Highly variable skeletal muscle histo-immunocytochemical and ultrastructural features in titin-related myopathies

Titin is the largest protein in humans, and is expressed in both skeletal and cardiac muscle. Mutations in the Titin gene (TTN) have been associated with a large spectrum of clinical conditions ranging from myopathies to isolated cardiomyopathy. Aiming at reevaluating the TTN-related skeletal muscle morphological, immunocytochemical and ultrastructural alterations, we analyzed 19 patients with heterogeneous phenotypes: (a) 2-Tibial muscle dystrophy (TMD); (b) 1-Limb girdle dystrophy (LGMD2J); (c) 4-Hereditary myopathy with early respiratory failure (HMERF); (d) 7-Retractile myopathies (RM); (e) 5-Neonatal onset myopathies with or without arthrogryposis. All patients harbored pathogenic AD or AR mutations located in different Titin domains. Variable histological findings were found: (a) mild structure disorganization and nuclear internalization in TMD and LGM2J; (b) cytoplasmic bodies (CB) ± rimmed vacuoles (RV) in HMERF; (c) mild or moderate necrosis/regeneration process (with RV and CB in 2/7 cases) in RM; (d) type 1 fiber predominance, with nuclear internalization and focal structural disorganizations (2/3 multicore) in neonatal onset myopathies. At ultrastructural level, we evidenced different changes: (a) Inclusions or aggregates (rods, tubular filamentous and CB; (b) Vacuoles (rimmed or autophagic); (c) Diverse sarcomeric disorganizations (focal areas of deranged sarcomeres, Z-line streaming, minicore or disruption of the central part of sarcomeres with Z-line preservation; (d) minor ultrastructural changes. Immunocytochemistry using titin-N-ter-antibodies did not show any abnormalities compared to healthy controls, demonstrating that the identified mutations did not impact on protein stability or localization. In summary, this study describes TTN-related myopathies associated with a wide spectrum of morphological findings. Based on these findings, we now aim to establish a genotype/phenotype correlation for each patient cohort with TTN mutations.