The evolution of addiction: a case study of nicotine dependence

A variety of studies have shown that the tendency toward nicotine dependence has a genetic component. The work described in this thesis addresses three separate questions: i) are there unidentified SNPs in the nicotinic receptors or other genes that contribute to the risk for nicotine dependence; ii) is there evidence of ongoing selection at nicotinic receptor loci; and, iii) since nicotine dependence is unlikely to be the phenotype undergoing selection, is a positive effect on memory or cognition the selected phenotype. I first undertook a genome –wide association scan of imputed data using samples from the Collaborative Study of the Genetics of Nicotine Dependence (COGEND). A novel association was found between nicotine dependence and SNPs at 13q31. The genes at this newly associated locus on chromosome 13 encode a group of micro-RNAs and a member of the glypican gene family. These are among the first findings to implicate a non-candidate gene in risk for nicotine dependence. I applied several complimentary methods to sequence data from the 1000 Genomes Project to test for evidence of selection at the nicotinic receptor loci. I found strong evidence for selection for alleles in the nicotinic receptor cluster on chromosome 8 that confer risk of nicotine dependence. I then used the dataset from the Collaborative Studies on the Genetics of Alcoholism (COGA) and looked for an association between neuropsychological phenotypes and SNPs conferring risk of nicotine dependence. One SNP passed multiple test correction for association with WAIS digit symbol score. This SNP is not itself associated with nicotine dependence but is in reasonable (r 2 = 0.75) LD with SNPs that are associated with nicotine dependence. These data suggest at best, a weak correlation between nicotine dependence and any of the tested cognitive phenotypes. Given the reproducible finding of an inverse relationship between SNPs associated with risk for nicotine dependence and cocaine dependence, I hypothesize that the apparently detrimental phenotype of nicotine dependence may confer decreased risk for cocaine dependence. As cocaine use impairs the positive rewards associated with social interactions, reducing the risk of cocaine addiction may be beneficial to both the individual and the group.