Differential involvement of sarcomeric proteins in myofibrillar myopathies: a morphological and immunohistochemical study

Myofibrillar myopathies (MFMs) are rare inherited or sporadic progressive neuromuscular disorders with considerable clinical and genetic heterogeneity. In the current study, we have analyzed histopathological and immunohistochemical characteristics in genetically identified MFMs. We performed a morphological and morphometrical study in a cohort of 24 genetically identified MFM patients (12 desmin, 6 alpha B-crystallin, 4 ZASP, 2 myotilin), and an extensive immunohistochemical study in 15 of these patients, using both well-known and novel antibodies directed against distinct compartments of the muscle fibers, including Z-disc and M-band proteins. Our morphological data revealed some significant differences between the distinct MFM subgroups: the consistent presence of 'rubbed-out' fibers in desminopathies and alpha B-crystallinopathies, an elevated frequency of vacuoles in ZASPopathies and myotilinopathies, and the presence of a few necrotic fibers in the two myotilinopathy patients. Immunohistochemistry showed that in MFM only a subset of Z-disc proteins, such as filamin C and its ligands myotilin and Xin, exhibited significant alterations in their localization, whereas other Z-disc proteins like alpha-actinin, myopodin and tritopodin, did not. In contrast, M-band proteins revealed no abnormalities in MFM. We conclude that the presence of 'rubbed-out' fibers are a suggestive feature for desminopathy or alpha B-crystallinopathy, and that MFM is not a general disease of the myofibril, but primarily affects a subgroup of stress-responsive Z-disc proteins.