Myofibrillar Aggregates Research Articles

MYOFIBRILLAR AND DISTAL MYOPATHIES: P.254Pyroxd1 mutations cause recessive adult-onset slowly progressive LGMD

Limb-girdle muscular dystrophy (LGMD) is a group of inherited muscle diseases with slowly progressive, predominantly proximal myopathy. The phenotype of LGMD ranges from mild late-onset to severe early-onset disease. At least 31 different genes have been implicated in LGMD. Mutations in PYROXD1 gene was recently reported to cause recessive muscle disease. The described phenotype of PYROXD1 disease was congenital myofibrillar myopathy with symptom onset between infancy and 8 years. Internalized nuclei and myofibrillar disorganization were described as specific histopathologic features. We report on four Finnish patients, of whom two are siblings, with homozygous or compound heterozygous variants in PYROXD1 underlying adult-onset LGMD. Three of our patients were homozygous for the previously described variant p.Asn155Ser, and presented with muscle weakness in their teens or early thirties but have remained ambulant to over 60 years of age. Our fourth patient was compound heterozygous for the p.Asn155Ser variant and a previously unknown variant p.Tyr354Cys. He had the onset of proximal muscle weakness as late as aged 49 years. Thus the disease onset in our patients, particularly in the compound heterozygous patient, was considerably later than in the previously reported individuals with PYROXD1 mutations. Muscle biopsies showed dystrophic changes, excess of fat and fibrosis, necrosis and atrophy but no clear myofibrillar aggregates that would be consistent with myofibrillar myopathy. On muscle MRI the most severe fatty degenerative change was seen in gluteal region and anterior compartment of the thigh with relative sparing of rectus femoris. Our results show that p.Asn155Ser is a common pathogenic PYROXD1 variant, leading to myopathy with highly variable ages of disease onset. These findings extend the phenotypes caused by PYROXD1 mutations to adult-onset LGMD.

Variants in the Oxidoreductase PYROXD1 Cause Early-Onset Myopathy with Internalized Nuclei and Myofibrillar Disorganization

This study establishes PYROXD1 variants as a cause of early-onset myopathy and uses biospecimens and cell lines, yeast, and zebrafish models to elucidate the fundamental role of PYROXD1 in skeletal muscle. Exome sequencing identified recessive variants in PYROXD1 in nine probands from five families. Affected individuals presented in infancy or childhood with slowly progressive proximal and distal weakness, facial weakness, nasal speech, swallowing difficulties, and normal to moderately elevated creatine kinase. Distinctive histopathology showed abundant internalized nuclei, myofibrillar disorganization, desmin-positive inclusions, and thickened Z-bands. PYROXD1 is a nuclear-cytoplasmic pyridine nucleotide-disulphide reductase (PNDR). PNDRs are flavoproteins (FAD-binding) and catalyze pyridine-nucleotide-dependent (NAD/NADH) reduction of thiol residues in other proteins. Complementation experiments in yeast lacking glutathione reductase glr1 show that human PYROXD1 has reductase activity that is strongly impaired by the disease-associated missense mutations. Immunolocalization studies in human muscle and zebrafish myofibers demonstrate that PYROXD1 localizes to the nucleus and to striated sarcomeric compartments. Zebrafish with ryroxD1 knock-down recapitulate features of PYROXD1 myopathy with sarcomeric disorganization, myofibrillar aggregates, and marked swimming defect. We characterize variants in the oxidoreductase PYROXD1 as a cause of early-onset myopathy with distinctive histopathology and introduce altered redox regulation as a primary cause of congenital muscle disease.

Distinct Muscle Biopsy Findings in Genetically Defined Adult-Onset Motor Neuron Disorders.

The objective of this study was to characterize and compare muscle histopathological findings in 3 different genetic motor neuron disorders. We retrospectively re-assessed muscle biopsy findings in 23 patients with autosomal dominant lower motor neuron disease caused by p.G66V mutation in CHCHD10 (SMAJ), 10 X-linked spinal and bulbar muscular atrophy (SBMA) and 11 autosomal dominant c9orf72-mutated amyotrophic lateral sclerosis (c9ALS) patients. Distinct large fiber type grouping consisting of non-atrophic type IIA muscle fibers were 100% specific for the late-onset spinal muscular atrophies (SMAJ and SBMA) and were never observed in c9ALS. Common, but less specific findings included small groups of highly atrophic rounded type IIA fibers in SMAJ/SBMA, whereas in c9ALS, small group atrophies consisting of small-caliber angular fibers involving both fiber types were more characteristic. We also show that in the 2 slowly progressive motor neuron disorders (SMAJ and SBMA) the initial neurogenic features are often confused with considerable secondary “myopathic” changes at later disease stages, such as rimmed vacuoles, myofibrillar aggregates and numerous fibers reactive for fetal myosin heavy chain (dMyHC) antibodies. Based on our findings, muscle biopsy may be valuable in the diagnostic work-up of suspected motor neuron disorders in order to avoid a false ALS diagnosis in patients without clear findings of upper motor neuron lesions.

Diagnostically important muscle pathology in DNAJB6 mutated LGMD1D.

IntroductionLimb girdle muscular dystrophies are a large group of both dominantly and recessively inherited muscle diseases. LGMD1D is caused by mutated DNAJB6 and the molecular pathogenesis is mediated by defective chaperonal function leading to impaired handling of misfolded proteins which normally would be degraded. Here we aim to clarify muscle pathology of LGMD1D in order to facilitate diagnostic accuracy.After following six Finnish LGMD1D families, we analysed 21 muscle biopsies obtained from 15 patients at different time points after the onset of symptoms. All biopsies were obtained from the lower limb muscles and processed for routine histochemistry, extensive immunohistochemistry and electron microscopy.ResultsHistopathological findings were myopathic or dystrophic combined with rimmed vacuolar pathology, and small myofibrillar aggregates. These myofibrillar inclusions contained abnormal accumulation of a number of proteins such as myotilin, αB-crystallin and desmin on immunohistochemistry, and showed extensive myofibrillar disorganization with excess of Z-disk material on ultrastructure. Later in the disease process the rimmed vacuolar pathology dominated with rare cases of pronounced larger pleomorphic myofibrillar aggregates. The rimmed vacuoles were reactive for several markers of defect autophagy such as ubiquitin, TDP-43, p62 and SMI-31.ConclusionsSince DNAJB6 is known to interact with members of the chaperone assisted selective autophagy complex (CASA), including BAG3 – a known myofibrillar myopathy causing gene, the molecular muscle pathology is apparently mediated through impaired functions of CASA and possibly other complexes needed for the maintenance of the Z-disk and sarcomeric structures. The corresponding findings on histopathology offer clues for the diagnosis.

Myotilin overexpression enhances myopathology in the LGMD1A mouse model

Missense mutations in the myotilin gene cause limb-girdle muscular dystrophy type 1A (LGMD1A). We set out to examine the effect of overexpression of wild-type myotilin in an LGMD1A mouse model by crossing wild-type and mutant transgenic mice. Compared to single-transgenic mutant mice, double-transgenic mice overexpressing myotilin showed more severe muscle degeneration, enhanced myofibrillar aggregation, and earlier onset of aggregation. These data suggest that strategies aimed at lowering total myotilin levels in LGMD1A patients may be an effective therapeutic approach.

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.