CYP2A6 associates with respiratory disease risk and younger age of diagnosis: a phenome-wide association Mendelian Randomization study

CYP2A6, a genetically variable enzyme, inactivates nicotine, activates carcinogens, and metabolizes many pharmaceuticals. Variation in CYP2A6 influences smoking behaviors and tobacco-related disease risk. This phenome-wide association study examined associations between a reconstructed version of our weighted genetic risk score (wGRS) for CYP2A6 activity with diseases in the UK Biobank (N = 395 887). Causal effects of phenotypic CYP2A6 activity (measured as the nicotine metabolite ratio: 3'-hydroxycotinine/cotinine) on the phenome-wide significant (PWS) signals were then estimated in two-sample Mendelian Randomization using the wGRS as the instrument. Time-to-diagnosis age was compared between faster versus slower CYP2A6 metabolizers for the PWS signals in survival analyses. In the total sample, six PWS signals were identified: two lung cancers and four obstructive respiratory diseases PheCodes, where faster CYP2A6 activity was associated with greater disease risk (Ps < 1 × 10-6). A significant CYP2A6-by-smoking status interaction was found (Psinteraction < 0.05); in current smokers, the same six PWS signals were found as identified in the total group, whereas no PWS signals were found in former or never smokers. In the total sample and current smokers, CYP2A6 activity causal estimates on the six PWS signals were significant in Mendelian Randomization (Ps < 5 × 10-5). Additionally, faster CYP2A6 metabolizer status was associated with younger age of disease diagnosis for the six PWS signals (Ps < 5 × 10-4, in current smokers). These findings support a role for faster CYP2A6 activity as a causal risk factor for lung cancers and obstructive respiratory diseases among current smokers, and a younger onset of these diseases. This research utilized the UK Biobank Resource.

Natural and induced variations in transcriptional regulator genes result in low-nicotine phenotypes in tobacco

In tobacco, the homologous ETHYLENE RESPONSE FACTOR (ERF) transcription factors ERF199 and ERF189 coordinate the transcription of multiple metabolic genes involved in nicotine biosynthesis. Natural alleles at the NIC1 and NIC2 loci greatly affect alkaloid accumulation and overlap with ERF199 and ERF189 in the tobacco genome, respectively. In this study, we identified several low-nicotine tobacco varieties lacking ERF199 or ERF189 from a tobacco germplasm collection. We characterized the sequence of these new nic1 and nic2 alleles, as well as the previously defined alleles nic1-1 and nic2-1. Moreover, we examined the influence of different nic alleles on alkaloid contents and expression levels of genes related to nicotine biosynthesis. We also demonstrated that the deletion of a distal genomic region attenuates ERF199 expression, resulting in a moderately negative effect on the alkaloid phenotype. Our study provides new insights into the regulation of nicotine biosynthesis and novel genetic resources to breed low-nicotine tobacco.

NtMYB305a binds to the jasmonate-responsive GAG region of NtPMT1a promoter to regulate nicotine biosynthesis

MYB transcription factors play essential roles in regulating plant secondary metabolism and jasmonate (JA) signaling. Putrescine N-methyltransferase is a key JA-regulated step in the biosynthesis of nicotine, an alkaloidal compound highly accumulated in Nicotiana spp. Here we report the identification of NtMYB305a in tobacco (Nicotiana tabacum) as a regulatory component of nicotine biosynthesis and demonstrate that it binds to the JA-responsive GAG region, which comprises a G-box, an AT-rich motif, and a GCC-box-like element, in the NtPMT1a promoter. Yeast one-hybrid analysis, electrophoretic mobility shift assay and chromatin immunoprecipitation assays showed that NtMYB305a binds to the GAG region in vitro and in vivo. Binding specifically occurs at the ∼30-bp AT-rich motif in a G/C-base-independent manner, thus defining the AT-rich motif as previously unknown MYB-binding element. NtMYB305a localized in the nucleus of tobacco cells where it is capable of activating the expression of a 4×GAG-driven GUS reporter in an AT-rich motif-dependent manner. NtMYB305a positively regulates nicotine biosynthesis and the expression of NtPMT and other nicotine pathway genes. NtMYB305a acts synergistically with NtMYC2a to regulate nicotine biosynthesis, but no interaction between these two proteins was detected. This identification of NtMYB305a provides insights into the regulation of nicotine biosynthesis and extends the roles played by MYB transcription factors in plant secondary metabolism.

Effect of the over-dominant expression of proteins on nicotine heterosis via proteomic analysis

Heterosis is a common biological phenomenon that can be used to optimize yield and quality of crops. Using heterosis breeding, hybrids with suitable nicotine content have been applied to tobacco leaf production. However, the molecular mechanism of the formation of nicotine heterosis has never been explained from the perspective of protein. The DIA proteomics technique was used to compare the differential proteomics of the hybrid Va116 × Basma, showing strong heterosis in nicotine content from its parent lines Va116 and Basma. Proteomics analysis indicated that 65.2% of DEPs showed over-dominant expression patterns, and these DEPs included QS, BBL, GS, ARAF and RFC1 which related to nicotine synthesis. In addition, some DEPs (including GST, ABCE2 and ABCF1 and SLY1) that may be associated with nicotinic transport exhibited significant heterosis over the parental lines. These findings demonstrated that the efficiency of the synthesis and transport of nicotine in hybrids was significantly higher than that in the parent lines, and the accumulation of over-dominant expression proteins may be the cause of heterosis of nicotinic content in hybrids.

Genetics of substance use disorders: a review

Substance use disorders (SUDs) are prevalent and result in an array of negative consequences. They are influenced by genetic factors (h2 = ~50%). Recent years have brought substantial progress in our understanding of the genetic etiology of SUDs and related traits. The present review covers the current state of the field for SUD genetics, including the epidemiology and genetic epidemiology of SUDs, findings from the first-generation of SUD genome-wide association studies (GWAS), cautions about translating GWAS findings to clinical settings, and suggested prioritizations for the next wave of SUD genetics efforts. Recent advances in SUD genetics have been facilitated by the assembly of large GWAS samples, and the development of state-of-the-art methods modeling the aggregate effect of genome-wide variation. These advances have confirmed that SUDs are highly polygenic with many variants across the genome conferring risk, the vast majority of which are of small effect. Downstream analyses have enabled finer resolution of the genetic architecture of SUDs and revealed insights into their genetic relationship with other psychiatric disorders. Recent efforts have also prioritized a closer examination of GWAS findings that have suggested non-uniform genetic influences across measures of substance use (e.g. consumption) and problematic use (e.g. SUD). Additional highlights from recent SUD GWAS include the robust confirmation of loci in alcohol metabolizing genes (e.g. ADH1B and ALDH2) affecting alcohol-related traits, and loci within the CHRNA5-CHRNA3-CHRNB4 gene cluster influencing nicotine-related traits. Similar successes are expected for cannabis, opioid, and cocaine use disorders as sample sizes approach those assembled for alcohol and nicotine.

Association of NRG3 and ERBB4 gene polymorphism with nicotine dependence in Turkish population

BACKGROUND

Nicotine dependence (ND) is characterized by regular smoking, anxiety, irritation, difficulty concentrating, impatience, restlessness, tremor, dizziness, hunger, nicotine demand, and the individual's reluctance to quit despite knowing the health risks of smoking. Recently, it has been reported that the Neuregulin 3 (NRG3)/Erb-B2 receptor tyrosine kinase 4 (ERBB4) signaling pathway plays a role in ND. NRG3, which is activated after nicotine intake, binds to ERBB4 and causes GABA release. GABA reduces anxiety and tension, which are one of the nicotine withdrawal symptoms. Therefore we aimed to investigate the relationship between NRG3 and ERBB4 gene polymorphisms and ND.

MATERIALS AND METHODS

The study population was comprised of patients with ND (n = 200) and healthy non-smoker control subjects (n = 200) who were matched for age, sex, and compared for comorbidity factors such as alcohol, smoking, duration, and education (age range 18-60). Genotypes were detected by Real-Time PCR using TaqMan technology. The Fagerström Nicotine Dependence Test (FTND) score was 5 and above for the patient group and 0 for the control group. DNA was obtained from whole peripheral blood and six polymorphisms of Neuregulin 3 (NRG3) (rs1836724, rs7562566, and rs10048757) and Erb-B2 Receptor Tyrosine Kinase 4 (ERBB4) (rs1764072, rs6584400, and rs10883934) genes were analyzed by real-time PCR method.

RESULTS

Our findings show that the six selected SNPs are not significantly associated with ND in the Turkish population and no correlation with dependence levels (p > 0.05).

CONCLUSION

Although our findings do not show a relationship between ND and these polymorphisms, it is the first study to investigate these single nucleotide polymorphisms (SNPs) for the first time in ND and to find some genotypes in the Turkish population when compared to other populations. Also, our findings are important in terms of their contribution to the literature and forensic genetics.

Genetic attenuation of alkaloids and nicotine content in tobacco (Nicotiana tabacum)

MAIN CONCLUSION

The role of six alkaloid biosynthesis genes in the process of nicotine accumulation in tobacco was investigated. Downregulation of ornithine decarboxylase, arginine decarboxylase, and aspartate oxidase resulted in viable plants with a significantly lower nicotine content. Attenuation of nicotine accumulation in Nicotiana tabacum was addressed upon the application of RNAi technologies. The approach entailed a downregulation in the expression of six different alkaloid biosynthesis genes encoding upstream enzymes that are thought to function in the pathway of alkaloid and nicotine biosynthesis. Nine different RNAi constructs were designed to lower the expression level of the genes that encode the enzymes arginine decarboxylase, agmatine deiminase, aspartate oxidase, arginase, ornithine decarboxylase, and SAM synthase. Agrobacterium-based transformation of tobacco leaves was applied, and upon kanamycin selection, T0 and subsequently T1 generation seeds were produced. Mature T1 plants in the greenhouse were topped to prevent flowering and leaf nos. 3 and 4 below the topping point were tested for transcript levels and product accumulation. Down-regulation in arginine decarboxylase, aspartate oxidase, and ornithine decarboxylase consistently resulted in lower levels of nicotine in the leaves of the corresponding plants. Transformants with the aspartate oxidase RNAi construct showed the lowest nicotine level in the leaves, which varied from below the limit of quantification (20 μg per g dry leaf weight) to 1.3 mg per g dry leaf weight. The amount of putrescine, the main polyamine related to nicotine biosynthesis, showed a qualitative correlation with the nicotine content in the arginine decarboxylase and ornithine decarboxylase RNAi-expressing transformants. A putative early senescence phenotype and lower viability of the older leaves was observed in some of the transformant lines. The results are discussed in terms of the role of the above-mentioned genes in the alkaloid biosynthetic pathway and may serve to guide efforts to attenuate nicotine content in tobacco leaves.

Genetic and environmental risk factors in the non-medical use of over-the-counter or prescribed analgesics, and their relationship to major classes of licit and illicit substance use and misuse in a population-based sample of young adult twins

BACKGROUND AND AIMS

The non-medical use of over-the-counter or prescribed analgesics (NMUA) is a significant public health problem. Little is known about the genetic and environmental etiology of NMUA and how these risks relate to other classes of substance use and misuse. Our aims were to estimate the heritability NMUA and sources of genetic and environmental covariance with cannabis and nicotine use, cannabis and alcohol use disorders and nicotine dependence in Australian twins.

DESIGN

Biometrical genetic analyses or twin methods using structural equation univariate and multivariate modeling.

SETTING

Australia.

PARTICIPANTS

A total of 2007 young adult twins [66% female; μage  = 25.9, standard deviation (SD) = 3.6, range = 18-38] from the Brisbane Longitudinal Twin Study retrospectively assessed between 2009 and 2016.

MEASUREMENTS

Self-reported NMUA (non-opioid or opioid-based), life-time nicotine, cannabis and opioid use, DSM-V cannabis and alcohol use disorders and the Fagerström Test for Nicotine Dependence.

FINDINGS

Life-time NMUA was reported by 19.4% of the sample. Univariate heritability explained 46% [95% confidence interval (CI) = 0.29-0.57] of the risks in NMUA. Multivariate analyses revealed that NMUA is moderately associated genetically with cannabis (rg  = 0.41) and nicotine (rg  = 0.45) use and nicotine dependence (rg  = 0.34). In contrast, the genetic correlations with cannabis (rg  = 0.15) and alcohol (rg  = 0.07) use disorders are weak.

CONCLUSIONS

In young male and female adults in Australia, the non-medical use of over-the-counter or prescribed analgesics appears to have moderate heritability. NMUA is moderately associated with cannabis and nicotine use and nicotine dependence. Its genetic etiology is largely distinct from that of cannabis and alcohol use disorders.

Genetically determined metabolism of nicotine and its clinical significance

Enzymes of the cytochrome P-450 (CYP 450) which belong to the family of oxidase enzymes, are present in cells of all organisms and play a major role in the first phase of xenobiotic metabolism. There are several isoenzymes of CYP 450 that show differences in the speed of metabolism: poor-, extensive- and ultra-rapid. Nicotine undergoes biotransformation in the liver mainly by the CYP2A6 isoform of CYP 450. There are many polymorphic isoforms of CYP2A6 affecting the metabolism of nicotine. There are also several CYP2A6 activity inhibitors and inducers among commonly used drugs. The ability of CYP2A6 isozymes to activate certain procancerogenic substances present in cigarette smoke makes their polymorphism more significant. Moreover, some isoforms may have also influence on the risk of lung cancer development by affecting the enzymatic activation of tobacco-specific nitrosamines. Metabolism of nicotine, mainly through CYP2A6, has also many clinical implications, such as efficacy and safety of the nicotine replacement therapy (NRT) or occurrence of several diseases. In summary, type of the nicotine metabolism may be a potential predictor of the clinical outcomes in patients with cardiovascular disease, addicted to nicotine and in those using NRT. The purpose of this work is to summarize current knowledge on variation in genetically determined metabolism of nicotine and its clinical significance.

Ethylene response factor NtERF91 positively regulates alkaloid accumulations in tobacco (Nicotiana tabacum L.)

Tobacco alkaloid metabolism is regulated by various transcription factors (TFs). Here, we have characterized a non-NIC2 locus gene, Ethylene Response Factor 91 (ERF91), function in regulation of alkaloid accumulation in tobacco. NtERF91 was preferentially expressed in roots and induced by jasmonic acid. Additionally, NtERF91 was able to in vitro bind to the NtPMT2 and NtQPT2 promoters via directly targeting the GCC-box elements and transactivate NtQPT2 gene expression. Ectopic overexpression of NtERF91 not only increased the expression of most nicotine biosynthetic genes, but also altered alkaloid accumulation profile, resulting in dramatically anatabine accumulation. We conclude that NtERF91 plays an overlapped but distinct role in regulating tobacco alkaloid accumulations.

The effect of body mass index on smoking behaviour and nicotine metabolism: a Mendelian randomization study

Given clear evidence that smoking lowers weight, it is possible that individuals with higher body mass index (BMI) smoke in order to lose or maintain their weight. We performed Mendelian randomization (MR) analyses of the effects of BMI on smoking behaviour in UK Biobank and the Tobacco and Genetics Consortium genome-wide association study (GWAS), on cotinine levels and nicotine metabolite ratio (NMR) in published GWAS and on DNA methylation in the Avon Longitudinal Study of Parents and Children. Our results indicate that higher BMI causally influences lifetime smoking, smoking initiation, smoking heaviness and also DNA methylation at the aryl-hydrocarbon receptor repressor (AHRR) locus, but we do not see evidence for an effect on smoking cessation. While there is no strong evidence that BMI causally influences cotinine levels, suggestive evidence for a negative causal influence on NMR may explain this. There is a causal effect of BMI on smoking, but the relationship is likely to be complex due to opposing effects on behaviour and metabolism.

From genes to treatments: a systematic review of the pharmacogenetics in smoking cessation

Smoking cessation treatment outcomes may be heavily influenced by genetic variations among smokers. Therefore, identifying specific variants that affect response to different pharmacotherapies is of major interest to the field. In the current study, we systematically review all studies published in or after the year 1990 which examined one or more gene-drug interactions for smoking cessation treatment. Out of 644 citations, 46 articles met the inclusion criteria for the systematic review. We summarize evidence on several genetic polymorphisms (CHRNA5-A3-B4, CYP2A6, DBH, CHRNA4, COMT, DRD2, DRD4 and CYP2B6) and their potential moderating pharamacotherarpy effects on patient cessation efficacy rates. These findings are promising and call for further research to demonstrate the effectiveness of genetic testing in personalizing treatment decision-making and improving outcome.

Novel approach for selective reduction of NNN in cigarette tobacco filler and mainstream smoke

Research conducted during past decades to reduce the level of the tobacco specific nitrosamine N-nitrosonornicotine (NNN) and its precursor nornicotine in tobacco yielded identification of three tobacco genes encoding for cytochrome P450 nicotine demethylases converting nicotine to nornicotine. We carried out trials to investigate the effect of using tobaccos containing three non-functional nicotine demethylase genes on the selective reduction of NNN in cigarette tobacco filler and mainstream smoke. Our results indicate that the presence of non-functional alleles of the three genes reduces the level of nornicotine and NNN in Burley tobacco by 70% compared to the level observed in currently available low converter (LC) Burley tobacco varieties. The new technology, named ZYVERT™, does not require a regular screening process, while a yearly selection process is needed to produce LC Burley tobacco seeds for NNN reduction. The reduction of NNN observed in smoke of blended prototype cigarettes is proportional to the inclusion level of tobacco having ZYVERT™ technology. Inclusion of Burley tobacco possessing the new trait into a typical American blend resulted in a selective reduction of NNN in cigarette smoke, while the levels of other Harmful and Potentially Harmful Constituents (HPHC) currently in the abbreviated list provided by the US Food and Drug Administration are statistically equivalent in comparison with the levels obtained in reference prototype cigarettes containing LC Burley.

Wild tobacco genomes reveal the evolution of nicotine biosynthesis

Nicotine, the signature alkaloid of Nicotiana species responsible for the addictive properties of human tobacco smoking, functions as a defensive neurotoxin against attacking herbivores. However, the evolution of the genetic features that contributed to the assembly of the nicotine biosynthetic pathway remains unknown. We sequenced and assembled genomes of two wild tobaccos, Nicotiana attenuata (2.5 Gb) and Nicotiana obtusifolia (1.5 Gb), two ecological models for investigating adaptive traits in nature. We show that after the Solanaceae whole-genome triplication event, a repertoire of rapidly expanding transposable elements (TEs) bloated these Nicotiana genomes, promoted expression divergences among duplicated genes, and contributed to the evolution of herbivory-induced signaling and defenses, including nicotine biosynthesis. The biosynthetic machinery that allows for nicotine synthesis in the roots evolved from the stepwise duplications of two ancient primary metabolic pathways: the polyamine and nicotinamide adenine dinucleotide (NAD) pathways. In contrast to the duplication of the polyamine pathway that is shared among several solanaceous genera producing polyamine-derived tropane alkaloids, we found that lineage-specific duplications within the NAD pathway and the evolution of root-specific expression of the duplicated Solanaceae-specific ethylene response factor that activates the expression of all nicotine biosynthetic genes resulted in the innovative and efficient production of nicotine in the genus Nicotiana Transcription factor binding motifs derived from TEs may have contributed to the coexpression of nicotine biosynthetic pathway genes and coordinated the metabolic flux. Together, these results provide evidence that TEs and gene duplications facilitated the emergence of a key metabolic innovation relevant to plant fitness.

Alternative splicing of basic chitinase gene PR3b in the low-nicotine mutants of Nicotiana tabacum L. cv. Burley 21

Two unlinked semi-dominant loci, A (NIC1) and B (NIC2), control nicotine and related alkaloid biosynthesis in Burley tobaccos. Mutations in either or both loci (nic1 and nic2) lead to low nicotine phenotypes with altered environmental stress responses. Here we show that the transcripts derived from the pathogenesis-related (PR) protein gene PR3b are alternatively spliced to a greater extent in the nic1 and nic2 mutants of Burley 21 tobacco and the nic1nic2 double mutant. The alternative splicing results in a deletion of 65 nucleotides and introduces a premature stop codon into the coding region of PR3b that leads to a significant reduction of PR3b specific chitinase activity. Assays of PR3b splicing in F2 individuals derived from crosses between nic1 and nic2 mutants and wild-type plants showed that the splicing phenotype is controlled by the NIC1 and NIC2 loci, even though NIC1 and NIC2 are unlinked loci. Moreover, the transcriptional analyses showed that the splicing patterns of PR3b in the low-nicotine mutants were differentially regulated by jasmonate (JA) and ethylene (ET). These data suggest that the NIC1 and NIC2 loci display differential roles in regulating the alternative splicing of PR3b in Burley 21. The findings in this study have provided valuable information for extending our understanding of the broader effects of the low-nicotine mutants of Burley 21 and the mechanism by which JA and ET signalling pathways post-transcriptionally regulate the activity of PR3b protein.

Genetic determinants of cytochrome P450 2A6 activity and biomarkers of tobacco smoke exposure in relation to risk of lung cancer development in the Shanghai cohort study

Cytochrome P450 2A6 (CYP2A6) catalyzes nicotine metabolism and contributes to the metabolism of the tobacco-specific lung carcinogen, NNK. Genetic variation in CYP2A6 may affect smoking behavior and contribute to lung cancer risk. A nested case-control study of 325 lung cancer cases and 356 controls was conducted within a prospective cohort of 18,244 Chinese men in Shanghai, China. Quantified were 4 allelic variants of CYP2A6 [*1(+51A), *4, *7, and *9] and urinary total nicotine, total cotinine, total trans-3'-hydroxycotinine (3HC) and total NNAL (an NNK metabolite). Calculated were total nicotine equivalents (TNE), the sum of total nicotine, total cotinine and total 3HC and the total 3HC:total cotinine ratio as a measure of CYP2A6 activity. The nicotine metabolizer status (normal, intermediate, slow and poor) was determined by CYP2A6 genotypes. The smoking-adjusted odds ratios (95% confidence intervals) of lung cancer for the highest vs lowest quartile of total nicotine, total cotinine, total 3HC, TNE and total NNAL were 3.03 (1.80-5.10), 4.70 (2.61-8.46), 4.26 (2.37-7.68), 4.71 (2.61-8.52), and 3.15 (1.86-5.33) (all Ptrend  < 0.001), respectively. Among controls CYP2A6 poor metabolizers had a 78% lower total 3HC:total cotinine ratio and 72% higher total nicotine (Ptrend ≤ 0.002). Poor metabolizers had an odds ratio of 0.64 (95% confidence interval = 0.43-0.97) for lung cancer, which was statistically nonsignificant (odds ratio = 0.74, 95% confidence interval = 0.48-1.15) after adjustment for urinary TNE and smoking intensity and duration. The lower lung cancer risk observed in CYP2A6 poor metabolizers is partially explained by the strong influence of CYP2A6 genetic polymorphisms on nicotine uptake and metabolism.

Genetic Factors for Enhancement of Nicotine Levels in Cultivated Tobacco

Nicotine has practical applications relating to smoking cessation devices and alternative nicotine products. Genetic manipulation for increasing nicotine content in cultivated tobacco (Nicotiana tabacum L.) may be of value for industrial purposes, including the possibility of enhancing the efficiency of nicotine extraction. Biotechnological approaches have been evaluated in connection with this objective, but field-based results are few. Here, we report characterization of two genes encoding basic-helix-loop-helix (bHLH) transcription factors (TFs), NtMYC2a and NtMYC2b from tobacco. Overexpression of NtMYC2a increased leaf nicotine levels in T1 transgenic lines approximately 2.3-fold in greenhouse-grown plants of tobacco cultivar 'NC 95'. Subsequent field testing of T2 and T3 generations of transgenic NtMYC2a overexpression lines showed nicotine concentrations were 76% and 58% higher than control lines, respectively. These results demonstrated that the increased nicotine trait was stably inherited to the T2 and T3 generations, indicating the important role that NtMYC2a plays in regulating nicotine accumulation in N. tabacum and the great potential of NtMYC2a overexpression in tobacco plants for industrial nicotine production. Collected data in this study also indicated a negative feedback inhibition of nicotine biosynthesis. Further enhancement of nicotine accumulation in tobacco leaf may require modification of the processes of nicotine transport and deposition.

Regulation of Nicotine Biosynthesis by an Endogenous Target Mimicry of MicroRNA in Tobacco

The interaction between noncoding endogenous target mimicry (eTM) and its corresponding microRNA (miRNA) is a newly discovered regulatory mechanism and plays pivotal roles in various biological processes in plants. Tobacco (Nicotiana tabacum) is a model plant for studying secondary metabolite alkaloids, of which nicotine accounts for approximately 90%. In this work, we identified four unique tobacco-specific miRNAs that were predicted to target key genes of the nicotine biosynthesis and catabolism pathways and an eTM, novel tobacco miRNA (nta)-eTMX27, for nta-miRX27 that targets QUINOLINATE PHOSPHORIBOSYLTRANSFERASE2 (QPT2) encoding a quinolinate phosphoribosyltransferase. The expression level of nta-miRX27 was significantly down-regulated, while that of QPT2 and nta-eTMX27 was significantly up-regulated after topping, and consequently, nicotine content increased in the topping-treated plants. The topping-induced down-regulation of nta-miRX27 and up-regulation of QPT2 were only observed in plants with a functional nta-eTMX27 but not in transgenic plants containing an RNA interference construct targeting nta-eTMX27. Our results demonstrated that enhanced nicotine biosynthesis in the topping-treated tobacco plants is achieved by nta-eTMX27-mediated inhibition of the expression and functions of nta-miRX27. To our knowledge, this is the first report about regulation of secondary metabolite biosynthesis by an miRNA-eTM regulatory module in plants.

A Genome-Wide Association Study of a Biomarker of Nicotine Metabolism

Individuals with fast nicotine metabolism typically smoke more and thus have a greater risk for smoking-induced diseases. Further, the efficacy of smoking cessation pharmacotherapy is dependent on the rate of nicotine metabolism. Our objective was to use nicotine metabolite ratio (NMR), an established biomarker of nicotine metabolism rate, in a genome-wide association study (GWAS) to identify novel genetic variants influencing nicotine metabolism. A heritability estimate of 0.81 (95% CI 0.70-0.88) was obtained for NMR using monozygotic and dizygotic twins of the FinnTwin cohort. We performed a GWAS in cotinine-verified current smokers of three Finnish cohorts (FinnTwin, Young Finns Study, FINRISK2007), followed by a meta-analysis of 1518 subjects, and annotated the genome-wide significant SNPs with methylation quantitative loci (meQTL) analyses. We detected association on 19q13 with 719 SNPs exceeding genome-wide significance within a 4.2 Mb region. The strongest evidence for association emerged for CYP2A6 (min p = 5.77E-86, in intron 4), the main metabolic enzyme for nicotine. Other interesting genes with genome-wide significant signals included CYP2B6, CYP2A7, EGLN2, and NUMBL. Conditional analyses revealed three independent signals on 19q13, all located within or in the immediate vicinity of CYP2A6. A genetic risk score constructed using the independent signals showed association with smoking quantity (p = 0.0019) in two independent Finnish samples. Our meQTL results showed that methylation values of 16 CpG sites within the region are affected by genotypes of the genome-wide significant SNPs, and according to causal inference test, for some of the SNPs the effect on NMR is mediated through methylation. To our knowledge, this is the first GWAS on NMR. Our results enclose three independent novel signals on 19q13.2. The detected CYP2A6 variants explain a strikingly large fraction of variance (up to 31%) in NMR in these study samples. Further, we provide evidence for plausible epigenetic mechanisms influencing NMR.

Stress-induced expression of NICOTINE2-locus genes and their homologs encoding Ethylene Response Factor transcription factors in tobacco

Plants have evolved diverse defense metabolites as adaptations to biotic and abiotic stresses. The defense alkaloid nicotine is produced in Nicotiana tabacum (tobacco) and its biosynthesis is elicited by jasmonates in the roots. At least seven jasmonate-responsive genes that encode transcription factors of the Ethylene Response Factor (ERF) family are clustered at the nicotine-regulatory locus NICOTINE2 (NIC2) in the tobacco genome. A subset of the NIC2-locus ERFs and their homologs, including ERF189 and ERF199, have been shown to be most effective in controlling nicotine biosynthetic pathway genes. Herein reported is that the ERF genes of this group, other than ERF189 and ERF199, were strongly induced by NaCl in tobacco hairy roots, although salt stress had no effect on expression of nicotine biosynthesis genes. Abscisic acid and osmotic stress also increased expression of a subset of these NaCl-inducible ERF genes. Promoter expression analysis in transgenic tobacco hairy roots confirmed that while methyl jasmonate (MJ) activated the promoters of ERF29, ERF210 and ERF199, salt stress up-regulated the promoters of only ERF29 and ERF210, but not ERF199. The protein biosynthesis inhibitor cycloheximide induced expression of the ERFs, and simultaneous addition of MJ and cycloheximide showed synergistic effects. These results indicate that, after several gene duplication events, the NIC2-locus ERFs and possibly their homologs appear to have diverged in their responses to jasmonates and various environmental inputs, including salt stress, and may have evolved to regulate distinct metabolic processes and cellular responses.

Unpredictability of nectar nicotine promotes outcrossing by hummingbirds in Nicotiana attenuata

Many plants use sophisticated strategies to maximize their reproductive success via outcrossing. Nicotiana attenuata flowers produce nectar with nicotine at concentrations that are repellent to hummingbirds, increasing the number of flowers visited per plant. In choice tests using native hummingbirds, we show that these important pollinators learn to tolerate high-nicotine nectar but prefer low-nicotine nectar, and show no signs of nicotine addiction. Nectar nicotine concentrations, unlike those of other vegetative tissues, are unpredictably variable among flowers, not only among populations, but also within populations, and even among flowers within an inflorescence. To evaluate whether variations in nectar nicotine concentrations increase outcrossing, polymorphic microsatellite markers, optimized to evaluate paternity in native N. attenuata populations, were used to compare outcrossing in plants silenced for expression of a biosynthetic gene for nicotine production (Napmt1/2) and in control empty vector plants, which were antherectomized and transplanted into native populations. When only exposed to hummingbird pollinators, seeds produced by flowers with nicotine in their nectar had a greater number of genetically different sires, compared to seeds from nicotine-free flowers. As the variation in nectar nicotine levels among flowers in an inflorescence decreased in N. attenuata plants silenced in various combinations of three Dicer-like (DCL) proteins, small RNAs are probably involved in the unpredictable variation in nectar nicotine levels within a plant.

Root-to-shoot translocation of alkaloids is dominantly suppressed in Nicotiana alata

In tobacco (Nicotiana tabacum), nicotine and related pyridine alkaloids are produced in the root, and then transported to the aerial parts where these toxic chemicals function as part of chemical defense against insect herbivory. Although a few tobacco transporters have been recently reported to take up nicotine into the vacuole from the cytoplasm or into the cytoplasm from the apoplast, it is not known how the long-range translocation of tobacco alkaloids between organs is controlled. Nicotiana langsdorffii and N. alata are closely related species of diploid Nicotiana section Alatae, but the latter does not accumulate tobacco alkaloids in the leaf. We show here that N. alata does synthesize alkaloids in the root, but lacks the capacity to mobilize the root-borne alkaloids to the aerial parts. Interspecific grafting experiments between N. alata and N. langsdorffii indicate that roots of N. alata are unable to translocate alkaloids to their shoot system. Interestingly, genetic studies involving interspecific hybrids between N. alata and N. langsdorffii and their self-crossed or back-crossed progeny showed that the non-translocation phenotype is dominant over the translocation phenotype. These results indicate that a mechanism to retain tobacco alkaloids within the root organ has evolved in N. alata, which may represent an interesting strategy to control the distribution of secondary products within a whole plant.

Three nicotine demethylase genes mediate nornicotine biosynthesis in Nicotiana tabacum L.: functional characterization of the CYP82E10 gene

In most tobacco (Nicotiana tabacum L.) plants, nornicotine is a relatively minor alkaloid, comprising about 2-5% of the total pyridine alkaloid pool in the mature leaf. Changes in gene expression at an unstable locus, however, can give rise to plants that produce high levels of nornicotine, specifically during leaf senescence and curing. Minimizing the nornicotine content in tobacco is highly desirable, because this compound serves as the direct precursor in the synthesis of N'-nitrosonornicotine, a potent carcinogen in laboratory animals. Nornicotine is likely produced almost entirely via the N-demethylation of nicotine, in a process called nicotine conversion that is catalyzed by the enzyme nicotine N-demethylase (NND). Previous studies have identified CYP82E4 as the specific NND gene responsible for the unstable conversion phenomenon, and CYP82E5v2 as a putative minor NND gene. Here, by discovery and characterization of CYP82E10, a tobacco NND gene, is reported. PCR amplification studies showed that CYP82E10 originated from the N. sylvestris ancestral parent of modern tobacco. Using a chemical mutagenesis strategy, knockout mutations were induced and identified in all three tobacco NND genes. By generating a series of mutant NND genotypes, the relative contribution of each NND gene toward the nornicotine content of the plant was assessed. Plants possessing knockout mutations in all three genes displayed nornicotine phenotypes that were much lower (∼0.5% of total alkaloid content) than that found in conventional tobacco cultivars. The introduction of these mutations into commercial breeding lines promises to be a viable strategy for reducing the levels of one of the best characterized animal carcinogens found in tobacco products.

cDNA libraries for virus-induced gene silencing

Virus-induced gene silencing (VIGS) exploits endogenous plant antiviral defense mechanisms to posttranscriptionally silence the expression of targeted plant genes. VIGS is quick and relatively easy to perform and therefore serves as a powerful tool for high-throughput functional genomics in plants. Combined with the use of subtractive cDNA libraries for generating a collection of VIGS-ready cDNA inserts, VIGS can be utilized to screen a large number of genes to determine phenotypes resulting from the knockdown/knockout of gene function. Taking into account the optimal insert design for VIGS, we describe a methodology for producing VIGS-ready cDNA libraries enriched for inserts relevant to the biological process of interest.

[Searching for genes influencing tobacco smoking susceptibility]

Numerous studies indicate genetical background of susceptibility to tobacco smoking. The current review presents new achievements and perspectives in searching for genes involved in tobacco smoking and dependence, which have been described in recent years as a result of the development of molecular techniques. It has been emphasized that tobacco smoking is a complex, polygenic behavior. Presence of multiple gene-gene and gene-environment interactions makes it difficult to find a strong genetic association. The causes of inconsistency in the results of candidate gene association studies, as well as the new perspectives in searching for genetic determinants of tobacco smoking, are also widely discussed. In particular, the focus is on the genome-wide association studies which allow the complex analysis of links between thousands of single nucleotide polymorphisms with smoking phenotypes.

Physiological integration of roots and shoots in plant defense strategies links above- and belowground herbivory

Roots play a critical, but largely unappreciated, role in aboveground anti-herbivore plant defense (e.g. resistance and tolerance) and root-leaf connections may therefore result in unexpected coupling between above- and belowground consumers. Using the tobacco (Nicotiana tabacum) system we highlight two examples of this phenomenon. First, the secondary metabolite nicotine is produced in roots, yet translocated aboveground for use as a foliar resistance trait. We demonstrate that nematode root herbivory interferes with foliar nicotine dynamics, resulting in positive effects on aboveground phytophagous insects. Notably, nematode-induced facilitation only occurred on nicotine-producing plants, and not on nicotine-deficient mutants. In the second case, we use stable isotope and invertase enzyme analyses to demonstrate that foliar herbivory elicits a putative tolerance response whereby aboveground nutritional reserves are allocated to roots, resulting in facilitation of phytoparasitic nematodes. Thus, plants integrate roots in resistance and tolerance mechanisms for leaf defense, and such root-leaf connections inherently link the dynamics of above- and belowground consumers.

Smoking behavior and related cancers: the role of CYP2A6 polymorphisms

Smoking exerts complex central and peripheral nervous system, behavioral, cardiovascular, and endocrine effects in humans and is a primary risk factor for various cancers. Nicotine, a major constituent of tobacco, is the compound that is responsible for the development and maintenance of tobacco dependence. The absorbed nicotine is rapidly and extensively metabolized to inactive cotinine by CYP2A6 in human livers, which has a major impact on nicotine clearance. Progress has been made in understanding the relationship between the inter-individual variability in nicotine metabolism and genetic polymorphisms of CYP2A6. Recent findings have increased our knowledge concerning ethnic differences in the allele frequencies of the CYP2A6 variants, nicotine metabolism, and cancer risk. In this review, the potential associations between the CYP2A6 polymorphisms and smoking behavior or the risk of cancer are also discussed.

Pharmacogenetics and smoking cessation with nicotine replacement therapy

Nicotine replacement therapies (NRTs), including transdermal patch, gum, lozenge, nasal spray and inhaler, are widely used for the treatment of tobacco dependence; however, only one-quarter to one-third of smokers who utilise NRTs to quit smoking are able to maintain long-term abstinence from tobacco use. Pharmacogenetic studies of NRT may be useful to identify subgroups of smokers who respond more favourably to specific NRTs, and to determine the optimal dose and duration of NRT. To date, pharmacogenetic studies have examined genes coding for nicotine metabolising enzymes, as well as proteins in neurotransmitter pathways that mediate the effects of nicotine. Initial findings suggest that polymorphisms in nicotine metabolising enzymes, and selected genes in the dopaminergic and opioidergic pathways, may have predictive validity for NRT response; however, independent replication is necessary before translation to clinical practice. Larger-scale investigations that incorporate pathway-based or genome-wide analysis, as well as intermediate measures of nicotine dependence (i.e. 'endophenotypes'), may be necessary to capture the complexity of pharmacogenetic effects.

Will nicotine genetics and a nicotine vaccine prevent cigarette smoking and smoking-related diseases?

Hall argues that the preventive use of genetic and vaccine biotechnologies is a superficially attractive tobacco policy option of doubtful efficacy, cost-effectiveness, and ethicality.

Genetic polymorphisms in human CYP2A6 gene causing impaired nicotine metabolism

AIMS

Previously, we determined the phenotyping of in vivo nicotine metabolism and the genotyping of the CYP2A6 gene (CYP2A6*1 A, CYP2A6*1B, CYP2A6*2, CYP2A6*3, CYP2A6*4 and CYP2A6*5 ) in 92 Japanese and 209 Koreans. In the study, we found one Korean and four Japanese subjects genotyped as CYP2A6*1B/CYP2A6*4 who revealed impaired nicotine metabolism, although other many heterozygotes of CYP2A6*4 demonstrated normal nicotine metabolism (CYP2A6*4 is a whole deletion type). After our previous report, several CYP2A6 alleles, CYP2A6*6 (R128Q), CYP2A6*7 (I471T), and CYP2A6*8 (R485L), have been reported. The purpose of the present study was to clarify whether the impaired nicotine metabolism can be ascribed to these CYP2A6 alleles. Furthermore, we also determined whether the subjects possessing CYP2A6*1x2 (duplication) reveal higher nicotine metabolism.

METHODS

Genotyping of CYP2A6 alleles, CYP2A6*6, CYP2A6*7, CYP2A6*8, and CYP2A6*1x2 was determined by PCR.

RESULTS

The five poor metabolizers were re-genotyped as CYP2A6*7/CYP2A6*4, suggesting that a single nucleotide polymorphism (SNP) causing I471T decreases nicotine metabolism in vivo. Furthermore, we found that two subjects out of five with a lower potency of nicotine metabolism possessed SNPs of CYP2A6*7 and CYP2A6*8 simultaneously. The novel allele was termed CYP2A6*10. In the 92 Japanese and 209 Koreans, the CYP2A6*6 allele was not found. The allele frequencies of CYP2A6*7, CYP2A6*8, and CYP2A6*10 were 6.5%, 2.2%, and 1.1%, respectively, in Japanese, and 3.6%, 1.4%, and 0.5%, respectively, in Koreans. The CYP2A6*1x2 allele was found in only one Korean subject (0.5%) whose nicotine metabolic potency was not very high.

CONCLUSIONS

It was clarified that the impaired in vivo nicotine metabolism was caused by CYP2A6*7 and CYP2A6*10 alleles.

Nicotine metabolism variability and nicotine addiction

Individual variation in nicotine metabolism may play a role in a person's level of smoking, as well as in the transition from initiation to maintenance of a smoking behavior pattern. Since there is a paucity of research on nicotine metabolism in youth that smoke, a brief review of salient aspects of nicotine metabolism in adults provides a basis from which to extrapolate. We do know that factors influencing the rate of nicotine metabolism include differences in nicotine intake and absorption, inhalation patterns, genetic polymorphisms of pertinent enzymes, as well as daily activities such as meal consumption. Variability is illustrated with differences in cotinine levels identified in African-American and Caucasian women and in menthol and nonmenthol smokers. There are a number of areas where more information to improve understanding the initiation and maintenance of smoking behavior is needed. Characterization of nicotine metabolism and smoking topography in youth from multiple ethnic groups who are engaged in smoking initiation is currently lacking. Important measures of smoke constituent exposure such as carbon monoxide, nicotine and cotinine, as well as puff volume and duration and respiratory movements should be addressed. While there are numerous factors that impact initiation and maintenance of smoking behavior, nicotine metabolism may represent one important aspect.

Mecamylamine-precipitated nicotine-withdrawal aversion in Lewis and Fischer 344 inbred rat strains

Many studies have indicated that Lewis and Fischer 344 inbred rat strains show marked differences in behavioral responses to abused drugs. In the present study, we sought to determine whether these two strains of rats show different responses in mecamylamine-precipitated nicotine-withdrawal aversion using the conditioned place preference paradigm. Rats were treated subcutaneously with 10 mg/kg/day nicotine for 7 days using an osmotic minipump. After chronic nicotine infusion, the nicotinic receptor antagonist mecamylamine produced a significant place aversion in Lewis, but not in Fischer 344 rats. These results suggest that mecamylamine-precipitated nicotine-withdrawal aversion is strongly regulated by genetic factors.

Gene structures and properties of enzymes of the plasmid-encoded nicotine catabolism of Arthrobacter nicotinovorans

Arthrobacter nicotinovorans is a Gram-positive aerobic soil bacterium able to grow on nicotine as its sole source of carbon and nitrogen. The initial steps of nicotine catabolism are catalyzed by nicotine dehydrogenase, the l- and d-specific 6-hydroxynicotine oxidases, and ketone dehydrogenase. The genes encoding these enzymes reside on a 160 kb plasmid, pAO1. The cccDNA of this plasmid was isolated in high purity and reasonable yield. It served as template material for the construction of a lambda-phage DNA library of the plasmid. The genes coding for 6-hydroxy-l-nicotine oxidase and for the subunits of the heterotrimeric ketone dehydrogenase were identified, subcloned and sequenced. The 6-hlno gene was identified as a 1278 bp open reading frame; its regulatory elements were also recognized. The derived primary structure of the monomer of apo-6-hydroxy-l-nicotine oxidase (46,264.5 Da) agrees with the data obtained by partial amino acid sequencing. 6-Hydroxy-l-nicotine oxidase and 6-hydroxy-d-nicotine oxidase were expressed in Escherichia coli and obtained in a state of high purity and crystallized. Ketone dehydrogenase (KDH) was found to be a heterotrimer with subunits of molecular mass 89,021.71, 26,778.65 and 17,638.88. The genes of KDH-A and KDH-B are juxtaposed; the A of the stop codon of KDH-A is used in the start codon of KDH-B, eliciting a frame shift. KDH-C is separated from KDH-A by 281 bp.