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SIR, I read with great interest the article by Suzuki et al. [1] on clinical utility of anti-signal recognition particle (anti-SRP) antibody in the differential diagnosis of myopathies. Suzuki et al. [1] demonstrated that anti-SRP antibody is positive in 8.3% of the patients with polymyositis but in none of the patients with muscular dystrophy. About 60% of patients with polymyositis due to anti-SRP antibody depicted lymphocytic infiltration on their muscle biopsies [1]. The authors conclude that anti-SRP antibody is useful for discriminating polymyositis from muscular dystrophy [1]. It is known that muscle biopsy from patients with muscular dystrophy typically depicts necrotic fibres with a certain degree of inflammatory reaction; however, a number of subtypes of muscular dystrophy have a pathological mimicry to myositis and are potentially misdiagnosed as inflammatory myopathy. These myositis-mimicking muscular dystrophies include dysferlinopathy, calpainopathy and facioscapulohumeral muscular dystrophy (FSHD) [2–4]. Dysferlinopathy covers a variety of skeletal muscle disorders caused by DYSF gene mutations including limb-girdle muscular dystrophy type 2B (LGMD2B), Miyoshi myopathy and distal myopathy with anterior tibial onset (DMAT). About 70% of patients with dysferlinopathy have significant lymphocytic infiltration on the muscle biopsy samples [2] and 25% of the patients were initially diagnosed with polymyositis [5]. These findings highlight the clinical and pathological resemblances between dysferlinopathy and polymyositis. The recent report of an LGMD2B patient with Addison disease and sarcoidosis and the known susceptibility of SJL/J mice, the animal model of dysferlinopathy, to autoimmune disorders strengthen the link between dysferlin and the inflammatory mechanism [6, 7]. Despite the discovery of the role of dysferlin in membrane-repair machinery, it remains unclear how dysferlin mutations trigger these inflammatory changes [7]. It would be interesting if Suzuki et al. [1] could clarify whether antibody-negative LGMD patients included in their study carry dysferlin mutations. Although Selva-O’Callaghan et al. [6] reported the absence of myositis-specific autoantibodies in a single case of LGMD2B patient with Addison disease and sarcoidosis who initially was diagnosed with polymyositis, it needs a study in a larger scale before drawing any conclusions. Further study of myositis-specific autoantibodies in these particular myositis-mimicking muscular dystrophies would not only shed the light on the pathomechanism but also give the clue for the potential therapeutic strategies. Moreover, the screening for dysferlinopathy by either immunohistochemical study or western blot analysis in the steroid-unresponsive or autoantibody-negative polymyositis patients may identify a surprising number of patients with muscular dystrophies.

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