Early, sustained efficacy of adeno-associated virus vector-mediated gene therapy in glycogen storage disease type Ia

Gene therapy for glycogen storage diseases

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

Glycogen storage disease type I: Genetic etiology, clinical manifestations, and conventional and gene therapies

Glycogen storage disease type I (GSDI) is an inherited metabolic disorder characterized by a deficiency of enzymes or proteins involved in glycogenolysis and gluconeogenesis, resulting in excessive intracellular glycogen accumulation. While GSDI is classified into four different subtypes based on molecular genetic variants, GSDIa accounts for approximately 80%. GSDIa and GSDIb are autosomal recessive disorders caused by deficiencies in glucose-6-phosphatase (G6Pase-α) and glucose-6-phosphate-transporter (G6PT), respectively. For the past 50 years, the care of patients with GSDI has been improved following elaborate dietary managements. GSDI patients currently receive dietary therapies that enable patients to improve hypoglycemia and alleviate early symptomatic signs of the disease. However, dietary therapies have many limitations with a risk of calcium, vitamin D, and iron deficiency and cannot prevent long-term complications, such as progressive liver and renal failure. With the deepening understanding of the pathogenesis of GSDI and the development of gene therapy technology, there is great progress in the treatment of GSDI. Here, we review the underlying molecular genetics and the current clinical management strategies of GSDI patients with an emphasis on promising experimental gene therapies.

Genome editing using Staphylococcus aureus Cas9 in a canine model of glycogen storage disease Ia

Glycogen storage disease type Ia (GSD Ia) is the inherited deficiency of glucose-6-phosphatase (G6Pase), associated with life-threatening hypoglycemia and long-term complications, including hepatocellular carcinoma formation. Gene replacement therapy fails to stably reverse G6Pase deficiency. We attempted genome editing using two adeno-associated virus vectors, one that expressed Staphylococcus aureus Cas9 protein and a second containing a donor transgene encoding G6Pase, in a dog model for GSD Ia. We demonstrated donor transgene integration in the liver of three adult-treated dogs accompanied by stable G6Pase expression and correction of hypoglycemia during fasting. Two puppies with GSD Ia were treated by genome editing that achieved donor transgene integration in the liver. Integration frequency ranged from 0.5% to 1% for all dogs. In adult-treated dogs, anti-SaCas9 antibodies were detected before genome editing, reflecting prior exposure to S. aureus. Nuclease activity was low, as reflected by a low percentage of indel formation at the predicted site of SaCas9 cutting that indicated double-stranded breaks followed by non-homologous end-joining. Thus, genome editing can integrate a therapeutic transgene in the liver of a large animal model, either early or later in life, and further development is warranted to provide a more stable treatment for GSD Ia.

Studies on glycogen storage disease type 1a animal models: a brief perspective

Glycogen storage disease type 1 (GSD1) is a rare hereditary monogenic disease characterized by the disturbed glucose metabolism. The most widespread variant of GSD1 is GSD1a, which is a deficiency of glucose-6-phosphatase-ɑ. Glucose-6-phosphatase-ɑ is expressed only in liver, kidney, and intestine, and these organs are primarily affected by its deficiency, and long-term complications of GSD1a include hepatic tumors and chronic liver disease. This article is a brief overview of existing animal models for GSD1a, from the first mouse model of 1996 to modern CRISPR/Cas9-generated ones. First whole-body murine models demonstrated exact metabolic symptoms of GSD1a, but the animals did not survive weaning. The protocol for glucose treatment allowed prolonged survival of affected animals, but long-term complications, such as hepatic tumorigenesis, could not be investigated. Next, organ-specific knockout models were developed, and most of the metabolic research was performed on liver glucose-6-phosphate-deficient mice. Naturally occuring mutation was also discovered in dogs. All these models are widely used to study GSD1a from metabolic and physiological standpoints and to develop possible treatments involving gene therapy. Research performed using these models helped elucidate the role of glycogen and lipid accumulation, hypoxia, mitochondrial dysfunction, and autophagy impairment in long-term complications of GSD1a, including hepatic tumorigenesis. Recently, gene replacement therapy and genome editing were tested on described models, and some of the developed approaches have reached clinical trials.

Novel Gene-Correction-Based Therapeutic Modalities for Monogenic Liver Disorders

The majority of monogenic liver diseases are autosomal recessive disorders, with few being sex-related or co-dominant. Although orthotopic liver transplantation (LT) is currently the sole therapeutic option for end-stage patients, such an invasive surgical approach is severely restricted by the lack of donors and post-transplant complications, mainly associated with life-long immunosuppressive regimens. Therefore, the last decade has witnessed efforts for innovative cellular or gene-based therapeutic strategies. Gene therapy is a promising approach for treatment of many hereditary disorders, such as monogenic inborn errors. The liver is an organ characterized by unique features, making it an attractive target for in vivo and ex vivo gene transfer. The current genetic approaches for hereditary liver diseases are mediated by viral or non-viral vectors, with promising results generated by gene-editing tools, such as CRISPR-Cas9 technology. Despite massive progress in experimental gene-correction technologies, limitations in validated approaches for monogenic liver disorders have encouraged researchers to refine promising gene therapy protocols. Herein, we highlighted the most common monogenetic liver disorders, followed by proposed genetic engineering approaches, offered as promising therapeutic modalities.

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

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

A case report of acute pancreatitis with glycogen storage disease type IA in an adult patient and review of the literature

RATIONALE

Glycogen storage disease type IA (GSD IA) is an inherited disorder of glycogen metabolism characterized by fasting hypoglycemia, hyperuricemia, and hyperlipidemia including hypertriglyceridemia (HTG). Patients have a higher risk of developing acute pancreatitis (AP) because of HTG. AP is a potentially life-threatening disease with a wide spectrum severity. Nevertheless, almost no reports exist on GSD IA-induced AP in adult patients.

PATIENT CONCERNS

A 23-year-old male patient with GSD 1A is presented, who developed moderate severe AP due to HTG.

DIAGNOSES

The GSD 1A genetic background of this patient was confirmed by Sanger sequencing. Laboratory tests, along with abdominal enhanced-computed tomography, were used for the diagnosis of HTG and AP.

INTERVENTIONS

This patient was transferred to the intensive care unit and treated by reducing HTG, suppressing gastric acid, inhibiting trypsin activity, and relieving hyperuricemia and gout.

OUTCOMES

Fifteen days after hospital admission, the patient had no complaints about abdominal pain and distention. Follow-up of laboratory tests displayed almost normal values. Reexamination by computed tomography exhibited a reduction in peripancreatic necrotic fluid collection compared with the initial stage.

LESSONS

Fast and long-term reduction of triglycerides along with management of AP proved effective in relieving suffering of an adult GSD IA-patient and improving prognosis. Thus, therapeutic approaches have to be renewed and standardized to cope with all complications, especially AP, and enable a better outcome so that patients can master the disease.

Fenofibrate rapidly decreases hepatic lipid and glycogen storage in neonatal mice with glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD Ia) is caused by autosomal mutations in glucose-6-phosphatase α catalytic subunit (G6PC) and can present with severe hypoglycemia, lactic acidosis and hypertriglyceridemia. In both children and adults with GSD Ia, there is over-accumulation of hepatic glycogen and triglycerides that can lead to steatohepatitis and a risk for hepatocellular adenoma or carcinoma. Here, we examined the effects of the commonly used peroxisomal proliferated activated receptor α agonist, fenofibrate, on liver and kidney autophagy and lipid metabolism in 5-day-old G6pc -/- mice serving as a model of neonatal GSD Ia. Five-day administration of fenofibrate decreased the elevated hepatic and renal triglyceride and hepatic glycogen levels found in control G6pc -/- mice. Fenofibrate also induced autophagy and promoted β-oxidation of fatty acids and stimulated gene expression of acyl-CoA dehydrogenases in the liver. These findings show that fenofibrate can rapidly decrease hepatic glycogen and triglyceride levels and renal triglyceride levels in neonatal G6pc -/- mice. Moreover, since fenofibrate is an FDA-approved drug that has an excellent safety profile, our findings suggest that fenofibrate could be a potential pharmacological therapy for GSD Ia in neonatal and pediatric patients as well as for adults. These findings may also apply to non-alcoholic fatty liver disease, which shares similar pathological and metabolic changes with GSD Ia.

Pathogenesis of Hepatic Tumors following Gene Therapy in Murine and Canine Models of Glycogen Storage Disease

Glycogen storage disease type Ia (GSD Ia) is caused by mutations in the glucose-6-phosphatase (G6Pase) catalytic subunit gene (G6PC). GSD Ia complications include hepatocellular adenomas (HCA) with a risk for hepatocellular carcinoma (HCC) formation. Genome editing with adeno-associated virus (AAV) vectors containing a zinc-finger nuclease (ZFN) and a G6PC donor transgene was evaluated in adult mice with GSD Ia. Although mouse livers expressed G6Pase, HCA and HCC occurred following AAV vector administration. Interestingly, vector genomes were almost undetectable in the tumors but remained relatively high in adjacent liver (p < 0.01). G6Pase activity was decreased in tumors, in comparison with adjacent liver (p < 0.01). Furthermore, AAV-G6Pase vector-treated dogs with GSD Ia developed HCC with lower G6Pase activity (p < 0.01) in comparison with adjacent liver. AAV integration and tumor marker analysis in mice revealed that tumors arose from the underlying disorder, not from vector administration. Similarly to human GSD Ia-related HCA and HCC, mouse and dog tumors did not express elevated α-fetoprotein. Taken together, these results suggest that AAV-mediated gene therapy not only corrects hepatic G6Pase deficiency, but also has potential to suppress HCA and HCC in the GSD Ia liver.

Gene therapy for glycogen storage diseases

The focus of this review is the development of gene therapy for glycogen storage diseases (GSDs). GSD results from the deficiency of specific enzymes involved in the storage and retrieval of glucose in the body. Broadly, GSDs can be divided into types that affect liver or muscle or both tissues. For example, glucose-6-phosphatase (G6Pase) deficiency in GSD type Ia (GSD Ia) affects primarily the liver and kidney, while acid α-glucosidase (GAA) deficiency in GSD II causes primarily muscle disease. The lack of specific therapy for the GSDs has driven efforts to develop new therapies for these conditions. Gene therapy needs to replace deficient enzymes in target tissues, which has guided the planning of gene therapy experiments. Gene therapy with adeno-associated virus (AAV) vectors has demonstrated appropriate tropism for target tissues, including the liver, heart and skeletal muscle in animal models for GSD. AAV vectors transduced liver and kidney in GSD Ia and striated muscle in GSD II mice to replace the deficient enzyme in each disease. Gene therapy has been advanced to early phase clinical trials for the replacement of G6Pase in GSD Ia and GAA in GSD II (Pompe disease). Other GSDs have been treated in proof-of-concept studies, including GSD III, IV and V. The future of gene therapy appears promising for the GSDs, promising to provide more efficacious therapy for these disorders in the foreseeable future.

Challenges of Gene Therapy for the Treatment of Glycogen Storage Diseases Type I and Type III

Glycogen storage diseases (GSDs) type I (GSDI) and type III (GSDIII), the most frequent hepatic GSDs, are due to defects in glycogen metabolism, mainly in the liver. In addition to hypoglycemia and liver pathology, renal, myeloid, or muscle complications affect GSDI and GSDIII patients. Currently, patient management is based on dietary treatment preventing severe hypoglycemia and increasing the lifespan of patients. However, most of the patients develop long-term pathologies. In the past years, gene therapy for GSDI has generated proof of concept for hepatic GSDs. This resulted in a recent clinical trial of adeno-associated virus (AAV)-based gene replacement for GSDIa. However, the current limitations of AAV-mediated gene transfer still represent a challenge for successful gene therapy in GSDI and GSDIII. Indeed, transgene loss over time was observed in GSDI liver, possibly due to the degeneration of hepatocytes underlying the physiopathology of both GSDI and GSDIII and leading to hepatic tumor development. Moreover, multitissue targeting requires high vector doses to target nonpermissive tissues such as muscle and kidney. Interestingly, recent pharmacological interventions or dietary regimen aiming at the amelioration of the hepatocyte abnormalities before the administration of gene therapy demonstrated improved efficacy in GSDs. In this review, we describe the advances in gene therapy and the limitations to be overcome to achieve efficient and safe gene transfer in GSDs.

A Liver-Specific Thyromimetic, VK2809, Decreases Hepatosteatosis in Glycogen Storage Disease Type Ia

Background: Glycogen storage disease type Ia (GSD Ia), also known as von Gierke disease, is the most common glycogen storage disorder. It is caused by the deficiency of glucose-6-phosphatase, the enzyme that catalyzes the final step of gluconeogenesis and glycogenolysis. The accumulation of glucose-6-phosphate leads to increased glycogen and triglyceride levels in the liver. Patients with GSD Ia can develop steatohepatitis, cirrhosis, and increased risk for hepatocellular adenomas and carcinomas. We previously showed that animal models of GSD Ia had defective autophagy and dysfunctional mitochondria. In this study, we examined the effect of VK2809, a liver-specific thyroid hormone receptor β agonist, on hepatic steatosis, autophagy, and mitochondrial biogenesis in a mouse model of GSD Ia. Methods:G6pc^-/--deficient (GSD Ia) mice were treated with VK2809 or vehicle control by daily intraperitoneal injection for four days. The hepatic triglyceride and glycogen were determined by biochemical assays. Autophagy and mitochondrial biogenesis were measured by Western blotting for key autophagy and mitochondrial markers. Results: VK2809 treatment decreased hepatic mass and triglyceride content in GSD Ia mice. VK2809 stimulated hepatic autophagic flux as evidenced by increased microtubule-associated protein light chain 3-II (LC3B-II), decreased p62 protein levels, activation of AMP-activated protein kinase (AMPK), inhibition of the mammalian target of rapamycin (mTOR) signaling, enhancement of protein levels of ATG5-ATG12, and increased lysosomal protein expression. VK2809 also increased the expression of carnitine palmitoyltransferase 1a (CPT1α) and fibroblast growth factor 21 (FGF21), as well as mitochondrial biogenesis to promote mitochondrial β-oxidation. Conclusions: In summary, VK2809 treatment decreased hepatic triglyceride levels in GSD Ia mice through its simultaneous restoration of autophagy, mitochondrial biogenesis, and β-oxidation of fatty acids. Liver-specific thyromimetics represent a potential therapy for hepatosteatosis in GSD Ia as well as nonalcoholic fatty liver disease.

Bezafibrate induces autophagy and improves hepatic lipid metabolism in glycogen storage disease type Ia

Glucose-6-phosphatase α (G6Pase) deficiency, also known as von Gierke's Disease or Glycogen storage disease type Ia (GSD Ia), is characterized by decreased ability of the liver to convert glucose-6-phosphate to glucose leading to glycogen accumulation and hepatosteatosis. Long-term complications of GSD Ia include hepatic adenomas and carcinomas, in association with the suppression of autophagy in the liver. The G6pc-/- mouse and canine models for GSD Ia were treated with the pan-peroxisomal proliferator-activated receptor agonist, bezafibrate, to determine the drug's effect on liver metabolism and function. Hepatic glycogen and triglyceride concentrations were measured and western blotting was performed to investigate pathways affected by the treatment. Bezafibrate decreased liver triglyceride and glycogen concentrations and partially reversed the autophagy defect previously demonstrated in GSD Ia models. Changes in medium-chain acyl-CoA dehydrogenase expression and acylcarnintine flux suggested that fatty acid oxidation was increased and fatty acid synthase expression associated with lipogenesis was decreased in G6pc-/- mice treated with bezafibrate. In summary, bezafibrate induced autophagy in the liver while increasing fatty acid oxidation and decreasing lipogenesis in G6pc-/- mice. It represents a potential therapy for glycogen overload and hepatosteatosis associated with GSD Ia, with beneficial effects that have implications for non-alcoholic fatty liver disease.

Bezafibrate Enhances AAV Vector-Mediated Genome Editing in Glycogen Storage Disease Type Ia

Glycogen storage disease type Ia (GSD Ia) is a rare inherited disease caused by mutations in the glucose-6-phosphatase (G6Pase) catalytic subunit gene (G6PC). Absence of G6Pase causes life-threatening hypoglycemia and long-term complications because of the accumulations of metabolic intermediates. Bezafibrate, a pan-peroxisome proliferator-activated receptor (PPAR) agonist, was administered in the context of genome editing with a zinc-finger nuclease-containing vector (AAV-ZFN) and a G6Pase donor vector (AAV-RoG6P). Bezafibrate treatment increased survival and decreased liver size (liver/body mass, p < 0.05) in combination with genome editing. Blood glucose has higher (p < 0.05) after 4 h of fasting, and liver glycogen accumulation (p < 0.05) was lower in association with higher G6Pase activity (p < 0.05). Furthermore, bezafibrate-treated mice had increased numbers of G6PC transgenes (p < 0.05) and higher ZFN activity (p < 0.01) in the liver compared with controls. PPAR-α expression was increased and PPAR-γ expression was decreased in bezafibrate-treated mice. Therefore, bezafibrate improved hepatocellular abnormalities and increased the transduction efficiency of AAV vector-mediated genome editing in liver, whereas higher expression of G6Pase corrected molecular signaling in GSD Ia. Taken together, bezafibrate shows promise as a drug for increasing AAV vector-mediated genome editing.

Long-term complications of glycogen storage disease type Ia in the canine model treated with gene replacement therapy

BACKGROUND

Glycogen storage disease type Ia (GSD Ia) in dogs closely resembles human GSD Ia. Untreated patients with GSD Ia develop complications associated with glucose-6-phosphatase (G6Pase) deficiency. Survival of human patients on intensive nutritional management has improved; however, long-term complications persist including renal failure, nephrolithiasis, hepatocellular adenomas (HCA), and a high risk for hepatocellular carcinoma (HCC). Affected dogs fail to thrive with dietary therapy alone. Treatment with gene replacement therapy using adeno-associated viral vectors (AAV) expressing G6Pase has greatly prolonged life and prevented hypoglycemia in affected dogs. However, long-term complications have not been described to date.

METHODS

Five GSD Ia-affected dogs treated with AAV-G6Pase were evaluated. Dogs were euthanized due to reaching humane endpoints related to liver and/or kidney involvement, at 4 to 8 years of life. Necropsies were performed and tissues were analyzed.

RESULTS

Four dogs had liver tumors consistent with HCA and HCC. Three dogs developed renal failure, but all dogs exhibited progressive kidney disease histologically. Urolithiasis was detected in two dogs; uroliths were composed of calcium oxalate and calcium phosphate. One affected and one carrier dog had polycystic ovarian disease. Bone mineral density was not significantly affected.

CONCLUSIONS

Here, we show that the canine GSD Ia model demonstrates similar long-term complications as GSD Ia patients in spite of gene replacement therapy. Further development of gene therapy is needed to develop a more effective treatment to prevent long-term complications of GSD Ia.

Overexpression of Mutant FKRP Restores Functional Glycosylation and Improves Dystrophic Phenotype in FKRP Mutant Mice

Autosomal recessive homozygous or compound heterozygous mutations in FKRP result in forms of muscular dystrophy-dystroglycanopathy varying in age of onset, clinical presentation, and disease progression, ranging from the severe Walker-Warburg, type A,5 (MDDGA5), muscle-eye-brain (MDDGB5) with or without cognitive deficit, to limb-girdle type 2I (MDDGC5). Phenotypic variation indicates degrees of functionality of individual FKRP mutation, which has been supported by the presence of residual expression of functionally glycosylated α-dystroglycan (DG) in muscles of both animal models and patients. However, direct evidence showing enhancement in glycosylation of α-DG by mutant FKRP is lacking. Using AAV9-mediated overexpression of mutant human FKRP bearing the P448L mutation (mhFKRP-P448L) associated with severe congenital muscular dystrophy (CMD), we demonstrate the restoration of functional glycosylation of α-DG and reduction in markers of disease progression. Expression of mhFKRP-P448L also corrects dystrophic phenotypes in the models of L276I mutation with mild disease phenotype and causes no obvious histological or biomarker alteration in C57BL/6 normal mice. Our results confirm the existing function of mutant FKRP. The results also suggest that mutant FKRP could be an alternative approach for potential gene therapy should normal FKRP gene products be immunogenic.

Hepatic mitochondrial dysfunction is a feature of Glycogen Storage Disease Type Ia (GSDIa)

Glycogen storage disease type Ia (GSDIa, von Gierke disease) is the most common glycogen storage disorder. It is caused by the deficiency of glucose-6-phosphatase, an enzyme which catalyses the final step of gluconeogenesis and glycogenolysis. Clinically, GSDIa is characterized by fasting hypoglycaemia and hepatic glycogen and triglyceride overaccumulation. The latter leads to steatohepatitis, cirrhosis, and the formation of hepatic adenomas and carcinomas. Currently, little is known about the function of various organelles and their impact on metabolism in GSDIa. Accordingly, we investigated mitochondrial function in cell culture and mouse models of GSDIa. We found impairments in oxidative phosphorylation and changes in TCA cycle metabolites, as well as decreased mitochondrial membrane potential and deranged mitochondrial ultra-structure in these model systems. Mitochondrial content also was decreased, likely secondary to decreased mitochondrial biogenesis. These deleterious effects culminated in the activation of the mitochondrial apoptosis pathway. Taken together, our results demonstrate a role for mitochondrial dysfunction in the pathogenesis of GSDIa, and identify a new potential target for the treatment of this disease. They also provide new insight into the role of carbohydrate overload on mitochondrial function in other hepatic diseases, such as non-alcoholic fatty liver disease.

Systemic Correction of Murine Glycogen Storage Disease Type IV by an AAV-Mediated Gene Therapy

Deficiency of glycogen branching enzyme (GBE) causes glycogen storage disease type IV (GSD IV), which is characterized by the accumulation of a less branched, poorly soluble form of glycogen called polyglucosan (PG) in multiple tissues. This study evaluates the efficacy of gene therapy with an adeno-associated viral (AAV) vector in a mouse model of adult form of GSD IV (Gbe1^ys/ys). An AAV serotype 9 (AAV9) vector containing a human GBE expression cassette (AAV-GBE) was intravenously injected into 14-day-old Gbe1^ys/ys mice at a dose of 5 × 10¹¹ vector genomes per mouse. Mice were euthanized at 3 and 9 months of age. In the AAV-treated mice at 3 months of age, GBE enzyme activity was highly elevated in heart, which is consistent with the high copy number of the viral vector genome detected. GBE activity also increased significantly in skeletal muscles and the brain, but not in the liver. The glycogen content was reduced to wild-type levels in muscles and significantly reduced in the liver and brain. At 9 months of age, though GBE activity was only significantly elevated in the heart, glycogen levels were significantly reduced in the liver, brain, and skeletal muscles of the AAV-treated mice. In addition, the AAV treatment resulted in an overall decrease in plasma activities of alanine transaminase, aspartate transaminase, and creatine kinase, and a significant increase in fasting plasma glucose concentration at 9 months of age. This suggests an alleviation of damage and improvement of function in the liver and muscles by the AAV treatment. This study demonstrated a long-term benefit of a systemic injection of an AAV-GBE vector in Gbe1^ys/ys mice.

A Modified Enzymatic Method for Measurement of Glycogen Content in Glycogen Storage Disease Type IV

Deficiency of glycogen branching enzyme in glycogen storage disease type IV (GSD IV) results in accumulation of less-branched and poorly soluble polysaccharides (polyglucosan bodies) in multiple tissues. Standard enzymatic method, when used to quantify glycogen content in GSD IV tissues, causes significant loss of the polysaccharides during preparation of tissue lysates. We report a modified method including an extra boiling step to dissolve the insoluble glycogen, ultimately preserving the glycogen content in tissue homogenates from GSD IV mice. Muscle tissues from wild-type, GSD II and GSD IV mice and GSD III dogs were homogenized in cold water, and homogenate of each tissue was divided into two parts. One part was immediately clarified by centrifugation at 4°C (STD-prep); the other part was boiled for 5 min then centrifuged (Boil-prep) at room temperature. When glycogen was quantified enzymatically in tissue lysates, no significant differences were found between the STD-prep and the Boil-prep for wild-type, GSD II and GSD III muscles. In contrast, glycogen content for GSD IV muscle in the STD-prep was only 11% of that in the Boil-prep, similar to wild-type values. Similar results were observed in other tissues of GSD IV mice and fibroblast cells from a GSD IV patient. This study provides important information for improving disease diagnosis, monitoring disease progression, and evaluating treatment outcomes in both clinical and preclinical clinical settings for GSD IV. This report should be used as an updated protocol in clinical diagnostic laboratories.

Preclinical Development of New Therapy for Glycogen Storage Diseases

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

In Vivo Zinc Finger Nuclease-mediated Targeted Integration of a Glucose-6-phosphatase Transgene Promotes Survival in Mice With Glycogen Storage Disease Type IA

Glycogen storage disease type Ia (GSD Ia) is caused by glucose-6-phosphatase (G6Pase) deficiency in association with severe, life-threatening hypoglycemia that necessitates lifelong dietary therapy. Here we show that use of a zinc-finger nuclease (ZFN) targeted to the ROSA26 safe harbor locus and a ROSA26-targeting vector containing a G6PC donor transgene, both delivered with adeno-associated virus (AAV) vectors, markedly improved survival of G6Pase knockout (G6Pase-KO) mice compared with mice receiving the donor vector alone (P < 0.04). Furthermore, transgene integration has been confirmed by sequencing in the majority of the mice treated with both vectors. Targeted alleles were 4.6-fold more common in livers of mice with GSD Ia, as compared with normal littermates, at 8 months following vector administration (P < 0.02). This suggests a selective advantage for vector-transduced hepatocytes following ZFN-mediated integration of the G6Pase vector. A short-term experiment also showed that 3-month-old mice receiving the ZFN had significantly-improved biochemical correction, in comparison with mice that received the donor vector alone. These data suggest that the use of ZFNs to drive integration of G6Pase at a safe harbor locus might improve vector persistence and efficacy, and lower mortality in GSD Ia.

Induction of autophagy improves hepatic lipid metabolism in glucose-6-phosphatase deficiency

BACKGROUND & AIMS

Glucose-6-phosphatase (G6Pase α, G6PC) deficiency, also known as von Gierke's disease or GSDIa, is the most common glycogen storage disorder. It is characterized by a decreased ability of the liver to convert glucose-6-phosphate (G6P) to glucose leading to glycogen and lipid over-accumulation progressing to liver failure and/or hepatomas and carcinomas. Autophagy of intracellular lipid stores (lipophagy) has been shown to stimulate fatty acid β-oxidation in hepatic cells. Thus, we examined autophagy and its effects on reducing hepatic lipid over-accumulation in several cell culture and animal models of GSDIa.

METHODS

Autophagy in G6PC-deficient hepatic cell lines, mice, and dogs was measured by Western blotting for key autophagy markers. Pro-autophagic Unc51-like kinase 1 (ULK1/ATG1) was overexpressed in G6PC-deficient hepatic cells, and lipid clearance and oxidative phosphorylation measured. G6PC(-/-) mice and GSDIa dogs were treated with rapamycin and assessed for liver function.

RESULTS

Autophagy was impaired in the cell culture, mouse, and canine models of GSDIa. Stimulation of the anti-autophagic mTOR, and inhibition of the pro-autophagic AMPK pathways occurred both in vitro and in vivo. Induction of autophagy by ULK1/ATG1 overexpression decreased lipid accumulation and increased oxidative phosphorylation in G6PC-deficient hepatic cells. Rapamycin treatment induced autophagy and decreased hepatic triglyceride and glycogen content in G6PC(-/-) mice, as well as reduced liver size and improved circulating markers of liver damage in GSDIa dogs.

CONCLUSIONS

Autophagy is impaired in GSDIa. Pharmacological induction of autophagy corrects hepatic lipid over-accumulation and may represent a new therapeutic strategy for GSDIa.

Development of hepatocellular adenomas and carcinomas in mice with liver-specific G6Pase-α deficiency

Glycogen storage disease type 1a (GSD-1a) is caused by a deficiency in glucose-6-phosphatase-α (G6Pase-α), and is characterized by impaired glucose homeostasis and a high risk of developing hepatocellular adenomas (HCAs). A globally G6Pase-α-deficient (G6pc(-/-)) mouse model that shows pathological features similar to those of humans with GSD-1a has been developed. These mice show a very severe phenotype of disturbed glucose homeostasis and rarely live beyond weaning. We generated liver-specific G6Pase-α-deficient (LS‑G6pc(-/-)) mice as an alternative animal model for studying the long-term pathophysiology of the liver and the potential treatment strategies, such as cell therapy. LS‑G6pc(-/-) mice were viable and exhibited normal glucose profiles in the fed state, but showed significantly lower blood glucose levels than their control littermates after 6 hours of fasting. LS‑G6pc(-/-) mice developed hepatomegaly with glycogen accumulation and hepatic steatosis, and progressive hepatic degeneration. Ninety percent of the mice analyzed developed amyloidosis by 12 months of age. Finally, 25% of the mice sacrificed at age 10-20 months showed the presence of multiple HCAs and in one case late development of hepatocellular carcinoma (HCC). In conclusion, LS‑G6pc(-/-) mice manifest hepatic symptoms similar to those of human GSD-1a and, therefore, represent a valid model to evaluate long-term liver pathogenesis of GSD-1a.

Large animal models and new therapies for glycogen storage disease

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

Enhanced gene expression of systemically administered plasmid DNA in the liver with therapeutic ultrasound and microbubbles

Ultrasound-mediated delivery (USMD) of novel therapeutic agents in the presence of microbubbles is a potentially safe and effective method for gene therapy offering many desired characteristics, such as low toxicity, potential for repeated treatment, and organ specificity. In this study, we tested the capability of USMD to improve gene expression in mice livers using glycogen storage disease Type Ia as a model disease under systemic administration of naked plasmid DNA. Image-guided therapeutic ultrasound was used in two studies to provide therapeutic ultrasound to mice livers. In the first study, involving wild-type mice, control animals received naked plasmid DNA (pG6Pase 150 μg) via the tail vein, followed by an infusion of microbubbles; the treated animals additionally received therapeutic ultrasound (1 MHz). Following the procedure, the animals were left to recover and were subsequently euthanized after 2 d and liver samples were extracted. Reverse transcription polymerase chain reaction (RT-PCR) assays were performed on the samples to quantify mRNA expression. In addition, Western blot assays of FLAG-tagged glucose-6-phosphatase (G6Pase) were performed to evaluate protein expression. Ultrasound-exposed animals showed a 4-fold increase in G6Pase RNA in the liver, in comparison with control animals. Furthermore, results from Western blot analysis demonstrated a 2-fold increased protein expression in ultrasound-exposed animals after two days ( p < 0.05). A second pilot study was performed with G6Pase knockout mice, and the animals were monitored for correction of hypoglycemia over a period of 3 weeks before tissue analysis. The RT-PCR assays of samples from these animals demonstrated increased G6Pase RNA in the liver following ultrasound treatment. These results demonstrate that USMD can increase gene expression of systemically injected naked pDNA in the liver and also provide insight into the development of realistic approaches that can be translated into clinical practice.

In search of proof-of-concept: gene therapy for glycogen storage disease type Ia

The emergence of life threatening long-term complications in glycogen storage disease type Ia (GSD-Ia) has emphasized the need for new therapies, such as gene therapy, which could achieve biochemical correction of glucose-6-phosphatase deficiency and reverse clinical involvement. We have developed gene therapy with a novel adeno-associated virus (AAV) vector that: 1) prevented mortality and corrected glycogen storage in the liver, 2) corrected hypoglycemia during fasting, and 3) achieved efficacy with a low number of vector particles in G6Pase-deficient mice and dogs. However, the gradual loss of transgene expression from episomal AAV vector genomes eventually necessitated the administration of a different pseudotype of the AAV vector to sustain dogs with GSD-Ia. Further preclinical development of AAV vector-mediated gene therapy is therefore warranted in GSD-Ia.

Long-term efficacy following readministration of an adeno-associated virus vector in dogs with glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD-Ia) is the inherited deficiency of glucose-6-phosphatase (G6Pase), primarily found in liver and kidney, which causes life-threatening hypoglycemia. Dogs with GSD-Ia were treated with double-stranded adeno-associated virus (AAV) vectors encoding human G6Pase. Administration of an AAV9 pseudotyped (AAV2/9) vector to seven consecutive GSD-Ia neonates prevented hypoglycemia during fasting for up to 8 hr; however, efficacy eventually waned between 2 and 30 months of age, and readministration of a new pseudotype was eventually required to maintain control of hypoglycemia. Three of these dogs succumbed to acute hypoglycemia between 7 and 9 weeks of age; however, this demise could have been prevented by earlier readministration an AAV vector, as demonstrated by successful prevention of mortality of three dogs treated earlier in life. Over the course of this study, six out of nine dogs survived after readministration of an AAV vector. Of these, each dog required readministration on average every 9 months. However, two were not retreated until >34 months of age, while one with preexisting antibodies was re-treated three times in 10 months. Glycogen content was normalized in the liver following vector administration, and G6Pase activity was increased in the liver of vector-treated dogs in comparison with GSD-Ia dogs that received only with dietary treatment. G6Pase activity reached approximately 40% of normal in two female dogs following AAV2/9 vector administration. Elevated aspartate transaminase in absence of inflammation indicated that hepatocellular turnover in the liver might drive the loss of vector genomes. Survival was prolonged for up to 60 months in dogs treated by readministration, and all dogs treated by readministration continue to thrive despite the demonstrated risk for recurrent hypoglycemia and mortality from waning efficacy of the AAV2/9 vector. These preclinical data support the further translation of AAV vector-mediated gene therapy in GSD-Ia.

Treatment of newborn G6pc(-/-) mice with bone marrow-derived myelomonocytes induces liver repair

BACKGROUND & AIMS

Several studies have shown that bone marrow-derived committed myelomonocytic cells can repopulate diseased livers by fusing with host hepatocytes and can restore normal liver function. These data suggest that myelomonocyte transplantation could be a promising approach for targeted and well-tolerated cell therapy aimed at liver regeneration. We sought to determine whether bone marrow-derived myelomonocytic cells could be effective for liver reconstitution in newborn mice knock-out for glucose-6-phosphatase-α.

METHODS

Bone marrow-derived myelomonocytic cells obtained from adult wild type mice were transplanted in newborn knock-out mice. Tissues of control and treated mice were frozen for histochemical analysis, or paraffin-embedded and stained with hematoxylin and eosin for histological examination or analyzed by immunohistochemistry or fluorescent in situ hybridization.

RESULTS

Histological sections of livers of treated knock-out mice revealed areas of regenerating tissue consisting of hepatocytes of normal appearance and partial recovery of normal architecture as early as 1 week after myelomonocytic cells transplant. FISH analysis with X and Y chromosome paints indicated fusion between infused cells and host hepatocytes. Glucose-6-phosphatase activity was detected in treated mice with improved profiles of liver functional parameters.

CONCLUSIONS

Our data indicate that bone marrow-derived myelomonocytic cell transplant may represent an effective way to achieve liver reconstitution of highly degenerated livers in newborn animals.

Acoustic accessibility investigation for ultrasound mediated treatment of glycogen storage disease type Ia patients

Glycogen storage disease type Ia (GSDIa) is caused by an inherited defect in the glucose-6-phosphatase gene. The recent advent of targeted ultrasound-mediated delivery (USMD) of plasmid DNA (pDNA) to the liver in conjunction with microbubbles may provide an alternative treatment option. This study focuses on determining the acoustically accessible liver volume in GSDIa patients using transducer models of various geometries with an image-based geometry-driven approach. Results show that transducers with longer focal lengths and smaller apertures (up to an f/number of 2) are able to access larger liver volumes in GSDIa patients while still being capable of delivering the required ultrasound dose in situ (2.5 MPa peak negative pressure at the focus). With sufficiently large acoustic windows and the ability to use glucose to easily assess efficacy, GSD appears to be a good model for testing USMD as proof of principle as a potential therapy for liver applications in general.

Recombinant AAV-directed gene therapy for type I glycogen storage diseases

INTRODUCTION

Glycogen storage disease (GSD) type Ia and Ib are disorders of impaired glucose homeostasis affecting the liver and kidney. GSD-Ib also affects neutrophils. Current dietary therapies cannot prevent long-term complications. In animal studies, recombinant adeno-associated virus (rAAV) vector-mediated gene therapy can correct or minimize multiple aspects of the disorders, offering hope for human gene therapy.

AREAS COVERED

A summary of recent progress in rAAV-mediated gene therapy for GSD-I; strategies to improve rAAV-mediated gene delivery, transduction efficiency and immune avoidance; and vector refinements that improve expression.

EXPERT OPINION

rAAV-mediated gene delivery to the liver can restore glucose homeostasis in preclinical models of GSD-I, but some long-term complications of the liver and kidney remain. Gene therapy for GSD-Ib is less advanced than for GSD-Ia and only transient correction of myeloid dysfunction has been achieved. A question remains as to whether a single rAAV vector can meet the expression efficiency and tropism required to treat all aspects of GSD-I, or if a multi-pronged approach is needed. An understanding of the strengths and weaknesses of rAAV vectors in the context of strategies to achieve efficient transduction of the liver, kidney and hematopoietic stem cells is required for treating GSD-I.

Therapeutic in vivo gene transfer for genetic disease using AAV: progress and challenges

In vivo gene replacement for the treatment of inherited disease is one of the most compelling concepts in modern medicine. Adeno-associated virus (AAV) vectors have been extensively used for this purpose and have shown therapeutic efficacy in a range of animal models. Successful translation to the clinic was initially slow, but long-term expression of donated genes at therapeutic levels has now been achieved in patients with inherited retinal disorders and haemophilia B. Recent exciting results have raised hopes for the treatment of many other diseases. As we discuss here, the prospects and challenges for AAV gene therapy are to a large extent dependent on the target tissue and the specific disease.

Hepatorenal correction in murine glycogen storage disease type I with a double-stranded adeno-associated virus vector

Glycogen storage disease type Ia (GSD-Ia) is caused by the deficiency of glucose-6-phosphatase (G6Pase). Long-term complications of GSD-Ia include life-threatening hypoglycemia and proteinuria progressing to renal failure. A double-stranded (ds) adeno-associated virus serotype 2 (AAV2) vector encoding human G6Pase was pseudotyped with four serotypes, AAV2, AAV7, AAV8, and AAV9, and we evaluated efficacy in 12-day-old G6pase (-/-) mice. Hypoglycemia during fasting (plasma glucose <100 mg/dl) was prevented for >6 months by the dsAAV2/7, dsAAV2/8, and dsAAV2/9 vectors. Prolonged fasting for 8 hours revealed normalization of blood glucose following dsAAV2/9 vector administration at the higher dose. The glycogen content of kidney was reduced by >65% with both the dsAAV2/7 and dsAAV2/9 vectors, and renal glycogen content was stably reduced between 7 and 12 months of age for the dsAAV2/9 vector-treated mice. Every vector-treated group had significantly reduced glycogen content in the liver, in comparison with untreated G6pase (-/-) mice. G6Pase was expressed in many renal epithelial cells of with the dsAAV2/9 vector for up to 12 months. Albuminuria and renal fibrosis were reduced by the dsAAV2/9 vector. Hepatorenal correction in G6pase (-/-) mice demonstrates the potential of AAV vectors for the correction of inherited diseases of metabolism.

Rescue administration of a helper-dependent adenovirus vector with long-term efficacy in dogs with glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD-Ia) stems from glucose-6-phosphatase (G6Pase) deficiency and causes hypoglycemia, hepatomegaly, hypercholesterolemia and lactic acidemia. Three dogs with GSD-Ia were initially treated with a helper-dependent adenovirus encoding a human G6Pase transgene (HDAd-cG6Pase serotype 5) on postnatal day 3. Unlike untreated dogs with GSD-Ia, all three dogs initially maintained normal blood glucose levels. After 6-22 months, vector-treated dogs developed hypoglycemia, anorexia and lethargy, suggesting that the HDAd-cG6Pase serotype 5 vector had lost efficacy. Liver biopsies collected at this time revealed significantly elevated hepatic G6Pase activity and reduced glycogen content, when compared with affected dogs treated only by frequent feeding. Subsequently, the HDAd-cG6Pase serotype 2 vector was administered to two dogs, and hypoglycemia was reversed; however, renal dysfunction and recurrent hypoglycemia complicated their management. Administration of a serotype 2 HDAd vector prolonged survival in one GSD-Ia dog to 12 months of age and 36 months of age in the other, but the persistence of long-term complications limited HDAd vectors in the canine model for GSD-Ia.

Perinatal gene transfer to the liver

The liver acts as a host to many functions hence raising the possibility that any one may be compromised by a single gene defect. Inherited or de novo mutations in these genes may result in relatively mild diseases or be so devastating that death within the first weeks or months of life is inevitable. Some diseases can be managed using conventional medicines whereas others are, as yet, untreatable. In this review we consider the application of early intervention gene therapy in neonatal and fetal preclinical studies. We appraise the tools of this technology, including lentivirus, adenovirus and adeno-associated virus (AAV)-based vectors. We highlight the application of these for a range of diseases including hemophilia, urea cycle disorders such as ornithine transcarbamylase deficiency, organic acidemias, lysosomal storage diseases including mucopolysaccharidoses, glycogen storage diseases and bile metabolism. We conclude by assessing the advantages and disadvantages associated with fetal and neonatal liver gene transfer.

Glycogen storage disease type I and G6Pase-β deficiency: etiology and therapy

Glycogen storage disease type I (GSD-I) consists of two subtypes: GSD-Ia, a deficiency in glucose-6-phosphatase-α (G6Pase-α) and GSD-Ib, which is characterized by an absence of a glucose-6-phosphate (G6P) transporter (G6PT). A third disorder, G6Pase-β deficiency, shares similarities with this group of diseases. G6Pase-α and G6Pase-β are G6P hydrolases in the membrane of the endoplasmic reticulum, which depend on G6PT to transport G6P from the cytoplasm into the lumen. A functional complex of G6PT and G6Pase-α maintains interprandial glucose homeostasis, whereas G6PT and G6Pase-β act in conjunction to maintain neutrophil function and homeostasis. Patients with GSD-Ia and those with GSD-Ib exhibit a common metabolic phenotype of disturbed glucose homeostasis that is not evident in patients with G6Pase-β deficiency. Patients with a deficiency in G6PT and those lacking G6Pase-β display a common myeloid phenotype that is not shared by patients with GSD-Ia. Previous studies have shown that neutrophils express the complex of G6PT and G6Pase-β to produce endogenous glucose. Inactivation of either G6PT or G6Pase-β increases neutrophil apoptosis, which underlies, at least in part, neutrophil loss (neutropenia) and dysfunction in GSD-Ib and G6Pase-β deficiency. Dietary and/or granulocyte colony-stimulating factor therapies are available; however, many aspects of the diseases are still poorly understood. This Review will address the etiology of GSD-Ia, GSD-Ib and G6Pase-β deficiency and highlight advances in diagnosis and new treatment approaches, including gene therapy.

Long-term amelioration of feline Mucopolysaccharidosis VI after AAV-mediated liver gene transfer

Mucopolysaccharidosis VI (MPS VI) is caused by deficient arylsulfatase B (ARSB) activity resulting in lysosomal storage of glycosaminoglycans (GAGs). MPS VI is characterized by dysostosis multiplex, organomegaly, corneal clouding, and heart valve thickening. Gene transfer to a factory organ like liver may provide a lifetime source of secreted ARSB. We show that intravascular administration of adeno-associated viral vectors (AAV) 2/8-TBG-felineARSB in MPS VI cats resulted in ARSB expression up to 1 year, the last time point of the study. In newborn cats, normal circulating ARSB activity was achieved following delivery of high vector doses (6 × 10(13) genome copies (gc)/kg) whereas delivery of AAV2/8 vector doses as low as 2 × 10(12) gc/kg resulted in higher than normal serum ARSB levels in juvenile MPS VI cats. In MPS VI cats showing high serum ARSB levels, independent of the age at treatment, we observed: (i) clearance of GAG storage, (ii) improvement of long bone length, (iii) reduction of heart valve thickness, and (iv) improvement in spontaneous mobility. Thus, AAV2/ 8-mediated liver gene transfer represents a promising therapeutic strategy for MPS VI patients.

Adeno-associated virus-mediated correction of a canine model of glycogen storage disease type Ia

Glycogen storage disease type Ia (GSDIa; von Gierke disease; MIM 232200) is caused by a deficiency in glucose-6-phosphatase-alpha. Patients with GSDIa are unable to maintain glucose homeostasis and suffer from severe hypoglycemia, hepatomegaly, hyperlipidemia, hyperuricemia, and lactic acidosis. The canine model of GSDIa is naturally occurring and recapitulates almost all aspects of the human form of disease. We investigated the potential of recombinant adeno-associated virus (rAAV) vector-based therapy to treat the canine model of GSDIa. After delivery of a therapeutic rAAV2/8 vector to a 1-day-old GSDIa dog, improvement was noted as early as 2 weeks posttreatment. Correction was transient, however, and by 2 months posttreatment the rAAV2/8-treated dog could no longer sustain normal blood glucose levels after 1 hr of fasting. The same animal was then dosed with a therapeutic rAAV2/1 vector delivered via the portal vein. Two months after rAAV2/1 dosing, both blood glucose and lactate levels were normal at 4 hr postfasting. With more prolonged fasting, the dog still maintained near-normal glucose concentrations, but lactate levels were elevated by 9 hr, indicating that partial correction was achieved. Dietary glucose supplementation was discontinued starting 1 month after rAAV2/1 delivery and the dog continues to thrive with minimal laboratory abnormalities at 23 months of age (18 months after rAAV2/1 treatment). These results demonstrate that delivery of rAAV vectors can mediate significant correction of the GSDIa phenotype and that gene transfer may be a promising alternative therapy for this disease and other genetic diseases of the liver.

Neonatal gene therapy of glycogen storage disease type Ia using a feline immunodeficiency virus-based vector

Glycogen storage disease type Ia (GSD-Ia), also known as von Gierke disease, is caused by a deficiency of glucose-6-phosphatase-alpha (G6Pase), a key enzyme in glucose homeostasis. From birth, affected individuals cannot maintain normal blood glucose levels and suffer from a variety of metabolic disorders, leading to life-threatening complications. Gene therapy has been proposed as a possible option for treatment of this illness. Vectors have been constructed from feline immunodeficiency virus (FIV), a nonprimate lentivirus, because the wild-type virus does not cause disease in humans. Previously, we have shown that these vectors are capable of integrating stably into hepatocyte cell lines and adult murine livers and lead to long-term transgene expression. In the current work, we have assessed the ability to attenuate disease symptoms in a murine model of GSD-Ia. Single administration of FIV vectors containing the human G6Pase gene to G6Pase-alpha(-/-) mice did not change the biochemical and pathological phenotype. However, a double neonatal administration protocol led to normalized blood glucose levels, significantly extended survival, improved body weight, and decreased accumulation of liver glycogen associated with the disease. This approach shows a promising paradigm for treating GSD-Ia patients early in life thereby avoiding long-term consequences.

Complete normalization of hepatic G6PC deficiency in murine glycogen storage disease type Ia using gene therapy

Glycogen storage disease type Ia (GSD-Ia) patients deficient in glucose-6-phosphatase-alpha (G6Pase-alpha or G6PC) manifest disturbed glucose homeostasis. We examined the efficacy of liver G6Pase-alpha delivery mediated by AAV-GPE, an adeno-associated virus (AAV) serotype 8 vector expressing human G6Pase-alpha directed by the human G6PC promoter/enhancer (GPE), and compared it to AAV-CBA, that directed murine G6Pase-alpha expression using a hybrid chicken beta-actin (CBA) promoter/cytomegalovirus (CMV) enhancer. The AAV-GPE directed hepatic G6Pase-alpha expression in the infused G6pc(-/-) mice declined 12-fold from age 2 to 6 weeks but stabilized at wild-type levels from age 6 to 24 weeks. In contrast, the expression directed by AAV-CBA declined 95-fold over 24 weeks, demonstrating that the GPE is more effective in directing persistent in vivo hepatic transgene expression. We further show that the rapid decline in transgene expression directed by AAV-CBA results from an inflammatory immune response elicited by the AAV-CBA vector. The AAV-GPE-treated G6pc(-/-) mice exhibit normal levels of blood glucose, blood metabolites, hepatic glycogen, and hepatic fat. Moreover, the mice maintained normal blood glucose levels even after 6 hours of fasting. The complete normalization of hepatic G6Pase-alpha deficiency by the G6PC promoter/enhancer holds promise for the future of gene therapy in human GSD-Ia patients.

Intensive pharmacological immunosuppression allows for repetitive liver gene transfer with recombinant adenovirus in nonhuman primates

Repeated administration of gene therapies is hampered by host immunity toward vectors and transgenes. Attempts to circumvent antivector immunity include pharmacological immunosuppression or alternating different vectors and vector serotypes with the same transgene. Our studies show that B-cell depletion with anti-CD20 monoclonal antibody and concomitant T-cell inhibition with clinically available drugs permits repeated liver gene transfer to a limited number of nonhuman primates with recombinant adenovirus. Adenoviral vector-mediated transfer of the herpes simplex virus type 1 thymidine kinase (HSV1-tk) reporter gene was visualized in vivo with a semiquantitative transgene-specific positron emission tomography (PET) technique, liver immunohistochemistry, and immunoblot for the reporter transgene in needle biopsies. Neutralizing antibody and T cell-mediated responses toward the viral capsids were sequentially monitored and found to be repressed by the drug combinations tested. Repeated liver transfer of the HSV1-tk reporter gene with the same recombinant adenoviral vector was achieved in macaques undergoing a clinically feasible immunosuppressive treatment that ablated humoral and cellular immune responses. This strategy allows measurable gene retransfer to the liver as late as 15 months following the first adenoviral exposure in a macaque, which has undergone a total of four treatments with the same adenoviral vector.

A detailed characterization of the adult mouse model of glycogen storage disease Ia

Glycogen storage disease type Ia (GSDIa) is caused by a genetic defect in the hepatic enzyme glucose-6-phosphatase (G6Pase-alpha), which manifests as life-threatening hypoglycemia with related metabolic complications. A G6Pase-alpha knockout (KO) mouse model was generated to study potential therapies for correcting this disorder. Since then, gene therapy studies have produced promising results, showing long-term improvement in liver histology and glycogen metabolism. Under existing protocols, however, untreated KO pups seldom survived weaning. Here, we present a thorough characterization of the G6Pase-alpha KO mouse, as well as the husbandry protocol for rearing this strain to adulthood. These mice were raised with only palliative care, and characterized from birth through 6 months of age. Once KO mice have survived the very frail weaning period, their size, agility, serum lipids and glycemic control improve dramatically, reaching levels approaching their wild-type littermates. In addition, our data reveal that adult mice lacking G6Pase-alpha are able to mate and produce viable offspring. However, liver histology and glycogen accumulation do not improve with age. Overall, the reliable production of mature KO mice could provide a critical tool for advancing the GSDIa field, as the availability of a robust enzyme-deficient adult offers a new spectrum of treatment avenues that would not be tolerated by the frail pups. Most importantly, our detailed characterization of the adult KO mouse provides a crucial baseline for accurately gauging the efficacy of experimental therapies in this important model.

AAV2/8-mediated correction of OTC deficiency is robust in adult but not neonatal Spf(ash) mice

Ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder, is associated with severe hyperammonemia accompanied by a high risk of neurological damage and death in patients presenting with the neonatal-onset form. Contemporary therapies, including liver transplantation, remain inadequate with considerable morbidity, justifying vigorous investigation of alternate therapies. Clinical evidence suggests that as little as 3% normal enzyme activity is sufficient to ameliorate the severe neonatal phenotype, making OTC deficiency an ideal model for the development of liver-targeted gene therapy. In this study, we investigated metabolic correction in neonatal and adult male OTC-deficient Spf(ash) mice following adeno-associated virus (AAV)2/8-mediated delivery of the murine OTC complementary DNA under the transcriptional control of a liver-specific promoter. Substantially supraphysiological levels of OTC enzymatic activity were readily achieved in both adult and neonatal mice following a single intraperitoneal (i.p.) injection, with metabolic correction in adults being robust and life-long. In the neonates, however, full metabolic correction was transient, although modest levels of OTC expression persisted into adulthood. Although not directly testable in Spf(ash) mice, these levels were theoretically sufficient to prevent hyperammonemia in a null phenotype. This loss of expression in the neonatal liver is the consequence of hepatocellular proliferation and presents an added challenge to human therapy.

Activation of glycolysis and apoptosis in glycogen storage disease type Ia

The deficiency of glucose-6-phosphatase (G6Pase) underlies glycogen storage disease type Ia (GSD-Ia, von Gierke disease; MIM 232200), an autosomal recessive disorder of metabolism associated with life-threatening hypoglycemia, growth retardation, renal failure, hepatic adenomas, and hepatocellular carcinoma. Liver involvement includes the massive accumulation of glycogen and lipids due to accumulated glucose-6-phosphate and glycolytic intermediates. Proteomic analysis revealed elevations in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and other enzymes involved in glycolysis. GAPDH was markedly increased in murine G6Pase-deficient hepatocytes. The moonlighting role of GAPDH includes increasing apoptosis, which was demonstrated by increased TUNEL assay positivity and caspase 3 activation in the murine GSD-Ia liver. These analyses of hepatic involvement in GSD-Ia mice have implicated the induction of apoptosis in the pathobiology of GSD-Ia.

Emerging therapies for glycogen storage disease type I

Glycogen storage disease type I (GSD I) is caused by deficiency of the glucose-6-phosphatase catalytic subunit in type Ia or of glucose-6-phosphate transporter in type Ib. The cellular bases for disruptions of homeostasis have been increasingly understood in GSD I, including those for anemia, renal failure and neutropenia. Advances in the understanding of cellular abnormalities in GSD I have provided rationales for new therapy, and recent developments in gene therapy have led to potential curative treatments for GSD I. These advances will benefit patients with GSD I in the future, improving both quality of life and survival, as well as illuminating the molecular effects of altered metabolism upon multiple organ systems.

Sexually dimorphic patterns of episomal rAAV genome persistence in the adult mouse liver and correlation with hepatocellular proliferation

Recombinant adeno-associated virus vectors (rAAVs) show exceptional promise for liver-targeted gene therapy, with phenotype correction in small and large animal disease models being reported with increasing frequency. Success in humans, however, remains a considerable challenge that demands greater understanding of host-vector interactions, notably those governing the efficiency of initial gene transfer and subsequent long-term persistence of gene expression. In this study, we examined long-term enhanced green fluorescent protein (eGFP) expression and vector genome persistence in the mouse liver after rAAV2/8-mediated gene transfer in early adulthood. Two intriguing findings emerged of considerable scientific and clinical interest. First, adult female and male mice showed distinctly different patterns of persistence of eGFP expression across the hepatic lobule after exhibiting similar patterns initially. Female mice retained a predominantly perivenous pattern of expression, whereas male mice underwent inversion of this pattern with preferential loss of perivenous expression and relative retention of periportal expression. Second, these changing patterns of expression correlated with sexually dimorphic patterns of genome persistence that appear linked both spatially and temporally to underlying hepatocellular proliferation. Observation of the equivalent phenomenon in man could have significant implications for the long-term therapeutic efficacy of rAAV-mediated gene transfer, particularly in the context of correction of liver functions showing metabolic zonation.

A preclinical animal model to assess the effect of pre-existing immunity on AAV-mediated gene transfer

Hepatic adeno-associated virus (AAV)-serotype 2-mediated gene transfer results in sustained transgene expression in experimental animals but not in human subjects. We hypothesized that loss of transgene expression in humans might be caused by immune memory mechanisms that become reactivated upon AAV vector transfer. Here, we tested the effect of immunological memory to AAV capsid on AAV-mediated gene transfer in a mouse model. Upon hepatic transfer of an AAV2 vector expressing human factor IX (hF.IX), mice immunized with adenovirus (Ad) vectors expressing AAV8 capsid before AAV2 transfer developed less circulating hF.IX and showed a gradual loss of hF.IX gene copies in liver cells as compared to control animals. This was not observed in mice immunized with an Ad vectors expressing AAV2 capsid before transfer of rAAV8-hF.IX vectors. The lower hF.IX expression was primarily linked to AAV-binding antibodies that lacked AAV-neutralizing activity in vitro rather than to AAV capsid-specific CD8(+) T cells.

Biochemical correction of very long-chain acyl-CoA dehydrogenase deficiency following adeno-associated virus gene therapy

We report the development of a gene replacement strategy for very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. VLCAD is a mitochondrial enzyme involved in fatty acid beta-oxidation, a key step in energy production during times of fasting or stress. Deficiency of VLCAD classically presents as hepatic dysfunction, hypoglycemia, cardiomyopathy, rhabdomyolysis, and/or sudden death. While dietary therapy for VLCAD deficiency has proven beneficial in preventing some symptoms, a risk of metabolic catastrophic decompensation remains throughout life during times of increased energy demand. We designed a recombinant adeno-associated virus (AAV) expressing the human VLCAD gene (AAV8-hVLCAD). To demonstrate its in vivo activity, AAV8-hVLCAD was administered via the tail vein to VLCAD-knockout mice. A reduction in accumulated serum long-chain acylcarnitines and increased fasting tolerance judged on blood glucose concentrations were observed as of 11 days postinjections through >100 days. Western analysis of liver, skeletal muscle, and heart extracts using PEP1 anti-hVLCAD antibody revealed short-term hVLCAD expression in the liver and muscle and longer-term expression in heart. This demonstrates the ability of human VLCAD to correct the biochemical phenotype of VLCAD-deficient mice.