Cystic fibrosis (CF) is a multisystem disorder that originates in the respiratory system of individuals. It is caused by a malfunction of the cystic fibrosis transmembrane receptor protein (CFTR). Improved understanding of the CFTR gene has opened doors to better understand the disease itself through research and experimental procedures. Understanding the pathophysiology of the CFTR gene itself has also raised questions about the potential benefits of gene therapy in cystic fibrosis patients. Treatment for cystic fibrosis is prioritized on the improvement of life-threatening symptoms of the disease and the elimination of the things causing these symptoms. Overall, treatments for cystic fibrosis are complex and there are several gaps in research evidence. The purpose of this review study is to explore the pathophysiology of cystic fibrosis presented in scientific literatures and provide recent information on the diagnosis and treatment therapies.
Cystic fibrosis (CF) is a genetic disease that affects the respiratory system, and consequentially the digestive, reproductive, immune, and endocrine systems of the individuals effected. CF is caused by an inherited mutation in the cystic fibrosis transmembrane regulator (CFTR) gene (Donnelley & Parsons, 2018). As a recessive genetic disorder, CF is inherited when two carrier parents (who have one normal gene, and one gene with the mutation) each pass the abnormal CFTR gene to their child. The most common mutation is a deletion of phenylalanine at position 508 (ΔF508), which accounts for 70% of cases. (Nicholson & Sheppard, 2014). Cystic fibrosis is a persistent, life threatening disease that causes an accumulation of mucous in the lungs, leading to chronic symptoms and most commonly pulmonary disease (Cystic Fibrosis Overview, 2019). The thick and sticky mucous produced in the lungs of individuals with CF harbors bacteria and makes it extremely difficult to breath. This life-threatening illness is terminal; with most patients living until around age forty (Donnelley & Parsons, 2018). Though there is no cure for this disease, research advances within the last decade have provided further understanding of the disease and the improved development of therapeutic approaches in order to manage cystic fibrosis and its associated symptoms.
Cystic fibrosis effects at least 30,000 people in the United States and between 900 and 1,000 new cases are diagnosed every year. One in 29 people of Caucasian ancestry is an unaffected carrier (heterozygous) of the CF gene mutation. In the United States, cystic fibrosis occurs at a rate of 1 in 3,400 births, with most cases associated with Caucasian racial background. Most individuals with cystic fibrosis were diagnoses by five months of age; however, the average age of diagnosis was five years. Many are not even diagnosed until adulthood (Cystic Fibrosis, 2014).
A mutation, or alteration, in the host genome affects the production of certain proteins needed for important biological functions. Almost 2,000 mutations in the CFTR gene have been found to cause cystic fibrosis (Hodges & Conlon, 2019). When there is a mutation in the CFTR gene of individuals, the production of the CFTR protein may be affected. Individuals that have cystic fibrosis disease possess these mutations in the CFTR gene. These mutations either result in the complete absence of, or reduction of the CFTR protein, or an incorrect protein with the wrong configuration (Gene Therapy for Cystic Fibrosis, 2019). The ultimate role of the CFTR gene is to maintain the flow of salt (sodium and chloride) and water across the cells that line the respiratory, digestive, and reproductive tracts. A mutation in this gene therefore decreases the flow of salts through the epithelia of multiple organs, including the lungs, sweat glands, vas deferens, liver, and intestines. (Pranke et al., 2019) The decreased flow of salt ions results in a reduction of water in the fluid lining the airways. The lack of water production in these airways drives the accumulation of abnormally thick and sticky mucous that underlies chronic lung inflammation and recurrent bacterial infections, leading to continuous lung deterioration. (Pranke et al., 2019). The inactive flow of water and salt ions allows for increased inflammation and thus inflammatory response. The neutrophils secreted by the body’s immune system further enhances inflammation because they produce elastase and proinflammatory cytokines (Nichols and Chmiel, 2015). This vicious cycle of airway obstruction, inflammation, and persistent infection leads to decrease in lung function, eventually leading to respiratory failure and death. Clogged mucus secretions can lead to additional problems in other organ systems in the body as well (Cystic Fibrosis, 2014).
There are various symptoms associated with cystic fibrosis disease. Among the most common and consequentially the most life-threatening of symptoms are those associated with the respiratory system, ultimately leading to the individual’s inability to breath. This is a result of damaged airways, or bronchiectasis, which makes it difficult for the transfer of air in and out of the lungs and clear the mucous from the airways. The thickened mucous provides an ideal breeding ground for bacteria and fungi which could cause additional infections such as: bronchitis, sinus infections, or pneumonia. Patients with more progressive forms of cystic fibrosis have hemoptysis, or coughing up blood. This happens due to the thinning of the airway walls over time. Other respiratory associated symptoms include pneumothorax which involves chest pain and breathlessness, and respiratory failure, which happens as a result of severe damage to the lung tissues that hinders the function of the lungs (Cystic Fibrosis, 2019).
Cystic fibrosis also causes digestive system complications such as: nutritional deficiencies, diabetes, blocked bile duct, intestinal obstructions, and distal intestinal obstruction syndrome (DIOS). Nutritional deficiencies happen as a result of thickened mucous preventing the transfer of digestive enzymes from pancreas to the intestines; this inhibits the body’s ability to absorb protein, fats, and some vitamins. Many CF patients also develop diabetes because the pancreas is unable to produce insulin—the hormone necessary to use sugar. Blocked bile ducts prevent the transfer of bile from the liver to the gallbladder, leading to liver problems and sometimes gallstones. That malfunctions of particular components of the digestive system could lead to even more complication in and, in later stages of cystic fibrosis, organ failure. (Cystic Fibrosis, 2019).
Most CF patients die from infection present within the lower respiratory tract due to the accumulation of harmful bacteria within the viscous mucous in the lungs. The thickened mucosal lining of the airways harbors pathogens such as Staphylococcus aureaus, Haemophilus influenzae, and Pseudomonas aeruginosa. (Sheppard & Nicholson, 2014) The resulting immune response generated by the body to fight the infections imposed by these pathogens ultimately leads to pulmonary disease due to the lung damage caused by the increased inflammatory response.
Several studies have been conducted to eradicate the detrimental side effects of bacterial infections on CF patients in an attempt to treat the symptoms of the disease and increase lifespan by slowing or eliminating pulmonary disease. One of the main causes of reoccurring pulmonary infections and the resulting life-threatening lung conditions in CF patients is caused by the bacteria Staphylococcus aureus; within the first three years of life. (Ranzenbacher et al., 2019). Two strains of this bacterium block the respiratory pathways of infected individuals: methicillin-sensitive Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus (MRSA). The most detrimental of these strains is MRSA considering the difficulty in treating the resulting infection with antibiotics. (Ranzenbacher et al., 2019).
In an attempt to eradicate MRSA and MSSA in CF patients, studies were conducted analyzing the effect of S. aureus colonization in the airways of CF patients and the different procedures for antimicrobial treatment. One specific study focused on the long-term antibiotics, rifampicin and fucidin in the attempt to eradicate the MRSA and MSSA. Eradication proved successful in patients exhibiting intermittent (less than 50 percent) bacterial colonization. With this, the data showed that the process of eradication of S. aureus in particular can improve patients most in early stages of CF when the percent of S. aureus colonization is lower than that of patients possessing a more progressed form of CF. (Esposito et al., 2019).
Apart from Staphylococcus aureus infection, Pseudomonas aeruginosa is another life-threatening bacterium that resides in the respiratory systems of cystic fibrosis patients. P. aeruginosa is noted as the major cause of mortality in CF patients. The gram-negative bacterium possesses virulence factors that secrete toxins for overcoming immune response produces by the host. (Bhagirath et al., 2016).
In patients with CF, their immune systems are compromised, making it an especially favorable environment for pathogens like P. aeruginosa. With this, there have been few experimental attempts to eradicated P. aeruginosa in CF patients. A 2014 study cultured the respiratory microbiomes of 279 CF patients, several patients exhibited the eradication or the slowed growth of P. aeruginosa after administration of several antibiotics and additional treatment methods (Kenny et al., 2014). These procedures are termed antibiotic eradication therapy (AET). The use of aggressive AET proved successful in the majority of adults exhibiting first-time P. aeruginosa infection (Kenny et al., 2014). Ultimately, this is important because the number of patients with CF entering adult care with no history of P. aeruginosa infection is increasing; therefore, these patients have an increased need for AET in order to help prevent chronic infections produced by P. aeruginosa and the resulting progression of lung damage and the disease in general.
Scientists are exploring several strategies concerning gene therapy in patients with cystic fibrosis. With advancing technologies, the mechanism of gene therapy has become more achievable with new methods for genome editing such as Zinc-finger, CRISPER, and TALENs (Vargas et al., 2016). Gene therapy is the process by which the correct gene is incorporated into the host genome in an attempt to allow the host to construct more normal copies of the wild-type gene. In this case, the mutated form of CFTR gene would coexist with the normal CFTR gene, and the presence of the normal gene relocates to the epithelial cell layer in the airways with the goal of replacing the mutated gene and expressing the functional CFTR protein (Gene Therapy for Cystic Fibrosis, 2019). Although gene therapy has potential to make a significant impact in the lives of individuals with CF and possibly cure the disease as a whole, there are several limitations associated with gene therapy. First, determining the correct and most relevant plasmid DNA molecule model is important. Second, natural barriers such as mucus, immune responses, and intracellular limitations inhibit gene transfer into the lungs. Finally, gene therapy would need to be administered several times throughout the lifetime of the individual due to the fact that the epithelium lining the airways is constantly regenerating. (Pranke et al., 2019). Another disadvantage of gene therapy in CF patients is that this strategy only works in the specific cells that receive the therapy. This being said, gene therapy could be used to treat the lungs, but it will not help the cells in other organ systems throughout the body that are affected by CF, such as the digestive system (Gene Therapy for Cystic Fibrosis, 2019). Taking these disadvantages into consideration, the selection of the appropriate method of gene therapy is essential for success of the treatment. There are three types of gene therapies that have potential to treat cystic fibrosis: integrating gene therapy, non-integrating gene therapy, and RNA therapy (Gene Therapy for Cystic Fibrosis, 2019).
Integrating gene therapy involves the incorporation of DNA containing the corrected CTFR gene into the host genome. This is a permanent integration which could be beneficial in that the patient may only need to receive the therapy a small number of times; however, there is little control in where exactly the gene will be inserted into the host genome associated with this therapy type (Gene Therapy for Cystic Fibrosis, 2019). There are several forms of integrating gene therapy that have been used in the treatment of patients with certain cancers, however, research is still underway in order to determine a beneficial effect of this type of gene therapies in CF patients. Recent studies have examined lentiviral vectors and non-viral vectors as integrating vectors into CF patient genomes (Cooney et al., 2018).
Non-integrating gene therapy involves the incorporation of the correct copy of the CTFR gene into the host genome, without disrupting the genome itself. The gene of interest is simply placed within the genetic information of the individual, but still remains separate from the individual. This form of gene therapy can be beneficial concerning the limited side effects that occur. Though, a disadvantage to this type of therapy is that is not permanent because the new CTFR is not in direct correlation with the rest of the host genome. For this reason, non-integrating gene therapy needs to be administered several times during treatment in order to be beneficial.
In recent years, use of several different types of viral vectors are of high interest in the research of gene therapy for cystic fibrosis patients. A study published in March of 2019 examined the use of lentiviral vectors in the potential prevention of cystic fibrosis lung disease. The lentiviral vector is a type of retrovirus that can transduce in dividing and non-dividing cells. Successful therapies using this type of vector would be beneficial in CF patients especially because they can provide long-term transgene expression. In the particular study in 2019, the lentivirus was evaluated based on successful platforms that vector produced in earlier studies with HIV, FIV, EIAV, and SIV (Marquez Loza et al., 2019). Furthermore, a study in 2016 worked to identify a variety of pre-clinical trial components associated with the lentiviral vectors: mapped integration sites, characterized transduced cell types, assessed acute toxicity, determined the effects of pre-existing immunity on transduction efficiency and toxicity, assessed CFTR function of our lead vector, and quantified vector stability in delivery devices suitable for a first-in-man trial. After careful evaluation of all these necessary components for gene therapy, the study supported the proposed use of the particular lentiviral vector in a future first-in-man clinical trial (Alton et al., 2016).
Cystic Fibrosis is a detrimental disease that comes with numerous complications associated with several different organ systems within the human body. The etiology, pathophysiology, and treatments provide a solid foundation to the understanding of Cystic Fibrosis. As a genetic disease, there is no cure; however, research is directed toward therapies that target the symptoms of the disorder in order to make the disease more manageable for individuals with CF. With the development of new genetic and biomedical technologies, future direction aims to use gene therapy as a means for treating the disease. There is still a lot that is unknown about gene therapy, and though there is no cure for cystic fibrosis, there is hope that developing research will eventually provide answers to fix the mutated CFTR gene permanently. The ultimate goal is to change CF from cystic fibrosis to cure found.
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