CF

Question 1

CFTR protein function

Answer 1

CFTR protein acts as an ion channel
Located in apical membrane of epithelial cells
Regulated transport of Chloride and Sodium

Question 2

Mutations tested for in NBS for CF

Answer 2

F508del
c.621+1G>T
R542X
G551D

Question 3

CFTR poly T tract

Answer 3

String of T’s in intron 8
Can have 5T, 7T and 9T
5T reduces efficiency of intron 8 splicing > transcripts lack exon 9

Question 4

When should polyTG tract be analysed?

Answer 4

When polyT tract shows 5T because a longer TG tract with a short polyT (ie 5T) has reduced intron 8 splicing (and thus more transcripts lacking exon 9)

Question 5

When should polyT tract be analysed?

Answer 5

CFTR related disorders = can be caused by severe CF mutation in trans with 5T allele
CBAVD = can be caused by above OR homozygosity of 5T12TG in the absence of other mutations
R117H = can make this mut pathogenic when in cis

Question 6

p.Arg117His and polyT tract

Answer 6

5T in cis with R117H is pathogenic; R117H not usually pathogenic alone
R117H/5T + CF mut = classical CF
R117H/7T + CF mut - CFTR-related illness such as CBAVD

Question 7

Why is the frequency of CF stable in the population?

Answer 7

CF carriers have selective advantage to typhoid/cholera

CF present gene frequency will be maintained if CF carriers have 2% more children than normal people

Question 8

Principle of ARMs PCR

Answer 8

Allele specific PCR. Principle is that oligonucleotide primers with 3’ mismatched residue will not function as PCR primers under specific conditions
Design primers for mut and WT

Question 9

Underlying pathogenesis of CF

Answer 9

When normally functioning CFTR is activated, chloride ions are secreted out of the cell. CFTR also inhibits epithelial sodium channel (ENaC) and less sodium is absorbed into the cell, leaving a greater combined ionic gradient to allow water to leave the cell by osmosis providing fluid for epithelial tissue secretions.
Absent or malfunctioning CFTR = less water leaves cell by osmosis, mucus becomes dehydrated, thick and sticky

Question 10

CF; common reasons for referral

Answer 10

?CF
?CF related disease
family history of CF
Azoospermia/CBAVD
Fetal echogenic bowel
Newborn screening

Question 11

Apparent homozygosity for a mut - causes

Answer 11

SNP under primer binding site for WT allele
Deletion of CFTR exon(s)
UPD7 (RSS)

Always perform biparental testing where possible to confirm homozygosity

Question 12

Frequency of 5T allele in general population

Answer 12

5%

Question 13

Therapies in CF with type 1 mutations

Answer 13

TYPE 1 = no synthesis = drugs that permit readthrough of PTC

Question 14

Therapies in CF with type 2 mutations

Answer 14

TYPE 2 = block in processing = Lumacaftor

Question 15

Therapies in CF with type 3 mutations

Answer 15

TYPE 3 = defect in channel activation = Ivacaftor (potentiator - increases functional CFTR at the membrane)

Question 16

Severity of the different classes of CF mutation

Answer 16

Types 1 (no synthesis) and 2 (block in processing) are SEVERE - classical CF
Types 3-5 are milder as some residual CFTR functionality remains