Electron microscopy in myofibrillar myopathies reveals clues to the mutated gene

We studied the ultrastructural characteristics in patients with myofibrillar myopathy (MFM) and differentiated between MFM-subtypes using electron microscopic (EM) findings. The ultrastructural findings in 19 patients with different genetically proven MFMs (9 desmin, 5 αB-crystallin, 3 ZASP, 2 myotilin) were analyzed. In one ZASPopathy, we additionally performed an immunoEM study, using antibodies against desmin, αB-crystallin, ZASP and myotilin. The ultrastructural findings in desminopathies and αB-crystallinopathies were very similar and consisted of electrondense granulofilamentous accumulations and sandwich formations. They differed in the obvious presence of early apoptotic nuclear changes in αB-crystallinopathies. ZASPopathies were characterized by filamentous bundles (labeled with the myotilin antibody on immunoEM), and floccular accumulations of thin filamentous material. Tubulofilamentous inclusions in sarcoplasm and myonuclei in combination with filamentous bundles were characteristic for myotilinopathies. We conclude that MFMs ultrastructural findings can direct diagnostic efforts towards the causal gene mutated, and that EM should be included in the diagnostic workup of MFMs.