Genetics

Question 1

What are the general reasons to do a cytogenetic study?

15

Answer 1

to diagnose constitutional disorders present at birth

add further diagnostic or prognostic information to a diagnosis of an acquired disorder (usually malignancies)

Question 2

What are the different specimen types you can do cytogenetics on?

Answer 2

blood
bone marrow
products of conception
skin
solid tumors
prenatal specimens (chroionic villus, amniotic fluid)

Question 3

Why does cytogenetic testing require cells to be cultured?

Answer 3

they need to undergo mitosis

(so they add a mitogen called PHA to stimulate cells to replicate DNA while in culture - especially need to do this for lymphocytes that will not normally undergo spontaneous division)

Question 4

What drug can be used to synchronize culture division? How?

Answer 4

Methotrexate because it will block cells in S phase, allowing more cells in the culture to enter and remain there

Question 5

What drug will release the methotrexate block to allow cells to synchronously proceed to mitosis?

Answer 5

Thymidine

Question 6

What third drug is then added to prevent the synthesis of spindle fibers, thus stopping mitosis in metaphase?

Answer 6

Colcemid

Question 7

Why do we want to stop them in metaphase?

Answer 7

it’s the best time of the cell cycle to see and examine chromosomes

Question 8

In general ,what are the two types of chromosomal abnormalities?

Answer 8

numerical (too many or some missing)

structural abnormalities

Question 9

What governing body is in charge of the standardized grammatical rules for the designation of cytogenetic findings?

Answer 9

International System for Human Cyctogenetic Nomenclature (ISCN 2013)

Question 10

THe debate over miscarriage risk after amniocentesis continues. What’s the rate?

Answer 10

Depends on the study you look at. The only large randomized trial had an increased risk of 1%, a meta-analysis of >68,000 put risk at 0.6%

others found no statistically significant risk

Question 11

What does a quad screen screen for?

Answer 11

trisomy 21
trisomy 18
open neural tube defects

Question 12

What four analytes does a quad screen measure?

Answer 12

AFP
Estriol
HCG
Inhibin

Question 13

What are the ultrasound screening markers that can be used in the first trimester?

Answer 13

nuchal translucency
nasal bone
tricuspid regurg
fronto-maxillary angle

Question 14

What are the first trimester biochemistry screening markers?

Answer 14

PAPP-A
free-B hCG
total hCG
hyperglycosylated hCG
ADAM12

Question 15

Right now we have two main options: a screening test that has no risk for miscarriage, but isn’t definitive; and amnio which is definitive, but carries a risk of miscarriage.

What upcoming test may get us the best of both worlds?

Answer 15

cell-free fetal DNA in maternal plasma

Question 16

What percentage of the total free DNA in maternal plasma is from the fetus?

Answer 16

less than 10%

Question 17

How long do the fetal DNA fragments last in circulation?

Answer 17

We think a very short half-life on the order of 16 minutes

Question 18

How do you specifically analyze the fetal portion of the cell free DNA and not the maternal portion?

Answer 18

1. Look for fetal alleles that are known to be absent in mom
2. analyze mRNA of placenta-pecific genes only
3. Massively parallel sequencing (do all the free DNA withotu distinguishing fetal from maternal and identify trisomies)

Question 19

If you do the massively parallel sequencing, how do you identify trisomies? Do you care about the absolute number of reads mapping to a particular chromosome?

Answer 19

No, because that number will vary from run to run. WHat you care about is the proportion of the total reads mapping to a specific chromosomes. Chromosome 21 for example should only have about 1% of the maps, so if there’s a higher percentage, that suggests there is a trisomy. (note - these are converted to Z-scores)

Question 20

If you have a patient with a significant family hx of early breast cancer, in what situation can genetic testing definitively exclude a familial risk for this patient?

Answer 20

You would need to identify the family genetic mutation in a relative who was affected and then show that the patient did not inherit the allele.

Otherwise, the patient will need to be considered high risk regardless of the genetic test results