Dr Flotte received his undergraduate degree in the biological sciences from the University of New Orleans in 1982, and his medical degree from the Louisiana State University School of Medicine in 1986. After serving his residency in pediatrics at Johns Hopkins University, he completed a pediatric pulmonary fellowshipa and postdoctoral training in molecular virology there in 1992.
In 1995, Dr. Flotte and his colleagues at Johns Hopkins became the first to use adeno-associated virus, or AAV, as a vehicle to deliver corrective genes to targeted sites in the body, including the damaged airways of adults with cystic fibrosis.
In 1996, Dr Flotte joined the faculty of the University of Florida and was appointed Associate Director of UF’s Powell Gene Therapy Center. In 2000, he was named Director of the Powell Center and founding Director of the newly established UF Genetics Institute, a cross-campus multidisciplinary unit encompassing gene therapy, human genetics, agricultural genetics and comparative genomics. In 2002, Flotte stepped down from these roles to accept the position of Chair of the Department of Pediatrics.
An internationally known pioneer in human gene therapy, Flotte is currently investigating the use of gene therapy for genetic diseases that affect children, including cystic fibrosis, alpha-1 antitrypsin (AAT) deficiency, type I diabetes, and disorders of fatty acid oxidation. He is currently conducting Phase I trials with rAAV expressing alpha-1antitrypsin in AAT-deficient patients. He has also focused on the study of adeno-associated virus (AAV) vectors. Dr. Flotte's laboratory has also focused on the mechanisms of AAV persistence, since these represent the basis for a more profound understanding of the potential for long-term safe and effective gene therapy.
Recombinant adeno-associated virus (AAV) gene therapy vectors for, cystic fibrosis (CF), alpha-1-antitrypsin deficiency (AAT) and fatty oxidation disorders (FAO)
AAV is a non-pathogenic human parvovirus which is commonly isolated from the respiratory tract of humans (Figure 1A). AAV’s life cycle includes a unique mechanism for persistence in human cells by means of site-specific integration into a region of chromosome 19, the AAVS1 site (Figure 1B). In 1993, our group published the results of the first successful in vivo gene transfer with AAV. In that study, AAV vectors carrying the human CF transmembrane conductance regulator (CFTR) gene were delivered to the bronchial epithelium of rabbits. Efficient gene transfer was observed which persisted for over 6 months without any detectable toxicity. Subsequently, we studied AAV-CFTR gene transfer in rhesus macaques. Our studies in rhesus served two purposes: 1.We performed the primary toxicology study to justify beginning our phase I trial of AAV-CFTR administration in humans, and 2. we studied vector integration and persistence in vivo for the first time. Since then our laboratory has gone on to perform phase I clinical trials delivering AAV2 expressing the CFTR gene in CF patients. Much has been learned form the early trials including that paucity of AAV2 receptors on the luminal side of the airways and the weak promoter activity in early vectors. This limited the efficiency and efficacy of the viral vectors used in the early trials. To overcome this, the lab has designed a more robust expression cassette delivering a CFTR ‘mini’ gene and has determined that AAV1 is far superior to AAV2 at transducing airways. These new generation viral vectors for CF gene therapy are currently undergoing pre-clinical testing and will soon be tested in patients. Our laboratory has also pioneered phase I clinical studies using intra-muscular administration of rAAV2 as well as this more novel AAV1 serotype expressing the gene for alpha-1-antitrypsin (AAT) in AAT deficient patients. We are also developing hybrid rAAV-RNAi approaches to treat AAT deficient liver disease in which a toxic gain of function of the mutant protein contributes to the pathology. The laboratory is also involved in gene therapy for fatty oxidation disorders. Specifically we are developing gene delivery for the Acyl-CoA Dehydrogenases family enzymes that initiate the first step of the B-oxidation pathway.
Current Studies in Cystic Fibrosis
Cystic fibrosis (CF), the most common lethal, single-gene disorder affecting Northern Europeans and North Americans, is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR is a chloride channel and a regulator of other ion channels, and many aspects of the CF phenotype are directly related to ion channel abnormalities attributable to CFTR mutation. What remains less clear, however, is how CFTR mutation leads to persistent endobronchial infection with Pseudomonas aeruginosa and severe airway inflammation which are the hallmarks of the lung pathology in CF. One theory, which remains controversial, contends that CFTR mutation primes CF cells to release greater quantities of pro-inflammatory cytokines than non-CF cells. One line of evidence in support of this theory is a Pseudomonas agarose bead airway infection model, in which CFTR knockout mice demonstrate greater weight loss and mortality and higher levels of pro-inflammatory cytokines than control mice. One of the limitations of this model has been its technical complexity and a wide variability of responses among individual animals. Because of the need for a technically simpler mouse model for inflammatory lung disease in CF, our laboratory has recently developed a new approach based on Aspergillus fumigatus (Af) sensitization and challenge. This model reproduces certain key immunologic and pathologic aspects of allergic bronchopulmonary aspergillosis (ABPA) an inflammatory lung phenotype that is much more common in CF patients than in any other clinical context. In the animal model studies, CFTR knockout mice demonstrated up-regulated levels of IL4 prior to sensitization, and subsequent to sensitization developed a hyper-IgE response and markedly divergent cytokine expression (including increased IL13, IL4, IL2, IL10, and KC) as compared with non-CF. While this CFTR-dependent inflammatory phenotype is potentially very promising, it has raised more questions than it answered. In particular, the divergence of expression of cytokines that are predominantly produced within lymphocytes, macrophages and other non-epithelial cells raises the possibility of an important role of CFTR itself in non-epithelial cells. Therefore our lab is currently studying which cell types contribute primarily to the CF-dependent inflammatory lung disease phenotype in Aspergillus-sensitized CF mice? What pathways or other mechanisms might be aberrantly regulated in affected cells?
Gene and RNAi Therapy for Alpha-1 Antitrypsin
Alpha-1 antitrypsin (AAT) is one of the primary circulating serum antiproteases in humans. It inhibits a variety of serine proteinases, with neutrophil elastase being one of the most physiologically important, as well as inhibiting a number of metallo-proteinases and other pro-inflammatory and pro-apoptotic molecules. AAT is normally produced within hepatocytes and macrophages, where hepatocyte-derived AAT forms the bulk of the physiologic reserve of AAT. Approximately 4% of the North American and Northern European populations possess at least one copy of a mutant allele, known as PI*Z, which results from a single amino acid substitution of Lys for Glu at position 342. In the homozygous state, this mutation leads to severe deficiency of AAT, and can result in two distinct pathologic states: a lung disease which is primarily due to the loss of antiprotease function, and a liver disease (present to a significant degree in only 10% of patients) which is due to a toxic gain of function of the Z-AAT mutant protein. The mutation of Z-AAT leads to the loss of a crucial salt-bridge in a beta sheet region of the protein, allowing for the insertion of the reactive loop of a neighboring AAT molecule in between the two beta sheets. The structure of the resultant very stable loop-sheet polymers has been solved and provides insight into how the mutant may accumulate in a misfolded configuration. This accumulation within hepatocytes consistently results in a state of serum deficiency due to inefficient secretion. In individuals affected by AAT liver disease, it also triggers a cascade of aberrant signals within the hepatocyte, most likely the result of an unfolded protein response. However, the downstream details remain unclear, as does the reason for the disparity between those Z-homozygotes who develop liver disease and those who do not. Regardless of the precise mechanism by which Z-AAT conformational changes lead to liver disease, there is general consensus that molecular therapies for AAT liver disease should act by down-regulating Z-AAT. Our group has developed two different investigational clinical gene therapy products for gene augmentation of AAT as a potential therapy for the lung disease (rAAV2-AAT and rAAV1-AAT) (1, 4), and we have a third vector in development that is much more efficient for delivery of wild-type (M) AAT to hepatocytes (rAAV8-AAT). Given the need for down-regulation of Z-AAT, we have also begun to examine a number of innovative approaches to long-term expression of therapeutic RNAs using the recombinant adeno-associated virus (rAAV) platform, including spliceosome-mediated RNA trans-splicing (SMaRT), and rAAV-mediated in vivo delivery of RNAi in Z-AAT transgenic mice, a project which was recently published from our laboratory. The constructs in that paper utilized a pol III promoter (U6) to express short hairpin RNAs (shRNAs) targeting three different sites on the AAT molecule, but were not specific to the mutant allele. While it is feasible to direct silencing agents to the liver to decrease Z-AAT expression, while directing gene augmentation to other sites, the hepatocyte is most likely the optimal target for augmentation as well. Our lab is currently focusing on systematically developing RNAi-based approaches to Z-AAT down-regulation within hepatocytes and to devise appropriate strategies that might allow this to be combined with M-AAT gene augmentation
Gene Therapy for Fatty Oxidation Disorders
Genetic disorders of mitochondrial fatty acid oxidation (FAO) represent a relatively common class of metabolic disorders, exceeding more than 1 in 15,000 newborns. These disorders are associated with genes involved in the mitochondrial b-oxidation of fatty acids, which provide fuel once glycogen stores are depleted during prolonged fasting or during times of increased energy demands and physiological stress. During these circumstances liberation from triglycleride stores in adipose tissue causes free fatty acids circulate to the liver and striated muscle. After the fatty acids diffuse (short chain) or are actively transported (long chain) across the cellular and mitochondrial membranes, the oxidation of fatty acids occurs within the mitochondrial matrix. b-oxidation proceeds in a cyclical fashion resulting in the removal of sequential 2-carbon units as acetyl-CoA, for entry into the TCA cycle
Importantly, the first step of each cycle in the mitochondrial matrix is catalyzed by an acyl-CoA dehydrogenase. This rate limiting function is performed by a family of enzymes that differ in their substrate specificity based on the carbon chain length of the acyl CoA molecule. The acyl-CoA dehydrogenases (ACDs) are a family of 5 mitochondrial enzymes involved in fatty acid and amino acid metabolism that catalyze the transfer of electrons from various acyl-CoA esters to electron transfer flavoprotein. Very long, medium and short chain acyl-CoA dehydrogenases (VLCAD, MCAD and SCAD) catalyze the first step in the b-oxidation cycle with substrate specificities of 16-, 8- and 4- carbon chains, respectively.
Currently our lab is applying rAAV-based transduction of skeletal muscle for those FAO deficiencies that are most common in humans, medium chain acyl CoA dehydrogenase (MCAD) and very long chain acyl CoA dehydrodgenase (VLCAD). Over the past 5 years, mouse models have become available for each of these disorders, and mass newborn screening for FAO disorders has been initiated in many states. Our prior work has relied heavily on rAAV1 pseudotyped vectors for skeletal muscle transduction. The emergence of rAAV9-based vectors now provides the potential for even greater efficiency in using transduction of skeletal muscle and cardiac muscle as a platform for gene therapy of systemic genetic diseases. We are investigating to compare rAAV1 and rAAV9 in the context of a translational application to VLCAD and MCAD deficiencies. Specifically, the feasibility of molecular and biochemical correction of VLCAD and MCAD deficiencies is tested in mouse models of these deficiencies. Our primary focus is on determining whether rAAV9 (vs. rAAV1)-mediated transduction of skeletal and cardiac muscle will result in greater improvement of acyl carnitine profiles (i.e., clearance of abnormally high levels of fatty acyl carnitine metabolites that are characteristic of this disorder) and MRS profiles. We are also going to develop formal preclinical toxicology studies of muscle delivery of rAAV9 or rAAV1-VLCAD and rAAV9 or rAAV1-MCAD which will be completed in anticipation of new phase I clinical trials of gene therapy for these FAO disorders.