Congenital myotonic dystrophy: fiber type abnormalities in two cases

Abnormalities in Skeletal Muscle Myogenesis, Growth, and Regeneration in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) and 2 (DM2) are autosomal dominant degenerative neuromuscular disorders characterized by progressive skeletal muscle weakness, atrophy, and myotonia with progeroid features. Although both DM1 and DM2 are characterized by skeletal muscle dysfunction and also share other clinical features, the diseases differ in the muscle groups that are affected. In DM1, distal muscles are mainly affected, whereas in DM2 problems are mostly found in proximal muscles. In addition, manifestation in DM1 is generally more severe, with possible congenital or childhood-onset of disease and prominent CNS involvement. DM1 and DM2 are caused by expansion of (CTG•CAG)n and (CCTG•CAGG)n repeats in the 3' non-coding region of DMPK and in intron 1 of CNBP, respectively, and in overlapping antisense genes. This critical review will focus on the pleiotropic problems that occur during development, growth, regeneration, and aging of skeletal muscle in patients who inherited these expansions. The current best-accepted idea is that most muscle symptoms can be explained by pathomechanistic effects of repeat expansion on RNA-mediated pathways. However, aberrations in DNA replication and transcription of the DM loci or in protein translation and proteome homeostasis could also affect the control of proliferation and differentiation of muscle progenitor cells or the maintenance and physiological integrity of muscle fibers during a patient's lifetime. Here, we will discuss these molecular and cellular processes and summarize current knowledge about the role of embryonic and adult muscle-resident stem cells in growth, homeostasis, regeneration, and premature aging of healthy and diseased muscle tissue. Of particular interest is that also progenitor cells from extramuscular sources, such as pericytes and mesoangioblasts, can participate in myogenic differentiation. We will examine the potential of all these types of cells in the application of regenerative medicine for muscular dystrophies and evaluate new possibilities for their use in future therapy of DM.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

Use of Clinical and Electrical Myotonia to Differentiate Childhood Myopathies

We retrospectively reviewed 2030 childhood electromyograms performed over an 11-year period (2004-2014). Twenty children (1%) with myotonic discharges were identified and placed into 2 groups. Group A (electrical and clinical myotonia) comprised 9 children (8 with myotonia congenita and 1 with paramyotonia congenita); all of them had diffuse myotonic discharges without clinical weakness or elevated creatine kinase. Group B (electrical myotonia without clinical myotonia) comprised 11 children (4 with inflammatory myopathy; 3, congenital myopathy, 3, muscular dystrophy; and 1, congenital muscular dystrophy). Clinical weakness was demonstrated in all of them and elevated creatine kinase in 6; all had a myopathic electromyogram and scattered myotonic discharges. We conclude that myotonic discharges are a rare but characteristic spontaneous discharge identified during electrodiagnostic studies in children. The presence of electrical and clinical myotonia provides helpful clues to differentiate between various muscle disorders in children.

(CTG)n repeats markedly inhibit differentiation of the C2C12 myoblast cell line: implications for congenital myotonic dystrophy

Although the mutation for myotonic dystrophy has been identified as a (CTG)n repeat expansion located in the 3'-untranslated region of a gene located on chromosome 19, the mechanism of disease pathogenesis is not understood. The objective of this study was to assess the effect of (CTG)n repeats on the differentiation of myoblasts in cell culture. We report here that C2C12 myoblast cell lines permanently transfected with plasmid expressing 500 bases long CTG repeat sequences, exhibited a drastic reduction in their ability to fuse and differentiate into myotubes. The percentage of cells fused into myotubes in C2 C12 cells (53.4+/-4.4%) was strikingly different from those in the two CTG repeat carrying clones (1.8+/-0.4% and 3.3+/-0. 7%). Control C2C12 cells permanently transfected with vector alone did not show such an effect. This finding may have important implications in understanding the pathogenesis of congenital myotonic dystrophy.

Association of Krabbe leukodystrophy and congenital fiber type disproportion

Hypotonia and weakness developed in a 12-month-old boy whose psychomotor development had previously been normal. The muscle biopsy demonstrated a disparity in the mean diameters of type 1 and type 2 fibers and satisfied major histologic criteria for diagnosis of congenital fiber type disproportion (CFTD). However, deterioration of motor and mental function, which developed subsequently, strongly suggested progressive encephalopathy. Examination of leukocyte cerebral enzymes at 15 months of age revealed a complete lack of galactosylceramide-beta-galactosidase. Selective type 1 fiber atrophy with type 1 fiber predominance has been observed in various conditions, including Krabbe disease. We report an additional case of Krabbe leukodystrophy associated with CFTD. The finding on the molecular level will resolve the dilemma of whether CFTD is a congenital myopathy or whether these patterns of disproportion may result from a number of different processes that interfere with the maturation of the developing motor unit.

Mitochondrial DNA does not appear to influence the congenital onset type of myotonic dystrophy

Neither the maternal inheritance pattern nor the early onset of congenital myotonic dystrophy are fully explained. One possible mechanism is that mitochondrial DNA (mtDNA) mutations might interact with the DM gene product, producing an earlier onset than would otherwise occur. We have used Southern hybridisation to show that high levels of major rearrangements of mtDNA are not present in muscle of five and in blood of 35 patients with congenital myotonic dystrophy. We used sequence analysis to show that no one particular mtDNA morph appears to cosegregate with congenital onset. A minor degree of depletion of mtDNA compared with nuclear DNA was present in the muscle of five patients with congenital DM, but we propose that this is not the primary cause of the muscle pathology but secondary to it. We have not found evidence that mtDNA is involved in congenital myotonic dystrophy.

Variation in mitochondrial DNA levels in muscle from normal controls. Is depletion of mtDNA in patients with mitochondrial myopathy a distinct clinical syndrome

Recent studies have identified a group of patients with cytochrome oxidase (COX) deficiency presenting in infancy associated with a deficiency of mtDNA in muscle or other affected tissue (Moraes et al 1991). We used a novel approach to compare the level of mitochondrial (mtDNA) compared to nuclear DNA in skeletal muscle from a group of patients and controls, based on dot blots that were hybridized with a mtDNA probe labelled with 35S[dCTP] and a reference nuclear DNA probe labelled with [32P]dCTP. The ratio of mtDNA to nuclear DNA varied in samples from different muscles of the same individual. Secondly, fetal muscle had very low levels of mtDNA compared to nuclear DNA, and data from older controls (cross-sectional rather than sequential) suggest that this increases rapidly over the first 3 months after birth and thereafter more slowly. Four patients with COX deficiency had levels of mtDNA that were below the age-specific range defined by 'normal' quadriceps muscle. The clinical features to two of these patients were similar to earlier case reports of mtDNA depletion. In three patients the clinical course was relatively benign compared to cases that have previously been described. Levels of mtDNA in skeletal muscle from some patients with other forms of muscle disease were also found to be low, suggesting that mtDNA depletion, possibly related to depletion of mitochondria, may be a relatively non-specific response of muscle to various pathological processes. However, there does appear to be a distinctive group of young patients with reduced cytochrome oxidase activity in muscle, in whom marked mtDNA depletion reflects the primary defect.

New insights into the pathogenesis of congenital myopathies

Congenital myopathies are developmental disorders of muscle that are best understood in the context of ontogenesis. Segmental amyoplasia results from a defective somite, usually because of lack of induction by the notochord and neural tube; the connective tissue matrix of the muscle is derived from lateral mesoderm and is present, but the myocytes are derived from somitic mesoderm and are replaced by adipose cells. Generalized amyoplasia is due to defective myogenic regulatory genes. X-linked recessive myotubular myopathy is associated with overexpression of vimentin and desmin, fetal intermediate filaments that attach to nuclear, mitochondrial, and inner sarcolemmal membranes and Z-bands of sarcomeres to preserve the morphologic organization of the myotube. Neonatal myotonic dystrophy is a true maturational delay in muscle development. Congenital muscle fiber-type disproportion is a syndrome of multiple etiologies but in some cases is associated with cerebellar hypoplasia and may be the result of abnormal suprasegmental stimulation of the developing motor unit at 20 to 28 weeks' gestation, mediated through bulbospinal pathways but not the corticospinal tract. Maturational delay of muscle in late developmental stages is less specific than in stages before midgestation. The Proteus syndrome is a muscular dysgenesis; abnormal paracrine growth factors and perhaps altered genes that regulate muscle differentiation and growth, such as myoD and myogenin, are the suspected cause. Focal proliferative myositis may be another example of a "paracrine myopathy."

Early neurological manifestations and brain anomalies in Marden-Walker syndrome

We report on an infant with the Marden-Walker syndrome. In addition to the consistent neurological abnormalities described previously in this syndrome, the infant had a striking neurological constellation, absence of primitive reflexes, jerky eye movements, failure to habituate to repeated stimuli, inadequate behavior development, and absence of orientation responses to visual or auditory stimuli. Muscle biopsy showed a similar pattern to the congenital fiber-type disproportion. Ultrasonograms and magnetic resonance imaging of his brain demonstrated absence of corpus callosum, colpocephaly, hypoplastic brainstem, hypoplasia of the inferior vermis and of the cerebellar hemispheres. These findings further delineate this syndrome and suggest that prenatal central nervous system (CNS) dysfunction, mainly of the cerebellum and brainstem, may play a significant role in the pathogenesis of the Marden-Walker syndrome.

Vimentin/desmin immunoreactivity of myofibres in developmental myopathies

Immunoreactivity for the intermediate filament proteins vimentin and desmin was studied in muscle biopsies of 33 children with neuromuscular diseases and in postmortem muscle of 15 fetuses and neonates at 8-42 weeks gestation. Fetal myotubes exhibited strong reactions for vimentin and desmin; reactivity was still present, though weaker, by 31 weeks and was no longer demonstrable at term. In X-linked myotubular myopathy (5 cases) myofibres showed strong reactivity for both vimentin and desmin; in myotonic dystrophy desmin but not vimentin had strong reactivity in myofibres of neonates and children. A similar but much weaker pattern of desmin reactivity was seen in nemaline rod disease and in congenital muscle fibre-type disproportion. The small myofibres in spinal muscular atrophy were reactive for both vimentin and desmin, as were regenerating myofibres in Duchenne muscular dystrophy and dermatomyositis. Acridine orange fluorochrome distinguished vimentin/desmin-reactive myofibres that were regenerating from those of developmental myopathies because the RNA fluorescence was strong in regenerating myofibres and in fetal myotubes, but was absent from myofibres in developmental disorders of muscle. A failure to regress of fetal cytoskeletal proteins may contribute to the apparent arrest in morphogenesis of myofibres. These stains are useful in studying the muscle biopsies of children with developmental myopathies because they demonstrate an aspect of muscle maturation not detected by standard histochemical methods.

A serial muscle biopsy study in a case of congenital fiber-type disproportion associated with progressive respiratory failure

The muscle fiber diameter and distribution were studied in serial muscle biopsy specimens taken at the ages of 21 months and 8 years in a girl with congenital fiber type disproportion. The patient was floppy from birth and showed delayed motor development. Progressive respiratory failure developed from the age of 8, which required artificial respiration during the night. The mean diameters of type 1 fibers at the ages of 21 m and 8 y were 10 mu and 21 mu, and those of type 2 fibers 32 mu and 61 mu, respectively. Whereas most type 1 fibers were hypotrophic in both biopsy specimens, a small number of type 1 fibers (approximately 15%) were normal-sized or hypertrophic, measuring 50 to 90 mu, in the second biopsy specimen, which were thought to have arisen on the maturation of normal-sized or hypertrophic type 1 fibers measuring 15 to 35 mu in the first biopsy specimen. Excessive hypotrophy of type 1 fibers was considered to be responsible for the progressive respiratory failure due to weakness of the diaphragm. The pattern of type 1 fiber evolution from the first to the second biopsy specimen suggested that dysmaturation of spinal motoneurons innervating type 1 muscle fibers would be involved in the pathophysiology of the fiber type disproportion in this case.

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: its consequences and its origin

Skeletal muscle maturation is impaired in children with congenital myotonic dystrophy. This immaturity is characterized at the light microscopy level by an abnormal presence of myotubes, small fascicles of muscle fibers, thin myofibers, and delayed muscle fiber type differentiation with a peripheral halo lacking mitochondrial oxidative enzyme activity. At an ultrastructural level, the characteristics are a paucity of myofibrils with a peripheral rim devoid of mitochondria and myofibrils in the fibers. In time the muscle is able to gain a certain degree of maturity as shown in one of our cases who had two successive muscle biopsies. The muscle, however, never becomes normal but retains discrepancies in fiber size and fiber type distribution and shows some fiber necrosis. Maturation of the motoneurons is normal, which may explain necrosis of immature muscle fibers. In an experimental study carried out to look for evidence of a circulatory factor in mothers of children with congenital myotonic dystrophy, it was found that sera from these mothers administered intra-peritoneally to newborn rats does in fact impair muscle maturation, whereas rats injected similarly with sera from control women showed normal muscle maturation.

Muscle fibre type composition, motoneuron firing properties, axonal conduction velocity and refractory period for foot extensor motor units in dystrophia myotonica

Seven patients with dystrophia myotonica were investigated using neurophysiological combined with histochemical techniques to elucidate motor unit properties in foot extensor muscles, which are often involved in the early stages of this disorder. For the 25 extensor digitorum brevis motor units studied the axonal conduction velocity, the axonal refractory period and the voluntary firing properties were within normal limits. However, high threshold motor units were not observed and the mean value of the axonal conduction velocities was lower (p less than 0.02) for the dystrophia myotonica motor units when compared with corresponding data from healthy subjects. There were also signs of impaired impulse propagation in the terminal part of the motor unit. In muscle biopsy specimens from the anterior tibial muscle, fibre type composition and structure were demonstrated using enzyme histochemical techniques for adenosine-triphosphate and immunohistochemical techniques for identification of the types of myosin isoform present. The histochemical findings indicated a type I fibre dominance, which was most obvious in the more seriously affected muscles. Neonatal myosin was observed preferentially in small but also in some normal sized fibres. Furthermore, some ring fibres were present and these showed staining with antineonatal myosin in their superficial portion. This indicates that an abnormal regeneration is one cause of the myopathic appearance of the muscle fibres in dystrophia myotonica. These investigations show that there is a reduced proportion of type II motor units in foot extensor muscles involved in the myopathy in dystrophia myotonica although it cannot definitely be established whether this is due to a loss of high threshold type II motor units or type II to type I transformation.

Congenital myotonic dystrophy. Changes in muscle pathology with ageing

Undifferentiated type 2C fibers and satellite cells were increased in number in younger patients with congenital myotonic dystrophy (CMD) indicating immaturity in muscle fiber growth. The changes found in a 38-year-old man with CMD were identical to those described in late onset myotonic dystrophy. Type 1 fibers were found to become predominant with age. This suggests that in this disorder fiber type transformation progresses with age, presumably due to abnormal neural influences or aberrant sarcolemmal responses.

Hyperinsulinemia in myotonic dystrophy: identity of the maternal factor causing the neonatal myotonic dystrophy syndrome

An environmental factor acting on the fetus is thought to cause a neonatal syndrome characterized by marked muscular hypotonia, lack of respiratory drive and feeding difficulties, in some infants born to mothers with myotonic dystrophy. Mortality is high, especially amongst those babies born prematurely, but muscle strength and tone improve rapidly in survivors. Nevertheless, most survivors have physical deformities and mental retardation and are thought to develop myotonic dystrophy later. We propose that alterations in maternal insulin secretion (usual in myotonic dystrophy subjects) alter fetal blood glucose and amino acid levels and retard growth and maturation of fetal skeletal muscle. This leads to severe muscular hypotonia in affected infants. Also, we suggest that infants who die during the perinatal period may not have inherited the defective autosomal dominant gene that causes myotonic dystrophy.

X-linked recessive myotubular myopathy: II. Muscle morphology and human myogenesis

The classification of centronuclear myotubular myopathies is controversial. Within this group of disorders, congenital X-linked recessive myotubular myopathy (XLMTM), characterized by marked cell hypotrophy and structural resemblance to fetal myotubes, represents a distinct entity. The histologic findings in verified and probable cases of XLMTM are reviewed. In addition, the ultrastructural features of muscle in one case of XLMTM are compared with those of normal fetal muscle at various developmental ages. In XLMTM both muscle and nerve show evidence of immaturity. Proliferation of the sarcotubular organelles in XLMTM, not observed in normal fetal muscle, may be due to impaired innervation.

Morphologic and morphometric analyses of muscle in the neonatal myotonic dystrophy syndrome

Autopsy studies of three premature siblings who died soon after birth with the neonatal myotonic dystrophy syndrome revealed pulmonary hypoplasia and congenital pleural effusions. Neither of these findings has been described previously in this condition. New ultrastructural findings include focal diaphragmatic myofiber degeneration and necrosis, which were attributed to over-stretching of the fetal diaphragm. In addition, abnormally small stores of free and intravesicular glycogen were observed in skeletal muscle fibers. The morphometric features of control fetal and neonatal skeletal muscle were recorded for comparison with muscle fiber measurements in the three infants. Fiber diameters in the latter were much smaller than expected for body weights. The morphologic and morphometric findings support the concept that fetal muscle maturation is severely retarded in this syndrome.

Familial myopathy associated with Marfanoid features and multicores

The cases of a mother and son with a slowly progressive generalised myopathy and Marfanoid features are described. Muscle biopsies showed atrophy, type 1 fibre preponderance, excessive variation in fibre size and shape, and multiple foci of myofibrillar disorganisation with loss of oxidative enzyme activity of the type found in multicore disease.

Skeletal muscle in preterm infants with congenital myotonic dystrophy. Morphologic and histochemical study

The skeletal muscle in 3 preterm infants (27, 34, 37 weeks gestation age) born to mothers with myotonic dystrophy showed a syncytial pattern at 27 weeks and a decreasing percentage of satellite cells and central nuclei at 34 and 37 weeks gestation. The fiber type differentiation was observed only at 37 weeks of gestational age. In all 3 cases muscle fibers with multiple acid phosphatase positive were seen. The muscle spindles also had thick capsules and showed lack of morphologic and histochemical differentiation into fiber types. These findings suggest immaturity of skeletal muscle in comparison to the normal. The immaturity of the skeletal muscle correlated well with the prognosis of the patients.