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Prenatal genetics > Week 3: Diagnostic testing > Flashcards

Week 3: Diagnostic testing Flashcards

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1
Q

Differentiate screening vs diagnostic testing

A

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

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2
Q

When can chorionic villus sampling be performed? How is it performed?

A

-Timing: 10w0d-13w6d
-Negative pressure with syringe used to aspirate small amount of placental villi
-Transcervical or transabdominal

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3
Q

What is the primary advantage of CVS over amniocentesis?

A

It can be performed earlier in gestation, time for management options

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4
Q

What is the risk for complications for CVS leading to miscarriage?

A

1:500

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5
Q

How often does placental mosaicism occur? How often does it occur in the placenta vs in the fetus?

A

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)

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6
Q

What causes placental mosaicism?

A

-Trisomy rescue
-Mitotic nondisjunction

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7
Q

What other physical risks (not mosaicism) are related to CVS?

A
  1. Limb reduction defects: increased risk when CVS performed <10 wks
  2. Vaginal spotting or bleeding: may occur in up to 32% of pts after transcervical CVS
  3. <0.5% chance of culture failure, amniotic fluid leakage, or infection post CVS
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8
Q

When and how is an amniocentesis performed?

A

-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

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9
Q

What is the risk for complications related to amniocentesis and what are the two primary risks?

A

1:900

Risk that complications would lead to preterm labor or miscarriage

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10
Q

Describe how preparing a sample using direct preparation (interphase) differs from using cell culture (metaphase)?

A

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

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11
Q

A sample taken using CVS that is prepared using long term culture originates from what tissues?

A

Blastocyst ->
Inner cell mass -> hypoblast -> chorionmesoderm -> CVS long term culture

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12
Q

A sample taken using CVS and prepared using direct preparation originates from what tissues?

A

Blastocyst ->
Trophoblast ->
CVS direct preparation

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13
Q

A sample taken with amniocentesis originates from what tissue layer?

A

blastocyst ->
inner cell mass ->
Epiblast ->
amnionectoderm ->
amniocentesis

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14
Q

How does FISH work? What does a typical prenatal FISH panel include?

A

-FISH uses fluorescent probes to count number of select chromosomes by targeting the centromere
-Typical prenatal FISH includes chromosomes 21, 18, 13, X, Y

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15
Q

What type of sample preparation is used for FISH samples? What is the primary advantage of this prep method?

A

-Can be performed in interphase (direct)
-Allows for rapid results

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16
Q

T/F FISH can detect genetic mechanisms and/or other chromosome abnormalities

A

False!

17
Q

What is a karyotype? What can it detect?

A

-Chromosome spread that analyzes the number of chromosomes
-Can detect structural rearrangements, large dup/del

18
Q

What type of cell preparation method is used for karyotype?

A

-Cell culture (metaphase)
-Takes time!!

19
Q

T/F karyotypes can be used to determine mechanism?

A

True! Important for recurrence risk!

20
Q

Approximately what proportion of Down syndrome is due to de novo vs inheritance of unbalanced chromosomal segment in balanced carrier parent?

A

1/3 of cases due to inheriting unbalanced chromosomal segment from balanced translocation carrier parent

2/3 de novo

21
Q

Recurrence risk for Down syndrome for parents who are translocation carrier is dependent on what?

A

The type of translocation and the sex of the carrier parent

22
Q

Is the risk higher to have a child with T21 due to unbalanced translocation if the mother or father is a balanced translocation carrier?

A

Higher chance if mother/egg is carrier!

23
Q

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?

A

-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

23
Q

What is the recurrence risk to have a child with T21 for a de novo translocation?

A

<1%

24
Q

What is the risk to have a child with translocation Down syndrome when the male is the translocation carrier?

A

~1%

24
Q

What is the risk to have a liveborn child with T21 with translocation Down syndrome when the female is the translocation carrier?

A

~10%

25
Q

What does a microarray detect? How does the diagnostic yield compare to karyotype?

A

-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

26
Q

What sample preparation method is used for microarray?

A

direct or cultured

27
Q

What can’t a microarray detect?

A

-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

28
Q

What are the three types of molecular testing and what is an example of indication for each?

A
  1. Known familial variant testing: parents have sickle cell trait, test pregnancy for sickle cell disease
  2. Panels (based on US findings): on US fetus have short long bones and abnormal bowing -> skeletal dysplasia panel
  3. Exome sequencing: non-specific findings with major concern for genetic syndrome
29
Q

What is AF-AFP and what does it screen for?

A

-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

30
Q

When US is abnormal, what is the typical flow of testing?

A

-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

Prenatal genetics (9 decks)

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