Study Design
This research utilized a two-sample MR approach to explore the causal relationship between NETs-associated inflammatory traits and T2DM and its complications. The MR framework relies on three key assumptions20: 1) the genetic variant must be robustly associated with the exposure, 2) the genetic variant must only influence the outcome through the exposure (exclusion of pleiotropy), and 3) the genetic variant must not be associated with any confounding factors. For a detailed overview of this study’s process, please refer to Figure 1. The manuscript adheres to the MR-STROBE guidelines for reporting MR studies, ensuring rigorous standards of transparency and reproducibility21.
Data Sources
The Genome-Wide Association Studies (GWAS) data utilized in this MR analysis were sourced from the GWAS public databases. In terms of the summary-level GWASs concerning T2DM, incorporating the largest meta-analysis of a European population, comprising 74,124 cases and 824,006 controls, alongside a prospective nested case-cohort study conducted in Europe, featuring 9,978 cases and 12,348 controls22. For diabetic complications, including T2DM with renal complications and T2DM with peripheral circulatory complications, this study retrieved GWAS summary statistics from studies in which cases were T2DM patients with the specific complication and controls were individuals without T2DM. Detailed information on data sources can be found in Supplementary Table 1.
GWAS data for NETs-associated inflammatory traits were obtained from the GWAS catalog (see Supplementary Table 2). To address the heterogeneity in biological specificity, this study categorized the included exposures into two distinct groups based on their functional roles in neutrophil biology: (1) Core NETosis-related Factors: This category includes markers directly involved in the structural formation of NETs or the enzymatic process of chromatin decondensation. Specifically, this study included NETs themselves, Myeloperoxidase (MPO), Neutrophil Elastase (NE), and the MPO-DNA complex. MPO and NE are essential enzymes for histone degradation and chromatin decondensation, while the MPO-DNA complex is a specific surrogate marker for NETs23. (2) NETs-associated Inflammatory Mediators: This category comprises cytokines and mediators that act as upstream regulators or downstream effectors closely linked to NETosis but are also involved in broader inflammatory pathways. This group includes Interleukin-6 (IL-6)24, Tumor Necrosis Factor-alpha (TNF-α)25, Neutrophil gelatinase-associated lipocalin levels (NGAL)26, and Cellular Communication Network Factor 1 (CCN1)27.
The data used in this study were sourced from open-access databases or GWAS published in previous studies; therefore, ethical approval was not required for this research.
Selection of NETs-Related Genetic Instruments
Given that NETs formation represents a dynamic biological process not directly measured in conventional GWAS, this study employed a gene-based approach to identify genetic instruments for NETs-associated inflammatory traits. A comprehensive set of 257 genes known to be critically involved in NETs formation and regulation, such as MPO, along with genes encoding histones and neutrophil granule proteins, was curated from established mechanistic studies and published literature28.
Instrumental Variable Selection
To identify robust genetic instruments for NETs-associated inflammatory traits, this study implemented a sequential filtering workflow using GWAS summary statistics. The specific operational steps were as follows:
Initial Screening: Genetic variants significantly associated with the exposure traits were extracted29 based on a significance threshold of P < 5 × 10⁻6.
Minor allele frequency (MAF) Filtering: To ensure statistical power, Single Nucleotide Polymorphisms (SNPs) with a MAF ≤ 0.01 were excluded30.
Linkage Disequilibrium (LD) Clumping: To mitigate linkage disequilibrium (LD) confounding, independent SNPs were selected using the clumping function with parameters set to r2 < 0.001 within a 10,000 kb window31.
Instrument Strength Assessment: The strength of each remaining IV was quantified using the F-statistic, calculated as F = R2 × (N-2) / (1-R2). Only SNPs with an F-statistic > 10 were retained to minimize weak instrument bias32.
Proxy SNP Identification and Data Harmonization
To address missing SNPs in the outcome dataset, this study performed a proxy search and data harmonization process:
Proxy Substitution: When a target SNP from the exposure GWAS was unavailable in the outcome GWAS, this study utilized the LDproxy() function based on the 1,000 Genomes Project European reference panel. A candidate proxy SNP was selected only if it exhibited high LD with the original SNP (r2> 0.8). If no qualified proxy was found, the SNP was excluded.
Harmonization: Exposure and outcome datasets were aligned using the harmonise_data() function from the TwoSampleMR package (R version 4.0.5). This study set the parameter action = 2 to automatically align all SNPs to the forward strand and remove palindromic SNPs with ambiguous strand orientation.
Verification: Post-harmonization, this study manually inspected the harmonized dataset to confirm that the effect allele frequencies were consistent between the exposure and outcome data.
MR Analysis and Sensitivity Tests
Causal inference was conducted using the TwoSampleMR package (R version 4.0.5).
Primary and Secondary Analyses: This study applied the Inverse Variance Weighted (IVW) method as the primary approach33. Complementary analyses were performed using MR-Egger34, weighted median, and weighted mode methods to ensure robustness35.
Sensitivity Assessments: Heterogeneity among IVs was evaluated using Cochran’s Q test36 via the mr_heterogeneity() function. Horizontal pleiotropy was assessed using the MR-Egger34 intercept test (mr_pleiotropy_test()).
Outlier Detection: This study employed the MR-PRESSO package to detect potential outliers37. The mr_presso() function was executed with 1,000 simulations. Identified outlier SNPs (P < 0.05) were removed, and the causal estimates were recalculated to verify result stability. Additionally, a leave-one-out analysis was performed to ensure that the causal association was not driven by a single SNP38.
Statistical Correction
To account for multiple testing, P-values obtained from the MR analyses were corrected using the False Discovery Rate (FDR) method. This was implemented using the P.adjust() function in R with the parameter method = "fdr". Associations with a corrected P-value (PFDR) < 0.05 were considered statistically significant.