Sporadic inclusion body myositis (sIBM) is a
late onset disorder of unkown aetiology. Mitochondrial changes such as
cytochrome oxidase deficient fibres are a well recognised feature and
mitochondrial DNA (
mtDNA) deletions have also been reported, but not consistently. Since
mtDNA deletions are not present in all cases, we investigated whether other types of
mtDNA abnormality were responsible for the mitochondrial changes. We studied 9 patients with sIBM. To control for fibre loss or replacement with inflammatory cells, we compared sIBM patients with necrotising
myopathy (n = 4) as well as with healthy controls. Qualitative anlysis for
mtDNA deletions and quantitative measurement of
mtDNA copy number showed that muscle from patients with sIBM contained on average 67% less
mtDNA than healthy controls (P = 0.001). The level of
mtDNA was also significantly depleted in sIBM when compared to necrotising
myopathy. No significant difference in copy number was seen in patients with necrotising
myopathy compared to controls. Deletions of
mtDNA were present in 4 patients with sIBM, but not all. Our findings suggest that
mtDNA depletion is a more consistent finding in sIBM, and one that may be implicated in the pathogenesis of the disease.