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

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.

Arrest is a regulator of fiber-specific alternative splicing in the indirect flight muscles of Drosophila

The RNA-binding protein Arrest occupies a novel intranuclear domain and directs flight muscle–specific patterns of alternative splicing in flies.

Drosophila melanogaster flight muscles are distinct from other skeletal muscles, such as jump muscles, and express several uniquely spliced muscle-associated transcripts. We sought to identify factors mediating splicing differences between the flight and jump muscle fiber types. We found that the ribonucleic acid–binding protein Arrest (Aret) is expressed in flight muscles: in founder cells, Aret accumulates in a novel intranuclear compartment that we termed the Bruno body, and after the onset of muscle differentiation, Aret disperses in the nucleus. Down-regulation of the aret gene led to ultrastructural changes and functional impairment of flight muscles, and transcripts of structural genes expressed in the flight muscles became spliced in a manner characteristic of jump muscles. Aret also potently promoted flight muscle splicing patterns when ectopically expressed in jump muscles or tissue culture cells. Genetically, aret is located downstream of exd (extradenticle), hth (homothorax), and salm (spalt major), transcription factors that control fiber identity. Our observations provide insight into a transcriptional and splicing regulatory network for muscle fiber specification.

Pleural effusions in the neonate

INTRODUCTION

Pleural effusions are rare in the neonate and may be associated to several clinical conditions. Only a few series of pleural effusions in the fetus and newborn are described in the literature.

AIM

This study was undertaken to determine more accurately the causes and prognostic significance of pleural effusions in a population of high-risk neonates.

MATERIALS AND METHODS

A retrospective chart review of 62 neonates admitted to the neonatal intensive care unit of six medical centers in the north of Portugal, between 1997 and 2004, that presented the diagnosis of pleural effusion.

RESULTS

33M/29F newborns; preterms 47 (76%); GA 33 (25-40) wk; BW 1830 (660-4270) g; C-section 39 (63%). Pleural effusions were congenital in 20 (32%) newborns and acquired in 42 (68%). Congenital pleural effusions occurred as fetal hydrops in 11 (18%) patients and as chylothorax in 9 (15%). In four cases of hydrops, the cause was a congenital chylothorax. Congenital chylothorax (n=13) was the most common (65%) congenital pleural effusion in this study. The incidence of congenital chylothorax was 1:8.600 deliveries and male:female ratio was 2:1. Mortality occurred in five newborns due to pulmonary hypoplasia. Traumatic (iatrogenic) were the most frequent (n=31) acquired pleural effusions. These included 8 (13%) cases of hemothorax and 8 (13%) cases of total parenteral nutrition leakage. Pleural effusions after intra-thoracic surgery were mainly (79%) chylothoraces. There were 11 (26%) non-iatrogenic acquired pleural effusions. No mortality was associated with acquired pleural effusions.

CONCLUSIONS

Congenital pleural effusions usually occur as hydrops or congenital chylothorax. Traumatic (iatrogenic) are the most frequent acquired pleural effusions in a tertiary NICU. Pleural effusions after intra-thoracic surgery are mainly chylothoraces. Non-iatrogenic acquired pleural effusions are associated to several clinical conditions, and mortality is usually associated to the underlying condition.

Transgenic mice expressing CUG-BP1 reproduce splicing mis-regulation observed in myotonic dystrophy

Myotonic dystrophy type I (DM1) is an RNA-mediated disease caused by a non-coding CTG repeat expansion. A key feature of the RNA-mediated pathogenesis model for DM is the disrupted splicing of specific pre-mRNA targets. A link has been established between splicing regulation by CUG-BP1, a member of the CELF family of proteins, and DM1 pathogenesis. To determine whether increased CUG-BP1 function was sufficient to model DM, transgenic mice overexpressing CUG-BP1 (MCKCUG-BP1) in heart and skeletal muscle, two tissues affected in DM1, were generated. Histological and electron microscopic analyses of skeletal muscle reveal common pathological features with DM tissues: chains of central nuclei, degenerating fibers and centralized NADH reactivity. MCKCUG-BP1 mice have disrupted splicing of three CELF target pre-mRNAs, cardiac troponin T (Tnnt2), myotubularin-related 1 gene (Mtmr1) and the muscle-specific chloride channel (Clcn1), consistent with that observed in DM heart and skeletal muscle. The results are consistent with a mechanism for DM pathogenesis in which expanded repeats result in increased CUG-BP1 activity and/or other CELF family members and have trans-dominant effects on specific pre-mRNA targets.

Presentation, clinical course, and outcome of the congenital form of myotonic dystrophy

We report the clinical experience of 18 patients with the congenital form of myotonic dystrophy, the majority of whom were diagnosed during the neonatal period and monitored from 5 to 14 years. Prematurity associated with congenital myotonic dystrophy gives rise to the severest clinical manifestations. Among them, respiratory involvement is common and is the leading cause of death in the neonatal period. Weakness and foot deformities secondary to muscle involvement are the predominant clinical features of this group of patients from birth to age 3 or 4 years. Once muscle strength improves, learning disabilities and behavioral disturbances become the main clinical problems. All our patients, when tested after 5 years of age, had intelligence quotients under 65, clearly below the average intelligence quotient of their mothers (IQ = 80). There is no relationship between the degree of mothers' and patients' disease. No patient has presented problems with routine immunizations, and no complications were observed in the 7 patients who underwent surgery under general anesthesia. Among the surviving patients, no correlation can be established between severity of disease in the neonatal period and the magnitude of sequelae as teenagers. Mental and behavioral disturbances are the factors which mainly influence the long-term management and prognosis of this cohort of individuals.

DNA confirmation of congenital myotonic dystrophy in non-immune hydrops fetalis

We report on the prenatal diagnosis of congenital myotonic dystrophy in a case of non-immune hydrops fetalis. DNA analysis of amniocytes revealed expansion of the trinucleotide (CTG) repeat within the myotonin PK gene, associated with myotonic dystrophy. The fetus was found to have approximately 1730 copies, while the normal population has between 5 and 27 copies, and minimally affected individuals have at least 50 copies.

Neuroradiological findings in children with congenital myotonic dystrophy

We studied seven children with congenital myotonic dystrophy, aged 2.1-8.3 years, and the results of computed tomography and magnetic resonance imaging of the brain were analyzed and neurological development was assessed from the neonatal period. We found that ventricular dilatation that had been seen on the first day of life in two of three infants had not progressed in sequential follow-up computed tomography scans taken at intervals of one to six years. Also, in T2-weighted magnetic resonance imagings, areas of periventricular hyperintensity were identified in all children, as well as areas of subcortical hyperintensity in one child. Further, an asphyxial episode had occurred at birth in five patients and the extent of the periventricular hyperintensity was found to correlate significantly with Apgar scores, indicating that the degree of perinatal asphyxia that had occurred was responsible for the abnormalities uncovered by the magnetic resonance imagings. However, there was no correlation between the neurodevelopment outcome and the extent of the periventricular hyperintensity or ventriculomegaly. Therefore, in patients with congenital myotonic dystrophy, a neonatal episode of asphyxia can be responsible for a finding of periventricular hyperintensity, but it is unlikely that an integral part of the mental retardation is attributable to brain damage due to perinatal asphyxia.

Diaphragmatic contraction band necrosis in a perinatal and infantile autopsy population

A morphologic study of the anterolateral costal diaphragm in 125 newborns and infants who died suddenly showed that contraction band necrosis is a common finding. In cases that showed the most extensive lesions, acute asphyxia was the usual mode of death; within eight diagnostic categories birth asphyxia (11 of 26 cases) and sudden infant death syndrome (19 of 30 cases) had the highest frequency of lesions. It was more frequent than myocardial contraction band necrosis and myocardial coagulation necrosis among the cases studied. The morphologic age and, if present, the stage of healing in each case suggested that the diaphragmatic lesion commenced at or shortly before death or at the time of the cardiac arrest that led to death. Thus, the lesion appeared to represent a very early event after a lethal injury, but it had no specificity for the nature of the injury. Because skeletal muscle of the respiratory diaphragm structurally and functionally resembles myocardium, the pathogenesis of contraction band necrosis may be similar in the two muscle types.

Neonatal form of nemaline myopathy, muscle immaturity, and a microvascular injury

An infant with a neonatal form of nemaline myopathy showed ultrastructural features of muscle immaturity. Immaturity was characterized by an abnormal presence of myotubes, as well as cells in clusters within a common basement membrane and a great number of satellite cells adhering to very small muscle fibers. In addition, degenerative changes and a severe microvascular lesion were observed. The pathologic findings in the muscle of this patient were those of neonatal nemaline myopathy complicating severe microvascular injury, possibly induced by an unknown toxic agent.

Congenital myotonic dystrophy: respiratory function at birth determines survival

The clinical features of 14 neonates with congenital myotonic dystrophy were retrospectively reviewed. These babies represent all the new cases of congenital myotonic dystrophy seen in this department since 1982. Twelve babies were referred because of either difficulties in diagnosis or difficulties in the management of their respiratory problems. Of the 14 babies, 13 had birth asphyxia, 11 were premature, and four had intrauterine growth retardation. Ten babies required artificial ventilation from birth. Abnormalities on chest radiography included thin ribs (n =9) and raised right hemidiaphragms (n = 5). Recurrent episodes of collapse and consolidation of the lungs secondary to poor swallowing occurred in all ventilated babies. All babies ventilated for longer than four weeks died of respiratory complications before the age of 15 months. One baby was successfully extubated after diaphragmatic plication, but he died a few months later. Duration of ventilation was the best guide to prognosis.

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.

Perinatal pulmonary hyperplasia due to laryngeal atresia

A premature newborn boy, who died at birth because of cartilaginous laryngeal atresia, showed lung development that was far more advanced than normal for gestational age. The lungs, which were histologically normal, were three times the expected weight and showed a degree of alveolarization appropriate for 3 months' postnatal age. The lungs crowded the chest cavity so that the diaphragm was flattened and immobilized; the massive ascites, documented by ultrasound 5 weeks prior to delivery, appeared to be due to obstructed venous return. Thus, the pulmonary hyperplasia, a new finding in perinatal pathology, caused severe fetal ascites. Subsequently, the hyperplasia led to acute polyhydramnios that, ultrasonographically, was observed to develop 2 weeks prior to delivery. The observation that fetal lungs may be hyperplastic has a bearing on the known relationship between fetal lung growth and retention of lung fluid.

Pulmonary hypoplasia in neonatal hypophosphatasia

Morphological, biophysical, and biochemical parameters of lung growth were studied at autopsy on a male infant with hypophosphatasia who died with asphyxia immediately after birth. The lungs were hypoplastic because of a marked decrease in airspace formation but lung maturation was normal for gestational age by all the parameters used. Diaphragmatic development, assessed by weight and fiber measurement, was in keeping with the decreased chest size. The proposed mechanism for this late onset type of pulmonary hypoplasia, attributed to decreased thoracic volume, is correlated with antenatal ultrasonographic observations of normal fetal breathing movements in the affected infant.

Fetal breathing movements are not a reliable predictor of continued lung development in pregnancies complicated by oligohydramnios

Fetal breathing activity was monitored by ultrasonography in 14 patients with oligohydramnios (4 with fetal anuria and 10 with premature rupture of the membranes). 45 patients with intact membranes, with normal amniotic fluid volume, and matched for gestational age were the controls. Fetal breathing movements were seen in all 8 patients who died in the perinatal period with a necropsy diagnosis of lung hypoplasia. While patients with premature rupture of the membranes (irrespective of outcome) spent the same or less time breathing than controls, fetuses with renal anomalies, oligohydramnios, and lung hypoplasia spent more time breathing than controls. The observation of fetal breathing activity is not helpful in identifying patients with oligohydramnios at risk of lung hypoplasia.

The nature of host tissue destruction in tumor invasion. An experimental investigation on carcinoma and sarcoma xenotransplants

The nature of host tissue destruction in tumor invasion was investigated in experimentally induced carcinomas and sarcomas, xenografted into skeletal muscle. By means of light and electron microscopy it was shown that in both carcinomas and sarcomas the confrontation of host tissue with the invading tumor cells does not result in immediate destruction of host tissue but in a transitory state of coexistence which gradually proceeds to progressive host tissue atrophy. This process of progressive atrophy, which finally results in the total disappearance of the invaded host tissue, is considered to be caused mainly by the increasing pressure and competitive withdrawal of oxygen and nutrients by the invading and proliferating tumor cells. Morphological changes suggesting an active enzymatic breakdown of host tissue cells by tumor cells were not observed during any stage of tumor invasion.

Morphologic and morphometric analysis of muscle in X-linked myotubular myopathy

The X-linked form of myotubular myopathy is highly lethal in neonates. Several autopsy-derived muscles from two probands of a new kindred who survived for 100 days because of intensive supportive care were analyzed by light microscopy, morphometry, enzyme histochemistry, and electron microscopy. The results were compared with a similar analysis of muscle from control fetal and neonatal subjects. The findings, in addition to the characteristic centronucleated hypotrophic myofibers, included widespread myofiber degeneration and focal contraction band necrosis that differed from the types seen in other myopathic and dystrophic muscle diseases. A high frequency of degenerating nuclei that often contained large nucleoli was observed. Because of the paradoxic nuclear morphology, nuclear failure (in migration and myofibrillogenesis) is believed to be of central importance in the pathogenesis of this disease.