Mitochondrial abnormalities in myofibrillar myopathies

Histological mitochondrial changes are generally found to be associated with late onset myofibrillar myopathies (MFMs). How these changes contribute to the pathogenesis of MFMs is unknown. Mitochondrial changes, including COX-deficient fibers (n = 8), biochemical activities of respiratory chain complexes (n = 7), and multiple mtDNA deletions by long-range PCR (n = 9) were examined in patients with genetically confirmed MFMs [MYOT (n = 2), DES (n = 1), ZASP (n = 2), FLNC (n = 4)] and compared with age and sex matched normal controls (n = 27) and patients with a mitochondrial disorder with multiple mtDNA deletions due to nuclear genetic defects (n = 8). In 2 MFM patients, micro dissected fibers were analyzed for multiple mtDNA deletions by nested long-range PCR. The COX-deficient fibers only partly corresponded with fibers containing myofibrillar accumulations. In total, there was no difference in the percentage of COX-deficient fibers in MFM patients and normal controls. However, the percentage of COX-deficient fibers was significantly higher in 3 MFM patients. Two MFM patients but none of the controls had multiple mtDNA deletions. Nested long-range PCR detected multiple mtDNA deletions only in COX-deficient fibers. Citrate synthase activities in MFM patients were 1.5-fold increased by compared to those in controls, suggesting initiation of mitochondrial alterations. However, it is unclear whether this is a direct consequence of MFM pathology. *both authors contributed equally to the manuscript.