0316: Increased Synovial Inflammation, Xist Escape Gene Expression and Knee Pain in Females with Osteoarthritis

Background/Purpose: MRI evidence of synovitis and bone marrow lesions, but not cartilage degeneration associate with pain in osteoarthritis (OA). In this work, we integrated clinical, histological, and transcriptomic analyses of synovial tissue from patients with knee OA to explore correlations of OA pain and sex related pain differences.

Methods: We enrolled 135 patients undergoing total knee arthroplasty who met 1986 ACR classification criteria for knee OA. Demographic, clinical data, patient-reported pain outcomes, and Osteoarthritis Research Society International (OARSI) cartilage scores and synovial histology features measured by board-certified pathologists were collected and analyzed (Table 1). Cell density and cell aggregate number and size were quantified using a previously established computer vision pipeline. Bulk RNA-seq was performed on RNAlater-preserved synovial tissues, using a KAPA-Stranded RNAseqKit with RiboErase for library preparation, and sequenced on a NovaSeq with 150 base pair paired-end reads targeting 100M reads per sample. Reads were pseudoaligned to Hg38 with Kallisto, and Limma was used to test for differentially expressed genes (DEGs). Gene set enrichment analysis (GSEA) was used to test for enrichment in GO pathways.

Results: Kellgren Lawrence (KL) grades, BMI, and symptom duration were similar between sexes. Females had higher Erythrocyte Sedimentation Rate (ESR) and reported worse Knee Injury and Osteoarthritis Outcome Score (KOOS) pain scores (p=0.01), particularly rest pain (at night, while in bed) (p=0.02) (Table 1). Rest pain was less common than mechanical pain (e.g., with twisting or stair use) but, when present, it correlated with higher total KOOS pain scores. Synovial histology revealed increased plasma cells, binucleate plasma cells, Russell bodies, and size and number of cellular aggregates in females. Females with synovial plasma cells reported increased rest pain. OARSI cartilage scores were similar in the most severely eroded femoral condyles between sexes, but females had significantly higher scores in the less eroded condyles (Figure 1). RNA-seq revealed 25 DEGs between sexes, and a pronounced enrichment in Xist escape genes in females. GSEA identified 132 pathways significantly enriched in females, including “adaptive immune response” and “complement activation”. Among females, genes associated with inflammation, fibrosis, and neurogenesis pathways were positively associated with rest pain, while genes associated with DNA damage response and progenitor cells were negatively associated with rest pain. There were no genes associated with rest pain in males.

Conclusion: Compared to males, females have significantly increased systemic and synovial tissue inflammation, along with increased multicompartmental cartilage degeneration. Among females, rest pain is positively associated with synovial inflammation, fibrosis, and neurogenesis pathway gene expression and negatively associated with DNA damage response and progenitor cell gene expression. This analysis provides insights into the molecular underpinnings of the clinical observation that MRI evidence of synovitis is associated with pain in knee OA.