Progress in desmin-related myopathies

Clinical, immunohistochemical, and genetic characterization of splice-altering biallelic DES variants: Therapeutic implications

Pathogenic variants in the DES gene clinically manifest as progressive skeletal muscle weakness, cardiomyopathy with associated severe arrhythmias, and respiratory insufficiency, and are collectively known as desminopathies. While most DES pathogenic variants act via a dominant mechanism, recessively acting variants have also been reported. Currently, there are no effective therapeutic interventions for desminopathies of any type. Here, we report an affected individual with rapidly progressive dilated cardiomyopathy, requiring heart transplantation at age 13 years, in the setting of childhood-onset skeletal muscle weakness. We identified biallelic DES variants (c.640-13 T>A and c.1288+1 G>A) and show aberrant DES gene splicing in the affected individual's muscle. Through the generation of an inducible lentiviral system, we transdifferentiated fibroblast cultures derived from the affected individual into myoblasts and validated this system using RNA sequencing. We tested rationally designed, custom antisense oligonucleotides to screen for splice correction in these transdifferentiated cells and a functional minigene splicing assay. However, rather than correctly redirecting splicing, we found them to induce undesired exon skipping. Our results indicate that, while an individual precision-based molecular therapeutic approach to splice-altering pathogenic variants is promising, careful preclinical testing is imperative for each novel variant to test the feasibility of this type of approach for translation.

Influences of desmin and keratin 19 on passive biomechanical properties of mouse skeletal muscle

In skeletal muscle fibers, forces must be transmitted between the plasma membrane and the intracellular contractile lattice, and within this lattice between adjacent myofibrils. Based on their prevalence, biomechanical properties and localization, desmin and keratin intermediate filaments (IFs) are likely to participate in structural connectivity and force transmission. We examined the passive load-bearing response of single fibers from the extensor digitorum longus (EDL) muscles of young (3 months) and aged (10 months) wild-type, desmin-null, K19-null, and desmin/K19 double-null mice. Though fibers are more compliant in all mutant genotypes compared to wild-type, the structural response of each genotype is distinct, suggesting multiple mechanisms by which desmin and keratin influence the biomechanical properties of myofibers. This work provides additional insight into the influences of IFs on structure-function relationships in skeletal muscle. It may also have implications for understanding the progression of desminopathies and other IF-related myopathies.

Primary desminopathies

Mutations of the human desmin gene on chromosome 2q35 cause a familial or sporadic form of skeletal myopathy frequently associated with cardiac abnormalities. Skeletal and cardiac muscle from patients with primary desminopathies characteristically display cytoplasmic accumulation of desmin-immunoreactive material and myofibrillar changes. However, desmin-positive protein aggregates in conjunction with myofibrillar abnormalities are also the morphological hallmark of the large group of secondary desminopathies (synonyms: myofibrillar myopathies, desmin-related myopathies), which comprise sporadic and familial neuromuscular conditions of considerable clinical and genetic heterogeneity. Here, we will give an overview on the functional role of desmin in striated muscle as well as the main clinical, myopathological, genetic and patho-physiological aspects of primary desminopathies. Furthermore, we will discuss recent genetic and biochemical advances in distinguishing primary from secondary desminopathies.

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.

Protein aggregate myopathies

Protein aggregate myopathies (PAMs) based on the morphologic phenomenon of aggregation of proteins within muscle fibers may occur in children (selenoproteinopathies, actinopathies, and myosinopathies) or adults (certain myofibrillar myopathies and myosinopathies). They may be mutation related, which includes virtually all childhood forms but certain other forms as well, or sporadic, which are largely seen in adults. Their classification as myofibrillar or desmin-related myopathies, actinopathies, or myosinopathies is based on the identification of respective mutant proteins, most of them components of the sarcomeres. Recognition of PAM requires muscle biopsy and an extensive immunohistochemical and electron microscopic workup of the biopsied muscle tissue after which molecular analysis of morphologically ascertained proteins should ensue to permit recognition of individual entities and genetic counseling of patients and families. Because pathogenetic principles in PAMs are still incompletely known, causative therapy, at this time, is not available.

Diagnostic immunohistochemistry in neuromuscular disorders

Most neuromuscular disorders display only non-specific myopathological features in routine histological preparations. However, a number of proteins, including sarcolemmal, sarcomeric, and nuclear proteins as well as enzymes with defects responsible for neuromuscular disorders, have been identified during the past two decades, allowing a more specific and firm diagnosis of muscle diseases. Identification of protein defects relies predominantly on immunohistochemical preparations and on Western blot analysis. While immunohistochemistry is very useful in identifying abnormal expression of primary protein abnormalities in recessive conditions, it is less helpful in detecting primary defects in dominantly inherited disorders. Abnormal immunohistochemical expression patterns can be confirmed by Western blot analysis which may also be informative in dominant disorders, although its role has yet to be established. Besides identification of specific protein defects, immunohistochemistry is also helpful in the differentiation of inflammatory myopathies by subtyping cellular infiltrates and demonstrating up-regulation of subtle immunological parameters such as cell adhesion molecules. The role of immunohistochemistry in denervating disorders, however, remains controversial in the absence of a reliable marker of muscle fibre denervation. Nevertheless, as well as the diagnostic value of immunocytochemical analysis it may also widen understanding of muscle fibre pathology as well as help in the development of therapeutic strategies.

The enlarging spectrum of desminopathies: new morphological findings, eastward geographic spread, novel exon 3 desmin mutation

A 52-year-old man, who had developed distal muscle weakness in legs and arms, was found to have distal muscle atrophy as well as cardiac arrhythmia. His 10-year younger brother developed restrictive cardiomyopathy at the age of 20 years, which required cardiac transplantation at the age of 41 years. Skeletal muscle biopsy specimens of the older brother revealed granulofilamentous material and plaques containing numerous proteins, foremost desmin, as did cardiac biopsy tissue. The explanted heart of the younger brother showed similar protein-rich plaques and granulofilamentous material within cardiac myocytes. A novel heterozygous Glu245Asp (E245D) missense mutation in exon 3 of the desmin gene (DES) at 2q35 was found in the older brother. While clinical data and muscle biopsy pathology of the older brother conform to the nosological spectrum of desminopathies, the early-onset cardiomyopathy, a similar cardiac pathology as in skeletal muscle tissues and a novel missense mutation in the DES gene, enlarge the nosological spectrum of desminopathies.

Congenital myopathies in the new millennium

Few medical disciplines have benefited so enormously from the molecular revolution as myology. Whereas the congenital myopathies have flourished from enzyme histochemistry and electron microscopy, defining individual congenital myopathies by structural abnormalities, genetic research has only recently focused on congenital myopathies. However, a number of congenital myopathies have been molecularly elucidated: central and multiminicore diseases, nemaline myopathy, myotubular myopathy, and congenital myopathy marked by aggregation of proteins, giving rise to the concept of protein aggregate myopathies, to which now desminopathies, alpha-B crystallinopathies, selenoproteinopathy, myotilinopathy, actinopathies, and myosinopathies belong. Based on recent identification of mutations in respective genes, the principle "from morphology, that is, immunohistochemistry, to molecular analysis" through recognition of certain accrued proteins within muscle fibers and subsequent analysis of their respective genes has resulted in a wealth of genetic data and in reconsidering classification and nosologic interpretation of certain congenital myopathies. This heuristic principle needs to be further applied to other genetically still obscure congenital myopathies.

Desmin aggregate formation by R120G alphaB-crystallin is caused by altered filament interactions and is dependent upon network status in cells

The R120G mutation in alphaB-crystallin causes desmin-related myopathy. There have been a number of mechanisms proposed to explain the disease process, from altered protein processing to loss of chaperone function. Here, we show that the mutation alters the in vitro binding characteristics of alphaB-crystallin for desmin filaments. The apparent dissociation constant of R120G alphaB-crystallin was decreased while the binding capacity was increased significantly and as a result, desmin filaments aggregated. These data suggest that the characteristic desmin aggregates seen as part of the disease histopathology can be caused by a direct, but altered interaction of R120G alphaB-crystallin with desmin filaments. Transfection studies show that desmin networks in different cell backgrounds are not equally affected. Desmin networks are most vulnerable when they are being made de novo and not when they are already established. Our data also clearly demonstrate the beneficial role of wild-type alphaB-crystallin in the formation of desmin filament networks. Collectively, our data suggest that R120G alphaB-crystallin directly promotes desmin filament aggregation, although this gain of a function can be repressed by some cell situations. Such circumstances in muscle could explain the late onset characteristic of the myopathies caused by mutations in alphaB-crystallin.

Actin-related myopathy without any missense mutation in the ACTA1 gene

Actinopathies are defined by missense mutations in the ACTA1 gene coding for sarcomeric actin, of which some 70 families have, so far, been identified. Often, but not always, muscle fibers carry large patches of actin filaments. Many such patients also have nemaline myopathy, qualifying actinopathies as a subgroup of nemaline myopathies. This article concerns a then newborn, now 2 1/2-year-old boy, the first and single child of nonconsanguineous parents, who was born floppy, requiring immediate postnatal assisted ventilation. A quadriceps muscle biopsy revealed large patches of thin myofilaments reacting at light and electron microscopic levels with antibodies against actin but only a few sarcoplasmic rods and no intranuclear rods. DNA analysis of the patient's and both parents' blood did not reveal any missense mutation in the ACTA1 gene. Thus, this congenital myopathy can be caused by a new type of ACTA1 gene mutation, a new non-ACTA1 gene mutation, or no mutation at all, designating it as an actin-related myopathy, perhaps a new type of congenital myopathy and a new member of protein aggregate myopathies marked by aggregation of proteins within muscle fibers, among them desminopathies, alpha-beta crystallinopathies, other desmin-related myopathies (also termed myofibrillar myopathies), actinopathies and, now, actin-related myopathies.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

Roles of mu-calpain in cultured L8 muscle cells: application of a skeletal muscle-specific gene expression system

The goal of this work was to characterize the roles of mu-calpain in skeletal muscle protein degradation. Three approaches were developed to alter mu-calpain activity in rat myotubes. These included over-expression of antisense mu-calpain (mu-AS), dominant negative mu-calpain (mu-DN) and the antisense 30-kDa calpain subunit (30-AS). Constructs were expressed in rat L8 myotubes, and their effects on protein degradation and on concentrations of intact and/or degraded fodrin, desmin and tropomyosin were examined. An ecdysone-inducible expression system, in which we replaced a constitutively active CMV promoter with a skeletal muscle-specific alpha-actin promoter, was used to drive expression. Cell lines were evaluated by expression of the gene-of-interest following addition of ponasterone A (PA; ecdysone analog) to culture medium. Changes in calpain activity were assessed by evaluating fodrin degradation. 30-AS, which should alter both mu- and m-calpain activities, increased intact fodrin concentration. mu-DN and mu-AS reduced fodrin degradation products. mu-DN reduced total protein degradation by 7.9% (P<0.01) at 24 h and by 10.6% (P<0.01) at 48 h. mu-AS reduced total protein degradation by 6.4% at 24 h (P<0.05). 30-AS reduced total protein degradation by 13.4% (P<0.05) and 7.3% (P<0.05) following 24 and 48 h of PA administration, respectively. We assessed effects of mu-DN, mu-AS and 30-AS on concentrations of desmin and tropomyosin. Inhibition of calpains stabilized desmin, but had no effect on tropomyosin. These data indicate that fodrin and desmin are mu-calpain substrates and that mu-calpain accounts for a small proportion of total protein degradation in muscle cells. Tropomyosin is not degraded by calpain in muscle cells.

Filamin C accumulation is a strong but nonspecific immunohistochemical marker of core formation in muscle

Filamin C is the muscle isoform of a group of large actin-crosslinking proteins. On the one hand, filamin C is associated with the Z-disk of the myofibrillar apparatus and binds to myotilin; on the other hand, it interacts with the sarcoglycan complex at the sarcolemma. Filamin C may be involved in reorganizing the cytoskeleton in response to signalling events and in muscle it may, in addition, fulfill structural functions at the Z-disk. An examination of biopsies from patients with multi-minicore myopathy, central core myopathy and neurogenic target fibers with core-like target formations (TF) revealed strong reactivity of all the cores and target formations with two different anti-filamin C antibodies. In all three conditions, the immunoreactivity in the cores for filamin C was considerably stronger than that for desmin. Only for alphaB-crystallin were comparable levels of immunoreactivity detected. There was no difference in intensity for filamin C between the three pathological conditions. Thus, filamin C along with alphaB-crystallin is a strong and robust, but nonspecific marker of core formation. The reason why filamin C accumulates in cores is unclear at present, but we postulate that it may be critically involved in the chain of events eventually leading to myofibrillar degeneration.

Protein kinase C-mediated desmin phosphorylation is related to myofibril disarray in cardiomyopathic hamster heart

The cardiomyopathic (CM) Syrian golden hamster (strain UM-X7.1) exhibits a hereditary cardiomyopathy, which causes premature death resulting from congestive heart failure. The CM animals show extensive cardiac myofibril disarray and myocardial calcium overload. The present study has been undertaken to examine the role of desmin phosphorylation in myofibril disarray observed in CM hearts. The data from skinned myofibril protein phosphorylation assays have shown that desmin can be phosphorylated by protein kinase C (PKC). There is no significant difference in the content of desmin between CM and control hamster hearts. However, the desmin from CM hearts has a higher phosphorylation level than that of the normal hearts. Furthermore, we have examined the distribution of desmin and myofibril organization with immunofluorescent microscopy and immunogold electron microscopy in cultured cardiac myocytes after treatment with the PKC-activating phorbol ester, 12-O-tetradecanylphorbol-13-acetate (TPA). When the cultured normal hamster cardiac cells are treated with TPA, desmin filaments are disassembled and the myofibrils become disarrayed. The myofibril disarray closely mimics that observed in untreated CM cultures. These results suggest that disassembly of desmin filaments, which could be caused by PKC-mediated phosphorylation, may be a factor in myofibril disarray in cardiomyopathic cells and that the intermediate filament protein, desmin, plays an important role in maintaining myofibril alignment in cardiac cells.

Human synemin gene generates splice variants encoding two distinct intermediate filament proteins

Intermediate filament (IF) proteins are constituents of the cytoskeleton, conferring resistance to mechanical stress, and are encoded by a dispersed multigene family. In man we have identified two isoforms (180 and 150 kDa) of the IF protein synemin. Synemin alpha and beta have a very short N-terminal domain of 10 amino acids and a long C-terminal domain consisting of 1243 amino acids for the alpha isoform and 931 amino acids for the beta isoform. An intronic sequence of the synemin beta isoform is used as a coding sequence for synemin alpha. Both mRNA isoforms (6.5 and 7.5 kb) result from alternative splicing of the same gene, which has been assigned to human chromosome 15q26.3. Analyses by Northern and Western blot revealed that isoform beta is the predominant isoform in striated muscles, whereas both isoforms (alpha and beta) are present in almost equal quantities in smooth muscles. Co-transfection and immunolabeling experiments indicate that both synemin isoforms are incorporated with desmin to form heteropolymeric IFs. Furthermore synemin and desmin are found aggregated together in certain pathological situations.

Congenital myopathies and congenital muscular dystrophies

Congenital myopathies and congenital myopathic dystrophies are distinct groups of inherited diseases of muscle, genetically heterogeneous, that manifest in early life or infancy. Congenital myopathic dystrophy is characterized by a dystrophic pattern, whereas no necrotic or degenerative changes are present in congenital myopathies. Much progress has been made in recent years in clarifying the classification of the congenital myopathies. This is a clinically and genetically heterogeneous group of conditions originally classified according to unique morphological changes seen in muscle. Not unlike the later-onset muscular dystrophies, the discovery of the genetic aetiology of many of the congenital myopathies has led to a revamping of how these conditions can now be diagnosed and this should enable physicians to give a more accurate prognosis to patients and their families. New mutations in the ryanodine receptor, slow tropomyosin, troponin T1, actin, and nebulin genes have been described in the last 2 years. Clinical and genetic guidelines for conditions like nemaline rod myopathy and central core disease have been suggested. The notion of minus and surplus protein myopathies has been developed. Several groups of congenital myopathic dystrophy have been identified. In the first category, without intellectual impairment or major structural brain abnormalities, half of the cases are merosin deficient due to mutations of the laminin alpha 2 chain gene. If generally the muscular phenotype is severe, mild allelic variants have been reported with early onset dystrophies and partial merosin deficiency. Among other pure congenital myopathic dystrophies unlinked to the laminin alpha 2 gene, one form has been assigned to chromosome 1q42. In the group of congenital myopathic dystrophies associated with mental retardation and structural brain abnormalities, two main entities are genetically characterized: (1) Fukuyama congenital myopathic dystrophy, affecting the Japanese population, is due to fukutin gene mutations, and (2) the muscle eye brain syndrome assigned to chromosome 1p32-34. In several cases, the gene localization remains unknown.