Serum levels of manganese superoxide dismutase in patients with mixed connective tissue disease.

Sirs, Oxygen free radicals are implicated in the pathogenesis of many human diseases (1, 2). This can cause several abnormalities such as endothelial cell damage or enhanced platelet activation, leading to an upregulation of the expression of adhesion molecules or the secretion of inflammatory or fibrogenic cytokines including PDGF and TGF-β. Superoxide dismutase (SOD) is a family of enzymes that catalyze the dismutation of O2 to H2O2 and O2, protecting aerobic cells from damage by oxygen free radicals. Several different SOD have evolved to inactivate both intracelluar and extracellular superoxide (3). Human tissues have three major SOD isozymes: copperz i n c SOD, manganese SOD (Mn SOD), and extracelluar SOD. The former two isozymes have different intracellular distributions, and the elevation of their serum levels has been demonstrated in various kinds of diseases such as acute leukemias, diabetis mellitus, or polymyositis/dermatomyositis (PM/ DM), besides systemic sclerosis (SSc) (1, 4). However, there has been no report demonstrating the levels of SOD in patients with mixed connective tissue disease (MCTD). In this study, we determined the serum levels of Mn SOD in patients with MCTD, and investigated their clinical significance in this disease. Twenty-three patients with MCTD who had received no treatment were included in this study. Serum samples, clinical manifestations and laboratory findings were collected or evaluated in their first visit. The patients with MCTD, diagnosed according to the criteria of Alarcon-Segovia (5), had clinical features of systemic lupus erythematosus (SLE), SSc and PM/DM and anti-U1RNP antibodies, and did not satisfy the criteria for other connective tissue diseases as described previously (6). And the diagnosis of Sjögren syndrom (SjS) was based on a revised version of the European criteria proposed by the American-European Consensus Group (7). We also collected control serum samples from 20 healthy age/sexmatched volunteers. Institutional review board approval and written informed consent were obtained before patients and healthy volunteers were entered into this study according to the Declaration of Helsinki. Levels of Mn SOD were measured with a specific ELISA kit (Amersham Co. UK), according to the manufacturer's instructions. Values greater than 99th percentile of the normal control subjects were regarded as elevated. Statistical analysis was carried out with Mann-Whitney test for the comparison of medians and Fisher’s exact probability test for the analysis of the incidence. P values less than 0.05 were considered significant. The serum Mn SOD levels in the patients with MCTD were significantly higher than those in the healthy controls (median; 28.6 ng/ml vs 137.6ng/ml, 25th-75th percentile; 16.3–85.8 ng/ml vs 69.2–333.5 ng/ml, p < 0.0005). When the cut-off value was set as 132.4ng/ml (99th percentile of the controls), elevated serum Mn SOD levels was observed in 13 of 23 patients (56.5%) with MCTD. The clinical and serological features of the patients with elevated Mn SOD levels, as well as those with normal levels, are shown in Table I. The patients with the elevated serum Mn SOD levels had SjS at a significantly higher incidence than those without (87.5% versus 33.3%, p < 0.05). Sicca symptoms in patients with MCTD were reported to be common, occurring in 41.8% of MCTD patients (8). Ohtsuka et al. reported that the prevalence of sialectasia or secondary SjS in MCTD was significantly higher than in any other connective tissue diseases, but that there were no significant differences in clinical manifestations or laboratory findings between MCTD patients with sialectasia or SjS and those without (9). A d d i t i o n a l l y, we previously reported that there was no significant association between sicca symptom and anti-SS-Aantibody in patients with MCTD (10). Thus, our results show Mn SOD may be a rare marker of secondary SjS in MCTD.