Characterization of the enzymatic lesion in inherited phosphofructokinase deficiency in the dog: an animal analogue of human glycogen storage disease type VII

Preclinical Research in Glycogen Storage Diseases: A Comprehensive Review of Current Animal Models

GSD are a group of disorders characterized by a defect in gene expression of specific enzymes involved in glycogen breakdown or synthesis, commonly resulting in the accumulation of glycogen in various tissues (primarily the liver and skeletal muscle). Several different GSD animal models have been found to naturally present spontaneous mutations and others have been developed and characterized in order to further understand the physiopathology of these diseases and as a useful tool to evaluate potential therapeutic strategies. In the present work we have reviewed a total of 42 different animal models of GSD, including 26 genetically modified mouse models, 15 naturally occurring models (encompassing quails, cats, dogs, sheep, cattle and horses), and one genetically modified zebrafish model. To our knowledge, this is the most complete list of GSD animal models ever reviewed. Importantly, when all these animal models are analyzed together, we can observe some common traits, as well as model specific differences, that would be overlooked if each model was only studied in the context of a given GSD.

A Great Catch for Investigating Inborn Errors of Metabolism-Insights Obtained from Zebrafish

Inborn errors of metabolism cause abnormal synthesis, recycling, or breakdown of amino acids, neurotransmitters, and other various metabolites. This aberrant homeostasis commonly causes the accumulation of toxic compounds or depletion of vital metabolites, which has detrimental consequences for the patients. Efficient and rapid intervention is often key to survival. Therefore, it requires useful animal models to understand the pathomechanisms and identify promising therapeutic drug targets. Zebrafish are an effective tool to investigate developmental mechanisms and understanding the pathophysiology of disorders. In the past decades, zebrafish have proven their efficiency for studying genetic disorders owing to the high degree of conservation between human and zebrafish genes. Subsequently, several rare inherited metabolic disorders have been successfully investigated in zebrafish revealing underlying mechanisms and identifying novel therapeutic targets, including methylmalonic acidemia, Gaucher’s disease, maple urine disorder, hyperammonemia, TRAPPC11-CDGs, and others. This review summarizes the recent impact zebrafish have made in the field of inborn errors of metabolism.

The proliferating cell hypothesis: a metabolic framework for Plasmodium growth and development

•

The hypothesis offers a framework to explain the atypical features of parasite metabolism.

•

Aerobic glycolysis is hypothesised to meet the biosynthetic demands of rapid proliferation.

•

Differentiation may be epigenetically regulated in response to nutrient-linked metabolism.

We hypothesise that intraerythrocytic malaria parasite metabolism is not merely fulfilling the need for ATP generation, but is evolved to support rapid proliferation, similar to that seen in other rapidly proliferating cells such as cancer cells. Deregulated glycolytic activity coupled with impaired mitochondrial metabolism is a metabolic strategy to generate glycolytic intermediates essential for rapid biomass generation for schizogony. Further, we discuss the possibility that Plasmodium metabolism is not only a functional consequence of the ‘hard-wired’ genome and argue that metabolism may also have a causal role in triggering the cascade of events that leads to developmental stage transitions. This hypothesis offers a framework to rationalise the observations of aerobic glycolysis, atypical mitochondrial metabolism, and metabolic switching in nonproliferating stages.

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.

Animal models of glycogen storage disorders

Glycogen is a polymer of glucose needed to provide for a continuous source of glucose during fasting. Glycogen synthesis and degradation are tightly controlled by complex regulatory mechanisms and any disturbance in this regulation can lead to an inadequate reservoir of glycogen or an accumulation of excess or abnormal glycogen stored either in the cytosol or in the lysosomes. Problems in the degradation or synthesis of glycogen are referred to as glycogen storage disorders (GSDs), which individually are rare diseases, yet collectively are a major category of inborn errors of metabolism in humans. To date, 11 distinct forms of GSDs are represented in animal models. These models provide a means to understand the mechanisms that regulate and execute the synthesis and degradation of glycogen. In this review, we summarize animal models that have arisen spontaneously in nature or have been engineered in the laboratory by recombinant DNA techniques, and categorize the disorders of glycogen metabolism as disorders of either synthesis or degradation.

Phosphofructo-1-kinase deficiency leads to a severe cardiac and hematological disorder in addition to skeletal muscle glycogenosis

Mutations in the gene for muscle phosphofructo-1-kinase (PFKM), a key regulatory enzyme of glycolysis, cause Type VII glycogen storage disease (GSDVII). Clinical manifestations of the disease span from the severe infantile form, leading to death during childhood, to the classical form, which presents mainly with exercise intolerance. PFKM deficiency is considered as a skeletal muscle glycogenosis, but the relative contribution of altered glucose metabolism in other tissues to the pathogenesis of the disease is not fully understood. To elucidate this issue, we have generated mice deficient for PFKM (Pfkm^−/−). Here, we show that Pfkm^−/− mice had high lethality around weaning and reduced lifespan, because of the metabolic alterations. In skeletal muscle, including respiratory muscles, the lack of PFK activity blocked glycolysis and resulted in considerable glycogen storage and low ATP content. Although erythrocytes of Pfkm^−/− mice preserved 50% of PFK activity, they showed strong reduction of 2,3-biphosphoglycerate concentrations and hemolysis, which was associated with compensatory reticulocytosis and splenomegaly. As a consequence of these haematological alterations, and of reduced PFK activity in the heart, Pfkm^−/− mice developed cardiac hypertrophy with age. Taken together, these alterations resulted in muscle hypoxia and hypervascularization, impaired oxidative metabolism, fiber necrosis, and exercise intolerance. These results indicate that, in GSDVII, marked alterations in muscle bioenergetics and erythrocyte metabolism interact to produce a complex systemic disorder. Therefore, GSDVII is not simply a muscle glycogenosis, and Pfkm^−/− mice constitute a unique model of GSDVII which may be useful for the design and assessment of new therapies.

Type VII glycogen storage disease (GSDVII), or Tarui disease, is a rare genetic disorder characterized by glycogen accumulation in skeletal muscle. The molecular cause is loss of activity of the muscle isoform of phosphofructokinase (PFK), which phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate, commiting glucose to glycolysis. Entry of fructose-6-phosphate into glycolysis is thus blocked, increasing glycogen synthesis and accumulation. Clinical manifestations of the disease are heterogeneous, ranging from exercise intolerance to early childhood death. To further understand the human pathology, we generated mice lacking muscle PFK. As in human patients, these mice showed severe exercise intolerance, hemolysis, and most died young. Lack of glycolysis in skeletal muscle also causes alterations in bioenergetics and compensatory changes in key metabolic genes. Additionally, although erythrocytes retained 50% of normal PFK activity, their overall functionality was impaired, aggravating the muscle dysfunction. Moreover, marked metabolic alterations in the heart lead to chronic hypertrophy, suggesting that cardiac pathology in GSDVII may be underestimated or misdiagnosed. This study indicates that this disease is more complex than a muscle glycogenosis and that symptoms other than those classically described should be taken into consideration. Finally, this animal model will enable us to develop new therapeutic approaches and better diagnostic tools.

Hemolysis, myopathy, and cardiac disease associated with hereditary phosphofructokinase deficiency in two Whippets

Two male castrated Whippet littermates were presented at 1 year of age for pallor, tachycardia, systolic heart murmur, dark yellow to orange feces, intermittent lethargy, pigmenturia, and muscle shivering or cramping after exercise. Persistent macrocytic hypochromic anemia with marked reticulocytosis and metarubricytosis was found when CBC results were compared with reference values for Whippets. Increased serum creatine kinase activity and hyperkalemia also were sometimes present over the 4-year period of evaluation. Progressively increasing serum concentrations of N-terminal prohormone brain natriuretic peptide suggested cardiac disease. Erythrocytes from the whippets were less osmotically fragile but more alkaline fragile than those from control dogs. Erythrocyte phosphofructokinase (PFK) activities and 2,3-diphosphoglycerate concentrations were decreased. Restriction enzyme-based DNA test screening and DNA sequencing revealed the same mutation in the muscle-PFK gene of the Whippets as seen in English Springer Spaniel dogs with PFK deficiency. This is the first report of PFK deficiency in Whippet dogs. In addition to causing hemolysis and exertional myopathy, heart disease may be a prominent clinical component of PFK deficiency in this breed and has not been previously recognized in PFK-deficient English Springer Spaniels.

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.

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.

Molecular basis of canine muscle type phosphofructokinase deficiency

Muscle type phosphofructokinase (M-PFK) deficiency is a rare inherited glycogen storage disease in humans that causes exertional myopathy and hemolysis. The molecular basis of canine M-PFK deficiency, the only naturally occurring animal homologue, was investigated. Lack of M-PFK enzyme activity was caused by a nonsense mutation in the penultimate exon of the M-PFK gene, leading to rapid degradation of a truncated (40 amino acids) and therefore unstable M-PFK protein. A polymerase chain reaction-based test was devised to identify M-PFK-deficient and carrier animals. This represents one of only a few inborn errors of metabolism where the molecular defect has been identified in a large animal model which can now be used to develop and assess novel therapeutic strategies.

Myophosphorylase deficiency associated with rhabdomyolysis and exercise intolerance in 6 related Charolais cattle

A Charolais calf presented to the Veterinary Medical Teaching Hospital with a history of recumbency following forced exercise. The calf was unable to stand, and had severe rhabdomyolysis, dehydration, and electrolyte imbalance. Blood selenium concentrations were within normal limits. A complete absence of histochemical staining for phosphorylase was apparent in muscle biopsies. Five other animals in the herd also had exercise intolerance and had a complete absence of phosphorylase staining in muscle biopsies. Biochemical analyses confirmed a deficiency of myophosphorylase (range 0-0.3 mumol/g per minute: normals 15-27) with normal to slightly elevated muscle glycogen concentrations. Pedigrees from all affected animals showed a common ancestor on the sire's and dam's side of each phosphorylase-deficient animal, suggesting an autosomal recessive transmission. Although myophosphorylase deficiency was described in humans (McArdle's disease) over 40 years ago, these cattle represent the first animal model for this disease.

Characterization of phosphofructokinase-deficient canine erythrocytes

Dogs homozygously affected with muscle-type phosphofructokinase (PFK) deficiency had about 20% of normal erythrocyte PFK activity and exhibited a compensated haemolytic anaemia. Erythrocyte glucose-6-phosphate and fructose-6-phosphate concentrations were increased and dihydroxyacetone phosphate and 2,3-bisphosphoglycerate values were below normal in affected dogs. Other intermediates distal to the PFK step were not significantly below normal and fructose-1,6-bisphosphate was even above normal. Erythrocyte ATP was higher than normal in affected dogs owing to the reticulocytes present. Abnormal adenylate metabolism was demonstrated by low ATP/AMP and ADP/AMP ratios and the inability to maintain ATP content when affected erythrocytes were incubated with cyanide. Glucose-1,6-bisphosphate content was normal, and fructose-2,6-bisphosphate content in affected canine erythrocytes was higher than normal. Studies of erythrocyte PFK isozymes revealed altered enzyme kinetic properties in affected dogs which appeared to be due to the loss of the M-type subunit.

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.

Metabolic myopathy in canine muscle-type phosphofructokinase deficiency

In vivo 31phosphorus nuclear magnetic resonance spectroscopy (P-NMR) of the anterior tibialis muscle was used to investigate the metabolic myopathy of inherited muscle-type phosphofructokinase (PFK) deficiency in four (homozygous) dogs who had mild exercise intolerance, rare muscle cramps, increased serum creatine kinase activity, but no myoglobinuria. During isometric muscle work induced by indirect electrical stimulation, and subsequent recovery, changes in the ratio of phosphocreatine (PCr) and inorganic phosphates (Pi) were comparable in muscle of PFK-deficient and normal dogs and indicated a large capacity for arobic oxidative phosphorylation in canine muscle. The progressive accumulation of sugar phosphates (PME) during graded exercise clearly demonstrated the glycolytic block in PFK-deficient dogs. During a muscle contracture, induced by acute muscle stimulation, PFK-deficient muscle became completely depleted of PCr and ATP, accumulated large amounts of PME, and recovered very slowly. We conclude that PFK-deficient dogs have a metabolic myopathy that demonstrated some but not all the features recognized in the human disorder.

Characterization of the enzymatic defect in late-onset muscle phosphofructokinase deficiency. New subtype of glycogen storage disease type VII

Human phosphofructokinase (PFK) exists in tetrameric isozymic forms, at least in vitro. Muscle and liver contain homotetramers M4 and L4, respectively, whereas red cells contain five isozymes composed of M (muscle) and L (liver) type subunits, i.e., M4, M3L, M2L2, and ML3, and L4. Homozygous deficiency of muscle PFK results in the classic glycogen storage disease type VII characterized by exertional myopathy and hemolytic syndrome beginning in early childhood. The genetic lesion results in a total and partial loss of muscle and red cell PFK, respectively. Characteristically, the residual red cell PFK from the patients consists of isolated L4 isozyme; the M-containing hybrid isozymes are completely absent. In this study, we investigated an 80-yr-old man who presented with a 10-yr history of progressive weakness of the lower limbs as the only symptom. The residual red cell PFK showed the presence of a few M-containing isozymes in addition to the predominant L4 species, indicating that the genetic lesion is a "leaky" mutation of the gene coding for the M subunit. The presence of a small amount of enzyme activity in the muscle may account for the atypical myopathy in this patient.

Autosomal recessive inherited phosphofructokinase deficiency in English springer spaniel dogs

Three families of English springer spaniel dogs with phosphofructokinase (PFK) deficiency causing haemolysis were studied. Four male dogs and one female dog with chronic haemolysis and haemolytic crises were found to have markedly reduced PFK activity in erythrocytes (8-20% of control English springer spaniels). PFK-deficient erythrocytes exhibited an extreme alkaline and sucrose lysis. The oxygen dissociation curve of erythrocyte suspensions was shifted to the left with a 50% saturation of haemoglobin at a partial oxygen pressure of 16-17 mmHg (normal 26-31 mmHg). Muscle wasting and mildly increased serum creatine phosphokinase activity were also noted. Six clinically normal first degree relatives of affected dogs had erythrocyte PFK activities that were 38-51% of controls. In these family members, there was an erythrocytosis and mild reticulocytosis probably due to a mildly enhanced haemoglobin-oxygen affinity but no increase in serum creatine phosphokinase. These studies confirm the familial nature of muscle-type PFK deficiency in English springer spaniels and support the conclusion that this animal model of the human glycogen storage disease type VII is inherited as an autosomal recessive trait.