04 - Cytogenetics

Question 1

What types of sample can be taken for a Prenatal Test

Answer 1

1 - cffDNA
2 - CVS
3 - Amniotic Fluid
4 - Fetal Blood

Question 2

What tissue is take during a CV biopsy

Answer 2

• Chronic Villus
• Placental tissue made up of trophoblast + mesenchyme cells
• Villi need to be dissected to reduce risk of MCC

Question 3

what cells are found in Amniotic Fluid Sample

Answer 3

cells derived from the foetus, mostly epithelial (skin and gastrointestinal tract)

Question 4

Mosaicism can be detected prenatally. What are the types and levels which help determine between culture artefact vs real mosaicism

Answer 4

1) Level 1 - Single cell in a single culture
2) Level 2 - Abnormality found in >2 cells, from a single culture
3) Level 3 - abnormality found in >2 cells in 2 or more cultures

Level 3 most likely represents TRUE mosaicism

Question 5

Confined Placental Mosaicism is an issue in CVS samples. What proportion of mosaic autosome trisomy are actually CPM?

Answer 5

80%

Question 6

what are the 2 types of triploidy (n=69)

Answer 6

> Diandry (2x paternal contribution)

> Digyny (2x maternal contribution)

Question 7

What are the types of hydatidiform moles? what type of disease re they?

Answer 7

Hydratidiform moles are a form of gestational trophoblastic disease with a risk of malignancy

Can be with a Complete Mole or a Partial Mole:

Question 8

What is a complete Mole?

Answer 8

Complete Mole:

• 46 chr (diploid)
• androgenetic - BOTH sets of chromosomes com from paternal side
• NO fetal development
• mostly caused by fertilisation of an empty egg by either a single (80%) or two (20%) sperm
• * 15% risk of malignancy ***

Question 9

What is a partial mole?

Answer 9

Partial Mole:

• 69 chr triploid
• diandry (2x pat, 1x mat)
• some metal development
• ultrasounds = looks like bunch of grapes
• ** 1% risk of malignancy ***

Question 10

NIPT and NIPD rely on the detection of cell free fetal DNA (cffDNA). At what gestation can this be reliably detected

Answer 10

7wks

Question 11

What is the fragment size of cffDNA

Answer 11

< 300bp

Question 12

List 4 clinical applications of NIPD

Answer 12

1) Fetal Sex determination in pregnancies at high risk of an X-linked disorder (e.g. DMD) or gender-linked concern (e.g. CAH - treatment of female fetuses to prevent virilisation)
2) Diagnosis of certain singe gene disorders through the detection of a paternally inherited mutation
3) Detection of fetal aneuploidy - small but detectable change in the ratio of chromosomes (e.g. chr21).
4) Detection of high risk allele using RHDO (relative haplotype dosage analysis) - effectively linkage testing

Question 13

What is the currently test strategy for fetal structural anomalies detected in a prenatal setting

Answer 13

> Rapid testing for aneuploidy (QF-PCR or FISH)

> Microarray (replaced karyotyping)

Question 14

What is the PAGE project

Answer 14

Prenatal Assessment of Genome + Exome project.

Funded by Wellcome Trust and UK Department of Health. Aim is to explore utility of Exome + genome use in prenatal setting.

Performed Exomes (development disorder panel of 1600 genes) on 600 fetus samples in trio (following using CNV and trisomy testing)

Diagnostic genetic variant found in 8.5% cases.

Fetuses with multiple abnormalities = 15% detection rate