A further aim of this study was to determine whether the reported genomic regions contributed to nicotine dependence specifically or to addiction more generally by showing evidence of linkage to both alcohol and nicotine dependence. The former conclusion was supported as no overlap between linkage signals reported in the present study were observed with those reported in a previous linkage study of alcohol dependence to chromosomes 1 at 11cM, 2 at 287 cM, 8 at 163 cM, and 18 at 48 cM (Gizer et al., unpublished observation). Further, supplementary genome-wide linkage scans of nicotine dependence utilizing alcohol dependence diagnoses alternatively as a covariate and as an additional predictor in a bivariate analysis showed little evidence of linkage between alcohol dependence and the regions reported herein. This suggests that the susceptibility loci identified in the present study are specifically involved in the etiology of nicotine dependence and are unrelated to alcohol dependence.
