Cystic Fibrosis: The Role of Gene Therapy in Management
1. Introduction
Cystic fibrosis (CF) is an autosomal recessive disease of epithelial cell metabolism, characterized by abnormal transport function of a family of proteins that includes a defect in chloride transport. While this protein is present in many organ systems, its pathology is most notably present in the lungs where CF’s characteristic viscous mucus affects the conduction of airways, blocks airflow, and becomes a culture medium for many types of bacteria seen in chronic CF infections like Staphylococcus aureus, Pseudomonas aeruginosa, Hemophilus influenzae, and Streptococcus milleri. There is a high mortality rate with end-stage lung disease accounting for most of these deaths. Defects in the pancreas, intestine, hepatobiliary tract, male reproductive tract have also been described. Successful resuscitation of a fetus with multiple hyperinsulinism episodes in utero has also been reported using in utero gene replacement. Lastly, its fatal nature reduces the incidence of the characteristic genetic defect in many populations around the globe.
An otherwise healthy 30-year-old man presents with chronic nasal congestion and chronic occasional cough productive of thick mucus over the past six months. There is no associated fever, nocturnal cough, or wheezing. Laryngoscopy is normal and rigid sinusoscopy shows polypoid mucosa in the bilateral maxillary sinuses as well as thick, cloudy mucus. He also had a ratio of weight to height of 88% before surgery to remove the polyps. Addition of routine twice-a-day rhinocort, normal saline nasal irrigations, and oral montelukast 10 milligrams per day decrease his sinus symptoms and decrease the need for in-office polyp irrigations but the gross mucus problem persisted. Scans of the paranasal sinuses again showed chronic pansinusitis, increased mucosal thickening in the maxillary sinuses, a small amount of opacification present within all paranasal sinuses, and complete opacification of the clip and maxillary ostia bilaterally. The patient was asking for more effective and long-term treatment to his problem so that he can avoid antibiotic therapy and nasal steroid irrigations on a regular basis.
2. Understanding Cystic Fibrosis
CF is the most common life-shortening genetic disease in the Caucasian population, affecting approximately 1 in 2000-3000 live births. The incidence of the condition decreases across the globe, mirroring the frequency of different CFTR mutations in various populations. Early genetics research led to the discovery that mutations in the CFTR gene are responsible for the pathological changes in CF, and the identification of CF as an autosomal recessive condition. Since the identification of the CFTR gene in 1989, over 2000 mutations of the gene have been identified, of which up to 330 have been described as disease-causing or confer no known adverse effect. The CFTR protein is a large molecule and bores through the cell membrane utilizing a number of segments or ‘domains’ to do so. Failure of the protein to be trafficked appropriately, e.g., remaining within the cell and incapable of transporting chloride, results in a lack of functional CFTR at the cell surface, rendering the cell’s ability to transport chloride as being ‘dysfunctional’. Dysfunctional CFTR is implicated in the development of the characteristic multi-organ pathologies seen in individuals with cystic fibrosis.
Cystic fibrosis (CF) is a life-limiting autosomal recessive genetic disease caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein. The CFTR is expressed in a range of cells and tissues, including the cells that line the airways in the lung where it plays a role in maintaining a liquid lining of the airway surface allowing mucus to be cleared from the airways. In CF, this process is impaired, resulting in dehydrated and ‘sticky’ mucus which is not adequately cleared from the airways. The mucus provides an ideal environment for microorganisms, particularly bacteria, to thrive, leading to chronic airway inflammation and infection. Importantly, if not treated appropriately, respiratory failure and death ensue. Other serious complications of CF affect the gastrointestinal, hepatobiliary, reproductive, and musculoskeletal systems, increasing the complexity of management and the need for multidisciplinary teams to provide care to individuals affected by this condition.
3. Gene Therapy Approaches
In 1986, urinary epithelial cells were removed from a patient with what was then known to solely cause Male Sexual Reversal (MSR), and adenovirus was used to deliver a healthy gene to the cells. Subsequently, transducing the cells with this caused them to express an androgen receptor once more, therefore resuming male sexual characteristics. Ultimately, in late 2011, Phase III trials showed that patients responded to a treatment that either stabilized or modestly improved disease condition. At the time of this research, the longest clinical trial had been performed for 15 years on the same patient, which initially started with monthly treatments and then spaced out to biannual treatments. Strikingly, this is an excellent proof of concept for the fact that CF is certifiably and believably amenable to lifelong gene therapy, which bypasses the difficulty in achieving efficacy in RDEBs.
Nature’s way of preventing the propagation of deleterious mutations within a gene pool is through a strong biological force remaining dominant and in majority over a variant that could lead to demise. This sentinel obligation is often weakened or fails under prevailing conditions, like in bacterial genomes or in the event of hybridization in plants. Unlike the natural design of life forms, synthetic methods, unlike by-products of chance mutations, can be used deliberately to introduce and remove mutations. With the advent of precision genome editing methods like Pluripotent Stem Cells (PSCs), Zinc Finger Nuclease (ZFN), Transcription Activator-Like Effector Nuclease (TALEN), and CRISPR-Cas9, the tenacity of heritability is increasing while the genetic variation is decreasing.
4. Current Challenges and Future Directions
Cystic fibrosis patients often have significant extra-pulmonary disease, so further work in gene therapy to address these issues is required. Indeed, in the absence of optimized methods of delivery and gene transfection, successful gene therapy in CF may be elusive. A highly effective, stable vector system capable of crossing the mucus layer, entering ciliated cells, evading ABPA and allowing long-term expression of CFTR continues to be the major challenge. Progress made in developing vectors of choice for CF gene therapy continues to be modest. The recent advances in DNA manipulation from a variety of approaches may lead to highly efficient vectors for CFTR gene delivery in the near future.
It was originally envisaged that to be successful, a gene therapy approach would need to have a strong expression of the exogenous gene in the affected cell types. This has been shown to be very difficult to achieve in airway cells, particularly in the ciliated cells of the respiratory epithelium. Many hurdles need to be overcome to achieve good levels of gene transfer and expression in the airway epithelium. Apical entry of the vector and vector inactivation by mucus, loss of cilia, submucosal arteries and slow airway clearance were found to reduce lung transfection. Increasing the genotype of confounding factors perturbing gene delivery to the lungs and absence of molecular tracers enabling to follow gene transfer were significant limitations to further improve the procedure.
5. Conclusion
Despite its many potential benefits and promise at the onset of gene therapy for CF, it has not been an engine pushed to the brink of success yet. None of the proposed gene therapy approaches have yet restored significant levels of CFTR function in the lungs of a CF patient. Nor have there been any instances of stable gene transfer and gene protection. Furthermore, underway in China are designed to bring 3rd generation CFTR gene transfer systems to finished products that are both non-toxic and sufficiently efficient, as well as enhancing the ability of CF diagnosis and a foil for use as an effective pharmacogenomic tool. The realization of RNAi has the potential for both treating CF and other diseases that result from the malfunction of cell membrane proteins. Furthermore, gene therapy, specifically for CF, will only be maximized when we understand CFTR function control, cell biology, and the airway environment. Since each of these constitutes aspects of the core cystic fibrosis lesions. Finally, the need to develop gene therapy combined with optimal pharmacologic treatments for CF is acknowledged to develop the ultimate CF disease-modifying strategy. Such combination therapies may finally be able to conquer CF and related polluting lung diseases.
This article aims to highlight the role of gene therapy in the management of cystic fibrosis. From the review of the relevant articles, it was observed that gene therapy holds promise for the therapy of cystic fibrosis. Among the current gene therapy modalities being tested, liposome-mediated CFTR gene transfer has demonstrated some preliminary evidence of clinical efficacy, despite transgene limitations. Notably, the reduction in pulmonary inflammation and concomitant clinical status improvement resulting from aerosolized liposome-mediated CFTR gene transfer indicated that gene therapy could provide benefit to individuals receiving the treatment. The need to improve this therapeutic system by incorporating rational design strategies that target remaining bottlenecks in gene transfer efficiency during clinical development is recognized. These limitations have been partly attributed to the lack of a suitable animal model for predicting the efficacy and safety of next-generation reagents in clinical trials. Gene therapy for CF is currently in the clinical trial phase. Initial phase 1 and 2 studies have shown it is safe for administration and exhibited evidence of clinical efficacy. Future studies incorporating large populations, different reagents, and modifying existing genetic conditions are critical. Recent successful lentivirus-mediated clinical trials in other gene therapy areas, e.g., treating ADA-SCID, could become a guide for improving gene therapy for CF.
