Introduction: Low back pain (LBP) is the leading cause of disability and is commonly associated with spine pathologies including the diagnoses of disc herniation, disc degeneration, and spinal stenosis. Some studies have identified correlations between systemic inflammatory cytokine levels and painful related disc pathologies. However, the relationship between symptom severity and systemic inflammation is not well defined. A previous study found transcriptional differences in pain, ECM, and immune-related genes in patients with LBP that originated or transitioned to a chronic pain state, in comparison to patients with acute pain and healthy controls. However, transcriptional differences as a function of symptom severity were not examined and the effect of disability level on the systemic environment has not been examined. The goal of this study is to characterize global transcriptomic changes associated with chronic pain and disability in the blood of patients with IVD pathology using bulk RNA sequencing.
Methods: Subjects (n=29) were recruited from adult patients being evaluated for treatment in the lumbar spine region for diagnosis of disc herniation, spinal stenosis, or disc degeneration based on MRI imaging. Patient demographic and patient reported outcomes (VAS, ODI) were collected. Whole blood samples was also collected and processed for RNA isolation. RNA sequencing was then performed and differentially expressed genes (DEGs) between subjects reported high vs. low ODI or high vs. low VAS were determined, with padj<0.05 considered significant. Gene Set Enrichment
Analysis (GSEA) was utilized for pathway analysis of comparisons resulting in significant DEGs.
Results: In comparison of high versus low ODI, 2691 DEGs were identified; however, the expression of these genes led to only modest hierarchical clustering. Since a significant difference was observed in the distribution of male and female subjects, differential expression analysis was performed controlling for gender as a covariate. This analysis resulted in 87 DEGs between high and low ODI and improved hierarchical clustering. GSEA of high versus low ODI, with control for gender as a covariate, resulted in 79 significant Reactome pathways. Results indicate that low ODI subjects exhibit an enrichment for pathways related to histone modification and senescence-related pathways. Interestingly, the innate immune response appeared dampened in high ODI subjects compared to low ODI demonstrated by a downregulation of pathways related to interferon signaling and complement activation.
Discussion: This study demonstrates that subjects with higher disability levels exhibit significantly lower expression of genes related to inflammation and immune activation compared to subjects with lower disability. Expression of interferon signaling, complement activation and cell senescence were specifically found to contribute to the suppressed inflammatory profile in high ODI. Moreover, significant differences in the transcriptomic profiles in whole blood of female and male subjects were observed, indicating gender as a significant covariate to consider in further analyses at the systemic level. Finally, a significant correlation between ODI and VAS within this patient population was observed, indicating that these findings in the comparison of high and low ODI may translate to pain severity as well.