POS0232 SYNOVIAL FLUID CRTAC1: A POTENTIAL INDICATOR OF KNEE OSTEOARTHRITIS SEVERITY
Authors: A. Prokopcova, K. Bubova, J. Baloun, A. Navrátilová, L. Ondrejčáková, J. Juhaszova, P. Fulín, R. Ballay, O. Růžičková, O. Sleglova, J. Gatterova, M. Tomčík, J. Vencovský, L. Šenolt
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Cartilage Acidic Protein (CRTAC1) is a chondrocyte-produced protein associated with osteoarthritis (OA) pathophysiology. Elevated levels of CRTAC1 were observed in both cartilage and plasma samples from OA patients and can potentially predict OA occurrence or progression to joint replacement. <h3>Objectives:</h3> This study aims to compare CRTAC1 levels in synovial fluid (SF) and plasma samples collected from patients diagnosed with knee OA and rheumatoid arthritis (RA) in comparison to unpaired samples from healthy controls (HCs). Secondly, it aims to explore the specificity of plasma CRTAC1 for OA compared to other immune-mediated rheumatic diseases. Lastly, the study aims to explore the association between CRTAC1 levels and the OA severity or other clinical assessments. <h3>Methods:</h3> Paired plasma and SF samples were collected from 24 OA patients and eight RA patients presenting knee effusion who underwent therapeutic arthrocentesis. Control samples were acquired from 15 healthy individuals with post-traumatic knee effusion. Additionally, plasma samples were obtained from a cohort of 84 HCs, 114 patients with knee, hip, and/or hand OA, 80 patients with RA, 20 patients with axial spondyloarthritis (axSpA) and 44 patients with idiopathic inflammatory myopathies (IIM). CRTAC1 levels were assessed using the CRTAC1 ELISA (Ray-Biotech) and Pepti Quant (MRM Proteomics). Knee radiographs were evaluated by an unbiased assessor utilizing the Kellgren-Lawrence grading scale. Patients completed assessments including the Visual Analog Scale (VAS) for pain, the Western Ontario and McMaster Universities Arthritis Index (WOMAC), the AUStralian CANadian Osteoarthritis Hand Index (AUSCAN), and the Health Assessment Questionnaire (HAQ). <h3>Results:</h3> Levels of CRTAC1 in both SF and plasma were found to be elevated in OA patients compared to HCs (p&lt;0.01). Regression analysis revealed a positive correlation between CRTAC1 concentrations in SF and the severity of knee OA (p=0.02; β=2.39) but not in plasma (p=0.15, β=1.48). Furthermore, a positive correlation was observed between CRTAC1 levels and leukocyte count in SF (p=0.03, β=2.25). Notably, patients treated with nonsteroidal anti-inflammatory drugs exhibited a reduction in CRTAC1 levels in both SF and plasma (SF, p=0,007; plasma, p=0.04). In a broader cohort of patients with other rheumatic conditions, plasma CRTAC1 levels were still higher in patients with OA compared to individuals with RA, axSpA, IIMs and HCs (all p&lt;0.001). Regression analysis revealed a positive association between CRTAC1 and the number of affected joints (p=0.002; β=3.08). However, no significant variations in CRTAC1 levels were identified among different OA subtypes or associations with other clinical assessments. <h3>Conclusion:</h3> This study presents preliminary findings on the elevated localized and systemic presence of CRTAC1 in OA patients. The levels of CRTAC1 detected in synovial fluid potentially provide a more accurate insight into the extent of joint involvement compared to plasma, potentially reflecting different physiological or pathophysiological mechanisms. Additionally, this study suggests a correlation between CRTAC1 and joint inflammation. <h3>REFERENCES:</h3> [1] Styrkarsdottir, U.;2022. The CRTAC1 Protein in Plasma Is Associated With Osteoarthritis and Predicts Progression to Joint Replacement: A Large-Scale Proteomics Scan in Iceland. Arthritis &amp; Rheumatology 73, 2025–2034.. doi: 10.1002/art.41793. <h3>Acknowledgements:</h3> Supported by GAUK-266523, MHCR 023728, SVV 260 638. <h3>Disclosure of Interests:</h3> <b>None declared.</b>