An introduction to Mendelian randomization with applications in neurology

Causal relationship between genetically predicted mental disorders and frailty: a bidirectional and multivariable mendelian randomization study

BACKGROUND

In observational studies, frailty has been strongly associated with mental disorders. However, the mechanisms underlying the association between frailty and mental disorders remain unclear.

METHODS

We conducted a two-sample Mendelian randomization (MR) study to assess the causal relationship between frailty, as measured by the frailty index (FI), and ten common mental disorders. The datasets involved European ancestry individuals and included measurements of the FI (N = 175,226), schizophrenia (SCZ; N = 320,404), major depressive disorder (MDD; N = 143,265), bipolar disorder (N = 337,199), insomnia (N = 462,341), obsessive-compulsive disorder (N = 33,925), anxiety disorders (N = 463,010), autism spectrum disorder (N = 46,351), anorexia nervosa (N = 14,477), opioid-related mental and behavioral disorders (N = 215,650), and mental and behavioral disorders due to use of other stimulants including caffeine (N = 215,570).

RESULTS

Two-sample MR analyses were performed using inverse variance weighting followed by various sensitivity and validation analyses. Genetically predicted SCZ (odds ratio [OR] = 1.019, 95% confidence interval [CI] 1.005-1.033) and MDD (OR = 1.211, 95% CI 1.092-1.343) had significant causal effects on FI. In the reverse MR analysis, we discovered that MDD was significantly and causally affected by FI (OR = 1.290, 95% CI 1.133-1.469). No causal links were identified between the FI and the other eight common mental disorders. In the Multivariable MR, the estimated MDD effect on FI is comparable to the univariate IVW estimate (OR = 1.298; 95% CI, 1.175 to 1.435), while the estimated SCZ effect on FI fails to be significant compared to the univariate estimate. The results of the sensitivity and validation analyses confirmed stabilization.

CONCLUSIONS

Our study found evidence of a causal relationship between SCZ, MDD, and frailty and explored the underlying mechanisms.

Dissecting the mediating role of inflammatory factors in the interaction between metabolites and sepsis: insights from bidirectional Mendelian randomization

Sepsis, a life-threatening condition, involves complex interactions among metabolic alterations, inflammatory mediators, and host responses. This study utilized a bidirectional Mendelian randomization approach to investigate the causal relationships between 1400 metabolites and sepsis, and the mediating role of inflammatory factors. We identified 36 metabolites significantly associated with sepsis (p < 0.05), with AXIN1, FGF-19, FGF-23, IL-4, and OSM showing an inverse association, suggesting a protective role, while IL-2 exhibited a positive correlation, indicating a potential risk factor. Among these metabolites, Piperine and 9-Hydroxystearate demonstrated particularly interesting protective effects against sepsis. Piperine's protective effect was mediated through its interaction with AXIN1, contributing to a 16.296% reduction in sepsis risk. This suggests a potential pathway where Piperine influences sepsis outcomes by modulating AXIN1 levels. 9-Hydroxystearate also exhibited a protective role against sepsis, mediated through its positive association with FGF-19 and negative association with IL-2, contributing 9.436% and 12.565%, respectively, to its protective effect. Experimental validation confirmed significantly elevated IL-2 levels and reduced FGF-19, AXIN1, piperine, and 9-hydroxyoctadecanoic acid levels in sepsis patients compared to healthy controls. Piperine levels positively correlated with AXIN1, while 9-hydroxyoctadecanoic acid levels negatively correlated with IL-2 and positively correlated with FGF-19, supporting the Mendelian randomization findings. Our findings provide insights into the molecular mechanisms of sepsis, highlighting the unique roles and contributions of specific metabolites and their interactions with inflammatory mediators. This study enhances our understanding of sepsis pathophysiology and opens avenues for targeted therapeutic interventions and biomarker development for sepsis management. However, further research is essential to validate these pathways across diverse populations and fully explore the roles of these metabolites in sepsis.

Gut Microbiota Composition Is Causally Linked to Multiple Sclerosis: A Mendelian Randomization Analysis

Accumulating evidence links the microbial communities inhabiting the gut to the pathophysiological processes underlying multiple sclerosis (MS). However, most studies on the microbiome in MS are correlative in nature, thus being at risk of confounding and reverse causality. Mendelian randomization (MR) analyses allow the estimation of the causal relationship between a risk factor and an outcome of interest using genetic variants as proxies for environmental exposures. Here, we performed a two-sample MR to assess the causality between the gut microbiome and MS. We extracted genetic instruments from summary statistics from three large genome-wide association studies (GWASs) on the gut microbiome (18,340, 8959, and 7738 subjects). The exposure data were derived from the latest GWAS on MS susceptibility (47,429 patients and 68,374 controls). We pinpointed several microbial strains whose abundance is linked with enhanced MS risk (Actinobacteria class, Bifidobacteriaceae family, Lactobacillus genus) or protection (Prevotella spp., Lachnospiranaceae genus, Negativibacillus genus). The largest risk effect was seen for Ruminococcus Torques (OR, 2.89, 95% C.I. 1.67-5, p = 1.51 × 10-4), while Akkermansia municiphila emerged as strongly protective (OR, 0.43, 95% C.I. 0.32-0.57, p = 1.37 × 10-8). Our findings support a causal relationship between the gut microbiome and MS susceptibility, reinforcing the relevance of the microbiome-gut-brain axis in disease etiology, opening wider perspectives on host-environmental interactions for MS prevention.

Causal relationship between genetically determined plasma metabolites and skin cancer: a two-sample Mendelian randomization study

We aimed to unveil the underlying pathogenic mechanisms of skin cancer in relation to metabolic factors and pathway mechanisms. This study utilized the TwoSample Mendelian randomization (MR) method to investigate the causal relationship between 1400 plasma metabolites and skin cancer. The primary method employed was the inverse variance weighting (IVW). Through IVW analysis, we found 105 plasma metabolites associated with Basal Cell Carcinoma (BCC), with the highest association observed for Prolylglycine levels (OR [95% CI]: 1.1902 [1.0274, 1.3788]). For Malignant Melanoma of Skin (MSS), 68 plasma metabolites were linked, with the highest causal relationship seen for 3-Hydroxybutyrate levels (OR [95% CI]: 1.0030 [1.0013, 1.0048]). Regarding actinic keratosis (AK), and the highest association observed for Hexadecadienoate (16:2n6) levels (OR [95% CI]: 1.3302 [1.0333, 1.7125]). Glycerol to palmitoylcarnitine (16: n6) levels (OR [95% CI]: 1.3302 [1.0333, 1.125]) were found to be significant for BCC and AK. Palmitoylcarnitine (C16) had the most positive causal effect for BCC (OR [95% CI]: 1.1777 [1.0493, 1.3218]), while 5-hydroxy-2-methylpyridine sulfate levels had the highest effect for AK (OR [95% CI]: 1.1788 [1.0295, 1.3498]). And 4-guanidinobutanoate levels had the largest positive causal effect (OR [95% CI]: 1.0857 [1.0417, 1.1317]) for BCC, and X-11880 levels for MSS (OR [95% CI]: 1.0013 [1.0000, 1.0025]). The study revealed a positive association between hereditary Glycerol to palmitoylcarnitine (C16) and 5-hydroxy-2-methylpyridine sulfate levels with the risk of developing BCC and AK. Additionally, 4-guanidinobutanoate levels and X 11880 levels were found to be positively associated with the risk of BCC and MMS.

Disturbance of skin sensation and autism spectrum disorder: A bidirectional Mendelian randomization study

BACKGROUND AND AIM

Patients with autism spectrum disorder (ASD) commonly experience aberrant skin sensation sensitivity; however, the causal relationship is not yet clear. This study uses a bidirectional Mendelian randomization (MR) method to explore the relationship between disturbance of skin sensation (DSS) and ASD.

METHODS

Single-nucleotide polymorphisms (SNPs) extracted from the summary data of genome-wide association studies were used as genetic instruments. MR was performed using the inverse-variance-weighted method, with alternate methods (e.g., weighted median, MR-Egger, simple mode, weighted mode, and MR-pleiotropy residual sum and outlier) and multiple sensitivity analyses to assess horizontal pleiotropy and remove outliers.

RESULTS

The results of the analysis using six SNPs as genetic instruments showed that the DSS is associated with an increased risk of ASD (odds ratio = 1.126, 95% confidence interval = 1.029-1.132; p = .010). The results of the sensitivity analyses were robust with no evidence of pleiotropy. The reverse MR analyses showed no causal effects of ASD on DSS.

CONCLUSION

This study's findings suggest that DSS has potential causal effects on ASD, whereas ASD has no effect on DSS. Thus, skin sensitivity may represent a behavioral marker of ASD, by which some populations could be subtyped in the future.

The causality between obstructive sleep apnea and ventricular structure and function: a bidirectional Mendelian randomization study

Background: Multiple observational studies have discovered a substantial link between obstructive sleep apnea (OSA) and ventricular dysfunction. However, conventional observational studies are vulnerable to causal reversal and confounding, making it challenging to infer the causes of effects and their direction. Methods: With the help of a bidirectional, two-sample Mendelian randomization (MR) study, we assessed the potential causality between OSA and left and right ventricular (LV, RV) structure and function. We conducted our analysis utilizing summary data from genome-wide association studies of OSA (16,761 cases and 201,194 controls) in the FinnGen Study, as well as LV (36,041 participants) and RV (29,506 participants) in the UK Biobank cardiovascular magnetic resonance research. The inverse variance weighted (IVW) was selected as the main strategy, with the MR-Egger and weighted median methods serving as supplements. Other methods were employed as sensitivity analysis tools to look at heterogeneity and pleiotropy, including MR-Egger intercept, Cochran Q statistic, MR-PRESSO, and leave-one-out analysis. Results: In the primary IVW analysis, genetically predicted OSA was strongly causative on LV end-diastolic volume (β = 0.114, 95% CI = 0.034-0.194, p = 0.006) and LV stroke volume (β = 0.111, 95% CI = 0.031-0.191, p = 0.007), and genetically predicted LV ejection fraction was linked to an increased risk of OSA (OR = 1.161, 95% CI = 1.029-1.309, p = 0.015). However, there was no connection found between OSA and any RV parameters. Conclusion: Our genetic analysis raises a potential causative link between OSA and ventricular structure and function, which may improve the knowledge of OSA as a risk factor for cardiovascular disease by demonstrating a direct impact on cardiac structure and function.

Causal effect of psychiatric disorders on epilepsy: A two-sample Mendelian randomization study

BACKGROUND

This study aims to explore the relationship between psychiatric disorders and the risk of epilepsy using Mendelian randomization (MR) analysis.

METHODS

We collected summary statistics of seven psychiatric traits from recent largest genome-wide association study (GWAS), including major depressive disorder (MDD), anxiety disorder, autism spectrum disorder (ASD), bipolar disorder (BIP), attention deficit hyperactivity disorder (ADHD), schizophrenia (SCZ), and insomnia. Then, MR analysis estimates were performed based on International League Against Epilepsy (ILAE) consortium data (n_case  = 15,212 and n_control  = 29,677), the results of which were subsequently validated in FinnGen consortium (n_case  = 6260 and n_control  = 176,107). Finally, a meta-analysis was conducted based on the ILAE and FinnGen data.

RESULTS

We found significant causal effects of MDD and ADHD on epilepsy in the meta-analysis of the ILAE and FinnGen, with corresponding odds ratios (OR) of 1.20 (95% CI 1.08-1.34, p = .001) and 1.08 (95% CI 1.01-1.16, p = .020) by the inverse-variance weighted (IVW) method respectively. MDD increases the risk of focal epilepsy while ADHD has a risk effect on generalized epilepsy. No reliable evidence regarding causal effects of other psychiatric traits on epilepsy was identified.

CONCLUSIONS

This study suggests that major depressive disorder and attention deficit hyperactivity disorder may causally increase the risk of epilepsy.

Genetically predicted childhood obesity and adult atrial fibrillation: A mendelian randomization study

BACKGROUND AND AIMS

It is unclear whether the association of childhood obesity with adult atrial fibrillation observed in observational studies reflects causal effects. The aim of this study was to evaluate the association of childhood obesity with adult atrial fibrillation using genetic instruments.

METHODS AND RESULTS

We used a two-sample Mendelian randomization (MR) design to evaluate the association between childhood obesity and adult atrial fibrillation. Two sets of genetic variants (15 single nucleotide polymorphisms [SNPs] for childhood body mass index [BMI] and 12 SNPs for dichotomous childhood obesity) were selected as instruments. Summary data on SNP-childhood obesity and SNP-atrial fibrillation associations were obtained from recently published genome-wide association studies. Effect estimates were evaluated using inverse-variance weighted (IVW) methods. Other MR analyses, including MR-Egger, simple and weighted median, weighted MBE and MR-PRESSO methods were performed in sensitivity analyses. The IVW models showed that both a genetically predicted one-standard deviation increase in childhood BMI (kg/m²) and higher log-odds of childhood obesity were associated with a substantial increase in the risk of atrial fibrillation (OR = 1.22, 95% CI: 1.11-1.34, P < 0.001; OR = 1.09, 95% CI: 1.04-1.14, P < 0.001). MR-Egger regression showed no evidence of genetic pleiotropy for childhood BMI (intercept = 0.000, 95% CI: -0.024 to 0.023), but for childhood obesity (intercept = -0.036, 95% CI: -0.057 to -0.015). Similar results were observed using leave-one-out and other MR methods in sensitivity analyses.

CONCLUSIONS

This MR analysis found a consistent association between genetically predicted childhood obesity and an increased risk of adult atrial fibrillation. Further research is warranted to validate our findings.

Causal Effect of Obstructive Sleep Apnea on Atrial Fibrillation: A Mendelian Randomization Study

Background

Obstructive sleep apnea (OSA) has shown to be associated with an increased risk of atrial fibrillation in observational studies. Whether this association reflect causal effect is still unclear. The aim of this study was to evaluate the causal effect of OSA on atrial fibrillation.

Methods and Results

We used a 2‐sample Mendelian randomization (MR) method to evaluate the causal effect of OSA on atrial fibrillation. Summary data on genetic variant‐OSA association were obtained from a recently published genome‐wide association studies with up to 217 955 individuals and data on variant‐atrial fibrillation association from another genome‐wide association study with up to 1 030 836 individuals. Effect estimates were evaluated using inverse‐variance weighted method. Other MR analyses, including penalized inverse‐variance weighted, penalized robust inverse‐variance weighted, MR‐Egger, simple median, weighted median, weighted mode‐based estimate and Mendelian Randomization Pleiotropy Residual Sum and Outlier methods were performed in sensitivity analyses. The MR analyses in both the fixed‐effect and random‐effect inverse‐variance weighted models showed that genetically predicted OSA was associated with an increased risk of atrial fibrillation (odds ratio [OR], 1.21; 95% CI, 1.12–1.31, P<0.001; OR, 1.21; 95% CI, 1.11–1.32, P<0.001) using 5 single nucleotide polymorphisms as the instruments. MR‐Egger indicated no evidence of genetic pleiotropy (intercept, −0.014; 95% CI, −0.033 to 0.005, P=0.14). Results were robust using other MR methods in sensitivity analyses.

Conclusions

This MR analysis found that genetically predicted OSA had causal effect on an increased risk of atrial fibrillation.

Interplay between stress-related genes may influence Alzheimer's disease development: The results of genetic interaction analyses of human data

Emerging evidence from experimental and clinical research suggests that stress-related genes may play key roles in AD development. The fact that genome-wide association studies were not able to detect a contribution of such genes to AD indicates the possibility that these genes may influence AD non-linearly, through interactions of their products. In this paper, we selected two stress-related genes (GCN2/EIF2AK4 and APP) based on recent findings from experimental studies which suggest that the interplay between these genes might influence AD in humans. To test this hypothesis, we evaluated the effects of interactions between SNPs in these two genes on AD occurrence, using the Health and Retirement Study data on white indidividuals. We found several interacting SNP-pairs whose associations with AD remained statistically significant after correction for multiple testing. These findings emphasize the importance of nonlinear mechanisms of polygenic AD regulation that cannot be detected in traditional association studies. To estimate collective effects of multiple interacting SNP-pairs on AD, we constructed a new composite index, called Interaction Polygenic Risk Score, and showed that its association with AD is highly statistically significant. These results open a new avenue in the analyses of mechanisms of complex multigenic AD regulation.

Sleep/wake cycle alterations as a cause of neurodegenerative diseases: A Mendelian randomization study

Sleep and/or wake cycle alterations are common in neurodegenerative diseases (ND). Our aim was to determine whether there is a causal relationship between sleep and/or wake cycle patterns and ND (Parkinson's disease (PD) age at onset (AAO), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS)) using two-sample Mendelian Randomization (MR). We selected 12 sleep traits with available Genome-Wide Association Study (GWAS) to evaluate their causal relationship with the ND risk through Inverse-Variance Weighted regression as main analysis. We used as outcome the latest ND GWAS with available summary-statistics: PD-AAO (N = 17,996), AD (N = 21,235) and ALS (N = 40,136). MR results pointed to a causal effect of subjective and objective-measured morning chronotype on later PD-AAO (95%CI:0.33-1.81, p = 8.47×10^-09 and 95%CI:-7.28 to -4.44, p = 5.87×10^-16, respectively). Sleep efficiency was causally associated with a decreased AD risk (95%CI:-20.408 to -0.66, p = 0.04) and daytime sleepiness with an increased ALS risk (95%CI:0.15 to 1.61, p = 0.01). Our study suggests that sleep and/or wake patterns have causal relationship with ND. Given that sleep and/or wake patterns are modifiable risk factors, sleep interventions should be investigated as a potential treatment in PD-AAO, AD and ALS.

Non-Genetic Risk Factors for Parkinson's Disease: An Overview of 46 Systematic Reviews

BACKGROUND

Numerous systematic reviews (SRs) and meta-analyses on non-genetic risk factors for Parkinson's disease (PD) development have been published with inconsistent conclusions.

OBJECTIVE

This overview of SRs aimed to summarize evidence on non-genetic factors for the development of PD from the published SRs, and explore the reasons behind the conflicting results.

METHODS

Three international databases were searched for SRs with meta-analyses summarized evidence on non-genetic factors for PD development. The Assessing the Methodological Quality of Systematic Reviews 2 tool was used to appraise the methodological quality of included SRs. Pooled effect estimations were extracted from each meta-analysis.

RESULTS

Forty-six SRs covered six categories, and more than 80 factors were included in this overview. Thirty-nine SRs (84.7%) were judged to be of critically low methodological quality. Evidence from prospective studies showed that physical activity, smoking, coffee, caffeine, tea, fat intake, ibuprofen use, calcium channel blocker use, statin use, thiazolidinediones, and high serum urate levels significantly reduced the risk of PD, while dairy intake, diabetes, hormone replacement therapy, depression, mood disorder, bipolar disorder, and aspirin use significantly increased the risk of PD. Differences in study designs (e.g., cohort studies, case-control studies) accounted for the conflicting results among included SRs.

CONCLUSION

Modifiable lifestyle factors such as physical activity and tea and coffee drinking may reduce the risk of PD, which may offer PD prevention strategies and hypotheses for future research. However, the designs of primary studies on PD risk factors and related SRs need to be improved and harmonized.

Genetically Elevated LDL Associates with Lower Risk of Intracerebral Hemorrhage

OBJECTIVE

Observational studies point to an inverse correlation between low-density lipoprotein (LDL) cholesterol levels and risk of intracerebral hemorrhage (ICH), but it remains unclear whether this association is causal. We tested the hypothesis that genetically elevated LDL is associated with reduced risk of ICH.

METHODS

We constructed one polygenic risk score (PRS) per lipid trait (total cholesterol, LDL, high-density lipoprotein [HDL], and triglycerides) using independent genomewide significant single nucleotide polymorphisms (SNPs) for each trait. We used data from 316,428 individuals enrolled in the UK Biobank to estimate the effect of each PRS on its corresponding trait, and data from 1,286 ICH cases and 1,261 matched controls to estimate the effect of each PRS on ICH risk. We used these estimates to conduct Mendelian Randomization (MR) analyses.

RESULTS

We identified 410, 339, 393, and 317 lipid-related SNPs for total cholesterol, LDL, HDL, and triglycerides, respectively. All four PRSs were strongly associated with their corresponding trait (all p < 1.00 × 10-100 ). While one SD increase in the PRSs for total cholesterol (odds ratio [OR] = 0.92; 95% confidence interval [CI] = 0.85-0.99; p = 0.03) and LDL cholesterol (OR = 0.88; 95% CI = 0.81-0.95; p = 0.002) were inversely associated with ICH risk, no significant associations were found for HDL and triglycerides (both p > 0.05). MR analyses indicated that 1mmol/L (38.67mg/dL) increase of genetically instrumented total and LDL cholesterol were associated with 23% (OR = 0.77; 95% CI = 0.65-0.98; p = 0.03) and 41% lower risks of ICH (OR = 0.59; 95% CI = 0.42-0.82; p = 0.002), respectively.

INTERPRETATION

Genetically elevated LDL levels were associated with lower risk of ICH, providing support for a potential causal role of LDL cholesterol in ICH. ANN NEUROL 2020 ANN NEUROL 2020;88:56-66.

Vitamin D polygenic score is associated with neuroticism and the general psychopathology factor

Vitamin D, used here to refer to both 25-hydroxyvitamin D, the main circulating form of the vitamin, and 1,25-hydroxyvitamin D, the biologically active form, has been shown to influence brain development and function. Consistent with these findings, low levels of vitamin D have been implicated in various mental disorders, including depression, schizophrenia, and autism. Recently, a shared variance across multiple categories of mental health disorders has been identified and shown to be genetically influenced. This shared variance, thought to represent a general risk for psychopathology, has been termed the p factor. Individuals with high p factor scores are characterized by high neuroticism and low agreeableness and conscientiousness. Here, we investigated the links between vitamin D polygenic scores - derived from the latest genome-wide association study of circulating vitamin D (25-hydroxyvitamin D) levels - the Big Five personality traits (neuroticism, agreeableness, conscientiousness, openness-to-experience, and extraversion), and the p factor, in a sample of 522 (278 women, mean age 20 ± 1 years) non-Hispanic Caucasians. Vitamin D polygenic scores were significantly and negatively associated with neuroticism and the p factor, even after correcting for multiple comparisons, and controlling for sex, age, ancestry, socioeconomic status, and body mass index. Based on previous research implicating neuroticism as a risk factor for psychopathology, mediation was tested. Results showed a significant indirect effect from the vitamin D polygenic score to the p factor via neuroticism. Our findings support a genetic link between vitamin D levels, neuroticism, and the p factor, but due to the cross-sectional nature of our data, future studies are needed to clarify the causal associations between these phenotypes.

Causal associations of insulin resistance with coronary artery disease and ischemic stroke: a Mendelian randomization analysis

INTRODUCTION

The relationship between insulin resistance (IR) and cardiovascular diseases is unclear. We aimed to examine the causal associations of IR with cardiovascular diseases, including coronary artery disease, myocardial infarction, ischemic stroke and its subtypes, using Mendelian randomization.

RESEARCH DESIGN AND METHODS

Due to low sample size for gold standard measures and in order to well reflect the underlying phenotype of IR, we used 53 single nucleotide polymorphisms associated with IR phenotypes (ie, fasting insulin, high-density lipoprotein cholesterol and triglycerides) from recent genome-wide association studies (GWASs) as instrumental variables. Summary-level data from four GWASs of European individuals were used. Data on IR phenotypes were obtained from meta-analysis of GWASs of up to 188 577 individuals and data on the outcomes from GWASs of up to 446 696 individuals. Mendelian randomization (MR) estimates were calculated with inverse-variance weighted, simple and weighted-median approaches and MR-Egger regression was used to explore pleiotropy.

RESULTS

Genetically predicted 1-SD increase in IR phenotypes were associated with a substantial increase in risk of coronary artery disease (OR=1.79, 95% CI: 1.57 to 2.04, p<0.001), myocardial infarction (OR=1.78, 95% CI: 1.54 to 2.06, p<0.001), ischemic stroke (OR=1.21, 95% CI: 1.05 to 1.40, p=0.007) and the small-artery occlusion subtype of stroke (OR=1.80, 95% CI: 1.30 to 2.49, p<0.001), but not associated with the large-artery atherosclerosis and cardioembolism subtypes of stroke. There was no evidence of pleiotropy. Results were broadly consistent in sensitivity analyses using simple and weighted-median approaches accounting for potential genetic pleiotropy.

CONCLUSIONS

This study provides evidence to support that IR was causally associated with risk of coronary artery disease, myocardial infarction, ischemic stroke and the small-artery occlusion subtype of stroke.

Investigation of Causal Effect of Atrial Fibrillation on Alzheimer Disease: A Mendelian Randomization Study

Background

Atrial fibrillation (AF) has been shown to be associated with an increased risk of dementia as well as Alzheimer disease in observational studies. Whether this association reflects causal association is still unclear. The purpose of this study was to examine the causal association of AF with Alzheimer disease.

Methods and Results

We used a 2‐sample Mendelian randomization approach to evaluate the causal effect of AF on Alzheimer disease. Summary data on the association of single nucleotide polymorphisms with AF were obtained from a recently published genome‐wide association study with up to 1 030 836 individuals and data on single nucleotide polymorphism‐Alzheimer disease association from another genome‐wide association study with up to 455 258 individuals. AF was mainly diagnosed according to International Classification of Diseases, Ninth Revision (ICD‐9 or ICD‐10) and Alzheimer disease was mainly diagnosed according to clinical criteria (eg, National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association [NINCDS‐ADRDA] criteria). Effect estimates were calculated using the inverse‐variance weighted method. The Mendelian randomization analysis showed nonsignificant association of genetically predicted AF with risk of Alzheimer disease (odds ratio=1.002, 95% CI: 0.996–1.009, P=0.47) using 93 single nucleotide polymorphisms as the instruments. Mendelian randomization‐Egger indicated no evidence of genetic pleiotropy (intercept=0.0002, 95% CI: −0.001 to 0.001, P=0.70).

Conclusions

This Mendelian randomization analysis found no evidence to support causal association between AF and Alzheimer disease.

Vitamin D and the nervous system

Objective: to summarise the activities that Vitamin D (VD) carries out in the brain and to clarify the potential role of VD in neurological diseases. Methods: a literature research has been performed in Pubmed using the following keywords: 'Vitamin D', 'nervous system', 'brain'. Results: the studies reviewed show that VD contributes to cerebral activity in both embryonic and adult brain, helping the connectivity of neural circuits responsible for locomotor, emotional and reward-dependent behavior. Low VD serum levels have been found in patients affected by Alzheimer Disease, Parkinson Disease, Multiple Sclerosis, Autism Spectrum Disorders, Sleep Disorders and Schizophrenia. Discussion: findings are controversial and should be interpreted with caution, since most of the studies performed have observational study set and few interventional studies are available, producing conflicting results. Overall, it can be stated that the potential role of Vitamin D in neurological diseases is mostly unclear and further randomised controlled trials are needed to understand better whether Vitamin D supplementation treatment can be useful in brain disorders.