We conducted a 1000 Genomes–imputed genome-wide association study (GWAS) meta-analysis for nicotine dependence, defined by the Fagerström Test for Nicotine Dependence in 17 074 ever smokers from five European-ancestry samples. We followed up novel variants in 7469 ever smokers from five independent European-ancestry samples. We identified genome-wide significant association in the alpha-4 nicotinic receptor subunit (CHRNA4) gene on chromosome 20q13: lowest P=8.0 × 10−9 across all the samples for rs2273500-C (frequency=0.15; odds ratio=1.12 and 95% confidence interval=1.08–1.17 for severe vs mild dependence). rs2273500-C, a splice site acceptor variant resulting in an alternate CHRNA4 transcript predicted to be targeted for nonsense-mediated decay, was associated with decreased CHRNA4 expression in physiologically normal human brains (lowest P=7.3 × 10−4). Importantly, rs2273500-C was associated with increased lung cancer risk (N=28 998, odds ratio=1.06 and 95% confidence interval=1.00–1.12), likely through its effect on smoking, as rs2273500-C was no longer associated with lung cancer after adjustment for smoking. Using criteria for smoking behavior that encompass more than the single ‘cigarettes per day' item, we identified a common CHRNA4 variant with important regulatory properties that contributes to nicotine dependence and smoking-related consequences.

The histidine triad nucleotide-binding protein-1 gene (HINT1) is implicated in schizophrenia and in the behavioral effects of morphine and amphetamine. Because nicotine dependence (ND) is highly comorbid with schizophrenia and other substance abuse, we examined the association of HINT1 with ND. Association analyses from two independent samples show that HINT1 gene variants are associated with ND phenotypes. Furthermore, human postmortem mRNA expression shows that smoking status and genotype influence HINT1 expression in the brain. In animal studies, western blot analyses show an increase of HINT1 protein level in the mouse nucleus accumbens (NAc) after chronic nicotine exposure. This increase was reduced after treatment with the nicotinic-receptor antagonist mecamylamine, and 24 and 72 h after cessation of nicotine treatment. These results indicate a genetic association between HINT1 variants and ND, and indicate that nicotine-induced modulation of HINT1 level may be involved in mechanisms of excess smoking.

BACKGROUND AND PURPOSEAbrupt discontinuation of nicotine, the main psychoactive component in tobacco, induces a withdrawal syndrome in nicotine-dependent animals, consisting of somatic and affective signs, avoidance of which contributes to drug maintenance. While blockade of fatty acid amide hydrolase, the primary catabolic enzyme of the endocannabinoid arachidonoylethanolamine (anandamide), exacerbates withdrawal responses in nicotine-dependent mice, the role of monoacylglycerol lipase (MAGL), the main hydrolytic enzyme of a second endocannabinoid 2-arachidonylglycerol (2-AG), in nicotine withdrawal remains unexplored. EXPERIMENTAL APPROACHTo evaluate the role of MAGL enzyme inhibition in nicotine withdrawal, we initially performed a genetic correlation approach using the BXD recombinant inbred mouse panel. We then assessed nicotine withdrawal intensity in the mouse after treatment with the selective MAGL inhibitor, JZL184, and after genetic deletion of the enzyme. Lastly, we assessed the association between genotypes and smoking withdrawal phenotypes in two human data sets. KEY RESULTSBXD mice displayed significant positive correlations between basal MAGL mRNA expression and nicotine withdrawal responses, consistent with the idea that increased 2-AG brain levels may attenuate withdrawal responses. Strikingly, the MAGL inhibitor, JZL184, dose-dependently reduced somatic and aversive withdrawal signs, which was blocked by rimonabant, indicating a CB1 receptor-dependent mechanism. MAGL-knockout mice also showed attenuated nicotine withdrawal. Lastly, genetic analyses in humans revealed associations of the MAGL gene with smoking withdrawal in humans. CONCLUSIONS AND IMPLICATIONSOverall, our findings suggest that MAGL inhibition maybe a promising target for treatment of nicotine dependence.

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