Lecture 6 Atypical CF Flashcards
What percentage of CF patients don’t possess 2 CFTR mutations
1-2% in Northern Europ
What is the significance of some patients with CF not having a mutation in both CF alleles yet still having symptoms
CF is recessive and carriers should be asymptomatic. 1 or no CFTR mutation should cause symptoms in these patients
Atypical CF usually has less severe symptoms T or F
T
What would be the predicted hypothesis for how mutations in ENaC could cause CF-like symptoms
A gain of function in ENaC would cause more Na+ to be absorbed in the airways. This would cause a decrease in the height of the ASL and subsequent CF symptoms
Mutations in which ENaC subunits have been identified in atypical CF patients
Α β and γ subunits as well as other genes
What are the two ENaC α subunit common genetic backgrounds
T663 is what is normally seen in the ENaC α subunit (wild type) however A663 is a common background seen in the population. A663 doesn’t impact the overall function of ENaC
Give some examples of ENaC α subunit gain of function mutations
V114I (in A663 background) L180L-R181W (in A663 background) and W493R (in both backgrounds)
Which ENaC α subunit mutations seems to give the most severe gain of function effects
W493R
Which ENaC α subunit mutations have been found to be loss of function
F61L and A334T (in the A663 background)
What can be said about the total levels of the ENaC α subunit protein in the loss of function and gain of function mutations
Overall protein levels remain the same in both the gain of function and loss of function mutations. Hence there is no change in the total number of channels
How could a loss of function in ENaC account for atypical CF symptoms
If there is a loss of ENaC function then decrease Na+ reabsorption could lead to an increase in the height of the PCL. This prevents the cilia from beating as effectively and may result in defects in the ability to clear mucus.
What was the effect of amiloride on the nasal potential of patients with classical CF compared to healthy patients why was this the case
There was a much bigger positive shift in nasal potential with amiloride compared to healthy patients. This means that the function of the ENaC channel must be greater in the patients. This is because the CFTR mutant protein can’t supress the function of ENaC thus resulting in a greater ENaC function in CF patients
What was the effect on the nasal potential of switching to a Cl- free environment in patients with classical CF compared to healthy patients why was this the case
Low Cl- caused no change in the nasal potential of the classical CF patients. This is because they have no functional CFTR protein hence there will be no effect on extracellular Cl- concentrations on the membrane potential
What was the effect of amiloride on the nasal potential of patients with atypical CF compared to healthy patients and those with classical CF why was this the case
There was a bigger positive shift in nasal potential with amiloride compared to healthy patients this was a similar degree as the patients with classical CF. This is most likely through a more direct gain of function mechanism in the ENaC channel itself rather than the effects of mutant CFTR as these patients still retained one wild type copy of the gene
What was the effect on the nasal potential of switching to a Cl- free environment in patients with atypical CF compared to healthy patients and those with classical CF why was this the case
Low Cl- a change in the nasal potential reminiscent of the healthy patients. This is because they have retained 50% of functional CFTR protein sufficient for normal functioning of the channel
Describe the subunit structure and composition of ENaC
ENaC is a channel consisting of 4 subunits made up of 2 transmembrane domains and an extracellular loop
Where is the W493R mutation in the ENaC α subunit found
In the extracellular loop of the subunit
Using the data below discuss the impact of the W493R mutation in the ENaC α subunits on the currents generated in both the genetic backgrounds
The αW493R mutation causes increased ENaC currents. These bigger currents can be seen in both ENaC backgrounds