INTRODUCTION: Modic changes (MC) are vertebral bone marrow lesions seen on magnetic resonance imaging (MRI). MC are adjacent to degenerated intervertebral discs and are a specific source of chronic low back pain (CLBP). The etiology of MC remains largely unknown. The degenerating disc has been accused to trigger MC by releasing pro-inflammatory cytokines. Biomarker studies showed inconsistent association of MC with pro-inflammatory serum cytokines. A recent study identified very‑low‑density lipoprotein as risk factor and potential biomarker for MC. The aim of this study was to identify serum biomarkers for MC using an unbiased quantitative mass spectrometry screening approach.

METHODS: One hundred serum samples from the Northern Finland Birth Cohort 1966 have been selected. Patient data including demographic data, comorbidities, psychosocial factors, pain and disability scores were retrieved from the cohort database. Four conditions (n=25 each) were compared: (i) CLBP + MC, (ii) CLBP + no-MC, (iii) no-CLBP + MC, (iv) no-CLBP + no-MC. Serum samples were analyzed using data-independent-acquisition (DIA)-based sequential window acquisition of all theoretical mass spectra (SWATH-MS). DIA-based SWATH-MS is a systematic and unbiased acquisition that combines deep proteome coverage capabilities with quantitative consistency and accuracy. All detected proteins were analyzed with two-way ANOVA for effects of CLBP and MC. Gene ontology analysis and pathway analysis was run to identify enriched pathways. In a sub-population analysis of the 50 subjects with CLBP, proteins with significant changes due to MC were identified. Level of significance was α=0.05. Power to predict MC in the CLBP population was calculated as area-under-the-curve (AUC) of the receiver-operating-characteristics (ROC).

RESULTS: With DIA-based SWATH-MS 1099 proteins were identified. Two-way ANOVA identified 37 significantly dysregulated proteins due to MC and 15 significantly dysregulated proteins due to CLBP. “Lipid transport” (p=1.3e-5) was the top enriched biological process and “Complement and coagulation cascade” (p=4.2e-3) as top enriched pathway. Top differentially expressed proteins in “lipid transport” were apolipoprotein D (APOD), apolipoprotein C3 (APOC3), apolipoprotein F (APOF), insulin receptor (INSR), and very low-density lipoprotein receptor (VLDLR). In the clinically relevant subpopulation with CLBP, the ratio of APOD/APOC3 was significantly higher in MC than no-MC (p=6.82e-5, Fig.1) and had a predictive power for MC of 0.84 (confidence interval: 0.73-0.95, Fig.2)

DISCUSSION: Population-based serum proteomics indicates that lipid transport is dysregulated in MC. This suggests that MC is not only a local inflammatory reaction to degenerated discs but that systemic metabolic factors are risk factors and potential biomarkers for MC. Serum lipoprotein measures could enhance a MC biomarker profile, yet robustness of lipid transport biomarkers for MC with respect to demographics, comorbidities, medication, and psychosocial factors need to be tested. This study lays ground for larger mechanistic and biomarker studies on the clinically relevant MC phenotype of CLBP.