These observations suggest that the BAG3 variant of myofibrillar myopathy may result from a spontaneous mutation at an early point of embryonic development and that transmission from a mosaic parent may occur more than once.
To our knowledge this is the first study reporting, in a BAG3 MFM, the simultaneous presence of genetic variants in the BAG3 and FHL1 genes (previously described as independently associated with MFMs) and linking the NRAP gene to MFM for the first time.
Myofibrillar myopathies (MFM) are a group of disorders associated with mutations in DES, CRYAB, MYOT, ZASP, FLNC, or BAG3 genes and characterized by disintegration of myofibrils and accumulation of degradation products into intracellular inclusions.
Most of them were identified, as expected, on TTN (29 DCM probands), but truncating variants were also identified on myofibrillar myopathies causing genes in 17 DCM patients (7.7% of the DCM cohort): 10 variations on FLNC and 7 variations on BAG3 .
It has been documented that mutations in the human desmin gene lead to a severe type of myofibrillar myopathy, termed more specifically desminopathy, which affects cardiac and skeletal as well as smooth muscle.
One sporadic patient had a de novo R406W mutation in the desmin gene, and 1 patient with autosomal dominant MM had a single amino acid deletion at position 366 in the desmin gene.
Variants in the desmin gene (DES) are associated with desminopathy; a myofibrillar myopathy mainly characterized by muscle weakness, conduction block, and dilated cardiomyopathy.
Dependent on the MFM causing mutation, different sets of proteins were revealed as genuine (accumulated) plaque components in independent technical replicates: (i) αB-crystallin, desmin, filamin A/C, myotilin, PRAF3, RTN2, SQSTM, XIRP1, and XIRP2 (patient with defined MFM mutation distinct from FHL1) or (ii) desmin, FHL1, filamin A/C, KBTBD10, NRAP, SQSTM, RL40, XIRP1, and XIRP2 (patient with FHL1 mutation).
Three mutant desmin variants were detected directly on the protein level as components of the aggregates, suggesting their direct involvement in aggregate-formation and demonstrating for the first time that proteomic analysis can be used for direct identification of a disease-causing mutation in myofibrillar myopathy.
Desminopathies belong to a family of muscle disorders called myofibrillar myopathies that are caused by Desmin mutations and lead to protein aggregates in muscle fibers.
Here, we analyse myoblasts behaviour in the context of myofibrillar myopathy resulting from p.D399Ydesmin mutation which disorganizes the desmin IF network in muscle cells.
While MFMs are partly caused by mutations in genes encoding for extramyofibrillar proteins (desmin, alphaB-crystallin, plectin) or myofibrillar proteins (myotilin, Z-band alternatively spliced PDZ-containing protein, filamin C, Bcl-2-associated athanogene-3, four-and-a-half LIM domain 1), a large number of these diseases are caused by still unresolved gene defects.