Irritable bowel syndrome and migraine: evidence from Mendelian randomization analysis in the UK Biobank

Rome Foundation Working Team Report on overlap in disorders of gut-brain interaction

In patients with disorders of gut-brain interaction (DGBI), overlapping non-gastrointestinal conditions such as fibromyalgia, headaches, gynaecological and urological conditions, sleep disturbances and fatigue are common, as is overlap among DGBI in different regions of the gastrointestinal tract. These overlaps strongly influence patient management and outcome. Shared pathophysiology could explain this scenario, but details are not fully understood. This overlap has been shown to be of great relevance for DGBI. In addition, symptoms considered to be caused by a DGBI could have a detectable organic cause, and in patients with a diagnosed organic gastrointestinal disease, symptoms not clearly explained by the pathology defining this organic disease are common. Thus, the aims of this Rome Foundation Working Team Report were to review the literature on overlapping conditions among patients with paediatric and adult DGBI and, based on the available epidemiological and clinical evidence, make recommendations for the current diagnostic and therapeutic approach, and for future research. Specifically, we focused on other DGBI in the same or different gastrointestinal anatomical region(s), DGBI overlap with organic bowel diseases in remission, and DGBI overlap with non-gastrointestinal, non-structural conditions.

Spectrum, Time Course, Stages, and a Proposal for the Diagnosis of Histamine Intolerance in General Practice: A Nonrandomized, Quasi-Experimental Study

Background/Objectives: Limited research has explored histamine intolerance from the perspective of primary caregivers. Our objective was to develop a practical symptom profile from the standpoint of general practice. We also aimed to gather data on the frequency and timing of disease progression and to establish a staging system. Methods: This study utilized a nonrandomized, quasi-experimental design. An in-depth interview was conducted with 217 patients involving 120 questions. To evaluate associations between food intake and symptoms, we recommended either an exclusion diet or a low-histamine diet. A follow-up questionnaire was subsequently administered. We also analyzed 3831 doctor-patient meetings involving upper respiratory symptoms. Results: Symptoms in 77 patients were associated with histamine-rich meals. The most characteristic symptoms included respiratory symptoms (95%), bloating (94%), headache (91%), fatigue (83%), postprandial drowsiness (81%), skin symptoms (81%), diarrhea/loose stool (77%), psychological symptoms (77%), dyspepsia (69%), and muscle/eyelid twitching (61%). Patients with suspected histamine intolerance visited primary care three times more often with upper respiratory symptoms than those without suspected histamine intolerance. The symptom spectrum of histamine intolerance involves multiple organ systems and occurs in distinct, repeating patterns. Symptoms can be described by their duration, sequence, and severity level, which is the key focus of this research, including visual representations. In its most severe stages, histamine intolerance may potentially involve mast cell activation. A personalized diet is associated with a gradual reduction in both the intensity and frequency of symptoms. Conclusions: The spectrum of histamine intolerance can be characterized by specific symptom patterns with defined frequencies, timelines, and symptom stages.

Modifiable factors for irritable bowel syndrome: evidence from Mendelian randomisation approach

ABSTRACT

Background

The potential modifiable factors influencing irritable bowel syndrome (IBS) have not been thoroughly documented. We aimed to systematically investigate the modifiable factors associated with IBS, while accounting for the impact of unobserved confounders and coexisting disorders.

Methods

Genetic correlation and Mendelian randomisation (MR) analyses were integrated to identify potential modifiable factors and coexisting disorders linked to IBS. Subsequently, multiresponse MR (MR2) was employed to further examine these associations. Summary-level genome-wide association data were used. Modifiable factors and coexisting disorders (ie, gastrointestinal and psychiatric disorders) were identified based on evidence from cohort studies and meta-analysis. In all analyses, IBS was the primary outcome, while in the MR2 analysis, coexisting disorders were also treated as outcomes alongside IBS.

Results

Most identified modifiable factors and coexisting disorders exhibited genetic correlations with IBS. MR analyses revealed strong causation between IBS and multisite chronic pain (OR=2.20, 95% CI 1.82 to 2.66), gastro-oesophageal reflux disease (OR=1.31, 95% CI 1.23 to 1.39), well-being spectrum (OR=0.17, 95% CI 0.13 to 0.21), life satisfaction (OR=0.31, 95% CI 0.25 to 0.38), positive affect (OR=0.30, 95% CI 0.24 to 0.37), neuroticism score (OR=1.20, 95% CI 1.16 to 1.25) and depression (OR=1.50, 95% CI 1.37 to 1.66). Additionally, smoking, alcohol frequency, college or university degree, intelligence, childhood maltreatment, frailty index, diverticular disease of the intestine and schizophrenia were suggestively associated with IBS. Robust associations were found between multisite chronic pain and both IBS and coexisting disorders.

Conclusions

Our study identified a comprehensive array of potential modifiable factors and coexisting disorders associated with IBS, supported by genetic evidence, including genetic correlation and multiple MR analyses. The presence of multisite chronic pain may offer a promising avenue for the concurrent prevention of IBS and its coexisting disorders.

Serum iron status and the risk of female infertility in European populations: A two-sample Mendelian randomization study

The relationship between iron status and female infertility has been observed in several studies, yet its causal nature remains ambiguous. We employed univariate Mendelian randomization (MR) analyses to explore the potential causal connection between these 2 factors. For our analysis, genetic instrumental variables pertaining to iron status were selected using data from the Iron Status Genetics Consortium, encompassing 48,972 individuals of European descent from 19 cohorts (11 discovery and 8 replication). For female infertility data, we referred to FinnGen Consortium Release 9, which includes 11,442 cases and 107,564 controls. Our MR approach utilized both a conservative strategy (involving single nucleotide polymorphisms pertinent to general iron status) and a liberal strategy (encompassing single nucleotide polymorphisms linked to any iron status indicator). The conservative approach relied on inverse variance-weighted methods, whereas the liberal strategy integrated inverse variance weighted with MR-Egger regression, the weighted median approach, and simple mode techniques. The conservative strategy did not reveal a significant link between iron status and female infertility risk. Conversely, the liberal strategy indicated a positive correlation specifically between serum iron levels and female infertility risk (odds ratio from MR: 1.225; 95% confidence interval: 1.064-1.410; P = .030), while no significant associations were found for other iron indicators (P > 0.05). Our MR investigation suggests a potential positive association between serum iron levels and the risk of female infertility, while other iron markers do not appear to significantly influence this risk. These findings highlight the need for further research into the possible connection between serum iron status and female infertility risk.

A thorough investigation into the correlation between migraines and the gut microbiome: an in-depth analysis using Mendelian randomization studies

Objective

A growing body of evidence underscores a significant association between neurological disorders, particularly migraines, and the gut microbiota. However, a research gap persists in understanding the cause-and-effect dynamics between these elements. Therefore, we employed robust methodologies aimed at thoroughly exploring the causal relationship between the gut microbiome and migraines.

Methods

Employing bidirectional Two Sample Mendelian Randomization (TSMR) analysis, we investigated the causal association between the composition of the gut microbiota and migraines. Data summarizing the relationship between gut microbiota and migraines were extracted from one or more genome-wide association studies. The TSMR analysis employed five methods to assess the correlation between the gut microbiota and migraines, with the inverse variance-weighted method serving as the primary approach for analyzing causal links. Sensitivity analyses were applied to address horizontal pleiotropy and heterogeneity. Simultaneously, a meta-analysis was performed to strengthen the robustness of the findings. Additionally, a reverse TSMR was carried out to explore potential occurrences of reverse causal relationships.

Results

The ongoing TSMR analysis identified a collection of 14 bacterial taxa connected to migraines. Among these, 8 taxa exhibited a protective effect, while 5 taxa had a detrimental impact, and 1 taxon maintained a neutral relationship. The reverse Mendelian randomization analysis highlighted stable outcomes for only one bacterial taxonomic group.

Conclusion

The study confirms a causal relationship between the gut microbiota and migraines, offering a new perspective for migraine research. Strategically targeting specific bacterial taxa with dysregulation may be effective in both preventing and treating migraines, thus opening new avenues for therapeutic strategies.

Associations of modifiable factors with risk of irritable bowel syndrome

Background

Modifiable factors were found to be associated with the risk of irritable bowel syndrome (IBS) in observational studies, but whether these associations are causal is uncertain. We conducted a Mendelian randomization (MR) study to systematically explore the causal associations of modifiable factors with IBS.

Methods

Summary-level statistical data for IBS was obtained from a genome-wide association study (GWAS) meta-analysis of UK Biobank (40,548 cases and 293,220 controls) and the international collaborative Bellygenes initiative (12,852 cases and 139,981 controls). Genetic instruments associated with the exposures at the genome-wide significance (p < 5 × 10-8) level were selected from previous GWASs. Mendelian randomization was performed using inverse-variance weighted (IVW) method, supplemented with several sensitivity analyses to evaluate potentially causal relationships between identified contributing factors and IBS. Furthermore, we applied another database from FinnGen (8,116 IBS cases and 276,683 controls) to testify the reliability of the significant associations.

Results

Seven convincing modifiable factors were significantly associated with IBS after correction for multiple testing. Genetically predicted smoking initiation (OR = 1.12, 95% CI = 1.06-1.18, p = 1.03 × 10-4), alcohol consumption (OR = 0.47, 95% CI = 0.34-0.64, p = 3.49 × 10-6), sedentary behavior (OR = 1.17, 95% CI = 1.07-1.28, p = 4.02 × 10-4), chronotype (OR = 0.92, 95% CI = 0.88-0.96, p = 4.42 × 10-4), insomnia (OR = 1.19, 95% CI = 1.15-1.24, p = 7.59 × 10-19), education (OR = 0.80, 95% CI = 0.74-0.88, p = 5.34 × 10-7), and visceral adiposity (OR = 1.15, 95% CI = 1.06-1.24, p = 7.96 × 10-4). We additionally identified several suggestive factors, including serum magnesium, serum phosphorus, physical activity, lifetime smoking, intelligence, lean body mass, and body mass index (BMI). After pooling the effect estimates from FinnGen, the associations remained significant except for chronotype.

Conclusion

This MR analysis verified several modifiable risk factors for IBS, thus prevention strategies for IBS should be considered from multiple perspectives on these risk factors.

Genetics of migraine: complexity, implications, and potential clinical applications

Migraine is a common neurological disorder with large burden in terms of disability for individuals and costs for society. Accurate diagnosis and effective treatments remain priorities. Understanding the genetic factors that contribute to migraine risk and symptom manifestation could improve individual management. Migraine has a strong genetic basis that includes both monogenic and polygenic forms. Some distinct, rare, familial migraine subtypes are caused by pathogenic variants in genes involved in ion transport and neurotransmitter release, suggesting an underlying vulnerability of the excitatory-inhibitory balance in the brain, which might be exacerbated by disruption of homoeostasis and lead to migraine. For more prevalent migraine subtypes, genetic studies have identified many susceptibility loci, implicating genes involved in both neuronal and vascular pathways. Genetic factors can also reveal the nature of relationships between migraine and its associated biomarkers and comorbidities and could potentially be used to identify new therapeutic targets and predict treatment response.

Major depressive disorder and irritable bowel syndrome risk: A Mendelian randomization study

BACKGROUND

Previous studies have revealed a connection between major depressive disorder (MDD) and irritable bowel syndrome (IBS), but it remains obscure if the two diseases are related causally. Mendelian randomization was utilized in this investigation to ascertain whether MDD contributed to the emergence of IBS.

METHODS

To examine possible connections between MDD and IBS, we used two-sample Mendelian randomization (MR) utilizing summary data from genome-wide association studies (GWAS). The Psychiatric Genomics Consortium (PGC) provided information on genetic associations with MDD (cases: 135,458; controls: 344,901). The Medical Research Council Integrative Epidemiology Unit (MRC-IEU) provided information on genetic associations with IBS (cases:10,939; controls:451,994). Inverse Variance Weighted (main analyses), MR-Egger regression, Weighted mode, and Weighted Median were the four MR methods used in this investigation. In addition, we also performed multiplicity and heterogeneity analyses to eliminate possible biases.

RESULTS

In the standard Inverse Variance Weighting (IVW) method, an increased risk of IBS was linked to a genetic susceptibility to MDD (OR: 1.01; 95% CI: 1.006 to 1.014, p = 1.02E-07). In addition, neither significant heterogeneity (IVW Q = 24.80, p = 0.73) nor horizontal pleiotropy (MR Egger p = 0.17; MRPRESSO p = 0.54) were detected in this MR analysis. The bidirectional analysis, however, did not show a genetic link between IBD and MDD (p steiger <0.01).

CONCLUSION

A direct causal relationship between MDD and IBS was revealed by Mendelian randomization study, which contributes to the effective clinical management of both diseases.

Cellular and Molecular Roles of Immune Cells in the Gut-Brain Axis in Migraine

Migraine is a complex and multi-system dysfunction. The realization of its pathophysiology and diagnosis is developing rapidly. Migraine has been linked to gastrointestinal disorders such as irritable bowel syndrome and celiac disease. There is also direct and indirect evidence for a relationship between migraine and the gut-brain axis, but the exact mechanism is not yet explained. Studies have shown that this interaction appears to be influenced by a variety of factors, such as inflammatory mediators, gut microbiota, neuropeptides, and serotonin pathways. Recent studies suggest that immune cells can be the potential tertiary structure between migraine and gut-brain axis. As the hot interdisciplinary subject, the relationship between immunology and gastrointestinal tract is now gradually clear. Inflammatory signals are involved in cellular and molecular responses that link central and peripheral systems. The gastrointestinal symptoms associated with migraine and experiments associated with antibiotics have shown that the intestinal microbiota is abnormal during the attacks. In this review, we focus on the mechanism of migraine and gut-brain axis, and summarize the tertiary structure between immune cells, neural network, and gastrointestinal tract.

Genetic associations and potential mediators between psychiatric disorders and irritable bowel syndrome: a Mendelian randomization study with mediation analysis

Objective

Potential causal associations between psychiatric disorders and irritable bowel syndrome have been demonstrated in observational studies; however, these studies are susceptible to underlying confounding and reverse causation biases. We aimed to assess the causal effects of psychiatric disorders on irritable bowel syndrome (IBS) and the potential mediators from a genetic perspective by conducting a Mendelian randomization (MR) study with mediation analysis.

Method

Genetic instruments associated with psychiatric disorders, potential mediators, and IBS were obtained from large-scale genome-wide association studies (GWAS). Three MR methods - the inverse-variance weighted (IVW) method, MR-Egger method, and weighted median method, were used to investigate causal association estimates. Heterogeneity among different genetic instrumental variables (IVs) was assessed using Q tests. Additionally, the MR-PRESSO and MR-Pleiotropy methods were used to verify horizontal pleiotropy and detect outliers that might bias the results, which were removed from further analysis. Consequently, we used MR mediation analysis to investigate potential mediators in the causal associations between psychiatric disorders and IBS.

Results

MR provided evidence of the causal effects of genetically predicted broad depression, major depressive disorder (MDD), anxiety disorder, post-traumatic stress disorder (PTSD), and schizophrenia on IBS. The results of MR mediation analysis demonstrated that the reduction in acetate levels mediated 12.6% of the effects of broad depression on IBS; insomnia mediated 16.00%, 16.20%, and 27.14% of the effects of broad depression, MDD, and PTSD on IBS, respectively; and the increase in blood β-hydroxybutyrate levels mediated 50.76% of the effects of schizophrenia on IBS.

Conclusion

Our study confirmed the brain-gut axis involvement and potential modulators in the pathophysiology of psychiatric disorder-induced IBS from a genetic perspective, and suggests potential therapeutic targets for the disrupted brain-gut axis.

Association of plasma branched-chain amino acid with multiple cancers: A mendelian randomization analysis

BACKGROUND

Studies have suggested a possible relevance between branched-chain amino acid (BCAA) catabolic enzymes and cancers. However, few studies have explored the variation in circulating concentrations of BCAAs. Our study used bi-directional, two-sample Mendelian randomization (MR) analysis for predicting the causality between the BCAA levels and 9 types of cancers.

METHODS

The largest genome-wide association studies (GWAS) provided data for total BCAAs, valine, leucine, and isoleucine from the UK Biobank. Data on multiple cancer endpoints were collected from various sources, such as the International Lung Cancer Consortium (ILCCO), the Pancreatic Cancer Cohort Consortium 1 (PanScan1), the Breast Cancer Association Consortium (BCAC), the FinnGen Biobank, and the Ovarian Cancer National Alliance (OCAC). The mainly analysis method was the inverse-variance-weighted (IVW). For assessing horizontal pleiotropy, the researchers performed MR-Egger regression and MR-PRESSO global test. Finally, the Cochran's Q test served for evaluating the heterogeneity.

RESULTS

Circulating total BCAAs levels (OR 1.708, 95%CI 1.168, 2.498; p = 0.006), valine levels (OR 1.747, 95%CI 1.217, 2.402; p < 0.001), leucine levels (OR 1.923, 95%CI 1.279, 2.890; p = 0.002) as well as isoleucine levels (OR 1.898, 95%CI 1.164, 3.094; p = 0.010) positively correlated with the squamous cell lung cancer risk. Nevertheless, no compelling evidence was found to support a causal link between BCAAs and any other examined cancers.

CONCLUSIONS

Increased circulating total-BCAAs levels, leucine levels, isoleucine levels and valine levels had higher hazard of squamous cell lung cancer. No such associations were found for BCAAs with other cancers.

Causal association between psycho-psychological factors, such as stress, anxiety, depression, and irritable bowel syndrome: Mendelian randomization

BACKGROUND

Pathogenesis, diagnosis, and treatment of irritable bowel syndrome (IBS) have been reported to be challenging hotspots in clinical practice. Previous observational studies have found that stress, anxiety, depression, and other mental and psychological diseases are closely associated with IBS. This study aimed to further explore the causal relationships of these associations through Mendelian randomization (MR).

METHODS

The data needed for MR were obtained from publicly published genome-wide association databases. We performed a bidirectional, 2-sample MR analysis using instrumental variables (IV) associated with stress, anxiety, and depression, and other mental and psychological factors as exposures and IBS as the outcome. A reverse MR analysis with IBS as exposure and stress, anxiety, depression, and other mental and psychological factors as the outcomes was also performed. The inverse variance weighting (IVW) method was adopted as the main method of MR, and the causal effect between stress, anxiety, depression, and other mental and psychological factors and IBS was evaluated as the main result of the study. In addition, a series of sensitivity analyses was conducted to comprehensively evaluate the causal relationship between them.

RESULTS

Stress, anxiety, depression, and other mental and psychological factors were the underlying etiologies for IBS (odds ratio [OR] = 1.06, 95% confidence interval [CI]: 1.03-1.08), and they were positively correlated. Univariate analysis further supported the above conclusions (Depression, [OR = 1.31, 95% CI: 1.05-1.63, P = .016], Anxiety, [OR = 1.53, 95% CI: 1.16-2.03, P = .003]). However, in reverse MR analysis, we found that IBS did not affect stress, anxiety, depression, or other mental and psychological factors and that there was no causal relationship between IBS and stress, anxiety, depression, or other mental and psychological factors (P > .05).

CONCLUSION

This study demonstrates that mental and psychological factors are the underlying etiologies for IBS. These findings may provide important information for physicians regarding the clinical treatment of IBS.

Combined GWAS and single cell transcriptomics uncover the underlying genes and cell types in disorders of gut-brain interaction

Disorders of gut-brain interaction (DGBIs), formerly known as functional gastrointestinal disorders, are extremely common and historically difficult to manage. This is largely because their cellular and molecular mechanisms have remained poorly understood and understudied. One approach to unravel the molecular underpinnings of complex disorders such as DGBIs is performing genome wide association studies (GWASs). However, due to the heterogenous and non-specific nature of GI symptoms, it has been difficult to accurately classify cases and controls. Thus, to perform reliable studies, we need to access large patient populations which has been difficult to date. Here, we leveraged the UK Biobank (UKBB) database, containing genetic and medical record data of over half a million individuals, to perform GWAS for five DGBI categories: functional chest pain, functional diarrhea, functional dyspepsia, functional dysphagia, and functional fecal incontinence. By applying strict inclusion and exclusion criteria, we resolved patient populations and identified genes significantly associated with each condition. Leveraging multiple human single-cell RNA-sequencing datasets, we found that the disease associated genes were highly expressed in enteric neurons, which innervate and control GI functions. Further expression and association testing-based analyses revealed specific enteric neuron subtypes consistently linked with each DGBI. Furthermore, protein-protein interaction analysis of each of the disease associated genes revealed protein networks specific to each DGBI, including hedgehog signaling for functional chest pain and neuronal function and neurotransmission for functional diarrhea and functional dyspepsia. Finally, through retrospective medical record analysis we found that drugs that inhibit these networks are associated with an increased disease risk, including serine/threonine kinase 32B drugs for functional chest pain, solute carrier organic anion transporter family member 4C1, mitogen-activated protein kinase 6, and dual serine/threonine and tyrosine protein kinase drugs for functional dyspepsia, and serotonin transporter drugs for functional diarrhea. This study presents a robust strategy for uncovering the tissues, cell types, and genes involved in DGBIs, presenting novel predictions of the mechanisms underlying these historically intractable and poorly understood diseases.

Genetics of migraine: where are we now?

Migraine is a complex brain disorder explained by the interaction of genetic and environmental factors. In monogenic migraines, including familial hemiplegic migraine and migraine with aura associated with hereditary small-vessel disorders, the identified genes code for proteins expressed in neurons, glial cells, or vessels, all of which increase susceptibility to cortical spreading depression. The study of monogenic migraines has shown that the neurovascular unit plays a prominent role in migraine. Genome-wide association studies have identified numerous susceptibility variants that each result in only a small increase in overall migraine risk. The more than 180 known variants belong to several complex networks of "pro-migraine" molecular abnormalities, which are mainly neuronal or vascular. Genetics has also highlighted the importance of shared genetic factors between migraine and its major co-morbidities, including depression and high blood pressure. Further studies are still needed to map all of the susceptibility loci for migraine and then to understand how these genomic variants lead to migraine cell phenotypes.

Migraine genetics: Status and road forward

BACKGROUND

Migraine is considered a multifactorial genetic disorder. Different platforms and methods are used to unravel the genetic basis of migraine. Initially, linkage analysis in multigenerational families followed by Sanger sequencing of protein-coding parts (exons) of genes in the genomic region shared by affected family members identified high-effect risk DNA mutations for rare Mendelian forms of migraine, foremost hemiplegic migraine. More recently, genome-wide association studies testing millions of DNA variants in large groups of patients and controls have proven successful in identifying many dozens of low-effect risk DNA variants for the more common forms of migraine with the number of associated DNA variants increasing steadily with larger sample sizes. Currently, next-generation sequencing, utilising whole exome and whole genome sequence data, and other omics data are being used to facilitate their functional interpretation and the discovery of additional risk factors. Various methods and analysis tools, such as genetic correlation and causality analysis, are used to further characterise genetic risk factors.

FINDINGS

We describe recent findings in genome-wide association studies and next-generation sequencing analysis in migraine. We show that the combined results of the two most recent and most powerful migraine genome-wide association studies have identified a total of 178 LD-independent (r² < 0.1) genome-wide significant single nucleotide polymorphisms (SNPs), of which 99 were unique to Hautakangas et al., 11 were unique to Choquet et al., and 68 were identified by both studies. When considering that Choquet et al. also identified three SNPs in a female-specific genome-wide association studies then these two recent studies identified 181 independent SNPs robustly associated with migraine. Cross-trait and causal analyses are beginning to identify and characterise specific biological factors that contribute to migraine risk and its comorbid conditions.

CONCLUSION

This review provides a timely update and overview of recent genetic findings in migraine.

The Causal Association of Irritable Bowel Syndrome with Multiple Disease Outcomes: A Phenome-Wide Mendelian Randomization Study

BACKGROUND

This study aimed to identify novel associations between irritable bowel syndrome (IBS) and a broad range of outcomes.

METHODS

In total, 346,352 white participants in the U.K. Biobank were randomly divided into two halves, in which a genome-wide association study (GWAS) of IBS and a polygenic risk score (PRS) analysis of IBS using GWAS summary statistics were conducted, respectively. A phenome-wide association study (PheWAS) based on the PRS of IBS was performed to identify disease outcomes associated with IBS. Then, the causalities of these associations were tested by both one-sample (individual-level data in U.K. Biobank) and two-sample (publicly available summary statistics) Mendelian randomization (MR). Sex-stratified PheWAS-MR analyses were performed in male and female, separately.

RESULTS

Our PheWAS identified five diseases associated with genetically predicted IBS. Conventional MR confirmed these causal associations between IBS and depression (OR: 1.07, 95%CI: 1.01-1.14, p = 0.02), diverticular diseases of the intestine (OR: 1.13, 95%CI: 1.08-1.19, p = 3.00 × 10^-6), gastro-esophageal reflux disease (OR: 1.09, 95%CI: 1.05-1.13, p = 3.72 × 10^-5), dyspepsia (OR: 1.21, 95%CI: 1.13-1.30, p = 9.28 × 10^-8), and diaphragmatic hernia (OR: 1.10, 95%CI: 1.05-1.15, p = 2.75 × 10^-5). The causality of these associations was observed in female only, but not men.

CONCLUSIONS

Increased risks of IBS is found to cause a series of disease outcomes. Our findings support further investigation on the clinical relevance of increased IBS risks with mental and digestive disorders.

Genetic Causal Association between Iron Status and Osteoarthritis: A Two-Sample Mendelian Randomization

Objective: Observational studies have shown the association between iron status and osteoarthritis (OA). However, due to difficulties of determining sequential temporality, their causal association is still elusive. Based on the summary data of genome-wide association studies (GWASs) of a large-scale population, this study explored the genetic causal association between iron status and OA. Methods: First, we took a series of quality control steps to select eligible instrumental SNPs which were strongly associated with exposure. The genetic causal association between iron status and OA was analyzed using the two-sample Mendelian randomization (MR). Inverse-variance weighted (IVW), MR-Egger, weighted median, simple mode, and weighted mode methods were used for analysis. The results were mainly based on IVW (random effects), followed by sensitivity analysis. IVW and MR-Egger were used for heterogeneity testing. MR-Egger was also used for pleiotropy testing. Leave-one-SNP-out analysis was used to identify single nucleotide polymorphisms (SNPs) with potential impact. Maximum likelihood, penalized weighted median, and IVW (fixed effects) were performed to further validate the reliability of results. Results: IVW results showed that transferrin saturation had a positive causal association with knee osteoarthritis (KOA), hip osteoarthritis (HOA) and KOA or HOA (p < 0.05, OR > 1), and there was a negative causal association between transferrin and HOA and KOA or HOA (p < 0.05, OR < 1). The results of heterogeneity test showed that our IVW analysis results were basically free of heterogeneity (p > 0.05). The results of the pleiotropy test showed that there was no pleiotropy in our IVW analysis (p > 0.05). The analysis results of maximum likelihood, penalized weighted median and IVW (fixed effects) were consistent with our IVW results. No genetic causal association was found between serum iron and ferritin and OA. Conclusions: This study provides evidence of the causal association between iron status and OA, which provides novel insights to the genetic research of OA.

Dural Immune Cells, CGRP, and Migraine

Migraine is the most common neurological disorder in the world, affecting 12% of the population. Migraine involves the central nervous system, trigeminal nerves and meninges. Recent advances have shown that targeting calcitonin gene-related peptide (CGRP) through either antibodies or small molecule receptor antagonists is effective at reducing episodic and chronic migraine episodes, but these therapeutics are not effective in all patients. This suggests that migraine does not have a singular molecular cause but is likely due to dysregulated physiology of multiple mechanisms. An often-overlooked part of migraine is the potential involvement of the immune system. Clinical studies have shown that migraine patients may have dysregulation in their immune system, with abnormal plasma cytokine levels either during the attack or at baseline. In addition, those who are immunocompromised appear to be at a higher risk of migraine-like disorders. A recent study showed that migraine caused changes to transcription of immune genes in the blood, even following treatment with sumatriptan. The dura mater is densely packed with macrophages, mast and dendritic cells, and they have been found to associate with meningeal blood vessels and trigeminal afferent endings. Recent work in mice shows activation and morphological changes of these cells in rodents following the migraine trigger cortical spreading depression. Importantly, each of these immune cell types can respond directly to CGRP. Since immune cells make up a large portion of the dura, have functional responses to CGRP, and interact with trigeminal afferents, CGRP actions on the dural immune system are likely to play key roles in migraine.

Kynurenine Pathway of Tryptophan Metabolism in Migraine and Functional Gastrointestinal Disorders

Migraine, the leading cause of disability in the population aged below 50, is associated with functional gastrointestinal (GI) disorders (FGIDs) such as functional nausea, cyclic vomiting syndrome, and irritable bowel syndrome (IBS). Conversely, changes in intestinal GI transit may cause diarrhea or constipation and are a component of the autonomic symptoms associated with pre- and post-dorsal phases of migraine attack. These mutual relationships provoke a question on a common trigger in migraine and FGIDs. The kynurenine (l-kyn) pathway (KP) is the major route for l-tryptophan (l-Trp) metabolism and transforms l-Trp into several neuroactive compounds. Changes in KP were reported in both migraine and FGIDs. Migraine was largely untreatable, but several drugs approved lately by the FDA, including monoclonal antibodies for calcitonin gene-related peptide (CGRP) and its receptor, create a hope for a breakthrough in migraine treatment. Derivatives of l-kyn were efficient in pain relief with a mechanism including CGRP inhibition. KP products are important ligands to the aryl hydrocarbon receptor (AhR), whose activation is implicated in the pathogenesis of GI and migraine. Toll-like receptors (TLRs) may play a role in migraine and IBS pathogeneses, and KP metabolites detected downstream of TLR activation may be an IBS marker. The TLR4 signaling was observed in initiating and maintaining migraine-like behavior through myeloid differentiation primary response gene 88 (MyD88) in the mouse. The aim of this review is to justify the view that KP modulation may provide common triggers for migraine and FGIDs with the involvement of TLR, AhR, and MyD88 activation.