Conspicuous involvement of desmin tail mutations in diverse cardiac and skeletal myopathies.

Myofibrillar myopathy (MFM) encompasses a genetically heterogeneous group of human diseases caused by mutations in genes coding for structural proteins of muscle. Mutations in the intermediate filament (IF) protein desmin (DES), a major cytoskeletal component of myocytes, lead to severe forms of "desminopathy," which affects cardiac, skeletal, and smooth muscle. Most mutations described reside in the central alpha-helical rod domain of desmin. Here we report three novel mutations--c.1325C>T (p.T442I), c.1360C>T (p.R454W), and c.1379G>T (p.S460I)--located in desmin's non-alpha-helical carboxy-terminal "tail" domain. We have investigated the impact of these and four--c.1237G>A (p.E413K), c.1346A>C (p.K449T), c.1353C>G (p.I451M), and c.1405G>A (p.V469M)--previously described "tail" mutations on in vitro filament formation and on the generation of ordered cytoskeletal arrays in transfected myoblasts. Although all but two mutants (p.E413K, p.R454W) assembled into IFs in vitro and all except p.E413K were incorporated into IF arrays in transfected C2C12 cells, filament properties differed significantly from wild-type desmin as revealed by viscometric assembly assays. Most notably, when coassembled with wild-type desmin, these mutants revealed a severe disturbance of filament-formation competence and filament-filament interactions, indicating an inherent incompatibility of mutant and wild-type protein to form mixed filaments. The various clinical phenotypes observed may reflect altered interactions of desmin's tail domain with different components of the myoblast cytoskeleton leading to diminished biomechanical properties and/or altered metabolism of the individual myocyte. Our in vitro assembly regimen proved to be a very sensible tool to detect if a particular desmin mutation is able to cause filament abnormalities.

Variable pathogenic potentials of mutations located in the desmin alpha-helical domain.

Mutations in the desmin gene have been recognized as a cause of desminopathy, a familial or sporadic disorder characterized by skeletal muscle weakness, often associated with cardiomyopathy or respiratory insufficiency. Distinctive histopathologic features include aberrant intracytoplasmic accumulation of desmin (DES). We present here comparative phenotypic, molecular, and functional characteristics of four novel and three previously reported, but not fully characterized, desmin mutations localized in desmin alpha-helical domain. The results indicate that the c.638C>T (p.A213V), c.1178A>T (p.N393I), and to some extent the c.1078G>C (p.A360P) mutations exhibit pathogenic potentials only if combined with other mutations in desmin or other genes and should therefore be considered conditionally pathogenic. The c.1009G>C (p.A337P), c.1013T>G (p.L338R), c.1195G>T (p.D399Y), and c.1201G>A (p.E401K) mutations make desmin filaments dysfunctional and are capable of causing disease. The pathogenic potentials of desmin mutations correlate with the type and location of the disease-associated mutations in the relatively large and structurally and functionally complex desmin molecule. Mutations within the highly conserved alpha-helical structures are especially damaging since the integrity of the alpha-helix is critical for desmin filament assembly and stability.