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When nutrient transport goes wrong: Cystic Fibrosis
Learning Objective: To develop a deeper understanding of membrane transport by
discovering what can happen when nutrients are not moving across the membrane as they
should and further your appreciation for the effects of a single DNA mutation on the
overall physiology of a complex organism.
Background on Cystic Fibrosis (CF):
*Cystic Fibrosis is the most common recessive genetic disease of Caucasians affecting 1
in 2500. The disease is most associated with chronic coughing but also results in
digestive pathologies as well. The respiratory problems are due to the fact that the mucus
that lines the lungs to protect them is too thick. This thickened mucus can be detected as
a white haze on lung x-rays. Healthy people clear mucus from their lungs when they
cough. CF patients have trouble clearing the thick mucus (hence the coughing spells).
*CF is caused by a mutation in the Cystic Fibrosis Transmembrane Regulator (CFTR).
This is a cell membrane transport protein. This protein moves Cl- ions from the inside
of cells to the outside. The reason for the lung and digestive pathologies (and not brain,
muscle, etc.) is that CFTR gene is primarily expressed in cells that line the lungs and
digestive tract. So, for example, the lung cells are supposed to pump Cl- ions to the
outside fluid. This “outside fluid” is essentially the mucus that lines the outer surface of
the lungs and becomes too thick in CF patients.
*There are over 1000 mutations found in CFTR that can cause CF, but one in particular is
responsible for 70% of all CF cases. This mutation (delta F508) results in the cell still
being able to make CFTR. The problem is that this mutation causes a slight misfolding
in the CFTR protein. Before in class, I said that misfolded proteins can’t function, but
this is a little different: This one can function if the cell allows it to get to the cell
membrane after it is made, but that’s where the problem lies. All cells have a check-point
so if cells detect a misfolded protein, it can degrade it. This is usually a good thingprevents bad proteins from sticking around. Sometimes this isn’t always 100% though.
The result is that the cell degrades some of the mutated CFTR, but some of it can sneak
past this check-point and still make it to the surface. The CFTR that does make it the
surface is able to transport Cl-ions to the outside of the cell just fine even though it
is slightly misfolded. Unforunately, the cell doesn’t know that so it gets rid of CFTR that
would function just fine.
Case Study
As a second year PA student on rotations, your mentor introduces you to Max. Max is a
5-year old Caucasian boy. Max has had a horrible cough for three weeks and he is
wheezing so badly that his parents can hear it without a stethoscope. You speculate that
Max has pneumonia, cystic fibrosis or asthma. You find out that the nebulizer treatments
that typically work for asthma have not helped Max in the last week. You order the lab
test for pneumonia but while you are waiting for the results, you decide to start
determining if it could be CF-- starting with an x-ray of his lungs.
The x-ray of Max’s lungs shows a white haze over a large part of Max’s lungs (see Fig
1). You run to your mentor exclaiming that you have figured out that Max has CF. Your
mentor looks less than pleased and hands you an x-ray of another patient of hers with
pneumonia. You find your patient’s lungs look remarkably like the patient with
pneumonia. After blaming “Scrubs” for making medicine look so easy, you go back to
your books and find another diagnostic test for CF.
This test is a DNA analysis test. Although there are over 1000 mutations in CFTR that
can cause CF, you decide to start with the most common mutation. This is called the
deltaF508 mutation where 3 nucleotides from the CFTR gene are deleted resulting in
deletion of an amino acid which is phenylanine (abbreviated “Phe” or “F”) at the 508th
amino acid position (hence the name of the mutation). See figure 2 for the results of the
DNA analysis.
Question 1.
You quickly ask Max’s parent to come in to explain the results. Max’s parents are college
graduates but have not had a biology course since their Gen Ed Bio senior year in
college. In the space below, provide an explanation to Max’s parents when they ask,
“How can one of those DNA letter things mess up a big protein like CFTR. I heard on
PBS that we have 3 billion of those letters in our cells; can’t he spare a screw-up in one
of them? You said his cells are making some CFTR protein, so why is he sick? (Reread
my background on CF to help you if having trouble, but use your own words).
Figure 1.
Figure 1. A: Lungs of a
healthy 5 yr old boy.
B: Lungs of a 5 yr old boy
with pneumonia.
C: Max’s lungs.
C
Figure 2: DNA sequence analysis and translated amino acid sequence of a region of CFTR
Healthy Person
DNA:
Amino Acid Sequence:
ATC
Ile
ATC
Ile
TTT
Phe
Max
DNA:
Amino Acid Sequence
ATC
Ile
ATC
Ile
GGT GTT
Gly Val
GGT GTT
Gly Val
Because DNA tests for Cystic Fibrosis are only mildly conclusive and you are inspired by
Dr. House to be more investigate, you decide to look at Max’s lung cells to see if you can
see further evidence of CF. You find the following:
*Elevated Cl- ion levels inside of the cells compared to cells from a
healthy person.
*Lower Cl- ion levels outside of the cells compared to cells from a
healthy person.
Question 2: Below are models of 2 cells; Max’s and one from healthy individual. The
cylinder shaped objects represent CFTR proteins in the membrane.
To the drawing, add the following:
Relative amounts of Cl- ions inside and outside of cell, direction of Cl- ion flow
through the CFTR.
Max
Healthy
individual
While these results interesting, you regret sleeping in Pathology Class during the CF
lecture so you still don’t understand how high levels of Cl- inside cell and low levels
outside cell cause the thickened mucus lining the lungs. After more research, you find
that thickened mucus is actually a result of not enough water being secreted from the
lung cells to thin out the mucus.
Question 3: Using what you learned about osmosis as movement of water through the
membrane to reach an equilibrium with other molecules, draw a model of a Max’s cell
and that of a healthy individual. The cells should include the following:
CFTR, relative amounts of Cl- ions inside and outside of cell, relative amounts of water
molecules inside and outside of cell, direction of Cl- flow, direction of water flow and the
mucus layer showing the relative thickness. Start by drawing your models from the
previous question and then add in the water molecules, water flow and mucus layer.
Question 4: Referring to your model above, give a brief explanation to Max’s parents for
how a poorly functioning CFTR protein causes thick mucus lining their son’s lungs.
Question 5: In healthy people, the fluid outside of the lungs typically has a higher
concentration of Cl- ions than that the inside of the cell. So, in healthy people, what type
of transport is used by the CFTR (Facilitated Diffusion or Active Transport?). Explain
your answer.
Question 6: Finally, after brushing up on their college bio, Max’s parents ask the
following question. Below, provide a brief answer to their question using what you
learned in Monday’s lecture about permeability.
1. “We understand that his CFTR is messed but Cl- is such a small ion, why can’t it just
leave the cell by moving through the cell membrane. We know that other small molecules
can get through the cell membrane?”