Carbohydrate-remodelled acid alpha-glucosidase with higher affinity for the cation-independent mannose 6-phosphate receptor demonstrates improved delivery to muscles of Pompe mice

Glycosylation: impact, control and improvement during therapeutic protein production

The emergence of the biopharmaceutical industry represented a major revolution for modern medicine, through the development of recombinant therapeutic proteins that brought new hope for many patients with previously untreatable diseases. There is a ever-growing demand for these therapeutics that forces a constant technological evolution to increase product yields while simultaneously reducing costs. However, the process changes made for this purpose may also affect the quality of the product, a factor that was initially overlooked but which is now a major focus of concern. Of the many properties determining product quality, glycosylation is regarded as one of the most important, influencing, for example, the biological activity, serum half-life and immunogenicity of the protein. Consequently, monitoring and control of glycosylation is now critical in biopharmaceutical manufacturing and a requirement of regulatory agencies. A rapid evolution is being observed in this context, concerning the influence of glycosylation in the efficacy of different therapeutic proteins, the impact on glycosylation of a diversity of parameters/processes involved in therapeutic protein production, the analytical methodologies employed for glycosylation monitoring and control, as well as strategies that are being explored to use this property to improve therapeutic protein efficacy (glycoengineering). This work reviews the main findings on these subjects, providing an up-to-date source of information to support further studies.

Glycosylation-independent lysosomal targeting of acid α-glucosidase enhances muscle glycogen clearance in pompe mice

We have used a peptide-based targeting system to improve lysosomal delivery of acid α-glucosidase (GAA), the enzyme deficient in patients with Pompe disease. Human GAA was fused to the glycosylation-independent lysosomal targeting (GILT) tag, which contains a portion of insulin-like growth factor II, to create an active, chimeric enzyme with high affinity for the cation-independent mannose 6-phosphate receptor. GILT-tagged GAA was taken up by L6 myoblasts about 25-fold more efficiently than was recombinant human GAA (rhGAA). Once delivered to the lysosome, the mature form of GILT-tagged GAA was indistinguishable from rhGAA and persisted with a half-life indistinguishable from rhGAA. GILT-tagged GAA was significantly more effective than rhGAA in clearing glycogen from numerous skeletal muscle tissues in the Pompe mouse model. The GILT-tagged GAA enzyme may provide an improved enzyme replacement therapy for Pompe disease patients.

A bacterial glycosidase enables mannose-6-phosphate modification and improved cellular uptake of yeast-produced recombinant human lysosomal enzymes

Lysosomal storage diseases are treated with human lysosomal enzymes produced in mammalian cells. Such enzyme therapeutics contain relatively low levels of mannose-6-phosphate, which is required to target them to the lysosomes of patient cells. Here we describe a method for increasing mannose-6-phosphate modification of lysosomal enzymes produced in yeast. We identified a glycosidase from C. cellulans that 'uncaps' N-glycans modified by yeast-type mannose-Pi-6-mannose to generate mammalian-type N-glycans with a mannose-6-phosphate substitution. Determination of the crystal structure of this glycosidase provided insight into its substrate specificity. We used this uncapping enzyme together with α-mannosidase to produce in yeast a form of the Pompe disease enzyme α-glucosidase rich in mannose-6-phosphate. Compared with the currently used therapeutic version, this form of α-glucosidase was more efficiently taken up by fibroblasts from Pompe disease patients, and it more effectively reduced cardiac muscular glycogen storage in a mouse model of the disease.

Aerobic training as an adjunctive therapy to enzyme replacement in Pompe disease

BACKGROUND

Aerobic exercise may be used in conjunction with enzyme replacement therapy (ERT) to attenuate cardiovascular deconditioning, skeletal muscle wasting, and loss of motor function in Pompe disease (glycogen storage disease type II; GSDII), but the effects on lysosomal glycogen content and macroautophagy have not been defined to date.

PURPOSE

The main objectives of this study were to determine if acute aerobic exercise enhances 24-h uptake of recombinant human enzyme (rhGAA; Myozyme® [aim 1]) and if endurance training improves disease pathology when combined with ERT [aim 2] in Pompe mice.

METHODS

For the first aim in our study, Pompe mutant mice (6(neo)/6(neo)) were grouped into ERT (Myozyme® injection only [40 mg/kg]) and ERT+EX (Myozyme® injection followed by 90 min treadmill exercise) cohorts, and enzyme uptake was assessed in the heart and quadriceps 24h post injection. For the second aim of our study, mutant mice were randomized into control, endurance-trained, enzyme-treated, or combination therapy groups. Exercised animals underwent 14 weeks of progressive treadmill training with or without biweekly Myozyme® injections (40 mg/kg) and tissues were harvested 1 week post last treatment.

RESULTS

Myozyme® uptake (GAA activity) was not improved in ERT+EX over ERT alone at 24-h post injection. Endurance exercise training, with or without ERT, improved aerobic capacity and normalized grip strength, motor function, and lean mass (P<0.05), but did not reduce glycogen content or normalize macroautophagy beyond traditional enzyme replacement therapy.

CONCLUSIONS

Endurance training is beneficial as an adjunctive therapy to ERT in Pompe disease, although it works by mechanisms independent of a reduction in glycogen content.

Biochemical evidence for superior correction of neuronal storage by chemically modified enzyme in murine mucopolysaccharidosis VII

Enzyme replacement therapy has been used successfully in many lysosomal storage diseases. However, correction of brain storage has been limited by the inability of infused enzyme to cross the blood-brain barrier (BBB). We recently reported that PerT-GUS, a form of β-glucuronidase (GUS) chemically modified to eliminate its uptake and clearance by carbohydrate-dependent receptors, crossed the BBB and cleared neuronal storage in an immunotolerant model of murine mucopolysaccharidosis (MPS) type VII. In this respect, the chemically modified enzyme was superior to native β-glucuronidase. Chemically modified enzyme was also delivered more effectively to heart, kidney, and muscle. However, liver and spleen, which express high levels of carbohydrate receptors, received nearly fourfold lower levels of PerT-GUS compared with native GUS. A recent report on PerT-treated sulfamidase in murine MPS IIIA confirmed enhanced delivery to other tissues but failed to observe clearance of storage in neurons. To confirm and extend our original observations, we compared the efficacy of 12 weekly i.v. infusions of PerT-GUS versus native GUS on (i) delivery of enzyme to brain; (ii) improvement in histopathology; and (iii) correction of secondary elevations of other lysosomal enzymes. Such correction is a recognized biomarker for correction of neuronal storage. PerT-GUS was superior to native GUS in all three categories. These results provide additional evidence that long-circulating enzyme, chemically modified to escape carbohydrate-mediated clearance, may offer advantages in treating MPS VII. The relevance of this approach to treat other lysosomal storage diseases that affect brain awaits confirmation.

Adjunctive β2-agonists reverse neuromuscular involvement in murine Pompe disease

Pompe disease has resisted enzyme replacement therapy with acid α-glucosidase (GAA), which has been attributed to inefficient cation-independent mannose-6-phosphate receptor (CI-MPR) mediated uptake. We evaluated β2-agonist drugs, which increased CI-MPR expression in GAA knockout (KO) mice. Clenbuterol along with a low-dose adeno-associated virus vector increased Rotarod latency by 75% at 4 wk, in comparison with vector alone (P<2×10(-5)). Glycogen content was lower in skeletal muscles, including soleus (P<0.01), extensor digitorum longus (EDL; P<0.001), and tibialis anterior (P<0.05) following combination therapy, in comparison with vector alone. Glycogen remained elevated in the muscles following clenbuterol alone, indicating an adjunctive effect with gene therapy. Elderly GAA-KO mice treated with combination therapy demonstrated 2-fold increased wirehang latency, in comparison with vector or clenbuterol alone (P<0.001). The glycogen content of skeletal muscle decreased following combination therapy in elderly mice (P<0.05). Finally, CI-MPR-KO/GAA-KO mice did not respond to combination therapy, indicating that clenbuterol's effect depended on CI-MPR expression. In summary, adjunctive β2-agonist treatment increased CI-MPR expression and enhanced efficacy from gene therapy in Pompe disease, which has implications for other lysosomal storage disorders that involve primarily the brain.

The pharmacological chaperone AT2220 increases recombinant human acid α-glucosidase uptake and glycogen reduction in a mouse model of Pompe disease

Pompe disease is an inherited lysosomal storage disease that results from a deficiency in the enzyme acid α-glucosidase (GAA), and is characterized by progressive accumulation of lysosomal glycogen primarily in heart and skeletal muscles. Recombinant human GAA (rhGAA) is the only approved enzyme replacement therapy (ERT) available for the treatment of Pompe disease. Although rhGAA has been shown to slow disease progression and improve some of the pathophysiogical manifestations, the infused enzyme tends to be unstable at neutral pH and body temperature, shows low uptake into some key target tissues, and may elicit immune responses that adversely affect tolerability and efficacy. We hypothesized that co-administration of the orally-available, small molecule pharmacological chaperone AT2220 (1-deoxynojirimycin hydrochloride, duvoglustat hydrochloride) may improve the pharmacological properties of rhGAA via binding and stabilization. AT2220 co-incubation prevented rhGAA denaturation and loss of activity in vitro at neutral pH and 37°C in both buffer and blood. In addition, oral pre-administration of AT2220 to rats led to a greater than two-fold increase in the circulating half-life of intravenous rhGAA. Importantly, co-administration of AT2220 and rhGAA to GAA knock-out (KO) mice resulted in significantly greater rhGAA levels in plasma, and greater uptake and glycogen reduction in heart and skeletal muscles, compared to administration of rhGAA alone. Collectively, these preclinical data highlight the potentially beneficial effects of AT2220 on rhGAA in vitro and in vivo. As such, a Phase 2 clinical study has been initiated to investigate the effects of co-administered AT2220 on rhGAA in Pompe patients.

The role of immune tolerance induction in restoration of the efficacy of ERT in Pompe disease

Pompe disease is a lysosomal storage disorder caused by deficiency in the enzyme acid α-glucosidase (GAA). Pompe disease is characterized by the accumulation of glycogen, predominantly in muscle tissue, leading to progressive muscle weakness, loss of motor, respiratory, and, in the infantile-onset form, cardiac function. Disease progression is highly variable depending on phenotype, but premature death due to respiratory complications occurs in most patients. Beginning in 2006, approved alglucosidase alfa enzyme replacement therapies [recombinant human (rh) GAA] have been available to treat Pompe patients. Treatment of classic infantile-onset patients, who manifest the severest form of the disease, with alglucosidase alfa (Myozyme®) has led to extended survival and an evolving understanding of the pathophysiology and course of the disease. Moreover, such treatment has brought to light the role of the immune response in abrogating the efficacy of rhGAA in classic infantile-onset patients with severe genetic mutations. Thus, optimization of treatment for such patients includes development and utilization of strategies to prevent or eliminate immune responses, including modulating the immune system (prophylactic and therapeutic immune tolerance induction regimens) and engineering the enzyme to be less immunogenic and more effective. Future research is also critical for evaluating and mitigating novel disease-associated pathologies uncovered by prolonged survival of infantile-onset patients including development of novel therapeutics, and for protein design strategies to increase delivery of enzyme replacement therapy to critical target tissues. Such efforts would be greatly bolstered by further development of predictive animal models and biomarkers to facilitate clinical trials and patient management. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

New insights into therapeutic options for Pompe disease

Glycogen storage disease type II or Pompe disease (GSD II, MIM 232300) is a rare inherited metabolic myopathy caused by a deficiency of lysosomal acid α-glucosidase or acid maltase (GAA; EC 3.2.1.20), resulting in a massive lysosomal glycogen accumulation in cardiac and skeletal muscles. Affected individuals exhibit either severe hypotonia associated with hypertrophic cardiomyopathy (infantile forms) or progressive muscle weakness (late-onset forms). Even if enzyme replacement therapy has recently become a standard treatment, it suffers from several limitations. This review will present the main results of enzyme replacement therapy and the recent findings concerning alternative treatments for Pompe disease, such as gene therapy, enzyme enhancement therapy, and substrate reduction therapy.

Enhanced efficacy of enzyme replacement therapy in Pompe disease through mannose-6-phosphate receptor expression in skeletal muscle

Enzyme replacement therapy (ERT) with acid α-glucosidase has become available for Pompe disease; however, the response of skeletal muscle, as opposed to the heart, has been attenuated. The poor response of skeletal muscle has been attributed to the low abundance of the cation-independent mannose-6-phosphate receptor (CI-MPR) in skeletal muscle compared to heart. To further understand the role of CI-MPR in Pompe disease, muscle-specific CI-MPR conditional knockout (KO) mice were crossed with GAA-KO (Pompe disease) mice. We evaluated the impact of CI-MPR-mediated uptake of GAA by evaluating ERT in CI-MPR-KO/GAA-KO (double KO) mice. The essential role of CI-MPR was emphasized by the lack of efficacy of ERT as demonstrated by markedly reduced biochemical correction of GAA deficiency and of glycogen accumulations in double KO mice, in comparison with the administration of the same therapeutic doses in GAA-KO mice. Clenbuterol, a selective β(2)-agonist, enhanced the CI-MPR expression in skeletal tissue and also increased efficacy from GAA therapy, thereby confirming the key role of CI-MPR with regard to enzyme replacement therapy in Pompe disease. Biochemical correction improved in both muscle and non-muscle tissues, indicating that therapy could be similarly enhanced in other lysosomal storage disorders. In summary, enhanced CI-MPR expression might improve the efficacy of enzyme replacement therapy in Pompe disease through enhancing receptor-mediated uptake of GAA.

Strategies for Neoglycan conjugation to human acid α-glucosidase

Engineering proteins for selective tissue targeting can improve therapeutic efficacy and reduce undesired side effects. The relatively high dose of recombinant human acid α-glucosidase (rhGAA) required for enzyme replacement therapy of Pompe disease may be attributed to less than optimal muscle uptake via the cation-independent mannose 6-phosphate receptor (CI-MPR). To improve muscle targeting, Zhu et al. (1) conjugated periodate oxidized rhGAA with bis mannose 6-phosphate bearing synthetic glycans and achieved 5-fold greater potency in a murine Pompe efficacy model. In the current study, we systematically evaluated multiple strategies for conjugation based on a structural homology model of GAA. Glycan derivatives containing succinimide, hydrazide, and aminooxy linkers targeting free cysteine, lysines, and N-linked glycosylation sites on rhGAA were prepared and evaluated in vitro and in vivo. A novel conjugation method using enzymatic oxidation was developed to eliminate side oxidation of methionine. Conjugates derived from periodate oxidized rhGAA still displayed the greatest potency in the murine Pompe model. The efficiency of conjugation and its effect on catalytic activity were consistent with predictions based on the structural model and supported its use in guiding selection of appropriate chemistries.

Suppression of autophagy permits successful enzyme replacement therapy in a lysosomal storage disorder--murine Pompe disease

Autophagy, an intracellular system for delivering portions of cytoplasm and damaged organelles to lysosomes for degradation/recycling, plays a role in many physiological processes and is disturbed in many diseases. We recently provided evidence for the role of autophagy in Pompe disease, a lysosomal storage disorder in which acid alphaglucosidase, the enzyme involved in the breakdown of glycogen, is deficient or absent. Clinically the disease manifests as a cardiac and skeletal muscle myopathy. The current enzyme replacement therapy (ERT) clears lysosomal glycogen effectively from the heart but less so from skeletal muscle. In our Pompe model, the poor muscle response to therapy is associated with the presence of pools of autophagic debris. To clear the fibers of the autophagic debris, we have generated a Pompe model in which an autophagy gene, Atg7, is inactivated in muscle. Suppression of autophagy alone reduced the glycogen level by 50–60%. Following ERT, muscle glycogen was reduced to normal levels, an outcome not observed in Pompe mice with genetically intact autophagy. The suppression of autophagy, which has proven successful in the Pompe model, is a novel therapeutic approach that may be useful in other diseases with disturbed autophagy.

Protein glycosylation and its impact on biotechnology

Glycosylation is a post-translational modification that is of paramount importance in the production of recombinant pharmaceuticals as most recombinantly produced therapeutics are N- and/or O-glycosylated. Being a cell-system-dependent process, it also varies with expression systems and growth conditions, which result in glycan microheterogeneity and macroheterogeneity. Glycans have an effect on drug stability, serum half-life, and immunogenicity; it is therefore important to analyze and optimize the glycan decoration of pharmaceuticals. This review summarizes the aspects of protein glycosylation that are of interest to biotechnologists, namely, biosynthesis and biological relevance, as well as the tools to optimize and to analyze protein glycosylation.

Fiber type conversion by PGC-1α activates lysosomal and autophagosomal biogenesis in both unaffected and Pompe skeletal muscle

PGC-1α is a transcriptional co-activator that plays a central role in the regulation of energy metabolism. Our interest in this protein was driven by its ability to promote muscle remodeling. Conversion from fast glycolytic to slow oxidative fibers seemed a promising therapeutic approach in Pompe disease, a severe myopathy caused by deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA) which is responsible for the degradation of glycogen. The recently approved enzyme replacement therapy (ERT) has only a partial effect in skeletal muscle. In our Pompe mouse model (KO), the poor muscle response is seen in fast but not in slow muscle and is associated with massive accumulation of autophagic debris and ineffective autophagy. In an attempt to turn the therapy-resistant fibers into fibers amenable to therapy, we made transgenic KO mice expressing PGC-1α in muscle (tgKO). The successful switch from fast to slow fibers prevented the formation of autophagic buildup in the converted fibers, but PGC-1α failed to improve the clearance of glycogen by ERT. This outcome is likely explained by an unexpected dramatic increase in muscle glycogen load to levels much closer to those observed in patients, in particular infants, with the disease. We have also found a remarkable rise in the number of lysosomes and autophagosomes in the tgKO compared to the KO. These data point to the role of PGC-1α in muscle glucose metabolism and its possible role as a master regulator for organelle biogenesis - not only for mitochondria but also for lysosomes and autophagosomes. These findings may have implications for therapy of lysosomal diseases and other disorders with altered autophagy.

Analysis of the dispersity in carbohydrate loading of synthetic glycoproteins using MALDI-TOF mass spectrometry

Statistical correlation of mass spectrum peak broadening with product dispersity in protein conjugation reactions allows more detailed characterization of putative therapeutic conjugates.

Neonatal gene transfer using lentiviral vector for murine Pompe disease: long-term expression and glycogen reduction

Pompe disease results from the deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA), leading to accumulated glycogen in the heart and the skeletal muscles, which causes cardiomyopathy and muscle weakness. In this study, we tested the feasibility of gene therapy for Pompe disease using a lentivirus vector (LV). Newborn GAA knockout mice were treated with intravenous injection of LV encoding human GAA (hGAA) through the facial superficial temporal vein. The transgene expression in the tissues was analyzed up to 24 weeks after treatment. Our results showed that the recombinant LV was efficient not only in increasing the GAA activity in tissues but also in decreasing their glycogen content. The examination of histological sections showed clearence of the glycogen storage in skeletal and cardiac muscles 16 and 24 weeks after a single vector injection. Levels of expressed hGAA could be detected in serum of treated animals until 24 weeks. No significant immune reaction to transgene was detected in most treated animals. Therefore, we show that LV-mediated delivery system was effective in correcting the biochemical abnormalities and that this gene transfer system might be suitable for further studies on delivering GAA to Pompe disease mouse models.

Glycoengineered acid alpha-glucosidase with improved efficacy at correcting the metabolic aberrations and motor function deficits in a mouse model of Pompe disease

Improving the delivery of therapeutics to disease-affected tissues can increase their efficacy and safety. Here, we show that chemical conjugation of a synthetic oligosaccharide harboring mannose 6-phosphate (M6P) residues onto recombinant human acid alpha-glucosidase (rhGAA) via oxime chemistry significantly improved its affinity for the cation-independent mannose 6-phosphate receptor (CI-MPR) and subsequent uptake by muscle cells. Administration of the carbohydrate-remodeled enzyme (oxime-neo-rhGAA) into Pompe mice resulted in an approximately fivefold higher clearance of lysosomal glycogen in muscles when compared to the unmodified counterpart. Importantly, treatment of immunotolerized Pompe mice with oxime-neo-rhGAA translated to greater improvements in muscle function and strength. Treating older, symptomatic Pompe mice also reduced tissue glycogen levels but provided only modest improvements in motor function. Examination of the muscle pathology suggested that the poor response in the older animals might have been due to a reduced regenerative capacity of the skeletal muscles. These findings lend support to early therapeutic intervention with a targeted enzyme as important considerations in the management of Pompe disease.

Medical bioremediation of age-related diseases

Catabolic insufficiency in humans leads to the gradual accumulation of a number of pathogenic compounds associated with age-related diseases, including atherosclerosis, Alzheimer's disease, and macular degeneration. Removal of these compounds is a widely researched therapeutic option, but the use of antibodies and endogenous human enzymes has failed to produce effective treatments, and may pose risks to cellular homeostasis. Another alternative is "medical bioremediation," the use of microbial enzymes to augment missing catabolic functions. The microbial genetic diversity in most natural environments provides a resource that can be mined for enzymes capable of degrading just about any energy-rich organic compound. This review discusses targets for biodegradation, the identification of candidate microbial enzymes, and enzyme-delivery methods.

Pompe's disease

Pompe's disease, glycogen-storage disease type II, and acid maltase deficiency are alternative names for the same metabolic disorder. It is a pan-ethnic autosomal recessive trait characterised by acid alpha-glucosidase deficiency leading to lysosomal glycogen storage. Pompe's disease is also regarded as a muscular disorder, but the generalised storage of glycogen causes more than mobility and respiratory problems. The clinical spectrum is continuous and broad. First symptoms can present in infants, children, and adults. Cardiac hypertrophy is a key feature of classic infantile Pompe's disease. For a long time, there was no means to stop disease progression, but the approval of enzyme replacement therapy has substantially changed the prospects for patients. With this new development, the disease is now among the small but increasing number of lysosomal storage disorders, for which treatment has become a reality. This review is meant to raise general awareness, to present and discuss the latest insights in disease pathophysiology, and to draw attention to new developments about diagnosis and care. We also discuss the developments that led to the approval of enzyme replacement therapy with recombinant human alpha-glucosidase from Chinese hamster ovary cells (alglucosidase alfa) by the US Food and Drug Administration and European Medicines Agency in 2006, and review clinical practice.

Therapeutic approaches in glycogen storage disease type II/Pompe Disease

Glycogen storage disease type II (GSDII)/Pompe disease is an autosomal recessive multi-system disorder due to a deficiency of the glycogen-degrading lysosomal enzyme, acid alpha-glucosidase. Without adequate levels of alpha-glucosidase, there is a progressive accumulation of glycogen inside the lysosome, resulting in lysosomal expansion in many tissues, although the major clinical manifestations are seen in cardiac and skeletal muscle. Pompe disease presents as a continuum of clinical phenotypes. In the most severe cases, disease onset occurs in infancy and death results from cardiac and respiratory failure within the first 1 or 2 years of life. In the milder late-onset forms, cardiac muscle is spared and muscle weakness is the primary symptom. Weakness of respiratory muscles is the major cause of mortality in these cases. Enzyme replacement therapy (ERT) with alglucosidase alfa (Myozyme; Genzyme Corp., Framingham, MA) is now available for all forms of glycogen storage disease type II. ERT has shown remarkable success in reversing pathology in cardiac muscle and extending life expectancy in infantile patients. However, skeletal muscle has proven to be a more challenging target for ERT. Although ERT is less effective in skeletal muscle than was hoped for, the lessons learned from both clinical and pre-clinical ERT studies have greatly expanded our understanding of the pathogenesis of the disease. A combination of fundamental studies and clinical follow-up, as well as exploration of other therapies, is necessary to take treatment for glycogen storage disease type II to the next level.

Biochemical and pharmacological characterization of different recombinant acid alpha-glucosidase preparations evaluated for the treatment of Pompe disease

Pompe disease results in the accumulation of lysosomal glycogen in multiple tissues due to a deficiency of acid alpha-glucosidase (GAA). Enzyme replacement therapy for Pompe disease was recently approved in Europe, the U.S., Canada, and Japan using a recombinant human GAA (Myozyme, alglucosidase alfa) produced in CHO cells (CHO-GAA). During the development of alglucosidase alfa, we examined the in vitro and in vivo properties of CHO cell-derived rhGAA, an rhGAA purified from the milk of transgenic rabbits, as well as an experimental version of rhGAA containing additional mannose-6-phosphate intended to facilitate muscle targeting. Biochemical analyses identified differences in rhGAA N-termini, glycosylation types and binding properties to several carbohydrate receptors. In a mouse model of Pompe disease, glycogen was more efficiently removed from the heart than from skeletal muscle for all enzymes, and overall, the CHO cell-derived rhGAA reduced glycogen to a greater extent than that observed with the other enzymes. The results of these preclinical studies, combined with biochemical characterization data for the three molecules described within, led to the selection of the CHO-GAA for clinical development and registration as the first approved therapy for Pompe disease.

Pompe disease: current state of treatment modalities and animal models

Pompe disease is a rare autosomal recessive lysosomal storage disease caused by deficiency of acid-alpha-glucosidase (GAA). This deficiency results in glycogen accumulation in the lysosomes, leading to lysosomal swelling, cellular damage and organ dysfunction. In early-onset patients (the classical infantile form and juvenile form) this glycogen accumulation leads to death. The only therapy clinically available is enzyme replacement therapy, which compensates for the missing enzyme by i.v. administration of recombinant produced enzyme. The development of clinically relevant animal models gained more insight in the disease and allowed evaluation of recombinant enzyme therapy. Several therapies are currently under investigation for Pompe disease, including gene therapy. This review gives an overview of the available knockout mouse models, of the in vitro and in vivo studies performed using recombinant produced enzyme. Furthermore, it describes current therapeutic approaches for Pompe disease as well as experimental therapies like gene correction therapy.

A review of treatment of Pompe disease in infants

The glycogen storage disease type II (GSD-II), or Pompe disease, is due to the deficit of lysosomal glycogen degradation enzyme acid alpha-glucosidase (GAA). In infants, Pompe disease is characterized by prominent hypotonia, muscle weakness, motor delay, feeding problems, and respiratory and cardiac insufficiency. In a retrospective study, the median age at death was 8.7 months. Enzyme replacement therapy with recombinant human GAA is recently used to treat patients with Pompe disease, and has been shown to prolong survival, reverse cardiomyopathy, and improve motor function. This article briefly reviews the history and manifestations of Pompe disease, and then focuses on the development of the drug for Pompe disease, alglucosidase alfa. Current status of treatment and future developments are also discussed.

Differential muscular glycogen clearance after enzyme replacement therapy in a mouse model of Pompe disease

Glycogen storage disease in the alpha-glucosidase knockout(6neo(-)/6neo(-)) (GAA KO) mouse, a model of Pompe disease, results in the pathologic accumulation of glycogen primarily within skeletal myocytes and cardiomyocytes. Intravenous administration of recombinant human alpha-glucosidase (rhGAA, Myozyme, aglucosidase alfa) can result in significant glycogen clearance from both cardiomyocytes and skeletal myocytes, however, the degree of clearance varies from one skeletal muscle type to another. We sought to determine what role muscle fiber type predominance played in this variability. To examine this question in the GAA KO mouse model we delivered intravenous doses of 100 mg/kg rhGAA on Day 1, and Day 14, and harvested a variety of fast and slow twitch muscles on Day 28. We measured glycogen clearance, muscle fiber type content and capillary density by light microscopy with computer morphometry. Recombinant human-GAA administration resulted in differential clearance of glycogen in the various muscles examined. Slow twitch-predominant muscles cleared glycogen significantly more efficiently than fast twitch-predominant muscles. There was a strong correlation between capillary density and glycogen clearance (r=0.55), suggesting that at the high doses used in this study the differential glycogen clearance observed between muscles is largely due to differential bioavailability of rhGAA regulated by blood flow.

Hyaluronidase increases the biodistribution of acid alpha-1,4 glucosidase in the muscle of Pompe disease mice: an approach to enhance the efficacy of enzyme replacement therapy

Pompe disease (glycogen storage disease type II) is a glycogen storage disease caused by a deficiency of the lysosomal enzyme, acid maltase/acid alpha-1,4 glucosidase (GAA). Deficiency of the enzyme leads primarily to intra-lysosomal glycogen accumulation, primarily in cardiac and skeletal muscles, due to the inability of converting glycogen into glucose. Enzyme replacement therapy (ERT) has been applied to replace the deficient enzyme and to restore the lost function. However, enhancing the enzyme activity to the muscle following ERT is relatively insufficient. In order to enhance GAA activity into the muscle in Pompe disease, efficacy of hyaluronidase (hyase) was examined in the heart, quadriceps, diaphragm, kidney, and brain of mouse model of Pompe disease. Administration of hyase 3000 U/mouse (intravenous) i.v. or i.p. (intraperitoneal) and 10 min later recombinant human GAA (rhGAA) 20 mg/kg i.v. showed more GAA activity in hyase i.p. injected mice compared to those mice injected with hyase via i.v. Injection of low dose of hyase (3000 U/mouse) or high dose of hyase (10,000 U/mouse) i.p. and 20 min or 60 min later 20 mg/kg rhGAA i.v. increased GAA activity into the heart, diaphragm, kidney, and quadriceps compared to hyase untreated mice. These studies suggest that hyase enhances penetration of enzyme into the tissues including muscle during ERT and therefore hyase pretreatment may be important in treating Pompe disease.

Chinese hamster ovary cell-derived recombinant human acid alpha-glucosidase in infantile-onset Pompe disease

OBJECTIVE

To conduct an open-label, multinational, multicenter study examining the safety and efficacy of recombinant human acid alpha-glucosidase (rhGAA) in treatment of infantile-onset Pompe disease.

STUDY DESIGN

We enrolled 8 infant patients who had Pompe disease with GAA activity <1% of normal, cardiomyopathy, and hypotonia. In the 52-week initial phase, rhGAA was infused intravenously at 10 mg/kg weekly; an extension phase continued survivors' treatment with 10 to 20 mg/kg of rhGAA weekly or 20 mg/kg every 2 weeks for as long as 153 weeks. Safety measurements included adverse events, laboratory tests, and anti-rhGAA antibody titers. Efficacy evaluations included survival, ventilator use, echocardiograms, growth, and motor and cognitive function.

RESULT

After 52 weeks of treatment, 6 of 8 patients were alive, and 5 patients were free of invasive ventilator support. Clinical improvements included ameliorated cardiomyopathy and improved growth and cognition. Five patients acquired new motor milestones; 3 patients walked independently. Four patients died after the initial study phase; the median age at death or treatment withdrawal for all patients was 21.7 months, significantly later than expected for patients who were not treated. Treatment was safe and well tolerated; no death was drug-related.

CONCLUSION

rhGAA improved ventilator-free survival, cardiomyopathy, growth, and motor function in patients with infantile-onset Pompe disease compared with outcomes expected for patients without treatment.

Dysfunction of endocytic and autophagic pathways in a lysosomal storage disease

OBJECTIVE

To understand the mechanisms of skeletal muscle destruction and resistance to enzyme replacement therapy in Pompe disease, a deficiency of lysosomal acid alpha-glucosidase (GAA), in which glycogen accumulates in lysosomes primarily in cardiac and skeletal muscles.

METHODS

We have analyzed compartments of the lysosomal degradative pathway in GAA-deficient myoblasts and single type I and type II muscle fibers isolated from wild-type, untreated, and enzyme replacement therapy-treated GAA knock-out mice.

RESULTS

Studies in myoblasts from GAA knock-out mice showed a dramatic expansion of vesicles of the endocytic/autophagic pathways, decreased vesicular movement in overcrowded cells, and an acidification defect in a subset of late endosomes/lysosomes. Analysis by confocal microscopy of isolated muscle fibers demonstrated that the consequences of the lysosomal glycogen accumulation are strikingly different in type I and II muscle fibers. Only type II fibers, which are the most resistant to therapy, contain large regions of autophagic buildup that span the entire length of the fibers.

INTERPRETATION

The vastly increased autophagic buildup may be responsible for skeletal muscle damage and prevent efficient trafficking of replacement enzyme to lysosomes.

Identification of Sites of Mannose 6-Phosphorylation on Lysosomal Proteins

Most newly synthesized soluble lysosomal proteins contain mannose 6-phosphate (Man-6-P), a specific carbohydrate modification that is recognized by Man-6-P receptors (MPRs) that direct targeting to the lysosome. A number of proteomic studies have focused on lysosomal proteins, exploiting the fact that Man-6-P-containing forms can be purified by affinity chromatography on immobilized MPRs. These studies have identified many known lysosomal proteins as well as many proteins not previously classified as lysosomal. The latter are of considerable biological interest with potential implications for lysosomal function and as candidates for lysosomal storage diseases of unknown etiology. However, a significant problem in interpreting the biological relevance of such proteins has been in distinguishing true Man-6-P glycoproteins from simple contaminants and from proteins associated with true Man-6-P glycoproteins (e.g. protease inhibitors and lectins). In this report, we describe a mass spectrometric approach to the verification of Man-6-phosphorylation based upon LC-MS of MPR-purified proteolytic glycopeptides. This provided a useful tool in validating novel MPR-purified proteins as true Man-6-P glycoproteins and also allowed identification of low abundance components not observed in the analysis of the total Man-6-P glycoprotein mixture. In addition, this approach allowed the global mapping of 99 Man-6-phosphorylation sites from 44 known lysosomal proteins purified from mouse and human brain. This information is likely to provide useful insights into protein determinants for this modification and may be of significant value in protein engineering approaches designed to optimize protein delivery to the lysosome in therapeutic applications such as gene and enzyme replacement therapies.