The Genetic Landscape of Plasma P-Selectin Glycoprotein Ligand Levels and Bidirectional Mendelian Randomization to Assess Role in Proinflammatory Cytokine Levels

Background: Polymorphonuclear (PMN) leukocyte recruitment to activated pulmonary endothelium is a central mechanism in acute respiratory distress syndrome (ARDS). This process is mediated by selectins and their counter-ligand, P-selectin glycoprotein ligand-1 (PSGL-1), encoded by SELPLG. Genetic variation in SELPLG has been associated with ARDS susceptibility, while disruption of PSGL-1/P-selectin interactions attenuates lung injury in preclinical models. Because inflammatory stimuli increase both SELPLG expression and circulating PSGL-1 levels, PSGL-1 represents a promising biomarker and therapeutic target. We sought to define the genetic determinants of plasma PSGL-1 levels and evaluate their causal relationships with key inflammatory and endothelial biomarkers. Methods: Genome-wide association study (GWAS) summary statistics for plasma PSGL-1 levels were obtained from the UK Biobank Pharma Proteomics Project (n = 35,571) and the SCALLOP consortium (n = 21,758 across 13 cohorts). Associated variants underwent functional annotation and in silico analyses to identify potential effects on protein structure and gene regulation. Bidirectional Mendelian randomization (MR) was performed using GWAS summary statistics for C-reactive protein (CRP), E-selectin, GlycA, and soluble intercellular adhesion molecule-1 (sICAM-1) to assess potential causal relationships with PSGL-1 levels. Results: Multiple cis- and trans-acting loci were significantly associated with plasma PSGL-1 concentrations. Three coding SELPLG variants (rs201689859, rs74792300, and rs139943851) were predicted to alter PSGL-1 protein structure and were associated with lower circulating PSGL-1 levels. Four promoter variants (rs1420663, rs1833245, rs1420664, and rs8179110) were linked to altered transcriptional activity, including a potential effect of rs1420664 on hypoxia-inducible factor binding. Bidirectional MR demonstrated that genetically predicted CRP, E-selectin, GlycA, and sICAM-1 levels were associated with increased plasma PSGL-1 concentrations. Additional loci implicated pathways related to immune signaling, cell adhesion, and protein stability. Conclusions: Large-scale GWAS and Mendelian randomization analyses identified genetic variants that regulate plasma PSGL-1 levels and demonstrated causal links between inflammatory and endothelial biomarkers and PSGL-1 expression. These findings provide new insights into the genetic regulation of leukocyte trafficking pathways and support a role for PSGL-1 in inflammatory diseases, including ARDS, sepsis, and cardiovascular disorders.