Abstract
BACKGROUND: The aging population and advancements in medical science have heightened the focus on fractures, which affect over 150 million individuals annually, with substantial health and economic consequences. OBJECTIVE: This study investigated the potential causal relationships between 731 immune cells, 1400 metabolites, and nine fracture types using Mendelian randomization (MR). METHODS: A combination of bidirectional MR, two-sample MR, and mediation MR was employed to assess potential causal links. Sensitivity analysis was performed using MR-PRESSO. Bioinformatics analyses, including functional enrichment and protein-protein interaction (PPI) network analysis, were conducted. Colocalization analysis was used to examine associations between key genes and fractures. RESULTS: Bidirectional MR identified 7 immune cell subtypes (e.g., B cells, Tregs, and monocytes) and 11 metabolite classes (e.g., lipids, amino acids) with significant MR-supported associations with fracture risk, with effects varying by skeletal site. Mediation analysis revealed that the increased wrist fracture risk associated with CD28+CD45RA-CD8br T cells was mediated by 5-methylthioadenosine (19.6%), while the reduced foot fracture risk linked to CD28-CD8dim T cells was mediated via the taurine-to-cysteine ratio (20.9%). SNP nearest gene integration highlighted enriched pathways related to immune response, cell adhesion, and metabolism. PPI network analysis pinpointed 9 hub genes, six of which (CD8A, PRKACA, IL-6, ITGB1, ITPR1, and STAT3) showed strong colocalization evidence with fractures. Moreover, DNA methylation at cg09664550 (ITGB1) showed the most significant negative impact on thoracic spine fractures (OR = 1.986), whereas cg18112163 (STAT3) conferred the strongest protective effect against foot fractures (OR = 0.602; all p < 0.05). CONCLUSIONS: The findings suggest that immune cells and metabolites may have genetically predicted effects on fracture risk, with metabolites potentially serving as key mediators. Critical pathways, hub genes, and fracture-associated SNPs were identified, along with potential epigenetic regulation via methylation sites. These preliminary insights offer novel directions for future research into the underlying mechanisms of fracture risk and intervention.