Introduction: Alcohol dependence and habitual smoking frequently co-occur and possibly mutually influence each other. Both have been related to alterations of dopaminergic neurotransmission. The aim of this analysis of the Munich Gene Data Bank for Alcoholism(MGBA) was to re-evaluate the potential relation between D2 receptor and dopamine transporter gene haplotypes and quantity-related phenotypes of alcohol consumption (average daily alcohol intake before admission for treatment) and smoking (average units smoked per day).
Methods: A total of 333 inpatients (265 males) were enrolled in the study, all of who met the ICD10 diagnosis of alcohol dependence. Mild and strong quantity drinkers and smokers were separated into groups by median split. A number of genetic markers were chosen across D2 dopamine receptor gene (-141 Ins/Del, Taq1B, Taq1D, Ser311Cys; rs1079594 (intron 7); Taq1A) and dopamine transporter (40bp variable number of tandem repeat; rs2617605 (intron 2); rs37022 (intron 7); rs40184 (intron 14)). Genotyping was performed using PCR.
Results: Strong drinkers reported significantly higher amounts of smoking and vice versa. While no association was detected for dopamine transporter genetic variants, a number of D2 receptor gene single nucleotide polymorphisms were related to both smoking- and drinking-related behaviours. Subsequent analysis of D2 receptor gene haplotypes revealed that two common haplotypes had a significant association with quantitative phenotypes of regular drinking (Ins-C-G-C-A1) and smoking (Ins-T-G-A-A2).
Discussion: The finding of an association between common D2 dopamine receptor gene haplotypes with the quantity of drinking and smoking corroborates with results from previous studies suggesting a relationship between the dopamine system and alcohol and substance use disorders. Furthermore, it makes D2 dopamine receptor a candidate gene significantly influencing both alcohol and nicotine dependence.