Spheroid body myopathy revisited

An Italian case of hereditary myopathy with early respiratory failure (HMERF) not associated with the titin kinase domain R279W mutation

Hereditary myopathy with early respiratory failure (HMERF) is a rare disorder characterized by severe respiratory involvement at onset, muscle weakness starting in the early adulthood, and cytoplasmic bodies with peculiar immunohistochemical reactivity on muscle biopsy. Here we describe a patient who presented with hypercapnic coma at age 32. A detailed light and electron microscopy analysis on muscle biopsy was performed and, together with clinical data, led to the diagnosis. The R279W mutation in the TTN gene was excluded. This report expands the geographical region of incidence and encourages additional studies to clarify the genetic heterogeneity of the condition.

Transgenic mice expressing the myotilin T57I mutation unite the pathology associated with LGMD1A and MFM

Myotilin is a muscle-specific Z-disc protein with putative roles in myofibril assembly and structural upkeep of the sarcomere. Several myotilin point mutations have been described in patients with limb-girdle muscular dystrophy type 1A (LGMD1A), myofibrillar myopathy (MFM), spheroid body myopathy (SBM), three similar adult-onset, progressive and autosomal dominant muscular dystrophies. To further investigate myotilin's role in the pathogenesis of these muscle diseases, we have characterized three independent lines of transgenic mice expressing mutant (T57I) myotilin under the control of the human skeletal actin promoter. Similar to LGMD1A and MFM patients, these mice develop progressive myofibrillar pathology that includes Z-disc streaming, excess myofibrillar vacuolization and plaque-like myofibrillar aggregation. These aggregates become progressively larger and more numerous with age. We show that the mutant myotilin protein properly localizes to the Z-disc and also heavily populates the aggregates, along with several other Z-disc associated proteins. Whole muscle physiological analysis reveals that the extensor digitorum longus muscle of transgenic mice exhibits significantly reduced maximum specific isometric force compared with littermate controls. Intriguingly, the soleus and diaphragm muscles are spared of any abnormal myopathology and show no reductions in maximum specific force. These data provide evidence that myotilin mutations promote aggregate-dependent contractile dysfunction. In sum, we have established a promising patho-physiological mouse model that unifies the phenotypes of LGMD1A, MFM and SBM.

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.

Congenital myopathies

Most congenital myopathies have been defined on account of the morphological findings in enzyme histochemical preparations. In effect, the diagnosis of this group of diseases continues to be made on the histological pattern of muscle biopsies. However, progress has been made in elucidating the molecular genetic background of several of the congenital myopathies. In this updated review we address those congenital myopathies for which gene defects and mutant proteins have been found (central core disease, nemaline myopathies, desminopathy, actinopathy, certain vacuolar myopathies, and myotubular myopathy) and the other disease with central nuclei (centronuclear myopathy).

Cytoplasmic body myopathy masquerading as motor neuron disease

Cytoplasmic body myopathy (CBM) is characterized by proteinaceous inclusion bodies in muscle tissue. A 43-year-old woman presented with rapidly progressive weakness and dysphagia. Electromyography (EMG) elsewhere demonstrated lower-limb chronic partial denervation. Muscle biopsy showed fiber size variation without diagnostic features. A diagnosis of possible motor neuron disease was made and the patient was commenced on riluzole. Subsequently, the patient's condition stabilized, prompting reassessment. Repeat EMG demonstrated no features of denervation and was more suggestive of a myopathic process. Review of the original muscle biopsy showed cytoplasmic bodies. The case highlights a further diagnostic possibility in the assessment of patients with "possible" motor neuron disease. The clinical features of CBM are briefly reviewed.

Desmin-related myopathy: clinical, electrophysiological, radiological, neuropathological and genetic studies

Ten Spanish patients from six unrelated families diagnosed with desmin-related myopathy (DRM) were studied. The pattern of DRM inheritance was autosomal dominant in three families, autosomal recessive in one, and there was no family history in two cases. The disease onset was in early adulthood. Cardiac myopathy was the initial presentation in two patients, respiratory insufficiency in one, and lower limb weakness in all others. Cardiac involvement was observed in four patients. Lens opacities were found in four. CK level was normal or slightly elevated, and electrophysiological examination was consistent with myopathy. Muscle biopsies identified intracytoplasmic desmin-immunoreactive inclusions. In addition to desmin, synemin, actin, gelsolin, ubiquitin, alphaB-crystallin and amyloid betaA4 were also present in the deposits. Ultrastructural examination revealed areas of myofibrillary disruption, abnormal electron-dense structures and accumulations of granulofilamentous material. A missense R406W mutation and a novel single amino acid deletion in the desmin gene were identified in two patients; the other patients did not show mutations in desmin, synemin, syncoilin or alphaB-crystallin genes. Analysis of 10 Spanish DRM cases illustrates a wide clinical, myopathological and genetic spectrum of DRM, reinforcing the need for further exploration of genetic causes for this group of disorders.

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.

Protein surplus myopathies and other rare congenital myopathies

The protein surplus myopathies have emerged as a newly recognized subgroup of morphologically defined myopathies within the spectrum of congenital myopathies because of the accumulation of protein aggregates, some of them mutant proteins. Currently, nosologic, including molecular criteria include desmin-related myopathies, actinopathies, and hereditary inclusion body myopathies, whereas hyaline body myopathy is still a putative form of protein surplus myopathy because of lack of any molecular data. The congenital myopathies (CM), foremost including nemaline and myotubular myopathies, have given evidence that, despite their epidemiologic rarity, the molecular age has dawned in CM and has even revealed surprising new nosologic features requiring reassessment and reclassification of certain CM. It is to be expected that a recently updated ENMC Consortium on "Protein surplus and other congenital myopathies" may procure important new information.

Passive stiffness is increased in soleus muscle of desmin knockout mouse

The effects of an absence of desmin on passive stiffness of skeletal muscle were analyzed using soleus muscles from desmin knockout mice. Stiffness was evaluated by analyzing the passive tension induced by a ramp-and-hold extension test. This test showed that passive resistance to stretch was largely increased for muscles lacking desmin. This study could facilitate interpretation of changes in muscle mechanics observed in humans affected by desmin-related myopathies.

Progress in desmin-related myopathies

Desmin-related myopathies are sporadic and familial neuromuscular conditions of considerable clinical heterogeneity uniformly marked by the pathologic accretion of desmin, often in a filamentous fashion. A large variety of other proteins, some of them cytoskeletal, also accrue. Morphologically, two types may be distinguished, one characterized by inclusions such as cytoplasmic and spheroid bodies or desmin-dystrophin plaques and another marked by granulofilamentous material. The genetic spectrum of desmin-related myopathies is quite diverse in that missense mutations and deletions in the desmin gene and a missense mutation in the alpha-B crystallin gene have been detected and several genes on other chromosomes have been mapped; the encoded protein products of these genes, however, are unknown. Accumulation of desmin and other proteins appears to be due to impaired nonlysosomal proteolysis. Mutant desmin that appears to be hyperphosphorylated seems to act as a seed protein for filament aggregation, inducing formation of inclusions and granulofilamentous material in these conditions. This condition is part of the group of disorders known as "surplus protein myopathies."

Gene-related protein surplus myopathies

Numerous muscular dystrophies, such as dystrophinopathies, sarcoglycanopathies, and emerino- and laminopathies, are marked by the absence or reduction of mutant transsarcolemmal or nuclear proteins. In addition to these recently identified minus-proteinopathies, there are a growing number of plus-proteinopathies among neuromuscular disorders marked by a surplus or excess of endogenous proteins within muscle fibers of different, i.e., nontranssarcolemmal and nonnuclear types. These proteins are often filamentous; for example, desmin and actin accrue in respective desmin-related myopathies, among which are entities marked by mutant desmin, true desminopathies, and actinopathy, the latter often seen as a subgroup in nemaline myopathies. Desmin-related myopathies consist largely of those marked by desmin-containing inclusions and those characterized by desmin-containing granulofilamentous material. When mutations in the desmin gene can be identified, the mutant desmin is thought to form the major myopathological lesion. Together with desmin, other proteins often accumulate. The spectrum of these proteins is quite diverse and encompasses such proteins as dystrophin, nestin, vimentin, alphaB-crystallin, ubiquitin, amyloid precursor protein, and beta-amyloid epitopes, as well as gelsolin and alpha(1)-antichymotrypsin. Among these associated proteins, one, alphaB-crystallin, has been found mutant in one large family, justifying the term alphaB-crystallinopathy as a separate condition among the desmin-related myopathies. Other proteins accruing with desmin have not yet been identified as mutant in desmin-related myopathies. Mutations in the desmin gene entail missense mutations and small deletions. The formation of mutant actin may lead to aggregates of actin filaments which may or may not be associated with formation of sarcoplasmic and/or intranuclear nemaline bodies. A considerable number of missense mutations in the sarcomeric actin gene ACTA1 have been discovered in patients with nemaline myopathy and also in a few patients without myopathological evidence of nemaline bodies in biopsied skeletal muscle fibres. Apart from alphaB-crystallin, no other proteins coaggregating with actin in actin filament aggregates of actinopathy or the actin mutation type of nemaline myopathy have so far been identified. Two further candidates for protein surplus myopathies are hyaline body myopathy, which is marked by accumulation of granular nonfilamentous material within muscle fibers that is rich in myosin and adenosine triphosphatase activities, and hereditary inclusion body myopathies, which are marked by accumulation of tubulofilaments similar to the helical filaments of Alzheimer neurofibrillary tangles. These tubulofilaments consist of diverse proteins as well, though no mutant protein has yet been discovered. So far, no genes responsible for familial hyaline body and hereditary inclusion body myopathies have been identified. The discovery of mutant proteins, desmin, alphaB-crystallin, and actin, as components of surplus or excess proteins accumulating in muscle fibers in certain neuromuscular conditions is responsible for the recent emergence of this new concept of gene-related protein surplus myopathies.

A dysfunctional desmin mutation in a patient with severe generalized myopathy

Mice lacking desmin produce muscle fibers with Z disks and normal sarcomeric organization. However, the muscles are mechanically fragile and degenerate upon repeated contractions. We report here a human patient with severe generalized myopathy and aberrant intrasarcoplasmic accumulation of desmin intermediate filaments. Muscle tissue from this patient lacks the wild-type desmin allele and has a desmin gene mutation encoding a 7-aa deletion within the coiled-coil segment of the protein. We show that recombinant desmin harboring this deletion cannot form proper desmin intermediate filament networks in cultured cells, nor is it able to assemble into 10-nm filaments in vitro. These findings provide direct evidence that a mutation in desmin can cause human myopathies.

Congenital myopathies with inclusion bodies: a brief review

Based on morphological abnormalities, congenital myopathies can be classified into several categories: (1) enzyme histochemically abnormal appearance without structural pathology, e.g., congenital fibre type disproportion or congenital fibre type uniformity; (2) abnormally placed nuclei, e.g. myotubular and centronuclear myopathies; (3) disruption of normal intrinsic structures, largely sarcomeres, e.g. central cores and minicores; (4) abnormal inclusions within muscle fibres. Several such inclusions are derived from pre-existing structures, most notably rods or nemaline bodies. Other derivatives of Z-band material are cytoplasmic bodies and possibly related inclusions as spheroid bodies, sarcoplasmic bodies or Mallory body-like inclusions. These inclusions share accumulation of desmin, the muscle fibre-specific intermediate filament, and of other proteins, some of them physiological, but others quite abnormal. Inclusions without identified precursors are fingerprint bodies, reducing bodies, cylindrical spirals, and Zebra bodies. Experimental models and tissue culture reproduction are necessary to further clarify significance of these inclusions in congenital myopathy pathology.