1-33 Clinical Applications: Cystic Fibrosis and Fragile X Syndrome Flashcards
Current clinical lab tests for cystic fibrosis
detection of delta F508 mutation via PCR then run on gel to separate sequences
- linkage with intragenic markers
- whole genetic sequencing
- sanger sequencing
- pyrosequencing
pyrosequencing
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.
current clinical lab tests for fragile x
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.
premutation fragile x carryin females with show southern blot bands
4 bands. as they have methylated and unmethylated sequences on both the active and inactive X. All four sequecnes contain differnt numbers of bases
MOI of cystic fibrosis
autosomal recessive with significant allelic heterogeneity
MOI of fragile x
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
fragile x is caused by
disease caused by triplet repeat expansion
CF is characterized by
pancreatic insufficiency
Chronic lung disease (mucus dehydrated)
blockage of chloride channels produces salty sweat
what is allelic heterogeneity?
the same disease (CF) is caused by different mutations of the same gene
fragile X is the most
common cause of inherited mental retardation
presentation of fragile x
retardation, large ears, long face
99% of fragile X cases are caused by
CGG triplet repeats in non-coding, fragile region and methylation of FMR1 gene
almost all boys with CF will be
infertile due to vas deferens issue
mutations at the gene CFTR are also found in about half of patients with
congenital bilateral absence of vas deferens
Mutations of DMD?
deletions
CF mutations?
point mutations
classic cystic fibrosis is characteerized by
progressive chronic lung disease and pancreatic insufficicency
CF results from a deficiency in the
CFTR protein, which acts as a chloride ion channel in endocrine cells
the lungs in CF become colonized by..
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
one of the earliest signs of CF?
the meconium plug detected in fetal imaging
a straightforward way to detect delta F508?
run the PCR product on acylamide gel which can detect the 3bp difference in aplicon length
CF gene sequencing of a CF patient should detect
two CF mutations (homo or compound hetero)
What makes Fragile X MOI odd?
although it is X-linked and seen more often in boys, it is only seen twice as much.
carrier females
some, but not all are symptomatic
the best MOI model of fragile x
x-linked dominant with a reduced penetrance (80% in males, 30% in females)
it is thought that the disease penetrance of fragile x
is dependant on the x-inactivation pattern in the critical brain tissues
cause of FRAXA?
CGG repeat in the FMR1 gene
the expansion from premutation to mutation only occurs
during transmission from a FEMALE, but not a male, premutation carrier
inhertitance of the full mutation?
the full mutation is not inherited from premutation males, selection against sperm carryin gthe full mutation
where is the risk of having a child with FRAXA?
is in the children of of premutation carrier females and
grandchildren of premutation males.
how should you counsel a patient who is a fragile X normal transmitting male
that he has a 1/3 chance of developing FXTAS
a modest increase in size of the marker
normal transmitting male
Fragile X demonstrates
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.
triplet repeats account for the phenomenon of
anticipation
cystic fibrosis is best analyzed with…
microarray to detect deletions
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
newborn screening is not disease confirmation. a diagnostic test is needed to confirm.
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?
nope
HD is due to a _____ of function, FRAXA is due to a _____ of function
HD - gain of function
FRAXA - loss of function due to expansion of a CGG repeat.
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
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.
What is the approximate risk of sickle cell disease (SCD) in a child from a couple, both of whom have first cousins with SCD?
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
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
d
PCR of fragile x
has difficulty amplifying the full mutation and larger pre-mutations
allelic dropout can also happen
southern blot of fragile x
can detect the expansion in the high premutation and full mutation ranges until about 200 repeats
how many bands will premutation fragile x females show on southern blot?
4 - methylated and unmethylated sequences of both active and inactive X.
