A transcriptional regulatory element critical for CHRNB4 promoter activity in vivo

Evidence for Association Between Low Frequency Variants in CHRNA6/CHRNB3 and Antisocial Drug Dependence

Common SNPs in nicotinic acetylcholine receptor genes (CHRN genes) have been associated with drug behaviors and personality traits, but the influence of rare genetic variants is not well characterized. The goal of this project was to identify novel rare variants in CHRN genes in the Center for Antisocial Drug Dependence (CADD) and Genetics of Antisocial Drug Dependence (GADD) samples and to determine if low frequency variants are associated with antisocial drug dependence. Two samples of 114 and 200 individuals were selected using a case/control design including the tails of the phenotypic distribution of antisocial drug dependence. The capture, sequencing, and analysis of all variants in 16 CHRN genes (CHRNA1-7, 9, 10, CHRNB1-4, CHRND, CHRNG, CHRNE) were performed independently for each subject in each sample. Sequencing reads were aligned to the human reference sequence using BWA prior to variant calling with the Genome Analysis ToolKit (GATK). Low frequency variants (minor allele frequency < 0.05) were analyzed using SKAT-O and C-alpha to examine the distribution of rare variants among cases and controls. In our larger sample, the region containing the CHRNA6/CHRNB3 gene cluster was significantly associated with disease status using both SKAT-O and C-alpha (unadjusted p values <0.05). More low frequency variants in the CHRNA6/CHRNB3 gene region were observed in cases compared to controls. These data support a role for genetic variants in CHRN genes and antisocial drug behaviors.

Response to Transdermal Selegiline Smoking Cessation Therapy and Markers in the 15q24 Chromosomal Region

INTRODUCTION

Current treatments for smoking cessation have limited efficacy. A potential pharmaceutical treatment for smoking cessation is selegiline, a selective and irreversible monoamine oxidase B inhibitor. A few clinical trials have been carried out using selegiline but the results have been mixed. We sought to determine if genetic markers in cholinergic loci in the 15q24 chromosomal region predict response to smoking cessation therapy with selegiline.

METHODS

We performed an 8-week double-blind, placebo-controlled clinical trial of the selegiline transdermal system in heavy smokers, with follow-up at weeks 25 and 52. Eight single nucleotide polymorphisms (SNPs) in the 15q24 region, which contains the genes for the nicotinic acetylcholine receptor subunits CHRNA5, CHRNA3, and CHRNB4, were investigated for association with treatment response.

RESULTS

The CHRNB4 promoter SNP rs3813567 was associated with both point prevalence abstinence and post-quit craving. Carriers of the minor C allele treated with selegiline showed lower rates of abstinence and higher levels of craving than selegiline-treated non-carriers, indicating that the rs3813567 C allele adversely affects abstinence in selegiline-treated smokers. This effect was not present among placebo-treated smokers. Selegiline-treated smokers with the CHRNA5 rs680244 GG genotype had lower post-quit craving, and unlike placebo-treated GG-carrying smokers, did not experience a post-quit increase in depressive symptoms.

CONCLUSIONS

Variants in genes encoding cholinergic receptors affect abstinence, craving and mood in selegiline-treated smokers. Selegiline primarily affects dopamine levels in the brain, but cholinergic input affects nicotine-induced dopaminergic activity. These markers may have value in identifying those likely to respond to selegiline for smoking cessation.

Nicotine-induced expression of low-density lipoprotein receptor in oral epithelial cells

Background

Nicotine use is one of the most important risk factors for the development of cardiovascular and periodontal diseases. Numerous reports have suggested the possible contribution of disturbed lipid metabolism for the development of both disease groups. Despite these observations, little is known about the relationship between tobacco smoking and the development of these diseases. Our previous microarray data revealed that nicotine induced low-density lipoprotein receptor (LDLR) expression in oral epithelial cells (OECs). The aim of the present study was to confirm nicotine-mediated LDLR induction and to elucidate the signaling mechanisms leading to the augmented expression of LDLR in OECs.

Methods and Results

LDLR and nicotinic acetylcholine receptor (nAChR) subunit expression was detected by real-time PCR. The production of LDLR was demonstrated by immunofluorescence staining. nAChR-mediated LDLR induction was examined by pre-incubation of the cells with its specific inhibitor, α-bungarotoxin (α-BTX). The functional importance of transcription factor specific protein 1 (Sp1) was examined by luciferase assay, mithramycin pre-incubation or by small interfering RNA (siRNA) transfection. The specific binding of Sp1 to R3 region of LDLR 5’-untranslated region was demonstrated with electrophoretic mobility shift assay (EMSA) and streptavidin-agarose precipitation assay followed by western blotting. The results confirmed that nicotine induced LDLR expression at the transcriptional level. Nicotine was sensed by nAChR and the signal was transduced by Sp1 which bound to the R3 region of LDLR gene. Augmented production of LDLR in the gingival epithelial cells was further demonstrated by immunofluorescence staining using the gingival tissues obtained from the smoking patients.

Conclusions

Taken together, the results suggested that nicotine might contribute to the development of both cardiovascular and periodontal diseases by inducing the LDLR in OECs thereby disturbing lipid metabolism.

Alternative CHRNB4 3'-UTRs mediate the allelic effects of SNP rs1948 on gene expression

Common genetic factors strongly contribute to both nicotine, the main addictive component of tobacco, and alcohol use. Several lines of evidence suggest nicotinic acetylcholine receptors as common sites of action for nicotine and alcohol. Specifically, rs1948, a single-nucleotide polymorphism (SNP) located in the CHRNB4 3′-untranslated region (UTR), has been associated to early age of initiation for both alcohol and tobacco use. To determine the allelic effects of rs1948 on gene expression, two rs1948-containing sequences of different lengths corresponding to the CHRNB4 3′-UTR were cloned into pGL3-promoter luciferase reporter vectors. Data obtained showed that the allelic effects of SNP rs1948 on luciferase expression are mediated by the length and species of transcripts generated. In addition, it was found that miR-3157 increased the overall luciferase expression while miR-138, a microRNA known to play a role in neuroadaptation to drug abuse, decreased luciferase expression when compared to basal conditions. These findings demonstrate the importance of SNP rs1948 on the regulation of CHRNB4 expression and provide the first evidence of CHRNB4 down-regulation by miR-138.

Aversion to nicotine is regulated by the balanced activity of β4 and α5 nicotinic receptor subunits in the medial habenula

Nicotine dependence is linked to single nucleotide polymorphisms in the CHRNB4-CHRNA3-CHRNA5 gene cluster encoding the α3β4α5 nicotinic acetylcholine receptor (nAChR). Here we show that the β4 subunit is rate limiting for receptor activity, and that current increase by β4 is maximally competed by one of the most frequent variants associated with tobacco usage (D398N in α5). We identify a β4-specific residue (S435), mapping to the intracellular vestibule of the α3β4α5 receptor in close proximity to α5 D398N, that is essential for its ability to increase currents. Transgenic mice with targeted overexpression of Chrnb4 to endogenous sites display a strong aversion to nicotine that can be reversed by viral-mediated expression of the α5 D398N variant in the medial habenula (MHb). Thus, this study both provides insights into α3β4α5 receptor-mediated mechanisms contributing to nicotine consumption, and identifies the MHb as a critical element in the circuitry controlling nicotine-dependent phenotypes.