Cystic fibrosis, also known as mucoviscidosis, is an autosomal recessive disorder.1,2,22 It is the most common inherited disease in the Caucasian population affecting 1 in 3000 children in Western Europe.3 It is a multiorgan disorder caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein, which is located on the long arm of chromosome 7 and encodes for a special chloride ion channel.4,5 The vast majority of mutations involve three or fewer nucleotides and result in primarily amino acid substitutions, frameshifts, splice site, or nonsense mutations.11 Of more than 800 identified CFTR mutations, the 3 base pair deletion of phenylalanine at position 508 is found worldwide in 70% of cystic fibrosis sufferers, therefore making F508 CFTR the most common deadly mutant in the Caucasian populations.6 Since cystic fibrosis has a genetic origin the opportunity to treat by replacing the defective gene with a normal healthy gene (gene therapy) offers a ‘novel therapeutic approach’ for sufferers.7 The estimated survival age of cystic fibrosis sufferers is 33.4 years (Fig 1). In this essay we will discuss the aetiology and symptoms of cystic fibrosis and the current available treatments, with particular emphasis on gene therapy and furanones, which prevent the build up of bacterial biofilms and thus reduce lung infection.

Mutations in the CF gene can disrupt CFTR function within epithelial cells in different ways, ranging from complete loss of protein to surface expression with poor chloride conductance.12 The five major mechanisms by which CFTR function is altered are summarized in Figure 2. Class one mutations produce premature transcription termination signals resulting in unstable, truncated, or no protein expression. Class two mutations, including F508, cause the protein to fold incorrectly leading to premature degradation and failure to reach the apical cell membrane except for special conditions such as low temperature. Class three mutations, reach the cell membrane but the channel is not properly activated. Class 4 mutations, reach the cell surface and the channel can be activated but have decreased chloride conductance. Class 5 mutations result in decreased abundance of CFTR, as exemplified by incorrect splicing. With some Class 5 mutations a small percentage of correctly spliced mRNA are produced, resulting in a milder phenotype.13

Cystic fibrosis affects the entire body and is age dependant. Symptoms of the disease are shown in the table 1.

Table 1. Signs and symptoms of cystic fibrosis
Lung and sinus disease • clogging and infection of the airways due to inflammation
• persistent coughing
• abundant phlegm production
• decreased ability to exercise
• coughing up blood
• bronchiectasis
• pulmory hypertension
• heart failure
• difficulty getting enough oxygen to body
• blocking of sinus passage
• facial pain
• fever
• nasal drainage headaches
• nasal polys
Gastrointestinal, liver and pancreatic disease • Meconium ileus
• Rectal prolapse – caused by increased faecal volume, malnutrition and increased intra-abdominal pressure due to coughing
• Pancreatitis
• Malabsorption malnutrition
• Poor growth and development
• Vitamin A, D, E and K defieciency
• Heart burn
• Intestinal blocking by intussusceptions and constipation
• Distal intestinal obstruction syndrome
• Blocking of bile ducts causing liver damage
• Cirrhosis.
Endocrine disease and growth • Diabetes – caused by loss of islets of Langerhans by damaged pancreas.
• Osteoporosis.
• Clubbing of fingers – caused by chronic illness and low oxygen (Figure 3).
Infertility • Men make normal sperm but the tube which connects the ejaculatory ducts of the penis to the testes are missing.
• Women are infertile as a result of thickened mucus which interferes with the passage of sperm. In severe cases, ovulation can be disrupted due to malnutrition causing amenorrhea. The sweat glands, airways, and pancreas are each composed of epithelia and in cystic fibrosis these epithelia have decreased anion permeability.10 The most serious problem facing cystic fibrosis patients is obstruction of the lungs by heavy, thick mucus. As this mucus cannot be cleared effectively, the lungs are highly susceptible to infection by bacteria such as Pseudomonas aeruginosa.8,14 P. aeruginosa produce a number of virulence determinants that give rise to colonization and infection of the respiratory tract which is responsible for their high morbidity and mortality.9,15 Although intensive antibiotic treatment allows 90% of CF patients to survive chronic P. aeruginosa lung infections for more than 10 years, the chronic infection cannot be eradicated by antibiotics.16 Bacterial quorum-sensing influences bacterial behavioural processes such as the ability of bacteria to form biofilms.17,18 Bacteria in biofilms are resistant to disinfectants, antibiotics, and the action of host immune defences. Biofilm formation plays an important role in bacterial pathogenicity during chronic infections.18,19 Furanones, a synthetic group of chemicals, can be used to interfere with the formation of Pseudomonas biofilms by blocking the signals sent by the bacteria, thus preventing the formation of a colony.20 The synthetic furanon that is found to be so effective against the formation of biofilms is similar chemically to a furanon produced by a type of marine algae, Delisea pulchra.15 This synthetic furanon prevents the formation of new colonies, and also has been found to promote the detachment of bacteria from existing biofilms and hinder the ability of bacteria to cause disease and infection.15,23

The identification of the CFTR protein in 1989 allowed for the possibility of correcting the cystic fibrosis phenotype close to reality.21 The aim of gene therapy is to introduce a normal copy of a gene into cells and restore normal gene function.7 The normal CFTR protein is expected to exert a dominant function over the mutant CFTR, which makes it a good candidate disorder for treatment, by gene therapy.7 In this case the introduction of a normal CFTR protein into the cystic fibrosis cells.14 Some possible ways to deliver the protein are aerosolised liposomes, retrovirus-based vectors, and virus-mediated plasmid delivery. These can be divided into viral and non viral systems.28 The advantages and disadvantages are of these are shown in Table 2. Gene therapy attacks at the heart of the problem which is the genes themselves, rather than just treating the symptoms of the disease.24

Table 2. Advantages and disadvantages of possible ways to deliver the CFTR protein Advantages Disadvantages
Viral vectors Safe, limited toxicity, potential for cure Low efficiency to date, side effects owing to recombinant DNA, potential for insertational mutagenesis
Non viral vectors Limited immune response, improved gene transduction Low efficiency, systematic side effects owing to recombinant DNA
Gene rescue Corrects only abnormal cells Mutation specific, side effects of drugs
Transcomplementation Corrects abnormal cells, multiple possible vectors Mutation specific

Retroviruses are used as a gene shuttle for therapy of foetuses in the uterus while the lungs are developing. They are however unsuitable for fully developed lungs of an adult patient. Retrovirus-based vectors efficiently integrate the preferred gene into the genome of the host cell. The retrovirus vector genome is able to integrate into the host cell of the DNA, therefore offers an opportunity for long term expression and maybe even a cure for cystic fibrosis.34 However this process has a risk of integrational mutagenesis and tumour formation. Further low rates of epithelial cell production in normal human lungs have made the retrovirus vectors objectionable in gene transfer to the lungs.34

Adenovirus has been extensively studied as a vector because it infects non-replicating cells, and shows tropism for airway cells. It also contains a double stranded DNA genome which means that it can easily be genetically manipulated.29 Deletion of the early region one (E1) on the adenovirus vector creates enough room to insert the CFTR gene.32 In vitro and in vivo adenoviruses have shown to be efficient however are known to have immunological properties that may potentially limit their usefulness in human gene transfer.33

More recent studies have used an adeno-associated virus (AAV) to deliver a complimentary CFTR protein to respiratory epithelia.25 Dissimilar to the adenovirus, AAV is not known to cause any illness in humans.26 It requires a helper virus to replicate and without this helper virus wild-type AAV can produce a latent infection by integrating its genome onto the human chromosome.27 A clinical trial using AAV-2, containing the whole sequence coding for the CFTR protein, showed no evidence of inflammation. However the main risk factor with the AAV vectors is insertational mutagenesis. Dissimilar to wild type AAV, which integrates onto specific sites on chromosome 19, AAV vectors appear to assimilate into random sites.30 Nonetheless AAV-CFTR vectors are promising because they seem to infect and express in non dividing cells until DNA repair occurs allowing integration.31

Cystic fibrosis is showing to be a challenging disease, despite the advancements over the previous few years. However even if current strategies of gene therapy are not yet optimised, active research in this area is showing a bright future and realistic treatment options in the near future.

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