Fat and carbohydrate metabolism during exercise in late-onset Pompe disease

Defect in degradation of glycogenin-exposed residual glycogen in lysosomes is the fundamental pathomechanism of Pompe disease

Pompe disease is a lysosomal storage disorder that preferentially affects muscles, and it is caused by GAA mutation coding acid alpha-glucosidase in lysosome and glycophagy deficiency. While the initial pathology of Pompe disease is glycogen accumulation in lysosomes, the special role of the lysosomal pathway in glycogen degradation is not fully understood. Hence, we investigated the characteristics of accumulated glycogen and the mechanism underlying glycophagy disturbance in Pompe disease. Skeletal muscle specimens were obtained from the affected sites of patients and mouse models with Pompe disease. Histological analysis, immunoblot analysis, immunofluorescence assay, and lysosome isolation were utilized to analyze the characteristics of accumulated glycogen. Cell culture, lentiviral infection, and the CRISPR/Cas9 approach were utilized to investigate the regulation of glycophagy accumulation. We demonstrated residual glycogen, which was distinguishable from mature glycogen by exposed glycogenin and more α-amylase resistance, accumulated in the skeletal muscle of Pompe disease. Lysosome isolation revealed glycogen-free glycogenin in wild type mouse lysosomes and variously sized glycogenin in Gaa-/- mouse lysosomes. Our study identified that a defect in the degradation of glycogenin-exposed residual glycogen in lysosomes was the fundamental pathological mechanism of Pompe disease. Meanwhile, glycogenin-exposed residual glycogen was absent in other glycogen storage diseases caused by cytoplasmic glycogenolysis deficiencies. In vitro, the generation of residual glycogen resulted from cytoplasmic glycogenolysis. Notably, the inhibition of glycogen phosphorylase led to a reduction in glycogenin-exposed residual glycogen and glycophagy accumulations in cellular models of Pompe disease. Therefore, the lysosomal hydrolysis pathway played a crucial role in the degradation of residual glycogen into glycogenin, which took place in tandem with cytoplasmic glycogenolysis. These findings may offer a novel substrate reduction therapeutic strategy for Pompe disease. © 2024 The Pathological Society of Great Britain and Ireland.

Long-term benefits of physical activity in adult patients with late onset Pompe disease: a retrospective cohort study with 10 years of follow-up

BACKGROUND

In 2011 a 12 weeks personalized exercise training program in 23 mildly affected adult late onset Pompe patients (age 19.6-70.5 years) improved endurance, muscle strength and function. Data on long-term effects of this program or of other physical activity in Pompe disease are absent. This retrospective cohort study aimed to explore effects of long-term healthy physical activity according to the WHO norm and the former exercise training program on the disease course.

RESULTS

A total of 29 adult late onset Pompe patients were included: 19 former exercise training program participants and 10 comparable control patients. Patients, who based on interviews, met the 2010 WHO healthy physical activity norm (active, n = 16) performed better on endurance (maximal cardiopulmonary exercise test), muscle strength and function compared to patients not meeting this norm (inactive, n = 13) (p < 0.05). Majority of the outcomes, including endurance and manually tested muscle strength, tended to be higher in the active patients of the 2011 training cohort who continued the program compared to active control patients (p > 0.05).

CONCLUSION

In Pompe disease long-term healthy physical activity according to the 2010 WHO norm leads to physical benefits and a personalized exercise training program may have additional favorable effects and both should be recommended as standard of care.

Metabolic Myopathies in the Era of Next-Generation Sequencing

Metabolic myopathies are rare inherited disorders that deserve more attention from neurologists and pediatricians. Pompe disease and McArdle disease represent some of the most common diseases in clinical practice; however, other less common diseases are now better-known. In general the pathophysiology of metabolic myopathies needs to be better understood. Thanks to the advent of next-generation sequencing (NGS), genetic testing has replaced more invasive investigations and sophisticated enzymatic assays to reach a final diagnosis in many cases. The current diagnostic algorithms for metabolic myopathies have integrated this paradigm shift and restrict invasive investigations for complicated cases. Moreover, NGS contributes to the discovery of novel genes and proteins, providing new insights into muscle metabolism and pathophysiology. More importantly, a growing number of these conditions are amenable to therapeutic approaches such as diets of different kinds, exercise training protocols, and enzyme replacement therapy or gene therapy. Prevention and management-notably of rhabdomyolysis-are key to avoiding serious and potentially life-threatening complications and improving patients' quality of life. Although not devoid of limitations, the newborn screening programs that are currently mushrooming across the globe show that early intervention in metabolic myopathies is a key factor for better therapeutic efficacy and long-term prognosis. As a whole NGS has largely increased the diagnostic yield of metabolic myopathies, but more invasive but classical investigations are still critical when the genetic diagnosis is unclear or when it comes to optimizing the follow-up and care of these muscular disorders.

Metabolic Myopathies

PURPOSE OF REVIEW

Metabolic myopathies are disorders that affect skeletal muscle substrate oxidation. Although some drugs and hormones can affect metabolism in skeletal muscle, this review will focus on the genetic metabolic myopathies.

RECENT FINDINGS

Impairments in glycogenolysis/glycolysis (glycogen storage disease), fatty acid transport/oxidation (fatty acid oxidation defects), and mitochondrial metabolism (mitochondrial myopathies) represent most metabolic myopathies; however, they often overlap clinically with structural genetic myopathies, referred to as pseudometabolic myopathies. Although metabolic myopathies can present in the neonatal period with hypotonia, hypoglycemia, and encephalopathy, most cases present clinically in children or young adults with exercise intolerance, rhabdomyolysis, and weakness. In general, the glycogen storage diseases manifest during brief bouts of high-intensity exercise; in contrast, fatty acid oxidation defects and mitochondrial myopathies usually manifest during longer-duration endurance-type activities, often with fasting or other metabolic stressors (eg, surgery, fever). The neurologic examination is often normal between events (except in the pseudometabolic myopathies) and evaluation requires one or more of the following tests: exercise stress testing, blood (eg, creatine kinase, acylcarnitine profile, lactate, amino acids), urine (eg, organic acids, myoglobin), muscle biopsy (eg, histology, ultrastructure, enzyme testing), and targeted (specific gene) or untargeted (myopathy panels) genetic tests.

SUMMARY

Definitive identification of a specific metabolic myopathy often leads to specific interventions, including lifestyle, exercise, and nutritional modifications; cofactor treatments; accurate genetic counseling; avoidance of specific triggers; and rapid treatment of rhabdomyolysis.

A Systematic Review investigating the Effectiveness of Exercise training in Glycogen Storage Diseases

Introduction

Glycogen storage diseases (GSDs) are rare inborn errors of carbohydrate metabolism typically with skeletal muscle and liver involvement. In those with skeletal muscle involvement, the majority display symptoms of exercise intolerance which can cause profound exercise limitation and impair everyday living and quality of life (QoL). There are no curative treatments for GSDs, thus therapeutic options, such as exercise training, are aimed at improving QoL by alleviating signs and symptoms. In order to investigate the effectiveness of exercise training in adults with GSDs, we systematically reviewed the literature.

Methods

In this review we conducted searches within SCOPUS and MEDLINE to identify potential papers for inclusion. These papers were independently assessed for inclusion and quality by two authors. We identified 23 studies which included aerobic training, strength training or respiratory muscle training in patients with McArdles (n = 41) and Pompe disease (n = 139).

Results

In McArdle disease, aerobic exercise training improved aerobic capacity (VO2 peak) by 14-111% with further benefits to functional capacity and well-being. Meanwhile, strength training increased muscle peak power by 100-151% and reduced disease severity. In Pompe disease, a combination of aerobic and strength training improved VO2 peak by 9-10%, muscle peak power by 64%, functional capacity and well-being. Furthermore, respiratory muscle training (RMT) improved respiratory muscular strength [maximum inspiratory pressure (MIP) increased by up to 65% and maximum expiratory pressure (MEP) by up to 70%], with additional benefits shown in aerobic capacity, functional capacity and well-being.

Conclusion

This adds to the growing body of evidence which suggests that supervised exercise training is safe and effective in improving aerobic capacity and muscle function in adults with McArdle or Pompe disease. However, the literature base is limited in quality and quantity with a dearth of literature regarding exercise training in other GSD subtypes.

Energy metabolism during exercise in patients with β-enolase deficiency (GSDXIII)

AIM

To investigate the in vivo skeletal muscle metabolism in patients with β-enolase deficiency (GSDXIII) during exercise, and the effect of glucose infusion.

METHODS

Three patients with GSDXIII and 10 healthy controls performed a nonischemic handgrip test as well as an incremental cycle ergometer test measuring maximal oxidative consumption (VO2max) and a 1-hour submaximal cycle test at an intensity of 65% to 75% of VO2max. The patients repeated the submaximal exercise after 2 days, where they received a 10% iv-glucose supplementation.

RESULTS

Patients had lower VO2max than healthy controls, and two of three patients had to stop prematurely during the intended 1-hour submaximal exercise test. During nonischemic forearm test, all patients were able to produce lactate in normal amounts. Glucose infusion had no effect on patients' exercise capacity.

CONCLUSIONS

Patients with GSDXIII experience exercise intolerance and episodes of myoglobinuria, even to the point of needing renal dialysis, but still retain an almost normal anaerobic metabolic response to submaximal intensity exercise. In accordance with this, glucose supplementation did not improve exercise capacity. The findings show that GSDXIII, although causing episodic rhabdomyolysis, is one of the mildest metabolic myopathies affecting glycolysis.

Inherited Neuromuscular Disorders: Which Role for Serum Biomarkers?

Inherited neuromuscular disorders (INMD) are a heterogeneous group of rare diseases that involve muscles, motor neurons, peripheral nerves or the neuromuscular junction. Several different lab abnormalities have been linked to INMD: sometimes they are typical of the disorder, but they usually appear to be less specific. Sometimes serum biomarkers can point out abnormalities in presymtomatic or otherwise asymptomatic patients (e.g., carriers). More often a biomarker of INMD is evaluated by multiple clinicians other than expert in NMD before the diagnosis, because of the multisystemic involvement in INMD. The authors performed a literature search on biomarkers in inherited neuromuscular disorders to provide a practical approach to the diagnosis and the correct management of INMD. A considerable number of biomarkers have been reported that support the diagnosis of INMD, but the role of an expert clinician is crucial. Hence, the complete knowledge of such abnormalities can accelerate the diagnostic workup supporting the referral to specialists in neuromuscular disorders.

A New Glycogen Storage Disease Caused by a Dominant PYGM Mutation

OBJECTIVE

Glycogen storage diseases (GSDs) are severe human disorders resulting from abnormal glucose metabolism, and all previously described GSDs segregate as autosomal recessive or X-linked traits. In this study, we aimed to molecularly characterize the first family with a dominant GSD.

METHODS

We describe a dominant GSD family with 13 affected members presenting with adult-onset muscle weakness, and we provide clinical, metabolic, histological, and ultrastructural data. We performed exome sequencing to uncover the causative gene, and functional experiments in the cell model and on recombinant proteins to investigate the pathogenic effect of the identified mutation.

RESULTS

We identified a heterozygous missense mutation in PYGM segregating with the disease in the family. PYGM codes for myophosphorylase, the enzyme catalyzing the initial step of glycogen breakdown. Enzymatic tests revealed that the PYGM mutation impairs the AMP-independent myophosphorylase activity, whereas the AMP-dependent activity was preserved. Further functional investigations demonstrated an altered conformation and aggregation of mutant myophosphorylase, and the concurrent accumulation of the intermediate filament desmin in the myofibers of the patients.

INTERPRETATION

Overall, this study describes the first example of a dominant glycogen storage disease in humans, and elucidates the underlying pathomechanisms by deciphering the sequence of events from the PYGM mutation to the accumulation of glycogen in the muscle fibers. ANN NEUROL 2020;88:274-282.

Adapted physical activity and therapeutic exercise in late-onset Pompe disease (LOPD): a two-step rehabilitative approach

Aerobic exercise, training to sustain motor ability, and respiratory rehabilitation may improve general functioning and quality of life (QoL) in neuromuscular disorders. Patients with late-onset Pompe disease (LOPD) typically show progressive muscle weakness, respiratory dysfunction and minor cardiac involvement. Characteristics and modalities of motor and respiratory rehabilitation in LOPD are not well defined and specific guidelines are lacking. Therefore, we evaluated the role of physical activity, therapeutic exercise, and pulmonary rehabilitation programs in order to promote an appropriate management of motor and respiratory dysfunctions and improve QoL in patients with LOPD. We propose two operational protocols: one for an adapted physical activity (APA) plan and the other for an individual rehabilitation plan, particularly focused on therapeutic exercise (TE) and respiratory rehabilitation.

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.

Muscle glycogen synthesis and breakdown are both impaired in glycogenin-1 deficiency

OBJECTIVE

To study fat and carbohydrate metabolism during exercise in patients with glycogenin-1 (GYG1) deficiency, and to study whether IV glucose supplementation can alleviate exercise intolerance in these patients.

METHODS

This is a case-control study with 4 patients with GYG1 deficiency and 4 healthy controls. Patients performed 1 hour of cycling at 50% of their maximal workload capacity, while controls cycled at the same absolute workloads as patients. Heart rate was measured continuously, and production and utilization of fat and glucose was assessed by stable isotope technique. The following day, patients repeated the exercise, this time receiving an IV 10% glucose supplement.

RESULTS

Glucose utilization during exercise was similar in patients and controls, while palmitate utilization was greater in patients compared to controls. However, exercise-induced increases in lactate were attenuated to about half normal in patients. This was also the case during a handgrip exercise test. Glucose infusion improved exercise tolerance in patients, and lowered heart rate by on average 11 beats per minute during exercise.

CONCLUSIONS

The findings suggest that patients with GYG1 deficiency not only have abnormal formation of glycogen, but also have impaired muscle glycogenolysis, as suggested by impaired lactate production during exercise and improved exercise tolerance with glucose infusion.

Skeletal muscle metabolism during prolonged exercise in Pompe disease

OBJECTIVE

Pompe disease (glycogenosis type II) is caused by lysosomal alpha-glucosidase deficiency, which leads to a block in intra-lysosomal glycogen breakdown. In spite of enzyme replacement therapy, Pompe disease continues to be a progressive metabolic myopathy. Considering the health benefits of exercise, it is important in Pompe disease to acquire more information about muscle substrate use during exercise.

METHODS

Seven adults with Pompe disease were matched to a healthy control group (1:1). We determined (1) peak oxidative capacity (VO_2peak) and (2) carbohydrate and fatty acid metabolism during submaximal exercise (33 W) for 1 h, using cycle-ergometer exercise, indirect calorimetry and stable isotopes.

RESULTS

In the patients, VO_2peak was less than half of average control values; mean difference -1659 mL/min (CI: -2450 to -867, P = 0.001). However, the respiratory exchange ratio increased to >1.0 and lactate levels rose 5-fold in the patients, indicating significant glycolytic flux. In line with this, during submaximal exercise, the rates of oxidation (ROX) of carbohydrates and palmitate were similar between patients and controls (mean difference 0.226 g/min (CI: 0.611 to -0.078, P = 0.318) and mean difference 0.016 µmol/kg/min (CI: 1.287 to -1.255, P = 0.710), respectively).

CONCLUSION

Reflecting muscle weakness and wasting, Pompe disease is associated with markedly reduced maximal exercise capacity. However, glycogenolysis is not impaired in exercise. Unlike in other metabolic myopathies, skeletal muscle substrate use during exercise is normal in Pompe disease rendering exercise less complicated for e.g. medical or recreational purposes.

Investigation on acute effects of enzyme replacement therapy and influence of clinical severity on physiological variables related to exercise tolerance in patients with late onset Pompe disease

Exercise intolerance is one of the clinical hallmarks of late-onset Pompe disease (LOPD). We studied the acute effects of ERT on the physiological variables associated with exercise tolerance in patients chronically ERT treated. Moreover, we assessed the influence of clinical severity on the investigated variables. The day before (B) and the day after (A) ERT injection, 11 LOPD patients performed on a cycle-ergometer an exercise tolerance test to voluntary exhaustion; VO₂, HR, RPE, and GAA activity were determined in B and A. The disease severity was characterized by Walton scale, 6MWT, and pulmonary function tests. No significant differences in the variables related to exercise tolerance were found in A vs B, despite a significant increase in GAA activity in peripheral lymphocytes. No differences in VO₂ peak were observed between patients with only skeletal muscle impairment and patients with both skeletal and respiratory muscle impairment. Distance walked at 6MWT was significantly higher than VO₂ peak expressed as percentage of normal values. In conclusion, in LOPD patients the exercise tolerance test is not acutely affected by ERT administration; the peripheral muscle component seems more prominent in determining the VO₂ peak decrease than the respiratory component; VO₂ peak might be more sensitive than 6MWT in estimating exercise tolerance in LOPD.

Glycogen metabolism in humans

In the human body, glycogen is a branched polymer of glucose stored mainly in the liver and the skeletal muscle that supplies glucose to the blood stream during fasting periods and to the muscle cells during muscle contraction. Glycogen has been identified in other tissues such as brain, heart, kidney, adipose tissue, and erythrocytes, but glycogen function in these tissues is mostly unknown. Glycogen synthesis requires a series of reactions that include glucose entrance into the cell through transporters, phosphorylation of glucose to glucose 6-phosphate, isomerization to glucose 1-phosphate, and formation of uridine 5'-diphosphate-glucose, which is the direct glucose donor for glycogen synthesis. Glycogenin catalyzes the formation of a short glucose polymer that is extended by the action of glycogen synthase. Glycogen branching enzyme introduces branch points in the glycogen particle at even intervals. Laforin and malin are proteins involved in glycogen assembly but their specific function remains elusive in humans. Glycogen is accumulated in the liver primarily during the postprandial period and in the skeletal muscle predominantly after exercise. In the cytosol, glycogen breakdown or glycogenolysis is carried out by two enzymes, glycogen phosphorylase which releases glucose 1-phosphate from the linear chains of glycogen, and glycogen debranching enzyme which untangles the branch points. In the lysosomes, glycogen degradation is catalyzed by α-glucosidase. The glucose 6-phosphatase system catalyzes the dephosphorylation of glucose 6-phosphate to glucose, a necessary step for free glucose to leave the cell. Mutations in the genes encoding the enzymes involved in glycogen metabolism cause glycogen storage diseases.

Lack of robust satellite cell activation and muscle regeneration during the progression of Pompe disease

Introduction

Muscle stem cells termed satellite cells are essential for muscle regeneration. A central question in many neuromuscular disorders is why satellite cells are unable to prevent progressive muscle wasting. We have analyzed muscle fiber pathology and the satellite cell response in Pompe disease, a metabolic myopathy caused by acid alpha-glucosidase deficiency and lysosomal glycogen accumulation. Pathology included muscle fiber vacuolization, loss of cross striation, and immune cell infiltration.

Results

The total number of Pax7-positive satellite cells in muscle biopsies from infantile, childhood onset and adult patients (with different ages and disease severities) were indistinguishable from controls, indicating that the satellite cell pool is not exhausted in Pompe disease. Pax7/Ki67 double stainings showed low levels of satellite cell proliferation similar to controls, while MyoD and Myogenin stainings showed undetectable satellite cell differentiation. Muscle regenerative activity monitored with expression of embryonic Myosin Heavy Chain was weak in the rapidly progressing classic infantile form and undetectable in the more slowly progressive childhood and adult onset disease including in severely affected patients.

Conclusions

These results imply that ongoing muscle wasting in Pompe disease may be explained by insufficient satellite cell activation and muscle regeneration. The preservation of the satellite cell pool may offer a venue for the development of novel treatment strategies directed towards the activation of endogenous satellite cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0243-x) contains supplementary material, which is available to authorized users.

Exercise in muscle glycogen storage diseases

Glycogen storage diseases (GSD) are inborn errors of glycogen or glucose metabolism. In the GSDs that affect muscle, the consequence of a block in skeletal muscle glycogen breakdown or glucose use, is an impairment of muscular performance and exercise intolerance, owing to 1) an increase in glycogen storage that disrupts contractile function and/or 2) a reduced substrate turnover below the block, which inhibits skeletal muscle ATP production. Immobility is associated with metabolic alterations in muscle leading to an increased dependence on glycogen use and a reduced capacity for fatty acid oxidation. Such changes may be detrimental for persons with GSD from a metabolic perspective. However, exercise may alter skeletal muscle substrate metabolism in ways that are beneficial for patients with GSD, such as improving exercise tolerance and increasing fatty acid oxidation. In addition, a regular exercise program has the potential to improve general health and fitness and improve quality of life, if executed properly. In this review, we describe skeletal muscle substrate use during exercise in GSDs, and how blocks in metabolic pathways affect exercise tolerance in GSDs. We review the studies that have examined the effect of regular exercise training in different types of GSD. Finally, we consider how oral substrate supplementation can improve exercise tolerance and we discuss the precautions that apply to persons with GSD that engage in exercise.

Role of the cardio-pulmonary exercise test and six-minute walking test in the evaluation of exercise performance in patients with late-onset Pompe disease

In patients with late-onset Pompe disease, we explored the role of the Cardiopulmonary Exercise Test (CPET) and the Six-Minute Walking Test (6MWT) in the assessment of exercise capacity and in the evaluation of the effects of enzyme replacement therapy (ERT). Eight patients affected by late-onset Pompe disease, followed up at the Centre for Neuromuscular Diseases and treated with ERT, underwent a baseline evaluation with a spirometry, a CPET and a 6MWT. Four of them were restudied after 36 months of treatment. Three patients showed a reduction in exercise capacity as evaluated by peak oxygen uptake (VO2) measured at the CPET and Distance Walked (DW) measured at the 6MWT (median % predicted: 67.1 [range 54.3-99.6] and 67.3 [56.6-82.6], respectively). Cardiac and respiratory limitations revealed by the CPET were correlated to peak VO2, but not to the DW. Nevertheless, percent of predicted values of peak VO2 and DW were strongly correlated (rho = 0.85, p = 0.006), and close to identity. In the longitudinal evaluation forced vital capacity decreased, while peak VO2 and DW showed a trend to a parallel improvement. We concluded that although only the CPET revealed causes of exercise limitation, which partially differed among patients, CPET and 6MWT showed a similar overall degree of exercise impairment. That held true in the longitudinal assessment during ERT, where both tests demonstrated similar small improvements, occurring despite deterioration in forced vital capacity.

New insights in the field of muscle glycogenoses

PURPOSE OF REVIEW

This review highlights recent contributions regarding clinical heterogeneity, pathogenic mechanisms, therapeutic trials, and animal models of the muscle glycogenoses.

RECENT FINDINGS

Most recent publications have dealt with the clinical effects of enzyme replacement therapy (ERT) in glycogenosis type II (Pompe disease), including the cognitive development of children with the infantile form who have reached school age. Standardized exercise testing has shown the similarity between McArdle disease and one of the most recently described muscle glycogenoses, phosphoglucomutase deficiency. Cycle ergometry in patients with glycogenosis type III (debrancher deficiency) without overt weakness has documented exercise intolerance relieved by glucose infusion, consistent with the glycogenolytic block. A mouse model of McArdle disease faithfully recapitulates most features of the human disease and will prove valuable for a better understanding of pathogenesis and therapeutic modalities. Polyglucosan body myopathy with cardiomyopathy has been associated with mutations in RBCK1, a ubiquitin ligase, which have also been reported in children with early-onset immune disorder. The role of polyglucosan storage in muscle and in both central and peripheral nervous systems has been confirmed in the infantile and late-onset forms of glycogenosis type IV (brancher enzyme deficiency). Additional novel findings include the involvement of the heart in one patient with phosphofructokinase (PFK) deficiency and the presence of tubular aggregates in a manifesting heterozygote with phosphoglycerate mutase deficiency.

SUMMARY

Important recent developments in the field of muscle glycogenoses include a new disease entity, a new animal model of McArdle disease, and better knowledge of the pathogenesis in some glycogenoses and of the long-term effects of enzyme replacement therapy in Pompe disease.

Extended phenotype description and new molecular findings in late onset glycogen storage disease type II: a northern Italy population study and review of the literature

Glycogen storage disease type II (GSDII) is a lysosomal storage disorder caused by acid alpha-1,4-glucosidase deficiency and associated with recessive mutations in its coding gene GAA. Few studies have provided so far a detailed phenotypical characterization in late onset GSDII (LO-GSDII) patients. Genotype-phenotype correlation has been previously attempted with controversial results. We aim to provide an in-depth description of a cohort (n = 36) of LO-GSDII patients coming from the north of Italy and compare our population's findings to the literature. We performed a clinical record-based retrospective and prospective study of our patients. LO-GSDII in our cohort covers a large variability of phenotype including subtle clinical presentation and did not differ significantly from previous data. In all patients, molecular analysis disclosed GAA mutations, five of them being novel. To assess potential genotype-phenotype correlations we divided IVS1-32-13T>G heterozygous patients into two groups following the severity of the mutations on the second allele. Our patients harbouring "severe" mutations (n = 21) presented a strong tendency to have more severe phenotypes and more disability, more severe phenotypes and more disability, higher prevalence of assisted ventilation and a shorter time of evolution to show it. The determination of prognostic factors is mandatory in order to refine the accuracy of prognostic information, to develop follow-up strategy and, more importantly, to improve the decision algorithm for enzyme replacement therapy administration. The demonstration of genotype-phenotype correlations could help to reach this objective. Clinical assessment homogeneity is required to overcome limitations due to the lack of power of most studies.

Fat and carbohydrate metabolism during exercise in phosphoglucomutase type 1 deficiency

CONTEXT

Phosphoglucomutase type 1 (PGM1) deficiency is a rare metabolic myopathy in which symptoms are provoked by exercise.

OBJECTIVE

Because the metabolic block is proximal to the entry of glucose into the glycolytic pathway, we hypothesized that iv glucose could improve the exercise intolerance experienced by the patient.

DESIGN

This was an experimental intervention study.

SETTING

The study was conducted in an exercise laboratory.

SUBJECTS

Subjects were a 37-year-old man with genetically and biochemically verified PGM1 deficiency and 6 healthy subjects.

INTERVENTIONS

Cycle ergometer, peak and submaximal exercise (70% of peak oxygen consumption), and exercise with an iv glucose infusion tests were performed.

MAIN OUTCOME MEASURES

Peak work capacity and substrate metabolism during submaximal exercise with and without an iv glucose infusion were measured.

RESULTS

Peak work capacity in the patient was normal, as were increases in plasma lactate during peak and submaximal exercise. However, the heart rate decreased 11 beats minute⁻¹, the peak work rate increased 12.5%, and exercise was rated as being easier with glucose infusion in the patient. These results were in contrast to those in the control group, in whom no improvements occurred. In addition, the patient tended to become hypoglycemic during submaximal exercise.

CONCLUSIONS

This report characterizes PGM1 deficiency as a mild metabolic myopathy that has dynamic exercise-related symptoms in common with McArdle disease but no second wind phenomenon, thus suggesting that the condition clinically resembles other partial enzymatic defects of glycolysis. However, with glucose infusion, the heart rate decreased 11 beats min⁻¹, the peak work rate increased 12.5%, and exercise was considered easier by the patient.

Enzyme replacement therapy and fatigue in adults with Pompe disease

BACKGROUND

Pompe disease is a hereditary metabolic myopathy, for which enzyme replacement therapy (ERT) has been available since 2006. We investigated whether ERT reduces fatigue in adult patients with Pompe disease.

METHODS

In this prospective international observational survey, we used the Fatigue Severity Scale (FSS) to measure fatigue. Repeated measures ANOVA was used to analyze the data over time. In a subgroup of patients, we also evaluated muscle strength using the Medical Research Council Scale, measured pulmonary function as Forced Vital Capacity, and assessed depression using the Hospital Anxiety and Depression Scale.

RESULTS

We followed 163 patients for a median period of 4 years before ERT and for 3 years during ERT. Before ERT, the mean FSS score remained stable at around 5.3 score points; during ERT, scores improved significantly by 0.13 score points per year (p < 0.001). Fatigue decreased mainly in women, in older patients and in those with shorter disease duration. Patients' improvements in fatigue were moderately correlated with the effect of ERT on depression (r 0.55; CI 95% 0.07 to 0.70) but not with the effect of ERT on muscle strength or pulmonary function.

CONCLUSIONS

Fatigue is a common and disabling problem in patients with early and advanced stages of Pompe disease. Our finding that ERT helps to reduce fatigue is therefore important for this patient population, irrespective of the mechanisms underlying this effect.

Patients with medium-chain acyl-coenzyme a dehydrogenase deficiency have impaired oxidation of fat during exercise but no effect of L-carnitine supplementation

BACKGROUND

It is not clear to what extent skeletal muscle is affected in patients with medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD). l-Carnitine is commonly used as a supplement in patients with MCADD, although its beneficial effect has not been verified.

DESIGN

We investigated (1) fuel utilization during prolonged low-intensity exercise in patients with MCADD and (2) the influence of 4 weeks of oral l-carnitine supplementation on fuel utilization during exercise.

METHODS

Four asymptomatic patients with MCADD and 11 untrained, healthy, age- and sex-matched control subjects were included. The subjects performed a 1-hour cycling test at a constant workload corresponding to 55% of Vo2max, while fat and carbohydrate metabolism was assessed, using the stable isotope technique and indirect calorimetry. The patients ingested 100 mg/kg/d of l-carnitine for 4 weeks, after which the cycling tests were repeated.

RESULTS

At rest, palmitate oxidation and total fatty acid oxidation (FAO) rates were similar in patients and healthy control subjects. During constant workload cycling, palmitate oxidation and FAO rates increased in both groups, but increased 2 times as much in healthy control subjects as in patients (P = .007). Palmitate oxidation and FAO rates were unchanged by the l-carnitine supplementation.

CONCLUSION

Our results indicate that patients with MCADD have an impaired ability to increase FAO during exercise but less so than that observed in patients with a number of other disorders of fat oxidation, which explains the milder skeletal muscle phenotype in MCADD. The use of carnitine supplementation in MCADD cannot be supported by the present findings.