L7 - Respiratory Channelopathies

Question 1

What are the statistics on cystic fibrosis?

Answer 1

Autosomal recessive 	
Disease of electrolyte transport in epithelial tissue 
1 in 2,500 births    		
1 in 20 a carrier
50 years ago survival past 1 year rare

Question 2

What are the symptoms of cystic fibrosis in the many different tissues affected?

Answer 2

Airways – clogging and infection
Liver – blockage of small bile ducts and problems with liver function in 5%
Pancreas – blockage of the ducts prevents secretion of digestive enzymes in 65%
Small intestine – obstructions due to thick content in 10%
Reproductive tract – absence of vas deferens in 95% males making them infertile
- Small number of infertile women
Skin – excess secretion of NaCl via sweat glands

Question 3

What are the clinical manifestations of cystic fibrosis?

Answer 3

Meconium ileus – 1 day 
Pancreatic insufficient – 1 week 
Distal intestinal obstruction syndrome – 3 weeks
Respiratory disease – 1 month 
Liver disease – 2 years 
Male infertility – 3 years

Question 4

What is the inheritance of cystic fibrosis?

Answer 4

Autosomal recessive (both copies don’t require the same mutation)
If two carrier parents
- 2 will be carriers (50% normal CF protein)
- 1 will be normal
- 1 will have CF

Question 5

What is NBD1/NBD2?

Answer 5

Nucleotide binding domain
Binding of nucleotide regulates whether the ion channels are open or closed
Found in the cystic fibrosis protein

Question 6

What is the cystic fibrosis protein structure?

Answer 6

NBD1/NBD2
Transmembrane spanning domains and intracellular/extracellular binding loops
- Mutations found throughout these areas
1 protein = 1 functional channel

Question 7

Where are many cystic fibrosis mutations found?

Answer 7

> 1900 mutations in cystic fibrosis transmembrane conductance regulator (CFTR)

Question 8

What 5 things do cystic fibrosis mutations impact on?

Answer 8

Conduction – ion channel doesn’t allow Cl ions to move
Regulation – by nucleotides and phosphorylation means it never opens
Trafficking – protein is made but is not transported to membrane
Processing – mutant protein is misfolded so doesn’t get sent to membrane
Production - protein not made

Question 9

What are some common cystic fibrosis mutations?

Answer 9

F508 (found in NBD1) – 90% allelic frequency
G551 – 15% allelic frequency in the UK
W1282X
G542X
N1303K
Last three - 1-3% allelic frequency

Question 10

What is the lung pathology of a cystic fibrosis patient?

Answer 10

Viscous airway mucous
Recurrent bacterial infections – due to inability to clear mucus
Antibiotic resistance bacteria
Inflammation – triggered by infections
Tissue degeneration – due to over inflammation
Damage to lung tissue is a common cause of death

Question 11

What channels are found on the upper airways of the lung on the basolateral membrane?

Answer 11

NKCC1
NaKATPase
K channel

Question 12

What channels are found on the upper airways of the lung on the apical membrane?

Answer 12

ENaC

CFTR (not present in CF patient)

Question 13

What does NKCC1 do?

Answer 13

Na and K recycle
Cl builds up in the cell
- Cl moves out the cell across the liquid layer

Question 14

What does CFTR d0?

Answer 14

Normally when CFTR is active it inhibits ENaC

Question 15

What happens to CFTR in CF patient?

Answer 15

1. Cannot secrete Cl so cannot control the height of the liquid layer
2. CFTR does not inhibit ENaC so influx of Na into the cell
3. Further reduces the height of the liquid layer
4. Cilia bend over and can no longer beat and remove the mucus

Question 16

What is F508 involved in?

Answer 16

Normal conductance and regulation

Mutations in trafficking, processing and production

Question 17

How do you get colonic protection from CF?

Answer 17

Bottom two thirds of the crypt contains CFTR - control Cl secretion and H2O follows
- This regulates water content of the faeces
Enterotoxins activate CFTR leading diarrhoea
Not in a carrier – as only 50% of the proteins are affected by enterotoxins

Question 18

What are the different treatments for CF?

Answer 18

Physiotherapy – move mucus 
Bronchodilator drugs – open airways 
Antibiotics 
Steroids – reduces inflammation 
Mucolytics – enzymes that break down mucus
Gene therapy

Question 19

How can gene therapy be used to treat CF?

Answer 19

Delivery of CFTR DNA to target cells
DNA transcribed to mRNA and protein produced
Challenging
Poor success rate
Expensive

Question 20

What are some new approaches as ways to treat CF?

Answer 20

CFTR modulators

• Read-through agents
• Correctors
• Potentiators

Question 21

How can read through agents treat CF?

Answer 21

Force production of full length CFTR when premature stop or nonsense mutations
E.g. aminoglycoside antibiotics

Question 22

How can correctors treat CF?

Answer 22

Force mutant CFTR protein to the cell membrane
If the mutant is functional then Cl- secretion is restored
E.g. F508

Question 23

How can potentiators treat CF?

Answer 23

Increase the Po of the CFTR channels, which must be trafficked normally
E.g. G551D mutation is a target for VX-770

Question 24

What is the impact of potentiators in vitro?

Answer 24

Very expensive
Effective – height of liquid layer increases
Improvement of lung function by 10% after 2 weeks