Identification of fibroblast-specific causal genes in atrial fibrillation through single-cell transcriptomics and Mendelian randomization

Background Atrial fibrillation (AF) is the most common persistent arrhythmia. Atrial fibrosis driven by fibroblast activation is its key pathological basis, but AF pathogenic genes that specifically act on cardiac fibroblasts have not been fully elucidated at present. Methods This study integrated single-cell RNA sequencing (scRNA-seq) data of human atrial tissue and Mendelian randomization (MR) analysis, constructed a cardiac cell atlas to identify AF-related fibroblast subsets and differentially expressed genes, and meanwhile carried out cell-cell communication analysis to evaluate the signal transduction characteristics of fibroblasts. The expression quantitative trait loci of fibroblast differentially expressed genes were used as instrumental variables, and the summary statistics of AF genome-wide association studies were used as outcome variables. Subsequent protein-protein interaction network construction, functional enrichment analysis and virtual gene knockout analysis were further carried out. Results Fibroblasts are the main cell population in atrial tissue and the core signal hub in the cardiac microenvironment. MR analysis identified 2 fibroblast-specific pathogenic genes: HLA-DRB5 is a risk gene (OR = 1.052, 95% CI: 1.006-1.100, P  = 0.026), and NUSAP1 is a protective gene (OR = 0.909, 95% CI: 0.843–0.980, P  = 0.013). The causal effect inferred by MR was consistent with the single-cell expression pattern, and sensitivity analysis verified the robustness of the results. The functional enrichment effect was mainly driven by HLA-DRB5, concentrated in the antigen processing and presentation pathway. After virtual knockout of HLA-DRB5, the expression network of complement factor D (CFD) was significantly disturbed. Conclusions This study identified that HLA-DRB5 and NUSAP1 are 2 AF susceptibility genes with genetic causal evidence in atrial fibroblasts, and their effect directions are opposite. Virtual knockout analysis suggests that CFD may be a downstream effector molecule of HLA-DRB5. The above research results provide insights from the genetic causal level for revealing the molecular mechanism of atrial fibrosis.