Neuronal nicotinic acetylcholine receptors: From gene to disease and back

Nicotine addiction has frequently been observed to be comorbid with alcoholism (True et al., 1999). Despite recent investigations into the common genetic influences between nicotine and alcohol dependence in humans, comparatively little progress has been made in identifying the genes or gene families contributing to this comorbidity. Only a handful of studies on human nicotinic receptor variation exist in the literature, both of which focus only on genetic variation of the beta-2 subunit and the subsequent role that such variation plays in nicotine dependence (not alcoholism). To address this issue, we have investigated two genes in the nicotinic receptor gene family for the role that these genes play in the comorbidity of both nicotine and alcohol dependence and for which prior experiments or theory support a role in these specific disorders. The experimental approach used to achieve our objective is four-fold: (1) develop, test, and evaluate a high-volume, uniform screening strategy for the rapid identification of DNA sequence variation and specifically of gene variants; (2) apply such a strategy to screen for novel polymorphisms in a sample set that is both clinically and ethnically diverse; (3) determine the frequencies and distribution of both known and newly-discovered polymorphic loci via a PCR-RFLP or 5'exonuclease genotyping analysis, and (4) conduct formal association analysis to evaluate the specific contribution of variant alleles (or groups thereof) to two specific clinical diagnoses and phenotypes: alcohol and smoking. Here we present data for eight polymorphic loci in CHRNA4 and CHRNB2 as part of a linkage and association study in a population of Finns and Plains Indians. This study represents the first study in humans which assesses genetic variation at both the alpha-4 and beta-2 loci and the first which assesses the role that such genetic variation at both loci play in the specific phenotypes of alcohol dependence and smoking. It is also among the most robust in terms of number of participants. Two out of the eight polymorphisms examined are novel by virtue of having been identified using a pooled-sample DHPLC method and by not having been identified or investigated with respect to the aforementioned populations and phenotypes. The remaining polymorphisms were selected for analysis based on their potential functional significance. All polymorphisms have been mass-genotyped and the potential contribution of CHRNA4 and CHRNB2 genetic variation assessed.