Metabolic myopathy in canine muscle-type phosphofructokinase deficiency

Gene therapy for glycogen storage diseases

Glycogen storage disorders (GSDs) are inherited disorders of metabolism resulting from the deficiency of individual enzymes involved in the synthesis, transport, and degradation of glycogen. This literature review summarizes the development of gene therapy for the GSDs. The abnormal accumulation of glycogen and deficiency of glucose production in GSDs lead to unique symptoms based upon the enzyme step and tissues involved, such as liver and kidney involvement associated with severe hypoglycemia during fasting and the risk of long-term complications including hepatic adenoma/carcinoma and end stage kidney disease in GSD Ia from glucose-6-phosphatase deficiency, and cardiac/skeletal/smooth muscle involvement associated with myopathy +/- cardiomyopathy and the risk for cardiorespiratory failure in Pompe disease. These symptoms are present to a variable degree in animal models for the GSDs, which have been utilized to evaluate new therapies including gene therapy and genome editing. Gene therapy for Pompe disease and GSD Ia has progressed to Phase I and Phase III clinical trials, respectively, and are evaluating the safety and bioactivity of adeno-associated virus vectors. Clinical research to understand the natural history and progression of the GSDs provides invaluable outcome measures that serve as endpoints to evaluate benefits in clinical trials. While promising, gene therapy and genome editing face challenges with regard to clinical implementation, including immune responses and toxicities that have been revealed during clinical trials of gene therapy that are underway. Gene therapy for the glycogen storage diseases is under development, addressing an unmet need for specific, stable therapy for these conditions.

EHBP1L1 Frameshift Deletion in English Springer Spaniel Dogs with Dyserythropoietic Anemia and Myopathy Syndrome (DAMS) or Neonatal Losses

Hereditary myopathies are well documented in dogs, whereas hereditary dyserythropoietic anemias are rarely seen. The aim of this study was to further characterize the clinical and clinicopathological features of and to identify the causative genetic variant for a dyserythropoietic anemia and myopathy syndrome (DAMS) in English springer spaniel dogs (ESSPs). Twenty-six ESSPs, including five dogs with DAMS and two puppies that died perinatally, were studied. Progressive weakness, muscle atrophy—particularly of the temporal and pelvic muscles—trismus, dysphagia, and regurgitation due to megaesophagus were observed at all ages. Affected dogs had a non-regenerative, microcytic hypochromic anemia with metarubricytosis, target cells, and acanthocytes. Marked erythroid hyperplasia and dyserythropoiesis with non-orderly maturation of erythrocytes and inappropriate microcytic metarubricytosis were present. Muscle biopsies showed centralized nuclei, central pallor, lipocyte infiltrates, and fibrosis, which was consistent with centronuclear myopathy. The genome sequencing of two affected dogs was compared to 782 genomes of different canine breeds. A homozygous frameshift single-base deletion in EHBP1L1 was identified; this gene was not previously associated with DAMS. Pedigree analysis confirmed that the affected ESSPs were related. Variant genotyping showed appropriate complete segregation in the family, which was consistent with an autosomal recessive mode of inheritance. This study expands the known genotype–phenotype correlation of EHBP1L1 and the list of potential causative genes in dyserythropoietic anemias and myopathies in humans. EHBP1L1 deficiency was previously reported as perinatally lethal in humans and knockout mice. Our findings enable the genetic testing of ESSP dogs for early diagnosis and disease prevention through targeted breeding strategies.

Preclinical Development of New Therapy for Glycogen Storage Diseases

Glycogen storage disease (GSD) consists of more than 10 discrete conditions for which the biochemical and genetic bases have been determined, and new therapies have been under development for several of these conditions. Gene therapy research has generated proof-of-concept for GSD types I (von Gierke disease) and II (Pompe disease). Key features of these gene therapy strategies include the choice of vector and regulatory cassette, and recently adeno-associated virus (AAV) vectors containing tissue-specific promoters have achieved a high degree of efficacy. Efficacy of gene therapy for Pompe disease depend upon the induction of immune tolerance to the therapeutic enzyme. Efficacy of von Gierke disease is transient, waning gradually over the months following vector administration. Small molecule therapies have been evaluated with the goal of improving standard of care therapy or ameliorating the cellular abnormalities associated with specific GSDs. The receptor-mediated uptake of the therapeutic enzyme in Pompe disease was enhanced by administration of β2 agonists. Rapamycin reduced the liver fibrosis observed in GSD III. Further development of gene therapy could provide curative therapy for patients with GSD, if efficacy from preclinical research is observed in future clinical trials and these treatments become clinically available.

Large animal models and new therapies for glycogen storage disease

Glycogen storage diseases (GSD), a unique category of inherited metabolic disorders, were first described early in the twentieth century. Since then, the biochemical and genetic bases of these disorders have been determined, and an increasing number of animal models for GSD have become available. At least seven large mammalian models have been developed for laboratory research on GSDs. These models have facilitated the development of new therapies, including gene therapy, which are undergoing clinical translation. For example, gene therapy prolonged survival and prevented hypoglycemia during fasting for greater than one year in dogs with GSD type Ia, and the need for periodic re-administration to maintain efficacy was demonstrated in that dog model. The further development of gene therapy could provide curative therapy for patients with GSD and other inherited metabolic disorders.

Hereditary phosphofructokinase deficiency in wachtelhunds

Hereditary phosphofructokinase (PFK) deficiency was diagnosed in two Wachtelhund dogs and suspected in three related Wachtelhund dogs with exercise intolerance, hemolytic anemia, and pigmenturia. Severe, persistent reticulocytosis in light of only mild anemia together with hemoglobinuria after strenuous exercise suggested PFK deficiency. Low erythrocyte PFK activity together with low 2,3-diphosphoglycerate concentrations and a high hemoglobin-oxygen affinity confirmed the diagnosis. The PFK deficiency is due to a single missense mutation in the muscle-type PFK M-PFK gene in English springer and American cocker spaniels, whippets, and mixed-breed dogs; however, these PFK-deficient Wachtelhunds do not have the same PFK mutation.

A complex rearrangement in GBE1 causes both perinatal hypoglycemic collapse and late-juvenile-onset neuromuscular degeneration in glycogen storage disease type IV of Norwegian forest cats

Deficiency of glycogen branching enzyme (GBE) activity causes glycogen storage disease type IV (GSD IV), an autosomal recessive error of metabolism. Abnormal glycogen accumulates in myocytes, hepatocytes, and neurons, causing variably progressive, benign to lethal organ dysfunctions. A naturally occurring orthologue of human GSD IV was described previously in Norwegian forest cats (NFC). Here, we report that while most affected kittens die at or soon after birth, presumably due to hypoglycemia, survivors of the perinatal period appear clinically normal until onset of progressive neuromuscular degeneration at 5 months of age. Molecular investigation of affected cats revealed abnormally spliced GBE1 mRNA products and lack of GBE cross-reactive material in liver and muscle. Affected cats are homozygous for a complex rearrangement of genomic DNA in GBE1, constituted by a 334 bp insertion at the site of a 6.2 kb deletion that extends from intron 11 to intron 12 (g. IVS11+1552_IVS12-1339 del6.2kb ins334 bp), removing exon 12. An allele-specific, PCR-based test demonstrates that the rearrangement segregates with the disease in the GSD IV kindred and is not found in unrelated normal cats. Screening of 402 privately owned NFC revealed 58 carriers and 4 affected cats. The molecular characterization of feline GSD IV will enhance further studies of GSD IV pathophysiology and development of novel therapies in this unique animal model.

Neuromuscular complications in endocrine and metabolic disorders

Many of the endocrine and metabolic myopathies have no unique features, and for most clinicians, it is not possible to remember the clinical nuances of all the specific abnormalities and deficiencies responsible for these myopathies. This can make this group of diseases difficult to suspect. It is more important to recognize the general features of myopathic disease and to consider muscle biopsies as a preliminary diagnostic technique, with the potential for further investigation if a myopathy is confirmed.

Haemolytic anaemia and exercise intolerance due to phosphofructokinase deficiency in related springer spaniels

Phosphofructokinase (PFK) deficiency is an autosomal recessive inherited disorder in dogs causing haemolytic crises and exertional myopathy. The clinical signs may be confused with those of recurrent immune-mediated haemolytic anaemia. The deficiency has been commonly observed in field trial (working) English springer spaniels (ESSPs), but also in the conformation line of ESSPs in the USA over the past two decades. This report documents the first family of ESSPs found with PFK deficiency in Europe. Two related adult ESSPs in Denmark had intermittent signs of pigmenturia after exercise (hunting) and had evidence of a regenerative haemolytic anaemia. Based upon DNA sequencing data, both dogs had the previously described nonsense point mutation in the muscle-type PFK gene (delta2228G-->A). Study of 17 related family members using a simple and accurate PFK-DNA test revealed one additional PFK-deficient dog (with minor exercise intolerance), nine carriers and seven normal (or 'clear') ESSPs. Recently, the authors have also identified PFK carriers and affected ESSPs in the UK. Screening for PFK deficiency is recommended for ESSPs with suspicious clinical signs and before using any for field trials or breeding in order to prevent the further spread of this hereditary disorder.

Insights into muscle diseases gained by phosphorus magnetic resonance spectroscopy

Phosphorus magnetic resonance spectroscopy (P-MRS) has now been used in the investigation of muscle energy metabolism in health and disease for over 15 years. The present review describes the basics of the metabolic observations made by P-MRS including the assumptions and problems associated with the use of this technique. Extramuscular factors, which may affect the P-MRS results, are detailed. The important P-MRS observations in patients with mitochondrial myopathies, including the monitoring of experimental therapies, are emphasized. The findings in other metabolic myopathies (those associated with glycolytic defects or endocrine disturbances) and in the destructive myopathies (the dystrophies and the inflammatory myopathies) are also described. Observations made in normal and abnormal fatigue, fibromyalgia, and malignant hyperthermia are considered. Finally, a summary of the possible diagnostic use of P-MRS in exercise intolerance is provided.

MR spectroscopy and imaging in metabolic myopathies

Magnetic resonance spectroscopy and imaging of muscle and brain offers new possibilities for noninvasive diagnosis of metabolic myopathies. These functional techniques allow assessment of the pathophysiology of these disorders and also can be used for monitoring disease evolution and response to therapy. In this article, the authors review the magnetic resonance spectroscopy and imaging features of mitochondrial encephalomyopathies, glycolytic disorders, and hypothyroidism.

In vivo determination of altered hemoglobin saturation in dogs with M-type phosphofructokinase deficiency

Muscle-type phosphofructokinase (M-PFK) deficiency causes an exertional myopathy and chronic hemolysis in affected humans and dogs, the only animal model available. Deficient individuals have impaired glycolytic metabolism, impaired oxidative metabolism, and increased hemoglobin-oxygen (HbO2) affinity as a result of low 2,3-diphosphoglycerate (2,3-DPG) levels. The purpose of this study was to determine if PFK-deficient muscle has abnormal oxygen saturation during exercise. Oxygen saturation of hemoglobin/myoglobin was measured noninvasively in skeletal muscle during progressive muscle activation using near-infrared spectroscopy (NIRS). Muscle metabolites were also measured using magnetic resonance spectroscopy (MRS). PFK-deficient and normal dogs were anesthetized and the cranial tibial muscles stimulated for 6 min at each of four different rates (1, 2, 4, and 8 Hz). With increasing stimulation, muscles from normal dogs showed progressive decrease in hemoglobin saturation. In contrast, PFK-deficient dogs exhibited either an increase in hemoglobin saturation or an initial decrease with no further change. PFK-deficient muscles accumulated 11.1 +/- 3.5 mmol/L of sugar phosphate which was not seen in normal muscle and had higher calculated [ADP] levels at each stimulation level, indicating impaired oxidative metabolism. These findings are consistent with the hypothesis that these animals have impaired oxidative metabolism and impaired muscle O2 extraction from hemoglobin due to increased HbO2 affinity. NIRS appears to be a useful noninvasive method of monitoring tissue oxygen saturation in normal or disease conditions.

Impaired aerobic glycolysis in muscle phosphofructokinase deficiency results in biphasic post-exercise phosphocreatine recovery in 31P magnetic resonance spectroscopy

Using 31P magnetic resonance spectroscopy, energy metabolism in calf muscles of two patients with biochemically and genetically proven muscular phosphofructokinase deficiency, and an asymptomatic heterozygote was monitored during isometric foot plantarflexion performed under aerobic and anaerobic conditions and in the aerobic recovery phases. In the heterozygote only a moderate alteration from normal was found in terms of an elevated ATP demand during exercise. In the homozygote, hexose phosphates, indicated as phosphomonoesters, increased dramatically during contraction. Phosphomonoester accumulation resulted in consumption of free inorganic phosphate (P(i)). During ischemic exercise the absence of glycolytic ATP formation resulted in a linear time course of phosphocreatine breakdown and a moderate alkalinization. During the recovery, phosphocreatine resynthesis showed a biphasic time course, indicating that mitochondrial function itself was not directly affected. At first glance, the early depletion of P(i) below initial resting levels and the rate of phosphate splitting from sugar phosphates seemed to become the limiting factor for the rate of the oxidative phosphorylation and creatine kinase reaction. However, the actual concentrations of P(i) and ADP estimated at the onset of delay were too high to exclusively explain the dramatic delay in PCr resynthesis. For this reason, a reduced turnover of the citric acid cycle was assumed, which was caused by the complete absence of glycolysis in PFK deficiency patients. Furthermore, results from PFK deficiency patients were compared with previous findings from myophosphorylase deficiency patients in the literature.

Congenital erythrocyte enzyme deficiencies

Congenital hemolytic anemias resulting from PK, PFK, and G6PD enzyme deficiencies have been reported in domestic animals. Dogs with PFK deficiency may have episodes of intravascular hemolysis with hemoglobinuria in addition to a persistent compensated hemolytic anemia. Patients with mild G6PD deficiency are not anemic but may show increased susceptibility to oxidant-induced erythrocyte injury. Persistent methemoglobinemia has been reported in dogs and cats with methemoglobin reductase enzyme deficiency. Affected animals have cyanotic-appearing mucous membranes but show no or only mild clinical signs attributable to hypoxemia. Enzyme assays are usually done after acquired causes of hemolytic anemia and methemoglobinemia have been ruled out.

In vivo 31P NMR studies of paraplegics' muscles activated by functional electrical stimulation

The bioenergetics of paralyzed muscles of spastic paraplegic patients under functional electrical stimulation (FES) was studied in vivo using 31P NMR. The protocol included rest, 3 min of induced tetanic isometric contraction through surface electrodes and 40 min of recovery. The continuous stimulation, the force recording and the 31P NMR measurements were sampled simultaneously inside the whole body imager. Normal values were found for the phosphorous metabolite ratios at rest. During contraction, prominent changes were detected including: a) accumulation of inorganic phosphate (Pi) accompanied by an unusually strong signal in the phosphomonoester (PME) region, b) phosphocreatine (PCr) decline, and c) a decrease in the intracellular pH. In the following recovery period the physiological state of the muscle was monitored and quantitated by 31P NMR. No metabolic and mechanical irreversible damage was detected in the paraplegics' muscles activated by FES under our experimental conditions.

Rhabdomyolysis in childhood. A primer on normal muscle function and selected metabolic myopathies characterized by disordered energy production

Patients with rhabdomyolysis present an important clinical problem. In acute episodes immediate treatment may be necessary to prevent significant morbidity and mortality. Evaluation of affected patients necessitates an understanding of basic muscle pathophysiology and of the variety of disturbances that can interfere with muscle energy metabolism. The physician must then pursue a systematic stepwise evaluation (Table 6) that includes obtaining relevant history and laboratory studies, as well as arranging for appropriate provocative testing and muscle biopsy. Once the diagnosis is established, patient and family counseling is necessary, particularly in genetic disorders. Unfortunately, specific therapies have not proven entirely successful, and treatment generally has been directed at reducing the severity of rhabdomyolytic episodes.

Canine X-linked muscular dystrophy studied with in vivo phosphorus magnetic resonance spectroscopy

Duchenne muscular dystrophy (DMD) is an X-linked disease characterized by progressive muscle weakness and degeneration. Dystrophin is the product of the missing gene in this disorder. However, the cause of the dystrophic process is not understood. Transient muscle injury is normally seen after muscle exercise, and may be a necessary process in muscle growth and preservation. We, therefore, chose to evaluate the role of exercise in Duchenne dystrophy by studying the canine X-linked animal model (CXMD). These dogs also lack dystrophin and have clinical signs similar to humans. Exercise was initiated by electrical stimulation, and muscle metabolism was monitored with phosphorus magnetic resonance spectroscopy (P-MRS). Dogs with CXMD had abnormal muscle pathology and markedly elevated serum CK. The inorganic phosphate (Pi) to phosphocreatine (PCr) ratio was increased in CXMD dogs at rest compared with normal dogs (Pi/(Pi + PCr) = 0.166 +/- 0.054 for CXMD and 0.073 +/- 0.017 for normals, mean +/- SE). No changes in resting ATP, pH, phosphomonoesters (PME), and phosphodiesters (PDE) were seen. The mean Pi/(Pi + PCr) and pH values during stimulation were normal in the CXMD dogs. Two to three days after electrical stimulation, resting Pi/(Pi + PCr) ratios were significantly increased in the CXMD dogs (0.127 +/- 0.029 compared with 0.172 +/- 0.054, mean +/- SD). Normal dogs showed no increase in Pi/(Pi + PCr) following stimulation. There was a 50-fold greater increase in serum CK in CXMD compared with normal dogs following exercise. These results indicate greater muscle injury in CXMD muscle, and suggest that in the absence of dystrophin, exercise-induced muscle injury may play a role in the dystrophic process.

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.

Polysaccharide storage myopathy in canine phosphofructokinase deficiency (type VII glycogen storage disease)

A severe, progressive myopathy developed in an 11-year-old, phosphofructokinase (PFK)-deficient, male, English Springer Spaniel dog. Results from a routine neurological examination were normal. Examination of histologic sections of skeletal muscle revealed large accumulations of material in some myofibers. These deposits were pale, basophilic, somewhat flocculent, and slightly granular with hematoxylin and eosin stain. Most fascicles examined in sections of limb and trunk muscles were affected to some degree, with up to 10% of muscle fibers being involved. Deposits stained strongly with periodic acid-Schiff and were resistant to digestion by alpha amylase but were removed by incubation with gamma amylase. Deposits were faintly positive with Gomori's methenamine silver technique and alcian blue (pH 2.5) and were brown-gray with Lugol's iodine solution but were negative with other stains. Based on staining characteristics, the deposits seemed to consist primarily of an amylopectin-like polysaccharide(s). Alcian blue staining (pH 2.5) was removed by treatment with neuraminidase but not with hyaluronidase, indicating that some sialic acid residues were also present. Electron microscopically, the deposits were composed of short granular filaments, small granules and amorphous material. They were not membrane bound. The morphologic appearance and staining characteristics of the deposits were remarkably similar to deposits previously described in human PFK-deficient myopathy. As expected, total PFK activities were markedly reduced when assayed in skeletal muscles of this dog. In contrast with other PFK-deficient dogs, muscle glycogen in this animal was not increased above that of normal dogs.

Muscle bio-energetics in acute glycolytic block: in vivo phosphorus-nuclear magnetic resonance study of iodo-acetate injected rats

In vivo phosphorus nuclear magnetic resonance spectroscopy of muscle was performed at rest, during work and during postexercise recovery in rats injected with iodo-acetate (IA) (35-40 mg.kg-1, intra-arterially), in order to follow bio-energetic changes in muscle with acute glycolytic block. Three animals with contracture had very low ratios of phosphocreatine:inorganic phosphate (PCr:Pi) at rest (0.5-0.9). The PCr:Pi were normal at rest (6.9 +/- 2.0, +/- 2 SD) in all other rats. Exercise-induced continuous accumulation of phosphomonoesters (PME), the characteristic finding of glycolytic block, was observed. The end-exercise levels of PME correlated with the degree of block measured in vitro. During steady-state work, induced by nerve stimulation at four frequencies, PCr:Pi values were significantly lower (p less than 0.02) than the control values at 0.25, 1.0 and 2.0 Hz. The ATP levels fell during exercise to reach 75% +/- 7% of initial values. The recovery of PCr:Pi from exercise and the disappearance of PME were slow. Two animals which survived the IA injection demonstrated much lower PME accumulation 18 h later. It is concluded that in acute muscle glycolytic block: (1) energy metabolism is impaired during exercise and also at rest, (2) accumulating PME can serve as an indicator of the degree of glycolytic block, (3) ATP levels fall during work, and (4) postexercise recovery is slow. The findings are compared with 31P-NMR observations in chronic muscle glycolytic disorders.