1-33 Clinical Applications: Cystic Fibrosis and Fragile X Syndrome

Question 1

Current clinical lab tests for cystic fibrosis

Answer 1

detection of delta F508 mutation via PCR then run on gel to separate sequences

• linkage with intragenic markers
• whole genetic sequencing
• sanger sequencing
• pyrosequencing

Question 2

pyrosequencing

Answer 2

based on luminscence

Pyrosequencing - produces light when nucleotide encorporated and sequence extended.

Nucleotides are dispensed one at a time in a defined order. Excess are removed before dispensing next one.

If the complement of the nucleotide dispensed is present int eh template, light is produced.

Light produced is converted into a peak (pyrogram). Height = proportional to number of nucleotides incorporated.

Question 3

current clinical lab tests for fragile x

Answer 3

PCR has difficulty amplifying the full mutation due to allelic dropout.

southern blot is an option, it detects the expansion in the high premutation and full mutation.

Question 4

premutation fragile x carryin females with show southern blot bands

Answer 4

4 bands. as they have methylated and unmethylated sequences on both the active and inactive X. All four sequecnes contain differnt numbers of bases

Question 5

MOI of cystic fibrosis

Answer 5

autosomal recessive with significant allelic heterogeneity

Question 6

MOI of fragile x

Answer 6

disease caused primary by dynamic mutations at a chromosomal fragile site

but it is best modeled as x-linked dominant disease with reduced penetrance for females

Question 7

fragile x is caused by

Answer 7

disease caused by triplet repeat expansion

Question 8

CF is characterized by

Answer 8

pancreatic insufficiency
Chronic lung disease (mucus dehydrated)

blockage of chloride channels produces salty sweat

Question 9

what is allelic heterogeneity?

Answer 9

the same disease (CF) is caused by different mutations of the same gene

Question 10

fragile X is the most

Answer 10

common cause of inherited mental retardation

Question 11

presentation of fragile x

Answer 11

retardation, large ears, long face

Question 12

99% of fragile X cases are caused by

Answer 12

CGG triplet repeats in non-coding, fragile region and methylation of FMR1 gene

Question 13

almost all boys with CF will be

Answer 13

infertile due to vas deferens issue

Question 14

mutations at the gene CFTR are also found in about half of patients with

Answer 14

congenital bilateral absence of vas deferens

Question 15

Mutations of DMD?

Answer 15

deletions

Question 16

CF mutations?

Answer 16

point mutations

Question 17

classic cystic fibrosis is characteerized by

Answer 17

progressive chronic lung disease and pancreatic insufficicency

Question 18

CF results from a deficiency in the

Answer 18

CFTR protein, which acts as a chloride ion channel in endocrine cells

Question 19

the lungs in CF become colonized by..

Answer 19

the mucus of the lungs gets thick, lungs get colonized by P.aeruginosa and other bacteria - this leads to an immune response that further degrades pulmonary function

Question 20

one of the earliest signs of CF?

Answer 20

the meconium plug detected in fetal imaging

Question 21

a straightforward way to detect delta F508?

Answer 21

run the PCR product on acylamide gel which can detect the 3bp difference in aplicon length

Question 22

CF gene sequencing of a CF patient should detect

Answer 22

two CF mutations (homo or compound hetero)

Question 23

What makes Fragile X MOI odd?

Answer 23

although it is X-linked and seen more often in boys, it is only seen twice as much.

Question 24

carrier females

Answer 24

some, but not all are symptomatic

Question 25

the best MOI model of fragile x

Answer 25

x-linked dominant with a reduced penetrance (80% in males, 30% in females)

Question 26

it is thought that the disease penetrance of fragile x

Answer 26

is dependant on the x-inactivation pattern in the critical brain tissues

Question 27

cause of FRAXA?

Answer 27

CGG repeat in the FMR1 gene

Question 28

the expansion from premutation to mutation only occurs

Answer 28

during transmission from a FEMALE, but not a male, premutation carrier

Question 29

inhertitance of the full mutation?

Answer 29

the full mutation is not inherited from premutation males, selection against sperm carryin gthe full mutation

Question 30

where is the risk of having a child with FRAXA?

Answer 30

is in the children of of premutation carrier females and

grandchildren of premutation males.

Question 31

how should you counsel a patient who is a fragile X normal transmitting male

Answer 31

that he has a 1/3 chance of developing FXTAS

Question 32

a modest increase in size of the marker

Answer 32

normal transmitting male

Question 33

Fragile X demonstrates

Answer 33

anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation.

Question 34

triplet repeats account for the phenomenon of

Answer 34

anticipation

Question 35

cystic fibrosis is best analyzed with…

Answer 35

microarray to detect deletions

Question 36

Which of the following is NOT a characteristic of newborn screening?

a. biochemical disorder
b. disease confirmation
c. easy to perform
d. high sensitivity
e. high specificity

Answer 36

newborn screening is not disease confirmation. a diagnostic test is needed to confirm.

Question 37

joyce is from population in which autosomal recessive disorder is in HW equilibrium and has a birth incidence of 1/1,300. her husband is phenotypically normal but has a newphew effected wtih the disorder. Since joyce is pregnant, what do you tell her is the risk to the fetus?

Answer 37

nope

Question 38

HD is due to a _____ of function, FRAXA is due to a _____ of function

Answer 38

HD - gain of function

FRAXA - loss of function due to expansion of a CGG repeat.

Question 39

Bernard and his pregnant wife, Joan, are phenotypically normal. Joan’s brother is affected with a rare disorder. Which of the following modes of inheritance would result in the highest risk that Joan’s fetus will manifest (i.e. be affected with) this disorder?

A. Autosomal recessive

B. Autosomal dominant.

C. X-linked benign recessive

D. X-linked dominant

E. X-linked genetic-lethal recessive

Answer 39

C. Since Joan is phenotypically normal, autosomal or X-linked dominant inheritance is unlikely. If autosomal recessive, Bernard would also have to be a carrier, which is unlikely. If X-linked recessive benign, Joan’s carrier risk is ½ and the chance that the fetus is affected would be ½ x ½ x ½ = 1/8. Due to Haldane’s Rule, if it was an X-linked lethal, the risk would be less than 1/8.

Question 40

What is the approximate risk of sickle cell disease (SCD) in a child from a couple, both of whom have first cousins with SCD?

Answer 40

Each has a 1/ 4 carrier risk, multiplied by the 1/ 4 risk they both pass it on gives 1/64.

one of grandparents has it
1/2 chance parent carries
1/4 chance they carry

1/41/41/4 = 1/64

Question 41

Match the situation described with the term that best defines it. There are over 2,000 disease-causing mutations already described for cystic fibrosis.
Selected Answer:
Incorrect

A. Locus heterogeneity

B. Anticipation

C. New mutation

D. Allelic heterogeneity

E. Non-penetrant

Answer 41

d

Question 42

PCR of fragile x

Answer 42

has difficulty amplifying the full mutation and larger pre-mutations

allelic dropout can also happen

Question 43

southern blot of fragile x

Answer 43

can detect the expansion in the high premutation and full mutation ranges until about 200 repeats

Question 44

how many bands will premutation fragile x females show on southern blot?

Answer 44

4 - methylated and unmethylated sequences of both active and inactive X.