To systematically explore the causes and consequences of atrial fibrillation, we conducted a phenome-wide, bidirectional MR analysis of atrial fibrillation, spanning thousands of traits. In this study, PheWAS and two-sample MR analyses found robust evidence in support of associations of genetically predicted AF with risk of digoxin, direct current cardioversion, warfarin, pulse rate and personal history of long-term (current) use of anticoagulants in of which direct current cardioversion, warfarin and personal history of long-term (current) use of anticoagulants have a bidirectional causal relationship.
In this work, multiple risk factors showed potential evidence of a role in the aetiology of AF and most of them were related to different phenotypes of the heart, which is consistent with the findings reported in previous study. Liu et al. successfully pinpointed numerous genes and DNAm sites that potentially exert causal influences on atrial fibrillation (AF) [9]. Concurrently, extensive genome-wide association studies (GWAS) have revealed over a hundred loci linked to AF, offering crucial insights for devising future research endeavors aimed at elucidating the functional mechanisms underlying the pathogenesis of AF triggered by DNA mutations. Furthermore, our research substantiates the existence of a bidirectional causality between atrial fibrillation and direct current cardioversion, a relationship that has seldom been reported in the previous MR study. Direct current cardioversion is a common rhythm control strategy in patients with symptomatic atrial fibrillation, which is capable of delivering high-energy electrical impulses to the myocardial tissue within an extremely brief timeframe, resulting in the simultaneous depolarization of the majority of myocardial fibers (approximately over 75%) [10]. A multicenter randomized controlled trial has demonstrated that patients treated with baseline cardioversion appeared to have a significantly lower mortality risk during follow-up after adjustment for known confounders (event rates per 100 patient years 2.52 v 3.87, weighted hazard ratio 0.75, 95% confidence interval 0.67 to 0.85, P < 0.001) [11]. However, the results of another study indicate that a higher incidence of intracardiac thrombus is observed following direct current cardioversion treatment [12]. Despite the high success rate in restoring sinus rhythm, the incidence of rapid and slow arrhythmia complications in patients with atrial fibrillation is significantly elevated compared to the control group. Notably, the participants in this study were in high cardiovascular risk (e.g. cardiac amyloidosis), which is different from the UK Biobank individuals with a generally good health status.
Warfarin, a derivative of dicumarol, exerts its anticoagulant effect by inhibiting the interconversion of vitamin K and its 2,3-epoxide (vitamin K epoxide) [13]. According to previous MR study, it has been hypothesized that warfarin administration may induce pathological changes in the skin cells of patients with AF, subsequently precipitating chronic myelogenous leukemia [14]. However, to date, no definitive literature exists to report a direct causal relationship between AF and warfarin administration. Based on PheWAS, a bidirectional causal relationship has been ascertained between AF and the risk associated with warfarin, which partly supported the association between AF and warfarin as reported by previous MR studies [15]. Furthermore, Current guidelines in the United States and Europe recommend the use of warfarin in AF, but between 2010 and 2014, the use of warfarin decreased from 32.8% in 2010 to 22.9% in 2014, which may be related to significant differences in age, gender, income level, insurance status, geographical region, stroke risk, use of anticoagulants, and choice of anticoagulants [16, 17]. In another registry study based on the multi-center nature of the United States, even after controlling for clinical and socio-demographic characteristics, black patients were significantly less likely to receive direct-acting oral warfarin than white patients, with no difference between white and Hispanic patients [18]. It is noteworthy that the studies were conducted exclusively within the United States, exhibiting certain distinctions from the data of the UK Biobank.
Additionally, it has been observed that personal history of long-term (current) use of anticoagulants exhibits a bidirectional causative relationship with atrial fibrillation. Previous network meta-analyses have revealed that the strategy of administering standard-dose anticoagulants for the long-term treatment of patients with AF significantly reduces the risk of stroke/systemic embolism, intracranial hemorrhage, and all-cause mortality, however, no statistically significant differences were observed in the incidence of major bleeding or other complications [19]. Although current research has not directly demonstrated a causal relationship between personal history of long-term (current) use of anticoagulants and the onset of AF, it has been observed that the history of long-term oral anticoagulation therapy is strongly associated with the prognosis of atrial fibrillation, an association that was independent of gender and age [20]. Concurrently, current therapeutic guidelines stipulate that patients with atrial fibrillation were required to undergo long-term anticoagulant therapy, which was also in accordance with the conclusions that we have arrived at.
Digoxin is a cardiac glycoside used as drug in case of heart problems, including congestive heart failure, atrial fibrillation or flutter, and certain cardiac arrhythmias [21]. Our study reveals a unidirectional causal relationship between AF and digoxin, a finding not previously demonstrated in previous MR studies. However, due to the minor positive effect (odds ratio = 1.002), it is postulated that this may be associated with limitations related to ethnicity and geographical regions. The number of digoxin users in the United States declined by 47% from 766 users per 100,00 adults in 2010–2011 to 402 users per 100,000 adults in 2016–2017, however, digoxin remains prevalent among Asian and Hispanic populations in the United States [22]. Furthermore, more than 20 years following the publication of trials related to digoxin, there exist many reports concerning the safety of digoxin treatment that are characterized by contradictory data [23]. Despite the aforementioned controversies and discrepant data, digoxin remains a universally utilized medication. It continues to be an excellent choice for the pharmacological control of heart rate in patients with atrial fibrillation.
Current observational studies and MR research have demonstrated a causative relationship between pulse rate and AF, which is consistent with the results of our research. Existing research findings support an inverse causality between genetically determined resting heart rates and AF, for those below 65 beats per minute [24]. As the same time, additional research has demonstrated that pulse rate can predicts AF events in a large, population-based cohort [25]. In summary, the resting pulse rate may constitute a useful component in elucidating the intricate mechanisms underlying the initiation of AF.
In previous Mendelian randomization studies, several other health outcomes have been causally associated with atrial fibrillation. These associations encompass renal function [26], stroke [27], and the causal effects of obstructive sleep apnea [28], as well as the frequency of its episodes, among others. However, these findings were not captured in our PheWAS analysis, potentially due to the lack of power in our study to detect such associations, such as diseases with low prevalence or small case numbers among the UK Biobank participants, including multiple sclerosis, Alzheimer's disease, and Parkinson's disease. Future research with larger sample sizes and independent study populations is required to replicate and validate these reported MR findings.
The strength of this study lies in the employment of a hypothesis-free approach to assess the bidirectional causal effects of the full phenotypic traits of atrial fibrillation. To address the issue of multiple testing, we utilized the Bonferroni correction and compared the consistency of results across three distinct MR methods. We acknowledge that sample overlap between exposure and outcome GWAS may induce overfitting in cases of weak instrumentation. However, considering the adequate F-statistics for risk factors in univariate MR models, the potential for bias should be minimal.