Flashcards in Genetics Deck (20)
What are the general reasons to do a cytogenetic study?
to diagnose constitutional disorders present at birth
add further diagnostic or prognostic information to a diagnosis of an acquired disorder (usually malignancies)
What are the different specimen types you can do cytogenetics on?
products of conception
prenatal specimens (chroionic villus, amniotic fluid)
Why does cytogenetic testing require cells to be cultured?
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)
What drug can be used to synchronize culture division? How?
Methotrexate because it will block cells in S phase, allowing more cells in the culture to enter and remain there
What drug will release the methotrexate block to allow cells to synchronously proceed to mitosis?
What third drug is then added to prevent the synthesis of spindle fibers, thus stopping mitosis in metaphase?
Why do we want to stop them in metaphase?
it's the best time of the cell cycle to see and examine chromosomes
In general ,what are the two types of chromosomal abnormalities?
numerical (too many or some missing)
What governing body is in charge of the standardized grammatical rules for the designation of cytogenetic findings?
International System for Human Cyctogenetic Nomenclature (ISCN 2013)
THe debate over miscarriage risk after amniocentesis continues. What's the rate?
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
What does a quad screen screen for?
open neural tube defects
What four analytes does a quad screen measure?
What are the ultrasound screening markers that can be used in the first trimester?
What are the first trimester biochemistry screening markers?
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?
cell-free fetal DNA in maternal plasma
What percentage of the total free DNA in maternal plasma is from the fetus?
less than 10%
How long do the fetal DNA fragments last in circulation?
We think a very short half-life on the order of 16 minutes
How do you specifically analyze the fetal portion of the cell free DNA and not the maternal portion?
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)
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?
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)