GPNMB is a potential biomarker for arthroplasty aseptic loosening stages

Methods: Four patient cohorts (primary and revision arthroplasty of hip or knee) were defined. Aseptic loosening of implants was assessed by the standardised approach of the Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System (KSRESS) which is based on radiolucent lines. Synovial fluid, bone marrow, and blood were collected from a total of 96 patients. Bulk RNA-sequencing (RNA-seq) of mesenchymal stem cells (MSCs) isolated from the bone marrow of patients was performed for 28 samples. The data was analysed by bioinformatic tools. Quantitative real-time PCR served to validate and extend RNA-seq data. Western blots were performed for molecular differentiation. ELISAs were used to quantitate selected protein targets. Results: RNA-seq of MSCs identified glycoprotein non-metastatic melanoma protein B (GPNMB) as significantly upregulated in the cells from revision patients compared to patients with primary implantations. The protein encoded by this gene is plasma membrane-standing; its extracellular domain may be cleaved and secreted. In synovial fluid plasma, the soluble GPNMB protein exhibited increased levels in patients undergoing revision surgery compared to patients subjected to primary arthroplasty. No significant differences were observed in bone marrow plasma and blood plasma. The molecular species detected by Western blotting were similar in the plasma fractions of synovial fluid, bone marrow and blood. Conclusions: The use of GPNMB as a biomarker may be of prime importance for early detection of implant loosening, ahead of radiographic failure demonstration. Translational potential of this article: GPNMB has the potential to act as a prospective and quantitative parameter to assess different stages of implant loosening. Radiography and histo-pathological classification of the periprosthetic membrane, on the contrary, are used retrospectively for diagnosis and are subjective measures, performed mainly at the end of a full loosening process.