Can the Common Cold Cure Cystic Fibrosis?
The ciliated epithelium that lines the airways of the lung normally functions to transport hydrated mucus secretions out of the airways to maintain respiratory sterility. Cystic fibrosis (CF) lung disease results from reduced airway surface hydration leading to decreased mucus clearance that precipitates bacterial infection and progressive obstructive lung disease. CF is a genetic disease, and the mutant protein is a chloride ion channel (CFTR) that normally regulates ion and fluid transport on the airway surface. Restoration of corrected CFTR function to the airway epithelium of CF patients by delivering a new CFTR gene to airway epithelial cells has long been envisioned as a therapeutic strategy for CF lung disease. In 1989 scientists identified the gene mutation that causes cystic fibrosis (CF), which led to the hope that CF lung disease could be “cured” using gene therapy. The premise of gene therapy is that modified viruses or other gene-based systems could be used to deliver a corrected version of a gene into affected tissues. However, the projected cure has been hampered by the natural ability of the lung to limit the introduction of foreign genes into its cells.
Now scientists have found what may be the most efficient way to deliver a corrected gene to lung cells derived from CF patients, renewing hope that gene therapy for CF lung disease could be a successful future treatment. While cystic fibrosis is a multiple organ disease, it most devastatingly affects the lung. In people with CF the airways are clogged with mucus that is dehydrated and thicker than normal. The inability to clear mucus from the lung increases the susceptibility of CF patients to lung infections, which results in lung damage. Over the last two decades scientists have developed a variety of viral and non-viral vector systems suitable 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. However, the efficiency of gene delivery achieved in the laboratory has not borne out in the clinical studies, suggesting that the cell models used in the laboratory do not represent the status of the cells in patients’ lungs. Scientists have since developed laboratory models of human lung cells derived from CF patients that recapitulate the architecture and function of the cells present in the human lung. Studies using such cell models have revealed that previously used vector systems cannot deliver the corrected CF gene to enough lung cells to be of clinical benefit to CF patients.
In this new study, scientists took a different approach and used parainfluenza virus, a virus known to infect human lung cells and to cause common colds. The researchers 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. This study demonstrates efficient and efficacious CFTR delivery to CF ciliated airway epithelium and that CFTR delivered to approximately 25% of the surface epithelial cells restores normal levels of airway surface hydration and mucus transport. These studies serve as a benchmark for the efficiency of CFTR gene delivery to CF airways for future CF gene therapy studies in vivo.
This is the first demonstration in which medicine has been able to execute delivery in an efficient manner to a tissue that resembles what is present in the lung. 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. Now the researchers must work to ensure the safety of the delivery system. In a pleasant surprise, simply adding the CF gene to the virus significantly attenuated it, potentially reducing its ability to cause an inflammatory reaction. But the scientists may need to alter the virus further. Although they have not generated a vector that that can be used in patients right now, researchers are slowly but surely moving forward towards this goal. It is going to require a long term commitment from the CF gene therapy field that has achieved so much this far.
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
Dysfunction of CFTR in cystic fibrosis (CF) airway epithelium perturbs the normal regulation of ion transport, leading to a reduced volume of airway surface liquid (ASL), mucus dehydration, decreased mucus transport, and mucus plugging of the airways. CFTR is normally expressed in ciliated epithelial cells of the surface and submucosal gland ductal epithelium and submucosal gland acinar cells. Critical questions for the development of gene transfer strategies for CF airway disease are what airway regions require CFTR function and how many epithelial cells require CFTR expression to restore normal ASL volume regulation and mucus transport to CF airway epithelium? An in vitro model of human CF ciliated surface airway epithelium (CF HAE) was used to test whether a human parainfluenza virus (PIV) vector engineered to express CFTR (PIVCFTR) could deliver sufficient CFTR to CF HAE to restore mucus transport, thus correcting the CF phenotype. PIVCFTR delivered CFTR to .60% of airway surface epithelial cells and expressed CFTR protein in CF HAE approximately 100-fold over endogenous levels in non-CF HAE. This efficiency of CFTR delivery fully corrected the basic bioelectric defects of Cl2 and Na+ epithelial ion transport and restored ASL volume regulation and mucus transport to levels approaching those of non-CF HAE. To determine the numbers of CF HAE surface epithelial cells required to express CFTR for restoration of mucus transport to normal levels, different amounts of PIVCFTR were used to express CFTR in 3%–65% of the surface epithelial cells of CF HAE and correlated to increasing ASL volumes and mucus transport rates. These data demonstrate for the first time, to our knowledge, that restoration of normal mucus transport rates in CF HAE was achieved after CFTR delivery to 25% of surface epithelial cells. In vivo experimentation in appropriate models will be required to determine what level of mucus transport will afford clinical benefit to CF patients, but we predict that a future goal for corrective gene transfer to the CF human airways in vivo would attempt to target at least 25% of surface epithelial cells to achieve mucus transport rates comparable to those in non-CF airways.
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Tags: Biology, Biotechnology, gene therapy, Genetics, Health, Medicine, Microbiology, Science, Virology, virus
