Cystic Fibrosis Transmembrane Conductance Regulator

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)-Gene Transmission through Viral and Non-viral Vectors for Treating Cystic Fibrosis (CF) Lung Disease

Ramy Gadalla

George Mason University

In this paper I would like to highlight few aspects in the utility of viral and non-viral vectors to treat Cystic fibrosis (CF) lung disease. CF is a recessive genetic disease caused by the malfunction of the chloride ion-transport channels in the epithelial cells lining the airway (Genetics, 2013, p. 106). The protein that functions in the transport channel is termed, cystic fibrosis transmembrane conductance regulator (CFTR). Persons with CF have a mutated, dysfunctional form of the CFTR that causes the channel to stay closed, and so chloride ions build up in the cell. As a result of imbalanced ion and water movement, airways dehydrate. This affects mucociliary clearance and causes accumulation of dehydrated sputum, leading to chronic bacterial colonization, inflammation and ultimately organ failure. Major health complications resulting from plugging up of the airways and clogging of the ducts leading from the pancreas to the intestine include frequent respiratory infections, pneumonia, digestive problems, and often diabetes (Cleveland Clinic, 2012). Gene therapy trials have investigated different vectors at carrying the CFTR gene and making it enter the host cell’s nucleus in order to express the gene for the correct protein that would ultimately lead to treating the malfunctioning ion-transport channel. This paper will shed the light on two methods of transmission, the first is through a viral vector, called adenovirus (Av) and the second is via a non-viral vectors.

Av is non-enveloped, non-segmented, linear, and double-stranded type of virus. It has natural tropism for lung and respiratory infection and can cause the common cold, pneumonia, and bronchitis, but when it is used as a gene transfer vector it is based on type 5, which is devoid of most viral genes and causes mild respiratory infection (Griesenbach and Alton, 2012, p. 646).

Conclusions from the Av trials in CF revealed: (a) Low level gene transfer based on detection of vector-specific CFTR mRNA and protein, (b) partial correction of the CF-specific ion transport defect, specifically the chloride transport in nasal epithelium, (c) drawbacks from administration, such as lung-inflammation, that were dose-dependent, (d) humoral and cellular immune-responses affecting the efficacy after re-administration, and (e) low efficacy compared to the predictions by pre-clinical models (Griesenbach and Alton, 2012, p. 646).

Some advances have been made leading to improved efficacy for Av-mediated gene transfer: (a) Researchers have gained access to the coxsackie Av receptor (CAR) on the basolateral membrane of epithelial cells, through opening the tight junctions through various pharmacological agents. But, this is controversial to repeat in humans’ lungs due to the heavy bacterial colonization that could lead to systemic invasion. (b) Researchers have administered immunosuppressants and corticosteroids to transiently block CD4+ T cells and reduce cellular and humoral immune responses to the virus. However, success was limited and re-administration was only twice, leading to reduced and finally absent transgene expression. Kolb et al. were an exception, where they re-administered the virus at least five times without loss of transfection efficiency. (c) They developed helper-dependent adenoviral vectors, which, devoid of all viral genes, promised to be less immuno-stimulatory, and, when administered with tight junction openers, lead to high-level and longer lasting gene expression and reduced inflammation compared to previous generations of the virus. Problems related to re-administrations, however, persisted. Yet, interestingly, Cao et al have shown transient immune suppression and were able to transmit two doses of the virus without loss of efficacy (Griesenbach and Alton, 2012, p. 646).

The second possible method of transmission is via non-viral vectors. The two main components of non-viral vectors are (a) the plasmid, carrying the CFTR cDNA and appropriate regulatory elements, and (b) the carrier molecule, such as cationic polymers and cationic lipids, that attaches to the negatively charged human DNA through charge interactions to generate lipoplexes and polyplexes. It is thought that lipoplexes and polyplexes then bind to the cell membrane, are endocytosed and subsequently escape from endosomes by inducing rupture of the endosomal membrane. Researchers are improving the two non-viral vectors’ main components, the plasmid and the carrier molecule, by utilizing promoters and removing methylation, respectively. Non-viral vectors are inherently less efficient because they did not evolve the strategies for cell entry, endosomal escape, movement through the cytoplasm and nuclear uptake, which viruses have evolved. However, they may be better for reducing the re-administration drawbacks (immune-response and inflammation) because of their simpler composition, being devoid of non-human protein components (Griesenbach and Alton, 2012, p. 655).

In summary, viral vectors have evolved strategies for gene transfer than non-viral vectors. However, researchers are examining the possibility of utilizing non-viral vectors for treating long-term illnesses, like CF, enhancing the two non-viral vectors main components, the plasmid and the carrier molecule. This is to avoid complications.

References

A. Pierce, Benjamin. (2013) Genetics: A Conceptual Approach. New York: W. H. Freeman.

Cleveland Clinic. (2012). Cystic Fibrosis. Retrieved from http://my.clevelandclinic.org/health/diseases_conditions/hic_Cystic_Fibrosis

Griesenbach, Uta and Alton, Eric W. F. W.. (2012). Progress in Gene and Cell Therapy for Cystic Fibrosis Lung Disease. Current Pharmaceutical Design, 18(642-662), 642-658. http://benthamscience.com/journal/abstracts.php?journalID=cpd&articleID=76319.