Role of intermediate filament desmin in development of desmin-related myopathy

Sammanfattning: Desmin is a major intermediate filament of muscle cells, serving to transmit mechanical forces and propagate mechanochemical signals, to coordinate contraction and relaxation cycles, and to stabilize the positioning of cellular organelles, e.g. mitochondria. Around 70 desmin gene mutations have been reported in conjunction with desmin-related myopathy. Desmin-related myopathy can be described as pathophysiological complex, accompanied by desmin intracellular aggregate accumulation and impairment of desmin interactions with structural proteins, signal molecules, and cell organelles. However, the precise molecular mechanism underlying desmin-related myopathy have not been described yet. There are speculations if it is connected with toxic effects of desmin aggregates or with violation of desmin mechanotransduction functions. The general aim of the present PhD project was to extend existing knowledge about the molecular machinery on how desmin gene mutations lead to the development of desmin-related myopathy, with an emphasis on development of cardiomyopathies. To address this aim the following research questions were stated: (i) genetic study of a group of patients with cardiomyopathies in order to describe novel mutations in the desmin gene, and to assess the frequency of DES A213V; (ii) genetic study by means of next-generation sequencing approach of a group of patients with idiopathic restrictive cardiomyopathy in order to describe novel genetic variants associated with disease; (iii) functional study of desmin gene point mutations effect on mitochondrial properties. The main findings regarding genetic background were: (i) DES A213V represents a disease-modifying polymorphism, rather than disease-related mutation, since it was found both in patients and healthy donors; (ii) combination of disease-related– disease-modifying or disease-related–disease-related genetic variants, rather than single disease-related mutation, determined the development of idiopathic restrictive cardiomyopathy. Most proteins of these combinations belonged to four functional groups: sarcomeric contractile proteins, mechanosensing Z-disc proteins, nuclear membrane, and outer mitochondrial membrane proteins. Functional studies of the impact of desmin mutations on mitochondria showed that aggregate-prone mutations decreased mitochondrial calcium uptake, as well as depressed maximal oxygen consumption rate and spare respiratory capacity. In contrast, non-aggregate-prone mutations did not disturb mitochondrial calcium. They did, however, result in the reduction of maximal oxygen consumption rate and affected spare respiratory capacity. To conclude, (i) distortion of desmin mechanotransduction functions plays an important role in desmin-related myopathy onset, affecting mitochondrial properties; (ii) combination of mutations in genes encoding sarcomeric contractile and mechanosensing proteins, rather than a single mutation, predisposes to the development of cardiomyopathy. These data facilitate understanding of molecular pathways underlying desmin-related myopathy development, and increase existing knowledge of intracellular interactions within the muscle cell.