Molecular mechanisms, symptoms and treatments in Cystic Fibrosis
Abstract
INTRO
MOLECULAR MECHANISMS
In order to find effective treatments and mitigate the consequences of cystic fibrosis, it is first necessary to understand the reasons for its clinical manifestation.
Firstly, Cystic fibrosis is an autosomal recessive disease meaning that in order to inherit the disease; the offspring must be born of two parents both carrying at least one CF allele- to which the lowest possibility of being a sufferer is 25%.
The development of cystic fibrosis results from a CF gene defect or mutation that leads to a malfunctioning, misfolded or absent cystic fibrosis conductance regulator (CFTR). With over 1500 possible CFTR mutations that could occur; the malfunctioning of CTFR proteins can be classified into 6 different classes of mutations.
Class I: Problems in protein production
Due to premature termination codons (known as truncated proteins) and unstable mRNAs due to abdnormalities in the splicing process (e.g. frameshifts in insertion/deletion) this leads to dramatic reduction in ctfr production as the protein is becomes undetectable.
As all class I mutations produce very little if not no full-length fully functional protein, the CTFR CL- channel is not produced in the epithelia of affected cells. Class I contributes to roughly 5% of mutations.
Class II: Problems with processing of proteins
Most of the total mutations (~75%) will be caused by the mis-location of CTFR-associated proteins meaning that they do not go to the necessary cellular location and therefore
Most commonly mutation DF508, caused by the deletion of a Phe codon. The CTFR protein in effect are misfolded and eventually degraded in the endoplasmic reticulum.
Class III: Defective regulation
In the proteins that do reach the plasma membrane, certain mutations may occur in the nucleotide-binding regions and regulatory domain. This leads to defective, malfunctioning gating and regulation (i.e. although protein reaches cell surface, the channel remains closed). This often severe disease
Class IV: Problems in conduction
Mutations occurring in the transmembrane region means CFTR molecules correctly assembled and processed but exhibit altered channel functions (e.g cannot move chloride ions through efficiently… less severe disease with no pancreatic dysfunction
Suprisingly only 50% of newly synthesized CTFR is correctly folded and trafficked to the cell surface
Despite the several vast number and classes of mutations; only a few are known to have functional importance. The class II mutation, ΔF508 in particular- which occurs due to the deletion of phenylalanine (Phe508del) at the position of 508 of the polypeptide- counts for nearly 70% of all mutated alleles in the chromosomes of Europe and North American populations.
The mutation causes abnormal biogenesis and premature degradation of the CTFR protein as it reaches the stage where by the cells are quality controlled. As a result, there is an abnormal chloride conductance on the apical membrane of the epithelial cell in organs throughout the human body. This means that throughout the CF epithelia, the permeability of chloride ions decrease significantly.
There are various hypotheses on how the dysfunction of CFTR leads to the phenotypic disease, however it is generally unknown whether one or more in combination of these hypotheseses cause the disease. The four that are main contributes to the pathogensis of cystic fibrosis are highlighted below :
1. Low-volume hypothesis
Claims that excess sodium and water reabsorption- which ultimately leads to the dehydration of airway surface materials- is due to the loss of inhibition of epithelial sodium channels caused by CTFR mutation. The normal ciliary is inhibited as the water volume decreases
2. Salt Hypothesis
Initially, it was believed that the absence/malfunctioning of CTFR caused a change in the airway surface liquid composition (ASL) most noticeably an NaCl increase. The high level of salt in would thus cause the inhibition of hBD-1- a salt-sensitive protein with antimicrobial properties which would leave the epithelia unprotected thus susceptible to infection. It has also recently been showed that there is a osmolality difference between CF and normal ASL which would further support this hypothesis.
3. Dysregulation of host inflammatory mediators
There is evidence that dysfunctioning cystic fibrosis can also just be a dysregulation of host inflammatory mediators as several experiments have shown extremely high concentrations of inflammatory mediators present in cell cultures infected by cystic fibrosis when compared to those uninfected.
This hypothesis is also supported by the conclusion made by lung lavage studies where inflammation occurred in children
4. Increased presence of asiolo-GM1
The final hypothesis states that chronic bacterial infection is the reason for the occurrence of cystic Fibrosis. Normally, bacteria P.aeruginosa readily binds to the functional CTFR protein initiating a rapid and self-limiting innante immune response in order to combat the infection. However due to the increase of aisolo-GM1 – a receptor in the apical cell membranes- that enhances the binding of P.aeruginosa and bacteria S. aureus to airway epithelium. In the absence of a functional CTFR protein, the immune response is compromised and therefore occurs without initiation as the CTFR- P-aeruginosa binding can no-longer occur to generate an innate response. Along with the inability to produce clear mucus due to abnormal chloride conductance in the epithelial cells of the lungs, this can lead to severe infection.
Therefore, research carried out on CF has focused on how a defective innate defense system i.e. defective ion channels can lead to disease.
SYMPTOMS
CF was previously considered a pulmonary and digestive disease however it is now known to affect most of the body systems.
The innate defence mechanisms such as the mucous lining in the respiratory tract- providing a physical barrier is no longer sufficient in protecting a cystic fibrosis sufferer.
Diagnosis of cystic fibrosis
Pulmonary disease
Symptoms linked to Cystic Fibrosis vary from patient to patient, however most of the morbidity and mortality of CF is caused by respiratory disease. The lungs, at birth, function normally however they eventually become chronically infected as well as inflamed, initially thought to be due to the pathogens Staphyloccocus aureus and Haemophilus influenza in particular Psuedomonas aeruginosa. These pathogens are supposedly responsible in damaging the epithelial surface cells making it possible for other bacteria to attach to the surface- however this has been widely debated by scientists. The Bronchoalveolar fluid (BAL) was found to have high concentrations of Interleukin 8 (IL-8), an inflammatory marker, peroxidases along with their oxidants in a study investigating cystic fibrosis in infants who were found to be CF sufferers after a newborn screening. This concentration further increased despite not yet experiencing any symptoms or bacterial colonization. However, alternative studies claim that control subjects (i.e. non-sufferers of CF) had similar BAL compositional profiles suggesting that it is rather the elevated neutrophil count that causes the symptoms in the respiratory system and not inflammation caused by the bacterial infection of Staphylococcus aureus.
TREATMENTS
Due to growing knowledge in how CTFR mutations cause lung disease, there has been many recent attempts for new treatments. Previously, treatments for cystic fibrosis concentrated mainly on the downstream effects of CFTR dysfunction (e.g. sputum retention, infection and inflammation) however therapies for the CF abnormality caused mainly by basic defects (i.e. Class I-IV mutations) has instead drawn attention.
FUTURE OF CYSTIC FIBROSIS
Conclusion
Cystic fibrosis can essentially act as a paradigm of how more understanding of an underlying disease process may be able to lead on to new and potentially more effective treatments.