Integrative Analysis Coupled with In Vitro Validation Reveals KAZN and SUPT3H as Shared Negative Regulators in Osteosarcopenia

Osteosarcopenia, which is the coexistence of sarcopenia and osteoporosis, is being increasingly recognized as a systemic musculoskeletal aging syndrome. However, shared molecular regulators of bone–muscle deterioration remain unclear. In this study, we integrated bulk transcriptomic datasets from sarcopenic skeletal muscle and osteoporosis peripheral blood mononuclear cells to identify shared differentially expressed genes, followed by two-sample Mendelian randomization using osteoporosis genome-wide association study summary statistics to prioritize genes with potential causal relevance. Furthermore, diagnostic performance, functional enrichment, immune infiltration, single-cell RNA sequencing, regulatory network reconstruction, compound prediction, and siRNA-mediated validation were conducted in C2C12 and MC3T3-E1 cells. Overall, 122 shared differentially expressed genes were preliminarily screened, and KAZN and SUPT3H were tentatively proposed as candidate genes genetically associated with osteoporosis risk. Both genes were upregulated in the disease groups and exhibited weak to modest diagnostic performance. Furthermore, Kazn or Supt3h knockdown promoted myogenic differentiation in C2C12 cells as well as osteogenic differentiation and mineralization in MC3T3-E1 cells, supporting their roles as negative regulators of lineage differentiation. Moreover, functional analyses linked KAZN mainly to mitochondrial-related programs and SUPT3H to immune signaling, whereas single-cell analyses localized these genes to stromal, progenitor, and immune-related compartments. These hypothesis-generating findings suggest that KAZN and SUPT3H participate in shared bone–muscle dysfunction and generate candidate genes and mechanistic hypotheses for subsequent functional validation and translational research.