Cystic fibrosis

Question 1

CF carrier frequency?

Answer 1

1/25

Question 2

CF incidence?

Answer 2

1 in 2500 live births

Question 3

Where is CFTR and what size?

Answer 3

7q31.2, 27 exons

Question 4

What cells are affected in classical CF?

Answer 4

Mucus-producing cells in exocrine glands

Question 5

Organs which are affected?

Answer 5

Lungs and respiratory tract, pancreas, gut, male genital tract

Question 6

Condition which occurs in 10-20% of newborns with known CF?

Answer 6

Meconium ileus- failure to pass meconium due to “blocked” intestine

Question 7

Diagnostic requirements for CF?

Answer 7

One or more phenotypic feature + either 2 known disease causing mutations, or 2x sweat test with Cl greater than 60

Question 8

Main cause of morbidity and mortality?

Answer 8

Pulmonary disease

Question 9

Mean survival?

Answer 9

40 years (56 years if pancreatic function intact)

Question 10

Genotype phenotype correlation:

a) pancreatic sufficiency
b) pulmonary disease
b) male infertility

Answer 10

a) good
b) poor
c) good

Question 11

Strcture and function of CFTR protein?

Answer 11

Three domain intramembrane protein which regulates movement of Cl ions

Question 12

Commonest mutation in:

a) white UK
b) Ashkenazi jews

Answer 12

a) F508

b) W1282X (nonsense)

Question 13

Majority of CF mutations are what type?

Answer 13

Single nucleotide changes

Question 14

Classes of CF mutation? (1-5)

Answer 14

1- affects translation (G542X)
2- affects protein maturation (F508del)
3- affects function of protein at cell surface
4- causes a decrease in Cl conductance (R1347P)
5- cause decrease in synthesis of CFTR

Question 15

Mutation classes associated with a) classical phenotype b) milder phenotype

Answer 15

a) 1, 2, some 3

b) 4, 5, some 3

Question 16

Why are CF mutations common? (2)

Answer 16

Founder effect- loss of genetiuc variation that occurs when a new population is established by a small number of individuals from a larger population
Heterozygote advantage- when the heterozygote has a greater relative fitness than either class of homozygote

Question 17

CF-related disease which may be seen in men with CF, and in those who have mutations in CFTR gene but don’t present as classical CF

Answer 17

Congenital bilateral absence of the vas deferens (CBAVD)

Question 18

Partially-functioning allelels associated with CBAVD?

Answer 18

R117H or 5T

Question 19

What should be checked when “obstructive azoospermia” is written on the reqest form, or when R117H has been identified?

Answer 19

poly T status

Question 20

What affects the efficiency of splicing adjacent to the exon 9 acceptor splice site?

Answer 20

Tract of thymidine residues which can be 5, 7 or 9 long. 5T is least efficient- reduced production of functional CFTR protein. 9T is the common allele

Question 21

Combinations of thymidine residues and mutations:

a) F508del on one chromosome, R117H in cis with 5T
b) F508del on one chromosome, R117H in cis with 7T
c) 5T on one chromosome, 5T or CF mutation in trans
d) F508 on one chromosome, R117H in cis wit 9T

Answer 21

a) classical CF
b) no symptoms or should show CF-related disease
c) benign or could show CF related disease
d) rarely found, benign

Question 22

F508del is usually found in cis with…

Answer 22

9T

Question 23

What other site affects splicing efficiency? What is the commonest size of this?

Answer 23

Upstream of exon 8 polyT, a TG repeat tract. 11 repeats is normal

Question 24

What is the worst combination involving TG repeats?

Answer 24

12 or 13 repeats in cis with FT on one chromosome, severe CF mutation in trans

Question 25

Testing for CF?

Answer 25

ARMS

Question 26

Which CF kit can test poly T status?

Answer 26

CFEU2 (the other one is CF29)

Question 27

Mutations tested by CF29?

Answer 27

29 mutations, uses gel electrophoresis. 90% of European Caucasian mutations

Question 28

Mutations tested by CFEU2?

Answer 28

50% mutations and uses capillary electrophoresis

Question 29

What do Tube A and Tube B in CFEU2 contain?

Answer 29

Tube A- all mutation specific primers and WT F508. Primers for Poly T
Tube B- all other WT primers- only required to determine zygosity if mutation identified in Tube A and it is not F508

Question 30

Dye coloures in CFEU2:

a) WT
b) mutant
c) hypervariable markers

Answer 30

a) green
b) blue
c) red- if these are absent the test has failed, can also use as an identity check

Question 31

How is testing for rare CF mutations carried out?

Answer 31

Gene screen by means of sequecning, MLPA or linkage analysis (can put markers around cf gene to identify the high risk haplotype)

Question 32

How could false negatives/heterozygotes being wrongly called as homozygotes result from eithr CF mutation assays?

Answer 32

snp under primer binding site

Question 33

Mutation which is more prevalent in Asian populations? What is the issue with testing for this?

Answer 33

Y569D. Don’t get ethnicity details on referral cards and using surname is not an accurate method of determining ethnicity

Question 34

What age does an individual need to be in order to be tested for CF?

Answer 34

at least 16

Question 35

What is the risk of the asymptomatic sibling of a patient being a carrier?

Answer 35

2/3