INTRODUCTION: The aim was to evaluate S100A4 protein as a biomarker of disease activity and potential cancer development in patients with myositis. METHODS: Serum levels of S100A4 were determined in 43 dermatomyositis (DM), 39 polymyositis (PM) and 22 cancer associated myositis (CAM) patients as well as in 77 healthy controls. The associations between S100A4 levels, inflammation, disease activity, muscle strength and cancer development were evaluated. RESULTS: All myositis patients had significantly higher serum levels of S100A4 protein compared to healthy controls (median (IQR): 31.5 (17.4 to 59.5) versus 23.8 (14.5 to 33.7) ng/ml, P <0.05). In patients with PM, serum levels of S100A4 protein were significantly higher than in healthy controls (41.6 (24.2 to 123.1) versus 23.8 (14.5 to 33.7) ng/ml; P <0.001) as well as in patients with DM (26.7 (11.3 to 47.5) ng/ml; P <0.05). The levels of S100A4 were comparable between myositis with and without cancer. In all myositis patients, serum S100A4 levels correlated with MYOsitis disease ACTivity assessment (MYOACT) score (r = 0.34; P = 0.001), constitutional (r = 0.30; P = 0.003), pulmonary (r = 0.43; P = 0.0001) and extramuscular disease activity (r = 0.36; P = 0.0001), as well as with creatine phosphokinase (r = 0.27; P = 0.015) and lactate dehydrogenase (r = 0.37; P = 0.002) or c-reactive protein (CRP) levels (r = 0.24; P = 0.038). Multiple regression analysis showed significant association between S100A4 serum levels and extramuscular disease activity (beta = 0.552; P = 0.002) in PM patients and with MYOACT (beta = 0.557; P = 0.003) and CRP levels (beta = 0.391; P = 0.029) in DM patients. CONCLUSIONS: Circulating levels of S100A4 are elevated in patients with myositis and associate with several disease activity parameters, particularly with extramuscular components. No relation between S100A4 levels and presence of cancer associated myositis was found.

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