Pharmacogenetics of alcohol response and alcoholism: the interplay of genes and environmental factors in thresholds for alcoholism

Recent advances in neuroscience and genetics have enabled a better understanding of genetically influenced differences in ethanol ("alcohol")-related responses and differential vulnerability to alcohol dependence at the cellular and molecular levels. Heritability studies reveal that the role of genetic factors in alcoholism is largely substance-specific, with the exception of nicotine. One focus of genetic research in alcoholism is the study of functional polymorphisms influencing alcohol metabolism, such as the aldehyde dehydrogenase type 2 Glu487Lys and alcohol dehydrogenase type 2 His47Arg polymorphisms, which affect vulnerability to alcoholism via pharmacokinetic mechanisms, and cross-population studies have begun to reveal important gene-environment interactions. The other focus is on functional genetic variants of proteins involved in the neuronal response to alcohol, including alcohol sensitivity, reward, tolerance, and withdrawal. Studies on the roles of GABA(A) alpha6-amino acid substitutions in rodents in alcohol and benzodiazepine sensitivity, and potential roles in human alcohol and benzodiazepine sensitivity are reviewed. These studies, together with recently developed knowledge on a GABA(A) receptor gene cluster at a quantitative trait loci for alcohol withdrawal on mouse chromosome 11, indicate that research investigation of variation at GABA(A) neurotransmission is a promising area in the pharmacodynamics of alcohol and in differential susceptibility to alcoholism. Genes for proteins involved in alcohol-mediated reward include genes for transporters and receptors for dopamine, serotonin, opioids, and GABA. These genes and their functional variants also represent important targets for understanding alcohol's effects in humans. Identification of genes for alcoholism vulnerability is important in the near future, not only for prevention, but also for development and targeting treatments.

Relationship between a GABAA alpha 6 Pro385Ser substitution and benzodiazepine sensitivity

OBJECTIVE

In humans, interindividual variation in sensitivity to benzodiazepine drugs may correlate with behavioral variation, including vulnerability to disease states such as alcoholism. In the rat, variation in alcohol and benzodiazepine sensitivity has been correlated with an inherited variant of the GABAA alpha 6 receptor. The authors detected a Pro385Ser [1236C > T] amino acid substitution in the human GABAA alpha 6 that may influence alcohol sensitivity. In this pilot study, they evaluated the contribution of this polymorphism to benzodiazepine sensitivity.

METHOD

Sensitivity to diazepam was assessed in 51 children of alcoholics by using two eye movement measures: peak saccadic velocity and average smooth pursuit gain. Association analysis was performed with saccadic velocity and smooth pursuit gain as dependent variables and comparing Pro385/Ser385 heterozygotes and Pro385/Pro385 homozygotes.

RESULTS

The Pro385Ser genotype was associated with less diazepam-induced impairment of saccadic velocity but not with smooth pursuit gain.

CONCLUSIONS

The Pro385Ser genotype may play a role in benzodiazepine sensitivity and conditions, such as alcoholism, that may be correlated with this trait.

Selective genotyping for the role of 5-HT2A, 5-HT2C, and GABA alpha 6 receptors and the serotonin transporter in the level of response to alcohol: a pilot study

BACKGROUND

The vulnerability to alcohol dependence appears to be genetically influenced through a variety of mechanisms. One potentially genetically mediated channel may be a low level of response (LR) to alcohol, which has been seen in children of alcoholics and noted to predict future alcohol abuse and dependence. This pilot study uses a case and control genetic association approach to evaluate the possible role of five genotypes in both LR and alcoholism in informative subgroups of men with high and low LR scores documented 15 years earlier.

METHODS

As part of a larger study, 41 men, about 39 years old, were selected from among the first 113, completed 15-year follow-ups in a prospective study. The 17 subjects whose LRs at age 20 were in the lower third were compared on five polymorphisms of four genes with 24 men whose reactions to alcohol had been above the median.

RESULTS

The 14 men with the LL genotype of the serotonin transporter (5-HTT) polymorphism and the seven with the Pro/Ser genotype of the GABAA alpha 6 polymorphism had demonstrated lower LR scores at about age 20, and had significantly higher proportions of alcoholics than the other genotypes for those loci. All four subjects with combined LL and Pro/Ser genotypes had developed alcoholism and demonstrated the lowest LR scores overall. There was no evidence that two polymorphisms of the 5-HT2A receptor gene and one of the 5-HT2C receptor gene were related to LR or alcoholism in this sample.

CONCLUSIONS

These results are consistent with animal and human studies suggesting a possible role for genetic variation in the GABAA alpha 6 and the serotonin transporter in the reaction to alcohol and the alcoholism risk.

Theory of mind and pragmatic understanding following right hemisphere damage

It has been maintained that 3-year-olds' difficulties in correctly predicting the undesired outcome of false beliefs reflects difficulties in interpreting the implications of conversations rather than a conceptual limitation in their theory of mind. As the right hemisphere has been seen to be responsible for the interpretation of the pragmatic aspects of communication, right-hemisphere-damaged (RHD) and left-hemisphere-damaged (LHD) adult patients in our study were compared on their ability to correctly draw inferences in false belief tasks. The RHD but not the LHD patients were found to have difficulties similar to those of young children in understanding the conversational implications test questions. Most reported that a central story character would look for a pet in the place where it was really located instead of where the character believed it was located. However, when then asked in a control question where the pet really was, the RHD patients often switched their answer to the test question and referred to the believed location. Removal of the need to infer the questioner's meaning enabled both RHD and LHD subjects to make correct false belief predictions. The results are discussed in terms of the effects of brain damage on spatial memory and the pragmatic demands of theory of mind tasks.