Barriers identified to using AAV vectors in gene therapy for cystic fibrosis
Researcher: Jeffrey Bartlett, Children's Hospital of Columbus
Contact: Katie Pakel, 614-722-4595
Embargo: Sunday, May 14, 4:45 pm
In ongoing clinical studies, researchers are working on ways to replace defective cystic fibrosis genes with normal genes. While the research looks promising, there have been some challenges to using viral vectors as a delivery system. A clinical study, presented at the 2000 Pediatric Academic Societies and American Academy of Pediatrics Joint Meeting Sunday, May 14, identified the barriers to using adeno-associated virus (AAV) viral vectors in gene therapy for cystic fibrosis. Columbus Children's Hospital researcher Jeffrey S. Bartlett, Ph.D., presented the findings, which will be important in improving the effectiveness of gene therapy for cystic fibrosis.
The study concluded that AAV-mediated gene delivery to the lung is blocked at least at two key steps. First, the receptors that the virus uses to enter cells are not expressed on the surface of the cells that face the inside of the airway, where the virus is administered and where the cystic fibrosis gene must be expressed to be therapeutically beneficial. Instead, the receptors are expressed on the wrong side of the cells, so the virus does not attach to the cells. Second, the study found that if the virus does get into the cells, it does not make it into the nucleus very efficiently. However, the reason is unclear. If the virus does not deliver the genes to the nucleus, they cannot be expressed.
In simple terms, gene therapy involves the replacement of a gene within the body that does not function properly with a normal copy of that same gene, so that normal function can be restored. AAV is a non-pathogenic (non-disease causing) virus that has been used in studies to "carry" or deliver normal copies of cystic fibrosis genes to the lungs of patients with cystic fibrosis lung disease. Cystic fibrosis is the most common inherited genetic disease in the Caucasian population, occurring once in every 2,000 live births. Patients with cystic fibrosis are born with cystic fibrosis genes that do not work properly. Previous clinical studies have suggested that the disease could be corrected by replacing the defective cystic fibrosis genes with normal genes through the process of gene therapy. The Cystic Fibrosis Foundation supported the study and Dr. Bartlett collaborated with colleagues at the University of North Carolina at Chapel Hill.
Recombinant AAV is an appealing vector system for cystic fibrosis gene therapy. In fact, clinical trials are currently underway using this vector system for the treatment of cystic fibrosis. However, these trials have been less successful, because it appears AAV may have certain inherent limitations in regard to its ability to transfer and express genes in the lung. Bartlett's study was designed to understand the biology of AAV infection of the airway epithelium and ultimately increase the effectiveness of AAV vector-mediated cystic fibrosis gene transfer.
"AAV works very well as a vector system in other instances such as transferring genes to muscle, vasculature, liver or brain, but it has been less than effective in the lung, which is the target organ for cystic fibrosis gene therapy," explained Dr. Bartlett. "Our study was designed to identify and understand the problems using AAV as a gene-delivery vehicle. We knew that if we could determine the flaws in the vector system, we could eventually overcome them and increase its effectiveness."
The key experiments in this study involved fluorescent-labeled virus, which was used to tag and visualize individual viral particles in the lung tissue and cells. This process enabled Children's researchers to assess the virus' ability to attach to lung cells, enter these cells, deliver the genes to appropriate sites in the cell (the cell nucleus) and express the genes. In previous studies, the success of gene experiments could only be determined by the final outcome-the expression of the transferred gene. With this new technique, Dr. Bartlett and his team were able to dissect the series of events necessary to achieve successful gene transfer.
Children's Hospital of Columbus, Ohio, a 108-year-old institution devoted to the health of children, is among the top 10 free-standing children's hospitals in the country that receives NIH funding for research. Physicians and scientists at Children's are actively engaged in basic, applied, and patient-oriented research in a wide range of disciplines. In serving central Ohio and surrounding states, Children's Hospital has more than 400,000 patient visits a year, from birth through age 21. Children's offers specialty programs and services, including more than 18 support groups and 300 public education classes. As a tertiary teaching hospital affiliated with The Ohio State University, Children's also serves as the training ground for hundreds of pediatric healthcare providers each year.
More information on Children's Hospital of Columbus is available by calling (614) 722-KIDS (5437) or through the hospital's Web site at http://www.childrenscolumbus.org.
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