Genetic variants in nicotinic receptors and smoking cessation in Parkinson's disease

The Role of α7-Nicotinic Acetylcholine Receptors in the Pathophysiology and Treatment of Parkinson's Disease

The α7 nicotinic acetylcholine receptor (α7-nAChR) is a pivotal regulator of neurotransmission, neuroprotection, and immune modulation in the central nervous system. This review explores its structural and functional attributes, highlighting its therapeutic potential in neurodegenerative disorders, particularly Parkinson's disease (PD). α7-nAChRs mediate synaptic plasticity, modulate inflammatory responses, and influence dopamine release, positioning them as a promising pharmacological target. Positive allosteric modulators (PAMs) enhance α7-nAChR activity mainly by reducing desensitization, offering a superior therapeutic approach compared with direct agonists. Emerging preclinical studies suggest that α7-nAChR activation mitigates dopaminergic neurodegeneration, improves L-dopa-induced dyskinesia, and reduces neuroinflammation. Despite promising findings, clinical trials have yielded mixed results, necessitating further research into optimizing α7-targeted therapies. This review underscores the significance of α7-nAChRs in PD pathophysiology and highlights future directions for their translational potential in neuroprotection and symptomatic relief.

Exploring the Nicotine-Parkinson's disease link - Insights from the UK Biobank

INTRODUCTION

Evidence suggests an inverse correlation between smoking and Parkinson's disease (PD), yet the mechanisms remain unclear. This study examines the changing risk relationship between smoking and PD diagnosis using alcohol consumption, another reward-driven behavior, as a comparative measure.

METHODS

A nested case-control study was conducted using the UK Biobank (UKBB) database. Participants in the prediagnostic phase of PD were identified, and self-reported data on tobacco and alcohol use were analyzed employing conditional binary logistic regression. Polynomial and piece-wise regression models were employed to discern shifting associations with PD over time.

RESULTS

Of 502,304 participants (63 ± 5.3 years, 63 % male), 3049 prediagnostic PD cases were identified. Non-smokers had a heightened PD risk, and this association strengthened closer to diagnosis. The odds ratio (OR [95 % CI]) associated with PD in non-smokers was 2.02 [1.07-3.81] 1-4 years before diagnosis, compared to 1.36 [1.02-1.83] at >10-year intervals (linear trend, p = 0.012). The time trajectory of ORs was best depicted by a quadratic function, identifying a shift in risk 7.5 years before diagnosis documentation. Similar patterns emerged among alcohol non-consumers, with an 8.5-year interval inflection point.

CONCLUSION

This study identified two disparate risk trajectories among non-smokers: an initial low-amplitude increased risk at prolonged prediagnostic intervals possibly related to genetic/personality factors, followed by a sharp escalation in risk association commencing 7-8 years before diagnosis, possibly propelled by reverse causality. Similar trends in alcohol consumption reinforce these conclusions. These findings could suggest that smoking cessation may serve as an early indicator of PD.

Update: Protective and risk factors for Parkinson disease

We review the epidemiologic literature on potential protective and risk factors in Parkinson's Disease (PD). Prior research identified numerous possible protective and risk factors. Potential protective factors include tobacco abuse, physical activity, urate levels, NSAID use, calcium channel blocker use, statin use, and use of some α1-adrenergic antagonists. Some potential protective factors could be products of reverse causation, including increased serum urate, tobacco abuse, and coffee-tea-caffeine consumption. Potential risk factors include traumatic brain injury, pesticide exposure, organic solvent exposure, lead exposure, air pollution, Type 2 Diabetes, some dairy products, cardiovascular disease, and some infections including Hepatitis C, H. pylori, and COVID-19. Potential non-environmental risk factors include bipolar disorder, essential tremor, bullous pemphigoid, and inflammatory bowel disease. There is an inverse relationship with PD and risk of most cancers. Though many potential protective and risk factors for PD were identified, research has not yet led to unique, rigorous prevention trials or successful disease-modifying interventions. While efforts to reduce exposure to some industrial toxicants are well justified, PD incidence might be most effectively reduced by mitigation of risks, such as Type 2 Diabetes, air pollution, traumatic brain injury, or physical inactivity, that are general public health intervention targets.

Transdermal Nicotine Treatment and Progression of Early Parkinson's Disease

BACKGROUND: Epidemiologic studies show that smokers have a lower incidence of Parkinson’s disease. Nicotine has been hypothesized to slow progression in early Parkinson’s disease. METHODS: In a double-blind, placebo-controlled multicenter trial, we randomly assigned patients with Parkinson’s disease, diagnosed within 18 months, who were in Hoehn and Yahr disease stage less than or equal to 2 (range from 0 to 5; higher scores indicate greater impairment), who were therapy naïve (except for stable monoamine-oxidase-B inhibition), and not requiring dopaminergic therapy, to transdermal nicotine or placebo. The primary end point was change in Unified Parkinson’s Disease Rating Scale parts I–III (Total UPDRS) score (range from 0 to 172; higher scores indicate greater impairment) between baseline and 60 weeks (52 weeks of trial therapy, 8 weeks of washout). The first secondary end point was change in Total UPDRS from baseline to 52 weeks. Differences between groups were estimated using the Hodges–Lehmann (HL) method and tested with the exact two-sided stratified Mann–Whitney–Wilcoxon test according to the intention-to-treat principle. RESULTS: Among 163 participants, 101 were assessed for the primary end point. Mean worsening of Total UPDRS was 3.5 in the placebo versus 6.0 in the nicotine group (HL-difference with 95% CI: –3 [–6 to 0], P=0.06). For the first secondary end point, analysis of 138 participants showed a mean worsening of 5.4 in the placebo versus 9.1 in the nicotine group (HL-difference with 95% CI: –4 [–7 to –1]). Dropout was mainly because of early treatment discontinuation or adverse events. Cutaneous adverse effects at the patch application site were common. In all, 34.6% of participants initiated dopaminergic therapy during participation. CONCLUSIONS: One-year transdermal nicotine treatment did not slow progression in early Parkinson’s disease. (Funded by the Michael J. Fox Foundation for Parkinson’s Research and others; ClinicalTrials.gov number, NCT01560754; EudraCT number, 2010-020299-42.)

Evaluating the causal effect of tobacco smoking on white matter brain aging: a two-sample Mendelian randomization analysis in UK Biobank

BACKGROUND AND AIMS

Tobacco smoking is a risk factor for impaired brain function, but its causal effect on white matter brain aging remains unclear. This study aimed to measure the causal effect of tobacco smoking on white matter brain aging.

DESIGN

Mendelian randomization (MR) analysis using two non-overlapping data sets (with and without neuroimaging data) from UK Biobank (UKB). The group exposed to smoking and control group consisted of current smokers and never smokers, respectively. Our main method was generalized weighted linear regression with other methods also included as sensitivity analysis.

SETTING

United Kingdom.

PARTICIPANTS

The study cohort included 23 624 subjects [10 665 males and 12 959 females with a mean age of 54.18 years, 95% confidence interval (CI) = 54.08, 54.28].

MEASUREMENTS

Genetic variants were selected as instrumental variables under the MR analysis assumptions: (1) associated with the exposure; (2) influenced outcome only via exposure; and (3) not associated with confounders. The exposure smoking status (current versus never smokers) was measured by questionnaires at the initial visit (2006-10). The other exposure, cigarettes per day (CPD), measured the average number of cigarettes smoked per day for current tobacco users over the life-time. The outcome was the 'brain age gap' (BAG), the difference between predicted brain age and chronological age, computed by training machine learning model on a non-overlapping set of never smokers.

FINDINGS

The estimated BAG had a mean of 0.10 (95% CI = 0.06, 0.14) years. The MR analysis showed evidence of positive causal effect of smoking behaviors on BAG: the effect of smoking is 0.21 (in years, 95% CI = 6.5 × 10-3 , 0.41; P-value = 0.04), and the effect of CPD is 0.16 year/cigarette (UKB: 95% CI = 0.06, 0.26; P-value = 1.3 × 10-3 ; GSCAN: 95% CI = 0.02, 0.31; P-value = 0.03). The sensitivity analyses showed consistent results.

CONCLUSIONS

There appears to be a significant causal effect of smoking on the brain age gap, which suggests that smoking prevention can be an effective intervention for accelerated brain aging and the age-related decline in cognitive function.

Parkinson's disease population-wide registries in the United States: Current and future opportunities

Parkinson's disease (PD) is a neurodegenerative disease with both genetic and environmental risk factors. Efforts to understand the growing incidence and prevalence of PD have led to several state PD registry initiatives in the United States. The California PD Registry (CPDR) is the largest state-wide PD registry and requires electronic reporting of all eligible cases by all medical providers. We borrow from our experience with the CPDR to highlight 4 gaps to population-based PD registries. Specifically we address (1) who should be included in PD registries; (2) what data should be collected in PD case reports; (3) how to ensure the validity of case reports; and (4) how can state PD registries exchange and aggregate information. We propose a set of recommendations that addresses these and other gaps toward achieving a promise of a practical, interoperable, and scalable PD registry in the U.S., which can serve as a key health information resource to support epidemiology, health equity, quality improvement, and research.

A systematic review of genetic variation within nicotinic acetylcholine receptor genes and cigarette smoking cessation

BACKGROUND

Nicotine produces its effects by binding to nicotinic acetylcholine receptors (nAChRs). Variants of genes encoding properties of nAChRs are candidates for affecting likelihood of smoking cessation.

METHODS

A systematic review was conducted summarizing evidence of associations between single nucleotide polymorphisms (SNPs) of nAChR genes and smoking cessation. From 24 articles meeting inclusion criteria, summary odds ratios (ORs) for associations between nine SNPs and smoking cessation were calculated from 26 studies (N = 233-29,072) stratified by gene, ancestry, study design, and pharmacotherapy; SNPs in linkage disequilibrium were pooled. Results for a tenth SNP from two GWAS were summarized.

RESULTS

People of European ancestry with minor alleles of CHRNA5 rs16969968 and CHRNA3 rs1051730 had longer time to cessation [HR = 0.90, 95 % CI 0.88 - 0.92 (n = 2 studies)] and lower odds of cessation [OR = 0.88, 95 % CI 0.80 - 0.97 (n = 5 cohort studies), OR = 0.64, 95 % CI 0.45 - 0.90 (n = 4 placebo arms)]. Risk of persistent smoking associated with these alleles was attenuated in smokers receiving nicotine replacement therapy (NRT). Recipients of bupropion alone or with NRT with these alleles had higher, though not statistically significant, odds of cessation. Results for CHRNA5 rs588765 and rs680244 were similar to rs16969968/rs1051730 findings. Evidence was limited for other SNPs.

CONCLUSION

Evidence consistently indicates the minor alleles of four SNPs within CHRNA3 or CHRNA5 are risk alleles for cessation failure. Analysis by pharmacotherapy revealed bupropion may be the most efficacious intervention for people with these alleles.

Age at Onset of Parkinson's Disease Among Ashkenazi Jewish Patients: Contribution of Environmental Factors, LRRK2 p.G2019S and GBA p.N370S Mutations

BACKGROUND

Both genetic and environmental factors contribute to Parkinson's disease (PD) risk.

OBJECTIVE

We investigated the potential association of several relevant variables with PD age at onset (AAO), focusing on LRRK2 p.G2019S and GBA p.N370S mutations.

METHODS

Ashkenazi Jewish (AJ) PD patients, screened for LRRK2 and GBA mutations, underwent an interview regarding exposure to the following environmental and lifestyle factors: cigarette smoking, consumption of coffee, tea and alcohol, head injury and rural living. Multivariate linear regression (adjusted for sex) was used to examine the association with AAO, and models included LRRK2 p.G2019S and GBA p.N370S mutation status (carrier/non-carriers), single environmental variable and their interactions terms, as independent variables.

RESULTS

225 Israeli AJ PD patients were enrolled: 65 LRRK2 p.G2019S mutation carriers, 60 GBA p.N370S carriers and 100 non-carries of these mutations. In the dichotomized exposure/non-exposure analyses, positive LRRK2 p.G2019S status was associated with younger AAO in all models, at nominal or near significant levels (p = 0.033-0.082). Smoking was associated with older AAO (p = 0.032), and the interaction between GBA p.N370S and history of head injury was associated with younger AAO (p = 0.049), both at nominal significance. There was no indication of a consistent main effect for GBA p.N370S status or significant LRRK2 p.G2019S-environmental factor interaction. In the dose-dependent analyses, increased coffee and tea consumption levels were associated with older AAO (p = 0.001 and p = 0.002, respectively).

CONCLUSIONS

Our results suggest that genetic and environmental factors may affect AAO in PD patients, but validation in additional samples is required.

Neuroticism, Smoking, and the Risk of Parkinson's Disease

Background:

The relationship among neuroticism, smoking, and Parkinson’s disease (PD) is less examined.

Objective:

To examine the causal associations between neuroticism, smoking initiation, and the risk of PD.

Methods:

We performed a two-sample Mendelian randomization (MR) design in a network framework. Summary statistics from meta-analyses of genome-wide association studies (GWAS) were based on large cohorts of European ancestry. Study participants were from various cohort studies for neuroticism and smoking initiation, and case-control studies or cohort studies of PD from previously published GWAS meta-analyses. Patients with PD were ascertained from either clinical visit or self-reported.

Results:

The two-sample MR analysis showed no evidence for a causal association between neuroticism and PD risk (odds ratio [OR] 0.86, 95%confidence intervals [CIs] 0.67 to 1.12). While we did not find a significant association between neuroticism and PD, one SNP, rs58879558 (located in MAPT region), was associated with both neuroticism and PD. We found a significant association of neuroticism on smoking initiation (OR: 1.10, 95%CI: 1.05 to 1.14). Further, our results provided evidence for a protective effect of smoking initiation on the risk of PD (OR: 0.75, 95%CI: 0.62 to 0.91).

Conclusion:

These findings do not support a causal association of neuroticism on PD risk. However, they provide evidence for a causal relationship between neuroticism and smoking initiation and a strong causal effect of smoking initiation on a reduced risk of PD.