*2.5. Histopathology, Immunohistochemistry, Desmin Western Blot, and Electron Microscopy*

In 5 probands (P1–P4, P6), formalin-fixed paraffin-embedded samples of myocardium were available either from endomyocardial biopsy (P2, P3) and/or from hearts explanted during transplantation (P1, P2, P6) or post-mortem (P4). The samples were snap frozen in liquid nitrogen and stored at −70 ◦C. Resin-embedded myocardial samples for electron microscopy were analyzed in 4 patients (P1, P2, P4, and P6). A biopsy of skeletal muscle was performed in 3 individuals with clinical signs of myopathy (P4 and P5: Soleus-, P6: Deltoid muscle). In P4, also we obtained samples of intercostal muscles post-mortem. The excisions from the skeletal muscle (approx. 10 × 5 × 5 mm in size) were snap frozen in isopentane (2-methylbutane; Merck, Kenilworth, NJ, USA) and cooled in liquid nitrogen. Cryosections were examined by routine hematoxylin–eosin staining and a conventional spectrum of histochemical reactions, including myofibrillary ATPase, nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR), succinate dehydrogenase (SDH), and cytochrome c oxidase (COX), as described elsewhere [23].

Desmin immunohistochemistry and electron microscopy were performed on both skeletal muscle and myocardium samples according to standard protocols (Supplementary Materials).
