Small deletions disturb desmin architecture leading to breakdown of muscle cells and development of skeletal or cardioskeletal myopathy.

Desmin ( DES) mutations have been recognized as a cause of desmin-related myopathy (OMIM 601419), or desminopathy, a disease characterized by progressive limb muscle weakness and accumulation of desmin-reactive granular aggregates in the myofibers. We have studied three families with skeletal or cardioskeletal myopathy caused by small in-frame deletions in the desmin gene. The newly identified in-frame deletions E359_S361del and N366del alter the heptad periodicity within a critical 2B coiled-coil segment. Structural analysis reveals that the E359_S361 deletion introduces a second stutter immediately downstream of the naturally occurring stutter, thus doubling the extent of the local coiled-coil unwinding. The N366del mutation converts the wild-type stutter into a different type of discontinuity, a stammer. A stammer, as opposed to a stutter, is expected to cause an extra overwinding of the coiled-coil. These mutations alter the coiled-coil geometry in specific ways leading to fatal damage to desmin filament assembly. Expression studies in two cell lines confirm the inability of desmin molecules with this changed architecture to polymerize into a functional filamentous network. This study provides insights into molecular pathogenetic mechanisms of desmin mutation-associated skeletal and cardioskeletal myopathy.

Respiratory insufficiency in desminopathy patients caused by introduction of proline residues in desmin c-terminal alpha-helical segment.

Mutations in desmin gene have been identified in patients with cardiac and skeletal myopathy characterized by intracytoplasmic accumulation of desmin-reactive deposits and electron-dense granular aggregates. We characterized two new desminopathy families with unusual features of adult-onset, slowly progressive, diffuse skeletal myopathy and respiratory insufficiency. Progressive reduction of respiratory muscle strength became clinically detectable between the 3rd and the 8th years of illness and led to recurrent chest infections and death in one of the patients. Novel mutations, A357P and L370P, predicted to introduce proline residue into a highly conserved alpha-helical region of desmin, were identified. Proline is known to disrupt the alpha-helix. In addition, the A357P mutation distorts a unique stutter sequence that is considered to be critically important for proper filament assembly. Functional assessment in two cell-lines, one of which does and the other of which does not constitutively produce type III intermediate filaments, demonstrated the inability of mutant desmin carrying either the A357P or the L370P mutation to polymerize and form an intracellular filamentous network. The results of this study indicate that respiratory insufficiency is an intrinsic feature of disease associated with specific desmin mutations; in some patients, respiratory weakness may present as a dominant clinical manifestation and a major cause of disability and death.

Progressive skeletal myopathy, a phenotypic variant of desmin myopathy associated with desmin mutations.

Desmin myopathy is a familial or sporadic disorder characterized by the presence of desmin mutations that cause skeletal muscle weakness associated with cardiac conduction block, arrhythmia and heart failure. Distinctive histopathologic features include intracytoplasmic accumulation of desmin-reactive deposits and electron-dense granular aggregates in skeletal and cardiac muscle cells. We describe two families with features of adult-onset slowly progressive skeletal myopathy without cardiomyopathy. N342D point mutation was present in the desmin helical rod domain in patients of family 1, and I451M mutation was found in the non-helical tail domain in patients of family 2. Of interest, the same I451M mutation has previously been reported in patients with cardiomyopathy and no signs of skeletal myopathy. Some carriers of the I451M mutation did not develop any disease, suggesting incomplete penetrance. Expression studies demonstrated inability of the N342D mutant desmin to form cellular filamentous network, confirming the pathogenic role of this mutation, but the network was not affected by the tail-domain I451M mutation. Progressive skeletal myopathy is a rare phenotypic variant of desmin myopathy allelic to the more frequent cardio-skeletal form.

Siderophore-mediated iron uptake in Saccharomyces cerevisiae: the SIT1 gene encodes a ferrioxamine B permease that belongs to the major facilitator superfamily.

Uptake of iron from various siderophores by a deltafet3deltafet4 strain of Saccharomyces cerevisiae was investigated. The catecholate enterobactin and the hydroxamate coprogen were taken up by the cells by passive diffusion, whereas the hydroxamates ferrioxamine B (FOB) and ferricrocin (FC) were taken up via a high-affinity energy-dependent mechanism. The kinetics of FOB and FC uptake showed reciprocal competitive inhibition. The transport was regulated by iron availability, but was independent of the Aft1p and Mac1p transcriptional activators. Mutants affected in the transport of FOB were isolated. The transport of FC was not impaired in these mutants. Functional complementation of one mutant allowed the identification of the SIT1 gene (Siderophore Iron Transport) encoding a putative permease belonging to the major facilitator superfamily. The Sit1 protein is probably a permease specific for the transport of ferrioxamine-type siderophores. The evidence suggests that the uptake of ferrichrome-type siderophores like FC involves other specific permease(s), although there seems to be a common handling of FOB and FC following their internalization by the cell.