory, estimation procedures, bias, and precision of each method and review recent advances in the dissection of genetic variation of complex traits in human populations. By using genome-wide data, it is now established that SNPs in total account for far more of the genetic variation than the statistically highly significant SNPs that have been detected in genome-wide association studies. All SNPs together, however, do not account for all of the genetic variance estimated by pedigree-based methods. We explain possible reasons for this remaining "missing heritability."

[Characteristics of smoking and secondhand smoke exposure in monozygotic and dizygotic twins: results from an international twin study]

INTRODUCTION

Zygosity differences in smoking exposure are unclear in twins.

AIMS

To assess smoking and secondhand smoke exposure characteristics in twins.

METHODS

151 monozygotic and 62 dizygotic Hungarian and American adult twin pairs (age 43.8±16.5 years, mean±SD) participated in the study.

RESULTS

Monozygotic twins started smoking 1.8 years earlier compared to dizygotic twins (p = 0.08). Dizygotic twins smoked longer (p<0.01) and suffered more parental smoke exposure during childhood (p<0.05). Monozygotic twins reported stricter smoking restrictions at home and workplaces (p<0.005) and less smoke exposure in indoor public places (p<0.01). 85.7% of monozygotic twins were ex or active smokers, while only 69.5% of the dizygotics (p<0.01). Lesser difference was observed in the self-reported smoke exposure rate in monozygotic compared to dizygotic pairs concerning restaurants and cafés (p<0.05) which was not present regarding bars, pubs and transportation facilities.

CONCLUSIONS

Different psychological family orientation may be present across zygosity. Preventive parental care is warranted in twin families exposed to smoking.

Genetic and environmental transmission of body mass index fluctuation

This study sought to determine the relationship between body mass index (BMI) fluctuation and cardiovascular disease phenotypes, diabetes, and depression and the role of genetic and environmental factors in individual differences in BMI fluctuation using the extended twin-family model (ETFM). This study included 14,763 twins and their relatives. Health and Lifestyle Questionnaires were obtained from 28,492 individuals from the Virginia 30,000 dataset including twins, parents, siblings, spouses, and children of twins. Self-report cardiovascular disease, diabetes, and depression data were available. From self-reported height and weight, BMI fluctuation was calculated as the difference between highest and lowest BMI after age 18, for individuals 18-80 years. Logistic regression analyses were used to determine the relationship between BMI fluctuation and disease status. The ETFM was used to estimate the significance and contribution of genetic and environmental factors, cultural transmission, and assortative mating components to BMI fluctuation, while controlling for age. We tested sex differences in additive and dominant genetic effects, parental, non-parental, twin, and unique environmental effects. BMI fluctuation was highly associated with disease status, independent of BMI. Genetic effects accounted for ~34 % of variance in BMI fluctuation in males and ~43 % of variance in females. The majority of the variance was accounted for by environmental factors, about a third of which were shared among twins. Assortative mating, and cultural transmission accounted for only a small proportion of variance in this phenotype. Since there are substantial health risks associated with BMI fluctuation and environmental components of BMI fluctuation account for over 60 % of variance in males and over 50 % of variance in females, environmental risk factors may be appropriate targets to reduce BMI fluctuation.

A systematic review of the A118G (Asn40Asp) variant of OPRM1 in relation to smoking initiation, nicotine dependence and smoking cessation

Candidate gene studies on smoking behaviors mainly focused on dopaminergic and serotonergic genes, but genes within the µ-opioid system might also be involved. The A118G variant within the OPRM1 gene has been most often examined in relation to smoking, yielding inconsistent findings. It is largely unknown which of the alleles increases susceptibility for smoking behaviors. The aim of this review was to merge findings of OPRM1 gene studies in relation to smoking behaviors and to elaborate on the underlying biological mechanism of the A118G variant. It appeared that A118 was more likely to increase susceptibility to smoking behaviors than 118G, especially with regard to nicotine dependence, but less with smoking initiation and cessation. The proposed functioning of the OPRM1 gene is further explained.

Genetics of smoking and depression

Smoking and depression are significant public health problems with multiple etiological dimensions and outcomes. Although each condition is important by itself, they are important because they often potentiate each other. Consequently, it is also essential to understand the nature their relationship. This representative review focuses on the genetic etiology of the relationship in the context of reviewing first the epidemiology of depression and smoking, and then by exploring behavioral and molecular genetic studies, and other psychiatric and medical comorbidities. At this point, epidemiological evidence for a relationship between depression and smoking/nicotine dependence is compelling. Although behavioral genetic results differ somewhat by gender and in accordance with specific definitions of depression and smoking variables, recent studies show converging evidence for common genetic factors underlying the relationship, often in addition to non-shared environmental factors. The search for underlying genes and genetic mechanisms is at an early stage, but shows promising candidate genes and genetic approaches for future studies.

Delay discounting by adolescents experimenting with cigarette smoking

AIMS

To evaluate delay discounting and self-reported impulsive behavior in a sample of adolescents experimenting with cigarette smoking compared with adolescents who had never smoked or were daily smokers.

DESIGN

A cross-sectional design was used to compare smoking-status groups.

SETTING

Columbus, Ohio, a city of approximately 780,000 people.

PARTICIPANTS

A sample of 141 male and female adolescents with a mean age of 15.37 (standard deviation = 1.09) years.

MEASUREMENTS

Primary measures included a computerized assessment of delay discounting, a self-report assessment of impulsivity [Barratt Impulsiveness Scale-adolescent (BIS-11-A)] and verifications of cigarette smoking status (breath carbon monoxide and urinary cotinine level).

FINDINGS

Smokers discounted more by delay and had higher impulsivity scores than non-smokers. Experimenters had scores intermediate to those of smokers and non-smokers on both measures. In some analyses the difference between experimenters and non-smokers was significant, with experimenters showing greater delay discounting, but in no case did experimenters differ significantly from the smokers.

CONCLUSIONS

Young people who experiment with cigarettes appear to be similar to those who smoke regularly in terms of tendency to discount future gains and report impulsive tendencies, and generally higher in these traits than non-smokers.

Cigarette experimentation in Mexican origin youth: psychosocial and genetic determinants

BACKGROUND

Established psychosocial risk factors increase the risk for experimentation among Mexican origin youth. Now, we comprehensively investigate the added contribution of select polymorphisms in candidate genetic pathways associated with sensation seeking, risk taking, and smoking phenotypes to predict experimentation.

METHODS

Participants (N = 1,118 Mexican origin youth) recruited from a large population-based cohort study in Houston, TX, provided prospective data on cigarette experimentation over 3 years. Psychosocial data were elicited twice-baseline and final follow-up. Participants were genotyped for 672 functional and tagging variants in the dopamine, serotonin, and opioid pathways.

RESULTS

After adjusting for gender and age, with a Bayesian False Discovery Probability set at 0.8 and prior probability of 0.05, six gene variants were significantly associated with risk of experimentation. After controlling for established risk factors, multivariable analyses revealed that participants with six or more risk alleles were 2.25 [95% confidence interval (CI), 1.62-3.13] times more likely to have experimented since baseline than participants with five or fewer. Among committed never-smokers (N = 872), three genes (OPRM1, SNAP25, HTR1B) were associated with experimentation as were all psychosocial factors. Among susceptible youth (N = 246), older age at baseline, living with a smoker, and three different genes (HTR2A, DRD2, SLC6A3) predicted experimentation.

CONCLUSIONS

Our findings, which have implications for development of culturally specific interventions, need to be validated in other ethnic groups.

IMPACT

These results suggest that variations in select genes interact with a cognitive predisposition toward smoking. In susceptible adolescents, the impact of the genetic variants appears to be larger than committed never-smokers.

Parental criticism and externalizing behavior problems in adolescents: the role of environment and genotype-environment correlation

Genetic factors are important for the association between parental negativity and child problem behavior, but it is not clear whether this is due to passive or evocative genotype-environment correlation (rGE). In this study, we applied the extended children-of-twins model to directly examine the presence of passive and evocative rGE as well as direct environmental effects in the association between parental criticism and adolescent externalizing problem behavior. The cross-sectional data come from the Twin and Offspring Study in Sweden (N = 909 pairs of adult twins) and from the Twin Study of Child and Adolescent Development (N = 915 pairs of twin children). The results revealed that maternal criticism was primarily due to evocative rGE emanating from their adolescent's externalizing behavior. On the other hand, fathers' critical remarks tended to affect adolescent problem behavior in a direct environmental way. This suggests that previously reported differences in caretaking between mothers and fathers also are reflected in differences in why parenting is associated with externalizing behavior in offspring.

A latent class analysis of DSM-IV and Fagerström (FTND) criteria for nicotine dependence

BACKGROUND

Nicotine dependence is associated with considerable morbidity and mortality. Two predominant classification systems, the Diagnostic and Statistical Manual (DSM-IV) and Fagerström Test for Nicotine Dependence (FTND), have been used to measure liability to nicotine dependence, yet few studies have attempted to simultaneously examine both sets of criteria.

METHODS

Using a sample of 624 regular smoking individuals who are offspring of Vietnam Era Twin fathers ascertained for an offspring of twin study, we applied latent class analysis to the 7 DSM-IV and the 6 FTND criteria to classify individuals by their nicotine dependence symptom profiles. Post-hoc across-class comparisons were conducted using a variety of smoking-related variables and aspects of psychopathology. Whether a single class identified offspring at high genetic and environmental vulnerability was also investigated.

RESULTS

The cross-diagnosis kappa was .30. A 4-class solution fit these data best. The classes included a low DSM-low FTND class and a high DSM-high FTND class; a moderate DSM-moderate FTND class, which was distinguished by moderate levels of smoking and intermediate levels of comorbid psychopathology; and a light smoking-moderate FTND class consisting primarily of lighter smokers with a more recent onset of regular smoking. High genetic and environmental vulnerability to nicotine dependence was noted in all classes with no statistically significant across-class differences.

CONCLUSIONS

In general, the DSM-IV and FTND criteria performed similarly to define a continuum of risk for nicotine dependence. The emerging class of light smokers should be further investigated to assess whether they transition to another class or remain as such.

Interplay between heritability of smoking and environmental conditions? A comparison of two birth cohorts

BACKGROUND

Attitudes and policy towards smoking changed over the past years in many countries including the Netherlands. Generally, this led to a decrease in smoking prevalence. As demonstrated in twin and family studies, individual differences in smoking behavior are partly influenced by genetic factors. We explore whether the current change in environmental conditions has influenced the genetic architecture of smoking. This would constitute evidence for Gene × Environment (G×E) interaction.

METHODS

Data on smoking were available from 2 cohorts of young adult twins (18-25 year) registered with the Netherlands Twin Register. The first cohort completed a survey in 1993-1995 (n = 2669) and the second in 2009-2010 (n = 2339). Prevalence and genetic architecture of smoking were compared across cohorts using structural equation models in MX.

RESULTS

Smoking prevalence decreased from 40-51% to 22-23% between 1993-1995 and 2009-2010. Genetic analyses, making use of the different genetic resemblance in monozygotic and dizygotic twins, showed that the heritability was the same in both cohorts.

CONCLUSIONS

The change in policy and smoking attitudes that led to a decrease in prevalence of smoking did not change the heritability of smoking and thus no evidence was found for GxE interaction.

Behavior matters

Behavior has a broad and central role in health. Behavioral interventions can be effectively used to prevent disease, improve management of existing disease, increase quality of life, and reduce healthcare costs. A summary is presented of evidence for these conclusions in cardiovascular disease/diabetes, cancer, and HIV/AIDS as well as with key risk factors: tobacco use, poor diet, physical inactivity, and excessive alcohol consumption. For each, documentation is made of (1) moderation of genetic and other fundamental biological influences by behaviors and social-environmental factors; (2) impacts of behaviors on health; (3) success of behavioral interventions in prevention; (4) disease management; (5) quality of life, and (6) improvements in the health of populations through behavioral health promotion programs. Evidence indicates the cost effectiveness and value of behavioral interventions, especially relative to other common health services as well as the value they add in terms of quality of life. Pertinent to clinicians and their patients as well as to health policy and population health, the benefits of behavioral interventions extend beyond impacts on a particular disease or risk factor. Rather, they include broad effects and benefits on prevention, disease management, and well-being across the life span. Among priorities for dissemination research, the application of behavioral approaches is challenged by diverse barriers, including socioeconomic barriers linked to health disparities. However, behavioral approaches including those emphasizing community and social influences appear to be useful in addressing such challenges. In sum, behavioral approaches should have a central place in prevention and health care of the 21st century.

Association of the calcyon neuron-specific vesicular protein gene (CALY) with adolescent smoking initiation in China and California

Although previous investigations have indicated a role for genetic factors in smoking initiation, the underlying genetic mechanisms are still unknown. In 2,339 adolescents from a Chinese Han population in the Wuhan Smoking Prevention Trial (Wuhan, China, 1998-1999), the authors explored the association of 57 genes in the dopamine pathway with smoking initiation. Using a conservative approach for declaring significance, positive findings were further examined in an independent sample of 603 Caucasian adolescents followed for up to 10 years as part of the Children's Health Study (Southern California, 1993-2009). The authors identified 1 single nucleotide polymorphism (rs2298122) in the calcyon neuron-specific vesicular protein gene (CALY) that was positively associated with smoking initiation in females (odds ratio = 2.21, 95% confidence interval: 1.49, 3.27; P = 8.4 × 10(-5)) in the Wuhan Smoking Prevention Trial cohort, and they replicated the association in females from the Children's Health Study cohort (hazard rate ratio = 2.05, 95% confidence interval: 1.27, 3.31; P = 0.003). These results suggest that the CALY gene may influence smoking initiation in adolescents, although the potential roles of underlying psychological characteristics that may be components of the smoking-initiation phenotype, such as impulsivity or novelty-seeking, remain to be explored.

Addiction as a systems failure: focus on adolescence and smoking

OBJECTIVE

Scientific advances in the field of addiction have forever debunked the notion that addiction reflects a character flaw under voluntary control, demonstrating instead that it is a bona fide disease of the brain. The aim of this review is to go beyond this consensus understanding and explore the most current evidence regarding the vast number of genetic, developmental, and environmental factors whose complex interactions modulate addiction risk and trajectory.

METHOD

Focusing on childhood and adolescent smoking as a paradigm, we review the important risk factors for the development of addictions, starting at the level of genetics and closing with a focus on sociocultural and policy factors.

RESULTS

A critical review of the pertinent literature provides a detailed view of the cumulative power of risk and protection factors across different phenomenological levels to modulate the risk of undesirable outcomes, particularly for young people. The result represents a compelling argument for the need to engage in comprehensive, multilevel approaches to promoting health.

CONCLUSIONS

Today, the field of medicine understands more about disease than about health; however it need not be that way. The view of drug addiction as a systems failure should help refocus our general approach to developing dynamic models and early comprehensive interventions that optimize the ways in which we prevent and treat a complex, developmental disorder such as drug addiction.

Genetically informative research on adolescent substance use: methods, findings, and challenges

OBJECTIVE

To provide an overview of the genetic epidemiology of substance use and misuse in adolescents.

METHOD

A selective review of genetically informative research strategies, their limitations, and key findings examining issues related to the heritability of substance use and substance use disorders in children and adolescents is presented.

RESULTS

Adoption, twin, and extended-family designs have established there is a strong heritable component to liability to nicotine, alcohol, and illicit drug dependence in adults. However, shared environmental influences are relatively stronger in youth samples and at earlier stages of substance involvement (e.g., use). There is considerable overlap in the genetic influences associated with the abuse/dependence across drug classes, and shared genetic influences contribute to the commonly observed associations between substance-use disorders and externalizing and, to a lesser extent, internalizing psychopathology. Rapid technologic advances have made the identification of specific gene variants that influence risks for substance-use disorders feasible, and linkage and association (including genomewide association studies) have identified promising candidate genes implicated in the development of substance-use disorders.

CONCLUSIONS

Studies using genetically informative research designs, including those that examine aggregate genetic factors and those examining specific gene variants, individually and in interaction with environmental influences, offer promising avenues not only for delineating genetic effects on substance-use disorders but also for understanding the unfolding of risk across development and the interaction between environmental and genetic factors in the etiology of these disorders.

Tobacco, cannabis, and other illicit drug use among Finnish adolescent twins: causal relationship or correlated liabilities?

OBJECTIVE

Among Finnish adolescent twins, we compared (a) a model that describes a direct impact of liability to tobacco use on cannabis and other illicit drug use with (b) a model that included a shared underlying liability for these substances. Furthermore, the extent to which genetic and environmental influences contribute to the covariation between liabilities to use these substances was examined.

METHOD

Tobacco and illicit drug use were assessed at age 17.5 years. Twin data on 3,744 individuals were analyzed using standard biometrical methods. Two alternative multivariate models were fi t and compared with Mx, a statistical program for genetic model fitting.

RESULTS

The multivariate model, including a direct impact of the initiation of tobacco use on illicit drug use, provided the best fit to the data. In this model, the total variation in the initiation of illicit drugs was decomposed to genetic factors (32%), common environmental factors (20%), unique environmental factors (8%), and a component due to initiation of smoking (40%). Most variation in the progression of illicit drug use was the result of initiation of smoking and illicit drug use (83%).

CONCLUSIONS

Liability to initiate smoking directly affects illicit drug use in our best-fitting model. Our findings suggest that several common genetic influences may be related to tobacco use and illicit drugs but that a search for specific genes underlying illicit drug use is justifi ed as well. Such specific genes may hold a key to understanding biological vulnerabilities that lead to illicit drug use, which could aid in the development of targeted interventions.

Heritability of smoking, alcohol use, and psychological characteristics among adolescent twins in Qingdao, China

BACKGROUND

Previous studies of genetic and environmental influences on adolescent substance use have been limited to Western samples.

METHODS

This study assesses genetic and environmental contributions to cigarette smoking, alcohol use, and psychological variables (depression, anxiety, aggression, hostility) among 602 pairs of adolescent twins, 11 to 19 years old, in Qingdao, China.

RESULTS

Heritable influences were more pronounced for alcohol use than cigarette smoking. In univariate analyses, no heritable effects were found for depression or aggression, and modest heritability was found for anxiety. Hostility was relatively more heritable in girls than boys. Bivariate associations between substance use and psychological measures could be attributed to a combination of common genetic and environmental factors.

CONCLUSIONS

Among Chinese adolescents, experimentation with tobacco is familial, and experimentation with alcohol is heritable. The genetic and environmental architecture of hostility differs by gender. Consistency of univariate results with Western adolescent samples appears limited to the alcohol use measures.

Putting the 'human' back in genetics: modeling the extended kinships of twins

Few papers have attempted to address the potential of extended kinships of twins to resolve the complexities of biological and cultural inheritance in humans. Since the mid-80s these issues have largely been buried beneath the quest for specific genes that contribute to individual differences in complex traits and liability to disease. The articles of Keller et al. and Medland and Keller (current issue) revisit these issues. History will decide whether these new papers represent the final gasp of a paradigm superceded by the 'new' genetics or contain the timely seeds of new birth in the face of a 30-year genetic research program otherwise poised on the cusp of degeneration (Lakatos & Musgrave, 1970).

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.

Delay discounting by the children of smokers and nonsmokers

This research compared delay discounting in mothers and their children (12 or 13 years of age). Half of the mothers (n=15) were current smokers, and the other half (n=15) reported never smoking. Considerable research has shown that adult smokers discount more by delay than nonsmokers, and that parent smoking is a risk factor for adolescent smoking. Thus, it was hypothesized that the mothers who smoked would discount more by delay than the mothers who had never smoked. Also, it was expected that children at increased risk for smoking (i.e., mother is smoker) would discount more by delay than children at lower risk for smoking (i.e., mother is nonsmoker). The results confirmed these hypotheses: mothers who smoked discounted significantly more than nonsmoking mothers; and, in a parallel fashion, children with mothers who smoked discounted significantly more than children of nonsmokers. These findings indicate that delay discounting may be a behavioral risk factor for adolescent cigarette smoking that predates any substantial use of nicotine.

Testing different types of genotype-environment correlation: an extended children-of-twins model

This study presents an extended children-of-twins model, which allowed the authors to test the direction of the association between parenting and child adjustment. Three mechanisms were examined: direct phenotypic influence of parenting on child behavior (controlling for both parental and child genotype), passive genotype-environment correlation, and evocative genotype-environment correlation. This model was tested with Monte Carlo simulations. The authors generated data sets consisting of 1,000 twin parent pairs together with their children and 1,000 twin children pairs together with their parents. These simulated data sets were then used to estimate the model, and the procedure was repeated 1,000 times. The simulation results showed that this model recovered the true values of parameters with high precision. The model was also applied to an observed data set to analyze, as a first example, the association between maternal emotional overinvolvement and child internalizing problems. The results showed that this association was best explained by evocative genotype-environment correlation.

Gamma-aminobutyric acid receptor genes and nicotine dependence: evidence for association from a case-control study

AIMS

The gamma-aminobutyric acid receptor A (GABRA) gene clusters on chromosomes 4 and 5 have been examined previously for their association with alcohol and drug dependence phenotypes. Compelling evidence suggests that GABRA2 is associated with alcohol and drug dependence. However, no study has investigated whether genes in the GABA(A) gene clusters are associated with nicotine dependence, an important phenotype with a high correlation to persistent smoking, the single most preventable cause of mortality world-wide.

DESIGN

Using data on 1050 nicotine-dependent cases and 879 non-dependent smoking controls, we used logistic regression to examine the association between single nucleotide polymorphisms (SNPs) in 13 genes in the GABA(A) receptor system as well as GABBR2 (a GABA(B) gene).

FINDINGS

We found evidence for association between four SNPs in GABRA4, two SNPs in GABRA2 and one SNP in GABRE with nicotine dependence. These included a synonymous polymorphism in GABRA2 (rs279858), lying in a highly conserved region, which has been shown previously to be associated with alcohol and drug dependence. A non-synonymous polymorphism (rs16859834/rs2229940) in GABRA4, also highly conserved, was associated at P-value of 0.03. Significant haplotypes associated with nicotine dependence were found for GABRA2. No evidence for epistatic interactions were noted. Our study did not find evidence for an association between GABBR2 gene and nicotine dependence.

CONCLUSIONS

Given the potential role of compounds that enhance GABAergic neurotransmission in smoking cessation research, these findings have enormous potential for informing the wider field of addiction research.

Differences in smoking habits of MZ and DZ twins: a commentary on Tishler and Carey

Tishler and Carey (this issue, 2007) have analyzed the difference in ever smoking prevalence between monozygotic (MZ) and dizygotic (DZ) twins in 8 published twin studies. They conclude that the lower prevalence in MZ twins might invalidate conclusions from classic twin models. I critique the conducted meta-analysis and suggest additional analyses to be conducted. A reanalysis of the Finnish Twin Cohort 1975 questionnaire data (included in Tisher and Carey) indicates that when intrapair correlations are correctly taken into account, the MZ-DZ difference in that study is no longer statistically significant. Even if the Tisher/Carey finding holds up in further analyses, it pertains only to the observed trait, the effect is small in size, and the mechanism underlying it needs to be explored.

Does smoking among friends explain apparent genetic effects on current smoking in adolescence and young adulthood?

We used data from a prospective cohort study of twins to investigate the influence of unmeasured genetic and measured and unmeasured environmental factors on the smoking behaviour of adolescents and young adults. Twins were surveyed in 1988 (aged 11-18 years), 1991, 1996 and 2004 with data from 1409, 1121, 732 and 758 pairs analysed from each survey wave, respectively. Questionnaires assessed the smoking behaviour of twins and the perceived smoking behaviour of friends and parents. Using a novel logistic regression analysis, we simultaneously modelled individual risk and excess concordance for current smoking as a function of zygosity, survey wave, parental smoking and peer smoking. Being concordant for having peers who smoked was a predictor of concordance for current smoking (P<0.001). After adjusting for peer smoking, monozygotic (MZ) pairs were no more alike than dizygotic pairs for current smoking at waves 2, 3 and 4. Genetic explanations are not needed to explain the greater concordance for current smoking among adult MZ pairs. However, if they are invoked, the role of genes may be due to indirect effects acting through the social environment. Smoking prevention efforts may benefit more by targeting social factors than attempting to identify genetic factors associated with smoking.