{'fraction_non_alphanumeric': 0.027174336214280153, 'fraction_numerical': 0.019232266604375926, 'mean_word_length': 5.1454545454545455, 'pattern_counts': {'":': 0, '<': 4, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
The Association of Waist Circumference and the Risk of Deep Vein Thrombosis

Objective In this study, we aimed to use a two sample Mendelian randomization (MR) method to identify a potentially causality between waist circumference and the risk of deep vein thrombosis (DVT). Methods With a two‐sample MR approach, we analyzed the summary data. The main analysis was performed by using the summary genetic data from two large consortium cohorts. Three MR approaches were used to explore MR estimates of waist circumference for DVT (inverse‐variance weighted [IVW] approach, weighted median method and MR‐Egger method). A total of 224 single nucleotide polymorphisms (SNPs) were identified associated with the level of waist circumference at statistical significance (P < 5*10−8; linkage disequilibrium r2 < 0.1). Results The result of IVW indicated the positive association between waist circumference and the risk of DVT (OR 1.012, 95% CI 1.009–1.014, P 7.627E-17). The other two methods were observed with consistent result. MR-Egger regression analysis indicated that no evidence for the presence of directional horizontal pleiotropy. Additionally, DVT was not a causal factor for waist circumference. Conclusion In summary, we used the GWAS genetic data from two large consortium cohorts and indicated the positive association between waist circumference and DVT. Further researches are needed to investigate potential mechanism and clarify the role of waist circumference on DVT.


Introduction
As a subset of venous thromboembolism (VTE), deep vein thrombosis (DVT) is the medical condition when thrombus formation occurs in deep veins, occupying two-third of VTE cases. 1,2 Once the DVT falls off, it may block the pulmonary artery and form a fatal pulmonary embolism, which is an important cause of abnormal death in hospitalized patients. 3 Coronavirus disease 2019 (COVID- 19) is an ongoing outbreak of respiratory illness worldwide. 4 Coagulation abnormalities and thromboembolism are becoming common complications in critically ill patients with COVID-19. 5,6 The high incidence of DVT in patients with COVID-19 has attracted the attention of researchers and clinicians. Exploring the risk factors of the DVT is even more important at present.
Obesity has been demonstrated with increased risk of DVT in several researches. [7][8][9] Recently, some studies have found that abdominal obesity might be a better predictor of DVT while waist circumference was the crucial indicator to evaluate abdominal obesity. 10,11 Relevant reports were mostly conventional retrospective observation studies, which are inevitably interfered by reverse causality and confounding factors. 12 Randomized controlled trials (RCTs) often require lots of time and money, and may involve ethical issues. 13 In recent years, Mendelian randomization (MR) have been widely used in causal inferences of exposure factors and outcomes. [14][15][16] Compared with RCTs, MR is more economical and cost-effective. However, there is no relevant reports of MR studies on abdominal obesity and DVT.
In this study, we aimed to determine the potential causal relationship between waist circumference and the risk of DVT by using MR analysis.

Genetic Variants Associated with Waist Circumference
The genetic variants associated with waist circumference was assessed from the Neale lab consortium, which consisted of 336,639 participants and 10,894,596 single nucleotide polymorphisms (SNPs). The detail of the consortium was shown in Table 1. As with most MR studies, the selection criteria for SNP was set as "P < 5×10-8, linkage disequilibrium r 2 < 0.1" to decrease the impact of linkage disequilibrium. 14,15 Finally, there were 229 SNPs met the above criteria.

Genetic Variants Associated with DVT
The summary data for DVT was extracted from Medical research council-Integrative Epidemiology Unit (MER-ICU) consortium. There were 9241 DVT patients and 453,692 controls in this consortium. Apart from 5 SNPs (rs11208779, rs11666480, rs12335914, rs13264909, rs1454687) not found in MER-ICU, the remaining 224 SNPs were included in the analysis.

Estimation of the Causal Relationship
Two-sample MR can be used to analysis the data without contacting with clinical individual patients. 17 We utilized a R (version 3.4.2) package "TwoSampleMR" (version 0.3.4) to implement operations. We estimated the causal effect of waist circumference on DVT and harnessed the statistical/6 power of pre-existing GWAS analyses with the SNP-exposure effects and the SNP-out-come effects which are obtained from different studies. Summary data were extracted from GWAS through MR-Base platform. 18 Three MR approaches were used to explore MR estimates of waist circumference for DVT (inverse-variance weighted [IVW] approach, weighted median method and MR-Egger method). At first, we carried out a random-effects IVW meta-analysis by regressing the SNP-waist circumference associations against the SNP-DVT associations. The inverse-variance weighted mean of ratio estimates was calculated from 224 instruments. Fixed effects IVW assumed none of the SNPs exhibit horizontal pleiotropy while random effects IVW allows each SNP having different mean effects. 18,19 Secondly, by using weighted median methods, we found the weighted empirical distribution function of ratio estimates of SNPs selected. Median-

9274
based estimator had the advantage that only half of the SNPs needed to be valid instruments, which meant that the other SNPs might exhibit no horizontal pleiotropy, no association with confounders or robust associations with the exposure, to make sure the causal effect to be unbiased. Moreover, it allowed stronger SNPs to contribute more towards the estimate. 18,20 Third, MR-Egger analysis was implemented to assume that the horizontal pleiotropy had no association with the SNP-exposure effects. 20 When adapting the IVW analysis, MR-Egger allowed a non-zero intercept and unbalanced horizontal pleiotropy across all SNPs. MR-Egger regression could return an unbiased causal effect even all the SNPs were invalid instruments. It helped to figure out weighted linear regression of SNP-waist circumference risk against SNP-DVT effect estimates. The causal effect of DVT on waist circumference was also investigated by these three methods.

Sensitivity Analysis
Leave-one-out method that eliminated the included SNPs one by one and calculated the effect of the remaining instrumental variables to find the decisive SNPs, was applied in the sensitivity analysis. The intercept in MR-Egger was calculated to check the presence of directional horizontal pleiotropy.

Sensitivity Analysis
For the MR-Egger regression, the presence of directional horizontal pleiotropy was not observed because the intercept was close to zero and P values was larger than 0.05 (intercept = −3.9E-05, P = 0.616) ( Table 4). The method of leave-one-analysis indicated that there was no decisive SNP to reverse the result of causal inference ( Figure 3).

Causal Effect of DVT on Waist Circumference
As presented in

Discussion
DVT is a common and frequently-occurring disease in clinical practice, and the incidence is increasing year by year. People with DVT might lead to disability and cause

9279
death for severe cases, which seriously affect the prognosis and quality of life of patients. In present study, we explored the causal association between waist circumference and DVT through the two-sample MR analysis. The result indicated that higher level of waist circumference was causally associated with a higher risk of DVT while DVT did not contribute to the level of waist circumference. The finding highlighted the great importance of prevention and screening in the patients with abdominal obesity, especially during the COVID-19 pandemic.
Against the background of a sharp increase in obesity incidence worldwide, obesity has been the independent risk factors of DVT for long. 22,23 The MR research from Denmark demonstrated the causal relationship between obesity and risk of DVT while similar result was observed in another MR study. 24,25 Recently, abdominal obesity was recommended as the more suitable predictor for DVT in several researches. Yuan and his colleagues adjusted the factor of waist circumference and found that the association between body mass index (BMI) and DVT was   relativity weakened, indicating that waist circumference might be the preferable indicator of DVT. 10 A Swedish study found that abdominal obesity was an independent risk factor for middle-aged men in community. 26 14,27,28 Till now, we were the first to investigate the association between waist circumference and DVT through the method of two-sample MR analysis. Moreover, a large sample size (more than 400 thousand) reduced the bias from weak instrumental variables and provided enough power to robust causal detection.
Although the mechanism of abdominal obesity and DVT remains unclear, several possible mechanisms may explain the causal association. More than a century ago, Rudolf Virchow came up with three critical factors, venous stasis, activation of blood clotting, and venous damage. 29 As the key component in metabolic syndrome, abdominal obesity plays a role in the insulin resistance while body fat distribution is a determined factor in insulin resistance. 30 In comparison with subcutaneous adipose tissue, the abdominal fat had greater ability in insulin resistance, in which the balance of nitric oxide (NO) production and endothelin-1 secretion is broken, leading to the damage of endothelial cells. 31 Adipocytes secrete inflammatory factors, such as IL-6, MCP-1, MCP-1 and so on. Overexpression of MCP-1 leads to increased free fatty acids in plasma, increases the recruitment of macrophages and the expression of inflammatory cytokines, then the coagulation system is activated. 32 In addition, central obesity has been demonstrated with elevated intra-abdominal pressure and decreased flow velocity of venous blood, thus is more likely to form DVT. 33,34 The patients with abdominal obesity are more likely with less physical activity, which might lead to the formation of DVT. Fat mass and obesity-associated gene (FTO) rs11642015 polymorphism was found significantly associated with risk of DVT in present study. Elevated FTO expression can decrease the expression of AKT phosphorylation in endothelial cells, 35 which might lead to the dysfunction of endothelial and the development of DVT. It is a remarkable fact that the role of the other SNPs is still not clear. Therefore, further researches are needed to explore the potentially biological pathways in the progression of DVT.
Our analysis had several important strengths. First, compared with the traditional observational studies, MR analysis can avoid the reverse causality between exposure and outcome and not be affected by classical confounding factors. Larger sample size can bring more accurate estimation of causality. Second, three statistical approaches (IVW random-effect, weighted median and MR-Egger regression) were performed to test the causal relationship and make our finding more reliable. Horizontal pleiotropy might affect the validity of result. The consistency in the findings from these four different methods helps to adjust for pleiotropy. Third, our analysis is more economical and time-saving. There were also some limitations in this analysis. First, 224 SNPs were finally included in this analysis and invalid instrument variables might arise, which results in the biased estimates for causal effect and increases type I error rates. 36 However, the result of MR Egger was coincident with the other methods, indicating the robustness of the findings. Second, for the limitation of summary data, we were not able to perform further subgroup analysis or mechanism of action. Third, the data for both exposure and outcome were extracted from European consortiums. Therefore, variations in genetic background are various among different populations and ethnicities, further researches are needed to investigate whether the conclusion can be genialized to other races. 37

Conclusion
In summary, we used the GWAS genetic data from two large consortium Cohorts and indicated the positive association between waist circumference and the risk of DVT. Further researches are needed to investigate potential mechanism and clarify the role of waist circumference in DVT.

Ethics Approval and Informed Consent
There were no patients involved in the research design, recruitment or conduct, so the ethical approval was waived by ethic committee of Guangdong Provincial People's Hospital. No participants were requested to advise on interpretation or writing up of results. There are no plans to disseminate the results of the research to study participants or the relevant patient community.