Constitutional Cytogenetics

Question 1

Name some indications for constitutional cytogenetic studies.

Answer 1

1. Prenatal diagnosis (eg trisomies)
2. Postnatal diagnosis (abnormalities of development)
3. Recurrent infertility or miscarriages

Question 2

What specimens are suitable for constitutional cytogenetic studies?

Answer 2

Prenatal: Amniotic fluid, chorionic villi, maternal blood
POC: Fetal tissue, chorionic villi, FFPE blocks (FISH-only)
Recurrent pregnancy loss: Peripheral blood

Question 3

What mitogens are used to culture T cells? B cells?

Answer 3

T-cells: Phytohemagglutinin (PHA)

B-cells: Pokeweed extract

Question 4

What method of banding is used in the US? What does it stain? What resolution does it confer?

Answer 4

G-banding; stains AT-rich regions (gene-poor heterochromatin) darkly to a resolution of 5-10 Mb

Question 5

What is C banding?

Answer 5

Staining of constitutive heterochromatin, for example all centromeres and the heterochromatic region of Y

Question 6

What is NOR staining?

Answer 6

Staining of the “Nucleolar Organizer Regions”; essentially rRNA that is present on satellite stalks.

Question 7

What are acrocentric chromosomes? Which are they?

Answer 7

Acrocentric chromosomes only have one arm; the other comprised of “stalks” of rRNA which form the nucleolus?

13/14/15, 21/22

Question 8

What does Aneuvision detect?

Answer 8

Copy numbers of chromosomes 13, 18, 21, X, and Y.

Question 9

A prenatal specimen appears to demonstrate trisomy 21. What could this represent if not a true diagnosis?

Answer 9

Contamination by normal maternal cells, confined placental mosaicism, or chimerism.

Question 10

What are the general turn-around times for karyotypes for: Bloods, prenatal samples, and POCs/skins?

What about for FISH?

Answer 10

Bloods: 28d (7d if STAT)
Prenatals: 14d
POCs / Skin biopsies: 42d

FISH: 7-10d

Question 11

Order the most common constitutional trisomies / monosomies by their relative frequency.

Answer 11

+21 > XXY/XYY/XXX > XO > +18 > +21

Question 12

Which chromosomes are parentally silenced?

Answer 12

6 (paternal)
7 (maternal)
11 (both)
14 (both)
15 (both)
20 (both)

Question 13

Recall which form of uniparental disomy results in the following diseases:

• Beckwith-Wiedemann syndrome
• Prader-Willi syndrome
• Angelman syndrome

Answer 13

BWS: Paternal 11
PWS: Paternal 15
Angelman: Maternal 15

Question 14

Recall which form of uniparental disomy results in the following diseases:

• Transient neonatal diabetes
• Pseudohypoparathyroidism
• Silver-Russell syndrome

Answer 14

Transient neonatal DM: Paternal 6
Pseudohypoparathyroidism: Paternal 20
Silver-Russell: Maternal 7 or 11

Question 15

What is a ring chromosome?

What is a dicentric chromosome?

What is a double minute chromosome?

Answer 15

Ring: Loss of telomeres with fusion of each end.

Dicentric: A chromosome with two centromeres.

Double minute: Fragments of extrachromosomal DNA, mostly seen in tumors and conferring drug resistance.

Question 16

What is a robertsonian translocation?

Answer 16

Fusion of two acrocentric chromosomes resulting in one large metacentric or submetacentric chromosome.

Question 17

What is the difference between a pericentric and paracentric inversion?

Answer 17

Paricentric inversions involve the centromere, whereas paracentric inversions are limited to one arm.

Question 18

What is an isochromosome and how does it form?

Answer 18

Isochromosomes are structural rearrangements resulting from mis-division of the centromere, resulting in mirror long and short arms.

Question 19

What is normally responsible for X-chromosome inactivation?

Answer 19

The XIST gene, which is itself on the X chromosome, promotes trimethylation of histone H3 at Lysine 27.

Question 20

How may small deletions (eg < 5 Mb) be detected?

Answer 20

FISH or SNP microarray analysis.

Question 21

Challenge: Recall the deleted regions in the following diseases.

1. TAR syndrome
2. SOTOS syndrome
3. HNPP
4. Smith-Magenis syndrome

Answer 21

1. 1q21
2. 5q35
3. 17p12
4. 17p11.2