Genetic researchers at University of North Carolina at Chapel Hill School of Medicine have made a breakthrough in the development of a gene therapy for cystic fibrosis. The mutation that causes cystic fibrosis, (CF) discovered in 1989, is a defect in the cystic fibrosis transmembrane regulator (CFTR) gene.
The idea behind a CF gene therapy is simple enough; use modified viruses to deliver a corrected version of the CFTR gene into affected tissues. But this hoped for cure has been stymied by the natural ability of the lung to limit the introduction of foreign genes into its cells.
In a study published in PLOS Biology July 21, 20091, scientists successfully used parainfluenza virus, which causes common colds and targets ciliated airway epithelium cells. They engineered this virus to contain the corrected CF gene and found that it could deliver this gene to 60-70% of lung cells although only 25% of cells needed to be targeted to restore normal function back to the tissue model.
Great Leap Forward
“This is the first demonstration in which we’ve been able to execute delivery in an efficient manner to a tissue that resembles what is present in the lung,” said Ray Pickles, Ph.D., associate professor of Microbiology and Immunology at the Cystic Fibrosis Research and Treatment Center. “When you consider that in past gene therapy clinical trials, the targeting efficiency has been somewhere around 0.1 percent of cells at best, you can see this is a giant leap forward”.
“We discovered that if you take a virus that has evolved to infect the human airways, and you engineer a normal CF gene into it, you can use this virus to correct hallmark CF features in the model system that we used,” Pickles said. For example, the experiment restored the cells’ ability to hydrate and transport mucus secretions, allowing the CF cells to function, in effect, like normal cells.
Although Cystic Fibrosis is a multiple organ disease, it most seriously affects the lungs2. CF patient’s airways are clogged with mucus that is thicker than normal and dehydrated. Inability to clear mucus from the lung increases susceptibility to lung infections, which can result in lung damage.
Over the last twenty years, scientists have developed a range of viral and non-viral vector systems appropriate for delivering a corrected CF gene back into lung cells grown in the laboratory.
Several of these vectors systems have been tested in human clinical trials, but the efficacy of gene delivery in the laboratory achieved has not borne out in the clinical studies3-5. This suggests that the cell models used in the laboratory do not characterize the workings of the cells in patients’ lungs.
Since then, lab models of human lung cells derived from CF patients have been created which reconstruct the architecture and function of cells present in the human lung. Studies using the newer cell models have shown that previously used vector systems do not deliver the corrected CF gene to enough lung cells to be of clinical benefit to CF patients.
Researchers must now work to guarantee the safety of the delivery system. Unexpectedly, simply adding the CF gene to the virus considerably attenuated it, possibly reducing its ability to cause an inflammatory reaction. But they may need to alter the virus further.
“We haven’t generated a vector that we can go out and give to patients right now,” Pickles said, “but we are slowly but surely moving forward towards this goal” Pickles says. “It is going to require a long term commitment from the CF gene therapy field that has achieved so much this far and it’s only a matter of time until we understand how to do this reproducibly and safely”.
1. Zhang L, Button B, Gabriel SE, Burkett S, Yan Y, et al. (2009) CFTR Delivery to 25% of Surface Epithelial Cells Restores Normal Rates of Mucus Transport to Human Cystic Fibrosis Airway Epithelium. PLoS Biol 7(7): e1000155. doi:10.1371/journal.pbio.1000155
2. Koch C, Hoiby N (1993) Pathogenesis of cystic fibrosis. Lancet 341: 1065–1069.
3. Knowles M. R, Hohneker K. W, Zhou Z, Olsen J. C, Noah T. L, et al. (1995) A controlled study of adenoviral-vector-mediated gene transfer in the nasal epithelium of patients with cystic fibrosis [see comments]. N Engl J Med 333: 823–831.
4. Noone P. G, Hohneker K. W, Zhou Z, Johnson L. G, Foy C, et al. (2000) Safety and biological efficacy of a lipid-CFTR complex for gene transfer in the nasal epithelium of adult patients with cystic fibrosis. Mol Ther 1: 105–114.
5. Moss R. B, Rodman D, Spencer L. T, Aitken M. L, Zeitlin P. L, et al. (2004) Repeated adeno-associated virus serotype 2 aerosol-mediated cystic fibrosis transmembrane regulator gene transfer to the lungs of patients with cystic fibrosis: a multicenter, double-blind, placebo-controlled trial. Chest 125: 509–521.