Adolescent Drinking Behavior and Genes

Gene - Environment Interaction
by National Institute of Health

(Page 5 of 5)

Although the contribution of genetic influences to alcohol use increases during adolescence, environmental factors do not become irrelevant but play an important role in moderating the influence of genetic predispositions. For example, findings from the FinnTwin studies demonstrated that urban or rural residency has a significant impact on the relative contributions of genetic and other environmental factors which affect adolescent drinking. Genetic influences on drinking frequency were much stronger among twins living in urban settings, where they accounted for 34 percent of the variance in drinking at age 16, than among twins residing in rural Finland, where they accounted for 18 percent of the variance in drinking. Conversely, common environmental effects accounted for more than 50 percent of the variation in drinking patterns among rural youth, but for only one-third of the variance in youth living in urban settings. This finding has been replicated recently in a twin study conducted in Minnesota.

Subsequent analyses have attempted to identify the processes underlying this phenomenon. What characteristics of urban settings allow for greater expression of genetic effects? Why do environmental influences exert a larger influence in rural settings? To explore these questions, researchers have expanded the classic twin model to incorporate more continuous measures of the environment and examine a series of socioregional variables that they hypothesized to be more closely related to the interaction effect.

Finland offers an opportunity to explore such questions because the country is divided into municipalities - local government units that levy taxes and assume responsibility for education, health care, and social services. For these municipalities, whether in the metropolitan Helsinki area or in sparsely populated areas of Finnish Lapland and the Åland Islands, a wealth of sociodemographic information is available that may be relevant to risk for adolescent drinking.

To study the influence of community- level factors on adolescent drinking outcomes, Dick and colleagues linked this sociodemographic information to each twin pair's residency code. By doing so, the investigators were able to identify characteristics of the communities in which each twin pair lived that could be related to risk. The specific sociodemographic variables assessed to characterize the urban/rural interaction effect included the following:

• Percentage of young adults, ages 20 - 24, in a municipality.
• Neighborhood instability, as indicated by the percentage of migration into and out of a given region.
• Per capita expenditure on alcohol in each region, relative to the mean amount spent in the entire country.

The researchers found that each of these variables had dramatic effects on the genetic and environmental influences on drinking behavior, with a more than fivefold difference in the magnitude of genetic effects between environmental extremes. Thus, communities characterized by more young adult role models, greater social mobility, and higher alcohol sales allowed for increased expression of genetic dispositions that contribute to individual differences in adolescent drinking. On the other hand, communities with more social stability created an environment in which common environmental effects, within families and within communities, assumed greater importance.

Analyses of other twin data sets also have demonstrated the importance of environmental factors in moderating genetic and environmental influences on drinking patterns. In Australian twin data, marital status moderated the importance of genetic effects on alcohol consumption in women - that is, living in a marriage-like relationship reduced the impact of genetic influences on drinking. In this study, genetic factors accounted for only half as much variance in drinking among married women as among unmarried women.

In another study, on the effect of religiosity on alcohol use among women, genetic factors played a significant role in women who had not had a religious upbringing, dwarfing the influence of shared environment. By contrast, there was no evidence of genetic influence in women with a religious upbringing, but shared environment had a substantial impact.

Lastly, researchers recently have begun to explore the potential moderating role of parenting factors on adolescent alcohol use. Preliminary analyses have provided strong evidence that changes in the relevance of genetic and environmental factors are related to differences in parental monitoring, involvement, and support.

Complementary twin designs, such as studies of the offspring of adult twins, also provide rich opportunities to study gene - environment interaction and the potential moderating role of environmental factors, such as parenting practices and parental models.

Molecular Genetic Advances in Understanding Drinking Patterns

Advances in genetic analysis and in the technology used to determine a person's genetic makeup now make it possible to move beyond estimates of unmeasured sources of genetic variance. Researchers have begun to identify specific genes involved in complex behavioral outcomes such as alcohol use.

Using a candidate gene approach, which examines the influence of specific genes suspected of playing a role in the behavior under investigation, two studies recently reported associations between specific gene variants and drinking behavior among college students. The first study investigated the role of variation in the gene encoding a protein called the serotonin transporter. This study found that in a sample of White students, those with a particular version of this gene engaged in binge drinking more often, drank to intoxication more often, and consumed more alcoholic drinks per drinking occasion than students with other variants of the gene.

Another study focused on the gene that encodes aldehyde dehydrogenase, an enzyme involved in the breakdown of alcohol in the body. This study reported that Asian American college students who carried a particular version of the ALDH gene which results in less efficient alcohol breakdown were less likely to be regular drinkers and engage in binge-drinking episodes; they also reported a lower number of maximum drinks consumed in a 24-hour period than Asian students with other ALDH variants.

Such studies are complemented by large-scale efforts to identify genes that contribute to alcoholism. One of these efforts, funded by the National Institute on Alcohol Abuse and Alcoholism, is the Collaborative Study on the Genetics of Alcoholism. COGA researchers recently published reports of several genes associated with alcohol dependence in adults, and some of these findings already have been replicated by other investigators.

The next challenge will be to expand these findings to children and adolescents to better understand how specific predisposing genes influence trajectories of drinking and other behavioral problems across development. The identification of specific genes influencing alcohol use and abuse will enable researchers to begin to decipher how the effects of these genes are modified by specific environmental influences. Elucidating the influence of specific genes on various aspects of drinking, the actions of these genes, and their interactions with specific environmental risk factors should dramatically enhance our understanding of the development of alcohol use and abuse.

Conclusions and Future Outlook

Both genetic and environmental factors contribute to the development of patterns of alcohol use during adolescence. The relative importance of genetic and environmental factors, however, varies for different phases of alcohol use and different stages of development. Environmental influences originating both within the family and in the child's school and neighborhood greatly influence the initiation of alcohol use. Once drinking has been initiated, however, genetic influences assume increasing importance across development, as individual differences in patterns of drinking emerge. The degree to which genetic effects contribute to adolescent drinking patterns also is subject to moderation by the environment, with factors such as religiosity, marital status, and regional residency all contributing to the relative importance of genetic and environmental effects on alcohol use.

Other recent efforts have been directed toward identifying specific genes contributing to variation in drinking patterns. As scientists begin to understand more about the nature of gene - environment interactions, molecular genetic studies can be designed that take this emerging information into account. The potential of the molecular genetics approach has been illustrated by a recent study that found an association between a variant of the serotonin transporter gene and major depression, but only in people who had experienced stressful life events. Large-scale collaborative projects have begun to identify genes involved in alcohol dependence and related behaviors.

Once researchers have identified specific genes associated with complex disorders such as alcoholism, they can begin to address questions regarding developmental effects or gene - environment interactions associated with these genes. Such questions include whether genetic variants affect different behaviors at different developmental stages, whether particular genetic variants are more or less important at various ages, or whether the risk associated with a specific gene varies according to environmental context. Integrating the knowledge obtained from behavior-genetic studies with findings emerging from the areas of statistical and molecular genetics promises to enhance our understanding of the factors that contribute to drinking among adolescents and young adults.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.