Anti-SAE dermatomyositis: clinical and histologic characteristics from a monocentric Italian cohort

Objectives: Multiple myositis-specific antibodies have been identified, each associated with different clinical subsets of dermatomyositis (DM). Anti-SAE associated DM is considered the least studied subset. Our study aimed to evaluate the clinical and histological characteristics of DM patients with anti-SAE antibodies. As reference, patients with anti-Mi2 antibodies associated DM, representing a well-characterised subset, were analysed. Methods: We recorded data from our DM cohort in the INflammatory MYositis REgistry (INMYRE). Patients were divided into two groups: those positive for anti-SAE and those positive for anti-Mi2 antibodies. Clinical characteristics, including skin, muscle, and extra-muscular involvements, were recorded. Available muscle biopsies were compared between the two groups. Results: Of 92 DM patients, 10 (10.9%) were positive for anti-SAE and 17 (18.5%) for anti-Mi2. Anti-SAE positive DM patients showed classic DM findings but were characterised by a higher prevalence of skin itching (60% vs. 11.8%, p<0.01), shawl sign (40% vs. 5.9%, p<0.05) and lung involvement (30% vs. 0%, p<0.05) compared to anti-Mi2 positive patients. Furthermore, anti-SAE positive DM patients showed lower creatine kinase levels than those with anti-Mi2 (median [IQR]: 101 [58-647] vs. 1984 [974-3717], p<0.05) and a lower percentage of muscle fibre degeneration and necrosis (1.5%±1.7 vs. 5.9%±3.2, p<0.05) in muscle biopsies. No other differences were observed. Conclusions: Anti-SAE DM represents a disease subset characterised by classic cutaneous involvement often associated with itching, less severe muscle involvement, but potential pulmonary involvement that should always be investigated in these patients.