Adjunctive β2-agonist treatment reduces glycogen independently of receptor-mediated acid α-glucosidase uptake in the limb muscles of mice with Pompe disease.

Bursac's research interests include: Stem cell, tissue engineering, and gene based therapies for heart and muscle regeneration; Cardiac electrophysiology and arrhythmias; Organ-on-chip and tissue engineering technologies for disease modeling and therapeutic screening; Small and large animal models of heart and muscle injury, disease, and regeneration.

The focus of my research is on application of pluripotent stem cells, tissue engineering, and gene therapy technologies for: 1) basic studies of striated muscle biology and disease in vitro and 2) regenerative therapies in small and large animal models in vivo. For in vitro studies, micropatterning of extracellular matrix proteins or protein hydrogels and 3D cell culture are used to engineer rodent and human striated muscle tissues that replicate the structure-function relationships present in healthy and diseased muscles. We use these models to separate and systematically study the roles of structural and genetic factors that contribute cardiac and skeletal muscle function and disease at multiple organizational levels, from single cells to tissues. Combining cardiac and skeletal muscle cells with primary or iPSC-derived non-muscle cells (endothelial cells, smooth muscle cells, immune system cells, neurons) allows us to generate more realistic models of healthy and diseased human tissues and utilize them to mechanistically study molecular and cellular processes of tissue injury, vascularization, innervation, electromechanical integration, fibrosis, and functional repair. Currently, in vitro models of Duchenne Muscular Dystrophy, Pompe disease, dyspherlinopathies, and various cardiomyopathies are studied in the lab. For in vivo studies, we employ rodent models of volumetric skeletal muscle loss, cardiotoxin and BaCl2 injury as well as myocardial infarction and transverse aortic constriction to study how cell, tissue engineering, and gene (viral) therapies can lead to safe and efficient tissue repair and regeneration. In large animal (porcine) models of myocardial injury and arrhythmias, we are exploring how human iPSC derived heart tissue patches and application of engineered ion channels can improve cardiac function and prevent heart failure or sudden cardiac death.

Paul M. Yen currently is Professor at Duke-NUS Graduate Medical School in Singapore and Head of the Laboratory of Hormonal Regulation in the Cardiovascular and Metabolic Disorders Program. He also is Professor of Medicine at Duke University School of Medicine, Durham, NC and a member of the Duke Molecular Physiology Institute. He received his B.A. in Chemistry from Amherst College and his M.D. from Johns Hopkins. He completed his residency in internal medicine at University of Chicago and fellowship in endocrinology at National Institutes of Health, Bethesda, MD. He was formerly Assistant Professor at Harvard Medical School, Chief of the Neuro-endocrinology and Molecular Regulation Section of the Clinical Endocrinology Branch at NIDDK (at the National Institutes of Health, Bethesda, MD), and Associate Professor of Medicine and Pharmacology at Johns Hopkins University School of Medicine. He has served on the editorial boards of Endocrinology, Molecular Endocrinology, and Thyroid. He also is a U.S. board-certified physician in internal medicine and endocrinology. He is listed as a top 2% scientist worldwide by Stanford University and a leading World Expert on thyroid hormone by Expertscape. He has served as an Asia-Oceanic Thyroid Association (AOTA) Council Member and the AOTA delegate to the World Thyroid Foundation and Singapore Representative to the International Iodine Global Network. He was awarded the 2020 Nagataki-Fujifilm Prize for his contributions to basic and clinical thyroid hormone research in Asia by AOTA. At Duke-NUS, he has served as Master of Sheares Medical College since 2010. He also has served as the clinical faculty advisor for the Duke Overseas Volunteer Expedition (DOVE) program in which medical students deliver primary care in neighbouring underdeveloped countries since its inception in 2010.. His laboratory uses molecular biological and genomic approaches to study hormonal regulation of transcription, autophagy, and metabolism as well as searching for ways to improve the diagnosis and treatment of non-alcoholic fatty liver disease (NAFLD).

As a clinical biochemical geneticist and a director of the Duke Biochemical Genetics laboratory, my research interests are focused on improving laboratory diagnostics for rare inherited disorders of metabolism. I am actively involved in the development of assays using mass spectrometry and other analytical techniques. My current research on biomarkers for lysosomal storage disorders, such as Fabry and Pompe disease and the mucopolysaccharidoses includes monitoring the response to novel therapies in patients. I also have an interest in neurometabolic disorders such as the creatine deficiency syndromes and sulfite oxidase and molybdenum cofactors. These disorders can be diagnosed using liquid chromatography-tandem mass spectrometric assays that measure biomarkers in urine. Guanidinoacetate methyltransferase deficiency is a disorder that can be detected in the newborn period and is amenable to dietary therapy, and is thus a good candidate for newborn screening.

As a physician-scientist practicing clinical and biochemical genetics, I am highly motivated to seek improved therapy for my patients with inherited disorders of metabolism. The focus of our research has been the development of gene therapy with adeno-associated virus (AAV) vectors, most recently by genome editing with CRISPR/Cas9. We have developed gene therapy for inherited disorders of metabolism, especially glycogen storage disease (GSD) and phenylketonuria (PKU). 
1) GSD Ia: Glucose-6-phosphatase (G6Pase) deficient animals provide models for developing new therapy for GSD Ia, although early mortality complicates research with both the murine and canine models of GSD Ia. We have prolonged the survival and reversed the biochemical abnormalities in G6Pase-knockout mice and dogs with GSD type Ia, following the administration of AAV8-pseudotyped AAV vectors encoding human G6Pase. More recently, we have performed genome editing to integrate a therapeutic transgene in a safe harbor locus for mice with GSD Ia, permanently correcting G6Pase deficiency in the GSD Ia liver. Finally, we have identified reduced autophagy as an underlying hepatocellular defect that might be treated with pro-autophagic drugs in GSD Ia.
2) GSD II/Pompe disease: Pompe disease is caused by the deficiency of acid-alpha-glucosidase (GAA) in muscle, resulting in the massive accumulation of lysosomal glycogen in striated muscle with accompanying weakness. While enzyme replacement has shown promise in infantile-onset Pompe disease patients, no curative therapy is available. We demonstrated that AAV vector-mediated gene therapy will likely overcome limitations of enzyme replacement therapy, including formation of anti-GAA antibodies and the need for frequent infusions. We demonstrated that liver-restricted expression with an AAV vector prevented antibody responses in GAA-knockout mice by inducing immune tolerance to human GAA. Antibody responses have complicated enzyme replacement therapy for Pompe disease and emphasized a potential advantage of gene therapy for this disorder. The strategy of administering low-dose gene therapy prior to initiation of enzyme replacement therapy, termed immunomodulatory gene therapy, prevented antibody formation and increased efficacy in Pompe disease mice. We are currently conducting a Phase I clinical trial of immunomodulatory gene therapy in adult patients with Pompe disease. Furthermore, we have developed drug therapy to increase the receptor-mediated uptake of GAA in muscle cells, which provides adjunctive therapy to more definitively treat Pompe disease.
3) PKU: In collaboration with researchers at OHSU, we performed an early gene therapy experiment that demonstrated long-term biochemical correction of PKU in mice with an AAV8 vector. PKU is a very significant disorder detected by newborn screening and currently inadequately treated by dietary therapy. Phenylalanine levels in mice were corrected in the blood, and elevated phenylalanine causes mental retardation and birth defects in children born to affected women, and gene therapy for PKU would address an unmet need for therapy in this disorder.

Currently we are developing methods for genome editing that will stably correct the enzyme  deficiency in GSD Ia and in Pompe disease.  Our long-term goal is to develop efficacious genome editing for glycogen storage diseases, which will allow us to treat these conditions early in life with long-term benefits.