Role of cigarette smoking in the development of ischemic stroke and its subtypes: a Mendelian randomization study

Appraising the Causal Role of Risk Factors in Coronary Artery Disease and Stroke: A Systematic Review of Mendelian Randomization Studies

BACKGROUND Mendelian randomization (MR) offers a powerful approach to study potential causal associations between exposures and health outcomes by using genetic variants associated with an exposure as instrumental variables. In this systematic review, we aimed to summarize previous MR studies and to evaluate the evidence for causality for a broad range of exposures in relation to coronary artery disease and stroke. METHODS AND RESULTS MR studies investigating the association of any genetically predicted exposure with coronary artery disease or stroke were identified. Studies were classified into 4 categories built on the significance of the main MR analysis results and its concordance with sensitivity analyses, namely, robust, probable, suggestive, and insufficient. Studies reporting associations that did not perform any sensitivity analysis were classified as nonevaluable. We identified 2725 associations eligible for evaluation, examining 535 distinct exposures. Of them, 141 were classified as robust, 353 as probable, 110 as suggestive, and 926 had insufficient evidence. The most robust associations were observed for anthropometric traits, lipids, and lipoproteins and type 2 diabetes with coronary artery; disease and clinical measurements with coronary artery disease and stroke; and thrombotic factors with stroke. CONCLUSIONS Despite the large number of studies that have been conducted, only a limited number of associations were supported by robust evidence. Approximately half of the studies reporting associations presented an MR sensitivity analysis along with the main analysis that further supported the causality of associations. Future research should focus on more thorough assessments of sensitivity MR analyses and further assessments of mediation effects or nonlinearity of associations.

Mendelian randomization analysis implicates bidirectional associations between brain imaging-derived phenotypes and ischemic stroke

Brian imaging-derived phenotypes (IDPs) have been suggested to be associated with ischemic stroke, but the causality between them remains unclear. In this bidirectional two-sample Mendelian randomization (MR) study, we explored the potential causal relationship between 461 imaging-derived phenotypes (n = 33,224, UK Biobank) and ischemic stroke (n = 34,217 cases/406,111 controls, Multiancestry Genome-Wide Association Study of Stroke). Forward MR analyses identified five IDPs associated with ischemic stroke, including mean diffusivity (MD) in the right superior fronto-occipital fasciculus (1.22 [95% CI, 1.11-1.34]), MD in the left superior fronto-occipital fasciculus (1.30 [1.17-1.44]), MD in the anterior limb of the right internal capsule (1.36 [1.22-1.51]), MD in the right anterior thalamic radiation (1.17 [1.09-1.26]), and MD in the right superior thalamic radiation (1.23 [1.11-1.35]). In the reverse MR analyses, ischemic stroke was identified to be associated with three IDPs, including high isotropic or free water volume fraction in the body of corpus callosum (beta, 0.189 [95% confidence interval, 0.107-0.271]), orientation dispersion index in the pontine crossing tract (0.175 [0.093-0.257]), and volume of the third ventricle (0.219 [0.138-0.301]). This bidirectional two-sample MR study suggested five predictors and three diagnostic markers for ischemic stroke at the brain-imaging level. Further studies are warranted to replicate our findings and clarify underlying mechanisms.

Attention-deficit/hyperactivity disorder and ischemic stroke: A Mendelian randomization study

BACKGROUND

Observational studies have found an association between attention-deficit/hyperactivity disorder (ADHD) and ischemic stroke.

AIMS

The purpose of this study was to investigate whether genetic liability to ADHD has a causal effect on ischemic stroke and its subtypes.

METHODS

In this two-sample Mendelian randomization (MR) study, genetic variants (nine single-nucleotide polymorphisms; P < 5 × 10^-8) using as instrumental variables for the analysis was obtained from a genome-wide association study of ADHD with 19,099 cases and 34,194 controls. The outcome datasets for stroke and its subtypes were obtained from the MEGASTROKE consortium, with 40,585 cases and 406,111 controls. MR inverse variance-weighted method was conducted to investigate the effect of genetic liability to ADHD on ischemic stroke and its subtypes. Sensitivity analyses (median-based methods, MR-Egger, MR-robust adjusted profile scores, MR-pleiotropy residual sum and outlier) were also utilized to assess horizontal pleiotropy and remove outliers. Multivariable MR (MVMR) analyses were conducted to explore potential mediators.

RESULTS

Genetically determined ADHD (per 1 SD) was significantly associated with a higher risk of any ischemic stroke (AIS) (odds ratio (OR) = 1.15, 95% confidence interval (CI) = 1.05-1.25, P = 0.002) and large-artery atherosclerotic stroke (LAS) (OR = 1.40, 95% CI = 1.10-1.76, P = 0.005). The significant association was also found in sensitivity analyses and MVMR analyses.

CONCLUSIONS

Genetic liability to ADHD was significantly associated with an increased risk of AIS, especially LAS. The association between ADHD and LAS was independent of age of smoking initiation but mediated by coronary artery disease.

Genetically Predicted Smoking and Alcohol Consumption and Functional Outcome After Ischemic Stroke

BACKGROUND AND OBJECTIVES

Smoking and alcohol consumption have been adversely associated with poststroke outcome in traditional epidemiologic studies. The present study explored the association of genetically predicted smoking and alcohol consumption on poststroke outcomes using the mendelian randomization (MR) framework.

METHODS

Instrumental variables for smoking initiation and alcohol consumption were selected from a genome-wide association study data of European ancestry individuals. Summary-level data for functional outcome after ischemic stroke were obtained from the Genetics of Ischemic Stroke Functional Outcome network study of European ancestry patients. The univariable and multivariable inverse-variance weighted MR methods were performed to obtain the causal estimates. The weighted median, MR-robust adjusted profile score, and MR-Egger regression approaches were adopted as sensitivity analyses. Q and I 2 statistics were used to evaluate heterogeneity in MR estimates across variants.

RESULTS

Genetic predisposition to smoking initiation was associated with worse functional outcome after ischemic stroke in univariable inverse-variance weighted MR analysis (odds ratio [OR] 1.48; 95% CI 1.08-2.01, p = 0.013). This association remained significant when adjusting for genetically predicted alcohol consumption in multivariable MR analyses (OR 1.56; 95% CI 1.05-2.32, p = 0.027). Genetically predicted alcohol consumption was not associated with functional outcome after ischemic stroke (p > 0.05). Sensitivity analyses with other approaches and in analyses restricted to models without adjustment for baseline stroke severity produced similar results, and no evidence of heterogeneity in MR estimates between variants was detected (p > 0.05).

DISCUSSION

Our results provide genetic support for a causal association of smoking with worse functional outcome after ischemic stroke and have important implications for poststroke recovery. Smoking cessation and avoidance should be promoted in patients with ischemic stroke.

Genome-Wide Studies in Ischaemic Stroke: Are Genetics Only Useful for Finding Genes?

Ischaemic stroke is a complex disease with some degree of heritability. This means that heritability factors, such as genetics, could be risk factors for ischaemic stroke. The era of genome-wide studies has revealed some of these heritable risk factors, although the data generated by these studies may also be useful in other disciplines. Analysis of these data can be used to understand the biological mechanisms associated with stroke risk and stroke outcome, to determine the causality between stroke and other diseases without the need for expensive clinical trials, or to find potential drug targets with higher success rates than other strategies. In this review we will discuss several of the most relevant studies regarding the genetics of ischaemic stroke and the potential use of the data generated.

Genetics of common cerebral small vessel disease

Cerebral small vessel disease (cSVD) is a leading cause of ischaemic and haemorrhagic stroke and a major contributor to dementia. Covert cSVD, which is detectable with brain MRI but does not manifest as clinical stroke, is highly prevalent in the general population, particularly with increasing age. Advances in technologies and collaborative work have led to substantial progress in the identification of common genetic variants that are associated with cSVD-related stroke (ischaemic and haemorrhagic) and MRI-defined covert cSVD. In this Review, we provide an overview of collaborative studies - mostly genome-wide association studies (GWAS) - that have identified >50 independent genetic loci associated with the risk of cSVD. We describe how these associations have provided novel insights into the biological mechanisms involved in cSVD, revealed patterns of shared genetic variation across cSVD traits, and shed new light on the continuum between rare, monogenic and common, multifactorial cSVD. We consider how GWAS summary statistics have been leveraged for Mendelian randomization studies to explore causal pathways in cSVD and provide genetic evidence for drug effects, and how the combination of findings from GWAS with gene expression resources and drug target databases has enabled identification of putative causal genes and provided proof-of-concept for drug repositioning potential. We also discuss opportunities for polygenic risk prediction, multi-ancestry approaches and integration with other omics data.

Impact of risk factors for major cardiovascular diseases: a comparison of life-time observational and Mendelian randomisation findings

Background

This study compared the strength and causality of associations between major risk factors for cardiovascular disease (CVD) and the four major CVDs: myocardial infarction, ischaemic stroke, heart failure and atrial fibrillation. Both a long-term follow-up in an observational cohort and Mendelian randomisation (MR) were used for this aim.

Methods

In the Uppsala Longitudinal Study of Adult Men study, 2322 men, all aged 50 years, were assessed for CVD risk factors and then followed for four decades regarding incident CVDs. The two-sample MR part used public available Genome-Wide Association Study (GWAS) data.

Results

In multivariate analyses, systolic blood pressure was overall by far the most important risk factor, since it was related to all four CVDs, both in observational and MR analyses. Body mass index was the second most overall important risk factor, being linked to all four CVDs, except ischaemic stroke, both in observational and MR analyses. Smoking was an important risk factor for ischaemic stroke and heart failure, both in observational and MR analyses, while low-density lipoprotein-cholesterol mainly was related to myocardial infarction. Diabetes was mainly a causal risk factor for incident myocardial infarction and heart failure. Neither HDL-cholesterol nor triglycerides were of major importance as risk factors in these multivariable models.

Conclusion

By combining long-term observational data with genetic data, we show that the impact and causal role of specific established cardiovascular risk factors varies between different major CVDs. Systolic blood pressure was causally related to all four cardiovascular outcomes and was therefore, overall, the most important risk factor.

Mendelian Randomization in Stroke: A Powerful Approach to Causal Inference and Drug Target Validation

Stroke is a leading cause of death and disability worldwide. However, our understanding of its underlying biology and the number of available treatment options remain limited. Mendelian randomization (MR) offers a powerful approach to identify novel biological pathways and therapeutic targets for this disease. Around ~100 MR studies have been conducted so far to explore, confirm, and quantify causal relationships between several exposures and risk of stroke. In this review, we summarize the current evidence arising from these studies, including those investigating ischemic stroke, hemorrhagic stroke, or both. We highlight the different types of exposures that are currently under study, ranging from well-known cardiovascular risk factors to less established inflammation-related mechanisms. Finally, we provide an overview of future avenues of research and novel approaches, including drug target validation MR, which is poised to have a substantial impact on drug development and drug repurposing.

Disparities between Asian and Non-Asian Thrombolyzed Acute Ischemic Stroke Patients in the Enhanced Control of Hypertension and Thrombolysis Stroke Trial

BACKGROUND AND PURPOSE

As outcomes for acute ischemic stroke (AIS) vary according to clinical profile and management approaches, we aimed to determine disparities in clinical outcomes between Asian and non-Asian participants of the international, Enhanced Control of Hypertension and Thrombolysis Stroke study (ENCHANTED).

METHODS

ENCHANTED was a multicenter, prospective, partial-factorial, randomized, open trial of low-dose (0.6 mg/kg) versus standard-dose (0.9 mg/kg) alteplase, and intensive (target systolic blood pressure [SBP] 130-140 mm Hg) or guideline-recommended (<180 mm Hg) BP management, in thrombolysis-eligible AIS patients. Logistic regression models were used to examine the associations with outcomes of death or disability (modified Rankin scale [mRS] scores 2-6), major disability (mRS 3-5), death, and intracranial hemorrhage (ICH), with adjustment prognostic factors, alteplase dose, and mean SBP over 1-24 h.

RESULTS

Among 4,551 thrombolyzed AIS patients (mean age 66.7 years, 37.8% female), there were 65.4% Asians who were younger, fewer female, and with less atrial fibrillation, hypercholesterolemia, premorbid symptoms, and concomitant antihypertensive, antithrombotic and statin treatment, and more prior stroke, compared to non-Asians. Frequencies of hypertension, coronary artery disease, and diabetes mellitus were comparable between groups. Asian patients were less likely to be admitted to an acute stroke unit and receive early mobilization by a therapist or rehabilitation but more likely to receive intensive care. There were no significant differences between Asians and non-Asians in functional outcome (defined by mRS scores 2-6 or 3-5; adjusted odds ratio [OR] 1.00, 95% confidence interval [CI] 0.85-1.19 [p = 0.958] and OR 0.95, 95% CI 0.80-1.13 [p = 0.572], respectively), or death (OR 1.25, 95% CI 0.95-1.65; p = 0.116), despite Asians having greater odds of ICH (OR 1.51, 95% CI 1.23-1.86; p = 0.0001) and neurological deterioration within 24 h (OR 1.58, 95% CI 1.18-2.12; p = 0.002).

CONCLUSIONS

Within the context of an international clinical trial of thrombolyzed AIS patients, demography, risk factors, management, and odds of early neurological deterioration and ICH, all differ between Asian and non-Asian participants. However, patterns of functional recovery are similar between these major regional groups.

Smoking influences outcome in patients who had thrombolysed ischaemic stroke: the ENCHANTED study

Background and purpose

As studies vary in defining the prognostic significance of smoking in acute ischaemic stroke (AIS), we aimed to determine the relation of smoking and key outcomes in patient participants who had thrombolysed AIS of the international quasi-factorial randomised Enhanced Control of Hypertension and Thrombolysis Stroke Study (ENCHANTED).

Methods

Post-hoc analyses of ENCHANTED, an international quasi-factorial randomised evaluation of intravenous alteplase-dose comparison and levels of blood pressure control in patients who had thrombolysed AIS. Multivariable logistic regression models with inverse probability of treatment weighting (IPTW) propensity scores were used to determine associations of self-reported smoking status and clinical outcomes, according to 90-day modified Rankin Scale (mRS) scores and symptomatic intracerebral haemorrhage (sICH).

Results

Of 4540 patients who had an AIS, there were 1008 (22.2%) current smokers who were younger and predominantly male, with more comorbidities of hypertension, coronary artery disease, atrial fibrillation and diabetes mellitus, and greater baseline neurological impairment, compared with non-smokers. In univariate analysis, current smokers had a higher likelihood of a favourable shift in mRS scores (OR 0.88, 95% CI 0.77 to 0.99; p=0.038) but this association reversed in a fully adjusted model with IPTW (adjusted OR 1.15, 95% CI 1.04 to 1.28; p=0.009). A similar trend was also apparent for dichotomised poor outcome (mRS scores 2–6: OR 1.18, 95% CI 1.05 to 1.33; p=0.007), but not with the risk of sICH across standard criteria.

Conclusion

Smoking predicts poor functional recovery in patients who had thrombolysed AIS.

Trial registration number

NCT01422616.

Assessment Causality in Associations Between Serum Uric Acid and Risk of Schizophrenia: A Two-Sample Bidirectional Mendelian Randomization Study

Purpose

Although increasing lines of evidence showed associations between serum uric acid (UA) levels and schizophrenia, the causality and the direction of the associations remain uncertain. Thus, we aimed to assess whether the relationships between serum UA levels and schizophrenia are causal and to determine the direction of the association.

Patients and Methods

Two-sample bidirectional Mendelian randomization (MR) analyses and various sensitivity analyses were performed utilizing the summary data from genome-wide association studies within the Global Urate Genetics Consortium and the Psychiatric Genomics Consortium. Secondary MR analyses in both directions were conducted within summary data using genetic risk scores (GRSs) as instrumental variables.

Results

Three MR methods provided no causal relationship between serum UA and schizophrenia. Furthermore, GRS approach showed similar results in the three MR methods after adjustment for heterogeneity. By contrast, inverse variance weighted method, weighted median and GRS approach suggested a causal effect of schizophrenia risk on serum UA after adjustment for heterogeneity (per 10-symmetric percentage increase in schizophrenia risk, beta: -0.039, standard error (SE): 0.013, P = 0.003; beta: -0.036, SE: 0.018, P = 0.043; beta: -0.039, SE: 0.013, P = 0.002; respectively). Moreover, in both directions' analyses, the heterogeneity and sensitivity tests suggested no strong evidence of bias due to pleiotropy.

Conclusion

Schizophrenia may causally affect serum UA levels, whereas the causal role of serum UA concentrations in schizophrenia was not supported by our MR analyses. These findings suggest that UA may be a useful potential biomarker for monitoring treatment or diagnosis of schizophrenia rather than a therapeutic target for schizophrenia.