Muscle glycogenoses.

There are 11 hereditary disorders of glycogen metabolism affecting muscle alone or together with other tissues, and they cause two main clinical syndromes: episodic, recurrent exercise intolerance with cramps, myalgia, and myoglobinuria; or fixed, often progressive weakness. Great strides have been made in our understanding of the molecular bases of these disorders, all of which show remarkable genetic heterogeneity. In contrast, the pathophysiological mechanisms underlying acute muscle breakdown and chronic weakness remain unclear. Although glycogen storage diseases have been studied for decades, new biochemical defects are still being discovered, especially in the glycolytic pathway. In addition, the pathogenesis of polyglucosan deposition is being clarified both in traditional glycogenoses and in disorders such as Lafora's disease. In some conditions, combined dietary and exercise regimens may be of help, and gene therapy, including recombinant enzyme replacement, is being actively pursued.     

Denervation and developmental alterations of glycogen synthase and glycogen phosphorylase in mammalian skeletal muscle.

The activities of glycogen synthase and glycogen phosphorylase were determined in normal and denervated fast and slow muscle and in developing skeletal muscle of the rabbit. Normal fast muscle (16.0 nmol glucose/min/mg) showed increased specific activity for glycogen synthase compared to slow muscle (9.5 nmol glucose/min/mg). Glycogen synthase in both muscle types was activated approximately twofold by 10 mm glucose 6-phosphate. After 7 days of denervation, glycogen synthase decreased 78% in both muscle types. Glycogen phosphorylase activity of fast muscle (1088 nmol P_i min/mg) was eightfold greater than slow muscle (141 nmol P_i/min/mg). Denervation resulted in a 33% decline in phosphorylase activity in fast muscle and 52% in slow muscle. Denervation had no effect on either fast or slow muscle glycogen content. Neonatal skeletal muscle exhibited relatively lower values of glycogen synthase (1.8 nmol glucose/min/mg) and glycogen phosphorylase (284 nmol P_i/min/mg) than adult muscle. Adult activity levels of each enzyme occurred at approximately the 4th postnatal week for glycogen synthase and the 6th postnatal week for glycogen phosphorylase. The data suggest, on a qualitative basis, that denervation induced a decrease in activity of key enzymes in glycogen metabolism similar to that observed in newborn skeletal muscle.     

Increased PFK activity and GLUT4 protein content in McArdle's disease

Inborn errors of metabolism represent an opportunity to conduct studies in order to understand compensatory adaptations to a defective metabolic pathway. We evaluated the molecular and biochemical adaptations in substrate metabolism (glycolysis, electron transport chain, tricarboxylic acid cycle, beta-oxidation) in response to myophosphorylase deficiency in skeletal muscle from 13 patients with McArdle's disease (MD) and 13 age-matched controls. MD muscle had higher phosphofructokinase protein content and activity as well as glucose transporter 4 (GLUT4) protein content and lower GLUT4 mRNA content than controls. At the protein level, skeletal muscle adaptations suggest an augmented glucose transport and glycolytic flux as a compensatory metabolic strategy to a chronic absence of muscle glycogen phosphorylase. These results support previous findings of increased glucose uptake during exercise and alleviation of symptoms with oral sucrose in patients with MD.     

Defects of mitochondrial beta-oxidation: a growing group of disorders.

Two disorders of fatty acid metabolism were described in 1973. Since then, at least 22 different inborn errors of metabolism affecting beta-oxidation in skeletal muscle and other tissues have been identified. Neurological findings are prominent in many of these, including hypotonia, myopathy (often with lipid storage), and peripheral neuropathy. Recurrent rhabdomyolysis and hypoglycemia are frequent clinical problems. In many cases, a correct diagnosis will only be made if these disorders are specifically considered and appropriate tests are obtained, since screening tests which detect other inborn errors of metabolism are often normal in patients with beta-oxidation defects under many circumstances. Clinical symptoms, diagnostic testing, and issues of newborn screening for this important group of disorders are discussed.     

Phosphorus magnetic resonance spectroscopy (31P MRS) in neuromuscular disorders.

Phosphorus magnetic resonance spectroscopy monitors muscle energy metabolism by recording the ratio of phosphocreatine to inorganic phosphate at rest, during exercise, and during recovery from exercise. In mitochondrial diseases, abnormalities may appear during some or all these phases. Low phosphocreatine-inorganic phosphate ratios at rest are not disease-specific, but can be increased by drug therapy in several myopathies. Phosphorus magnetic resonance spectroscopy can also record intracellular pH and thus identify disorders of glycogen metabolism in which the production of lactic acid is blocked during ischemic exercise. The measurements of accumulated sugar phosphate intermediates further delineate glycolytic muscle defects. Myophosphorylase deficiency responds to intravenous glucose administration with improved exercise bioenergetics, but no such response is seen in phosphofructokinase deficiency. The muscular dystrophies show no specific bioenergetic abnormality; however, elevation of phospholipids metabolites and phosphodiesters was detected in some cases. While phosphorus magnetic resonance spectroscopy remains primarily a research tool in metabolic myopathies, it will be clinically useful in identifying new therapies and monitoring their effects in a variety of neuromuscular disorders.     

Genetic and clinical characteristics of skeletal and cardiac muscle in patients with lamin A/C gene mutations

Alterations of the lamin A/C (LMNA) gene are associated with different clinical entities, including disorders that affect skeletal and cardiac muscle, peripheral nerves, metabolism, bones, and disorders that cause premature aging. In this article we review the clinical and genetic characteristics of cardiac and skeletal muscle diseases related to alterations in the LMNA gene. There is no single explanation of how LMNA gene alterations may cause these disorders; however, important goals have been achieved in understanding the pathogenic effects of LMNA gene mutations on cardiac and skeletal muscle. Muscle Nerve, 48: 161–170, 2013     

Rhabdomyolysis and myoglobinuria

Rhabdomyolysis is a disorder characterized by acute damage of the sarcolemma of the skeletal muscle leading to release of potentially toxic muscle cell components into the circulation, most notably creatine phosphokinase (CK) and myoglobin, and is frequently accompanied by myoglobinuria. Therefore, the term myoglobinuria is often used interchangeably with the term rhabdomyolysis. This disorder may result in potential life-threatening complications such as acute myoglobinuric renal failure, hyperkalemia and cardiac arrest, disseminated intravascular coagulation, and compartment syndrome. The condition is etiologically heterogeneous and may result from a large variety of diseases affecting muscle membranes, membrane ion channels, and muscle energy supply including acquired causes (e.g., exertion, crush injury and trauma, alcoholism, drugs, and toxins) and hereditary causes (e.g., disorders of carbohydrate metabolism, disorders of lipid metabolism, or diseases of the muscle associated with malignant hyperthermia). In many patients with idiopathic recurrent rhabdomyolysis, specific inherited metabolic defects have not been recognized up to now.     

Potential new roles for glycogen in epilepsy

Seizures often originate in epileptogenic foci. Between seizures (interictally), these foci and some of the surrounding tissue often show low signals with ¹⁸fluorodeoxyglucose (FDG) positron emission tomography (PET) in many epileptic patients, even when there are no radiologically detectable structural abnormalities. Low FDG-PET signals are thought to reflect glucose hypometabolism. Here, we review knowledge about metabolism of glucose and glycogen and oxidative stress in people with epilepsy and in acute and chronic rodent seizure models. Interictal brain glucose levels are normal and do not cause apparent glucose hypometabolism, which remains unexplained. During seizures, high amounts of fuel are needed to satisfy increased energy demands. Astrocytes consume glycogen as an additional emergency fuel to supplement glucose during high metabolic demand, such as during brain stimulation, stress, and seizures. In rodents, brain glycogen levels drop during induced seizures and increase to higher levels thereafter. Interictally, in people with epilepsy and in chronic epilepsy models, normal glucose but high glycogen levels have been found in the presumed brain areas involved in seizure generation. We present our new hypothesis that as an adaptive response to repeated episodes of high metabolic demand, high interictal glycogen levels in epileptogenic brain areas are used to support energy metabolism and potentially interictal neuronal activity. Glycogenolysis, which can be triggered by stress or oxidative stress, leads to decreased utilization of plasma glucose in epileptogenic brain areas, resulting in low FDG signals that are related to functional changes underlying seizure onset and propagation. This is (partially) reversible after successful surgery. Last, we propose that potential interictal glycogen depletion in epileptogenic and surrounding areas may cause energy shortages in astrocytes, which may impair potassium buffering and contribute to seizure generation. Based on these hypotheses, auxiliary fuels or treatments that support glycogen metabolism may be useful to treat epilepsy.     

Lipid storage myopathies

PURPOSE OF REVIEW: The aim of this review is to provide an update on disorders of lipid metabolism affecting skeletal muscle exclusively or predominantly and to summarize recent clinical, genetic, and therapeutic studies in this field. RECENT FINDINGS: Over the past 5 years, new clinical phenotypes and genetic loci have been described, unusual pathogenic mechanisms have been elucidated, and novel pharmacological approaches have been developed. At least one genetic defect responsible for the myopathic form of CoQ10 deficiency has been identified, causing a disorder that is allelic with the late-onset riboflavine-responsive form of multiple acyl-coenzyme A dehydrogenation deficiency. Novel mechanisms involved in the lipolytic breakdown of cellular lipid depots have been described and have led to the identification of genes and mutations responsible for multisystemic neutral lipid storage disorders, characterized by accumulation of triglyceride in multiple tissues, including muscle. SUMMARY: Defects in lipid metabolism can affect either the mitochondrial transport and oxidation of exogenous fatty acid or the catabolism of endogenous triglycerides. These disorders impair energy production and almost invariably involve skeletal muscle, causing progressive myopathy with muscle weakness, or recurrent acute episodes of rhabdomyolysis triggered by exercise, fasting, or infections. Clinical and genetic characterization of these disorders has important implications both for accurate diagnostic approach and for development of therapeutic strategies.     

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.     

Glycogen storage disease: clinical, biochemical, and molecular heterogeneity

Glycogen storage diseases (GSDs) are characterized by abnormal inherited glycogen metabolism in the liver, muscle, and brain and divided into types 0 to X. GSD type I, glucose 6-phosphatase system, has types Ia, Ib, Ic, and Id, glucose 6-phosphatase, glucose 6-phosphate translocase, pyrophosphate translocase, and glucose translocase deficiencies, respectively. GSD type II is caused by defective lysosomal alpha-glucosidase (GAA), subdivided into 4 onset forms. GSD type III, amylo-1,6-glucosidase deficiency, is subdivided into 6 forms. GSD type IV, Andersen disease or amylopectinosis, is caused by deficiency of the glycogen-branching enzyme in numerous forms. GSD type V, McArdle disease or muscle phosphorylase deficiency, is divided into 2 forms. GSD type VI is characterized by liver phosphorylase deficiency. GSD type VII, phosphofructokinase deficiency, has 2 subtypes. GSD types VIa, VIII, IX, or X are supposedly caused by tissue-specific phosphorylase kinase deficiency. GSD type 0, glycogen synthase deficiency, is divided into 2 subtypes.     

CNS disorders caused by metabolic disorders

A guideline for early diagnosis of metabolic disorders affecting central nervous system during neonatal and early infancy was presented. Clinical manifestations associated with inborn errors of metabolism in the neonatal period are poor feeding, vomiting, diarrhea, abnormalities in muscle tonus, dyspnea, convulsion, coma and so on, and these are not specific to each disorder. However, such symptoms or signs as described below have often intimate relation to metabolic disorders: (1) previous children died of undetermined causes during early infancy; (2) complication of sepsis; (3) onset in the early neonatal period; (4) developmental and growth retardation. When newborns and infants have these symptoms or signs, we should start simple screening studies immediately for metabolic disorders, including CBC, hepatic function tests, blood glucose, lactate, pyruvate, ketone bodies, ammonia, blood gas analysis, urinalysis (including non-glucose reducing substance tests and FeCl3 reaction) and so on. As for CBC, we have to make our own effort to find spherocytosis and vacuoles in lymphocytes. Family history, especially the mother's personal history, is indispensable. During physical examinations, we must pay attention to facial appearance, skin color, macroglossia, hair abnormalities, peculiar odor of the urine and hepatosplenomegaly. When abnormality is found in these clinical signs or simple laboratory examinations, we should not hesitate to start dietary treatment even if special examinations for differential diagnosis are on the way.     

Progression in nemaline myopathy

Summary Four of seven patients with nemaline myopathy had severe, rapidly progressing symptoms. These four showed an increase in acid phosphatase activity in muscle fibers demonstrated by histochemistry and cathepsin B&L activity by biochemical measurement. On electron microscopy, nemaline bodies, occasionally disorganized myofibrils and autophagic vacuoles containing sarcoplasmic debris and glycogen particles were seen. Focal myofibrillar degeneration, through an unknown pathogenetic mechanism, induces an increase in lysosomal enzymes in the skeletal muscles which may be closely correlated with a rapid aggravation of muscle weakness in nemaline myopathy.     

Glycogen storage myopathies

The glycogen storage myopathies are caused by enzyme defects in the glycogenolytic or in the glycolytic pathway affecting skeletal muscle alone or in conjunction with other tissues. The authors review recent findings in this area, including a new entity, aldolase deficiency, and the wealth of molecular genetic data that are rapidly accumulating. Despite this progress, genotype-phenotyp3 correlations are still murky in most glycogen storage myopathies.     

Myopathies Related to Glycogen Metabolism Disorders

Most of the glycogen metabolism disorders that affect skeletal muscle involve enzymes in glycogenolysis (myophosphorylase (PYGM), glycogen debranching enzyme (AGL), phosphorylase b kinase (PHKB)) and glycolysis (phosphofructokinase (PFK), phosphoglycerate mutase (PGAM2), aldolase A (ALDOA), β-enolase (ENO3)); however, 3 involve glycogen synthesis (glycogenin-1 (GYG1), glycogen synthase (GSE), and branching enzyme (GBE1)). Many present with exercise-induced cramps and rhabdomyolysis with higher-intensity exercise (i.e., PYGM, PFK, PGAM2), yet others present with muscle atrophy and weakness (GYG1, AGL, GBE1). A failure of serum lactate to rise with exercise with an exaggerated ammonia response is a common, but not invariant, finding. The serum creatine kinase (CK) is often elevated in the myopathic forms and in PYGM deficiency, but can be normal and increase only with rhabdomyolysis (PGAM2, PFK, ENO3). Therapy for glycogen storage diseases that result in exercise-induced symptoms includes lifestyle adaptation and carefully titrated exercise. Immediate pre-exercise carbohydrate improves symptoms in the glycogenolytic defects (i.e., PYGM), but can exacerbate symptoms in glycolytic defects (i.e., PFK). Creatine monohydrate in low dose may provide a mild benefit in PYGM mutations.     

Cerebral blood flow and glucose metabolism in mitochondrial disorders

OBJECTIVE: To investigate cerebral metabolism by 2-[18F]fluorodeoxy-d-glucose (FDG) uptake using PET and cerebrovascular reverse capacity by transcranial Doppler sonography (TCD) in different mitochondrial diseases (mitochondrial myopathy; mitochondrial encephalopathy, lactacidosis, and stroke-like episodes [MELAS]; and chronic external ophthalmoplegia). BACKGROUND: Previous studies on individual patients with mitochondriopathies revealed abnormal accumulations of mitochondria in endothelium, smooth muscle cells, and pericytes of blood vessels in different parts of the nervous system (cerebrum, cerebellum, sural nerve) and skeletal muscle. On this basis, some investigators suggested a pathogenic role of vascular involvement in the MELAS syndrome and other encephalopathies. smhd1 DESIGN/METHODS: The authors investigated neuronal metabolism and cerebrovascular involvement with PET in 5 cases and with TCD with acetazolamide stimulation in 15 cases. The patients were divided into three groups: 1) interictal MELAS (n = 4); 2) progressive external ophthalmoplegia (n = 6); and 3) pure mitochondrial myopathy and neuropathy (n = 5). The results were compared with those from matched normal control subjects. The diagnoses were based on clinical phenotype as well as histopathologic and molecular analysis. RESULTS: Cerebral glucose uptake was impaired in all patients, both with and without CNS symptoms, particularly in the occipital and temporal lobes. The vasoreactivity of the small arterioles to acetazolamide did not differ significantly between the patients and healthy control subjects or between the different groups of mitochondrial disorders. CONCLUSIONS: MELAS does not appear to be a functional disturbance of arterioles leading to an ischemic vascular event. The clinical symptoms in MELAS are not the result of a mitochondrial angiopathy but are the consequences of a mitochondrial cytopathy affecting neurons or glia. There is no correlation between the decreased glucose metabolism and the duration of the disease.     

Voltage-gated ion channelopathies of the nervous system

Mutations in genes coding for voltage-gated ion channels cause a diverse group of disorders affecting heart, skeletal muscle, and brain. Mutant channels alter the electrical excitability of cells, which increases the susceptibility to paroxysmal symptoms including cardiac arrhythmia, periodic paralysis, myotonia, seizures, migraine headache, and episodic ataxia. This review provides an update on the genetics and physiology of diseases of skeletal muscle and brain caused by mutations in voltage-gated in ion channel genes. The discovery of specific ion channel defects provides a rational basis for designing pharmacological intervention, as ion channels are the molecular targets of many drugs in clinical use. Moreover, the advent of a molecular genetic-based diagnosis provides an important tool for clarifying the natural history and effectiveness of intervention in these disorders.     

Congenital disorder of glycosylation type 1T with a novel truncated homozygous mutation in PGM1 gene and literature review

The congenital disorders of glycosylation are a group of clinically and biochemically heterogeneous diseases characterized by multisystem involvement due to glycosylation defect of protein and lipid. Here we report a 49-year-old man with exercise-induced fatigue and pain of muscle, tachypnea, cleft palate and bifid uvula. Exercise induced elevation of serum creatine kinase (CK), ammonia and lactic acid was recorded. The abnormal levels of myoglobin, CK-MB and LDH as well as S-T elevation in electrocardiogram were observed in repeated hospitalization recordings. Electromyography showed myopathic damage. Repetitive nerve stimulation test of low rates showed decrement in the left deltoid muscle. He was identified with a novel homozygous frameshift variant in Phosphoglucomutase type 1 gene (c.405delT p.N135Kfs*9) by whole exome sequencing. Muscle biopsy exhibited minimal variation in fiber size without abnormal glycogen accumulation. Compared with controls’, the patient's sample showed no signal at ～61?kDa using N- or C-terminus antibody of Phosphoglucomutase type 1 in western blotting. A signal at ～20?kDa was detected in patient using N-terminus antibody. Immunofluorescence revealed trace expression of C-terminus and a much lower expression of N-terminus on the sarcolemma than normal. Our findings indicate that c.405delT encodes a truncated protein with abnormal distribution and expression in skeletal muscle. In conclusion, genes associated with congenital disorders of glycosylation should be analyzed in patients with maxillofacial dysplasia, exertional weakness, cardiac involvement and exercise-induced-ammoniemia, without glycogen storage in skeletal muscle.     

Physiologic principles underlying ion channelopathies

The ion channelopathies are a diverse array of human disorders caused by mutations in genes coding for ion channels. More than 40 different channelopathies have been identified, with representative disorders from every major class of ion channel and affecting all electrically excitable tissues: brain, peripheral nerve, skeletal muscle, smooth muscle, and heart. This review provides an overview of ion channel classification, structure, and function as a framework for understanding which ion channel properties are altered by disease-associated mutations and how these changes disrupt cellular excitability for channelopathies affecting skeletal muscle and the CNS.