Induction of tolerance to a recombinant human enzyme, acid alpha-glucosidase, in enzyme deficient knockout mice

Gene Therapy Developments for Pompe Disease

Pompe disease is an inherited neuromuscular disorder caused by deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA). The most severe form is infantile-onset Pompe disease, presenting shortly after birth with symptoms of cardiomyopathy, respiratory failure and skeletal muscle weakness. Late-onset Pompe disease is characterized by a slower disease progression, primarily affecting skeletal muscles. Despite recent advancements in enzyme replacement therapy management several limitations remain using this therapeutic approach, including risks of immunogenicity complications, inability to penetrate CNS tissue, and the need for life-long therapy. The next wave of promising single therapy interventions involves gene therapies, which are entering into a clinical translational stage. Both adeno-associated virus (AAV) vectors and lentiviral vector (LV)-mediated hematopoietic stem and progenitor (HSPC) gene therapy have the potential to provide effective therapy for this multisystemic disorder. Optimization of viral vector designs, providing tissue-specific expression and GAA protein modifications to enhance secretion and uptake has resulted in improved preclinical efficacy and safety data. In this review, we highlight gene therapy developments, in particular, AAV and LV HSPC-mediated gene therapy technologies, to potentially address all components of the neuromuscular associated Pompe disease pathology.

Long-term enzyme replacement therapy improves neurocognitive functioning and hippocampal synaptic plasticity in immune-tolerant alpha-mannosidosis mice

Alpha-mannosidosis is a glycoproteinosis caused by deficiency of lysosomal acid alpha-mannosidase (LAMAN), which markedly affects neurons of the central nervous system (CNS), and causes pathognomonic intellectual dysfunction in the clinical condition. Cognitive improvement consequently remains a major therapeutic objective in research on this devastating genetic error. Immune-tolerant LAMAN knockout mice were developed to evaluate the effects of enzyme replacement therapy (ERT) by prolonged administration of recombinant human enzyme. Biochemical evidence suggested that hippocampus may be one of the brain structures that benefits most from long-term ERT. In the present functional study, ERT was initiated in 2-month-old immune-tolerant alpha-mannosidosis mice and continued for 9months. During the course of treatment, mice were trained in the Morris water maze task to assess spatial-cognitive performance, which was related to synaptic plasticity recordings and hippocampal histopathology. Long-term ERT reduced primary substrate storage and neuroinflammation in hippocampus, and improved spatial learning after mid-term (10weeks+) and long-term (30weeks+) treatment. Long-term treatment substantially improved the spatial-cognitive abilities of alpha-mannosidosis mice, whereas the effects of mid-term treatment were more modest. Detailed analyses of spatial memory and spatial-cognitive performance indicated that even prolonged ERT did not restore higher cognitive abilities to the level of healthy mice. However, it did demonstrate marked therapeutic effects that coincided with increased synaptic connectivity, reflected by improvements in hippocampal CA3-CA1 long-term potentiation (LTP), expression of postsynaptic marker PSD-95 as well as postsynaptic density morphology. These experiments indicate that long-term ERT may hold promise, not only for the somatic defects of alpha-mannosidosis, but also to alleviate cognitive impairments of the disorder.

Pompe disease in adulthood: effects of antibody formation on enzyme replacement therapy

PURPOSE

To determine the effect of antibodies against recombinant human acid α-glucosidase (rhGAA) on treatment efficacy and safety, and to test whether the GAA genotype is involved in antibody formation.

METHODS

We used enzyme-linked immunosorbent assay (ELISA) to determine anti-rhGAA antibody titers at baseline and at 6, 12, and 36 months of rhGAA treatment. We measured the capacity of antibodies to neutralize rhGAA enzymatic activity or cellular uptake and the effects on infusion-associated reactions (IARs), muscle strength, and pulmonary function.

RESULTS

Of 73 patients, 45 developed antibodies. Maximal titers were high (≥1:31,250) in 22% of patients, intermediate (1:1,250-1:31,250) in 40%, and none or low (0-1:1,250) in 38%. The common IVS1/delex18 GAA genotype was absent only in the high-titer group. The height of the titer positively correlated with the occurrence and number of IARs (P ≤ 0.001). On the group level, antibody titers did not correlate with treatment efficacy. Eight patients (11%) developed very high maximal titers (≥156,250), but only one patient showed high sustained neutralizing antibodies that probably interfered with treatment efficacy.

CONCLUSIONS

In adults with Pompe disease, antibody formation does not interfere with rhGAA efficacy in the majority of patients, is associated with IARs, and may be attenuated by the IVS1/delex18 GAA genotype.Genet Med 19 1, 90-97.

Chronic enzyme replacement therapy ameliorates neuropathology in alpha-mannosidosis mice

OBJECTIVE

The lysosomal storage disease alpha-mannosidosis is caused by the deficiency of the lysosomal acid hydrolase alpha-mannosidase (LAMAN) leading to lysosomal accumulation of neutral mannose-linked oligosaccharides throughout the body, including the brain. Clinical findings in alpha-mannosidosis include skeletal malformations, intellectual disabilities and hearing impairment. To date, no curative treatment is available. We previously developed a beneficial enzyme replacement therapy (ERT) regimen for alpha-mannosidase knockout mice, a valid mouse model for the human disease. However, humoral immune responses against the injected recombinant human alpha-mannosidase (rhLAMAN) precluded long-term studies and chronic treatment.

METHODS

Here, we describe the generation of an immune-tolerant alpha-mannosidosis mouse model that allowed chronic injection of rhLAMAN by transgenic expression of a catalytically inactive variant of human LAMAN in the knockout background.

RESULTS

Chronic ERT of rhLAMAN revealed pronounced effects on primary substrate storage throughout the brain, normalization of lysosomal enzyme activities and morphology as well as a decrease in microglia activation. The positive effect of long-term ERT on neuronal lysosomal function was reflected by an improvement of cognitive deficits and exploratory activity. in vivo and in vitro uptake measurements indicate rapid clearance of rhLAMAN from circulation and a broad uptake into different cell types of the nervous system.

INTERPRETATION

Our data contribute to the understanding of neurological disorders treatment by demonstrating that lysosomal enzymes such as rhLAMAN can penetrate into the brain and is able to ameliorate neuropathology.

Nonclinical safety assessment of recombinant human acid sphingomyelinase (rhASM) for the treatment of acid sphingomyelinase deficiency:the utility of animal models of disease in the toxicological evaluation of potential therapeutics

Recombinant human acid sphingomyelinase (rhASM) is being developed as an enzyme replacement therapy for patients with acid sphingomyelinase deficiency (Niemann-Pick disease types A and B), which causes sphingomyelin to accumulate in lysosomes. In the acid sphingomyelinase knock-out (ASMKO) mouse, intravenously administered rhASM reduced tissue sphingomyelin levels in a dose-dependent manner. When rhASM was administered to normal rats, mice, and dogs, no toxicity was observed up to a dose of 30mg/kg. However, high doses of rhASM≥10mg/kg administered to ASMKO mice resulted in unexpected toxicity characterized by cardiovascular shock, hepatic inflammation, adrenal hemorrhage, elevations in ceramide and cytokines (especially IL-6, G-CSF, and keratinocyte chemoattractant [KC]), and death. The toxicity could be completely prevented by the administration of several low doses (3mg/kg) of rhASM prior to single or repeated high doses (≥20mg/kg). These results suggest that the observed toxicity involves the rapid breakdown of large amounts of sphingomyelin into ceramide and/or other toxic downstream metabolites, which are known signaling molecules with cardiovascular and pro-inflammatory effects. Our results suggest that the nonclinical safety assessment of novel therapeutics should include the use of specific animal models of disease whenever feasible.

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.

Specific antibody titer alters the effectiveness of intrathecal enzyme replacement therapy in canine mucopolysaccharidosis I

Intrathecal enzyme replacement therapy is an experimental option to treat central nervous system disease due to lysosomal storage. Previous work shows that MPS I dogs receiving enzyme replacement with recombinant human alpha-l-iduronidase into the cisterna magna showed normal brain glycosaminoglycan (GAG) storage after three or four doses. We analyzed MPS I dogs that received intrathecal enzyme in a previous study using an assay that detects only pathologic GAG (pGAG). To quantify pGAG in MPS I, the assay measures only those GAG which display terminal iduronic acid residues on their non-reducing ends. Mean cortical brain pGAG in six untreated MPS I dogs was 60.9±5.93 pmol/mg wet weight, and was 3.83±2.64 in eight normal or unaffected carrier animals (p<0.001). Intrathecal enzyme replacement significantly reduced pGAG storage in all treated animals. Dogs with low anti-iduronidase antibody titers showed normalization or near-normalization of pGAG in the brain (mean 8.17±6.17, n=7), while in dogs with higher titers, pGAG was reduced but not normal (mean 21.9±6.02, n=4). Intrathecal enzyme therapy also led to a mean 69% reduction in cerebrospinal fluid pGAG (from 83.8±26.3 to 27.2±12.3 pmol/ml CSF). The effect was measurable one month after each dose and did not differ with antibody titer. Prevention of the immune response to enzyme may improve the efficacy of intrathecal enzyme replacement therapy for brain disease due to MPS I.

The impact of antibodies on clinical outcomes in diseases treated with therapeutic protein: lessons learned from infantile Pompe disease

PURPOSE

Enzyme replacement therapy with rhGAA (Myozyme®) has lead to improved survival, which is largely attributable to improvements in cardiomyopathy and skeletal muscle function. However, crossreactive immunologic material-negative patients have a poor clinical response to enzyme replacement therapy secondary to high sustained antibody titers. Furthermore, although the majority of crossreactive immunologic material-positive patients tolerize or experience a downtrend in anti-rhGAA antibody titers, antibody response is variable with some crossreactive immunologic material-positive infants also mounting high sustained antibody titers.

METHODS

We retrospectively analyzed 34 infants with Pompe disease: 11 crossreactive immunologic material-negative patients, nine high-titer crossreactive immunologic material-positive patients, and 14 low-titer crossreactive immunologic material-positive patients. Clinical outcome measures were overall survival, ventilator-free survival, left ventricular mass index, Alberta Infant Motor Scale score, and urine Glc(4) levels.

RESULTS

Clinical outcomes in the high-titer crossreactive immunologic material-positive group were poor across all areas evaluated relative to the low-titer crossreactive immunologic material-positive group. For the crossreactive immunologic material-negative and high-titer crossreactive immunologic material-positive groups, no statistically significant differences were observed for any outcome measures, and both patient groups did poorly.

CONCLUSIONS

Our data indicate that, irrespective of crossreactive immunologic material status, patients with infantile Pompe disease with high sustained antibody titer have an attenuated therapeutic response to enzyme replacement therapy. With the advent of immunomodulation therapies, identification of patients at risk for developing high sustained antibody titer is critical.

Intrathecal tripeptidyl-peptidase 1 reduces lysosomal storage in a canine model of late infantile neuronal ceroid lipofuscinosis

Late infantile neuronal ceroid lipofuscinosis (LINCL) is caused by mutations in the gene encoding tripeptidyl-peptidase 1 (TPP1). LINCL patients accumulate lysosomal storage materials in the CNS accompanied by neurodegeneration, blindness, and functional decline. Dachshunds homozygous for a null mutation in the TPP1 gene recapitulate many symptoms of the human disease. The objectives of this study were to determine whether intrathecal (IT) TPP1 treatment attenuates storage accumulation and functional decline in TPP1-/- Dachshunds and to characterize the CNS distribution of TPP1 activity. TPP1 was administered to one TPP1-/- and one homozygous wild-type (WT) dog. An additional TPP1-/- and WT dog received vehicle. Four IT administrations of 32 mg TPP1 formulated in 2.3 mL of artificial cerebrospinal fluid (aCSF) or vehicle were administered monthly via the cerebellomedullary cistern from four to seven months of age. Functional decline was assessed by physical and neurological examinations, electrophysiology, and T-maze performance. Neural tissues were collected 48 h after the fourth administration and analyzed for TPP1 activity and autofluorescent storage material. TPP1 was distributed at greater than WT levels in many areas of the CNS of the TPP1-/- dog administered TPP1. The amount of autofluorescent storage was decreased in this dog relative to the vehicle-treated affected control. No improvement in overall function was observed in this dog compared to the vehicle-treated TPP1-/- littermate control. These results demonstrate for the first time in a large animal model of LINCL widespread delivery of biochemically active TPP1 to the brain after IT administration along with a decrease in lysosomal storage material. Further studies with this model will be necessary to optimize the dosing route and regimen to attenuate functional decline.

Cross-reactive immunologic material status affects treatment outcomes in Pompe disease infants

Deficiency of acid alpha glucosidase (GAA) causes Pompe disease, which is usually fatal if onset occurs in infancy. Patients synthesize a non-functional form of GAA or are unable to form native enzyme. Enzyme replacement therapy with recombinant human GAA (rhGAA) prolongs survival in infantile Pompe patients but may be less effective in cross-reactive immunologic material (CRIM)-negative patients. We retrospectively analyzed the influence of CRIM status on outcome in 21 CRIM-positive and 11 CRIM-negative infantile Pompe patients receiving rhGAA. Patients were from the clinical setting and from clinical trials of rhGAA, were 6 months of age, were not invasively ventilated, and were treated with IV rhGAA at a cumulative or total dose of 20 or 40 mg/kg/2 weeks. Outcome measures included survival, invasive ventilator-free survival, cardiac status, gross motor development, development of antibodies to rhGAA, and levels of urinary Glc(4). Following 52 weeks of treatment, 6/11 (54.5%) CRIM-negative and 1/21 (4.8%) CRIM-positive patients were deceased or invasively ventilated (p<0.0001). By age 27.1 months, all CRIM-negative patients and 4/21 (19.0%) CRIM-positive patients were deceased or invasively ventilated. Cardiac function and gross motor development improved significantly more in the CRIM-positive group. IgG antibodies to rhGAA developed earlier and serotiters were higher and more sustained in the CRIM-negative group. CRIM-negative status predicted reduced overall survival and invasive ventilator-free survival and poorer clinical outcomes in infants with Pompe disease treated with rhGAA. The effect of CRIM status on outcome appears to be mediated by antibody responses to the exogenous protein.

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.

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.

Intraventricular enzyme replacement improves disease phenotypes in a mouse model of late infantile neuronal ceroid lipofuscinosis

Late infantile neuronal ceroid lipofuscinosis (LINCL) is an autosomal recessive neurodegenerative disease caused by mutations in CLN2, which encodes the lysosomal protease tripeptidyl peptidase 1 (TPP1). LINCL is characterized clinically by progressive motor and cognitive decline, and premature death. Enzyme-replacement therapy (ERT) is currently available for lysosomal storage diseases affecting peripheral tissues, but has not been used in patients with central nervous system (CNS) involvement. Enzyme delivery through the cerebrospinal fluid is a potential alternative route to the CNS, but has not been studied for LINCL. In this study, we identified relevant neuropathological and behavioral hallmarks of disease in a mouse model of LINCL and correlated those findings with tissues from LINCL patients. Subsequently, we tested if intraventricular delivery of TPP1 to the LINCL mouse was efficacious. We found that infusion of recombinant human TPP1 through an intraventricular cannula led to enzyme distribution in several regions of the brain of treated mice. In vitro activity assays confirm increased TPP1 activity throughout the rostral-caudal extent of the brain. Importantly, treated mice showed attenuated neuropathology, and decreased resting tremor relative to vehicle-treated mice. This data demonstrates that intraventricular enzyme delivery to the CNS is feasible and may be of therapeutic value.

Induction of tolerance to human arylsulfatase A in a mouse model of metachromatic leukodystrophy

A deficiency of arylsulfatase A (ASA) causes metachromatic leukodystrophy (MLD), a lysosomal storage disorder characterized by accumulation of sulfatide, a severe neurological phenotype and early death. The efficacy of enzyme replacement therapy (ERT) has previously been determined in ASA knockout (ASA-/-) mice representing the only available animal model for MLD. Repeated intravenous injection of human ASA (hASA) improved the nervous system pathology and function, but also elicited a progressive humoral immune response leading to treatment resistance, anaphylactic reactions, and high mortality. In contrast to ASA-/- mice, most MLD patients express mutant hASA which may entail immunological tolerance to substituted wildtype hASA and thus protect from immunological complications. To test this notion, a cysteine-to-serine substitution was introduced into the active site of the hASA and the resulting inactive hASA-C69S variant was constitutively expressed in ASA-/- mice. Mice with sub-to supranormal levels of mutant hASA expression were analyzed. All mice, including those showing transgene expression below the limit of detection, were immunologically unresponsive to injected hASA. More than 100-fold overexpression did not induce an overt new phenotype except occasional intralysosomal deposition of minor amounts of glycogen in hepatocytes. Furthermore, long-term, low-dose ERT reduced sulfatide storage in peripheral tissues and the central nervous system indicating that high levels of extracellular mutant hASA do not prevent cellular uptake and lysosomal targeting of substituted wildtype hASA. Due to the tolerance to hASA and maintenance of the MLD-like phenotype, the novel transgenic strain may be particularly advantageous to assess the benefit and risk of long-term ERT.

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.

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.

Autophagy and mistargeting of therapeutic enzyme in skeletal muscle in Pompe disease

Enzyme replacement therapy (ERT) became a reality for patients with Pompe disease, a fatal cardiomyopathy and skeletal muscle myopathy caused by a deficiency of glycogen-degrading lysosomal enzyme acid alpha-glucosidase (GAA). The therapy, which relies on receptor-mediated endocytosis of recombinant human GAA (rhGAA), appears to be effective in cardiac muscle, but less so in skeletal muscle. We have previously shown a profound disturbance of the lysosomal degradative pathway (autophagy) in therapy-resistant muscle of GAA knockout mice (KO). Our findings here demonstrate a progressive age-dependent autophagic buildup in addition to enlargement of glycogen-filled lysosomes in multiple muscle groups in the KO. Trafficking and processing of the therapeutic enzyme along the endocytic pathway appear to be affected by the autophagy. Confocal microscopy of live single muscle fibers exposed to fluorescently labeled rhGAA indicates that a significant portion of the endocytosed enzyme in the KO was trapped as a partially processed form in the autophagic areas instead of reaching its target--the lysosomes. A fluid-phase endocytic marker was similarly mistargeted and accumulated in vesicular structures within the autophagic areas. These findings may explain why ERT often falls short of reversing the disease process and point toward new avenues for the development of pharmacological intervention.

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.

Fully deleted adenovirus persistently expressing GAA accomplishes long-term skeletal muscle glycogen correction in tolerant and nontolerant GSD-II mice

Glycogen storage disease type II (GSD-II) patients manifest symptoms of muscular dystrophy secondary to abnormal glycogen storage in cardiac and skeletal muscles. For GSD-II, we hypothesized that a fully deleted adenovirus (FDAd) vector expressing hGAA via nonviral regulatory elements (PEPCK promoter/ApoE enhancer) would facilitate long-term efficacy and decrease propensity to generate anti-hGAA antibody responses against hepatically secreted hGAA. Intravenous delivery of FDAdhGAA into GAA-tolerant or nontolerant GAA-KO mice resulted in long-term hepatic secretion of hGAA. Specifically, nontolerant mice achieved complete reversal of cardiac glycogen storage and near-complete skeletal glycogen correction for at least 180 days and tolerant mice for minimally 300 days coupled with the preservation of muscle strength. Anti-hGAA antibody levels in both mouse strains were significantly less relative to those previously generated by CMV-driven hGAA expression in nontolerant GAA-KO mice. However, plasma GAA levels decreased in nontolerant GAA-KO mice despite long-term intrahepatic GAA expression from the persistent vector. This intriguing result is discussed in light of other examples of "tolerance" induction by gene-transfer-based approaches.

Evasion of immune responses to introduced human acid alpha-glucosidase by liver-restricted expression in glycogen storage disease type II

Glycogen storage disease type II (GSD-II; Pompe disease) is caused by a deficiency of acid alpha-glucosidase (GAA; acid maltase) and manifests as muscle weakness, hypertrophic cardiomyopathy, and respiratory failure. Adeno-associated virus vectors containing either a liver-specific promoter (LSP) (AAV-LSPhGAApA) or a hybrid CB promoter (AAV-CBhGAApA) to drive human GAA expression were pseudotyped as AAV8 and administered to immunocompetent GAA-knockout mice. Secreted hGAA was detectable in plasma between 1 day and 12 weeks postadministration with AAV-LSPhGAApA and only from 1 to 8 days postadministration for AAV-CBGAApA. No anti-GAA antibodies were detected in response to AAV-LSPhGAApA (<1:200), whereas AAV-CBhGAApA provoked an escalating antibody response starting 2 weeks postadministration. The LSP drove approximately 60-fold higher GAA expression than the CB promoter in the liver by 12 weeks following vector administration. Furthermore, the detected cellular immunity was provoked by AAV-CBhGAApA, as detected by ELISpot and CD4+/CD8+ lymphocyte immunodetection. GAA activity was increased to higher than normal and glycogen content was reduced to essentially normal levels in the heart and skeletal muscle following administration of AAV-LSPhGAApA. Therefore, liver-restricted GAA expression with an AAV vector evaded immunity and enhanced efficacy in GSD-II mice.

Efficacy of an adeno-associated virus 8-pseudotyped vector in glycogen storage disease type II

Glycogen storage disease type II (GSD-II; Pompe disease) causes death in infancy from cardiorespiratory failure. The underlying deficiency of acid alpha-glucosidase (GAA; acid maltase) can be corrected by liver-targeted gene therapy in GSD-II, if secretion of GAA is accompanied by receptor-mediated uptake in cardiac and skeletal muscle. An adeno-associated virus (AAV) vector encoding human (h) GAA was pseudotyped as AAV8 (AAV2/8) and injected intravenously into immunodeficient GSD-II mice. High levels of hGAA were maintained in plasma for 24 weeks following AAV2/8 vector administration. A marked increase in vector copy number in the liver was demonstrated for the AAV2/8 vector compared to the analogous AAV2/2 vector. GAA deficiency in the heart and skeletal muscle was corrected with the AAV2/8 vector in male GSD-II mice, consistent with receptor-mediated uptake of hGAA. Male GSD-II mice demonstrated complete correction of glycogen storage in heart and diaphragm with the AAV2/8 vector, while female GSD-II mice had correction only in the heart. A biomarker for GSD-II was reduced in both sexes following AAV2/8 vector administration. Therefore, GAA production with an AAV2/8 vector in a depot organ, the liver, generated evidence for efficacious gene therapy in a mouse model for GSD-II.

Impact of Humoral Immune Response on Distribution and Efficacy of Recombinant Adeno-Associated Virus-Derived Acid α-Glucosidase in a Model of Glycogen Storage Disease Type II

Glycogen storage disease type II (GSDII) is a lysosomal storage disease caused by a deficiency in acid alpha-glucosidase (GAA), and leads to cardiorespiratory failure by the age of 2 years. In this study, we investigate the impact of anti-GAA antibody formation on cross-correction of the heart, diaphragm, and hind-limb muscles from liver-directed delivery of recombinant adeno-associated virus (rAAV)5- and rAAV8-GAA vectors. GAA(-/-) mice receiving 1 x 10(12) vector genomes of rAAV5- or rAAV8-DHBV-hGAA were analyzed for anti-GAA antibody response, GAA levels, glycogen reduction, and contractile function. We demonstrate that restoration of GAA to the affected muscles is dependent on the presence or absence of the antibody response. Immune-tolerant mice had significantly increased enzyme levels in the heart and skeletal muscles, whereas immune-responsive mice had background levels of GAA in all tissues except the diaphragm. The increased levels of activity in immune-tolerant mice correlated with reduced glycogen in the heart and diaphragm and, overall, contractile function of the soleus muscle was significantly improved. These findings highlight the importance of the immune response to rAAV-encoded GAA in correcting GSDII and provide additional understanding of the approach to treatment of GSDII.

Replacing acid α-glucosidase in Pompe disease: recombinant and transgenic enzymes are equipotent, but neither completely clears glycogen from type II muscle fibers

Pompe disease (type II glycogen storage disease) is an autosomal recessive disorder caused by a deficiency of lysosomal acid alpha-glucosidase (GAA) leading to the accumulation of glycogen in the lysosomes primarily in cardiac and skeletal muscle. The recombinant human GAA (rhGAA) is currently in clinical trials for enzyme replacement therapy of Pompe disease. Both clinical data and the results of preclinical studies in our knockout model of this disease show that rhGAA is much more effective in resolving the cardiomyopathy than the skeletal muscle myopathy. By contrast, another form of human GAA--transgenic enzyme constitutively produced in liver and secreted into the bloodstream of knockout mice (Gaa-/-)--completely prevented both cardiac and skeletal muscle glycogen accumulation. In the experiments reported here, the transgenic enzyme was much less efficient when delivered to skeletal muscle after significant amounts of glycogen had already accumulated. Furthermore, the transgenic enzyme and the rhGAA have similar therapeutic effects, and both efficiently clear glycogen from cardiac muscle and type I muscle fibers, but not type II fibers. Low abundance of proteins involved in endocytosis and trafficking of lysosomal enzymes combined with increased autophagy in type II fibers may explain the resistance to therapy.

Enzyme replacement therapy in the mouse model of Pompe disease

Deficiency of acid alpha-glucosidase (GAA) results in widespread cellular deposition of lysosomal glycogen manifesting as myopathy and cardiomyopathy. When GAA-/- mice were treated with rhGAA (20 mg/kg/week for up to 5 months), skeletal muscle cells took up little enzyme compared to liver and heart. Glycogen reduction was less than 50%, and some fibers showed little or no glycogen clearance. A dose of 100 mg/kg/week resulted in approximately 75% glycogen clearance in skeletal muscle. The enzyme reduced cardiac glycogen to undetectable levels at either dose. Skeletal muscle fibers with residual glycogen showed immunoreactivity for LAMP-1/LAMP-2, indicating that undigested glycogen remained in proliferating lysosomes. Glycogen clearance was more pronounced in type 1 fibers, and histochemical analysis suggested an increased mannose-6-phosphate receptor immunoreactivity in these fibers. Differential transport of enzyme into lysosomes may explain the strikingly uneven pattern of glycogen removal. Autophagic vacuoles, a feature of both the mouse model and the human disease, persisted despite glycogen clearance. In some groups a modest glycogen reduction was accompanied by improved muscle strength. These studies suggest that enzyme replacement therapy, although at much higher doses than in other lysosomal diseases, has the potential to reverse cardiac pathology and to reduce the glycogen level in skeletal muscle.