Respiratory channelopathies (L7) Flashcards
What is the prevalence of cystic fibrosis?
1 in 2500 births, 1 in 20 people are carriers
What does cystic fibrosis effect?
It’s an autosomal recessive disease which effects epithelial tissue and its electrolyte transport - 50 years ago it was very rare for someone with CF to live past 1-year-old, but now with the right treatment people live until past 40.
What are the symptoms of cystic fibrosis?
Airways - clogging and infection of the airways (respiratory tract infections) - severity varies but this is common among all patients
- Blockage of small bile ducts and problems with liver function in 5% of patients
- Blockage of pancreatic ducts which prevents secretion of digestive enzymes - in 65% of patients
Small intestine - obstruction due to thick content - in 10% of newborns, this can cause death in newborns so is checked as soon as a CF baby is born.
Reproductive tract- Absence of Vas-deferens, makes 95% of males infertile- a small number of women are also infertile
Skin- excess secretion of NaCl via sweat glands - sweat chloride test is a classic diagnostic test.
Why do carriers show no symptoms of cystic fibrosis?
50% of the protein is sufficient for normal function and the mutation is not dominant negative so the protein made functions properly.
What gene undergoes mutation to cause cystic fibrosis? What is its structure and where is it mutated?
The cystic fibrosis transmembrane conductance regulator (CFTR). It is a chloride transporter with 12 transmembrane domains. The NBD (nucleotide binding domain) binds things like ATP, which regulates the channel (open and closes it). The R domain regulates NBD as its where phosphorylation occurs. 70% of patients have a mutation in NBD1 (F508 mutation). There are over 1900 mutations that cause CF
F508 has 90% allelic frequency, G551D, W1282X, G542X, N1303K makeup 1-3% allelic frequency and all the others have less than 0.1% freq.
Why do the symptoms of CF vary so much?
The mutations have a variable penetrance (how much they cause the symptoms) because of environmental factors e.g. medication and diet. As well as genetic factors (variation in non-coding regions)
How is the CFTR gene impacted by mutations?
Production - mRNA is unstable so breaks down and is not translated
Processing - e.g. is misfolded
Trafficking - the protein (even if folded right) is not trafficked to the membrane properly
Conduction - the protein makes it tot he membrane but doesn’t form a pore, so can’t transport chloride even if regulation is okay
Regulation - the protein forms a pore but isn’t properly regulated so can’t be opened (even if it mechanically can open)
F508 mutations usually cause a problem with the trafficking and processing, so if there’s a way to get the protein to the membrane, it would theoretically be able to function normally.
Explain the lung pathology present in CF
Viscous airway mucous - doesn’t move easily
Recurrent bacterial infections because the mucus can’t be flushed out easily.
This leads to frequent antibiotics being needed which increases the risk of antibiotic resistance.
Inflammation - from all the infections, causes damage to the tissue due to pressure.
CF patients also get an increased immune response due to a non-functioning CFTR on immune cells (normally it would act as a suppressor of the response
Tissue degeneration sie to inflammation is a common cause of death
Explain why a loss of CFTR causes problems in the airways.
So, a transporter on the basolateral side of the cell moves 1 potassium, 2 chlorides and 1 sodium into the cell. The K and Na are recycled back out across the basolateral membrane. Usually, the chloride would move out through CFTR which is on the apical side. On the apical side outside of the cell cilia project out there’s a liquid layer. Above the liquid layer is a mucous layer. The cilia move in the liquid layer and this moves the mucus along (they can’t move in the mucous layer)
The balance of sodium (into the cell through apical ENAC) and chloride out sets the height of the liquid layer. In CF patients, there is less chloride transport across the apical membrane, so the height of the liquid layer is decreased (to less than 7 microns), meaning the cilia become bent and can’t move the mucus along.
You can grow bronchial cell cultures and then use EM to see the varied height of the liquid layer between wt and mutant cells.
Explain colonic protection from CF
There’s a high prevalence of CF in european populations
This is thought to be because of the colonic protection you get when you have a mutated CFTR gene. In lower mid-crypt cells, chloride secretion (using CFTR) is followed by water. When you get an infection in your gut, the bacteria secrete enterotoxins which activate CFTR and causes diarrhoea (secrete more Cl so secrete more water). However, if you have CF (or are a carrier of CFTR you are much less likely to get diarrhoea, which increases survival esp in the olden days when people died of dehydration. Therefore, there is now a higher prevalence of carriers.
What are the current treatments available for CF?
Physiotherapy - to dislodge mucus in the airways - decreases the risk of infection
Bronchodilatory drugs
Antibiotics when necessary
Steroids - anti-inflammatory -protects tissue from pressure during an infection
Mucolytics e.g. CNase which breaks down the mucous
Gene therapy - Delivery of CFTR DNA to target cells, healthy DNA is transcribed to mRNA, this is delivered so the cells can produce the healthy protein (poor success with this tho).
CFTR modulators:
- read through agents force production of full-length CFTR when a premature stop/non-sense mutation is present. e.g. gentamicin
- correctors force mutant CFTR protein to the cell membrane, if the mutant is functional then CL- secretion is restored
- potentiators - these increase Po of CFTR which most be trafficked normally, the GG551D is a target for Ivacaftor
Combinations of potentiators can correctors are under investigation in clinical studies (potentiators are very effective but are also very expensive)
