Mitochondrial and inflammatory changes in sporadic inclusion body myositis

Sporadic inclusion body myositis (sIBM) is the most common late onset muscle disease causing progressive weakness. In light of the lack of effective treatment, we investigated potential causes underlying muscle wasting. We hypothesized that accumulation of mitochondrial respiratory deficiency in muscle fibres may lead to fibre atrophy and degeneration, contributing to muscle mass reduction.

Histochemical and immunohistochemical analyses were performed on muscle biopsies from 16 sIBM patients to detect activity of mitochondrial enzymes and expression of mitochondrial respiratory chain proteins along with inflammatory markers respectively. Mitochondrial DNA mutations were assessed in single muscle fibres using real-time PCR.

We identified respiratory-deficient fibres at different stages of mitochondrial dysfunction, with downregulated expression of complex I of mitochondrial respiratory chain being the initial feature. We detected mitochondrial DNA rearrangements in the majority of individual respiratory-deficient muscle fibres. There was a strong correlation between number of T lymphocytes and macrophages residing in muscle tissue and the abundance of respiratory-deficient fibres. Moreover, we found that respiratory-deficient muscle fibres were more likely to be atrophic compared with respiratory-normal counterparts.

Our findings suggest that mitochondrial dysfunction has a role in sIBM progression. A strong correlation between the severity of inflammation, degree of mitochondrial changes and atrophy implicated existence of a mechanistic link between these three parameters. We propose a role for inflammatory cells in the initiation of mitochondrial DNA damage, which when accumulated, causes respiratory dysfunction, fibre atrophy and ultimately degeneration of muscle fibres.