HNRNPDL-related muscular dystrophy: expanding the clinical, morphological and MRI phenotypes.

Autosomal dominant limb girdle muscular dystrophy D3 HNRNPDL-related is a rare dominant myopathy caused by mutations in HNRNPDL. Only three unrelated families have been described worldwide, a Brazilian and a Chinese carrying the mutation c.1132G>A p.(Asp378Asn), and one Uruguayan with the mutation c.1132G>C p. (Asp378His), both mutations occurring in the same codon. The present study enlarges the clinical, morphological and muscle MRI spectrum of AD-HNRNPDL-related myopathies demonstrating the significant particularities of the disease. We describe two new unrelated Argentinean families, carrying the previously reported c.1132G>C p.(Asp378His) HNRNPDL mutation. There was a wide phenotypic spectrum including oligo-symptomatic cases, pure limb girdle muscle involvement or distal lower limb muscle weakness. Scapular winging was the most common finding, observed in all patients. Muscle MRIs of the thigh, at different stages of the disease, showed particular involvement of adductor magnus and vastus besides a constant preservation of the rectus femoris and the adductor longus muscles, defining a novel MRI pattern. Muscle biopsy findings were characterized by the presence of numerous rimmed vacuoles, cytoplasmic bodies, and abundant autophagic material at the histochemistry and ultrastructural levels. HNRNPDL-related LGMD D3 results in a wide range of clinical phenotypes from the classic proximal form of LGMD to a more distal phenotype. Thigh MRI suggests a specific pattern. Codon 378 of HNRNPDL gene can be considered a mutation hotspot for HNRNPDL-related myopathy. Pathologically, the disease can be classified among the autophagic rimmed vacuolar myopathies as with the other multisystem proteinopathies.

Dynamin-2 mutations linked to Centronuclear Myopathy impair actin-dependent trafficking in muscle cells.

Dynamin-2 is a ubiquitously expressed GTP-ase that mediates membrane remodeling. Recent findings indicate that dynamin-2 also regulates actin dynamics. Mutations in dynamin-2 cause dominant centronuclear myopathy (CNM), a congenital myopathy characterized by progressive weakness and atrophy of skeletal muscles. However, the muscle-specific roles of dynamin-2 affected by these mutations remain elusive. Here we show that, in muscle cells, the GTP-ase activity of dynamin-2 is involved in de novo actin polymerization as well as in actin-mediated trafficking of the glucose transporter GLUT4. Expression of dynamin-2 constructs carrying CNM-linked mutations disrupted the formation of new actin filaments as well as the stimulus-induced translocation of GLUT4 to the plasma membrane. Similarly, mature muscle fibers isolated from heterozygous knock-in mice that harbor the dynamin-2 mutation p.R465W, an animal model of CNM, exhibited altered actin organization, reduced actin polymerization and impaired insulin-induced translocation of GLUT4 to the sarcolemma. Moreover, GLUT4 displayed aberrant perinuclear accumulation in biopsies from CNM patients carrying dynamin-2 mutations, further suggesting trafficking defects. These results suggest that dynamin-2 is a key regulator of actin dynamics and GLUT4 trafficking in muscle cells. Our findings also support a model in which impairment of actin-dependent trafficking contributes to the pathological mechanism in dynamin-2-associated CNM.