Transcript CFTR and chloride channel disease

Essay marking scheme
0: No answer
2 : Fail. Very incomplete or very inaccurate answer, or answer
indicating substantial and potentially dangerous misunderstanding.
3: Borderline Fail. Poor attempt, inadequate in some respects (for
example, with inaccuracies, omissions or irrelevant information)
4: Borderline Pass: information is relevant and largely accurate, but
indicating little more than a reasonable understanding of the Core
5: Pass: answers with good (and relevant) content extending
significantly beyond the Core
Answers scoring 5 can be given up to three bonus marks :
1 mark: for a well-organised, clear and direct answer to the question,
1 or 2 marks: for the following as they may be possible according to
the question set:
the appropriate use of experimental evidence, reference to relevant
clinical matters, an integrative (as opposed to a narrow) approach, a
perceptive/original and relevant discussion.
The CF Transmembrane Conductance
Regulator protein and cystic fibrosis
Syllabus:
1.6.5 Transporter structure and function
8.2.4 Tissues of respiratory system
9.5.3 GI secretion
9.5.5 Intestinal electrolyte transport
14.7.4 Secretin control via cAMP of Cl-/HCO3- exchange
Genetics of cystic fibrosis
Cystic fibrosis (CF) commonest monogenic disease
1 in 2500 Caucasian births
1 in 15 carriers
7000 people in UK
Autosomal recessive disease
Most common (~70%) phe506del (DF508)
>1600 other mutations
The sweat gland and CF
“a child that taste salty when kissed will soon die”
Alonso y de los Ruyzes 1606
Quinton 2007 Physiology
The sweat gland and CF
Quinton 2007 Physiology
Livraghi & Randell 2010 Toxocol Path
CF is primarily a lung disease…
…but not exclusively
• Pancreas
– Exocrine
– Endocrine (CF related diabetes)
• GI tract
– Constipation due to thickened faeces
– Malabsorption of vitamin D  osteoporosis
• Liver (cirrhosis)
– Thickened bile
• Infertility in males
– vas deferens absent
See Davies et al (15th Dec, 2007) BMJ for more extensive symptom list
Davies et al 2007 BMJ
Lung disease
in CF
Life
expectancy
<40 yrs
Diagnosis of CF
All babies are given Guthrie blood spot test
• measures ↑ immunoreactive trypsinogen
• if positive  genetic screening & sweat testing
Advantages of early diagnosis:
nutritional benefits, early access to care, relatively
quick diagnosis, appropriate counselling
The CF Transmembrane
Conductance Regulator protein
CFTR 1480 amino acids
Member of ABC family
The CFTR protein
CFTR – how does it work?
Not a classical ABC transporter
cAMP regulated Cl channel
Role of ATP?
Mutations in CF
Mutations in CFTR can result in trafficking and/or
Class IV
function defects
Defective
conduction
Class III
Defective
regulation
Class V
Reduced
mRNA
and/or
protein
often due
to splicing
defects
Class II
Defective
processing
Class I
Defective protein
production
DF508 is the most common CF
mutation
DF508 is primarily a class II mutation
Accounts for ~70% of CF
Protein retained in ER and then degraded by proteosome
F508 is in NBD1, and its mutation makes the domain
less temperature sensitive
• can increase surface expression by reducing
temperature (Denning et al 1992)
• ‘correctors’
Nature, 1992
Questions
What was the rationale for looking at the effects of
temperature?
What cells were used?
What are the key results from figure 1, 2 and 4?
What is the potential clinical significance of the
study?
‘Correctors’ increase surface
expression
‘Correctors’ act as chaperones to rescue cell surface
expression
Chemical chaperones: e.g. glycerol, taurine,
can suppress protein-folding defects
Disadvantages: need high (mM) concs as nonspecific effect
More specific correctors have been found by
screening of small molecule libraries
(pharmacochaperones)
Work by enhancing folding, decreasing degradation,
increasing plasma membrane stability
‘Correctors’
2. Targeting endogenous molecular chaperones
Could try to either
- Pharmacologically decrease interaction with
chaperones that signal degradation
- Knock down chaperones with antisense RNA
Mixed/controversial results…
DF508 is also a class III mutant
Even when at the membrane, the conductance of
DF508 is lower than WT
Need potentiator (drug which increases conductance)
- act directly on CFTR
- open basolateral K+ channels
- open an alternative route for apical Cl- exit
Pharmacology currently seen as more feasable
route that gene therapy…
Difficult to target specific cells, cell turnover relatively high needing
continual treatment
UK Cystic fibrosis Gene Therapy consortium performing clinical trials
Reading primary literature
• Introduction
– What was known and not known
– Why did they do the experiment
– What is the fundamental question addressed by the experiment
• Methods Techniques
– Overview of techniques; what did they use?
– What are their limitations?
• Results
– Each figure is a mini-paper: state why they did it, what was the
technique, what the results show, what do the results mean
• Discussion
– Overall merit of paper; clinical significance of findings
– What remains to be done?