Clinical utility of anti-signal recognition particle antibody in the differential diagnosis of myopathies

Objective. Auto-antibodies to signal recognition particle (SRP) are known to be specific to PM among rheumatic disorders, but the specificity in myopathic diseases remains unclear. The clinical utility of anti-SRP antibody in the differential diagnosis of myopathies has not been studied. The aim of the present study was to elucidate whether detection of anti-SRP antibody can discriminate of PM from muscular dystrophy (MD). Methods. We report a patient with a childhood onset myopathy, in whom it was clinically difficult to make a differential diagnosis of PM or MD for 21 yrs, despite repeated muscle biopsies. Myositis-specific auto-antibodies to RNA-associated antigens were screened in this particular case as well as in 105 serum samples from various types of MD and 84 from PM patients using RNA immunoprecipitation. The MD and PM serum samples were obtained from different institutions. The presence of anti-SRP antibody was confirmed by RNA immunoprecipitation combined with immunodepletion of SRP from the antigen. Results. Anti-SRP antibody was positive in the present patient, supporting the diagnosis of PM. Anti-SRP antibody was detected in seven (8.3%) patients with PM, but in none of the patients with MD. Myositis-specific auto-antibodies were not detected in any of the patients with MD. Conclusion. Anti-SRP antibody is useful for discriminating PM from MD among patients with myopathies.