BACKGROUND: Nonischemic cardiomyopathies are frequently caused by genetic mutations in about 100 different genes. The cardiomyopathy-associated gene DES encodes the intermediate filament protein desmin, which is important for the structural integrity of the cardiomyocytes. METHODS: Using a next-generation sequencing approach, we performed cascade screening of a previously identified heterozygous DES missense mutation (c.380G>C, p.R127P) segregating in a large 6-generation Kuwaiti family, where several members died from sudden cardiac death or developed different cardiomyopathies, partially in combination with conduction disease and atrial fibrillation. DES -p.R127P affects a highly conserved position and is absent or super rare in different genetic human population databases. In silico predictions support the pathogenicity of DES -p.R127P. To investigate the detrimental impact of desmin-p.R127P, we performed cell transfection experiments using different cell lines and cardiomyocytes derived from induced pluripotent stem cells in combination with confocal microscopy. RESULTS: These experiments demonstrated a severe desmin filament assembly defect leading to aberrant cytoplasmic desmin aggregates, even when co-expressed with wild-type desmin. Atomic force microscopy analysis supported the filament assembly defect of mutant recombinant desmin-p.R127P. To investigate the physicochemical impact of the amino acid at this position, we generated a set of 20 different variants and analyzed their filament formation in cell culture. Most of these variants disturbed the filament assembly comparable to p.R127P. CONCLUSIONS: In summary, we present and characterize a likely pathogenic missense mutation DES -p.R127P, which causes high cardiac mortality and morbidity in the described family. Our study has relevance for the interpretation and classification of further DES variants and might be helpful for the genetic counseling of patients and their relatives in future cases.

Background: The DES gene encodes the intermediate filament protein desmin, which connects different multi-protein complexes like the cardiac desmosomes and is highly important for the structural integrity of cardiomyocytes. Pathogenic DES -mutations cause filament assembly defects leading to cardiomyopathies. However, most DES -variants listed in genetic disease databases are currently classified as variants of unknown significance. Here, we functionally characterized 21 different DES -variants of unknown significance and 18 additional proline variants, localized in a highly conserved stretch at the C-terminus of the desmin coil-2 subdomain. Methods: We inserted desmin variants via site-directed-mutagenesis and investigated the filament assembly in transfected cell lines and cardiomyocytes derived from induced pluripotent stem cells by confocal microscopy. In addition, we purified recombinant wild-type and mutant desmin and analyzed the filament formation by atomic force microscopy. Co-expression with wild-type desmin delivered by adeno-associated virus was used to model the heterozygous status of cardiomyopathy patients. Results: Twelve DES -variants of unknown significance formed cytoplasmic aggregates, which was likewise verified by atomic force microscopy. Of note, these twelve variants disturb the filament assembly even when co-expressed with wild-type desmin. Using a proline screen, we showed that proline residues localized at nearly each of the positions in this stretch cause filament assembly defects. By modelling the tetrameric structure of desmin, we demonstrated that specific heptad positions as well as positions of intra- and intermolecular ion bridge sites are particularly susceptible mutations that promote desmin aggregation. Conclusion: In summary, our study demonstrated that the highly conserved stretch at the C-terminus of the coil-2 subdomain is a hotspot region, where several pathogenic DES -mutations cause an aberrant desmin aggregation. Based on our functional data we suggest to re-classify the aggregate-forming variants as likely-pathogenic mutations rather than variants of unknown significance. Our study may have relevance for the genetic counselling of cardiomyopathy patients with similar DES -variants.

BACKGROUND: TMEM43 (transmembrane protein 43) is a ubiquitously expressed 4-transmembrane-protein localized in the endoplasmic reticulum and nuclear lamina. The missense mutation TMEM43 -p.S358L causes fully penetrant ARVC5 (arrhythmogenic right ventricular cardiomyopathy type 5) especially in males. The TMEM43 function of the protein and the pathomechanisms of TMEM43-p.S358L remain poorly understood. We analyzed carrier-derived human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), human myocardial tissue from TMEM43-wild-type, and TMEM43-p.S358L and identified differentially interacting proteins. Here we provide evidence for a novel pathomechanism contributing to the onset of ARVC5. METHODS: Microsomes of human wild-type myocardium were separated by sucrose-gradient ultracentrifugation and characterized by mass-spectrometry to identify potential interacting proteins. Proteome and metabolome analyses of a TMEM43 -p.S358L explanted human myocardium were performed. hiPSC-derived cardiomyocytes of TMEM43 -p.S358L carrier and a corresponding isogenic control were generated. A 3’-end HA-Tag was introduced in TMEM43 for pull-down experiments under optimized conditions. Lipidomics, proteomics, contractility, and ATP-content were measured in hiPSC-CMs. RESULTS: Pull-down analyses of TMEM43-WT and mutant showed altered interacting proteins involved in metabolic pathways. Lipidomics revealed the accumulation of lipids and decreased lipid metabolism capacity in mutant hiPSC-CMs. The ATP to ADP ratio was lower in mutant hiPSC-CMs and could be associated with diminished contraction frequency. The human TMEM43 -p.S358L myocardial proteome revealed altered protein-expression of metabolic pathways comparable to mutant hiPSC-CMs. Metabolic remodeling was also found in the mutant human myocardium. Ultracentrifugation fraction with the highest protein amount of TMEM43 and pull-down experiments of hiPSC-CMs revealed differentially interacting proteins of TMEM43-p.S358L from endoplasmic reticulum and mitochondrial membranes. CONCLUSIONS: We suggest differential interaction of mutant TMEM43 with proteins of mitochondria and endoplasmic reticulum influences endoplasmic reticulum-mitochondrial contact sites. TMEM43 -p.S358L primarily contributes to changes in mitochondrial function affecting lipid homeostasis and energy supply.

DES encodes the muscle-specific intermediate filament protein desmin, which is highly relevant to the structural integrity of cardiomyocytes. Mutations in this gene cause different cardiomyopathies including dilated cardiomyopathy. Here, we functionally validate DES-p.L112Q using SW-13, H9c2 cells, and cardiomyocytes derived from induced pluripotent stem cells by confocal microscopy in combination with deconvolution analysis. These experiments reveal an aberrant cytoplasmic aggregation of mutant desmin. In conclusion, these functional analyses support the re-classification of DES-p.L112Q as a likely pathogenic variant leading to dilated cardiomyopathy.

DES encodes the muscle specific intermediate filament protein desmin and mutations in this gene cause different cardiomyopathies. Here, we functionally validate DES-p.L112Q using SW-13, H9c2 cells and cardiomyocytes derived from induced pluripotent stem cells by confocal microscopy. These experiments reveal an aberrant cytoplasmic aggregation of mutant desmin. In conclusion, these functional analyses support the re-classification of DES-p.L112Q as a likely pathogenic variant leading to dilated cardiomyopathy.

BACKGROUND: The DES gene encodes the IF (intermediate filament) protein desmin, which connects different multiprotein complexes, such as the cardiac desmosomes, and is highly important for the structural integrity of cardiomyocytes. Pathogenic DES mutations cause filament assembly defects leading to cardiomyopathies. However, most DES variants listed in genetic disease databases are currently classified as variants of unknown significance. Here, we characterized 21 different DES variants of unknown significance and 18 additional proline variants, localized in a highly conserved stretch at the C terminus of the desmin coil-2 subdomain. METHODS: We inserted desmin variants via site-directed mutagenesis and investigated the filament assembly in transfected cell lines and cardiomyocytes derived from induced pluripotent stem cells by confocal microscopy. In addition, we purified recombinant wild-type and mutant desmin and analyzed the filament formation by atomic force microscopy. Coexpression with wild-type desmin delivered by adeno-associated virus was used to model the heterozygous status of cardiomyopathy patients. RESULTS: Twelve DES variants of unknown significance formed cytoplasmic aggregates, which were likewise verified by atomic force microscopy. Of note, these 12 variants disturb the filament assembly even when coexpressed with wild-type desmin. Using a proline screen, we showed that proline residues localized at nearly each of the positions in this stretch cause filament assembly defects. By modeling the tetrameric structure of desmin, we demonstrated that specific heptad positions, as well as positions of intramolecular and intermolecular ion bridge sites, are particularly susceptible to mutations that promote desmin aggregation. CONCLUSIONS: In summary, our study demonstrated that the highly conserved stretch at the C terminus of the coil-2 subdomain is a hotspot region, where several pathogenic DES mutations cause an aberrant desmin aggregation. Based on our molecular data, we suggest reclassifying the aggregate-forming variants as likely pathogenic mutations rather than variants of unknown significance. Our study may have relevance for the genetic counseling of cardiomyopathy patients with similar DES variants.