Week 3: Diagnostic testing

Question 1

Differentiate screening vs diagnostic testing

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What is the primary advantage of CVS over amniocentesis?

Answer 3

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

Question 4

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

Answer 4

1:500

Question 5

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

Answer 5

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)

Question 6

What causes placental mosaicism?

Answer 6

-Trisomy rescue
-Mitotic nondisjunction

Question 7

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

Answer 7

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

Question 8

When and how is an amniocentesis performed?

Answer 8

-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

Question 9

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

Answer 9

1:900

Risk that complications would lead to preterm labor or miscarriage

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

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

Answer 12

Blastocyst ->
Trophoblast ->
CVS direct preparation

Question 13

A sample taken with amniocentesis originates from what tissue layer?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

False!

Question 17

What is a karyotype? What can it detect?

Answer 17

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

Question 18

What type of cell preparation method is used for karyotype?

Answer 18

-Cell culture (metaphase)
-Takes time!!

Question 19

T/F karyotypes can be used to determine mechanism?

Answer 19

True! Important for recurrence risk!

Question 20

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

Answer 20

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

2/3 de novo

Question 21

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

Answer 21

The type of translocation and the sex of the carrier parent

Question 22

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

Answer 22

Higher chance if mother/egg is carrier!

Question 23

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?

Answer 23

-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

Question 23

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

Answer 23

<1%

Question 24

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

Answer 24

~1%

Question 24

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

Answer 24

~10%

Question 25

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

Answer 25

-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

Question 26

What sample preparation method is used for microarray?

Answer 26

direct or cultured

Question 27

What can't a microarray detect?

Answer 27

-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

Question 28

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

Answer 28

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

Question 29

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

Answer 29

-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

Question 30

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

Answer 30

-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