Week 3: Diagnostic testing Flashcards
Differentiate screening vs diagnostic testing
Screening: identifies those at increased risk, results are high or low risk, does not rule out a genetic condition
Diagnostic: determines whether a condition is present, for sure yes or no, definitive info regarding condition
When can chorionic villus sampling be performed? How is it performed?
-Timing: 10w0d-13w6d
-Negative pressure with syringe used to aspirate small amount of placental villi
-Transcervical or transabdominal
What is the primary advantage of CVS over amniocentesis?
It can be performed earlier in gestation, time for management options
What is the risk for complications for CVS leading to miscarriage?
1:500
How often does placental mosaicism occur? How often does it occur in the placenta vs in the fetus?
1-2% of cases
When there is placental mosaicism:
10% of time occurs in the fetus, 90% of time occurs in the placenta
Amnio recommended as follow up
Confined placental mosaicism may cause fetal growth restriction (FGR)
What causes placental mosaicism?
-Trisomy rescue
-Mitotic nondisjunction
What other physical risks (not mosaicism) are related to CVS?
- Limb reduction defects: increased risk when CVS performed <10 wks
- Vaginal spotting or bleeding: may occur in up to 32% of pts after transcervical CVS
- <0.5% chance of culture failure, amniotic fluid leakage, or infection post CVS
When and how is an amniocentesis performed?
-Timing: after 15 or 16wks (depending on fusion of membrane) until end of pregnancy
-Technique for withdrawing amniotic fluid from uterine cavity with a needle guided by a ultrasound probe transabdominally
What is the risk for complications related to amniocentesis and what are the two primary risks?
1:900
Risk that complications would lead to preterm labor or miscarriage
Describe how preparing a sample using direct preparation (interphase) differs from using cell culture (metaphase)?
Direct preparation (interphase):
-cells from tissue/fluid can be used directly without culturing
-Advantage: test results available quicker
Cell culture (metaphase):
-using cells from procedure and placing in artificial environment to foster cell growth to expand cells to perform testing
-Requires additional time
A sample taken using CVS that is prepared using long term culture originates from what tissues?
Blastocyst ->
Inner cell mass -> hypoblast -> chorionmesoderm -> CVS long term culture
A sample taken using CVS and prepared using direct preparation originates from what tissues?
Blastocyst ->
Trophoblast ->
CVS direct preparation
A sample taken with amniocentesis originates from what tissue layer?
blastocyst ->
inner cell mass ->
Epiblast ->
amnionectoderm ->
amniocentesis
How does FISH work? What does a typical prenatal FISH panel include?
-FISH uses fluorescent probes to count number of select chromosomes by targeting the centromere
-Typical prenatal FISH includes chromosomes 21, 18, 13, X, Y
What type of sample preparation is used for FISH samples? What is the primary advantage of this prep method?
-Can be performed in interphase (direct)
-Allows for rapid results
T/F FISH can detect genetic mechanisms and/or other chromosome abnormalities
False!
What is a karyotype? What can it detect?
-Chromosome spread that analyzes the number of chromosomes
-Can detect structural rearrangements, large dup/del
What type of cell preparation method is used for karyotype?
-Cell culture (metaphase)
-Takes time!!
T/F karyotypes can be used to determine mechanism?
True! Important for recurrence risk!
Approximately what proportion of Down syndrome is due to de novo vs inheritance of unbalanced chromosomal segment in balanced carrier parent?
1/3 of cases due to inheriting unbalanced chromosomal segment from balanced translocation carrier parent
2/3 de novo
Recurrence risk for Down syndrome for parents who are translocation carrier is dependent on what?
The type of translocation and the sex of the carrier parent
Is the risk higher to have a child with T21 due to unbalanced translocation if the mother or father is a balanced translocation carrier?
Higher chance if mother/egg is carrier!
If a child with Down syndrome has standard T21 (47, +21/46 mosaic) is it necessarily routine to study the parents’ chromosomes? How does a child with standard T21 affect their recurrence risk?
-It is unnecessarily routine to study the parents’ chromosomes, one can assume they will type 46, XX and 46, XY
-Risk of recurrence of T21 or different aneuploidy is typically small but above that of a same age maternal population
What is the recurrence risk to have a child with T21 for a de novo translocation?
<1%
What is the risk to have a child with translocation Down syndrome when the male is the translocation carrier?
~1%
What is the risk to have a liveborn child with T21 with translocation Down syndrome when the female is the translocation carrier?
~10%
What does a microarray detect? How does the diagnostic yield compare to karyotype?
-Detects copy number variants across all chromosomes
-Per ACOG, this should be first line of testing in event of abnormal ultrasound
-Increased diagnostic yield over karyotype
What sample preparation method is used for microarray?
direct or cultured
What can’t a microarray detect?
-CANNOT detect balanced rearrangements!
-May also detect regions of homozygosity (UPD, parents are related)
-May identify CNVs that are VUSs
-Incidental findings that may not explain US findings
What are the three types of molecular testing and what is an example of indication for each?
- Known familial variant testing: parents have sickle cell trait, test pregnancy for sickle cell disease
- Panels (based on US findings): on US fetus have short long bones and abnormal bowing -> skeletal dysplasia panel
- Exome sequencing: non-specific findings with major concern for genetic syndrome
What is AF-AFP and what does it screen for?
-Screen to measure AFP in amniotic fluid (not CVS sample) to screen for ONTD
-if AFP is increased, acetylcholinesterase (AChE) is measured
-AChE is typically only present in amniotic fluid when there is an ONTD
When US is abnormal, what is the typical flow of testing?
-Often FISH performed on all samples regardless of indication
-If FISH is positive/abnormal: karyotype to find mechanism
-If FISH negative/normal: microarray
-If FISH/karyotype/microarray all normal molecular testing can be considered
