Cystic Fibrosis

Question 1

Define a CF class 1 mutation with example

Answer 1

Biosynthesis of CFTR affected. mRNA processing interrupted by premature translation or splicing. eg Gly542X

Question 2

Define a CF class 2 mutation with example

Answer 2

Incorrect folding. Protein destroyed sometimes by ER-associated degradation pathway thereby reducing amount of CFTR at cell surface eg F508del

Question 3

Define a CF class 3 mutation with example

Answer 3

Affects chloride channel gating. CFTR improperly activated as mutations affect binding and hydroylsis of ATP or phosphrylation of the R-domain eg Gly551Asp

Question 4

Define a CF class 4 mutation with example

Answer 4

Defective conduction through pore. CFTR does not allow prorper chloride flux due to defective conduction through the pore eg R117h

Question 5

Define a CF class 5 mutation

Answer 5

Affects regulation of other ion channels eg ENaC

Question 6

Name some clinical features of CF

Answer 6

CFPANCREAS
Chronic cough and wheezing
Failure to thrive
Pancreatic insufficiency (symptoms of malabsorption like steatorrhea)
Alkalosis and hypotonic dehydration
Neonatal intestinal obstruction (meconium ileus)/ Nasal polyps
Clubbing of fingers/ Chest radiograph with characteristic changes
Rectal prolapse
Electrolyte elevation in sweat, salty skin
Absence or congenital atresia of vas deferens
Sputum with Staph or Pseudomonas (mucoid)

Question 7

Name clinical feaures of CFTR-RD

Answer 7

infertility CBAVD, bronchiectasis, idiopathic pancreatitis

Question 8

What is the pathophysiology of CF

Answer 8

hyperviscous mucous blocks membrane clinical picture similar to chronic obstructive airways disease, exocrine pancreas produces digestive enzyes with reduced volume, become activated before reaching duodenum causing inflammation and damage, malabsorption of fats,

Question 9

What is the function of CFTR

Answer 9

•CFTR is a cyclic AMP-activated chloride channel located in the plasma membrane of secretory epithelial cells in the kidney, pancreas, intestine, heart, lungs, vas deferens and sweat ducts.Transmembrane chloride channel. Chloride OUT, ENaC also inhibited by CFTR and less sodium absorbed by cell. Sodium chloride builds up outside cell, water leaves providing fluid for epithelial tissue secretion. In CF these are hyperviscous.

Question 10

What is the structure of CFTR>?

Answer 10

Has 12 transmembrane domains. cyclic-AMP - increasing levels of c-AMP inside cell triggers activation of protein kinase A which binds the phosphorylation site on the regulatory (R-domain) thus opening the channel. It draaws intr and extracellular annions towards positively charged transmembrane domains inside the channel.

Question 11

Describe fetal echogenic bowel

Answer 11

occurs in 0.2-1.8% pregnanices 3% due to CF. Intestinal obstruction due to abnormal meconium. Grade 2 or above CF 2-4% of CF referrals. If one parent carrier risk to fetus 1/7 assuming incidence of 2% for CF in FEB and mutation sensitivity 90%.

Question 12

Why might you get apparent homozygosity in CF testing if not real?

Answer 12

Confirm in both parents, rare DNA variant under probe or deletion on other allele, cross reactivity, UPD7.

Question 13

what is the poly T tract significance

Answer 13

Intron 8 poly T tract significant in infertility CBAVD patients. c.1210_12T (5_9). Adjacent to CFTR exon 9 splice acceptor site affects the efficiency of the splice site. 5T is least efficient and therefore high levels of RNA lacking exon 9 leading to reduced production of functional protein.

Question 14

What is the significance of 5T and R117H?

Answer 14

5T modifies the expression of R117H when in cis. R117H more variable and can be benign, R117H/9T very rare and benign.

Question 15

What polyT tract is F508del in strong linkage disequilibrium with?

Answer 15

9T

Question 16

What is the clinical outcome of R117H/5T and another known mutation?

Answer 16

CF variable severity

Question 17

What is the clinical outcome of R117H/7T and another known mutation?

Answer 17

CFTR-related disease, maybe benign.

Question 18

What is the clinical outcome of R117H/7T and 5T?

Answer 18

Maybe CFTR-related disease, maybe benign.

Question 19

What is the clinical outcome of 5T and known mutation?

Answer 19

Maybe CFTR-related disease, maybe benign.

Question 20

What is the clinical outcome of 5T and 5T?

Answer 20

maybe CFTR-RD more likely benign

Question 21

what is the significance of Intron 8 polyTG tract?

Answer 21

9-13 repeats. Increasing length of intron 8 polyTG has an additional impact on efficiency of exon 9 splicing. 11 most common. TG12, TG13 in cis with 5T = increased skipping of exon 9. TG12 or 13/5T and severe mutation more likely to have CFTR-RD.

Question 22

What are the CFTR protein domains?

Answer 22

CFTR protein domains: 2 transmembrane domains (TM1, TM2), 2 nucleotide binding domains (NBD1, NBD2), a regulatory domain (R) that is phosphorylated by protein kinase C, and several protein-protein interaction domains in the C-terminus (a PDZ domain and 2 endocytic motifs) involved in assembly of the signalling complex.

Question 23

What is IRT

Answer 23

(immunoreactive trypsinogen (IRT) is an inactive precursor to the proteolytic enzyme trypsin. Produced in the pancreas and transported to the small intestine where it is converted to trypsin in the intestinal mucosa. In CF patients mucous plugs can block the pancreatic ducts, preventing trypsinogen reaching the intestine. Newborns with CF may have raised levels of trypsinogen in their blood for several months). Blood spots are taken on Guthrie cards at day 5 and analysed for the presence of immunoreactive trypsinogen (IRT) – if levels of IRT are observed to be above the 99.5th centile, this can be indicative of CF

Question 24

What is the CF sweat test?

Answer 24

Sweat test (quantitative pilocarpine iontophoresis sweat chloride) –Gold standard test for confirmation of diagnosis. Individuals with CF will lose more Cl- in sweat than unaffected people. Two abnormal readings above 60 meq/l are indicative of CF and more than 90% of CF patients will produce abnormal sweat chloride readings.

Question 25

Describe the variable clinical phenotypes that arise due to mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR). What are the molecular determinants of phenotypic variability due to these mutations?

Answer 25

KLJ