Expression of transglutaminase 2 does not differentiate focal myositis from generalized inflammatory myopathies

Objectives - Idiopathic inflammatory myopathies (IIM), including dermatomyositis (DM), polymyositis (PM), sporadic inclusion-body myositis (s-IBM) and focal myositis (FM) are a heterogeneous group of autoimmune disorders of skeletal muscle. An increased transglutaminase 2 (TG2) expression has been found in DM, PM and s-IBM. The aim of our study was to investigate TG2 expression in FM in comparison with other IIM. Materials and methods - We re-examined tissue material we have gathered in the course of our previous studies on IIM, investigating muscle expression of TG2 in patients with FM in comparison with DM, PM and s-IBM using immunocytochemistry and real-time RT-PCR. Results - Immunocytochemistry revealed an increased TG2 signal in endomysial vessels, in atrophic and degenerating/regenerating muscle fibres in PM, DM, s-IBM and FM; in s-IBM, some vacuoles were immunostained too. Real-time RT-PCR study confirmed a significantly increased expression of TG2 in all IIM muscles examined. Conclusions - Our study demonstrates the presence of TG2 in FM muscles. The study suggests that TG2 expression does not represent a distinctive marker to differentiate FM from generalized IIM. TG2 over-expression in inflamed skeletal muscle does not seem have a pathogenetic role in such a disease, but it could represent a way to contain the inflammatory process.