Gene Expression Remains More Altered In Synovium Than In Cartilage One Year Following ACL Injury

ACL injuries are known to predispose patients to post-traumatic osteoarthritis (PTOA), regardless of whether surgical reconstruction is performed or not. The changes in articular cartilage and synovium gene expression during the development of PTOA have not yet been fully characterized, and it is unknown which of these tissues plays the primary role in driving the development of PTOA. PURPOSE: To gauge whether gene expression profiles of articular cartilage and synovium at one year after ACL injury are different in knees developing PTOA vs uninjured control knees. METHODS: Thirty-six Yucatan minipigs underwent unilateral ACL transection and received ligament reconstruction, bridge-enhanced ligament repair, or no further treatment. One year after surgery, cartilage and synovium samples were collected from each surgical joint and from 12 intact contralateral joints. Samples underwent bulk RNA-sequencing, and unsupervised clustering was performed on gene expression values across three Gene Ontology (GO) terms chosen for their relevance in osteoarthritis literature. Two-sample t-tests with Bonferroni correction (significance p < .05) were used to assess separation of surgical (T) and control (C) samples between the two largest clusters. RESULTS: Cartilage gene expression was not significantly different in T vs C samples for all three GO terms as shown by sample type mixing in the figure (immune clusters: 67% T vs 89% T, p = .37; collagen clusters: 72% T vs 81% T, p = 1; hypoxia clusters: 75% T vs 75% T, p = 1). Synovium gene expression was significantly different in T vs C knees for all three GO terms as shown by sample type separation in the figure (immune clusters: 94% T vs 17% T, p = 8.7e-5; collagen clusters: 21% T vs 97% T, p = 7.3e-5; hypoxia clusters: 68% T vs 100% T, p = 1.1e-3). CONCLUSION: The synovium is a more reliable transcriptomic indicator of prior knee injury one year following injury than the articular cartilage. Supported by NIH grants R01-AR065462, R01-AR056834.